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 forlean NOx reduction in the low-temperature regime relevant for diesel and lean-burnengines and to investigate methanol as reducing agent. The lean NOx reduction oversupported 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 andevaluated 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 influenceof the silver loading in Ag-Al2O3, increasing the silver loading (0 – 4 wt%) extends orshifts the active temperature window towards lower temperatures, at the same time asthe NOx reduction in the high-temperature region decreases. Ag-Al2O3 with 3 wt% Agshows the most promising results. It is active for lean NOx reduction in a broadtemperature interval (200 – 500 \ub0C) with maximum activity at relatively lowtemperature (300 \ub0C). This is likely owing to high amounts of small oxidized silverspecies, compared to the amount of metallic silver particles.When hydrogen is added to the methanol-SCR gas feed the most pronounced effect isseen over Ag-Al2O3, where the active temperature interval broadens towards lowertemperatures and the maximum NOx reduction increases. Furthermore, the addition ofhydrogen results in formation of gas phase species with a higher carbon oxidationstate. 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 ratiosresult in poor selectivity to N2 over Ag-Al2O3.Characterization by UV-vis spectroscopy shows no obvious differences between theAg-ZSM-5 and Ag-Al2O3 samples. Both are found to contain silver ions (Ag+) andsmall silver clusters (Agnδ+). One possible explanation for the low NOx reduction overAg-ZSM-5 is its strong acidity (observed in NH3-TPD experiments), likely promotingdehydration of methanol to formaldehyde
