Interaction of Hydrogen with Cerium Dioxide─Adsorption–Desorption Equilibria and Vibrational Frequencies of Hydroxyl Groups Based on Computational Modeling

The paper reports a density functional study of hydrogen adsorption on a series of cerium dioxide models with different degrees of reduction and analyzes the energetics of the interactions, the relative stability of the formed hydroxyl groups, and their vibrational frequencies. Based on reaction energetics, we have shown that ceria samples may be reduced to a degree involving the removal of oxygen centers with oxygen vacancy formation energy below the decomposition enthalpy of water, 2.52 eV. The opposite process, generation of hydrogen from water on reduced ceria, may occur only on oxygen vacancies with formation energy above this value. Calculations for model ceria nanoparticles and the CeO2(111) surface suggest that the adsorption of a hydrogen molecule, leading to the reduction of ceria and the formation of two hydroxyl groups, is an energetically favorable process. It is more than 2.0 eV more exothermic than the heterolytic dissociation and hydrogen adsorption, which causes oxidation of reduced ceria containing up to 25% reduced cerium ions. Three factors are suggested to affect the relative stability of the surface hydroxyl groups formed after ceria reduction by hydrogen: the coordination of the oxygen, the reducibility of the ceria, and the degree of reduction of the model. The hydroxyl groups at 2-fold coordinated oxygen centers are more stable than hydroxyls at 3-fold coordinated oxygen centers. According to the simulated vibrational frequencies, the two types of hydroxyls cannot be distinguished by their O–H stretching vibrations. A specific case is the bulk model of reduced ceria, for which both the reduction and oxidation of ceria upon hydrogen adsorption are endothermic, with a minor preference for the former process.