CO oxidation over Au/TiO2 prepared from metal-organic gold complexes

Although the early publications by Bond et al. [1,2] and Parravano et al. [3,4] reported that small gold particles exhibited catalytic activity for some reactions, such as hydrogenation of olefins and oxygen transfer reactions, gold was generally considered to be catalytically inactive [5]. Over the past 20 years, the volume of literature on the application of gold catalysts has been steadily increasing since Haruta reported that highly dispersed gold particles supported on metal oxides are extremely active for CO oxidation [6,7]. Studies on highsurface-area catalysts have shown that the catalytic performance of gold catalysts depends strongly on the size of the Au particles, with a maximum activity found for particles with an average diameter 3 nm [8]. Valden et al. [9] found that the size dependence of CO oxidation on model catalysts is similar to that on highsurface-area catalysts, and explained this size dependence as arising from quantum size effects. For the preparation of highly dispersed gold catalysts, specific methods have been used, such as co-precipitation, deposition-precipitation, and chemical vapor deposition [10]. Conventional impregnation methods, which are widely used to prepare supported metal catalysts, were thought to be ineffective in producing active gold catalysts. Haruta reported that gold catalysts prepared by the impregnation method using HAuCl4 as a precursor, had a particle size larger than 30 nm, and were catalytically inactive for CO oxidation [10]. Metal-organic and organometallic complexes have been widely used in the synthesis of catalysts, however, the use of metal-organic or organometallic gold complexes has been limited. Gates and coworkers reported that a supported mononuclear gold complex was active for ethylene hydrogenation at 353 K [11]. Recently, Iwasawa and coworkers [12–16] prepared highly active CO oxidation catalysts by grafting Au-phosphine complexes onto as-precipitated Ti(OH)4. They found that the as-precipitated Ti(OH)4 was essential for obtaining highly dispersed gold catalysts and the use of a conventional TiO2 support resulted in a much less active catalyst with a particle size greater than 15 nm. Previous work from our laboratories used a [Au6(PPh3)6](BF4)2 complex to prepare Au catalysts by the impregnation method on a conventional TiO2 support; the resulting catalyst was highly dispersed (<5 nm) and active for CO oxidation [17]. Here we report a series of Au/TiO2 catalysts prepared from Aun (n = 2–4, number of gold atoms in the complex) metal-organic complexes together with their catalytic activity for CO oxidation.