Employing controlled periodic illumination in identifying limiting factors of heterogeneous catalytic processes: A case study of photocatalytic generation of H2O2 and H2

Semiconductor photocatalysis is a versatile process that has been utilised in a wide range of applications from water treatment to sustainable energy generation and storage. In a heterogeneous photocatalytic reaction, identifying the limiting factor in a reaction is crucial in optimising operating parameters such as catalyst loading or irradiation intensity. With the application of Controlled Periodic Illumination (CPI), it is possible to control the intensity of photons entering the system. In this paper, two processes were evaluated and monitored by employing CPI to identify limiting factors: photocatalytic generation of H2O2 and H2. The relationship between catalyst loading and light intensity on H2/H2O2 generation constant rate was explored using CPI. In photocatalytic generation of H2O2, two commercial catalysts, TiO2 and ZnO were used under identical experimental conditions and the limiting factors on them differed. The effect of CPI varied greatly between both catalysts with TiO2 demonstrating that up to 50% less light is required to achieve comparable H2O2 formation rates at lower catalyst loadings (0.1 g L−1). In contrast, for ZnO photocatalysis, the CPI experiments highlighted the impact of adsorbed O2 levels, catalyst loading, and light intensity on H2O2 yields. The H2O2 formation rate constant plateaued beyond a duty cycle of 0.6 regardless of the tested catalyst loading. For photocatalytic generation of H2 on Pt/TiO2 from glucose, the generation rate constant increased linearly at low duty cycles and plateaued with different catalyst loading, which helped to identify different limiting factors for each condition. Therefore, exploring H2O2 and H2 generation via CPI is reported for the first time and the results show the importance of monitoring the interdependency of photocatalytic parameters and prove the validity of CPI on determining the limiting factor.