New innovative gold catalysts prepared by an improved incipient wetness method

We described 0.3% Au/Al2O3 catalysts prepared by the incipient wetness impregnation method using hydrogen tetrachloroaurate dissolved in different aqueous solutions as an impregnation solution. Hydrogen tetrachloroaurate hydrolyses in aqueous solution and the composition of the gold complexes depend on the pH value and the chloride concentration; therefore, different impregnation solutions were prepared by dissolving the hydrogen tetrachloroaurate in water and aqueous solutions of hydrochloric acid, nitric acid, sodium chloride, and sodium hydroxide. The resulting gold complexes were evaluated by UV/vis spectroscopy. Gold catalyst precursors were activated via gas-phase reduction with hydrogen. In addition, the catalyst precursors and the catalysts were investigated by TPR and TEM analysis, respectively, and tested in glucose oxidation. The gold catalyst prepared with a hydrochloric acid solution exhibited the highest activity in glucose oxidation and the smallest gold particles (mean diameter, 1–2 nm). In fact, the catalysts prepared from precursor solutions in which the tetrachloroaurate ion was stabilized demonstrated the highest activities. This is a significant finding, because residual chloride in the catalyst precursor has been considered detrimental to the preparation of active catalysts. In addition, the gold catalyst prepared with 2 M HCl was tested in repeated batches and exhibited excellent long-term stability for the conversion of highly concentrated glucose solutions.