Selective NH3 oxidation on (110) and (111) iridium surfaces

Ammonia oxidation was studied in situ on Ir(110) and Ir(111) under low-pressure image conditions. NH3 does not dissociate on a flat Ir(111) surface, but surface defects and Oad facilitate NH3 ad decomposition. High-energy resolution fast XPS measurements were used to monitor the surface coverage during reaction on Ir(110), and temperature-programmed reaction measurements were applied to reveal the gas phase reaction products during reaction. The steady-state NH3 oxidation reaction starts between 350 and 500 K on both surfaces, which also show similar selectivity. Below 600 K, N2 and H2O are the principal reaction products. Above 600 K, the selectivity changes to NO and H2O, with the exact temperature depending on the NH3:O2 pressure ratio. The surface population changes from NHad/Nad to Oad around 500 K, about 200 K lower than the selectivity change from N2 to NO observed in the gas phase. This behavior can be explained by considering the activation energies for N2 and NOad formation. We present a model to explain why Ir is more selective toward N2 than Pt.