Published: Kem. Ind. 54 (9) (2005) 389–397
Paper reference number: KUI-31/2004
Paper type: Review
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Gold as a Catalyst
This article describes possibilities of using gold as a catalyst. Because of its chemically inert character, gold was not considered as an active catalyst until the mid-eighties in the last century. Then, M. Haruta revealed in his article that gold is a very active catalyst for CO oxidation if it is dispersed at some suitable support (oxides of 3d-transition metals, hydroxides of alkaline earth metals or activated carbon) with diameters of its particles smaller than 10 nm. What followed was an explosion of publications about gold catalysis (Fig. 1). Before the great discovery was made by M. Haruta, gold catalysts were usually prepared by impregnation which did not provide samples with a dispersion as high as those for Pt-group metals because of a much lower melting point of gold (1336 K). Therefore, new methods of preparation had to be found that would make a strong connection between supports and particles of gold. Very active gold catalyst will be prepared if it is dispersed at supports with specific surface areas larger than 50 m2 g–1. The best catalysts were prepared with diameters of gold particles from 2–3 nm. It is possible to achieve this using the following methods: a) deposition-precipitation: immersing a metal oxide into an aqueous solution of HAuCl4 · 3 H2O (pH 6–10) and aging for about one hour, b) co-precipitation: simultaneous precipitation of aqueous solutions of HAuCl4 ·3H2O and metal nitrate by Na2CO3 or NH4OH, c) Iwasawa’s method: reaction of Au-phosphine complexes with freshly precipitated supports of (Ti(OH)4, Mn(OH)2, Co(OH)2, Fe(OH)3, Zn(OH)2, Mg(OH)2…), d) co-sputtering: simultaneous sputter-deposition of gold and metal oxide on a suitable substrate to form a film in an atmosphere containing oxygen, e) chemical vapour deposition: adsorption and decomposition of an organogold compound in a form of vapour onto an metal oxide. Methods a) and b) are most frequently used. HAuCl4 · 3 H2O or phosphine complexes Au(PPh3)(NO3) and [Au9(PPh3)8](NO3)3 are usually used as gold precursors. Gold loading should not be more than 20 at.% [at. % Au = 100 Au/(Au + Fe)], and during precipitation pH must be greater than 6. During deposition in alkaline solutions, AuCl4– transforms to Au(OH)3Cl– which precipitates as Au(OH)3. Calcination at temperatures above 473 K causes transformations of Au(OH)3 to Au2O3 and Au2O3 to metallic gold. Before characterization or activity test, gold catalysts are usually pretreated in different ways to achieve bigger activity and stability. But, the catalysts with best performance were prepared through different pretreatments (oxidation, reduction, no pretreatment). The investigations have tried to explain the roles of supports and gold particles. Those results made the divergences even bigger. Some results have shown that there supports had no importance, whereas the others have shown that supports had a crucial role. As for the gold component, some scientists believe that metallic gold is the most active, whereas the others consider oxidized forms of gold more active. Many investigators think that perimeter of gold particles is a place that takes the main role for the most of reactions, so the main aim during preparation should be smaller gold particles. The role of humidity is very important because of practical use of those catalysts. However, the results about it are also divergent. Despite these differences and a restraining from making any generalizations, we can conclude that during the last decade a great introduction of gold as a heterogeneous and homogeneous catalyst for many reactions has been made (Fig. 4 and 5). The most significant patents and prototypes, like air purification from CO (Mintek), three-way catalyst (Toyota), fuel cells (Mitsubishi), addition of alcohols to alkynes (BASF), show that an era of gold as a very important catalyst for conventional and unconventional processes has started.
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catalysis, gold, supporters, preparation methods, patents