Theoretical Investigation Into The Reaction Mechanism Of Reverse Water Gas Shift Reaction On Fe₃C-Based SAC Catalysts

In recent years,Fe₃C has attracted much attention as the active phase in iron-based Fischer-Tropsch synthesis（FTS）and reverse water-gas shift（RWGS）reactions.However,the morphology change of Fe₃C catalyst under FTS condition is not clear,and the mechanism of how single atom regulates the reverse water-gas shift reaction is unknown.Based on this,the stability of Fe₃C particles in CO atmosphere was studied by using density functional theory（DFT）and atomic thermodynamics,and then the mechanism of the reaction of CO₂to CO catalyzed by Fe₃C-based single atom catalysts（SACs）was studied.In the first part of this thesis,the adsorption of CO on seven Fe₃C surfaces with high coverage is studied.It is found that CO preferentially adsorbs near the surface iron atoms,and the saturation coverage on different surfaces was 0.5 to 0.833 ML.It is also found that on the surface of Fe₃C（001）,（111）,（100）and（010）,CO preferentially binds on top of iron atom;on the surface of Fe₃C（011）,CO prefers to adsorb on the bridge sites of two iron atoms;while on the surface of Fe₃C（101）and（110）,CO preferentially adsorbed in the hollow site.Using atomic thermodynamics,we have plotted thermodynamic phase diagrams of CO on different Fe₃C surfaces,which enable us to easily determine the CO concentration at different pressures and temperatures.Furthermore,the adsorption mechanism of CO on different surfaces is analyzed by the DOS and charge density difference.Finally,using Wulff construction,it is found that adsorbed CO molecules could lead to changes in the morphology of Fe₃C nanoparticles.In the second part of this thesis,the mechanism of CO₂reduction reaction on clean Fe₃C surface and four Fe₃C-based SAC catalysts is investigated.It is found that the carboxyl group path is more favorable on the clean Fe₃C（011）surface and Cu@Fe₃C（011）（the Fe₃C（011）surface with one iron atom replaced by one Cu）,the redox path has obvious advantages on Ni@Fe₃C（011）and Ru@Fe₃C（011）surfaces,while formic acid path has obvious advantage on the surface of Pt@Fe₃C（011）.By comparing the reaction energies of the clean Fe₃C surface and the Fe₃C-based SAC catalyst surface,it is found that the desorption of CO*is the decisive step of the whole RWGS,and the Pt@Fe₃C（011）surface is favorable for the occurrence of RWGS.The d-band center theory explains that Pt@Fe₃C（011）surfaces are prone to RWGS because their d-band centers are far away from the Fermi level,which favors the desorption of CO.This study provides a theoretical basis for the design of high performance RWGS iron carbide-based SACs catalysts.