Determination of surface coverage of catalysts: Temperature programmed experiments on platinum and iridium sponge catalysts after low temperature ammonia oxidation

The activity of iridium and platinum sponge catalysts was studied in the lo w temperature gas phase oxidation of ammonia with oxygen. Under the reactio n conditions used, iridium was found to be more active and more selective t o nitrogen than platinum. Furthermore it was established from activity meas urements that both catalysts lose activity as a function of time on stream due to inhibition of the surface by reaction intermediates. The used cataly sts were studied with XPS and temperature programmed techniques. It was fou nd that the surface of the catalysts had a high coverage of NH and OH and s ome additional NH2. It seems most likely that the reaction mechanism procee ds through a stepwise dehydrogenation of the ammonia molecule. It appears t hat the last dehydrogenation step (NH by OH to N and water) is the rate det ermining step. The high selectivity of iridium to nitrogen can be explained by the higher activity of iridium in dissociating NO. (C) 1999 Academic Pr ess.