A model for the role of carbon on carbochlorination of TiO2

The carbochlorination of TiO2 is characterized by two successive reaction stages, named the first and the second stage. In both stages, reaction rate increases as carbon mass fraction increases. This kinetic effect of carbon was interpreted on the basis of a model developed to describe the rate of each reaction stage as a function of carbon mass fraction in TiO2-C powder mixture. The model considers, for both reaction stages, two global steps: [I] the formation of gaseous intermediates at the carbon surface and [II] the reaction of these intermediates with TiO2 to form the reaction products. The mathematical treatment from these steps involves the adaptation of the general model applied to catalytic gas-solid reactions. The first reaction stage occurs, for high carbon contents, under strong external chlorine transport control and, for low enough carbon content, under internal mixed control. The second reaction stage occurs out of external gaseous influence and a net chemical control operates, both at low temperatures and for the lowest carbon content. The intrinsic activation energies for the first and second reaction stages are 40 and 117 kJ-mol−1, respectively.