DFT calculations of ammonia oxidation reactions on bimetallic clusters of platinum and iridium

Density functional theory calculations were performed on four platinum-iridium clusters, Pt3-xIrx (x = 03) to examine ammonia oxidation in alkaline media. The adsorption of NH3-x (x = 0-3) on these clusters and the effect of cluster composition on the adsorption were investigated. The hybrid B3LYP level of theory was used in Gaussian 09 along with the LANL2DZ and 6-311++g basis sets. The HOMO-LUMO energy gap on bare metal clusters showed that increasing the iridium concentration decreased the energy gap, a sign of a more reactive catalyst. The relative adsorption energy showed more stability of NH3-x on the Ir-3 cluster and less stability as the number of platinum atoms increased in the cluster. These results combined with activation and dissociation energy calculations of sequential dehydrogenation reactions showed that Ir-3 is more active than Pt-3 for ammonia oxidation, and the addition of iridium to platinum makes a more favorable pathway for the ammonia oxidation reaction. The computational calculations suggest the possibility of two different mechanisms of ammonia oxidation on platinum and iridium electrocatalysts. (C) 2016 Elsevier B.V. All rights reserved.