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.

N-Phenylpyrrole (PP) was studied in an argon matrix at 14−30 K. The absorption and fluorescence excitation spectra are structureless, while the emission spectrum shows distinct vibrational structure. When a small amount of the polar molecule acetonitrile is added to the PP/argon matrix, UV excitation leads to two separate emission bands the relative intensity of which is strongly excitation wavelength dependent. Control experiments using a nonpolar molecule (ethylene) and dimethylaminobenzonitrile help to assign the spectra. Both emission bands are assigned to transitions from an electronically excited charge-transfer state that has two distinct minima.

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 fluorescence spectrum of PBN in a neat argon matrix is excitation-wavelength-dependent: at short excitation wavelengths, it consists of dual emission assigned to a charge-transfer (CT) state and a much weaker band assigned to the locally excited (LE) state. The CT emission is broad and almost completely devoid of vibrational structure, whereas the LE band is characterized by vibrationally resolved emission. At long excitation wavelengths, only CT emission is observed, indicating that the CT state is populated directly by light absorption and not via the LE state. Comparison with jet-cooled spectra of the bare molecule allows the unambiguous assignment of the LE spectrum and the location of the 0,0 band. The matrix LE emission spectrum is blue-shifted with respect to that of the gas phase, showing that the dipole moment of the LE state is smaller than that of the ground state. The fluorescence spectrum of PBN in an argon matrix does not change appreciably when acetonitrile (AN) is added to the matrix, in contrast to the case of N-phenylpyrrol (PP) (Schweke, D.; Haas, Y. J. Phys. Chem. A 2003, 107, 9554), for which addition of AN results in the appearance of two well separated emission bands. The different photophysical behaviors of PP and PBN in an argon matrix (and in supersonic jets) are analyzed by a simple model that considers the restriction of large-amplitude motions in the matrix. The implications of these low-temperature studies for understanding the properties of these systems in liquid solution are discussed.

Two different molecular dynamics-based models are compared with respect to their ability to predict the number and the distribution of trapping sites of a molecule in a rare-gas matrix. The two approaches are applied to the same problem:  anthracene molecules trapped in an argon matrix. Both methods give a small number of trapping sites with similar structures, but the distributions of sites in each model are different. In all stable sites, the molecule was found to lie on either the {001} or the {111} plane of the crystalline argon. We propose a structure for the most stable site in which anthracene lies in the 6 substitutional site in the {001} plane.

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Understanding the interplay between thermodynamics and kinetics is of high importance for the optimization of catalytic reactions involving the adsorption of CO 2 on CeO 2 (ceria). The present study explores the interaction of CO 2 with ceria powder in near-realistic conditions by correlating adsorption and thermal desorption analyses. Activation energies for desorption, E a , and kinetic parameters (adsorption time constants, τ, and sticking coefficients, s 0 ) are determined using a new methodology based on surface science models. The sticking coefficients obtained for CO 2 on ceria powder are significantly lower than those observed for CO 2 on flat surfaces. CO 2 is found to adsorb most rapidly on sites attributed to surface defects. CO 2 adsorption is slower on nondefective active sites, leading to the formation of various carbonate species. The desorption analysis indicates that each peak in the CO 2 -TPD profiles is composed of several subpeaks, resulting from various binding sites for CO 2 on the polycrystalline powder. The distribution of the chemisorbed CO 2 species between the different sites, the corresponding adsorption energies, and the influence of coverage on those energies are thus determined. In addition, the correlation between adsorption and desorption analyses indicates the influence of heating on the distribution of the chemisorbed species.

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