Selective Hydrogenation of Cinnamaldehyde, Furanic, and Aromatic Aldehydes over a Z-Scheme Heterojunction Photocatalyst Constituted of Pd NPs Supported on g-C3N4/ZnO Nanocomposite

In this study, a Z-scheme heterojunction photocatalyst is fabricated by systematically manipulating and integrating g-C3N4 and ZnO. The Z-scheme g-C3N4(x)/ZnO(y) exhibits enhanced charge separation and migration, confirmed through photoluminescence (PL), time-correlated single-photon counting spectroscopy, and electrochemical impedance spectroscopy. Pd NPs, with an elevated work function and low Fermi energy level, are decorated onto the heterojunction, further amplifying charge separation, as revealed by ultraviolet photoelectron spectroscopy. The resulting Pd-decorated photocatalysts (Z%Pd@g-C3N4(x)/ZnO(y)) are evaluated for the selective reduction of α–β unsaturated compound, cinnamaldehyde. The catalysts with lower Pd NPs (0.5 or 1%) afford ∼100% yield for hydrocinnamaldehyde, while higher Pd NPs (3%Pd@g-C3N4(0.73)/ZnO(0.27)) afford ∼100% yield for hydrocinnamyl alcohol. Biomass-derived furanic aldehydes afforded ring-reduced and side-chain-reduced products under different Pd loading, whereas no ring-reduced products were obtained for the aromatic aldehydes. Interface interactions impact charge migration, leading to longer lifetimes for photogenerated charge pairs and improved separation of electrons and holes in photocatalytic processes. Characterizations and control experiments provide valuable insights into the structure–activity relationship, ultimately contributing to formulating a plausible reaction mechanism for photocatalytic reduction.