Reactivity Control of Rhodium Cluster Ions by Alloying with Tantalum Atoms

Gas phase, bielement rhodium and tantalum clusters, Rh_nTa_m⁺ (n + m = 6), were prepared by the double laser ablation of Rh and Ta rods in He carrier gas. The clusters were introduced into a reaction gas cell filled with nitric oxide (NO) diluted with He and were subjected to collisions with NO and He at room temperature. The product species were observed by mass spectrometry, demonstrating that the NO molecules were sequentially adsorbed on the Rh_nTa_m⁺ clusters to form Rh_nTa_m⁺N_xO_x (x = 1, 2, 3, ...) species. In addition, oxide clusters, Rh_nTa_m⁺O₂, were also observed, suggesting that the NO molecules were dissociatively adsorbed on the cluster, the N atoms migrated on the surface to form N₂, and the N₂ molecules were released from Rh_nTa_m⁺N₂O₂. The reactivity, leading to oxide formation, was composition dependent: oxide clusters were dominantly formed for the bielement clusters containing both Rh and Ta atoms, whereas such clusters were hardly formed for the single-element Rh_n⁺ and Ta_m⁺ clusters. DFT calculations indicated that the Ta atoms induce dissociation of NO on the clusters by lowering the dissociation energy, whereas the Rh atoms enable release of N₂ by lowering the binding energy of the N atoms on the clusters.