Model heterogeneous catalysts have been synthesized and studied to better understand how the surface structure of noble metal nanoparticles affects catalytic performance. In this project, monodisperse rhodium and platinum nanoparticles of controlled size and shape have been synthesized by solution phase polyol reduction, stabilized by polyvinylpyrrolidone (PVP).
The NE Pt nanoparticles of controlled size were used to study particle size dependent reactivity for hydrogenation reactions. The influence of particle size on selectivity was examined with crotonaldehyde hydrogenation. For the hydrogenation of crotonaldehyde at temperatures near 373 K and pressures of 200 Torr H2 and 8 Torr crotonaldehyde, the selectivity towards crotyl alcohol (selective hydrogenation of the aldehyde functionality) increases as particle size increases.
Furthermore, simultaneous hydrogenation and dehydrogenation of cyclohexene in excess H2 was examined over the same size range of Pt nanoparticles between 273 and 650 K and at 10 Torr cyclohexene in 200–600 Torr H2. The conversion of cyclohexene in the presence of excess H2 is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (< 423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (> 573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size.
In order to study the influence of particle shape on catalyst selectivity, a nitric acid etching procedure was developed that effectively removes the Ag from the catalysts and increases the rate of ethylene hydrogenation by up to 3 orders of magnitude. Nitric acid etching of Ag from the Pt nanoparticles is > 90% effective when [HNO3] is greater than 7 M, but is ineffective below this concentration. This process was monitored with elemental analysis and ethylene hydrogenation. HRTEM and CTEM confirmed that the Pt nanoparticles themselves do not etch and the shapes of the particles are preserved throughout the etching procedure.
A series of Rh nanoparticles between 1.9 and 11.3 nm was also supported on mesoporous silica SBA-15 for a continued study of the size dependence of CO oxidation as well as a study of the role of PVP in gas phase catalysis. The deposition of the particles on SBA-15 allows for the ability to treat the nanoparticles at high temperatures in O2 and/or H2 without sintering of the particles, which occurs easily on the silicon wafers. PVP stabilized Rh NP catalysts were studied for CO oxidation before and after calcination in O2 at ∼ 673 K. Uncalcined PVP stabilized Rh NP catalysts exhibit a higher turnover frequency for CO oxidation than calcined catalysts. Additionally, the turnover frequency for CO oxidation increases with decreasing particle size from 11 to 2 nm for uncalcined catalysts, but is particle size independent for calcined catalysts. CO adsorbs at bridge sites before the catalysts are calcined, but on atop sites after calcination, with or without reduction in H2 as monitored by infrared spectroscopy. By comparing infrared studies to turnover frequency measurements for CO oxidation, we propose that PVP affects how CO binds to Rh and thus affects the turnover frequency for CO oxidation.
This dissertation has focused on the synthesis, characterization, and reaction studies of model noble metal heterogeneous catalysts. Careful control of particle size and shape has been accomplished though solution phase synthesis of Pt and Rh nanoparticles in order to elucidate further structure-reactivity relationships in noble metal catalysis. (Abstract shortened by UMI.)