An investigation on the promoting effect of Pr modification on SO2 resistance over MnOx catalysts for selective reduction of NO with NH3

Pr-modified MnOₓ catalyst was synthesized through a facile co-precipitation process, and the results showed that MnPrOₓ catalyst exhibited much better selective catalytic reduction (SCR) activity and SO₂ resistance performance than pristine MnOₓ catalyst. The addition of Pr in MnOₓ catalyst led to a complete NO conversion efficiency in 120–220 °C. Moreover, Pr-modified MnOₓ catalyst exhibited a superior resistance to H₂O and SO₂ compared with MnOₓ catalyst. After exposing to SO₂ and H₂O for 4 h, the NO conversion efficiency of MnPrOₓ catalyst could remain to 87.6%. The characterization techniques of XRD, BET, hydrogen-temperature programmed reduction (H₂-TPR), ammonia-temperature programmed desorption (NH₃-TPD), XPS, TG and in situ diffuse reflectance infrared spectroscopy (DRIFTS) were adopted to further explore the promoting effect of Pr doping in MnOₓ catalyst on SO₂ resistance performance. The results showed that MnPrOₓ catalyst had larger specific surface area, stronger reducibility, and more L acid sites compared with MnOₓ catalyst. The relative percentage of Mn⁴⁺/Mnⁿ⁺ on the MnPrOₓ-S catalyst surface was also much higher than those of MnOₓ catalyst. Importantly, when SO₂ exists in feed gas, PrOₓ species in MnPrOₓ catalyst would preferentially react with SO₂, thus protecting the Mn active sites. In addition, the introduction of Pr might promote the reaction between SO₂ and NH₃ rather than between SO₂ and Mn active sites, which was also conductive to protect the Mn active sites to a great extent. Since the presence of SO₂ in feed gas had little effect on NH₃ adsorption on the MnPrOₓ catalyst surface, and the inhibiting effect of SO₂ on NO adsorption was alleviated, SCR reactions could still proceed in a near-normal way through the Eley-Rideal (E-R) mechanism on Pr-modified MnOₓ catalyst, while SCR reactions through the Langmuir-Hinshelwood (L-H) mechanism were suppressed slightly.