Density-functional studies of tungsten trioxide, tungsten bronzes, and related systems

Tungsten trioxide adopts a variety of structures which can be intercalated with charged species to alter the electronic properties, thus forming `tungsten bronzes'. Similar optical effects are observed upon removing oxygen from WO_3, although the electronic properties are slightly different. Here we present a computational study of cubic and hexagonal alkali bronzes and examine the effects on cell size and band structure as the size of the intercalated ion is increased. With the exception of hydrogen (which is predicted to be unstable as an intercalate), the behaviour of the bronzes are relatively consistent. NaWO_3 is the most stable of the cubic systems, although in the hexagonal system the larger ions are more stable. The band structures are identical, with the intercalated atom donating its single electron to the tungsten 5d valence band. Next, this was extended to a study of fractional doping in the Na_xWO_3 system (0 < x < 1). A linear variation in cell parameter, and a systematic change in the position of the Fermi level up into the valence band was observed with increasing x. In the underdoped WO_3-x system however, the Fermi level undergoes a sudden jump into the conduction band at around x = 0.2. Lastly, three compounds of a layered WO_4&#215;a,wdiaminoalkane hybrid series were studied and found to be insulating, with features in the band structure similar to those of the parent WO_3 compound which relate well to experimental UV-visible spectroscopy results.Comment: 12 pages, 16 figure

Influence of Phase Composition of Bulk Tungsten Vanadium Oxides on the Aerobic Transformation of Methanol and Glycerol

[EN] A series of W-V-O catalysts with different m-WO3 and h-WO3 phase contents were hydrothermally synthesized by employing different tungsten, vanadium, and ammonium precursors and characterized by powder XRD, N-2 adsorption, SEM, X-ray energy-dispersive spectroscopy, thermogravimetric analysis, Raman and FTIR spectroscopy, NH3 temperature programmed desorption, H-2 temperature-programmed reduction, and XPS. Finally, the acid/redox properties were analyzed by using aerobic transformation of methanol as a characterization reaction. A correlation between phase composition as well as acid and redox properties was observed, which were correlated to the catalytic performance of the title materials in a one-pot oxydehydration reaction of glycerol. The hexagonal tungsten bronze (h-WO3) phase shows a significantly higher concentration of acid sites than monoclinic m-WO3, so that the acid properties of W-V-O oxides are directly related to the presence of h-WO3 crystals. The presence of a higher concentration of acid sites in V-containing h-WO3 crystals is a key factor to achieve high selectivity to both acrolein and acrylic acid during one-pot glycerol oxydehydration. Also, V sites in h-WO3 show higher selectivity in the consecutive reaction (partial oxidation of acrolein to acrylic acid), while V sites in the m-WO3 phase fundamentally lead to the formation of carbon oxides.The authors acknowledge the DGICYT in Spain, CTQ2015-68951-C3-1-R and CTQ2015-68951-C3-3-R. Authors from ITQ also thank Project SEV-2016-0683 for financial support. D. D. thanks MINECO and Severo Ochoa Excellence Program for his fellowship (SVP-2014-068669). The research group of Prof. Fabrizio Cavani (University of Bologna, Italy)and Consorzio INSTM (Firenze) are gratefully acknowledged for a PhD grant to A. C. Authors also thank the Electron Microscopy Service of Universitat Politecnica de Valencia for their support.Delgado-Muñoz, D.; Chieregato, A.; Soriano Rodríguez, MD.; Rodríguez-Aguado, E.; Ruiz-Rodríguez, L.; Rodriguez-Castellon, E.; López Nieto, JM. (2018). Influence of Phase Composition of Bulk Tungsten Vanadium Oxides on the Aerobic Transformation of Methanol and Glycerol. European Journal of Inorganic Chemistry. 10:1204-1211. https://doi.org/10.1002/ejic.201800059S1204121110GUO, J.-D., & WHITTINGHAM, M. S. (1993). TUNGSTEN OXIDES AND BRONZES: SYNTHESIS, DIFFUSION AND REACTIVITY. International Journal of Modern Physics B, 07(23n24), 4145-4164. doi:10.1142/s0217979293003607Long, H., Zeng, W., & Zhang, H. (2015). Synthesis of WO3 and its gas sensing: a review. Journal of Materials Science: Materials in Electronics, 26(7), 4698-4707. doi:10.1007/s10854-015-2896-4Haldolaarachchige, N., Gibson, Q., Krizan, J., & Cava, R. J. (2014). 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On the Origin of the Spontaneous Formation of Nanocavities in Hexagonal Bronzes (W,V)O3

[EN] Hexagonal (W,V)O3−x oxides of high thermal stability have been synthesized hydrothermally through the intermediate products Nax(W,V)O3·zH2O and (NH4)0.33−x(W,V)O3−y. The obtained crystals show nanostructured surface via the formation of a dense population of polyhedral nanocavities self-distributed along particular crystallographic directions. Nanocavities present a regular size that ranges from 5 to 10 nm in both length and width. The synthesis process involves a significant topotactic relationship between the as-synthesized product and the desired final product and this relationship is suggested as the origin of the observed surface nanostructure. The comparison of our results with observations in different solids has allowed us to suggest that the formation of nanocavities is an extensive spontaneous process when materials are obtained by the chemical reactions of solids leading to products with defined crystallographic orientation with respect to the original compound. The characterization provides evidence regarding the potential relevance of nanocavities in the functional properties of the resulting solids.Authors acknowledge the financial support from DGICYT in Spain through projects MAT2010-19837-C06-05 and CTQ2012-37925-C03-1. Authors are also grateful to the Centro de Microscopia Electronica (UCM) for facilities.García-González, E.; Soriano Rodríguez, MD.; Urones-Garrote, E.; López Nieto, JM. (2014). On the Origin of the Spontaneous Formation of Nanocavities in Hexagonal Bronzes (W,V)O3. Dalton Transactions. 43:14644-14652. https://doi.org/10.1039/C4DT01465KS14644146524