Frequency-Resolved Modulation Excitation Spectroscopy Methodology for Identifying Surface Reaction Species in Ethanol Oxidation on Gold Catalysts

This study used in situ modulation excitation spectroscopy (MES) with varying frequencies in a single experiment to identify surface species during ethanol oxidation on Au/SiO2, Au/TiO2, Au/ZnO, and Au/SrTiO3. Fixed-bed reactor (FBR) tests (1 kPa ethanol, 1.5 kPa O2, 513 K) showed that Au/SiO2 and Au/SrTiO3 had higher ethanol conversions. Au/SiO2 favored acetaldehyde, while Au/SrTiO3 yielded more acetates (acetic acid and ethyl acetate). Operando modulation excitation (ME)–phase sensitive detection (PSD)–DRIFTS, with ethanol and oxygen modulation, identified surface ethanol, acetaldehyde, acetates, ethoxy, and hydroxyl species. Oxygen modulation showed charge transfer to supports in Au/TiO2 and Au/ZnO. At the fundamental frequency (f0 = 1/90 Hz), ME–PSD–DRIFTS showed minimal adsorbed ethanol on Au/SiO2, indicating high ethanol conversion. Au/SrTiO3 had higher acetaldehyde consumption, correlating with increased acetates, consistent with FBR results. ME–PSD–DRIFTS at lower frequencies (0.07f0, 0.5 f0) and higher harmonics (2f0, 3f0) showed rapid ethoxy formation/decomposition, and slower acetaldehyde reactions, confirming acetaldehyde as a primary product and acetates as secondary products. Oxygen modulation revealed rapid hydrogen spillover and hydroxyl changes. Overall, operando spectroscopy via mass spectrometry confirmed the FBR findings.