Trends in Ammonia Electro-Oxidation On Model Transition Metal Surfaces

The electro-oxidation of ammonia (NH₃) is a fundamentally important reaction for a number of applications including electrochemical wastewater cleanup, chemical sensing, and hydrogen generation. In addition, there has been interest in utilizing ammonia as an alternative feed for direct fuel cell applications.¹  The mechanism for this reaction is not well understood,² complicating the design of improved electrocatalysts.  Gerischer and Mauerer suggested a mechanism where NH₃ is oxidized to some NH_x intermediate, which reacts with another NH_y intermediate to form an N₂H_x+y intermediate.³ This intermediate is subsequently oxidized to N₂. Rosca and Koper have demonstrated that the activity on Pt(100) is significantly higher than Pt(111) and they attribute this to the preferential stabilization of NH₂ on Pt(100), facilitating dimerization to N₂H₄.² Here we present a first principles, periodic density functional theory analysis of  the electrochemical oxidation of NH₃on the (111) and (100) facets of Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh to understand: (i) the reaction mechanism and (ii) the experimentally observed structure sensitivity. 

[1] Vidal-Iglesias, F. J.; Solla-Gullon, J.; Montiel, V.; Feliu, J. M.; Aldaz, A., Journal of Power Sources 2007, 171, 448.

[2] Rosca, V.; Koper, M. T. M., Physical Chemistry Chemical Physics 2006, 8, 2513.

[3] Gerischer, H.; Mauerer, A., Journal of Electroanalytical Chemistry 1970, 25, 421.