Effects of different Zr/Ti ratios on NH3–SCR over MnOx/ZryTi1-yO2: Characterization and reaction mechanism

Abstract

A series of ZryTi1-yO2 (y=0.1, 0.4, 0.5, 0.6 and 0.9) are prepared by co-precipitation method and Mn(NO3)2 is impregnated on them to achieve a 5wt% Mn loading. The catalytic activity of MnOx/ZryTi1-yO2 for the selective reduction of NO with ammonia (NH3–SCR) in the range of 80–360°C is evaluated by a fixed-bed reactor. The prepared catalysts are characterized by N2 adsorption-desorption, XRD, XPS, H2–TPR, NH3–TPD and in situ DIRFTS. The results showed that different Zr/Ti ratios influence the NH3–SCR activity of MnOx/ZryTi1-yO2, but they have high N2 selectivity at low temperature and similar changes in the range of 80–360°C. MnOx/Zr0.6Ti0.4O2 achieves 80% of NO removal efficiency at 140°C and can keep 100% from 200 to 360°C, and it exhibits an excellent resistance of H2O at 200°C and a certain tolerance to SO2. The pores of all prepared catalysts are composed of many mesopores with pore width centered at 4nm and some micropores with pore width centered at 1.75nm. MnOx/Zr0.4Ti0.6O2 has 344m2/g of BET surface area, 0.42cm3/g of total pore volume with 0.075cm3/g of micropore volume, but MnOx/Zr0.6Ti0.4O2 has the highest contents of Mn4+ (52.0%) and Oβ (32.8%) and more Lewis acid sites. Zr/Ti ratios change Mn valence and O species in MnOx/ZryTi1-yO2, and Mn4+ can promote the adsorption of NH3 and accelerates the formation of NH4NO3 to improve NO removal with NH3 by L–H mechanism.