On the promotion of Ag-ZSM-5 by cerium for the SCR of NO by methane
- Tufts Univ., Medford, MA (United States). Dept. of Chemical Engineering
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{sub 4}-SCR of NO{sub x} over the temperature range of 450--600 C. Rate measurements of NO reduction to N{sub 2} under SCR conditions and of the competing CH{sub 4} oxidation by O{sub 2} (CH{sub 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{sub 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{sub 2}, SCR of NO{sub 2} by CH{sub 4}, and CH{sub 4} combustion as well as catalyst characterization by STEM/EDS, HRTEM/EDS, XPS, and UV-VIS DRS techniques. Silver existed mainly as dispersed Ag{sup +} ions in low Ag-content (Ag/Al < 0.5--0.6) Ce-Ag-ZSM-5 samples, while nanoparticles of silver of {approximately}10 nm size were also found on the surface of high Ag-content samples. The dispersed Ag{sup +} 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{sub 2}, suppress the CH{sub 4} combustion, and stabilize silver in dispersed Ag{sup +} state.
- OSTI ID:
- 338550
- Journal Information:
- Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 2 Vol. 182; ISSN 0021-9517; ISSN JCTLA5
- Country of Publication:
- United States
- Language:
- English
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