Metal-supported catalysts for methane combustion under wet conditions, process for their preparation and uses thereof

Abstract

The present invention relates to a process for obtaining a metal-supported catalyst, as well as to the metal-supported catalyst as such, for methane combustion under wet conditions. The invention also relates to the use of said metal-supported catalyst in methane combustion, preferably in methane combustion under wet conditions. Furthermore, the invention is related to a process of methane combustion under wet conditions, which involves the use of a metal-supported catalyst according to the invention.

Description

[0001] METAL-SUPPORTED CATALYSTS FOR METHANE COMBUSTION UNDER WET CONDITIONS, PROCESS FOR THEIR PREPARATION AND USES THEREOF

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a process for obtaining a metal-supported catalyst, as well as to the metal-supported catalyst as such, which is particularly suitable for methane combustion under wet conditions. The invention also relates to the use of said metal-supported catalyst in methane combustion, preferably in methane combustion under wet conditions. Furthermore, the invention is related to a process of methane combustion under wet conditions, which involves the use of the metal-supported catalyst according to the invention.

[0004] BACKGROUND OF THE INVENTION

[0005] A significant source of methane emission originates from large stationary natural gas engines used for power production and maritime applications. Furthermore, the more traditional fossil fuels are being replaced with sustainable sources of energy such natural gas, the more amounts of methane will be obtained, since natural gas contains more than 90% of methane. However, methane is a potent greenhouse gas with a global warming potential around 28 times higher than CO2.

[0006] It thus becomes absolutely necessary to find an efficient way of managing these methane amounts, and it is in that context that methane oxidation becomes of particular relevance. The main drawback of methane oxidation catalysts (MOCs) such as those based on platinum group metals, though, is their lack of stability in a stream of methane and water.

[0007] Solutions based on Pd / ALOs methane oxidation catalysts have received significant attention through time but, despite vast research, there still exists a dire need for methane oxidation catalysts which remain stable in the presence of water. Deactivation is typically ascribed to Pd sintering, water adsorption, hydroxyl formation and sulfate formation when sulfur is in the gas stream.

[0008] Furthermore, for a successful catalytic application in methane combustion, the catalytic systems must possess high catalytic activities at low-intermediate reaction temperatures ( / .e., temperatures about and below 550°C). However, known methane oxidation catalysts typically undergo severe deactivation in the presence of water, e.g., when the reaction stream contains water. In this regard, it is generally known that methane oxidation catalyst systems are negatively affected by the presence of water, which dramatically inhibits or modifies the catalyst performance under such low- intermediate reaction temperatures, as described, for example, by Hoque et al. (Korean J. Chem. Eng. 2014, 31, 1316). On the other hand, it is known in the art that ceria and cerium-based mixed oxides typically provide suitable supports onto which platinum group metals can be deposited to form metal-supported catalytic systems. Different ceria particles having either rod- or cube-type morphologies have been recently evaluated in the literature as catalyst supports for methane combustion, though all these studies were carried out under dry conditions: Guo et al. (RSC Adv. 2018, 8, 38641-38647), Dong et al. (Chin. J. Catal. 2021, 42, 2234-2241), Zhou et al. (Chinese patent application CN113274999) and Chen et al. (ACS Catal. 2021, 11, 5666-5677). However, it is not possible to extrapolate results achieved under dry conditions to those obtained under wet conditions due to the dramatic effect water exerts over catalytic performance in methane combustion as described in previously mentioned Hoque et al.

[0009] Thus, there exists the need for a technical solution which provides new metal-supported catalytic systems which may advantageously provide high catalytic activities in methane combustion reactions taking place under wet conditions, while also having significant resistance to deactivation under those working conditions.

[0010] SUMMARY OF THE INVENTION

[0011] In a first aspect of the invention, a process for obtaining a metal-supported catalyst is provided, wherein said process comprises the following steps: a) adding at least one supported transition metal (M) selected from any of the groups 5 to 11 of the periodic table of the elements to a support material, wherein the support material comprises at least one oxide selected from the group consisting of ceria, a ceria-based mixed oxide, and any combination thereof; b) drying, preferably under homogenization conditions; and c) thermally treating the solid obtained in step b) at a temperature from about 250°C to about 850°C; wherein the at least one oxide is substantially present in crystalline form, has an average particle size from 5 to 20 nm as determined by HRTEM, and exhibits a specific surface area from 50 to 140 m2 / g as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours; and wherein the molar ratio of the at least one supported transition metal (M) selected from any of the groups 5 to 11 of the periodic table of the elements (M) to surface oxygen in the support material (i.e., [at least one supported transition metal (M)]:[surface oxygen in the support material] molar ratio; or (M / OSUrf) molar ratio) is from 0.4 to 1.4 as determined by Hz temperatureprogrammed reduction (Hj-TPR). In the context of the present invention, the Hj-TPR analysis was carried out on the metal-free support, wherein "metal-free" shall be herein understood, throughout the patent application, as being devoid of the at least one supported transition metal (M).

[0012] Throughout this patent application, the expressions "molar ratio of the at least one supported transition metal (M) to surface oxygen in the support material" and "M / OSUrf" are interchangeably used. Similarly, the expressions "molar ratio of palladium to surface oxygen in the support material" and "Pd / Osurf" are interchangeably used.

[0013] Furthermore, when calculating the molar ratio of the at least one supported transition metal (M) to surface oxygen in the support material, the total number of moles corresponding to the supported transition metal(s) shall be considered, that is, if more than one supported transition metal is added in step a) in the process according to the first aspect of the invention, the total number of moles of all supported transition metals shall be considered for calculating the M / OSUrf value.

[0014] According to a second aspect of the invention, a metal-supported catalyst for methane combustion under wet conditions is provided, which comprises: a support material comprising at least one oxide selected from the group consisting of ceria, a ceria-based mixed oxide, and any combination thereof; and at least one supported transition metal (M) selected from any of the groups 5 to 11 of the periodic table of the elements; wherein the at least one oxide is substantially present in crystalline form, has an average particle size from 5 to 20 nm as determined by HRTEM, and exhibits a specific surface area from 50 to 140 m2 / g as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours.

[0015] The expression "methane combustion under wet conditions", as used throughout the specification, shall be understood as meaning that methane combustion is carried out using a reaction feed having a volume percentage of water which is in the range from 0.5% to 30%, based on the total volume of the reaction feed. In a preferred embodiment, the wet conditions employed during methane combustion may be understood as meaning that methane combustion is carried out using a reaction feed having a volume percentage of water which is in the range from 5% to 15%, based on the total volume of the reaction feed.

[0016] In a third aspect of the invention, a process of methane combustion under wet conditions is provided, said process comprising the following steps: a) introducing a metal-supported catalyst, as defined according to the second aspect of the invention, in a reactor; b) feeding the reactor with a gas reaction feed which comprises methane, from 5% to 15 % (v / v) water and at least one inert gas, wherein the sum of all components in the gas reaction feed is 100 % (v / v); and c) allowing the metal-supported catalyst to remain in contact with the gas reaction feed in the reactor for a time sufficient and at a temperature sufficient for methane combustion to take place.

[0017] In a fourth aspect of the invention, the use of the metal supported catalyst according to the second aspect of the invention in methane combustion is provided, preferably the use in methane combustion under wet conditions.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 - HRTEM images of the nanosized ceria supports employed for the metal-supported catalysts prepared according to the present invention.

[0020] FIG. 2 - Hz Temperature-programmed reduction profiles of the nanosized ceria supports employed for the metal-supported catalysts prepared according to the present invention.

[0021] FIG. 3 - Light-off type experiments to evaluate the catalytic performance for the methane combustion reaction under wet conditions employing different nanoceria supports containing a 3 wt.% Pd content, which have been prepared according to the Examples 1-4 of the present invention.

[0022] FIG. 4 - Long-term steady-state methane combustion experiments to evaluate the catalytic performance for the methane combustion reaction under wet conditions and dynamic high- temperature variations (400-600°C) employing different nanoceria supports containing a 3 wt.% Pd content, which have been prepared according to the Examples 1-4 of the present invention.

[0023] FIG. 5 - Light-off type experiments to evaluate the catalytic performance for the methane combustion reaction under wet conditions employing the nanoceria Ce2 as support (see Table 1) containing different Pd contents, which have been prepared according to the Example 1 (3 wt.% Pd), Example 5 (5 wt.% Pd) and Example 6 (1 wt.% Pd) of the present invention.

[0024] FIG. 6 - Long-term steady-state methane combustion experiments to evaluate the catalytic performance for the methane combustion reaction under wet conditions and dynamic high- temperature variations (400-600°C) employing nanoceria Ce2 as support (see Table 1) containing different Pd contents, which have been prepared according to Example 1 (3 wt.% Pd), Example 5 (5 wt.% Pd) and Example 6 (1 wt.% Pd) of the present invention.

[0025] FIG. 7 - Long-term steady-state methane combustion experiments to evaluate the catalytic performance for the methane combustion reaction under wet conditions and dynamic high- temperature variations (400-600°C) employing nanoceria Cel and Ce2 as supports (see Table 1) containing different Pd contents, which have been prepared according to Example 1 (Ce2_3 wt.% Pd), Example 3 (Cel_3 wt.% Pd), Example 5 (Ce2_5 wt.% Pd) and Example 7 (Cel_5 wt.% Pd) of the present invention.

[0026] FIG. 8 - Long-term steady-state methane combustion experiment to evaluate the catalytic performance for the methane combustion reaction under wet conditions employing the bimetallic catalyst according to Example 8 of the present invention.

[0027] FIG. 9 - Long-term steady-state methane combustion experiment to evaluate the catalytic performance for the methane combustion reaction under wet conditions employing the catalyst according to Example 9 of the present invention.

[0028] DETAILED DESCRIPTION OF THE INVENTION

[0029] In a first aspect of the invention, a process for obtaining a metal-supported catalyst is provided, wherein said process comprises the following steps: a) adding at least one supported transition metal (M) selected from any of the groups 5 to 11 of the periodic table of the elements to a support material, wherein the support material comprises at least one oxide selected from the group consisting of ceria, a ceria-based mixed oxide, and any combination thereof; b) drying, preferably under homogenization conditions; and c) thermally treating the solid obtained in step b) at a temperature from about 250°C to about 850°C; wherein the at least one oxide is substantially present in crystalline form, has an average particle size from 5 to 20 nm as determined by HRTEM, and exhibits a specific surface area from 50 to 140 m2 / g as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours; and wherein the molar ratio of the at least one supported transition metal (M) to surface oxygen in the support material (M / OSUrf) is from 0.4 to 1.4 as determined by Hj-TPR, wherein the Hj- TPR analysis was carried out on the metal-free support. Hj-TPR analysis on the metal-free ceria support allows for the estimation of the surface oxygen species found in the support material (OSUrf) as shown, for example, in FIG. 2, by quantifying the Hz consumption peaks between 250 and 550°C. Theoretical values of the supported transition metal(s) which is deposited onto the support material may be established considering that the experimentally deposited supported transition metal(s) will remain in the final catalyst. In any case, the amount of experimentally deposited supported transition metal(s) may also be optionally confirmed with inductively coupled plasma (ICP) analyses.

[0030] The inventors have surprisingly found that by controlling the properties of the support material such as the available surface oxygen, especially the above-indicated M / OSUrf ratio, the specific surface area and the average particle size, it was not only possible to achieve an efficient deposition of the transition group metal(s), but also the catalytic activity and stability of the resulting metal-supported catalyst were advantageously maximized under wet reaction conditions. Specifically, the aboveindicated M / Osurf ratio was found to be a critical parameter during the catalyst synthetic process for enhancing such stability and activity of the metal-supported catalysts for the methane combustion reaction in wet conditions. Furthermore, those nanosized metal-supported catalysts of the invention, which may be obtained according to the first aspect of the invention, provided improved stability during longer time-on-stream (TOS) periods and also after being subjected to dynamic high- temperature variations compared to other known catalysts.

[0031] The at least one supported transition metal (M), which is added in step a) of the process according to the first aspect of the invention, may be provided in the form of a metal salt or metal complex, or it may be alternatively provided as a metal-based alloy. Said addition of the at least one at least one supported transition metal (M) to the support material may be preferably performed by wet impregnation, incipient volume impregnation or deposition-precipitation, which are methods widely known in the art. In a preferred embodiment, wherein a plurality of supported transition metals is used to produce the metal-supported catalysts of the invention according to the first aspect, they may all be added in step a) of the process, so that subsequent steps b) and c) need to be carried out only once onto the previously formed mixture of that plurality of supported transition metals to produce the metal-supported catalyst. These embodiments were found to be particularly advantageous since they may avoid longer multi-step processes known in the art wherein each supported transition metal is independently subject to all steps a) to c), therefore requiring several drying and calcination steps to produce the final catalyst.

[0032] In preferred embodiments, the at least one supported transition metal (M) is added in step a) of the process according to the first aspect of the invention, so that the total amount of the transition metal supported onto the support material is equal to or more than 2 wt.%, based on the total weight of the resulting metal-supported catalyst. In other embodiments, the at least one supported transition metal (M) is added in step a) of the process according to the first aspect of the invention, so that the total amount of the transition metal supported onto the support material is more than 2 wt.%, from 2 wt.% to 5 wt.%, from 2.5 wt.% to 5 wt.%, from 3 wt.% to 5 wt.%, or from more than 2 wt.% to 3 wt.%, based on the total weight of the resulting metal-supported catalyst

[0033] In some embodiments, the metal-supported catalyst may comprise at least one supported transition metal (M) as a metal-based alloy and provide a plurality of supported transition metals (M). In those embodiments, the metal-supported catalyst according may preferably comprise equal to or more than 2 wt.% of at least one supported transition metal (M) as a metal-based alloy, based on the total weight of the metal-supported catalyst or, alternatively, more than 2 wt.%, from more than 2 wt.% to 5 wt.%, from 2.5 wt.% to 5 wt.%, from more than 2 wt.% to 4 wt.% or from more than 2 wt.% to 3 wt.% of the at least one supported transition metal (M) as a metal-based alloy, based on the total weight of the metal-supported catalyst.

[0034] The at least one supported transition metal (M) may be particularly selected from the group consisting of Pd, Pt, Rh, Ir, Cu, Ru, Co, Ag, Nb and any mixture thereof; preferably, the at least one supported transition metal (M) may be selected from the group consisting of Pd, Pt, Rh, Ir, Ru and any mixture thereof; more preferably, the at least one supported transition metal (M) may be selected from the group consisting of Pd, Pt, Rh, Ru and any mixture thereof; even more preferably, the at least one supported transition metal (M) may be selected from the group consisting of Pd, Pt, Ru and any mixture; still even more preferably, the at least one supported transition metal (M) may be Pd or Pt. In another preferred embodiment, the at least one supported transition metal (M) is or consists of Pd. Examples of suitable transition metal salts or complexes include, but are not limited to, palladium propionate, palladium butyrate, tris(dibenzylideneacetone)dipalladium (0), tris(tribenzylideneacetylacetone)tripalladium (0), palladium benzoate, palladium acetylacetonate, palladium nitrate, palladium sulfate, palladium chloride, palladium acetate, platinum acetylacetonate, platinum acetate, dinitrodiammine platinum nitrate, sodium chloroplatinate, sodium hexachloroplatinate, tetraamineplatinum nitrate, rhodium nitrate, rhodium chloride, iridium tribromide, iridium chloride, idirium potassium chloride, ruthenium nitrate, ruthenium chloride, ammonium chlororuthenate or potassium ruthenate.

[0035] Throughout the specification, the expression "metal-based alloy" in reference to the at least one supported transition metal (M) mainly refers to an alloy comprising at least one supported transition metal element selected from any of the groups 5 to 11 of the periodic table of the elements as main constituent, that is, the alloy comprises at least 80 wt.% ( / .e., equal to or higher than 80 wt.%) of the at least one supported transition metal element, based on the total weight of the alloy. In a preferred embodiment, a suitable metal-based alloy comprises at least 90 wt.% (i.e., equal to or higher than 90 wt.%) of the at least one supported transition metal element, based on the total weight of the alloy. In some embodiments, the metal-based alloy may comprise two supported transition metal (M) elements, one of them being the main constituent. Examples of suitable metalbased alloys include, but are not limited to, Pd-Pt alloys, Pt-Au alloys, Pt-Ru alloys (e.g., Pt with 5 wt.% Ru) and Pt-Rh alloys (e.g., Pt with 10 wt.% Rh).

[0036] The support material employed in the process according to the first aspect of the invention comprises at least one oxide selected from the group consisting of ceria, a ceria-based mixed oxide, and any combination thereof, and the at least one oxide is substantially present in crystalline form. The expression "substantially present in crystalline form", as used throughout the specification, refers to that at least 90 wt.% of the at least one oxide, based on the total weight of the at least one oxide, is in crystalline form (i.e., up to 10 wt.% of the at least one oxide, based on its total weight, may be in amorphous form). According to some embodiments, at least 95 wt.% of the at least one oxide, based on the total weight of the at least one oxide, is provided in crystalline form (i.e., up to 5 wt.% of the at least one oxide, based on its total weight, may be in amorphous form). In some other embodiments, the at least one oxide may be in crystalline form in its entirety. Preferably, the support material comprises at least 95 wt.% of the at least one oxide, based on the total weight of the at least one oxide, in crystalline form, said crystalline form being fluorite crystalline structure. In another embodiment, the support material may comprise at least one oxide, based on the total weight of the at least one oxide, wherein the at least one oxide is in crystalline form in its entirety, and has fluorite crystalline structure.

[0037] It is worth noting that the support material comprising at least one ceria-based mixed oxide, which is employed throughout the different aspects of the present invention, is defined by specific morphology, size and / or shape features, and may be produced by synthetic methods known in the art, such as hydrothermal synthesis, co-precipitation or by sol-gel methods. A suitable example thereof is cerium oxide referred to as "Ce3" throughout the present patent application, which has been prepared by a method known in the art and has the characteristics detailed in Table 1 below. Alternatively, support materials comprising at least one ceria-based mixed oxide may be commercially available: e.g., cerium oxide with ref. AC106-011, Lot # 06-0117 (purchased from NanoScale Corporation, Cel); cerium oxide with ref. ITQ. 3A, Lot # 0633984 (purchased from Solvay, Ce2); and cerium oxide with ref. 544841-25G, Lot #MKCK1143 (purchased from Sigma-Aldrich, Ce4); wherein those commercial ceria-based mixed oxides have the following characteristics summarized in Table 1:

[0038] Table 1

[0039] (a) Measured from N2adsorption; (b) Measured from H2-TPR

[0040] In an embodiment, the support material comprises at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises less than 40 mol % (i.e., higher than 0 mol % but less than 40 mol %) of at least one metal element which is capable of forming a ceria-based mixed oxide. The support material may comprise at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide, said at least one metal element being preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %. The molar percentage of the at least one metal element is calculated relative to the total number of moles of the at least one ceria-based mixed oxide, which means that, if the support material over which the at least one platinum group metal is added in step a) comprises a plurality of ceria-based mixed oxides, the molar percentage of the at least one metal element shall be calculated relative to the total number of moles of the plurality of ceria-based mixed oxides.

[0041] The at least one element capable of forming a ceria-based mixed oxide may be preferably selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y and any combination thereof. In a preferred embodiment, the at least one element capable of forming a ceria-based mixed oxide may be selected from the group consisting of Zr, La, Y and any combination thereof. In another preferred embodiment, the at least one element capable of forming a ceria-based mixed oxide may be selected from the group consisting of Zr, La and any combination thereof. More preferably, the at least one element capable of forming a ceria-based mixed oxide is Zr.

[0042] The support material may preferably comprise at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises less than 40 mol % (i.e., higher than 0 mol % but less than 40 mol %) of at least one metal element capable of forming a ceria-based mixed oxide, wherein the at least one metal element is selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y and any combination thereof. In another preferred embodiment, the support material comprises at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide which is preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %, and wherein the at least one metal element is selected from the group consisting of Zr, La, Y and any combination thereof. In other preferred embodiments, the support material comprises at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide which is preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %, and wherein the at least one metal element is selected from the group consisting of Zr, La and any combination thereof.

[0043] In an embodiment, the support material may comprise at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises less than 40 mol % (i.e., higher than 0 mol % but less than 40 mol %) of at least one metal element capable of forming a ceria-based mixed oxide, and wherein the at least one metal element is or consists of Zr. Preferably, the support material of the metal-supported catalyst of the invention may comprise, in an embodiment, at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide which is preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %, and wherein the at least one metal element is or consists of Zr.

[0044] The support material may comprise a plurality of ceria-based mixed oxides, wherein each ceria-based mixed oxide may be independently defined as already indicated in any of the previously described embodiments.

[0045] In the process according to the first aspect of the invention, the support material may further have a molar ratio of surface oxygen to cerium from 0.06 to 0.20, as determined by Hj-TPR. Said molar ratio of surface oxygen to cerium may preferably be from 0.07 to 0.15 or, still more preferably, said molar ratio may be from 0.08 to 0.14, as determined by Hj-TPR.

[0046] In the process according to the first aspect of the invention, the molar ratio of the at least one supported transition metal (M) to surface oxygen in the support material (M / OSUrf) may be from 0.4 to 1.4, as determined by Hj-TPR, wherein the Hj-TPR analysis is carried out on the metal-free support. In an embodiment, the support material may further have a molar ratio of surface oxygen to cerium from 0.06 to 0.20, as determined by Hj-TPR, and the molar ratio of the at least one supported transition metal (M) to surface oxygen may be from 0.4 to 1.4, as also determined by Hj-TPR, wherein the Hj-TPR analysis is carried out on the metal-free support.

[0047] In a preferred embodiment, the molar ratio of the at least one supported transition metal (M) to surface oxygen in the support material (M / OSUrf) may be from 0.4 to 1.2, from 0.5 to 1.1 or from 0.5 to 1.0, as determined by Hj-TPR, wherein the Hj-TPR analysis is carried out on the metal-free support. In some embodiments, the molar ratio of the at least one supported transition metal (M) to surface oxygen in the support material (M / OSUrf) may be from 0.4 to 1.4, and / or the support material may further have a molar ratio of surface oxygen to cerium from 0.06 to 0.20, wherein the molar ratio of the at least one supported transition metal (M) to surface oxygen and / or surface oxygen to cerium are each one determined by Hj-TPR, wherein the Hj-TPR analysis is carried out on the metal- free support.

[0048] In the process according to the first aspect of the invention, the support material may preferably exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours.

[0049] In the process according to the first aspect of the invention, the support material may preferably have an average particle size from 5 to 20 nm, as determined by HRTEM, or, more preferably, the support material may have an average particle size from 5 to 18 nm, from 5 to 15 nm, from 6 to 15 nm, from 8 to 15 nm, from 8 to 12 nm or from 5 to 10 nm, as determined by HRTEM, Alternatively, particle size may also be determined using Field Scanning Electron Microscopy (FESEM).

[0050] In an embodiment, the support material employed in the process according to the first aspect of the invention may have an average particle size from 5 to 20 nm, as determined by HRTEM, and exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In another embodiment, the support material employed in the process according to the first aspect of the invention may have an average particle size from 5 to 18 nm, as determined by HRTEM, and exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In another embodiment, the support material employed in the process according to the first aspect of the invention may have an average particle size from 5 to 15 nm, as determined by HRTEM, and exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In still another embodiment, the support material employed in the process according to the first aspect of the invention may have an average particle size from 8 to 15 nm, as determined by HRTEM, and exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In other embodiments, the support material employed in the process according to the first aspect of the invention may have an average particle size from 8 to 12 nm, as determined by HRTEM, and exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In still other embodiments, the support material employed in the process according to the first aspect of the invention may have an average particle size from 5 to 10 nm, as determined by HRTEM, and exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours.

[0051] In step b) of the process according to the first aspect of the invention, drying is preferably carried out under homogenization conditions. In the context of the process according to the first aspect of the invention "homogenization conditions" shall be understood as conditions which are sufficient to ensure uniform dispersion of the at least one platinum group metal on the surface of the support material and / or in the lattice of the support material, more particularly, in the lattice of the ceria, cerium mixed oxide or any combination thereof which is comprised by the support material.

[0052] In a preferred embodiment, drying step b) is carried out under homogenization conditions which comprise or consist of stirring, such as manual stirring (e.g., using a spatula) or stirring using a magnetic stirrer. In another embodiment, after step a), but before step b), homogenization may be preferably carried out by stirring, e.g., by manual stirring (e.g., using a spatula) or stirring using a magnetic stirrer. In still another embodiment, after step a) but before step b), and also during step b), homogenization may also be carried out, preferably by stirring, e.g., by manual stirring (e.g., using a spatula) or stirring using a magnetic stirrer. Drying step b) of the process is preferably carried out at a temperature from 50 to 110°C, more preferably a temperature from 80 to 100°C.

[0053] In step c) of the process according to the first aspect of the invention, according to a preferred embodiment, the solid obtained in step b) may be thermally treated (i.e., calcinated) at a temperature from about 300°C to about 850°C and, more preferably, at a temperature from about 350°C to about 800°C. In another preferred embodiment, in step c) of the process according to the first aspect of the invention, the solid obtained in step b) may be thermally treated at a temperature from about 400°C to about 700°C.

[0054] The term "about" when used throughout the specification preceding a number and referring to it, is to be understood as disclosing the particular value and designating any value lying within the range defined by the number ±5%, more preferably a range defined by the number ±2%. For example, the expression "about 1" shall be construed as "within the range of 0.95 to 1.05", preferably as "within the range of 0.98 to 1.02".

[0055] The process according to the first aspect of the invention may optionally further comprise, after step b) but before step c), a step wherein the dried solid obtained in step b) is: applied in or onto an inert substrate (e.g., a monolithic substrate or monolithic structure), preferably a honeycomb inert substrate; or shaped into pellets.

[0056] The choice on whether the dried solid obtained in step b) is either applied in or onto an inert substrate or, alternatively, whether it is shaped into pellets, typically depends on the requirements of the application for which the metal-supported catalyst resulting from the synthetic process according to the invention is intended. Examples of suitable inert substrates include, but are not limited to, silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania. In a particular embodiment, the inert substrate comprises or consists of silica, alumina, silica- alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania or any combination thereof.

[0057] In a preferred embodiment, the process according to the first aspect of the invention may further comprise, after step b) but before step c), a step wherein the dried solid obtained in step b) is applied in or onto a honeycomb inert substrate. Said honeycomb inert substrate may be a honeycomb corrugated substrate, which may preferably be a metal foil substrate or a glass fibre substrate, such as a non-woven e-glass fibre substrate, or a ceramic extruded substrate, such as cordierite. The expression "honeycomb inert substrate", as used throughout the specification, shall be understood as a referring to an inert substrate having a honeycomb lattice structure, wherein said lattice structure may be hexagonal, square, triangular or wavy. Preferably, said honeycomb lattice structure is hexagonal, triangular or wavy.

[0058] In the process according to the first aspect of the invention, the thermally treated solid resulting from step c) may be optionally subjected to a reduction step. In particular, the thermally treated solid resulting from step c) may be subjected to a reduction step wherein it is contacted with a reducing agent, said reducing agent being preferably selected from the group consisting of a reducing gas (e.g., Hz or CO), a hypophosphite salt, a borohydride salt (e.g., sodium borohydride), hydrazine, formic acid, formaldehyde, citric acid and any combination thereof. Reducing gases may be preferably employed at temperatures normally fixed between 100 and 600 °C, optionally also in the presence of additional well-known reducing components, in order to facilitate the formation of metal particles. Examples of these reducing components include, but are not limited to, CO, hypophosphite salts, borohydride salts such as sodium borohydride, hydrazines, formic acid, formaldehyde, citric acid or any combination thereof.

[0059] In some embodiments, the process according to the first aspect of the invention may further comprise, after step b) but before step c), the step wherein the dried solid obtained in step b) is applied in / onto an inert substrate, in addition to the additional step wherein the thermally treated solid resulting from step c) may be subjected to a reduction step, in particular with a reducing agent which is preferably selected from the group consisting of a reducing gas (e.g., Hz or CO), a hypophosphite salt, a borohydride salt, hydrazine, formic acid, formaldehyde, citric acid and any combination thereof. In these particular embodiments, it will become apparent that the application of the dried solid obtained in step b) in / onto the inert substrate may be carried out before, after, or simultaneously with the optional reduction step.

[0060] The process according to the first aspect of the invention may optionally further comprise, after step b) but before step c), the step wherein the dried solid obtained in step b) is shaped into pellets, in addition to the additional step wherein the thermally treated solid resulting from step c) may be subjected to a reduction step, in particular with a reducing agent which is preferably selected from the group consisting of a reducing gas (e.g., H? or CO), a hypophosphite salt, a borohydride salt, hydrazine, formic acid, formaldehyde, citric acid and any combination thereof. In these particular embodiments, it will become apparent that the shaping of the dried solid obtained in step b) into pellets may be carried out before, after, or simultaneously with the optional reduction step.

[0061] According to a second aspect of the invention, it is provided a metal-supported catalyst for methane combustion under wet conditions, characterized by comprising: a support material comprising at least one oxide selected from the group consisting of ceria, a ceria-based mixed oxide, and any combination thereof; and at least one supported transition metal (M) selected from any of the groups 5 to 11 of the periodic table of the elements; wherein the at least one oxide is substantially present in crystalline form, has an average particle size from 5 to 20 nm as determined by HRTEM, and exhibits a specific surface area from 50 to 140 m2 / g as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours.

[0062] The support material of the metal-supported catalyst according to this second aspect of the invention may preferably comprise at least one oxide selected from the group consisting of ceria, a ceria-based mixed oxide, and any combination thereof, wherein at least 90 wt.% of the at least one oxide, based on the total weight of the at least one oxide, is in crystalline form ( / .e., up to 10 wt.% of the at least one oxide, based on its total weight, may be in amorphous form). According to another embodiment, at least 95 wt.% of the at least one oxide, based on the total weight of the at least one oxide, is provided in crystalline form ( / .e., up to 5 wt.% of the at least one oxide, based on its total weight, may be in amorphous form). In some other embodiments, the at least one oxide is in crystalline form in its entirety.

[0063] The support material of the metal-supported catalyst according to this second aspect of the invention may comprise at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises less than 40 mol % (i.e., higher than 0 mol % but less than 40 mol %) of at least one metal element which is capable of forming a ceria-based mixed oxide. The support material may comprise at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide, said at least one metal element being preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %. The molar percentage of the at least one metal element is calculated relative to the total number of moles of the at least one ceria-based mixed oxide, which means that, if the support material comprises a plurality of ceria-based mixed oxides, the molar percentage of the at least one metal element shall be calculated relative to the total number of moles of the plurality of ceria-based mixed oxides.

[0064] The at least one element capable of forming a ceria-based mixed oxide may be preferably selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y and any combination thereof. In a preferred embodiment, the at least one element capable of forming a ceria-based mixed oxide may be selected from the group consisting of Zr, La, Y and any combination thereof. In another preferred embodiment, the at least one element capable of forming a ceria-based mixed oxide may be selected from the group consisting of Zr, La and any combination thereof. More preferably, the at least one element capable of forming a ceria-based mixed oxide is Zr.

[0065] The support material may preferably comprise at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises less than 40 mol % (i.e., higher than 0 mol % but less than 40 mol %) of at least one metal element capable of forming a ceria-based mixed oxide, wherein the at least one metal element is selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y and any combination thereof. In another preferred embodiment, the support material comprises at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide which is preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %, and wherein the at least one metal element is selected from the group consisting of Zr, La, Y and any combination thereof. In other preferred embodiments, the support material comprises at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide which is preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %, and wherein the at least one metal element is selected from the group consisting of Zr, La and any combination thereof.

[0066] In an embodiment, the support material may comprise at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises less than 40 mol % (i.e., higher than 0 mol % but less than 40 mol %) of at least one metal element capable of forming a ceria-based mixed oxide, and wherein the at least one metal element is or consists of Zr. Preferably, the support material of the metal-supported catalyst of the invention may comprise, in an embodiment, at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises an amount of at least one metal element capable of forming a ceria-based mixed oxide which is preferably in the range from 1 mol % to less than 40 mol %, more preferably in the range from 5 mol % to 30 mol %, and wherein the at least one metal element is or consists of Zr.

[0067] In one embodiment, the support material of the metal-supported catalyst of the invention may comprise a plurality of ceria-based mixed oxides, wherein each ceria-based mixed oxide may be independently defined as already indicated in any of the previously described embodiments.

[0068] In a preferred embodiment, the support material of the metal-supported catalyst according to the second aspect of the invention may exhibit a specific surface area from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g, from 80 to 130 m2 / g, from 100 to 140 m2 / g or from 100 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours.

[0069] The support material of the metal-supported catalyst according to the second aspect of the invention may preferably have an average particle size from 5 to 20 nm and, more preferably, the support material may have an average particle size from 5 to 18 nm, from 5 to 15 nm, from 6 to 15 nm, from 8 to 15 nm, from 8 to 12 nm or from 5 to 10 nm, as determined by HRTEM or, alternatively, by FESEM.

[0070] The support material of the metal-supported catalyst according to the second aspect of the invention may preferably have an average particle size which is in the range from 5 to 20 nm, as determined by HRTEM, and exhibit a specific surface area from 50 to 140 m2 / g, from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g or from 80 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In a preferred embodiment, the support material of the metal-supported catalyst may have an average particle size which is in the range from 5 to 18 nm, as determined by HRTEM, and exhibit a specific surface area from 50 to 140 m2 / g, from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g or from 80 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In another preferred embodiment, the support material of the metal-supported catalyst may have an average particle size which is in the range from 8 to 15 nm, as determined by HRTEM, and exhibit a specific surface area from 50 to 140 m2 / g, from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g or from 80 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours. In still another preferred embodiment, the support material of the metal- supported catalyst may have an average particle size which is in the range from 5 to 15 nm, as determined by HRTEM, and exhibit a specific surface area from 50 to 140 m2 / g, from 60 to 130 m2 / g, from 65 to 140 m2 / g, from 50 to 120 m2 / g, from 75 to 125 m2 / g or from 80 to 130 m2 / g, as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours.

[0071] The metal-supported catalyst according to the second aspect of the invention comprises at least one supported transition metal (M) which may be preferably selected from the group consisting of Pd, Pt, Rh, Ir, Cu, Ru, Co, Ag, Nb and any mixture thereof. Preferably, the at least one supported transition metal (M) may be selected from the group consisting of Pd, Pt, Rh, Ir, Ru and any mixture thereof. Still more preferably, the at least one supported transition metal (M) may be selected from the group consisting of Pd, Pt, Rh, Ru and any mixture thereof. Even more preferably, the at least one supported transition metal (M) may be selected from the group consisting of Pd, Pt, Ru and any mixture thereof; and still even more preferably, the at least one supported transition metal (M) may be Pd or Pt. In still another embodiment, the at least one supported transition metal (M) is or consists of Pd.

[0072] A suitable metal-based alloy for the metal-supported catalyst according to the second aspect of the invention may comprise at least 80 wt.% ( / .e., equal to or higher than 80 wt.%) of the at least one supported transition metal element, based on the total weight of the alloy, or at least 90 wt.% ( / .e., equal to or higher than 90 wt.%) of the at least one supported transition metal element, based on the total weight of the alloy. In some embodiments, the metal-based alloy may comprise two supported transition metal elements, one of them being the main constituent. Examples of suitable metal-based alloys include, but are not limited to, Pd-Pt alloys, Pt-Au alloys, Pt-Ru alloys (e.g., Pt with 5 wt.% Ru) and Pt-Rh alloys (e.g., Pt with 10 wt.% Rh).

[0073] The metal-supported catalyst according to the second aspect of the invention may be provided in the form of extrudates or pellets (i.e., shaped into pellets). Alternatively, the metal-supported catalyst may be supported on an inert substrate. Examples of suitable inert substrates include, but are not limited to, silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania. In a particular embodiment, the inert substrate comprises or consists of silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania or any combination thereof. The metal-supported catalyst according to the second aspect of the invention may further comprise an inert substrate, said inert substrate preferably being a honeycomb inert substrate. Said honeycomb inert substrate may be a honeycomb corrugated substrate, which may preferably be a metal foil substrate or a glass fibre substrate, such as a non-woven e-glass fibre substrate, or a ceramic extruded substrate, such as cordierite.

[0074] The metal-supported catalyst according to the second aspect of the invention may preferably comprise equal to or more than 2 wt.% of the at least one supported transition metal (M), more than 2 wt.% of the at least one supported transition metal (M), from 2 wt.% to 5 wt.% of the at least one supported transition metal (M), from 2.5 wt.% to 5 wt.% of the at least one supported transition metal (M), from 3 wt.% to 5 wt.% of the at least one supported transition metal (M), from 2 wt.% to 4 wt.% of the at least one supported transition metal (M), or from more than 2 wt.% to 3 wt.% of the at least one supported transition metal (M), based on the total weight of the metal-supported catalyst.

[0075] In some embodiments, the metal-supported catalyst according to the second aspect of the invention may comprise at least one supported transition metal (M) as a metal-based alloy, wherein the provision of a plurality of supported transition metals (M) may advantageously boost the catalytic activity at low-intermediate temperatures. In those embodiments, the metal-supported catalyst according to the second aspect of the invention may preferably comprise equal to or more than 2 wt.% of at least one supported transition metal (M) as a metal-based alloy, based on the total weight of the metal-supported catalyst or, alternatively, more than 2 wt.%, from more than 2 wt.% to 5 wt.%, from 2.5 wt.% to 5 wt.%, from more than 2 wt.% to 4 wt.% or from more than 2 wt.% to 3 wt.% of the at least one supported transition metal (M) as a metal-based alloy, based on the total weight of the metal-supported catalyst.

[0076] In preferred embodiments according to the second aspect of the invention, the metal-supported catalyst is obtainable or obtained by the process according to the first aspect of the invention.

[0077] In a third aspect of the invention, a process of methane combustion under wet conditions is provided, said process comprising the following steps: a) introducing a metal-supported catalyst, as defined according to the second aspect of the invention, in a reactor; b) feeding the reactor with a gas reaction feed which comprises methane, from 5% to 15 % (v / v) water and at least one inert gas, wherein the sum of all components in the gas reaction feed is 100 % (v / v); and c) allowing the metal-supported catalyst to remain in contact with the gas reaction feed in the reactor for a time sufficient and at a temperature sufficient for methane combustion to take place.

[0078] The expression "reaction feed" in the context of the present invention refers to a gas stream which is fed to a combustion reactor. The gas stream may preferably consist of methane, 0.5-30 vol.% water, and a remaining amount adding up to 100 vol.% which comprises at least one inert gas, such as nitrogen, and which may optionally comprise other gas components such as NO, NO2, CO, CO2 and / or O2. More preferably, the gas stream may consist of methane (e.g., 0.01-0.2 vol.% methane, which corresponds to 100-2000 ppm methane), 5-15 vol.% water, and a remaining amount adding up to 100 vol.% which comprises at least one inert gas, such as nitrogen, and which may optionally comprise other gas components such as NO, NO2, CO, CO2 and / or O2.

[0079] In some embodiments, the process according to the third aspect of the invention may further comprise, before step a), subjecting the metal-supported catalyst to a reduction step with a reducing agent selected from the group consisting of a reducing gas, a hypophosphite salt, a borohydride salt, hydrazine, formic acid, formaldehyde, citric acid, and any combination thereof. Should that reduction step be carried out, the process may optionally subsequently comprise an oxidation step, which can be carried out before, simultaneously or after step a).

[0080] In a fourth aspect of the invention, the use of the metal supported catalyst according to the second aspect of the invention in methane combustion is provided, preferably the use in methane combustion under wet conditions.

[0081] Throughout the description and the claims, the word "comprise" and variations thereof are not intended to exclude other technical features, ingredients or steps. Additional advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention without undue burden.

[0082] EXAMPLES

[0083] The following examples are provided by way of illustration and shall not be construed as limiting the invention. Numerous variations are possible, and it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described in the following examples. In the Examples provided below, Brunauer-Emmett-Teller (BET) values of the corresponding samples have been calculated from Nj isotherms at 77 K on a volumetric Micromeritics ASAP™ 2020 analyzer, after activation at 400°C and under vacuum for 3 hours. High-Resolution Transmission Electron Microscopy (HRTEM) analyses have been carried out using a 200 kV JEOL-Model JEM2100F microscope.

[0084] Hj-TPR profiles were obtained using Autochem 2910 with a thermal conductivity detector (TCD) under the following conditions: Among 50 mg of solids (granulometry: 0.2-0.4 mm) was placed in a quartz tube, maintained at room temperature for 15 min in Ar (50 ml / min), and then reduced in a stream of a mixture of 10% Hj / Ar (50 ml / min) at a heating rate of 10°C / min to 800°C. The expression surface oxygen (OSUrf) of the support materials has been defined and quantified considering the Hz consumption of the first reduction peaks (between 250 and 550°C) from the Hj-TPR profiles obtained according to the previous experimental considerations.

[0085] Unless indicated otherwise, in each one of the catalytic performance tests provided below, 50 mg of a pelletized catalyst sample were diluted in 1.5 g of pelletized silicon carbide before being used in each methane combustion reaction.

[0086] Furthermore, all methane combustion reactions were independently conducted in a fixed bed, quartz tubular reactor of 1.2 cm of diameter and 53 cm of length. 100 mL / min of air were passed through a water saturator, which was heated at 46°C, and the resultant saturated air current was mixed with 50 mL / min of a 3,000-ppm methane current diluted in N?.

[0087] Additionally, unless indicated otherwise, the following specific feed conditions were employed in each catalytic test case:

[0088] Methane combustion reactions under dry conditions: 0.1 vol.% CH4and 14.0 vol.% O? in N?; total flow 150 mL / min; and GHSV = 180,000 mL / gcafh (GHSV = gas hourly space velocity);

[0089] Methane combustion reactions under wet conditions: 0.1 vol.% CH4, 14.0 vol.% O? and 6.1 vol.% HjO in N2; total flow 150 mL / min; and GHSV = 180,000 mL / gcafh.

[0090] EXAMPLE 1 - Synthesis of an exemplary 3 wt.% Pd-containing Ce2 metal-supported catalyst (Pd / OSurf=0.56)

[0091] An exemplary metal-supported catalyst according to the invention, which comprises 3 wt.% palladium as supported platinum group metal, was prepared according to the process of the invention, as follows:

[0092] A commercially available ceria (see Ce2 in Table 1) has been considered to prepare this metal- supported catalyst. Ce2 is a commercially available ceria (purchased from Solvay, ref. ITQ. 3A, Lot # 0633984), which presents 7-9 nm average particle sizes (see HRTEM image for Ce2 in FIG. 1), 98 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.087 (obtained from H2-TPR, see Ce2 in FIG. 2).

[0093] 0.85 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was mixed with 50 mg of Milli-Q water. The resulting solution was subsequently added dropwise using a syringe on 1 g of the Ce2 support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0094] For this metal-support, the Pd / OSUrf parameter is 0.56, which has been estimated considering both the incorporated Pd amount (3 wt.% Pd) and the previous measured molar ratio of surface oxygen to cerium of 0.087 (obtained from H2-TPR, see Ce2 in FIG. 2).

[0095] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 3 wt.% Pd-containing Ce2 material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow. The measured BET surface area by N2adsorption was ~93 m2 / g. Average particle size of 8-10 nm was determined using HRTEM.

[0096] EXAMPLE 2 - Synthesis of an exemplary 3 wt.%t Pd-containing Ce3 metal-supported catalyst (Pd / Qsurf=0.58)

[0097] An exemplary metal-supported catalyst according to the invention, which comprises 3 wt.% palladium as supported platinum group metal, was prepared according to the process of the invention, as follows:

[0098] A particular cerium oxide (see Ce3 in Table 1) prepared according to known methods in the art has been considered to prepare this metal-supported catalyst. Ce3 is a ceria which presents 6-8 nm average particle sizes (see HRTEM image for Ce3 in FIG. 1), 120 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.083 (obtained from H2-TPR, see Ce3 in FIG. 2).

[0099] 0.85 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was mixed with 50 mg of Milli-Q water. The resulting solution was subsequently added dropwise using a syringe on 1 g of the Ce3 support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0100] For this metal-support, the Pd / OSUrf parameter is 0.58, which has been estimated considering both the incorporated Pd amount (3 wt.% Pd) and the previous measured molar ratio of surface oxygen to cerium of 0.083 (obtained from Hj-TPR, see Ce3 in FIG. 2).

[0101] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 3 wt.% Pd-containing Ce3 material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0102] EXAMPLE 3 - Synthesis of a comparative 3 wt.% Pd-containing Cel metal-supported catalyst (Pd / Qsurf=0.35)

[0103] A comparative metal-supported catalyst, which comprises 3 wt.% palladium as supported platinum group metal, was prepared according to the process of the invention, as follows:

[0104] A commercially available ceria (see Cel in Table 1) has been considered to prepare this metal- supported catalyst. Cel is a commercially available ceria (purchased from NanoScale Corporation, ref. AC106-011, Lot # 06-0117), which presents 5-7 nm average particle sizes (see HRTEM image for Cel in FIG. 1), 73 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.139 (obtained from Hj-TPR, see Cel in FIG. 2).

[0105] 0.85 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was mixed with 50 mg of Milli-Q. water. The resulting solution was subsequently added dropwise using a syringe on 1 g of the Cel support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0106] For this metal-support, the Pd / OSUrf parameter is 0.35, which has been estimated considering both the incorporated Pd amount (3 wt.% Pd) and the previous measured molar ratio of surface oxygen to cerium of 0.139 (obtained from H2-TPR, see Cel in FIG. 2).

[0107] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 3 wt.% Pd-containing Cel material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0108] EXAMPLE 4 - Synthesis of a comparative 3 wt.% Pd-containing Ce4 metal-supported catalyst (Pd / Qsurf=0.86)

[0109] A comparative metal-supported catalyst, which comprises 3 wt.% palladium as supported platinum group metal, was prepared according to the process of the invention, as follows:

[0110] A commercially available ceria (see Ce4 in Table 1) has been considered to prepare this metal- supported catalyst. Ce4 is a commercially available ceria (purchased from Sigma-Aldrich, ref. 544841- 25G, Lot #MKCK1143), which presents 20-30 nm average particle sizes (see HRTEM image for Ce4 in FIG. 1), 40 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.056 (obtained from Hj-TPR, see Ce4 in FIG. 2).

[0111] 0.85 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was mixed with 50 mg of Milli-Q. water. The resulting solution was subsequently added dropwise using a syringe on 1 g of the Ce4 support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0112] For this metal-support, the Pd / OSUrf parameter is 0.86, which has been estimated considering both the incorporated Pd amount (3 wt.% Pd) and the previous measured molar ratio of surface oxygen to cerium of 0.056 (obtained from H2-TPR, see Ce4 in FIG. 2).

[0113] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 3 wt.% Pd-containing Ce4 material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0114] EXAMPLE 5 - Synthesis of an exemplary 5 wt.% Pd-containing Ce2 metal-supported catalyst (Pd / Qsurf=0.93)

[0115] An exemplary metal-supported catalyst according to the invention, which comprises 5 wt.% palladium as supported platinum group metal, was prepared according to the process of the invention, as follows: A commercially available ceria (see Ce2 in Table 1) has been considered to prepare this metal- supported catalyst. Ce2 is a commercially available ceria (purchased from Solvay, ref. ITQ 3A, Lot # 0633984), which presents 7-9 nm average particle sizes (see HRTEM image for Ce2 in FIG. 1), 98 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.087 (obtained from H2-TPR, see Ce2 in FIG. 2).

[0116] 1.4 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was added dropwise using a syringe on 1 g of the Ce2 support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0117] For this metal-support, the Pd / OSUrf parameter is 0.93, which has been estimated considering both the incorporated Pd amount (5 wt.% Pd) and the previous measured molar ratio of surface oxygen to cerium of 0.087 (obtained from H2-TPR, see Ce2 in FIG. 2).

[0118] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 5 wt.% Pd-containing Ce2 material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0119] EXAMPLE 6 - Synthesis of a comparative 1 wt.% Pd-containing Ce2 metal-supported catalyst (Pd / Osurf=0.19)

[0120] A comparative metal-supported catalyst according to the invention, which comprises 5 wt.% palladium as supported platinum group metal, was prepared according to the process of the invention, as follows:

[0121] A commercially available ceria (see Ce2 in Table 1) has been considered to prepare this metal- supported catalyst. Ce2 is a commercially available ceria (purchased from Solvay, ref. ITQ 3A, Lot # 0633984), which presents 7-9 nm average particle sizes (see HRTEM image for Ce2 in FIG. 1), 98 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.087 (obtained from H2-TPR, see Ce2 in FIG. 2).

[0122] 2.28 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was mixed with 0.62 g of Milli-Q water. The resulting solution was subsequently added dropwise using a syringe on 1 g of the Ce2 support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0123] For this metal-support, the Pd / OSUrf parameter is 0.19, which has been estimated considering both the incorporated Pd amount (1 wt.% Pd) and the previous measured molar ratio of surface oxygen to cerium of 0.087 (obtained from Hj-TPR, see Ce2 in FIG. 2).

[0124] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 1 wt.% Pd-containing Ce2 material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0125] EXAMPLE 7 - Synthesis of an exemplary 5 wt.% Pd-containing Cel metal-supported catalyst (Pd / Qsurf=0.58)

[0126] A comparative metal-supported catalyst, which comprises 5 wt.% palladium as supported platinum group metal, was prepared according to the process of the invention, as follows:

[0127] A commercially available ceria (see Cel in Table 1) has been considered to prepare this metal- supported catalyst. Cel is a commercially available ceria (purchased from NanoScale Corporation, ref. AC106-011, Lot # 06-0117), which presents 5-7 nm average particle sizes (see HRTEM image for Cel in FIG. 1), 73 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.139 (obtained from Hj-TPR, see Cel in FIG. 2).

[0128] 1.4 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was added dropwise using a syringe on 1 g of the Cel support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0129] For this metal-support, the Pd / OSUrf parameter is 0.58, which has been estimated considering both the incorporated Pd amount (5 wt.% Pd) and the previous measured molar ratio of surface oxygen to cerium of 0.139 (obtained from H2-TPR, see Cel in FIG. 2).

[0130] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 5 wt.% Pd-containing Cel material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0131] EXAMPLE 8 - Synthesis of an exemplary PdPt-containing Ce2 metal-supported catalyst (Pd / Qsurf=0.66)

[0132] An exemplary metal-supported catalyst according to the invention, which comprises 3 wt.% palladium and 1 wt.% platinum as supported platinum group metals, was prepared according to the process of the invention, as follows:

[0133] A commercially available ceria (see Ce2 in Table 1) has been considered to prepare this metal- supported catalyst. Ce2 is a commercially available ceria (purchased from Solvay, ref. ITQ. 3A, Lot # 0633984), which presents 7-9 nm average particle sizes (see HRTEM image for Ce2 in FIG. 1), 98 m2 / g of BET surface area and a measured molar ratio of surface oxygen to cerium of 0.087 (obtained from H2-TPR, see Ce2 in FIG. 2).

[0134] 0.85 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) and 0.02 g tetraammineplatinum (II) nitrate (purchased from Sigma-Aldrich; CAS nr. 20634-12-2) were mixed. The resulting solution was subsequently added dropwise using a syringe on 1 g of the Ce2 support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0135] For this metal-support, the (PdRt) / OSurf parameter is 0.66, which has been estimated considering both the incorporated Pd and Pt amount (3 wt.% Pd and 1 wt.% Pt) and the previous measured molar ratio of surface oxygen to cerium of 0.087 (obtained from H2-TPR, see Ce2 in FIG. 2).

[0136] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 3 wt.% Pd and 1 wt.% Pt-containing Ce2 material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0137] EXAMPLE 9 - Synthesis of an exemplary 5 wt.% Pd-containing CeZr metal-supported catalyst (Pd / Qsurf=0.56) 1

[0138] An exemplary metal-supported catalyst according to the invention, which comprises 5 wt.% palladium as supported platinum group metals, was prepared according to the process of the invention, as follows:

[0139] 66 g of ammonium hydroxide solution (NH4OH, 25% NH3 in H2O, Sigma-Aldrich) is dissolved in 134 mL of MilliQ water. Then, 10,42 gr of cerium nitrate (CefNOah-GHjO, Alfa-Aesar) and 1.29 gr of zirconium(IV) oxynitrate (ZrONOa, Sigma-Aldrich) are added and the resultant mixture is stirred for 30 minutes. In parallel, 6 g of a hydrogen peroxide solution (50 wt. % in H2O, Sigma-Aldrich) was diluted in 24 g of MilliQ water. Then, this H2O2 solution is added dropwise over the previous mixture, and the mixture is maintained under stirring for 45 min. Afterwards, the mixture is washed and separated by filtration with water several times, followed by drying at 100°C overnight. The obtained solids are introduced in 300 ml of isopropanol, maintaining the mixture under reflux and stirring conditions at 100°C for 5 h. The solids are recovered by filtration and washed with water and finally calcined in a muffle at 500°C for 4 h (heating ramp 2°C / min).

[0140] The resultant CeZr support presents 6-10 nm average particle sizes, 79 m2 / g of BET surface area and a measured molar ratio of surface oxygen to CeZr of 0.135 (obtained from H2-TPR).

[0141] 1.4 g of tetraamminepalladium (II) nitrate solution (10 wt.% solution in H2O, purchased from Merck; CAS 13601-08-6) was added dropwise using a syringe on 1 g of the CeZr support (which had been previously deposited on a flat-bottom porcelain crucible). Homogenization was then performed manually by using a spatula. Afterwards, the flat-bottom porcelain crucible with the wet-solid was deposited on top of a hot plate and let dry at 100°C. During that drying process, the wet solid was subject to continuous homogenizing with a spatula every minute until reaching complete dryness.

[0142] For this metal-support, the Pd / OSUrf parameter is 0.56, which has been estimated considering both the incorporated Pd amount (5 wt.% Pd) and the previous measured molar ratio of surface oxygen to CeZr of 0.135 (obtained from H2-TPR).

[0143] The resulting dry solid was then pelletized (pellet size: 0.2 - 0.4 mm) by using a manual hydraulic press at 4 bars. Subsequently, a thermal treatment ( / .e., calcination) was carried out using a vertical flow-bed reactor. 150 mg of the resulting pelletized 5 wt.% Pd-containing CeZr material was then calcinated with an air flow of 175 mL / min at 650°C for 4 h (heating ramp 2°C / min). Finally, the sample was cooled in air flow.

[0144] EXAMPLE 10 - Catalytic evaluation of 3 wt.% Pd / nanoceria materials using different nanoceria supports (Examples 1-4) The resulting 3 wt.% Pd-containing nanoceria catalysts have been tested for the methane combustion reaction under wet conditions at different reaction temperatures following classical light-off curve experiments (from 200 to 600°C, see FIG. 3).

[0145] For the light-off experiments, the catalyst was first activated with air (100 ml / min) for 1 h. Then, the feed mixture under wet conditions was admitted over the catalyst with an overall flow of 150 ml / min and the following composition (0.1 vol% CH4, 14.0 vol% O2, 6 vol% H2O in N2, GHSV=180 000 ml / gcat-h). The reaction temperature was increased stepwise between 200 and 600°C. The methane combustion activity was measured under steady-state conversion at each temperature by gaschromatography (Agilent Technologies 7890B.GC system with a TCD detector using a SP2100 precolumn and PLOTQ and Molsieve 5A capillary columns).

[0146] The catalysts synthesized according to the Examples 1 and 2 of the present invention were found to perform better for the methane combustion reaction in wet conditions, compared to those comparative catalysts prepared according to Examples 3 and 4 of the present invention (see FIG. 3). It is worth noting that both catalysts present values of the Pd / OSUrf parameter ~0.56-0.58 and particle sizes between 6-9 nm and BET surface areas between 90 and 120 rri2 / g. The catalyst prepared according to Example 3 presents a lower value for the Pd / OSUrf parameter -0.35, while the catalyst prepared according to Example 4 presents a higher value for the Pd / OSUrf parameter -0.86 but its particle sizes are above 20 nm and BET surface areas below 50 m2 / g.

[0147] In addition to the light-off type experiments at different temperatures, the long-term stability of these catalysts when exposed to wet feeds under dynamic high-temperature conditions (between 400 and 600°C) for long time-on-stream (TOS) was also evaluated. For the steady-state long TOS experiments, the catalysts were first activated with air (100 ml / min) for 1 h and, afterwards, with the feed mixture under dry conditions (150 ml / min, 0.1 vol% CH4, 14.0 vol% O2, in N2, GHSV=180 000 ml / gcat-h), the reaction temperature was increased up to 400°C with a heating ramp of ~14°C / min, and maintained at 400°C under dry conditions for 1 h. At this moment, the feed was changed to wet conditions (0.1 vol% CH4, 14.0 vol% O2, 6 vol% H2O in N2, GHSV=180 000 ml / gcafh) and maintained for -40 h. Later, dynamic high-temperature variations were studied by increasing the reaction temperature to 600°C under wet conditions for 1 h and posteriorly lowered down to 400°C and maintained for an additional 1 h. The methane conversion was measured by gas-chromatography (Agilent Technologies 7890B.GC system with a TCD detector using a SP2100 pre-column and PLOTQ and Molsieve 5A capillary columns).

[0148] The two catalysts performing better for the steady-state long TOS at 400°C under wet conditions were found to be, as occurred previously for the light-off experiments, those prepared according to Examples 1 and 2, presenting both catalysts almost unaltered the methane conversion values not only for 40 h under wet conditions at 400°C (see Examples 1 and 2 in section (I) of FIG. 4), but also after being subjected to dynamic high-temperature variations between 400 and 600°C (see Examples 1 and 2 in section (III) of FIG. 4). These results clearly highlight the great activity and stability offered by these two 3 wt.% Pd-containing catalysts, where the nanoceria supports present particle sizes between 6-9 nm, BET surface areas between 90-120 m2 / g area and Pd / OSUrf values ~0.6.

[0149] EXAMPLE 11 - Catalytic evaluation of different metal contents in Ce2 support (Examples 1, 5 and 6)

[0150] In order to see the influence of the metal content, different amounts of Pd on Ce2 support ( / .e., 1 wt.% from Example 6, 3 wt.% from Example 1 and 5 wt.% from Example 5) were deposited, followed by a simple activation treatment in air at 650°C. The Pd / OSUrf parameter was calculated for the three catalysts, resulting in the following values: 0.19 (1 wt.% Pd, Example 6), 0.56 (3 wt.% Pd, Example 1) and 0.93 (5 wt.%, Example 5).

[0151] These three catalysts have been first tested under wet conditions following the classical light-off type experiments (see Example 10 for experimental details on the light-off type reaction test). Increasing the amount of Pd in the catalysts resulted in an enhancement of the methane conversion profiles (see FIG. 5). Interestingly, most significant differences were observed when evaluating these catalysts for long TOS under wet feeds and dynamic high-temperature reaction conditions (see Example 10 for experimental details on the steady-stated long TOS experiments). As seen in FIG. 6, the two catalysts prepared according to the Examples 1 and 5 (3 and 5% Pd contents) showed similar catalytic performance at 400°C and wet conditions to the results obtained during the short light-off type experiments at 400°C (see FIG. 5), and, in addition, both catalysts showed almost unaltered methane conversion values not only for 40 h under wet conditions at 400°C (see section (I) in FIG. 6), but also after being subjected to dynamic high-temperature variations between 400 and 600°C (see section (III) in FIG. 6). The calculated turnover frequency (TOF, methane conversion normalized by metal content) values for both catalysts prepared according to Examples 1 and 5 of the present invention was found to be 8.4 and 9.6 h1(calculated from the long TOS experiments after 2000 min). These results clearly highlight the great activity and stability offered by this nanoceria support, even when increasing the metal charge to 5 wt.%. However, the 1 wt.% Pd-containing nanoceria clearly showed a minor methane conversion after being maintained for long TOS periods at 400°C in presence of steam (see Example 6 in section (I) of FIG. 6). In fact, the calculated TOF value after 2000 min for this catalyst was low (0.4 h1), highlighting the negative role of low Pd / OSUrf ratios (-0.2), even in a nanoceria support with average particle sizes between 7-9 and BET surface area of 97 m2 / g.

[0152] EXAMPLE 12 - Catalytic evaluation of different metal contents in Cel support (Examples 3 and 7) In order to see the influence of the metal content on an additional nanoceria support with higher measured surface oxygen species, Cel (see Table 1), different amounts of Pd on this support (i.e., 3 wt.% from Example 3 and 5 wt.% from Example 7) were deposited, followed by a simple activation treatment in air at 650°C. The Pd / OSUrf parameter was calculated for the two catalysts, resulting in the following values: 0.35 (3%wt Pd, Example 3) and 0.58 (5 wt.%, Example 7).

[0153] As seen in FIG. 7, the methane conversion achieved by the catalyst prepared according to Example 7 was notably higher compared to the catalyst prepared according to Example 3 when tested for long TOS-type experiments under wet conditions and dynamic high-temperature variations (see experimental details for the steady-state long TOS in Example 10). The calculated TOF value after 2000 min of reaction and wet conditions at 400°C for the catalyst prepared according to Example 7 of the present invention is ~7.0 h1, value that approaches the calculated TOF for the catalyst prepared according to Example 1 at the same conditions, ~8.4 h1, both catalysts presenting similar Pd / OSUrf ratios (~0.56-0.58). Another important conclusion from the catalytic results obtained when testing the catalyst prepared according to Example 7 for long TOS conditions is that this catalyst shows an excellent stability after being subjected to dynamic high-temperature variations (400-600-400°C) under wet conditions (see Example 7 in section (III) of FIG. 7).

[0154] EXAMPLE 13 - Catalytic evaluation of PdPt in Ce2 support (Example 8)

[0155] The methane conversion for long TOS-type experiments under wet conditions (see experimental details for the steady-state long TOS in Example 10) achieved by the bimetallic catalyst prepared according to Example 8 of the present invention shows not only an excellent stability after 40 hours of reaction (see FIG. 8), but also a similar catalytic performance to the catalyst prepared according to Example 1.

[0156] EXAMPLE 14 - Catalytic evaluation of Pd in CeZr support (Example 9)

[0157] The methane conversion for long TOS-type experiments under wet conditions (see experimental details for the steady-state long TOS in Example 10) achieved by the 5 wt.% Pd containing CeZr catalyst prepared according to Example 9 presents an excellent stability after 40 hours of reaction (see FIG. 8), as well as a similar catalytic performance to the catalyst prepared according to Example 7 with similar Pd content and Pd / OSUrf ratio.

Claims

CLAIMS1. A process for obtaining a metal-supported catalyst, wherein the process is characterized by comprising the following steps: a) adding at least one supported transition metal (M) selected from any of the groups 5 to 11 of the periodic table of the elements to a support material, wherein the support material comprises at least one oxide selected from the group consisting of ceria, a ceriabased mixed oxide, and any combination thereof; b) drying, preferably under homogenization conditions; and c) thermally treating the solid obtained in step b) at a temperature from about 250°C to about 850°C; wherein the at least one oxide is substantially present in crystalline form, has an average particle size from 5 to 20 nm as determined by HRTEM, and exhibits a specific surface area from 50 to 140 m2 / g as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours; and wherein the molar ratio of the at least one supported transition metal (M) to surface oxygen in the support material (M / OSUrf) is from 0.4 to 1.4 as determined by Hj-TPR.

2. The process according to claim 1, wherein the at least one oxide has a molar ratio of surface oxygen to cerium from 0.05 to 0.20, as determined by Hj-TPR.

3. The process according to claim 2, wherein the addition of the at least one supported transition metal (M) to the support material is performed by wet impregnation, incipient volume impregnation or deposition-precipitation.

4. The process according to any one of claims 1-3, wherein the at least one supported transition metal (M) is selected from the group consisting of Pd, Pt, Rh, Cu, Ru, Co, Ag, Nb and any mixture thereof.

5. The process according to any one of claims 1-4, wherein the at least one supported transition metal (M) is added in step a) in the form of a metal salt or metal complex, or as a metal-based alloy.

6. The process according to any one of claims 1-5, wherein the support material comprises at least one ceria-based mixed oxide, wherein the at least one ceria-based mixed oxide comprises less than 40 mol % of at least one metal element which is capable of forming a ceria-based mixed oxide.

7. The process according to claim 6, wherein the at least one metal element capable of forming a ceria-based mixed oxide is selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y and any combination thereof.

8. The process according to any one of claims 1-7, wherein the process further comprises, after step b) but before step c), a step wherein the dried solid obtained in step b) is applied in or onto an inert substrate, preferably a honeycomb inert substrate; or shaped into pellets.

9. The process according to any one of claims 1-8, wherein the thermally treated solid resulting from step c) is subjected to a reduction step wherein it is contacted with a reducing agent selected from the group consisting of a reducing gas, a hypophosphite salt, a borohydride salt, hydrazine, formic acid, formaldehyde, citric acid and any combination thereof.

10. Metal-supported catalyst for methane combustion under wet conditions, characterized by comprising: a support material comprising at least one oxide selected from the group consisting of ceria, a ceria-based mixed oxide, and any combination thereof; and at least one supported transition metal (M) selected from any of the groups 5 to 11 of the periodic table of the elements; wherein the at least one oxide is substantially present in crystalline form, has an average particle size from 5 to 20 nm as determined by HRTEM, and exhibits a specific surface area from 50 to 140 m2 / g as measured by BET method using nitrogen gas adsorption after activation at 400 °C under vacuum for 3 hours; and wherein the metal-supported catalyst comprises more than 2 wt.% of the at least one supported transition metal (M), based on the total weight of the metal-supported catalyst.

11. The metal-supported catalyst according to claim 10, which is obtainable by the process according to any one of claims 1-9.

12. Process of methane combustion under wet conditions, characterized by comprising the following steps: a) introducing a metal-supported catalyst, as defined according to any one of claims 10-11, in a reactor; b) feeding the reactor with a gas reaction feed which comprises methane, from 5% to 15 % (v / v) water and at least one inert gas, wherein the sum of all components in the gas reaction feed is 100 % (v / v); andc) allowing the metal-supported catalyst to remain in contact with the gas reaction feed in the reactor for a time sufficient and at a temperature sufficient for methane combustion to take place.

13. The process, according to claim 12, which further comprises, before step a), subjecting the metal-supported catalyst to a reduction step with a reducing agent selected from the group consisting of a reducing gas, a hypophosphite salt, a borohydride salt, hydrazine, formic acid, formaldehyde, citric acid, and any combination thereof.

14. The process, according to claim 13, wherein the metal-supported catalyst is subject to an oxidation step, after having been subject to the reduction step, and wherein said oxidation step is carried out before step b) and before, simultaneously or after step a).

15. Use of a metal-supported catalyst, as defined according to any one of claims 10-11, in methane combustion, preferably in methane combustion under wet conditions.

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