Interactions between O(2) and CeO(2) are examined experimentally using in situ Raman spectroscopy and theoretically using density-functional slab-model calculations. Two distinct oxygen bands appear at 825 and 1131 cm(-1), corresponding to peroxo- and superoxo-like species, respectively, when partially reduced CeO(2) is exposed to 10 % O(2). Periodic density-functional theory (DFT) calculations aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO(2). The O(2) adsorption energies on unreduced CeO(2) surfaces are endothermic (0.91

Reaction mechanisms for the interactions between CeO(2)(111) and (110) surfaces are investigated using periodic density functional theory (DFT) calculations. Both standard DFT and DFT+U calculations to examine the effect of the localization of Ce 4f states on the redox chemistry of H(2)-CeO(2) interactions are described. For mechanistic studies, molecular and dissociative local minima are initially located by placing an H(2) molecule at various active sites of the CeO(2) surfaces. The binding energies of physisorbed species optimized using the DFT and DFT+U methods are very weak. The dissociative adsorption reactions producing hydroxylated surfaces are all exothermic; exothermicities at the DFT level range from 4.1 kcal mol(-1) for the (111) to 26.5 kcal mol(-1) for the (110) surface, while those at the DFT+U level are between 65.0 kcal mol(-1) for the (111) and 81.8 kcal mol(-1) for the (110) surface. Predicted vibrational frequencies of adsorbed OH and H(2)O species on the surfaces are in line with available experimental and theoretical results. Potential energy profiles are constructed by connecting molecularly adsorbed and dissociatively adsorbed intermediates on each CeO(2) surface with tight transition states using the nudged elastic band (NEB) method. It is found that the U correction method plays a significant role in energetics, especially for the intermediates of the exit channels and products that are partially reduced. The surface reduction reaction on CeO(2)(110) is energetically much more favorable. Accordingly, oxygen vacancies are more easily formed on the (110) surface than on the (111) surface.

Spin-polarized density functional theory with the inclusion of on-site Coulomb correction (DFT+U) calculation is carried out to study the oxygen vacancy and migration of Ce(1-x)Zr(x)O(2) in a series of Ce/Zr ratios. Substitution of Zr(4+) ion in CeO(2) creates activated oxygen in Ce(1-x)Zr(x)O(2), leading to higher oxygen storage capacity (OCS) compared to CeO(2) due to its structural and electronic modifications. It is found that the oxygen vacancy formation energy (E(f)) is lowered even by small amounts of zirconia; the oxide with a content of 50% zirconia exhibits the lowest E(f) and the best OCS. This indicates that the O vacancy is most easily created near the Zr centers. In addition, the activation energy calculations for oxygen vacancy migration around Zr dopant show facile oxygen migration through the Ce(1-x)Zr(x)O(2) materials, especially for 50% Zr-doped ceria. The detailed electronic analysis is also carried out to gain insights into the higher OCS of the Ce(1-x)Zr(x)O(2) catalyst.

The interaction and mechanism for CO oxidation on a Ru-modified CeO 2 surface have been investigated by using periodic density functional theory calculations corrected with the on-site Coulomb interaction via a Hubbard term (DFT + U). Our calculations showed that (i) the Ru dopant facilitates oxygen vacancy formation, while the Ru adatoms may suppress oxygen vacancy formation. (ii) Physisorbed CO, physisorbed CO 2 , and chemisorbed CO (carbonite, CO 2 − ) species are observed on the Ru-doped CeO 2 (111) surface; in contrast, only physisorbed CO is found on the clean CeO 2 (111) surface. The vibrational frequency calculations are carried to characterize these species. (iii) Incorporating Ru ions into the ceria lattice as substitutional point defects can instead sustain a full catalytic cycle for CO oxidation and catalyst regeneration. The Ru dopant promotes CO oxidation without any activation energy leading to O vacancy formation and CO 2 desorption. Molecular O 2 adsorbs at the O vacancy forming O adspecies that then drive CO oxidation and recover the stoichiometric Ru-doped CeO 2 surface. The Bader charge analysis is carried to characterize the oxidation state of Ru ions along the catalytic cycle.

The mechanisms for the oxidation of CO catalyzed by Fe-modified CeO2 surfaces have been investigated using periodic density functional theory calculations corrected for the on-site Coulomb interaction by a Hubbard term (DFT + U). The following findings were made: (i) Fe is stable both as an adsorbed atom, Feδ+ (δ < 2), on the surface and as a dopant, Fe3+, in the surface region. (ii) The Fe dopant facilitates oxygen vacancy formation, whereas Fe adatoms might suppress oxygen vacancy formation. (iii) In addition to physisorbed CO as on the clean surface, physisorbed CO2 and chemisorbed CO (carbonate, CO3 2–) species are observed on the Fe-doped CeO2(111) surface. Vibrational frequency calculations were carried to characterize these species. (iv) CeO2(111) containing positively charged Fe ions, either as supported isolated Feδ+ adatoms [or small Fe x δ+ (x = 2–5) clusters] or as substitutional Fe3+ ions, was found to catalyze the conversion of CO to CO2. Incorporating Fe3+ ions into the ceria lattice as substitutional point defects can instead sustain a full catalytic cycle for CO oxidation and catalyst regeneration. The Fe dopant promotes multiple oxidations of CO without any activation energy, leading to O vacancy formation and CO2 desorption. Molecular O2 adsorbs at the O vacancy, forming O adspecies that then drive CO oxidation and recover the stoichiometric Fe-doped CeO2 surface. A Bader charge analysis was carried to characterize the oxidation state of Fe ions along the catalytic cycle.

The kinetics and mechanisms for the unimolecular decomposition reactions of formic acid and oxalic acid have been studied computationally by the high-level G2M(CC1) method and microcanonical RRKM theory. There are two reaction pathways in the decomposition of formic acid: The dehydration process starting from the Z conformer is found to be the dominant, whereas the decarboxylation reaction starting from the E conformer is less competitive. The predicted rate constants for the dehydration and decarboxylation reactions are in good agreement with the experimental data. The calculated CO/CO2 ratio, 13.6-13.9 between 1300 and 2000 K, is in close agreement with the ratio of 10 measured experimentally by Hsu et al. (In The 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1983; p 89). For oxalic acid, its isomer with two intramolecular hydrogen bonds is the most stable structure, similar to earlier reports. Two primary decomposition channels of oxalic acid producing CO2+HOCOH with barriers of 33-36 kcal/mol and CO2+CO+H2O with a barrier of 39 kcal/mol were found. At high temperatures, the latter process becomes more competitive. The rate constant predicted for the formation of CO2 and HOCOH (the precursor of HCOOH) agrees well with available experimental data. The mechanism for the isomerization of HOCOH to HCOOH is also discussed.

The mechanism for H 2 S-CeO 2 (111) interactions in solid oxide fuel cells (SOFCs) has been investigated by using periodic density functional theory (DFT) calculations. In order to properly characterize the effect of the localization of Ce 4f states on the interactions, DFT + U calculations were applied. Adsorption of H 2 S, SH, and atomic S was initially examined to locate energetically favorable intermediates. The species adsorb favorably at the Ce-top, O-top, and Ce-O bridging sites, respectively. Potential energy profiles for the H 2 SCeO 2 (111) interactions along the three product channels producing H 2 , H 2 O, and SO 2 were constructed using the nudged elastic band (NEB) method. Calculations show that H 2 S weakly bounds on CeO 2 (111) with a small binding energy, followed by dehydrogenation processes, forming surface sulfur species with an exothermicity of 29.9 kcal/mol. Molecular-level calculations demonstrated that the SO 2 -forming pathway is energetically most favorable.

The interaction and mechanism for CO oxidation on the Mn/ CeO 2 (111) surface have been studied by using periodic density functional theory calculations corrected with the on-site Coulomb interaction via a Hubbard term (DFT + U). It is found that the Mn dopant facilitates oxygen vacancy formation, while the Mn adatoms may restrain oxygen vacancy formation. In addition, physisorbed CO, physisorbed CO 2 ,and chemisorbed CO (carbonite, CO 2 − ) species are observed on the Mn-doped CeO 2 (111) surface, in contrast, only physisorbed CO is found on the pure CeO 2 (111) surface. The vibrational frequency calculations as well as the calculated adsorption energies are carried to characterize these species. The Mn dopant promotes CO oxidation without any activation energy leading to O vacancy formation and CO 2 desorption. The Bader charge analysis is carried to characterize the oxidation state of Mn ions along the catalytic cycle.