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.