Reactivity of the Fe2O3(0001) Surface for Methane Oxidation: A GGA plus U Study

CH4 oxidation by an oxygen carrier, such as iron oxide, continues to be involved in many important valuable industrial catalytic processes, including chemical looping combustion. In this paper, reaction pathways of complete and partial oxidations of CH4 on thermodynamically stable hematite (alpha-Fe2O3) (0001) facets are investigated with periodic GGA + U calculations. Upon Fe-O-3-Fe-termination, initial CH4 decomposition proceeds via C-H bond activation on the Fe site, with an energy barrier of 1.04 eV. Subsequent decomposition and oxidation of the CHx species (x = 1, 2, 3) exploit the lattice O species according to the Mars-van Krevelan mechanism, forming CHxO in more thermodynamically and kinetically favorable pathways. The reduced iron oxide can be reoxidized with O-2 as the oxidant, allowing V-o to greatly facilitate O-2 dissociation, i.e., dramatically lowering the O-2 dissociation barrier by 2.83 eV, for active site regeneration. Furthermore, CH4 oxidation chemistry involving the ferryl O (i.e., oxygen species adsorbed on the Fe site) is also investigated. The ferryl O species are highly energetic and able to activate the C-H bond via direct oxygen insertion, thereby producing methanol with an energy barrier of 0.37 eV. However, the role of surface ferryl O in iron oxide is limited due to its high reactivity and low concentration indicated by the high surface energy of the O-Fe-O-3-terminated surface.