The Theoretical Calculation Study On Carbon Dioxide Hydrogenation Over Iron-based Catalysts

The catalytic conversion of carbon dioxide(CO2)to value-added chemicals,especially high-carbon chain hydrocarbons,is of great academic value.Iron-based catalysts have been widely used in experimental research,but the active species and reaction mechanisms over the active site of the catalyst are not yet clear.Theoretical computational research plays an important role in elucidating the active species and reaction mechanisms over the active site of the catalyst.In this thesis,the density functional theory(DFT)method was used to study the reaction mechanism of carbon dioxide hydrogenation on different active phases of iron-based catalysts(e.g.metal and metal carbides),to clarify the active sites for carbon dioxide and hydrogen adsorption,activation and its electronic interaction with adsorbed species.The key intermediates and reaction pathways for CO2 hydrogenation to olefins are investigated to identify the important factors that affect the selectivity of products.By exploring the correlationship between active site or electronic effect-reaction pathway-activity or selectivity,combined with micro-kinetic model,it is revealed that the active site promotes the conversion of carbon dioxide to light olefins by controlling specific reaction paths.These key factors provide a theoretical basis for the design of catalysts.The results are summarized as follows.By constructing different facet models of iron,the influence of the surface on the adsorption and activation of carbon dioxide and hydrogen was investigated.The charge transferred to carbon dioxide and hydrogen atoms from different iron surfaces is positively correlated with carbon dioxide and hydrogen adsorption energy,indicating that the adsorption of small molecules is caused by the electron transfer to carbon dioxide and hydrogen atoms through the catalyst.Surface H*coverage impacts the adsorption stability of CO2.When the H*coverage becomes higher than specific monolayer,CO2 adsorption energy substantially decreases.Therefore,a suitable surface carbon-hydrogen ratio has an important effect on carbon dioxide adsorption.Different dissociation and hydrogenation barriers were identified over iron surfaces.By adjusting the surface of the catalyst,the carbon dioxide hydrogenation reaction path can be effectively changed,thereby adjusting product distribution of carbon dioxide hydrogenation reaction.By designing iron-based catalyst models with different transition metal doping ratios on different crystal facets,the effect of transition metal doping on the adsorption and activation of carbon dioxide and hydrogen was investigated.Transition metal doping can adjust the surface active site and electronic structure of the iron-based catalyst,thereby affecting the adsorption energy of carbon dioxide and hydrogen atoms,changing the initial activation and hydrogenation pathway of carbon dioxide,thus regulating the relative selectivity of the reaction products.Different transition metal doping has different effects on carbon dioxide adsorption and hydrogenation.The effect of the transition metal on the adsorption energy of carbon dioxide on iron is greater than that of the hydrogen atom on iron.Among them,the doping of Co and Ni shows less influence on the adsorption energy of carbon dioxide than the doping of Cu and Pd.The doping of Cu and Pd can reduce the adsorption energy of carbon dioxide and regulate the ratio of carbon to hydrogen on the surface of the catalyst.An appropriate ratio of Cu-doped Fe(100)surface can reduce the carbon-carbon coupling barrier.After Cu doping,it can promote the growth of C2 hydrocarbons and improve the selectivity of C2 hydrocarbons.The results of the theoretical calculation are in general agreement with the experimental results.The complete reaction pathways of CO2 hydrogenation to methane,ethylene,ethane and carbon monoxide were systematically investigated on the ?-Fe3C catalyst model.Combining DOS,differential charge density and ELF electronic structure analysis methods,carbon dioxide tends to dissociate directly on the ?-Fe3C(031)catalyst to form CO*(and O*)intermediates,which tends to desorb due to the higher energy barrier for CO*conversion.Carbon monoxide is formed instead of further conversion to hydrocarbons.Combined with the micro-kinetic model,the coverage of H*species on ?-Fe3C(031)catalyst is high,which can effectively inhibit the further oxidation of the catalyst,indicating that the catalyst has a certain stability.Carbon monoxide has the highest relative selectivity in experimental work in general,which also confirms the data of reaction energy barrier.The reaction pathways leading to methane,ethylene,ethane and carbon monoxide by hydrogenation of carbon dioxide on the ?-Fe5C2(510)surface were systematically investigated.Using COHP,differential charge density and other analytical methods,the dominant reaction path for generating hydrocarbons is determined by hydrogenation of carbon dioxide to generate HCOO*intermediates,continuous dissociation to generate CH*species,and further hydrogenation or carbon-carbon coupling to generate methane and ethylene,ethane.To determine the kinetic control steps of carbon dioxide hydrogenation and the relative selectivity of key products,micro-kinetic models were built based on the results derived from DFT.During the hydrogenation of carbon dioxide,O*species accumulate on the carbide surface and can oxidize the iron carbide catalyst.By constructing the iron oxide/iron carbide interfacial site or introducing potassium promoter,the reaction energy barrier for hydrogenation of O*species can be effectively reduced.The removal of O*species can be promoted,the oxidation resistance of the catalyst can be improved by introducing the iron oxide/iron carbide interfacial site or potassium promoter,thus provide more active sites and improve the selectivity of higher hydrocarbons.