| 1. |
- Härelind, Hanna, 1973, et al.
(författare)
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Silver-alumina catalysts for lean NOx reduction
- 2015
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Ingår i: Presented at the 10th International Congress on Catalysis and Automotive Pollution Control, CAPoC10, Brussels, Belgium, October 28-30, 2015..
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 2. |
- Härelind, Hanna, 1973, et al.
(författare)
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Silver-alumina catalysts for lean NOx reduction
- 2014
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Ingår i: Presented (oral) at the 8th International Conference on Environmental Catalysis (ICEC), Asheville, North Carolina, USA, August 24-27, 2014..
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 3. |
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| 4. |
- Männikkö, Marika, 1970, et al.
(författare)
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Characterization of the active species in the silver/alumina system for lean NOx reduction with methanol
- 2016
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 267, s. 76-81
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Tidskriftsartikel (refereegranskat)abstract
- Low-temperature activity and selectivity for lean NOx reduction over silver/alumina is strongly dependent on the composition of surface silver species. This motivates the present investigation of the role of the supported silver species for the lean NOx reduction with methanol. The catalyst samples, with different composition of silver species, are characterized by temperature programmed desorption of ammonia (NH3-TPD), temperature programmed reduction with hydrogen (H-2-TPR) and temperature programmed desorption with NO (NO-TPD) in oxygen excess. The small differences in acidity do not significantly influence the lean NOx reduction with methanol. However, comparison of results from H-2-TPR experiments with previous characterization by UV-vis spectroscopy shows that fairly small silver species are reduced by hydrogen, possibly small silver clusters. These small silver species are likely, in addition to others, involved in the lean NOx reduction reactions. NO-TPD experiments, in the presence of oxygen and hydrogen, reveal a shift in temperature for one of the desorption peaks from the different samples. This peak is likely related to the shift in temperature for NOx reduction during methanol-SCR conditions for the compared samples. (C) 2016 Elsevier B.V. All rights reserved.
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| 5. |
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| 6. |
- Männikkö, Marika, 1970, et al.
(författare)
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Effect of silver loading on the lean NOx reduction with methanol over Ag-Al2O3
- 2013
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Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1572-9028 .- 1022-5528. ; 56:1-8, s. 145-150
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Tidskriftsartikel (refereegranskat)abstract
- 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 . © 2013 Springer Science+Business Media New York.
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| 7. |
- Männikkö, Marika, 1970
(författare)
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Lean NOx reduction over supported silver catalysts with methanol as reductant - Influence of silver loading, support material, hydrogen addition and C/N molar ratio
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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 °C) with maximum activity at relatively lowtemperature (300 °C). 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.
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| 8. |
- Männikkö, Marika, 1970
(författare)
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Lean NOx reduction with methanol over supported silver catalysts
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 9. |
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| 10. |
- Männikkö, Marika, 1970, et al.
(författare)
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Methanol Assisted Lean NOx Reduction Over Ag-Al2O3-Influence of Hydrogen and Silver Loading
- 2015
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Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1572-9028 .- 1022-5528. ; 58:14-17, s. 977-983
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Tidskriftsartikel (refereegranskat)abstract
- The influence of hydrogen and silver loading on lean NOx reduction with methanol over Ag-Al2O3 are investigated by flow-reactor experiments, H-2-TPR, TEM and XPS. Reduction of silver at low temperatures over Ag-Al2O3 with high silver loading, suggests involvement of silver species/particles that are large (which is supported by the TEM analysis) and/or loosely bound to the support. The silver species reduced by hydrogen cannot be directly associated with the activity for NOx reduction under methanol-SCR conditions. Furthermore, the activity for NOx reduction increases slightly at low temperatures when hydrogen is added to the feed. However, also without co-fed hydrogen, a high low-temperature activity is achieved for methanol assisted lean NOx reduction over Ag-Al2O3.
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| 11. |
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| 12. |
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| 13. |
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| 14. |
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| 15. |
- Männikkö, Marika, 1970, et al.
(författare)
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Role of hydrogen formation and silver phase for methanol-SCR over silver/alumina
- 2015
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 258, s. 454-460
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Tidskriftsartikel (refereegranskat)abstract
- Formation of reaction products over silver/alumina during lean NOx reduction with methanol was studied, with and without addition of hydrogen to the feed. The silver phase in silver/alumina was analyzed by UV–vis spectroscopy in order to elucidate the influence of silver loading and different gas environments. Formation of molecular hydrogen and reduction of supported silver species were observed during selective catalytic reduction (SCR) with methanol, without hydrogen in the feed. This availability of hydrogen and the reduction of silver are suggested to contribute to the high low-temperature activity connected to oxygenates (like alcohols) as reducing agents for NOx.
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| 16. |
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| 17. |
- Männikkö, Marika, 1970, et al.
(författare)
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Selective catalytic reduction of NOx with methanol over supported silver catalysts
- 2012
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 119-120, s. 256-266
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Tidskriftsartikel (refereegranskat)abstract
- Methanol is a potential renewable fuel for the transport sector and is thus interesting to study as reducing agent for NOx in hydrocarbon assisted selective catalytic reduction (HC SCR). The effect of catalyst composition on the lean NOx reduction during methanol SCR conditions was investigated. In particular silver supported on alumina and ZSM-5 were in focus and parameters that can improve the NOx reduction performance, i.e. the C/N molar ratio and the addition of hydrogen, were specifically studied. Five catalysts were prepared (H ZSM 5, Ag/H ZSM 5, Pd/Ag/H ZSM-5, γ-Al2O3 and Ag-Al2O3) and compared in flow reactor experiments. The Ag-Al2O3 (2 wt% Ag, sol-gel) sample was found to give higher NOx reduction compared to the other tested samples, where important factors are suggested to be the support material, the preparation method and the Ag loading. The NOx reduction over Ag Al2O3 with methanol is proposed to proceed by adsorbed nitrates or nitrites (or gas phase NO2) reacting with adsorbed acetate or formaldehyde like species forming adsorbed R NO2 or R ONO, with subsequent conversion into NCO, -CN, R-NH2 or NH3, with final desorption of N2 and COx (or HCHO). The addition of hydrogen and an increase of the supplied methanol concentration resulted in an increased NOx reduction mainly over the Ag-Al2O3 sample, where the addition of hydrogen also extended the active temperature interval towards lower temperatures. Another effect of the addition of hydrogen was formation of more oxidized carbon containing reaction products. The Ag-Al2O3 catalyst prepared by the sol-gel method, including freeze-drying of the formed gel, is concluded to be the most promising candidate of the tested catalysts for methanol-SCR.
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| 18. |
- Männikkö, Marika, 1970, et al.
(författare)
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Silver/alumina for methanol-assisted lean NOx reduction - on the influence of silver species and hydrogen formation
- 2016
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 180, s. 291-300
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Tidskriftsartikel (refereegranskat)abstract
- High low-temperature activity for lean NOx reduction can be achieved for silver/alumina by using an oxygenated reducing agent. In this system the catalytic reactions, including the H2 formation previously observed during methanol-SCR conditions, are strongly dependent on the composition of surface silver species. With the aim to increase the understanding of the role of supported silver species in combination with the methanol-SCR reactions, catalysts with the same silver loading but different composition of silver species are prepared by utilizing different preparation methods. The supported silver species are characterized by UV-vis spectroscopy and transmission electron microscopy, while the catalytic performance for lean NOx reduction with methanol is investigated in flow reactor experiments and surface species studied by DRIFT spectroscopy. The results indicate that hydrogen atoms are abstracted mainly from the methyl group during the conversion of surface species formed from methanol. The hydrogen atoms could contribute to reduction of the catalyst or affect the catalytic reactions in other ways, before they react to form H2 or H2O. Here, more H2O is formed over the samples containing more silver nanoparticles. The released hydrogen atoms are suggested to explain the high NOx reduction at low temperature associated with oxygenated reducing agents, rather than the subsequently formed gaseous H2. Furthermore, the results show that lean NOx reduction with methanol is determined by silver species in the size range from small silver clusters to small silver nanoparticles (
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