Methane oxidation catalyst and method for producing the same

Abstract

To provide a methane oxidation catalyst that can selectively attach a required amount of heteroatoms, which contribute to suppressing creation of a substance which is inactive to methane oxidation, efficiently to a specified area of a noble metal particle which contributes to catalytic reaction, and a method for producing methane oxidation catalyst.SOLUTION: Methane oxidation catalyst includes noble metal particles on the surface of which heteroatom is attached, and carriers carrying the noble metal particles, and an amount of heteroatom for noble metal based on mass is 0.001 to 0.020 in a molar ratio.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a methane oxidation catalyst and a method for producing the same, and more particularly to a methane oxidation catalyst in which heteroatoms are attached to precious metal particles and a method for producing the same. [Background technology]

[0002] Lean-burn engines, such as diesel engines, operate at a high air-fuel ratio under lean fuel conditions, achieving excellent fuel economy. However, diesel engine exhaust gas contains emissions such as particulate matter (PM), unburned hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Therefore, exhaust gas purification is performed to remove or reduce these emissions. For example, as a method for removing unburned hydrocarbons, particularly methane, contained in lean-burn exhaust gas, catalysts (methane oxidation catalysts) have been proposed, which consist of palladium (Pd) alone or palladium (Pd) with platinum (Pt) or rhodium (Rh) added to a support such as alumina or zirconia.

[0003] It is known that catalysts for purifying such exhaust gases have low catalytic activity immediately after the initial cold start of engine operation, etc. For example, it has been reported that in palladium (Pd)-based catalysts, at low temperatures, a portion of the Pd generates Pd(OH) species due to HO contained in the gas atmosphere, making the catalyst inactive in methane oxidation (Non-Patent Documents 1, 2, etc.).

[0004] To address the above problem, it has been reported that adding nickel (Ni) to Pd-based catalysts contributes to suppressing the generation of Pd(OH)2 species (Non-Patent Documents 3 to 6). For example, Non-Patent Document 4 reports that Pd-Ni nanoparticles produced from Pd and Ni are supported on an Al2O3 support. Non-Patent Document 5 reports that Ni-containing Pd nanoparticles are supported on an Al2O3 support using PVP, and that Ni or Pd is supported on an Al2O3 support by an impregnation method. Non-Patent Document 6 reports that nanoparticles in which Pd is supported on Ni particles by a two-stage polyol reduction method are supported on an Al2O3 support. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] ChemCatChem 2021, 13, 3765-3771 [Non-patent document 2] ChemCatChem 2020, 12, 944-952 [Non-patent document 3] Catalysis Today 47 (1999)95-101 [Non-patent document 4] J. Am Chem. Soc. 2017, 139, 11989-11997 [Non-patent document 5] ACS Catal. 2015, 5, 2916-2920 [Non-patent document 6] ACS Catal. 2017, 7, 1615-1625 Summary of the Invention [Problem to be solved by the invention]

[0006] When Ni and Pd particles are attached by the impregnation method or by incorporating Ni into a support as described above, in a catalyst in which Pd particles are highly dispersedly supported on the surface of a support such as alumina, an excessive amount of Ni is required to selectively attach Ni by the impregnation method to specific locations of the Pd particles where the Pd particles efficiently contribute to the catalytic reaction (for example, the ridges that are the boundaries of the crystal planes when the Pd particles have a shape similar to a regular octahedral crystal). Furthermore, with the impregnation method, Ni also adheres to the support surface other than the Pd particles, which does not contribute to the catalytic reaction. Furthermore, Ni adhered to the support surface at positions away from the Pd particles is thought to be wasted because it does not contribute to suppressing the generation of Pd(OH)2 species. Furthermore, when Ni is supported on Pd by a method other than those described above, Ni is randomly supported on the Pd particles, and an excessive amount of Ni is still required to selectively attach Ni to specific locations on the Pd particles.

[0007] Therefore, an object of the present invention is to provide a methane oxidation catalyst in which a necessary amount of heteroatoms that can contribute to suppressing the formation of substances that are inactive in methane oxidation (for example, Pd(OH)2 species when Pd is used as the precious metal) are selectively attached to specific locations on precious metal particles that efficiently contribute to the catalytic reaction, and a method for producing the same. [Means for solving the problem]

[0008] The inventors have discovered that by attaching heteroatoms to the surfaces of precious metal particles using chemical vapor deposition (CVD) instead of impregnation, it is possible to preferentially attach heteroatoms to specific locations on the precious metal particles, and to efficiently attach the required amount of heteroatoms to the precious metal particles. They have also found that chemical vapor deposition can suppress the attachment of transition metal elements to the support surface. The present invention is based on these findings. The gist of the present invention is as follows.

[0009] [1] Noble metal particles with foreign atoms attached to their surfaces; a support on which the noble metal particles are supported; Including, A methane oxidation catalyst, wherein the molar ratio of the heteroatom to the noble metal is 0.001 to 0.020 by mass. [2] The methane oxidation catalyst according to [1], wherein the noble metal is at least one selected from the group consisting of palladium, platinum, and rhodium. [3] The methane oxidation catalyst according to [1], wherein the heteroatom is at least one selected from the group consisting of nickel, iron, vanadium, molybdenum, tungsten, manganese, technetium, and rhenium. [4] The methane oxidation catalyst according to [1], which is used to purify exhaust gas from an internal combustion engine. [5] A step of preparing a precious metal catalyst in which precious metal particles are supported on a support; and a heterometal deposition step of depositing heteroatoms on the noble metal particles by chemical vapor deposition; A method for producing a methane oxidation catalyst, comprising: [6] The method according to [5], wherein the chemical vapor deposition method includes contacting a gas of a carbonyl compound containing a different metal. [7] The method according to [5], wherein the chemical vapor deposition method is carried out at a temperature of 220 to 260°C. [8] The method according to [5], further comprising a heat treatment step after the noble metal catalyst preparation step. [9] The manufacturing method according to [8], wherein the heat treatment step is performed at a temperature of 900°C or less.

[10] The method according to [5], further comprising a reduction treatment step after the noble metal catalyst preparation step. [Effects of the Invention]

[0010] According to the present invention, it is possible to obtain a methane oxidation catalyst in which a necessary amount of heteroatoms capable of contributing to suppressing the formation of substances inactive in methane oxidation is attached to precious metal particles. [Brief explanation of the drawings]

[0011] [Figure 1]Figure 1 shows STEM-EDS mapping of a methane oxidation catalyst subjected to a 20-hour Ni chemical vapor deposition process. [Figure 2] Figure 2 shows STEM-EDS mapping of a methane oxidation catalyst subjected to a 100-hour Ni chemical vapor deposition process. [Figure 3] Figure 3 shows the STEM-EDS mapping of a methane oxidation catalyst with 1300 ppm of Ni deposited by impregnation. [Figure 4] Figure 4 shows the STEM-EDS mapping of a methane oxidation catalyst with 17,400 ppm of Ni deposited by impregnation. [Figure 5] Figure 5 shows STEM-EDS mapping of a methane oxidation catalyst subjected to a 100-hour Ni chemical vapor deposition process without heat treatment. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Methane oxidation catalyst and method for producing the same] The methane oxidation catalyst of the present invention includes precious metal particles having heteroatoms attached to their surfaces and a support on which the precious metal particles are supported, wherein the molar ratio of the heteroatoms to the precious metal is 0.001 to 0.020 by mass. The method for producing the methane oxidation catalyst of the present invention includes preparing a precious metal catalyst in which precious metal particles are supported on a support and depositing heteroatoms on the precious metal particles by chemical vapor deposition. Here, "heteroatoms" refers to elements other than the precious metal supported on the support. Using Pd as an example of a precious metal and Ni as an example of a heteroatom, the method of the present invention can produce a methane oxidation catalyst in which the Pd particles are coated with a sufficient amount of Ni to suppress the formation of Pd(OH)2 species, which are inactive in methane oxidation. This significantly reduces the amount of Ni used while maintaining catalytic performance compared to when Ni is deposited by conventional impregnation. The methane oxidation catalyst and its production method are described in detail below.

[0013] [Preparation process for precious metal catalyst] First, a precious metal catalyst is prepared in which precious metal particles are supported on a support. Examples of precious metal sources for producing the precious metal catalyst include nitrates, acetates, or chlorides of precious metals (e.g., palladium nitrate). The support is immersed in a solution containing the precious metal source to impregnate the support with a predetermined amount of the solution, and then calcined to support the precious metal particles on the support.

[0014] Examples of the noble metal include palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os). At least one selected from the group consisting of Pd, Pt, and Rh is preferred.

[0015] From the viewpoint of catalytic activity, the particle diameter of the precious metal particles supported on the carrier is preferably 1 nm to 100 nm, more preferably 2 nm to 50 nm. If the particle diameter is too small, the particles tend to become an oxide state that does not exhibit catalytic activity, while if the particle diameter is too large, the amount of active sites decreases, and the catalytic activity per unit amount of precious metal catalyst tends to decrease. The particle diameter of the precious metal particles can be adjusted by controlling the concentration of the solution containing the precious metal source, the impregnation amount of the solution, and the calcination conditions (temperature and time).

[0016] The noble metal catalyst is a catalyst in which noble metal particles are supported on a carrier, but may contain catalytically active species components other than the noble metal particles. However, the proportion of the noble metal components as catalytically active species is 50% by mass or more. Even when noble metal particles contain catalytically active species components other than the noble metal particles, the proportion of the noble metal components is preferably 70% by mass or more, more preferably 90% by mass or more, and particularly preferably 95% by mass or more (including 100% by mass).

[0017] The carrier for supporting the above-mentioned noble metal particles is not particularly limited, and known carriers can be used. However, inorganic oxides are preferred from the viewpoint of durability. Examples include alumina (Al2O3), such as α, γ, δ, and θ, zirconia (ZrO2), titania (TiO2), silica (SiO2), ceria (CeO2), and aluminosilicates. These may be used alone or in combination, or may be composite oxides of these. Among these, alumina is preferred from the viewpoint of durability. When alumina is used as a carrier, any of α-alumina, γ-alumina, δ-alumina, and θ-alumina can be used. Among these, γ-alumina, which has a porous structure and a high BET specific surface area, is preferred.

[0018] In the present invention, from the viewpoint of uniformly dispersing the noble metal particles on the support, the support used is preferably a porous body. 2 / g~800m 2 To prepare a porous body, a carrier is prepared by a conventionally known method as described below.

[0019] The support may contain a transition element such as chromium, cobalt, iron, nickel, titanium, manganese, or copper. The transition elements may be used alone or in appropriate combination of two or more. When a transition element is contained, its content is not particularly limited, but is preferably 0.01 to 10 mass%, more preferably 0.1 to 5 mass%, and even more preferably 0.5 to 3 mass%, calculated as the total amount of the transition element oxide relative to the total amount of the support.

[0020] The carrier is preferably in a particulate form. In this case, the average particle diameter (D50) can be appropriately set depending on the desired performance and is not particularly limited, but from the viewpoint of maintaining a large specific surface area, improving heat resistance, and increasing the number of catalytically active sites, it is preferably 0.5 μm to 30 μm, more preferably 1 μm to 20 μm, and even more preferably 3 μm to 15 μm. In this specification, the average particle diameter D50 means the median diameter measured using a laser diffraction particle size distribution analyzer.

[0021] The firing treatment may be carried out in a conventional manner and is not particularly limited. The firing atmosphere may be any of an oxidizing atmosphere, a reducing atmosphere, and an air atmosphere. The firing temperature and treatment time vary depending on the desired composition and its stoichiometric ratio, but from the viewpoint of productivity, they are generally 150 to 1300°C for 30 minutes to 12 hours, and more preferably 350 to 800°C for 30 minutes to 2 hours. Prior to the firing treatment, reduced-pressure drying may be carried out using a vacuum dryer or the like.

[0022] [Heat treatment process] After producing a noble metal catalyst in which noble metal particles are supported on a carrier as described above, the noble metal catalyst may be heat-treated. Heat treatment can promote crystal growth of the noble metal particles and enlarge the crystal faces of the noble metal particles. When Pd is used as the noble metal, heat treatment can transform the Pd particles, which were previously approximately spherical, into a shape closer to a polyhedron. As described below, when Ni is attached to Pd particles as a heteroatom by chemical vapor deposition, it has been found that Ni selectively attaches to the ridges (boundaries between faces) of the polyhedrons of the Pd particles. This is presumably because, as described below, when Ni(CO)4 is thermally decomposed and Ni is deposited on Pd particles by chemical vapor deposition, CO selectively attaches to the crystal faces during the thermal decomposition of Ni(CO)4, resulting in Ni attaching to the boundaries between the crystal faces (the ridges of the polyhedrons). This selective attachment of Ni to Pd particles allows the necessary amount of Ni to be attached to the Pd particles, which can contribute to suppressing the generation of Pd(OH)2 species.

[0023] The heat treatment of the noble metal catalyst is preferably carried out at 700 to 900°C, more preferably at 750 to 850°C. The heat treatment atmosphere may be air or an inert gas atmosphere. For example, the heat treatment can be carried out at 850°C for 10 hours using an apparatus such as an electric furnace.

[0024] [Reduction treatment process] After producing a noble metal catalyst in which noble metal particles are supported on a carrier as described above, the noble metal catalyst may be subjected to a reduction treatment. This reduction treatment allows for effective deposition of heteroatoms by chemical vapor deposition, which will be described later. The reduction treatment can be performed by heating the noble metal catalyst at a temperature of 200°C to 300°C while passing a reducing gas (e.g., a mixed gas of hydrogen and argon).

[0025] [Foreign atom attachment process by chemical vapor deposition] A noble metal catalyst having noble metal particles supported on a support is prepared as described above, and heteroatoms are deposited on the noble metal particles by chemical vapor deposition. The heteroatoms can be at least one selected from the group consisting of nickel (Ni), iron (Fe), vanadium (V), molybdenum (Mo), tungsten (W), manganese (Mn), technetium (Tc), and rhenium (Re). Among these, Ni or Fe is preferred. Depositing the heteroatom can be achieved by contacting the catalyst with a carbonyl compound gas containing the heteroatom. For example, if the noble metal is Pd and the transition metal is Ni, Ni(CO)4 gas can be contacted with the Pd catalyst, the Ni(CO)4 is thermally decomposed, and the Ni transition metal element can be deposited on the Pd noble metal particles by chemical vapor deposition. Ni(CO)4 is thermally decomposed into Ni and CO species, and the CO species tends to selectively adhere to the crystal surfaces of the Pd particles. The Ni species then selectively adhere to locations other than the crystal faces of the Pd particles (for example, to the ridges that are the boundaries of the crystal faces when the Pd particles have a shape similar to a regular octahedron crystal).

[0026] On the other hand, the active sites for the reaction between the solid-supported catalyst and the gas phase gas are atoms exposed on the surface of the precious metal particles, and among these, atoms with a low coordination number are particularly highly reactive, so it is thought that the reaction occurs more easily at atoms located at the boundaries of the crystal planes than at the crystal planes. The catalyst of the present invention is a catalyst in which heteroatoms are selectively supported at the boundaries of the crystal planes. For example, in a Pd-Ni catalyst for methane oxidation, by supporting Ni at the boundaries of the Pd crystal planes, it is possible to effectively prevent the Pd near the Ni from reacting with water (HO) to produce Pd(OH)2.

[0027] In conventional impregnation methods, Ni is supported on the Pd crystal faces and the support, but also on the boundaries of the Pd crystal faces. This means that more Ni than necessary must be supported to achieve the required amount of Ni at the boundaries of the Pd crystal faces to suppress the formation of Pd(OH)2. In other words, in conventional impregnation methods, Ni is supported on the Pd crystal faces and the support, which are not involved in the suppression of Pd(OH)2 formation. In contrast, the chemical vapor deposition process employed in the present invention results in almost no Ni adhering to the support surface or to Pd crystal faces that are not significantly involved in the reaction, significantly reducing the amount of Ni required to suppress the formation of Pd(OH)2 species.

[0028] On the other hand, since the active site for the methane oxidation reaction is ultimately Pd, if Ni is selectively supported on all of the Pd atoms at the boundaries of the highly reactive crystal planes, it is assumed that the catalytic reaction will be inhibited and activity will decrease.In fact, in a methane oxidation catalyst (Ni-CVD-100h_850℃-Pd / Al2O3) that underwent a 100-hour Ni chemical vapor deposition process (described below), excessive Ni adhered to the Pd atoms at the boundaries of the crystal planes, resulting in a decrease in catalytic reactivity (reference example).

[0029] The locations of the noble metal particles to which the foreign atoms are attached by the chemical vapor deposition process can be confirmed, for example, by performing elemental distribution analysis using a scanning transmission electron microscope with energy dispersive X-ray spectroscopy (STEM-EDS).

[0030] The chemical vapor deposition process can employ a conventionally known method. For example, when the heteroatom is Ni, a process gas is generated from Ni(CO)4, and Ni can be deposited on the surface of the precious metal particles by a thermal chemical vapor deposition process. Specifically, Ni(CO)4 is heated to a predetermined temperature and evaporated (sublimated) to generate a process gas, which is then circulated together with a carrier gas (e.g., an inert gas such as He, Ne, Ar, Kr, or Xe) to bring the Ni(CO)4 gas into contact with the precious metal catalyst. The precious metal catalyst is then heated to a predetermined temperature while adjusting the temperature, thereby performing chemical vapor deposition of Ni on the precious metal particles. The heating temperature of the precious metal particles during the chemical vapor deposition process can be 220°C to 260°C.

[0031] The amount of heteroatoms attached to precious metal particles by chemical vapor deposition can be adjusted by the flow rates of the process gas and carrier gas, the deposition time, the deposition temperature, etc. In the present invention, the heteroatom / precious metal ratio (molar ratio of heteroatoms to precious metal) is preferably 0.001 to 0.020, and more preferably 0.003 to 0.010, on a mass basis. The ratio of precious metal to heterometal can be confirmed by dissolving Al2O3 powder on which precious metal particles with heterometals attached are dispersed and carrying the resulting solution through inductively coupled plasma (ICP) analysis.

[0032] Furthermore, although it depends on the amount of precious metal particles supported on the support, the amount of foreign metals in the entire obtained methane oxidation catalyst (i.e., a supported catalyst in which precious metal particles having foreign atoms attached thereto are supported on a support) is preferably 17 wtppm to 310 wtppm, and more preferably 40 wtppm to 150 wtppm.

[0033] [Catalyst structure] When the methane oxidation catalyst described above is used, for example, as a catalyst for purifying exhaust gas from an internal combustion engine, it is preferably used as a catalyst structure in which a hydrocarbon oxidation catalyst is supported as a catalyst layer on a substrate. The substrate is not particularly limited as long as it is made of a material that can support the catalyst layer described below and has a certain degree of fire resistance, and conventionally known substrates can be used. Examples of substrate materials include ceramics such as alumina, silica, mullite (alumina-silica), cordierite, cordierite-alpha alumina, zircon-mullite, alumina-silica magnesia, zircon silicate, sillimanite, magnesium silicate, zircon, petalite, aluminosilicates, aluminum titanate, silicon carbide, and silicon nitride, as well as metal materials such as refractory metals, e.g., corrosion-resistant alloys such as stainless steel or iron-based ferritic stainless steel. The inorganic or metallic materials described above may be used alone or in combination of two or more. Among the above, alumina, silica, mullite, cordierite, stainless steel, and silicon carbide are preferred, and those containing cordierite, stainless steel, and silicon carbide are more preferred. In this case, it is recommended to use a substrate in which preferably 80 mass % or more, more preferably 90 mass % or more, and particularly preferably 99 mass % or more (including 100 mass %) of the substrate is made of the above material.

[0034] The substrate may contain other components in addition to the above-mentioned materials as the main components. For example, Fe2O3, SiO2, Na2O, etc., which are known to improve the heat resistance of the support, may be added to the above-mentioned materials.

[0035] The shape of the substrate is not particularly limited, and various shapes such as spheres, cylinders, beads, pellets, prisms, tablets, needles, membranes, and honeycomb monoliths can be used depending on the application. Among these, beads, pellets, and honeycomb monoliths are preferred. Therefore, the substrate according to a preferred embodiment of the present invention is made of alumina, silica, mullite, cordierite, or stainless steel, and is preferably in the shape of beads, pellets, or honeycomb monoliths, with cordierite honeycomb monoliths being more preferred.

[0036] When the substrate is in the form of beads or pellets, the average diameter of the substrate is preferably 0.5 mm to 10 mm, more preferably 0.7 mm to 5 mm, and even more preferably 1 mm to 3 mm, from the viewpoints of handleability and fluidity in the container. Furthermore, when the support is spherical, the average diameter of the support is preferably 1 mm to 10 mm, more preferably 1 mm to 5 mm, from the viewpoints of handleability and fluidity in the container. The average diameter of the support can be determined by observing with an optical microscope or the like, measuring the major and minor axes of 100 randomly selected supports (catalyst particles), calculating the average of the major and minor axes as the particle diameter, and then calculating the average particle diameter of the 100 individual particles.

[0037] In one embodiment of a substrate having a honeycomb monolith shape, the length of the substrate is approximately 25.4 mm to 400 mm, depending on the outer diameter. The outer diameter of the substrate is approximately 76.5 mm to 400 mm. When the cross section of the flow passage 40 defined by the wall is square, the size of each side is approximately 0.77 mm to 1.5 mm. Specifically, a honeycomb monolith substrate having 200 to 900 cells is preferred.

[0038] As an example of the catalyst structure, a honeycomb catalyst with a catalyst layer can be obtained by mixing the above-mentioned methane oxidation catalyst with a binder, a dispersant, and a solvent as needed to prepare a slurry solution, impregnating a substrate with the slurry solution by a washcoating method or the like, and firing the substrate impregnated with the slurry solution.

[0039] Examples of binders used in preparing the slurry solution include various sols such as boehmite, alumina sol, titania sol, silica sol, and zirconia sol. Soluble salts such as aluminum nitrate, aluminum acetate, titanium nitrate, titanium acetate, zirconium nitrate, and zirconium acetate can also be used as binders. Acids such as acetic acid, nitric acid, hydrochloric acid, and sulfuric acid can also be used as binders.

[0040] The coating amount can be appropriately set depending on the desired performance and is not particularly limited. However, taking into consideration the influence of pressure loss, engine output, fuel consumption, etc., the coating amount is preferably 10 g to 500 g per unit volume (1 L) of substrate, more preferably 20 g to 400 g, and even more preferably 50 g to 300 g. In this specification, "per unit volume (1 L) of substrate" refers to the total bulk volume of 1 L, including not only the net volume of the substrate but also the volume of voids formed inside the substrate. Furthermore, when cells are partitioned and formed inside the substrate, this refers to the total bulk volume of 1 L, including the net volume of the substrate, the volume of the voids, and the volume of the cells.

[0041] The content of the methane oxidation catalyst in the catalyst layer can be appropriately set depending on the desired performance and is not particularly limited, but is preferably 50 to 100 mass %, more preferably 70 to 95 mass %, and even more preferably 90 to 95 mass %, relative to the total amount of the catalyst layer.

[0042] The methane oxidation catalyst of the present invention can be used as a catalyst for purifying exhaust gases, particularly methane gas, from diesel engines, gasoline engines, jet engines, boilers, gas turbines, and the like. When used for exhaust gas purification, the methane oxidation catalyst can be disposed in the exhaust system of various engines. The number and locations of the catalysts can be appropriately designed depending on exhaust gas regulations. For example, when exhaust gas regulations are strict, two or more catalysts can be disposed in the underfloor position immediately downstream of the exhaust system catalyst. Furthermore, the methane oxidation catalyst of the present invention can oxidize hydrocarbons such as methane contained in exhaust gas with high efficiency even under low-temperature conditions. [Example]

[0043] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples in any way as long as the gist of the invention is not exceeded. In other words, the materials, amounts used, proportions, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate as long as they do not deviate from the spirit of the present invention.

[0044] [Preparation of noble metal catalyst] Palladium (II) nitrate solution (20% by mass Pd equivalent) was used as a precursor and impregnated into Al2O3 powder (Puralox TH130 / 130: manufactured by Sasol) at a loading of 3 wt%. The impregnated powder was then dried in air at 100°C for 3 hours and calcined at 550°C for 1 hour to obtain a Pd catalyst. The Pd catalyst obtained in this manner is hereinafter referred to as "AS-Pd / Al2O3."

[0045] Subsequently, the AS-Pd / Al2O3 was heat-treated in an electric furnace at 850°C for 10 hours. The Pd catalyst obtained in this manner is hereinafter referred to as "850°C-Pd / Al2O3."

[0046] [Preparation of methane oxidation catalyst (Example)] First, 100 g of Ni powder (<50 μm, 99.7% trace metals basis, Aldirch) was mixed with 150 g of pure water to prepare a 40 wt% Ni slurry. Next, a 300-cell, 5-mil cordierite honeycomb measuring φ25.4 × 50 mm was immersed in the stirred Ni slurry for 10 seconds, then removed and centrifuged at 1000 rpm for 1 minute to separate the excess slurry. The Ni powder-impregnated honeycomb thus obtained was dried at 180°C for 1.5 hours to obtain a Ni powder-supported honeycomb carrying 1.8 g of Ni powder. The resulting honeycomb was cut into a 3-cell x 3-cell channel array for installation in a sample holder. The sample holder was a 550 mm long, 3 / 8 inch diameter stainless steel tube. A recess was made in the sample holder (SUS tube) at a position 315 mm from the inlet side, and using the recess as a stopper, 0.1 g of glass wool, 0.3 g of 850°C-Pd / Al2O3 powder, and 0.1 g of glass wool were filled into the SUS tube from the inlet side in that order. Next, a recess was made in the SUS tube at a position 215 mm from the inlet side, and the Ni powder-supported honeycomb obtained as described above was placed in the inlet side.

[0047] The sample holder filled with the Pd catalyst and Ni powder-supported honeycomb as described above was placed in a 300 mm long electric furnace (ARF-30KC, manufactured by Asahi Rika Seisakusho Co., Ltd.). First, the sample holder was placed so that the Ni honeycomb was positioned at the center of the electric furnace. A mixed gas flow of hydrogen gas at a flow rate of 5 mL / min and Ar gas at 50 mL / min was introduced into the inlet side of the sample holder. The sample holder was heated from room temperature to 250°C at a rate of 10°C / min, and treated at 250°C for 20 minutes. The temperature at the Ni honeycomb position during this process was 55 to 170°C. Next, for the reduction treatment of the 850°C-Pd / Al2O3 powder, the sample holder was moved so that the 850°C-Pd / Al2O3 powder was positioned at the center of the electric furnace, and the mixture gas was introduced into the inlet side of the sample holder in the same manner as above, while the powder was treated at 250°C for 20 minutes. The mixed gas flowing into the sample holder was then changed to CO gas, and a chemical vapor deposition process was performed for a predetermined time. Ni was deposited on the 850°C-Pd / Al2O3 powder via thermal decomposition of Ni(CO)4. The flow rate was then changed to Ar gas at 50 mL / min, and the temperature was lowered to room temperature. The sample holder was then removed from the furnace, cut, and the 850°C-Pd / Al2O3 powder from which Ni had been deposited was recovered. The methane oxidation catalyst obtained as described above is hereafter referred to as "Ni-CVD-Xh_850°C-Pd / Al2O3," where X represents the chemical vapor deposition time. For example, a methane oxidation catalyst subjected to a 20-hour Ni chemical vapor deposition process is referred to as "Ni-CVD-20h_850°C-Pd / Al2O3."

[0048] For each methane oxidation catalyst obtained as described above, the Ni and Pd contents in the sample were measured by ICP analysis (Shimadzu Corporation, ICPS-8100). The Ni / Pd molar ratio (the amount of Ni relative to Pd) was measured from the Ni and Pd contents obtained by ICP analysis. The Ni content (the amount of Ni attached to the methane oxidation catalyst) was also calculated from the ICP analysis. The measurement results are shown in Table 1 below. [Table 1]

[0049] In addition, elemental analysis of each methane oxidation catalyst was performed using a STEM-EDS (JEOL, JEM-ARM200F). Figure 1 shows the STEM-EDS elemental analysis results for a methane oxidation catalyst (Ni-CVD-20h_850°C-Pd / Al2O3) that underwent a 20-hour Ni chemical vapor deposition process. Figure 2 shows the elemental analysis results for a methane oxidation catalyst (Ni-CVD-100h_850°C-Pd / Al2O3) that underwent a 100-hour Ni chemical vapor deposition process. As is clear from Figures 1 and 2, the Ni concentration was low after the 20-hour chemical vapor deposition process, making it difficult to determine the location of Ni support. However, in the 100-hour process, Ni selectively adhered to the peaks, which are the boundaries of the crystal planes of the Pd particles (which have a shape similar to a regular octahedron).

[0050] [Preparation of methane oxidation catalyst by impregnation method (comparative example)] A 10 wt% nickel(II) acetate solution was prepared by dissolving it in pure water. 300 mg of 850°C-Pd / Al2O3 was added to the solution, stirred, dried at 80°C, and then calcined in an electric furnace at 550°C for 1 hour to prepare a Ni-attached methane oxidation catalyst. Two types of methane oxidation catalysts were prepared by adjusting the amount of 10 wt% nickel(II) acetate solution added.

[0051] The two impregnated methane oxidation catalysts obtained as described above were also analyzed by ICP in the same manner as above. The Ni loadings were calculated to be 1300 wtppm and 17400 wtppm, respectively. The Ni / Pd ratios were also calculated to be 0.084 and 1.151, respectively. The methane oxidation catalyst with a Ni loading of 1300 wtppm is referred to as "Ni1300_850°C-Pd / Al2O3," and the methane oxidation catalyst with a Ni loading of 17400 wtppm is referred to as "Ni17400_850°C-Pd / Al2O3." In addition, STEM-EDS elemental analysis was also performed on a methane oxidation catalyst (Ni1300_Pd / Al2O3) with 1300 ppm of Ni attached by impregnation and a methane oxidation catalyst (Ni17400_850℃-Pd / Al2O3) with 17400 ppm of Ni attached by impregnation. Figure 3 shows the results of STEM-EDS elemental analysis of Ni1300_Pd / Al2O3. Figure 4 shows the results of STEM-EDS elemental analysis of Ni17400_Pd / Al2O3. As is clear from Figures 3 and 4, Ni is not only uniformly attached to the entire Pd particle, but also to the Al2O3 support.

[0052] [Catalyst performance evaluation] A gas simulating exhaust gas under lean conditions consisting of 0.1% methane, 2.0% water, 8.0% oxygen, and the remainder helium (He) was passed through 50 mg of each methane oxidation catalyst. The reaction tube was heated to 600 °C at a heating rate of 10 °C / min. The gas passing through the reaction tube was analyzed using Fourier transform infrared spectroscopy (FT-IR) to evaluate the degree of methane decomposition. Note that T20 refers to the temperature at which the methane decomposition rate reaches 20%. The lower these values, the better the catalyst's methane decomposition properties at low temperatures. The T20 for "850 °C-Pd / Al2O3" was 348 °C. The results of the evaluation of the increase or decrease in T20 relative to this 348 °C are shown in Table 2 below.

[0053] [Table 2]

[0054] As is clear from the evaluation results in Table 2, the methane oxidation catalysts (Examples 1 and 2) prepared by the manufacturing method of the present invention have catalytic performance equivalent to or superior to that of methane oxidation catalysts prepared by conventional impregnation methods (Comparative Examples 1 and 2), despite using a fractional to several tenths of the amount of Ni. Furthermore, the methane oxidation catalyst (Ni-CVD-100h_850°C-Pd / Al2O3) of the Reference Example, which underwent a long chemical vapor deposition process, had excessive Ni deposited at the boundaries of the crystal planes of the Pd particles, as shown in Figures 1 and 2. This resulted in reduced catalytic reactivity compared to the methane oxidation catalyst (Ni-CVD-1h_850°C-Pd / Al2O3) of Example 1, which was prepared using a chemical vapor deposition process for 1 hour, and the methane oxidation catalyst (Ni-CVD-1h_850°C-Pd / Al2O3) of Example 2, which was prepared using a chemical vapor deposition process for 6 hours.

[0055] In addition, a methane oxidation catalyst (Ni-CVD-1h-AS-Pd / Al2O3) was prepared in the same manner as in Example 1, except that heat treatment was not performed, and a methane oxidation catalyst (Ni-CVD-100h-AS-Pd / Al2O3) was prepared in the same manner as in Reference Example. For the methane oxidation catalyst prepared as described above without heat treatment, the Ni / Pd molar ratio was measured and the Ni content was calculated in the same manner as in Example 1. The measurement results are shown in Table 3 below. Figure 5 shows the results of STEM-EDS elemental analysis of the methane oxidation catalyst (Ni-CVD-100h-Pd / Al2O3) prepared without heat treatment.

[0056] [Table 3]

[0057] Furthermore, three types of methane oxidation catalysts were prepared in the same manner as in Comparative Example 1, except that the amount of 10 wt% nickel(II) acetate aqueous solution added was changed and the methane oxidation catalyst used was changed to AS-Pd / Al2O3. These three impregnation methane oxidation catalysts were also subjected to ICP analysis in the same manner as above, and the Ni deposition amounts were calculated to be 4100 wtppm, 16000 wtppm, and 66000 wtppm, respectively. The Ni / Pd ratios were also calculated to be 0.248, 0.967, and 3.989, respectively. These three methane oxidation catalysts are referred to as "Ni4100_AS-Pd / Al2O3," "Ni16000_AS-Pd / Al2O3," and "Ni66000_AS-Pd / Al2O3," respectively.

[0058] Furthermore, the catalytic performance of a methane oxidation catalyst prepared without heat treatment of "AS-Pd / Al2O3" and the three types of methane oxidation catalysts obtained by the impregnation method were evaluated in the same manner as above. The T20 of "AS-Pd / Al2O3" was 316°C, and the evaluation results of the increase or decrease in T20 based on this 316°C are shown in Table 4 below.

[0059] [Table 4]

[0060] As is clear from the evaluation results in Table 4, the methane oxidation catalysts prepared without heat treatment (Examples 3 and 4) also had catalytic performance equivalent to or superior to that of the methane oxidation catalysts to which Ni was attached by the conventional impregnation method (Comparative Examples 3 to 5), despite the amount of Ni used being several tenths to several tenths.

[0061] That is, according to the present invention, it is possible to attach a necessary amount of dissimilar metal to precious metal particles, which can contribute to suppressing the production of substances inactive in methane oxidation, while maintaining catalytic performance, compared to when dissimilar metals are attached by conventional impregnation methods.

Claims

1. Noble metal particles having heteroatoms attached to their surfaces; a support on which the noble metal particles are supported; Including, A methane oxidation catalyst, wherein the molar ratio of the heteroatom to the noble metal is 0.001 to 0.020 on a mass basis.

2. 2. The methane oxidation catalyst according to claim 1, wherein the noble metal is at least one selected from the group consisting of palladium, platinum, and rhodium.

3. 2. The methane oxidation catalyst according to claim 1, wherein the heteroatom is at least one selected from the group consisting of nickel, iron, vanadium, molybdenum, tungsten, manganese, technetium, and rhenium.

4. 2. The methane oxidation catalyst according to claim 1, which is used for purifying exhaust gas from an internal combustion engine.

5. A step of preparing a precious metal catalyst in which precious metal particles are supported on a support; a heterometal deposition step of depositing heteroatoms on the noble metal particles by chemical vapor deposition; A method for producing a methane oxidation catalyst, comprising:

6. The method according to claim 5 , wherein the chemical vapor deposition method includes contacting a gas of a carbonyl compound containing a different metal.

7. The method of claim 5, wherein the chemical vapor deposition is carried out at a temperature of 220 to 260°C.

8. The method according to claim 5 , further comprising a heat treatment step after the step of preparing the noble metal catalyst.

9. The manufacturing method according to claim 8 , wherein the heat treatment step is performed at a temperature of 900° C. or less.

10. The method according to claim 5 , further comprising a reduction treatment step after the noble metal catalyst preparation step.

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