Catalytic activity of physically deposited metal clusters and their atomic structure study

[摘要] The deposition of preformed clusters onto suitable support materials represents a new technique for the precision synthesis of heterogeneous catalysts. Benefitting from the successful scale-up of the cluster production, this thesis directly explores the catalytic performance of physically produced metal clusters for liquid-phase and vapour-phase model reactions, particularly under realistic conditions. The catalyst powders used in the experiments were produced using two kinds of cluster beam sources: a magnetron sputtering gas condensation cluster source and a matrix assembly cluster source (MACS). The cluster atomic structure, chemical composition and size evolution due to the reaction were characterized by aberration-corrected scanning transmission electron microscopy (STEM), coupled with energy dispersive X-ray spectroscopy (EDS), and correlated with the cluster chemical activities.In the first part, Au/Cu binary clusters with variable controlled composition were deposited onto magnesium oxide supports using a magnetron sputtering gas condensation cluster source. It was found that Au/Cu cluster catalysts are highly active for the liquid-phase 4-nitrophenol reduction. Electron microscopy revealed that the Au/Cu clusters produced have an alloy structure, which results in a random distribution of Au and Cu atoms on the cluster surface. Combined with theoretical calculations of the binding energies, the interplay between Au and Cu atoms at the cluster surface, resulting in an enhanced catalytic activity, is proposed.In the second part, we demonstrated the catalyst preparation with a new type of cluster beam source, MACS. Pd nanoclusters were deposited onto diced carbon tape supports and used for catalyzing vapour-phase selective 1-pentyne hydrogenation. The catalysis results showed that the Pd cluster catalyst from MACS is more active (per unit weight) than the Pd reference sample synthesized by traditional wet impregnation. Cluster size evolution before and after reaction suggested that the superior activity derived from the smaller cluster size and better stability against sintering compared to the Pd reference sample. In addition, no synergetic effect was found in Pd/Au clusters. The observed similar cluster activity (per surface atom) for Pd and Pd/Au cluster catalysts may be due to the oxidation in air, which could drive Pd atoms to the cluster surface, thus leading to a Pd cluster surface as pure Pd clusters.Finally, a new method to prepare metal colloids from the cluster beam technique is introduced, in which metal clusters made using MACS were directly deposited onto soluble polymer films, followed by dissolving the polymer films in suitable solvents. The Pd colloids produced were also used to catalyze the reduction of 4-nitrophenol to demonstrate the catalytic performance. Only a small activity was observed which was attributed to the protecting polymers blocking a high fraction of the active sites on the cluster surface.

[发布机构] University:University of Birmingham;Department:School of Physics and Astronomy, Nanoscale Physics Research Laboratory

[关键词] Q Science;Q Science (General)