DFT Theoretical Studies Of Syngas Conversion Mechanism On MoP Catalyst

The synthesis of lower alcohol from syngas is one of the important chemical reactions in the synthesis industry.The molybdenum phosphide（MoP）catalysts have attracted widespread attention in recent years due to its low cost,high chemical stability and excellent hydrogenation performance.It has been found that MoP catalysts have high catalytic activity in the conversion of syngas to produce lower alcohols,and the selectivity of C₂₊oxygenates is high,especially for the modified MoP catalysts.However,the mechanism of synthesis gas conversion on MoP catalysts is still unclear.The previous theoretical researches mainly focus on the adsorption of small molecules on MoP（001）and（100）surfaces.Therefore,in this paper,the density functional theory calculation method is used to systematically study the CO adsorption and dissociation properties on the different crystal planes of MoP（（101）,（112）,（100）,（111）,（110）,（102）,and（001））,the co-adsorption of CO and H₂ on the MoP（101）and（001）surfaces and the reaction mechanism of methane and methanol formation from syngas conversion.the following conclusions were mainly obtained.（1）Our investigations demonstrated that among different MoP surfaces with diversity terminations,CO has very strong adsorption and high dissociation energy barrier on the Mo and Mo/P terminated surfaces.Furthermore,each surface has quite different saturation coverage,i.e.,the Mo-termination of（001）surface has one monolayer（1 ML）,（001）-P has4/9 ML,（101）has 1.5 ML,（111）and（110）surfaces have 2 ML,（100）has 4/3 ML,while（112）and（102）surfaces have 2.5 ML.In addition,analyzing with the atomistic thermodynamics method,the full CO desorption temperature is about 220,540 and 630K on（001）-P,（001）-Mo and（102）surface,as well as 390-490K on（101）,（110）,（111）,（100）and（112）surfaces.（2）We studied the co-adsorption of CO and H₂ on MoP（101）and（001）surfaces.The results show that CO adsorbs much stronger than dissociative H₂ on（101）and（001）-Mo surfaces but competitively with dissociative H₂ on the（001）-P surface,and the hydrogen saturation coverage decreases with increasing CO pre-coverage.Ab initio atomistic thermodynamics analysis indicates the quite different CO and H₂ coadsorption manners on three surfaces under syngas atmosphere,i.e.,the Mo/P-terminated（101）surface and the Mo-terminated（001）surface have more versatile surface CO and H₂ ratios.However,the P-terminated（001）surface has only hydrogen adsorption at a wide range of conditions,which plays a role of hydrogen reservoir.（3）By comparing the reaction mechanism of CH₃OH and CH₄ formation from CO hydrogenation on MoP（101）and MoP（001）-Mo surfaces,it is found that,on both surfaces,the optimal route for CO hydrogenation to CH₃OH is CO→CHO→CH₂O→CH₃O→CH₃OH.and the rate-determining step of the reaction is CH₃O+H→CH₃OH.The optimal path for CH₄ formation on the MoP（101）surface is CO→CHO→CH₂O→CH₂→CH₃→CH₄,but the optimal path for CH₄ formation on the MoP（001）-Mo surface is CO→C→CH→CH₂→CH₃→CH₄.Comparing the potential energy surface of CH₃OH and CH₄formation on the two surfaces,it can be seen that the formation of CH₄ on both surfaces is easier than that of CH₃OH.Furthermore,no matter the formation of CH₃OH or CH₄,MoP（101）has lower energy barrier and lower reaction heat.