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.