Oxidative chemistry on gold : unraveling molecular transformations at surfaces
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Gold has been considered catalytically inert due to its resistance to oxidation and corrosion. However, decades ago, it was discovered that gold nano-particles (<5nm) on metal oxides demonstrate superior chemical activity towards many reactions. These seminal findings spurred considerable interest in investigations of the mechanistic details of oxidative reactions on gold-based catalysts. However, the active site and structure of supported Au nanoclusters as well as the active oxygen species remains elusive. Achieving high selectivity toward partial oxidation products also remains a challenge. In this dissertation, an oxygen-covered Au(111) crystal under ultra vacuum conditions was used as a model system to gain insights into oxidative reactions in gold-based catalysis. I have been able to demonstrate that (i) surface-bound oxygen atoms are metastable at low temperature; (ii) the oxygen atoms participate in surface reactions as a Brønsted base or a nucleophilic base; and (iii) the acid-base reactions that have been observed on silver and copper may also occur on gold. Low temperature CO oxidation and the associated mechanistic aspects are investigated. CO reacts with hydroxyls formed from water-oxygen interactions to produce CO₂ on Au(111) populated with atomic oxygen at low temperatures. Directing an ¹⁶O beam toward C¹⁸O₂ pre-adsorbed Au(111), the formation of carbonate is significantly enhanced. This reaction is suggested to follow a hot-precursor-mediated mechanism. The identification of reaction pathways in oxidation of N-containing molecules such as ammonia and propylamine is presented. Abstraction of hydrogen from ammonia or propylamine by O atoms is the initial step in the surface decomposition of NHx (or RNHx-1) on Au(111). Atomic oxygen or hydroxyl-assisted dehydrogenation steps have lower barriers than the recombination steps under relevant conditions. 100% selectivity of N₂ or propionitrile can be obtained if the oxygen coverage is below the stoichiometric value. The surface oxidative chemistry of alcohols on Au(111) is also investigated. Except for methanol that is fully oxidized, alcohols initially undergo O-H bond cleavage (producing alcoxides) followed by selective β-C-H bond activation to form aldehydes or ketones. This finding reveals that the interaction of Au with the metal oxide support might not be essential to facilitate the reactions if active oxygen species are readily present, particularly at low temperatures.