Palladium-platinum bimetallic catalyst as well as preparation method and application thereof

Abstract

The invention discloses a palladium-platinum bimetallic catalyst and a preparation method thereof.The catalyst comprises precious metal palladium, platinum and a supporting material thereof, the molar ratio of the precious metal palladium to the platinum is 1: 1-1: 5, the supporting material is commercial modified aluminum oxide with the large surface area, and the key feature of the catalyst is that most of the precious metal palladium is dispersed on the surface of the catalyst in an oxidation state; specifically, the proportion of oxidation-state palladium in surface palladium is larger than or equal to 50%. Through control of the preparation method, generation of active species and effective dispersion of precious metal nanoparticles on the surface of the catalyst can be stimulated and promoted, the preparation process is controllable, and the preparation method is easy to implement in the industrial production process. The catalyst disclosed by the invention can tolerate a water-containing atmosphere and a high-temperature environment, is suitable for a powdery or granular or honeycomb integral catalyst application scene, and can be applied to tail gas post-treatment of natural gas automobiles, gasoline cars and hybrid electric vehicles and tail gas purification of combustors taking natural gas, coal gas and liquefied petroleum gas as fuels.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and relates to a palladium-platinum bimetallic catalyst and a preparation method thereof, in particular to a supported palladium-platinum bimetallic catalyst for wet methane oxidation and a preparation method thereof. Background Art

[0002] Due to its rich reserves, low price and mature technology, natural gas is the most potential alternative fuel in the transportation field.

[0003] Compared with gasoline and diesel vehicles, natural gas vehicles can effectively reduce the emissions of toxic and harmful substances such as CO2 and NO x , PM, etc., but inevitably emit a certain concentration of unburned CH4. As the second largest greenhouse gas, the warming potential per unit volume of CH4 is 21 times that of CO2, and its residence time in the atmosphere is generally 10 years. Its emission will make the already serious global warming situation even more severe. Therefore, the emission control of CH4 in the exhaust gas of natural gas vehicles is one of the most important technical challenges in the popularization and utilization of natural gas.

[0004] The catalytic oxidation method is the most effective way for the "end treatment" of methane tail gas in natural gas vehicles. Noble metal catalysts are the most active catalytic oxidation materials for CH4, with high low-temperature catalytic activity and good sulfur poisoning resistance. Among noble metal catalysts, the research on Pd catalysts is the most extensive. Since it is generally believed that the CH4 oxidation reaction catalyzed by Pd follows the Mars-van Krevelen mechanism, PdO is considered to be the active phase for CH4 low-temperature oxidation.

[0005] The literature JACS (2013) 15425-15442 reported the activation paths of C-H bonds in CH4 molecules on Pd clusters, Pd clusters saturated with oxygen adsorption, and PdO clusters, and found that on the PdO surface, the C-H bond undergoes heterolytic cleavage through σ-bond substitution to form a stable (H3C δ+ ···Pd ox ···H δ+ ···O ox ) four-center transition state. Since the (H3C δ+ ···Pd ox ···H δ+ ···O ox ) four-center transition state increases with the increase of oxygen chemical potential, and the increase amplitude of the CH4 oxidation reaction rate is greater. Therefore, it can be determined that PdO clusters are more conducive to the dissociation and activation of CH4 molecules.

[0006] ACS Cat. (2020) 1381 - 1387 reported the morphological structures of Pd on different metal oxide supports such as θ - Al2O3, γ - Al2O3, ZrO2, CeO2, MgO, La2O3, TiO2, SnO2, and Nb2O5. It was found that Pd on MgO and La2O3 supports presented an oxidized state (PdO), Pd on θ - Al2O3, γ - Al2O3, ZrO2, and CeO2 supports presented a Pd core - PdO shell structure, and Pd on TiO2, SnO2, and Nb2O5 supports presented a metallic state (Pd).

[0007] CN113042038A discloses a palladium - platinum catalyst, its preparation method and application, and studied the influence of the impregnation order on the stability of PdO and the wet methane conversion activity. It was found that platinum was loaded first to prepare a platinum - based catalyst, and then palladium was impregnated and loaded on the surface of the platinum - based catalyst. The active substance PdO on the prepared palladium - platinum catalyst surface was more stable and had higher wet methane conversion activity, but the methane conversion rate was still less than 50% in the temperature drop range of 700 - 500 °C.

[0008] It can be seen that how to control the oxidized state of Pd on a large - specific - surface - area, commercial support to improve the wet methane oxidation performance of the catalyst is still an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to overcome the above - mentioned deficiencies and provide a supported palladium - platinum bimetallic catalyst for wet methane oxidation and its preparation method. In this catalyst, the noble metal palladium is mostly in an oxidized state, and the efficient utilization of the active metal can be achieved, overcoming the problems of low methane catalytic activity of the existing methane oxidation catalysts for motor vehicles in a water - containing atmosphere and poor hydrothermal stability of the catalysts.

[0010] The palladium - platinum bimetallic catalyst of the present invention is specifically a supported palladium - platinum bimetallic catalyst, including the noble metals palladium, platinum and their supporting materials.

[0011] The molar ratio of the noble metals palladium and platinum is 1:1 - 1:5. The noble metal palladium is dispersed on the surface of the catalyst in the vast majority of oxidized states, and the proportion of its oxidized palladium is ≥50%.

[0012] The supporting material is a large - surface - area, commercial modified alumina.

[0013] The catalyst of the present invention can withstand a water - containing atmosphere and a high - temperature environment, is suitable for the application scenarios of powdery, granular or honeycomb monolithic catalysts, and can be applied to the post - treatment of the exhaust gas of natural gas vehicles, gasoline vehicles, hybrid vehicles and the purification of the exhaust gas of burners fueled by natural gas, coal gas, and liquefied petroleum gas.

[0014] The present invention also provides a method for preparing a palladium-platinum bimetallic catalyst, which specifically includes the following steps:

[0015] (1) Dissolve a calculated amount of palladium salt and platinum salt and a certain amount of protective agent in ethylene glycol under vigorous stirring;

[0016] (2) Slowly heat the protective agent-noble metal-ethylene glycol mixture to 120-150 °C, and then introduce a certain amount of reducing agent into the synthesis solution under vigorous stirring;

[0017] (3) Slowly heat the mixed solution added with the reducing agent to the condensation reflux temperature of ethylene glycol, and then maintain it at this temperature for 2-4 h to completely reduce the noble metal;

[0018] (4) Precipitate the colloidal solution containing noble metal palladium and platinum nanoparticles obtained in step (3) with acetone, and deposit it on a calculated amount of modified alumina support by wet impregnation. After standing overnight at room temperature, carry out suction filtration and washing, and then transfer the obtained cake-like substance to an oven at 100-120 °C to dry overnight and calcine it in a muffle furnace at 400-800 °C for 3-6 h.

[0019] Further, the palladium salt is palladium nitrate, palladium chloride, palladium acetate or palladium acetylacetonate, and the platinum salt is platinum nitrate, platinum chloride, platinum acetate, chloroplatinic acid or platinum acetylacetonate.

[0020] Further, the protective agent is polyvinylpyrrolidone or polyvinyl alcohol.

[0021] Further, the reducing agent is sodium borohydride, sodium citrate or lithium aluminum hydride.

[0022] Advantages of the present invention:

[0023] (1) For the palladium-platinum supported bimetallic catalyst of the present invention, most of the noble metal palladium is in the oxidized state, and it can achieve the efficient utilization of the active metal, can tolerate the water-containing atmosphere and high-temperature environment, is suitable for the application scenarios of powdery, granular or honeycomb monolithic catalysts, and can be applied to the exhaust gas aftertreatment of natural gas vehicles, gasoline vehicles, hybrid vehicles and the exhaust gas purification of burners fueled by natural gas, coal gas and liquefied petroleum gas;

[0024] (2) For the preparation method of the present invention, through the preparation and process control, it can stimulate and promote the generation of the active species PdO and the effective dispersion of noble metal nanoparticles on the surface of the modified alumina. The preparation process of this method is controllable and easy to be realized in the industrial production process;

[0025] (3) Compared with the preparation method disclosed in CN113042038A, the present invention uses palladium-platinum bimetallic nanoparticles supported on the surface of commercially modified alumina with a large surface area. The interaction between palladium and platinum is stronger, which is more conducive to the formation and stability control of the wet methane oxidation active species PdO and the efficient oxidation of wet methane. Description of the Drawings

[0026] Figure 1 XPS spectra of the fresh samples of Comparative Example 1, Comparative Example 2, Example 1, Example 2, and Example 3.

[0027] Figure 2 XPS spectra of the hydrothermally aged samples of Comparative Example 1, Comparative Example 2, Example 1, Example 2, and Example 3. Detailed Embodiments

[0028] The present invention will be further described below in conjunction with comparative examples and examples, but it does not limit the present invention.

[0029] Comparative Example 1

[0030] A preparation method of a Pd catalyst includes:

[0031] 0.669 g of PdCl2 (Pd content 59.8%) dissolved in dilute hydrochloric acid was wet-impregnated onto 39.6 g of La-Al2O3 powder (specific surface area 127 m 2 / g). After standing overnight, suction filtration was carried out, and it was washed with a mixed solution of deionized water and absolute ethanol until no more precipitate was formed when AgNO3 solution was added to the filtrate. Then it was transferred to an oven at 100 °C and dried overnight and calcined in a muffle furnace at 600 °C for 3 h. Then the above two samples were mechanically mixed in a mortar and ground evenly to obtain a fresh Pd catalyst.

[0032] Part of the fresh sample was hydrothermally aged for 10 h in a wet oxidation atmosphere (10% O2, 10% H2O, balanced with N2, total flow rate 2 L / min) at 850 °C to obtain a hydrothermally aged Pd catalyst.

[0033] Comparative Example 2

[0034] A preparation method of a Pd-Pt bimetallic catalyst includes:

[0035] 0.234 g of PdCl2 (Pd content 59.8%) dissolved in dilute hydrochloric acid was impregnated onto 19.8 g of La-Al2O3 powder (specific surface area 127 m 2 / g), and 0.684 g of H2PtCl6·6H2O (Pt content 38%) was impregnated onto another 19.8 g of La-Al2O3 powder (specific surface area 127 m 2 / g), stir each evenly, let stand overnight, then perform suction filtration, wash with a mixed solution of deionized water and absolute ethanol until no precipitate is formed when adding AgNO₃ solution to the filtrate, then transfer it to an oven at 100 °C to dry overnight and calcine in a muffle furnace at 600 °C for 3 h, and then mechanically mix the above two samples in a mortar and grind evenly to obtain a fresh Pd-Pt bimetallic catalyst.

[0036] Hydrothermally age a portion of the fresh sample in a wet oxidation atmosphere (10% O₂, 10% H₂O, balanced with N₂, total flow rate 2 L / min) at 850 °C for 10 h to obtain a hydrothermally aged Pd-Pt bimetallic catalyst.

[0037] Example 1

[0038] A preparation method of a palladium-platinum bimetallic catalyst, comprising:

[0039] Dissolve 0.234 g of PdCl₂, 0.684 g of H₂PtCl₆·6H₂O and 8.375 g of polyvinylpyrrolidone (PVP) in a 500 mL three-necked round-bottom flask containing 200 mL of ethylene glycol under vigorous stirring. Slowly heat the PVP-noble metal-ethylene glycol mixture to 140 °C, and then introduce 3.567 g of NaBH₄ into the synthesis solution under vigorous stirring. Then heat the reaction system to the condensation reflux temperature of ethylene glycol and maintain it at this temperature for 2 h for complete reduction. The PVP-stabilized Pd-Pt nanoparticles prepared in ethylene glycol are precipitated with acetone and deposited on 39.6 g of La-Al₂O₃ powder (specific surface area 127 m 2 / g), let stand overnight, then perform suction filtration and washing, and then transfer it to an oven at 100 °C to dry overnight and calcine in a muffle furnace at 600 °C for 3 h to obtain a fresh Pd-Pt bimetallic catalyst.

[0040] Hydrothermally age a portion of the fresh sample in a wet oxidation atmosphere (10% O₂, 10% H₂O, balanced with N₂, total flow rate 2 L / min) at 850 °C for 10 h to obtain a hydrothermally aged Pd-Pt bimetallic catalyst.

[0041] Example 2

[0042] A preparation method of a palladium-platinum bimetallic catalyst, comprising:

[0043] 0.415 g of PdCl2, 0.399 g of H2PtCl6·6H2O and 8.375 g of polyvinylpyrrolidone (PVP) were dissolved in a 500 mL three-necked round-bottom flask containing 200 mL of ethylene glycol under vigorous stirring. The PVP-noble metal-ethylene glycol mixture was slowly heated to 140 °C, and then 3.567 g of NaBH4 was introduced into the synthesis solution under vigorous stirring. Then the reaction system was heated to the condensation reflux temperature of ethylene glycol and maintained at this temperature for 2 h for complete reduction. The PVP-stabilized Pd-Pt nanoparticles prepared in ethylene glycol were precipitated with acetone and deposited on 39.6 g of La-Al2O3 powder (specific surface area 127 m 2 / g) by wet impregnation. After standing overnight, suction filtration and washing were carried out, and then it was transferred to an oven at 100 °C and dried overnight and calcined in a muffle furnace at 600 °C for 3 h to obtain a fresh Pd-Pt bimetallic catalyst.

[0044] Part of the fresh sample was hydrothermally aged for 10 h in a wet oxidation atmosphere (10% O2, 10% H2O, N2 balance, total flow rate 2 L / min) at 850 °C to obtain a hydrothermally aged Pd-Pt bimetallic catalyst.

[0045] Example 3

[0046] A preparation method of a palladium-platinum bimetallic catalyst, comprising:

[0047] 0.490 g of PdCl2, 0.282 g of H2PtCl6·6H2O and 8.375 g of polyvinylpyrrolidone (PVP) were dissolved in a 500 mL three-necked round-bottom flask containing 200 mL of ethylene glycol under stirring. The PVP-noble metal-ethylene glycol mixture was slowly heated to 140 °C, and then 3.567 g of NaBH4 was introduced into the synthesis solution under vigorous stirring. Then the reaction system was heated to the condensation reflux temperature of ethylene glycol and maintained at this temperature for 2 h for complete reduction. The PVP-stabilized Pd-Pt nanoparticles prepared in ethylene glycol were precipitated with acetone and deposited on 39.6 g of La-Al2O3 powder (specific surface area 127 m 2 / g), and after standing overnight, suction filtration and washing were carried out, and then it was transferred to an oven at 100 °C and dried overnight and calcined in a muffle furnace at 600 °C for 3 h to obtain a fresh Pd-Pt bimetallic catalyst.

[0048] Part of the fresh sample was hydrothermally aged for 10 h in a wet oxidation atmosphere (10% O2, 10% H2O, N2 balance, total flow rate 2 L / min) at 850 °C to obtain a hydrothermally aged Pd-Pt bimetallic catalyst.

[0049] Surface elemental and valence analysis was performed on the catalyst samples of Comparative Examples 1 and 2 and Examples 1-3, including:

[0050] The valence information of the metal element (Pd) on the catalyst surface was obtained by the K-Alpha type X-ray photoelectron spectrometer (XPS) of Thermo Fisher Scientific, USA, with a monochromatized AlKα source (energy: 1486.6 eV, tube voltage: 12 kV, tube current: 6 mA), at a system vacuum of approximately 2×10 + mbar. The binding energy of the test element was corrected with reference to the C1s (284.8 eV) of surface contamination. For the quantitative distribution of the valence state of specific Pd species, the Pd 3d peak was first subjected to peak fitting during the XPS data processing. The relative percentage content of Pd in the catalyst was calculated from the ratio of the fitted peak areas Pd -7 / (Pd 2+ +Pd 2+ ). 2+ +Pd 0 )

[0051] The activities of the catalyst samples of Comparative Examples 1 and 2 and Examples 1-3 were evaluated, including:

[0052] The above fresh and hydrothermally aged catalysts were taken for activity evaluation in a fixed-bed reactor with a reaction gas composition of: methane: 1000 ppm, oxygen: 10%, water: 8%, nitrogen: balance gas, and space velocity: 80000 h-1.

[0053] The results of the surface valence state distribution and activity evaluation of Pd elements in Comparative Examples 1 and 2 and Examples 1-3 are shown in Table 1.

[0054] Table 1

[0055]

[0056]

[0057] As can be seen from Table 1 and Figure 1 , Figure 2 compared with the single Pd catalyst provided in Comparative Example 1 and the Pd-Pt bimetallic catalyst provided in Comparative Example 2, the Pd-Pt bimetallic catalysts provided in Examples 1, 2, and 3 have a richer surface Pd 2+ species on the surface, and the proportion of PdO is >50% in both fresh and aged states, showing more excellent wet methane oxidation activity and stability.

[0058] It should be noted that the above specific embodiments of the present invention are only used for illustrative explanation or interpretation of the principles of the present invention, and do not constitute a limitation on the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention shall be included within the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all variations and modifications that fall within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A preparation method of a palladium-platinum bimetallic catalyst, characterized in that, It includes the following steps: (1) Dissolve a calculated amount of palladium salt and platinum salt and a certain amount of protective agent in ethylene glycol under vigorous stirring; (2) Slowly heat the protective agent-noble metal-ethylene glycol mixture to 120-150 °C, and then introduce a certain amount of reducing agent into the synthesis solution under vigorous stirring; (3) Slowly heat the mixed solution added with the reducing agent to the condensation reflux temperature of ethylene glycol and keep it to completely reduce the noble metal, obtaining a colloidal solution containing noble metal palladium and platinum nanoparticles; (4) Precipitate the colloidal solution with acetone, deposit it on a calculated amount of modified alumina support by wet impregnation, let it stand overnight at room temperature, then carry out suction filtration and washing, and then transfer the obtained cake-like substance to an oven for drying and calcination to obtain the palladium-platinum bimetallic catalyst.

2. The preparation method of a palladium-platinum bimetallic catalyst according to claim 1, characterized in that, The palladium salt is palladium nitrate, palladium chloride, palladium acetate or palladium acetylacetonate.

3. The preparation method of a palladium-platinum bimetallic catalyst according to claim 1, characterized in that, The platinum salt is platinum nitrate, platinum chloride, platinum acetate, chloroplatinic acid or platinum acetylacetonate.

4. The preparation method of a palladium-platinum bimetallic catalyst according to claim 1, characterized in that, The protective agent is polyvinylpyrrolidone or polyvinyl alcohol.

5. The preparation method of a palladium-platinum bimetallic catalyst according to claim 1, characterized in that, The reducing agent is sodium borohydride, sodium citrate or lithium aluminum hydride.

6. The preparation method of a palladium-platinum bimetallic catalyst according to claim 1, characterized in that, In step (3), keep it for 2-4 h to completely reduce the noble metal.

7. The preparation method of a palladium-platinum bimetallic catalyst according to claim 1, characterized in that, In step (4), the oven temperature is 100-120 °C, then dry it overnight and calcine it in a muffle furnace at 400-800 °C for 3-6 h.

8. A palladium-platinum bimetallic catalyst prepared by the preparation method of the palladium-platinum bimetallic catalyst according to any one of claims 1-7, characterized in that, In the palladium of the catalyst, the proportion of PdO is greater than 50%, and the PdO is dispersed on the surface of the catalyst.

9. A palladium-platinum bimetallic catalyst according to claim 8, characterized in that, The molar ratio of the noble metals palladium and platinum is 1:1-1:5, and the supported material is a large-surface-area, commercial modified alumina.

10. The palladium-platinum bimetallic catalyst according to claim 8 or 9 is applied to the post-treatment of the exhaust gas of natural gas vehicles, gasoline vehicles, and hybrid vehicles and the purification of the exhaust gas of burners fueled by natural gas, coal gas, and liquefied petroleum gas in a water-containing atmosphere and a high-temperature environment.

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