Potassium niobate (KNbO3) microcubes with orthorhombic and tetragonal phases were hydrothermally prepared and characterized by powder X-ray diffraction, nitrogen adsorption-desorption, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, diffuse reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The photoreactivity of the as-prepared KNbO3 samples was evaluated regarding the hydrogen evolution from aqueous methanol under UV, and the results were compared with that of cubic KNbO3 microcubes. The photocatalytic reactivity was shown to be phase-dependent, following the order cubic > orthorhombic > tetragonal. Insight into the phase-dependent photocatalytic properties was gained by first-principles density functional calculations. The best photocatalytic performance of cubic KNbO3 is ascribed to it having the highest symmetry in the bulk structure and associated unique electronic structure. Further, the surface electronic structure plays a key role leading to the discrepancy in photoreactivity between orthorhombic and tetragonal KNbO3. The results from this study are potentially applicable to a range of perovskite-type mixed metal oxides useful in water splitting as well as other areas of heterogeneous photocatalysis.

C-halogen) have been computed by using five density functional methods (B3LYP, MPW1PW91, B3PW91, B3P86, and MPW1P86). The quality of these methods is comprehensively evaluated on the basis of the available experimental bond dissociation enthalpies, and it is found that the MPW1P86 has the best agreement, while B3LYP performs the largest deviations. Large deviations also are found at the sophisticated CCSD(T) level of theory. The restricted open-shell method underestimates the radical stability.

An computational study using density functional theory and grand-canonical Monte Carlo simulation that explore the adsorption mechanism of C 2 H 2 , CO 2 , and CH 4 to metal−organic frameworks (MOFs) with coordinatively unsaturated metal sites (M-MOF-74, M = Mg and Zn) has been carried out. The theoretical studies reveal that open metal sites have important roles in adsorption. The high CO 2 adsorption ability of M-MOF-74 is due to the strong Lewis acid and base interactions between metal ions and oxygen atom of CO 2 , as well as carbon atom of CO 2 with oxygen atoms in organic linkers. Meanwhile, the high C 2 H 2 adsorption for M-MOF-74 is contributed by the strong complexation between the metal ions and the π orbital of C 2 H 2 . The different adsorption mechanisms of CO 2 , C 2 H 2 , and CH 4 in M-MOF-74 can qualitatively explain the high CO 2 selectivity in CO 2 /CH 4 mixture and high C 2 H 2 selectivity in C 2 H 2 /CH 4 mixture. Energy decomposition analysis reveals that electrostatic energy, exchange energy, and repulsive energy are key factors in the binding strength of gas molecules on M-MOF-74. The preferential adsorption sites are confirmed to be located near the five-coordinate metal ions decorating the edges of the hexagonal channels. The elucidation of the adsorption mechanism at the molecular level provides key information for designing novel MOFs with high capacity and selectivity for CO 2 from light hydrocarbon mixtures.

An amine-functionalized metal organic framework, TEPA-MIL-101, was prepared by grafting tetraethylenepentamine (TEPA) on the coordinatively unsaturated Cr(III) sites of MIL-101 for the selective adsorption of CO 2 over CO. The adsorbents were characterized using various techniques. The results indicate that the TEPA molecule was successfully grafted on Cr(III) without destroying the intrinsic structure of MIL-101. Isotherms for CO 2 and CO adsorption on MIL-101 and TEPA-MIL-101 were obtained to determine the effects of the grafted TEPA on the CO 2 adsorption capacity and selectivity. The results show that the CO 2 capacity on TEPA-MIL-101 was higher than that on MIL-101 at lower pressures, whereas the CO capacity sharply decreased. The selectivity for CO 2 over CO was clearly improved from 1.77 to 70.2 at 298 K and total pressure 40 kPa. The density functional theory calculation for the adsorption of CO 2 and CO on TEPA indicates that the bonding energy of CO 2 is obviously higher than that of CO. Analysis of the cyclic adsorption performance reveals the high stability of the adsorbent. On the basis of the experimental and simulation results, the grafting of amines on coordinatively unsaturated sites of metal organic frameworks is an effective method of achieving selective adsorption of CO 2 over CO.

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The structure and properties of small neutral and cationic CrGen(0,+) clusters, with n from 1 to 5, were investigated using quantum chemical calculations at the CASSCF/CASPT2 and DFT/B3LYP levels. Smaller clusters prefer planar geometries, whereas the lowest-lying electronic states of the neutral CrGe4, CrGe5, and cationic CrGe5+ forms exhibit nonplanar geometries. Most of the clusters considered prefer structures with high-spin ground state and large magnetic moments. Relative to the values obtained for the pure Gen clusters, fragmentation energies of doped CrGen clusters are smaller when n is 3 and 4 and larger when n = 5. The averaged binding energy tends to increase with the increasing number of Ge atoms. For n = 5, the binding energies for Ge5, CrGe5, and CrGe5+ are similar to each other, amounting to approximately 2.5 eV. The Cr atom acts as a general electron donor in neutral CrGen clusters. Electron localization function (ELF) analyses suggest that the chemical bonding in chromium-doped germanium clusters differs from that of their pure or Li-doped counterparts and allow the origin of the inherent high-spin ground state to be understood. The differential DeltaELF picture, obtained in separating both alpha and beta electron components, is consistent with that derived from spin density calculations. For CrGen, n = 2 and 3, a small amount of d-pi back-donation is anticipated within the framework of the proposed bonding model.

Four density-functional methods (B3LYP, B3PW91, MPW1PW91, and B3P86) are employed to compute the C-C homolytic bond dissociation enthalpies (BDE) of a set of aromatic hydrocarbons related to coal structures with aliphatic linkages. In comparison with the available experimental data, the B3P86 method can provide reasonably reliable BDE values for these model compounds. The BDE values for large aromatic hydrocarbon systems of interest are computed, and the substituent effects are discussed.

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