Kinetics and mechanism of the reduction of colloidal manganese dioxide by d-fructose

Kinetics of the reduction of water-soluble colloidal manganese dioxide by d-fructose was investigated in acidic perchlorate media at different temperatures. The observed kinetics is explained by assuming fast adsorption of d-fructose on the surface of colloidal MnO2. The reaction is fractional-order each in [d-fructose] and [H+] with a unit order in [MnO2]0. The results suggest formation of an adsorption complex between d-fructose and MnO2. The complex decomposes in a rate-determining step, leading in the formation of a free radical, which again reacts with the colloidal MnO2 in a subsequent fast step to yield the products. The reaction rate decreased by addition of F−, P2O74− and Mn2+ ions. The rates of Mn2+ and P2O74− inhibited reactions further increase with increasing the concentrations of those ions. The Arrhenius equation was valid for the reaction between 30 and 60 °C. On the basis of various observations and product characterization a plausible mechanism has been envisaged for the reaction taking place at the colloid surface. The colloidal MnO2 has been found to be reduced by gum arabic (a protective colloid). The adsorption hypothesis viz. Freundlich isotherm for the mechanism of the reaction was tested and the Freundlich exponents (n = 0.57 and n′ = 0.69) have been calculated.