To enhance the chlorine tolerance of copper ion exchanged mordenite (CuHM) catalyst under NH3-SCR reaction condition when HCl exists in the feed gas stream, mainly from a waste incinerator, a second metal, particularly cerium, has been additionally exchanged onto the catalyst along with Cu. It prevents Cu(II) ion from being evaporated from the catalyst surface by HCl and significantly improves the catalyst deactivation. The loss of copper from CeCuHM catalyst has been observed at a reaction temperature of 450 degrees C, while that from CuHM catalyst without Ce begins at 350 degrees C. Ce simply stabilizes Cu(II) ion on the catalyst surface, generally recognized as an active reaction site for the present reaction system. X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), synchrotron radiation X-ray diffraction (SR-XRD), and X-ray photoelectron spectroscopy (XPS) have been employed to identify the stabilization of Cu(II) ion on CuHM catalyst by Ce. As confirmed by SR-XRD, the lattice parameters of the unit cell of CeCuHM catalyst are less altered than those of CuHM catalyst without Cc during the course of the catalyst deactivation. Similarly, as identified by XPS, Cu(II) on the surface of CeCuHM catalyst is less transformed to Cu(I) ions. Ce ions and Ce-Cl compounds formed on the catalyst surface may decrease the average electronegativity of the catalyst and increase the charge of the lattice oxygen in the zeolite framework, resulting in an enhancement of the stability of the Cu-O bond in the present catalytic system. (C) 2006 Elsevier B.V. All rights reserved.