Catalytic reactions on neutral Rh oxide clusters more efficient than on neutral Rh clusters

Abstract

Gas phase catalytic reactions involving the reduction of N₂O and oxidation of CO were observed at the molecular level on isolated neutral rhodium clusters, Rh_n (n = 10–28), using mass spectrometry. Sequential oxygen transfer reactions, Rh_nO_m−1 + N₂O → Rh_nO_m + N₂ (m = 1, 2, 3,…), were monitored and the rate constant for each reaction step was determined as a function of the cluster size. Oxygen extraction reactions by a CO molecule, Rh_nO_m + CO → Rh_nO_m−1 + CO₂ (m = 1, 2, 3,…), were also observed when a small amount of CO was mixed with the reactant N₂O gas. The rate constants of the oxygen extraction reactions by CO for m ≥ 4 were found to be two or three orders of magnitude higher than the rate constants for m ≤ 3, which indicates that the catalytic reaction proceeds more efficiently when the reaction cycles turn over around Rh_nO_m (m ≥ 4) than around bare Rh_n. Rhodium clusters operate as more efficient catalysts when they are oxidized than non- or less-oxidized rhodium clusters, which is consistent with theoretical and experimental studies on the catalyticCO oxidation reaction on a rhodium surface.