Kinetics and mechanism of free-surface decomposition of solid and melted AgNO3 and Cd(NO3)2 analyzed thermogravimetrically by the third-law method

Abstract

The third-law method and the retardation effect of the excess of gaseous product in the reactor were used for thermogravometric (TG) investigation of kinetics and mechanisms of thermal decomposition of AgNO3 and Cd(NO3)2. The most important results of this study are formulated as follows. (i) In contrast to AgNO3, the evolution of oxygen in the process of decomposition of Cd(NO3)2 proceeds in the form of free atoms (O). This explains, in particular, the absence of O2 molecules in primary products of Cd(NO3)2 decomposition revealed earlier by quadrupole mass spectroscopy (QMS). (ii) The decomposition rate is reduced after reactant melting because of the absence of product/reactant interface and associated transfer of condensation energy to the reactant. This conclusion is supported by a rise of the E parameter of the Arrhenius equation by about 20kJmol−1 after melting of AgNO3 and Cd(NO3)2. (iii) The absence of layer of solid product in case of decomposition of melt provides conditions for a free evaporation in vacuum of all primary products including the low-volatility species. This phenomenon was observed in experiments on decomposition of AgNO3 melt in vacuum.