WO2019004938A1

WO2019004938A1 - Nanostructured zinc oxide and application in warm fuel gas desulfurization

Info

Publication number: WO2019004938A1
Application number: PCT/SG2018/050312
Authority: WO
Inventors: Wen Da OH; Junxi LEI; Andrei VEKSHA; Apostolos GIANNIS; Wei-Chung Chang; Xiao Hu
Original assignee: Nanyang Technological University
Priority date: 2017-06-27
Filing date: 2018-06-27
Publication date: 2019-01-03

Abstract

Disclosed herein is a composite material made from a honeycomb substrate coated with ZnO, particularly in the form of nanosheets or nano rods that is able to remove sulfur-containing compounds from a fuel gas, amongst other applications. Also disclosed herein are a method to make the composite material and its application to the removal of said sulphur-containing compounds from a fuel gas in need thereof.

Description

Nanostructured zinc oxide and application in warm fuel gas

desulfurization

Field of Invention This invention relates to the synthesis of nanostructured ZnO loaded on a honeycomb support, as well as the resulting material and their use as a sorbent in warm fuel gas desulfurization.

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. Gasification is a promising technology for solid waste treatment and energy generation. The gasification process is used to convert solid waste into a useful fuel gas called syngas. Syngas, which predominantly contains carbon dioxide, carbon monoxide and hydrogen, can be utilized in gas engines, gas turbines or fuel cells for power generation. However, prior to its application for power generation, the fuel gas (syngas) needs to be purified to remove corrosive gases (particularly, sulfur compounds such as H₂S and COS) for corrosion protection of the downstream equipment (e.g. see K. Sato ef a/., J. Chem. Eng. Jpn., 40 (2007) 860-868 and W. Torres et ai, Catalysis Reviews, 49 (2007) 407-456). Depending on the feedstock, the amount of sulfur compounds present varies from tens of parts per million by volume (ppmv; in gas from biomass and municipal solid waste) to above 6000 ppmv (in gas from coal), which exceeds acceptable levels for downstream applications (M. Chomiak et ai, Fuel Process. Technoi, 144 (2016) 64-70). For application in gas turbines and fuel cells, the total sulfur concentration should be lower than 20 ppmv and 1 ppmv, respectively (see U. Arena, Process and technological aspects of municipal solid waste gasification. A review, Waste Manage. (Oxford), 32 (2012) 625-639; K. Liu et ai, Hydrogen and syngas production and purification technologies, John Wiley & Sons, 2009; and Y. Ohtsuka ef al, Powder Technoi., 190 (2009) 340-347).

Various methods for control of sulfur compounds have been patented previously, including: sulfur removal by caustic scrubbing at low temperature (e.g. <100 °C) (US 3435590); washing with adsorbents such as piperazine (US 4336233) or methyl isopropyl ethers (US 4330305); and metal oxide sorbents (US 4442078). Among these methods, the warm desulfurization process (above 200 °C) is desirable because of its significantly higher thermal efficiency compared to wet methods (e.g. see D. Vamvuka et al., Environ. Eng. Sci., 21 (2004) 525-548). Several other techniques, such as the Rectisol™ process, the Selexol™ process/solvent and amine scrubbing are also commercially available, but these processes can only be used at low temperature (<100°C). US 8658321 describes the use of a zeolite Y adsorbent for selective sulfur removal from a fuel stream at relatively low temperature (<100°C).

To avoid heat loss associated with the cooling of fuel gas for desulfurization, it is advantageous to conduct the desulfurization process at higher temperatures. US 5494880 describes the use of ZnO composite pellets (which consist of titania, silica and a binder) for coal gas desulfurization. US 3441370 and US 4442078 claim the use of ZnO for removing sulfur compounds from gases. Although this approach is feasible, the use of pellets/powders of ZnO for desulfurization can cause an undesirable pressure drop and plugging of the reactor, which can increase the maintenance and operational cost of the plants.

Thus, there remains a need for new materials for use as sorbents of sulfur compounds in fuel gases that can overcome the problems discussed above. In particular, there remains a need for sorbents that can operate at a higher temperature (e.g. around 400 °C) and which avoid issues relating to pressure drops and/or plugging of the reactor. In addition, it is desirable that any such new materials can be made by a method that is readily amendable to scaling-up.

Summary of Invention It has been surprisingly found that a composite material containing ZnO can be used to more effectively remove sulphur compounds from a fuel gas stream. As such, in a first aspect of the invention, there is provided a composite material suitable for use to remove sulfur- containing compounds from a fuel gas, the composite material comprising:

a honeycomb substrate material having a surface suitable for contacting a fuel gas; and

a ZnO material coating whole or part of the surface of the substrate, wherein the ZnO is provided on the surface of the substrate in the form of nanosheets or nano rods.

In embodiments of the first aspect of the invention, include those in which:

(1) the honeycomb substrate material may be a ceramic and/or metallic honeycomb substrate material (e.g. the ceramic may be a cordierite-mullite ceramic); (2) the ZnO material further comprises one or more additive materials that may be selected from the group consisting of a transition metal and a lanthanoid metal;

(3) the amount of ZnO material in the composite material may be from 0.5 to 3 wt% relative to the entire weight of the composite material (e.g. from 0.6 to 1.6 wt%, such as from 0.6 to 1.1 wt% relative to the entire weight of the composite material).

In certain embodiments, the composite material may be one in which the ZnO is in the form of nanosheets having a height on the surface of the substrate of from 0.5 to 3 pm (e.g. from 0.8 to 1.2 pm, such as around 1 pm). In such embodiments:

(1a) the ZnO nanosheets may substantially wholly cover the surface of the substrate;

(1 b) the composite material may have a total sulfur capacity at breakthrough of from 37.6 to 100 mg S/g of ZnO, such as from 40 to 59.8 mg S/g ZnO, such as 48.7 mg S/g ZnO; (1c) the composite material may have a ZnO efficiency at total sulfur breakthrough of from 9 to 20 mol%, such as from 9.6 to 20 mol%, such as from 10.5 to 15.2 mol%, such as 12.4 mol%;

(1d) the nanosheets may have a smallest measureable dimension (i.e thickness) of from 1 to less than 100 nm, or between 1 to 100 nm.

In certain embodiments, the composite material may be one in which the ZnO is in the form of nano rods where:

(2a) the nano rods have a height of from 0.5 to 2 pm on the surface of the substrate, such as around 1 pm; and/or

(2b) each nano rod has an internal diameter of from 0.1 to 1 pm, such as from 0.1 to 0.5 pm. In such embodiments:

(2aa) the ZnO nano rods partly cover the surface of the substrate;

(2bb) the composite material may have a total sulfur capacity at breakthrough of from 6.9 to 50 mg S/g of ZnO, such as from 7.1 to 20 mg S/g ZnO, such as from 9.0 to 17.1 mg S/g

ZnO, such as from 10.9 to 12.0 mg S/g ZnO;

(2cc) the composite material may have a ZnO efficiency at total sulfur breakthrough of from 1 to 9 mol%, such as from 1.8 to 5 mol%, such as from 2.3 to 4.4 mol% , such as from 2.8 to 3.1 mol%.

In a second aspect of the invention, there is provided a method of manufacturing a composite material comprising a honeycomb substrate material having a surface suitable for contacting a fuel gas and a ZnO material coating whole or part of the surface of the substrate, where the amount of ZnO material in the composite material is from 0.5 to 3 wt% relative to the entire weight of the composite material and the ZnO is provided on the surface of the substrate in the form of nanosheets, the method comprising the steps of:

(a) providing a seeded honeycomb substrate material that has a ZnO covering on a surface of the substrate suitable for contacting a fuel gas;

(b) immersing the seeded honeycomb material in a crystal growth solution comprising a first zinc oxide source and an amine base at a temperature of from 50 to 150 °C for a first period of time;

(c) after step (b) aging the resulting mixture for a second period of time at room temperature to form a ZnO nanosheet coating on the surface of the honeycomb substrate.

In embodiments of this method:

(aa) the first zinc oxide source may be Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0 or, more particularly, Zn(CH₃C0₂)2 and solvates thereof, such as Zn(CH₃- C0₂)₂-2H₂0;

(bb) the first zinc oxide source may be present in the crystal growth solution at a concentration of from 0.05 to 0.5 M, such as around 0.1 M;

(cc) the amine base may be hexamethylenetetramine;

(dd) the amine base may be present in the crystal growth solution at a concentration of from 0.05 to 0.5 M, such as around 0.2 M;

(ee) the molarity ratio of the amine base to first zinc oxide source may be from 1 :2 to 3:1 , such as around 2: 1 ;

(ff) the temperature of step (b) of the second aspect of the invention may be from 60 to 100 °C, such as around 90 °C;

(gg) the first period of time may be greater than or equal to 3 hours, such as greater than or equal to 5 h, for example from 3 to 10 h, such as around 5 h or from greater than 5 h to 8 ;

(hh) the second period of time may be greater than or equal to 8 hours, such as greater than or equal to 18 h, for example from 8 to 24 h, such as around 18 h;

(ii) the honeycomb substrate material may be a ceramic and/or metallic honeycomb substrate material, optionally wherein the ceramic may be a cordierite-mullite ceramic;

(jj) the ZnO material further comprises one or more additive materials that may be selected from the group consisting of a transition metal and a lanthanoid metal;

(kk) the amount of ZnO material in the composite material may be from 0.6 to 1.6 wt%, such as from 0.6 to 1.1 wt% relative to the entire weight of the composite material;

(II) the ZnO may be in the form of nanosheets having a height on the surface of the substrate of from 0.5 to 3 pm; (mm) the ZnO nanosheets may have a height on the surface of the substrate of from 0.8 to 1.2 μιτι, such as around 1 μιη;

(nn) the ZnO nanosheets may substantially wholly cover the surface of the substrate;

(00) the composite material may have a total sulfur capacity at breakthrough of from 37.6 to 100 mg S/g of ZnO, such as from 40 to 59.8 mg S/g ZnO, such as 48.7 mg S/g ZnO;

(pp) the composite material may have a ZnO efficiency at total sulfur breakthrough of from 9 to 20 mol%, such as from 9.6 to 20 mol%, such as from 10.5 to 15.2 mol%, such as 12.4 mol%. In a third aspect of the invention, there is provided a method of manufacturing a composite material comprising a honeycomb substrate material having a surface suitable for contacting a fuel gas and a ZnO material coating whole or part of the surface of the substrate, where the amount of ZnO material in the composite material is from 0.5 to 3 wt% relative to the entire weight of the composite material and the ZnO is provided on the surface of the substrate in the form of nano rods, the method comprising the steps of:

(i) providing a seeded honeycomb substrate material that has a ZnO covering on a surface of the substrate suitable for contacting a fuel gas;

(ii) immersing the seeded honeycomb material in a crystal growth solution comprising a first zinc oxide source and an amine base at a temperature of from 50 to 150 °C for a period of time.

In embodiments of this method:

(1) the first zinc oxide source may be Zn(CH₃C0₂)₂ and solvates thereof, such as Zn(CH₃C0₂)₂-2H₂0 or, more particularly, Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0;

(II) the first zinc oxide source may be present in the crystal growth solution at a concentration of from 0.05 to 0.5 M, such as around 0.1 M;

(III) the amine base may be hexamethylenetetramine;

(IV) the amine base may be present in the crystal growth solution at a concentration of from 0.05 to 0.5 M, such as from 0.05 to 0.2 M;

(V) the molarity ratio of the amine base to first zinc oxide source may be from 0.1 :1 to 3: 1 , such as from 0.5: 1 to 2:1 ;

(VI) the temperature of step (ii) in the third aspect of the invention may be from 60 to 100 °C, such as around 90 °C;

(VII) the period of time may be greater than or equal to 3 hours, such as greater than or equal to 5 h, for example from 3 to 10 h, such as around 5 h or from greater than 5 h to 8 h; (VIII) the honeycomb substrate material may be a ceramic and/or metallic honeycomb substrate material, optionally wherein the ceramic is a cordierite-mullite ceramic;

(IX) the ZnO material further comprises one or more additive materials that may be selected from the group consisting of a transition metal and a lanthanoid metal;

(X) the amount of ZnO material in the composite material may be from 0.6 to 1.6 wt%, such as from 0.6 to 1.1 wt% relative to the entire weight of the composite material;

(XI) the nano rods may have a height of from 0.5 to 2 pm on the surface of the substrate, such as around 1 pm;

(XII) each nano rod may have an internal diameter of from 0.1 to 1 pm, such as from 0.1 to 0.5 pm;

(XIII) the ZnO nano rods may partly cover the surface of the substrate;

(XIV) the composite material may have a total sulfur capacity at breakthrough of from 6.9 to 50 mg S/g of ZnO, such as from 7.1 to 20 mg S/g ZnO, such as from 9.0 to 17.1 mg S/g ZnO, such as from 10.9 to 12.0 mg S/g ZnO;

(XV) the composite material may have a ZnO efficiency at total sulfur breakthrough of from 1 to 9 mol%, such as from 1.8 to 5 mol%, such as from 2.3 to 4.4 mol%, such as from 2.8 to 3.1 mol%.

In embodiments of the second and third aspects of the invention, the seeded honeycomb substrate material may be provided by the steps of:

(ia) immersing a honeycomb substrate material in a seeding solution comprising a second zinc oxide source, a solvent and a base; and

(ib) drying the resulting pre-seeded honeycomb substrate at first temperature for a period of time and then calcinating it at a second temperature for a period of time to provide the seeded honeycomb substrate. In embodiments that use this method:

(iaa) the second zinc oxide source may be Zn(CH₃C0₂)2 and solvates thereof, such as Zn(CH₃C0₂)2-2H₂0 and/or Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0; and/or (ibb) the second zinc oxide source may be present in the seeding solution at a concentration of from 0.1 to 0.5 M, such as from 0.3 to 0.4 M; and/or

(ice) the amine base may be diethanolamine; and/or

(idd) the molar ratio of the amine base to second zinc oxide source may be from 1 :2 to 3:1 , such as around 1 : 1 ; and/or

(iee) the first temperature may be from 50 to 100 °C, such as around 60 °C; and/or (iff) the second temperature may be from 300 to 500 °C, such as around 400 °C; and/or (igg) the period of time may be from 1 to 5 h, such as around 2 h; and/or

(ihh) the solvent in step (ia) above is ethanol; and/or

(iii) the heating rate of the calcination step may be from 1 to 4 °C/min, such as 2 °C/min. In a fourth aspect of the invention, there is provided a method of desulfurizing a fuel gas in need thereof, the method comprising the step of passing a fuel gas in need of desulfurization through a composite material according to the first aspect of the invention and any technically sensible combination of its embodiments at a temperature of from 200 to 600 °C to provide a desulfurized fuel gas, wherein:

the fuel gas in need of desulfurization is a fuel gas containing from 30 to 10,000 ppmv of sulfur compounds; and

the desulfurized fuel gas contains less than 20 ppmv of sulfur compounds.

In embodiments of this process:

(zi) the temperature may be around 400 °C;

(zii) the fuel gas in need of desulfurization may be a fuel gas containing from 40 to 6,000 ppmv of sulfur compounds, such as from 50 to 500 ppmv of sulfur compounds;

(ziii) the fuel gas passing through the composite material may have a gas hourly space velocity of from 500 to 20,000 h^" , such as from 3,000 to 10,000 h^"1 ;

(ziv) the method may further comprise a step of regenerating the composite material by passing a regenerating gas mixture through it at a temperature of from 500 °C to 800 °C, such as 650 °C, at a gas hourly space velocity of from 2,000 to 5,000 h^"1 , where the regenerating gas mixture comprises from 0.5 vol% to 20 vol% oxygen, such as from 1 vol% to 5 vol% oxygen, such as around 2 vol%.

Further aspects and embodiments of the invention are provided in the numbered clauses below.

1. This invention relates to the synthesis and application of nanostructured ZnO loaded on honeycomb support for the removal of sulfur compounds (H₂S and COS) from warm fuel gas.

2. A sorbent structure, comprising:

a) a honeycomb support; and

b) zinc oxide nanosheets or nano rods loaded on the honeycomb support to form the sorbent, where the sorbent can be used for desulfurization at 200-600°C.

3. A method to prepare a sorbent structure, comprising the following steps:

a) cleaning a honeycomb support in ethanol and drying it; b) immersing the honeycomb support in a solution containing 20 mmol of zinc acetate dihydrate for nanosheets (or zinc nitrate hexahydrate for nano rods) in 50 ml. ethanol and 2 ml_ diethanoamine to form a coated honeycomb;

c) drying the coated honeycomb in an oven;

d) calcining the dried coated honeycomb at 400-500 °C for 2 - 5 h with a heating rate of 2 °C min^"1 to form a seeded honeycomb;

e) immersing the seeded honeycomb in a solution containing 0.05-0.2 M hexamethylenetetramine and 0.1 M of zinc acetate dihydrate (or zinc nitrate hexahydrate for nano rods) at 90 °C for > 5 h to form zinc oxide crystal; and

f) cooling the solution to 25 °C or room temperature and aging the solution for > 18 h to form zinc oxide nanosheets on the honeycomb support (this step is not needed for nano rods).

4. In one embodiment, the invention can be used to remove >80% of sulfur compounds from fuel gas at 400 °C, thereby avoiding the need to reduce the operational temperature (better thermal efficiency). The purified fuel gas can be used for electricity generation.

5. In another embodiment, the invention can minimize the risk of pressure drop due to the obstruction in gas flow thereby reducing the maintenance and operational costs. Drawings

Fig. 1 depicts FESEM images of: (a) surface view of ZnO-nR2; (b) cross sectional view of ZnO-nR2; (c) surface view of ZnO-nS; and (d) cross sectional view of ZnO-nS. Fig. 2 depicts SEM images of: (A) surface of support following seeding with ZnO; (B) surface of (A) following ZnO growth solution treatment; and (C) surface following ageing to provide ZnO nanosheet.

Fig. 3 depicts the XRD patterns of ZnO-nS and ZnO-nR1.

Fig. 4: Experimental setup for the desulfurization process experiments.

Fig. 5: Breakthrough curves for (a) H₂S, (b) COS and (c) total sulfur removal from a model fuel gas using various ZnO sorbents. Fig. 6: FESEM micrographs of ZnO-nanosheets on the surface of the honeycomb support prepared with Zn(AC)₂-2H₂0 at different HMX concentrations. The FESEM micrographs show the top view of the ZnO nanosheets. Fig. 7: FESEM micrographs of a partly-formed ZnO-nanosheet after 6 hours of the aging step using 0.2 M HMX.

Fig. 8: FESEM micrographs of ZnO loaded on the surface of the honeycomb support prepared with Zn(N0₃)₂.6H₂0 at different HMX concentrations. The FESEM micrographs show the top view of the ZnO nano rods.

Fig. 9: Cross-section FESEM micrographs of (a) ZnO-nR2, and (b) ZnO-nS; (c) and (d) EDX elemental distribution of ZnO-nS; (e) top view and (f) cross section of ZnO-nS-3. Fig. 10: FESEM micrograph of ZnO nano rods prepared with 3 growth cycles.

Fig. 11 : Breakthrough curves for (a) H₂S and (b) total sulfur removal from model syngas using various ZnO sorbents. Conditions: T = 400 °C, GHSV = 9230 h^~1 , C_H2s = 50 ppmv. FIG. 12: Breakthrough curve for (a) H₂S and (b) total sulfur removal at different space velocities using ZnO-nS and commercial sorbent. Conditions: T = 400 °C, and CH2S ⁼ 50 ppmv.

Fig. 13: Breakthrough curve for (a) H₂S and (b) total sulfur removal at different H₂S concentrations using ZnO-nS and commercial sorbent. Conditions: T = 400 °C and GHSV = 9230 h^~1.

Fig. 14: Breakthrough curve for (a) H₂S and (b) total sulfur removal at various cycles. Conditions: T = 400 °C, C_H2s = 50 ppmv, and GHSV=9230 h^"1.

Fig. 15: XRD (A), FESEM (B, after desulfurization and E after regeneration) and EDX mapping (C and D after desulfurization and F after regeneration) of ZnO-nS. Regeneration conditions: 10% air and nitrogen (balance) at 650 °C and GHSV of 3415 h^"1. Description

A ZnO sorbent can react with H₂S at moderate temperature (400 °C) to produce ZnS as follows:

ZnO + H₂S→ZnS + H₂0 (1)

The ZnO can also be used to remove COS from fuel gas as follows: ZnO + COS→ ZnS + C0₂ (2)

As disclosed hereinbefore, the current invention relates to a composite material suitable for use to remove sulfur-containing compounds from a fuel gas, the composite material comprising:

a ZnO material coating whole or part of the surface of the substrate, wherein the ZnO is provided on the surface of the substrate in the form of nanosheets or nano rods. It has been surprisingly found that, for an efficient fuel gas desulfurization, it is advantageous to load a ZnO sorbent onto a honeycomb support for unobstructed fuel gas flow (lower pressure drop) and application at higher temperatures (better thermal efficiency). The synthetic parameters used to manufacture the resulting composite material are important to obtain a nanostructured sorbent with increased desulfurization efficiency. In addition, the preparation method disclosed herein is facile and therefore allows for easy scale-up.

The resulting composite material can be used for desulfurization of warm fuel gas. Some possible applications of the composite material include, but are not limited to, the following applications.

1. Coal/biomass power plants using gasification technology. In order to increase electrical efficiency, the fuel gas should be purified for utilization in gas engine or gas turbine.

2. Waste-to-energy (WtE) plants to enhance electrical efficiency.

3. Gasification plants equipped with fuel cells.

4. Gasification plants producing syngas for chemical synthesis (e.g., methanol, Fisher- Tropsch), which utilize catalysts sensitive to sulfur poisoning. 5. Wastewater treatment plants with anaerobic digesters that produce biogas with high concentration of H₂S. For high end use of biogas, it should be purified from sulfur compounds. As noted above, one application of the invention is for warm fuel gas desulfurization (i.e., H₂S and COS removal) at 200-600 °C. The application temperature is significantly higher than other technologies (e.g. the temperature used in a caustic scrubber), which results in a better thermal efficiency. As shown below, the ZnO nanosheets perform 10 times better than commercially available ZnO sorbents (based on total sulfur removal at breakthrough point at 20 ppmv). The advantages of employing a nanostructured ZnO supported on a honeycomb substrate include:

(i) higher desulfurization efficiency (compared to commercial ZnO);

(ii) lower pressure drop (unobstructed fuel gas flow path);

(iii) better function and robustness (nanostructured material); and

(iv) ease of scaling-up (fabrication protocol).

As will be appreciated, the composite material can be used for corrosion control of downstream gas turbine, fuel cell and other equipment.

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.

When used herein "honeycomb substrate" refers to a substrate material suitable for exposure to a fluid at high temperatures that is composed of honeycomb-like structures with hundreds, if not thousands, of parallel channels running through the material. The walls of these channels provide a surface that can be used to support the ZnO, which is then exposed to the fluid run through the honeycomb in operation. Any suitable honeycomb substrate material may be used, for example a ceramic and/or metallic honeycomb substrate material. Particular examples of honeycomb materials that may be mentioned herein include, but are not limited to aluminium, cordierite, mullite and, more particularly, cordierite-mullite ceramic honeycomb substrate materials. When referred to herein, the term "honeycomb substrate surface" refers to the portion of the surface of the honeycomb substrate that will be exposed to a fluid (e.g. a gas). As will be appreciated, the physical dimensions, bulk density, surface area, channel width, wall thickness, cell density, free space and packing density may be selected to suit the particular application in mind. Suitable, non-limiting values, for most of the parameters above are provided below.

• Bulk density: 1.5 to 3.0 g/cm³

· Surface area: 500 to 1 ,500 m²/m³

• Channel width: 1 to 10 mm

• Wall thickness: 0.1 to 2 mm

• Cell density: 50-3000 cpsi (cells/inch² honeycomb cross-section)

• Free space: 60-75%

· Packing density: 500 to 1 ,500 kg/m³

As will be appreciated, the composite material may also contain additive materials on the surface of the honeycomb substrate. Said additive materials may be selected from one or more transition metals and/or lanthanoids. Transition metals that may be mentioned herein include Sc, Ti, V, Cr, Mn Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg. Lanthanoids that may be mentioned herein include, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. As will be appreciated, the additive materials used herein may be provided in a predominantly stable isotopic form (e.g. based on the natural abundance of stable isotopes or based on selection of the stable isotopes). Particular additive materials that may be mentioned herein include, but are not limited to, Cr, Fe, Co, Ni, Cu, Mo, Ce and combinations thereof. Any suitable loading amount of the additive material may be used.

The amount of ZnO deposited on the surface of the honeycomb substrate may be any amount that will provide the desired catalytic effect. Particular amounts that may be mentioned herein include, but are not limited to an amount of from 0.5 to 3 wt% relative to the entire weight of the composite material (i.e. [[(Weight of honeycomb + ZnO)-(Weight of honeycomb)]/(Weight of honeycomb + ZnO)] x 100). Examples of other suitable loading values of ZnO include from 0.6 to 1.6 wt%, such as from 0.6 to 1.1 wt% relative to the entire weight of the composite material. As noted above, the ZnO on the surface of the honeycomb substrate may be provided in the form of nanosheets or nano rods.

When used herein, the term "nanosheet" is a two-dimensional nanostructure having three measurable dimensions (height, width and thickness), where the thickness dimension is measured in a range of from 1 to less than 100 nm (or between 1 to 100 nm). As will be appreciated, in the current invention the nanosheets are deposited perpendicularly on the surface of the substrate and are attached to the substrate by their smallest (thickness) dimension, such that the nanosheets point out from the plane of the substrate with a measurable height (e.g. see Fig. 1 c). These nanosheets, when measured from the surface of the substrate to the top of the nanosheets, have a height of from 0.5 to 3 pm, such as from 0.8 to 1.2 pm, such as around 1 pm on the surface of the substrate. The nanosheets (and nano rods) are bound to the surface of the substrate by a strong metal-support interaction. As discussed hereinbelow, it was found that the application of a single nanosheet formation treatment (providing nanosheets having a height of from 0.8 to 1.2 pm, such as around 1 Mm on the surface of the substrate) provided the best results. Without wishing to be bound by theory, it is noted that further applications of nanosheets onto the surface result in a denser and larger in size, potentially resulting in fewer active sites being exposed for sorption. However, the addition of additives may moderate or reverse this effect, such that denser and larger nanosheets become more effective.

While the nanosheets may cover any proportion of the surface of the honeycomb substrate, it has been found that the nanosheets may substantially wholly cover the surface of the substrate (i.e. the active surface that will be exposed to a fluid in use). When used herein "substantially wholly cover" may refer to a coverage portion of greater than or equal to 75% of the surface of the substrate, such as greater than or equal to 85%, such as greater than or equal to 90%, such as greater than or equal to 95%, such as greater than or equal to 99%, such as greater than or equal to 99.9999% of the surface of the substrate intended to be in contact with a fluid. The percentages mentioned herein may be calculated based on estimating the overall surface covered by ZnO (e.g. based on the averaged coverage analysis of FESEM images (e.g. 5 to 10 images)) divided by the total surface of the substrate that is accessible by a fluid in use.

Composite materials formed using ZnO nanosheets may have one or more of the following properties:

(a) a total sulfur capacity at breakthrough of from 37.6 to 100 mg S/g of ZnO, such as from 40 to 59.8 mg S/g ZnO, such as 48.7 mg S/g ZnO; and (b) a ZnO efficiency at total sulfur breakthrough of from 9 to 20 mol%, such as from 9.6 to 20 mol%, such as from 10.5 to 15.2 mol%, such as 12.4 mol%.

As noted above, the ZnO in the composite material may be provided in the form of nano rods instead of nanosheets. When used herein "nano rod" refers to a material having a rod- /needle-like appearance when examined using imaging techniques (e.g. FESEM). The ZnO nano rods on the surface of the substrate may have a height of from 0.5 to 2 pm on the surface of the substrate, such as around 1 pm. Additionally or alternatively, the ZnO nano rods may have an averaged internal diameter of from 0.1 to 1 pm, such as from 0.1 to 0.5 pm or, more particularly, each ZnO nano rod may have an internal diameter of from 0.1 to 1 pm, such as from 0.1 to 0.5 pm.

When in the form of nano rods, the ZnO may only cover part of the substrate's available surface (i.e. the active surface that will be exposed to a fluid in use). When used herein "partly cover" may refer to a coverage portion of from less than or equal to 75% to 10 % of the surface of the substrate, such as from less than or equal to 70% to 20%, such as from less than or equal to 65% to 35%, such as from less than or equal to 60% to 45% of the surface of the substrate intended to be in contact with a fluid. Composite materials formed using ZnO nano rods may have one or more of the following properties:

(a) a total sulfur capacity at breakthrough of from 6.9 to 50 mg S/g of ZnO, such as from 7.1 to 20 mg S/g ZnO, such as from 9.0 to 17.1 mg S/g ZnO, such as from 10.9 to 12.0 mg S/g ZnO; and

(b) a ZnO efficiency at total sulfur breakthrough of from 1 to 9 mol%, such as from 1.8 to 5 mol%, such as from 2.3 to 4.4 mol% , such as from 2.8 to 3.1 mol%.

It has been found that the manufacture of the composite materials mentioned may be accomplished using simple methods of synthesis. For example, disclosed herein is a method of manufacturing a composite material comprising a honeycomb substrate material having a surface suitable for contacting a fuel gas and a ZnO material coating whole or part of the surface of the substrate, where the amount of ZnO material in the composite material is from 0.5 to 3 wt% relative to the entire weight of the composite material and the ZnO is provided on the surface of the substrate in the form of nanosheets, the method comprising the steps of:

(a) providing a seeded honeycomb substrate material that has a ZnO covering on a surface of the substrate suitable for contacting a fuel gas; (b) immersing the seeded honeycomb material in a crystal growth solution comprising a first zinc oxide source and an amine base at a temperature of from 50 to 150 °C for a first period of time;

It will be appreciated that the resulting material is a composite material comprising the ZnO nanosheets discussed hereinbefore. As such, the properties of the resulting product of this process are as described hereinabove with reference to composite materials having ZnO in the form of nanosheets.

The seeded honeycomb substrate mentioned above has been coated with a thin layer of ZnO on the surface of the substrate. The coating layer is less than or equal to 15 nm, such as less than or equal to 10 nm, such as from 0.1 nm to 9.99 nm. The purpose of this thin layer of ZnO on the surface of the substrate is to enable a more even growth of ZnO of the surface of the substrate in the immersion and aging steps.

While any suitable source of ZnO may be used, the first ZnO source may be Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0 or Zn(CH₃C0₂)₂ and solvates thereof, such as Zn(CH₃C0₂)₂-2H₂0. It is particularly preferred that Zn(CH₃C0₂)₂ and solvates thereof, such as Zn(CH₃C0₂)₂-2H₂0 are used as the first ZnO source for the formation of nanosheets. In any event, the first zinc oxide source may be present in the crystal growth solution at any suitable concentration, such as a concentration of from 0.05 to 0.5 M, such as around 0.1 M. Any suitable amine base may be used in the process. A particular amine base that may be mentioned herein is hexamethylenetetramine. Any suitable concentration of the amine base in the crystal growth solution may be used, such a suitable concentration range may be from 0.05 to 0.5 M, such as around 0.2 M. It is possible to change the ZnO nanosheet coating's properties by changing the molarity ratio of the amine base to first zinc oxide source in the crystal growth solution. For example, the molarity ratio of the amine base to first zinc oxide source may be from 1 :2 to 3: 1 , for example around 2:1. In the immersion phase mentioned above for the formation of nanosheets, the temperature of the immersion may be from 60 to 100 °C, such as around 90 °C. In this immersion phase, the first period of time for the formation of nanosheets may be greater than or equal to 3 hours, such as greater than or equal to 5 h, for example from 3 to 10 h, such as around 5 h or from greater than 5 h to 8 h.

When forming nanosheets it is important to then have an aging step which is conducted at ambient temperature (i.e. without heating of the crystal growth mixture) for a second period of time. When mentioned herein, "room temperature" is intended to mean the ambient temperature of the environment that the process is being conducted in, this will be typically from 10 to 40 °C, such as from 15 to 35 °C, such as from 20 to 30 °C, such as around 25 °C. As will be appreciated the aging step involves leaving the substrate within the liquid for a suitable period of time in order to obtain nanosheets. A suitable period of time for this aging step may be greater than or equal to 8 hours, such as greater than or equal to 18 h, for example from 8 to 24 h, such as around 18 h.

Also disclosed herein is a method of manufacturing a composite material comprising a honeycomb substrate material having a surface suitable for contacting a fuel gas and a ZnO material coating whole or part of the surface of the substrate, where the amount of ZnO material in the composite material is from 0.5 to 3 wt% relative to the entire weight of the composite material and the ZnO is provided on the surface of the substrate in the form of nano rods, the method comprising the steps of:

(ii) immersing the seeded honeycomb material in a crystal growth solution comprising a first zinc oxide source and an amine base at a temperature of from 50 to

150 °C for a period of time.

It will be appreciated that the resulting material is a composite material comprising the ZnO nano rods discussed hereinbefore. As such, the properties of the resulting product of this process are as described hereinabove with reference to composite materials having ZnO in the form of nano rods. As will be appreciated, no aging step is needed to form the nano rod materials.

The seeded honeycomb material described in this process is the same as that described above for the manufacture of composite materials having ZnO in the form of nanosheets. While any suitable source of ZnO may be used, the first ZnO source may be Zn(CH₃C0₂)₂ and solvates thereof, such as Zn(CH₃C0₂)₂-2H₂0 or Zn(N0₃)2 and solvates thereof, such as Zn(N0₃)₂-6H₂0. It is particularly preferred that Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0 are used as the first ZnO source for the formation of nano rods. In any event, the first zinc oxide source may be present in the crystal growth solution at any suitable concentration, such as a concentration of from 0.05 to 0.5 M, such as around 0.1 M. Any suitable amine base may be used in the process. A particular amine base that may be mentioned herein is hexamethylenetetramine. Any suitable concentration of the amine base in the crystal growth solution may be used, such a suitable concentration range may be from 0.05 to 0.5 M, such from 0.05 to 0.2 M, such as around 0.2 M. It is possible to change the ZnO nano rod's coating's properties by changing the molarity ratio of the amine base to first zinc oxide source in the crystal growth solution. For example, the molarity ratio of the amine base to first zinc oxide source may be from 0.1 :1 to 3: 1 , for example from 0.5: 1 to 2:1 or 0.5:1 or 2: 1. In the immersion phase mentioned above, the temperature of the immersion may be from 60 to 100 °C, such as around 90 °C. In this immersion phase, the first period of time may be greater than or equal to 3 hours, such as greater than or equal to 5 h, for example from 3 to 10 h, such as around 5 h or from greater than 5 h to 8 h. In both of the above processes, a seeded honeycomb substrate material is used. This material may be obtained by the steps of:

(ib) drying the resulting pre-seeded honeycomb substrate at first temperature for a period of time and then calcinating it at a second temperature for a period of time to provide the seeded honeycomb substrate.

The second zinc oxide source may be Zn(CH₃C0₂)2 and solvates thereof, such as Zn(CH₃- C0₂)₂-2H₂0 and/or Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0. The second zinc oxide source is present in the seeding solution at a concentration of from 0.1 to 0.5 M, such as from 0.3 to 0.4 M.

Any suitable amine base may be used in the process of forming the seeded substrate. A particular amine base that may be mentioned herein is diethanolamine. Any suitable concentration of the amine base in the seeding solution may be used, such a suitable concentration range may be from 1 :2 to 3: 1 , such as around 1 : 1. After immersion, the treated substrate is first dried and then calcinated. The drying step may be conducted at a temperature of from 50 to 100 °C, such as around 60 °C and the calcination step may be conducted at a temperature of from 300 to 500 °C, such as around 400 °C. As will be appreciated, the drying step may be conducted until the substrate has been dried and, coincidentally, in both the drying and calcination steps, the time required is similar, such as from 1 to 5 h, such as around 2 h. For the calcination step, the heating rate of the calcination step may be from 1 to 4 °C/min, such as 2 °C/min.

Any suitable solvent may be used as for the seeding solution. A particular solvent that may be mentioned herein is ethanol.

As mentioned herein, the composite materials described herein are particularly useful in removing sulphur compounds from a fluid stream, such as a fuel gas fluid stream. As such, there is also provided a method of desulfurizing a fuel gas in need thereof, the method comprising the step of passing a fuel gas in need of desulfurization through a composite material as described hereinbefore at a temperature of from 200 to 600 °C to provide a desulfurized fuel gas, wherein:

the desulfurized fuel gas contains less than 20 ppmv of sulfur compounds.

As will be appreciated the above-mentioned process allows the fuel gas to be purified to a level that makes it useable in a fuel cell without fear of poisoning said fuel cell. One of the advantages of the currently claimed system is that the temperatures used may be maintained at (or close to) the operational temperature of the fuel cell, thereby reducing the waste of heat energy. For example, as noted above, the purification may be conducted at a temperature of from 200 to 600 °C, such as 400 °C.

The composite materials disclosed herein are able to remove sulphur compounds so that the resulting purified fuel gas has less than 20 ppmv of sulphur compounds in it. As will be appreciated, any level of sulphur compounds within the fuel gas can in principle be purified using the composite materials disclosed herein, such as from 30 to 10,000 ppmv of sulfur compounds in the fuel gas. However, as will be appreciated, as the concentration of sulphur compounds in the fuel gas increases, the length of time to saturate a particular absorbent decreases. With that in mind, the composite materials disclosed herein may be particularly suitable for use in the purification of fuel gases that contain from 40 to 6,000 ppmv of sulfur compounds, such as from 50 to 500 ppmv of sulfur compounds. The velocity of the gas passing through the composite material has to be selected to allow the ZnO particles on the surface of the material to have sufficient time to interact with and adsorb the sulphur compounds. For the composite materials disclosed herein, the fuel gas passing through the composite material may have a gas hourly space velocity of from 500 to 20,000 IT¹ , such as from 3,000 to 10,000 IT¹.

Advantageously, the composite materials disclosed herein are capable of being regenerated. This may be accomplished by passing a regenerating gas mixture through the composite material in need of regeneration at a temperature of from 500 °C to 800 °C, such as 650 °C, at a gas hourly space velocity of from 2,000 to 5,000 h^"1 , where the regenerating gas mixture comprises from 0.5 vol% to 20 vol% oxygen, such as from 1 vol% to 5 vol% oxygen, such as around 2 vol%. Further aspects and embodiments of the invention will now be described with reference to the following non-limiting examples.

Examples

Materials

Cordierite-mullite honeycomb (Adena ceramics, Shanghai) was used as the support material. The honeycomb was 14 mm in diameter, 20 mm in height, with 1.5 mm ^χ 1.5 mm square channels. The total surface area of the square channels in the honeycomb was approximately 43.2 cm². All honeycombs were sonicated in ethanol and dried in an oven at 60 °C for 24 h prior to use for the immobilization of ZnO.

The chemicals used in this study were Zn(N0₃)2.6H₂0 (Sigma-Aldrich), Zn(CH₃COO)2.2H₂0 (Sigma-Aldrich), hexamethylenetetramine (HMX, Sigma-Aldrich), ethanol (Merck), diethanolamine (Merck), phosphorous pentoxide (Merck), and commercial ZnO sorbent (Liaoning Haitai Sci-Tech Development Co. Ltd., China).

Methods Field emission scanning electron microscopy (FESEM JEOL 7600F) equipped with the energy dispersive X-ray spectroscopy (EDX, Oxford Xmax80 LN2 Free) was used to investigate the surface morphology and elemental distribution of the ZnO-loaded honeycomb. The X-ray diffractometer (Bruker AXS D8 Advance, operated with Cu-Κα source at λ = 1.5418 A) was employed to obtain the X-ray diffraction (XRD) pattern.

Example 1

Synthesis of nanostructured ZnO sorbent with nanosheet morphology

ZnO with nanosheet morphology loaded on honeycomb support was prepared via a two-step seeding-growth method followed by aging. In a typical synthesis protocol, the cordierite- mullite honeycomb was first seeded by immersing it in a seeding solution containing 20 mmol of Zn(CH₃COO)2.2H₂0 in 50 ml. ethanol and 2 mL diethanolamine. The immersed honeycomb was then dried in an oven at 60 °C, followed by calcination at 400 °C in air for 2 h (heating rate 2 °C min^"1). The seeded honeycomb was then immersed in an aqueous crystal growth solution containing 0.2 M hexamethylenetetramine (HMX) and 0.1 M of Zn(CH₃COO)2.2H₂0 at 90 °C for 5 h to promote ZnO crystal growth.

HMX was used as a base to promote hydrolysis and condensation of Zn²⁺ to form nanostructured ZnO. Afterwards, the solution was cooled naturally to 25 °C and aged for 18 h to obtain the resultant ZnO with nanosheet morphology loaded on the honeycomb support.

Results/Characterisation

Several honeycombs with ZnO having nanosheet (ZnO-nS) morphologies (Figs. 1c and d) were obtained. The aging time after crystal growth was crucial to obtain the nanosheet morphology.

During the aging process, the acetate ions remained in the crystal growth solution altered the ZnO morphology through controlled partial dissolution of surface ZnO to produce the nanosheet morphology. Without the aging process, large nano rod-like morphology was obtained instead. A pictographic illustration of the preparation process is presented in Fig. 2. Fig. 2 depicts the seeded support (A) before it was subjected to the growth solution, while (B) depicts the surface after exposure to the growth solution (0.2 M hexamethylenetetramine (HMX) and 0.1 M of Zn(CH₃COO)₂.2H₂0) at 90 °C for 5 h. Subsequently subjecting the intermediate material (B) to aging in the growth solution for 18 hours at room temperature results in the nanosheet morphology depicted in (C). The XRD patterns of the ZnO-nS is shown in Fig. 3. The XRD patterns show that well- crystalline ZnO (JCPDS 36-1451) was formed on the surface of the honeycomb support.

The average ZnO loadings, for ZnO nanosheets on the supported honeycombs were 0.6±0.1 % w/w. The weight percentages calculated here were calculated as follows:

[(Weight of honeycomb + ZnO)— (Weight of honeycomb)]

(Weight of honeycomb + ZnO) °

Example 2

Synthesis of nanostructured ZnO sorbent with nano rod morphology

ZnO with nano rod morphology was also prepared using a similar method to Example 1 , but with Zn(N0₃)2-6H₂0 as the precursor for the crystal seeding-growth process and without aging. In addition, in the second step, the molar ratio HMX to Zn²⁺ was controlled at either 1 :2 or 1 :0.5.

ZnO nano rods of 0.5 pm in diameter were obtained using the 1 :2 ratio of HMX to Zn and are referred to herein as ZnO-nR1. ZnO nano rods of 0.1 Mm in diameter were obtained using the 1 :0.5 ratio of HMX to Zn²⁺ and are referred to herein as ZnO-nR2. The FESEM images for ZnO-nR2 are depicted in Figs. 1a and b, and its XRD pattern is depicted in Fig. 3. The XRD pattern for ZnO-nR2 shows that well-crystalline ZnO (JCPDS 36-1451) was formed on the surface of the honeycomb support. In the ZnO-nR2 XRD pattern, several additional peaks associated with the XRD patterns of the cordierite-mullite structure can also be observed. This appears to be due to a poorer surface coverage and crystallinity of the ZnO nano rods compared with ZnO nanosheets.

The average ZnO loadings, for both kinds of ZnO nano rods on the supported honeycombs in this example were also 0.6±0.1 % w/w. Example 3

Desulfurization Study

Fig. 4 presents a schematic illustration of the experimental setup for the desulfurization study. The desulfurization process took place in a vertical fixed-bed quartz reactor 47, having a 15- mm inner diameter and 400-mm height. One of the as-prepared ZnO nanosheets/nano rods loaded on a honeycomb of Examples 1 and 2 (48) was fixed in the middle of the quartz reactor 47. The honeycomb was wrapped in a thin layer of ceramic fibre that contacted the reactor wall to prevent bypass gas flow. A commercial ZnO sorbent was prepared by mixing commercial ZnO particles (~18-20 mg, particle size 0.212-0.56 mm) and cordierite-mullite particles (~3 g, particle size 0.212-0.56 mm) uniformly. The total weight was equivalent to nanostructured ZnO honeycomb sorbent to maintain a consistent space velocity during experiments.

Prior to the commencement of each experiment, the reactor was purged with N₂ gas at 100 mL/min for 10 min to remove the air inside the reactor. The composition of the fuel gas was controlled by individual gas cylinders 41 using mass flow controllers 43, while the H₂0 (moisture) flow rate was controlled by a syringe pump 44. As will be appreciated, one gas cylinder was used for each gas introduced into the system and the system involved a number of suitable valves 42, 45.

The H₂S source used in the study was a H₂S/N₂ matrix with 500 ppmv H₂S. The fuel gas composition was set at 50 ppmv H₂S, 8 vol% CO, 15 vol% C0₂, 10 vol% H₂, 26 vol% H₂0 and N₂ balance (this corresponds to the typical fuel gas composition in a gasification plant), with a total flow rate set at 200 mL/min at normal temperature and pressure. The fuel gas sample was preheated to 120 °C after mixing and was then purged into the fixed-bed tubular reactor 47, this included preheating all gas lines to the same temperature. The temperature of the reactor was set at 400 °C (using a furnace 46) for the desulfurization studies. The gas line at the outlet of the reactor was also heated to 120 °C to prevent moisture condensation and H₂S absorption by liquid water. The temperature of each portion of the system was controlled by thermocouples 414 and a controller 412 directed by a computer 413.

A fuel gas sample after desulfurization was first condensed in a trap 49 filled with phosphorus pentoxide (P₂0₅) to remove moisture and then it was split into two equal portions:

(a) one for sulfur compound sampling using gas chromatography coupled to a flame photometric detector (411 ; GC-FPD, Agilent); and

(b) the other for sulfur compound absorption by alkaline solution 410.

A by-pass was set parallel to the fixed-bed reactor for determination of initial H₂S concentration before desulfurization.

Sulfur capacity (SC) at breakthrough point was calculated by the following equation: 10 x M

SC (mg S/g ZnO) = GHSV x x

600000000 x V_mol x s x W J ( n C_out)dt where:

GHSV, Gas hourly space velocity, h

M, Atomic weight of sulfur, 32 g mol^"1

Vmoi, Molar volume of syngas under 298 K and 1 atm, 24.5 L mol^"1

Ps, Density of ZnO-honeycomb sorbent, g cm^-3, calculated as a ratio of sorbent mass to sorbent volume (exclusive of empty space, not bulk density)

W, Mass fraction of ZnO in ZnO honeycomb, %

Cm, Inlet concentration of sulfur compound, ppmv

Cut, Outlet concentration of sulfur compound, ppmv

The space velocity was preliminary set at 9230 h^"1 for this invention and can be adjusted by changing the total fuel gas flow rate to investigate the influence of space velocity on sulfur removal. Results

Figs. 5a-c presents the breakthrough curves of H₂S, COS and total sulfur for the model fuel gas using commercially available ZnO, the ZnO nanosheet of Example 1 (ZnO-nS) and the ZnO nano rods of Example 2 (ZnO-nR1 and ZnO-nR2), while Table 1 summarizes the breakthrough time (BT) and sulfur capacity of ZnO sorbents. The breakthrough time is defined as the maximum limit of sulfur content for safe operation of gas turbine (20 ppmv). There was negligible sulfur removal in the control study conducted with blank honeycomb (without ZnO). Among the sorbents, the ZnO-nS exhibited the best desulfurization performance with breakthrough time at 88 min with total sulfur capacity of 55 mg/g ZnO. Other ZnO sorbents (ZnO-nR1 and ZnO-nR2) showed significantly lower breakthrough time (24-26 min) and capacity (9-12 mg/g ZnO), while the commercial ZnO presented the lowest sulfur capacity at 5 mg/g ZnO. Compared with the commercial ZnO (with particle size of 0.2- 0.6 mm), the nanostructured ZnO having higher density of active sites enhances desulfurization reactions. The ZnO-nS performed at least 3 times better than ZnO-nR1 and ZnO-nR2. This could be due to a better surface coverage by the ZnO nanosheets (as indicated by the XRD results) suggesting that the ZnO morphology is important for obtaining complete surface coverage on the support. The said invention of ZnO nanosheets on the honeycomb support offers almost complete surface coverage for enhanced desulfurization of warm fuel gas. The nanosheets and nano rods can be regenerated after use via thermal treatment at 500-600 °C with air/oxygen (e.g. see M. Mureddu et al., Fuel, 102 (2012) 691- 700).

Table 1. Breakthrough time (BT), sulfur capacity (S capacity) and ZnO utilization for H₂S and total sulfur removal by various ZnO sorbents.

Example 4

Synthetic Variation Studies The procedures used in Example 1 and Example 2 were repeated using variable concentrations of HMX (<0.01 M to 0.6M) in the growth solutions. In additional studies, the nanosheets and nano rods were re-subjected to two further rounds of growth solution under the conditions mentioned in Example 1 and Example 2. Finally, the aging time for the nanosheet synthesis was also varied in studies to observe the effects associated with this step.

Nanosheet Variations

Fig. 6 provides the FESEM micrographs of nanosheets grown using different concentrations of HMX. For ZnO nanosheets preparation (with Zn(Ac)₂-2H₂0 as the precursor), both HMX concentration and aging time after the growth process were essential to obtain the desired morphology. The metal Ac^~ precursor could promote fast crystallization and better ZnO distribution. Based on Fig. 6, the optimum HMX concentration required to obtain the ZnO nanosheets with well-distributed quasi-porous structure (denoted as ZnO-nS) was 0.2 M. At lower HMX concentrations (< 0.2 M), ZnO nanosheets with lower interparticle distance were observed. A narrow interparticle distance and compacted ZnO structure may not be desirable due to the higher diffusion limitation. For ZnO-nS synthesis, it is postulated that the Zn(Ac)₂-2H₂0 provided Ac^" ions, which competed with the HMX for attachment onto the a- and b-axes and inhibited the HMX from suppressing the lateral ZnO nano rods growth along the a- and b-axes. This was evidenced by the observed compacted and merged ZnO growth, producing a uniform layer of ZnO nano rods (0.05-0.20 M HMX) with l_d= 200 nm compared to the Zn(N0₃)-6H₂0 precursor (l_d =100-500 nm depending on the HMX concentration). Upon completion of the growth process, the Ac^" (acting as conjugate base) remaining in the reaction vessel altered the ZnO morphology through controlled partial dissolution (etching) of ZnO via complexation of Ac^" with the surface Zn²⁺ during the 18 h aging process.

As seen in Fig. 7, the dissolution of ZnO nano rods was observed during the intermittent aging time (t =6 h), further supporting the above. This resulted in products having a nanosheet morphology. Furthermore, the Ac^" ions could potentially act as a Zn²⁺ ligand for controlled ZnO formation with nanosheets morphology.

When used herein, the a, b and c axes are conventionally used in the field of crystallography to represent the x, y and z axes, respectively.

Nano rod Variations

The optimum HMX concentration for ZnO nano rods formation (using Zn(N0₃)₂-6H₂0 as the precursor) was between 0.05-0.20 M. The internal diameter (Id) of the ZnO nano rods was linearly dependent on the HMX concentration employed during synthesis (Fig. 8). At 0.05 M HMX, the estimated Id was -500 nm (denoted as ZnO-nR1) while at 0.20 M HMX, the estimated Id was ~100 nm (denoted as ZnO-nR2). This indicates that the higher HMX concentration inhibited the ZnO lateral growth along the a- and b-axes without affecting the ZnO axial growth along the c-axis. The HMX molecules were attached preferentially to the exposed oxygen at (100) and (101) facets resulting in vertical ZnO nano rods growth with smaller internal diameter. However, excessive HMX (>0.2 M) suppressed both axial and lateral growth of ZnO leading to the observed transitional change from nano rods to irregular nanoparticles. The distance between two adjacent nano rods was relatively smaller for ZnO- nR1 compared with ZnO-nR2, as the nano rods in ZnO-nR1 were closely compacted and merged with the adjacent nano rods. This may not be desirable considering the compacted structure could increase the mass transfer resistance for gas sorption application.

Discussion

The seeding step is critical as it affects the growth, crystallinity, and morphology of the resulting ZnO. There was no ZnO growth on the honeycomb surface without the seed layer. Similarly, the HMX plays a significant role as capping agent for ZnO morphological control, and as base to promote direct formation of the crystalline ZnO through the following reactions:

C₆H₁₂N₄ (HMX) + 6H₂0→6HCHO + 4NH₃ (A)

Zn²⁺ + 20IT ^ H₂0+ ZnO (C)

There was also no significant ZnO growth at HMX concentration <0.01 M, regardless of the Zn precursor used, due to the low availability of OH for reaction (C) to proceed favorably.

Figs. 9a and b shows the cross-sectional areas of ZnO-nR2 and ZnO-nS revealing that the ZnO height in both sorbents are ~1 pm. However, the ZnO nano rods appear to have a poorer surface coverage and adhesion compared to the ZnO nanosheets. Further investigation of ZnO-nS using EDX elemental mapping (Fig. 9c and d) revealed a well- defined ZnO nanosheet layer on the honeycomb surface. The estimated ZnO loading (for both nano rods and nanosheets morphologies) on the honeycomb calculated from the masses of honeycomb before and after the growth process was ~1.0 ± 0.1 mg mrrf¹ per honeycomb. In both ZnO-nR2 and ZnO-nS, the ZnO growth process was repeated twice more, each time with a fresh ZnO growth solution to investigate the effect of higher ZnO loading on the morphology. However, for ZnO-nR2, the repeated ZnO growth process did not result in longer ZnO nano rods but instead, a mixture of immobilized (also ~1 pm) and free nano rods on the honeycomb surface was produced (Fig. 10). On the contrary, the well- distributed ZnO nanosheets in ZnO-nS allowed multiple stacking of ZnO layers. As indicated in Fig. 9e and f (3 layers of ZnO, denoted as ZnO-nS-3), each layer of the ZnO nanosheets was ~1 pm in height with the total height of ~3 pm and the resultant ZnO loading was ~1.5 ± 0.1 mg mrrf¹ honeycomb. However, the ZnO nanosheets in ZnO-nS-3 were significantly larger and denser than the single layer ZnO nanosheets. Example 5

Further Desulfurization Studies and Regenerability

Using the same equipment as Example 3, further desulfurization studies were conducted.

Prior to the commencement of each experiment, the reactor was purged with N₂ gas at 100 ml_ min^-1 for 10 min to remove air. The model syngas contained CO, C0₂, H₂, N₂ and H₂S (500 ppmv)/N₂ and was supplied from individual gas cylinders using mass flow controllers. The steam was generated from Dl water supplied by a syringe pump.

All the gas lines before and after the fixed-bed tubular reactor were preheated to 120- 150 °C to avoid moisture condensation and H₂S absorption by liquid water. The model syngas composition was 30-100 ppmv H₂S, 8 vol% CO, 15 vol% C0₂, 10 vol% H₂, 26 vol% H₂0 and N₂ balance, which is typical to syngas composition from municipal solid wastes. The total flow rates of model syngas were set at 100, 150 and 200 ml. min ¹ at STP, resulting in a gas hourly space velocities (GHSV, volume based) of 4615, 6923 and 9230 h ¹, respectively. All desulfurization studies were carried out at 400 °C and a pressure range of 20-21 psi. After desulfurization, the syngas was passed through a P₂0₅ trap to remove the moisture and analysed by gas chromatograph (GC) equipped with a Flame Photometric Detector (FPD) (Agilent 7890 GC, USA). There was no significant absorption of H₂S and COS by the P₂0₅ in the control study. Before and after desulfurization experiments, the gas was purged through the by-pass until stable response of GC was obtained to measure the H₂S content. The inlet H₂S and COS concentration was calculated as an average of two measurements which were conducted before and after desulfurization process from the by- pass path. In this study, the concentrations of H₂S and total sulfur (sum of H₂S and COS) were reported. The H₂S (and total sulfur) removal was calculated by comparing the H₂S (and total sulfur) concentration in the inlet and outlet of the reactor. The breakthrough time was defined as the time from the beginning of desulfurization until elution of 20 ppmv of H₂S and total sulfur from the reactor, which is the maximum limit of sulfur content for safe operation of gas turbines (20 ppmv).

The sulfur capacity was calculated as for Example 3.

All experiments were triplicated. The error analysis was carried out by calculating the standard deviation of the triplicate experimental runs. To study the regenerability of synthesized ZnO, the selected sorbent was treated with 10% air and 90% nitrogen mixture in the same reactor at 650 °C and GHSV of 3415 h Regeneration was considered complete when S0₂ content in the effluent gas was below the detection limit of gas chromatograph. Results Comparative Performance

In the gasification process, sulfur is mainly present as H₂S but can also exist as other sulfur compounds such as COS. The presence of COS in the model syngas is expected despite no COS was introduced in the inlet due to the reaction of H₂S with CO and C0₂.

H₂S + CO <→ COS + H₂ (D)

H₂S + C0₂ ^ COS + H₂0 (E)

As the model syngas used in this study contained H₂S, CO and C0₂, COS (several ppmv) was also detected in the gas stream. Figs. 11 a and b show the performance of various ZnO sorbents for H₂S and total sulfur (H₂S + COS) removal - these figures include the results from Example 3 above, as well as for the material ZnO-ns-3.

All of the nanostructured ZnO sorbents removed ~3-11 times more sulfur compounds compared to the commercial ZnO sorbent. The ZnO-nS exhibited the highest desulfurization performance with a total sulfur breakthrough time (BTTS) and sulfur capacity (SC) of 75.4 min and 48.7 mg g^"1 ZnO, respectively. Both ZnO-nR1 and ZnO-nR2 had shorter BTTS (23- 29 min) and lower total sulfur SC (9-12 mg g^"1 ZnO) than ZnO-nS while the commercial ZnO had the shortest BTTS and lowest total sulfur SC of 6.8 min and 4.6 mg g ^{~ 1} ZnO, respectively. This was attributed to the fact that nanostructured ZnO sorbents had a higher density of active sites which were readily available for surface desulfurization reactions compared with larger-size commercial ZnO sorbent (0.2-0.6 mm).

It was also observed that the ZnO-nS performed more than three times better than ZnO-nR1 and ZnO-nR2 in terms of BTTS. This was due to the better ZnO nanosheets surface coverage and higher ZnO crystallinity, resulting in higher ZnO conversion efficiency. The ZnO conversion efficiency of ZnO-nS was ~12 and ~4 times higher than that of the commercial ZnO sorbent and ZnO-nR, respectively. The H₂S SC for ZnO-nS was also higher than other reported SC for supported ZnO prepared using the wetness impregnation method (ZnO/Si0₂, SC =26 mg g^~1 sorbent (Dhage P et a!., PCCP 2011 ;13:2179-87); ZnO/SBA-15, SC = 53 mg g^~1 sorbent (Mureddu M et al, J. Mater. Chem. 2014;2:19396-406); and ZnO/monolith, SC =9.5-14.5 mg g ¹ sorbent (Novochinskii II et al., Energy Fuels 2004;18:584-9)). These results suggest that ZnO morphology on support is crucial for effective syngas desulfurization. Surprisingly, it was found that the single layer ZnO-nS outperformed ZnO-nS-3 (SC =11.6 mg g ¹ ZnO, BTTS= 24.6 min) despite ZnO-nS-3 having higher ZnO loading than ZnO-nS. This could be attributed to the continuous ZnO growth during repeated growth process causing the compaction of ZnO layers and thus decreasing the density of exposed active sites for desulfurization reaction. Besides, the densification of ZnO layers increased the internal mass transfer resistance due to decreased interlayer spacing. The results suggest that the height of the ZnO layer was important to preserve the advantages of having nanostructured materials and alleviate the internal mass transfer resistance.

To further improve the nanostructured ZnO loading, alternative design with denser and longer multi-channel honeycomb would be more effective than increasing the ZnO layer height.

Effect of GHSV Fig. 12 presents the effect of GHSV on the performance of ZnO-nS and commercial ZnO sorbent indicating that the BTTS increased gradually with decreasing GHSV from 9230 to 4615 h^" . The decrease of GHSV from 9230 to 4615 h^~1 extended the ZnO sorbent lifespan (BTTS) by ~2 times due to the larger residence time. The differences between BTTS and SCs of ZnO-nS and commercial ZnO sorbents decreased with decreasing the GHSV indicating that the desulfurization performance by the commercial ZnO was likely limited at high GHSV due to slower mass transfer. Comparison of the effect of GHSV on the performance of ZnO-nS and commercial ZnO sorbents showed that the difference in the BTTS of the two sorbents were more pronounced at higher GHSV (~5 and ~2 times difference for 9230 and 6923 h^"1, respectively) than at lower GHSV (4615 h^"1 , comparable BTTSs). This indicates that at higher GHSV, the performance of commercial ZnO sorbent was severely affected by the slower mass transfer and limited contact time for desulfurization reaction. The nanostructured ZnO-nS was less susceptible to the diffusion limitation (lower internal mass transfer) than the commercial ZnO sorbent further highlighting the advantages of nanostructured ZnO-nS at higher GHSV. Effect of H₂S concentration

The effect of H₂S concentration on the performance of ZnO-nS and commercial ZnO sorbent is presented in Fig. 13. The results indicate that increasing the H₂S concentration from 30 to 100 ppmv resulted in exponential decrease of the H₂S breakthrough time (BTH₂S) for ZnO- nS. The performance of ZnO-nS was significantly higher than the commercial ZnO at all H₂S concentrations. For the commercial ZnO, the BTH₂S was generally achieved in less than 10 min at high H₂S concentration. The ZnO conversion efficiency of commercial ZnO sorbent was considerably low at higher H₂S concentration (-12.5% at 50 ppmv vs. 6.1% at 100 ppmv) indicating that at higher H₂S flux, the H₂S molecules could pass through the honeycomb unreacted. The likely reasons for the observed phenomenon are mass transfer limitations. Considering the ZnS lattice is significantly larger than that of the ZnO, the chemisorption process leads to the expansion of the crystal lattice. Consequently, the rapid lattice expansion (which was more pronounced at higher H₂S concentration) could result in lower permeability of the syngas thus reducing the SC and increasing the internal mass transfer resistance.

Regeneration ofZnO-nS The breakthrough curves of ZnO-nS up to 3 cycles are shown in Fig. 14, while the SCs at each cycle are presented in Table 2. The ZnO-nS was regenerated with 10% air in nitrogen (balance) at 650 °C, corresponding to 2% oxygen to avoid sulfate formation. The XRD pattern of spent ZnO-nS (Fig. 15A) shows no detectable ZnS peaks while EDX elemental mapping (Fig. 15 C) reveals that S was evenly distributed across the ZnO nanosheets with Zn to S molar ratio of 28:1. The plausible reasons are (i) the content of ZnS formed is relatively low and below the detection limit of the XRD, and (ii) the chemisorption process formed amorphous ZnS on the outer surface of the ZnO. After the regeneration process, no S was detected by EDX elemental mapping (Fig. 15E, F and Table 3) and the crystalline ZnO was restored (as evidenced by the XRD pattern of the regenerated ZnO-nS) indicating successful regeneration process. Comparison of the slopes of the breakthrough curves from cycle 1 to 3 in Fig. 14 indicates that the performance of ZnO-nS progressively became comparable (in BTTS and SC) to that of the pristine commercial ZnO catalyst. As observed in the FESEM micrographs of the spent and regenerated ZnO-nS, this was attributed to the change in the nanoarchitecture of the ZnO-nS probably as a consequence of rapid expansion and contraction of the sorbent lattice, leading to the increased ZnO particle size. The changes in the nanoarchitecture resulted in the increased mass transfer resistance, affecting the kinetics of reaction but not the overall SC. However, despite the small deterioration in the H₂S removal rate, the ZnO-nS can still be used after regeneration compared to the commercial ZnO, which completely lost its function after a single cycle. The results clearly indicate that the ZnO-nS performed significantly better (with advantages of lower mass transfer resistance, better regenerability, and higher SC) than the commercial ZnO sorbent.

Table 2. The sorption capacity of ZnO-nS at breakthrough point for various cycles

Table 3. Elemental Mapping of Regenerated ZnO-nS

Element Weight% Atomic%

C 3.81 10.35

0 24.12 49.24

Mg 0.42 0.57

Al 3.25 3.93

Si 2.62 3.04

Zn 65.79 32.87

Total 100.00 100.00

Claims

1. A composite material suitable for use to remove sulfur-containing compounds from a fuel gas, the composite material comprising:

2. The composite material according to Claim 1 , wherein the honeycomb substrate material is a ceramic and/or metallic honeycomb substrate material.

3. The composite material according to Claim 2, wherein the ceramic is a cordierite- mullite ceramic.

4. The composite material according to any one of the preceding claims, wherein the ZnO material further comprises one or more additive materials selected from the group consisting of transition metals and lanthanoids.

5. The composite material according to any one of the preceding claims, wherein the amount of ZnO material in the composite material is from 0.5 to 3 wt% relative to the entire weight of the composite material.

6. The composite material according to Claim 5, wherein the amount of ZnO material in the composite material is from 0.6 to 1.6 wt%, such as from 0.6 to 1.1 wt% relative to the entire weight of the composite material.

7. The composite material according to any one of the preceding claims, wherein the ZnO is in the form of nanosheets having a height on the surface of the substrate of from 0.5 to 3 μιη.

8. The composite material according to Claim 7, wherein the ZnO nanosheets have a height on the surface of the substrate of from 0.8 to 1.2 pm, such as around 1 pm.

9. The composite material according to Claim 7 or Claim 8, wherein the ZnO nanosheets substantially wholly cover the surface of the substrate.

10. The composite material according to any one of Claims 7 to 9, wherein, the composite material has:

(a) a total sulfur capacity at breakthrough of from 37.6 to 100 mg S/g of ZnO, such as from 40 to 59.8 mg S/g ZnO, such as 48.7 mg S/g ZnO; and/or

(b) a ZnO efficiency at total sulfur breakthrough of from 9 to 20 mol%, such as from 9.6 to 20 mol%, such as from 10.5 to 15.2 mol%, such as 12.4 mol%.

11. The composite material according to any one of Claims 1 to 6, wherein the ZnO is in the form of nano rods where:

(a) the nano rods have a height of from 0.5 to 2 μιη on the surface of the substrate, such as around 1 pm; and/or

(b) each nano rod has an internal diameter of from 0.1 to 1 pm, such as from 0.1 to 0.5 pm.

12. The composite material according to Claim 11 , wherein the ZnO nano rods partly cover the surface of the substrate.

13. The composite material according to Claim 11 or Claim 12, wherein, the composite material has:

(a) a total sulfur capacity at breakthrough of from 6.9 to 50 mg S/g of ZnO, such as from 7.1 to 20 mg S/g ZnO, such as from 9.0 to 17.1 mg S/g ZnO, such as from 10.9 to 12.0 mg S/g ZnO; and/or

14. A method of manufacturing a composite material comprising a honeycomb substrate material having a surface suitable for contacting a fuel gas and a ZnO material coating whole or part of the surface of the substrate, where the amount of ZnO material in the composite material is from 0.5 to 3 wt% relative to the entire weight of the composite material and the ZnO is provided on the surface of the substrate in the form of nanosheets, the method comprising the steps of:

(b) immersing the seeded honeycomb material in a crystal growth solution comprising a first zinc oxide source and an amine base at a temperature of from 50 to 150 °C for a first period of time; (c) after step (b) aging the resulting mixture for a second period of time at room temperature to form a ZnO nanosheet coating on the surface of the honeycomb substrate.

15. The method according to Claim 14, wherein:

(aa) the first zinc oxide source is Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0 or, more particularly, Zn(CH₃C0₂)2 and solvates thereof, such as Zn(CH₃- C0₂)₂-2H₂0; and/or

(bb) the first zinc oxide source is present in the crystal growth solution at a concentration of from 0.05 to 0.5M, such as around 0.1 M; and/or

(cc) the amine base is hexamethylenetetramine; and/or

(dd) the amine base is present in the crystal growth solution at a concentration of from 0.05 to 0.5M, such as around 0.2 M; and/or

(ee) the molarity ratio of the amine base to first zinc oxide source is from 1 :2 to 3:1 , such as around 2: 1 ; and/or

(ff) the temperature of step (b) in Claim 14 is from 60 to 100 °C, such as around 90 °C; and/or

(gg) the first period of time is greater than or equal to 3 hours, such as greater than or equal to 5 h, for example from 3 to 10 h, such as around 5 h or from greater than 5 h to 8 h; and/or

(hh) the second period of time is greater than or equal to 8 hours, such as greater than or equal to 18 h, for example from 8 to 24 h, such as around 18 h.

16. The method according to Claim 14 and Claim 15, wherein:

(aaa) the honeycomb substrate material is a ceramic and/or metallic honeycomb substrate material, optionally wherein the ceramic is a cordierite-mullite ceramic; and/or

(bbb) the ZnO material further comprises one or more additive materials selected from the group consisting of a transition metal and a lanthanoid; and/or

(ccc) the amount of ZnO material in the composite material is from 0.6 to 1.6 wt%, such as from 0.6 to 1.1 wt% relative to the entire weight of the composite material;

(ddd) the ZnO is in the form of nanosheets having a height on the surface of the substrate of from 0.5 to 3 pm; and/or

(eee) the ZnO nanosheets have a height on the surface of the substrate of from 0.8 to 1.2 pm, such as around 1 pm; and/or

(fff) the ZnO nanosheets substantially wholly cover the surface of the substrate; and/or (ggg) the composite material has a total sulfur capacity at breakthrough of from 37.6 to 100 mg S/g of ZnO, such as from 40 to 59.8 mg S/g ZnO, such as 48.7 mg S/g ZnO; and/or

(hhh) the composite material has a ZnO efficiency at total sulfur breakthrough of from 9 to 20 mol%, such as from 9.6 to 20 mol%, such as from 10.5 to 15.2 mol%, such as 12.4 mol%.

17. A method of manufacturing a composite material comprising a honeycomb substrate material having a surface suitable for contacting a fuel gas and a ZnO material coating whole or part of the surface of the substrate, where the amount of ZnO material in the composite material is from 0.5 to 3 wt% relative to the entire weight of the composite material and the ZnO is provided on the surface of the substrate in the form of nano rods, the method comprising the steps of:

18. The method according to Claim 17, wherein:

(I) the first zinc oxide source is Zn(CH₃C0₂)2 and solvates thereof, such as Zn(CH₃C0₂)₂-2H₂0 or, more particularly, Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0; and/or

(II) the first zinc oxide source is present in the crystal growth solution at a concentration of from 0.05 to 0.5M, such as around 0.1 M; and/or

(III) the amine base is hexamethylenetetramine; and/or

(IV) the amine base is present in the crystal growth solution at a concentration of from 0.05 to 0.5M, such as from 0.05 to 0.2 M; and/or

(V) the molarity ratio of the amine base to first zinc oxide source is from 0.1 :1 to 3:1 , such as from 0.5:1 to 2: 1 ; and/or

(VI) the temperature of step (ii) in Claim 17 is from 60 to 100 °C, such as around 90 °C; and/or

(VII) the period of time is greater than or equal to 3 hours, such as greater than or equal to 5 h, for example from 3 to 10 h, such as around 5 h or from greater than 5 h to 8 h.

19. The method according to Claim 17 and Claim 18, wherein: (la) the honeycomb substrate material is a ceramic and/or metallic honeycomb substrate material, optionally wherein the ceramic is a cordierite-mullite ceramic; and/or

(lb) the ZnO material further comprises one or more additive materials selected from the group consisting of a transition metal and a lanthanoid; and/or

(lc) the amount of ZnO material in the composite material is from 0.6 to 1.6 wt%, such as from 0.6 to 1.1 wt% relative to the entire weight of the composite material;

(Id) the nano rods have a height of from 0.5 to 2 pm on the surface of the substrate, such as around 1 pm; and/or

(le) each nano rod has an internal diameter of from 0.1 to 1 pm, such as from 0.1 to 0.5 pm; and/or

(If) the ZnO nano rods partly cover the surface of the substrate; and/or

(Ig) the composite material has a total sulfur capacity at breakthrough of from 6.9 to 50 mg S/g of ZnO, such as from 7.1 to 20 mg S/g ZnO, such as from 9.0 to 17.1 mg S/g

ZnO, such as from 10.9 to 12.0 mg S/g ZnO; and/or

(Ih) the composite material has a ZnO efficiency at total sulfur breakthrough of from 1 to 9 mol%, such as from 1.8 to 5 mol%, such as from 2.3 to 4.4 mol%, such as from

2.8 to 3.1 mol%.

20. The method according to any one of Claims 14 to 16 or the method according to any one of Claims 17 to 19, wherein the seeded honeycomb substrate material is provided by the steps of:

21. The method according to Claim 20, wherein:

(iaa) the second zinc oxide source is Zn(CH₃C0₂)2 and solvates thereof, such as Zn(CH₃C0₂)2-2H₂0 and/or Zn(N0₃)₂ and solvates thereof, such as Zn(N0₃)₂-6H₂0; and/or

(ibb) the second zinc oxide source is present in the seeding solution at a concentration of from 0.1 to 0.5 M, such as from 0.3 to 0.4 M; and/or

(ice) the amine base is diethanolamine; and/or

(idd) the molar ratio of the amine base to second zinc oxide source is from 1 :2 to 3:1 , such as around 1 : 1 ; and/or

(iee) the first temperature is from 50 to 100 °C, such as around 60 °C; and/or

(iff) the second temperature is from 300 to 500 °C, such as around 400 °C; and/or (igg) the period of time is from 1 to 5 h, such as around 2 h; and/or (ihh) the solvent in step (ia) of Claim 20 is ethanol; and/or

(iii) the heating rate of the calcination step is from 1 to 4 °C/min, such as 2 °C/min.

22. A method of desulfurizing a fuel gas in need thereof, the method comprising the step of passing a fuel gas in need of desulfurization through a composite material according to any one of Claims 1 to 13 at a temperature of from 200 to 600 °C to provide a desulfurized fuel gas, wherein:

the desulfurized fuel gas contains less than 20 ppmv of sulfur compounds.

23. The method according to Claim 22, wherein the temperature is around 400 °C.

24. The method according to Claim 22 or Claim 23, wherein the fuel gas in need of desulfurization is a fuel gas containing from 40 to 6,000 ppmv of sulfur compounds, such as from 50 to 500 ppmv of sulfur compounds.

25. The method according to any one of Claims 22 to 24, wherein the fuel gas passing through the composite material has a gas hourly space velocity of from 500 to 20,000 h^"1 , such as from 3,000 to 10,000 h^" .

26. The method according to any one of Claims 22 to 25, wherein the method further comprises a step of regenerating the composite material by passing a regenerating gas mixture through it at a temperature of from 500 °C to 800 °C, such as 650 °C, at a gas hourly space velocity of from 2,000 to 5,000 h^"1 , where the regenerating gas mixture comprises from 0.5 vol% to 20 vol% oxygen, such as from 1 vol% to 5 vol% oxygen, such as around 2 vol%.