A THERMOGRAVIMETRIC STUDY OF REDUCTION OF SILVER OXIDE UNDER NON-ISOTHERMAL CONDITIONS

Abstract

Thermal decomposition of a commercial powder of silver oxide (Ag2O, Merck, p.a.), with the mean particle diameter of 740 nm has been investigated under nonisothermal conditions in a reducing atmosphere of 25% H2 in argon. The process was conducted and controlled in a simultaneous TG/DTA analyzer, at heating rates of 2, 10 and 20 °C min. The residual mass was always around 93,2 % of the initial one, regardless of the heating rate, which, according to the oxide stoichiometry, corresponds to metallic silver as final product. The SEM microphotography indicated no significant change in particle size and morphology during reduction. The thermogravimetric data was used for kinetic analysis of reduction. Using Kissinger plot, the initial value of the activation energy was estimated to amount to 6.17 · 10 J mol. The initial values for kinetics parameters were further calculated using different methods of kinetic analysis. Both activation energy (E) as a function of conversion degree (α), and pre-exponential factor (A) were calculated by either the Friedman (Ea = 5,38 · 10 J mol, A = 1,49 · 10 s) or multi-heating rate CoastRedfern (Ea = 5,97 · 10 J mol, A = 3,90 · 10 s) isoconversional methods. A rather complex dependence of activation energy on the conversion degree was observed. A more complete approach to determine the kinetic parameters was done by means of nonlinear regression analysis (Ea = 6,00 · 10 J mol, A = 2,41 · 10s, n = 0,466 ).