Methane-Methanol Conversion Catalyzed by Hematite (α-Fe2O3) (0001) with Fe-O3-Fe- and O-Fe-O3- (Ferryl) Terminations: A DFT Study
Jia-Jun Tang$ and Bin Liu$
$Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506
Recent interest in Direct Methane To Methanol (DMTM) is driven by the strong need of low-scale technologies capable of operating on low resources, remote and unconventional gas fields. In DMTM, the direct partial oxidation of methane to methanol is of particular interest and deemed to be one of the most promising industrial processes. However, economically viable one-step oxidation of methane to methanol is elusive.
In this paper, we use density-functional theory approaches to study the catalytic properties of hematite (α-Fe2O3) (0001) for methane activation and its conversion to methanol. Specifically, the key step of methane activation, i.e. C-H bond activation of methane, is of the main focus. Specifically, two lowest energy terminations of hematite (α-Fe2O3) (0001) are considered, and their relative stabilities are compared to determine the surface stabilities at various oxygen chemical potentials. The results show that, at oxygen-rich conditions, experimentally observed Fe-O3-Fe- and ferryl (O-Fe-O3-) terminated surfaces are possible.
Fe-O3-Fe- and ferryl (O-Fe-O3-) terminations exhibit dramatically different behaviors towards methane activations. The activation energy of methane on bare Fe-O3-Fe- terminated surface is 1.04 eV, forming CH3 an H. Further oxidation with O adatom requires additional 1 eV, forming methoxy. Subsequent methanol formation from methoxy is relatively facile. Hence, methane and oxidation are considered as rate-limiting. On ferryl termination, the surface is, on the other hand, highly oxidative due to the presence of unsaturated O species. The activation energy of methane is reduced to 0.35 eV. Subsequent oxidations are also exothermic, with low methanol selectivity predicted. This study reveals that the key in realizing DMTM on iron oxide-based catalyst depends on the control oxygen activity. Structures of active sites and forms of oxidants shall play an important role.