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The production of ammonia for agricultural and energy demands has accelerated research for more environmentally-friendly synthesis options, particularly the electrocatalytic reduction of molecular nitrogen (nitrogen reduction reaction, NRR). Catalyst activity for NRR, and selectivity for NRR over the competitive hydrogen evolution reaction (HER), are critical issues for which fundamental knowledge remains scarce. Herein, we present results regarding the NRR activity and selectivity of sputter-deposited titanium nitride and titanium oxynitride films for NRR and HER. Electrochemical, fluorescence and UV absorption measurements show that titanium oxynitride exhibits NRR activity under acidic conditions (pH 1.6, 3.2) but is inactive at pH 7. Ti oxynitride is HER inactive at all these pH values. In contrast, TiN – with no oxygen content upon deposition – is both NRR and HER inactive at all the above pH values. This difference in oxynitride/nitride reactivity is observed despite the fact that both films exhibit very similar surface chemical compositions – predominantly Ti IV oxide – upon exposure to ambient, as determined by ex situ X-ray photoelectron spectroscopy (XPS). XPS, with in situ transfer between electrochemical and UHV environments, however, demonstrates that this Ti IV oxide top layer is unstable under acidic conditions, but stable at pH 7, explaining the inactivity of titanium oxynitride at this pH. The inactivity of TiN at acidic and neutral pH is explained by DFT-based calculations showing that N 2 adsorption at N-ligated Ti centers is energetically significantly less favorable than at O-ligated centers. These calculations also predict that N 2 will not bind to Ti IV centers due to a lack of π-backbonding. Ex situ XPS measurements and electrochemical probe measurements at pH 3.2 demonstrate that Ti oxynitride films undergo gradual dissolution under NRR conditions. The present results demonstrate that the long-term catalyst stability and maintenance of metal cations in intermediate oxidation states for pi-backbonding are critical issues worthy of further examination. 

The electrocatalytic nitrogen reduction reaction (NRR) is of significant interest as an environmentally friendly method for NH 3 production for agricultural and clean energy applications. Selectivity of NRR vis-à-vis the hydrogen evolution reaction (HER), however, is thought to adversely impact many potential catalysts, including Earth-abundant transition metal oxynitrides. Relative HER/NRR selectivities are therefore directly compared for two transition metal oxynitrides with different metal oxophilicities—Co and V. Electrocatalytic current–potential measurements, operando fluorescence, absorption, and GC measurements of H 2 and NH 3 production, ex situ X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined to directly compare NRR and HER activities under identical reaction conditions. Results show that cobalt oxynitrides – with Co primarily in the Co( ii ) oxidation state – are NRR active at pH 10, with electrochemical reduction of both lattice nitrogen and dissolved N 2 , the latter occurring without N incorporation into the lattice. Removal of lattice N then yields Co( ii ) oxide, which is still NRR active. These results are complemented by calculations showing that N 2 binding at Co( ii ) sites is energetically favored over binding at Co( iii ) sites. GC analysis demonstrates that H 2 production occurs in concert with ammonia production but at a far greater rate. In contrast, vanadium oxynitride films are HER inactive under the same (pH 10) conditions, as well as at pH 7, but are NRR active at pH 7. DFT calculations indicate that a major difference in the two materials is hindered O–H dissociation of H 2 O adsorbed at O-ligated Co vs. V cation centers. The combined studies indicate significant variation in HER vs. NRR selectivity as a function of employed transition metal oxynitrides, as well as different HER mechanisms in V and Co oxynitrides. 

The chemical structures of Co oxynitrides – in particular, interactions among N and O atoms bonded to the same cobalt – are of great importance for an array of catalytic and materials applications. X-ray diffraction (XRD), core and valence band X-ray photoelectron spectroscopy (XPS) and plane wave density functional theory (DFT) calculations are used to probe chemical and electronic interactions of nitrogen-rich CoO1-xNx (x > 0.7) films deposited on Si(100) using NH3 or N2 plasma-based sputter deposition or surface nitridation. Total energy calculations indicate that the zincblende (ZB) structure is energetically favored over the rocksalt (RS) structure for x > ~ 0.2, with an energy minimum observed in the ZB structure for x ~ 0.8 - 0.9. This is in close agreement with XPS-derived film compositions when corrected for surface oxide/hydroxide layers. XRD data indicate that films deposited on Si (100) at room temperature display either a preferred (220) orientation or no diffraction pattern, and are consistent with either rocksalt (RS) or zincblende (ZB) structure. Comparison between experimental and calculated X-ray excited valence band densities of states – also similar for all films synthesized herein – demonstrates a close agreement with a ZB, but not an RS structure. Core level XPS spectra exhibit systematic differences between films deposited in NH3 vs N2 plasma environments. Films deposited by N2 plasma magnetron sputtering exhibit greater O content as evidenced by systematic shifts in N 1s binding energies. Excellent agreement with experiment for core level binding energies is obtained for DFT calculations based on the ZB structure, but not for the RS structure. The agreement between theory and experiment demonstrates that these N-rich Co oxynitride films exhibit the ZB structure, and forms the basis of a predictive model for understanding how N and O interactions impact the electronic, magnetic and catalytic properties of these materials.