CuZSM-5, FeZSM-5 및 V2O5/TiO2촉매의 NH3에 의한 NOx 제거 성능에 탄화수소가 미치는 영향에 대한 연구

Abstract

Selective catalytic reduction (SCR) of NO with NH3 as a reductant is regarded as one of the most promising technologies to remove NOx from diesel engines. This technology is however still struggling with low temperature activity and catalyst deactivation due to SO2 poisoning and hydrothermal degradation in its application. A NH3/SCR reactor is commonly located right after DOC+DPF system to eliminate negative effect of particulate matter and hydrocarbons (HCs) from the diesel engines. A new issue for causing the catalyst deactivation recently raised may be the significant amount of unburned hydrocarbons slipped from an advanced combustion engine during its cold start, and even after engine warm up period upon the catalyst deactivation of the DOC. In the present study, the effect of C3H6 slip from the DOC system on the deNOx activity by NH3/SCR has been examined over the three representative commercial SCR catalysts including CuZSM-5, FeZSM-5 and V2O5/TiO2 catalysts. CuZSM-5 and FeZSM-5 catalysts were prepared by wet ion exchange method, while V2O5/TiO2 catalyst is commercial one supplied by a catalyst manufacturer. Particularly, two test modes were designed to examine the influence of HC on their NH3/SCR activities. To simulate the cold start period of engine emitting a large amount of HC, C3H6 was adsorbed onto the catalysts prior to the activity test. In order to simulate the commercial engine operating condition where the gradual slip of HC was expected with respect to the catalyst mileage of DOC, C3H6 was included in the feed gas stream during the course of NH3/SCR reaction. C3H6 were specifically employed as a representative HC from diesel engines in the present study. CuZSM-5 and FeZSM-5 catalysts showed the apparent decrease of the deNOx activity by C3H6 pre-adsorbed below 300 oC and 330 oC, respectively, probably due to the adsorption of C3H6 onto the catalyst surface by the competitive adsorption with NH3. The NH3/SCR activity of CuZSM-5 and FeZSM-5 catalyst was then particularly recovered at the temperature range higher than 330 oC and 390 oC, respectively. On the other hand, no deactivation of V2O5/TiO2 catalyst was observed over the reaction temperature region examined by C3H6 pre-adsorbed. These results imply that the NH3/SCR reaction over ZSM-5 based catalysts is more sensitive to the adsorption of C3H6 onto the catalysts than that of V2O5/TiO2, since the ZSM-5 based catalysts may absorb a large amount of C3H6. The deNOx performance of the catalysts peculiarly decreases and increases during the course of the NH3/SCR reaction with the continuous inclusion of C3H6 into feed gas stream over the three regions of the reaction temperature, low (150-210 oC), medium (270-330 oC) and high (390-500 oC) ranges. A unique trend of the NH3/SCR activity of CuZSM-5 was observed with respect to the reaction temperature. The NO conversion severely decreased in the low and medium temperature regions and then particularly increased in the high temperature region. It can suggest that another possible reaction such as HC/SCR reaction may occur during the course of NH3/SCR reaction with C3H6 included in feed stream. In fact, the deNOx activity of CuZSM-5 catalyst by HC/SCR reaction in the high temperature region was clearly determined in the present study. The NO conversion of FeZSM-5 catalyst also decreased over the wide range of the reaction temperature by the continuous inclusion of C3H6 into feed. Unlike CuZSM-5 catalyst, the deNOx activity of FeZSM-5 catalyst was not recovered in the temperature region higher than 390 oC, probably due to the strong adsorption of C3H6. V2O5/TiO2 catalyst showed no deactivation of the deNOx activity in the low temperature region, while the deNOx activity in both medium and high temperature regions markedly decreased. During the course of NH3/SCR reaction with C3H6, both NO and NH3 conversions decreased upon the inclusion of C3H6 into feed, while the NH3 conversion was always higher than the NO conversion, regardless of the catalysts. By the NH3 oxidation test over V2O5/TiO2 catalyst when C3H6 was included in the feed without NO, the NH3 conversion remarkably increased in both medium and high temperature regions. It can be speculated that NH3 is not only consumed by the NH3/SCR reaction but also by the side reaction with O2 and/or C3H6. To determine the adsorption capacity of C3H6 onto CuZSM-5, FeZSM-5, and V2O5/TiO2 catalysts, C3H6 TPD was conducted. The ZSM-5 based catalysts adsorbed a large amount of C3H6, while C3H6 was hardly adsorbed onto the surface of the V2O5/TiO2 catalyst. It can be immediately anticipated that C3H6 may compete with NH3 to be adsorbed onto the surface of CuZSM-5 and FeZSM-5 catalysts. C3H6 adsorbed then retarded the adsorption of NH3 as well as NO. On the other hand, no deactivation of V2O5/TiO2 catalyst by C3H6 was observed particularly in the low temperature region. In addition, the HC oxidation activity of the three catalysts was examined by C3H6 TPO and TG/DTA. The IR spectra for identifying the chemical species adsorbed onto the surface of the CuZSM-5, FeZSM-5, and V2O5/TiO2 catalysts were obtained by the sequential injection of the reactants including NH3, C3H6, O2 and NO. The advent of the IR peaks originating from nitrile species and/or organic nitrogen compounds over the catalysts suggested that a reaction of NH3 and C3H6 with O2 through ammoxidation may occur during the course of NH3/SCR reaction in the presence of C3H6. NH3 could be uselessly consumed by C3H6 through the ammoxidation and the NH3/SCR activity eventually decreased due to the shortage of the reductant. In addition, the formation of bulk acrylonitrile was clearly identified in the downstream of the reactor during the course of the NH3/SCR reaction with C3H6 by FT-IR equipped with a gas cell, regardless of the catalyst employed. At 450 oC, the acrylonitrile adsorbed onto CuZSM-5 catalyst disappeared by the introduction of O2 in the feed stream, whereas it was stable and hardly oxidized over FeZSM-5 catalyst. It could be one of the reasons why CuZSM-5 catalyst showed the sudden recovery of deNOx activity in the high temperature region, while FeZSM-5 catalyst was further deactivated. Over V2O5/TiO2 catalyst, the small amount of C3H6 fragments was observed on the catalyst surface in the medium regions where its NH3/SCR activity started to decrease by the continuous inclusion of C3H6, but not in the low temperature region. The C3H6/SCR reaction was examined over all the three catalysts to understand the sudden recovery of the NO conversion, particularly over CuZSM-5 catalyst in the high reaction temperature region. As expected, CuZSM-5 catalyst showed the deNOx performance by HC/SCR reaction in the high temperature region. On the other hand, no HC/SCR activity over FeZSM-5 and V2O5/TiO2 catalysts had been observed in the wide reaction temperature region covered in the present study. It may simply reveal that the sudden recovery of the deNOx activity over CuZSM-5 catalyst during the NH3/SCR activity test with C3H6 was basically attributed to the HC/SCR activity in the high temperature region. In order to develop the HC tolerant NH3/SCR catalyst, a variety of Cu-zeolite based catalysts have been prepared and their NH3/SCR activity by the pre-adsorption of C3H6 and by the continuous inclusion of C3H6 into feed gas stream was examined. Among the catalysts screened, CuHM catalyst showed the outstanding HC tolerance during NH3/SCR reaction with C3H6, especially in the medium temperature region where the rest of the catalysts were seriously deactivated. The relatively smaller amounts of C3H6 adsorbed onto the CuHM catalyst determined by C3H6 TPD may be one of the reasons for its strong HC tolerance. In addition, CuHM catalyst also showed the deNOx activity by HC/SCR reaction, but lower than that of CuZSM-5 catalyst. Although CuHM catalyst is promising as a commercial NH3/SCR catalyst with HC slip, the catalyst may still require further improvement of the deNOx performance, particularly the low temperature activity.