Metal-supported catalyst system for methane combustion under humid conditions, its preparation process, and its use

Abstract

The present invention relates to a metal-supported catalyst system for methane combustion under wet conditions, comprising a support material in the form of nanocubes substantially free of alkali metal cations (Alk), wherein the support material comprises at least one oxide selected from the group consisting of ceria, ceria-based mixed oxides, and any combination thereof, and at least one supported transition metal (M) selected from any of Groups 5 to 11 of the Periodic Table in an amount of less than 10% by weight based on the total weight of the catalyst system. The present invention also relates to a process for preparing the metal-supported catalyst system and the use of the metal-supported catalyst system in the catalytic combustion of methane under wet conditions.

Description

Technical Field

[0001] The present invention relates to a metal-supported catalyst system for methane combustion under wet conditions, comprising a supported material in the form of nanocubes substantially free of alkali metal cations (Alk), wherein the supported material comprises at least one oxide selected from the group consisting of ceria, ceria-based mixed oxides, and any combination thereof, and at least one supported transition metal (M). Also provided is a process for preparing the metal-supported catalyst system, and the use of the metal-supported catalyst system in the catalytic combustion of methane under wet conditions.

Background Art

[0002] In particular, the design of efficient methane combustion catalyst systems in natural gas combustion engines, gas turbines, or solid oxide fuel cells is a major concern for reducing the undesirable greenhouse effect associated with methane emissions. These emissions typically result from the incomplete combustion of natural gas.

[0003] Over the past few decades, various catalyst systems for methane combustion have been studied, and supported transition metals, particularly metals from groups 5-11 of the periodic table, and more specifically palladium, alone or in combination with other metals, are of particular interest. For successful catalytic applications in methane combustion, these catalyst systems necessarily require high catalytic activity at low to moderate reaction temperatures, i.e., temperatures around 550 °C and below, and high resistance to deactivation under operating conditions in the presence of water in the stream, and may also involve pressurized operating conditions.

[0004] On the one hand, it is known in the art that ceria and cerium-based mixed oxides typically provide suitable supports on which noble metals can be deposited to form metal-supported catalyst systems. In particular, various well-structured ceria particles have been evaluated in the literature in recent years as catalyst supports for methane combustion under dry conditions by 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 Publication No. 113274999), and Chen et al. (ACS Catal. 2021, 11, 5666 - 5677). Similarly, Yanyan Lei et al. (Fuel 233, 2018, 10 - 20) have studied the catalytic properties of palladium (Pd) nanocrystals supported on different forms of CeO 4 carriers for lean methane (CH 2 ) combustion. All of these prior art documents disclose the preparation of rod-shaped and cube-shaped ceria supports useful for manufacturing metal-supported catalysts, and all of these CeO 2 synthesis processes are carried out using alkali bases, particularly NaOH and KOH.

[0005] However, all of the above prior art studies have been conducted under dry conditions, and as a result, the influence of competitive adsorption of water and methane on catalytic results at low and medium temperatures has not been elucidated until now. However, regarding other different catalyst systems, it is known in the art that the presence of water can dramatically inhibit or change the catalytic performance for methane combustion, as described by Hoque et al. (Korean J. Chem. Eng. 2014, 31, 1316). Therefore, it is impossible to extrapolate the results achieved under dry conditions to those obtained under wet conditions. Summary of the Invention Problems to be Solved by the Invention

[0006] Accordingly, there is a need for a technical solution that provides a new metal-supported catalyst system that is particularly useful for methane combustion under humid conditions, which can advantageously provide high catalytic activity and at the same time provide significant resistance to deactivation under operating conditions.

Means for Solving the Problems

[0007] In a first aspect of the present invention, there is provided a metal-supported catalyst system for methane combustion under humid conditions, the metal-supported catalyst system comprising: - a support material comprising at least one oxide selected from the group consisting of ceria, ceria-based mixed oxides, and any combination thereof, - at least one supported transition metal (M) selected from any of Groups 5 to 11 of the Periodic Table of the Elements, the support material is in the form of nanocubes, substantially free of alkali metal cations (Alk), and the amount of at least one supported transition metal (M) is less than 10% by weight based on the total weight of the catalyst system.

[0008] According to a second aspect of the present invention, there is provided a process for preparing a metal-supported catalyst system, the process comprising the following steps: (i) preparing a mixture comprising water, at least one cerium source, at least one alkali metal cation (Alk) source, and optionally at least one source of a metal element (X) capable of forming a ceria-based mixed oxide, the resulting mixture having a composition with the following molar ratios, aCe:bX:cAlk:dH 2 O wherein, a is 1, b ranges from 0 to 0.4, c is greater than 0.01, d is greater than 1, step, (ii) subjecting the mixture to hydrothermal treatment at a temperature in the range of 50 °C to 250 °C for a predetermined period sufficient to form a crystalline material. (iii) A step of removing residual alkali metal cations (Alk) from the material obtained in step (ii) until the amount of residual alkali metal cations in the material is less than 0.3% by weight, and (iv) A step of incorporating at least one supported Group 5-11 transition metal (M) in a molar ratio Ce:M in the range of 1:0.001 to 1:0.5 respectively to provide a metal-supported catalyst system.

[0009] In a third aspect of the present invention, a metal-supported catalyst system for methane combustion under wet conditions is provided, which is obtained or can be obtained by the process of the second aspect of the present invention as further defined by any of the embodiments provided above.

[0010] According to a fourth aspect of the present invention, there is provided the use of a metal-supported catalyst system according to the first aspect of the present invention in the catalytic combustion of methane in which water is present in the reaction feed, more preferably in the catalytic combustion of methane under wet conditions.

[0011] In a fifth aspect of the present invention, there is provided the use of a metal-supported catalyst system obtained or obtainable by the process of the second aspect of the present invention in the catalytic combustion of methane in which water is present in the reaction feed, more preferably in the catalytic combustion of methane under wet conditions.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0013] According to a first aspect of the present invention, a metal-supported catalyst system for methane combustion under wet conditions, wherein the metal-supported catalyst system is - A supported material comprising at least one oxide selected from the group consisting of ceria, ceria-based mixed oxides, and any combination thereof, - and at least one supported transition metal (M) selected from any of Groups 5 to 11 of the Periodic Table of the Elements, The supported material is in the form of nanocubes, substantially free of alkali metal cations (Alk), and the amount of at least one supported transition metal (M) is less than 10% by weight based on the total weight of the catalyst system.

[0014] The expression "methane combustion under wet conditions" is understood to mean that the methane combustion is carried out using a reaction feed having a volume percentage of water in the range of 0.5% to 30% based on the total volume of the reaction feed. In a preferred embodiment, the wet conditions employed during methane combustion are understood to mean that the methane combustion is carried out using a reaction feed having a volume percentage of water in the range of 5% to 15% based on the total volume of the reaction feed.

[0015] The expression "reaction feed" in the context of the present invention refers to the gas stream supplied to the combustion reactor. In relation to the methane combustion reaction under wet conditions, the gas stream can preferably consist of methane, 0.5 to 30% by volume of water, and the balance up to a total of 100% by volume, which balance includes at least one inert gas such as nitrogen and, optionally, NO, NO 2 , CO, CO 2 , and / or O 2 and other gas components such as. More preferably, in relation to the methane combustion reaction under wet conditions, the gas stream can consist of methane (e.g., 0.01 to 0.2% by volume of methane, which corresponds to 100 to 2000 ppm of methane), 5 to 15% by volume of water, and the balance up to a total of 100% by volume, which balance includes at least one inert gas such as nitrogen and, optionally, NO, NO 2 , CO, CO 2 , and / or O 2 and other gas components such as.

[0016] The supported material according to the present invention is provided in the form of nanocubes. The term "nanocube" refers to nanoparticles having a cubic form (also known as a cubical form), which form is defined as any of the following shapes: (i) regular nanocubes surrounded by six {100} surface planes, or (ii) although not limited, to some extent irregular {110} / {110} sub-faceting in the {111} surface plane, and / or to some extent waviness (i.e., faceting) at the edges, and / or (c) nanocubes exposing their {110} and / or {111} surfaces by to some extent truncation at the edges and / or corners of the nanocubes, having no complete cubical form.

[0017] Surprisingly, the presence of alkali metal cations in the ceria and / or ceria-based mixed oxide(s) particles of the supported material, particularly those having a cubical form, has been found to significantly inhibit the catalytic performance during methane combustion. The new metal-supported catalyst system according to the present invention, wherein the supported material is in the form of nanocubes and substantially free of alkali metal cations (Alk), has been found to function as a highly efficient catalyst for undergoing the methane combustion reaction under wet conditions. Such catalytic performance has been found to be significantly higher than that observed in a comparative alkali-containing similar Pd-ceria catalyst system, or a comparative alkali-free Pd-ceria catalyst system based on nanorods (i.e., nanoparticles having a rod-like form), as exemplified by the experimental results provided herein.

[0018] Although not bound by theory, it is hypothesized that the unexpectedly high catalytic activity of the metal-supported catalyst system according to the present invention is greatly influenced by the specific crystalline form of the support material. The crystalline form is thought to have a great influence on the degree of metal-support interaction and thus on the intended catalytic use of the system, more specifically on the methane activation properties, especially when water is also present in the reaction medium. Different ceria-based forms expose different crystal planes and substantially modify the interaction between a support material comprising at least one oxide selected from the group consisting of ceria, ceria-based mixed oxides, and any combination thereof and at least one supported transition metal (M). In particular, the support material of the present invention in the form of nanocubes maximizes the {100} crystal plane, while other forms such as rod-shaped ceria forms maximize the {110} crystal plane and have been experimentally found to exhibit significantly different catalytic behavior.

[0019] The support material according to the present invention, particularly the nanocubes, can have an average primary particle size in the range of about 2 nm to about 800 nm, preferably about 2 nm to about 600 nm, more preferably about 2 nm to about 500 nm, even more preferably about 5 nm to about 400 nm, even more preferably about 5 nm to about 200 nm, and even more preferably about 5 nm to about 50 nm.

[0020] The term "about" is used in the context of the present invention in front of a number and, when referring to it, is to be understood as specifying a particular value and any value within a range defined by the number ±5%, more preferably within a range defined by the number ±2%. For example, the expression "about 1" is to be interpreted as being "within the range of 0.95 to 1.05", preferably "within the range of 0.98 to 1.02".

[0021] In one embodiment, the support material according to the present invention, particularly the nanocubes, can have an average primary particle size in the range of 2 nm to 800 nm, preferably 2 nm to 600 nm, more preferably 2 nm to 500 nm, even more preferably 5 nm to 400 nm, even more preferably 5 nm to 200 nm, and even more preferably 5 nm to 50 nm.

[0022] As used herein in this context, the expression "substantially free of" is understood to mean that the support material contains less than 0.3% by weight, preferably less than 0.2% by weight, more preferably less than 0.1% by weight of alkali metal cations (Alk) based on the total weight of the catalyst system.

[0023] In one embodiment, the support material of the metal-supported catalyst system of the present invention comprises at least one ceria-based mixed oxide, and at least one ceria-based mixed oxide contains less than 40 mol% (i.e., more than 0 mol% but less than 40 mol%) of at least one metal element (X) capable of forming the ceria-based mixed oxide. The support material may contain at least one ceria-based mixed oxide, and at least one ceria-based mixed oxide preferably contains at least one metal element (X) in an amount ranging from 1 mol% to less than 40 mol%, more preferably in the range of 5 mol% to 30 mol%, capable of forming the ceria-based mixed oxide. The mole percentage of at least one metal element (X) is calculated based on the total number of moles of at least one ceria-based mixed oxide, which means that when two or more ceria-based mixed oxides are present in the metal-supported catalyst system of the present invention, the mole percentage of at least one metal element (X) is calculated based on the total number of moles of the plurality of ceria-based mixed oxides.

[0024] At least one element (X) capable of forming the ceria-based mixed oxide may preferably be selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y, and any combination thereof. In a preferred embodiment, at least one element (X) capable of forming the ceria-based mixed oxide is selected from the group consisting of Zr, La, Y, and any combination thereof. In another preferred embodiment, at least one element (X) capable of forming the ceria-based mixed oxide is selected from the group consisting of Zr, La, and any combination thereof. More preferably, at least one element (X) capable of forming the ceria-based mixed oxide is Zr.

[0025] Preferably, the support material of the metal-supported catalyst system of the present invention contains at least one ceria-based mixed oxide, and the at least one ceria-based mixed oxide contains less than 40 mol% (i.e., more than 0 mol% but less than 40 mol%) of at least one metal element (X) capable of forming the ceria-based mixed oxide, and the at least one metal element (X) 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 of the metal-supported catalyst system of the present invention contains at least one ceria-based mixed oxide, and the at least one ceria-based mixed oxide preferably contains an amount in the range of 1 mol% to less than 40 mol%, more preferably in the range of 5 mol% to 30 mol% of at least one metal element (X) capable of forming the ceria-based mixed oxide, and the at least one metal element (X) is selected from the group consisting of Zr, La, Y, and any combination thereof. In another preferred embodiment, the support material of the metal-supported catalyst system of the present invention contains at least one ceria-based mixed oxide, and the at least one ceria-based mixed oxide preferably contains an amount in the range of 1 mol% to less than 40 mol%, more preferably in the range of 5 mol% to 30 mol% of at least one metal element (X) capable of forming the ceria-based mixed oxide, and the at least one metal element (X) is selected from the group consisting of Zr, La, and any combination thereof.

[0026] In one embodiment, the support material of the metal-supported catalyst system of the present invention may include at least one ceria-based mixed oxide in one embodiment. The at least one ceria-based mixed oxide contains less than 40 mol% (i.e., more than 0 mol% but less than 40 mol%) of at least one metal element (X) capable of forming the ceria-based mixed oxide, and the at least one metal element (X) is Zr or consists of Zr. Preferably, the support material of the metal-supported catalyst system of the present invention can include at least one ceria-based mixed oxide in one embodiment. The at least one ceria-based mixed oxide preferably contains an amount in the range of 1 mol% to less than 40 mol%, more preferably in the range of 5 mol% to 30 mol% of at least one metal element (X) capable of forming the ceria-based mixed oxide, and the at least one metal element (X) is Zr or consists of Zr.

[0027] In one embodiment, the support material of the metal-supported catalyst system of the present invention includes a plurality of ceria-based mixed oxides, and each ceria-based mixed oxide can be independently defined as shown in any of the foregoing embodiments.

[0028] The at least one supported transition metal (M) can be particularly selected from the group consisting of Pd, Pt, Rh, Cu, Ru, Co, Ag, Nb, and any mixture thereof. Preferably, the at least one supported transition metal (M) can be selected from the group consisting of Pd, Pt, Rh, Cu, and any mixture thereof, more preferably, the at least one supported transition metal (M) can be selected from the group consisting of Pd, Pt, and any mixture thereof, and even more preferably, the at least one supported transition metal is Pd or consists of Pd.

[0029] The metal-supported catalyst system according to the present invention can be further applied in or on an inert substrate (e.g., a monolithic substrate or structure), or can be further formed into pellets according to the requirements of the intended use. 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.

[0030] In certain embodiments, the metal-supported catalyst system according to the present invention further comprises an inert substrate, which comprises or consists of silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, or any combination thereof.

[0031] According to a second aspect of the present invention, there is provided a process for preparing a metal-supported catalyst system, the process comprising the following steps: (i) preparing a mixture comprising water, at least one cerium source, at least one alkali metal cation (Alk) source, and optionally at least one source of a metal element (X) capable of forming a ceria-based mixed oxide, wherein the resulting mixture has a composition with the following molar ratios, aCe:bX:cAlk:dH 2 O wherein a is 1, b ranges from 0 to 0.4, c is greater than 0.01, d is greater than 1, (ii) subjecting the mixture to hydrothermal treatment at a temperature in the range of 50 °C to 250 °C for a predetermined period sufficient to form a crystalline material, (iii) removing residual alkali metal cations (Alk) from the material obtained in step (ii) until the amount of residual alkali metal cations in the material is less than 0.3 wt%, and (iv) incorporating at least one supported Group 5-11 transition metal (M) in a molar ratio Ce:M in the range of 1:0.001 to 1:0.5, respectively, to provide a metal-supported catalyst system.

[0032] The process according to the present invention is particularly suitable for preparing the metal-supported catalyst system according to the first aspect of the present invention.

[0033] In step (i), any cerium source can be used as at least one cerium source. Preferably, at least one cerium source in step (i) can be cerium oxide and / or at least one cerium salt. In one embodiment, at least one cerium source in step (i) comprises cerium oxide and / or at least one cerium salt. The at least one cerium salt is preferably selected from the group consisting of cerium nitrate, cerium ammonium nitrate, cerium carbonate, cerium hydroxide, cerium sulfate, cerium oxalate, and any combination thereof. However, other cerium salts known in the art can also be suitably used.

[0034] In step (i), any source of alkali metal cations can be used as at least one source of alkali metal cations (Alk). Preferably, the at least one source of alkali metal cations (Alk) in step (i) is selected from alkali metal hydroxides, alkali metal halide salts, alkali metal nitrates, and any combination thereof. More preferably, the at least one source of alkali metal cations (Alk) in step (i) is selected from alkali metal hydroxides, alkali metal halide salts, and any combination thereof, and even more preferably, the at least one source of alkali metal cations (Alk) in step (i) is an alkali metal hydroxide.

[0035] In one embodiment, the at least one source of alkali metal cations (Alk) in step (i) comprises an alkali metal hydroxide, an alkali metal halide salt, an alkali metal nitrate, or any combination thereof. More preferably, the at least one source of alkali metal cations (Alk) in step (i) comprises an alkali metal hydroxide, an alkali metal halide salt, or any combination thereof, and even more preferably, the at least one source of alkali metal cations (Alk) in step (i) comprises an alkali metal hydroxide.

[0036] The alkali metal cation (Alk) is preferably selected from the group consisting of Li, Na, K, Rb, Cs, Fr, and any combination thereof. More preferably, the alkali metal cation (Alk) is preferably selected from the group consisting of Li, Na, K, and any combination thereof. Even more preferably, the alkali metal cation (Alk) is preferably selected from the group consisting of Li, Na, and any combination thereof. Even more preferably, the alkali metal cation (Alk) is Na.

[0037] In one embodiment, the alkali metal cation (Alk) includes Li, Na, K, Rb, Cs, Fr, or any combination thereof. More preferably, the alkali metal cation (Alk) includes Li, Na, K, or any combination thereof. Even more preferably, the alkali metal cation (Alk) includes Li, Na, or any combination thereof. Even more preferably, the alkali metal cation (Alk) includes Na.

[0038] In step (i), at least one source of the metal element (X) capable of forming the ceria-based mixed oxide can be used in the mixture to be prepared. The at least one metal element (X) is preferably selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y, and any combination thereof. More preferably, the at least one metal element (X) is preferably selected from the group consisting of Zr, La, Y, and any combination thereof. In another preferred embodiment, the at least one element (X) is selected from the group consisting of Zr, La, and any combination thereof, and even more preferably, the at least one element (X) is Zr.

[0039] Any source of the metal element (X) can be used in step (i), but the source of the at least one metal element (X) is preferably selected from the group consisting of metal hydroxides, metal salts, and any combination thereof. Preferred metal salts include metal halide salts, metal nitrates, or any combination thereof.

[0040] The mixture obtained from step (i) has the following molar ratios: aCe:bX:cAlk:dH 2 O which are defined by the molar values a, b, c and d, - a is 1, - b ranges from 0 to 0.4, - c is greater than 0.01 - d is greater than 1.

[0041] The value of b can preferably range from 0 moles to 0.3 moles, more preferably the value of b can range from 0.01 moles to 0.2 moles.

[0042] The value of c is preferably 0.2 or more, more preferably greater than 0.2, even more preferably 1 or more, even more preferably the value of c is greater than 1, particularly greater than 10, or may be greater than 20. In one embodiment, the value of c can range from 0.2 to 1,000, preferably from 1 to 300. The value of c can range from 5 to 300, from 5 to 200, or from 25 to 300.

[0043] The value of d is preferably 5 or more, more preferably greater than 5, even more preferably 10 or more, even more preferably the value of d may be greater than 10. In one embodiment, the value of d can be in the range of 5 to 10,000, preferably in the range of 10 to 10,000, more preferably in the range of 10 to 5,000, even more preferably in the range of 10 to 2,000. In one embodiment, the value of d can range from 50 to 2,000, from 100 to 1,000, or from 300 to 1,200. In another embodiment, the value of d can range from 500 to 2,000 or from 200 to 1,500.

[0044] Step (ii) can be carried out in a suitable reaction vessel, such as a polypropylene jar, an autoclave lined with Teflon® or a stainless steel autoclave, either under static conditions or under stirring conditions.

[0045] The expression "hot water treatment" refers to treatment conditions in a closed system that includes water as a reaction medium and has a temperature and pressure higher than ambient, usually significantly higher than ambient.

[0046] Preferably, in step (ii), the mixture is subjected to hot water treatment at a temperature in the range of about 50°C to about 250°C, preferably in the range of about 100°C to about 200°C, more preferably in the range of about 125°C to about 175°C, for a predetermined period sufficient to form a crystalline material. Such a predetermined period can be from about 1 hour to about 20 days, preferably 2 hours to 10 days, more preferably 2 hours to about 2 days.

[0047] In one embodiment, the hot water treatment in step (iii) is carried out at a temperature in the range of about 50°C to about 250°C for a period of about 1 hour to about 20 days, preferably 2 hours to 10 days, more preferably 2 hours to about 2 days. In another embodiment, the hot water treatment in step (iii) is carried out at a temperature in the range of about 100°C to about 200°C for a period of about 1 hour to about 20 days, preferably 2 hours to 10 days, more preferably 2 hours to about 2 days. In yet another embodiment, the hot water treatment in step (iii) is carried out at a temperature in the range of about 125°C to about 175°C for a period of about 1 hour to about 20 days, preferably 2 hours to 10 days, more preferably 2 hours to about 2 days.

[0048] In step (iii), the residual alkali metal cation (Alk) is removed from the previously obtained material until the amount of the residual alkali metal cation in the material is less than 0.3% by weight. Such removal can be carried out by any technical means known in the art. More preferably, in step (iii), the residual alkali metal cation (Alk) is removed from the previously obtained material by subjecting it to ion exchange, more specifically cation exchange, in order to selectively and substantially remove the alkali metal cation. Such cation exchange can be carried out, for example, using an aqueous solution of an ammonium salt, such as ammonium chloride. The amount of the residual alkali metal cation can be determined by any known technique that enables identification of the chemical composition of the material, such as inductively coupled plasma (ICP) spectrometry or field emission scanning electron microscopy with energy-dispersive X-ray spectrometry (FESEM-EDX).

[0049] In a preferred embodiment, after step (ii) and before step (iii), the process may further optionally include another step in which the crystalline material obtained from step (ii) is separated from and recovered from the mother liquor. The crystalline material can be separated by any means known in the art, such as decantation, filtration, ultrafiltration, centrifugation, or any other solid-liquid separation technique. In a more preferred embodiment, after step (ii) and before step (iii), the process may further optionally include another step in which the crystalline material obtained from step (ii) is first washed and then separated from and recovered from the mother liquor.

[0050] In step (iv), at least one supported Group 5-11 transition metal (M) can be incorporated to provide a metal-supported catalyst system. The expression "incorporated" should be understood to encompass any possible method of adding at least one supported Group 5-11 transition metal (M) to the support material formed in steps (i)-(iv) of this process. Preferably, at least one supported Group 5-11 transition metal (M) can be incorporated into the support material obtained from steps (i)-(iv) by, inter alia, impregnation, adsorption, ion exchange, precipitation, or vapor deposition or spray deposition. In certain embodiments, incorporating at least one supported Group 5-11 transition metal (M) in step (iv) means depositing at least one supported Group 5-11 transition metal (M), more specifically, depositing at least one supported Group 5-11 transition metal (M) onto the product obtained from step (iii) of the process according to the second aspect of the invention (i.e., a crystalline material substantially free of alkali metal cations).

[0051] The at least one supported transition metal (M) used in step (iv) can be selected, in particular, from the group consisting of Pd, Pt, Rh, Cu, Ru, Co, Ag, Nb, and any mixture thereof. Preferably, the at least one supported transition metal (M) can be selected from the group consisting of Pd, Pt, Rh, Cu, and any mixture thereof, more preferably, the at least one supported transition metal (M) can be selected from the group consisting of Pd, Pt, and any mixture thereof, and even more preferably, the at least one supported transition metal is Pd or consists of Pd.

[0052] At least one supported Group 5-11 transition metal can be incorporated into step (iv) of the process according to the invention in the form of a Group 5-11 transition metal precursor. The precursor is preferably a water-soluble Group 5-11 transition metal precursor, which can be incorporated directly into step (iv) or in the form of an aqueous solution that can be prepared prior to incorporation. By way of example, suitable gold precursors include, but are not limited to, sodium tetrachloroaurate, potassium tetrabromoaurate, hydrogen tetranitroaurate, sodium thioaurate malate, and any combination thereof. Suitable platinum metal precursors include, but are not limited to, hexachloroplatinic acid, sodium tetrachloroplatinate, platinum sulfate, and any combination thereof. Suitable palladium metal precursors include, but are not limited to, palladium chloride, sodium tetrachloropalladate, tetraamminepalladium nitrate, palladium sulfate, and any combination thereof. Suitable silver metal precursors include, but are not limited to, silver nitrate, silver perchlorate, silver sulfate, potassium silver cyanide, and any combination thereof. Examples of Cu precursors that can be used include Cu(acac) 2 (acac = acetylacetonate), Cu(thd) 2 (thd = tetrahydrodionate), hexafluoroacetylacetonate-copper-vinyltrimethylsilane, cyclopentadienyl (Cp) compounds (e.g., CpCu(CNMe), CpCu(CNCMe 3 ), Cp*CuCO, CpCuPR 3 (R = Me, Et, or Ph), CpCu(CSiMe 3 ) 2 , CuCN, (CuCl) 2 , (MeCN) 4 CuX (X = halide, alkyl, amine or phenyl group), Me 3 SiOCu(PMe 3 ) 3 , or Cu(C 4 H 4 S), but are not limited thereto.

[0053] After step (iv), the resulting supported metal catalyst system typically has a molar ratio of cerium to supported transition metal (Ce:M) in the range of 1:0.001 to 1:0.5, more preferably, the molar ratio Ce:M is in the range of 1:0.01 to 1:0.4, and even more preferably, the molar ratio Ce:M is in the range of 1:0.05 to 1:0.3. The molar ratio Ce:M corresponds to the molar ratio between cerium from at least one cerium source and at least one supported Group 5-11 transition metal (M). When multiple different supported Group 5-11 transition metals are used, the molar ratio of the supported Group 5-11 transition metals to be considered to establish this molar ratio Ce:M shall include the total molar number corresponding to all the different supported Group 5-11 transition metals used. Also, when multiple cerium sources are used, the molar ratio of cerium to be considered to establish this molar ratio Ce:M shall include the total number of cerium moles considering all the cerium sources used.

[0054] The process according to the second aspect of the present invention may further include step (v), and the supported metal catalyst obtained from step (iv) - is applied in or on an inert substrate (e.g., a monolithic substrate or structure), the substrate comprising silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, or any combination thereof, or - is formed into pellets.

[0055] The process according to the second aspect of the present invention may optionally further include an activation treatment that can be carried out after any of the above steps that are carried out after step (ii) but before step (iii) to recover the crystalline material obtained from step (ii), and after step (iii) and / or step (iv). This activation treatment can advantageously further enable control of the size and oxidative properties of the supported metal catalyst system, particularly when using noble metals such as Pd.

[0056] When the optional activation treatment is at a temperature of at least about 200 °C (i.e., about 200 °C or higher), preferably at least about 300 °C (i.e., about 300 °C or higher), more preferably at least about 370 °C (i.e., about 370 °C or higher) for at least 1 minute, but generally for a controlled period of 20 hours or less, it includes a step of heating. This optional activation treatment, particularly this heating step, is carried out under an oxidizing atmosphere, preferably in the presence of oxygen, under a reducing agent atmosphere, preferably in the presence of hydrogen, and / or under an inert atmosphere, preferably in the presence of N 2 or Ar.

[0057] In a preferred embodiment, after step (ii) and before step (iii), the process may further optionally include another step in which the crystalline material obtained from step (ii) (i.e., the untreated synthetic crystal) is separated from the mother liquor and recovered. The crystalline material can be separated by any means known in the art such as decantation, filtration, ultrafiltration, centrifugation, or any other solid-liquid separation technique. In a more preferred embodiment, the process may further optionally include another step in which the crystalline material obtained from step (ii) (i.e., the untreated synthetic crystal) is first washed and then separated from the mother liquor and recovered after step (ii) and before step (iii).

[0058] In a particular embodiment, the process may further include both the above-mentioned optional activation treatment and the above-mentioned optional step after step (ii) and before step (iii), the crystalline material obtained from step (ii) (i.e., the untreated synthetic crystal) is separated from the mother liquor and recovered, and the former occurs earlier than the latter, so that it is clear that it is possible to advantageously carry out further control of the size and oxidative properties of the metal-supported catalyst system before separation from the mother liquor and subsequent recovery.

[0059] In a third aspect of the present invention, a metal-supported catalyst system for methane combustion under wet conditions is provided, which is optionally obtained by, or obtainable by, the process of the second aspect of the present invention as further defined by any of the embodiments provided above.

[0060] In a specific embodiment of this third aspect of the present invention, a metal-supported catalyst system according to the first aspect of the present invention for methane combustion under wet conditions, which is obtained by, or obtainable by, the process of the second aspect of the present invention, is provided, and optionally, the metal-supported catalyst system, and / or the process for obtaining or being able to obtain the metal-supported catalyst system, may be further defined by any of the above embodiments.

[0061] According to a fourth aspect of the present invention, the use of a metal-supported catalyst system according to the first aspect of the present invention in the catalytic combustion of methane under wet conditions is provided.

[0062] According to a fifth aspect of the present invention, the use of a metal-supported catalyst system obtained by, or obtainable by, the process of the second aspect of the present invention in the catalytic combustion of methane under wet conditions is provided.

[0063] Throughout the specification and claims, the term "comprising" and its variations are not intended to exclude other technical features, components, or steps. Further advantages and features of the present invention will become apparent to those skilled in the art by considering the description, or may be learned by practicing the present invention without undue burden.

[0064] [Examples] The following examples are provided for illustrative purposes and should not be construed as limiting the present invention. Many variations are possible, and it should be understood that within the scope of the appended claims, the present invention may be practiced in ways other than specifically described in the following examples.

[0065] In the following examples provided, field emission scanning electron microscope (FESEM) analysis was carried out using a ZEISS-Model GeminiSEM 500 microscope equipped with an Inlens EsB detector operating at 2 - 5 kV. High-resolution transmission electron microscope (HR-TEM) analysis was carried out using a 200 kV JEOL-Model JEM2100F microscope. The corresponding Brunauer-Emmett-Teller (BET) values of the samples were calculated at 77 K using a volumetric Micromeritics ASAP™ 2020 analyzer after activation at 400 °C and under vacuum from the N 2 isotherms.

[0066] Example 1 An exemplary CeO in the form of a nanocube (cubic form) according to the invention, substantially free of alkali metal cations and prepared by steps (i) - (iii) of the process of the invention (i.e., including the alkali removal step). 2 Synthesis of the CeO-based supported material 4 g of cerium nitrate (Ce(NO 3 )) 3 ·6H 2 O was dissolved in 40 mL of water. Then, the cerium aqueous solution was added dropwise to 155 g of a 14.3 wt% NaOH aqueous solution, and the mixture was maintained under stirring for 30 minutes.

[0067] The mixture was introduced into a stainless-steel autoclave lined with Teflon® and maintained at 175 °C for 24 hours. After hydrothermal crystallization, the solid was washed several times with water, separated by filtration, and then dried overnight at 100 °C. The HRTEM image of the obtained solid revealed the formation of ceria crystals having a cubic form with an average particle size of 10 - 20 nm (see Figure 1). FESEM-EDX analysis of the obtained solid revealed the presence of about 1.3 wt% Na.

[0068] A 1 g sample of the Na-containing ceria with the obtained cubic morphology was exchanged with 10 g of 1 M aqueous ammonium chloride solution. The mixture was maintained at 80 °C for 10 h with stirring. Then, the solid was washed several times with water, separated by filtration, and then dried overnight at 100 °C. Finally, the dried solid was calcined in air at 500 °C for 3 h. FESEM-EDX analysis of the obtained solid revealed that Na cations had been removed from the sample (about 0.0 wt% Na). N 2 The BET surface area measured by adsorption was about 120 m 2 / g.

[0069] Example 2 Synthesis of an exemplary Pd-CeO 2 system supported material based on the cubic morphology CeO 2 system supported material of the present invention substantially free of alkali metal cations 1 g of the ceria-supported material according to the present invention prepared according to Example 1, having a cubic morphology and substantially free of alkali metal cations, was impregnated with about 3 wt% Pd. The Pd precursor used was tetraamminepalladium(II) nitrate solution (tetraamminepalladium(II) nitrate solution, 10 wt% in water, purchased from Sigma-Aldrich), and the following procedure according to the present invention was followed.

[0070] 0.842 g of Pd(NH 3 ) 4 (NO 3 ) 2 of 10 wt% aqueous solution was weighed, and water was added to make the total weight of the solution 1.5 g. Next, this solution was added dropwise to 1 g of cerium oxide and then dried overnight at 100 °C. Finally, the sample was calcined in air (100 mL / min) at 650 °C.

[0071] Example 3 Synthesis of an exemplary Zr-Ce mixed oxide system supported material in the form of nanocubes (cubic morphology) according to the present invention substantially free of alkali metal cations, prepared by steps (i)-(iii) (i.e., the alkali removal step) of the process of the present invention 5 g of cerium nitrate (Ce(NO 3 ) 3 ·6H 2 O) purchased from Alfa-Aesar and 0.4 g of zirconium(IV) oxynitrate hydrate (ZrO(NO 3 ) 2 hydrate) purchased from Sigma-Aldrich were dissolved in 83 mL of water. Next, this aqueous solution was added dropwise to 100 g of a 14.3 wt% NaOH aqueous solution, and the mixture was maintained under stirring for 30 minutes.

[0072] Subsequently, the mixture was introduced into a stainless-steel autoclave lined with Teflon (registered trademark) and maintained at 175 °C for 24 hours. After hydrothermal crystallization, the solid was washed several times with water, separated by filtration, and then dried overnight at 100 °C. The obtained solid showed cube-shaped Zr-containing ceria crystals having an average particle size of 20 - 40 nm as measured by HRTEM.

[0073] 1 g of the obtained solid having a cube-shaped form was exchanged with 10 g of a 1 M ammonium chloride aqueous solution. The mixture was maintained at 80 °C for 10 hours under stirring. Thereafter, the solid was washed several times with water, separated by filtration, and then dried overnight at 100 °C. FESEM-EDX analysis of the obtained solid revealed that the Na content was less than 0.3 wt% Na, more specifically about 0.0 wt% Na. Finally, the dried solid was calcined at 500 °C in air for 3 hours.

[0074] Comparative Example 1 Synthesis of a comparative Na-containing CeO 2 system-supported material in the form of nanocubes (cube-shaped form) without an alkali removal step 4 g of cerium nitrate (Ce(NO 3 ) 3 ·6H 2 O) purchased from Alfa-Aesar was dissolved in 40 mL of water. Next, the cerium aqueous solution was added dropwise to 155 g of a 14.3 wt% NaOH aqueous solution, and the mixture was maintained under stirring for 30 minutes.

[0075] The mixture was introduced into a stainless steel autoclave lined with Teflon (registered trademark) and maintained at 175 °C for 24 hours. After hydrothermal crystallization, the solid was washed several times with water, separated by filtration, and then dried overnight at 100 °C. The HRTEM image of the obtained solid revealed the formation of ceria crystals with a cubic morphology having an average particle size of 10 - 20 nm. The FESEM-EDX analysis of the obtained solid revealed the presence of about 1.3 wt% of Na.

[0076] Comparative Example 2 Synthesis of a comparative Na-containing Pd-CeO₂ metal-supported catalyst system based on the CeO₂-based supported material with a cubic morphology of Comparative Example 1 1 g of the Na-containing ceria-supported material with a cubic morphology prepared according to Comparative Example 1 was impregnated with about 3 wt% of Pd. The Pd precursor used was tetraamminepalladium(II) nitrate solution (tetraamminepalladium(II) nitrate solution, 10 wt% in water, purchased from Sigma-Aldrich), and the following procedure was followed.

[0077] 0.842 g of Pd(NH 3 ) 4 (NO 3 ) 2 of a 10 wt% aqueous solution was weighed, water was added to make a solution with a total weight of 1.5 g. Next, this solution was added dropwise to 1 g of cerium oxide and then dried overnight at 100 °C. Finally, the sample was calcined in air (100 mL / min) at 650 °C.

[0078] Comparative Example 3 Synthesis of a comparative CeO₂-based supported material in the form of nanorods substantially free of alkali metal cations 2 4.9 gr of cerium nitrate (Ce(NO 3 ) 3 ·6H 2 O, purchased from Alfa-Aesar and used without further purification) was dissolved in 84 mL of water. Then, the cerium aqueous solution was added dropwise to 149 g of a 6.4 wt% NaOH aqueous solution, and the mixture was maintained under stirring for 30 minutes.

[0079] The mixture was introduced into a stainless steel autoclave lined with Teflon (registered trademark) and maintained at 100 °C for 24 hours. After hydrothermal crystallization, the solid was washed several times with water, separated by filtration, and then dried overnight at 100 °C. The HRTEM image of the obtained solid revealed the formation of ceria crystals with a rod-like morphology having an average particle size of 40 nm × 5 nm (see Figure 2).

[0080] 1 g of the above Na-containing ceria sample having a rod-like morphology was exchanged with 10 g of a 1 M aqueous ammonium chloride solution. The mixture was maintained at 80 °C for 10 hours with stirring. Thereafter, the solid was washed several times with water, separated by filtration, and then dried overnight at 100 °C. Finally, the dried solid was calcined in air at 500 °C for 3 hours. FESEM-EDX analysis of the obtained solid revealed that the Na content was less than 0.3 wt% Na, more specifically 0.0 wt% Na. N 2 The BET surface area measured by adsorption was ~100 m 2 / g.

[0081] Comparative Example 4 A CeO having a rod-like morphology of Comparative Example 3, substantially free of alkali metal cations 2 Based on the supported material of the system, a comparative Pd-CeO 2 Synthesis of the metal-supported catalyst system 1 g of the Na-containing ceria sample having a rod-like morphology prepared according to Comparative Example 3 of the present invention was impregnated with about 3 wt% Pd. The Pd precursor used was a tetraamminepalladium(II) nitrate solution (tetraamminepalladium(II) nitrate solution, 10 wt% in water, purchased from Sigma-Aldrich), and the following procedure was followed.

[0082] 0.842 g of Pd(NH 3 ) 4 (NO 3 ) 2 A 10 wt% aqueous solution of was weighed, water was added to make a solution with a total weight of 1.5 g. Next, this solution was added dropwise to 1 g of cerium oxide, and then dried overnight at 100 °C. Finally, the sample was calcined in air (100 mL / min) at 650 °C.

[0083] Example 4 Methane combustion reaction The catalytic performance of the samples obtained from each of the following three examples (described in detail in their respective corresponding sections) in the methane combustion reaction was evaluated. - Example 2: A ceria-based supported material having a cubic form and substantially free of alkali metal cations, and CeO in the form of the nanocubes (cubic form) of Example 1 according to the present invention 2 A Pd-CeO metal-supported catalyst system according to the present invention prepared starting from the supported material. 2 metal-supported catalyst system. - Comparative Example 2: A Na-containing Pd-CeO metal-supported catalyst system having a ceria-based supported material having a cubic form 2 metal-supported catalyst system. - Comparative Example 4: A Pd-CeO metal-supported catalyst system having a ceria-based supported material having a rod form and substantially free of alkali metal cations 2 metal-supported catalyst system.

[0084] Each sample was pelletized by using a manual hydraulic press at 2 - 3 bar, and the resulting pellets were sieved using a sieve manufactured by Filtra Vibracion S.L. to obtain pellets having an average particle diameter of 0.25 - 0.42 mm. Subsequently, 50 mg of each pelletized sample was diluted with 1.5 g of pelletized silicon carbide before being used in each methane combustion reaction.

[0085] All methane combustion reactions were independently carried out in a fixed-bed quartz tubular reactor with a diameter of 2.2 cm and a length of 53 cm. For comparison, a sample prepared according to the teachings of one of the above three examples was tested under different experimental conditions, namely, (i) at four different reaction temperatures (i.e., 450 °C, 500 °C, 550 °C, and 600 °C), and (ii) under both dry and wet conditions, to evaluate the catalytic performance in the methane combustion reaction.

[0086] In the context of the methane combustion reaction carried out herein, the expression "dry conditions" is understood to mean that methane combustion is carried out using a reaction feed having a volume percentage of water of less than 0.5% based on the total volume of the reaction feed.

[0087] For the specific experimental tests carried out and summarized herein, the specific feed conditions used in each case are as follows. - Methane combustion reaction under dry conditions: N 2 with 0.1% by volume of CH 4 and 14.0% by volume of O 2 ; total flow rate 150 mL / min; and GHSV = 180000 mL / g cat ·h. - Methane combustion reaction under wet conditions: N 2 with 0.1% by volume of CH 4 , 14.0% by volume of O 2 , and 6.1% by volume of H 2 O; total flow rate 150 mL / min; and GHSV = 180000 mL / g cat ·h. *GHSV = Space velocity per hour of gas

[0088] The catalytic results obtained using different catalysts are summarized in Table 1 below.

[0089]

Table 1

[0090] As is clear from Table 1, the two reactions carried out on the sample corresponding to the Na-containing Pd-CeO 2 catalyst system having a ceria-based support material in cube form gave very low methane conversion rates even when tested at very high temperatures (see Entries 3 - 4) (i.e., 9.9% at 600 °C, see Table 1). However, surprisingly, the same Na-containing Pd-CeO 2When the sample is pre - subjected to cation - exchange treatment to substantially remove all alkali - metal cations (more specifically, Na species) by the process of the present invention, the obtained Pd - CeO 2 catalyst system advantageously shows, compared with the results obtained with the above - mentioned Na - containing Pd - CeO 2 catalyst system of Comparative Example 2, a substantial increase in methane conversion at all test temperatures (see Entries 1 - 2). These unexpected experimental results clearly demonstrate the inhibitory effect of alkali - metal cations in the Pd - CeO 2 catalyst showing a cube - type morphology.

[0091] On the other hand, Entries 5 - 6 in Table 1 further show the results obtained when the methane combustion reaction was carried out on samples corresponding to the Pd - CeO 2 catalyst system prepared by Comparative Example 4, which has a ceria - based support material in rod - type morphology and is substantially free of alkali - metal cations, more specifically Na species. The results prove that the methane conversion values obtained with this rod - type Pd - CeO 2 catalyst are significantly lower than those obtained with the cube - type Pd - CeO 2 catalyst according to the present invention (see Entries 1 - 2) (see Entries 5 - 6). More specifically, when both catalysts were evaluated under wet conditions, the Pd - CeO 2 catalyst showing a cube - type morphology according to the present invention (see Entry 2) was much more active than the comparative similar Pd - CeO 2 catalyst having a rod - type morphology (see Entry 6).

[0092] In addition, the methane conversion loss observed using the Pd - CeO 2 catalyst having a cube - type morphology of Example 2 according to the present invention (see Entries 1 and 2) was substantially lower than that of the comparative Pd - CeO 2 catalyst having a rod - type morphology at different tested reaction temperatures when comparing dry and wet conditions (see Entries 5 and 6). In particular, the Pd - CeO 2The methane conversion rate loss of the catalyst is about 50.8% and about 8.7% at 450 °C and 550 °C, respectively, when changing from dry conditions to wet conditions (see Entries 1-2). However, in the results obtained using the comparative Pd-CeO 2 catalyst having a rod-type morphology of Comparative Example 5 (see Entries 5-6), the methane conversion rate loss was significantly high (i.e., about 64.0% and about 45.9% at 450 °C and 550 °C, respectively, see Entries 5-6).

Claims

1. A metal-supported catalyst system for methane combustion under wet conditions, wherein the methane combustion is carried out using a reaction feed having a volume percentage of water in the range of 0.5% to 30% based on the total volume of the reaction feed, and the metal-supported catalyst system comprises - a support material comprising at least one oxide selected from the group consisting of ceria, ceria-based mixed oxides, and any combination thereof, - at least one supported transition metal (M) selected from any of Groups 5 to 11 of the Periodic Table of the Elements, the support material is in the form of nanocubes, substantially free of alkali metal cations (Alk), and the amount of the at least one supported transition metal (M) is less than 10% by weight based on the total weight of the catalyst system.

2. The metal-supported catalyst system according to Claim 1, wherein the support material has a primary particle size in the range of 2 nm to 800 nm.

3. The support material comprises at least one ceria-based mixed oxide, the at least one ceria-based mixed oxide comprises less than 40 mol% of at least one metal element (X) capable of forming the ceria-based mixed oxide, and the at least one metal element (X) is selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y, and any combination thereof. The metal-supported catalyst system according to Claim 1.

4. The metal-supported catalyst system according to Claim 1, wherein the support material comprises ceria.

5. The metal-supported catalyst system according to Claim 1, 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.

6. The metal-supported catalyst system according to Claim 1, further comprising an inert substrate comprising silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, or any combination thereof.

7. A process for preparing the metal-supported catalyst system according to Claim 1, comprising (i) preparing a mixture comprising water, at least one cerium source, at least one alkali metal cation (Alk) source, and optionally at least one source of a metal element (X) capable of forming a ceria-based mixed oxide, wherein the resulting mixture has a composition with the following molar ratios, aCe: bX: cAlk: dH ２ O where a is 1, b is in the range of 0 to 0.4, c is greater than 0.01, d is greater than 1, a process, (ii) subjecting the mixture to hydrothermal treatment at a temperature in the range of 50 ° C to 250 ° C for a predetermined period sufficient to form a crystalline material; (iii) removing the residual alkali metal cation (Alk) from the material obtained from step (ii) by subjecting the material to ion exchange until the amount of the residual alkali metal cation in the material is less than 0.3% by weight, and (iv) incorporating at least one supported Group 5-11 transition metal (M) in a molar ratio Ce:M in the range of 1:0.001 to 1:0.5 respectively to provide a metal-supported catalyst system, A process, characterized by comprising.

8. The process according to claim 7, wherein the cerium source is cerium oxide and / or at least one cerium salt.

9. The process according to claim 8, wherein the cerium salt is selected from the group consisting of cerium nitrate, cerium ammonium nitrate, cerium carbonate, cerium hydroxide, cerium sulfate, cerium oxalate, and any combination thereof.

10. The process according to claim 7, wherein the alkali metal cation (Alk) is selected from the group consisting of Li, Na, K, Rb, Cs, Fr, and any combination thereof.

11. The process according to claim 7, wherein the at least one alkali metal cation (Alk) source is selected from alkali metal hydroxides, alkali metal halide salts, alkali metal nitrates, and any combination thereof.

12. The process according to claim 7, wherein the at least one metal element (X) is selected from the group consisting of Zr, In, Sn, La, Pr, Nd, Gd, Y, and any combination thereof.

13. The process according to claim 7, wherein the at least one metal element (X) source is selected from metal hydroxides, metal salts, and any combination thereof.

14. Further comprising step (v), wherein the metal-supported catalyst obtained from step (iv) is - applied in or on an inert substrate, the substrate comprising silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, or any combination thereof, or - formed into pellets, the process according to claim 7.

15. Use of the metal-supported catalyst system according to any one of claims 1 to 6 or the metal-supported catalyst system obtained by the process according to claim 7 in the catalytic combustion of methane under wet conditions.

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