Mechanism of Ammonia Decomposition and Oxidation on Ir(100): A First-Principles Study

Density functional theory (DFT) calculations combined with microkinetic analysis were performed to study the behavior of ammonia on clean, oxygen- and hydroxyl-predosed Ir(100). It is shown that the predosed oxygen or hydroxyl promotes NH3 and NH dehydrogenation steps, while NH2 dehydrogenation is slightly inhibited relative to clean Ir(100). In both cases, the hydrogen transfer from NHx species to predosed O or OH is favored over thermal decomposition of NHx. Furthermore, the predosed O exhibits higher activity on NH3 and NH dehydrogenation steps than OH, while the case is reversed for NH2. On clean Ir(100), N + N pathway is the major N-2 formation pathway when TPD experiment starts from 200 K, and N + NH is also involved but less competitive; however, three pathways N + N, N + NH, and NH + NH are all possible with respect to TPD experiment starting from 410 K On O- and OH-predosed Ir(100), N + N pathway is the predominant pathway and is enhanced by the predosed O or OH. The microkinetic analysis further confirms that N-2 is the resulting product at different temperatures and ratios of NH3/O-2, and the formation of NO is unfavorable.