Hydrogen sulfide is highly toxic to humans and harmful to the environment. On the other hand, H₂S is produced in large quantities in some industrial processes like the refinery of crude oil and the sweetening of natural gas. Nowadays, H₂S is usually burned to sulfur and water in the Claussprocess wasting the energy of hydrogen stored in H₂S. Progress in the area of photocatalysis results in considerable development of the heterogeneous photocatalytic conversion of hydrogen sulfide into hydrogen gas. In this work, photocatalytic hydrogen production from alkaline sulfide solutions was investigated, utilizing various ZnS/CdS composites modified with PdS as cocatalyst. The highest photocatalytic activity was found at 1:1 molar ratio of CdS and ZnS. At this catalyst content the optimal PdS-loading was investigated in the range of 0-0.4% (n/n); the 0.05-0.10% (n/n) PdS content proved to be most efficient. The catalyst consisted of the agglomerates of 80-150 nm size particles. During long time illuminations (10 days) the size of the agglomerates increased, but the diameter of the individual particles and the photocatalytic activity did not noticeably change. The dependence of the rate of hydrogen production on the concentration of sulfide, sulfite, and thiosulfate ions was also studied. An increase of the amount of reactants resulted in an enhancement of the reaction rate, while the presence of thiosulfate ions lowered the catalytic activity. One of the possible reasons of this effect is the side reaction of thiosulfate ions by the conduction band electrons.
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