Heterogeneous Photocatalysis. VIII. Zinc Sulfide Catalyzed Hydrogen Formation from Water in the Presence of Sodium Formate

Article abstract of DOI:10.1246/bcsj.63.2378

A suspension of n-ZnS in aqueous sodium formate photocatalyzes formation of hydrogen (72percent) and carbon monoxide (27percent).The stoichiometry of the hydrogen producing reaction is determined as HCO2Na+H2O=H2+HCO3Na.The quantum yield at 290 nm is 0.24 mol H2/Einstein and its wavelength-dependence parallels that of the anodic photocurrent measured photoelectrochemically.Reaction rates increase linearly with increasing light-intensity and do not change when the temperature is increased up to 70 deg C.No gaseous products are formed when nonaqueous formic acid is employed insteda of formate, or when ZnS is dehydrated at 250 deg C before use or when CdS is substituted for ZnS.When THF or sulfur compounds are added to the solution, H2 is the only gaseous product.The dependence of the reaction rate on formate concentration points to a Langmuir-type adsorption of formate onto the semiconductor surface, K_ads=26 M^-1.In D2O/HCO2Na the initial rate does not change but the amount of CO increases to 60percent and photocorrosion is much stronger than in H2O; the initial isotopic composition of 85, 10, and 5percent changes to 63, 30, and 7percent of D2, HD, and H2, respectively, when 400 mL of gas has been produced.In H2O/DCO2Na the rate decreases by a factor of 1.3 and the amount of CO is only 15percent; only H2 but no D2 and HD are detectable.The hole scavenger KSCN does not influence the quantum yield of hydrogen formation while the electron scavenger NaNO3 induces a strong decrease and completely inhibits CO formation.Stern-Volmer analysis indicates that nitrate scavenges in addition to conduction band electrons also an intermediate CO2^- radical.The latter has been identified by spin-trapping experiments.