Surface Oxidation of Transition Metal Nitrides

The well-known Haber-Bosch process of NH3-production ishighly inefficient, with significant energy demand and high CO2 emissions. Alternative approaches like the electrochemical ammonia synthesis from N2 and H2 are attractive, but the sluggish nitrogen reduction reaction (NRR) that arises from the high energy input to activate stable N2 remains the significant challenge for NRR electrocatalysts. The Nitrogen-rich surface of transition metal nitrides (TMNs) can deliver one solution to this challenge, through a Mars-van Krevelen-like mechanism in which N vacancies form via hydrogenation steps and ammonia release, followed by vacancy filling through N2 activation. We recently showed that ZrN thin films deposited with metal-organic chemical vapour deposition (MOCVD) are rapidly oxidized when exposed to ambient and showed preliminary results, from ab initio molecular dynamics (aiMD) simulations, indicating surface oxidation is favourable. In this paper, we investigate in detail with aiMD the oxidation of ZrN and VN surfaces by ambient oxygen at various temperatures: 295K, 363K, 873K and 1023K. Results show that ZrN surfaces tend to form oxynitrides at lower temperatures and prefers to form a ZrOx layer interfaced with ZrN at higher temperature. By contrast, VN(111), forms VOx clusters on the surface and there is no significant migration of O species into bulk VN at all studied temperatures. We attribute the different oxidation processes of ZrN and VN to the relative strength of V-N/O bonds and Zr-O/N bonds - the bond dissociation energy of V-N (452 kJ/mol) is larger than Zr-N (339 kJ/mol), while the V-O bond (645 kJ/mol) is weaker than the Zr-O bond (776 kJ/mol). Experimental results on MOCVD nitride films, including Rutherford backscattering spectrometry in combination with nuclear reaction analysis (RBS/NRA), confirms that VN is less oxidized than ZrN at ambient conditions because VN forms a less stable, potentially volatile oxide layer, whereas ZrN has a stronger tendency to form a stable, protective ZrO₂ layer, promoting more complete oxidation at higher temperatures. This study defines a new degree of atomic scale understanding of the formation of oxynitride or separated oxide phase in TMNs at ambient oxygen conditions relevant for NRR electrocatalysis.