Kinetic Model for TiO2 Polymorphic Transformation from Anatase to Rutile
Abstract:We propose a distribution kinetics model for the polymorphic transformation (anatase to rutile) and coarsening of TiO2. Based on population balance equations for the size distributions of the dimorphs, the simplified model applies a first-order rate expression for transformation combined with Smoluchowski coalescence for coarsening of anatase and rutile particles. Two moments of the size distributions (number and mass of particles) lead to dynamic expressions for extent of reaction and average particle diameter. The model describes the time-dependent data of Banfield and colleagues fairly accurately, and provides activation energies for anatase coalescence and transformation. The equilibrium constant for the microscopically reversible transformation, occurring without coarsening, yields a small endothermic enthalpy change per mole of TiO2. This probably reflects contributions of the transformation enthalpy of the anhydrous phases at the given particle size, which is very close to 0, and the enthalpy associated with a small amount of dehydration (endothermic water evaporation) during transformation.
Document Type: Research Article
Affiliations: 1: Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India 2: Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 3: Thermochemistry Facility and NEAT ORU and Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616
Publication date: January 1, 2007