Discriminating photochemical and photothermal effects in heterogeneous photocatalysis

Abstract

An increased interest in photocatalytic reactions for various applications demands reliable methods and procedures for a valid quantification of photokinetic effects. When solid photocatalysts are irradiated with light, the dissipation of heat becomes an important issue that has to be addressed to avoid a potentially substantial hot-spot formation. This is especially relevant when the reaction is exothermic and the catalyst is also thermochemically active. When hot spots are formed, they can cause observable photothermal reaction rates which are much higher than the direct photochemical rate. This could even lead to an observable increase of the reaction rate due to irradiation for a catalyst that is not photoactive, which obviously obfuscates the true photoactivity of a catalyst and impedes its quantification. For this study, a one-dimensional model was developed to include mass and heat transport, as well as radiative energy transfer, in a catalyst for a simple, yet generally applicable reaction. This allowed the investigation of the general behaviour of a flat catalyst geometry, as is often used in photochemical investigations, under irradiation in different scenarios. These cases include a comparison of photoactive and photoinactive catalysts to showcase the potential erroneous quantification of a catalyst’s activity under irradiation. Further parameter variations illustrate the importance of the thermal integration of the catalyst in a reactor and indicate the catalyst layer thickness as the main driver for the potential appearance of photothermal effects. This broad variation of parameters and the use of a generic model may help to utilise the results of this investigation in a large number of different applications and improve the reliability of experimental quantifications.