TITANIUM-DIOXIDE PHOTOOXIDATION OF THIOCYANATE - (SCN)2- STUDIED BY DIFFUSE REFLECTANCE FLASH-PHOTOLYSIS

Diffuse reflectance flash photolysis has been used to study the kinetics of (SCN)2•- decomposition in aqueous suspensions of titanium dioxide (Degussa P-25). (SCN)2•- is formed when SCN•, from oxidation of thiocyanate, SCN-, by photoexcited TiO2, reacts with excess SCN-. The transient absorption spectra of (SCN)2•- obtained on transparent colloidal TiO2 or on TiO2 powder suspended in water have maxima at 480 nm and similar absorption band shapes. A two-parameter kinetic model, developed by Albery to describe kinetics in several heterogeneous systems, is applied for the first time to diffuse reflectance measurements. The kinetics of (SCN)2•- decomposition as a function of laser intensity, KSCN concentration, pH, and ionic strength (μ) have been investigated by using this model. The disappearance of (SCN)2•- on the surface of TiO2 powder suspended in water appears to follow first-order kinetics with a distribution of reaction rate constants. The average rate constant, kav, for a suspension of 1 g of TiO2 in 100 mL of 1 × 10-5 M KSCN, (2 ± 0.5) × 105 s-1, increases with increasing concentration of KSCN and decreases with increasing pH and inert electrolyte concentration. These results are consistent with the charged nature of the TiO2 particle surface. © 1990 American Chemical Society.