Photoreduction of bicarbonate catalyzed by supported cadmium sulfide

Cadmium sulfide supported on silica (CdS-chi/SiO2, chi = 4, 11, 17, 25%) and zinc sulfide (CdS-chi/ZnS, chi = 5, 10, 20, 30%) was prepared by impregnation with cadmium sulfate and subsequent addition of sodium sulfide. The specific surface areas of the silica and zinc sulfide supported powders are in the range o f 188-280 and 95-104 m2 g(-1), respectively. After sonication of an aqueous suspension of CdS-17/SiO2 the particle size distribution exhibited two maxima at 22 and 57 microm. In the presence of sodium sulfite the supported cadmium sulfides photocatalyze the reduction of bicarbonate to formate, formaldehyde, and oxalate. Upon polychromatic irradiation (lambda > or = 290 nm) the C1-products formate and formaldehyde were obtained in concentrations of 30-130 microM whereas the C2-product oxalate reached only 1-8 microM. Formaldehyde is not formed through reduction of intermediate formate whereas oxalate is produced via oxidation of the latter. The linear increase of oxalate concentration with coverage can be rationalized by the assumption that dimerization of the intermediate carbon dioxide radical anion does not occur on the cadmium sulfide but in solution or on the silica surface. For zinc sulfide supported samples the coverages of 10, 20, and 30% do not change the photocatalytic activity significantly whereas a loading of 5% induces a 40-fold and 16-fold increase as compared to unmodified cadmium and zinc sulfide, respectively. This strong enhancement suggests that in CdS-05/ZnS the efficiency of charge separation is strongly improved through interparticle electron transfer. The results demonstrate that both silica and zinc sulfide supports increase the photocatalytic activity of cadmium sulfide through the presence of a recently found electronic semiconductor-support interaction (SEMSI effect). Additionally, the low coverage sample CdS-05/ZnS combines this novel effect with the higher charge separation efficiency of a coupled semiconductor system.