High-Surface-Area Catalyst Design:  Synthesis, Characterization, and Reaction Studies of Platinum Nanoparticles in Mesoporous SBA-15 Silica

Abstract

Platinum nanoparticles in the size range of 1.7-7.1 nm were produced by alcohol reduction methods. A polymer (poly(vinylpyrrolidone), PVP) was used to stabilize the particles by capping them in aqueous solution. The particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations demonstrate that the particles have a narrow size distribution. Mesoporous SBA-15 silica with 9-nm pores was synthesized by a hydrothermal process and used as a catalyst support. After incorporation into mesoporous SBA-15 silica using low-power sonication, the catalysts were calcined to remove the stabilizing polymer from the nanoparticle surface and reduced by H2. Pt particle sizes determined from selective gas adsorption measurements are larger than those determined by bulk techniques such as XRD and TEM. Room-temperature ethylene hydrogenation was chosen as a model reaction to probe the activity of the Pt/SBA-15 materials. The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm. The hydrogenolysis of ethane on Pt particles from 1.7 to 7.1 nm was weakly structure sensitive with smaller particles demonstrating higher specific activity. Turnover rates for ethane hydrogenolysis increased monotonically with increasing metal dispersion, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C2H6 hydrogenolysis than the low index planes that dominate in large particles. An explanation for the structure sensitivity is suggested, and the potential applications of these novel supported nanocatalysts for further studies of structure-activity and structure-selectivity relationships are discussed.

Sub-10 nm Platinum Nanocrystals with Size and Shape Control: Catalytic Study for Ethylene and Pyrrole Hydrogenation

“…The Pt nanocrystals with different size and shape displayed similar turnover frequencies for ethylene hydrogenation, which was consistent with previous studies 3, 4 on Pt nanocrystals and single crystals. Normalized per mass of Pt, the activities of the 5 nm nanopolyhedra and nanocubes were higher than that of the 9 nm nanocubes, which demonstrated that the smaller Pt nanocubes were advantageous compared to the larger nanocubes due to its higher surface-to-bulk ratio.…”

“…After stirring for 5 min, the solution was aged at 40°C for 20 h under a quiescent condition. Then, 46 mg of NH 4 F was added and the solution was transferred to an autoclave and was aged at 100°C for another 24 h. The precipitate product was filtered, washed with water and ethanol, and calcined in air at 600 °C for 6 h. The white powder was kept in desiccator for further use. beams were spatially and temporally overlapped on the crystal surface at angles of incidence of 55° and 60°, respectively, with respect to the surface normal.…”

“…For example, alkane hydrogenolysis rates showed sensitivity to Pt nanocrystals size. [1][2][3][4] Our group further demonstrated the size sensitivity for carbon-nitrogen ring opening reactions. 5 Catalytic activity and selectivity dependence on nanocrystal shape has also been demonstrated.…”

“…[11][12][13][14][15][16][17][18] Previous research unambiguously showed that the structure dependency of activity and selectivity are more significant for nanocrystals smaller than 10 nm. [3][4][5] Also, smaller nanocrystals have higher dispersion than larger ones and this fact permits more efficient use of precious metals. The lack of a synthetic method for shaped nanocrystals in single-digit nanometer range reflects that size reduction while maintaining the shape is challenging.…”

Optimized reduction conditions for the microfluidic synthesis of 1.3 ± 0.3 nm Pt clusters

“…Typically, metal clusters provide highly active centers, and due to their higher surface energies and larger surfaces, they are not in a thermodynamically stable state [29,30]. To produce homogeneous and stable metal particles usually soluble polymers are widely employed as supports because of their availability, enhanced stabilization properties and resistance to particle sintering or agglomeration [31].…”