Kinetic analysis of the oxidative decomposition in gamma-zirconium and gamma-titanium phosphate intercalation compounds The case of 2,2 '-bipyridyl and its copper complex formed in situ

摘要

The thermal behaviour of the heterocyclic compound 2,2'-bipyridyl and its copper complex formed in situ, both intercalated between the layers of gamma-zirconium and gamma-titanium phosphates was studied by simultaneous thermogravimetry and differential scanning calorimetry techniques using non-isothermal constant heating rate conditions up to 873 K under a stream of air. In all the obtained intercalation materials, which are found to be stable up to 600K, the ligand shows an oxidative decomposition practically in only one step. Model-free isoconversional method of Ozawa-Flynn-Wall, applied to the vaporization of pure 2,2'-bipyridyl and to the oxidative decomposition step occurring in each intercalation material examined, yields practically constant activation energies for values of the fraction decomposed alpha <= 0.7. The activation energies calculated using the best fit between calculated and theoretical g(alpha) models do not differ significantly from the corresponding mean E values selected using the isoconversional OFW method in the range alpha <= 0.7. Activation energies derived by the Kissinger method show a good agreement with the mean values derived by the former method, and the Arrhenius rate constants determined using also the pre-exponential factor values enable to conclude that the bipyCu intercalation materials show a destabilizing effect with respect to the corresponding bipy intercalation materials (negligible difference in the oxidative decomposition temperatures, but a significant difference in the rate constant values: at least one order of magnitude). (C) 2007 Elsevier B.V. All rights reserved.