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

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


Bravo-Suarez, J. - Presenter, The University of Kansas
Torres-Velasco, A., The University of Kansas
Patil, B., The University of Kansas
Zhu, H., The University of Kansas
Qi, Y., Stevens Institute of Technology
Podkolzin, S. G., Stevens Institute of Technology
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.