Influence of WO_x on the performance of a CrO_x-WO_x catalyst for the selective catalytic reduction of NO_x with NH₃

A series of CrOx-WOx mixed-oxide catalysts was synthesized through the citric acid-assisted sol-gel method and used in the selective catalytic reduction of NOx with NH3 to eliminate NOx. The pristine CrOx displayed certain deNO(x) activity, achieving >90 % NOx conversion efficiency at the temperature range of 210 degrees C-230 degrees C. Its robust oxidative ability facilitated efficient activation of adsorbed NH3, contributing to relatively high low-temperature activity. However, at higher temperatures, the increased over-oxidation of NH3 led to additional NOx formation. Moreover, the comparatively weak acidity resulted in a lower NH3 coverage on the CrOx surface, causing a deficiency in reductant NH3 for NOx reduction and consequently a decrease in high-temperature activity. With WOx addition, the acidity of the catalysts was enhanced, which was primarily responsible for the high NH3 coverage on the catalysts' surfaces. Furthermore, the reduction in the reactivity of Cr6+ and the number of chemisorbed oxygen species slowed NH3 activation and inhibited NH3 over-oxidation, shifting the catalyst's operating temperature window toward higher ranges. At a fixed W/Cr molar ratio of 0.8, >90 % NOx was eliminated at the temperature range of 230 degrees C-390 degrees C. However, the relatively weak resistance of the CrOx-WOx mixed oxide to SO2 or Na, together with the irrecoverable deactivation upon long exposure to these two components, suggests the need for further modification to enhance the practicality of the catalyst.