(111b) Kinetics and in Situ Spectroscopy Studies of Ethanol Selective Oxidation on Supported Gold Catalysts

This work focusses on the kinetics of vapor phase oxidation of ethanol towards acetic acid over metal oxides (e.g., SiO₂, TiO₂, ZnO, SrTiO₃) supported gold catalysts assisted by catalytic activity and in situ spectroscopic studies (e.g., modulation excitation spectroscopy, surface plasmon resonance) to elucidate the molecular level interactions between gold nanoparticles and supports. All gold catalysts (~1 wt%) were prepared by ammonia deposition precipitation method with Au particle sizes of around 5 nm and support areas of ~15-70 m²/g. Initial screening results at temperatures from 150 to 400 ^oC, 1 kPa ethanol and 1.5 kPa O₂ in He at 101.3 kPa of total pressure indicated that Au(3nm)/TiO₂ and Au(5nm)/SrTiO₃ catalysts gave the highest acetic acid formation rates at 200-350 ^oC. Further kinetic measurements on both catalysts over a wider range of ethanol, O₂, and H₂O partial pressures at 240 ^oC showed that acetic acid and acetaldehyde formation depended weakly on O₂ and moderately on ethanol partial pressures, the latter which had a higher effect on acetaldehyde formation. It was found that that higher acetic acid selectivity was favored at higher O₂/ethanol ratio suggesting that either O₂ activation, whose surface species are favored at higher O₂ pressures, or that its interplay with ethanol surface reaction may be limiting the overall catalytic activity. Surprisingly, product formation rates were independent of co-fed water partial pressures (up to ~7 kPa) explained by the high reaction temperature. In situ gold surface plasmon resonance indicated that O₂ species are predominantly adsorbed at the gold-support perimeter interface. Overall, the results suggested a similarity in the reaction mechanism for ethanol oxidation on Au/TiO₂ and Au/SrTiO₃ which requires supports with n-type semiconducting properties for high activity to acetic acid and whose metal-support perimeter interface sites play a significant role in the activation of O₂ for acetic acid formation.