Gold nanoparticles encapsulated in porous carbonw

Gold was considered catalytically inactive until 1987, when Haruta and co-workers found that gold nanoclusters supported by metal oxides showed high activity toward CO oxidation even below room temperature. Since then, there has been intense interest in the use of Au nanoparticles (NPs) to catalyse a variety of reactions, including low temperature oxidation of CO, selective oxidation of alcohols, epoxidation of alkenes and reduction of nitroaromatic compounds. Due to their high surface energy, however, Au NPs readily sinter, resulting in a substantial decrease in catalytic activity. To solve this problem, encapsulated structures have been developed using various porous supports, including metal oxides, silica, dendrimers, organic polymers, and porous carbon, which have improved the catalytic activity and stability of Au NPs. Encapsulated Au NPs are currently prepared using deposition– precipitation or the coating of support materials on Au NPs followed by further chemical etching to introduce porosity into the supports. These preparation methods are usually cumbersome, require multiple steps, and use sacrificial templates or toxic etching reagents. Improvements are therefore still needed in both the synthetic methodology and the production of porous encapsulating supports. Ultrasonic spray pyrolysis (USP) is a continuous and one-step aerosol process for the production of various micro and nanostructured materials, including quantum dots, metal oxides, metal sulphides, and novel morphologies of high surface area carbons. We describe here the first preparation of porous carbon-encapsulated Au nanoparticles using USP (abbreviated USP Au/C) and report on their excellent catalytic activity for the reduction of both hydrophilic and hydrophobic nitroaromatics. The preparation of porous carbon-encapsulated metal nanoparticles using USP is illustrated in Fig. S1 (see ESIw). Ultrasound (1.7 MHz, 6 W cm ) is applied to generate an aerosol of precursor solution (0.005 M HAuCl4, 0.5 M sucrose and 1 M NaNO3) which is swept through a heated zone (e.g., 700 1C) by an inert gas (Ar) with a residence time of 10 s. The temperature is set high enough to decompose HAuCl4 into gold and sucrose into carbon in the presence of NaNO3 as catalyst and pore formation agent. The resulting USP Au/C catalyst contains 3.3 wt% Au. TEM and SEM micrographs (Fig. 1 and Fig. S2, ESIw) show that Au NPs are encapsulated in carbon microspheres, and that a furnace temperature of 700 1C gives the most highly dispersed Au NPs. XRD (Fig. 2) confirms the crystallinity of the Au NPs in an amorphous carbon matrix. The surface areas of the carbon supports are 770 m g , 40 m g , and 10 m g 1 for the products produced using furnace temperatures of 700 1C, 600 1C, 500 1C, respectively; at 800 1C, sintering of the Au NP occurs. The optimum reaction temperature in terms of porosity of support and minimal sintering is therefore 700 1C. The average crystallite size of the Au NPs in carbon spheres synthesized at 700 1C, calculated by the Debye–Scherrer equation, is 31 nm, which is slightly smaller than that measured directly from TEM images (33 nm) (Fig. S3, ESIw). This demonstrates that Au NPs are mostly single crystallites with minimal polycrystalline domains. The pore-size distribution of USP Au/C shows it to be a mesoporous material with most pores smaller than 10 nm (Fig. S4, ESIw). The pore volume calculated from pore-size distribution results (Fig. S4, ESIw) is 0.4 cm g 1 for USP Au/C synthesized at 700 1C. The density of porous carbon is thereby estimated to be 1.1 g cm 3 (i.e., roughly half the density of graphite). The volume occupied by Au with 3.3 wt% loading can then be calculated to be B0.19%. That is to say,