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.

Electroreduction of N2 to NH3 is an energy- and environmentally-friendly alternative to the Haber-Bosch process. Little is known, however, about reactive sites for electrochemical nitrogen reduction reaction (NRR) at Earth-abundant oxide or oxynitride surfaces. Here, we report N-free VIII/IV-oxide films, created by O2 plasma oxidation of polycrystalline vanadium, exhibiting N2 reduction at neutral pH with an onset potential of −0.16 V vs Ag/AgCl. DFT calculations indicate that N2 scission from O-supported V-centers is energetically favorable by ∼18 kcal mol−1 compared to N-supported sites. Theory and experiment yield fundamental insights concerning the effect of metal oxophilicity towards design of earth-abundant NRR electrocatalysts.

The electrocatalytic nitrogen reduction reaction (NRR) is of significant interest as an environmentally friendly method for NH3 production for agricultural and clean energy applications. Selectivity of NRR vis-à-vis the hydrogen...

Interactions of N2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction mechanisms. Interactions of N2 at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and total pressures up to 10-1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with V(III) and V(V) components. XPS spectra were acquired in environments of both pure N2 and equal pressures of N2 and H2O vapor. In pure N2, broad, partially resolved N1s features were observed at 401.0 eV and 398.7 eV binding energy, with relative intensities of ~ 3:1, respectively. These features remained upon subsequent pump down to 10-9 Torr. Observed maximum N surface coverage was ~ 1.5 x 1013 cm-2-a fraction of a monolayer. In the presence of equal pressures of H2O, the adsorbed N intensity at 10-1 Torr is ~ 25% of that observed in the absence of H2O. The formation of molecularly adsorbed H2O was also observed. Density functional theory-based calculations suggest favorable absorption energies for N2 bonding to both V(IV) and V(III) cation sites, but less so for V(V) sites. Hartree-Fock-based cluster calculations for N2-V end-on adsorption show that experimental XPS doublet features are consistent with calculated shake-up and normal, final ionic configurations, for N2 end-on bonding to V(III) sites, but not V(IV) sites. XPS spectra of vanadium oxide transferred in situ between electrochemical and UHV environments indicate that the oxide surfaces studied here are stable upon exposure to electrolyte under NRR-relevant conditions.

The electrocatalytic reduction of molecular nitrogen to ammonia—the nitrogen reduction reaction (NRR)—is of broad interest as an environmentally- and energy-friendly alternative to the Haber-Bosch process for agricultural and emerging energy applications. Herein, we review our recent findings from collaborative electrochemistry/surface science/theoretical studies that counter several commonly held assumptions regarding transition metal oxynitrides and oxides as NRR catalysts. Specifically, we find that for the vanadium oxide, vanadium oxynitride, and cobalt oxynitride systems, (a) there is no Mars-van Krevelen mechanism and that the reduction of lattice nitrogen and N2 to NH3 occurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice; and (b) that NRR and the hydrogen evolution reaction (HER) do occur in concert under the conditions studied for Co oxynitride, but not for V oxynitride. Additionally, these results highlight the importance of both O-ligation of the V or Co center for metal-binding of dinitrogen, and the importance of N in stabilizing the transition metal cation in an intermediate oxidation state, for effective N≡N bond activation. This review also highlights the importance and limitations of ex situ and in situ photoemission—involving controlled transfer between UHV and electrochemistry environments, and of operando Near Ambient Pressure photoemission coupled with in situ studies, in elucidating the complex chemistry relevant to the electrolyte/solid interface.