A kinetic and mechanistic study of the thermal decomposition of nickel malonate

Abstract

The kinetics of evolution of the main gaseous products (CO 2, CO and CH 4) of the isothermal decomposition (543–613 K) of nickel malonate, after preliminary dehydration, were studied in greater resolution than was possible previously, through use of plots of rate (dα/d t) against extent of reaction, α. An initial deceleratory process (α < 0.12) was followed by a brief acceleration to constant-rate (0.30 < α < 0.60). The subsequent deceleratory region (α > 0.60) contains a subsidiary small maximum at α = 0.92. Activation energies of 176 ± 10 kJ mol −1 were determined from rate measurements at fixed α values. Condensation of the principal gaseous product, CO 2, altered the kinetics by removing the initial deceleratory reaction and producing sigmoid α-time curves which were fitted by the Avrami-Erofe'ev equation with n = 4. Appreciable amounts (up to 20 mol-%) of acetate ion were shown by NMR to participate in the decomposition mechanism. The initial reaction is ascribed to reaction of residual water from the hydrated salt with the malonate ion to form acetate and CO 2. The sigmoid α-time curve, subsequently established, is the result of carboxylate breakdown promoted by the solid nickel carbide product, the reactivities of the acetate and malonate ions being similar. Scanning electron microscopic studies did not provide significant additional information concerning the mechanism.