Nonlinear Behavior in the NO−H2 Reaction over Ir(110) Studied by Fast X-ray Photoelectron Spectroscopy

NH3 adsorption and decomposition on Ir(110): A combined temperature programmed desorption and high resolution fast x-ray photoelectron spectroscopy study

The adsorption and decomposition of NH3 on Ir(110) has been studied in the temperature range from 80 K to 700 K. By using high-energy resolution x-ray photoelectron spectroscopy it is possible to distinguish chemically different surface species. At low temperature a NH3 multilayer, which desorbs at approximately 110 K, was observed. The second layer of NH3 molecules desorbs around 140 K, in a separate desorption peak. Chemisorbed NH3 desorbs in steps from the surface and several desorption peaks are observed between 200 and 400 K. A part of the NH3ad decomposes into NH(ad) between 225 and 300 K. NH(ad) decomposes into N(ad) between 400 K and 500 K and the hydrogen released in this process immediately desorbs. N2 desorption takes place between 500 and 700 K via N(ad) combination. The steady state decomposition reaction of NH3 starts at 500 K. The maximum reaction rate is observed between 540 K and 610 K. A model is presented to explain the occurrence of a maximum in the reaction rate. Hydrogenation of N(ad) below 400 K results in NH(ad). No NH2ad or NH3ad/NH3 were observed. The hydrogenation of NH(ad) only takes place above 400 K. On the basis of the experimental findings an energy scheme is presented to account for the observations.

The role of Oad in the decomposition of NH3 adsorbed on Ir(110): a combined TPD and high-energy resolution fast XPS study

High energy resolution fast XPS combined with TPD experiments were used to study the effect of chemisorbed oxygen on the adsorption and dissociation of NH(3) on Ir(110). Below 250 K the presence of O(ad) does not influence NH(3) decomposition. Above 250 K O(ad) enhances NH(3) dissociation, which results in three times as much N(2) formation and less molecular NH(3) desorption compared to the experiments without O(ad). The effect of O(ad) can be attributed to destabilization of NH(ad) on the surface, resulting in a further dehydrogenation towards N(ad). The presence of O(ad) on the surface lowers the temperature at which the N(ad) combination reaction takes place by as much as 200 K, due to repulsive interaction between N(ad) and O(ad). NO is formed above 450 K if both N(ad) and O(ad) are present on the surface.