탄화수소에 의한 NOx의 선택적 촉매환원반응 및 Ag/Al2O3 촉매의 반응속도론적 연구

Abstract

The selective catalytic reduction of NOx by hydrocarbons (HC/SCR) as reductants has been regarded as a potential alternative to the currently available urea/SCR and NSR (NOx storage and reduction) technologies for the removal of NOx from diesel engine. Among the HC/SCR catalysts proposed, Ag/Al2O3 catalyst might be one of the most promising catalysts due to its excellent NOx removal activity and selectivity under lean exhaust conditions, particularly in the high reaction temperature region above 350 oC. However, the deNOx performance of the current HC/SCR technology over the Ag/Al2O3 catalyst may not be sufficient for its commercial application to next generation vehicles including diesel engine driven automobiles. In the present study, the NOx reduction activity of Ag/Al2O3 catalyst has been systematically exmained, using a mixture of simulated diesel fuel (SD) and ethanol (E) as the reductant with the special interest for improving the low temperature deNOx performance below 350 oC. The Ag loading and catalyst operating conditions such as C1/NOx and E/SD feed ratios have been optimized in order to achieve the high deNOx performance of the Ag/Al2O3 catalyst. The optimal Ag content of the Ag/Al2O3 catalyst employed in the present catalytic study seems to be 3.8 wt.% in view of its NO-to-N2 conversion performance. When both the C1/NOx and the E/SD feed ratios were set at 4, the 3.8 wt.% Ag/Al2O3 catalyst showed the highest NOx-to-N2 conversion, 72% from 300 to 400 oC. Ammonia has been identified as the most abundant reaction intermediate over the Ag/Al2O3 catalyst under the optimized operating condition. To further enhance the deNOx performance by utilizing the NH3 formed over the Ag/Al2O3 catalyst, a dual-bed reactor system has been proposed with CuZSM5 catalyst in the rear bed consecutively following the front bed containing the Ag/Al2O3. The NOx-to-N2 conversion in this dual-bed reactor system increased up to 85% from 275 to 450 oC, mainly due to the NH¬3 oxidation to N2 by the CuZSM5 catalyst in the rear bed. The physicochemical characterization of the Ag/Al2O3 catalysts by UV-vis, TEM, H2-TPR and EELS has indicated that both ionic and metallic Ag formed on the catalyst surface play important roles for the high deNOx performance of the present catalytic system

the ionic Ag including Ag+ and Agδ+ is the active reaction site for the reduction of NOx to N2, while the metallic Ag is responsible for the partial oxidation of hydrocarbons (HCs) promoting the initiation of the HC/SCR process in the low temperature range. The optimum ionic/metallic (I/M) ratio of Ag species on Al2O3 support surface appears to be in the range of 1.4-1.7.The long-term stability and the alteration of the deNOx performance of Ag/Al2O3 and CuZSM5 catalysts upon aging were examined over both single- and dual-bed reactor systems. The initial NOx-to-N2 conversion over the Ag/Al2O3 catalyst has been maintained for 50 h, regardless of the reaction temperatures. However, the deNOx performance over the dual-bed system decreased about 10% at 275 oC after 10 h by the continuous catalyst longevity test, while the NH3 formation slightly increased, probably due to the inhibition of the NH3 oxidation to N2 reaction by hydrocarbons contained in the feed gas stream. The Ag/Al2O3 catalyst aged at 800 oC for 12 h under wet flow condition shows the lowest NOx-to-N2 conversion, 40 % in the temperature range from 300 to 350 oC. This is probably due to the alteration of the silver species on the catalyst surface by sintering. Moreover, the NOx-to-N2 conversion over the dual-bed catalytic system aged at 800 oC significantly decreased in a wide temperature range similar to that over the dual-bed system aged at 650 or 700 oC. In addition, the effect of SO2 in the exhaust gas stream from diesel engines on the deNOx performance of the single- and the dual-bed systems were also investigated at the variety of the SO2 feed concentrations (1, 5 and 15 ppm) and reaction temperatures (275, 350 and 450 oC) in a continuous deactivation test mode. When the concentration of SO2 increased from 1 to 15 ppm, the NOx-to-N2 conversion over the Ag/Al2O3 catalyst drastically decreased from 57 to 15% at 275 oC, mainly due to the sulfur poisoning of the active reaction sites for the reduction of NOx. The Ag/Al2O3 catalyst has been also deactivated by SO2 at 350 oC as the SO2 concentration increased. However, the NOx-to-N2 conversion over the Ag/Al2O3 catalyst increased by as much as ~ 20% at 450 oC, as the SO2 concentration increased from 0 to 15 ppm. This may be due to the moderation of the total oxidation reaction of hydrocarbons by SO2 at the high reaction temperature. The effects of SO2 feed concentration on the sulfur tolerance of both Ag/Al2O3 and CuZSM5 catalysts in the dual-bed system have been also investigated. The deactivation trend of the dual-bed system is similar to that over the Ag/Al2O3 catalyst at the identical reaction temperature (275, 350 or 450 oC) with respect to the SO2 feed concentration. The alteration of the characteristics of the Ag/Al2O3 and CuZSM5 catalysts were also examined by UV-vis, TG/DTA and SO2-TPD to determine the causes for the catalyst deactivation by sintering and sulfur poisoning. As the aging temperature increased from 650 to 800 oC, the intensities of the UV-vis spectra assigned to ionic Ag decreased, while the intensities of those to metallic Ag significantly increased. The primary cause for the catalyst deactivation by the hydrothermal aging may be attributed to the significant increase of the amount of the metallic Ag formed on the catalyst surface upon aging. The result of TG/DTA reveals that Al2(SO4)3 may be one of the possible deactivating agent formed on the catalyst surface, particularly on the surface of Ag/Al2O3 catalyst poisoned by SO2. Two distinct SO2 desorption peaks over the poisoned Ag/Al2O3 catalyst appear at 600 and 940 oC of the desorption temperatures by SO2-TPD. The high temperature peak at 940 oC may be attributed to the decomposition of Al2(SO4)3, whereas Ag2SO4 can be decomposed to SO2 from 450 to 800 oC. In addition, a primary desorption peak over the CuZSM5 catalyst has been also observed in the temperature range from 260 to 480 oC, probably due to the decomposition of CuSO4 also formed on the catalyst surface upon sulfur poisoning. To identify the commercial applicability of the catalytic system developed in the present study, both single- and double-layer monoliths washcoated with Ag/Al2O3 and CuZSM5 catalysts were prepared and tested in the single- and the dual-bed reactor systems to determine their NOx reduction activity. The NOx-to-N2 conversion as well as the formation of NH3 over the single-bed reactor containing a single-layer Ag/Al2O3 monolith was nearly identical to that over the pellet type Ag/Al2O3 catalyst. On the other hand, the deNOx performance over the double-layer monolith with Ag/Al2O3 catalyst in the top layer and CuZSM5 catalyst in the bottom layer was higher than that over the single-bed reactor containing the Ag/Al2O3 single-layer monolith in the temperature region above 350 oC, while the low temperature activity over the double-layer monolith reactor is much lower than that over the single-layer Ag/Al2O3 monolith reactor. In order to simulate and optimize the deNOx performance of the oxygenated hydrocarbon (OHC)/SCR process including Ag/Al2O3 catalyst, a detailed reaction kinetics for the selective catalytic reduction of NOx over Ag/Al2O3 catalyst with ethanol as the reductant has been developed. It may provide a guideline for developing the reaction kinetics for simulated diesel fuel (SD) and ethanol (E)/SCR process proposed in the present study. Ethanolamine (HOCH2CH2NH2), acetaldehyde (CH3CHO) and ammonia (NH3) have been identified as the primary reaction intermediates for the progress of OHC/SCR reaction. The rate expressions based upon LHHW mechanism were derived with the surface reaction as the rate determining step.Results of numerical simulation of the one-dimensional pseudo-homogeneous reactor model have demonstrated the capability of the model, describing reasonably well the general trends of the experimental data such as the conversions of NO and C2H5OH as well as the formation of N2, NH3, CO, CH3CHO, CO2, N2O and HOCH2CH2NH2 during the ethanol/SCR process with respect to the reaction temperature, reactor space velocity and C1/NOx feed ratio covered in the present study. After successful validation of the reactor model, the axial concentration profiles within the reactor may be used for the optimal design of a dual-bed catalytic system to enhance the deNOx performance of the ethanol/SCR by fully utilizing the reaction intermediates formed in the rear bed following the Ag/Al2O3 catalyst in the front bed. The methodology developed in the present study may provide useful guidance on the design of an OHC/SCR system using ethanol as the reductant, including, but not limited to, the diesel engine operating with E-diesel fuel.