Theoretical Study Of Active Structures Of Cu And Ag Catalysts In CO_x Conversions

In nowdays with serious environmental and energy problems,using harmful COx molecules as carbon resource for efficient catalytic conversions to target products is of particular importance,whose core lies at innovation of efficient catalysts.Identifications of catalytic active structures and catalytic reaction mechanisms can facilitate the R&D of efficient catalysts,but are difficult to be realized merely by experimental studies.In this thesis,density-function theory(DFT)-based theoretical calculations have been used,in collaboration with experimental studies,to construct model structures of working catalysts and calculate surface reaction networks for comprehensive studies of active structures and reaction mechanisms of ZnO/Cu and Ag/TiO2 catalysts in COx conversion reactions,with an aim at providing theoretical guidelines for structural designs of efficient catalysts.The main results are summarized as the following:1)Based on the experimental results that ZnO/Cu nanocrystal catalysts exhibit obvious facet-dependent activity in catalyzing water gas shift(WGS)reaction,CO2 hydrogenation to methanol reaction and CO2 hydrogenation to CO reaction(reverse water gas shift reaction,RWGS),model surfaces of ZnO/Cu(111),ZnO/Cu(100)and ZnO/Cu(110)are constructed,on which the reaction pathways of WGS and CO2 hydrogenation reaction are calculated in detail.Hydroxylated ZnO-Cu interface is the active structure for catalyzing the WGS reaction with the water dissociation as the rate-determining step,and the hydroxylated ZnO/Cu(100)interface is more active than the hydroxylated ZnO/Cu(111)interface;ZnO-Cu interface is the active structure for catalyzing the RWGS reaction with the decomposition of HOCO intermediate as the ratedetermining step,and the ZnO/Cu(100)interface is more active than the ZnO/Cu(110)and ZnO/Cu(111)interfaces;ZnO/Cu interface is the active structure for catalyzing CO2 hydrogenation to methanol reaction with the formation and desorption of CH3OH(a)intermediate as the key steps,and the ZnO/Cu(110)interface is more active than the ZnO/Cu(100)and ZnO/Cu(111)interfaces.2)Based on the experimental results that CuZn alloy is the active structure for catalyzing CO hydrogenation to methanol reaction and its formation and catalytic activity sensitively depend on the Cu structure,model surfaces of ZnO/Cu(111),ZnO/Cu(211),ZnO/Cu(100)and ZnO/Cu(611)are constructed,on which the formation energy of CuZn alloy and CO hydrogenation reaction network are calculated.The activation energy for CuZn alloy formation on various Zn/Cu surfaces follows an order of ZnO/Cu(111)>ZnO/Cu(100)>ZnO/Cu(211)>ZnO/Cu(611);meanwhile,CO hydrogenation to methanol reaction and CO hydrogenation to methane reaction compete on the CuZn surface,but the former is more favorable,the energetic favorable reaction path of CO hydrogenation reaction follows CO→HCO→H2CO→H3CO→CH3OH,CO hydrogenation to methanol reaction proceeds more facile on the ZnCu(611)alloy than on the ZnCu(211)alloy;On Cu(211)and Cu(611)surfaces,the energetic favorable reaction path of CO hydrogenation reaction follows CO→HCO→CHOH→H2COH→ H3COH→ CH3→CH4 and CO hydrogenation to methane proceeds more facilely on Cu(611)than on Cu(211).3)Based on the experimental results that Ag can wet TiO2 nanorods to form Ag@TiO2 core@shell structure with a higher catalytic activity than corresponding Ag/TiO2 catalysts with Ag-TiO2 interfaces in CO oxidation reaction,DFT calculations are carried out to study Ag growth on stoichiometric and defective TiO2 surfaces and CO oxidation on Ag film/TiO2x surface and Ag cluster/TiO2 surface.Surface oxygen vacancies on TiO2 surface exhibit greatly enhancd Ag-TiO2 interaction;Ag exhibit a twodimensional layer-by-layer growth mode on TiO2 surface with enough high coverage of surface oxygen vacancies to form wetting Ag film,but a threedimensional growth mode on stoichiometric TiO2 surface to form Ag clusters;The wetting Ag film on TiO2 surface with surface oxygen vacancies is more active in catalyzing CO oxidation than the Ag-TiO2 interface on Ag/TiO2 surface.