NO removal by CH4 on Co-NaX-CO and Ag-NaX catalysts in a dual-bed system

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 500 °C. 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 O2 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 N2 by CH4 in the presence of excess O2 and H2O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N2 in a deNO bed at 523 K and a deNO2 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.