We synthesize platinum nanoparticles with controlled average sizes of 2, 4, 6, and 8 nm and use them as model catalysts to study isopropanol oxidation to acetone in both the liquid and gas phases at 60 °C. The reaction at the solid/liquid interface is 2 orders of magnitude slower than that at the solid/gas interface, while catalytic activity increases with the size of platinum nanoparticles for both the liquid-phase and gas-phase reactions. The activation energy of the gas-phase reaction decreases with the platinum nanoparticle size and is in general much higher than that of the liquid-phase reaction which is largely insensitive to the size of catalyst nanoparticles. Water substantially promotes isopropanol oxidation in the liquid phase. However, it inhibits the reaction in the gas phase. The kinetic results suggest different mechanisms between the liquid-phase and gas-phase reactions, correlating well with different orientations of IPA species at the solid/liquid interface vs the solid/gas interface as probed by sum frequency generation vibrational spectroscopy under reaction conditions and simulated by computational calculations.

Utilizing Pt nanoparticles of varying sizes (2–7 nm), it was found that the oxidation of 1-propanol by molecular oxygen at 60 °C to propanal at the solid–gas and solid–liquid interfaces yielded significantly different results depending on Pt particle size and alcohol surface density. The reaction rate at the solid–gas interface was found to be 1 order of magnitude greater than that at the solid–liquid interface after normalizing concentration. In addition, catalytic activity increases with the size of Pt nanoparticles for both reactions. Moreover, water substantially promoted 1-propanol oxidation in the liquid phase, yet it inhibited the reaction in the gas phase. The gas phase and liquid phase reactions are believed to undergo different mechanisms due to differing kinetic results. This correlated well with different orientations of the 1-propanol species at the solid–gas interface versus the solid–liquid interface as probed by sum–frequency generation vibrational spectroscopy (SFGVS) under reaction conditions and simulated by computational density function theory calculations.

Catalytic oxidation of alcohols is an essential process for energy conversion, production of fine chemicals and pharmaceutical intermediates. Although it has been broadly utilized in industry, the basic understanding for catalytic alcohol oxidations at a molecular level, especially under both gas and liquid phases, is still lacking. In this paper, we systematically summarized our work on catalytic alcohol oxidation over size-controlled Pt nanoparticles. The studied alcohols included methanol, ethanol, 1-propanol, 2-propanol, and 2-butanol. The turnover rates of different alcohols on Pt nanoparticles and also the apparent activation energy in gas and liquid phase reactions were compared. The Pt nanoparticle size dependence of reaction rates and product selectivity was also carefully examined. Water showed very distinct effects for gas and liquid phase alcohol oxidations, either as an inhibitor or as a promoter depending on alcohol type and reaction phase. A deep understanding of different alcohol molecular orientations on Pt surface in gas and liquid phase reactions was established using sum-frequency generation spectroscopy analysis for in situ alcohol oxidations, as well as density functional theory calculation. This approach can not only explain the entirely different behaviors of alcohol oxidations in gas and liquid phases, but can also provide guidance for future catalyst/process design.

Platinum nanoparticles size range from 1 to 8 nm deposited on mesoporous silica MCF-17 catalyzed alcohol oxidations were studied in the gas and liquid phases. Among methanol, ethanol, 2-propanol and 2-butanol reactions, the turnover frequency increased with Pt nanoparticle size for all the alcohols utilized. The activation energies for the oxidations were 1 almost same among all alcohol species, but higher in the gas phase than those in the liquid phase.Water coadsorption poisoned the reaction in the gas phase, while it increased the reaction turnover rates in the liquid phase. Sum frequency generation (SFG) vibrational spectroscopy studies and DFT calculations revealed that the alcohol molecules pack horizontally on the metal surface in low concentrations and stand up in high concentrations, which affect the dissociation of β-hydrogen of the alcohols as the critical step in alcohol oxidations.

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.

1,3-Butadiene (1,3-BD) hydrogenation was performed on 4 nm Pt, Pd, and Rh nanoparticles (NPs) encapsulated in SiO2 shells at 20, 60, and 100 °C. The core-shells were grown around polyvinylpyrrolidone (PVP) coated NPs (Stöber encapsulation) prepared by colloidal synthesis. Sum frequency generation (SFG) vibrational spectroscopy was performed to correlate surface intermediates observed in situ with reaction selectivity. It is shown that calcination is effective in removing PVP, and the SFG signal can be generated from the metal surface. Using SFG, it is possible to compare the surface vibrational spectrum of Pt@SiO2 (1,3-BD is hydrogenated through multiple paths and produces butane, 1-butene, and cis/trans-2-butene) to Pd@SiO2 (1,3-BD favors one path and produces 1-butene and cis/trans-2-butene). In contrast to Pt@SiO2 and Pd@SiO2, SFG and kinetic experiments of Rh@SiO2 show a permanent accumulation of organic material.

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