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.

Automated summary

The valence state of Mo plays a critical role in the performance of catalysts for ammonia synthesis and nitrogen reduction reaction. However, the dissolution of Mo as MoO42- in alkaline electrolytes has hindered the study of the valence effect of Mo in nanoparticle catalysts. In this study, a MoO2+x catalyst enriched with surface Mo6+ was developed, which showed enhanced ammonia yield and Faradaic efficiency compared to the MoO2 catalyst without Mo6+. The stability and speciation of Mo6+ in the catalyst were also analyzed.