Multistep Kinetic Behavior of the Thermal Decomposition of Granular Sodium Percarbonate: Hindrance Effect of the Outer Surface Layer

Abstract

The kinetics and mechanism of the thermal decomposition of granular sodium percarbonate (SPC), which is used as a household oxygen bleach, were studied by thermoanalytical measurements under systematically changing conditions and morphological observation of the reactant solids at different reaction stages. A physico-geometrical kinetic behavior of the reaction that occurs in a core-shell structure composed of an outer surface layer and internal aggregates of SPC crystalline particles was illustrated through detailed kinetic analyses using the kinetic deconvolution method. Simultaneously, the hazardous nature of SPC as a combustion improver was evaluated on the basis of the kinetic behavior of the thermal decomposition. It was found that the outer surface layers of the SPC granules hinder the diffusional removal of product gases generated by the thermal decomposition of the internal SPC crystalline particles. The reaction rate decelerates because of an increase in the internal gaseous pressure as the reaction advances. However, the reaction rate accelerates once crack formation occurs in the outer surface layer at the midpoint of the reaction. Therefore, the overall reaction was empirically demonstrated to consist of two overlapping reaction steps owing to the changes in the self-generated reaction conditions in the interior of the SPC granules.

Exothermic Behavior of Thermal Decomposition of Sodium Percarbonate: Kinetic Deconvolution of Successive Endothermic and Exothermic Processes

“…The calculated temperature range of the overall reaction for each sample corresponds well to that of the mass-loss process of the thermal decomposition at the same β. 13,14 The optimized kinetic parameters were practically independent of β, as seen from the acceptably small standard deviations of the average kinetic parameters for those at different β shown in Table 3. Irrespective of the reaction step i, the optimized kinetic parameters do not have a large deviation from those determined for the mass-loss processes of thermal decom- The Journal of Physical Chemistry A Article position.…”

“…13,14 Parts a and b of Figure 3 show the DSC curves, recorded at different β, for the thermal decomposition of SPC crystalline particles and granules, respectively. The DSC curves indicate trends that correspond to the DTA curves of the respective samples.…”

“…Table 2 summarizes the reported kinetic parameters for the respective mass-loss steps of the thermal decomposition of SPC. 13,14 The four contributions (c endo,i , c exo,i ) i=1or2 in eq 11 were calculated using the values of c endo and c exo in Table 1 and c i values in Table 2 as (c endo c i , c exo c i ) i=1or2 . Irrespective of the endothermic and exothermic processes, the initial E a values for the first and second reaction steps were adopted from those determined for each mass-loss step ( Table 2).…”

“…6−10 In addition, reactions in solids can be composed of multiple constituent chemical processes that occur successively. Examples include the oxidative decomposition of solids, 11,12 the thermal decomposition of oxidizing solids, 13,14 and the successive thermal decomposition and reaction of the solid product with atmospheric gases. 15 In this type of reaction, each constituent chemical process contributes differently to the overall thermal effect of the reaction determined to be either exothermic or endothermic.…”

Sample‐Controlled analysis under high pressure for accelerated process studies

“…The CRTA methodology has been employed in previous works for studying different kinds of processes and for the synthesis of materials with controlled texture and structure . For example, the microstructure and the phase compositions of BaTiO 3 has been controlled using this technique for the thermal decomposition of different precursors …”