In Situ Mechanistic Investigation at the Liquid/Solid Interface by Attenuated Total Reflectance FTIR: Ethanol Photo-Oxidation over Pristine and Platinized TiO₂ (P25)

There is growing interest in applying photocatalysis to help solve both the energy crisis and effectively combat environmental contamination. However, it is difficult to investigate photocatalytic reactions at the liquid/solid interface to unravel the reaction mechanism by conventional (ex situ) surface analytical techniques. In this study, Attenuated Total Reflectance-FTIR spectroscopy, adapted for optical pumping, was used to observe in situ the surface of TiO₂ (Degussa P25, in both pristine and platinized forms) during photocatalytic oxidation of ethanol aqueous solution. It shows the feasibility to investigate not only the reaction pathway and the rate-determining step, but also the change in state of the catalyst under working conditions. During ethanol photo-oxidation over pristine TiO₂, band gap excitation caused the progressive accumulation of trapped electrons, as recognized by their characteristic quasi-continuum absorption, implying that photoreduction does not proceed at a significant rate under these conditions. Consistent with this view, only weak infrared features due to adsorbed intermediates were observed. Over platinized TiO₂, the noble metal nanodeposits promote photodehydrogenation and photoactivation of dioxygen. In addition to observing strong bands diagnostic of various intermediates, the presence or absence of a band around 2050 cm^–1, typical of Pt-CO_ad, served as a valuable spectroscopic marker of the instantaneous availability of the chemical oxidant. As such, mechanistic parallels were found between photoreforming (with H₂ generation) and photo-oxidation (to acetate), with acetaldehyde being a common intermediate in both processes. The rate-determining step in ethanol mineralization was found to be photodecomposition of adsorbed acetate.