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

Abstract

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 MnO 2. The reaction is fractional-order each in [ d-fructose] and [H +] with a unit order in [MnO 2] 0. The results suggest formation of an adsorption complex between d-fructose and MnO 2. The complex decomposes in a rate-determining step, leading in the formation of a free radical, which again reacts with the colloidal MnO 2 in a subsequent fast step to yield the products. The reaction rate decreased by addition of F −, P 2O 7 4− and Mn 2+ ions. The rates of Mn 2+ and P 2O 7 4− 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 MnO 2 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.