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Tungsten–niobium oxide bronzes: a bulk and surface structural study

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Delgado Muñoz,  Daniel
Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de Investigaciones Científicas;
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Trunschke,  Annette
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Delgado Muñoz, D., Concepción, P., Trunschke, A., & López Nieto, J. M. (2020). Tungsten–niobium oxide bronzes: a bulk and surface structural study. Dalton Transactions, 49(38), 13282-13293. doi:10.1039/D0DT02058C.


Cite as: http://hdl.handle.net/21.11116/0000-0007-45D7-9
Abstract
Materials from the WO3–Nb2O5 system, presenting bronze-type crystal structures, display outstanding functional properties for several applications as thermoelectric materials, lithium-ion battery electrodes, or catalysts. In this work, a series of W–Nb–O oxide bronzes have been synthesized by the hydrothermal method (with Nb/(W + Nb) ratios in the range of 0–1). A combination of bulk and surface characterisation techniques has been applied to get further insights into: (i) the effect of thermal treatments on as-prepared materials and (ii) the surface chemical nature of W–Nb–O oxide bronzes. Thermal treatments promote the following structural changes: (i) loss of emerging long-range order and (ii) the elimination of NH4+ and H2O species from the structural channels of the as-synthesized materials. It has been observed that W–Nb–O bronzes with Nb at% of <em>ca.</em> 50% are able to retain a long-range order after heat-treatments, which is attributed to the presence of a Cs0.5[W2.5Nb2.5O14]-type structure. Increasing amounts of Nb5+ in the materials (i) promote a phase transition to pseudocrystalline phases ordered along the <em>c</em>-axis; (ii) stabilize surface W5+ species (elucidated by XPS); and (iii) increase the proportion of surface Lewis acid sites (as determined by the FTIR of adsorbed CO). Results suggest that pseudocrystalline oxides (with a Nb at% ≥ 50%) are closely related to NbO7 pentagonal bipyramid-containing structures. The stabilisation of Lewis acid sites on these pseudocrystalline materials leads to a higher yield of heavy compounds, at the expense of acrolein formation, in the gas-phase dehydration of glycerol.