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

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, O₂^2-, O^-, and O^2-. 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.