Application of a high temperature aged palladium-based catalyst in methane combustion

Abstract

The present invention relates to a method for preparing a Pd-based catalyst that has been pretreated by high-temperature atmosphere aging and uses a nano-scale commercial γ-Al2O3 material as a carrier, and its catalytic application in a methane catalytic combustion process. The preparation method includes the loading of metal Pd on an Al2O3 carrier and a high-temperature atmosphere aging process of the catalyst. Compared with catalysts that have not been aged in a high-temperature atmosphere, the Pd / Al2O3 catalyst obtained in the present invention has significantly higher catalytic methane combustion activity at the same reaction temperature. Among them, the related species of metal Pd are the active centers of the reaction; Al2O3 plays a good role in dispersing the active centers and improving the reaction activity. The preparation method of the catalyst in the present invention has a simple process, readily available raw materials, is suitable for large-scale catalyst production, and can improve the catalyst activity on the basis of the existing catalyst preparation process.

Description

Technical Field

[0001] The present invention belongs to the field of industrial application, and relates to the preparation of a catalyst and the study of its activity on catalytic combustion of methane, and specifically to the preparation process of an Al2O3 carrier-supported nano-palladium particle catalyst pretreated by high-temperature atmosphere and its application in the catalytic combustion reaction of methane under lean combustion conditions. Background Art

[0002] With the rapid growth of energy demand and the increasingly serious environmental pollution problem, seeking clean energy to replace coal, oil and other highly polluting fossil energy has become the main development direction of the energy field. Although wind energy, photovoltaics, nuclear power, hydrogen energy, etc. have become hot topics in recent years, they are subject to practical difficulties such as unstable energy supply, large initial equipment investment, high potential safety risks, and preparation costs and storage and transportation efficiency that cannot meet actual needs. Fossil fuels will still be the main component of energy supply for a long time in the future. Among the three major fossil fuels: coal, oil, and natural gas, natural gas has become the focus of research and application in the energy field in recent decades because of its wide distribution, low price, multiple storage and transportation methods, and can be used directly as fuel and combined with the natural gas industry for deep utilization, and low pollution and low emissions during use.

[0003] As the main component of natural gas, methane accounts for more than 90% of the content in natural gas. It can not only directly provide energy for production and life through power generation, fire, driving natural gas vehicles and other transportation vehicles, but also become an important C1 raw material for the production of bulk chemicals through chemical processes such as direct conversion, oxidative coupling, anaerobic dehydrogenation, steam reforming and dry gas reforming. In addition, methane is not only present in natural gas. With the advancement of the "shale revolution" and "combustible ice" mining technology in the past 20 years, as well as the implementation of biogas technology from biomass, the scope and intensity of methane use as an important energy and chemical raw material will continue to expand. However, with the rapid increase in usage, methane leaked into the atmosphere through various channels has become a restricted object of emission regulations in many countries and regions due to its greenhouse effect potential 23 times that of CO2. Among the methane emission treatment options, catalytic combustion is considered to be the most practical and feasible treatment method. Although CO2 produced by catalytic combustion is still the main greenhouse gas, with the restoration of the natural ecological environment and the development of CO2 capture and utilization technology, the application prospects of methane catalytic combustion are still very broad.

[0004] Since the 1960s, Pd-based catalysts have been widely regarded as the most effective catalysts for methane combustion. Today, Pd-based catalysts commonly used in the exhaust treatment process of natural gas fueled vehicles often use alumina combined with a cerium-zirconium solid solution with strong oxygen storage and release capacity and high temperature resistance as a carrier, and precious metals such as Pt and Rh are added to improve the catalytic combustion performance of Pd-based catalysts under different working conditions. However, the actual working environment of the engine determines that the catalyst must undergo severe temperature changes, usually ranging from 200-1000°C; at the same time, in order to meet increasingly stringent emission laws and regulations and improve the thermal efficiency of the engine, the catalyst must achieve methane catalytic combustion at the lowest possible temperature; in addition, the engine exhaust often contains a large amount of water produced by combustion and a small amount of SOx from natural gas additives and engine lubricants. The catalyst must be able to withstand the activity reduction caused by water and SOx under high temperature conditions. With the deepening of research, more and more researchers have controllably changed the structure of the Pd active center and its interaction with the carrier through advanced preparation methods and functional materials, in order to achieve better thermal stability, low temperature activity, and resistance to water and SOx poisoning of the catalyst.

[0005] Based on the above research status, it is of great significance to seek a relatively simple method to prepare a highly active methane combustion catalyst and apply it to the methane catalytic combustion reaction. This patent aims to invent a highly active methane combustion catalyst containing Pd. Summary of the invention

[0006] An object of the present invention is to provide a palladium-based catalyst for methane catalytic combustion reaction, which catalyst has the characteristics of high activity for methane catalytic combustion reaction.

[0007] Another object of the present invention is to provide a high temperature atmosphere pretreatment method for preparing the catalyst of the present invention, which improves the catalyst activity without increasing the content of the metal Pd active component of the catalyst.

[0008] The preparation method of the palladium-based catalyst of the present invention is specifically achieved by the following steps:

[0009] 1. Carrier heat treatment: The carrier γ-phase Al2O3 with a high specific surface area is heated from room temperature to 600-900°C in still air and calcined for 1-5 hours to ensure that the carrier structure does not change during the subsequent preparation process.

[0010] 2. Loading of precious metal palladium: The calcined carrier γ-phase Al2O3 with a high specific surface area is immersed in a palladium salt precursor solution of a certain concentration, the palladium salt content depends on the metal palladium loading, and the mass percentage of palladium in the catalyst is 0.5-2wt%; rapidly stir for 5-30 minutes, let stand in room temperature air for 2-24 hours, and then place in a 30-120°C oven to dry for 2-24 hours; after fully grinding the above-mentioned dried solid for 10-30 minutes, calcine at 350-600°C in still air for 1-5 hours, and slowly cool to room temperature to obtain a fresh catalyst that has not been pretreated in a high-temperature atmosphere.

[0011] 3. High-temperature atmosphere pretreatment: The fresh catalyst prepared above is aged in an atmosphere with different methane and oxygen contents, wherein the volume percentage of methane is 0.1-4%, the volume percentage of oxygen is 0.1-20%, and the rest is inert gas He, Ar or N2, the aging temperature is 500-850°C, and the aging time is 0.5-12h.

[0012] The reaction conditions of methane catalytic combustion are as follows: raw gas volume ratio: methane volume percentage 0.2-2%, oxygen volume percentage 0.5-5%, water volume percentage 0-15%, carbon dioxide volume percentage 0-5%, and the rest are inert gases He, Ar or N2; reaction temperature 150-850°C, methane conversion rate 0-100%, and total reaction gas pressure 0.1-0.2MPa.

[0013] The catalyst provided in the present invention has mild preparation conditions and simple process and is suitable for large-scale production.

[0014] The catalyst high-temperature aging atmosphere pretreatment process provided in the present invention is simple, has good compatibility with existing industrial catalyst pretreatment equipment, and is suitable for large-scale catalyst preparation processes.

[0015] The Pd / Al2O3 catalyst obtained in the present invention has significantly higher catalytic methane combustion activity at the same reaction temperature than the catalyst that has not been aged in a high temperature atmosphere. Among them, the related species of metal Pd are the active centers of the reaction; Al2O3 plays a good role in dispersing the active centers and improving the reaction activity. The preparation method of the catalyst in the present invention has a simple process, the raw materials are easily available, it is suitable for large-scale catalyst production, and the catalyst activity can be improved on the basis of the existing catalyst preparation process.

[0016] The present invention is described in detail below by specific embodiments. It should be noted that these embodiments are only examples and do not constitute any limitation to the essence and scope of the present invention. The present invention can be implemented as long as the conditions described in the content of the present invention are met. Therefore, the protection scope of the present invention shall be subject to the claims of the application. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The catalytic methane combustion performance of Example 2 and Comparative Example 4 in the temperature range of 200-500°C.

[0018] Figure 2 This is a transmission electron microscope photograph of the nano-Al2O3 carrier in Example 1.

[0019] Figure 3 This is a transmission electron microscope photograph of the thin Al2O3 carrier in comparative example 1.

[0020] Figure 4 This is a scanning electron microscope photograph of the needle-shaped Al2O3 carrier of Comparative Example 2.

[0021] Figure 5 This is a scanning electron microscope photograph of the bulk Al2O3 carrier of comparative example 3.

[0022] Figure 6 The catalytic methane combustion performance of Example 2 and Comparative Examples 5-7 in the temperature range of 200-500°C.

[0023] Figure 7 The XRD diffraction spectra of Examples 1-3 and Comparative Examples 8-9 are shown.

[0024] Figure 8 The catalytic methane combustion performance of Examples 2-3 and Comparative Examples 8-9 in the temperature range of 200-500°C.

[0025] Fig. 9 The catalytic methane combustion performance of Examples 2-3 and Comparative Examples 8-9 within the catalytic reaction kinetic range of 300°C is shown.

[0026] Fig.10 This is the spherical aberration electron microscope photo of Example 3.

[0027] Fig.11 The catalytic methane combustion performance of Examples 3-5 within the catalytic reaction kinetic range of 300°C.

[0028] Fig.12 The catalytic methane combustion performance of Examples 3, 6-8 within the catalytic reaction kinetic range of 300°C. DETAILED DESCRIPTION

[0029] The technical solution of the present invention includes but is not limited to the following specific implementation methods:

[0030] Example 1.

[0031] Nano-Al2O3 carrier after high temperature heat treatment:

[0032] Accurately weigh 2.00g of commercial nano-scale γ-phase Al2O3, put it into a crucible, heat it to 850℃ in a muffle furnace at a heating rate of 2℃ / min, and naturally cool it to room temperature after roasting for 2h to obtain a high-temperature heat-treated nano-Al2O3 carrier (referred to as nano-Al2O3). The carrier composition is pure γ-phase Al2O3, and the carrier shape is nano-rod, 20-200nm long, and 5-25nm in diameter.

[0033] Example 2.

[0034] Preparation of fresh Pd / Al2O3 catalyst with nano-Al2O3 as carrier:

[0035] At room temperature (~25°C), take 0.106 mL (93.7 g Pd / L) Pd(NO3)2 solution is diluted to 10mL with water, and 1.00g of nano-Al2O3 heat-treated at high temperature in Example 1 is accurately weighed and added under stirring, and stirring is continued for 2h to allow Pd(NO3)2 to adsorb on the surface of the carrier. Heat the water bath to 80℃, and allow the water in the impregnation solution to evaporate naturally while stirring, and then put it into an 80℃ oven to dry in air for 12h. After drying, grind the sample and put it into a muffle furnace and heat it from room temperature (~25℃) to 500℃ at a heating rate of 2℃ / min and keep it for 4h, and then cool it naturally to room temperature (~25℃). That is, a fresh Pd / Al2O3 catalyst (referred to as Fresh) with nano-Al2O3 as the carrier is obtained.

[0036] Example 3.

[0037] Preparation of Pd / Al2O3 catalyst after aging in 0.5% methane and 2% oxygen atmosphere:

[0038] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are blocked with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pre-treated gas passes through the catalyst from top to bottom, and the gas composition is 0.5% (volume percentage) of methane, 2% (volume percentage) of oxygen, and He gas balance, and the total gas flow rate is 100 mL / min. Under the flow of pre-treated gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Then, it is cooled to 200°C at a cooling rate of 5°C / min, and finally cooled naturally to room temperature. The Pd / Al2O3 catalyst aged in 0.5% methane and 2% oxygen atmosphere (recorded as 0.5+2-aged) is obtained. The particle sizes of precious metal Pd and oxide PdO are 2-100nm, metal Pd is covered on the surface of PdO particles with a particle size of 10-100nm in the form of small particles with a particle size of 2-6nm, 20-60% of the outer surface of the PdO particles is covered with metal Pd, the mass ratio of metal Pd and oxide PdO is 1.5:1 to 2.5:1, the carrier composition is pure γ-phase Al2O3, and the carrier shape is nanorod-like, 20-200nm long and 5-25nm in diameter.

[0039] Example 4.

[0040] Preparation of Pd / Al2O3 catalyst after aging in 0.25% methane and 2% oxygen atmosphere:

[0041] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are blocked with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pre-treated gas passes through the catalyst from top to bottom, and the gas composition is 0.25% (volume percentage) of methane, 2% (volume percentage) of oxygen, and He gas balance, and the total gas flow rate is 100 mL / min. Under the flow of pre-treated gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Then, it is cooled to 200°C at a cooling rate of 5°C / min, and finally cooled naturally to room temperature. That is, the Pd / Al2O3 catalyst aged in 0.25% methane and 2% oxygen atmosphere (recorded as 0.25+2-aged) is obtained. The particle sizes of precious metal Pd and oxide PdO are 2-100nm, metal Pd is covered on the surface of PdO particles with a particle size of 10-100nm in the form of small particles with a particle size of 2-6nm, 20-60% of the outer surface of the PdO particles is covered with metal Pd, the mass ratio of metal Pd and oxide PdO is 1.5:1 to 2.5:1, the carrier composition is pure γ-phase Al2O3, and the carrier shape is nanorod-like, 20-200nm long and 5-25nm in diameter.

[0042] Example 5.

[0043] Preparation of Pd / Al2O3 catalyst after aging in 0.75% methane and 2% oxygen atmosphere:

[0044] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are blocked with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pretreated gas passes through the catalyst from top to bottom, and the gas composition is 0.75% (volume percentage) of methane, 2% (volume percentage) of oxygen, and He gas balance, and the total gas flow rate is 100 mL / min. Under the flow of pretreated gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Then, it is cooled to 200°C at a cooling rate of 5°C / min, and finally cooled naturally to room temperature. The Pd / Al2O3 catalyst aged in 0.75% methane and 2% oxygen atmosphere (recorded as 0.75+2-aged) is obtained. The particle sizes of precious metal Pd and oxide PdO are 2-100nm, metal Pd is covered on the surface of PdO particles with a particle size of 10-100nm in the form of small particles with a particle size of 2-6nm, 20-60% of the outer surface of the PdO particles is covered with metal Pd, the mass ratio of metal Pd and oxide PdO is 1.5:1 to 2.5:1, the carrier composition is pure γ-phase Al2O3, and the carrier shape is nanorod-like, 20-200nm long and 5-25nm in diameter.

[0045] Example 6.

[0046] Preparation of Pd / Al2O3 catalyst after aging in 0.5% methane, 1.5% oxygen atmosphere:

[0047] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are blocked with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pre-treated gas passes through the catalyst from top to bottom, and the gas composition is 0.5% (volume percentage) of methane, 1.5% (volume percentage) of oxygen, and He gas balance, and the total gas flow rate is 100 mL / min. Under the flow of pre-treated gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Then, it is cooled to 200°C at a cooling rate of 5°C / min, and finally cooled naturally to room temperature. That is, the Pd / Al2O3 catalyst aged in 0.5% methane and 1.5% oxygen atmosphere (recorded as 0.5+1.5-aged) is obtained. The particle sizes of precious metal Pd and oxide PdO are 2-100nm, metal Pd is covered on the surface of PdO particles with a particle size of 10-100nm in the form of small particles with a particle size of 2-6nm, 20-60% of the outer surface of the PdO particles is covered with metal Pd, the mass ratio of metal Pd and oxide PdO is 1.5:1 to 2.5:1, the carrier composition is pure γ-phase Al2O3, and the carrier shape is nanorod-like, 20-200nm long and 5-25nm in diameter.

[0048] Example 7.

[0049] Preparation of Pd / Al2O3 catalyst after aging in 0.5% methane and 3% oxygen atmosphere:

[0050] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are blocked with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pretreated gas passes through the catalyst from top to bottom, and the gas composition is 0.5% (volume percentage) of methane, 3% (volume percentage) of oxygen, and He gas balance, and the total gas flow rate is 100 mL / min. Under the flow of pretreated gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Then, it is cooled to 200°C at a cooling rate of 5°C / min, and finally cooled naturally to room temperature. The Pd / Al2O3 catalyst aged in 0.5% methane and 3% oxygen atmosphere (recorded as 0.5+3-aged) is obtained. The particle sizes of precious metal Pd and oxide PdO are 2-100nm, metal Pd is covered on the surface of PdO particles with a particle size of 10-100nm in the form of small particles with a particle size of 2-6nm, 20-60% of the outer surface of the PdO particles is covered with metal Pd, the mass ratio of metal Pd and oxide PdO is 1.5:1 to 2.5:1, the carrier composition is pure γ-phase Al2O3, and the carrier shape is nanorod-like, 20-200nm long and 5-25nm in diameter.

[0051] Example 8.

[0052] Preparation of Pd / Al2O3 catalyst after aging in 0.5% methane and 4% oxygen atmosphere:

[0053] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are blocked with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pretreated gas passes through the catalyst from top to bottom, and the gas composition is 0.5% (volume percentage) of methane, 4% (volume percentage) of oxygen, and He gas balance, and the total gas flow rate is 100 mL / min. Under the flow of pretreated gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Then, it is cooled to 200°C at a cooling rate of 5°C / min, and finally cooled naturally to room temperature. The Pd / Al2O3 catalyst aged in 0.5% methane and 4% oxygen atmosphere (recorded as 0.5+4-aged) is obtained. The particle sizes of precious metal Pd and oxide PdO are 2-100nm, metal Pd is covered on the surface of PdO particles with a particle size of 10-100nm in the form of small particles with a particle size of 2-6nm, 20-60% of the outer surface of the PdO particles is covered with metal Pd, the mass ratio of metal Pd and oxide PdO is 1.5:1 to 2.5:1, the carrier composition is pure γ-phase Al2O3, and the carrier shape is nanorod-like, 20-200nm long and 5-25nm in diameter.

[0054] Example 9.

[0055] Evaluation of Fresh Catalyst Activity in Example 2:

[0056] Catalytic reaction conditions: fixed bed microreactor, raw gas composition volume ratio of methane 0.5%, oxygen 2%, He gas balance, reaction pressure is atmospheric pressure, raw gas space velocity is 60000mL g -1 h -1 The reaction temperature ranges from 200 to 500°C. The reaction begins at 200°C, and the methane conversion rate increases with the increase of reaction temperature, and is completely converted at 450°C.

[0057] Example 10.

[0058] Activity evaluation of the 0.5+2-aged catalyst in Example 3: The same conditions as in Example 9, the reaction began to occur at 200°C, and the methane conversion rate increased with the increase of reaction temperature, and was completely converted at 450°C.

[0059] Example 11.

[0060] Evaluation of the activity of the Fresh catalyst in Example 2 under catalytic kinetic conditions:

[0061] Catalytic reaction conditions: fixed bed microreactor, raw gas composition volume ratio of methane 0.5%, oxygen 2%, He gas balance, reaction pressure is atmospheric pressure, raw gas space velocity is 3000000mL g -1 h -1 , reaction temperature 300℃, within 120min, the reaction rate is stable at 150μmol CH4 g Pd -1 s -1 .

[0062] Example 12.

[0063] Activity evaluation of the 0.5+2-aged catalyst in Example 3 under catalytic kinetic conditions: The same conditions as in Example 11, within 120 min, the reaction rate was stabilized at 340 μmol CH4 g Pd -1 s -1 .

[0064] Example 13.

[0065] Activity evaluation of the 0.25+2-aged catalyst in Example 4 under catalytic kinetic conditions: The same conditions as in Example 11, within 120 min, the reaction rate was stabilized at 250 μmol CH4 g Pd -1 s -1 .

[0066] Example 14.

[0067] Activity evaluation of the 0.75+2-aged catalyst in Example 5 under catalytic kinetic conditions: The same conditions as in Example 11, within 120 min, the reaction rate was stabilized at 335 μmol CH4 g Pd -1 s -1 .

[0068] Example 15.

[0069] Activity evaluation of the 0.5+1.5-aged catalyst in Example 6 under catalytic kinetic conditions: The same conditions as in Example 11, within 120 min, the reaction rate was stabilized at 375 μmol CH4 g Pd -1 s -1 .

[0070] Example 16.

[0071] Activity evaluation of the 0.5+3-aged catalyst in Example 7 under catalytic kinetic conditions: The same conditions as in Example 11, within 120 min, the reaction rate was stabilized at 185 μmol CH4 g Pd -1 s -1 .

[0072] Example 17.

[0073] Activity evaluation of the 0.5+4-aged catalyst in Example 8 under catalytic kinetic conditions: The same conditions as in Example 11, within 120 min, the reaction rate was stabilized at 190 μmol CH4 g Pd -1 s -1 .

[0074] Comparative Example 1.

[0075] High temperature heat treated flake Al2O3 carrier:

[0076] Accurately weigh 2.00 g of thin-flaked γ-phase Al2O3, put it into a crucible, heat it to 850°C in a muffle furnace at a heating rate of 2°C / min, and naturally cool it to room temperature after roasting for 2 hours to obtain a high-temperature heat-treated thin-flaked Al2O3 carrier (referred to as thin-flaked Al2O3).

[0077] Comparative Example 2.

[0078] High temperature heat treated needle-shaped Al2O3 carrier:

[0079] Accurately weigh 2.00 g of needle-shaped γ-phase Al2O3, put it into a crucible, heat it to 850°C in a muffle furnace at a heating rate of 2°C / min, and naturally cool it to room temperature after roasting for 2 hours to obtain a high-temperature heat-treated needle-shaped Al2O3 carrier (referred to as needle-shaped Al2O3).

[0080] Comparative Example 3.

[0081] High temperature heat treated bulk Al2O3 support:

[0082] Accurately weigh 2.00 g of bulk γ-phase Al2O3, put it into a crucible, heat it to 850°C in a muffle furnace at a heating rate of 2°C / min, and naturally cool it to room temperature after roasting for 2 hours to obtain a high-temperature heat-treated bulk Al2O3 carrier (referred to as bulk Al2O3).

[0083] Comparative Example 4.

[0084] Preparation of fresh Pd / Al2O3 catalyst using PdCl2 as precursor:

[0085] At room temperature (~25°C), take 0.106 mL (93.7 g Pd / L) PdCl2 solution is diluted with water to 10mL, and 1.00g of nano-Al2O3 heat-treated at high temperature in Example 1 is accurately weighed and added under stirring, and stirring is continued for 2h to allow PdCl2 to adsorb on the surface of the carrier. Heat the water bath to 80°C, and allow the water in the impregnation solution to evaporate naturally while stirring, and then put it into an 80°C oven to dry in air for 12h. After drying, grind the sample and put it into a muffle furnace and heat it from room temperature (~25°C) to 500°C at a heating rate of 2°C / min and maintain it for 4h, and then cool it naturally to room temperature (~25°C). That is, a fresh Pd / Al2O3 catalyst (referred to as Fresh-Cl) with PdCl2 as a precursor is obtained.

[0086] Comparative Example 5.

[0087] Preparation of fresh Pd / Al2O3 catalyst supported on thin Al2O3 flakes:

[0088] At room temperature (~25°C), take 0.106 mL (93.7 g Pd / L) Pd(NO3)2 solution is diluted to 10mL with water, and 1.00g of the Al2O3 flakes heat-treated at high temperature in Example 1 is added under stirring, and stirring is continued for 2h to allow Pd(NO3)2 to adsorb on the surface of the carrier. Heat to 80℃ in a water bath, and allow the water in the impregnation solution to evaporate naturally while stirring, and then place in an 80℃ oven to dry in air for 12h. After drying, grind the sample and place it in a muffle furnace and heat it from room temperature (~25℃) to 500℃ at a heating rate of 2℃ / min and maintain for 4h, and then cool it naturally to room temperature (~25℃). That is, a fresh Pd / Al2O3 catalyst (referred to as Fresh-flakes) with Al2O3 flakes as carrier is obtained.

[0089] Comparative Example 6.

[0090] Preparation of fresh Pd / Al2O3 catalyst with needle-shaped Al2O3 as support:

[0091] At room temperature (~25°C), take 0.106 mL (93.7 g Pd / L) Pd(NO3)2 solution is diluted with water to 10mL, and 1.00g of needle-shaped Al2O3 heat-treated at high temperature in Example 1 is accurately weighed and added under stirring, and stirring is continued for 2h to allow Pd(NO3)2 to adsorb on the surface of the carrier. Heat the solution in a water bath to 80°C, and allow the water in the impregnation solution to evaporate naturally while stirring, and then place it in an 80°C oven to dry in air for 12h. After drying, grind the sample and place it in a muffle furnace and heat it from room temperature (~25°C) to 500°C at a heating rate of 2°C / min and maintain it for 4h, and then cool it naturally to room temperature (~25°C). That is, a fresh Pd / Al2O3 catalyst (referred to as Fresh-Needle) with needle-shaped Al2O3 as the carrier is obtained.

[0092] Comparative Example 7.

[0093] Preparation of fresh Pd / Al2O3 catalyst supported on bulk Al2O3:

[0094] At room temperature (~25°C), take 0.106 mL (93.7 g Pd / L) Pd(NO3)2 solution is diluted with water to 10mL, and 1.00g of the bulk Al2O3 heat-treated at high temperature in Example 1 is added under stirring, and stirring is continued for 2h to allow Pd(NO3)2 to adsorb on the surface of the carrier. Heat in a water bath to 80°C, and allow the water in the impregnation solution to evaporate naturally while stirring, and then place in an 80°C oven to dry in air for 12h. After drying, grind the sample and place it in a muffle furnace and heat it from room temperature (~25°C) to 500°C at a heating rate of 2°C / min and maintain for 4h, then cool naturally to room temperature (~25°C). This is a fresh Pd / Al2O3 catalyst (referred to as Fresh-bulk) with bulk Al2O3 as the carrier.

[0095] Comparative Example 8.

[0096] Preparation of Pd / Al2O3 catalyst after aging in He atmosphere:

[0097] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are sealed with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pretreated gas passes through the catalyst from top to bottom, the gas composition is pure He, and the total gas flow rate is 100 mL / min. Under the flow of pretreated gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Thereafter, the temperature is reduced to 200°C at a cooling rate of 5°C / min, and finally naturally cooled to room temperature. The Pd / Al2O3 catalyst aged in He atmosphere (referred to as He-aged) is obtained.

[0098] Comparative Example 9.

[0099] Preparation of Pd / Al2O3 catalyst after aging in 10% oxygen atmosphere:

[0100] Accurately weigh 100 mg of the catalyst obtained in Example 2 and load it into a "U"-shaped quartz tube with an inner diameter of 8 mm. The bottom and top of the catalyst are sealed with quartz wool, and 2 g of 40-60 mesh quartz sand is loaded above the top quartz wool to preheat the gas. The pretreatment gas passes through the catalyst from top to bottom, and the gas composition is 10% (volume percentage) of oxygen and He gas balance, and the total gas flow rate is 100 mL / min. Under the flow of pretreatment gas, the catalyst is heated from room temperature (~25°C) to 800°C at a heating rate of 10°C / min and maintained for 2 hours. Thereafter, the temperature is reduced to 200°C at a cooling rate of 5°C / min, and finally naturally cooled to room temperature. The Pd / Al2O3 catalyst (recorded as 10% O 2- aged).

[0101] Comparative Example 10.

[0102] Evaluation of the activity of the Fresh-Cl catalyst in Comparative Example 4: Under the same conditions as in Example 9, the reaction began to occur at 250°C, and the methane conversion rate increased with the increase in reaction temperature, reaching a conversion rate of 49% at 500°C.

[0103] Comparative Example 11.

[0104] Activity evaluation of the Fresh-flake catalyst in Comparative Example 5: Under the same conditions as in Example 9, the reaction began to occur at 200°C, and the methane conversion rate increased with the increase of reaction temperature, and was completely converted at 450°C.

[0105] Comparative Example 12.

[0106] Activity evaluation of the fresh-needle catalyst in Comparative Example 6: Under the same conditions as in Example 9, the reaction began to occur at 200°C, and the methane conversion rate increased with the increase of reaction temperature. The methane conversion rate did not reach 100% at 500°C.

[0107] Comparative Example 13.

[0108] Activity evaluation of the Fresh-block catalyst in Comparative Example 7: Under the same conditions as in Example 9, the reaction began to occur at 200°C, and the methane conversion rate increased with the increase of reaction temperature. The methane conversion rate did not reach 100% at 500°C.

[0109] Comparative Example 14.

[0110] Activity evaluation of the He-aged catalyst in Comparative Example 8: Under the same conditions as in Example 9, the reaction began to occur at 300°C, and the methane conversion rate increased with the increase of reaction temperature. At 500°C, the methane conversion rate was close to 100%.

[0111] Comparative Example 15.

[0112] Activity evaluation of the 10% O2-aged catalyst in Comparative Example 9: Under the same conditions as in Example 9, the reaction began to occur at 300°C, and the methane conversion rate increased with the increase in reaction temperature, reaching 80% at 500°C.

[0113] Comparative Example 16.

[0114] Activity evaluation of the He-aged catalyst in Comparative Example 8 under catalytic kinetic conditions: Under the same conditions as in Example 11, within 120 min, the reaction rate was stable and slowly increased but did not exceed 80 μmol CH4 g Pd -1 s -1 .

[0115] Comparative Example 17.

[0116] Activity evaluation of 10% O2-aged catalyst in Comparative Example 9 under catalytic kinetic conditions: The same conditions as in Example 11, within 120 min, the reaction rate was stable at 25 μmol CH4 g Pd -1 s -1 .

[0117] Figure 1 The results of the catalytic performance of fresh Pd / Al2O3 catalysts prepared with Pd(NO3)2 and PdCl2 as precursors for methane combustion reaction as a function of temperature are given in Example 2 and Comparative Example 4. The total mass percentage of precious metal Pd and oxide PdO is 0.99wt% in terms of palladium. It can be seen that the activity of the catalyst prepared with Pd(NO3)2 as the precursor is significantly better than that of the catalyst prepared with PdCl2 as the precursor, indicating that the selection of the type of metal Pd precursor has an important influence on the catalyst activity. Combined with the experimental results, we prefer to prepare the catalyst with Pd(NO3)2 as the precursor.

[0118] Figure 2-5 The electron microscopy characterization results of four different morphologies of Al2O3 carriers in Example 1 and Comparative Examples 1-3 are given. Figure 6 The results of the catalytic performance of fresh catalysts prepared with 4 different morphologies of Al2O3 as carriers for methane combustion reaction as a function of temperature are given in Example 2 and Comparative Examples 5-7. It can be seen from the figure that the morphologies of different Al2O3 carriers are significantly different. The nanorod-shaped Al2O3 carrier in Example 1 is 20-200nm long and 5-25nm in diameter; the activity of the catalyst prepared with nanorod Al2O3 as carrier is significantly better than that of the catalyst prepared with other morphologies of Al2O3 as carrier, indicating that the morphology of the Al2O3 carrier has an important influence on the catalyst activity. Combined with the experimental results, we prefer nanorod-shaped Al2O3 as carrier to prepare the catalyst.

[0119] Figure 7 The XRD diffraction spectra of each sample prepared in Examples 1-3 and Comparative Examples 8-9 are given. It can be judged from the XRD diffraction spectra that the fresh catalyst without high-temperature atmosphere aging treatment has no obvious diffraction peaks of palladium-related species, indicating that the palladium is highly dispersed on the carrier surface; the He-aged catalyst shows a diffraction peak of the metal Pd (111) crystal plane at 2θ = 40.0°, indicating that the palladium in the catalyst after He aging treatment mainly exists in the metallic state; the 0.5+2-aged and 10% O2-aged catalysts show a diffraction peak of the PdO (101) crystal plane at 2θ = 33.9°, indicating that the crystalline palladium in these two samples mainly exists in the form of palladium oxide; in addition, the palladium-related species in all aged catalysts have better crystallinity and larger particle size.

[0120] Figure 8 The ignition curves of the methane catalytic combustion reaction of the catalysts prepared in Examples 2-3 and Comparative Examples 8-9 are given. It can be seen that the activity of the catalyst aged with He gas and 10% O2 is significantly reduced; while the activity of the 0.5+2-aged catalyst is not significantly changed compared with the fresh catalyst.

[0121] Fig. 9 The curves of the reaction rate of each catalyst prepared in Examples 2-3 and Comparative Examples 8-9 under the catalytic reaction kinetic conditions for catalyzing methane combustion reactions versus reaction time are given. It can be seen that 0.5+2-aged has a significantly higher catalytic activity for methane combustion than other catalysts, indicating that high temperature aging pretreatment in a mixed atmosphere of 0.5% methane and 2% oxygen can significantly improve the activity of Pd / Al2O3 catalysts.

[0122] Fig.10 The spherical aberration electron microscope photograph of the catalyst pretreated by high temperature aging in a methane and oxygen mixed atmosphere in Example 3 is given. It can be seen that after high temperature aging in a methane and oxygen mixed atmosphere, the particle size of the catalyst active phase PdO particles increases to 10-100nm, and a "raft"-shaped metal Pd covering layer appears on its surface. The particle size of the small metal Pd particles is 2-6nm, and 20-60% of the outer surface of the PdO particles is covered by metal Pd; this indicates that the Pd / Al2O3 catalyst prepared with nano-Al2O3 as a carrier will have a coexistence structure of metal Pd and PdO with high methane catalytic combustion activity after atmosphere aging; we believe that this special structure is the main reason for the improvement of catalyst activity.

[0123] Fig.11The comparison results of the reaction rates of the catalysts prepared in Examples 2, 4 and 5 when catalyzing the methane combustion reaction under the catalytic reaction kinetic conditions are given. It can be seen that under the lean combustion condition of a fixed oxygen volume percentage of 2%, appropriately increasing the methane / oxygen ratio in the aging atmosphere can further improve the catalyst performance, while reducing the methane / oxygen ratio will reduce the catalyst activity; therefore, when the oxygen volume percentage is 2%, the methane volume percentage is preferably 0.5-1%.

[0124] Fig.12 The comparison results of the reaction rates of the catalysts prepared in Examples 2, 6, 7 and 8 when catalyzing the methane combustion reaction under the catalytic reaction kinetic conditions are given. It can be seen that under the lean combustion condition of a fixed methane volume percentage of 0.5%, appropriately increasing the methane / oxygen ratio in the aging atmosphere has little effect on the catalyst activity, while reducing the methane / oxygen ratio will reduce the catalyst activity; therefore, when the methane volume percentage is 0.5%, the oxygen volume percentage is preferably 1.5-2%.

[0125] Combining the characterization data with the activity data, we believe that: the metal Pd precursor and Al2O3 support morphology used to prepare the catalyst have an important influence on the catalyst activity, among which Pd(NO3)2 and nanorod-shaped Al2O3 are the preferred precursors and supports, respectively; high-temperature aging pretreatment can change the particle size and crystal state of the active components Pd or PdO in the catalyst; aging pretreatment in an atmosphere of methane and oxygen with appropriate concentrations and proportions can significantly improve the catalytic performance of the Pd / Al2O3 catalyst in methane combustion reactions, and the special structure of the PdO particles produced during the aging process, in which the surface is partially covered with a "raft"-shaped metal Pd, is the main reason for the increased activity; the relative contents of methane and oxygen have a significant effect on the catalyst activity.

Claims

1. Application of a palladium-based catalyst pretreated with a high-temperature aging atmosphere in methane combustion, characterized in that: The palladium-based catalyst used is a supported catalyst with metal palladium and palladium oxide as active components, and the carrier is a nano-scale γ-Al2O3 material; the total mass percentage of palladium and palladium oxide in the catalyst is 0.5~5wt% based on palladium; The carrier composition is pure γ-phase Al2O3, and the carrier shape is nanorod-like, 20-200nm long and 5-25nm in diameter; Metal Pd and PdO are uniformly dispersed between carrier γ-Al2O3 nanorods in the form of nanoparticles. The particle sizes of Pd and PdO are 2-100 nm. Metal Pd is covered on the surface of PdO particles with a particle size of 10-100 nm in the form of small particles with a particle size of 2-6 nm. 20-60% of the outer surface of the PdO particles is covered by metal Pd. The mass ratio of metal Pd to PdO is 1.5:1~2.5:

1. Preparation method of Pd-based catalyst: The carrier nano-scale γ-Al2O3 is directly impregnated in a solution containing an active component metal palladium precursor Pd(NO3)2, and after impregnation, it is dried at 30-150°C for 2-24 hours, and air-baked at 400-600°C for 2-12 hours to obtain a palladium-based catalyst that has not been pretreated in a high-temperature aging atmosphere; High temperature aging atmosphere pretreatment method for palladium-based catalysts: The palladium-based catalyst that has not been pretreated in a high-temperature aging atmosphere is ground evenly and then pretreated at high temperature in an aging atmosphere. The aging atmosphere comprises 0.1-4% by volume of methane, 0.1-20% by volume of oxygen, and the remainder is one or more of He, Ar or N2. The pretreatment temperature is 500-850°C and the pretreatment time is 0.5-10 h.

2. The use according to claim 1, characterized in that: The total mass percentage of palladium and palladium oxide in the palladium-based catalyst is 0.8-2wt%; Calcinate in air at 500-550° C. for 2-12 hours to obtain a palladium-based catalyst that has not been pretreated with a high-temperature aging atmosphere; The aging atmosphere composition is 0.5-1% by volume of methane, 1.5-2% by volume of oxygen, the pretreatment temperature is 750-850° C., and the pretreatment time is 0.5-2 h.

3. The use according to claim 1, characterized in that: The raw gas ratio of the methane combustion is: methane volume percentage 0.2-2%, oxygen volume percentage 0.5-5%, water vapor volume percentage 0-15%, carbon dioxide volume percentage 0-5%, and the rest is one or more of He, Ar or N2.

4. The use according to claim 1 or 2, characterized in that: The reaction conditions of methane combustion are: reaction temperature 150-800°C, raw gas space velocity 60000-3000000mLg 催化剂 - 1 h -1 , the total pressure of reaction gas is 0.1-0.2MPa.

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