Ammonia oxidation kinetics on bimetallic clusters of platinum and iridium: A theoretical approach

Density functional theory was performed on Pt3-xIrx (x=0-3) clusters to investigate the ammonia electro-oxidation reaction to nitrogen. The adsorption of N2H4-x (X= 0-3) and the effect of cluster composition on the adsorption were investigated in Gaussian 09. N and NH were found to be the most stable intermediates on these clusters, with a pronounced stability caused by the presence of iridium. On a Pt cluster, the ammonia oxidation mechanism involves hydrazine formation followed by hydrazine dehydrogenation to molecular nitrogen; however, on an Ir cluster, the ammonia undergoes successive dehydrogenation to form atomic nitrogen, followed by N-N bond formation to N-2. Moreover, rate of reaction constants, activation, and free energy calculations showed further evidence that production of N-2 from its nitrogen atoms is sluggish and that the electro-catalyst may be considered as poisoned. Nonetheless, these calculations confirm that the onset potential for ammonia oxidation on iridium is lower than on platinum and this reaction starts at lower potential on Ir. In the presence of bimetallic catalysts, the iridium sites are more attractive for poisonous intermediates like NH and N, leaving platinum sites vacant for ammonia oxidation through hydrazine formation. (C) 2017 Elsevier B.V. All rights reserved.