New Trends in Gold Catalysts

Abstract

Gold is an element that has fascinated mankind for millennia. The catalytic properties of gold have been a source of debate, due to its complete chemical inertness when in a bulk form, while it can oxidize CO at temperatures as low as ~200 K when in a nanocrystalline state, as discovered by Haruta in the late 1980s [1]. Since then, extensive activity in both applied and fundamental research on gold has been initiated. The importance of the catalysis by gold represents one of the fasted growing fields in science and is proven by the promising applications in several fields, such as green chemistry and environmental catalysis, in the synthesis of single-walled carbon nanotubes, as modifiers of Ni catalysts for methane steam and dry reforming reactions and in biological and electrochemistry applications. The range of reactions catalyzed by gold, as well as the suitability of different supports and the influence of the preparation conditions have been widely explored and optimized in applied research [2]. Gold catalysts appeared to be very different from the other noble metal-based catalysts, due to their marked dependence on the preparation method, which is crucial for the genesis of the catalytic activity. Several methods, including deposition-precipitation, chemical vapor deposition and cation adsorption, have been applied for the preparation of gold catalysts over reducible oxides, like TiO2. Among these methods, deposition-precipitation has been the most frequently employed method for Au loading, and it involves the use of tetrachloroauric (III) acid as a precursor. On the other hand, the number of articles dealing with Au-loaded acidic supports is smaller than that on basic supports, possibly because the deposition of [AuCl4]− or [AuOHxCl4−x]− species on acidic supports is difficult, due to their very low point of zero charge. Despite this challenge, several groups have reported the use of acidic zeolites as supports for gold. Zeolites are promising supports for Au stabilization, because of the presence of ion-exchange sites, such as NH4+, that can be substituted by Au+ ions through the elimination of NH4Cl [3]. Moreover, zeolites, due to their high thermal stability, the presence of a large surface area and micropores, may hinder Au sintering. [...]

Hydrogen Production from Formic Acid over Au Catalysts Supported on Carbon: Comparison with Au Catalysts Supported on SiO2 and Al2O3

“…Nevertheless, carbon supported Au is relatively rarely studied. One of the reasons for this is the difficulty of synthesizing highly dispersed (<3 nm) gold nanoparticles with a narrow particle size distribution [4][5][6][7][8][9][10].…”

Selective Catalytic Activation of Acetylene by a Neutral Gold Cluster of Experimentally Known Gas-Phase Geometry

“…It is well-known that one of the most chemically inert metals, gold, is very active as a catalyst when it is dispersed in nanometric sizes less than 5 nm and when it is prepared in an appropriate manner. − This characteristic has been observed even for gold clusters deposited on an inert support, in solution, or in gaseous phase. − In general terms, the gold catalytic activity has been largely attributed to the high reactivity at the particles zones such as edges and corners, where the gold atoms have low coordination numbers. , In fact, it has been found that (a) the fraction of low-coordinated Au atoms, and not the particles surface area, scales approximately with the catalytic activity, (b) usually the Au catalytic activity is strongly dependent on the particle size and shape, increasing with the decrease in size, ,,− (c) very small neutral Au n clusters, with n ranges from 1 to 11, have outstanding catalytic activity ,,,− (d) small Au nanoparticles are sufficiently reactive to catalyze reactions at or even to very low temperatures, , and (e) the catalytic activity of gold is, in important cases, very selective. − …”

A Study of Catalytic Oxidation of a Library of C2 to C4 Alcohols in the Presence of Nanogold

“…In view of presented results, the 0.1% Au/SiO 2 catalyst seems to have prospects for wide applications for alcohol oxidation, and notably, SiO 2 appears to be a promising support material for nanogold particles. This is an intriguing finding taking into account the recent trends in the investigation for an optimal material for nano-Au support, which is one use of reducible metal oxides, usually Fe 3 O 4 , ZnO, CeO 2 , and TiO 2 [28]. In fact, the interactions between the active phase and active support (e.g., Au–TiO 2 , Au–Fe 3 O 4 ) via the formation of oxygen vacancies in reducible metal oxides are recognized as being one of the most effective ways to enhance the catalytic properties [29].…”