Kinetics of Photocatalytic Reactions at Stationary Illumination

Photoreduction of Methyl Orange on CdS Colloids in Deep Conversion 

The reduction of organic dye methyl orange (MO) over CdS colloids with the particles size d = 2R - 5 nm has appeared to be a convenient reaction for detail studying the kinetics of photocatalytic processes. This dye is readily reducible with no dimers formation. The MO adsorption spectrum in the pH range of 10-12, practically does not change. This allows simplifying the interpretation of the experiments on redox transformations of MO and considering the reaction of photostimulated reduction of MO as a model one. 

Sodium sulfide was used as the electron donor in the reaction under study. In aqueous solution with pH 10-12, the sulfide ion occurs mainly as HS anions [48]. Photochemical processes in the presence of CdS colloids are caused by the generation of nonequilibrium holes (h) and electrons (e) in CdS  

In the course of the photoinduced process, HS ion is oxidized by holes to give elementary sulfur 

Further, the formed sulfur is removed from the surface of CdS colloidal particle due to reaction with the sulfite ion added [49] 

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