디젤자동차에서 배출되는 NOx 저감을 위한 DOC-NH3/SCR 촉매 시스템의 반응속도론 연구

Abstract

A variety of reaction kinetics for describing the performance of the catalytic converters installed in a diesel-after treatment system, in particular for the abatement of NOx, have been developed over the last several decades. The kinetic model developed may save the time and resource required for optimizing the design of the catalytic converter. However, until now, most of the reaction kinetics developed is only focused on the single catalytic system, which may not comprehensively describe the catalytic behavior of the diesel after-treatment system consisting of multi reactor systems such as DOC, SCR and LNT. Thus, there has been the strong demand for developing the overall reaction kinetics for directly predicting the catalytic activity over the DOC-SCR combined system under the realistic diesel exhaust condition. In the present study, the kinetic model for the combined catalytic system consisting of DOC (La0.5Ag0.5MnO3) and SCR (CuSSZ13) reactors has been developed by integrating the reaction kinetics derived for each catalytic system based upon the reaction mechanism postulated. In order to derive the overall reaction kinetics for the DOC-SCR catalytic system, two sets of detailed reaction kinetics for the NO oxidation reaction over La0.5Ag0.5MnO3 and NH3/SCR reaction over CuSSZ13 have been independently developed in view of microkinetics. The catalytic activity over La0.5Ag0.5MnO3 and CuSSZ13 has been examined under the various operating conditions to collect the kinetic data for the NO oxidation and NH3/SCR reactions, respectively. The NO oxidation activity over La0.5Ag0.5MnO3 decreases from 80% to 20% when the reactor space velocity increased from 15,000 h-1 to 120,000 h-1 at the identical reaction temperature of 200 oC. Moreover, the conversion of NO to NO2 decreases in the temperature region above 250 oC, regardless of the reactor space velocity, mainly due to the thermodynamic equilibrium of NO-NO2. The NO oxidation activity of La0.5Ag0.5MnO3 decreased as the NO2 concentration in the reactor increased in the wide temperature window, which may be attributed to the pre-occupation of the catalyst active reaction site by NO2. The deNOx activity over CuSSZ13 also decreases especially in the low temperature range below 300 oC, when the reactor space velocity increases from 100,000 h-1 to 400,000 h-1. To determine the role of NO2 in the NH3/SCR reaction, the deNOx performance of CuSSZ13 has been examined as a function of the NO2/NOx feed ratio, while keeping the total inlet NOx concentration at 500 ppm and reactor space velocity at 400,000 h-1. The NOx conversion is dramatically promoted with the increase of NO2/NOx ratio up to 0.5 due to the “fast-SCR” reaction. However, the decline of deNOx activity is observed when the excess of NO2 (NO2/NOx ≥ 0.5) is included in the feed gas stream. The reaction mechanisms for the NO oxidation over the La0.5Ag0.5MnO3 and NH3/SCR reactions over CuSSZ13 have been investigated by TPD and DRIFT studies. NO and O2 may be adsorbed onto the two distinct active reaction sites formed on the surface of the La0.5Ag0.5MnO3, respectively, while O2 and H2O may occupy the identical site, which may be regarded as the catalyst oxygen vacant site. In addition, the result of DRIFT study reveals that the nitrate species formed by the surface reaction between nitrite and O2 may be the key reaction intermediate for the oxidation of NO to NO2. When the La0.5Ag0.5MnO3 catalyst is exposed to NO2 during the in-situ DRIFT study, the formation of nitrite species has been also observed, which may indicate that NO2 can be directly adsorbed onto the oxygen vacant site of the catalyst. The pre-occupation of NO2 onto this vacant site appears to inhibit the adsorption of O2 onto the catalyst surface, resulting in the decrease of its NO oxidation activity. On the surface of CuSSZ13, NOx may be adsorbed onto the Cu site, while NH3 is mainly adsorbed onto the Lewis and Brӧnsted acid sites. Based on the in-situ DRIFT study, the nitrate and nitrite species appeared to be the key reaction intermediate for the removal of NOx by NH3 over CuSSZ13, although the nitrite species formed cannot be directly observed, mainly due to their overlap with the strong zeolite structure peak of SSZ13. Furthermore, additional important surface reaction including the reduction of the nitrate species produced by NO and the formation/decomposition of NH4NO3 have been also determined. Based on the reaction mechanism postulated, the reaction kinetics to predict the oxidation activity of NO over La0.5Ag0.5MnO3 and the NOx removal activity by NH3 over CuSSZ13 have been developed, respectively. The model reasonably well predicted the oxidation activity of NO to NO2 under the various reactor space velocities employed. Moreover, the decline in the catalytic activity with the increase of inlet NO2 concentration can also be well described without the further adjustment of the kinetic parameters already estimated. On the other hand, the kinetic model for the NH3/SCR reaction also reasonably predicted the deNOx performance of the CuSSZ13 catalyst with respect to the reaction temperature, and reactor space velocity. In addition, the enhancement of the deNOx activity upon the addition of NO2 into the feed gas stream was also well described by the kinetic model developed. Finally, the two reaction kinetics independently derived have been integrated to develop the overall reaction kinetics for the DOC-SCR combined catalytic system. The overall reaction kinetics derived well predicted the catalytic deNOx performance over the combined catalytic system under the wide operating conditions without any further adjustment of kinetic parameters independently estimated for the NO oxidation and NH3/SCR reactions. In the present kinetic study, the detailed reaction kinetics for the combined catalytic system consisting of DOC and SCR reactors have been successfully developed in view of microkinetics on the basis of the reaction mechanism postulated over the La0.5Ag0.5MnO3 and CuSSZ13 catalysts. The kinetic model developed may provide the useful guideline for optimizing the reactor design for the catalytic converters simultaneously included in the commercial diesel after-treatment system.