The Preparation of Highly Dispersed Au/Al₂O₃ by Aqueous Impregnation

Abstract

In this work, we report an impregnation method for preparing Au supported on alumina from HAuCl₄. In the literature, impregnation under acidic conditions has been found to lead to poor dispersions of Au and the resulting catalysts are not as active as those prepared by deposition-precipitation. To overcome these problems, we have developed a two-step procedure: in the first step, the acidified Au solution is contacted with alumina to adsorb the Au chloride on the alumina. After washing off the excess Au precursor, we treat the solid with a strong base to convert the chloride to an absorbed hydroxide. Drying and calcination at 400 °C yields a catalyst with Au particles having a number average diameter of 2.4 nm. The reactivity for CO oxidation at room temperature is comparable to catalysts prepared by deposition-precipitation. These catalysts are stable to hydrothermal sintering, with average particle size around 4 nm after sintering in 10 mol% H₂O at 600 °C for 100 h. This work shows that stable Au/Al₂O₃ catalysts having a high reactivity for CO oxidation can be prepared by impregnation under acidic conditions.