Tungsten-Based Catalysts for Environmental Applications

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).

Simultaneous Desulfurization and Denitrification Using La-Ce-V-Cu-ZSM-5 Catalysts in an Electrostatic Precipitator

Different catalysts were loaded onto the collecting plate of an electrostatic precipitator to achieve the simultaneous removal of multiple pollutants from coal-fired gas. The synergistic desulfurization and denitrification effect of the catalyst and the effect of corona discharge on the activity of the catalyst were studied. The La(6%)-Ce(8%)-V(7%)-Cu(8%)-ZSM-5 catalyst prepared by successive impregnation methods had the optimum simultaneous desulfurization and denitrification efficiency at a roasting temperature of 600 °C. The desulfurization and denitrification rates reached 97.09 and 83.30%, respectively. BET and SEM characterization results showed that the loading of active components and additives improved the pore structure of the molecular sieve, which contributed to the high stability of the catalyst's internal structure and large surface area, as well as better desulfurization and denitrification efficiency. Corona discharge can significantly improve the catalytic effect.

Influence of the addition of vanadium to Pt/TiO2 catalyst on the selective catalytic oxidation of NH3 to N2

In this work, the effect of the addition of vanadium to the Pt/TiO₂ catalyst on the selective catalytic oxidation (SCO) of NH₃ to N₂ was investigated. It was found that the addition of vanadium significantly enhanced catalytic activity at all tested temperatures. The Pt/V/TiO₂ catalyst exhibited the highest NH₃ conversion (∼100%) and NH₃ to N₂ conversion (∼81%) at 250°C. The physicochemical characteristics of the catalysts were investigated via Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), NH₃ temperature-programmed desorption (TPD), NH₃ temperature-programmed oxidation (TPO), and in situ Fourier-transform infrared (FTIR) spectroscopy. It was found that the addition of V to the catalyst enhanced the conversion of NH₃ as a result of the formation of new acid sites. The increase in the number of acid sites resulted in increased NH₃ to N₂ conversion via the internal selective catalytic reduction (i-SCR) mechanism. This mechanism involves the SCR of NO_x, which are formed by the oxidation of NH₃. Based on experimental results and analyses of the catalysts modified by the addition of V, it was shown that there was a close relation between reaction selectivity and the surface oxygen species of the catalyst and N₂ yield. Furthermore, the addition of V increased the durability of SO₂ by inhibiting the formation of ammonium bisulfate (ABS).

Promotional effect of antimony on the selective catalytic reduction NO with NH3 over V-Sb/Ti catalyst

The effect of antimony on the selective catalytic reduction (SCR) performance and SO₂ durability of V-Sb/Ti was investigated. The physicochemical characteristics of catalyst were characterized by various techniques, including Brunauer-Emmett-Teller (BET) surface area analysis, X-ray diffraction (XRD), NH₃/SO₂-temperature programmed desorption (TPD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs), X-ray photoelectron spectroscopy (XPS), and H₂-temperature programmed reduction (H₂-TPR). The V-Sb/Ti catalyst showed excellent activity in the range 200-300°C (compared with V/Ti), with an optimum achieved for 2 wt.% antimony. The total amount of acidic sites and NH₃ adsorption characteristics did not affect the catalytic efficiency. The Sb⁵⁺ fraction was highest for V-2.0Sb/Ti and exhibited a positive correlation with the V⁴⁺ fraction. This phenomenon is related to the effect of synergistic between vanadium and antimony, promoting the conversion of V⁵⁺ to V⁴⁺ by Sb⁵⁺. Increasing the V⁴⁺ fraction in V-Sb/Ti increased the catalytic activity, which was mainly attributed to enhanced catalyst re-oxidation capability due to the addition of antimony. Furthermore, the addition of antimony delayed the adsorption of SO₂ onto the V-Sb/Ti catalyst surface, improving the resistance to this gas. Therefore, the addition of antimony to V/Ti improved NO_x conversion and SO₂ durability.