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

Abstract

CH 4 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 CH 4 on thermodynamically stable hematite (α-Fe 2 O 3 ) (0001) facets are investigated with periodic GGA + U calculations. Upon Fe–O 3 –Fe-termination, initial CH 4 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 CH x species ( x = 1, 2, 3) exploit the lattice O species according to the Mars–van Krevelan mechanism, forming CH x O 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, CH 4 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.