In Situ Pt Photodeposition and Methanol Photooxidation on Pt/TiO2: Pt-Loading-Dependent Photocatalytic Reaction Pathways Studied by Liquid-Phase Infrared Spectroscopy

We developed a top-irradiated, liquid-phase attenuated total reflectance Fourier transform infrared (ATR-FTIR) setup that allows time-resolved investigations of both Pt particle growth during in situ photodeposition via monitoring of the Pt0–COads band on TiO2 thin films as well as the photooxidation of methanol in aqueous environments. Obtained ATR-FTIR data sets were analyzed via multivariate curve resolution-alternating least squares (MCR-ALS), which enabled us to clearly differentiate various reaction pathways for different Pt loadings at otherwise fixed reaction conditions (i.e., methanol concentration, UV intensity). At the highest Pt loading (nominal concentration of 2.7 wt %), photooxidation of methanol occurs via direct oxidation through a formaldehyde intermediate to CO2, whereas the lower Pt loadings of 0.7 and 1.4 wt % favor a side reaction that includes methyl formate as an intermediate. These findings were correlated with the formation of different CO binding sites on Pt during photodeposition, and we presume that changes in the reaction pathway depend on the number rather than the nature of active available Pt sites. Complementary ex situ characterizations of the thin films by transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS) were performed, delivering information on the generated Pt nanoparticles and structural changes of TiO2. The presented optical setup paves the way for fundamental studies of heterogeneous catalytic reactions as close as possible to their actual use in aqueous systems.