The thermal decomposition in the solid phase (crystolysis) of silver malonate

Abstract

From the complementary consideration of microscopic and kinetic evidence we conclude that the thermal decomposition of silver malonate, CH₂(COOAg)₂, in the temperature range 453–485 K, occurs in the solid state by a nucleation-and-growth reaction. During this decomposition the small crystallites of reactant undergo no detectable textural change, except for some surface roughening. Isothermal fractional reaction (α) against time curves are sigmoid, and after subtraction of a subsidiary initial process the main decomposition obeys the Avrami–Erofe'ev equation [–log(1–[graphic omitted])]^1/n=kt where n= 2 (and in a few instances 3). This kinetic fit is ascribed to the growth of coherent but elongated nuclei (composed of Ag metal embedded in a carbonaceous material) in the rectangular but asymmetric (lath-shaped) reactant crystallites. Appreciable differences in the shapes of the α against time curves for decompositions of the five differently pretreated reactant samples studied are ascribed to variations in mean reaction geometry. However, it is concluded that the chemical changes occurring at the interface are identical for all preparations, since the Arrhenius parameters agree within experimental error. Mean values of the activation energy (E) and pre-exponential factor (A) were 170±10 kJ mol^–1 and 15.2±1.2 s^–1, respectively. Comparisons with the limited information available concerning the decomposition of silver acetate lead us to conclude that decomposition here is controlled by the breakdown of adsorbed radicals, probably including —CH₂CO₂— and related species, on the surface of the metallic silver product. The chemical steps probably involve heterogeneous catalytic-type interactions between several dissociatively adsorbed intermediates on the active metal at the reactant–product interface.