Semiconductor Photocatalysis.ZnS-Catalyzed Photoreduction of Aldehydes and Related Derivatives: Two-Electron-Transfer Reduction and Relationship with Spectroscopic Properties

Article abstract of DOI:10.1021/j100343a066

Photocatalytic activity and spectroscopic properties of ZnS suspensions for the two-electron reduction of aldehydes or related compounds in aqueous medium are described.The ZnS suspension (ZnS-0) prepared by cooling from aqueous ZnSO4 and Na2S solutions catalyzes photoredox reactions of acetaldehyde, giving ethanol without much H2 evolution as a two-electron-reduction product, and acetic acid, biacetyl, and acetoin as oxidation products.When the ZnS-0 suspension is refluxed (giving ZnS-100) or dried to powder, the resulting ZnS shows an increased activity for H2 evolution but a decreased activity for the two-electron reduction.The two-electron photoreduction is ascribed to the sequential transfer of active electrons in the conduction band of defect-free aggregates of ZnS microcrystallites (quantized ZnS).This mechanism is supported by product analysis, energetics at ZnS interfaces, the sharp and blue-shifted onset of absorption and excitation spectra, and the long-life band gap emission of the active ZnS-0 suspension.UV, emission, and ESR spectra, as well as the enhancement of the particle size and crystallinity, suggest that the activity change observed after heating or drying to powder is due to the formation of surface states which may trap active electrons.This interpretation is also supported by the generated activity of ZnS-100 for the H2 photoevolution under >350-nm irradiation.ZnS photocatalysis under >350-nm irradiation and relationship with spectroscopic properties are also discussed.