Shear-Induced Aggregation and Distribution in Photocatalysis Suspension System for Hydrogen Production

To improve the photocatalytic efficiency and realize the scaling up of photocatalytic hydrogen evolution reactions, the hydrodynamic distribution of particulate photocatalysts should be thoroughly understood through direct experimental observations and reasonable theoretical analysis. In this study, detailed investigations were conducted to reveal the shear-induced distribution of TiO2 photocatalyst suspensions dominated by laminar shear and gravity at different particle concentrations. Clusters with dense and compact structures were inevitably formed in an ultrasonication-pretreated TiO2 suspension without shear flow, owing to particle aggregation in the electrostatically controlled regime. Under the application of shear, with shear competing with the potential barrier, the particle aggregation would transition from a slow electrostatically controlled mode to a fast shear-controlled mode, with the formation of loose and fractal clusters, resulting in improved light absorption. Both the structure and size distributions of TiO2 clusters in suspensions could be determined by the applied shear via two effects, namely, cluster separation and restructuring, leading to reduced cluster sizes, and cluster collective settling, contributing to cluster growth. Considering these two rivalrous effects, the sizes of the formed clusters in suspensions initially increased and then decreased with the increasing shear rates. The applied shear could delay the cluster settling through a downward spiral trajectory, resulting in a long-term suspension of photocatalyst clusters for the prolonged solar illumination. Furthermore, at the increased particle concentrations, the cluster growth could be suppressed at relatively low shear rates, thus contributing to a homogeneous distribution of cluster sizes. This study deepens the understanding of particulate photocatalyst behaviors in hydrodynamic conditions and helps analyze potential strategies to optimize the solid-phase distribution in scaled-up photocatalytic water splitting systems.