Mechanisms of the various nitric oxide reduction reactions on a platinum-rhodium (100) alloy single crystal surface

Article abstract of DOI:10.1016/0039-6028(92)90463-G

The reduction of nitric oxide with hydrogen was studied over a Pt_0.25-Rh_0.75(100) alloy surface used as a model catalyst for the automotive three-way catalyst. This paper emphasizes the mechanisms of the different reactions leading to the products dinitrogen, ammonia and nitrous oxide. For this purpose the reaction was studied under various experimental conditions including reactivity measurements both in the 10^-7 mbar range under steady-state conditions and in the 10 mbar range with varying NO/H₂ ratio. In addition, the thermal decomposition of NO and the reactions of NO + NH₃ were investigated. ¹⁵NO and ¹⁵NH₃ were used in order to gather additional information concerning the mechanisms of the formation of the various N-containing products. The surface was characterized by using low-energy electron diffraction, Auger electron spectroscopy and thermal desorption spectroscopy. The main conclusions emerging from these studies are: (a) N₂ can be formed by combination of 2 N adatoms in the whole temperature range used (350-1300 K), provided that sufficient N adatoms are available; (b) below 600 K the main contribution to N₂ formation is via NO_ads + N_ads → N₂ + O_ads; at higher temperatures the dominant mechanisms is 2N_ads → N₂; (c) N₂O and NH₃ are formed via N_ads + NO_ads → N₂O, and N_ads + 3H_ads → NH₃, the contributions of which respectively decrease and increase with increasing temperature; (d) the selectivities to N₂, NH₃ and N₂O are determined by the relative concentrations of NO_ads, N_ads and H_ads, which vary with the experimental conditions such as the temperature.