Lean NOx reduction over supported silver catalysts with methanol as reductant - Influence of silver loading, support material, hydrogen addition and C/N molar ratio
The aim of this thesis is to find catalysts which show high activity and selectivity for
lean NOx reduction in the low-temperature regime relevant for diesel and lean-burn
engines and to investigate methanol as reducing agent. The lean NOx reduction over
supported silver catalysts is studied, focusing on the influence of the support material,
silver loading, addition of hydrogen and the C/N molar ratio. Alumina based (sol-gel)
and ZSM-5 based (ion-exchange) silver catalysts were prepared, characterized and
evaluated in flow reactor experiments.
Comparisons of Ag-Al2O3 (2 wt% Ag) and Ag-ZSM-5 (5 wt% Ag) during methanol-
SCR conditions show higher NOx reduction over Ag-Al2O3. In studies of the influence
of the silver loading in Ag-Al2O3, increasing the silver loading (0 – 4 wt%) extends or
shifts the active temperature window towards lower temperatures, at the same time as
the NOx reduction in the high-temperature region decreases. Ag-Al2O3 with 3 wt% Ag
shows the most promising results. It is active for lean NOx reduction in a broad
temperature interval (200 – 500 °C) with maximum activity at relatively low
temperature (300 °C). This is likely owing to high amounts of small oxidized silver
species, compared to the amount of metallic silver particles.
When hydrogen is added to the methanol-SCR gas feed the most pronounced effect is
seen over Ag-Al2O3, where the active temperature interval broadens towards lower
temperatures and the maximum NOx reduction increases. Furthermore, the addition of
hydrogen results in formation of gas phase species with a higher carbon oxidation
state. Increasing the C/N molar ratio enhances the lean NOx reduction over Ag-Al2O3,
while the activity of the ZSM-5 based samples is unchanged. Too high C/N ratios
result in poor selectivity to N2 over Ag-Al2O3.
Characterization by UV-vis spectroscopy shows no obvious differences between the
Ag-ZSM-5 and Ag-Al2O3 samples. Both are found to contain silver ions (Ag+) and
small silver clusters (Agnδ+). One possible explanation for the low NOx reduction over
Ag-ZSM-5 is its strong acidity (observed in NH3-TPD experiments), likely promoting
dehydration of methanol to formaldehyde.
C/N molar ratio
lean NOx reduction