Formation of Ammonia in the Reduction of Nitrogen Monoxide in Automotive Exhaust Simulated Gases over Rhodium and Ruthenium Catalysts

Abstract

Ammonia formation in the reduction of NO and the water-gas shift reaction over Rh and Ru catalysts have been studied by using two kinds of model gases (Table 1). The reaction rates were measured with a continuous flow reactor. The catalysts used were both "oxidized" and "reduced" ones.
Over Rh and Ru catalysts, NH₃ was found to be formed by the two reactions the reaction of NO and H₂ added in the model gas, and the reaction between NO and active hydrogen produced by the water-gas shift reaction (Figs.2, 3, 5 and 6). However, the former was predominant at high temperatures.
Most of the NO was converted into NH₃ over Rh catalysts. Oxidized Rh catalyst was more active for both NH₃ formation and the water-gas shift reaction than reduced Rh catalysts (Figs.2 and 4). The former catalyst decomposed NH₃ at high temperatures, while the latter didn't.
On the other hand, Ru catalysts yielded much less NH₃ (Figs.5 and 6) than Rh catalysts. Only a small difference was observed in the catalytic activities between oxidized Ru catalyst and reduced ones. Neither oxidized nor reduced Ru catalysts decomposed NH₃ formed over the catalysts (Figs.5 and 6). This suggests that NH₃ is not an intermediate in the gas phase.