Theoretical Study On CO Oxidative Coupling To Dimethyl Oxalate Over Pd-based Catalysts Modulated By Metaland Carbide

CO oxidative coupling to dimethyl oxalate?DMO?is a key step in the realizing conversion of coal to ethylene glycol.Pd-based catalysts are the current focus of research due to good catalytic performance.However,the relatively high price of Pd and dosage increase the production cost.Therefore,development some Pd-based catalysts with low amount of precious metal Pd and maintaining its high catalytic performance is a significant research content,which is with important theoretical and application value.The reaction of CO oxidative coupling to DMO over Pd-based catalysts including surface alloys about Pd monolayer,Pd stripe and Pd single atom supported on different the second metals,bulk alloys formed by the second metal and Pd,and Pd monolayer supported on carbide were studied by the density functional theory?DFT?method.The mechanisms of DMO formation on catalysts with different structures were clarified,as well as the Pd-based catalyst with low Pd dosage and high catalytic performance was obtained via DFT calculation combining with the micro-kinetic model.The main research contents and results are as follows:?1?The CO oxidative coupling to DMO was investigated on the main exposed surface Pd?111?,and its catalytic performance was used as the criterion for subsequent modification design and screening.The optimal path to generate DMO on Pd?111?was COOCH₃–COOCH₃coupling path,and the energy barrier of rate-controlling step was 120.6 k J·mol^-1.Micro-kinetic model analysis showed that DMO formation rate was much higher than that of DMC,which was consistent with the experimental results and demonstrated the reliability of the calculation results.?2?The first scheme to reduce the amount of precious metal Pd was to use the second metal?Al,Cu,Ag,Ni,Co?-supported Pd monolayer.The results showed that Al,Ni and Co-supported Pd monolayer improved catalytic performance for DMO formation,while Cu-supported Pd monolayer lowered the activity for DMO formation,and Ag-supported Pd monolayer reduced the selectivity for DMO generation.Additionally,the strain effect caused by different metal lattice constants and ligand effect caused by electron transfer on the activity for generating DMO were also investigated.It was found that the strain effect enhanced the activity,while the ligand effect decreased it.On catalysts of Al,Ni and Co-supported Pd monolayer,the strain effect dominated and resulted in the increase of the activity for DMO synthesis.?3?Based on the study about DMO formation on the second metal-supported Pd monolayer,it was found that catalytic active sites mainly located on the Pd stripe.The CO oxidative coupling to DMO was investigated on metal-supported Pd stripe catalysts.The results indicated that the catalytic performance of Cu and Co-supported Pd stripe were better than that of Cu,Co-supported Pd monolayer and Pd?111?,respectively,which could further reduce Pd amount and improve the performance.In which,the ligand effect dominated on Cu-supported Pd stripe and the activity was enhanced.Additionally,the activity of Ag and Ni-supported Pd stripe for DMO formation were weaker than that of Pd?111?.?4?Reducing Pd amount to the extreme,the Pd single atom surface alloys were studied.The formation of DMO on the second metal-supported Pd single atom Pd₁-M?111??M=Cu,Ag,Ni,Co?was investigated.It was obtained that the catalytic performances of Pd₁-Cu?111?and Pd₁-Ag?111?were superior to that of Cu and Ag-supported Pd monolayer,Cu and Ag-supported Pd stripe and Pd?111?,respectively,which could maximum reduce Pd amount and greatly improve the performance.The catalytic activity of Pd₁-Co?111?was lower than that Co-supported Pd stripe,but superior to that of Pd?111?.The catalytic activity of Pd₁-Ni?111?was lower than that of Pd?111?.In addition,the strain effect increased the activity for Pd₁-Cu?111?,and the ligand effect decreased it.The strain effect dominated,leading to the enhancement of activity.Both strain effect and ligand effect enhanced the activity for Pd₁-Ag?111?.?5?The influence of Pd-Co bulk alloys with diffident metal ratio on CO oxidative coupling to DMO was investigated.The results showed that the activity for DMO generation on Pd₃Co₁?111?was higher than that of Pd?111?,and it was more beneficial to the form DMO than DMC on Pd₃Co₁?111?.?6?Mo₂C has properties of like-noble metals,and SiC is an excellent carrier.Both are widely used in catalytic reactions.The CO oxidation to DMO on Mo₂C and SiC with different terminal-supported Pd monolayer were investigated.It was obtained that the catalytic performance on Mo₂C with Mo terminal,C terminal-supported Pd monolayer and SiC with C terminal-supported Pd monolayer were weaker than that on Pd?111?,while the catalytic performance on SiC with Si terminal-supported Pd monolayer was higher than that of Pd?111?.