https://doi.org/10.3390/ijms20040928
“Most of the TGA curves are fitted to the double exponential model, which is shown in Equation (5):
where y represents the weight gain of Li4SiO4 material after CO2 absorption; k1 and k2 denote two exponential constants for the chemical reaction-controlled stage and the diffusion-controlled stage, respectively; and two pre-exponential factors A and B are the intervals that control the corresponding stages [28].”
“Table 1 presents the kinetic parameters of the double exponential model fitted to the reaction between CO2 and Li4SiO4 [28]. As presented in Table 1, the values of k1 are usually one order of magnitude higher than those of k2, and B are always larger than A, indicating that CO2 absorption over the surface of Li4SiO4 controlled by chemical reaction is a rapid process, and CO2 absorption controlled by diffusion occurs in a large interval of time. Thus, CO2 absorption controlled by diffusion is the limiting step hindering the absorption of CO2 by Li4SiO4 [29,30].”
“Although the double exponential model is widely used due to its simplicity, Ortiz et al. [26] thought that this model was short of the theoretical mechanism to support its fitting with the experimental data. Zhang et al. [27] reported that the Avrami–Erofeev model was relevant to the reaction mechanism of the formation and growth of product crystals, which are shown as Equations (6) and (7):
where α refers to the degree of conversion; K denotes the kinetic constant; k equals to Kn; and n is the kinetic parameter; t represents the time. Equation (7) is an equation of a straight line with slope n in the coordinates ln (−ln (1 − α)) vs. ln t. If the value of n is higher than 1, the absorption reaction is controlled by the formation and growth of product crystals. When n equals to 0.5 approximately, the absorption reaction is controlled by the diffusion of ions [31].
