Manganese–niobium mixed oxide catalyst for the selective catalytic reduction of NOx with NH3 at low temperatures

• Excellent catalytic NH3-SCR performance was obtained over the Mn2Nb1Ox catalyst.
• The addition of niobium enhanced the acidity and decreased the oxidation ability of the manganese species.
• Combination of MnOx with strong redox capability and NbOx with high acidity led to high SCR activity over the Mn2Nb1Ox catalyst.

A series of manganese-niobium mixed oxide (Mn–Nb) catalysts were prepared by homogeneous precipitation method and tested for low temperature selective catalytic reduction of NOx with NH3 (NH3-SCR). These catalysts were characterized by N2 adsorption, X-ray diffraction, H2-TPR, X-ray photoelectron spectroscopy, NOx-TPD and NH3-TPD. Among the series of Mn–Nb catalysts, Mn2Nb1Ox calcined at 500 °C exhibited the best catalytic activity. MnOx had stronger redox capability than Mn2Nb1Ox, resulting in lower N2 selectivity due to the unselective oxidation of NH3 at relatively high temperatures. The addition of Nb to MnOx enhanced the acidity, especially the Brønsted acidity, which is responsible for the low temperature SCR activity. The combination of MnOx with strong redox capability and NbOx with high acidity simultaneously led to high NOx conversion and N2 selectivity over the Mn2Nb1Ox catalyst.