APT as Tungsten Source to Enhance V2O5/WO3-Tio2 Catalysts

Nitrogen oxides (NOx) are produced from the emissions of stationary sources and automobiles. Direct emission of NOx into the atmosphere without purification will lead to photochemical smog, acid rain and even global warming. The selective catalytic reduction (SCR) of NO with NH3 is widely employed for reduction of NOx emissions from stationary sources like gas, oil and coal- fired power plants. The most common catalyst is vanadium well-dispersed on a titania support as V2O5-TiO2.

Unfortunately, it has narrow and high reaction temperature window (350–400 °C). Moreover, the temperature of exhaust gas produced by many small and medium-sized industrial boiler plants and biomass burning plants is lower than 300 °C (even < 200 °C).

One promising route to overcome this is to dope heterogeneous components such as alumina, silica, zirconia. Among these introduced components, tungsten is an ideal promoter. Thus, tungsten has been doped to the V2O5-TiO2 as V2O5/WO3-TiO2 using ammonium paratungstate as tungsten source.

The hydrothermal synthesis of V2O5/WO3-TiO2 catalysts is as following procedures:

Metatitanic acid (H4TiO4) and ammonium paratungstate (APT) was mixed via the weight ratio of WO3:TiO2 = 5:95. The obtained suspension was stirred for 1 h and titrated by 25 wt% ammonium hydroxide (NH3·H2O) until pH = 9. The precipitate was stirred for 2 h, dried at 100 °C over night and calcined at 550 °C for 4 h in the muffle. The pristine TiO2 was prepared by the same process without adding APT. The WO3-TiO2 was obtained and would be used in next step.

V2O5/WO3-TiO2 samples with 1 wt% V2O5 were prepared as follows. 0.13 g ammonium metavanadate (NH4VO3) was dissolved into a mixed solution composed of 3 ml monoethanolamine (C2H7NO) and 5 ml deionized water. The obtained solution was added into 20 ml suspension contained 20 g WO3-TiO2. Finally, a hydrothermal treatment was performed at 750 °C for 24 h under a flowing air atmosphere with 10 vol% water vapor to obtain the aged samples. Finally, the enhanced  V2O5/WO3-TiO2 catalysts were obtained.

To sum up, the enhanced V2O5/WO3-TiO2 catalysts exhibited SCR performance with a wide temperature range, which is >90% of NOx conversion at 250–550 °C. The doping of WO3 had contributed to produce more polymeric surface VOx species. And the increased surface vanadium concentrations were also directly related to the higher SCR activity of the aged catalysts.

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