Decomposition of nitric oxide by rhodium cluster cations at high temperatures

Decomposition reactions of NO molecules on gas-phase Rh⁺ (n = 6-9) clusters were investigated by gas-phase thermal desorption spectrometry and density functional theory calculations. We found that NO adsorbs on the clusters, forming RhNO⁺ at room temperature. Upon heating, NO desorption was observed below 800 K. Above 800 K, while for n = 7 and 8, each of Rh₇N₃O₃⁺, Rh₇N₄O₄⁺, and Rh₈N₃O₃⁺ was found to release an N₂ molecule, no N₂ formation was clearly observed for Rh_6,9NO⁺. We considered that both Rh₇N₃O₃⁺ and Rh₈N₃O₃⁺ have at least two dissociated NO molecules, while Rh₆NO⁺ (x = 1-3) has one or less. Our computational results for Rh₈N₃O₃⁺ suggested that the formation of an N-N bond in the Rh₈N₃O₃⁺ structure must overcome an energy barrier of ∼2 eV, which is the highest among the suggested possible reaction pathways. These findings suggested that the size-dependent activity of NO decomposition is governed primarily by how NO molecules are adsorbed on Rh⁺ clusters, i.e. whether two or more N atoms from dissociated NO molecules exist in the NO adsorbed clusters, and secondly, by the readiness of the N-N bond formation.