Design Of Palladium-supported Alumina Catalysts Modified By Cobalt-containg Bimetal And Their Performance In Methane Combustion

Natural gas as an alternative energy source is widely used in automotive power fuels and power plants.The main component of natural gas is methane.However,the emission of unburned methane will cause serious greenhouse effect.Methane catalytic combustion is an effective way to reduce the emission of low-concentration methane.Generally,noble metal based catalysts,especially Pd/Al₂O₃ catalysts,demonstrate excellent low-temperature activity in methane catalytic combustion.However,Pd nanoparticles with low Taman temperature and high surface energy are prone to aggregate,sinter and grow up in the reaction process,especially at high temperatures.In addition,the Pd species are prone to react with water vapor,causing the loss of catalyst activity.Therefore,it is of great significance to design and synthesize catalysts with high activity,stability and water resistance at low temperature.In view of the above-mentioned drawback for Pd/Al₂O₃ catalysts,in this work,employing a sol-gel method,using block polymer P123 as template,aluminum isopropoxide as aluminum source and acetic acid as interface protector,mesoporous alumina co-doped with cobalt and different electronegative metal elements was facilely obtained by solvent evaporation self-assembly method.The performance of palladium-supported catalysts adopting the as-obtained mesoporous alumina as supports in CH₄combustion was investigated.The synergistic effect of cobalt with different electronegative metals on the structure and performance of catalysts was discussed.Meanwhile,the effects of texture property,surface acidity,valence state of active phase,redox property and metal-support interaction for as-synthesized catalysts on methane combustion performance were investigated.The influences of the introduction of Co-Mn and Co-Zr on the electronic structure of Pd O,the stability of active phase and the oxygen exchange capacity between the active phase and the support for Pd/Al₂O₃ catalyst were deeply explored to obtain high-performance palladium-based catalysts for catalytic methane oxidation.The main research contents and results are as follows:1、Taking advantage of the feature in the variable valence state of cobalt and the difference in the electronic structure of different electronegative metals,a facile sol-gel method was used to synthesize mesoporous alumina co-doped with cobalt and different electronegative metal elements X（X:Y,Mg,Zn,Ni,Sn）.The performance of as-obtained catalysts in methane combustion was studied.The effects of Co-X synergy on the electronic structure of the active phase,redox performance,oxygen migration ability and metal-support interaction for catalysts were explored.The significant differences were found on the electronic structure and existence state of Pd particles,the interaction between active components and supports among catalysts,making P/CX-A（X:Y,Mg,Zn,Ni,Sn）catalysts prepared by Co-X co-doped Al₂O₃ demonstrate different catalytic activity and stability towards methane combustion.In comparison with P/C-A,P/CX-A catalysts demonstrated better catalytic performance due to its less distortion of Pd O,higher palladium dispersion,and more active atoms exposed to the step/edge sites,lower activation energy.Among them,P/CMg-A catalyst exhibited superior catalytic activity(T₉₀=370℃),reusability,thermal stability and water-resistance,which was assigned to its higher proportion of Pd O,stronger oxygen migration ability,and lower surface acidity.2、The Co-Zr co-doped mesoporous alumina supported palladium catalysts were simply synthesized by the sol-gel approach combined with impregnation method,and their structural properties and catalytic methane oxidation behavior were studied.It was demonstrated that catalyst activity varied with the Zr doping amount.The catalyst with the adding amount of Zr at 5 wt.%performed the best activity,and its T₉₀（405℃）was lowered by 70℃compared with the catalyst with Co-adding alone.It was found that appropriate Zr doping induced higher surface-concentrations of oxygen vacancies,promoted the Pd O（?）Pd⁰ phase transition process,and enhanced the Pd O-support interaction and the reducibility of Pd O.Also,the Co-Zr synergistic effect delivered the increased effective charge of Pd particles,stabilized Pd O phase,and richer active sites exposed at the step/edge sites.Meanwhile,Zr-incorporation was conducive to accelerating the dehydroxylation process and inhibiting the accumulation of-OH on the catalyst surface.Hence,the tailored catalysts showed higher catalytic activity,excellent stability and water resistance in methane combustion.3、The Co-Mn binary oxides were introduced as promoters into Pd/Al₂O₃ system to construct efficient catalysts for methane combustion.The introduction of appropriate amount of manganese made Mn³⁺maximally enter into the Co₃O₄ spinel structure,conducive to the conversion of Co³⁺to Co²⁺and then the enhancement of lattice distortion,thereby generating more oxygen vacancies.An appropriate mole ratio of Mn/Co induced higher surface-concentrations of reactive Pd²⁺and O_ads species,along with the enhancement in oxygen exchange capability and stabilization of Pd²⁺species,which made the resulting catalyst with a molar Mn/Co ratio of 0.20 perform superior methane oxidation activity(T₉₀=360°C)and long-term stability.Moreover,owing to the lower surface acidity,which weakly coordination with-OH species can effectively inhibit the accumulation of hydroxyl groups on the surface of the catalyst.Meanwhile,the synergistic effect of Mn-Co can accelerate the removal of OH/H₂O accumulated on the surface of the active center,thus promoting the regeneration of Pd O and oxygen vacancies.Therefore,PCMA-0.20 catalyst also demonstrated remarkable water-resistance and a pronounced facilitative effect of water that the activity was further enhanced(T₉₀=350℃)after the removal of water.