Time-resolved in situXAS study of the preparation of supported gold clusters

Incipient-wetness impregnation of γ-Al2O3 with HAuCl4 and subsequent removal of chlorine with NaOH, and deposition-precipitation of HAuCl4 on TiO2 at pH 7 resulted in supported Au3+ species. Time-resolved in situ XAS at the Au L3 edge showed that the Al2O3-supported oxidic or hydroxidic species were reduced in hydrogen at 440 K to yield small metallic gold clusters. The Au3+ precursor decomposed to metallic gold in inert atmosphere at 573 K and in oxidizing atmosphere above 623 K. In all atmospheres, initially small clusters were formed that gradually grew with increasing temperature. The TiO2-supported species were considerably less stable. In hydrogen and carbon monoxide, Au0 clusters of 1 to 1.5 nm were formed at room temperature, which was the lowest temperature studied. In inert and oxidizing atmosphere, the Au3+ precursor decomposed fully to metallic gold at 530 K, as shown by XAS and temperature-programmed experiments. Large clusters were obtained already in the initial stage of reduction. Residual chlorine inhibited the reduction and led to sintering of the gold clusters. Exposure of the TiO2-supported catalyst precursor to light or the X-ray beam led to partial reduction, and STEM showed that storage of the reduced gold clusters under ambient conditions led to agglomeration and bimodal cluster-size distributions.