Abstract
Kinetic models used for the kinetic analysis of solid-state reactions assume ideal conditions that are very rarely fulfilled by real processes. One of the assumptions of these ideal models is that all sample particles have an identical size, while most real samples have an inherent particle size distribution (PSD). In this study, the influence of particle size distribution, including bimodal PSD, in kinetic analysis is investigated. Thus, it is observed that PSD can mislead the identification of the kinetic model followed by the reaction and even induce complex thermoanalytical curves that could be misinterpreted in terms of complex kinetics or intermediate species. For instance, in the case of a bimodal PSD, kinetics is affected up to the point that the process resembles a reaction driven by a multi-step mechanism. A procedure for considering the PSD in the kinetic analysis is presented and evaluated experimentally by studying the thermal dehydroxylation of kaolinite. This process, which does not fit any of the common ideal kinetic models proposed in the literature, was analyzed considering PSD influence. However, when PSD is taken into account, the process can be successfully described by a 3-D diffusion model (Jander’s equation). Therefore, it is concluded that the deviations from ideal models for this dehydroxylation process could be explained in terms of PSD.
Highlights
Solid-state kinetic models give a mathematical description of a process
The modification of the kinetic models caused by the presence in the sample of particles with different sizes needs to be considered, as a kinetic analysis based on the shape of f (α) without considering the particle size distribution (PSD) might lead to incorrect conclusions
To discriminate the kinetic model followed by the process, the combined kinetic analysis, which simultaneously analyzes all experimental data obtained under any heating conditions, was used
Summary
Solid-state kinetic models give a mathematical description of a process These algebraic expressions of simplified physical models are based on both mechanistic assumptions and geometrical considerations [1,2,3,4,5]. The modification of ideal kinetic models f (α) when considering PSD has been previously studied for some diffusion and interface reaction models [10,11,12]. We study how PSD affects the kinetic analysis in terms of the modification of the shape of interface reaction and diffusion models. We revisit the dehydroxylation of kaolinite considering PSD in this kinetic analysis, concluding that the apparent complexity of the process could be explained in terms of its PSD
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