WO2021150169A1

WO2021150169A1 - Catalytic sorbent material for chemical looping combustion and adsorption of impurities

Info

Publication number: WO2021150169A1
Application number: PCT/SG2021/050033
Authority: WO
Inventors: Haiming Wang; Andrei VEKSHA; Xiaomin Dou; Grzegorz LISAK; Guicai LIU
Original assignee: Nanyang Technological University
Priority date: 2020-01-22
Filing date: 2021-01-22
Publication date: 2021-07-29

Abstract

Disclosed herein is a catalytic sorbent material, comprising an oxygen carrier material having a surface and comprising a transition metal compound and a sorbent material comprising Al and Ba having a molar ratio of Al:Ba of from more than 0:1 to 12:1, where the sorbent material is dispersed on the surface of the oxygen carrier material. Also disclosed herein is a method of manufacturing said material and its use in industrial processes.

Description

Catalytic Sorbent Material for Chemical Looping Combustion and Adsorption of

Impurities

Field of Invention

The current invention relates to a catalytic sorbent material suitable for use in chemical looping combustion and the adsorption of impurities in a flue gas. The invention also relates to methods of making said material and its uses.

Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

The enhancement of living condition and development of industries has led to continuous and rapid rise in the production of municipal solid waste (MSW) which has become a worldwide problem. Gasification is one of the most efficient technologies for the reduction and waste-to- energy conversion of MSW. Through gasification, MSW can be converted into syngas mainly composed of H₂ and CO and the generated syngas in these processes can be utilized in advanced combustion systems to achieve higher energy efficiency with lower carbon footprint per unit electricity generated. However, the MSW-derived syngas contains impurities like tar and Cl-containing gases, which increases the risk of dioxin production. At the same time, the presence of HCI in the syngas could also cause hot corrosion of the boiler in combustion and decrease its stability and working life.

Chemical looping combustion (CLC) is a widely studied energy-efficient carbon capture technology that could address the problems associated with impure MSW syngas due to its ability to combust fuel in the absence of air. Unlike traditional combustion, CLC generally utilizes solid oxygen carriers (OCs) instead of air. The solid OCs are circulated between two reactors, a Fuel Reactor (FR) and an Air Reactor (AR), to separate the combustion into two stages, namely OCs supported combustion process in the FR and OCs regeneration process in the AR. CLC is operated at a temperature range of 800 - 1200 °C which allows direct introduction of generated raw syngas to FR for combustion with minimal energy penalty. At the same time, the hot air generated by exothermic AR is free of contaminants and thus is suitable for effective heat exchange to raise the pressure of generated steam for efficient power generation. As a result, CLC can achieve simultaneous flue gas combustion for power generation and carbon capture.

In the FR, fuels are fed and converted to CO₂ and H₂O by taking the lattice oxygen of the OCs. The reduced OCs are then transferred into the AR for oxygen replenishing in the atmosphere of air back to their initial states. Two streams of the flue gas are generated in this process. One is composed of mostly N₂ and residual O₂ coming out of the AR, and the other is comprised of CO₂ and H₂O coming out of the FR. After H₂O removal from the latter gas stream, pure CO₂ can be obtained, making CLC an effective carbon capture technique. Furthermore, since direct contact between fuel and air is avoided in the FR, energy-intensive downstream gas-gas separation of the flue gas for carbon capture is not required.

OCs play a vital role in the CLC process and their reactivity and oxygen carrying capacity are key parameters in evaluating their suitability for CLC process. High reactivity of the OCs is desired to achieve high combustion efficiency and reduce operation cost of the CLC system. Typically, metal oxides are used as OCs to provide the lattice oxygen for complete fuel combustion in the FR. However, the use of pure metal oxides is limited by poor fluidization ability, high sintering tendency, low mechanical strength, and/or low reactivity. Thus, metal oxides are often supported on inert materials and sometimes incorporated with promoters to improve their activity.

As mentioned earlier, the efficient utilization of generated MSW syngas is limited by high concentrations of impurities such as HCI, H₂S and alkali chlorides in the syngas which require purification by low temperature gas scrubbing systems. Furthermore, impurities such as HCI may cause deactivation of the OC catalyst and thus limit the application of the catalyst. However, it is possible to load the OCs with some sorbents to realise the removal of HCI in the syngas, and solve the problem of boiler corrosion. In particular, OCs that have high reactivity for chemical looping processes and the ability to remove HCI to purify the gas stream in situ are desirable. However, existing OCs are generally regarded as one-time used consumables, prone to losing its dichlorination function in multiple cycles of CLC and incapable of removing HCI at high temperatures. Therefore, there exists a need to develop suitable, low-cost OC catalysts that can simultaneously carry out efficient CLC and removal of HCI at high temperatures. Summary of Invention

Aspects and embodiments of the invention will now be described by reference to the following numbered clauses.

1. A catalytic sorbent material, comprising: an oxygen carrier material having a surface and comprising a transition metal compound; and a sorbent material comprising Al and Ba having a molar ratio of AI:Ba of from more than 0:1 to 12:1, wherein the sorbent material is dispersed on the surface of the oxygen carrier material.

2. The catalytic sorbent material according to Clause 1, wherein the molar ratio of AI:Ba in the sorbent material is from 0.5:1 to 12:1 or from 2:1 to 12:1.

3. The catalytic sorbent material according to Clause 2, wherein the molar ratio of AI:Ba in the sorbent material is from 2:1 to 8:1, such as from 4:1 to 8:1.

4. The catalytic sorbent material according to any one of the preceding clauses, wherein the sorbent material forms a layer on the surface of the oxygen carrier material.

5. The catalytic sorbent material according to any one of the preceding clauses, wherein the oxygen carrier material is in the form of particles having a surface and the sorbent material forms a coating on the surface of the oxygen carrier material.

6. The catalytic sorbent material according to Clause 5, wherein the coating of the sorbent material on the surface of the oxygen carrier material is from 50 to 1000 nm thick, such as from 60 to 500 nm thick, such as from 75 to 250 nm thick, such as about 100 nm thick.

7. The catalytic sorbent material according to Clause 5 or Clause 6, wherein the coating of the sorbent material on the surface of the oxygen carrier material comprises both nanorods and nanoplates.

8. The catalytic sorbent material according to any one of the preceding clauses, wherein the transition metal in the transition metal compound is selected from one or more of the group consisting of Fe, Ni, Cu, Mn, and Co. 9. The catalytic sorbent material according to any one of the preceding clauses, wherein the transition metal compound is a transition metal oxide.

10. The catalytic sorbent material according to Clause 9, wherein the transition metal oxide is selected from one or more of the group consisting of Fe₂O₃, NiO, CuO, Mn₂O₃, and CO₃O₄, optionally wherein, the transition metal oxide is Fe₂O₃ or a combination ofFe₂O₃ and CuO (e.g. in a ratio of CuO to Fe203 of from 4:6 to 0.1:1).

11. The catalytic sorbent material according to any one of the preceding clauses, wherein the sorbent further comprises a transition metal present in the oxygen carrier material, optionally wherein the transition metal is selected from one or more of the group consisting of Fe, Ni, Cu, Mn, and Co.

12. The catalytic sorbent material according to any one of the preceding clauses, wherein: the oxygen carrier material forms from 50 to 98 wt%; and the sorbent material forms from 2 to 50 wt% of the catalytic sorbent material, optionally wherein: the oxygen carrier material forms from 70 to 90 wt% (e.g. from 80 to 90 wt%); and the sorbent material forms from 10 to 30 wt% (e.g. from 10 to 20 wt%) of the catalytic sorbent material.

13. The catalytic sorbent material according to any one of the preceding clauses, wherein the catalytic sorbent material is in the form of particles having an average particle size of from 63 to 250 μm, such as from 150 to 212 μm.

14. The catalytic sorbent material according to any one of the preceding clauses, wherein the catalytic sorbent material self-activates during a chemical looping combustion process, optionally wherein the catalytic sorbent material self-activates during a redox reaction in the chemical looping combustion process.

15. The catalytic sorbent material according to any one of the preceding clauses, wherein the catalytic sorbent is thermally stable at an operating temperature of from 700 to 1,100 °C in a chemical looping process.

16. A method of forming a catalytic sorbent material, comprising the steps of:

(A) providing an oxygen carrier material having a surface and comprising a transition metal compound that is coated with a precursor material comprising Al and Ba having a molar ratio of AI:Ba of from more than 0:1 to 12:1 , such as from 0.5:1 to 12:1, such that the precursor material is dispersed on the surface of the oxygen carrier material; and (B) subjecting the oxygen carrier material coated with a precursor material to calcination for a period of time to form a catalytic sorbent material.

17. The method according to Clause 16, wherein the oxygen carrier material coated with a precursor material is formed by coating an oxygen carrier material having a surface and comprising a transition metal compound with a precursor material comprising Al and Ba having a molar ratio of AI:Ba of from more than 0:1 to 12:1, such as from 0.5:1 to 12:1 , such that the precursor material is dispersed on the surface of the oxygen carrier material.

18. The method according to Clause 17, wherein the coating is accomplished using one or more of physical mixing, wet impregnation, and co-precipitation.

19. The method according to any one of Clauses 16 to 18, wherein the precursor material comprising Al and Ba is formed by subjecting an aqueous solution comprising barium nitrate, aluminium nitrate and a base to a temperature of from 80 to 150°C (e.g. 100°C) for a period of time in a pressure resistant vessel (e.g. an autoclave).

20. The method according to Clause 19, wherein the base is ammonium carbonate or urea.

21. The method according to any one of Clauses 16 to 20, wherein the calcination step is conducted at a temperature of from 950 to 1100 °C for a period of from 5 to 12 hours.

22. The method according to any one of Clauses 16 to 21, wherein the catalytic sorbent material is a material as described in any one of Clauses 1 to 15.

23. A method of chemical looping combustion, which comprises the step of supplying a catalytic sorbent material according to any one of Clauses 1 to 15 as a bed material and running a chemical looping combustion process where a fuel is combusted.

24. The method according to Clause 23, wherein the fuel is selected from one or more of the group consisting of syngas, natural gas, coal, biomass, or combustible solid waste.

25. The method according to Clause 24, wherein the fuel is syngas. 26. The method according to any one of Clauses 23 to 25, wherein the catalytic sorbent material removes an impurity from the fuel, optionally wherein the impurity is selected from one or more of the group consisting of HCI, H₂S and alkali chlorides.

27. The method according to Clause 26, wherein the impurity is HCI.

28. The method according to any one of Clauses 23 to 27, wherein the chemical looping combustion is run at a temperature of from 700 to 1,100 °C.

29. A method for regenerating a catalytic sorbent material according to any one of Clauses 1 to 15 after it has been used in a chemical looping combustion process, the regeneration comprising subjecting a catalytic sorbent material according to any one of Clauses 1 to 15 that has been used in a chemical looping combustion process to an atmosphere comprising water and carbon dioxide and a temperature of from 800 to 1,100 °C for a period of time to regenerate the catalytic sorbent material.

Drawings

Figure 1 depicts the XRD pattern for the fresh IO and IO-10BA.

Figure 2 shows the FESEM images for fresh IO (A-1 to A-3) and IO-10BA (B-1 to B-4). A-3 and B-3 are the cross-section images of IO and IO-10BA, respectively.

Figure 3 illustrates EDS mapping of iron ore loaded with 10 wt% of BaAI₂O₄.

Figure 4 illustrates the XPS spectra of fresh IO and IO-BA before and after 20 cycles (a) Fe 2p; and (b) O 1s.

Figure 5 illustrates a schematic diagram of batch fluidized bed (bFB) reactor experimental setup.

Figure 6 shows the combustion efficiency for H₂ and CO with iron ore modified by 10% of BaAI₂O₄ at different temperatures, (a) 900 °C; (b) 800 °C; (c) 700 °C; and (d) combustion efficiency of the 20^th cycle at different temperatures. The error bars represent one standard error. Figure 7 (a)-(c) shows the variation of the combustion efficiency against reaction time in 20^th CLC cycle at the temperature of (a) 900 °C; (b) 800 °C; (c) 700 °C; (d) the equilibrium constant for iron ore calculated with Eq. (1); (e) the combustion efficiency against reaction time in 10^th CLC cycle at the temperature of 900 °C; and (f) oxygen content against reaction time in 10^th CLC cycle at the reaction temperature of 900 °C.

Figure 8 shows the combustion efficiency for (a) H₂ and (b) CO with iron ore modified by different contents of barium aluminate at the temperature of 900 °C.

Figure 9 illustrates oxygen transport ability, Ω, of different OCs within 20 cycles at the temperature of 900 °C.

Figure 10 shows the combustion efficiency for 50 cycles at a temperature of 900 °C.

Figure 11 illustrates the average oxygen transport ability, Ω, of 20 redox cycles for different OCs at different temperatures. (For IO-900 and IO-10BA-900, the average was taken for 50 cycles).

Figure 12 depicts the volume variation of the OCs after 50 cycles of redox reaction.

Figure 13 shows the FESEM-EDS images of OCs before and after 50 redox cycles. A1-A4: IO after 50 cycles; A3.1-A3.2: elements mapping for A3; B1-B4: IO-10BA after 50 cycles; B3.1- B3.4: elements mapping for B3. The cross section morphology of the IO are shown in A-3 and A-4; the cross section morphology of the IO-10BA are shown in B-3 and B-4.

Figure 14 depicts the XRD pattern of the oxygen carriers before and after redox reactions.

Figure 15 shows TGA and H₂-TPR data, (a) weight loss, X; (b) weight loss rate, x, for IO and IO-10BA before and after redox reactions; (c) weight loss ratio of fresh BaAI₂O₄ powder; and (d) H₂-TPR patterns.

Figure 16 shows SEM imaging of samples at a magnification of x20'000.

Figure 17 illustrates the fluidized bed reactor setup for syngas combustion/dechlorination and sorbent regeneration: 1-HCI solution; 2-mass flow controller; 3-transfer heating line; 4- pressure meter; 5-furnace; 6-reactor; 7-thermocouple; 8-OC sample; 9-control box; 10- impinger; 11- cooling bath; 12-gas sampling bag; 13-HCI sampling syringe. Figure 18 shows the gas concentration variation of H₂, CO, and CO₂ during the 60 min reaction in the presence of 500 ppm HCI.

Figure 19 shows HCI outlet concentration of the samples with lO- BaO ·AI₂O₃ compared to the baseline (i.e. without any catalysts used).

Figure 20 shows the gas combustion efficiency of (a) H₂ and (b) CO for lO- BaO ·AI₂O₃ prepared by two different methods.

Figure 21 shows the HCI outlet concentration during the CLC of syngas when using IO- BaO ·AI₂O₃ prepared by two different methods.

Figure 22 illustrates the experimental setup for real syngas.

Figure 23 depicts the real MSW syngas composition: 20-25 vol% H₂O, 40-50 vol% N₂, 5-11 vol% CO, 5-13 vol% H₂, 1-2 vol% CH4, 10-12 vol% CO₂.

Figure 24 shows the performance of (a) syngas combustion; and (b) HCI removal.

Figure 25 shows (a-b): CO and H₂ combustion efficiencies of catalytic sorbents in 8 cycles; (c-d) CO and H₂ concentrations of flue gas with catalytic sorbents in 8 cycles; and (e) HCI concentrations of flue gas during 8 cycles.

Figure 26 shows the FESEM images and EDS spectrum of fresh OCs.

Figure 27 depicts the XRD patterns of OCs loading with different sorbents.

Figure 28 depicts the XPS patterns of fresh OCs: (a) Fe 2p; (b) Ba 3d; (c) C 1s; and (d) 0 1s. Figure 29 depicts the XRD pattern of Ba-AI-based sorbent (BA0.5).

Figure 30 depicts the TGA results of CO oxidation with different oxygen carriers: (a-b) TG; (c- d) DTG.

Figure 31 shows the syngas conversion with different OCs: (a) CO; (b) H₂ and (c) CO2 production. Figure 32 shows HCI release concentration as a function of time at sorbent regeneration process.

Figure 33 shows the HCI removal efficiency at the beginning of 30 and 60 min while using different OCs.

Figure 34 shows the DQ for the reaction: AI2O3 + 6HCI → 2AICl₃ + 3H₂O.

Figure 35 depicts the XPS core spectra of IO-Ba0.5 after reaction: (a) Ba 3d; and (b) Cl 2p. Figure 36 shows the HCI breakthrough capacities of OCs and sorbent conversion.

Figure 37 shows the IO-Ba0.5 regeneration: (a) HCI desorption curves and (b) regeneration efficiency.

Figure 38 shows the FESEM images and EDS spectra of IO-BA0.5 after regeneration.

Figure 39 depicts the XRD patterns of IO-Ba0.5 after reduction and regeneration (1000°C).

Figure 40 depicts TGA data: (a) weight loss ratio, X, and (b) weight loss rate, x, for IO, IO- 10BA, and IO-10MA.

Figure 41 shows the comparison of the (a) H₂ and (b) CO combustion efficiency for IO, IO- 10BA, and IO-10MA.

Figure 42 shows the outlet HCI concentration for IO, IO-10BA, and IO-10MA.

Figure 43 depicts the TGA data: (a) weight loss ratio, X; and (b) weight loss rate, x, for 50Fe- 50BA and BaFe₅Al₇O₁₉.

Figure 44 shows the comparison of the (a) H₂ and (b) CO combustion efficiency for 50Fe- 50BA and BaFe₅Al₇O₁₉.

Figure 45 shows the outlet HCI concentration for 50Fe-50BA and BaFe₅Al₇O₁₉. Description

It has been surprisingly found that a composite material that combines an oxygen carrier material with a suitable sorbent material distributed on the surface of the oxygen carrier material can overcome one or more of the problems discussed hereinbefore, making it particularly suitable for use in chemical looping combustion. For example, natural ores containing transition metal oxides may be coated with a limited amount of an Al-Ba compound, which has a molar ratio of AI:Ba in the range of from more than 0:1 to 12:1 to improve their chemical looping combustion performance and equip the resulting catalyst with the ability to remove HCI at high temperature at the same time. Thus, in a first aspect of the invention, there is provided a catalytic sorbent material, comprising: an oxygen carrier material having a surface and comprising a transition metal compound; and a sorbent material comprising Al and Ba having a molar ratio of AI:Ba of from more than 0:1 to 12:1, wherein the sorbent material is dispersed on the surface of the oxygen carrier material.

In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of’ or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of’ or the phrase “consists essentially of’ or synonyms thereof and vice versa.

The phrase, “consists essentially of” and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 80% pure, greater than or equal to 85% pure, greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, reference to “an oxygen carrier” includes mixtures of two or more such oxygen carriers, reference to “the catalyst” includes mixtures of two or more such catalysts, and the like.

In conventional combustion, the chemical energy stored inside fuels such as coal has been utilized by combustion with oxygen with carbon dioxide and water as products. Similar reactions can be carried out if instead of oxygen, an oxygen carrier is used. Metal oxides such as an iron oxide can act as suitable oxygen carriers. However, unlike combustion of fuel with air, there is a relatively pure sequestration-ready carbon dioxide stream produced on combustion with the metal oxide carrier. The reduced form of the metal oxide (e.g. the metal) can then be reacted with air in a separate reaction chamber to liberate heat to produce electricity or reacted with water to form a relatively pure stream of hydrogen, which can then be used for a variety of purposes.

Thus, in embodiments of the invention that may be mentioned herein, the transition metal in the transition metal compound may be selected from one or more of the group consisting of Fe, Ni, Cu, Mn, and Co. For example, the transition metal compound may be a transition metal oxide, such as a transition metal oxide.

Particular transition metal compounds (e.g. in the form of transition metal oxides) that may be mentioned herein include, but are not limited to, Fe₂O₃, NiO, CuO, Mn₂O₃, and Co₃O₄ and combinations thereof. As will be appreciated, any suitable combination of these transition metal compounds may be presented. As an example, the transition metal compound may be a combination of Fe₂O₃ and CuO. Any suitable ratio of CuO to Fe₂O₃ may be used, but as an example, the ratio may be from 4:6 to 0.1:1. In particular embodiments of the invention that may be mentioned herein, the transition metal compound/oxide may be Fe₂O₃.

As noted above, the sorbent material may be used in chemical looping combustion and so it may also contain the transition metal present in the oxygen carrier material (i.e. the transition metal perse may be present as part of the sorbent material). This may be in the form of an impurity within the initial material used as the sorbent material, or it may be as a result of reaction between the original sorbent material and the oxygen carrier material during chemical looping combustion. For example, in embodiments of the invention that may be mentioned herein, the initial sorbent material may be BaAI₂O₄ and the oxygen carrier material may be Fe₂O_3. Following the chemical looping combustion process, the resulting sorbent material may comprise BaCO₃ and BaAIFe₁₁O₁₉. Without wishing to be bound by theory, it is believed that one or both of BaCO₃ and BaAIFe₁₁O₁₉ may help to remove impurities (e.g. HCI) from a flue gas used in the chemical looping combustion process. As will be appreciated, the transition metal may be present in the oxygen carrier material itself (e.g. as an impurity) or it may be present as the result of chemical looping combustion, resulting in some of the original transition metal oxide being reduced to the transition metal perse. Thus, the catalytic sorbent material may further comprise a transition metal present in the oxygen carrier material. For example, if the oxygen carrier material is formed from oxides of one or more of Fe, Ni, Cu, Mn, and Co, then these metals may be present in the oxygen carrier material too.

When the catalytic sorbent material is initially formed, the transition metal oxide may be in its fully oxidised form. However, after undergoing chemical looping combustion (or other processes), a portion of the transition metal oxide may be fully (as discussed above) or partially reduced. Therefore, there may also be present traces of other oxides of the transition metals mentioned above in the oxygen carrier - either as the result of small trace impurities within the oxygen carrier material itself, or as a result of the chemical looping combustion process.

It is noted that the oxygen carrier material may be provided as a natural ore of the transition metal that is used. As such, the oxygen carrier material may consist essentially of the transition metal oxides (and transition metals perse that may be present in the ore naturally or following reduction in chemical looping combustion), but it may also contain other materials that are typically found as part of that ore. As mentioned above, these impurities may form 20% or less (e.g. 15% or less) of the weight of the oxygen carrier material.

The oxygen carrier material may be provided in any suitable form. A suitable form that may be mentioned herein is where the oxygen carrier material may be provided in particulate form. For example, the oxygen carrier material may be provided as particles (e.g. crushed particles) having a size of from 100 to 500 μm, such as from 125 to 350 μm, such as from 150 to 250 μm. The degree of uniformity of the size range of the particles may be achieved by sieving the particles through sieves that exclude particles that are larger and/or smaller than the desired particle size range. However, it will be appreciated that such sieves may still allow particles that are slightly larger (or smaller) to be retained in the final sieved product.

As noted above, the sorbent material is a material that comprises aluminium (Al) and barium (Ba) in a molar ratio of from more than 0:1 to 12:1. In particular embodiments that may be mentioned herein, the molar ratio of AI:Ba in the sorbent material may be from 0.5:1 to 12:1 or from 2:1 to 12:1. In additional or alternative embodiments that may be mentioned herein, the molar ratio of AI:Ba in the sorbent material may be from 2:1 to 8:1, such as from 4:1 to 8:1.

For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, that the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges.

Thus, there is disclosed herein a molar ratio of AI:Ba of: from more than 0:1 to 0.5:1, from more than 0:1 to 2:1, from more than 0:1 to 4:1, from more than 0:1 to 8:1 , from more than 0:1 to 12:1; from 0.5:1 to 2:1, from 0.5:1 to 4:1 , from 0.5:1 to 8:1, from 0.5:1 to 12:1; from 2:1 to 4:1, from 2:1 to 8:1, from 2:1 to 12:1; from 4:1 to 8:1, from 4:1 to 12:1; and from 8:1 to 12:1.

As noted above, the sorbent material may be dispersed on the surface of the oxygen carrier material. This may be a dispersion where parts of the surface of the carrier material remain exposed, or it may be a dispersion where the entire surface of the carrier material has been covered by the sorbent material. For example, the sorbent material may form a layer on the surface of the oxygen carrier material. In embodiments of the invention, “coating” may be used to refer to a situation where the surface is partially covered by the dispersion or to a situation where the surface is fully coated. In the latter case, this may be specified by describing the coating as a “coating layer”.

As noted above, the oxygen carrier material may be provided in the form of particles. Each of these particles has a surface and the sorbent material may form a coating (e.g. a coating layer) on the surface of the oxygen carrier material. This coating may have any suitable thickness. For example, the coating may have a thickness of from 50 to 1000 nm, such as from as from 60 to 500 nm, such as from 75 to 250 nm, such as about 100 nm. It will be appreciated that the layer of the sorbent material referred to above may also have the same thicknesses. The coating of the sorbent material may take any suitable form on the surface of the oxygen carrier material. An example of a suitable coating may be one where the sorbent material is in the form of nanorods, or in the form of nanoplates, or where the sorbent material coating includes both nanorods and nanoplates. As will be appreciated, the layer of the sorbent material on the surface of the oxygen carrier material may also be in the form of nanorods, nanoplates or both. The distribution (and hence layers/coating) of the sorbent material may take any suitable form that produces a functional product. For example, the distribution may be a uniform distribution over the entire surface of the oxygen carrier material in the first instance. It will be appreciated that a slight degree of variation in the thickness and/or density of the sorbent material that covers the surface of the oxygen carrier material may occur even when said material is uniformly distributed (e.g. less than 10% variation, such as less than 5% variation, such as less than 1% variation). As will be appreciated, as the sorbent material is used and undergoes multiple redox cycles, the distribution of the sorbent material may change. For example, some of the sorbent material may migrate into the bulk of the particles.

Without wishing to be bound by theory, it is believed that the sorbent material (e.g. BaAhC ) may make the metal oxide more porous during the redox reactions and thus facilitate surface contact of fuel gas and the oxygen carrier material (e.g. the metal oxides described above), helping to improve the rates of reaction and thus combustion efficiency even over extended reaction cycles (e.g. 50 cycles or more).

Any suitable relative amount of the oxygen carrier material and the sorbent material may be present in the catalytic sorbent material of the invention. For example, the oxygen carrier material may form from 50 to 98 wt% and the sorbent material may form from 2 to 50 wt% of the catalytic sorbent material. In certain embodiments that may be mentioned herein, the oxygen carrier material may form from 70 to 90 wt% (e.g. from 80 to 90 wt%) and the sorbent material may form from 10 to 30 wt% (e.g. from 10 to 20 wt%) of the catalytic sorbent material.

The catalytic sorbent material may be provided in any suitable form. However, as one of the desired end applications for the catalytic sorbent material disclosed herein is in chemical looping combustion, it may be provided in the form of particles. For example, the catalytic sorbent material may be in the form of particles having an average particle size of from 63 to 250 μm, such as from 150 to 212 μm.

An advantage of the catalytic sorbent material disclosed herein is that it may be able to self- activate in a chemical looping combustion process. Thus, the catalytic sorbent material disclosed herein may be one that self-activates during a chemical looping combustion process. For example, the catalytic sorbent material may self-activate during a redox reaction in the chemical looping combustion process.

As the catalytic sorbent material may be used in chemical looping combustion, it may be a material that is thermally stable at temperatures used to conduct such processes. For example, the catalytic sorbent material may be thermally stable at an operating temperature of from 700 to 1,100 °C in a chemical looping process.

It is noted that the catalytic sorbents disclosed herein display increased activity during the combustion of H₂ and CO in syngas. Under optimized conditions (see experimental section), nearly complete combustion of H₂ and CO was achieved using a catalytic sorbent disclosed herein, compared to 74% and 85% combustion efficiency for CO and H₂, respectively, using iron ore without a coating containing Al and Ba. In addition, the catalytic sorbent materials disclosed herein also display an improved resistance to high-temperature sintering, higher stability of catalytic activity and increased solid utilization rates compared to conventional materials. The catalytic sorbents can also advantageously remove HCI gas at high temperatures (e.g. 800°C). As will be appreciated, by being able to reduce the presence of HCI in a flue gas, the corrosive nature of the flue gas will also be reduced, alleviating problems associated with corrosion in chemical looping combustion systems.

Other advantages of employing oxygen carrier materials that contain a sorbent material distributed on their surface include: i) ease of fabrication; ii) improved resistance to high-temperature sintering; iii) higher stability at different temperatures; and iv) an increased solid utilization rate.

As noted above, the materials used herein may be made easily from readily available starting materials. Thus, in a further aspect of the invention, there is provided a method of forming a catalytic sorbent material, comprising the steps of:

(A) providing an oxygen carrier material having a surface and comprising a transition metal compound that is coated with a precursor material comprising Al and Ba having a molar ratio of AI:Ba of from more than 0:1 to 12:1 , such as from 0.5:1 to 12:1, such that the precursor material is dispersed on the surface of the oxygen carrier material; and

(B) subjecting the oxygen carrier material coated with a precursor material to calcination for a period of time to form a catalytic sorbent material.

The oxygen carrier material coated with a precursor material may be formed by coating an oxygen carrier material having a surface and comprising a transition metal compound with a precursor material comprising Al and Ba having a molar ratio of AI:Ba of from more than 0:1 to 12:1, such as from 0.5:1 to 12:1, such that the precursor material is dispersed on the surface of the oxygen carrier material. As will be appreciated, low-cost natural ores containing transition metal oxides may be used as the basic catalyst (i.e. the oxygen carrier material). In addition, a relatively low amount of a sorbent material (e.g. a material containing Al and Ba) can be coated onto the surface of the oxygen carrier material (e g. the natural ores) to maintain a high oxygen transport ability of the catalyst during the chemical looping process. In addition, the preparation methods are diverse and can also be facile. Given this, the costs of materials and the costs associated with the synthesis of the desired catalytic sorbent material may be lower than conventional materials.

It will be appreciated that the method described above may generate the catalytic sorbent material described hereinbefore. Thus, in order to avoid repetition, features that have already been described above (e.g. the oxygen carrier material and the sorbent material) will not be described in detail again.

The coating of the oxygen carrier material with the sorbent material may be conducted using any suitable method to do so. For example, the coating may be accomplished using one or more of physical mixing, wet impregnation, and co-precipitation. Further details of how said methods may be used in the coating of the oxygen carrier material may be found in the examples below. When used herein, “physical mixing” refers to any method that may be used to mix the two components together using mechanical or manual intervention. For example, using mixing equipment.

The sorbent material used for coating may be prepared by any suitable method. For example, the sorbent material comprising Al and Ba is formed by subjecting an aqueous solution comprising barium nitrate, aluminium nitrate and a base to a temperature of from 80 to 150°C (e.g. 100°C) for a period of time in a pressure resistant vessel (e.g. an autoclave). It will be appreciated that the molar ratios of the barium nitrate and aluminium nitrate may be varied according to the desired molar ratio of Al and Ba in the sorbent material as discussed hereinbefore. Further details of how this hydrothermal reaction may be conducted are provided in the examples section below. Examples of suitable bases that may be used in this reaction include, but are not limited to, ammonium carbonate or, more particularly, urea.

Any suitable calcination temperature may be used. For example, the calcination temperature may be from 950 to 1100 °C. Any suitable period of time for the calcination may be used. For example, the period of time for the calcination may be from 5 to 12 hours. As noted hereinbefore, the catalytic sorbent material disclosed herein may be suitable for use in chemical looping combustion methods. Thus, in a further aspect of the invention, there is described a method of chemical looping combustion, which comprises the step of supplying a catalytic sorbent material as described hereinbefore as a bed material and running a chemical looping combustion process where a fuel is combusted. Examples of the use of the catalytic sorbent material in such processes are provided in the examples section below.

Any suitable fuel may be used in the method of chemical looping combustion. For example, the fuel may be selected from one or more of the group consisting of syngas, natural gas, coal, biomass, and combustible solid waste. In particular embodiments of the invention that may be mentioned herein, the fuel may be syngas.

As noted hereinbefore, an advantage associated with the catalytic sorbent materials disclosed herein is that they may be able to remove one or more impurities from the fuel simultaneously with the chemical looping combustion. This may result in a flue gas that is less corrosive than would otherwise have been the case. Thus, in embodiments of the invention that may be mentioned herein, the catalytic sorbent material may remove an impurity from the fuel. Examples of suitable impurities that may be removed include, but are not limited to one or more of the group consisting of HCI, H₂S and alkali chlorides. For example, the impurity may be HCI.

The chemical looping combustion may be run at any suitable temperature. For example, the chemical looping combustion may be run at a temperature of from 700 to 1 ,100 °C.

As will be appreciated, after the catalytic sorbent material has been used in a chemical looping combustion process it may need to be regenerated. Thus, in a further aspect of the invention, there is also disclosed a method for regenerating a catalytic sorbent material as described herein after it has been used in a chemical looping combustion process, the regeneration comprising subjecting a catalytic sorbent material as described herein that has been used in a chemical looping combustion process to an atmosphere comprising water and carbon dioxide and a temperature of from 800 to 1,100 °C for a period of time to regenerate the catalytic sorbent material. Further details of how the regeneration process may be conducted are discussed in the examples section below.

Possible commercial applications of the sorbent material disclosed herein may include the following, which may or may not be described elsewhere herein. • Chemical looping combustion of syngas, including MSW syngas, biomass syngas, coal syngas, etc., to achieve high combustion efficiency.

• Chemical looping combustion of natural gas to improve the oxygen carrier reactivity, reduce the solid inventory and solid circulating rates.

• Chemical looping in-situ gasification and combustion of solid fuels, including MSW, biomass, and coal.

• Catalytic conversion of gaseous products such as low concentration methane.

• Simultaneous chemical looping conversion of fuels and removal of acid gases.

Further aspects and embodiments of the invention may relate to the following numbered embodiments.

1. A catalytic sorbent for chemical looping combustion technology and the ability to remove HCI.

2. A catalytic sorbent, comprising: a. an oxygen carrier comprising a transition metal compound and a sorbent comprising Al and Ba with a molar ratio from 0 to 12 (e.g. >0 to 12); b. preferably the sorbent is uniformly dispersed on the surface of the oxygen carriers; c. the sorbent may further include the transition metal of the oxygen carriers; d. wherein the transition metal compound of the oxygen carriers may be a transition metal oxide (such as the oxides of Fe, Ni, Cu, Mn, Co and combination thereof); e. 2% to 50% of the sorbent and 50% to 98% of the oxygen carriers by mass of the catalytic sorbent; f. the catalytic sorbent may be formed by various preparation methods including physical mixing, wet impregnation, hydrothermal, co- precipitation, etc.; g. the catalytic sorbent may be used for the combustion of different kinds of fuel used in chemical looping combustion (CLC); h. the catalytic sorbent may have the ability to self-activate during the CLC redox reactions; i. the catalytic sorbent is thermally stable at different operation temperatures, such as from 700 °C to 1100 °C; j. the catalytic sorbent can simultaneously remove HCI during the CLC reaction; k. the catalytic sorbent may be used for different chemical looping processes; and I. the catalytic sorbent may be regenerated at 800-1100 °C in an atmosphere comprising water and carbon dioxide

3. In one embodiment, the catalytic sorbent can be used for complete combustion of syngas in the chemical looping process. The thermal stability, reactivity, and oxygen carrier utilization rate of the catalytic sorbent are high, thus being able to improve the system stability and lower the operation costs in the application.

4. In another embodiment, the catalytic sorbent can be used for the simultaneous chemical looping combustion of syngas and HCI removal. It, therefore, can be used as a potential catalyst in catalytic processing of flue gas containing HCI, due to its acid- resistance and HCI removal ability.

5. A method of forming a catalytic sorbent, comprising: a. providing a mixture of an oxygen carrier comprising a transition metal compound and a sorbent comprising Al and Ba with a molar ratio from 0 to 12 (e.g. >0 to 12); b. calcining the mixture at 950-1100 °C for 5-12 hours to form the catalytic sorbent; c. wherein the catalytic sorbent comprises 2% to 50% of the sorbent and 50% to 98% of the oxygen carrier by mass of the catalytic sorbent; and d. wherein the transition metal compound may comprise a transition metal oxide (such as the oxides of Fe, Ni, Cu, Mn, Co and combination thereof).

6. In particular, the method of forming a catalytic sorbent may comprise:

I. wet impregnation, comprising: a) mixing an oxygen carrier comprising a transition metal compound and a sorbent comprising Al and Ba with a molar ratio from 0 to 12 in a solvent at a temperature of 60-90 °C; b) evaporating most of the solvent to form a slurry; c) drying the slurry at 105-120 °C; d) calcining the slurry at 950-1100 °C for 5-12 hours to form the catalytic sorbent; e) wherein the catalytic sorbent comprises 2% to 50% of the sorbent and 50% to 98% of the oxygen carrier by mass of the catalytic sorbent; and f) wherein the transition metal compound may comprise a transition metal oxide (such as the oxides of Fe, Ni, Cu, Mn, Co and combination thereof).

II. physical mixing, comprising: a) mixing an oxygen carrier comprising a transition metal compound and a sorbent comprising Al and Ba with a molar ratio from 0 to 12 (e.g. >0 to 12) using a mixing equipment for 2-5 hours; b) wherein the mixing equipment comprises a rotator, shaking table or other similar devices; c) calcining the composites at 950-1100 °C for 5-12 hours to form the catalytic sorbent; d) wherein the catalytic sorbent comprises 2% to 50% of the sorbent and 50% to 98% of the oxygen carrier by mass of the catalytic sorbent; and e) wherein the transition metal compound may comprise a transition metal oxide (such as the oxides of Fe, Ni, Cu, Mn, Co and combination thereof)

III. hydrothermal method, comprising: a) heating a solution of barium nitrate to 60-90 °C to dissolve the barium nitrate; b) adding aluminum nitrate and a base to the heated solution to form a second solution; c) wherein the base may comprise ammonium carbonate, or preferably urea; d) transferring the second solution to an autoclave; e) heating the autoclave at 100-150 °C for 12-24 hours to form a sorbent comprising Al and Ba with a molar ratio from 0 to 12 (e.g. >0 to 12); f) mixing the sorbent with an oxygen carrier comprising a transition metal compound to form a mixture; g) drying and calcining the mixture at 950-1100 °C for 5-12 hours; h) wherein the sorbent may be in the form of nanostructures such as nano-rods and/or nano-plates; i) wherein the catalytic sorbent comprises 2% to 50% of the sorbent and 50% to 98% of the oxygen carrier by mass of the catalytic sorbent; and j) wherein the transition metal compound may comprise a transition metal oxide (such as the oxides of Fe, Ni, Cu, Mn, Co and combination thereof).

Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples.

Examples

Materials

Commercially available iron ore (10) from China (imported by SG Labware, Singapore) was used as the base metal oxide for the preparation of the oxygen carriers (OCs). The active compound for CLC, which is Fe₂O₃ in this case, makes up more than 85% of the material. Barium aluminum oxide ( BaAI₂O₄) and magnesium aluminium oxide (MgAI₂ O₄) were provided by Alfa Aesar with a purity higher than 99%. Barium carbonate (BaCO₃), nitric acid (HNO3), aluminum nitrate nonahydrate (AI(Nq3)3·9H₂q), barium nitrate (Ba(NO₃)₂), iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), and ammonium carbonate ((NH₄)CO₃) were provided by Sigma Aldrich. Copper nitrate hemi(pentahydrate) (Cu(NO₃)₂ 2.5H₂O) and urea were purchased from Sigma Aldrich. All of these chemicals were used without any further treatment.

Analysis Techniques

The OCs were characterized by complementary techniques. The composition of the OCs was analyzed by X-ray fluorescence (XRF) (PANalytical, Netherlands). X-ray powder diffraction (XRD) was used to determine the crystal phase change of the OCs. The XRD analyses were conducted using a XRD-6000 diffractometer (Shimadzu) with Cu-Ka radiation in continuous scan mode, with a step size of 0.02° in the 2θ range of 15° to 90°. To identify the surface composition and oxidation state of the various metal elements, X-ray photoelectron spectroscopy (XPS) was carried out on an AXIS Supra (Kratos), with monochromatic radiation from Al/Ag Ka source. The binding energy of C1s, which is fixed at 284.8 eV, was taken as an internal calibration standard for all tests. The morphology and surface elemental mapping of the OCs were characterized by field emission scanning electron microscope with energy dispersive spectroscopy (FESEM-EDS, JSM-7200F, JEOL). To identify the particle bulk morphology, cross sections of the OCs before and after redox reactions were examined by FESEM. Briefly, the OC particles were embedded in epoxy resin at room temperature (RT). Sandpaper (P1200, 3M) was then used for the initial polishing of the epoxy resin bed. A cross section polisher (SM-9020CP, JEOL) was finally applied to further fine-polish the particles cross section using an argon ion beam. The Brunauer-Emmett-Teller (BET) surface area and total pore volume of the OCs were analyzed based on the nitrogen adsorption-desorption isotherms measured at 77 K on a Quadrasorb SI (Quantachrome Instrument). Example 1

Before modification, IO was crushed and sieved to a size range of 150-250 μm. BaAI₂O₄ was impregnated onto the surface of the IO by wet-mixing, as described below.

200 g of IO was added into a beaker with 100 ml Dl water under stirring condition (500 rpm). Depending on the weight ratio of BaAI₂O₄ to IO, a known amount of BaAI₂O₄ (varying from 10 to 40 g) was added into the IO suspension. The two compounds were stirred vigorously at 70°C on a heating plate until most of the water was evaporated. Then, the slurry was transferred into a crucible and dried at 105°C overnight, followed by calcination at 950 °C for 5 h in air. The obtained oxygen carrier (OC) was crushed and sieved to the size range of 150- 250 pm. The selected mass ratio of BaAI₂O>₄4 to IO was 0:100, 5:100, 10:100, and 20:100. The IO loaded with BaAI₂O₄ are labelled as IO (for 0:100 BaAI₂O₄ to IO) or 10-xBA correspondingly, where x equals to 5, 10 or 20.

Characterisation

The composition of IO before and after loading with BaAI₂O₄, as measured by XRF, is listed in Table 1. The XRF analysis confirms that the unmodified IO contains more than 85% of Fe₂O₃.

Table 1 : Compositions of IO before and after the modification of BaAI₂O₄ given by XRF.

Calculated based on the content of BaO The XRD pattern of the fresh 10 and IO-1 OBA is shown in Figure 1. The major phase in fresh I0 was Fe₂O₃ (hematite, JCPDS 87-1166). The unidentified peaks with 2Q in the range of 25 to 30° may come from impurities containing oxides of Si, Ti, and/or Al as suggested by the results of XRF analysis (Table 1). In IO-10BA, the diffraction peaks of BaAI₂O₄ (JCPDS 72- 0387) can be clearly seen, thus suggesting loading of BaAI₂O₄ on the IO surface.

The surface morphology of the IO before and after loading with BaAI₂O₄ is shown in Figure 2. Figure 2 shows that fresh IO particles were non-porous materials with a dense surface structure, which caused the materials to have small surface area as shown in Table 2. Particularly, the morphology of fresh 10 featured the structure of small cells, e.g. as labelled in Figure 2(A-2). The cross section of the particle as depicted in Figure 2(A-3) shows the solid bulk of the OC before redox reactions. After modification, it was found that BaAI₂O₄, with a size of 1-2 μm, was uniformly distributed on the surface of the IO (Figure 2(B-2)). To distinguish the loaded BaAI₂O₄ from the IO, a selected surface of the IO-1 OBA was scanned by EDS, with the resulting element mapping shown in Figure 3. It is noted that the coating appears to result in a material with an increased particle size. Without wishing to be bound by theory, it is believed that the doping of small grains of BaAI₂O₄ on the 10 surface caused the surface area and pore volume to increase as observed in Table 2. Table 2. Bed height, surface area and pore volume of the OCs before and after redox cycles.

Figure 4 shows the XPS spectra of Fe 2p and O 1s for the fresh IO and IO-1 OBA. The high- resolution Fe 2p3/2 spectra were fitted using peaks corresponding to the Gupta and Sen (GS) multiplets (Grosvenor, A. P. et al., Surf. Interface Anal. 2004, 36, 1564-1574) as shown in Figure 4(a). These peaks have full widths at half-maximum (FWHM) ranging from 1.0 to 1.6 eV and with binding energy interval of ~1 eV. In addition to the multiplet structure, a surface peak and a satellite peak due to shake-up or charge transfer process were also used for fitting (Grosvenor, A. P. et al., Surf. Interface Anal, 2004, 36, 1564-1574). The multiplet parameters are summarized in Table 3. For the fresh samples, only the Fe³⁺quadruplet was observed, which is consistent with the observation in the XRD patterns.

High-resolution 0 1s spectra are shown in Figure 4(b). This spectrum can be deconvoluted to three different peaks corresponding to lattice oxygen (Oiat), physically adsorbed oxygen (Oads) , and hydroxyl oxygen (Ohyd) with binding energies of 529.8, 531.2, and 532.5 eV, respectively (Zheng Y. etal., Appl. Catal. B 2017, 202, 51-63; and Yang X. etal., Sci Rep. 2016, 6, 31591). It is generally accepted that the surface adsorbed oxygen, Oads, is mainly associated with the oxygen vacancies formed on the surface of the catalysts (Yang X. et al., Sci Rep. 2016, 6, 31591). Therefore, the ratio of O_ads/O_Iat can reflect the surface oxygen vacancies for the catalysts. When freshly calcined, the Oads/Oiat of IO-10BA (1.71) was much larger than that of IO (0.55). This indicated that the introduction of BaAI₂O₄ could possibly increase the oxygen vacancies on the IO surface and thus improve its activity. On the other hand, the surface exposure of lattice oxygen from IO would be reduced as it is coated with BaAI₂O₄, hence increasing the O_ads/O_Iat ratio. The variation of the surface oxygen vacancies caused by the BaAI₂O₄ modification will be discussed in detail below.

Table 3. Binding energy of the fitting peaks of Fe 2p3/2 spectra for IO and IO-BA*.

*Values in the brackets are full width at half-maximum (FWHM) in eV.

General Procedure 1

The batch fluidized bed reactor (bFB) experiment setup used herein is shown in Figure 5. The reactor was a quartz tube with an inner diameter of 15 mm. It was heated by an electrical furnace to the desired temperature (700-900 °C). In the middle of the quartz tube, a fritted silica disc was used as a sample holder as well as a gas distributor. Two type K thermocouples (TCs), 10 mm above and below the fritted quartz disc, respectively, were used to monitor the temperature inside the reactor. To make the performance results comparable, the amount of 10 used in each test was the same, i.e. 10 g, and without diluting sand, unless otherwise stated. A simulated MSW syngas consisting of ~10 vol% CO, ~10 vol% H₂, ~5 vol% CO₂, and 25 vol% H₂O, with N2 balance, was used as the reduction gas. This composition was based on a previous study by Chan et al. (Chan W. P. et a/., Appl Energy 2019, 237, 227-240). On the other hand, a concentration of 8 vol% O2 in N2 was used as the oxidation gas for the reduced OC. Steam was generated by injecting water into the reactor feed with a syringe pump at an infusion rate of 0.1 mL/min. To avoid water condensation, all the pipelines were heated to 150 °C. In each redox cycle, the syngas was introduced into the reactor for 5 min, followed by N2 purge for 3 min to avoid contact between reduction gas and oxidation gas. After purging, the OC was exposed to oxidation gas for 10 min. At all stages of a redox experiment, the total inlet gas flowrate was maintained at 500 seem (standard cubic centimeters per minute), which corresponds to 3.8 times the minimum fluidization velocity at 900 °C. The OC bed height was 23-27 mm when unfluidized. The superficial gas residence time was about 0.12 s. For each test, 20 to 50 redox cycles of the CLC process were performed for each sample. The syngas before and after the reaction was collected in gas bags for off-line analysis by a gas chromatograph (Agilent 7890A) equipped with thermal conductivity and flame ionization detectors (GC-TCD-FID, 7390B, Agilent), which is used for the calculation of the combustion efficiency of CO and H_2:

The concentration of steam was calculated based on a hydrogen balance. For the 1st, 10th, and 20th reduction cycle, the flue gas was sampled every minute. For other cycles, the average of the flue gas composition over every 5 min interval was sampled and analyzed. Each test was duplicated to validate the repeatability of the results.

Based on the analyzed gas compositions, the combustion efficiencies of CO and H₂, respectively, were calculated based on a nitrogen balance:

where η_i is the combustion efficiency for species; γ_i, is the molar fraction of species /, as measured by GC-TCD-FID; the subscript in indicates the inlet molar concentration. Example 2

The redox performance of the OC samples prepared in Example 1 were tested in bFB, as described in General Procedure 1.

Results and discussion

Figure 6 shows the effect of BaAl₂O₄ on CLC efficiency for CO and H₂ at 700, 800 and 900 °C. For unprocessed 10, the average efficiency of the 20 redox cycles for CO and H₂ at 800 °C was 76% and 66%, respectively. With a coating of 10 wt% BaAl₂O₄ , the efficiency increased to 91% and 83% for CO and H₂, respectively. A similar increase was observed at other operation temperatures. A more significant increment was observed at low reaction temperature (700 °C), indicating that the stability of the catalyst was also improved with varying operation temperature. It is worth noting that at 700 °C, the η_Η2 for IO was only ~30% (compared to ~70% at 900 °C for IO) due to decreased reaction rate and the water-gas shift (WGS) reaction (Eq. (3)) at low temperatures. The modification of IO with BaAI₂O₄ increased η_Η2 to about 60% at the 20^th cycle. This suggests that the unwanted WGS reaction in the CLC process was hindered by the loading of BaAl₂O₄ and it can be inferred that the modification of BaAl₂O₄ significantly enhanced the combustion efficiency of the syngas. In general, the combustion efficiency increased slightly for the unprocessed IO as the cycle numbers increased due to the activation of IO during the redox reactions. It appears that 10% BaAl₂O₄ doping not only improved the combustion efficiency remarkably but promoted the activation process of IO.

Generally, the stability of the OCs improved after modification (Figure 6). It is noteworthy that the increase in stability was especially significant in the case of CO. The CO combustion efficiency of IO-10BA remained above 80% in the temperature range of 700 to 900 °C, while that of IO varies between 60% to 80%. Figure 7(e) further supports this observation as η_αο of unprocessed IO decreased from ~0.7 to 0.5 within the first 5 min of reduction while the combustion efficiency of both CO and H₂ remained > 0.95 during the first 3 min for the 10% BaAl₂O₄ doped OC. Gradually, the Fe₂O₃ was converted into Fe₃O₄. Thus, the combustion efficiency decreased as a result of the reduced lattice oxygen activity in Fe₃O₄-FeO/Fe compared to that in Fe₂O₃- Fe₃O₄. In general, the reactivity of the OC with H₂ was higher than that with CO, which is in agreement with a previous study (Liu, W. etai, Chem. Eng. Sci. 2014, 120, 149-166). Between 2.5 and 5 min, decreased much faster than η_CΟ. Like above, this can be explained by the WGS reaction, which is catalyzed by Fe₃O₄ (Newsome, D. S., Catalysis Reviews Science and Engineering, 1980, 21, 275-318) consuming CO whilst generating additional H₂ (see Eq. (3)). Therefore, it is recommended that the OCs should be operated at the temperature higher than 800 °C since the H₂ combustion efficiency would drop significantly to -60% at 700 °C for IO-10BA.

The effect of WGS was more clearly shown by the η_CΟ and η_Η2 profiles in Figure 7. For 10, η_Η2 decreased from 94% to 24% over the 5 min reduction at 900 °C (Figure 7(a)). At 800 °C, some H₂ was generated during the last minute of the reduction, resulting in negative η_Η2 as shown in Figure 7(b). At 700 °C, about 50% more H₂ was generated by WGS (Figure 7(c)), whilst the onset of negative η_Η2 was much earlier in each cycle as compared to 800 °C. The rapid drop in η_Η2 with time was accompanied by the steady consumption of CO due to WGS.

In fact, by comparing the measured composition at the reactor’s outlet with the equilibrium composition (shown in Figure 7(d)) calculated based on Eq (4), it can be seen that chemical equilibrium was reached towards the end of the reduction stage at both 800 °C and 700 °C. Notably, in the presence of BaAI₂O₄, the onset of significant WGS was postponed, as shown in Figure 7(b) and (c), causing η_Η2 to be much higher.

Figure 7(f) shows the variation in the mole fraction of oxygen during the oxidation process of the 10^th redox cycle. The rate of oxidation was faster than that of reduction for both IO and IO- 10BA, showing complete oxygen uptake by the reduced OCs during the first several minutes (as will also be evidenced by the thermogravimetric analysis (TGA) test in the following section). From Figure 7(f), it can be seen that the oxygen breakthrough for IO-10BA was delayed compared to IO, suggesting higher lattice oxygen activity in the BaAI₂O₄ modified sample. Following breakthrough, the oxygen concentration quickly recovered to the inlet concentration level. Therefore, a 10 min reaction time is sufficient for completely regenerating the lattice oxygen in the OCs and the addition of BaAI₂O₄ significantly improved the redox activity and the durability of the IO-based OC. Example 3

The effect of BaAI₂O₄ loading on the combustion efficiencies and lattice oxygen transport of the OC samples prepared in Example 1 were tested in bFB as described in General Procedure 1.

The actual amount of lattice oxygen transported per unit mass of OCs in each cycle was defined as:

where Ω is the transported lattice oxygen ratio of the OCs; Υ_H2in and Y_co,in are the input mole flow rates of H₂ and CO, respectively, in [mol/min]; m is the mass of the OC used in the fluidized bed at its fully oxidized state, in [g]. Results and discussion

The combustion efficiencies of the simulated syngas with I0 modified by 5%, 10%, and 20% of BaAI₂O₄ are shown in Figure 8. For unmodified IO, the average η_Η2 over 20 cycles was 0.73.

No obvious improvement was observed after loading with 5% BaAI₂O₄. With 20% of BaAI₂O₄, the η_Η2 even dropped to 0.68, possibly owing to the coverage of the active Fe₂O₃ by BaAI₂O₄, hindering the contact between H₂ and IO. IO-10BA showed a distinct improvement compared to other OCs, with η_Η2 reaching up to 0.94 after 10 cycles. The addition of 10% BaAI₂0₄ rendered even more improvement to η_CΟ. For the first several cycles, η_CΟ increased from ~0.62 for IO to ~0.85 for IO-1 OBA. The performance of IO-1 OBA continued to improve after 5 cycles, with η_C0 exceeding 0.95 after 12 cycles. For all OCs, η_CΟ increased slightly with cycle numbers, suggesting some kind of activation process during the redox experiments (Adanez, J. et al., Energy Fuels 2010, 24, 1402-1413). These results show that the addition of 10% BaAI₂O₄ not only improved the combustion efficiency remarkably, but also accelerated the activation process.

The actual amounts of lattice oxygen transported by per unit mass of OCs in each cycle, Ω, is shown in Figure 9. For 10 (containing 85.45% Fe₂O₃), the theoretical lattice oxygen transport capacities for the Fe₂O₃/Fe₃O₄ and Fe₂O₃/FeO redox couples are 2.9% and 8.5%, respectively. Thermodynamically, only the Fe₂O₃/Fe₃O₄ redox couple could facilitate complete combustion. Indeed, for unmodified 10, the lattice oxygen transported per cycle was consistently below 2.4%, i.e. about 83% of the available lattice oxygen (for the Fe₂O₃/Fe₃O₄ conversion) was consumed during the 5-min syngas combustion in each cycle. Similar to the combustion efficiencies, the amount of lattice oxygen consumed also increased slightly with redox cycles. Indeed, BaAI₂O₄ doping significantly enhanced the Ω, especially in the cases of IO-5BA and IO-10BA. For 1O-10BA, Ω over 20 cycles was between 3.1 and 3.5%, consistently higher than the maximum capacity of the Fe₂O₃/Fe₃O₄ redox couple, but lower than 8.5% for Fe₂O₃/FeO. The above results showed that the addition of BaAI₂O₄ not only accelerated the redox reactions, but also promoted reducibility of the Fe₂O₃.

Example 4

The stability of IO-xBa prepared in Example 1 was tested in bFB at different temperatures as described in General Procedure 1.

Results and discussion

Figure 6 shows the combustion efficiencies of CO and H₂ by 10 and IO-10BA. It can be seen that the addition of BaAI₂O₄ significantly improved both η_co and η_H2 at all temperatures tested.

At 900 °C (Figure 6 (a)), the addition of BaAI₂O₄ increased the average η_co and η_H2 by 28.3% and 14.4%, respectively. In addition, the reactivity of IO-10BA increased gradually over the redox cycles, suggesting an apparent activation process, which is also apparent in Figure 8 and Figure 9. At lower temperatures of 800 and 700 °C, η_H2z values are accordingly lower. At

700 °C, η_H2 by unmodified IO was only ~30% (cf. -70% at 900 °C) while η_H2 by IO-10BA varied between 40% and 60% which is also much lower than the -90% measured at 900 °C.

The trend of variation of η_co with temperature differs from η_H2 Figure 6(d) summarizes the combustion efficiencies at the 20^th cycle measured at different temperatures. η_co appeared to be less dependent on temperature than η_H2 Generally, η_co remained at relatively high levels at 700 °C, 800 °C and 900 °C. For IO-10BA, η_co decreased slightly from 94% (900 °C) to 83% (700 °C). For unmodified IO, η_co increased first, then slightly decreased (63%→77%→72%) when the temperature was decreased from 900 to 700 °C. The variation of the apparent combustion efficiencies with temperature can be explained by the fact that WGS is favored at low temperatures. As mentioned above, CO can be consumed by WGS at low temperatures, thus resulting in the η_co to remain high. The long-term operation stability of 10 and IO-10BA was further evaluated by conducting 50 redox cycles at 900 °C. The results of the 50 cycles are presented in Figure 10. For unmodified IO, η_H2 and η_co increased gradually from 70% and 68% at the 1^st cycle to 85% and 74% at the 50^th cycle, respectively. IO-1 OBA showed better long-term performance as the combustion efficiency of IO-1 OBA increased from ~80% to >99.8% during the first 30 cycles and remained stable thereafter, achieving almost complete combustion of both H₂ and CO from the 30^th cycle onwards. For unmodified IO, η_H2 and h_co increased gradually from 70% and 68% at the 1^st cycle to 85% and 74% at the 50^th cycle, respectively. The growth rate was relatively slow as compared to the IO-1 OBA in its first 30 cycles. Hence, the modification of BaAI₂O₄ improved the reactivity of the IO and also accelerated the activation process of the IO during the redox reactions.

The average amounts of lattice oxygen transported by all the OCs, measured at different temperatures, are shown in Figure 11. The theoretical oxygen transport capacity for unmodified IO is also indicated for comparison. Under all tested conditions, the OCs modified with BaAI₂O₄ exhibited greater oxygen transport ability than the unmodified IO. The OCs with 10% BaAI₂O₄ coating showed the best performance (i.e. higher reactivity and stability) at 800 and 900 °C. The enhanced oxygen carrying capacity will be further discussed in the sections below.

Example 5

The characterisation of IO and IO-10BA, prepared in Example 1, after reactions and enhancement mechanism was carried out as described in Analysis Techniques.

Results and discussion

Interestingly, it was found that the total volume of the OCs increased significantly after 50 cycles, as shown in Figure 12, with the bed height for IO and IO-1 OBA expanding by 1.9 and 2.5 times, respectively. The OC particles became more porous after the redox cycles, especially in the case of IO-1 OBA. The surface area and pore volume for the IO and IO-1 OBA before and after 50 cycles are listed in Table 2. For the fresh OCs, the specific surface area and pore volume of the OCs increased slightly (from 0.21 to 0.40 m²/g) after loading with 10 wt% of BaAI₂O₄. After 50 cycles, the surface area of IO-1 OBA increased to 0.94 m²/g, which is 3.7 times that of IO. The addition of BaAI₂O₄ also increased the pore volume of the spent OC from 0.53 to 1.11 cm³/kg. The experimental results suggested that the modification of IO with BaAI₂O₄ facilitates the formation of pores in the bulk IO, which increases surface contact between the reduction gases and OC, leading to increased combustion efficiency, as also can be supported by the SEM images.

The morphology of the OCs after 50 redox cycles is shown in Figure 13. For IO without modification, the large pores on the surface (Figure 13(A-1)) are required for the reaction of active Fe203 with CO and H₂. Looking at the surface around those large pores (FigError! Reference source not found. ure 13(A-2)), one can find the IO surface remained dense even after 50 cycles. For IO-10BA, the newly formed pores after reaction were evenly distributed on the OC surface (Figure 13(B-1)), which is quite different from IO. Macropores with the size of several micrometers can be seen across the IO surface. The arborization structure formed gradually during the redox reactions, which increases the exposure of Fe₂O3 to the reduction gases.

This pore-generation phenomenon was further examined by FESEM (Figure 13). Different from the smooth and dense surface of the fresh IO (Figure 2(A-1) and (A-2)), macro-pore emerged on the surface of IO after 50 redox cycles. The generation of the additional macroporosity, which facilitated better access for syngas to react with the interior of the OC particles, may be a factor enabling the activation of the IO over redox cycles. Apart from the macropores, the surface texture of the cycled IO (Figure 13(A-2)) also became slightly more porous than that of the fresh sample (Figure 2(A-2)). The cross section of a cycled IO particle, shown in Figure 13(A-3), depicts the formation of large cavities inside the particle, encapsulated by a dense shell that was decorated with macropores.

With the loading of 10% BaAI₂O₄, the morphology of IO-10BA changed significantly after 50 cycles. Uniformly distributed macropores with size of several microns were formed on the surface of IO-10BA as shown in Figure 13(B-1). The surface became highly porous after 50 redox cycles (Figure 13(B-2)), contrasting the dense surface for spent IO particles (Figure 13(A-2)). The cross-sectional morphology of the IO-10BA is depicted in Figure 13(B-3) and it shows cavities with a variety of sizes inside the OC particle. Different from the cross section of IO, in which large lumps were observed inside the bulk (Figure 13(A-3)), clusters of small grains dominate the interior of the IO-10BA particle. A previous report suggested the strong influence of surface oxygen vacancy on the morphology of BaAI₂O₄ containing catalysts (Zhang, L. W. et al., Adv. Fund. Mater., 2007, 17, 3781-3790) and this may be an underlying factor causing the distinct structures of the IO-BA comparing to the unprocessed IO. The relative abundance of the oxygen vacancies can remarkably increase the rate of oxygen migration from bulk to surface through vacancy-enhanced n-type ionic diffusion (Zhu, Y. etal., Int. J. Hydrogen Energy, 2019, 44, 10218-10231). Consequently, the migration of the bulk lattice oxygen and the iron ions generates more vacancies in the bulk. The coalescence of these vacancies ultimately results in the formation of a highly porous structure (Knutsson, P. et a!., Appl. Energy, 2015, 157, 368-373) inside the particle. During the redox cycles, BaAI₂O₄ also migrates from the IO surface into the bulk (as supported by the EDS element mapping in Figure 13(B-3) and interacts with the newly exposed interior surface, which could possibly facilitate and accelerate the formation of pores inside the OC particles. The results of FESEM analysis are consistent with the larger surface area and pore volume of the cycled IO-10BA as obtained by BET analysis. The highly porous surface and inner structure of the cycled IO- 10BA greatly benefits intra-particle mass transfer, providing better contact of the gaseous reactants with OCs, as shown by the improved oxygen transport capacity of IO-BA (Figure 11).

The phase change of the OCs was evaluated with XRD. The XRD pattern of IO and IO-10BA before and after 20 redox cycles are shown in Figure 14. For both IO and IO-1 OBA, the conversion of Fe₂O₃ to Fe₃O₄ (JCPDS 88-0866) can be seen after reduction. After oxidation, the reduced samples were oxidized to their initial state with Fe₂O₃ as the major phase. No obvious diffraction peaks of FeO was identified in the XRD patterns for the reduced IO, implying that the reduction of Fe₂O₃ mostly terminated at Fe₃O₄. In contrast, FeO diffraction peaks were observed for the reduced IO-10BA sample. Thus, some of the Fe₃O₄ could be further reduced to FeO with the loading of BaAI₂O₄ during the reaction, causing enhanced oxygen transport capacity as observed in Figure 9 and Figure 11. The significantly reduced intensity of the BaAI₂O₄ diffraction peaks after 20 cycles suggests the possible chemical interaction between BaAI₂O₄and Fe₂O₃. This interaction may involve simple ionic exchanges, such as the one reported by Huang etal. (Huang, J. etal., Chem. Eng. J. 2017, 326, 470-476). However, no new crystal phase was observed in the cycled OCs, meaning that either the amount of new phase formed was insignificant or the new phase had low crystallinity. Nevertheless, given that the formation of barium ferrites is thermodynamically feasible (Siriwardane, R. etal., Appl. Energy 2016, 165, 952-966; and Huang, F. etal., J. Energy Chem. 2019, 29, 50-57), the possible interaction between iron oxides and BaAI₂O₄ merits further and more systematic investigations.

Figure 4 shows the XPS spectra of Fe2p and 01s for IO and IO-1 OBA before and after 20 redox cycles. Only the Fe³⁺quardruplet was observed for both the fresh and oxidized samples. However, for reduced OCs, the Fe²⁺2p3/2 triplet was also observed for both IO and IO-10BA. In the reduced 10, the ratio of Fe²7 Fe³⁺, calculated from the area under the corresponding multiplets, was 0.46 (shown in Table 3), which is slightly less than the theoretical ratio for Fe₃0₄ (0.5), implying the incomplete reduction of Fe₂O₃ to Fe₃O₄ after 5 min time-on-stream; this result is in line with the low oxygen transport ability of unmodified 10, as shown in Figure 9 and Figure 11. In the reduced IO-10BA, a ratio Fe²7 Fe³⁺ of 0.60 was obtained (shown in

Table 3), indicating that the presence of BaAl₂ O₄ promotes the reduction of Fe³⁺ to Fe²⁺ beyond the Fe₂O₃/Fe₃O₄ redox couple. This is also in accordance with the observation of FeO diffraction peaks of reduced IO-10BA shown in Figure 14(c). The high resolution O 1s spectra are shown in Figure 4(b). As aforementioned, the spectra can be deconvoluted to three different peaks corresponding to O_Iat, O_ads, and O_hyd. The ratio of Oads/Oiat reflects the surface oxygen vacancies before and after the reduction. The relative abundance of the three oxygen species (i.e. Oiat, O_ads, and O_hyd) on the surface of OCs before and after the redox cycles are listed in Table 4. After 20 cycles, O_ads/Oi_at for IO-10BA drastically decreased from 1.71 to 0.45, which can be ascribed to the loss of surface BaAI₂O₄ due to chemical interaction with the IO or surface abrasion during fluidization. Nevertheless, the O_ads/O_Iat of IO-10BA remained high compared to that of IO after the 20^th oxidation (0.39). After the 20^th reduction, O_ads/O_Iat increased to 0.69 and 0.88 for IO and IO-10BA, respectively, suggesting the formation of more surface oxygen vacancies as lattice oxygen was consumed. Because of its stronger oxygen transport ability, even in the oxidized state (after 20^th oxidation), the reduced IO-1 OBA exhibited higher O_ads/O_Iat ratio than the reduced IO, implying the promotional effect of BaAI₂O₄.

Table 4. 01s spectra derived oxygen species contents

General Procedure 2 The oxygen transport capacities of the fresh and used OCs were quantified by isothermal reduction-oxidation experiments carried out in TGA (STA 449 F3 Jupiter, NETZSCH) at 900 °C. During the redox cycle, the mass change of the OCs was measured with a precision of 0.1 μg. Approximately 17 vol% CO/N₂ was used as gaseous fuel for the reduction of OCs. The OC sample (20 ± 0.2 mg) was heated in 280 ml/min N₂ with a heating rate of 30 °C/min to 900 °C, purged with N₂ for 10 min and reduced in a mixture of 280 m/min N₂ and 60 ml/min CO for 60 min at 900 °C. After reduction, the OC was purged with N₂ for 10 min, and 250 ml/min air with 30 ml/min N₂ was introduced for the oxidation of the OC for 20 min. The tests were repeated two times for each sample and the results are reported as averages.

The mass loss, X (wt%), of the OCs during the redox reaction in TGA can be calculated as:

where m₀ and m_t are the masses (mg) of the OCs at their fully oxidized state and at time t during the isothermal redox stage, respectively.

The mass loss rate, x (%/min), was calculated as:

The oxygen transport capacity, R₀ (wt.%), was calculated based on the mass loss at the end of the 60 min reduction (in 17 vol% CO/N₂) in the TGA:

where m_r and mo are the OCs masses after reduction for 60 min and full oxidation, respectively.

Example 6

The lattice oxygen transport capacity and activity of IO-10BA prepared in Example 1 were evaluated by TGA and hydrogen temperature programmed reduction (H₂-TPR), as described in General Procedure 2 and below. H₂-TPR was performed on Autochem 2910 (Micromeritics, US) to evaluate the activities of the lattice oxygen in the fresh and used OCs. In a typical H₂-TPR, the OC powder (about 35 mg) was first degassed at 150 °C in N₂ for 1 h, followed by naturally cooling to room temperature. After the degassing, the OC particles were heated to 950 °C at a rate of 10 °C/min in 50 mL/min of 5 vol% H₂/N_2. The H₂ composition in the outlet was quantified by a thermal conductivity detector.

Results and discussion

The redox performance of the fresh and used OCs was further tested in TGA to demonstrate the effect of BaAl₂ O₄ on the reactivity of the lattice oxygen in the 10. Figure 15 shows the fractional weight loss and the rate of weight change of the OCs during the isothermal redox cycle at 900 °C. It can be seen that the addition of BaAI₂O₄ significantly improved the oxygen transport capacity during the 60 min reduction in 17 vol% CO/N₂ (shown in Figure 15(a)). At the same time, the rate of reaction was enhanced 2 to 3 times, especially for the reduction of Fe₃O₄ to Fe (shown in Figure 15(b)).

The reduction of Fe₂O₃ in IO and IO-10BA can be divided into three stages, viz. Fe₂O₃ to Fe₃O₄, Fe₃0_O to FeO, and FeO to Fe. Nevertheless, these three stages, especially the last two, usually overlap one another, as shown in Figure 15(b) and (d). Based on the elemental analysis of IO (shown in Table 1), the fractional weight loss curve can be mapped against the theoretical phase transitions, as shown in Figure 15(a). As BaAI₂O₄ is not redox active, it is anticipated that the theoretical weight loss of IO-10BA would be less than that of IO, due to dilution effect. In 17% CO/N₂, all OCs except fresh IO could be completely reduced past Fe₃O₄ in ~150 s. For fresh IO, the reduction mainly achieved the conversion of Fe₂O₃ to FeO in 60 min. The observed rate of reduction of IO was the lowest, mainly because of the low porosity and reactivity of the IO. After 20 and 50 redox cycles in bFB, the IO particles appear activated, achieving faster rates and reduction beyond FeO, i.e. final weight loss of 13.4% and 18.8% for IO cycled 20 and 50 times, respectively, compared to 7.8% for fresh IO. This can be explained by the generation of additional porosity and surface area, as clearly shown in Table 2 and Figure 13.

With BaAI₂O₄ loading, the reduction of fresh IO-10BA from Fe₂O₃ to Fe₃O₄ was almost twice as fast as that of fresh IO. The rate of the subsequent reduction stages (Fe₃O_O-Fe0-Fe) was also boosted significantly. This observation is in agreement with the enhanced performance seen in Figure 11. There was no significant difference in the performance of IO-10BA after 20 cycles and after 50 cycles, i.e. the reactivity of IO-10BA has stabilized after 20 redox cycles, and this is consistent with the bFB results shown in Figure 10. The oxygen transport capacity reached 23.9 wt% for IO-10BA after 50 cycles, which is close to the theoretical oxygen transport capacity for IO (Fe203-Fe, 25.6 wt%). Furthermore, the oxidation rate of the reduced OCs also improved with the addition of BaAl₂ O₄ (Figure 15(b)).

To further elucidate the role of BaAl₂ O₄ in the redox reactions, pure BaAl₂ O₄ was tested in TGA under the same operating conditions. The normalized weight loss curve is shown in Figure 15(c). Upon introducing the reduction gas, 17 vol% CO/N₂, a weight loss of ~0.25 wt% was observed, followed by a small weight gain. This suggested that a small fraction of the lattice oxygen in BaAl₂ O₄ can participate in the redox reaction using CO as a fuel. Although this weight change seems negligible compared to that of IO-10BA (shown in Figure 15(a)), the formation of these oxygen vacancies in BaAI₂O₄ under the reducing environment might facilitate enhanced mobility of the bulk lattice oxygen in the OCs, thus improving the redox reactivity. During the oxidation stage, BaAI₂O₄ rapidly gained an additional weight of 0.5 wt%, which also evidenced that BaAI₂O₄ can accommodate substantial oxygen non-stoichiometry during the redox cycles (Zhang, L. W. et al., Adv. Funct. Mater. 2007, 17, 3781-3790). The TGA tests of pure BaAI₂O₄ in a CO atmosphere proved that BaAI₂O₄ was seemingly redox active (Figure 15(c)) and could participate in the redox reaction as a promoter under a variety of operations conditions, including CO-TGA and H₂-TPR (Figure 15(d)). H₂-TPR was conducted to further examine the nature of the lattice oxygen in the OCs. Figure 15(d) shows the TPR profiles for IO and IO-10BA before and after redox cycles. Under identical experimental conditions, the reduction peaks shifted to lower temperatures for IO- 10BA, manifesting the improved reducibility and lattice oxygen activity. The intensity of the TCD signals for IO-1 OBA were also enhanced. Thus, IO-1 OBA has higher H₂ consumption and oxygen carrying ability than pristine IO.

For fresh IO, two reduction peaks ranging from 600 to 950 °C are present. The first peak around 800 °C can be assigned to the transformation of Fe₂O₃ to Fe₃O4. The second peak around 927 °C can be ascribed to the partial reduction of Fe304 to FeO. This is in line with the observation in TGA, where the successive reduction steps overlap with each other (Figure 15(b)). Only partial reduction of Fe₃O₄ to FeO was achieved at 900 °C. The weak peak observed at low temperature (308 °C) could be caused by surface adsorbed oxygen. After redox cycles in bFB, the TPR peaks of IO shifted to lower temperatures, revealing the increased reducibility, which is in agreement with the TGA results. For fresh IO-10BA, the peaks corresponding to the transformation of Fe₂O₃-Fe₃0₄ and Fe₃O₄- FeO shifted to 784 and 910 °C, respectively, which are lower in temperature than in the reduction of fresh IO. In addition, two new peaks appeared around 171 and 274 °C, which may be attributed to the release of lattice oxygen of BaAl₂ O₄, as shown in the TGA results (Figure 15(c)). After redox cycling, the low temperature peaks disappeared which suggested the loss of BaAl₂ O₄ from the OC surface. After 20 cycles, the Fe₂O₃-Fe₃O₄ peak of IO-10BA shifted to around 581 °C, which is much lower than that of IO. It was also observed that the Fe₃0₄ was further reduced to FeO with a major peak at 810 °C. Due to the short reaction time in TPR, the complete reduction peak of FeO-Fe after 850 °C was not observed, which was also reported previously when IO was used as OCs (Huang, Z. etai, Energy Fuels, 2013, 28, 183-191). It is worth noting that IO-10BA after 20 cycles showed an apparent H₂ consumption peak at 337 °C, which is possibly ascribed to the formation of perovskite type iron ferrite species, e.g. BaFe0₃, as a result of the chemical interaction between BaAI₂O₄ and Fe₂O₃. This assertion is supported by a previous study, which reported a broad H₂ consumption peak between 280 and 480 °C during the H₂-TPR of BaFeO₃ (Xian, H. etai, J. Phys. Chem. C 2010, 114, 11844-11852), producing Ba₂Fe₂O₅ and H₂0 (Ciambelli, P. et ai., Appl. Catal. B 2001 , 29, 239-250; and Rousseau, S. et ai., Appl. Catal. B 2009, 88, 438-447). The IO-10BA after 20 and 50 cycles showed similar TPR profiles except a slightly shift of the reduction peak to lower temperatures from 20 cycles to 50 cycles. This is in agreement with the TGA result suggesting that the performance of IO-10BA tended to be stable after 20 cycles.

Example 7

A hydrothermal method, was used to synthesize Ba-AI-loaded IO, or IO-BAx

(x = 2, 4.5 or 4.6, 8, and 12). Commercial Ba(NO₃)₂ can be used for the synthesis or Ba(N0₃)₂ can be prepared from other compounds using literature protocols. Urea was used as a precipitant and morphological regulator, for the formation of small size particles of Ba and Al compounds.

11.1 g of Ba(NO₃)₂, prepared from BaCO₃(99 % purity) and HN0₃ (65 % purity), was dissolved in 200 ml distilled water. The temperature of the solution was increased to 70 °C. 72.1 g of AI(NO₃)₃·9H₂O (98 % purity) was added to the reaction mixture, followed by 19.8 g urea (99.5 % purity) that was dissolved separately in 50 ml distilled water. Then, the solution was poured into an autoclave and placed into an oven at 100 °C for 18 h. The produced material was mixed with 280 g of IO (50-300 υm), dried at 100 °C and calcined at 950 °C for 5 h. The sample was crushed and sieved to obtain particles with sizes between 63 and 212 μm. The molar ratio of Al to Ba was in a range of 2 to 12 (Table 5). For this example, the Al and Ba are presented in their oxides form in the following section.

Table 5. Prepared samples for HCI adsorption test.

Characterisation

The morphology of the OC samples prepared in Example 7 was analyzed with FESEM as described in Analysis Techniques (Figure 16).

All samples contained the coating comprising Ba and Al elements. The coating was mainly found in the form of nano-rods and less in the form of nano-plates. At some parts, the nanostructure was sintered together resulting in a flat and dense coating surface. There was no significant difference in the coating morphology produced by varying the molar ratio of Al to Ba.

The following table shows the results of the porous properties of the samples with different Al to Ba ratios.

Table 6. Porous characteristics of the prepared samples.

General Procedure 3

The experimental setup for syngas combustion/dechlorination and sorbent regeneration is presented in Figure 17. The equipment comprises four parts: gas mixing module, reaction module, sampling module and monitor module. In the gas mixing module, five gas components are controlled by mass flow controller (MFC), and HCI are dissolved into deionized water, and simultaneously pumped into the heating transfer line through a syringe pump. From the outlet of syringe to the inlet of impinge, all the transfer lines and the connectors are bandaged by heater, and set at the temperature of 270°C, to avoid the condensation of H₂O and HCI at the transfer line. As for the reaction module, the reactor is a fluidized bed, made of Inconel, with the inert diameter of 25 mm. For the even heating of the reactor, the furnace is designed as three segments, and their temperatures are controlled by three TCs, respectively. The reactor length heated by a three zone electric furnace was ~90 cm, and the sample was placed 35 cm above the bottom of reactor. The temperature inside the reactor was monitored by two K- type TCs. The top TC tip was placed inside the particle bed, having a height of 3.5 cm (in a non-fluidized state). The tip of the second TC was located 3-5 mm below the sample bed holder and measured the gas temperature contacting the sample. The gas residence times before and inside sample were ~8.8 s and -0.88 s, respectively. The minimum fluidized velocity ( V mf) of the 63-212 μm sample particles was in the range of 0.011-0.125 m/s while the gas velocity (1000 ml/min) at 800 °C was 0.134 m/s. Due to the fluidized state of sample particles, the temperature distribution is relatively uniform at the experimental conditions and the temperature measured by the top TC could be regarded as the real sample temperature. A bypass was designed to collect the blank gas samples. In the sampling module, the HCI before or after reaction was collected through the absorption of deionized water in the impinger in the ice-water bath, and sampled with a small syringe at different times. At the same time, the gas products were collected through a gas sampling bag. For safe and stable operation, pressure gauges were set at the inlet and outlet of reactor. The reactor temperature at bottom and top part, as well as the pressure conditions were measured and controlled by the control box, connected and controlled by a computer.

In the reduction stage (syngas combustion/dechlorination), 30 g of the OCs were placed into the reactor, and was placed 30 cm above the inlet in the reactor. The simulated syngas components were set at: N₂ 455 ml/min, CO 90 ml/min, CO2 70 ml/min, H₂ 70 ml/min and H₂0/HCI solution 0.254 ml/min (315 ml/min in gas, with 300 ppmv HCI) (Chan, W. P. et ai, Appl. Energy 2019, 237, 227-240). Before introducing the model syngas into the reactor, the reactor was heated up to 800 °C and then purged with N₂ in 800 ml/min for 10 min, and the HCI sample was collected with the syringe. The reduction/sorption in the presence of simulated syngas was continued for 230 min. The temperature fluctuation in a typical run was ±1 °C, suggesting that endo- and exothermic reactions did not influence the sample temperature significantly. When the reaction starts, all the gas valves and syringe pump were turned on. The sampling time of HCI was designed as 0, 2, 5, 10, 20, 30, 45, 60, 90, 120, 150, 180, 210 and 230 min, while the gas sampling time was 1-1.5, 2-2.5, 3-3.5, 4-4.5, 7-7.5, 10-10.5, 30-30.5, 60-60.5 min, and the gas from bypass was also sampled after reaction. The baseline of gas concentrations was measured prior to the combustion/dechlorination using the bypass. The HCI solution and gas samples were analyzed using ion chromatograph (IC, Dionex ICS-1100) and gas chromatograph (GC, Agilent 7890B), respectively. The sorption tests were repeated at least two times and the results are reported as averages.

The HCI concentration (ppmv) at the reactor outlet, C_k, was calculated as follows:

where k is the label of each sample, K = 1 , 2, ... 14; V_k is the volume of solution in the impinger, ml_; V₀ is the initial volume, ml_; V_s is the sampling amount, ml_; V_w ,_k is the volume of water condensed into the impinger, mL ; c_k is the Cl- concentration of the kth sample, measured by IC, mg/L; Q is the total flow of model syngas, 1 L/min; t_k is the time of kth sampling, min.

The breakthrough of sorbent component of OCs was defined at the HCI concentration of 50 ppmv, and the HCI breakthrough capacity ( q_b , mmol/g) was defined as the HCI sorption amount per unit mass of OC at the breakthrough time, and calculated by:

where n_blk and n_b are the HCI amounts released from the reactor at the breakthrough time without and with OCs, respectively, mmol; m_oc is the mass of OC, g.

In the sorbent regeneration stage, the feasibility of using a flue gas from syngas combustion was investigated. The materials remained in the reactor after the completion of reduction stage and a model flue gas mixture was set at: N₂375 ml/min, CO₂200 ml/min, H₂O (I) 0.342 ml/min (425 ml/min in gas). Prior to the introduction of model flue gas, the reactor was adjusted to the regeneration temperature of 800 °C, 900 °C or 1000 °C and the reactor was purged with 800 ml/min N₂ for 10 min. The model flue gas mixture was introduced into the reactor for 170 min and the sampling time of HCI was 0, 2, 4, 6, 8, 10, 15, 20, 30, 40, 50, 60, 90, 120, 150 and 170 min. The HCI solution and gas samples were analyzed using ion chromatograph (IC, Dionex ICS-1100) and gas chromatograph (GC, Agilent 7890B), respectively.

For sorbent regeneration, the regeneration efficiency in 170 min (R_cl,170,%) was defined as the ratio of desorbed Cl to sorbed Cl during sorption:

where C_0.cl , Cci and C_r,cl are the Cl contents in the OC before reaction, after reduction and after regeneration stage, respectively, wt%.

Due to differences in the total gas volume rate before and after reaction, in the calculation of combustion efficiency in bFB tests, the flow rate of each component was calibrated with ISh. The N₂ flow remained constant during the reaction, whereas the N₂ concentration was measured by GC. The combustion efficiency (η) of CO and H₂ was calculated by:

where is the combustion efficiency of component / (CO or H₂) at time j, %; X_N2,0 and X_{N2 , j} are the measured concentrations of N₂ at the reactor inlet and outlet of time j, respectively; X_i,0 and X_i,j are the measured concentrations of the gas component / at reactor inlet and outlet of time j, respectively, vol%.

A CO2 production rate (Pco_{2 ’}% ) was adopted to compare the CO₂ amount produced in the reaction for different OCs, and calculated by:

Example 8

The OC samples prepared in Example 7 were tested for their CLC property in bFB as described in General Procedure 3.

Results and discussion The variation of the gas concentration for H₂, CO and C0₂ during the 60 min cycle is shown in Figure 18. For both IO and IO-BaO AI₂O₃, the combustion efficiency of H₂ and CO gradually decreased with reaction time due to the limited amount of lattice oxygen in the catalyst. However, the combustion efficiency of I0-BaO·Al₂O₃ remained higher than that of IO until the complete consumption of the applicable lattice oxygen, indicating that the reactivity of IO was improved. In this case, the applicable lattice oxygen in the catalyst was mostly consumed during the first 10 min. After which, the concentration of H₂ and CO in the outlet stabilized, as can be seen in Figure 18. Similar to the results observed in Figure 6, the loading of Ba0 AI₂O₃ on IO significantly enhanced the activity of IO even though the preparation method was different. It is worth noting that in the real chemical looping application, interconnected fluidized bed reactors are usually used, in which the solid catalysts are circulated in between the FR and AR. The lattice oxygen in the fresh catalyst is consumed in the FR by CO/H₂ and the replenishing of the lattice oxygen for the reduced catalyst is realized in the AR. If the applicable lattice oxygen is mostly consumed in the FR, the combustion efficiency of CO/H₂ will decrease dramatically, which is not desired. So to maintain a high CO/H₂ combustion efficiency in FR, the regenerated catalysts from the AR will be sent back to the FR to provide enough lattice oxygen for CO/H₂ conversion. The modification with Ba0 AI₂O₃ significantly improved the activity of the IO, especially for CO conversion. This means that with the same catalyst circulating rate between the FR and AR, much higher combustion efficiency can be achieved for IO-BA than the pristine IO, which will reduce the operation costs significantly.

Example 9

The OC samples prepared in Example 7 were tested for their HCI adsorption property in bFB as described in General Procedure 3.

Results and discussion

Figure 19 shows the removal of HCI by the samples prepared in Example 11. The HCI outlet concentration was back-calculated with the equations in General Procedure 3. The baseline was measured 4 times without any loading while the measurements with sorbents were duplicated and the results were reported as averages. In the absence of sorbent, the HCI concentration at the outlet of the reactor was 250-300 ppmv after 30 min of measurement. Compared to the baseline and IO, I0-Ba0-Al₂0₃ could decrease the concentration of HCI in the gas to lower levels indicating the ability of material to remove HCI at high temperature. Combining the results in Figures 18 and 19, it can be found that the CLC of syngas and the removal of HCI can be achieved effectively at the same time, which is the advantage of the catalytic sorbent in the current invention.

Example 10

To show the influence of preparation methods, the activities of modified OCs prepared by wetmixing method described in Example 1, and hydrothermal method described in Example 7, were investigated using General Procedure 3, and compared.

Results and discussion

The combustion efficiencies of H₂ and CO for the BaAI₂O₄-doped 10 samples prepared by hydrothermal (IO-BaO AI₂O₃) and wet-mixing methods (IO-BaAI₂O₄) were compared and shown in Figure 20. The BaO contents as the active dechlorination component in hydrothermal and wet-mixing methods are 1.97 wt% and 6.49 wt%, respectively. Although the Ba contents from hydrothermal method was lower than that from wet-mixing method, the syngas combustion efficiency of IO-Ba0·Al₂O₃ prepared by hydrothermal method were higher than IO-BaAI₂O₄ prepared by wet-mixing method for the initial duration of approximately 8 minutes.

For the simultaneous removal of HCI (shown in Figure 21), the sample prepared by hydrothermal method showed stronger ability in HCI adsorption because the HCI adsorption breakthrough time was longer (55 min) than the sample prepared by wet-mixing (27 min). Considering the results of the CLC and HCI removal tests, the hydrothermal method is more suitable for the preparation of the modified OCs. It is most likely that the distinct structure of the OCs prepared by hydrothermal method led to the better performance. As can be seen from the SEM morphology (Figure 2(B-4) and Figure 16), a structure containing both nanorod and nano-plate structures with a thickness of ~100 nm was formed on the IO surface for the OCs prepared by hydrothermal method while BaAI₂O₄ particles with a size of ~1 μm were decorated on the IO surface for the OCs prepared by wet-mixing method. This nano-plate structure may enhance the interaction of Fe₂O₃ with BaAI₂O₄ and facilitate the lattice oxygen diffusion during the CO/H₂ reduction reaction, thus resulting in a better performance.

Therefore, the hydrothermal method was used to synthesize the OCs used in the following examples. Example 11

The performance of I0-Ba0-Al₂0₃ prepared in Example 7 was tested in real MSW syngas. Figure 22 shows the experimental setup. The raw syngas contained 200-400 ppmv HCI and Figure 23 depicts the real MSW syngas composition: 20-25 vol% H₂O, 40-50 vol% N₂, 5-11 vol% CO, 5-13 vol% H₂, 1-2 vol% CH₄, 10-12 vol% CO_2.

Results and discussion

The total syngas combustion efficiency was 77-94% which increased gradually due to activation of the OC. The CO and H₂ combustion efficiencies of IO-BaO AI₂O₃ in real MSW syngas were within the range of 82-97% and 86-97%, respectively (Figure 24(a)). After 5 cycles, the combustion efficiencies of CO and H₂ were nearly stable at ~97%, and the total syngas combustion efficiency was maintained at 90-94%.

After reacting with I0-Ba0·Al₂O₃, the HCI concentration in the flue gas was reduced to an extremely low level (Figure 24(b)). In the initial 7 cycles, the HCI concentration could be controlled to the level of < 10 ppmv, but it started to increase at the 8th cycle, with a breakthrough time of ~30min. Therefore, the catalytic sorbent I0-Ba0-Al₂0₃ had high combustion efficiency and HCI removal capacity for real MSW syngas.

Example 12

To explore the effect of Cu doping, the activity of IO-BaO AI₂O₃ prepared in Example 7 and a catalytic sorbent comprising of Cu-doped IO and Ba-AI sorbent (l05Cu-BA) was tested. The experimental setup is as described in General Procedure 3 except that a model syngas composed of 10 vol% CO, 10 vol% H₂, 10 vol% CO2, 25 vol% H₂O, 45 vol% N₂ and 500 ppmv HCI (300 ml/min) was used to react with 20 g of catalytic sorbents. The reduction and oxidation time were 4 and 5 min in each cycle, respectively.

The preparation method of l05Cu-BA is similar to that described in Example 7. Ba(N0₃)₂ (8.52 g), AI(NO₃)₃·9H₂O (24.51 g), Cu(NO₃)₂· 2.5H₂O (14.56 g) and urea (60 g) were dissolved in Dl water (100 mL). The solution was poured into an autoclave and placed into an oven at 100 °C for 18 h. The produced material was mixed with IO (50-300 urn), dried at 100 °C and calcined at 950 °C for 5 h. The sample was crushed and sieved to obtain particles with sizes between 63 and 250 μm. In this catalytic sorbent sample, the OC is a combination of Fe₂O₃ and CuO, and the sorbent component is Ba0 AI₂O₃. The weight ratio of OC to sorbent is 91:9. Results and discussion

Both l05Cu-BA and I0-BaO·Al₂O₃ had high syngas combustion performance, with efficiency of >99.5% (Figure 25(a-b)). With the doping of Cu, the combustion efficiency was further improved to >99.8%. The Cu-doped catalytic sorbent reduced the emission concentrations of CO and H₂ from 300-400 ppmv to <150 ppmv (Figure 25 (c-d)). Both l05Cu-BA and IO- BaO· AI₂O₃ displayed HCI removal ability as they reduced the HCI concentration in flue gas from 500 ppmv to <20 ppmv, suggesting comparable performance (Figure 25(e)).

Therefore, the catalytic sorbent sample containing Cu did not have sintering problem in fluidized bed reactor, thus suggesting applicability for CLC.

Example 13

The OCs, IO-Ba0.5, IO-BA2, IO-BA4.5, IO-BA8, IO-BA12, were prepared using the hydrothermal method described in Example 7. In the preparation, the molar ratio of urea to NO₃- was 2. The weight ratio of sorbent/IO was 5%, and the molar ratio of Al/Ba in sorbent was 0.5, 2, 4.5, 8, 12, respectively (labeled as BA0.5, BA2, BA4.5, BA8 and BA12), which can be expressed in a general form of BaO· xAI₂O₃ (x=0.25, 1, 2.25, 4, and 6). For comparing before and after Ba-AI modification, IO after calcination and sieving into 63~212 μm was also prepared.

Characterisation

Fresh OCs were characterized as described in Analysis Techniques. In addition, the metal composition of OCs was measured by acid digestion and Inductively coupled plasma-optical emission spectrometry (ICP-OES, Perkin Elmer Optima 8300). The OCs were digested using 7.2 mL HNO₃ (60 wt%), 10 mL HCI (37 wt%), 1.2 mL HF (50 wt%) and 10 mL HBO 3 (4.5 wt%) in PTFE vessels heated in a microwave digestion unit (Ethos 1 , Milestone, Inc). The total Cl content in OCs after sorption and desorption was measured by dissolving samples in deionized water and characterizing Cl- in ion chromatography (IC, Dionex ICS-1100).

The composition of fresh OCs was characterized using ICP-OES and is shown in Table 7. It was found that iron was the main metal in OCs. Furthermore, based on the ~60 wt% Fe content in IO, the calculated content of Fe₂O₃ was ~85 wt%. Other identified IO components included Ti, Si, Ca, Mg and Mn. After the coating of IO with Ba-AI sorbent, the Fe content in OCs decreased to 57.2-60.0 wt%, subjected to the Ba-AI loading. The actual molar AI:Ba ratios in IO-Ba0.5, IO-BA2, IO-BA4.5, IO-BA8 and IO-BA12 were 0.8, 2.3, 3.7, 7.8, 12.7, respectively. Table 7. Composition of prepared OCs (wt%).

The surface morphology of the prepared OCs is shown in Figure 26. The calcined IO had some agglomeration on its surface. After the modification with Ba-AI sorbents, the IO surface changed significantly. The surface of the modified-IO was coated with rich nanorods which were identified with EDS to be the Ba-AI sorbents (shown in Figure 26(g)). As mentioned in previous examples, the Ba-AI nanorods facilitated the contact between OCs and syngas. On the other hand, there was no significant change in morphology among OCs with sorbents having different molar ratio of Al/Ba. Table 8 shows the surface areas and pore volumes of different OCs. It can be seen that IO had the lowest BET surface area (0.21 m²/g). After coating the IO with Ba-AI sorbents, the BET surface area increased to 0.44-0.56 m²/g. At the same time, the coating of Ba-AI sorbents improved the total pore volume of OCs.

Table 8. Surface area and pore volume of fresh OCs.

The XRD patterns of different OCs are shown in Figure 27. In all fresh OCs, the XRD peaks corresponding to the crystalline phase of Fe₂O₃ were predominant. Additional XRD peaks identified SiO₂ as an impurity. Due to the small ratio of Ba-AI sorbents, the phase of Ba-AI- containing compound was invisible, except for IO-Ba0.5, in which BaO·6Fe_2-xAl_xO₃ could be detected. Among the OCs decorated with the sorbent, IO-Ba0.5 that contained the highest Ba content (i.e. 3.78 wt%) and had additional XRD peaks identified as BaAIFe₁₁O₁₉. These results indicated that there were interactions between the Ba-AI sorbents and Fe₂O₃, thus causing the formation of a new species. The intensities of XRD peaks corresponding to BaAIFe₁₁O₁₉ in other OC samples were below instrumental detection level, owing to the low Ba content. Although the synthesized Ba-AI sorbents in this study were present as oxides rather than BaCO₃, the oxides would be gradually transformed into BaCO₃ after reacting with syngas, in which there is sufficient CO_2.

The XPS results of the prepared OCs are shown in Figure 28. The oxygen peak was separated into two peaks, corresponding with O_ad and O_la, respectively. Generally, the adsorbed oxygen peak is located at higher binding energy, presenting higher activity than the lattice oxygen in redox reaction. Material with higher content of adsorbed oxygen had higher reactivity in the oxidation of fuel. Table 9 shows the distribution and position of the two kinds of oxygen species on the surface of OCs. It was obvious that the modification of Ba-AI sorbent significantly increased the amount of adsorbed oxygen, and the O_ad content almost increased as the barium content increased. This means that the modification of Ba-AI sorbent might enhance the reactivity of OCs in CLC. At the same time, for the sample of IO-Ba0.5, IO-BA2 and IO- BA4.5, their O_ad content was similar. It indicated that the oxidation activity might not be enhanced significantly when the Ba content increased to a certain level. In the Fe 2p spectra, 2p 3/2, 2p 1/2 and satellite of all OCs were located at 711.0 eV, 725.0 eV and 719.5 eV (ascribed to Fe³⁺), respectively. The peaks for Fe²⁺ with the binding energy lower than 709.7 eV (Zeng, D. et al., Int. J. Hydrogen Energy, 2019, 44, 21290-21302) were not observed. Among the different OCs, the modified OCs had lower peak intensity, due to the lower Fe content, but there was no significant difference in their binding energy position. Hence, the modification did not result in the coordination condition of Fe³⁺.

Table 9. Oxygen species distribution on the surface of OCs.

In C 1s spectra, the decorated OCs had an obvious peak located at 289.1 eV as compared to the spectrum of IO. Since the C 1s in BaC03 was located at 289.1-289.4 eV (Dissanayake, D. et ai, J. Catal., 1993, 139, 652-663; and Christie, A. et al., Appl. Surf. Sci. 1983, 15, 224- 237), it can be inferred that BaC03 was one of the Ba compounds in the decorated OCs. The XRD pattern of the Ba-AI-based sorbent (BA0.5, prepared via the same method without

mixing with 10) in Figure 29 suggested that BaCO₃ can be formed during the synthesis of OCs. The carbonation of Ba compound was ascribed to the CO₂ generated from urea decomposition during sample preparation. However, BaCO₃ was not observed in the XRD patterns (Figure 27) which could be due to its low content in the OC samples. The remaining peaks located at lower binding energies (284.6 eV and 286.2 eV) were attributed to C-C and C-O from the external contamination of carbon, respectively. It was ascribed to the adsorption of CO₂ from air during the preparation and storage of samples.

In the Ba 3d spectra, the binding energies of all the OCs were located at 795.5 eV (3d 3/2) and 780.2 eV (3d 5/2). The binding energy of Ba 3d5/2 in BaCO₃ was reported as 779.8-780.4 eV (Christie, A. etal., Appl. Surf. Sci., 1983, 15, 224-237; Gauzzi, A. etal, Vacuum, 1990, 41, 870-874; and Wang, J. et ai, Catal. Commun., 2006, 7, 59-63), and that of Ba Fe₁₂O₁₉ (the same structure with BaAIFe₁₁O₁₉) was located at close binding energies of 779.6-780.1 eV (Wang, H. et ai., J. Alloys Compd., 2017, 710, 510-518; and Wang, L. et ai, J. Magn. Magn. Mater. 2015, 377 , 362-367). Thus, BaCO₃ and BaAIFe₁₁O₁₉ could not be distinguished from the Ba 3d spectra. Based on the discussion above, BaCO₃ and BaAIFe₁₁O₁₉ were the main Ba compounds in the OCs.

Example 14

The CLC performance of modified IO prepared in Example 13 was evaluated in TGA. The TGA tests were carried out as described in General Procedure 2, except at 800°C instead of 900 °C.

Results and discussion

Figure 30 shows the weight loss of OCs in the reduction of CO. According to the results, all of these OC samples could undergo complete reduction by CO and oxidation by air. For better comparison among different OCs, the weight loss rate of OCs in the second cycle was calculated and presented in Figure 30(b). According to the DTG curves, the reaction process could be generally separated into 2 stages based on the fact that Fe304 could be fully reduced into wustite before it is reduced into metallic iron (Pena, J. etal., Catal. Today, 2006, 116, 439- 444). In the first stage, there was a great weight loss rate, which corresponded to the conversion of Fe₂O₃→Fe₃O₄→FeO. Fe₂O₃ could rapidly release lattice oxygen, and this process is the conversion used in CLC process. The second stage mainly corresponded to the deep reduction of FeO→Fe, and it could be avoided in real CLC operation through controlling the dwell time of OC.

After modification of the 10 with Ba-AI sorbents, the weight loss, as well as the weight loss rate, significantly increased in the reduction process. This indicated that the modification of 10 with Ba-AI sorbents greatly improve its activity in the oxidation of CO. At the same time, in the first stage, as the Ba content in OCs increased, the weight loss rate of OC almost continuously increased. However, the weight loss rates of IO-Ba0.5, IO-BA2 and IO-BA4.5 were similar. Therefore, there is a limit for the Ba modification enhancement for the activity of 10.

Example 15

The CLC performance of the prepared OCs in Example 13 was tested in bFB as described in General Procedure 3.

Results and discussion

Due to the presence of steam and CO₂ in syngas, the OCs would be converted to Fe₃O₄ instead of Fe/FeO. According to the combustion efficiency results in Figure 31, the reaction could be divided into two stages, corresponding to the CO oxidation by OCs and syngas catalytic conversion by reduced OCs, respectively. In the first 4 min, the syngas was oxidized by OCs, and almost remained at a stable conversion rate. After 4 min, the combustion efficiency went down because of insufficient lattice oxygen. 30 min later, the conversion stabilized at a value. This indicated that there was still some ongoing reaction between the syngas and reduced OCs. Due to the active phase from Fe₃O₄(Wang, X. J. etal Top Catai, 2013, 56, 1899-1905), this effect could be the catalytic conversion of WGS (H₂O + CO → H₂ + CO₂), which converts H₂O and CO into H₂ and CO2.

Since the oxidation stage with sufficient oxygen (i.e. first 4 minutes of oxidation) was only adopted in CLC, the average combustion efficiency of CO and H₂ within 4 min was calculated and shown in Table 10. It is obvious that the combustion efficiency had been greatly improved with the modification of Ba-AI sorbents, especially for CO conversion, which increased from 62.17% into values higher than 95%. As for different sorbents, the syngas conversion rate was in the order of: IO-BA12 < IO-BA8 < IO-Ba0.5 < IO-BA2 < IO-BA4.5. It meant that the increase in Ba content in OC could enhance the conversion of syngas, but excess Ba content could not further improve the syngas combustion. In this work, IO-BA4.5 had the greatest syngas combustion efficiency. In addition, according to the combustion efficiency after 30 min, the modification increased CO conversion and decreased H₂ conversion, with an increase in CO2 production. The results also showed that Ba-AI modification enhanced the catalytic effects of IO on WGS, which could be the reason why CO combustion efficiency was higher than that of H_2.

Table 10. Average combustion efficiency within 4 min with different OCs (%).

Example 16

The HCI removal efficiency of the OCs prepared in Example 13 was tested in bFB as described in General Procedure 3.

Results and discussion

The breakthrough curves of HCI removal by OCs are shown in Figure 32. The breakthrough time was defined as the time when the concentration of HCI in the outlet gas reached 50 ppmv. The baseline experiments without OC showed that the HCI concentration stabilized after the first 30 min and was approximately 300 ppmv. The fresh IO showed negligible HCI removal. On the other hand, both the OCs, IO-BA12 and IO-BA8, showed a weak HCI adsorption effect at the beginning of 45 min. However, the OCs with higher Ba content displayed significant HCI adsorption effect. For IO-BA4.5, IO-BA2 and IO-Ba0.5, the concentration of HCI was kept at lower than 50 ppm at 45, 60 and 120 min, respectively, and the time of exhausting all the sorbent was 120, 150 and 230- min, respectively. Therefore, as the Ba content increased, the HCI removal efficiency increased.

Figure 33 shows the HCI removal efficiency at the beginning of 30 and 60 min for different OCs. The results showed that the order of different sorbents in HCI removal capacity was: IO < IO-BA12 < IO-BA8 < IO-BA4.5 < IO-BA2 < IO-Ba0.5. For BA12, BA8 and BA4.5, the removal efficiency decreased from 30 to 60 min, due to the consumption of the active component in the sorbent. For BA2 and BA0.5, HCI concentration was at a relatively stable stage between 30-60 min, and since their sorbent was not depleted, their removal efficiency increased at 60 min. For the sorbent BA0.5, the removal efficiency remained high and reached 92.4% at the beginning of 60 min. The HCI removal by these OCs could be attributed to the BaC03 and Ba-Fe-AI composite (e.g. BaAIFe₁₁O₁₉) on the surface of OCs since the formations of FeCl ₃ (or FeCl ₂) and AlCl ₃ during the dechlorination process were unfeasible in thermodynamics, according to the positive ΔG values of chlorination of AI₂O₃ (Figure 34), Fe203 and Fe₃O4 (Wang, J. et al., Combust. Flame 2015, 162, 3503-3515) at 600-1000 °C.

The XPS spectra of Ba 3d and Cl 2p are shown in Figure 35. According to Figure 35(a), both Ba 3d3/2 and Ba 3d5/2 spectra of spent IO-Ba0.5 could be separated into two groups of peaks: (1) 797.1 eV and 795.5 eV and (2) 782.0 eV and 780.2 eV. The higher binding energies of 797.1 eV (3d 3/2) and 782.0 eV (3d 5/2) were consistent with that of BaCl ₂ (i.e. 797.3 for 3d 3/2 and 782.0 eVfor3d5/2 (Kim, S.-.B. etal., J. Vac. Sci. Technol. A: Vac. Surf. Films, 2000, 18, 1381-1384)). Furthermore, the Cl 2p3/2 was located at the binding energy of 199.7 eV, which was close to the result (200.0 eV) reported in the literature (Kadono, K. et al., Phys. Chem. Glasses 1994, 35, 59-64). Therefore, it is reasonable to conclude that BaCl ₂ is the main product during the reactions between HCI and Ba compounds (i.e., BaCO₃ and BaAIFe₁₁O₁₉).

Figure 36 shows the HCI breakthrough capacities ( q_b ) of different sorbents loaded on 10 surface. The HCI breakthrough capacity gradually increased with the Ba loading, showing consistent results with breakthrough curves. The highest HCI breakthrough capacity was observed for IO-Ba0.5 (0.0345 mmol/g). The low HCI removal capacity was ascribed to the low loading content (5 wt%) of Ba compound on OCs. Since Cl was converted into BaCI₂, the sorbent conversion (/¾, the ratio of Ba compounds converted to BaCI₂ to that in the fresh sample) could be calculated by Eq. (15) and is shown in Figure 36.

where C_Ba is the Ba content in fresh OCs, wt%; M_Ba and M_Cl are the molar masses of Ba and Cl, respectively, mg/mol.

The results in Figure 36 indicate that only 6.51 ~17.33% of Ba reacted with HCI at 800 °C. The incomplete active compound conversion could be attributed to the diffusion resistance (including product layer, pore blocking and sintering) and gas-solid phase chemical equilibrium, which resulted in the saturation phenomenon (Liang, S. et al., Chem. Eng. J. 2020, 381, 122738; and Pachitsas, S. eta!., J. Environ. Chem. Eng. 2019, 7, 102959). →xample 17

In order to use the developed materials for simultaneous HCI sorption and CLC, it is necessary that the Ba-AI sorbent coated on OCs is regenerable. Therefore, the regeneration of the OCs prepared in Example 13 was explored in detail using General Procedure 3 .

Results and discussion

According to the results in Figure 33, IO-Ba0.5 demonstrated the strongest HCI removal property. Hence, it was adopted to study the regeneration of sorbent component at 800- 1000 °C.

The HCI desorption curves are shown in Figure 37(a. According to the results, HCI could be desorbed by a mixture of CO₂ and H₂O in the entire temperature range. The HCI concentration peaked at 8 min, 20 min and 90 min while regenerating at 1000°C, 900°C and 800°C, respectively. Thus, the time taken to reach the peak concentration of HCI was in the order of: 1000°C > 900°C > 800°C. Therefore, it is obvious that a higher temperature accelerates the regeneration process. This could be explained by the fact that the sorbent regeneration (BaCI₂ + H₂O + CO₂ ↔ BBCO₃ + HCI) is an endothermic process and higher temperature would favour the forward reaction in the equilibrium.

After regeneration for 170 min, the Cl- content in IO-Ba0.5 was quantified using ion chromatography and the regeneration efficiency was calculated (Figure 37(b)). It can be seen that the regeneration efficiency increased with temperature from 58.0% at 800 °C to 91.3% at 900 °C and finally, to 99.6% at 1000 °C. According to the XPS spectra of Ba 3d and Cl 2p in IO-Ba0.5 after 1000 °C regeneration (Figure 35), the Ba 3d binding energies of 3d3/2 and 3d5/2 shifted back to 795.5 eV and 780.2 eV, respectively, while Cl 2p peak became nearly invisible, indicating that the BaCI₂ in the spent OC was converted back to BaC03 via the following reaction BaCl ₂ + H₂O + CO₂ → BaCO₃ + 2HCI. The XPS results further confirmed the regeneration of the active compound (BaCO₃) necessary for HCI removal. Considering the short regeneration time and high regeneration efficiency, 1000 °C is the most suitable temperature for the regeneration of the sorbent component of OCs.

The structural changes in IO-Ba0.5 after regeneration process were studied using FESEM (Figure 44). The content of nanorods decreased drastically at 800 °C in comparison to the fresh sample and became negligible at higher temperatures. Part of the nanorods still remained on the sample’s surface at 800 °C, and the nanorods could be identified as the sorbent as the EDS mapping of selected areas suggested that both Ba and Al were present on the material surface. Thus, while the morphology was changed by the regeneration process at 800 °C, the chemical composition of the material was retained. When IO-Ba0.5 was regenerated at 900 °C, no significant sightings of nanorods was observed on the surface but some melting of the sorbent was seen. One of the possible reasons for the observed morphological transformation was the conversion of BaCI₂ into BaCO₃, which has a low melting point (811 °C (Ersoy, B. et al., Fuel 2008, 87, 2563-2571) and can also cause the sintering of OC components. After regeneration at 1000 °C, the granules of IO grew much bigger than that at 800°C and 900°C. The regenerated sorbent was also presented in melting form, and some melting Ba-AI sorbent even covered the entire surface in certain areas (shown in Figure 38(c)). Therefore, a higher temperature not only deteriorated the sintering problem of IO, but also caused the melting of Ba-AI sorbent. Although the findings suggest the feasibility of proposed strategy for OC regeneration, the influence of material sintering on the repeated use of material for HCI removal should be further addressed.

Example 18

To demonstrate the effect of Ba-AI sorbent on OC reactivity, TGA tests, as described in General Procedure 2, were conducted to compare the oxygen transport capacity (Cui, D. et al., Energy Convers. Manag. 2019, 202, 112209) and reducibility (Liu, S. et al., Energy Fuels 2016, 30, 4251-4262) of the different OCs prepared in Example 13.

Results and discussion

The oxygen transport capacity at 800 °C (Ro.soo) was calculated and is shown in Table 11. The weight loss of OC increased greatly from 6.81% to 12.52-17.21% with the loading of Ba-AI sorbents. According to the characterization in Example 15, the OCs contained BaCO₃ and BaAIFe₁₁ O₁₉ besides Fe₂O₃. Since there was no reaction between BaCO₃ and CO, BaCO₃ did not exhibit mass loss during the oxidation of CO. On the other hand, BaAIFe₁₁O₁₉ was one of the BaFe_xAl_12-xO₁₉ composites and it contains Fe³⁺. Therefore, both BaAIFe₁₁O₁₉ and Fe₂03 exhibited mass loss during the oxidation of CO.

Table 11. Oxygen transport capacity and the maximum weight loss rate of different OCs.

Generally, the reduction process of Fe₂O₃ could be divided into three stages Fe₂O₃ → Fe₃O₄ (R1), Fe₃O₄ → FeO (R2) and FeO → Fe (R3), which were ended at the Ro of 96.7%, 90% and 75% (according to their molar weights), respectively. According to Figure 30(b), the reduced state of IO was Fe₃O₄/FeO, while the Ba-AI-decorated OCs were mainly reduced to FeO/Fe. Since FeO could not be spontaneously reduced to metal Fe at 800 °C (negative AG in R3) and 947 °C was required for FeO ® Fe (Rao, Y., Metall. Trans., 1971, 2, 1439-1447), it is reasonable to infer that the formation of Fe-containing composites (shown in Figure 27) might affect the reduction pathway and directly reduce IO to metallic Fe omitting the intermediate stage of FeO → Fe.

The weight-loss rates ( R_w ) of prepared OCs are shown in Figure 30(c). For reaching a high combustion efficiency, Fe₂O₃ is generally reduced to Fe₃O₄ in a typical CLC process (Adanez, J. et a!., Prog. Energy Combust. Sci., 2012, 38, 215-282). Since the main DTG peak was located at the first stage of Fe₂O₃ → Fe₃O₄, the maximum weight-loss rates could be adopted to characterize the performance of OCs in CLC, as shown in Table 11. After coating the IO with Ba-AI, the reduction rates significantly increased. This indicated that the loading of Ba- Al sorbents could greatly improve the reactivity of OCs in CLC. At the same time, the R_w of OC increased gradually as the Ba content in OCs increased. The TGA data above suggested that in addition to HCI removal ability, the coating of IO with Ba and Al compounds had a positive synergistic effect on the CLC activity of the OCs.

Example 19

The TGA data above showed that the coating of IO with Ba and Al compounds improved the CLC activity of the OCs. Thus, the effect of Ba-AI sorbent on the activity of the OCs prepared in Example 13 was tested in bFB and in the presence of HCI as described in General Procedure 3.

Results and discussion

The experimental results are shown in Figure 31. In the presence of a large amount of H₂O and CO₂ in the syngas, which have oxidizing effects on FeO/Fe (FeO/Fe + H₂O → Fe₃O₄ + H₂) (Liu, G. et a/., Energy Convers. Manag. 2018, 160, 262-272), the OCs were mainly converted to Fe₃O₄ instead of Fe/FeO (shown in Figure 39), which was consistent with our previous study (Wang, H. et al., Appl. Energy 2020, 272, 115236). The combustion efficiency gradually declined after 4 min due to the consumption of lattice oxygen. Since high conversion of fuel is required in a typical CLC process, the combustion efficiencies of different OCs in the first 4 min were compared.

The average combustion efficiency for CO and H₂ within the first 4 min is listed in Table 12, showing the reactivity of the lattice oxygen of different OCs during the initial stage. The results suggested that the combustion efficiency was greatly improved with the loading of Ba-AI sorbents, especially for the CO conversion. More than 99% combustion efficiency was achieved for IO-BA4.5 compared to the ~62% for the pristine IO. For the different OCs, the syngas combustion efficiency was in the order of: IO-BA12 < IO-BA8 < IO-Ba0.5 < IO-BA2 < IO-BA4.5. The order was different from that of R_w in TGA tests, which could be due to the presence of HCI in the syngas. A greater Ba content in the OCs led to more BaCI₂ formed on the surface, which separated the barium from the Ba-Fe composites (e.g., BaAIFenOis), and the BaCI₂-modified catalyst exhibited lower reducibility than that modified with BaO (Au, C. et al., J. Catal., 1997, 171, 231-244). Hence, the combustion efficiency had a slight decline when Ba content was higher than that of IO-BA4.5. The CO₂ production from the syngas combustion is shown in Figure 31(c). The decorated OCs could produce more CO₂ from the syngas than pristine IO, thus demonstrating the improved activity of IO loaded with Ba-AI sorbents.

According to the combustion data after 30 min, negative combustion efficiency was observed for H₂, indicating the generation of H₂. This was caused by WGS reaction, which is catalyzed in the presence of Fe₃O₄ (Wang, X. J. et al., Top. Catal. 2013, 56, 1899-1905). The WGS reaction equilibrium constant calculated by HSC chemistry 9 was 1.059 at 800 °C, while the actual reaction constant K calculated by Eq. (4) for IO at 60 min was 0.76. On the other hand, the K values for IO-Ba0.5, IO-BA2, IO-BA4.5, IO-BA8 and IO-BA12 at 60 min were 0.98, 0.97, 1.02, 1.07 and 1.04, respectively. These data suggest that WGS reaction equilibrium was not reached for the IO, while it was close to the equilibrium state for the Ba-AI decorated OCs. Hence, the Ba-AI sorbent acted as a promoter for the WGS reaction in the OCs. This promotion effect could be the reason why CO combustion efficiency was higher than that of H_2.

Similar to TGA experiments, the above results demonstrated that the coating of 10 with Ba and Al species improved the activity of OCs during the CLC process. The promotion effect could be attributed to the interaction between 10 and Ba-AI compound. BaAIFenOis formed in the decorated OCs is a Fe-substituted hexaaluminate (BaFe_xAl_12-xO₁₉), consisting of alternate stacking of rigid spinel blocks and loosely packed mirror planes (Tian, M. etal., Catal. Sci. Technol. 2016, 6, 1984-2004). The Fe³⁺ was reported to occupy the Al position in the mirror plane, leading to more lattice oxygen associated with Fe, thus facilitating the lattice oxygen mobility (Huang, F. et al., J. Energy Chem. 2019, 29, 50-57). Therefore, it is reasonable to conclude that the formation of BaFe_xAl_12-xO₁₉ enhanced the reducibility of OCs.

Compared to the depleting time of lattice oxygen in Figure 31 , the breakthrough time (in Figure 32) of IO-Ba0.5 was much longer. It suggested that the OCs could maintain low HCI concentration during the reaction in FR. Due to the regenerable nature of the sorbent component, the OC might be able to maintain the HCI sorption capacity in the multiple CLC cycles.

Comparative Example 1

MgAI₂O₄ was reported to be a promoter or a support for the preparation of OCs for CLC process (Zafar, Q. et al., Energy Fuels 2006, 20, 34-44; and Arjmand, M. et al., Energy Fuels 2011 , 25, 5493-5502). In this example, both MgAI₂O₄ and BaAI₂O₄were used to modify the IO to demonstrate the effect of MgAI₂O₄ and BaAI₂O₄ on improving the redox performance of OCs. The mass ratio of BaAI₂O₄or MgAI₂O₄ to iron ore was kept at 10:100 and the IO modified with BaAI₂O₄ and MgAI₂O₄ were denoted as IO-1 OBA and IO-10MA, respectively. IO-1 OBA and IO- 10MA were prepared using the wet-mixing method described in Example 1. Briefly, to prepare IO-10MA, 20 g of MgAI₂O₄ was added into an IO suspension made by stirring 200 g of IO in 100 ml of Dl water. The redox performances of IO-10BA and IO-10MA were tested in bFB (described in General Procedure 3, except with 20 g of OCs) and TGA (described in General Procedure 2) while the HCI removal performance of IO-1 OBA and IO-10MA was tested in bFB only (described in General Procedure 3, except with 20 g of OCs).

Results and discussion

The TGA tests data are shown in Figure 40. From the TGA results, it can be seen that IO- 10BA has the highest oxygen transport capacity and reaction rate during the reduction reaction, which reveals the superiority of the IO-10BA over both IO and IO-10MA. As can be seen from the weight loss ratio in Figure 40(a), the IO sample modified by MgAl204 showed slightly higher weight loss comparing to the pristine IO. However, for IO-10BA, a significant weight loss was observed, indicating the remarkable improvement of oxygen transport capacity during the 60 min reduction of the IO due to loading of BaAI₂O₄. The oxygen transport capacity was about 9, 10, and 17 wt% for the IO, IO-10MA, and IO-1 OBA, respectively, during the 60 min of TGA testing. The weight loss rates for the three samples are shown in Figure 40(b). The weight loss rate for 10, IO-10MA, and IO-10BA are -0.99, -1.09, and -1.64 %/min, respectively. This indicated that the IO-1 OBA had the highest reaction rate among the three samples.

The combustion efficiency of simulated syngas (CO and H₂) for IO, IO-10MA, and IO-10BA was tested in a bFB reactor and the results are shown in Figure 41. For the combustion of H₂, the introduction of BaAI₂O₄ and MgAI₂O₄ both caused a decrease in the combustion efficiency. Notably, the decrease caused by MgAI₂O₄ was more significant. For the combustion of CO, IO-10MA showed a slight increase in the combustion efficiency, while a significant improvement was observed for IO-10BA. This result is consistent with the observation in TGA tests (Figure 40).

Figure 42 shows the outlet HCI concentration of the bFB reactor for IO, IO-10BA, and IO- 10MA. The loading with BaAI₂O₄ and MgAI₂O₄ caused an improved adsorption of IO, especially in the initial 50 min. After the HCI adsorption breakthrough, HCI concentration increased dramatically for both IO-10MA and IO-10BA. However, IO-10BA maintained a relatively higher HCI adsorption efficiency as compared to IO-10MA.

In general, IO modified with BaAI₂O₄ demonstrated a better performance in both CLC and HCI removal comparing to the IO modified with MgAI₂O₄.

Comparative Example 2

In patent CN 105056955, Fe was reacted to substitute some of the Al in the barium hexaaluminate structure to form BaFe_xAl_12-xO₁₉ (0<x<12) as the OCs. They were reported to have high combustion efficiency for CH₄ and high selectivity towards CO and H₂ in chemical looping dry reforming. However, these OCs are synthetic materials and cost more than readily- available natural ores. As they have a low content of active iron species for chemical looping, their oxygen transport ability is low and thus, the operation cost is increased as aforementioned. In addition, the OCs are incapable of removing HCI at high temperatures. In the current invention, instead of substituting Al with Fe, Ba and Al were directly incorporated with Fe₂O₃ to obtain the OC sample, 50%Fe₂O₃-50%BaAI₂O₄ (denoted as 50Fe-50BA hereafter). To compare the CLC and HCI removal performances of the samples prepared by these two methods, BaFe₅Al₇O₁₉ and 50Fe-50BA were synthesized and taken for bFB and TGA tests. .aFe₅Al₇O₁₉ was prepared based on the protocol described in CN105056955B. Briefly, Ba(NO₃)₂ (26.13 g), Fe(NO₃)₃·9H₂O (202 g) and AI(NO₃)₃-9H₂O O262.5 g) in a molar ratio of 1:5:7, were dissolved separately in Dl water at 60 °C to form 1 mol/L solution. Ba(NO₃)₂ and Fe(NO₃)₃·9H₂O solution were mixed uniformly and the pH was adjusted to 1 with 0.5 mol/L nitric acid. Following that, the AI(NO₃)₃ solution was added. After well-mixing, the combined solution was quickly added to excess saturated (NH₄)₂CO₃ solution at 60 °C under stirring at 300 rpm for 6 hours. The formed slurry was dried at 120 °C for 12 hours and then calcined at 500 °C for 4 hours and at 1100 °C for another 4 hours. The Fe composition was 30.8 wt% in the synthesized BaFe₅Al₇O₁₉ .

The preparation of 50Fe-50BA is similar to that described in Example 1. BaAI₂O₄ (25 g) was dissolved in 50 ml Dl water under stirring at 500 rpm at 60 °C. The Fe₂O₃ precursor, Fe(N0₃)₃·9H₂O (126.25 g), was dissolved in 100 ml Dl water at 60 °C to form a uniform solution. Then, the Fe(N0₃)₃ solution was added into the BaAI₂O₄ suspension under stirring at 500 rpm for 0.5 hour. Excess saturated (NH₄)₂CO₃ solution was then added into the mixture at 60 °C and stirred at 500 rpm for 6 hours. The formed slurry was dried at 105 °C for 12 hours and then calcined at 950 °C for 5 hours. The Fe composition was 35 wt% in the synthesized 50Fe- 50 BA.

The redox performances of BaFe₅Al₇O₁₉ and 50Fe-50BA were tested in bFB (described in General Procedure 3, except with 20 g of OCs) and TGA (described in General Procedure 2) while the HCI removal performance of BaFe₅Al₇O₁₉ and 50Fe-50BA was tested in bFB only (described in General Procedure 3, except with 20 g of OCs).

Results and discussion

The weight loss ratio and weight loss rate for 50Fe-50BA and BaFe₅Al₇O₁₉ obtained from TGA analysis are shown in Figure 43. The oxygen transport capacity was 16.1 and 12.2 wt% for 50Fe-50BA and BaFe₅Al₇O₁₉ , respectively. As aforementioned, the calculated Fe contents in 50Fe-50BA and BaFe₅Al₇O₁₉ were 35 and 30.8 wt%, respectively, corresponding to the theoretical mass loss of 16.2 and 14.9 wt% (consider the reduction of Fe₂O₃ to Fe). As observed, 50Fe-50BA almost reached its theoretical mass loss during the 60 min of reduction. However, a much lower conversion ratio of solid was obtained for BaFe₅Al₇O_19. The reaction rate of 50Fe-50BA was much higher than that of, BaFe₅Al₇O₁₉ at the Fe₂O₃-FeO-Fe reduction stage as indicated by Figure 43(b) during the reaction time of ~20 to 40 min. Taken together, both the oxygen transport capacity and reaction rate of 50Fe-50BA were better than that of BaFe₅Al₇O₁₉. Figure 44 shows the H₂ and CO combustion efficiency for 50Fe-50BA and BaFe₅Al₇O₁₉ in bFB. It can be seen that both the 50Fe-50BA and BaFe₅Al₇O₁₉ had high combustion efficiency for both H₂ and CO during the initial 2 min reaction, after which the efficiency decreased gradually as the reaction progressed. The decrease in efficiency for50Fe-50BA was much slower than that for BaFe₅Al₇O₁₉, indicating the higher activity of 50Fe-50BA. This agrees with the observed higher reaction rate for 50Fe-50BA in the TGA tests.

50Fe-50BA demonstrated a much stronger ability for HCI adsorption during the CLC process as shown in Figure 45. The HCI outlet concentration remained below 100 ppmv during the 240 min operation. In contrast, the HCI concentration started to increase significantly at 50 min for BaFe₅Al₇O₁₉.

Therefore, in general, a better performance in the CLC of syngas was observed for 50Fe- 50BA than BaFe₅Al₇O₁₉. A much higher HCI removal efficiency was obtained using 50Fe-50BA when comparing to BaFe₅Al₇O₁₉. This indicates the superiority of the proposed redox catalysts in this invention for the simultaneous CLC and HCI removal.

Claims

2. The catalytic sorbent material according to Claim 1, wherein the molar ratio of AI:Ba in the sorbent material is from 0.5:1 to 12:1 or from 2:1 to 12:1.

3. The catalytic sorbent material according to Claim 2, wherein the molar ratio of AI:Ba in the sorbent material is from 2:1 to 8:1, such as from 4:1 to 8:1.

4. The catalytic sorbent material according to any one of the preceding claims, wherein the sorbent material forms a layer on the surface of the oxygen carrier material.

5. The catalytic sorbent material according to any one of the preceding claims, wherein the oxygen carrier material is in the form of particles having a surface and the sorbent material forms a coating on the surface of the oxygen carrier material.

6. The catalytic sorbent material according to Claim 5, wherein the coating of the sorbent material on the surface of the oxygen carrier material is from 50 to 1000 nm thick, such as from 60 to 500 nm thick, such as from 75 to 250 nm thick, such as about 100 nm thick.

7. The catalytic sorbent material according to Claim 5 or Claim 6, wherein the coating of the sorbent material on the surface of the oxygen carrier material comprises both nanorods and nanoplates.

8. The catalytic sorbent material according to any one of the preceding claims, wherein the transition metal in the transition metal compound is selected from one or more of the group consisting of Fe, Ni, Cu, Mn, and Co.

9. The catalytic sorbent material according to any one of the preceding claims, wherein the transition metal compound is a transition metal oxide.

10. The catalytic sorbent material according to Claim 9, wherein the transition metal oxide is selected from one or more of the group consisting of Fe₂O₃, NiO, CuO, Mh2q3, and C03O4, optionally wherein, the transition metal oxide is Fe₂O₃ or a combination ofFe₂O₃ and CuO (e.g. in a ratio of CuO to Fe203 of from 4:6 to 0.1:1).

11. The catalytic sorbent material according to any one of the preceding claims, wherein the sorbent material further comprises a transition metal present in the oxygen carrier material, optionally wherein the transition metal is selected from one or more of the group consisting of Fe, Ni, Cu, Mn, and Co.

12. The catalytic sorbent material according to any one of the preceding claims, wherein: the oxygen carrier material forms from 50 to 98 wt%; and the sorbent material forms from 2 to 50 wt% of the catalytic sorbent material, optionally wherein: the oxygen carrier material forms from 70 to 90 wt% (e.g. from 80 to 90 wt%); and the sorbent material forms from 10 to 30 wt% (e.g. from 10 to 20 wt%) of the catalytic sorbent material.

13. The catalytic sorbent material according to any one of the preceding claims, wherein the catalytic sorbent material is in the form of particles having an average particle size of from 63 to 250 μm, such as from 150 to 212 pm.

14. The catalytic sorbent material according to any one of the preceding claims, wherein the catalytic sorbent material self-activates during a chemical looping combustion process, optionally wherein the catalytic sorbent material self-activates during a redox reaction in the chemical looping combustion process.

15. The catalytic sorbent material according to any one of the preceding claims, wherein the catalytic sorbent is thermally stable at an operating temperature of from 700 to 1,100 °C in a chemical looping process.

16. A method of forming a catalytic sorbent material, comprising the steps of:

17. The method according to Claim 16, wherein the oxygen carrier material coated with a precursor material is formed by coating an oxygen carrier material having a surface and comprising a transition metal compound with a precursor material comprising Al and Ba having a molar ratio of AI:Ba of from more than 0:1 to 12:1 , such as from 0.5:1 to 12:1, such that the precursor material is dispersed on the surface of the oxygen carrier material.

18. The method according to Claim 17, wherein the coating is accomplished using one or more of physical mixing, wet impregnation, and co-precipitation.

19. The method according to any one of Claims 16 to 18, wherein the precursor material comprising Al and Ba is formed by subjecting an aqueous solution comprising barium nitrate, aluminium nitrate and a base to a temperature of from 80 to 150°C (e.g. 100°C) for a period of time in a pressure resistant vessel (e.g. an autoclave).

20. The method according to Claim 19, wherein the base is ammonium carbonate or urea.

21. The method according to any one of Claims 16 to 20, wherein the calcination step is conducted at a temperature of from 950 to 1100 °C for a period of from 5 to 12 hours.

22. The method according to any one of Claims 16 to 21, wherein the catalytic sorbent material is a material as described in any one of Claims 1 to 15.

23. A method of chemical looping combustion, which comprises the step of supplying a catalytic sorbent material according to any one of Claims 1 to 15 as a bed material and running a chemical looping combustion process where a fuel is combusted.

24. The method according to Claim 23, wherein the fuel is selected from one or more of the group consisting of syngas, natural gas, coal, biomass, or combustible solid waste.

25. The method according to Claim 24, wherein the fuel is syngas.

26. The method according to any one of Claims 23 to 25, wherein the catalytic sorbent material removes an impurity from the fuel, optionally wherein the impurity is selected from one or more of the group consisting of HCI, H₂S and alkali chlorides.

27. The method according to Claim 26, wherein the impurity is HCI.

28. The method according to any one of Claims 23 to 27, wherein the chemical looping combustion is run at a temperature of from 700 to 1,100 °C.

29. A method for regenerating a catalytic sorbent material according to any one of Claims 1 to 15 after it has been used in a chemical looping combustion process, the regeneration comprising subjecting a catalytic sorbent material according to any one of Claims 1 to 15 that has been used in a chemical looping combustion process to an atmosphere comprising water and carbon dioxide and a temperature of from 800 to 1,100 °C for a period of time to regenerate the catalytic sorbent material.