THRESHOLD LIMITS AND KINETICS OF THE NONISOTHERMAL DECOMPOSITION OF AMMONIUM-NITRATE CATALYZED BY CHROMIUM IONS

Ammonium nitrate (AN) samples, containing varying amounts of Cr3+ cations were prepared and their mode of thermal decomposition was analysed under non-isothermal conditions in a thermogravimetric balance. Pure AN decomposes endothermically, but addition of Cr3+ in amounts larger than one mole of Cr3+ per 3000 moles of AN acts catalytically, changing the decomposition to an exothermic mode. At concentrations lower than this, the diffusion limitations prevent the condensation of Cr3+ species into Cr2O72- which are the catalytically active species. The rates of decomposition, described by a typical Coats-Redfern kinetic equation, show two stages. Satisfactory results were obtained using a modified Coats-Redfern equation and by taking into account the continuous increment of chromium catalyst in the melt. The second stage of the decomposition shows an increased activation energy compared with the first. This is probably caused by a change in the rate-determining step of the reaction path, which is most likely the decomposition of dichromates. © 1990.