Role of the different copper species on the activity of Cu/zeolite catalysts for SCR of NO_x with NH3

摘要

The SCR of NO_x with NH3 has been studied by using different Cu zeolite catalysts, prepared both with ZSM5 and BETA zeolite supports by ionic exchange or by impregnation. The catalysts were characterized by ICP-AES, N2 adsorption at -196°C, XRD, TEM, XPS and H2-TPR. The catalysts characterization confirmed the presence of different Cu(II) species on all catalyst (CuO and Cu(II) exchanged on tetrahe-dral and octahedral positions of the zeolites framework). Clear evidences of Cu(I) or Cu(0) species were not obtained. CuO was more abundant in high copper-content catalysts and in ZSM5 catalysts, due to its lower ionic exchange capacity, while isolated Cu(II) ions are more abundant in low copper-content catalysts and in BETA catalysts. It was concluded that CuO catalyzes the oxidation of NO to NO2, and this favors the reduction of NO_x at lower temperature (the NH3-NO2 reaction is faster than the NH3-NO reaction because NO2 is much more oxidizing than NO), whereas isolated Cu(II) ions maintain high NO_x conversion at high temperatures.