(478g) Support Effect on the Size of Au Particles on Functionalized Amorphous Silica

Catalysis by small gold particles has been the object of considerable attention in the past few years and their activity is well-known for size dependent behavior [1]. It is also widely recognized that oxide supports play key roles in the activity and performance of Au nanocatalysts [1-3]. The support structure and its nature can affect the nucleation of Au particles during deposition and the stabilization of the Au particles during subsequent treatments and reaction conditions. Additionally, the support can participate in the adsorption and exchange of oxygen via defect oxygen sites in the support lattice. Therefore, our goal is to investigate the effect of different support types on the size and stability of Au nanoparticles. Tailored non-porous and mesoporous silica materials with single and doublelayers of TiO₂ and Al₂O₃ were prepared via a layer-by-layer approach and used as Au support. X-ray Absorption Spectroscopy (XAS) was utilized to investigate both the Au particle size on functionalized amorphous silica support, and the coordination of TiO₂ in the support phase. Measurements of the Au LIII-edge and Ti K-edge were used to correlate changes of Au particle size with variations of titanium oxide structure.

Binary oxide monolayers of Al₂O₂ and TiO₂ on a silica support were prepared by using surface sol-gel [4]. The silica support used for this investigation was an amorphous silica material (Cab-O-Sil) and mesoporous silica (SBA-15). The catalysts investigated were Au supported on: a) single and double layer of TiO₂ on SiO₂; b) single and double layer of Al₂O₂ on SiO₂; and, c) doublelayers of Al₂O₂ and TiO₂ on SiO₂. The XAS data at the Au LIII-edge and Ti K-edge were collected at beamline X19A at the NSLS, Brookhaven National Laboratory. Catalysts were inserted in a quartz reactor tube that could be heated for sample pre-treatment or cooled for extended X-ray absorption fine structure (EXAFS) measurements at liquid nitrogen temperatures.

A protocol for reducing and calcining the catalysts at variable temperatures was used to follow the growth and stability of Au particles by monitoring the Au LIII-edge after each treatment step. Treatment with 4%H2/He at 150^oC was sufficient to reduce completely the Au particles and significant aggregation was observed after calcination at high temperatures (500^oC). However, the size and stability varied greatly with the different layer-by-layer support type. The Ti oxide structures on the layered TiO₂ and binary oxide TiO₂/Al₂O₂ were determined by observing the near-edge region (XANES) of the XAS spectra. Ti K-edge XANES has been widely used to determine the Ti coordination and disorder [5]. Both the pre-edge position and the normalized height are used to determine the Ti coordination number. Measurements were made before and after heating the catalysts in an inert atmosphere in order to cause dehydration of the oxide support. The results indicate the absence of fully 6-coordinated titanium, which demonstrates that TiO₂ is likely present as small crystallites mostly located at the surface of the layered catalysts.

Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. The National Synchrotron Light Source, Brookhaven National Laboratory, is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences.

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