Role of Surface Oxygen in α-MoC Catalyst Stability and Activity under Electrooxidation Conditions

Abstract

Transition metal carbides are attractive, low-cost alternatives to Pt group metals, exhibiting multifunctional acidic, basic, and metallic sites for catalysis. Their widespread applications are often impeded by a high surface affinity for oxygen, which blocks catalytic sites. However, recent reports indicate that the α-MoC phase is a stable and effective cocatalyst for reactions in oxidative or aqueous environments. In this work, we elucidate the factors affecting the stability and catalytic activity of α-MoC under mild electrooxidation conditions (0–0.8 V SHE) using density functional theory calculations, kinetics-informed surface Pourbaix diagram analysis, electronic structure analysis, and cyclic voltammetry. Both computational and experimental data indicate that α-MoC is significantly more resistant to electrooxidation by H₂O than β-Mo₂C. This higher stability is attributed to structural and kinetic factors, as the Mo-terminated α-MoC surface disfavors substitutional oxidation of partially exposed, less oxophilic C* atoms by hindering CO/CO₂ removal. The α-MoC surface exposes H₂O-protected [MoC₂O₂] and [MoC(CO)O₂] oxycarbidic motifs available for catalysis in a wide potential window. At higher potentials, they convert to unstable [Mo(CO)₂O₂], resulting in material degradation. Using formic acid as a probe molecule, we obtain evidence for Pt-like O*-mediated O–H and C–H bond activation pathways. The largest kinetic barrier, observed for the C–H bond activation, correlates with the hydrogen affinity of the site in the order O*/Mo_fcc > O*/C_top > O*/Mo_top. To mitigate the site-blocking effect of surface-bound H₂O and bidentate formate, doping with Pt was investigated computationally to make the surface less oxophilic and more carbophilic, indicating a possible design strategy toward more active and selective carbide electrocatalysts.