Oscillation in low-temperature NO–H2–O2 reactions over Pt catalysts supported on NOx-adsorbing material, TiO2–ZrO2

The oscillatory behavior of 0.08% NO–0.28% H2–10% O2 reactions in a flowing He (image) over 1 wt% Pt/TiO2–ZrO2 catalyst has been studied at 90 °C with attention to the NOx adsorption property of the support oxide. After an induction period of ca. 20 h in a steady-state flow, oscillations appeared with different periods and amplitudes depending on the concentration of H2 in the gas feed. During the oscillation, the conversions of both NO and H2 were decreased simultaneously from the steady state, whereas the corresponding N2/N2O yields were more than estimated from the reaction stoichiometry. The parallel TPD measurement revealed that the amount of nitrate (NO3) accumulation on the catalyst became largest when the oscillation started. In situ DRIFTS detected two different nitrates species with unidentate and bidentate structures, which adsorbed on basic sites forming at the boundary of two oxide components. Based on the fact that the surface coverage of nitrate on the support is correlated to the oscillation period, a mechanism is suggested in which the surface domains of accumulated nitrates block the adsorption of gaseous NO, leading to a sudden drop of the catalytic activity. However, with the progress of nitrate removal by the reaction with H2, the adsorption of NO onto Pt can restart the catalytic NO reduction.