Study of crystal growth and kinetic parameters of Zn/ZnO oxidation in the presence of H2O and CO2

Abstract

During the H2O/CO2 co-splitting, ZnO crystals were formed by oxidation of Zn giving gaseous product, H2/CO. Micrographs of Zn/ZnO and temperature program reaction (TPRx) of H2O/CO2 splitting showed that the crystal growth was occurred via two phenomena; (1) surface reaction: at temperature from 400 to 550 °C, H2O/CO2 oxidized Zn resulting in the outward growth of the ZnO crystals until the whole surface became inert and totally covered all the unreacted Zn inside the core. At this point, the production rates of H2/CO were temporarily paused; (2) phase changing and core-cracking/shrinking: at temperature 550–600 °C, Zn particles inside the core was phase-changed from solid to liquid/vapor due to the higher temperature. The higher pressure inside the core led to core-cracking, where the Zn travelled to the surface to continue reacting with H2O/CO2. Core-shrinking was then occurred due to the crack of ZnO shell and leakage of Zn. TPRx results showed that CO2 competed H2O in adsorption/dissociation on the Zn during temperature from 400 to 550 °C. Conversions and kinetic parameters were experimental and mathematical determined. The apparent activation energy of H2O and CO2 splitting on Zn/ZnO particles was estimated at 77.63 and 51.11 kJ/mol, while the pre-exponential factors were determined at 4.02 and 0.30 s−1, respectively.