UC Berkeley

Sum frequency generation (SFG) vibrational spectroscopy is utilized in this work to study the oxidation of alcohols on platinum catalysts. A brief overview of SFG spectroscopy is provided, including a short theoretical summary of the information pertinent to understand the technique. Background information on the oxidation of alcohols is also provided. A description of techniques and experimental setups is discussed, including detailed information on the preparation of catalysts and SFG laser setups.

The oxidation of 1-propanol to propanal by molecular oxygen at 60oC was studied at the solid-liquid and solid-gas interfaces using a range of nanoparticle sizes from 2-7 nm. The reaction rate at the solid-liquid interface was found to be two orders of magnitude greater than that at the solid-gas interface after normalizing to the concentration. In addition, catalytic activity increases with the size of platinum nanoparticles for both reactions. Moreover, water substantially promotes 1-propanol oxidation in the liquid phase, decreasing the activation energy by 21 kJ/mol. However, water inhibits the reaction in the gas phase, increasing the activation energy by 31 kJ/mol. The liquid-phase and gas-phase reactions appear to undergo different mechanisms due to differing kinetic results. This correlates well with different orientations of the 1-propanol species at the solid-liquid interface versus the solid-gas interface as probed by SFG spectroscopy under reaction conditions and simulated by computational calculations.

The oxidation of 2-butanol was carried out in both the liquid and the gas phase. Size-controlled platinum nanoparticles loaded into mesoporous silica MCF-17 were synthesized and studied in the oxidation reaction of 2-butanol with molecular oxygen in the gas and liquid phases. The turnover frequency values increased as the nanoparticle size became larger in both phases, with a consistently high selectivity toward 2-butanone. The activation energy in the gas phase was twice as much as that in the liquid phase. Water did not interrupt the reaction progress in the gas phase, while it poisoned the Pt surface fully to decrease the turnover rate significantly in the liquid phase. SFG spectra were taken of the gas-phase 2-butanol reaction on Pt in both reactive and inert conditions. Water was determined to have no significant effect on the 2-butanol spectra, supporting the reaction data. Combined with the DFT calculations, it was determined that the molecule tends to lay down in the low surface concentration but tends to stand up in the high surface concentration.

SFG studies were carried out to study the oxidation of 1-butanol on a platinum thin film. These studies examined the effects of oxygen and water on the gas-phase reaction. Spectra of 1-butanol were taken in both nitrogen and oxygen environments, providing information about both the reactive and inert conditions. Spectra were also taken of 1-butanol in the presence of oxygen and water in order to study the effect of water on the surface.

1,3-butadiene hydrogenation was performed on 4 nm Pt, Pd, and Rh nanoparticles encapsulated in SiO2 shells at 20, 60, and 100°C. The core-shells were grown around PVP-coated nanoparticles prepared by colloidal synthesis. SFG spectroscopy was performed to correlate surface intermediates observed under reaction conditions with reaction selectivity. Using SFG, it was possible to compare the surface vibrational spectra of Pt@ SiO2, Pd@ SiO2, and Rh@ SiO2. These studies that the calcination is effective at removing PVP, and that the SFG signal can be generated from the metal surface through the outer SiO2 shell. This work was then used to investigate Pt@SiO2 nanoparticles as potential catalysts for the liquid-phase oxidation of isopropanol. It was found that these nanoparticles were successful in the gas and liquid phases for reaction studies. However, despite several attempts, a clear SFG spectrum was not able to be obtained on a Langmuir-Blodgett film of these nanoparticles.