Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au(20) was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O(2) molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O(2) molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au(20) nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects.

Using p-MBRS experiments and TPR as well as FTIR measurements, it could be shown that dissociative oxygen activation occurs in the same temperature range for small clusters in comparison to faceted nanoparticles (NPs) and Pd single crystals. Surprisingly, CO poisoning does not take place on small Pd clusters at temperatures above 300 K. Furthermore, an oxygen activation has been found in this low temperature range which differs from the normal dissociative activation since it occurs only if CO is already adsorbed, before O 2 adsorbs. Hence, the reactivity is promoted by CO under these conditions. This is in contrast to the normal Langmuir-Hinshelwood mechanism which has previously been observed for single crystals and facted NPs. Cooperative effects in the adsorption of the reactants are most likely responsible for such unordinary behaviors.

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, 16 O 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 18 O 2 and 13 C 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 firstprinciples 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 2π* 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 30 O 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.

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd 13 clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd 13 cluster, leading to facile partial oxidation to form Pd 13 O 4 clusters with C 4v symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd 13 O 6 clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd 13 O 4 cluster. Subsequent reactions on the resulting lower-symmetry Pd 13 O x (x < 4) clusters entail lower activation energies. The nonsymmetric Pd 13 O 6 clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.