Electrocatalytic Reduction of Nitrogen to Ammonia: the Roles of Lattice O and N in Reduction at Vanadium Oxynitride Surfaces
Adaeze Osonkie, Ashwin Ganesan, Precious Chukwunenye, Fatima Anwar, Kabirat Balogun, Mojgan Gharaee, Ishika Rashed, Thomas R. Cundari, Francis D’Souza, Jeffry A. Kelber
Abstract
Vanadium oxynitride and other earth-abundant oxynitrides are of growing interest for the electrocatalytic reduction of nitrogen to NH3. A major unresolved issue, however, concerns the roles of lattice N and lattice O in this process. Electrochemistry and photoemission data reported here demonstrate that both lattice N and dissolved N2 are reduced to NH3 by cathodic polarization of vanadium oxynitride films at pH 7. These data also show that ammonia production from lattice N occurs in the presence or absence of N2 and involves the formation of V≡N: intermediates or similar unsaturated VN surface states on a thin vanadium oxide overlayer. In contrast, N2 reduction proceeds in the presence or absence of lattice N and without N incorporation into a vanadium oxide lattice. Thus, both lattice N and N2 reduction mechanisms involve oxide-supported V surface sites ([V]O) in preference to N-supported sites ([V]N). This result is supported by density functional theory-based calculations showing that the formation of V≡N:, V–N═N–H, and a few other plausible reaction intermediates is consistently energetically favored at [V]O rather than at [V]N surface sites. Similar effects are predicted for the oxynitrides of other oxophilic metals, such as Ti.