Theoretical study of reduction mechanism of Fe_(2)O_(3) by H_(2) during chemical looping combustion

摘要

An atomic-level insight into the H_(2)adsorption and oxidation on the Fe_(2)O_(3)surface during chemicallooping combustion was provided on the basis of density functional theory calculations in this study.The results indicated that H_(2)molecule most likely chemisorbs on the Fe_(2)O_(3)surface in a dissociative mode.The decomposed H atoms then could adsorb on the Fe and O atoms or on the two neighboring O atoms of the surface.In particular,the H_(2)molecule adsorbed on an O top site could directly form H_(2)O precursor on the O_(3)-terminated surface.Further,the newly formed H-O bond was activated,and the H atom could migrate from one O site to another,consequently forming the H_(2)O precursor.In the H_(2)oxidation process,the decomposition of H_(2)molecule was the rate-determining step for the O_(3)-terminated surface with an activation energy of 1.53 eV.However,the formation of H_(2)O was the ratedetermining step for the Fe-terminated surface with an activation energy of 1.64 eV.The Feterminated surface is less energetically favorable for H_(2)oxidation than that the O_(3)-terminated surface owing to the steric hindrance of Fe atom.These results provide a fundamental understanding about the reaction mechanism of Fe_(2)O_(3)with H_(2),which is helpful for the rational design of Fe-based oxygen carrier and the usage of green energy resource such as H_(2).