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Abstract

This collaboration has recently shown that infrared excitation can drive decomposition reactions of molecules on the surface of gas-phase transition metal clusters. We describe here a significant extension of this work to the study of bimolecular reactions initiated in a similar manner. Specifically, we have observed the infrared activated CO oxidation reaction (CO_(ads)+ O_(ads) → CO_2(g)) on isolated platinum oxide cations, Pt_nO_m⁺. Small platinum cluster oxides Pt_nO_m⁺ (n = 3–7, m = 2, 4), have been decorated with CO molecules and subjected to multiple photon infrared excitation in the range 400–2200 cm⁻¹ using the Free Electron Laser for Infrared eXperiments (FELIX). The Pt_nO_mCO ⁺ clusters have been characterised by infrared multiple photon dissociation spectroscopy using messenger atom tagging. Evidence is observed for isomers involving both dissociatively and molecularly adsorbed oxygen on the cluster surface. Further information is obtained on the evolution of the cluster structure with number of platinum atoms and CO coverage. In separate experiments, Pt_nO_mCO ⁺ clusters have been subjected to infrared heating via the CO stretch around 2100 cm⁻¹. On all clusters investigated, the CO oxidation reaction, indicated by CO₂ loss and production of Pt_nO_m-1⁺, is found to compete effectively with the CO desorption channel. The experimental observations are compared with the results of preliminary DFT calculations in order to identify both cluster structures and plausible mechanisms for the surface reaction.