Electro-Catalytic Reduction of Nitrogen to Ammonia By Vanadium Oxide and Vanadium Oxynitride Thin Films: The Roles of Metal Oxophilicity, and Lattice Oxygen and Nitrogen Towards NRR

Electro-catalytic reduction of N2 to NH3 using Earth-abundant oxide and oxynitrides is an energy- and environmentally-friendly alternative to the Haber-Bosch process. The present contribution summarizes our recent progress on vanadium oxide (VO) and vanadium oxynitride (VON) thin films1,2 as electrochemical nitrogen reduction reaction (NRR) catalysts at neutral pH conditions. In the case of VO films, the N-free VIII/IV-oxide, created by O2 plasma oxidation of polycrystalline vanadium, exhibited N2 reduction at an onset potential of -0.16 V vs. Ag/AgCl. DFT calculations indicated that N2 scission from O-supported V-centers is energetically favorable by ~18 kcal/mole as compared to N-supported sites. Interestingly, in the case of VON catalysts, both electrochemistry and photoemission data revealed the involvement of both lattice N and dissolved N2 in the NRR process. Our results also show that NH3 production from VON lattice N occurs in the presence or absence of N2 and involves the formation of V≡N: intermediates or similar unsaturated VN surface states. This is in contrast to VO where N2 reduction occurred in the presence or absence of lattice N, and without N incorporation into a vanadium oxide lattice. Thus, both lattice N and N2 reduction mechanisms involve oxide-supported V surface sites ([V]O) in preference to N-supported sites ([V]N). DFT calculations revealed the formation of V≡N:, V-N=N-H, plus other plausible reaction intermediates that are energetically favored at [V]O rather than at [V]N surface sites. Acknowledgment: This work was supported by the National Science Foundation through grants DMR-2112864 (to JAK, TRC, and FD) and partial support by CHE-1953547 (to TRC). Additional NSF support for the UNT CASCaM HPC cluster via Grant CHE-1531468 is also gratefully acknowledged. Ganesan, A. Osonkie, P. Chukwunenye, T. Cundari, F. D'Souza, J. Kelber. Electrochemical Reduction of N2 to ammonia by vanadium oxide thin films at neutral pH, J. Electrochem. Soc., 2021, 168, 026504. Osonkie, A. Ganesan, P. Chukwunenye, F. Anwar, K. Balogun, M. Gharee, I. Rashed, T. R. Cundari, F. D’Souza, and J. A. Kelber, Electro-catalytic reduction of nitrogen to ammonia: The roles of lattice O and N in reduction at vanadium oxynitride surfaces, ACS Adv. Mater & Interface, 2021, in revision.