Abstract

Operando IR spectroscopy and steady state transient isotopic exchange (SSITKA) are used to highlight the internal shading effect and surface sites heterogeneity in heterogeneous photocatalysis. Different TiO₂-P25 samples are prepared in different shapes and tested under the same experimental conditions. These samples show different kinetics (SSITKA cross points) for the surface species (formate) and gas phase products during ¹²CH₃OH/¹³CH₃OH photo-oxidation. The difference is assigned to the shading effect and is negligible when the photocatalyst layer is lower than 10μm (for I0 = 12mW/cm² at λirr = 366nm). Therefore, a homogeneous irradiation of the photocatalyst bed is achieved on TiO₂ thin film. In the second part of this work we show that the difference between SSITKA cross points for the surface and gas phase species may also result from surface sites heterogeneity. In this work we demonstrate that the reaction rate (per gram per unit time) often used in the photocatalysis community, has no meaning on its own in heterogeneous photocatalysis under soft irradiation for a relatively thick layer (>30μm). In other hand, including the irradiated surface in the calculation of the reaction rate seems more representative than the mass of the photocatalyst, for comparing efficiencies of heterogeneous photocatalysts for a given reaction.

Keywords:Photocatalysis, SSITKA, TOF, FTIR-Operando, TiO₂