Synthesis And Application Of Pd-Co Bimetallic Catalyst With High Activity

Natural gas has been recognized as clean energy due to its abundant reserves,high calorific value and low cost.As a high quality alternative energy,it has been widely applied in gas-fueled vehicles and gas power station.The major component of natural gas is methane,the incomplete combustion of it causes the emission of unburned methane during the use of natural gas,resulting in worse greenhous effect.Therefore,effective purification measures must be taken.The methane catalytic combustion has been considered as an effective route to reduce the unburned methane emitted from vehicle exhaust.At present,catalysts for methane catalytic combustion are as follows:noble metal,metal oxide,hexaaluminate and perovskite type catalysts.Metal oxide catalysts?such as nickle,cobalt,manganese oxide,etc?display relatively high catalytic activity for methane combustion due to its unique structure and electrical properties.Meanwhile,it has become the research hotpot for the low cost.However,their anti-sintering ability and stability during methane catalytic combustion need to be further improved.Hexaaluminate and perovskite type catalysts possess high thermal stability but low catalytic efficiency.Comparatively speaking,noble metal-based catalysts,especially supported palladium-based catalysts possessing low initiation temperature,excellent anti-poisoning performance and high thermal stability are widely studied,which are recognized as the most promissing catalyst at present.Nevertheless,the aggregation and growth of active components palladium as well as the decomposition and reconstruction of active components significantly affect the activity and stability of the catalysts at high temperature.Therefore,the preparation of Pd based catalysts with high catalytic activity and stability has important research value and practical significance.Cobalt-doped ordered mesoporous alumina?Co-OMA?,which had high specific surface area and catalytic active sites for methane combustion,can be synthesized by the sol-gel approach by adjusting the sol-system acidity and cobalt doping amount.Taking OMA and Co-OMA as support,we synthesized supported Pd catalyst by incipient wetness impregnation and investigated the effect of cobalt doping on the catalyst performance.Furthermore,taking the obtained OMA and Co-OMA as support,different synthetic methodes were exploited to synthesize Pd-Co bimetallic catalysts with same composition but different microstructure.We studied the effect of difference in microstructure on the surface acid-base properties,reducibility and interaction intensity between active component and support,tested the long-term stability and hydrothermal stability of the catalyst during methane combustion reaction.The following studies were carried out:1.Employing P123 as templete agent,and cobalt acetate as cobalt source,Co-OMA with high specific surface area and catalytic active sites for methane combustion can be synthesized by the sol-gel approach by adjusting the sol-system acidity?Hydrochloric acid or acetic acid as acidity regulator?to control the solvent evaporation rate and the sol polymerization rate.The effect of changing the sol-system acidity on the pore channle ordering and textural parameters of the samples as well as the effect of changing the Co dopoing content on the textural parameters,metal-support intensity,suface active species and oxygen vacancies content of the samples were investigated.2.Taking OMA and Co-OMA as support,the supported Pd based catalysts with 0.5wt%loading amount can be synthesized via incipient wetness impregnation.We analysized the effect of cobalt doping content on the intensity between PdO and support,the content of active palladium species and surface adsorbed oxygen,and conducted the evaluation of methane catalytic combustion activity.3.Taking OMA and Co-OMA as support,we adopted co-impregnation,sequential impregnation and organic matric combustion method?OMx?to synthesize Pd-Co bimetallic catalysts with same composition but different microstructure.The effects of the change of catalyst microstructure on the phase composition,surface acid-base properties of the catalyst and the adsorption properties of CO₂ were emphatically analyzed.In addition,the reducibility and stability of the catalyst were also studied.Pd-Co bimetallic catalysts with high catalytic activity and thermal stability were prepared.The following research results can be obtained through the above research work:1.When the sol-system pH=2,the pore channel order of Co-OMA-3?2?sample was better,and the specific surface area reached 328m²/g.Under the condition of sol-system pH=2,cobalt-doped ordered mesoporous alumina with different specific surface area,interaction intensity between cobalt species and alumina,the content of oxygen vacancies and surface active component can be synthesized by adjusting cobalt doping content.When the cobalt doping content was 6wt%,the prepared samples Co-OMA-6 still retain a high specific surface area of 276 cm²/g after being calcined at 800?.Additionally,Co-OMA-6 presented higher methane catalytic combustion activity for the higher content of Co²⁺,oxygen vacancies and surface adsorbed oxygen species as well as stronger interaction between cobalt species and support.2.Study on the effect of cobalt doping on the synergistic effect between Pd and Co,we can found that a series of supported Pd-based alumina catalysts with high catalytic activity were synthesized by incipient wetness impregnation method using the synthesized Co-OMA with high specific surface area and active sites for methane catalytic combustion as support.The Raman and XPS characterization showed that the Raman characteristic peak belonging to PdO of Pd/Co-OMA-6 catalyst located at 650cm^-1was at a higher wavenumber,which indicated that there was stronger interaction between palladium species and support in Pd/Co-OMA-6 catalyst.In addition,the catalyst had higher Pd²⁺/Pd⁰ and O_ads/O_lattatt values,more active palladium species and surface adsorbed oxygen.Therefore,the catalyst had higher catalytic activity for methane combustion,and its methane conversion temperature is 460?.3.A series of Pd-Co bimetallic catalysts were prepared by co-impregnation,sequential impregnation and organic matrix combustion methods using OMA and Co-OMA as support.Studying the phase composition,surface acid-base properties,adsorption capacity for CO₂ and reducibility of the catalysts can we found the catalysts Pd/6CoDEG/OMA,Pd/Co-OMA-6 and Pd/6Co/OMA showed higher catalytic activity for methane combustion.The complete conversion temperature of methane was 430,460 and 470?respectively.Pd/6CoDEG/OMA catalyst synthesized with diethylene glycol as complexing agent showed better catalytic performance.The better crystallization effect of?-Al₂O₃ in the catalyst promoted stronger interaction between palladium species and support,which helped to improve the reducibility of the catalyst.At the same time,the catalyst had better conversion effect for CO,which was the intermediate product of methane catalytic combustion,and CO was easily oxidized to CO₂.In addition,the catalyst had more surface acid amount and stronger acidity,less base amount and more basic strength.The adsorption capacity for methane catalytic combustion product CO₂ was weaker,and CO₂ was easy to remove on its surface.The above-mentioned influence factors synergistically made Pd/6CoDEG/OMA catalyst displayed higher catalytic activity for methane catalytic combustion,and had high stability and hydrothermal stabiblity.