(629c) How Ceria Redox Materials Split H2o and CO2 with Concentrated Solar Energy into H2, CO, and O2
Heterogeneous catalyst design has been revolutionized in the past several years by the discovery of adsorption-energy scaling relations on transition-metal catalysts. These and correlated activation energies of chemical reactions are understood to limit the catalytic activity of a surface for a number of interesting reactions, such as production of H2 and CO via splitting of H2O and CO2. Metal oxide redox materials exchange oxygen with a gas phase and store it in the solid state. This allows circumventing scaling relations that limit catalytic reactions, as a chemical reaction may be split into reduction and oxidation steps that are conducted at different process conditions. This work employs electronic structure calculations to investigate the reaction mechanism of CO2 and H2O splitting with ceria. Conceptually, non-stoichiometric ceria is reduced at elevated temperature with concentrated solar energy, liberating O2. In a second step, the lattice oxygen is replenished via oxidation with H2O and CO2, yielding renewable H2 and CO (syngas). Syngas is the precursor for synthetic liquid hydrocarbon fuel, such as diesel, gasoline and kerosene. The presented computations outline the effect of reduction and oxidation temperatures, partial pressure of O2, and metal doping at the ceria surface on the free energy landscape of the two studied chemical reactions. Limits of isothermal processing as well redox trade-offs when controlling the reduction potential of the oxide surface are discussed.