Interplay of material thermodynamics and surface reaction rate on the kinetics of thermochemical hydrogen production

- Oxidation kinetics transition from surface-to gas-limited with rising temperature.
- Oxidation rate is maximized at an intermediate temperature of ∼1000 °C.
- Addition of Rh on ceria enhances oxidation rate only in surface-limited regime.
- Reduction kinetics are gas-limited at temperature ramp rates up to 50 °C min−1.
- Adherence of oxide to quasi-equilibrium behavior simplifies reactor modeling.

Production of chemical fuels using solar energy has been a field of intense research recently, and two-step thermochemical cycling of reactive oxides has emerged as a promising route. In this process, the oxide of interest is cyclically exposed to an inert gas, which induces (partial) reduction of the oxide at a high temperature, and to an oxidizing gas of either H2O or CO2 at the same or lower temperature, which reoxidizes the oxide, releasing H2 or CO. Thermochemical cycling of porous ceria was performed here under realistic conditions to identify the limiting factor for hydrogen production rates. The material, with 88% porosity and moderate specific surface area, was reduced at 1500 °C under inert gas with 10 ppm residual O2, then reoxidized with H2O under flow of 600 sccm g−1 of 20% H2O in Ar to produce H2. The fuel production process transitions from one controlled by surface reaction kinetics at temperatures below ∼1000 °C to one controlled by the rate at which the reactant gas is supplied at temperatures above ∼1100 °C. The reduction of ceria, when heated from 800 to 1500 °C, is observed to be gas limited at a temperature ramp rate of 50 °C min−1 at a flow of 1000 sccm g−1 of 10 ppm O2 in Ar. Consistent with these observations, application of Rh catalyst particles improves the oxidation rate at low temperatures, but provides no benefit at high temperatures for either oxidation or reduction. The implications of these results for solar thermochemical reactors are discussed.

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