(100h) Spectrokinetic Studies of Intermediate Species during Ethanol Oxidation on Supported Gold Catalysts | AIChE

(100h) Spectrokinetic Studies of Intermediate Species during Ethanol Oxidation on Supported Gold Catalysts


Torres-Velasco, A. - Presenter, The University of Kansas
Bravo-Suarez, J. - Presenter, The University of Kansas
Patil, B., The University of Kansas
This work combines in situ modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform (ME-PSD-DRIFTS) and UV-Visible (ME-PSD-DR-UV-Vis) spectroscopies on Au/SiO2, Au/TiO2, Au/ZnO, and Au/SrTiO3 to elucidate the molecular level interactions between intermediate species, gold nanoparticles, and supports during vapor phase oxidation of ethanol. It was hypothesized that charge transfer (CT), a proxy for oxygen activation, would correlate with catalytic activity regardless of support. Fixed bed reactor catalytic activity measurements at 240 oC, EtOH = 1.5 kPa, and O2 = 1 kPa resulted in a relative decreasing production rate for acetates of: Au/SrTiO3(1.0)>Au/ZnO(0.51)> Au/TiO2(0.28)>Au/SiO2(0.18) and acetaldehyde of: Au/SiO2(1.0)>Au/SrTiO3(0.88)>Au/ZnO(0.75)> Au/TiO2(0.25). However, the relative absorbance change due to CT from high to low O2 partial pressure during ME-PSD-DRIFTS resulted in the relative decreasing order: Au/ZnO(1.0)>Au/TiO2(0.41)> Au/SrTiO3(0.002)≈Au/SiO2(0.005), indicating that support reducibility alone is not a sufficient descriptor to explain the activity trends. The ME-PSD-DR-UV-Vis spectra measured at similar conditions to those for O2 ME-PSD-DRIFTS showed a relative CT (at 900 nm) to the support in the vicinity of gold nanoparticles that followed the decreasing order: Au/ZnO(1.0)>Au/TiO2(0.53)> Au/SrTiO3(0.33)>Au/SiO2(0.03), which tracked with DRIFTS results, but with significantly higher sensitivity. A spectrokinetic methodology was developed to relate CT measurements from O2 (0-1.5kPa) ME-PSD-DR-UV-Vis spectra to different types of oxygen intermediate species including O2-, OOH, O22-, O-, and O2-. Operando spectroscopic measurements including online MS calibrated measurements allowed the determination of reaction rates during modulation at varying O2 partial pressures. A combination of kinetics, in situ Au plasmon, and ME spectroscopic observations indicated that ethanol oxidation on gold catalysts occurs in a region surrounding the gold-support periphery where ethanol and oxygen adsorb, that charge transfer occurs during ethanol dehydrogenation, and that intermediate oxygen species are likely to be O2- and OOH.