Vanadium oxide, vanadium oxynitride, and cobalt oxynitride as electrocatalysts for the nitrogen reduction reaction: a review of recent developments

Abstract

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.

Niobium Carbide and Tantalum Carbide as Nitrogen Reduction Electrocatalysts: Catalytic Activity, Carbophilicity, and the Importance of Intermediate Oxidation States

“…In summary, the data in Figure demonstrate that at potentials more positive than −1.0 V Ag/AgCl, NaOH-treated NbC is NRR active, while untreated NbC is electrochemically very similar to the FTO substrate. The FE of 3.15%, although relatively small compared to many reports in the literature, is substantial in comparison with results for other thin films with relatively low surface areas …”

“…Furthermore, calculated N 2 activation upon binding to niobium (as evidenced by lengthened NN bond lengths and a diminution of the NN stretching frequencies upon metal coordination) is more significant for Nb III than for Nb IV molecular models (e.g., r NbN = 2.041 Å, r NN = 1.178 Å, ν NN = 1901 cm –1 for Nb III and r NbN = 2.414 Å, r NN = 1.129 Å, ν NN = 2400 cm –1 for Nb IV models) . Thus, the data in Figure , together with previous calculations and the experimental data described above, demonstrate that π-backbonding between the transition metal site and N 2 is critically important not only for NN bond activation but also in N 2 -surface binding.…”

“…DFT-based computations indicate that Nb and Ta in +4 and lower oxidation states are more effective at N�N bond activation. 19 These calculations indicate the potential importance of metal d/N 2 π* backbonding, suggesting experimental studies of NRR for other transition metal oxides with cations in d 0 vs non-d 0 oxidation states. Electrochemical and UHV-EC data reported here also show that at highly cathodic potentials, Nb 2 O 5 regrowth occurs at the surface, thus limiting the effective potential range for NRR.…”

“…(At potentials more negative than −1.0 V vs Ag/AgCl, slight NRR activity is observed for ambient exposed, NaOH-treated TaC, but see below.) DFT-based computations indicate that Nb and Ta in +4 and lower oxidation states are more effective at NN bond activation . These calculations indicate the potential importance of metal d/N 2 π* backbonding, suggesting experimental studies of NRR for other transition metal oxides with cations in d 0 vs non-d 0 oxidation states.…”

“…The FE of 3.15%, although relatively small compared to many reports in the literature, 15 is substantial in comparison with results for other thin films with relatively low surface areas. 19 Corresponding data for similarly treated TaC are displayed in Figure 7, upon immersion in Ar-purged (blue) and N 2purged electrolytes (red). The LSVs (Figure 7a) show that the presence of N 2 induces little significant additional cathodic current at potentials below approximately −1.0 V vs Ag/AgCl, indicating negligible NRR activity in this range.…”