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

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.