Theoretical Studies On Methanol Synthesis Over Cu-and Pd-based Catalysts

Methanol is an important chemical raw material in the petrochemical industry.At present,the methanol synthesis process in the industry takes syngas（a mixed gas of CO,H₂ and a small amount of CO₂）as the reactant,and uses copper as a catalyst under high temperature and high pressure reaction conditions.The design of high-efficiency catalysts to increase the reactivity and selectivity of methanol synthesis from syngas has always been the focus in this field.It was found that Pd-based catalysts significantly improved the activity of catalytic syngas conversion and the selectivity of methanol compared with Cu-based catalysts.Therefore,this thesis combines density functional theory（DFT）and microkinetic modeling to systematically study the syngas conversion to methanol on the surface of Cu-based and Pd-based catalysts,including the following five parts:1.In view of the most widely used copper catalysts in the industry,this part combines DFT calculation and microkinetic modeling to study the effect of Cu（211）surface formate（HCOO）coverage on methanol formation rate.The DFT calculation results show that when there is a certain coverage of HCOO on the surface of Cu（211）,the energy of the adsorbate and the transition state in the reaction path will change,and the reaction potential energy disgram will also change.Further microkinetic modeling results show that the rate of methanol formation under different reaction conditions is different when different coverage of HCOO is pre-adsorbed on the catalyst surface,and the rate of methanol formation on the surface of clean Cu（211）is the lowest.2.Using DFT calculations,we studied the selectivity changes of methanol and methane formation in the CO hydrogenation reaction over Pd（211）surface before and after boron doping.It is found that the reaction mechanism of methanol and methane after boron atom doping is unchanged.We further estimate the effective barriers with the two-step model.The results show that the effective energy barrier of methanol formation on the surface of Pd（211）after boron atom doping is reduced,and the effective barrier for methane formation is basically constant before and after doping.This indicates that the doping of boron atoms enhances the activity and selectivity of methanol formation on the surface of Pd（211）.The origin is summarized as the lattice strain effect produced by boron atom doping and the electronic structure modification effect on surface Pd atoms.3.Through the combination of DFT calculations and microkinetic modeling,this part studied CO₂ hydrogenation over PdIn intermetallic catalyst surfaces.First,we selected two crystal planes,PdIn（110）and（211）,which are mainly distinguished by the difference in atomic arrangement between the surface Pd and In atoms.The results show that on both surfaces,the formation of methanol is obtained through the reaction pathway of HCOO and HCOOH intermediates;meanwhile,the by-product CO is obtained by the CO₂ direct bond-breaking mechanism on the surface of PdIn（110）,while on PdIn（211）CO is formed through a hydrogen assisted dissociation path with COOH as an intermediate.Microkinetic modelings show that the coverage of HCOO is rather high on both surfaces at steady state.Therefore,we estimate the methanol formation rate at different HCOO coverages based on the assumption that the rate controlling step is unchanged.The results show that when the coverage effect of formate on the surface of the catalyst is considered,the rate of methanol formation increases significantly,and is higher than the rate of methanol formation on the surface of Cu/ZnO/Al₂O₃ catalyst measured in the experiment,which is consistent with the results reported in the literature.4.Using PdIn（310）as a model,the mechanism of CO₂ hydrogenation to methanol at the defect site on the surface of PdIn catalyst was studied by the combination of DFT calculations and microkinetic modeling.The results show that on the surface of clean PdIn（310）,methanol generates from the hydrogenation of CO,which is produced through the dissociation of COOH.However,the microkinetic modeling results show that the surface Pd sites and In sites are almost completely occupied by CO and HCOO,respectively,so it is necessary to consider the coverage effect of the two in the simulations.On this basis,we found that when there are two（corresponding to 0.5 ML coverage）formate pre-adsorbed on the surface of the catalyst,the adsorption energy of the third formate at the Pd and In sites is equivalent,indicating that under this coverage,HCOO is likely to adsorb at both the Pd site and the In site.Further studies have found that when considering the pre-adsorption of formate on the surface,the reaction mechanism of methanol formation is the reaction path containing HCOO and HCOOH intermediates,and the rate of methanol formation also increases.5.Based on the above studies,it can be found that the coverage of the main adsorbates on the catalyst surface is crucial for us to understand the actual reaction from the perspective of simulation.Therefore,this thesis develops an iterative microkinetic modeling method based on DFT calculation for self-consistent surface species coverage.The method is described and illustrated by the example of CO₂ hydrogenation over GaPd₂（010）.First,we used DFT calculation and microkinetic modeling to study the reaction process on the clean GaPd2（010）.The results show that the coverage of CO on the surface at steady state is inconsistent with the coverage of CO in the DFT calculations,and the methanol selectivity is almost zero at this time,which is inconsistent with the experimental results.On this basis,we calculated the trend of CO adsorption energy with coverage,and found that when the coverage of pre-adsorbed CO reaches ¹/₆ monolayer,the adsorption energy starts to change.Therefore,we use this coverage as the starting point to gradually carry out the iterative surface species coverage analysis,and found that only when the surface species coverage is self-consistent,the reaction kinetics results become consistent with the experiment.