(584bh) Mechanistic Insights into Ethanol Oxidation on n-Type Oxide Supported Gold Catalysts Via Modulation Excitation Spectroscopy and Kinetics

This study investigates the role of n-type metal oxide supported gold catalysts in the selective oxidation of ethanol over Au catalysts. We hypothesized that supports enabling charge storage promote O species activation, enhancing selectivity toward acetaldehyde and acetates. Au nanoparticles (~5 nm, 1 wt%) were supported on SrTiO₃ and ZnO and evaluated under identical kinetic conditions (240–270 °C, 5–30 kPa O₂, 0.3–5 kPa ethanol). Ethanol conversion mainly yielded acetaldehyde (80–98%), with minor acetate formation (<10%), following the activity trend: Au/SrTiO₃ > Au/ZnO.

Operando frequency-resolved modulation excitation (FR-ME) with DRIFTS and MS identified ethanol and oxygen as positively contributing to acetaldehyde formation. Ethanol modulation correlated with acetaldehyde, water, and acetate signals, while oxygen modulation aligned with acetaldehyde, water, and ethyl acetate, indicating oxygen’s role in promoting oxidation. ME-DRIFTS showed adsorbed ethoxy, acetate, and water species. On Au/SrTiO₃, slow and fast pathways for acetaldehyde formation were observed, along with rapid adsorption/desorption of water and acetate. Au/ZnO exhibited faster charge transfer but limited water dynamics.

In situ UV-Vis-NIR via Au-MaPPS showed oxygen adsorption at the Au-support interface and fast charge transfer under reaction conditions. ZnO appeared more reducible, but SrTiO₃ promoted higher activity. Kinetic studies yielded ethanol and O₂ reaction orders of 0.65 and 0.09 for Au/SrTiO₃ (Ea = 57 kJ/mol), and 0.45 and 0.07 for Au/ZnO (Ea = 69 kJ/mol). These results support a Langmuir-Hinshelwood mechanism involving ethanol dissociative adsorption, O₂ activation at the interface, and oxidative dehydrogenation of acetaldehyde as the rate-determining step.

Overall, n-type supports enhance activity via charge transfer and O₂ activation, while selectivity is linked to intermediate adsorption and product desorption. Combined modulation excitation spectroscopy and kinetics offer critical insights for designing selective, efficient catalysts.