Electrocatalytic Reduction of Nitrogen to Ammonia: the Roles of Lattice O and N in Reduction at Vanadium Oxynitride Surfaces

Abstract

Vanadium oxynitride and other earth-abundant oxynitrides are of growing interest for the electrocatalytic reduction of nitrogen to NH 3 . 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 N 2 are reduced to NH 3 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 N 2 and involves the formation of VN: intermediates or similar unsaturated VN surface states on a thin vanadium oxide overlayer. In contrast, N 2 reduction proceeds in the presence or absence of lattice N and without N incorporation into a vanadium oxide lattice. Thus, both lattice N and N 2 reduction mechanisms involve oxide-supported V surface sites ([V] O ) in preference to Nsupported 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.

Electrocatalytic selectivity for nitrogen reduction vs. hydrogen evolution: a comparison of vanadium and cobalt oxynitrides at different pH values

“…Similar experiments on vanadium oxynitride at pH 10 reveal that V oxynitride films – similar to those previously examined for NRR at pH 7, 15 are inactive for NRR at pH 10, and inactive for HER at both pH 7 and pH 10. For the first time, these findings demonstrate that HER/NRR relative selectivity can vary substantially with TMON films for different transition metals under identical reaction conditions.…”

“…1b–d), approximating that of the FTO substrate. 15 All films used for experiments were deposited using similar parameters and had similar atomic ratios as determined from in situ AES and ex situ XPS data. Scanning electron microscopy/energy dispersive spectroscopy studies of a CoO o.8 N 0.2 film are shown in Fig.…”

“…Characterization (AFM, XRD, XPS) of V oxynitride films deposited by DC magnetron sputtering in this manner has been described previously. 15 Ex situ AES spectra are shown in Fig. S4† and correspond to a composition (after ambient exposure) of VO 0.4 N 0.6 .…”

“…28 Peak fitting employed Gaussian–Lorentzian components and was carried out as described previously. 15,24 XPS binding energies were calibrated to C 1s at 284.8 eV. 29…”

Metal Oxynitrides for the Electrocatalytic Reduction of Nitrogen to Ammonia

“…The role of lattice O and N in e-NRR on vanadium oxynitride (VON) surfaces has been studied by DFT . Plausible reaction intermediates were consistently predicted to be energetically favored at O-supported V surface sites rather than at N-supported sites.…”