Stability Study of Highly Dispersed Au Clusters Produced on Defected TiO 2 (110); Evidence from SEM and Olefin TPD
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Stability Study of Highly Dispersed Au Clusters Produced on Defected TiO2 (110); Evidence from SEM and Olefin TPD Y. Yang and E. McFarland Dept. Chemical Engineering, University of California, Santa Barbara, CA 93117, U.S.A. ABSTRACT The stability of Au nanoclusters created at defect sites on TiO2(110) surfaces is under investigation. Using Ar ion sputtering, 0 - 10% of the surface area of ideal surfaces are modified with defects at a controlled defect density. Defected surfaces are also produced using controlled vacuum heating to 1300 K for 25 minutes. Identical exposures (< 1Å) of Au were evaporated on each surface and results of propylene temperature programmed desorption (TPD) heating cycles to < 700 K are compared with ideal control surfaces. Results support the model that Au deposited on a sputtered surface is immobilized at defect sites and the clusters so formed are stable with respect to sintering. As previously observed, with increased sputtering time there is a decrease in the cluster size and increased defect density. The desorption peak of propylene from the Au-TiO2 surface is used as an in situ probe of cluster size stability. By cyclically performing TPD to 700 K for longer times and additional cycles (or higher temperature), the sintering stability can be assessed. On the sputtered TiO2 (110) surface, evaporated gold clusters sinter with only ~25% maintaining their initial size. On the unsputtered 1300 K annealed TiO2, Au is stable with respect to sintering for many cycles of 800 K heating. This phenomenon is attributed to both the surface stability and Au affinity for the thermal defects created on the surface. This work extends our previous investigations of Au cluster formation on defected TiO2 to their stability with respect to sintering. INTRODUCTION Transition metals supported on metal oxides are of widespread interest in catalysis. It is thought that both Lewis acidity and the more complex redox activity of the TiO2 surface give rise to its interesting catalytic activity [1]. Metals on TiO2 show strong support effects compared to other oxide supports [2,3]. For example, studies show that nanostructured gold on TiO2 has high catalytic activity for propylene epoxidation [4]. The detailed mechanism of such unusual activity is to be understood. It is noticed that when atomic flux of gold atoms is evaporated in UHV on titania, the nanoclusters tend to grow at defect sites [5]; on titania surfaces such defects seems to be oxygen vacancies, i.e., positively charged titanium centers [6]. Thus there is a model that due to the extremely small size of the nanocluster, transfer of even one electron from the cluster to the support may lead to a noticeable change in the electronic density of the entire nanocluster. A widely utilized model system for trying to understand the mechanism of such reactions is the in situ UHV evaporation of Au on single crystal TiO2 [3,7,8]. Studies of the individual components of the supported metal systems have been useful in trying to interpret the support effects. Dav
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