Study Of The Reaction Mechanism Of Ethylbenzene Dehydrogenation In The Presence Of Carbon Dioxide

Density functional theory has been performed to study the reaction mechanism of ethylbenzene dehydrogenation over Fe2O3 catalyst in the presence of CO2. Possible three terminations (Fe-, Ferryl- and O-termination) of Fe2O3(0001) surface under experimental conditions have been constructed to study catalytic activity, H removal and role of CO2. In order to study effects of reductive catalyst in the process of dehydrogenation, reactions of CO2 with H on the Fe3O4(111) surface are investigated.Through studying dehydrogenation of ethylbenzene on the Fe2O3(0001) surface, it is found that on all of the three terminations, the C-H activation in the methylene group followed by the dehydrogenation of the methyl group is energetically more favorable. The O-termination is much more highly reactive than other two terminations, but the produced styrene is hard to be desorbed from this termination, leading to the formation of side products and coke. On the Fe-terminated surface, H is removed in the form of H2 and H2O. Due to a lower energy barrier for H2O formation from reactions of CO2 with H, H2O should be dominant products. On the Ferryl- and O-terminated surface, H binds with the surface O atom to form H2O, then the desorbed H2O can result in 0 vacancy, following reductive surface. Due to so high energy barrier for CO2 oxidizing defective surface that the reductive surface cannot be reoxidized, the catalyst finally suffers deactivation. Overall analysis indicates that the Fe-terminated surface is dominant reactive surface under experimental conditions.The investigations on reactions of H with CO2 on the Fe3O4(111) surface show that the energy barrier for H2O formation from reactions of H with CO2 on the Fe3O4(111) surface is much lower than that on the Fe2O3(0001) surface. This result indicates that the partially reductive Fe2+ is favorable for H removal, consequently improving conversion of ethylbenzene. However, Fe3+ is activation center of ethylbenzene dehydrogenation, thus excess reduction will cause deactivation of catalyst. Hence, a certain ratio of Fe3+ to Fe2+ could make catalytic activity excellent.