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 CH(4)-SCR of NO(x) over the temperature range of 450-600 degrees C. Rate measurements of NO reduction to N(2) under SCR conditions and of the competing CH(4) oxidation by O(2) (CH(4) 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 CH(4) 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 NO(2), SCR of NO(2) by CH(4), and CH(4) 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 similar to 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 NO(2), suppress the CH(4) combustion, and stabilize silver in dispersed Ag+ state. (C) 1999 Academic Press.