Investigation On The Active Phases And Their Dynamic Evolution In Iron-based Catalysts During CO₂ Hydrogenation

The rising concentration of CO2 in the atmosphere has a strong impact on climate change such as global warming.Driven by "carbon peaking and carbon neutrality goals" of Chinese government,catalytic hydrogenation of CO2 to high value-added hydrocarbon chemicals and fuels is an effective strategy to alleviate related environmental and energy problems.Fe-based catalysts are active to catalyze CO2 hydrogenation to hydrocarbons.However,there are usually various Fe phases in Fe-based catalysts and all are dynamic during CO2 hydrogenation,which brings challenges to understanding the active centers and establishing the structureperformance relationship.Therefore,in this thesis,multiple in situ techniques and theoretical calculations were used to investigate the active phase in Fe catalysts and its dynamic evolution during CO2 hydrogenation.The main research contents and results are as follows:The bulk and surface structure of Fe(0)catalysts at different time on stream in CO2 hydrogenation were investigated by using multiple(quasi-)in situ techniques,and the correlation between CO2 hydrogenation performance and the dynamic structural evolution was established.Under reactant atmosphere,the surface of Fe(0)nanoparticle was carburized to Fe3C and then to the carbon-rich Fe5C2.Subsequently,the by-product water oxidized the surface,and the Fe3O4@(Fe5C2+Fe3O4)core-shell structure was formed at the steady state.The catalytic performance also exhibits the corresponding three stages,induction,deactivation and steady state,where CO2 conversion together with the selectivity of C2+ hydrocarbons increased,decreased and remained unchanged.Theoretical calculations and thermodynamic analysis reveal the interaction and interdependence of the dynamic surface phase transformation and the reactive microenvironment.The surface structure was determined by the chemical potential of carbon and oxygen and the dynamic balance of competitive carburization and oxidation.Since Fe(0)evolves into Fe5C2,Fe3C and Fe3O4 during CO2 hydrogenation,the corresponding catalysts were prepared by different in situ treatment on the Fe2O3 precursor.The CO2 conversion rate and product distribution were quantitatively analyzed and the reaction pathways were investigated.On Fe5C2 and Fe3C,CO2 can be converted into hydrocarbons by subsequent reverse water gas shift(RWGS)reaction and Fischer-Tropsch synthesis(FTS)process,and can also be directly hydrogenated to CH4 and C2+ hydrocarbons without undergoing CO intermediates.Fe3O4 can only catalyze RWGS reaction.The CO2 conversion rate over Fe5C2 is slightly higher than that over Fe3C,and 1-2 orders of magnitude higher than that over Fe3O4.Fe5C2 is shown as the main active phase,and the reaction networks of CO2 hydrogenation involving Fe3C,Fe5C2 and Fe3O4 is proposed.Taking the reactivity as a probe to determine the surface structure,the understandings were established that the surface of different Fe-based catalysts evolve into the mixture of FeCx and Fe3O4.The influence of the particle size of active phase Fe5C2 on CO2 hydrogenation performance was investigated on supported Fe catalysts.Within 2.5-12.9 nm,the CO2 conversion rate is low and the selectivity of by-product CO is high on small particles.The conversion rate and C2+selectivity are high on large particles.The formation rate(shown as TOF)of C2+hydrocarbons over Fe5C2 nanoparticles at 12.9 nm is 7.3×10-3 s-1,20 times higher than that at 2.5 nm.Based on the results of the reaction pathways and in situ characterizations,the particle size effect was deconvoluted into primary and secondary reactions.The primary reactions involve RWGS and methanation,and the large particles are prone to generate HCOO*species for CO2 methanation.The secondary reaction is FTS and the large particles have a higher chain growth ability for CC coupling.The effect of K promoters on the reactive microenvironment,the structural evolution of Fe catalysts and the formation and stabilization of active Fe5C2 was investigated.K can enhance CO2 adsorption,promoting the reaction of CO2 with the dissociated H on the neighboring Fe sites.The in situ characterizations showed that K promotes RWGS reaction and the product CO accelerates the carburization of Fe(0)to Fe3C and then to Fe5C2,and stabilizes Fe5C2 during CO2 hydrogenation.The addition of K promoters and CO pretreatment endow the Fe catalysts high formation rates of C2-C4 olefins and C5+ long-chain hydrocarbons,which are 32.8 and 35.5 mmolCO2·h-1·gFe-1,respectively.