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DESIGN AND STRUCTURAL CHARACTERIZATION OF MONOMETALLIC AND BIMETALLIC CLUSTERS FOR SELECTIVE HETEROGENEOUS CATALYSIS

Date

2022-03-14

Journal Title

Journal ISSN

Volume Title

Publisher

ORCID

0000-0003-2727-651X

Type

Thesis

Degree Level

Doctoral

Abstract

The development of catalysts for industrial use can take advantage of the design of clusters with a more abundant and relatively less expensive metal (i.e. silver (Ag)) rather than gold (Au)). Atom-precise Ag25(SR)18- clusters are attractive as they have a similar structure to the Au analogue, Au25(SR)18-, which has been well studied in the literature. This thesis documents the rational synthesis and structural characterization of Ag monometallic and silver-palladium (AgPd) bimetallic clusters for use as selective oxidation and hydrogenation catalysts. Clusters were synthesized and purified via solution-based synthetic strategies, followed by deposition of the clusters onto carbon supports. Atom-precise Ag25(SPhMe2)18- clusters on carbon supports were activated at a mild temperature of 250 °C without the formation of Ag2O or Ag2S, albeit with a slight growth in cluster size. A sequential deposition (SD) strategy towards the rational synthesis of supported-bimetallic clusters catalysts based on atom-precise monometallic Ag cluster precursors was investigated and contrasted with a direct synthesis approach. The SD approach resulted in AgPd bimetallic catalysts with cluster-in-cluster atomic arrangements and could incorporate variable amounts of dopant Pd atoms. In contrast, the direct synthesis approach predominantly formed atom-precise Ag24Pd1(SPhMe2)182- bimetallic clusters with the single Pd dopant occupying a non-central position in the clusters. These bimetallic clusters were used as model catalysts to reveal the correlation between catalyst performance and their structures. The isolation of active Pd sites suppressed the over-hydrogenation of alkynols to alkanols by Pd catalysts and the AgPd bimetallic catalysts prepared by both methods were selective for intermediate hydrogenation products. Electron microscopy and X-ray spectroscopy were found to be good characterization tools to elucidate structures of the synthesized Ag-based clusters before and after activation for catalysis. Pair distribution function (PDF) analyses based on total scattering data also allowed insight into the structural changes that occur during the thermal activation process. Furthermore, differential PDF analysis clearly shows the structural transformation of atom-precise Ag and Au clusters to nanoparticles with bulk fcc-like structures at intermediate temperatures, while the Ag system behaves significantly differently than the analogous Au system at high temperatures.

Description

Keywords

Bimetallic clusters, Heterogeneous catalysts, Pair distribution function analysis, X-ray absorption spectroscopy, Selectivity, Sequential deposition, Thermal activation, Total scattering measurements.

Citation

Degree

Doctor of Philosophy (Ph.D.)

Department

Chemistry

Program

Chemistry

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DOI

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