(677b) In Situ/Operando Spectroscopic, Computational, and Kinetic Study of Ethanol Partial Oxidation on Au/TiO2

This work focuses on understanding the mechanism of the vapor phase ethanol partial oxidation to acetaldehyde on Au/TiO₂ in the temperature range of 220-265 ^oC, ethanol and oxygen partial pressures of 0.3-5 kPa and 0.3-30 kPa, respectively. Fixed bed reactor kinetic studies indicated fractional reaction rate orders of ethanol and oxygen of 0.52-0.72 and 0.31-0.36, respectively, and minimum promoting/co-adsorption effect of co-fed water. Isotopic ethanol experiments also evidenced the equilibrated dissociative adsorption of ethanol and a rate determining step (rds) dominated by the C-H cleavage of the ethoxy group. In situ UV-vis spectroscopy of Au plasmon shifts revealed that oxygen species adsorb preferentially at the Au-TiO₂ support interface. A combination of operando UV-vis spectrokinetic analysis (from d-d transitions in the 900-100 nm region) of various charge transfer (CT) kinetic models combined with net CT changes derived from DFT calculations of adsorbed species on a Au₅/Ti(101) surface ruled out molecular oxygen as active species in the rate limiting step, revealed adsorption of ethanol on Au and confirmed stabilization of oxygen species at the Au-TiO₂ interface. Additionally, in situ isotopic ethanol-d6 modulation excitation--diffuse reflectance Fourier transform spectroscopy showed the presence of hydroxyls and ethanol derived intermediate species adsorbed on the Au/TiO₂ surface. Overall, these results provide strong experimental and theoretical evidence for a kinetic model where ethanol adsorbs dissociatively on Au and diffuses near the Au-support interface where the ethoxide reacts with oxygen active species to form acetaldehyde in a rate limiting step. Langmuir-Hinshelwood rate expressions based on dual active sites (Au and Au-TiO₂ interface) where oxygen active species are composed of atomic O* (derived from HOO*), HO*, or HOO* species were found to be consistent with the UV-vis and DRIFTS spectroscopic and DFT calculation results while correlating well with the fixed bed reactor experimental observations at the studied reaction conditions.