Influence of Reactor Configuration and Operating Conditions on Nanostructured Semiconductor Photocatalysts for Hydrogen Evolution: A Systematic Technical Review

Semiconductor-based photocatalytic water splitting is a promising pathway for sustainable hydrogen production; however, the reported performance depends not only on the intrinsic properties of the photocatalyst but also on reactor configuration and operating conditions. This systematic technical review examines the interplay between nanostructured semiconductor photocatalysts and the principal engineering variables governing photocatalytic hydrogen evolution. Particular attention is given to particle size, morphology, surface area, defect density, heterojunction design, cocatalyst incorporation, aggregation, and catalyst immobilization, as well as their interaction with reactor geometry, optical path length, photon distribution, catalyst loading, working volume, pH, sacrificial agents, mixing, thermal control, gas purging, and product quantification. The reviewed evidence indicates that these material and reactor parameters jointly determine light absorption, charge-carrier separation and transfer, suspension turbidity, mass transport, catalyst recovery, stability, and the measured hydrogen evolution rate. Batch slurry reactors remain the most widely used laboratory configuration, whereas annular, flat-panel, microreactor, fixed-bed, continuous-flow, and photofluidized systems offer specific advantages for photon utilization, catalyst reuse, product removal, and scale-up. The review also emphasizes the need to distinguish overall water splitting from sacrificial-agent-assisted hydrogen evolution. Standardized reporting of photocatalyst properties, irradiance, spectral distribution, illuminated area, reactor dimensions, reaction atmosphere, and gas-analysis procedures is essential to improve reproducibility and enable reliable comparisons among nanostructured photocatalytic systems.