BACKGROUNDAu valence for Au catalysts has been considered a vital parameter for catalytic activity towards CO oxidation. However, there is no effective method to prepare precise Au-0/Au+ molar ratios with the same Au nanoparticles (NPs) sizes. RESULTSA series of Au/TiO2 catalysts with tuneable Au-0/Au+ molar ratios were prepared by the combination of a biosynthesis method and a deposition-precipitation (DP) method, and characterized by XPS, TEM, DRUV-Vis techniques. First, biosynthesized metallic Au NPs were immobilized onto TiO2 supports to obtain Au/TiO2 catalysts with different Au loadings but with the same sized Au NPs. And then positively ionized Au species were supported onto the above Au/TiO2 catalysts by a DP method. The metallic Au and positively ionized Au species could be totally supported on the support. Therefore, the Au-0/Au+ molar ratios could be controlled exactly. Compared with Au/TiO2 catalysts with metallic and positively ionized Au species alone, the catalyst Au/TiO2-1:1 showed the best initial catalytic activity, suggesting enhanced catalytic performance due to the coexistence of Au-0 and Au+. CONCLUSIONThis study verifies that Au/TiO2 catalysts with accurate Au-0/Au+ molar ratios can be prepared by a combination of biosynthesis and a DP method. (c) 2015 Society of Chemical Industry