The Theoretical Studies Of Pd-M(M=Cu,Ag,Ni And Al) Bimetallic Catalysts For CO Oxidative Coupling To DMO

CO oxidative coupling to dimethyl oxalate?DMO?is the most critical step in the coal to ethylene glycol?CTEG?process,and the reaction realizes the important conversion of inorganic C1 to organic C2.At the same time,DMO is also an important chemical raw material with large market gap in China.For the coupling reaction,Pd-based catalysts exhibit excellent catalytic performance.However,the shortage of precious metal Pd resources and high price limit the wide application of CTEG in industry.Therefore,how to reduce the amount of Pd on the premise of maintaining the high catalytic performance of catalyst for the coupling reaction has become the focus of current research.In this paper,the CO oxidative coupling to DMO over Pd-based catalysts had been studied by density functional theory?DFT?.The effects of different surface structures,second metal doping and single-atom catalysts on the coupling reaction were studied in detail.And we hope this study can provide theoretical basis for the design of new Pd-based catalysts with low cost and high performance.The main conclusions are as follows:?1?For CO oxidative coupling to DMO,different Pd metal surfaces exhibit different catalytic properties.The reaction mechanism of CO oxidative coupling to DMO on Pd?100?with flat surface structure and Pd?110?and Pd?211?with defective?step?surface structure had been studied in detail by DFT method.Compared with Pd?111?surface,the effect of surface structure on the reaction had been clarified.Firstly,the key reaction species CO participates in the coupling reaction in different ways on the pure metal Pd surfaces,and shows a strong structural sensitivity,on Pd?111?and?110?surfaces,CO participates in the reaction mainly by adsorbing at the top site,while on Pd?100?and?211?surfaces,bridge site adsorption is the main way for CO to participate in the reaction.Secondly,the formation of C-C bond is also closely related to the surface morphology,on Pd?111?and?100?surfaces,C-C bond is mainly obtained by the coupling of COOCH₃-COOCH₃,while on Pd?110?and?211?surfaces,it is mainly achieved by the coupling of COOCH₃-CO.Finally,by comparing the catalytic performance of different surface structures for DMO synthesis,it can be concluded that:flat surface of Pd is more conducive to DMO synthesis,Pd?111?and?100?surfaces are both active surfaces for CO oxidative coupling to DMO.?2?In the alloy Pd-based catalysts,the type and proportion of the second metal will significantly affect the catalytic performance of catalysts for DMO synthesis.By calculating a series of reactions related to DMO synthesis on Pd-M?M=Cu,Ag,Ni and Al?bimetallic alloy catalysts with different proportions?3:1,1:1 and 1:3?,the reaction mechanism of CO oxidative coupling to DMO on these alloy catalysts had been elucidated,and the effect of the type and proportion of the second metal on the catalytic performance had also been clarified.The activity of Pd-Cu,Pd-Ag and Pd-Ni catalysts with appropriate proportions was better than that of pure metal Pd catalysts,while the activity and selectivity of Al-modified catalysts had been greatly reduced.Pd₃Cu₁?100?,Pd₂Ag₂?001?and Pd₂Ni₂?001?exhibited high activity and selectivity in a series of alloy catalysts,and the amount of Pd precious metal was reduced by 25%,50%and 50%,respectively.The formation of C-C bond on these three surfaces is achieved by the coupling of COOCH₃-COOCH₃.But the adsorption modes of CO involved in the coupling reaction are different:on Pd₃Cu₁?100?surface,CO participates in the reaction mainly by adsorbing at the top site of Pd;on Pd₂Ag₂?001?and Pd₂Ni₂?001?surfaces,CO mainly participates in the reaction by Pd-Pd bridge site adsorption.?3?Suitable single-atom Pd alloy catalyst can greatly improve the catalytic performance of DMO synthesis on Pd-based catalyst and minimize the use of Pd.The mechanism of CO oxidative coupling reaction on single-atom Pd catalyst had been clarified through calculation,which provided a new idea and theoretical basis for industrial preparation of high efficiency and low cost catalyst for DMO synthesis.Pd₁-Cu and Pd₁-Ag?100?single-atom catalysts can significantly improve the utilization of Pd atoms,significantly enhance the catalytic activity of the catalyst,and minimize the amount of precious metal Pd.Compared with Pd₁-Cu?100?,Pd₁-Ag?100?single-atom catalyst showed better selectivity to DMO.At the same time,the CO oxidative coupling reaction on single-atom catalysts also showed that:due to the active sites of single-atom catalysts are relatively single,in order to ensure the activity of single-atom catalysts for target reaction,the adsorption energies of all reaction species on the active atom should be ensured within an appropriate range.?4?During the synthesis of DMO,the coverage of COOCH₃ on the catalyst surfaces and the energy barrier of the speed-determining step in the optimal reaction path affect the formation rate of DMO,and further determine the activity and selectivity of the catalyst.