Sum Frequency Generation Study of the Interfacial Layer in Liquid-Phase Heterogeneously Catalyzed Oxidation of 2-Propanol on Platinum: Effect of the Concentrations of Water and 2-Propanol at the Interface

Sum frequency generation (SFG) vibrational spectroscopy was applied to study the solid–liquid interface in the heterogeneously catalyzed oxidation of 2-propanol to acetone by dissolved dioxygen in aqueous solution on platinum. The mole fraction of alcohol was varied from 0 to 1. At 2-propanol mole fractions less than 0.14 and above 0.23, the TOF for acetone is approximately 20 h–1. A 3-fold increase in the reaction rate is seen when 2-propanol is present with concentrations in this intermediate range (0.14–0.23). SFG spectra indicate that in aqueous mixtures of 2-propanol the solid–liquid interface is dominated by the alcohol, even at low mole fractions of alcohol, but resonant features from molecular 2-propanol not bound to the platinum surface appear only above 0.14 mole fraction. At 2-propanol concentrations where the highest reaction rates are observed, SFG shows the presence of water and alcohol at the catalyst interface, whereas, above and below these concentrations, either water or 2-propanol is not detectable at the surface. When water is excluded totally from the surface, the reaction rate is decreased. We attribute this correlation of surface concentrations to a dependence of the reaction rate on both alcohol and water, and our results demonstrate the importance of considering the interfacial concentration in liquid-phase heterogeneous catalysts.