Invited Paper HC+SA+SS-WeA11
Reactivity and Electronic Properties of Supported Metal Oxide and Sulfide Clusters

Wednesday, November 1, 2017, 5:40 pm, Room 24

Small clusters exhibit electronic and chemical properties that can differ significantly from that of the bulk and offer a unique opportunity for preparing novel catalysts whose reactivity can be modified at the atomic level. Here, we use mass-selected cluster deposition to prepare model “inverse” catalysts comprised of small metal oxide (M_xO_y: M = Ti, Nb, Mo, Ce, W) and sulfide (M_xS_y: M = Mo, W) clusters deposited on Cu, Cu₂O/Cu and Au surfaces for studies the water-gas-shift reaction (WGSR) and for CO/CO₂ activation. A key advantage of cluster deposition is that it allows control over cluster stoichiometry which provides a means of introducing oxygen/sulfur “vacancies” and varying the average cation oxidation state. Recent work has focused on the correlation of electron transfer at the cluster-support interface and activity for water dissociation, the latter being a key step in the WGSR mechanism. Electron transfer is probed by XPS core level spectra and 2PPE photoemission measurements of coverage-dependent work function shifts to extract surface dipoles. All the oxide clusters on Cu(111) exhibit negative surface dipoles, indicative of Cu to cluster charge transfer, with smaller dipoles for sub-stoichiometric and reducible oxides (Ti, Nb). Temperature programmed reaction (TPR) experiments show that the Ti_xO_y and Nb_xO_y clusters promote water dissociation on Cu(111), with the ‘reduced’ Ti_xO_y clusters being more active, while both stoichiometric and reduced Nb_xO_y clusters are active. Overall, these results suggest that local cation coordination is most important for determining water activity. Recent ambient pressure XPS (CO+H₂O, 100’s mTorr) measurements at NSLS-II show that small Ti_nO_2n (n = 3, 4, 5) clusters on Cu(111) are active for the WGSR reaction through the observation of reaction-induced O-vacancy formation (Ti³⁺ 2p) and the appearance of formate intermediates (C 1s) at room temperature. Results will also be presented on investigations of CO₂ activation on alkali modified surfaces of Mo₆S₈ clusters on Au(111), which had been previously predicted to be active for CO₂ hydrogenation to methanol. Combined TPR and XPS measurements show that co-deposition of K-atoms and Mo₆S₈ clusters strongly enhances CO₂ adsorption above room temperature, whereas the CO₂ is only weakly bound on the bare clusters. The results will be compared with DFT calculations of the possible CO₂ binding sites for the K-cluster-Au interfaces.

This work was performed at Brookhaven National Laboratory under Contract No. DE-SC0012704 with the U.S DOE, Office of Science, and supported by its Division of Chemical Sciences, Geosciences, and Biosciences.