Lean NOx reduction with methanol over supported silver catalysts

The oxygen rich and relatively cold exhaust gases from fuel-efficient combustion engines bring challenges for the reductive aftertreatment of NOx. In this context selective catalytic reduction over silver/alumina has shown positive results, especially with oxygenated reducing agents. In the present work, methanol, which is considered a promising renewable fuel, is evaluated as reducing agent for NOx over supported silver catalysts. The aim is to gain increased understanding of the catalytic processes, especially focusing on low-temperature activity and selectivity. The role of the supported silver species is studied, together with the influence of the support material and the gas composition, including the formation of hydrogen. For this purpose model catalysts were prepared, characterized and evaluated in flow-reactor experiments.The results show that the low-temperature activity for lean NOx reduction with methanol over silver/alumina is highly dependent on the composition of supported silver species and their interplay with adsorbed and gas phase species. This work provides new insights in the role of small silver species for the selectivity to N2 and the importance of somewhat larger silver species for catalytic activity at low temperature. These results were achieved by comparing silver/alumina samples of similar silver loading, but with different composition of silver species, as revealed by UV-vis spectroscopy, TEM and H2-TPR. Furthermore, comparisons of different support materials during methanol-SCR conditions, show a higher NOx reduction for silver supported on alumina than on ZSM-5. The NOx reduction over the alumina based catalyst is found to improve when the C/N ratio is moderately increased. The influence of the silver loading was studied and the results show that sol-gel silver/alumina with 3 wt% silver gives a high NOx reduction with methanol in a broad temperature interval, relevant for lean exhaust gases. Moreover, the observed formation of H2 from methanol gives an indication of that the high low-temperature activity associated with oxygenated reducing agents may be connected to the abstraction of hydrogen from the oxygenate, studied here for methanol by DRIFT spectroscopy. One effect of hydrogen, observed in the present work by UV-vis spectroscopy, is reduction of silver species

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

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

Effect of silver loading on the lean NOx reduction with methanol over Ag-Al2O3

The influence of silver loading on the lean NO x reduction activity using methanol as reductant has been studied for alumina supported silver catalysts. In general, increasing the silver loading (0-3 wt%), in Ag-Al 2 O 3 , shifts or extends the activity window, for lean NO x reduction towards lower temperatures. In particular Ag-Al 2 O 3 with 3 wt% silver is active for NO x reduction under methanol-SCR conditions in a broad temperature interval (200-500 C), with high activity in the low temperature range (maximum around 300 C) typical for exhaust gases from diesel and other lean burn engines. Furthermore, increasing the C/N molar ratio enhances the reduction of NO x . However, too high C/N ratios results in poor selectivity to N 2 . \ua9 2013 Springer Science+Business Media New York