(710c) The Effect of N* Coverage on the Electrocatalytic Oxidation of Ammonia on Pt(111): Insights from Theory

Pt-based electrocatalysts for the ammonia oxidation reaction (AOR) are generally characterized by easy deactivation, attributed to the high activation energy barrier associated with the N–N bond formation [1]. This specific molecular step has been investigated according to different mechanistic hypotheses. According to the Gerischer and Mauerer (GM) mechanism, the N–N bond is formed by the reaction of co-adsorbed NH_x (x=1–2) species. Instead, according to the N+N mechanism, the N–N bond formation takes place by the encounter of two co-adsorbed N atoms.

A mechanistic study on different close-packed transition metals surfaces was presented by Herron et al. [1] and more recently by Elnabawy et al. [2], by means of Density Functional Theory (DFT) calculations. They concluded that on Pt(111) the GM mechanism is kinetically preferred and N* acts as a surface poison. Independently, atomic N has been found to be the most abundant reaction intermediate on Pt(111), with coverage close to 0.6 ML [3-4]. As the presence of co-adsorbed N* can change the energetic landscape of the GM and N+N mechanisms, we used DFT calculations to assess the effect of N* coverage on the AOR mechanism on a model Pt(111) extended surface. We demonstrate that the presence of N* adatoms has a profound effect on the activation energy barriers associated with N–N bond formation steps, leading to easier N₂ evolution.

[1] Herron J A., Ferrin, P., Mavrikakis M., J. Phys. Chem. C, 119, 14692, (2005).

[2] Elnabawy A., Herron J. A., Karraker S., Mavrikakis M., J. Catal., 397, 137 (2021).

[3] Gootzen, J. F. E., Wonders, A. H., Visscher, W., van Santen, R. A., van Veen, J. A. R., Electrochim. Acta, 43, 1851 (1998).

[4] De Vooys A. C. A, Koper M. T. M., van Santen R. A., van Veen J. A. R., J. Electroanal. Chem., 506, 2, 126–137 (2001).