Effects of Nonstoichiometry on the Ambient Oxidation of Cubic Mo ₂ C and Impact on CO Hydrogenation

Hydrogenation of CO – Fischer–Tropsch synthesis (FTS), is of great interest for economic and environmental reasons to the synthetic fuel industry. A major challenge of prospective transition metal carbide catalysts, however, is the role that metal/carbon stoichiometry plays in catalyst surface oxidation and how this impacts catalyst activity and selectivity, which are hard to study with practical powdered catalysts. In this work, in situ X-ray Photoelectron Spectroscopy (XPS) and Constant Temperature Mass Spectrometry (CTMS) are used to investigate the response to oxidation of two cubic molybdenum carbide thin films synthesized by DC magnetron sputter deposition at two different temperatures: 300 and 700 K, giving rise to c-Mo1.3C and c-Mo2C, respectively. For c-Mo2C, a Mo(V) surface oxide is formed after ambient exposure and under reaction conditions (250 °C, PCO+H2 = 8 × 10–4 Torr, PCO/PH2 ∼ 2). c-Mo2C remains catalytically active yielding O2 and C3H8. In contrast, c-Mo1.3C forms a thicker surface oxide, including Mo(VI), upon ambient exposure, yielding CO2 without a trace of either O2 or C3H8 under the same reaction conditions. After surface Mo(VI) oxides were removed from c-Mo1.3C, O2 and C3H8 are formed under reaction conditions in the presence of Mo(V) oxide. Thus, Mo vacancies in c-Mo1.3C are specifically implicated in the formation of Mo(VI)─a d0 oxidation state─and thus the loss of FTS activity. In contrast, stoichiometric c-Mo2C is stabilized against oxidation to Mo(VI) both in ambient conditions and under the reaction conditions employed and is therefore a promising Earth-abundant FTS catalyst.