(665g) Phase-Dependent Promoting Effect of Surface Oxygen on Molybdenum Carbide Catalysts during Formic Acid Electrooxidation

Transition metal carbides (TMC) have long been heralded as inexpensive substitutes for Pt-group metals (PGMs) owing to similarities in chemical activity and electronic structure. TMCs offer vast catalyst design space owing to their particle-size-dependent polymorphism, tunable non-stoichiometric composition, and phase-dependent catalytic activity. Compared to Pt-group-based catalysts, TMCs exhibit higher stability and resistance to corrosion in their electrochemical operation, thus making them attractive low-cost electrocatalysts for sustainable energy generation and conversion. Unlike PGMs, TMCs are very oxophilic, which may limit their catalytic efficiency in oxidation and electrooxidation reactions due to the blocking of the catalytic sites by O* species. Contrary to this expectation, recent experimental findings reveal the significant co-catalytic activity of TMCs in the electrooxidation of methanol[1], ethanol, and formic acid[2]. The underlying mechanism for this unusual catalytic activity remains elusive.

In this work, we employ a combination of density functional theory (DFT), ab initio thermodynamics, and reaction rate theory to unravel the source of TMC catalytic activity in oxidation reactions. We investigate a model reaction system, namely formic acid electrooxidation on the meta-stable α-MoC_1-x phase, which was selected based on its reported excellent catalytic activity in low-temperature water-gas shift and methanol oxidation. The kinetics-cognizant surface Pourbaix diagram that we developed reveals in situ surface modification at varying electrochemical conditions. The origin of α-MoC_1-x high catalytic activity is attributed to unique oxycarbide motifs which weaken otherwise strong surface-adsorbate interactions as supported by the calibrated Bader charge analysis and energy decomposition analysis. We hypothesize that this counterintuitive promotional effect of in situ-deposited surface oxygen may be present in a range of electrocatalytic and thermocatalytic reactions on other TMCs.

References:

[1] L Lin et al., Nature 544 (7648), 80 (2017).

[2] M Yin et al., Journal of Power Sources 219, 106 (2012).