Hydrogenation of CO to produce alkanes is of great significance for synthetic fuel production. The formation of alkanes using non-petroleum sources yields fuels without significant S or N contamination and is therefore of environmental importance. A central problem for Earth-abundant catalysts, however, is surface oxidation. For existing Earth-abundant catalysts (Fe, Co), oxidation can significantly alter and degrade catalyst function. Here, we demonstrate that nearly stoichiometric mixture of cubic phase Mo_2-xC with interspersed MoO₂(c-Mo_1.9C/MoO₂) —produced by DC magnetron sputter deposition from a commercially available Mo₂C target at 700 K, and characterized by in situ Auger electron spectroscopy (AES), and ex situ x-ray photoelectron spectroscopy (XPS), x-ray diffraction XRD, atomic force microscopy (AFM) and scanning electron microscopy (SEM)-- is resistant to ambient oxidation to Mo⁺⁶. In contrast, deposition at room temperature yields a Mo-deficient phase (c-Mo_1.3C/MoO₂), which produces significant amounts of Mo⁺⁶ surface oxide upon ambient exposure. In situ XPS and temperature programmed reaction spectroscopy (TPRS) measurements demonstrate that c-Mo₂C/MoO₂ directly induces CO bond scission at 523 K, even in the absence of H₂. Reaction of CO with H₂ at 523K and a CO/H₂ partial pressure ratio of 2:1, and total pressure of 1 x 10^-4Torr, yields propane and smaller hydrocarbons. Such reactivity is observed both after ambient exposure and after surface cleaning by Ar ion bombardment in ultrahigh vacuum (UHV). In contrast, c-Mo_1.3C/MoO₂ when oxidized by ambient exposure, produces CO₂, under the same conditions. However, removal of the surface oxide phase by Ar ion bombardment in UHV results in production of propane and smaller hydrocarbons under the above reaction conditions. These results indicate that stoichiometric c-Mo₂C is oxidation-resistant and selective for alkane production under mild reaction conditions. Oxidation-resistant c-Mo₂C thus demonstrates a significant potential for C≡O activation/hydrogenation.

Renuka Rathnayake Mudiyanselage

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