Promoting effect of zirconium doping on Mn/ZSM-5 for the selective catalytic reduction of NO with NH3

Manganese–zirconium catalysts loaded on ZSM-5 (Si/Al = 25) were prepared using an ion-exchange method and investigated for the selective catalytic reduction of NO, with the aim of studying the effects of zirconium doping on the performance of Mn-based ZSM-5 catalysts. A series of characterization techniques, namely N2-sorption, X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), temperature-programmed reduction by hydrogen, and X-ray photoelectron spectroscopy (XPS), were used to characterize these samples. TEM and XRD results showed that the addition of zirconium increased the manganese dispersion and prevented manganese crystallization. XPS results demonstrated that the introduction of zirconium enhanced the manganese species enrichment on the ZSM-5 grain surfaces. After zirconium addition, some of the manganese ions were incorporated into the zirconia lattice. The strong interaction between Mn and Zr contributed to the formation of Zr3+ ions or oxygen vacancies around the Zr4+ ions, which enhanced the redox abilities of the Mn–Zr/ZSM-5 catalysts. The temperature range for efficient NO conversion was lowered after zirconium doping. The MnZr3/ZSM-5 (Mn 3.2 wt.% and Zr 8.6 wt.%) sample exhibited NOx conversions (>90%) over a wide temperature range (230–415 °C) and markedly superior to that of Mn/ZSM-5 (296–425 °C). Moreover, the formation of N2O was suppressed at low temperatures due to the presence of zirconium in the catalysts.