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◦C. Rate measurements of NO reduction to N2 under SCR conditions and of the competing CH4 oxidation by O2 (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 selective than those with high Ag loading, the latter being more active for CH4 combustion. To examine the role of silver and cerium in the Ce–Ag–ZSM-5 system, the kinetics of the SCR reaction were complemented by studies of NO oxidation to NO2, SCR of NO2 by CH4, and CH4 combustion as well as catalyst characterization by STEM/EDS, HRTEM/EDS, XPS, and UV-VIS DRS techniques. Silver 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 ∼10 nm size were also found on the surface of high Ag-content samples. The dispersed Ag+ state was more active for the SCR reaction, while silver particles more effectively catalyzed the methane combustion reaction. Cerium was present in both the 3+ and 4+ oxidation states. The major functions of cerium suggested 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