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-cont rolled" gold colloids of 2-nm mean particle size. They were prepared by red uction of chloroauric acid with tetrakis(hydroxymethyl)phosphonium chloride in an alkaline solution, followed by adsorption of gold colloids on TiO2 a nd ZrO2 at a pH lower than the isoelectric point of the metal oxides. Inves tigation of the size of the gold particles in solution by UV-vis spectropho tometry in combination with HRTEM indicated that the gold colloids are rath er stable in alkaline solution, during pH-change and purification with dial ysis. Ageing of the solutions showed that the particle size slowly increase d 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%, compar ed to the calculated nominal monolayer coverage of 45-55 wt%), incomplete a dsorption occurred, affording more inhomogeneous dispersion and some aggreg ation. After calcination at 673 K, both zirconia- and titania-based catalys ts containing 1.7 wt% Au exhibited high activity in low temperature CO oxid ation. Although the particle size on both supports was comparable, the Au/T iO2 catalyst showed significantly higher activity than the Au/ZrO2 catalyst . The uncalcined Au/TiO2 also exhibited high activity, whereas the uncalcin ed Au/ZrO2 was inactive under the same conditions, corroborating that not o nly the gold particle size but also the support plays a key role in CO oxid ation. (C) 1999 Academic Press.