Comparing the Catalytic Oxidation of Ethanol at the Solid–Gas and Solid–Liquid Interfaces over Size-Controlled Pt Nanoparticles: Striking Differences in Kinetics and Mechanism

Abstract

Pt nanoparticles with controlled size (2, 4, and 6 nm) are synthesized and tested in ethanol oxidation by molecular oxygen at 60°C to acetaldehyde and carbon dioxide both in the gas and liquid phases. The turnover frequency of the reaction is ∼80 times faster, and the activation energy is ∼5 times higher at the gas−solid interface compared to the liquid−solid interface. The catalytic activity is highly dependent on the size of the Pt nanoparticles; however, the selectivity is not size sensitive. Acetaldehyde is the main product in both media, while twice as much carbon dioxide was observed in the gas phase compared to the liquid phase. Added water boosts the reaction in the liquid phase; however, it acts as an inhibitor in the gas phase. The more water vapor was added, the more carbon dioxide was formed in the gas phase, while the selectivity was not affected by the concentration of the water in the liquid phase. The differences in the reaction kinetics of the solid−gas and solid−liquid interfaces can be attributed to the molecular orientation deviation of the ethanol molecules on the Pt surface in the gas and liquid phases as evidenced by sum frequency generation vibrational spectroscopy. KEYWORDS: Heterogeneous catalysis, ethanol oxidation, size control, platinum nanoparticles, sum frequency generation P latinum nanoparticles with controlled size are promising candidates for heterogeneous catalytic processes. The activity and selectivity of catalytic reactions can be highly dependent on the size of the nanoparticles. 1−3 For example, methanol oxidation by molecular oxygen over Pt nanoparticles with 1−6 nm sizes exhibited higher turnover frequency (TOF) values and higher selectivity toward carbon dioxide in the case of particles with larger sizes. 4 Alcohol oxidation reactions are used in industrial processes for energy conversion and as starting materials for synthesis of organic chemicals as well as for pharmaceuticals. For example, several highly effective fuel cells are based on the complete oxidation of low molecular weight alcohols, 5,6 while complex alcohols are used for production of drugs and fine chemicals. 7 There are numerous studies focused on the catalytic oxidation of alcohols with molecular oxygen. Usually liquid phase reactions are performed at near-ambient temperatures, 8 while elevated temperatures are used for gas phase alcohol oxidations. 9 Tuning the activity and selectivity of the reactions by varying the experimental conditions such as pressure, temperature, and reactant concentration has been extensively studied. 10,11 However, the effect of changing the phase from gas to liquid to the reaction performance under similar reaction conditions was only recently studied in our laboratory. We observed that the platinum catalyzed oxidation of isopropanol to acetone in gas and liquid phases showed dramatically different kinetics and mechanisms. 12 In this letter, we investigate the catalytic oxidation of ethanol by molecular oxygen to acetaldehyde and carbon dioxide in both gas and li...

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