Design And Performance Of High-efficiency Cerium-doped Alumina Supported Palladium Catalysts For Methane Catalytic Combustion

Natural gas,as a clean energy,has gradually augmented its proportion in the energy consumption structure,and has been mainly applied in power plants and automotive fuels.The primary constituent of natural gas is methane,and the greenhouse effect was exacerbated by the direct emission of unburned low-concentration methane during utilizing.At present,catalytic combustion technology has become an effective strategy for the methane abatement,meanwhile,the supported Pd catalyst with Al₂O₃as carrier（Pd/Al₂O₃）demonstrated the best application prospect for this reaction.Nevertheless,the palladium nanoparticles tended to coalescence and sintering during reaction,meanwhile small Pd O particles was prone to decompose into metal Pd at elevated temperature,resulting in severe deactivation.Therefore,improving the stability of Pd O phase and enhancing the anti-sintering performance of palladium species were critical to promote the catalytic performance of palladium-based catalyst.The excellent oxygen exchange and migration ability of CeO₂made it one of the most effective palladium oxidation promoters among oxides.In addition,the fluorite structure enabled CeO₂to form solid solution with lots of metal oxides.Therefore,it is expected to boost the activity and stability of catalyst by introducing the optimized promoters into Pd/Al₂O₃.Based on the above assumptions,herein aluminas co-doped by cerium and different metals were synthesized through a facile sol-gel method,and were adopted as carriers to construct high-efficiency and stable alumina composite oxide supported palladium catalysts for methane combustion by incipient wetness impregnation process.The primary research contents and results are as follows:（1）Inspired by the polymorphic characteristics of alumina and the excellent oxygen storage/release ability of CeO₂-Zr O₂solid solution,the Pd/CeO₂-Zr O₂-Al₂O₃（Pd/5CZA-x,x represents the calcination temperature of carrier）composite oxide catalyst was designed.The crystal structure and surface properties of catalysts treated at different temperatures and their performance in methane combustion were systematically explored,and the structure-activity relationship was described.The results showed that the calcination temperature of 1200-1300℃made Al₂O₃in carrier mainly exist in the form ofαphase,meanwhile promoted the formation of well-crystallized solid solution of cerium and zirconium components on catalyst.The weak interaction betweenα-Al₂O₃and palladium species on catalyst along with the strong interaction between CZ solid solution and palladium endowed Pd/5CZA-1300 with high-efficiency Pd/Pd O conversion,showing excellent low-temperature catalytic activity and favourable stability.Besides,due to a certain degree of surface hydrophobicity ofα-Al₂O₃,the water resistance of Pd/5CZA-1300 catalyst was boosted.（2）Thedesignofpalladiumcatalystsupportedon cerium-zirconium-magnesium-aluminum composite support was carried out.By introducing Mg into CeO₂-Zr O₂-Al₂O₃,the cerium-zirconium-magnesium-aluminum composite oxide supported palladium catalyst（Pd/5CZA-y M,y represents the mass fraction of magnesium）was prepared.The catalyst with excellent methane combustion performance was obtained by adjusting the amount of Mg.The influences of Mg content on the surface acid-base property,oxygen vacancy concentration,lattice oxygen mobility and active phase Pd?Pd O redox cycle were discussed.The results showed that more oxygen vacancies and surface adsorbed oxygen were generated,and migration of oxygen species was promoted due to the structural and electronic mismatch between Mg²⁺and Ce⁴⁺(or Zr⁴⁺).The Mg Al₂O₄spinel and oxygen vacancies on Mg-modified catalysts conjointly resulted in weaker Pd-O bonds and improved reducibility.Meanwhile,the efficient oxygen transfer between metal and support contributed to the stability of active Pd O phase,making palladium species maintain in a highly active oxidation state during reaction,which gave the Mg-containing catalysts with lower apparent activation energy and higher catalytic activity.Moreover,it was found that Pd particle size had a significant effect on the performance of catalyst.The catalytic activity of Pd/5CZA-y M showed a volcanic tendency with the increase of Pd particle size.The as-prepared Pd/5CZA-5M with a moderate Pd particle size（5.33±0.48 nm）demonstrated significantly boosted catalytic activity and stability,showing its T₉₉at 400℃.（3）Based on the above research,when CeO₂was introduced into alumina,Ti which could form tetravalent oxide,Y and La which were trivalent dopants and Sn⁴⁺/Sn²⁺,Sm³⁺/Sm²⁺which had variable valence states were further incorporated into alumina support to study the diverse of their effects on the generation of oxygen vacancy and surface oxygen species.Therefore,Ce-Sm,Ce-Sn,Ce-La,Ce-Y and Ce-Ti modified Al₂O₃supports were prepared to establish high-performance supported Pd catalysts（Pd/C-z A,z=Sm,Sn,La,Y and Ti）.The diversity of structure and performance of C-z bimetallic co-doping catalysts was revealed.This investigation may shed new light on the well design of high-efficiency multicomponent catalysts.Results demonstrated that the part ofγ-to-αphase transformation of Al₂O₃was induced in Ce-Ti co-doping catalyst,leading to a significant reduction of specific surface;Ce co-doping with Ti,Sm,La and Y promoted the as-prepared catalysts to generate well-crystallized CeO₂particles.The interaction between palladium and CeO₂provided active oxygen for palladium species,thereby increasing the Pd O thermal decomposition temperature and the reduction temperature of Pd O by H₂;when Ce-Sn co-doping,the presence of CeO₂-Sn O₂solid solution and the valence change of Sn^4+/2+at the tin site,facilitated the generation of oxygen vacancies and the migration of oxygen species,and thus improved reducibility was realized on Pd/C-Sn A.Amongst,Pd/C-Sm A exhibited excellent texture properties and suitable surface acid-base property,benefiting for the dispersion of active palladium and the adsorption and activation of methane.The optimized structure and surface properties endowed Pd/C-Sm A with more oxygen vacancies and surface oxygen species,stable Pd O phase and a higher proportion of Pd O particles with step sites,which reduced the activation energy barrier and boosted the low-temperature catalytic activity.