Gold catalysts, supported catalyst preparation

Highly active gold catalysts can be prepared by an appropriate selection of preparation methods such as CP, DP, DR, and SG with dimethyl Au(III) acetylacetonate, depending on the kind of support materials and reactions... 

J. Luo, M. M. Maye, N. N. Kariuki, L. Wang, P. Njoki, Y. Lin, M. Schadt, H. R. Naslund, and C. J. Zhong, Electrocatalytic oxidation of methanol Carbon-supported gold-platinum nanoparticle catalysts prepared by two-phase protocol, Catal. Today 99, 291-297 (2005). 

A. K. Sinha, S. Sedan, M. Okumura, T. Akita, S. Tsubota, M. Haruta, Three-dimensional meso-porous titanosilicates prepared by modified sol-gel method Ideal gold catalyst supports for enhanced propene epoxidation, J. Phys. Chem. B 109 (2005) 3956. 

Vapor-phase epoxidation of propylene using H2 and O2 was carried out over gold catalysts supported on mesoporous ordered (MCM-41) and disordered titanosilicates prepared hydrothermally or by modified sol-gel method. Gold nanoparticles were homogeneously dispersed on the titanosilicate supports by deposition-precipitation (DP) method. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. NaOH was found to be the best precipitant to prepare Au catalysts with optimum propylene oxide yields and H2 efficiency. The extent of catalysts washing during preparation was found to affect the activity of the catalyst. The activity and hydrogen efficiency was found to depend on the type of mesoporous support used. 

Gold catalysts supported on active carbon and alumina have been prepared by incipient wetness impregnation or by precipitation at basic pH using aqueous solutions of HAUCI4 [82]. 

The nature of the supporting material can determine the final catalytic performance. Interestingly, gold catalysts supported on various carbons displayed differences even in the same reaction, which implies that the catalytic performance was affected not only by the nature of the carbon but also by the preparation method [47]. One of the most relevant parameters for the choice ofthe support seems to be the type and distribution of surface groups, as these can favor or inhibit the grafting of the nanoparticles. Hence, acidic or basic treatments of supporting materials... 

It is useful first to summarise the main methods of preparation for gold catalysts supported on powdered oxides (so called real catalysts). Those used to prepare model catalysts, i.e. gold supported on single crystal oxides, are the same as for other metals, and are not repeated here. 

Andreeva et al. [126] prepared gold catalysts supported on Ce02 and Ce02-AI2O3 by CP by varying the ratio between Ce and Al. The catalytic activity... 

The goal of this chapter is to gather the methods most commonly used for the preparation of gold catalysts supported on common oxide powders. For the sake of brevity, deposition on more specific supports, such as structured mesoporous supports, zeolites, and carbon is barely reported in this chapter, and we refer to Chapter 4 of the book on gold catalysis [7]. The methods described are mainly those performed in the liquid phase. The preparations of model catalysts, that is, on films or single crystals and those of bimetallics are excluded. 

The previous examples are all supported gold catalysts. Supported bimetallic catalysts have been explored as well. Scurrell and coworkers developed a series of Au-M/ Fe Oj (M=Ag, Bi, Co, Cu, Mn, Ni, Pb, Ru, Sn, Tl) catalysts for WGS [70,71], These catalysts were prepared by deposition-coprecipitation using HAuCl, FeCNOjjj, and metal salts as precursors, followed by calcination in air at 400°C. Au-Ru/Fe Oj showed the highest activity. However, there is no evidence showing that the so-called bimetallic catalysts are bimetallic. The thermal decomposition of metal salts in air usually leads to the formation of metal oxides instead [61]. 

Gold catalysts supported on ceria, titania and iron oxide are reported in the literature as candidates for a low-temperature water-gas shift [164, 319], which may well be significantly active in the temperature range below 200 °C [320]. They are, however, very sensitive to the preparation conditions and gold particles generally suffer from sintering issues [164,321], which makes them not particularly applicable for practical applications to-date. 

Figure 10.7 WGS activity of gold catalysts supported on ceria and ceria modified by alumina, prepared by different methods.

Preparation of gold catalysts supported on Si02-Ti02 for the CO PROX reaction... 

CO conversions over Au/Ce02 catalyst were measured in the dry and wet condition as shown in Fig. 1. Similar to other supported gold catalysts, Au/Ce02 catalyst showed higher CO conversions in the presence of water vapor than in the absence of it at the same temperature. Catalytic activities for CO oxidation over Au/Ce02 catalysts prepared at different calcinations temperature were compared in the dry and wet condition as shown in Fig. 2. Au/Ce02 catalyst calcined at 473 K showed the highest initial CO conversion in the absence of water vapor. However, the CO conversion decreased steadily and reached a steady-state value over this catalyst. 

Gold catalysts containing NHC ligands can also promote cycloisomerisation reactions. Bicyclo[3.1.0]hexanes 137-139 can be prepared from the cycloisomerisation of 1,5-enynes bearing a propargyUc acetate (135) in the presence of catalytic amounts of [AuCl(lPr)]/AgBF (Scheme 5.36) [41]. The cycloisomerisation reaction of 135 occurs by a 1,3-OAc shift/aUene-ene cyclisation/l,2-OAc shift sequence. Experimental results with allenyl acetate 136 support this hypothesis as 139 is obtained in higher ratios than 137 and 138 [41b],... 

Figure 1. TEM image of a titania supported gold catalyst (1.7wt.% Au) prepared by deposition-precipitation (gold particle size = 5.3+ 0.3 nm, dispersion = 36%). (Reprinted from Reference [84], 2000, with permission from American Chemical Society).

---