On the promotion of Ag-ZSM-5 by cerium for the SCR of NO by methane

The promotion of Ag-ZSM-5 by cerium for the selective catalytic reduction ( SCR) of NO with methane in the presence of an excess of oxygen was studied in this work. Ce-Ag-ZSM-5 catalyzes the CH4-SCR of NOx over the temperature range of 450-600 degrees C. Rate measurements of NO reduction to N-2 under SCR conditions and of the competing CH4 oxidation by O-2 (CH4 combustion) show that incorporation of a small amount (1-1.5 wt%) of cerium into Ag-ZSM -5 enhances the activity and selectivity of the latter for the SCR reaction . Catalysts with low Ag exchange levels (<50%) were found to be more select ive than those with high Ag loading, the latter being more active for CH4 c ombustion. To examine the role of silver and cerium in the Ce-Ag-ZSM-5 syst em, the kinetics of the SCR reaction were complemented by studies of NO oxi dation to NO2, SCR of NO2 by CH4, and CH4 combustion as well as catalyst ch aracterization by STEM/EDS, HRTEM/EDS, XPS, and UV-VIS DRS techniques. Silv er existed mainly as dispersed Ag+ ions in low Ag-content (Ag/Al <0.5-0.6) Ce-Ag-ZSM-5 samples, while nanoparticles of silver of similar to 10 nm size were also found on the surface of high Ag-content samples. The dispersed A g+ state was more active for the SCR reaction, while silver particles more effectively catalyzed the methane combustion reaction. Cerium was present i n both the 3+ and 4+ oxidation states. The major functions of cerium sugges ted by the data presented in this paper are to catalyze the oxidation of NO to NO2, suppress the CH4 combustion, and stabilize silver in dispersed Ag state. (C) 1999 Academic Press.