53 PBE0/GTO 386 604 540 353 71 B3LYP/GTO 389 617 547 340 71 PBE+U(4.5)/pw 392 618 −1435 e 549 −834 e 221 −20 53 PBE/pw f 383 608 −1573 547 −988 403 162 53 PBE/pw g 385 604 −1589 547 −999 409 168 53 a Reference 81. b Zero-point vibrational energy and thermal contributions to the heats of formation are very small (2.1 and 1.4 kJ/mol for CeO 2 and Ce 2 O 3 , respectively) and even smaller for reaction 2.1 (0.8 kJ/mol). Therefore, we use ΔE f,r ≈ ΔH f,r 0 (values from Table 2). c Experimental formation enthalpy of water at 0 K (−239 kJ/mol) has been taken from the literature. 82 d Reference 63. e Reference 80. Since GGA+U yields only the less stable γ phase of cerium, the energy difference between α and γ phase (−3 kJ/mol) has been added to the heats of formation. f PAW ansatz to describe the electron−ion interaction using plane waves. g LAPW+LO ansatz to describe the electron−ion interaction using plane waves, which can be considered as the benchmark method in the solid-state community. Both, PAW and LAPW are so-called full-potential methods.

Covalent organic frameworks (COFs) have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction (HER)from water.T oo ptimize their photocatalytic activity,t ypically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However,t he effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor-acceptor (D-A) type imine-linked COFs can produce hydrogen with ar ate as high as 20.7 mmol g À1 h À1 under visible light irradiation, upon protonation of their imine linkages.Asignificant red-shift in light absorbance,largely improved charge separation efficiency,a nd an increase in hydrophilicity triggered by protonation of the Schiff-base moieties in the imine-linked COFs,a re responsible for the improved photocatalytic performance.

We examine (VO) k and (VO2) k (k = 1, 2, 3) species supported on CeO2(111) by periodic density functional theory as models for ceria-supported vanadia catalysts. We use the PBE functional and correct for onsite Coulomb correlation (PBE+U). As reactivity descriptors, we calculate oxygen defect formation and hydrogenation energies. In agreement with experiment, our results suggest that vanadyl-terminated monomers, that is, VO2, represent the most active species. This system has a remarkably low oxygen defect formation energy of 0.84 eV (clean surface: 1.84 eV), and hydrogenation proceeds more exothermic (−1.46 vs −1.07 eV for clean surface). The VO2 dimer, preferring an open, chain-like structure at the CeO2(111) surface, is the only other system with a similarly high reactivity. The active species are thermodynamically less favored compared with the VO2 trimer, which forms a ring structure and binds solely via vanadium atoms to CeO2(111). Thus, for this case, relaxation effects are minor. In contrast, the active species bind via oxygen atoms of the VO2 moiety to surface Ce atoms necessitating substantial surface relaxation. Calculated IR vibrational spectra of the supported VO2 monomer and trimer confirm the experimentally observed blue shift of the VO stretching mode upon aggregation.

When new covalent organic frameworks (COFs) are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage (imine, boroxine, vinyl, etc.) between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation, i.e., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine-linked donor-acceptor (D-A) COFs are synthesized, which are different in the orientation of the imine bonds (D-C=N-A (DCNA) and D-N=C-A (DNCA)). The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H2 evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COFs.

By virtue of periodic density functional theory, we investigate structure and thermodynamic stability of (VO)k and (VO2)k (k = 1, 2, 3) clusters deposited on the CeO2(111) surface, which serve as models for the very active sub-monolayer vanadia catalyst on a ceria support. We find V always completely oxidized (oxidation state +5) and coordinated to four O atoms. As a consequence, Ce4+ is (partially) reduced to Ce3+. Thus, localized Ce-4f states are populated, which requires an onsite U-term (PBE+U) to avoid over-delocalization off-electrons. Importantly, trimers of VO2 were found to be extraordinarily stable (agglomeration energy: -1.68 eV), whereas aggregation of VO species on CeO2(111) is thermodynamically clearly unfavourable (agglomeration energy: 3.45 eV). As a consequence a large area of the VnOm phase diagram (for relevant temperatures) is dominated by the VO2 trimer. The latter is less active towards reduction/oxidation than the active monomer and dimer of VO2, which are not present in the phase diagram at all, although directly observed by recent STM measurements. This suggests that kinetic effects hinder VO2 to grow into larger oligomers. The lowest migration energy barrier we found is as high as 1.95 eV, which indicates that adsorbed monomeric VO2 is "kinetically locked" at low temperatures and explains why monomers are stabilized on the ceria surface.

Structures and stabilities of vanadium oxide oligomers as well as two candidate structures for a monolayer on the CeO2(111) surface have been studied by density functional theory employing a genetic algorithm to determine the global energy minimum structures.These ceria-supported structures have predominantly four-fold coordinated V 5+ ions with V=O groups in common. The agglomeration of VO2 clusters deposited on the surface is a strongly exothermic process, particularly when ring structures with three or six VO2 units are formed that are commensurate with the close-packed surface-terminating oxygen layer. The VO2 and V2O5 monolayers feature larger coordination numbers (5, 6) of V and contain V atoms without V=O groups. Relative to oligomers, VO2 and V2O5 monolayer structures with and without oxygen defects are thermodynamically more stable. This, together with the fact that flat "monolayer" clusters are preferred to taller "bilayer" clusters, indicates the preference for a complete 2D wetting of the ceria support.3 Ceria-supported vanadia MLs have been extensively studied experimentally for powder catalysts, but detailed atomistic information has not become available yet. 9,18,[22][23] The complete ML represents an important limiting case in terms of activity. Beyond ML coverage, V2O5 crystallites are formed, which have a much lower active site density than VOx supported on CeO2(111). 6 Furthermore, the variation in activity with the support indicates that the V-O-M (M = support metal cation) interphase bond plays an important role. 8,[24][25][26] Different values for the vanadium content of the ML were obtained by different groups. Burcham and Wachs reported 5.7 V atoms/nm 2 corresponding to the highest loading for which no Raman bands typical of V2O5 crystallites were observed. 23 On the other hand, Feng and Vohs performed TPD experiments after exposing the (powder) catalyst to methanol. 9 They reported that the hightemperature CO desorption peak at ca. 615 K, characteristic for methanol oxidation on the employed pure ceria, does not appear for vanadia loadings higher than 9.5 V atoms/nm 2 .According to these authors, this indicates exhaustive (2D) coverage of the ceria surface and thus formation of a complete ML.The oxygen content of the ML, i.e. whether it corresponds to a fully oxidized V2O5 layer or to a layer with the composition of a reduced vanadium oxide phase like V2O3, is not directly accessible by experiment. As mentioned above, this is because vanadium is readily oxidized to V 5+ by ceria. 21 In refs. 19,21,24 and 27, physical vapor deposition was employed to deposit vanadia on the CeO2(111) films, while the work by Feng and Vohs 9 was accomplished for samples prepared via incipient wetness impregnation. According to their XPS results, nonreduced (so-called stoichiometric) CeO2 was present, i.e. no reduction to Ce 3+ upon vanadia deposition was observed. This indicates that the (electronic) structure of the vanadia ML catalyst may depend on the actual preparation technique.Density functional theor...

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The role of surface and subsurface O vacancies for gold adsorption on crystalline CeO2(111) films has been investigated by scanning tunneling microscopy and density functional theory. Whereas surface vacancies serve as deep traps for the Au atoms, subsurface defects promote the formation of characteristic Au pairs with a mean atom distance of two ceria lattice constants (7.6 Å). Hybrid density functional theory calculations reveal that the pair formation arises from a titration of the two Ce3+ ions generated by a single O vacancy. The Au-Ce3+ bond forms due to a strain effect, as the associated charge transfer from the spacious Ce3+ into the adgold enables a substantial relaxation of the ceria lattice. Also the experimentally determined Au-pair length is reproduced in the calculations, as we find a Ce3+-Ce3+ spacing of two ceria lattice parameters to be energetically preferred. Single Au atoms can thus be taken as position markers for Ce3+ ion pairs in the surface, providing unique information on electron-localization phenomena in reduced ceria.