Carbon monoxide adsorption and oxidation on monolayer films of cubic platinum nanoparticles investigated by infrared-visible sum frequency generation vibrational spectroscopy

The adsorption and oxidation of CO on monolayer films of cubic Pt nanoparticles synthesized by a modified solution-phase polyol process were examined by sum frequency generation (SFG) vibrational spectroscopy in total internal reflection (TIR) geometry. Extremely low incident laser power (similar to 5 mu J/pulse of infrared) yields sufficient SFG intensity in TIR geometry and reduces destructive interference. Because TIR-SFG spectroscopy does not require correction for bulk gas absorption, CO spectra can be collected over a wide pressure range (< 1 mTorr up to 700 Torr). Poly(vinylpyrrolidone)-capped Pt nanoparticles deposited on single-crystal sapphire were monitored under high-pressure reaction conditions in a combined spectroscopy-catalytic reactor cell. The effect of the capping polymer on the position and intensity of the CO peak was studied before and after low-temperature calcination. The polymer decreased the amount of CO adsorption and caused a slight red-shift of the atop CO band relative to a surface treated in oxygen at 373 K. Oxidation rates were determined by measuring the intensity of the atop CO peak as a function of time in the presence of flowing oxygen. The activation energy (similar to 19.8 kcal/mol) determined from the SFG data is close to that obtained from gas chromatography (GC) measurements of CO oxidation rates at different temperatures. The SFG and GC results are in good agreement with published data for Pt(100) surfaces.