Surface Valence State Effect of MoO2+x on Electrochemical Nitrogen Reduction

The valance of Mo is critical for FeMo cofactor in ambient ammonia synthesis. However, the valence effect of Mo has not been well studied in heterogeneous nanoparticle catalysts for electrochemical nitrogen reduction reaction (NRR) due to the dissolution of Mo as MoO42- in alkaline electrolytes. Here, a MoO2+x catalyst enriched with surface Mo6+ is reported. The Mo6+ is stabilized by a native oxide layer to prevent corrosion and its speciation is identified as (MoO3)(n) clusters. This native layer with Mo6+ suppresses the hydrogen evolution significantly and promotes the activation of nitrogen as supported by both experimental characterization and theoretical calculation. The as-prepared MoO2+x catalyst shows a high ammonia yield of 3.95 mu g mg(cat)(-1)h(-1) with a high Faradaic efficiency of 22.1% at -0.2 V versus reversible hydrogen electrode, which is much better than the MoO2 catalyst with Mo6+ etched away. The accuracy of experimental results for NRR is confirmed by various control experiments and quantitative isotope labeling.