A kinetic model for evolution of H2 and CO over Zr-doped ceria

Abstract

Among notable catalytic assisted processes by ceria-abased materials are the production reactions of H2 and CO from solar thermochemical splitting of water and carbon dioxide, respectively, for applications pertinent to hydrogen fuel cell and synthesis of syngas. Doping ceria with isovalent cations, most notably Zr, was shown to enhance the yield of O2, H2, and CO. Via sophisticated measurements techniques, literature provided several global-like rates for the overall evolutions of H2 and CO. However, these values remain without a mechanistic verification by means of first-principles calculations. Herein, we map out mechanisms and construct a detail chemistry kinetic model for the dissociative adsorption of H2O and CO2 over pure and Zr-decorated ceria surfaces. The overarching objective of the kinetic model is to deduce temperature-dependent profiles that account for the experimentally observed evolution of O2, H2, and CO during the redox cycles of ceria and to reveal the effect of Zr dopants. Ce surface atoms in the latter incur higher positive charges, and hence more affinity to capture gas phase molecules. Splitting of H2O into a gas phase H2 over the Zr-substituted surface ensues in two consecutive steps through activation energies of 77.7 kJ/mol and 191.2 kJ/mol. Our computed rate constant for evolution of H2 over ceria amounts to 2.67 mL/g. min, within the narrow range of the experimental values scattered between 1.5–5.4 mL/g. min. Formation of H2 and CO arises at a ∼ 100 K lower temperature over the Zr-doped surface in reference to pure ceria. The developed kinetic model is expected to describe - on a precise atomic basis - the H2O/CO2 redox cycles of ceria-based materials and the promoting effects of dopants.