Mechanism of ammonia decomposition and oxidation on Ir(110): a first-principles study.

Abstract

The mechanism of ammonia decomposition and oxidation on Ir(110) was studied on the basis of periodic density functional theory calculations and microkinetic modeling. The results indicate that NH3 dissociation is more favorable than desorption at atop site, while at top site NH3 desorption and dissociation are competitive. On the other hand, when O or OH is co-adsorbed, the NH3 dehydrogenation is slightly inhibited and mainly via hydrogen abstraction reaction rather than thermal decomposition, while it is reversed for NH2 dehydrogenation. The former mechanism is favored for O assisted NH dehydrogenation, while it changed to latter one for OH. On clean Ir(110), N + NH → N2 + H pathway is the major N2 formation pathway and N + N is also involved but less competitive, while N + N becomes the predominant one and is enhanced on O-predosed Ir(110). NO formation occurs only at higher temperature when N2 is desorbed from the surface. The microkinetic analysis further confirms that the dominant product is N2 at low temperature while becomes NO as temperature increases, and the temperature of NO formation decreases when O2 partial pressure increases. The present calculation results are in good agreement with the experimental observations.