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 NH3. 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. MnSb0.156/PG catalyst showed highest NO conversion at low temperatures in the presence of SO2 which reveals that addition of Sb oxides effectively enhances the SCR activity of catalysts. A SO2 step-wise study showed that MnSb0.156/PG catalyst displays higher durable resistance to SO2 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, SOx ad-species formed were mainly combined with SbOx rather than MnOx. This preferential interaction between SbOx and SO2 effectively shields the MnOx as active species from being sulfated by SO2 resulting in the improvement of SO2 tolerance on Sb-added catalyst. Multiple information support that, owing to the addition of Sb, original formed MnOx crystallite has been completely transformed into highly dispersed amorphous phase accounting for higher SCR activity.
Keywords: Antimony oxides; Low-temperature SCR; Manganese oxides; Metal sulfates; Sponge-like structure.
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