Heterogeneous catalysis by gold: The effect of oxide support, external conditions, and the metal/oxide interface.

摘要

Bulk Au is inert. However, when dispersed as small particles on an oxide support its chemistry changes dramatically, and can become active in many catalytic reactions. This unprecedented change in Au chemistry is still not well understood, and is the focus of this dissertation. A combined theoretical/experimental approach was used to investigate the active form of Au in low-temperature CO oxidation, how external conditions affect Au, and the role the oxide support.From literature, many forms of Au (Audelta-, Au 0, and Audelta+) have been proposed as the catalytically active form of Au in low-temperature CO oxidization. We found that experimental conditions directly affected the form of Au present, and, to some degree, all forms of Au could perform catalytically. Moreover, that Au/oxide interface played a critical role in producing charged Au species, which exhibited superior catalytic activity over metallic Au.Another key issue is the role of different forms of cationic Au. In literature it was argued that Audelta+ was both highly active and catalytically dead. We found that not all forms of cationic Au perform the same chemistry, and that only specific Audelta+ species could perform oxidation. Furthermore, we determined that common catalyst preparation procedures result in the deposition of AuClx or AuO x species, both cationic in nature, yet only AuO x is able to catalyze CO oxidation. Our results were further corroborated by experimental studies that directly tested the activity of AuClx and AuOx.Lastly we investigated the effect of the oxide support. We tested the effect of four oxides of different electronic character (SiO2, TiO2, SnO2, and IrO2), and found that the Au/oxide interface site was directly affected by the type of oxide present. Moreover, that the activity of the Au/oxide interface towards binding and dissociating O2 followed a volcano shaped curve with a maximum situated at the semi-conductor supported Au systems, i.e., TiO2 and SnO2. These results were found to be directly inline with the experimentally measured support effect, and indicated that the surface chemistry of Au/oxide may be rationally tuned.