Thermal decomposition of nickel squarate dihydrate, NiC4O4· 2H2O

Abstract

A kinetic and microscopic study has been made of the thermal decomposition of nickel squarate dihydrate in the solid state. Kinetic characteristics are sensitive to the availability of water vapour and behaviour varies with changes in reactant disposition within the reaction vessel. Reproducible data were obtained by standardization of technique and decomposition rates increased as the reactant mass was reduced. The activation energy for the reaction, found by extrapolation of decomposition rate coefficients to zero mass, was 163 ± 10 kJ mol^–1. Kinetic measurements did not, however, permit characterization of the geometry of interface development within individual reactant particles. Microscopic observations provided crucial evidence for the interpretation of rate data. Such evidence revealed that decomposition was accompanied by cracking of the approximately cubic crystallites of reactant and that preferential two-dimensional growth of superficial nuclei preceded penetration of the reaction interface into the separated reactant blocklets.

It is concluded that reaction involved two consecutive steps, dehydration followed by decomposition. A novel mechanism is proposed for the breakdown of the organic anion. It is suggested that nickel bonds to the four-membered unsaturated ring and bonds are then redistributed with the transitory intervention of nickel carbonyl, which rapidly disintegrates at reaction temperature (ca. 500 K) to give the predominant products, metallic nickel and carbon monoxide.