Gas-phase catalysis by micelle derived Au nanoparticles on oxide supports Ju Chou, Nathan R. Franklin, Sung-Hyeon Baeck, Thomas F. Jaramillo, and Eric W. McFarland * Department of Chemical Engineering, University of California, Santa Barbara, CA 93106 Received 4 December 2003; accepted 18 March 2004 The reactivity of gold clusters (8–22 nm diameter) supported on different metal oxides (titanium dioxide (TiO 2 ), zinc oxide (ZnO), zirconium oxide (ZrO 2 ), and silicon dioxide (SiO 2 )) was investigated in a continuous flow reactor. Clusters were encapsulated within polymer in toluene solution, impregnated onto the bulk supports, and reduced by calcination at 300 °C. Support dependent sintering (TiO 2 > ZrO 2 > ZnO) was observed following heating in air at 300 °C. For both CO oxidation and propylene hydrogenation, Au nanoclusters on TiO 2 exhibit the highest activity compared to other supports. KEY WORDS: nanoparticles; catalysis; CO oxidation; propylene hydrogenation. 1. Introduction Bulk gold has been considered catalytically inactive; however, gold nanoparticles (<10 nm) have shown high activity for several types of oxidation and reduction reactions [1–7]. Several studies have documented the profound size dependent selectivity and low temperature activity of supported Au nanoparticles [6–10]. In par- ticular, Haruta and colleagues showed data where Au clusters less than 2 nm were active for hydrogenation of propylene while clusters of 2–4 nm under the same conditions were selective for epoxidation [8,9]. Other work has verified the size dependence, albeit with somewhat different and unexplained product specificity [4]. What is clear is that to study and fully exploit Au nanoparticles for gas-phase catalysis, precise control and tuning of the nanoparticle size is desired. Numerous methods for the preparation of Au nano- particles have been employed including incipient wetness [11], deposition precipitation [2,8], organic capping [12] and micelle encapsulation [13–15]. Incipient wetness and deposition precipitation are very simple and scalable synthesis methods but suffer from several limitations such as precise particle size control and flexibility in oxide supports available for deposition, respectively. Organic capping has been employed with great success in producing Au nanoparticles with precise size control (5%). The size distribution of particles produced depends on the combination of synthesis parameters used, with particular emphasis on concentration, tem- perature and the organic capping agent selected. This method is able to give a narrow distribution of size selected particles, but tuning of the particle size to a new size can require large modifications of the synthetic procedure; making it very difficult to differentiate changes in catalytic activity due to size dependent effects and those which are due to differences in the synthetic method. In this paper, a micelle encapsulation method based on block copolymers is used to synthesize Au nanoparticles. This method has been used previously with great success to produce a wide range of nanopar- ticles, with particle size defined by the length of the hydrophilic block of the polymer [13–15]. These parti- cles are then typically deposited onto a planar substrate in a hexagonal pattern with the spacing defined by the length of the hydrophobic block of the copolymer. No previous work has investigated the activity of the clusters when used for gas-phase catalysis and here we have exploited the precise size control of the micelle derived clusters to prepare Au nanoparticle catalysts supported on different metal oxides where the cluster precursors are known to have a homogeneous distribu- tion. The particles were deposited onto several different oxide supports and evaluated for catalytic activity and selectivity in CO oxidation and propylene epoxidation. In doing so we attempt to answer two questions: (1) Can micelle derived Au nanoparticles be active in gas-phase catalysis reactions? (2) Is the catalytic activity dependent on the specific support composition? 2. Experimental HAuCl 4  3H 2 O, toluene, CDCl 3 were obtained from Sigma–Aldrich. Diblock copolymer [polystyrene 81,000 - block-poly(2-vinylpyridine) 14,200 ] was purchased from Polymer Source. Anatase TiO 2 (surface area 34.8 m 2 /g) was donated by Saint–Gobain Norpro Corp. The other metal oxides including zinc oxide (ZnO), zirconium dioxide (ZrO 2 ) and silicon dioxide (SiO 2 ) were all obtained from Sigma–Aldrich. *To whom Correspondence should be addressed. E-mail: mcfar@engineering.ucsb.edu Catalysis Letters Vol. 95, Nos. 3–4, June 2004 (Ó 2004) 107 1011-372X/04/0600–0107/0 Ó 2004 Plenum Publishing Corporation