Water at interfaces Faraday Discussion

Interfacial H 2 O reactivity during electrochemical NH 3 oxidation Francisco Sarabia, Mariana C.O. Monteiro, Daniel Escalera-Lopez, Sebastian Z. Öner, Beatriz Roldan Cuenya Interface Science Department of the Fritz Haber Institute of the Max Planck Society, Germany H 2 O is omnipresent in electrochemistry, yet, its effect on catalyst activity, selectivity and deactivation remains poorly understood. This is particularly true for the ammonia oxidation reaction (AOR, 2NH 3 + 6OH -> N 2 + 6H 2 O + 6e - ). By releasing the stored H + , here in the form of H 2 O, via the AOR, green NH 3 could serve a green hydrogen- carrier. However, the electrochemical AOR suffers from high overpotentials and rapid catalyst deactivation, the origin of which are still being debated in the literature. Here, we study the influence of OH - and H 2 O on the AOR activity and deactivation mechanism of polycrystalline Pt. In the absence of OH - , but in the presence of H 2 O, we observe strongly increasing overpotentials for NH 3 decomposition. This is consistent with previous studies 1,2 reporting a non-Nernstian pH dependence of the AOR overpotential. However, in contrast to previous explanations, we hypothesize that interfacial water dissociation might be inhibited on the NH x /N-covered Pt surface. When decorating the polycrystalline Pt surface with auxiliary metal oxides species (TiO X and NiO X ), we observe a substantially enhanced AOR rate, which we tentatively ascribe to enhanced water dissociation at freely accessible acid-base sites. Additionally, we perform extensive Pt deactivation studies, which indicate (i) two potential-dependent *N and *NO deactivation processes in aqueous media, the latter being closely linked to the potential-dependent OH adsorption on Pt, (ii) suppression of *NO but remaining *N poisoning with decreasing H 2 O concentration and (iii) potential- dependent N-stripping to N 2 in H 2 O-free conditions and stable NH 3 decomposition. Our findings highlight the crucial role of OH - adsorption and water dissociation in NH 3 oxidation and layout new pathways for more efficient and stable electrochemical NH 3 oxidation in the future. References 1. I. Katsounaros, T. Chen, A.A. Gewirth, N.M. Markovic, M.T.M. Koper, Evidence of Decoupled Electron and Proton Transfer in the Electrochemical Oxidation of Ammonia on Pt (100), J. Phys. Chem. Lett. 7, 3, 387-392 (2016). 2. I. Katsounaros, M.C. Figueiredo, F. Calle-Vallejo, H. Li, A.A. Gewirth, N.M. Markovic, M.T.M. Koper, On the mechanism of the electrochemical conversion of ammonia to dintrogen on Pt(100) in alkaline environment. Journal of Catalysis 359, 82-91 (2018).

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