Determination of aerosol kinetics of thermal ZnO dissociation by thermogravimetry

The thermal decomposition of ZnO is the high temperature solar step in a two-step water splitting process for sustainable H-2 production. To optimize aerosol solar reactor design, it is desired to understand the forward kinetics of this reaction in an aerosol configuration. Non-isothermal thermogravimetric (TG) experiments were conducted to determine the applicability of TG kinetic data to aerosol reactor environments. It was found that the differentiating heat and mass transfer factors-initial loaded mass, particle size, and heating rate-had no statistically significant effect on the activation energy or pre-exponential factor. This allowed TG data to be applied to the aerosol case. Isothermal TG experiments were subsequently performed to determine the kinetic rate parameters. Using the model expression d alpha/dt = k*e(Ea/R (1/T-1/T0)) (1-alpha)(2/3), with T-0 = 1895 K, E-a was found to be 353 +/- 25.9 kJ/mol, and k* was found to vary inversely with diffusion distance in the TG crucible. The specific rate constant for a diffusion distance of 1 in was k(0) = 3.15 x 10(6) +/- 5.54 x 10(5) s(-1). Both of these results are in agreement with L'vov theory, and a simple electrostatic dissociation mechanism was proposed. (C) 2007 Elsevier Ltd. All rights reserved.