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탄화수소에 의한 NOx 환원반응에서 저온 NOx 제거 효율 개선에 대한 연구

탄화수소에 의한 NOx 환원반응에서 저온 NOx 제거 효율 개선에 대한 연구
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Recently, diesel engines have been widely employed in Europe and Korea as an alternative to the conventional gasoline engine, mainly due to the low consumption of fuel and the low emission of CO2. However, the two primary pollutants of nitrogen oxides (NOx) and particulate matter (PM), which are extremely deleterious to environment and human health, are emitted from diesel engines under highly lean operating condition. Particularly, the emission regulation of NOx is more stringent around the world to prevent acid rain and smog and protect the depletion of the ozone layer around earth. The selective catalytic reduction of NOx by hydrocarbons (HC-SCR) is one of the most promising technologies for removing NOx from automotive engine exhaust under lean conditions, free from the drawbacks of the commercially available deNOx technologies such as urea-SCR and NOx storage reduction (NSR). HC-SCR technology uses HCs commonly included in the exhaust gas or fuel as a reductant for removing NOx. However, the deNOx activity of the HC-SCR technology, particularly its low-temperature activity, is still insufficient for its commercial application to the diesel engine. For the enhancement of the low-temperature deNOx efficiency over a Ag/Al2O3 catalyst, a variety of efforts such as the optimization of operating condition, the search for an effective reductant, and the development of suitable catalysts are still needed for its commercial application to diesel after-treatment system. In the present study, three approaches including the addition of H2 into the feed gas stream, the use of new reductant (monoethanolamine, MEA), and the modification of Ag/Al2O3 catalyst by using additives (AlF3, F, Nb, and B) have been extensively investigated to enhance the low-temperature deNOx activity of HC-SCR over the Ag/Al2O3 catalyst for its practical application to the diesel engine. The technical feasibility for enhancing the low-temperature deNOx activity of HC-SCR over the Ag/Al2O3 catalyst has been now successfully demonstrated. The enhanced deNOx performance of the Ag/Al2O3 catalyst by addition of H2 has been investigated for reducing NOx from diesel engine using a mixture of simulated diesel and ethanol as the reductant. The addition of H2 to the feed gas stream dramatically shifted the NOx reduction activity to lower temperatures, regardless of the Ag loading. Particularly, the Ag/Al2O3 catalyst with 3.8 wt.% Ag loading exhibited the highest NOx reduction activity in the temperature region below 300 oC under the standard reaction condition [(NO + HCs + O2 + H2O)/He flow] with or without H2. As H2 concentration increased in the feed gas stream from 0 to 1%, the low-temperature NOx reduction activity was remarkably enhanced. The addition of H2 promoted the oxidation of both NO and HCs, resulting in an increased formation of nitrates, enolic, and isocyanate species regarded as reaction intermediates. Result of the UV-vis indicated that H2 promoted the transformation of Ag+ to Agnδ+ and Ag0 on the surface of the Ag/Al2O3 catalyst, while those of H2-TPR, O2-TPD, in situ FTIR, and TGA revealed the formation of highly mobile and reactive oxygen species such as AgO, Ag2O3, Ag(OH)2, and Ag+(O2)- under the (O2 + H2) flow. A reaction mechanism was proposed to elucidate the role of H2 in the enhancement of deNOx performance of HC-SCR in view of morphological, chemical, and kinetic changes of the Ag/Al2O3 catalytic system. An autocatalytic kinetic synergism between the conventional HC- and NH3-SCR reactions has been observed during lean-NOx reduction with a bifunctional reductant (an amino alcohol) over a Ag/Al2O3 catalyst, resulting in a significant enhancement of the deNOx activity. MEA employed as the bifunctional reductant produced effective reductants such as H2, NH3, and isocyanate species, thereby transforming the single-functional Ag/Al2O3 catalyst for HC-SCR technology into a dual-functional catalyst for both the HC- and NH3-SCR technologies. The production of those highly reactive intermediate reductants by MEA initiated the autocatalytic deNOx process, while the kinetic synergy in the deNOx catalysis can be attributed to the formation of the (OH…N) hydrogen bonds in and among MEA molecules. Based on steady-state kinetic data, possible reaction pathways have been proposed for the MEA-SCR over the Ag/Al2O3 catalyst, and a method for the further improvement in deNOx activity has been demonstrated in a multiple-bed reactor system. For enhancing the low-temperature deNOx activity of HC-SCR, the Ag/Al2O3 catalysts modified by a variety of the catalyst additives such as B, Nb, F, and AlF3 have been particularly examined to increase the catalyst surface acidity hopefully promoting the decomposition of the reductant and/or product in the low temperature region. Among the catalysts modified, the Ag/AlF3/Al2O3 catalyst showed the highest deNOx activity and hydrothermal stability within the wide operating temperature window examined in the present study. Especially, the NOx reduction to N2 of the Ag(3.8)/AlF3(F: 2.7)/Al2O3 catalyst in the low temperature region below 400 oC was higher than that of the dual-bed reactor system [Ag(3.8)/Al2O3 + Cu(3.1)-ZSM5], while its formation of NH3 was negligible similar to the dual-bed reactor system. The addition of AlF3 to the Ag/Al2O3 catalyst promoted the oxidation of NH3 to N2, mainly due to the increase of surface acidity (NH3-TPD and pyridine-IR), indicating that the alteration of the catalyst surface acidity is one of the controlling factors for the enhanced NOx reduction to N2. Results of the UV-vis and XPS revealed that AlF3 promoted the alteration of Ag0 to Ag+ on the surface of the Ag/Al2O3 catalyst, while those of O2 chemisorption and H2-TPR showed the increase of Ag dispersion on the catalyst surface. The formation of Ag–(CN)x and Ag–NCO species recognized as the reaction intermediates for the reduction of NOx to N2 was observed by using in situ FTIR. Moreover, the formation of NH4+ species as a direct evidence of the NH3 oxidation to N2 was identified on the surface of the Ag(3.8)/AlF3(F: 2.7)/Al2O3 catalyst. Based on the observations obtained from XPS, 19F NMR, and FTIR, the surface structure of the Ag/AlF3/Al2O3 has been schematically illustrated. AlF3 locates on the type I and III of Al–OH groups on the catalyst surface forming terminal aluminum oxyfluoride (–OAlF2), and then Ag may be impregnated onto the type II of Al–OH groups. Reaction pathways for the HC-SCR over the Ag/AlF3/Al2O3 catalyst have been postulated and the multirole of AlF3 in the enhancement of the low-temperature deNOx activity on the basis of the formation of the reaction intermediates as well as the increase of the surface acidity and the active Ag site may be clearly understood. In the present study, the technical feasibility for enhancing the low-temperature deNOx performance of HC-SCR over the Ag/Al2O3 catalyst has been successfully proved by three approaches
the addition of H2 into the feed gas stream, the use of new reductant (MEA), and the modification of Ag/Al2O3 catalyst (Ag/AlF3/Al2O3). These approaches will provide a research guideline for developing a commercial HC-SCR technology to remove NOx from the diesel engine as a viable alternative to the existing deNOx technologies such as the current urea-SCR and NSR process.
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