title: Time-resolved in situ XAS study of the preparation of supported gold clusters.
abstract: Incipient-wetness impregnation of gamma-Al(2)O(3) with HAuCl(4) and subsequent removal of chlorine with NaOH, and deposition-precipitation of HAuCl(4) on TiO(2) at pH 7 resulted in supported Au(3+) species. Time-resolved in situ XAS at the Au L(3) edge showed that the Al(2)O(3)-supported oxidic or hydroxidic species were reduced in hydrogen at 440 K to yield small metallic gold clusters. The Au(3+) 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 TiO(2)-supported species were considerably less stable. In hydrogen and carbon monoxide, Au(0) clusters of 1 to 1.5 nm were formed at room temperature, which was the lowest temperature studied. In inert and oxidizing atmosphere, the Au(3+) 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 TiO(2)-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.
authors.affiliation: Institute for Chemical and Bioengineering, ETH Zurich, 8093, Zurich, Switzerland.
authors.forename: Eveline
authors.initials: E
authors.lastname: Bus
authors.forename: Roel
authors.initials: R
authors.lastname: Prins
authors.forename: Jeroen A
authors.initials: JA
authors.lastname: van Bokhoven
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry*; D002248:Carbon Monoxide / Q000737:chemistry; D002384:Catalysis; D002412:Cations; D002713:Chlorine / Q000737:chemistry; D006046:Gold / Q000737:chemistry*; D017612:Gold Compounds / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D049329:Nanostructures / Q000737:chemistry*; D010084:Oxidation-Reduction; D012972:Sodium Hydroxide / Q000737:chemistry; D013052:Spectrometry, X-Ray Emission / Q000379:methods*; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014025:Titanium / Q000737:chemistry
chemical_list: D002412:Cations; D017612:Gold Compounds; C038016:gold chloride; C009495:titanium dioxide; D002713:Chlorine; D012972:Sodium Hydroxide; D006046:Gold; D002248:Carbon Monoxide; D006859:Hydrogen; D014025:Titanium; D000537:Aluminum Oxide