Oxides are widely used for energy applications, as solid electrolytes in various solid oxide fuel cell devices or as catalysts (often associated with noble metal particles) for numerous reactions involving oxidation or reduction. Defects are the major factors governing the efficiency of a given oxide for the above applications. In this paper, the common defects in oxide systems and external factors influencing the defect concentration and distribution are presented, with special emphasis on ceria (CeO ) based materials. It is shown that the behavior of a variety of oxide systems with respect to properties relevant for energy applications (conductivity and catalytic activity) can be rationalized by general considerations about the type and concentration of defects in the specific system. A new method based on transmission electron microscopy (TEM), recently reported by the authors for mapping space charge defects and measuring space charge potentials, is shown to be of potential importance for understanding conductivity mechanisms in oxides. The influence of defects on gas-surface reactions is exemplified on the interaction of CO and H O with ceria, by correlating between the defect distribution in the material and its adsorption capacity or splitting efficiency.
The interaction of hydrogen with ceria is a fundamental process of high importance for various catalytic reactions. Theoretical calculations have been performed to identify the elementary steps in this reaction and the intermediate species, as they strongly influence the selectivity and efficiency of the hydrogenation reactions catalyzed by ceria. However, these studies do not provide direct information on the effect of temperature or H2 partial pressure on the reaction mechanism and intermediates. In the present work, thermogravimetric and differential thermal measurements were used to study the influence of temperature (in the 300–600 °C range), hydrogen partial pressure (0.2–3%), and the presence of oxygen vacancies on ceria–H2 interaction. In addition, temperature-programmed reduction (TPR) and desorption measurements were performed to monitor the adsorbing/desorbing gases as a function of temperature. It was found that at T ≤ 400 °C, H2 adsorbs chemically on stoichiometric ceria but nearly does not dissociate, whereas at higher temperatures, H2 adsorbs dissociatively, leading to hydroxyl formation followed by H2O desorption. Some of the hydroxyl formed remains on the surface at the end of the reduction stage, allowing very fast re-oxidation of ceria by O2 through H2O formation. Under the studied conditions, increasing H2 partial pressure led to a higher rate of H2 adsorption and H2O desorption but did not lead to an increased extent of reduction, suggesting that H2O formation/desorption is the rate-determining step in ceria reduction (with an activation energy of 105 ± 4 kJ/mol according to the TPR measurements). The presence of oxygen vacancies, before exposing ceria to H2, was found to strongly affect the characteristics of H2 adsorption and to lead to the penetration of hydrogen into the sub-surface and bulk, probably as hydridic hydrogen. Penetration of hydrogen into the lattice, competing with hydroxyl formation, was found to occur preferentially at low temperatures.
The Ce–U–O system raises growing interest due to its potential importance for water splitting at low temperatures. The variable possible oxidation states of Ce (Ce3+ and Ce4+) and U (U4+, U5+, and U6+) lead to the formation of point charged defects on the surface. These point charges are active sites for the chemisorption of H2O, which is the rate-determining step for H2 production. In the present work, the interaction of H2O with the surface of Ce1–x U x O2+δ oxides in a wide range of compositions (x = 0, 0.1, 0.25, 0.5, 0.75, and 1) is studied through the measurements of adsorption and calorimetric isotherms at room temperature. The oxides’ structure is determined by X-ray diffraction, and the charge distribution between the U and Ce cations is inferred from X-ray photoelectron spectroscopy (XPS) measurements. The adsorption thermodynamics is analyzed within the electronic theory of chemisorption on semiconducting surfaces. On the basis of this analysis, correlated to the XPS results and to density functional theory calculations on a representative oxide, an atomic scale understanding of the chemisorption process is proposed. Partial charge transfer between the Ce and U cations is shown to be a key factor for creating adsorption sites for H2O activation. This charge transfer is shown to occur most efficiently in the mixed oxides with low U content. The analysis proposed explains the adsorption behavior of the different mixed oxides and provides an explanation for the improved efficiency for H2O splitting reported on reduced Ce1–x U x O2 oxides with low U content.
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