Cr–MnOx mixed-oxide catalysts for selective catalytic reduction of NOx with NH3 at low temperature

Cr–Mn mixed-oxide based catalysts were prepared for the low-temperature selective catalytic reduction of NOx with ammonia in the presence of excess oxygen. It was found that the Cr(0.4)–MnOx showed the highest activity and yielded 98.5% NOx conversion at 120 °C. XRD, TPR and Raman data results suggested that a crystalline phase of CrMn1.5O4 was present in the Cr–MnOx catalysts, which contained the active species. XPS results of fresh, used and regenerated Cr(0.4)–MnOx catalysts illustrated clearly the presence of Mn2+, Mn3+, Mn4+ and Cr2+, Cr3+, Cr5+ oxidation states. Efficient electron transfer between Cr and Mn in the crystal of CrMn1.5O4 was thought to be the reason for the high activity and long lifetime of the Cr(0.4)–MnOx catalyst. In addition, the SCR activity was gradually suppressed in the presence of SO2, while such an effect was shown to be reversible after switching off the SO2 injection.