Kinetics and mechanism of solar-thermochemical H_2 and CO production by oxidation of reduced CeO_2

摘要

The combination of high thermal stability and fast reduction/oxidation kinetics has drawn attention to CeO2 for use in solar-driven, two-step thermochemical cycles for water and/or carbon dioxide splitting. In accordance with this renewed interest in CeO2, there is a need to better understand the gas splitting chemistry on the reduced oxide. In this study, we measured the H-2 production rates over ceria powder, thermally reduced by laser irradiation, during oxidation by H2O and CO2 gases in a stagnation flow reactor. The reaction kinetics was extracted using a model-based analytical approach to account for the effects of mixing and dispersion in the reactor. We find the rigor of this approach necessary in order to identify the rate controlling mechanism and assign parameters to the kinetic model, and to successfully model the production of H-2 and CO production over the entire envelope of the fuel curve. The water splitting reaction, in the range of 750-950 degrees C and 20-40 vol% H2O, can best be described by a first-order kinetic model with low apparent activation energy (29 kJ/mol); while the carbon dioxide splitting reaction, in the range of 650-875 degrees C and 10-40 vol% CO2, is a more complex surface mediated phenomena.