Structure and reactivity studies of size-selected transition metal sulfide nanoclusters in the gas phase and supported on gold(111).

摘要

The understanding of catalysts used in hydrodesulfurization (sulfur removal from petrochemical feedstock) is important from an environmental and economic standpoint as their efficiency directly correlates to atomospheric quality and fuel costs. Nanoparticles of MoS2 supported on Al2O 3 are used industrially for these reactions but the heterogeneity of such catalysts has limited the identification and in turn understanding of the catalytically active sites. Work presented here focuses on developing model catalytic systems where the chemical composition can be controlled in order to gain insight into those properties that are important for these catalytic processes and therefore can be tailored to increase selectivity.;Initial studies were directed towards generating MxS y+ (M=Mo,W) clusters in the gas phase via magnetron sputtering. Using tandem mass spectrometry and a gas collision cell we were able to size-select the clusters of interest and react them in the gas phase with probe molecules such as CO. The resulting cluster adducts provided information regarding the number of active metal sites and the geometry of the cluster. Density functional theory (DFT) calculations were used to search for the lowest energy structures of the bare MxSy+ clusters and to obtain their relative stability for sequential CO binding. The calculated trends in CO binding energies were then compared to the experimental adduct distributions for assigning the ground state structures.;A size-selected deposition investigation was also done on the Mo 6S8 cluster supported on Au(111). Carbonyl sulfide (OCS) undergoes a dissociation reaction on the Mo6S8 cluster that is substrate mediated as the Au(111) directly participates in the reaction. The OCS dissociatively adsorbs onto a molybdenum metal site, with the sulfur atom settling onto the Au surface and a CO molecule desorbing in the gas phase. This reaction is very unique in that the Au surface has an active role in the reaction mechanism and also lowers the barrier for OCS dissociation despite reports that the substrate is chemically inert. DFT calculations were done to observe local intermediates in order to generate a reaction pathway for the OCS dissociation on the Mo6S8/Au(111) system.