191 research outputs found
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×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). Superconducting properties of theKxWO3tetragonal tungsten bronze and the superconducting phase diagram of the tungsten bronze family. Physical Review B, 89(10). doi:10.1103/physrevb.89.104520Huang, Z.-F., Song, J., Pan, L., Zhang, X., Wang, L., & Zou, J.-J. (2015). Tungsten Oxides for Photocatalysis, Electrochemistry, and Phototherapy. Advanced Materials, 27(36), 5309-5327. doi:10.1002/adma.201501217Maiyalagan, T., & Viswanathan, B. (2008). Catalytic activity of platinum/tungsten oxide nanorod electrodes towards electro-oxidation of methanol. Journal of Power Sources, 175(2), 789-793. doi:10.1016/j.jpowsour.2007.09.106Weber, M. F., Bevolo, A. J., Shanks, H. R., & Danielson, G. C. (1981). Electrocatalytic Activity of Cubic Sodium Tungsten Bronze: I. Effects of Platinum Doping, Anodization, and Platinum Pre‐Electrolysis of the Electrolyte. Journal of The Electrochemical Society, 128(5), 996-1003. doi:10.1149/1.2127588Wickman, B., Wesselmark, M., Lagergren, C., & Lindbergh, G. (2011). 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Applied Catalysis B: Environmental, 163, 370-381. doi:10.1016/j.apcatb.2014.08.008Botella, P., Solsona, B., García-González, E., González-Calbet, J. M., & López Nieto, J. M. (2007). The hydrothermal synthesis of tetragonal tungsten bronze-based catalysts for the selective oxidation of hydrocarbons. Chemical Communications, (47), 5040. doi:10.1039/b711228aSoriano, M. D., Concepción, P., Nieto, J. M. L., Cavani, F., Guidetti, S., & Trevisanut, C. (2011). Tungsten-Vanadium mixed oxides for the oxidehydration of glycerol into acrylic acid. Green Chemistry, 13(10), 2954. doi:10.1039/c1gc15622eChieregato, A., Soriano, M. D., García-González, E., Puglia, G., Basile, F., Concepción, P., … Cavani, F. (2014). Multielement Crystalline and Pseudocrystalline Oxides as Efficient Catalysts for the Direct Transformation of Glycerol into Acrylic Acid. ChemSusChem, 8(2), 398-406. doi:10.1002/cssc.201402721Soriano, M. D., Chieregato, A., Zamora, S., Basile, F., Cavani, F., & López Nieto, J. M. (2015). Promoted Hexagonal Tungsten Bronzes as Selective Catalysts in the Aerobic Transformation of Alcohols: Glycerol and Methanol. Topics in Catalysis, 59(2-4), 178-185. doi:10.1007/s11244-015-0440-7Nagy, D., Nagy, D., Szilágyi, I. M., & Fan, X. (2016). Effect of the morphology and phases of WO3 nanocrystals on their photocatalytic efficiency. RSC Advances, 6(40), 33743-33754. doi:10.1039/c5ra26582gLin, S., Guo, Y., Li, X., & Liu, Y. (2015). Glycine acid-assisted green hydrothermal synthesis and controlled growth of WO3 nanowires. Materials Letters, 152, 102-104. doi:10.1016/j.matlet.2015.03.099Miao, B., Zeng, W., Hussain, S., Mei, Q., Xu, S., Zhang, H., … Li, T. (2015). Large scale hydrothermal synthesis of monodisperse hexagonal WO3 nanowire and the growth mechanism. Materials Letters, 147, 12-15. doi:10.1016/j.matlet.2015.02.020Marques, A. C., Santos, L., Costa, M. N., Dantas, J. M., Duarte, P., Gonçalves, A., … Fortunato, E. (2015). 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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
A Combined Multi-Technique In Situ Approach Used to Probe the Stability of Iron Molybdate Catalysts During Redox Cycling
The hydrogen bond in crystals IX. The isotope effect in acetylenedicarboxylic acid dihydrate
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