NO removal using CH4 as a reductant in a dual-bed system has been investigated with Co-NaX and Ag-NaX catalysts, which were prepared by Co2+-, Ag+-ion exchange into zeolite NaX, respectively, and activation for 5 h at 500degreesC. The experimental result has been compared with that of a Co-NaX-CO catalyst, additionally pre-treated under CO flow for the Co-NaX catalyst. The cobalt crystal structure of a Co-NaX-CO catalyst is Co3O4, which promotes NO oxidation to NO2 by excess O-2 at a low temperature (523 K). The mechanical mixture of Co-NaX-CO and Ag-NaX catalysts shows a synergy effect on NO reduction to N-2 by CH4 in the presence of excess O-2 and H2O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N-2 in a deNO bed at 523 K and a deNO(2) bed at 423 K, which are relatively lower than the reaction temperatures for common SCR systems, still remained at 67% even in a H2O 10% gas mixture after 160 min. (C) 2003 Elsevier B.V. All rights reserved.