Kinetic Peculiarities of Photocatalytic Processes on Ultradispersed CdS Colloids at Stationary Illumination

Kinetic Peculiarities of Photocatalytic Processes on Ultradisperse CdS Colloids at Stationary Illumination 

A typical experimentally observed kinetic curve of the MO reduction is composed of three sections section AB with the fast initial decay of the MO concentration (see Fig. 2.22), a linear section BC extending to the point of complete decomposition of the dye, and final section CH with fast decrease in the process rate. This kinetic dependence takes a more clear, typical stepped configuration at turning from coordinates optical density D (MO concentration) - time t to coordinates quantum yield cp - current MO concentration using the formulas for evaluation of quantum yield  

In our study, for all applied intensities of the irradiating light in the range 0.2-10 8 Einstein-s 1-cm 2 Io 1.6-10 8 Einstein-s 1-cm 2, the experimental-ly obtained initial reaction rate W0 was found to be proportional to the intensity I0 of irradiating light (Fig. 2.24), i.e., the initial quantum yield of the reaction cpo is independent of this intensity. 

A comparison of the experimentally obtained dependence of the initial quantum yield p0 on the MO concentration (Fig. 2.23) with the shape of the MO adsorption isotherm over an aqueous suspension reveals a qualitative similarity in the form of indicated curves. 

A characteristic increase of 90 after the horizontal section is apparently more pronounced when the potassium oxalate K2C2O4 is used as the electron donor instead of the sulfide ions (Fig. 2.25). A qualitative similarity of the adsorption isotherms and the MO concentration dependence on the initial quantum yield indicates that the adsorbed dye molecules take part in the reaction. Note that all kinetic curves attain the same value of the stationary quantum yield pst regardless of the initial MO concentration (Fig. 2.23). Therewith, the cpst/(po ratio depends on the nature of polymeric surfactant used for stabilization of CdS colloid. With PAA, this ratio equals ca. 0.5, and 0.6 with PVS. 

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