The research presented in this paper is devoted to the intriguing phenomenon of thermal decomposition that takes place during continuous cooling after being initiated by heating to higher temperature. This paper describes the principles of detecting this phenomenon and measuring its kinetics. As one of the possible ways, the process can be detected and its kinetics can be measured by means of differential scanning calorimetry provided that cooling is performed several hundred times slower than heating. By way of illustration, the thermal degradation of isotactic polystyrene and thermal dehydration of lithium sulfate monohydrate have been studied upon cooling and heating. The kinetics of both processes have been analyzed by means of the isoconversional methodology. For polystyrene, the kinetics of degradation upon cooling and heating have been similar. The thermal dehydration of lithium sulfate monohydrate has revealed that cooling kinetics differ significantly from the kinetics measured upon heating. It is proposed that such differences should be observed in multi-step processes whose activation energy varies with reaction progress.
Following the previous work (Phys. Chem. Chem. Phys., 2016, 18, 32021), this study continues to investigate the intriguing phenomenon of thermal decomposition during continuous cooling. The phenomenon can be detected and its kinetics can be measured by means of thermogravimetric analysis (TGA). The kinetics of the thermal decomposition of ammonium nitrate (NHNO), nickel oxalate (NiCO), and lithium sulfate monohydrate (LiSO·HO) have been measured upon heating and cooling and analyzed by means of the isoconversional methodology. The results have confirmed the hypothesis that the respective kinetics should be similar for single-step processes (NHNO decomposition) but different for multi-step ones (NiCO decomposition and LiSO·HO dehydration). It has been discovered that the differences in the kinetics can be either quantitative or qualitative. Physical insights into the nature of the differences have been proposed.
This
work explores the differences in the kinetics of reversible
thermal decomposition measured respectively during heating and cooling.
The kinetics of the process is measured by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). The thermal dehydrations
of calcium oxalate monohydrate (CaC2O4·H2O) and calcium sulfate dihydrate (CaSO4·2H2O) are studied as examples of reversible decomposition. The
kinetics is analyzed by an advanced isoconversional method that demonstrates
that on cooling the activation energy decreases with decreasing temperature,
whereas on heating it decreases with increasing temperature. This
qualitative difference can be understood by modifying an earlier proposed
kinetic model to account for a dependence of the equilibrium pressure
on conversion. The model is applied to the thermal dehydration of
CaC2O4·H2O, CaSO4·2H2O, and lithium sulfate monohydrate (Li2SO4·H2O) studied previously. The results
indicate that in the reversible decomposition on cooling the equilibrium
pressure has much stronger dependence on conversion than for the same
process on heating. The dramatic difference in the evolution of the
equilibrium pressure explains the qualitative difference in the temperature
dependencies of the activation energy evaluated respectively from
the cooling and heating data.
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