Abstract
This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).
Highlights
Tungsten is the metal having the highest melting point (3410 ◦C) and a density (19.3 g cm−3) close to that of gold (19.32 g cm−3). α-W is the crystallographic stable form of tungsten [1]. β-W is a metastable cubic form of tungsten, first found in products of WO3 reduction by hydrogen [2]. γ-W is an fcc form of tungsten only detected in thin film
The acidity of catalysts strongly depends on the obtained WOx surface species, which depend on the preparation protocol and the tungsten loading
The stronger acid sites are encountered on the WO3-ZrO2 system, where a condensation phenomenon between Lewis and Brønsted sites can occur during the calcination treatment
Summary
Tungsten is the metal having the highest melting point (3410 ◦C) and a density (19.3 g cm−3) close to that of gold (19.32 g cm−3). α-W (lattice type: body-centered cube) is the crystallographic stable form of tungsten (lattice parameter: 0.3165 nm) [1]. β-W is a metastable cubic (type A15) form of tungsten, first found in products of WO3 reduction by hydrogen [2]. γ-W is an fcc form of tungsten only detected in thin film. While group 8–11 elements are currently used as nanoparticles of metals in catalysis, W-based heterogeneous catalysts contain tungsten in the form of oxides, sulfides, carbides or heteropolytungstates [3]. Utilization of tungsten carbides for electrochemical applications was developed in the domain of hydrogen evolution reaction [9,10] Due to their high solubility in water, heteropolytungstates serve as precursors of tungsten in the preparation of W-based catalysts [11,12]. They are utilized in many organic syntheses, especially in oxidation reactions [13]. Tungsten-based catalysts are currently utilized for environmental applications (e.g., DeNOx, oxidation in gas and in liquid phases, sensors, photocatalysis). The energy bands and density of states from theoretical calculations confirm a metal-like behavior of WO2 [45]
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