Kinetics and mechanism of thermal decomposition of nickel, manganese, silver, mercury and lead oxalates

A model of dissociative evaporation of the reactant with simultaneous condensation of the low-volatility product has been invoked to interpret the reaction schemes of the thermal decompositions of nickel, manganese, silver, mercury and lead oxalates. A critical analysis of literature data and their comparison with theoretical calculations permitted selection of two different schemes: with the evolution of CO + CO2 + 1/2O(2) (or O) as primary products for Ag, Ni and Hg oxalates, and evolution of 2CO + 1/2O(2), for Mn and Pb oxalates. The Validity of the proposed decomposition schemes is supported by good agreement of experimental values of activation energies E-a and initial decomposition temperatures T-in with their theoretical values. As an additional criterion for correct selection of the primary-product composition, the depressing effect exerted on the decomposition rate by one of the assumed gaseous products was used. Some of the features of the oxalate decomposition were interpreted. Among them are the low values of decomposition coefficients; a correlation between T-in and E-a parameters; a difference between activation energies for induction and acceleratory periods in the decomposition of NiC2O4 and Ag2C2O4 and the existence of long induction periods in the process of decomposition of all the oxalates except for HgC2O4. (C) 2000 Elsevier Science B.V. All rights reserved.