Lean NOx reduction over supported silver catalysts with methanol as reductant - Influence of silver loading, support material, hydrogen addition and C/N molar ratio

Licentiate thesis, 2012

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.