H2 production through gas-phase photocatalysis remains limited by the poor light-harvesting ability and insufficient thermal activity of many conventional photocatalysts. In this work, Pd-loaded TiO2/C3N4 composites were prepared to investigate the complementary role of both semiconductors under photothermal conditions. The results show that C3N4 is mainly responsible for the photocatalytic contribution, improving visible-light absorption and enhancing H2 production from methanol and water under illumination. The catalyst containing 10 wt% C3N4 in TiO2 reached the highest light-induced H2 production rate of 7.2 mmol·gCAT−1·h−1, confirming its superior photoactive properties. In contrast, TiO2 provides higher thermocatalytic activity, favoring H2 production under thermal conditions. STEM-EDX and XPS analyses revealed support-dependent Pd distributions and metal-support interactions. In the composite, Pd preferentially localized at the C3N4/TiO2 interface, while XPS indicated stronger Pd-C3N4 interactions, helping to explain the different catalytic behaviors. These results demonstrate the complementary roles of TiO2 and C3N4 in gas-phase photothermal H2 production.
