Kinetic analysis of the crystallization processes in the protonated potassium polytitanates

Abstract

The kinetics of the TiO 2 crystalline phase formation in the powdered samples of protonated potassium polytitanate (PPPT) having an amorphous layered structure was studied by differential scanning calorimetry (DSC). The theoretical analysis was carried out using the THINKS free software, based on the methods of Kissinger, Friedman and Vyazovkin, as well as the method of combined kinetic analysis, and the methods of linear and non-linear regression. The obtained results have shown that the formation of the crystalline TiO 2 (anatase) in the investigated system occurs in two stages. The theoretical results were analyzed taking into account their correspondence to the kinetic models of crystallization process including: two independent flexible one-step stages; two sequentially flowing flexible single-step stages; two parallel flowing (competing) flexible single-step stages. The main kinetic parameters of the crystallization process are determined using the best satisfaction to the Bayesian fit criterion (BIC). Based on the results obtained, the mechanism of two sequentially flowing stages is proposed. The first stage of crystallization, involves the diffusion of surface-active components (H 2 O, H 3 O + , K + ) in the interlayer space to the surface of PPT particles and following desorption of water and transformation of the layered amorphous PPPT structural polyanions into the distorted crystalline anatase-like structure. At the second stage of the process, the distorted anatase structure is transformed into the traditional crystalline form, and, finally, the process of crystallization of protonated PPT is completed by the formation of small crystals of potassium hexatitanate onto the surface of particles with the participation of potassium ions previously diffused from the bulk.