Kinetics and Thermodynamics of H₂O Dissociation on Reduced CeO₂(111)

Abstract

We use density functional theory to investigate the reaction between reduced CeO 2−x (111) and water. H 2 O dissociation to hydroxyl is facile on surface vacancies and lattice oxygen, while subsequent decomposition of hydroxyl into H 2 has a high barrier, which results in reversible adsorption of H 2 O under ultra-high-vacuum conditions. The barrier to H 2 formation through hydroxyl decomposition decreases by 0.2 eV, while H 2 O formation becomes more difficult at high hydroxyl coverage. However, on isolated oxygen vacancies on a hydroxyl covered surface, H 2 may be produced through a CeH intermediate with a 1.14 eV barrier. Oxygen vacancies are found to be more stable in the subsurface than in the surface layer at all vacancy coverages and for hydroxyl coverages less than 25−50%. The competition of H 2 O desorption and vacancy diffusion from the subsurface to the surface may prevent formation of hydroxyl from H 2 O dosing at low temperature, while the highly stable hydroxyl phase may provide a thermodynamic driving force for further surface reduction in the presence of water. On the basis of our calculations we suggest substitutional doping with a cation that binds H stronger than Ce may improve the decomposition of hydroxyls into hydrogen.

Water‐Assisted Homolytic Dissociation of Propyne on a Reduced Ceria Surface

“…By overcoming a very small activation energy barrier of 0.12 eV, H 2 O dissociates and forms two hydroxy groups, one of which fills the oxygen vacancy. The dissociation process is highly exothermic with a total reaction energy of 2.1 eV, thus resulting in the strong adsorption of water on reduced ceria …”

Redox Kinetics Study of Fuel Reduced Ceria for Chemical-Looping Water Splitting

“…The surface water splitting and oxygen incorporation reactions are modeled using a two-step mechanism ( Figure 2a) 45,[50][51][52][53] :…”

“…Recently, there is an increasing effort towards a better understanding of the surface-oriented defect chemistry of ceria, owing to the growing interests in fuel cells, electrolyzers, and water splitting 45,50 . Density functional studies examined surface defect formation and the energy landscape of the redox process [51][52][53] . In-situ techniques, such as in-situ X-ray photoelectron spectroscopy (XPS), have emerged recently, and have been successfully applied to ceria [44][45][54][55][56] .…”

Giant Radiolytic Dissolution Rates of Aqueous Ceria Observed in Situ by Liquid‐Cell TEM

“…[30],b ut again not beyond one order of magnitude. Subsequently,C eO 2Àx could split water catalytically, [39] as proposed for commercialc eria-based H 2 -production setups. As the surfacee nergy is inverse to the radius of curvature, accelerated dissolution, as shown in Figure 2, is expected.…”