(181e) Reaction Kinetics of the High Temperature Zno Dissociation Step in a 2-Step Solar Thermochemical Water Splitting Process

The Zn/ZnO thermochemical water splitting process provides a method for generating clean burning hydrogen fuel from only water and solar thermal energy. Critical to its application is an understanding of the reaction kinetics of the high temperature solar step in the cycle, the dissociation of ZnO powder. A full understanding of the kinetics allows for optimal reactor design, maximizing solar efficiency and minimizing final cost of the hydrogen.

Thermogravimetric analysis (TGA) was used to study the kinetics of the solid state powder reaction. Varying amounts of initial powder mass and powder size were used in non-isothermal analysis to determine whether diffusion through the powder had a significant effect on the kinetics. This effect was found to be unimportant. Analysis of the non-isothermal TGA data using a shrinking core particle dissociation model showed compensation behavior in the Arrhenius parameters, with the activation energy and natural logarithm of the Arrhenius factor being linearly related.

Isothermal TGA experiments were conducted to determine if the compensation behavior was intrinsic to the reaction. Analysis of the data gave Arrhenius parameters consistent with L'vov solid sublimation theory. The activation energy was found to be 354 ± 4 kJ/mol, and the Arrhenius parameter was found to be dependent on diffusion distances in the gas phase within the instrument, but predictable from the L'vov theory.