Oxidation of magnesia-supported Pd30 nanoclusters and catalyzed CO combustion: Size-selected experiments and first-principles theory
Experimental and theoretical investigations of the oxidation reaction of CO to form carbon dioxide, catalyzed by size-selected Pd 30 clusters soft-landed on MgO(100), are described. The consequences of pretreatment of the deposited clusters with molecular oxygen, 16O 2, at a temperature of 370 K followed by annealing at around 450 K are explored. Subsequent to the above pretreatment stage, the system was cooled to 120 K, and after exposure to 18O 2 and 13C 16O the temperature was ramped and a temperature-programmed reaction (TPR) spectrum recorded. The onset of catalyzed combustion of CO starts at a temperature of 180 K, and the TPR spectrum shows oxidation to occur over a broad temperature range, up to 550 K. Using first-principles density-functional theory, the optimal adsorption geometry of the Pd 30 cluster on the MgO(100) surface is found to be a square-base pyramidal structure, with an excess electronic charge of 1.25e, originating from the underlying magnesia support, found to be localized near the interfacial region of the cluster with the supporting surface. Structural and energetic properties of a variety of oxygen adsorption sites on the supported palladium cluster and effects due to multiple adsorbed O 2 molecules were explored. It is found that the barriers for dissociation of the adsorbed molecules depend strongly on the locations of the adsorption sites, with very small (<0.1 eV) dissociation energy barriers found for adsorption sites on the Pd 30 cluster that are closer to its interface with the Mg(100) surface. This correlates with our finding that adsorption at these interfacial sites is accompanied by excess charge accumulation on the adsorbed molecule through excess partial (0.25e) occupation of the molecular antibonding 2* orbital, resulting in activation of the molecule to a peroxo-like state. This activation mechanism depletes the excess charge on the cluster, resulting in a self-limiting partial oxidation of the cluster. The information obtained through isotope labeling in the TPR experiments is explored through first-principles quantum simulations of various reaction pathways, with a focus on the multiple coadsorption system Pd 30O 10(CO) 13/MgO. These theoretical calculations allow us to correlate the measured isotope and temperature-dependent TPR features, with operative Langmuire-Hinshelwood, LH, and Mars-van Krevelen-type (MvK) reaction mechanisms, catalyzed by the partially oxidized cluster. The LH mechanism was found to contribute to the reaction at lower temperatures, while the MvK dominates for higher temperatures.