Size-dependent reactivity of Rh cationic clusters to reduce NO by CO in the gas phase at high temperatures

Abstract

The reactivity of the reduction of NO pre-adsorbed on Rh_2–9⁺ clusters by CO was investigated using a combination of an alternate on–off gas injection method and thermal desorption spectrometry. The reduction of Rh_nN_xO_y⁺ clusters by CO was evaluated by varying the CO concentration at T = 903 K. Among the Rh_nN_xO_x⁺ clusters, the Rh₃N₂O₂⁺ cluster exhibited the highest reduction activity, whereas the other clusters, Rh_2,4–9N_xO_x⁺, showed lower reactivity. Density functional theory (DFT) calculations for Rh₃⁺ and Rh₆⁺ revealed that the rate-determining step for NO reduction in the presence of CO was NO bond dissociation through the kinetics analysis using the RRKM theory. The reduction of Rh₃N₂O₂⁺ is kinetically preferable to that of Rh₆N₂O₂⁺. The DFT results were in qualitative agreement with the experimental results.