EP2451573A2

EP2451573A2 - Catalyst consisting of platinum supported on chemically promoted magnesium oxide and cerium dioxide towards h2-scr

Info

Publication number: EP2451573A2
Application number: EP10722966A
Authority: EP
Inventors: Angelos M. Efstathiou; George Olympiou
Original assignee: University of Cyprus
Current assignee: University of Cyprus
Priority date: 2009-06-16
Filing date: 2010-06-10
Publication date: 2012-05-16
Also published as: WO2010145777A3; WO2010145777A2; EP2269729A1

Abstract

This invention relates to a novel catalyst having excellent activity and selectivity for reducing nitric oxides (NO/NO₂) to nitrogen gas (N₂) with hydrogen (H₂) being used as a reducing agent under strongly oxidizing conditions (e.g., 2-10 vol % O₂) (H₂-SCR) in the 100-400°C range, but in particular to the 1 10-180°C low-temperature range. The inventive catalyst comprises of platinum and palladium nanoparticles of appropriate morphology and surface structure which are in contact with solid phases of a promoted with e.g. vanadium oxide (e.g. V₂O₅) mixed MgO and CeO₂ medium.

Description

Catalyst consisting of Platinum supported on chemically promoted Magnesium Oxide and Cerium Dioxide towards H?-SCR

This invention refers to a catalyst comprising platinum on a support, a process for the preparation of such a catalyst, and a use of such a catalyst.

This catalyst can be used in the selective conversion of nitric oxide (NO) or nitric dioxide (NO₂) produced in many industrial combustion processes, in the manufacture of nitric acid, etc., to N₂ gas using hydrogen (H₂) as a reducing agent (H₂-SCR). It is known that hydrogen is available in numerous industrial installations. Using the said catalyst, only a very small percentage of the available hydrogen is necessary for the reduction of NO_x to N₂ under strongly oxidizing conditions (H₂-SCR) in the low- temperature range of 100-200 ⁰C.

The selective catalytic reduction of NO with NH₃ (NH₃-SCR) in the presence of an excess of oxygen is at presently considered the state-of-the-art NO_x control catalytic technology for industrial stationary applications [1]. In this process, ammonia is used to convert NO_x into nitrogen and water reaction products with N₂ selectivities larger than 95% using vanadium pentoxide (V₂O₅) supported on oxides such as TiO₂, AI₂O₃ and SiO₂ and promoted with WO₃ and MoO₃ [2]. This catalyst is active in the temperature range 300-400 ⁰C [3], whereas other catalyst formulations suitable at lower temperatures (~200°C) have been commercialised [4]. Nevertheless, toxicity, handling of ammonia, leaks of unconverted NH₃ to the environment, corrosion and fouling of equipment (formation of ammonium bisulphate), poisoning of the catalyst by SO₂, and high investment costs constitute main problems and concerns nowadays for the use of NH₃-SCR as an industrial NO_x control technology [1 ,5].

Selective catalytic reduction (SCR) of NO_x from an industrial flue gas stream at low- temperatures (120-200⁰C) has many advantages over that at higher temperatures (e.g., T>250°C). For example, placement of the catalyst after the electrostatic dust precipitator unit implies that the partially cleaned flue gas from dust requires less soot blowing and catalyst cleaning, thus providing longer catalyst lifetime. Furthermore, low- temperature SCR process can reduce both the investment and operating costs since the SCR unit can be installed at the end of the stack gas train, thus minimising the need to run ductwork from a high-temperature region and then return the flue gas to the stack gas train. Also, less reheating of the flue gas from the de-SO_x to the SCR unit is required [4,5]. New low-temperature NO_x control SCR catalysts are also capable of retrofitting existing large utility boilers and installations (e.g. industrial furnaces) firing natural gas or refinery fuel gas, where better heat economy of the whole flue gas after- treatment process is achieved.

Current concerns regarding carbon dioxide emissions into the atmosphere and the problems resulting from the use of NH₃ as reducing agent [1 ,4-6] have encouraged a search for suitable molecules different from hydrocarbons for the selective catalytic reduction of NO in gaseous streams derived from combustion processes. It has been reported that hydrogen is a very effective reducing agent for the NO/H₂ reaction [7-17] and can potentially be used for reducing NO_x emissions derived from stationary combustion sources. Hydrogen is currently used in industrial processes of petroleum refining, such as hydrotreatment and hydrocracking [18-20], the production of methanol [21 ,22], the conversion of methanol to gasoline [23,24], and the synthesis of ammonia [25,26] and hydrocarbons (Fischer-Tropsch process) [27,28]. Therefore, hydrogen is available in many industrial installations wherein various processes are operated requiring a heat input. Furthermore, the progressive demand for hydrogen with a growth rate of approximately 10 % per year must be noted [29], which means that hydrogen availability in the industrial sector will be increasing further in the coming years.

Therefore, a low-temperature H₂-SCR of NO_x technology can be considered as breakthrough green and clean industrial NO_x control technology compared to the existing NH₃-SCR technology.

In the last years, a renewed interest in finding suitable cataiyst compositions for industrial low-temperature H₂-SCR of NO_x appeared [30-56]. In most of these publications, supported-platinum catalysts with different support chemical composition and platinum loading (wt%) were investigated. What is learned from these studies is that catalyst performance (NO conversion and N₂-selectivity) strongly depends on the combination of platinum metal loading and support chemical composition in a non- obvious way. Also, the temperature window of operation, ΔT₅₀ (the temperature range for which the NO conversion is at least equal to 50% of the maximum conversion obtained) was found to depend strongly on the latter parameters [49,50].

Supported-palladium catalysts have also been investigated towards H₂-SCR [57-63] but to a significantly less extent than supported-platinum catalysts. In addition, low- temperature NO_x control has been also studied with H₂/CO [64-67] and H₂/CO/CH₄ [68] reducing gas mixtures over supported-palladium catalysts. It appears from these reports that N₂-selctivity of H₂-SCR might be lower or higher than that obtained over supported-platinum catalysts at the same temperature in a non-obvious way, while NO conversion appears in general to be lower on supported-palladium compared to supported-platinum catalysts for the same experimental conditions.

EP 1 475 149 (2008) discloses a catalyst for NO_x control comprising platinum in an amount between 0.1 and 2.0 wt% dispersed on a pre-nitrated and pre-sulphated mixed metal oxide support of magnesium and cerium [55, 56]. The latter supported platinum catalyst provides a high activity (NO conversion larger than 90%) and N₂-selectivity (up to 82%) at low reaction temperatures (e.g. 140-160⁰C) when hydrogen is used as reducing agent at reaction temperatures between 100⁰C and 200⁰C.

DESCRIPTION OF THE INVENTION

The object of the present invention is the development of an alternative catalyst for the selective conversion of NO_x to N₂ by hydrogen (H₂) to that reported [55,56] with significantly better performance in the 110-180⁰C range and in the presence of large concentrations of water (ca. 20 vol%) and carbon dioxide (ca. 10 vol%) in the simulated flue gas stream to be purified.

This object is achieved by catalyst comprising platinum as noble metal, which is dispersed on a mixed metal oxide support of magnesium and cerium, charaterized in that the mixed metal oxide support of magnesium and cerium is chemically promoted by one of the following elements or compound of the following elements: vanadium, sodium, potassium, molybdenum or tungsten.

The inventive idea is the use of specific "chemical promoters" in combination with magnesia, ceria, and platinum in an appropriate composition (wt%) along with the application of an appropriate activation step to the final catalyst formed in a non- obvious way for achieving its effective H₂-SCR of NO_x performance.

The term "chemical promoter" here implies the addition of a chemical compound in the final catalyst composition that would enhance the chemical function of the catalyst [69]. Here, for the present invention as chemical function is meant the selective conversion of NO_x (NO/NO₂) into N₂ gas by the use of hydrogen from a gas mixture containing NO_x, oxygen, carbon dioxide, and water, and especially in the low-temperature range of 110-180⁰C. Thus, the role of "chemical promoter" is to enhance the rate of conversion of NO_x into N₂ than any other by-product (e.g. N₂O, NH₃).

It is well known in the catalytic science that the "chemical promoter" could interact chemically with one or more of the other catalyst constituents to form new more efficient catalytic centers, or alter the catalytic properties of other active centers present in other chemical compounds of the said catalyst composition. A very illustrative example is the use of potassium oxide (K₂O) as "chemical promoter" in the today's catalytic technology of ammonia synthesis using iron-based catalysts [70].

For the present inventive catalyst, the "chemical promoter" in the mixed metal oxide support of magnesium oxide and cerium dioxide could provide other centers for NO_x adsorption, thus changing the concentration and reactivity of the active surface absorbed NO_x species formed, which are then reduced by hydrogen into N₂ and H₂O [71 ,72]. Also, the "chemical promoter" could interact directly with the platinum, since some platinum is expected to be found on the surface of oxides or compounds of the "chemical promoter". Thus, different catalytic active components could have been expected to be formed. Furthermore, the "chemical promoter" may change the electronic structure of the catalytic surface which in turn could affect the catalysis at hand through alterations in the binding strength of adsorbed species or surface concentrations or energy barriers for hydrogen diffusion on the catalyst surface [71]. Therefore, very high conversion rates of NO_x into nitrogen (N₂-selectivity) by hydrogen in the temperature range of 110 - 180⁰C could be achieved.

Advantageously the mean primary crystal size of the mixed metal oxide support phases (MgO and CeO₂) is between 5 and 10 nm. Within the frame of this invention, the mean primary crystal size is determined by X-ray diffraction and using the Scherrer relationship [73].

According to preferred embodiments of the invention, the support is a mixture of vanadium-promoted magnesium oxide and vanadium-promoted cerium dioxide, whereby vanadium (V) oxide (V₂O₅) in an amount between 0.1 wt% and 12 wt%, preferably between 2 wt% and 8 wt%, most preferred between 3.0 wt% and 5 wt%, is used as chemical promoter. =Experiments provided evidence that very high N₂- selectivity (S_N2l %) values (> 90%) towards H₂-SCR in the 120-16O⁰C range are obtained by a catalyst comprising platinum in an amount between 0.01 and 2.0 wt%, wherein the platinum noble metal is dispersed on a vanadium-promoted MgO and CeO₂ mixed metal oxide of which the mean primary crystals are nano-crystals with a mean primary crystal size less than 10 nm, and after specific catalyst activation steps (e.g. calcination (O₂/He gas mixture) at a given temperature and time on stream followed by reduction (H₂/He gas mixture) at a given temperature and time on stream) are followed. Thereby vanadium (V) oxide in an amount as mentioned above was used.

Preferably platinum is dispersed in an amount between 0.01 and 2.0 wt% on the promoted mixed metal oxide support (MgO-CeO₂).

In another prefered embodiment of the invention, palladium (Pd) in an amount between 0.01 and 2.0 wt% as a second noble metal is dispersed on the mixed metal oxide support. Preferably 0.1 wt% of platinum and 0.05 wt% of palladium are dispersed on the promoted mixed metal oxide support. High N₂-selectivity values could be obtained by using a catalyst comprising of platinum and palladium between 0.01 and 2.0 wt%, wherein the platinum and palladium noble metal are dispersed on a preferably vanadium-promoted MgO and CeO₂ mixed metal oxide. In this case, the use of primary crystals with a mean crystal size larger than 40 nm is also advantageous.

According to a particular embodiment of the invention the promoted mixed metal oxide support consists of magnesium oxide and cerium dioxide solid phases in the ratio between 50:50 and 70:30 (w/w), respectively, depending on the primary crystal size of MgO and CeO₂ used. The present invention refers also to a process for obtaining catalyst comprising platinum dispersed on a mixture of promoted magnesium oxide and promoted cerium dioxide comprising the steps:

- impregnating the mixed metal oxide of magnesium oxide and cerium dioxide solid phases with an aqueous solution containing the desired quantity of the promoter precursor, whereby the promoter is one of the following elements or a compound of the following elements: vanadium (V), sodium (Na)₁ potassium (K), molybdenum (Mo) or tungsten (W),

- evaporation of water, drying, grinding and heating at 500°C in air flow for at least 2 hours,

- impregnating the resulting solid with an aqueous solution containing the desired quantity of platinum precursor,

- evaporation of water, drying at 12O⁰C for at least 12 h, grinding, and

- activation of the thus prepared catalyst first in a flow of air or O₂IHe gas mixture in the 500-650°C range for at least 1 h, and then in a flow of H₂/He gas mixture in the 200-

35O⁰C range for at least 1 h.

Also, the present invention refers to a process for obtaining a catalyst comprising platinum and palladium dispersed on a mixture of promoted magnesium oxide and promoted cerium dioxide comprising the steps:

- impregnating the mixed metal oxide of magnesium oxide and cerium dioxide solids with an aqueous solution containing the desired quantity of the promoter precursor, whereby the promoter is one of the following elements or a compound of the following elements: vanadium (V), sodium (Na), potassium (K), molybdenum (Mo) or tungsten (W)₁

- impregnating the resulting solid with an aqueous solution containing the desired quantity of platinum precursor, - evaporation of water, drying, grinding and heating between 500 and 600⁰C in air flow for at least 2 hours,

- impregnating the resulting solid with an aqueous solution containing the desired quantity of palladium precursor, - evaporation of water, drying, grinding and heating at 500⁰C in air flow for at least 2 hours for complete conversion of the platinum and palladium precursors into their respective oxide, and

- activation of the thus prepared catalyst first in a flow of air or O₂/He gas mixture in the 500-650⁰C range for at least 1 h, and then in a flow of H₂/He gas mixture in the 200-

35O⁰C range for at least 1 h.

The activation of the said catalyst compositions is crucial for the catalyst performance (activity, N₂-selectivity and stability) towards H₂-SCR, where optimum calcination and reduction conditions (gas compostion, temperature, time on stream) depend on Pt, Pd, and chemical promoter loading (wt%).

According to a particular embodiment of the invention, the temperature at which activation of the catalyst as prepared is conducted in a flow of O₂/He gas mixture (calcination step) followed by a flow of H₂/He gas mixture (reduction step), and the time on stream allowed for each activation step, are crucial parameters that strongly influence the activity (NO conversion) and N₂-selectivity of the H₂-SCR process.

In some cases it is useful to impregnate the promoted mixed metal oxide with an aqueous solution containing the desired quantity of platinum precursor and the desired quantity of palladium precursor.

If the catalyst should be used for the NO_x reduction in gas streams containing SO_x, an additional step for the pre-nitration and pre-sulphatation of the catalyst is advantageous. However, according to a particular embodiment, the presence of sulphates on the MgO-CeO₂ support of the present invented catalyst does not exhibit any chemical promoting effect.

According to a prefered embodiment of the invention, the "chemical promoter" is one of the elements of vanadium, sodium, potassium, molybdenum and tungsten.

According to an additional particular embodiment, a surface compound of magnesium is formed by interaction between NO_x (NO and NO₂) species present in the gaseous phase under reaction conditions and the oxide of magnesium present. According to an additional particular embodiment, a surface compound of cerium is formed by interaction between NO_x (NO and NO₂) species present in the gaseous phase under reaction conditions and the oxide of cerium present.

According to an additional particular embodiment, a surface compound of vanadium is formed by interaction between NO_x (NO and NO₂) species present in the gaseous phase under reaction conditions and the oxide of vanadium present.

According to an additional particular embodiment, surface compounds of platinum and palladium are formed by interaction between species present in the gaseous phase under reaction conditions (NO, NO₂, O₂) and metallic platinum and palladium present.

The present invention also refers to the selective reduction of nitric oxide, nitrogen dioxide and/or mixture of nitric oxide and nitrogen dioxide to N₂ gas using hydrogen as reducing agent in the presence of the catalysts described herein.

Preferably the inventive catalyst is used for the reduction of a chemical compound selected from the group consiting of nitrogen oxide, nitrogen dioxide or a mixture of both to nitrogen using hydrogen as reducing agent in the presence or absence of oxygen.

One embodiment of the invention also refers to a method of reducing a chemical compound selected from the group consisting of NO, NO₂ and/or a mixture of NO and NO₂ to N₂ gas using hydrogen as reducing agent in the presence of oxygen, and also in the presence of other gases, for example H₂O and CO₂, by a catalyst comprising Pt in an amount between 0.01 and 2.0 wt% and Pd in an amount between 0.01 and 2.0 wt%, dispersed on a vanadium-promoted MgO and CeO₂.

According to a particular embodiment in the mentioned method, a reactor selected from the group consisting of a fixed-bed and a monolithic-type reactor can be used.

The present invention provides a variety of advantages. First, the low-temperature (120-180°C) activity of the inventive catalyst comprising platinum dispersed on a promoted-MgO and promoted-CeO₂ mixed metal oxide was much better than the activity of a catalyst comprising platinum dispesed on a non promoted-MgO and CeO₂ mixed metal oxide according to prior art. Second, the N₂-selectivity of the inventive catalyst comprising platinum dispersed on a preferably vanadia-promoted MgO and preferably vanadia-promoted CeO₂ mixed metal oxide was remarkably increased, where N₂-selectivities above 90% were obtained.

In an preferred embodiment of the invention, the N₂-selectivity of a catalyst comprising platinum and palladium dispersed on a vanadium-promoted MgO and vanadium- promoted CeO₂ mixed metal oxide was remarkably increased by 10-30 percentage units in the H₂-SCR of NO at temperatures in the 120-160⁰C low-temperature range compared to platinum dispersed on a mixed metal oxide of MgO and CeO₂ according to prior art.

EXAMPLES OF EMBODIMENT OF THE INVENTION

In the following the present invention is described in more detail by examples of preferred embodiments of the invention. There can be no doubt that this detailed description is made by way of illustration only, and it does not limit the extent of the invention since there are many variations that can be made without detracting from the spirit of this invention.

Example 1

Pt/MgO-CeO₂, PW-MgO-CeO₂, and Pt-Pd/V-MgO-CeO₂ catalysts were prepared by means of the wet impregnation method as follows:

• PtZMgO-CeO₂ Catalyst

0.5 g of commercial nano-crystalline MgO (Aldrich, product no. 549649, mean primary crystal size 9.0 nm) or MgO with larger mean primary crystal size (Aldrich, product no. 288667, mean primary crystal size 44 nm) were mechanically mixed with 0.5 g of commercial nano-crystalline CeO₂ (Aldrich, product no. 544841 , mean primary crystal size 5 nm) or CeO₂ with larger mean primary crystal size (Aldrich, product no. 211575, mean primary crystal size 41 nm), and the resulting solid (1.0 g) was impregnated with 200 ml of an aqueous solution containing the desired quantity of hexachloroplatinic acid solution (Aldrich, product no. 262587) so as to yield the desired loading (wt%) of Pt. The excess of water was evaporated with continuous stirring at 60-70 ⁰C and the residue was dried in air at 120 ⁰C for ~ 12 h. The dry residue was then sieved and heated between 500 and 600 ⁰C in a flow of 20%O₂/He for at least 2 h in order to remove chlorine from the catalyst surface and convert Pt into PtO₂, cooled to room temperature and stored for further use. For H₂-SCR reaction applications, the catalyst is activated first in a flow of air or 10 vol% O₂/He gas mixture in the 500-650⁰C range for 1-2 h, and then reduced in a flow of 10-15 vol% H₂/He gas mixture in the 200-350 ⁰C range for at least 1 h. The content of metallic platinum varied in the 0.01-2.0 wt% range.

• PW-MgO-CeO₂ Catalyst

1.0 g of MgO-CeO₂ support prepared as described above was impregnated with 200 ml of an aqueous solution containing the desired quantity of ammonium metavanadate (Aldrich, product no. 31153) so as to yield the desired loading (wt%) of V. The excess of water was evaporated with continuous stirring at 60-70 ⁰C and the residue was dried at 120 ⁰C for ~12 h. The dry residue was sieved and heated at 500 ⁰C in a flow of air or 20 vol% O₂/He gas mixture for at least 2 h. The resulting solid was then impregnated with the desired quantity of hexachloroplatinic acid (Aldrich, product no. 262587) so as to yield the desired loading (wt%) of Pt. The excess of water was evaporated with continuous stirring at 60-70 ⁰C and the residue was dried at 120 ⁰C for ~ 12 h, cooled to room temperature and stored for further use. For H₂-SCR reaction applications, the catalyst is activated first in a flow of air or 10 vol% O₂/He gas mixture in the 500-650⁰C range for 1-2 h, and then reduced in a flow of 10-15 vol% H₂/He gas mixture in the 200- 350 ⁰C range for at least 1 h. The content of metallic platinum varied in the 0.01-2.0 wt% range.

• Pt-PdZV-MgO-CeO₂ Catalyst

1.0 g Of V-MgO-CeO₂, prepared according to the same procedure as in the case of PW-MgO-CeO₂ solid, was impregnated with 200 ml of an aqueous solution containing the desired quantity of hexachloroplatinic acid (Aldrich, product no. 262587). The excess of water was evaporated with continuous stirring at 60-70 ⁰C and the residue was dried at 120 ⁰C for ~ 12 h. The dry residue was sieved and heated at 500 ⁰C in a flow of air or 20%O₂/He gas mixture for at least 2 h. Following this step, the solid material was again impregnated with 200 ml of an aqueous solution containing the desired quantity of palladium(ll) nitrate solution (Aldrich, product no. 380040). The excess of water was evaporated with continuous stirring at 60-70 ⁰C and the residue was dried at 120 ⁰C for ~ 12 h. The dry residue was sieved and heated at 500 ⁰C in a flow of 20%O₂/He for at least 2 h in order to remove chlorine and nitrates from the solid surface and convert Pt and Pd into their respective metal oxides, cooled to room temperature and stored for further use. For H₂-SCR reaction applications, the catalyst is activated first in a flow of air or 10 vol% O₂/He gas mixture in the 500-650⁰C range for 1-2 h, and then reduced in a flow of 10-15 vol% H₂/He gas mixture in the 200-350 ⁰C range for at least 1 h. The content of metallic platinum (Pt) and palladium (Pd) was varied in the 0.01-2.0 wt% and 0.01-2.0 wt% range, respectively.

As will be shown in Example 9, the combination of Pt, Pd, V, MgO and CeO₂, the latter two solids having a primary crystal size larger than 40 nm, with the said chemical composition in a non-obvious way, resulted in remarkable improvements of N₂- selectivity (10-30 percentage units) in the low-temperature range of 120-180⁰C for the H₂-SCR process at the said feed gas composition as compared to the Pt/MgO-CeO₂ catalyst chemical composition previously reported by us [55,56]. Also, as will be shown in Examples 2 and 3, and Example 10, the combination of Pt, V, MgO and CeO₂, the latter two solids having a primary crystal size smaller than 10 nm, with the said chemical composition in a non-obvious way resulted also in significant improvements of N₂-selectivity (about 10 to 65 percentage units increase) in the low-temperature range of 120-15O⁰C for the H₂-SCR process at the said feed gas composition as compared to the unpromoted Pt/MgO-CeO₂ catalyst previously reported by us [55,56].

Example 2

The effect of 3.5 wt% V₂O₅ (2.0 wt% V) loading on the conversion of NO (X_N0, %) and N₂-selectivity (S_N2, %) for the H₂-SCR of NO obtained in the 130-180⁰C range, after 30- 32 h on reaction stream, is illustrated in Figure 1 over the 0.2 wt% Pt/3.5 wt%V₂O₅- MgO-CeO₂ and 0.2 wt% Pt/MgO-CeO₂ catalysts. The mean primary crystal size of MgO and CeO₂ support phases used was 9.0 and 5.0 nm, respectively, and their weight ratio was 70:30 (w/w). A catalyst sample of 0.62 g was placed in a fixed-bed quartz micro-reactor, and it was first activated in a flow of 10%O₂/He gas mixture (100 NmL/min) at 500⁰C for 1 h, followed by H₂ reduction (12.5%H₂/He, 100 NmL/min) at 250⁰C for 1 h before a reaction feed gas mixture of 325 ppm NO, 0.8 vol% H₂, 2.5 vol% O₂, 10 vol% CO₂, 20 vol% H₂O and 66.67 vol% He at a GHSV of 40,000 h ¹ (Lgas/Lca_{t .b}e_d/h) was used. Figure 1 clearly demonstrates that the use of 3.5 wt% V₂O₅-MgO-CeO₂ (magnesium oxide and cerium dioxide chemically promoted with vanadia) as a carrier to deposit 0.2 wt% Pt resulted in a remarkable improvement of both the NO conversion and N₂- selectivity, especially at the lowest temperatures (130-150⁰C) investigated, compared to the case of use of MgO-CeO₂ carrier alone. For example, at 13O⁰C the NO conversion (X_NO, %) was increased from 20.5% to 72.5% (an increase by a factor of ~ 3.5), whereas at 18O⁰C from 60.0 to 96.0% (an increase by a factor of 1.6). The high NO conversions (73.5 - 97.0%) obtained in the 130-180⁰C range under the given experimental conditions is noted which are representative of many industrial flue gas compositions and space velocities encountered.

In the case of N₂-selectivity of H₂-SCR reaction, the effect of vanadia was to siginificantly enhance N₂-selectivities (S_N2, %) to values of 90% in the 160-180⁰C range, and 72-87% in the 130-150⁰C range.

A pre-sulphated MgO-CeO₂ support loaded with 0.2 wt% Pt, prepared according to the methods described by us previously [55,56], was also tested under the same conditions corresponding to the results shown in Fig. 1. It was found that the NO- conversion and N₂-selectivity behaviour was practically the same or slightly lower than that shown in Fig. 1 for the 0.2 wt% Pt/MgO-CeO₂ catalyst. This result clearly illustrates the fact that pre-sulphation of support cannot be considered as a means of "chemical promotion" of the given MgO-CeO₂ support but rather as a means of preventing catalyst deactivation (loss of activity with time on stream) in the case only when SO₂ is present is the feed stream, as illustrated by us previously [55,56].

Example 3

The effect of 3.5 wt% V₂O₅ (2.0 wt% V) loading on the conversion of NO (X_N0, %) and N₂-selectivity (S_N2, %) for the H₂-SCR of NO obtained in the 130-180⁰C range, after 30- 32 h on reaction stream, is illustrated in Figure 2 over the 0.2 wt% Pt/3.5 wt% V₂O₅- MgO-CeO₂ and 0.2 wt% Pt/MgO-CeO₂ catalysts using a lower NO concentration (175 ppm) in the flue gas feed stream compared to that used in Example 2 (Fig. 1). The experimental conditions were the same as those presented in Example 2, except that now a feed flue gas composition of 175 ppm NO, 0.8 vol% H₂, 2.5 vol% O₂, 10 vol% CO₂, 20 vol% H₂O and 66.69 vol% He was used. Remarkably, the beneficial effect of vanadia promoter on the X_NO(%) and S_N2(%) is now larger compared to the previous Example 2 (use of 325 ppm NO in the feed stream). Very high NO conversions (85.0 - 98.7%) were seen in the 130-180⁰C range, and N₂- selectivities of 80-90% in the same temperature range were obtained. For example, at 13O⁰C the NO conversion (X_N0, %) was increased from 11.0 to 85.0% (increase by a factor of 7.7), whereas the N₂-selectivity (S_N2, %) from 14.0 to 80.0% (increase by a factor of 5.7).

Other vanadia loadings in the 0.1 - 12.0 wt% V₂O₅ range were investigated over various MgO-CeO₂ support compositions (0-100 wt% MgO) and using 0.1-2.0 wt% Pt loadings. For example, by increasing the vanadia loading from 3.5 to 5 wt% V₂O₅, an increase of NO conversion by 10-20 percentage units was seen at the lowest temperatures of 120-130⁰C for the 0.2 wt% Pt-supported catalyst with the MgO-CeO₂ support composition and feed gas compositions mentioned previously (Examples 2 and 3). Improvements up to 10-15 percentage units were seen in the N₂-selectivity for the given catalyst composition. The optimum vanadia loading for catalyst performance (X_NO(%) and S_N2(%)) was found to depend on the BET area of support composition, the primary crystal size of MgO and CeO₂ support, and the Pt loading (wt%) used.

Example 4

In this example, the effect of vanadia promoter used in the composition of the carrier (MgO-CeO₂) for preparing the supported-Pt catalyst for low-temperature H₂-SCR on its performance with time on stream (performance stability) is illustrated in Figure 3. It is clearly shown that the presence of vanadia in the support chemical composition prevented the loss of activity (NO conversion decrease) of the catalyst (0.2 wt% Pt/3.5wt%V₂0₅-Mg0-Ce0₂) with time on reaction stream. The former catalyst composition shows remarkable stability (in terms of both NO conversion and N₂- selectivity) under the feed gas composition: 175 ppm NO/2.5vol%O₂/0.8vol%H₂/10vol%CO₂/20 vol%H₂O/He (GHSV=40,000 h ¹) at the temperature of 16O⁰C (Fig. 3) for up to 70 h on reaction stream tested, as opposed to the case of catalyst consisting of 0.2 wt% Pt/MgO-CeO₂ (absence of vanadia promoter). It is concluded that the combination of vanadia, magnesia and ceria has promoted not only the catalytic activity and N₂-selectivity (Examples 2 and 3) of the supported-Pt solid, but also its stability with time on stream. The reasons for such behaviour is speculative, whether vanadia acts also as a "structural promoter" [69], preventing any changes of favourable Pt particle size and morphology that could be induced by the presence of large concentrations (e.g. 20 vol%) of water in the feed stream.

In Figure 3 it is shown that the absence of vanadia in the MgO-CeO₂ support causes a siginificant loss of activity (drop of X_NO(%) from 80 to 40%) within the first 70 h of continuous operation under the given reaction conditions. However, it should be noted that the absence of vanadia in the support composition does not influence the stability with time on stream of N₂-selectivity of the H₂-SCR process.

A pre-sulphated MgO-CeO₂ support used to deposit 0.2 wt% Pt (see Example 2) was also not able to prevent catalytic activity loss. However, in this case the activity loss was less than that observed with the 0.2 wt% Pt/MgO-CeO₂ (Fig. 3) catalyst and larger than that observed with the 0.2 wt% Pt/3.5wt%V₂O₅-MgO-CeO₂ catalyst (Fig. 3).

Example 5

A very important parameter that determines the catalytic performance of the PW₂O₅- MgO-CeO₂ solid towards its effective low-temperature H₂-SCR performance (NO conversion and N₂-selectivity) is the way the as prepared catalyst is activated before reaction (contact with the flue gas for NO_x purification). Catalyst activation includes calcination (use of O₂/He or air gas flow) and reduction (use of H₂/He gas mixture or pure hydrogen flow) steps at a given temperature and for a given time on stream. This parameter is illustrated by experimental data obtained and presented in Table 1 for the 0.2 wt% Pt/3.5wt%V₂O₅-MgO-CeO₂ catalyst, the same used in Examples 2-4. According to the results of Table 1 , the NO-conversion and N₂-selectivity versus reaction temperature (T, ⁰C) profile is strongly influenced by the activation steps performed on the given catalytic system before H₂-SCR reaction. For example, catalyst activation by H₂ reduction only performed at 300⁰C for 14 h results in high NO conversion values (88.5-97.0%) in the whole 125-18O⁰C range, but with N₂-selectivity values in the 66-80% range. On the other hand, activation of the catalyst using calcination (10%O₂/He gas flow) at 500⁰C for 1 h followed by H₂ reduction (12.5%H₂/He gas flow) for 1 h (see Table 1) results in a significantly improved N₂-selectivity versus T profile in the whole 125-18O⁰C temperature range. In particular, it is noted the high N₂- selectivity values of about 90% obtained in the 140-17O⁰C range as opposed to the values of 63-75% obtained with the H₂ activation step only, as previously mentioned. At the same time, the NO conversion is kept practically the same in the 140-180⁰C range and drops only by 7-10 percentage units at the two lowest temperatures of 125 and 13O⁰C.

By increasing the calcination temperature to 600⁰C, while keeping the H₂ reduction temperature and the time of activation in hydrogen flow the same, both the NO conversion and N₂-selectivity values obtained become significantly lower in the whole temperature range of 125-18O⁰C, when compared to the case of 500 ⁰C calcination. When compared to the activation step of H₂ reduction only, N₂-selectivities larger in the 160-180⁰C range but lower in the 125-15O⁰C range are obtained. At the same time, lower NO conversion values are obtained in the whole 125-18O⁰C range.

Another important consequence from the choice of the approprioate catalyst activation procedure is the influence on the H₂ combustion rate. As illustrated in Table 1 , the hydrogen conversion (X_H2, %) which is largely determined by the reaction of hydrogen combustion (H₂ + !4 O₂ → H₂O) taking place exclusively on the platinum (Pt) surface in this low-temperature range of 125-18O⁰C, is largely influenced by the chosen catalyst activation procedure. For example, catalyst activation by calcination at 500⁰C followed by reduction at 25O⁰C provided the lowest X_H2 (%) values in the whole reaction temperature range of 125-180⁰C as compared to the other two catalyst activation procedures (Table 1). At the same time, the former catalyst activation procedure resulted in the largest N₂-selectivity values in the same temperature range. This very important result proves that H₂-SCR technology can be operated using lower H₂ concentrations in the feed stream when proper activation of catalyst is made, thus lowering the operational cost of the technology.

Example 6

Pt loading (wt%) is a very important parameter that determines not only the catalytic performance (NO conversion and N₂-selectivity) of Pt/MgO-CeO₂ solid towards H₂-SCR [53-56] but also the economics of the associated technology for industrial applications. Figure 4 illustrates that using only 0.1 wt% R supported on the MgO-CeO₂ carrier, the latter promoted with 3.5 wt% V₂O₅, results in an excellent H₂-SCR performance in the 140-18O⁰C range, where both NO conversion and N₂-selectivity exchibit values larger than 90%. Only in the 120-130⁰C range these catalytic performance parameters exhibit values lower than 90% (X_NO=63-75%, S_N2=80-83%). These results relate to a feed gas composition of 110 ppm NO; 0.8%H₂; 2.5%O₂; 10%CO₂; 20%H₂O; He (balance) used at a GHSV of 40,000 h^'1. To our knowledge, this noble metal loading is the lowest ever reported in the patent literature for supported noble metals (e.g., Pt, Pd, Rh) towards H₂-SCR under the presently used severe flue gas compositions that contain 20 vol% H₂O and 10 vol% CO₂.

Example 7

The effect of increasing the vanadia loading from 3.5 to 5.0 wt% deposited on the same MgO-CeO₂ support used in the previous Examples 2-6, on the performance of 0.2 wt% supported-Pt catalyst is illustrated in Figure 5 after using a feed gas composition consisting of 325 ppm NO/0.8%H₂/2.5%O₂/10%CO₂/20%H₂O/He at a GHSV of 40,000 h^"1. A significant effect in the activity (NO conversion, XNO(%)) is obtained at the lowest temperatures of 125 and 130⁰C, while practically no effect is observed at temperatures in the 150-18O⁰C range. On the other hand, the N₂-selectivity stays practically unaffected after increasing the vanadia loading from 3.5 to 5.0 wt%. It was found that an optimum vanadia loading for the Pt/V₂O₅-MgO-CeO₂ catalytic system depends also on the Pt loading and feed gas composition.

Example 8

The effect of 2 wt% vanadium (V) loading (3.5 wt% V₂O₅) on the conversion of NO (X_N0, %) for the H₂-SCR of NO obtained in the 110-180⁰C range, after 8-10 h on reaction stream is illustrated in Figure 6 over the 0.1 wt% Pt/2 wt% V-Mg 0-CeO₂ and 0.1 wt% PtVMgO-CeO₂ catalysts. The mean primary crystal size of MgO and CeO₂ support phases used was 44 and 41 nm, respectively, as opposed to the previous cases of

Examples 2-7, where smaller primary crystals for the MgO-CeO₂ support were used. A catalyst sample of 0.3-g was placed in a fixed-bed quartz micro-reactor, and it was first calcined in 20%O₂/He gas flow (50 NmL/min) at 600⁰C for 2 h, followed by H₂ reduction (1 bar H₂, 50 NmLJmin) before a reaction feed gas mixture consisting of 500 ppm NO, 0.8 vol% H₂, 5 vol% O₂ and 94.15 vol% He or 500 ppm NO, 0.8 vol% H₂, 5 vol% O₂, 10 vol% CO₂, and 84.15 vol% He at a GHSV of 40,000 h ¹ was used.

Figure 6 clearly demonstrates that the use of 3.5 wt% V₂O₅-MgO-CeO₂ (magnesium oxide and cerium dioxide promoted with vanadia) as carrier to deposit 0.1 wt% Pt resulted in a remarkable improvement of NO conversion when 10 vol% CO₂ was also present in the reaction feed stream, compared to the case of use of MgO-CeO₂ carrier alone. For example, at 13O⁰C the NO conversion (%) was increased from 15% to 57% (an increase by a factor of 3.8), whereas at 140 and 16O⁰C an increase by a factor of 2.3 and 1.7, respectively, was obtained. Similar results to those depicted in Fig. 6 were obtained when 20 vol% H₂O was also present in the 500 ppm NO, 0.8 vol% H₂, 5 vol% O₂, 10 vol% CO₂, He reaction feed gas composition.

It is also stressed the fact that in the whole temperature range of 120-180⁰C the NO conversion was in the 80-100% range at the GHSV of 40,000 h^'1, one of the highest space velocities expected in industrial de-NO_x applications. Larger NO conversions by about 5-12 percentage units were measured at a GHSV of 20,000 h^"1 for the same reaction conditions.

Vanadium (V) loadings in the 0.1-12 wt% range were investigated, where it was found that the optimum loading for the 0.1 wt% Pt/x wt% V-MgO-CeO₂ catalytic system strongly depends not only on the reaction feed gas composition in NO, O₂ and H₂ gases but also on the primary crystal size of MgO and CeO₂ support used, as it is illustrated in the following Example 9.

Figure 6 presents also the effect of 2 wt% vanadium (V) loading on the N₂-selectivity (S_N2, %) of the H₂-SCR of NO under the same reaction conditions described for the NO-conversion versus T profile (500 ppm NO, 0.8 vol% H₂, 5 vol% O₂, 10 vol% CO₂, and 84.15 vol% He). It is seen that in the lowest reaction temperature range of 110- 13O⁰C, an increase in S_N2(%) between 6 and 15 percentage units was observed, whereas at higher temperatures S_N2(%) was practically the same. Siginificantly larger N₂-selectivity values were obtained at another V loading (wt%) and feed gas composition, as illustrated in the following Example 9, and also after using smaller primary crystals for MgO and CeO₂ support phases (see Examples 2 and 3).

It was also found that using these particular catalyst compositions the N₂-selectivity of the H₂-SCR obtained in the presence of CO₂ in the feed stream (NO/H₂/O₂/CO₂/He) was always larger by 5-15 percentage units compared to the case of absence of CO₂ from the reaction feed stream.

Pt loadings in the 0.01 - 2.0 wt% range were investigated, where an optimum Pt loading in the 0.1-0.3 wt% range was found, depending on vanadia loading, feed gas composition, and primary crystal size of MgO-CeO₂ support.

Example 9

In this example, the effect of combination of Pd and V chemical promoter on the N₂- selectivity (S_N2, %) of H₂-SCR of NO with Pt/MgO-CeO₂ is illustrated (Figure 7). Results refer to the development of a novel catalytic system which resulted as the combination of 0.1 wt% Pt and 0.05 wt% Pd with 8 wt% V-MgO-CeO₂ carrier (MgO:CeO₂=50:50 w/w). The mean primary crystal size of MgO and CeO₂ support phases was 44 and 41 nm, respectively. For comparison purposes, results obtained with the 0.1 wt% Pt/MgO- CeO₂ and 0.1 wt% Pt/8 wt% V-MgO-CeO₂ catalytic systems are also presented (Fig. 7). The amount of catalyst used was 0.3 g, and a reaction feed stream consisting of 500 ppm NO, 0.7 vol% H₂, 3 vol% O₂, and 96.25 vol% He at a GHSV of 40,000 h ¹ was used.

Remarkable improvements in S_N2(%) at the lowest temperature range of 120-150⁰C were obtained, ranging from 13 to 26 percentage units. In particular, at 12O⁰C an increase in S_N2(%) from 64% to 90% was obtained when the 0.1 wt% PfMgO-CeO₂ catalyst was promoted with 0.05 wt% Pd and 8 wt% V. It should be noted that NO conversions (X_No, %) larger than 85% were achieved in the 120-160⁰C range with the present Pt-PdAAMgO-CeO₂ catalyst for the conditions of the experiments presented in Fig. 7.

Pt and Pd loadings lower than 0.05 wt% or larger than 2.0 wt% did not result in better catalytic performance (S_N2, %) as that presented in Fig. 7. An optimum Pt and Pd loading was found to depend on the loading (wt%) of V, the mean primary crystal size of MgO and CeO₂, and also on their wheight ratio (w/w) used to deposit V, Pt and Pd.

Example 10 This example illustrates the effect of using nano-crystalline MgO and CeO₂ support phases (mean primary crystal size lower than 10 nm) to deposit the combination of Pt and V catalytic and chemical promoter components, respectively, on the low- temperature H₂-SCR of NO in terms of NO conversion, X_No(%) and N₂-selectivity, S_N2(%) at a feed containing larger NO concentrations (e.g. 500 ppm) as compared to the cases presented in Examples 2 and 3. A catalyst sample of 0.3 g and a feed gas composition of 500 ppm NO, 0.7 vol% H₂, 3 vol% O₂ and 96.25 vol% He at a GHSV of 40,000 h^"1 were used. As clearly illustrated in Fig. 8, remarkably high N₂-selectivity values (~ 88-90%) are obtained at the lowest reaction temperature range of 120-150⁰C, larger by about 10 percentage units when a combination of 0.1 wt% Pt and 8 wt% V are deposited on nano-crystalline MgO and CeO₂ carriers compared to 0.1 wt% Pt deposition alone. It is pointed out that the corresponding NO conversion observed in both catalytic systems is very similar since it varies only by 2-3 percentage units. In the case of use of larger MgO (44 nm) and CeO₂ (41 nm) primary crystals (Fig. 6, CO₂ present in the feed stream), the effect of V "chemical promoter" was to reduce slightly the S_N2(%) only at 18O⁰C, result opposite to that seen in the case of use of nano- crystalline MgO and CeO₂ support phases (Fig. 8). In Example 6 (Fig. 4) it was also illustrated that the use of small primary crystals for MgO and CeO₂ promoted with vanadia resulted in N₂-selectivities larger than 90% when using other feed gas compositions.

It is pointed out the wide temperature-window of operation achieved in the catalytic systems presented in Fig. 8, where only small variations in NO-conversion and N₂- selectivity are observed in the whole 120-180⁰C range. The same is true for the Pt- PdAAMgO-CeO₂ catalyst as illustrated in Fig. 7. This result has significant applications in industrial catalytic reactors where isothermal operation of exothermic reactions (as the present network of reactions in H₂-SCR technology) is not easy to control or the temperature of the flue gas to be purified from NO_x varies with process time. This behaviour is likely to be due to the formation of a number of different in structure active adsorbed NO_x species on the periphery of Pt and Pd nano-particles with the VO_x, MgO and CeO₂ crystals, as evidenced in the case of Pt/MgO-CeO₂ catalyst [71 ,72]. Also, the likely formation of Pt-Pd alloy nano-particles might enhance surface hydrogen diffusion from the noble metal to the metal-support interface, the latter step proved to be an important step in the mechanism of H₂-SCR over the Pt/MgO-CeO₂ catalyst [71 ,72]. Pt loadings in the 0.01-2 wt% range and V loadings in the 0.1-12 wt% range did not result in better catalytic performance in terms of N₂-selectivity to that presented in Fig. 8. However, N₂-selectivities larger than 90% could be obtained with lower NO feed concentrations (see Fig. 4).

Example 11

This example reports on the effect of using other than vanadium (V) chemical promoters on the catalytic performance of Pt/MgO-CeO₂ and Pt-Pd/MgO-CeO₂ solids towards H₂-SCR. Sodium (Na), potassium (K), tungsten (W), or molybdenum (Mo) were deposited on the MgO-CeO₂ mixed metal oxide support following exactly the same procedure as for vanadium (V) which is described in Example 1. Sodium and potassium loadings were varied in the 0.05-2.0 wt% range, tungsten loading in the 0.05-15 wt% range, and molybdenum loading in the 0.05-15 wt% range. In the case of use of 0.1 wt% PVMgO-CeO₂ in combination with one of these four chemical promoters (Na, K, W, Mo) in the loading range given above, and after using a feed gas composition consisting of 500 ppm NO/0.7 vol%H₂/3vol%0₂/He at a GHSV of 40,000 h^" ¹, a similar catalytic behavior compared to that obtained with vanadium (V) chemical promoter (Examples 9 and 10) was obtained, except of Mo, where smaller chemical promoting effect was observed. However, it should be pointed out that the use of any of these chemical promoters (Na, K, W₁ Mo) with Pt/MgO-CeO₂ or Pt-Pd/MgO-CeO₂ catalysts under different than the present feed gas compositions used with respect to NO, H₂, and O₂, could lead to better than vanadium (V) catalytic performance.

BRIEF DESCRIPTION OF THE FIGURES AND TABLE

Figure 1 illustrates the strong chemical promoting effect of vanadia when deposited on MgO and CeO₂ support solid phases (70 wt% MgO-30 wt% CeO₂) on the NO conversion, X_No(%), and N₂-selectivity, S_N2(%) of H₂-SCR in the low-temperature range of 130-18O⁰C for the 0.2 wt%Pt/3.5wt% V₂O₅-MgO-CeO₂ catalyst. Reaction conditions: NO = 325 ppm; H₂= 0.8 vol%; O₂ = 2.5 vol%; 10 vol%C0₂; 20 vol%H₂0; He balance gas. Mass of catalyst, W = 0.62 g; GHSV = 40,000 h^"1 (Lgas/Uatbed/h); P_tot = 1 0 bar; Primary crystal size of support used: MgO= 9 nm; CeO₂= 5 nm. Figure 2 illustrates the strong chemical promoting effect of vanadia when deposited on MgO and CeO₂ support solid phases (70 wt% MgO-30 wt% CeO₂) on the NO conversion, X_No(%), and N₂-selectivity, S_N2(%) of H₂-SCR in the low-temperature range of 130-180⁰C over 0.2 wt%Pt/3.5wt% V₂O₅-MgO-CeO₂ catalyst. Reaction conditions: NO = 175 ppm; H₂= 0.8 vol%; O₂ = 2.5 vol%; 10 vol%CO₂; 20 vol%H₂O; He balance gas. Mass of catalyst, W = 0.62 g; GHSV = 40,000 h^"1; P_tot = 1.0 bar; Primary crystal size of support used: MgO= 9 nm; CeO₂= 5 nm.

Figure 3 illustrates the strong chemical and likely structural promoting effect of vanadia when deposited on MgO and CeO₂ support solid phases (70 wt% MgO-30 wt% CeO₂) on the stability with time on stream (up to 70 h) of NO conversion, X_NO(%), and N₂- selectivity, S_N2(%) of H₂-SCR at 16O⁰C for the 0.2 wt%Pt/3.5wt% V₂O₅-MgO-CeO₂ catalyst. Reaction conditions: NO = 175 ppm; H₂= 0.8 vol%; O₂ = 2.5 vol%; 10 vol%CO₂; 20 vol% H₂O; He balance gas. Mass of catalyst, W = 0.62 g; GHSV = 40,000 h^"1; P_tot = 1.0 bar; Primary crystal size of support used: MgO= 9 nm; CeO₂= 5 nm.

Figure 4 illustrates the catalytic performance in terms of NO conversion, X_NO(%), and N₂-selectivity, S_N2(%) of H₂-SCR in the low-temperature range of 120-180⁰C over a low- loading 0.1 wt%Pt supported on vanadia-promoted support (3.5wt% V₂O₅-MgO-CeO₂). Reaction conditions: NO = 110 ppm; H₂= 0.8 vol%; O₂ = 2.5 vol%; 10 vol%CO₂; 20 vol%H₂O; He balance gas. Mass of catalyst, W = 0.62 g; GHSV = 40,000 h^"1; P_tot = 1 0 bar; Primary crystal size of support used: MgO= 9 nm; CeO₂= 5 nm.

Figure 5 illustrates the effect of increasing the vanadia loading from 3.5 to 5.0 wt% on the catalytic activity in terms of NO conversion, X_NO(%) of H₂-SCR in the low- temperature range of 120-180⁰C for the 0.2 wt%Pt supported on vanadia- promoted support (3.5 or 5.0 wt% V₂O₅-MgO-CeO₂). Reaction conditions: NO = 325 ppm; H₂= 0.8 vol%; O₂ = 2.5 vol%; 10 vol%CO₂; 20 vol%H₂O; He balance gas. Mass of catalyst, W = 0.62 g; GHSV = 40,000 h^"1; P_tot = 1.0 bar; Primary crystal size of support used: MgO= 9 nm; CeO₂= 5 nm.

Figure 6 illustrates the effect of 2 wt% vanadium deposited on MgO and CeO₂ support solid phases (50 wt% MgO-50 wt% CeO₂) on the NO conversion, X_No(%) and N₂- selectivity, S_N2(%) of H₂-SCR in the low-temperature range of 110-180⁰C for the 0.1 wt%Pt/2 wt% V-MgO-CeO₂ catalyst. Reaction conditions: NO = 500 ppm; H₂= 0.8 vol%; O₂ = 5 vol%; x vol%CO₂ (x=0, 10); He balance gas. Mass of catalyst, W = 0.3 g; GHSV = 40,000 h^"1; P_tot = 1.0 bar. For comparison purposes the X_No(%) versus reaction temperature, T(⁰C) behaviour over the 0.1 wt% Pt/MgO-CeO₂ (absence of 2 wt% V) is also presented. The mean primary crystal size of MgO and CeO₂ support phases used was 44 and 41 nm, respectively.

Figure 7 compares the N₂-selectivity, S_N2 (%) of H₂-SCR obtained in the low- temperature range of 120-18O⁰C over 0.1 wt% Pt supported on MgO-CeO₂ (■) or 8 wt% V-MgO-CeO₂ (•), and 0.1 wt% Pt - 0.05 wt% Pd supported on 8 wt% V-MgO-

CeO₂ ( A ) (d_Mgo = 44 nm; d_Ceo₂ = 41 nm). Reaction conditions: NO = 500 ppm; H₂= 0.7 vol%; O₂ = 3 vol%; He balance gas. Mass of catalyst, W = 0.3 g; GHSV = 40,000 h^'1; P_tot = 1.0 bar.

Figure 8 illustrates the effect of 8 wt% vanadium deposited on MgO and CeO₂ support solid phases (50 wt% MgO-50 wt% CeO₂) on the NO conversion, X_No(%) and N₂- selectivity, S_N2(%) of H₂-SCR in the low-temperature range of 120-180⁰C over 0.1 wt% Pt/8 wt% V-MgO-CeO₂ catalyst. Reaction conditions: NO = 500 ppm; H₂= 0.7 vol%; O₂ = 3 vol%; He balance gas. Mass of catalyst, W = 0.3 g; GHSV = 40,000 h^"1; P_tot = 1.0 bar. For comparison purposes the X_NO(%) and S_N2(%) versus the reaction temperature, T(⁰C) behaviour of H₂-SCR over the 0.1 wt% PtVMgO-CeO₂ (absence of 8 wt% V) is also presented. The mean primary crystal size of MgO and CeO₂ support phases used was 9.0 and 5 nm, respectively (nano-crystalline support phases).

Figure 9 illustrates the effect of catalyst (0.2wt%Pt/3.5wt%V₂O₅-MgO-CeO₂) activation conditions on its performance in terms of NO conversion (X_N0, %), N₂-selectivity (S_N2, %), and H₂ conversion (X_H2, %). Reaction Feed composition: 175 ppm NO/2.5%O₂/0.8%H₂/10%CO₂/20%H₂O/He; GHSV=40,000 h^"1; Reaction T: 125-18O⁰C; Time on stream: 25 h. Cited bibliography

1. G. Ertl, H. Knόzinger, J. Weitkamp, in Handbook of Heterogeneous Catalysis, VCH, Weinheim, Germany, 1997, p. 1633; F. Nakajima, I. Hamada, Catal. Today 29 (1996) 109; G. Busca, L. Lietti, G. Ramis, F. Berti, Appl. Catal. B: Environ. 18 (1998) 1 ; V.I. Parvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233.

2. C. J. Pereria, K.W. Phumlee, Catal. Today 13 (1992) 23.

3. A. Fritz, V. Pitchon, Appl. Catal. B: Environ. 13 (1997) 1.

4. L. Singoredjo, R. Korver, F. Kapteijn, J. Moulijn, Appl. Catal. B: Environ. 1 (1992) 297.

5. H. Gutberlet, B. Schallert, Catal. Today 16 (1993) 207.

6. B. Rausenberger, W. Swiech, A.K. Schmid, CS. Rastomjee, W. Emgel, A.M. Bradshaw, J. Chem. Soc, Faraday Trans. 94(7) (1998) 963.

7. R. Dumpelmannm, N. W. Cant, D. L. Trimm in A. Frennet and J. -M. Bastin (Eds.) 3^rd ICC and Automotive Pollution Control, Brussels, 2 (1994) 13.

8. K. Tomishige, K. Asakura, U. Iwasawa, J. Catal. 157 (1995) 472.

9. W.C. Hecker, AT. Bell, J. Catal. 92 (1985) 247.

10. A. Hornung, M. Muhler, G. Ertl, Catal. Lett. 53 (1998) 77.

1 1. T.P. Kobylinski, B.W. Taylor, J. Catal. 33 (1974) 376. 12. S.J. Huang, A.B. Walters, M.A. Vannice, J. Catal. Ϊ73 (1998) 229.

13. R. Burch, S. Squire, Catal. Lett. 27 (1994) 177.

14. T.M.Salama, R. Ohnishi, T. Shido, M. lchikawa, J. Catal. 162 (1996) 169.

15. K. Tanaka, H. Yokota, M. Doi, M. Sugiura, Chem. Lett. (1997) 273.

16. A. Lindsteld, D. Strόmberg, M.A. MiIh₁ Appl. Catal. 116 (1994) 109. 17. D. Ferri, L. Forni, MAP. Dekkers, B. E. Nieuwenhuys, Appl. Catal. B: Environ. 16 (1998) 339.

18. J. R. Rostrup-Nielsen, Catal. Today 18 (1993) 305.

19. I. Alstrup, J. Catal. 109 (1998) 241.

20. S.T.Ceyer, Q.Y. Yang, M. B. Lee, J. D. Beckerle, A.D. Johnson, Stud. Surf. Sci. Catal. 36 (1987) 51.

21. I. Alstrup, MT. Travers, J. Catal. 135 (1992) 147.

22. T.B. Beebe, Jr., D.W. Goddman, B. D. Kay, T.J. Yates, Jr., J. Chem. Phys. 87 (1987) 2305.

23. I. Alstrup, I. Chorkendorff, S. Ullmann, Surf. Sci. 234 (1990) 79. 24. H.J. Topfer, Gas Wasserfach 117 (1976) 412. 25. S. Tenner, Hydrocarbon Processing 66(7) (1987) 42.

26. AT. Ashcroft, A.K. Cheetham, M.L.H. Green, P.D.F. Vernon, Nature 352 (1991) 225.

27. JT. Richardson, S.A. Paripatyadar, Appl. Catal. 61 (1990) 293. 28. I. M. Bodrov, L.O. Apel'baum, Kinet. Katal. 8 (1967) 379.

29. M.A. Peήa, J. P. Gomez, J. L. G. Fierro, Appl. Catal. A: Chem. 144 (1996) 7.

30. B. Frank, G. Emig, A. Renken, Appl. Catal. B: Environ. 19 (1998) 45.

31. R. Burch, M. D. Coleman, Appl. Catal. B. Environ. 23 (1999) 115. 32. A. Ueda, T. Nakao, M. Azuma, T. Kobayashi, Catal. Today 45 (1998) 135.

33. K. Yokota, M. Fukui, T. Tanaka, Appl. Surf. Sci 121/122 (1997) 273.

34. M. Machida, S. Ikeda, D. Kurogi, T. Kijima, Appl. Catal. B: Environ. 35 (2001) 107. 35. R. Burch, P.J. Millington, A.P. Walker Appl. Catal B. Environ. 4 (1994) 160.

36. M. Fukui, K. Yokata, Shokubai, Catal. Catal. 36 (1994) 160.

37. J. Shibata, M. Hashimoto, K. Shimizu, H. Yoshida, T. Hattori, A. Satsuma, J. Phys. Chem. B 108 (2004) 18327. 38. L Fu, KT. Chuang, Energy Fuels 3 (1989) 740.

39. M. Machida, T. Watanabe, Appl. Catal. B: Environ. 52 (2004) 281.

40. T. Nanba, C. Kohno, S. Masukawa, J. Uchisawa, N. Nakayama, A. Obuchi, Appl. Catal. B: Environ. 46 (2003) 353.

41. M. Machida, Catal. Surveys Japan 5 (2002) 91. 42. T. Nanba, K. Sugawara, S. Masukawa, J. Uchisawa, A. Obuchi, Top. Catal. 42/43 (2007) 129.

43. S. Hamada, S. Hibarino, K. Ikeue, M. Machida, Appl. Catal. B: Environ. 74 (2007) 197.

44. S. Hamada, K. Ikeue, M. Machida, Appl. Catal. B: Environ. 71 (2007) 1. 45. Y. Hasegawa, M. Haneda, Y. Kintaichi, H. Hamada, Appl. Catal. B: Environ. 60 (2005) 41.

46. M. Engelmann-Pirez, P. Granger, G. Leclercq, Catal. Today 107/108 (2005) 315.

47. M. Machida, T. Watanabe, Appl. Catal. B: Environ. 52 (2004) 281.

48. J. Shibata, M. Hashimoto, K. Shimizu, H. Yoshida, T. Hattori, A. Satsuma, J. Phys. Chem. B 108 (2004) 18327. 49. CN. Costa, V.N. Stathopoulos, V.C. Belessi, A.M. Efstathiou, J. Catal. 197 (2001) 350.

50. CN. Costa, P. G. Savva, C. Andronikou, G. Lambrou, K. Polychronopoulou, V.N. Stathopoulos, V.C. Belessi, P.J. Pomonis, A.M. Efstathiou, J. Catal. 209 (2002) 456. 51. CN. Costa, A.M. Efstathiou, Environ. Chem. Lett. 2 (2004) 55.

52. CN. Costa, A.M. Efstathiou, J. Phys. Chem. B 108 (2004) 2620.

53. CN. Costa, A.M. Efstathiou, Appl. Catal. B: Environ. 72 (2007) 240.

54. CN. Costa, P. G. Savva, J.L.G. Fierro, A.M. Efstathiou, Appl. Catal. B: Environ. 75 (2007) 147. 55. A.M. Efstathiou, CN. Costa, J.L.G. Fierro, "Novel Catalyst for the Reduction of NO to N₂ with Hydrogen under NO_x Oxidation Conditions, Spanish Patent ES 2 192 985 B1 (2005); US-Patent 7,105,137 (2006).

56. A.M. Efstathiou, CN. Costa, J.L.G. Fierro, "Catalyst Containing Platinum on a Support Consisting of Magnesium Oxide and Cerium Oxide for the Reduction of NO to N₂ with Hydrogen Under NO_x Oxidation Conditions", EP-Patent No. 1 475 149 (2008).

57. K. Yokota, M. Fukui, T. Tanaka, Appl. Surf. Sci. 121/122 (1997) 273.

58. R. Burch, M. D. Coleman, Appl. Catal. B: Environ. 23 (1999) 115.

59. M. Machida, D. Kurogi, T. Kijima, Chem. Mater. 12 (2000) 3165.

60. M. Machida, D. Kurogi, T. Kijima, J. Phys. Chem. B 107 (2003) 196. 61. I. Twagirashema, M. Engelmann-Pirez, M. Frere, L. Burylo, L. Gengembre, C. Dujardin, P. Granger, Catal. Today 119 (2007) 100.

62. I. Twagirashema, M. Frere, L. Gengembre, C Dujardin, P. Granger, Top. Catal. 42/43 (2007) 171.

63. F. Dhainaut, S. Pietrzyk, P. Granger, Top. Catal. 42/43 (2007) 135. 64. N. Macleod, R.M. Lambert, Catal. Commun. 3 (2002) 61.

65. Y.-W. Lee, E. Gulari, Catal. Commun. 5 (2004) 499.

66. N. Macleod, R. Cropley, J. M. Keel, R.M. Lambert, J. Catal. 221 (2004) 20.

67. M. Konsolakis, M. Vrontaki, G. Avgouropoulos, T. loannides, I. V. Yentekakis, Appl. Catal. B: Environ. 68 (2006) 59. 68. J.A.Z. Pieterse, S. Booneveld, Appl. Catal. B: Environ. 73 (2007) 327.

69. R.J. Wijngaarden, A. Kronberg, K.R. Westerterp (Eds.), Industrial Catalysis - Optimizing Catalysts and Processes, p. 29, Wiley-VCH, 1998.

70. J. M. Thomas, W.J. Thomas (Eds.), Principles and Practice of Heterogeneous Catalysis, VCH, 1997. 71. CN. Costa, A.M. Efstathiou, J. Phys. Chem. C 111 (2007) 3010. 72. P.G. Sawa, A.M. Efstathiou, J. Catal. 257 (2008) 324.

73. A.L. Patterson, Phys. Rev. 56 (1939) 978.

Claims

1. Catalyst comprising platinum as noble metal, which is dispersed on a mixed metal oxide support of magnesium and cerium, charaterized in that the mixed metal oxide support of magnesium and cerium is chemically promoted by one of the following elements or compound of the following elements: vanadium (V), sodium

(Na), potassium (K), molybdenum (Mo) or tungsten (W).

2. Catalyst according to claim 1 , characterised in that the mean primary crystal size of the mixed metal oxide support phases is between 5 nm and 10 nm.

3. Catalyst according to claim 1 or 2, characterised in that the support is a mixture of promoted-magnesium oxide and promoted-cerium dioxide, whereby vanadium (V) oxide (V₂O₅) in an amount between 0.1 wt% and 12 wt%, preferably between 2 wt% and 8 wt%, most preferred between 3.0 wt% and 5 wt%, is used as chemical promoter.

4. Catalyst according to any of the claims 1 to 3, characterised in that platinum is dispersed in an amount between 0.01 and 2.0 wt% on the promoted mixed metal oxide support.

5. Catalyst according to any of the claims 1 to 4, characterised in that palladium in an amount between 0.01 and 2.0 wt% as a second noble metal is dispersed on the promoted mixed metal oxide support.

6. Catalyst according to claim 5, characterised in that 0.1 wt% platinum (Pt) and 0.05 wt% palladium (Pd) are dispersed on the promoted mixed metal oxide support.

7. Catalyst according to any of the claims 1 to 6, characterised in that the promoted mixed metal oxide support consists of magnesium oxide and cerium dioxide solid phases in the ratio between 50:50 and 70:30 (w/w).

8. Process for obtaining a catalyst comprising platinum dispersed on a mixture of promoted-magnesium oxide and promoted-cerium dioxide comprising the steps: - impregnating the mixed metal oxide of magnesium oxide and cerium dioxide solid phases with an aqueous solution containing the desired quantity of the promoter precursor, whereby the promoter is one of the following elements or a compound of the following elements: vanadium (V), sodium (Na), potassium (K), molybdenum (Mo) or tungsten (W),

- evaporation of water, drying, grinding and heating at 500⁰C in air flow for at least 2 hours,

- impregnating the resulting solid with an aqueous solution containing the desired quantity of platinum precursor, - evaporation of water, drying at 12O⁰C for at least 12 h and

- activation of the thus prepared catalyst first in a flow of air or O₂/He gas mixture in the 500-650⁰C range for at least 1 h, and then in a flow of H₂/He gas mixture in the 200-350⁰C range for at least 1 h.

Process for obtaining a catalyst comprising platinum (Pt) and palladium (Pd) dispersed on a mixture of promoted-magnesium oxide and promoted-cerium dioxide comprising the steps:

- impregnating the mixed metal oxide of magnesium oxide and cerium dioxide solids with an aqueous solution containing the desired quantity of the promoter precursor, whereby the promoter is one of the following elements or a compound of the following elements: vanadium (V), sodium (Na), potassium (K), molybdenum (Mo) or tungsten (W),

- evaporation of water, drying, grinding and heating at 500⁰C in air flow for at least 2 hours, - impregnating the resulting solid with an aqueous solution containing the desired quantity of platinum precursor,

- evaporation of water, drying, grinding and heating between 500 and 600⁰C in air flow for at least 2 hours,

- impregnating the resulting solid with an aqueous solution containing the desired quantity of palladium precursor,

- evaporation of water, drying, grinding and heating at 500⁰C in air flow for at least 2 hours for complete conversion of the platinum and palladium precursors into their respective oxide, and - activation of the thus prepared catalyst first in a flow of air or O₂/He gas mixture in the 500-650°C range for at least 1 h, and then in a flow of H₂/He gas mixture in the 200-350⁰C range for at least 1 h.

10. Use of a catalyst according to any of the claims 1 to 7 for the reduction of a chemical compound selected from the group consiting of nitrogen oxide, nitrogen dioxide or a mixture of both to nitrogen gas using hydrogen as reducing agent in the presence or absence of oxygen.