Mechanism of O2 Desorption during N2O Decomposition on an Oxidized Rh/USY Catalyst

Pulsed N2O decomposition on a reduced or oxidized Rh/USY catalyst has been carried out to study the dependence of oxygen coverage (θo) on activity and to study the mechanism of O2 desorption using an isotopic tracer technique. Decomposition activity decreased to the minimum (formation of only N2) at θo=0.5, but increased with increasing coverage (θo>0.5), and finally high activity with steady-state O2 production was attained at high θo (>1.2). In the isotope study, N216O was pulsed onto 18O/oxidized Rh catalyst at low temperatures (220–240°C), and desorbed O2 molecules were monitored by means of mass spectrometry. The 18O fraction in the desorbed oxygen had almost the same value as that on the surface oxygen. The result shows that O2 desorption does not proceed via the Eley–Rideal mechanism, but via the Langmuir–Hinshelwood mechanism, i.e., desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, O2-TPD measurements in He showed that desorption of oxygen from the Rh catalyst occurred at much higher temperatures. Therefore, it was proposed that reaction-assisted desorption of O2 occurs during N2O decomposition at low temperatures.