Preparation of supported gold catalysts for low-temperature CO oxidation via "size-controlled" gold colloids

Catalytically active gold model catalysts have been designed via "size-controlled" gold colloids of 2-nm mean particle size. They were prepared by reduction of chloroauric acid with tetrakis(hydroxymethyl)phosphonium chloride in an alkaline solution, followed by adsorption of gold colloids on TiO(2) and ZrO(2) at a pH lower than the isoelectric point of the metal oxides. Investigation of the size of the gold particles in solution by UV-vis spectrophotometry in combination with HRTEM indicated that the gold colloids are rather stable in alkaline solution, during pH-change and purification with dialysis. Ageing of the solutions showed that the particle size slowly increased over a time scale of 4 months. Analysis of the dried catalysts by XRD and HRTEM corroborated that the particle size was nearly preserved during the immobilization process. Only in the case of high loadings (16.6 wt%, compared to the calculated nominal monolayer coverage of 45-55 wt%), incomplete adsorption occurred, affording more inhomogeneous dispersion and some aggregation. After calcination at 673 K, both zirconia- and titania-based catalysts containing 1.7 wt% Au exhibited high activity in low temperature CO oxidation. Although the particle size on both supports was comparable, the Au/TiO(2) catalyst showed significantly higher activity than the Au/ZrO(2) catalyst. The uncalcined Au/TiO(2) also exhibited high activity, whereas the uncalcined Au/ZrO(2) was inactive under the same conditions, corroborating that not only the gold particle size but also the support plays a key role in CO oxidation. (C) 1999 Academic Press.