The effect of platinum loading on the efficiency of CdS-photocatalyzed hydrogen evolution from an aqueous solution of sodium sulfite and on the intensity of the green emission (510 nm) of CdS suspended in the same solution was studied. The Fermi level of the CdS photocatalyst at the working condition can be estimated from the intensity of the green emission of the platinum-loaded CdS powder and the luminescence-potential curve of a CdS electrode. When the amount of platinum deposited on CdS is increased, the Fermi level of CdS at the working stage is moved down toward more positive potential, leading to a stronger band bending, and the photocatalytic reaction is stimulated. Too much Pt loading, however, lowers the photocatalytic activity because of the enhanced rate of electron-hole recombination as well as light absorption by platinum. The temperature effect on the reactivity and the reason for the presence of an induction period of the reaction are also elucidated in relation with the band bending of the photocatalyst.

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Photocatalyzed hydrogen evolution on Pt-loaded CdS powder from aqueous solutions of sodium sulfite is enhanced by addition of a small amount of ethylenediaminetetraacetic acid (EDTA) to the solution. EDTA is hardly consumed by the reaction. It has been concluded from the measurements of the flat-band potential of CdS electrodes that EDTA and other chelating agents, such as 1,2-cyclohexanediaminetetraacetic acid and nitrilotriacetic acid, are adsorbed strongly on the surface of CdS and shift the conduction band energy toward the negative. The enhancement of the photocatalytic hydrogen evolution by the addition of EDTA is explained as being caused by the upward shift of the conduction band energy of CdS due to the negative charge of the chelating agents. The change of the conduction band energy by the adsorption of EDTA is observed also for CdSe electrodes. Although Pt-loaded CdSe powder is inactive for the hydrogen evolution from aqueous solutions of sodium sulfite, it generates hydrogen when EDTA is added to the solution.

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