NH3 (Urea)에 의한 NOx 환원 반응을 위한 저온 촉매 연구
- NH3 (Urea)에 의한 NOx 환원 반응을 위한 저온 촉매 연구
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- The diesel engine has been regarded as one of the most promising vehicle propulsion technologies due to its high fuel economy and low CO2 emission to atmosphere during operation. From an environmental point of view, however, the emissions of atmospheric pollutants including nitrogen oxides (NOx) and particulate matter (PM) remain as a critical environmental issue. In particular, the effective reduction of NOx from diesel engine exhaust has become a primary concern for the commercial implementation of the modern energy-efficient diesel engine technology in view of ever-tightening emission regulations for the recent automotive industry.
Among the promising NOx reduction technologies currently available, the selective catalytic reduction of NOx by urea (Urea/SCR) is the most proven and available technology for removing NOx from automotive engine exhaust under lean condition, free from the drawbacks of other deNOx technologies such as HC/SCR and NOx storage reduction (NSR). CuZSM5, V2O5-WO3/TiO2 and FeZSM5 catalysts have been recognized as the representative catalysts employed for the Urea/SCR technology. However, the strong demand for developing a new catalyst to enhance the low-temperature activity of Urea/SCR still remains, since the diesel exhaust temperature may further decrease when the advanced combustion technology including homogeneous charge compression ignition (HCCI) is employed.
In the present study, a series of promising low-temperature SCR catalysts including Mn/TiO2, Mn-Fe/ZSM5 and CuSSZ13 have been extensively examined for their successful implementation into the commercial Urea/SCR system. The deNOx performance of those catalytic systems has been systematically investigated under the realistic SCR reactor operating conditions to evaluate their feasibility to be applied to diesel after-treatment system. In addition, the physicochemical characteristics of the catalysts employed have been extensively examined by a variety of the catalyst characterization techniques including XRD, XPS, TEM, TPD, ESR and XAFS in order to understand and elucidate their high catalytic activity as well as deactivation behavior.
Mn/TiO2, recognized as a representative low-temperature SCR catalyst, have been prepared with respect to the Mn content and preparation method (sol-gel and impregnation). The deNOx activity of the s-Mn/TiO2 catalyst prepared by sol-gel method was much superior, particularly in the low temperature region, to the i-Mn/TiO2 catalyst prepared by conventional impregnation method. Based on the XRD and TEM results, Mn is basically incorporated into the matrix of Ti and becomes a structural component of s-MnO2/TiO2 mixed oxide, whereas by impregnation method it simply exists on the surface of i-Mn/TiO2. The well dispersed Mn on the s-Mn/TiO2 catalyst appeared to be the primary cause for its high deNOx performance. The addition of Fe onto the Mn/TiO2 catalyst improves the NO removal activity by NH3 in a wide operating temperature window, including the low temperature region less than 250 oC, regardless of the catalyst preparation methods. Including NO2 (NO2/NO=1) in the feed gas stream further enhances the low temperature activity of the Mn-Fe/TiO2 catalysts.
In order to overcome the drawbacks of the previous Mn-based SCR catalysts including Mn/TiO2, a new generation of Mn-Fe/ZSM5 catalyst has been developed for removing NOx from diesel engine exhausts. The excellent low-temperature SCR activity and N2 selectivity of Mn-Fe/ZSM5 catalyst demonstrated in comparison with other representative SCR catalysts including CuZSM5 and a CuSSZ13-based commercial catalyst (COM). Based on the XPS and NH3-TPD results, the well-dispersed Mn and the high NH3 adsorption capacity have been identified as the primary reason for the high deNOx activity over Mn-Fe/ZSM5. To determine the possibility of Mn-Fe/ZSM5 to be implemented in diesel after-treatment system, its hydrothermal stability and durability against HC, alkali metals and SO2 have been also examined and compared to those of the CuZSM5 and COM catalysts.
The hydrothermal stability of the Mn-Fe/ZSM5 catalyst has been improved upon the increase of Mn content and/or the addition of Er, the latter of which helps to stabilize the dispersion of Mn on the catalyst surface during aging. The deNOx activity of the Mn-Fe/ZSM5 and its Er-promoted counterpart was less affected by C3H6 poisoning, compared to the CuZSM5 and COM catalysts, mainly due to the excellent C3H6 oxidation activity of MnO2. No poisoning of the Mn-based ZSM5 and CuZSM5 catalysts has been observed upon the addition of 2wt.% of K+ and Ca2+ onto their surface, primarily attributed to the high NH3 adsorption capacity of the ZSM5 support, whereas the COM catalyst has been severely deactivated by the deposition of K+ and Ca2+.
The deNOx activity of the Mn-based ZSM5 catalyst, particularly the Er-promoted one, was less affected by SO2 compared to the CuZSM5 and COM catalysts, although it was hardly regenerated at 500 oC. Formation of MnSO4 on the catalytic surface appears to be the primary cause for the deactivation of the Mn-based ZSM5 catalysts in the presence of SO2 in the feed gas stream. The sulfur tolerance of the Mn-based ZSM5 catalysts has been significantly improved upon the addition of NO2 into the feed gas stream due to the “fast-SCR” reaction and/or the combination with SO2 trap in the front bed which contains a strong basic property to collect the acidic SO2.
A kinetic study for the selective catalytic reduction of NO by NH3 over the Mn-Fe/ZSM5 catalyst has been conducted using a packed-bed flow reactor system. According to the results of NO- and NH3-TPD experiments, the reaction rate expressions based upon a dual site Langmuir-Hinshelwood (LHHW) mechanism have been derived for the two main reactions including the NO reduction and NH¬3 oxidation. A kinetic model developed for the Mn-Fe/ZSM5 catalyst well predicted the experimental data with respect to the reaction temperature and reactor SVs, which may provide a guideline for designing the SCR reactor in the diesel after-treatment system.
In order to understand the exceptionally strong thermal stability of CuSSZ13, currently regarded as the most promising low-temperature SCR catalyst, the catalytic activity and chemical properties of a series of CuSSZ13 catalysts with respect to the Cu contents and Si/Al ratios have been investigated before and after hydrothermal aging. The decline of the deNOx performance after aging became more severe as the Cu loading and Si/Al ratio increased. Results of ESR, H2-TPR and DRIFT studies indicated that the D6R sites are exchanged first by Cu2+ ions (α species) up to their accommodation capacity, followed by the occupation of the CHA sites (β species) with the increase of the Cu contents and Si/Al ratios. The stability of Cu2+ ions on D6R appeared to be higher compared to that in CHA cage, due to its high coordination with nearby framework oxygen. The isolated Cu2+ ions in CHA cage may agglomerate more readily than those on D6R due to their less stable nature, leading to the formation of CuOx. The CuOx may grow to destroy the zeolite cage and channel, resulting in the collapse of the SSZ13 structure, which is believed to be the primary cause for the hydrothermal deactivation of the CuSSZ13 catalyst. Consequently, the deactivation of CuSSZ13 upon aging depends on the distribution of Cu2+ ions, and then the strong hydrothermal stability of CuSSZ13 may be mainly attributed to the presence of Cu2+ on D6R.
Based on the present study, the feasibility of three kinds of low-temperature SCR catalysts including Mn/TiO2, Mn-Fe/ZSM5 and CuSSZ13 to be directly applied to the commercial Urea/SCR technology has been comprehensively determined. The deNOx activity over those catalysts examined under a variety of reaction conditions and their physicochemical properties extensively studied may provide a research guideline for developing a new SCR catalyst, more appropriate to be implemented in the diesel after-treatment system.
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