Single-Atom Catalysts: Syntheses, Characterization, and Catalytic Evaluation in Selective Oxidation and Hydrogenation Reactions
The focus of this thesis is on the synthesis, characterization, and application of Pd-based single-atom catalysts (SACs). An emphasis will be placed on the characterization of these materials through X-ray Absorption Spectroscopy (XAS) and use as selective catalysts in oxidation and hydrogenation reactions. The first project discusses the characterization of quasi-homogeneous AuPd catalysts and their use in the selective oxidation of crotyl alcohol. The quasi-homogeneous catalysts were synthesized via two reduction routes, co- and sequential reduction, with varying Au/Pd ratios from 4/1 to 1/1. Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) analyses were performed to confirm the bimetallic nature of these catalysts. X-ray Absorption Near-Edge Structure (XANES) was used to probe the local atomic structure of Au and Pd in these systems, while Extended X-ray Absorption Fine Structure (EXAFS) data was modelled to quantitatively discuss the surrounding environment of Au and Pd in terms of coordination numbers (CN) and bond distances between neighbouring atoms and the metal center. An oxidation reaction of crotyl alcohol into three products, crotonaldehyde, 1-butanol, and 3-buten-1-ol, was performed to quantify the selectivity of each catalyst to crotonaldehyde. As the ratio of Au/Pd decreased towards unity, the selectivity decreased in both catalyst systems (i.e., co- and sequentially reduced AuxPd). The second project consists of the characterization of a heterogeneous Pd catalyst supported on graphitic carbon nitride (Pd/g-C3N4) and its use in the selective hydrogenation of an alkyne, 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE), an alkene. XANES and EXAFS analyses were performed to determine structural information with varying weight percentages of Pd (0.1%, 0.5%, and 2.0%). These Pd/gC3N4 catalysts were compared in the hydrogenation of MBY to see if selectivity towards MBE was affected. There was no semblance of selectivity, though as the amount of Pd in g-C3N4 was increased, the activity of the catalyst increased. A comparison was then performed between a 0.5% Pd/g-C3N4, Pd NP and co-Au4Pd catalyst in the hydrogenation of MBY. Results show that the supported Pd catalyst was most active, though none of the catalysts were selective for this reaction, suggesting there is no structure-sensitivity present. The loading of Pd in the heterogeneous catalyst was much lower than the co-Au4Pd, showing that metal loading can be decreased without sacrificing activity.
Single-atom, Catalysis, Nanomaterials
Master of Science (M.Sc.)