The role of molybdenum on the enhanced performance and SO2 resistance of V/Mo-Ti catalysts for NH3-SCR

The influence of molybdenum on the selective catalytic reduction (SCR) performance and SO₂ durability of V/Mo-Ti catalysts was investigated. The characteristics of the catalysts were investigated by Brunauer-Emmett-Teller (BET) surface area analysis, field-emission transmission electron microscopy (FE-TEM), Raman spectroscopy, NH₃-temperature programmed desorption (NH₃-TPD), in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTs), SO₂ temperature-programmed desorption (SO₂-TPD), and X-ray photoelectron spectroscopy (XPS). The V/Mo-Ti catalysts exhibited improved SCR performances and SO₂ resistances at 250 °C compared to those of V/W/Ti catalysts. The improved catalytic activity was shown to be imparted by amorphous MoOx. The formation of amorphous MoOx species with the addition of molybdenum to titanic acid was found to be more significant than that by the addition of molybdenum to crystalline TiO₂. The increased density of NH₃-adsorption and Brønsted-acid sites by addition of molybdenum was found to have a positive effect on catalyst performance. Since the reaction between SO₂ and VOx was suppressed by the addition of molybdenum, the area of the SO₂-TPD peak due to desorption of adsorbed SO₂ from the catalyst surface was reduced. This decrease in the amount of adsorbed SO₂ improved the SO₂ resistances of the catalysts. The V/Mo-Ti catalysts exhibited increasing activities and SO₂ resistances with increasing Mo⁶⁺ ratio (and Mo⁶⁺ atoms/cm³). Furthermore, they exhibited suppressed formation of ammonium sulfate salts due to decreased SO₂ adsorption, as the reaction of terminal VO groups and adsorbed SO₂ is inhibited by the addition of molybdenum. Thus, the V/Mo-Ti catalysts exhibited better catalytic activities and SO₂ durability than those of V/W/Ti catalysts.