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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. 

Tungsten based catalysts supported on silica (zWOX/SiO2) and silica promoted by titania (zWOX/yTiOX/SiO2) were studied for their catalytic activity towards propylene metathesis. The catalysts were prepared by a simple incipient wetness impregnation method using a large pore SiO2 of intermediate surface area (∼50 m2/g). Catalytic activity studies carried out in a fixed-bed reactor (723 K, 101 kPa propylene) indicated that propylene conversion increased with increasing W loading in zWOx/SiO2 catalysts (z = 0.5−6 W/nm2). It was shown that the catalytic activity of a poorly WOX dispersed 6WOX/SiO2 catalyst could be enhanced and maximized by an optimum titania promotion of 2 wt% TiO2 (∼3 Ti/nm2). In situ differential diffuse reflectance (DDR) UV-Vis spectroscopy at reaction conditions showed that TiOX domain size increased with increases in titania loading from isolated TiOX to TiOX clusters to TiO2 nanocrystals. The UV-Vis results also evidenced the existence of highly dispersed isolated WOX species, WOX clusters, and WO3 nanoparticles in the 6WOX/yTiOX/SiO2 (y = 0.5−6 wt% TiO2 or ∼0.7–9 Ti/nm2) catalysts. In situ DDR-UV-Vis, Raman, and mass spectrometry during propylene metathesis, and catalyst oxidation and reduction revealed the reasons for an optimum amount of titania promoter in 6WOX/2TiOX/SiO2. They were the result of a balanced interplay between two factors: (1) enhanced WOx species dispersion due to the presence of a trimeric TiOX cluster and (2) absence of catalyst deactivation (present at high TiO2 loadings) due to the trimeric TiOX cluster poor reactivity towards coke formation.