Improvement of the activity and SO tolerance of Sb-modified Mn/PG catalysts for NH-SCR at a low temperature.

A series of MnM/palygorskite (PG) (M = La, W, Mo, Sb, Mg) catalysts was prepared by the wetness co-impregnation method for low-temperature selective catalytic reduction (SCR) of NO with NH. Conversion efficiency followed the order Sb > Mo > La > W > Mg. A combination of various physico-chemical techniques was used to investigate the influence of Sb-modified Mn/PG catalysts. MnSb/PG catalyst showed highest NO conversion at low temperatures in the presence of SO which reveals that addition of Sb oxides effectively enhances the SCR activity of catalysts. A SO step-wise study showed that MnSb/PG catalyst displays higher durable resistance to SO than Mn/PG catalyst, where the sulfating of active phase is greatly inhibited after Sb doping. Scanning electron microscopy and X-ray diffraction results showed that Sb loading enhances the dispersion of Mn oxides on the carrier surface. According to the results of characterization analyses, it is suggested that the main reason for the deactivation of Mn/PG is the formation of manganese sulfates which cause the permanent deactivation of Mn-based catalysts. For Sb-doped Mn/PG catalyst, SO ad-species formed were mainly combined with SbO rather than MnO. This preferential interaction between SbO and SO effectively shields the MnO as active species from being sulfated by SO resulting in the improvement of SO tolerance on Sb-added catalyst. Multiple information support that, owing to the addition of Sb, original formed MnO crystallite has been completely transformed into highly dispersed amorphous phase accounting for higher SCR activity.