Mechanism of NOH2 reaction over Rh foil and the role of chemisorbed nitrogen atoms

The behaviour of adsorbed nitrogen and the overall catalytic reaction between NO and H₂ on Rh foil were investigated in a pressure region around 10^?5–10^?4 Pa and a temperature range between 400 and 1200 K, using the flash desorption technique and ultraviolet photoelectron spectroscopy. In a reducing condition, the NOH₂ reaction proceeded rapidly in the temperature range between 500 and 1000 K, and the reaction probability of NO was almost unity in the temperature region studied. The major product was N₂, but NH₃ was also formed around 500 K. Chemisorbed nitrogen was accumulated during the NO-H₂ reaction and also during the NH₃ decomposition reaction. In both cases, the dependence of the rate of N₂ formation upon the amount of N(ad) estimated during the reaction was similar to that in the case of N(ad) flash desorption, which indicates that N₂ is formed by recombination of N(ad) in both the NO-H₂ reaction and the NH₃ decomposition reaction. The rate constant for the second order desorption of N(ad) was estimated to be $\text{10}{}^{\mathrm{?6.8\; \pm \; 0.3}}\text{exp(?97 ± 5(}\text{kJ}\text{mol}\text{) RT) (}\text{cm}{}^2\text{atom·s)}$. The overall reaction of NO-H₂ on Rh proceeds in a similar manner to Pd previously reported, but the dissociation of NO takes place more easily over Rh and O(ad) is more stable, being liable to cause an inhibition of the NO-H₂ reaction, especially at lower temperature.