Theoretical Research Of CO Oxidative Coupling To DMO Over Core-shell And Supported Pd-based Catalysts

EG is one of crucial chemical raw materials with a global demand of about25 million tons each year,while about half of the market demand is in China.Coal to ethylene glycol?CTEG?,which has many advantages of the moderate reaction condition,high efficiency,green,atom economy and non-oil route,has drawn considerable attention.Moreover,it is also considered as one of the most important applications in C1 chemistry.In CTEG,The coupling of CO with nitrite esters to oxalates is the first step for the preparation of EG,which can be seen as a key step to realize the conversion of inorganic C1 to organic C2.Pd-based catalysts are widely used in experiments as high activity and selectivity for the CO oxidative coupling to oxalates with nitrite esters.Nevertheless,the shortage of noble metal resources is more and more rigorous in the world,and the high price of noble metal Pd hinders its large-scale application in industry.Therefore,how to reduce the amount of Pd and maintain its high activity and selectivity has become the focus of research.In the current work,CO oxidative coupling to DMO on Pd-based catalysts has been studied.The density functional theory?DFT?method has been used to systematically study effects of surface structures,support loading,sizes and second metal doping of Pd catalysts on catalytic activity and selectivity.Based on the above studies,we further obtain the important theoretical basis for the design of low-cost and high-performance Pd-based catalysts.The main conclusions are as follows:1.The effect of surface structures of catalysts on CO oxidative coupling to DMO:The surface strutures of Pd-based catalysts show structure-sensitivity for CO oxidative coupling to DMO.Pd_n?n=13,38 and 55?clustered catalysts and M_n@Pd_m?n=1,6,13,n+m=13,38,55?core-shell catalysts are selected.Among them,both of Pd₁₃,Pd₅₅,M@Pd₁₂ and M₆@Pd₃₂ catalysts exhibit the features of the Pd?111?surface,and Pd₃₈ and M₁₃@Pd₄₂ catalysts mainly represent Pd?100?surface.The results show that COOCH₃-COOCH₃ coupling path is the optimum pathway on the?111?surface of catalysts,while the optimum pathway to form DMO is different on the?100?surface of catalysts.Meanwhile,among the pure Pd cluster catalysts and the same kind of metal-doped core-shell catalysts,the catalysts with?111?features exhibit higher catalytic activity than those with?100?features.Therefore,the?111?plane is the reactive surface for CO oxidative coupling to DMO.Thus,in the preparation of Pd-based catalysts for CO oxidative coupling to DMO,the?111?plane of the crystal surface structure of the catalyst should be strictly controlled in order to improve the catalytic activity.2.The second non-noble metal,including Ti,Al,Fe,Cu and Ag atoms affect the species adsorption and catalytic properties towards CO oxidative coupling to DMO reaction:Non-noble metals Ti,Al,Fe,Cu,Ag have been doped in the center of Pd₁₃,Pd₃₈ and Pd₅₅ clusters to reduce the amount of precious metal Pd catalyst,clarifying the influence of the second metal on species adsorption and catalytic performance.The core atom of Pd₁₃ cluster replaced by Al-,Ti-,and Fe-doped to form Ti@Pd₁₂,Al@Pd₁₂ and Fe@Pd₁₂2 core-shell catalysts,and their activity is higher than that of Pd₁₃ catalyst;Cu-doped has little effect on the activity of Pd₁₃catalyst towards CO oxidative coupling to DMO;Ag@Pd₁₂ core-shell catalyst shows the lowest activity than Pd₁₃ catalyst.Similarly,Ti,Al and Fe replace Pd₅₅central atoms,and the activities of Ti₁₃@Pd₄₂,Al₁₃@Pd₄₂ and Fe₁₃@Pd₄₂ catalysts are higher than those of Pd₅₅.The second metal doped can change the d-band center of the Pd atom on catalyst surfaces,which in turn changes the catalytic activity of the catalyst.When the interaction between the core and the shell is stronger,the d-band center of the catalyst surface is farther away from the Fermi level,the adsorption energy of the reactants decreases,further increases the reaction activity of catalysts.3.The effect of different defect graphenes supporting Pd atom on catalytic activity of CO oxidative coupling to DMO:In order to further reduce the amount of Pd,Pd particles with smaller size have been loaded on different supports to explore the influence of carrier loading on the catalytic activity of the catalyst.the research shows:1)different defects of graphenes change the optimal pathway of CO oxidative coupling reaction.CO-COOCH₃ coupling path is the optimal path on Pd-DVG catalyst,while COOCH₃-COOCH₃ coupling path is the optimal path for the Pd-SVG,Pd-NVG and Pd-BVG catalysts;2)Pd-DVG and Pd-SVG catalysts show excellent high activity and selectivity of CO oxidative coupling to DMO,while Pd-NVG and Pd-NVG catalysts exhibit lower activity.4.The second metal doping and support loading are main methods of regulating the catalytic performance of catalysts.Ti₁₃@Pd₄₂ and Al₁₃@Pd₄₂ core-shell catalysts by doping Ti,Al atoms into Pd₅₅ clusters,and monoatomic Pd supported on DVG catalyst not only exhibit high catalytic activity and selectivity,but also can greatly reduce the amount of precious metal Pd.