Surface Oxidation of Transition Metal Nitrides

Kurzfassung

The well-known Haber-Bosch process for NH3 production is highly inefficient, with a significant energy demand and CO2 emissions. Alternative approaches, including 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 N2 remains a significant challenge for NRR electrocatalysis. The nitrogen-rich surface of transition metal nitrides (TMNs) can deliver one solution to this challenge. A Mars-van Krevelen-like mechanism is proposed that forms N vacancies via hydrogenation and ammonia release, followed by vacancy filling through N2 activation. We recently showed that ZrN thin films deposited with metal-organic chemical vapor deposition (MOCVD) are rapidly oxidized when exposed to ambient conditions during ex situ handling prior to analysis and showed preliminary results, from ab initio molecular dynamics (aiMD) simulations, indicating that surface oxidation is favorable. In this paper, we investigate in detail with aiMD the unintentional oxidation of ZrN and VN surfaces by oxygen present at ambient conditions at various temperatures: 295, 363, 873, and 1023 K. Results show that ZrN surfaces tend to form oxynitrides at lower temperatures and prefer 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 the O species into bulk VN at all studied temperatures. We attribute the different oxidation processes of ZrN and VN to the relative strengths of V-N/O bonds and Zr-O/N bonds-the bond dissociation energy of V-N (452 kJ/mol) is larger than that of 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), confirm 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.