0? order
For efficient diesel engines widely used in mobile equipment, the reduction of nitrogen oxides (NOx) in oxygen-enriched environment has always been one of the great challenges, especially to meet increasingly stringent emission standards. Since 2009, NH3 selective catalytic reduction (SCR) on Cu/ zeolite catalyst has been widely regarded as one of the most suitable technologies to meet the requirements of NOx emission. One of the biggest challenges faced by diesel engine aftertreatment system is how to quickly heat the catalyst to the working temperature (about 250℃) by using the exhaust gas of cold start diesel engine. ℃)。 Oxidation catalyst (DOC), diesel particulate trap (DPF) and SCR? Thermal integration of aftertreatment components is very important for the overall emission control performance, and the two functions have been combined into one device, such as SCR(SDPF) on particulate filter. This multifunctional equipment can not only provide fast warm-up time, but also reduce the volume, which is very attractive to small vehicles.
Compared with the known catalyst, it has CHA? Since 2009, Cu/SSZ- 13 with T-topology has been used as a standard SCR catalyst because of its better thermal durability. However, this catalyst may also be in 800? Thermal deactivation occurs above 100℃, which is due to the destruction of zeolite skeleton and the formation of copper oxide. According to NGK? Thermal insulation concept, when the soot accumulation is higher than 5? At g/L, the SCR catalyst coated on DPF can reach 800 at idle speed (DTI). The temperature is above 30℃. Therefore, the current SDPF system design limits the accumulation of soot to keep the SCR temperature below 800℃. Thereby protecting copper /SSZ- 13. When the accumulated smoke reaches 10? G/L causes DTI temperature to rise to 1? 100? ℃, which is acceptable to DPF Airlines itself. If the allowed soot accumulation increases, the interval of filter regeneration process can be extended, thus obviously improving fuel economy. This is the development of SCR with higher thermal stability. The powerful power of the catalyst.
On the other hand, compared with the previous New European Driving Cycle (NEDC), the introduction of new test modes, such as Global Uniform Light Vehicle Test Cycle (WLTC) and Actual Driving Emission (RDE), leads to the expansion of the operating temperature window. Due to SCR? It may not be possible to cover the NOx emission reduction process in the extended temperature range alone, so it is very necessary to combine NOx emission reduction technologies such as lean burn NOx trap (LNT)-SDPF system to meet the upcoming emission regulations. When is LNT? When the system is placed in front of the SDPF system, it is necessary to run the oil-rich engine regularly to operate at high temperature (650? Remove LNT in the environment above℃? Sulfur on the catalyst. In the process of desulfurization, saturation reaction will accelerate SCR? Is the catalyst near LNT? CuOx clusters form at the catalyst site, resulting in high temperature SCR? Because NH3 is oxidized to nitrogen oxides, the performance is significantly reduced. Actually, more than 600? It is very important to maintain the ability of reducing nitrogen oxides at the high temperature of 100℃ for practical driving conditions. For example, when SDPF is regenerated, the emission of nitrogen oxides tends to increase suddenly compared with normal operating conditions. Therefore, it is very necessary to keep the activity of SDPF system under the condition of high temperature and rich oil.
Recently, high-silica zeolite was successfully synthesized with benzyl imidazole cation as organic structure directing agent (OSDA). Copper exchange LTA in powder form? Even 900? 12 hydrothermal aging? After h, it can also show significant removal effect of nitrogen oxides. When the Cu content of Cu/SSZ- 13 is close to the molar ratio of Si/Al, although other catalytic properties seem to change, such as NH3 oxidation, it is only in a short aging time (3? H) will be effective. Consider improving Cu/LTA? Its high thermal stability makes its application in SDPF system reasonable. In the present work, aiming at the industrial application of SDPF system, we have explored Cu/LTA? It is more feasible and superior than the current commercial Cu/SSZ- 13. Prepared by Cu/LTA? Then compare the coated monolith with core size with Cu/SSZ- 13 catalyst and commercial SCR of the prior art? The catalysts were compared systematically, including Cu/LTA? Thermal durability of. In addition, the SCR performance of Cu/LTA in the presence of NO2, which can be produced by DOC or LNT, was also studied. Suggest Cu/LTA? And Cu/SSZ- 13 to further improve the low temperature SCR? Performance. Finally, Cu/LTA is verified by simulating dynamic WLTC transient modal test. The performance of.
1? test
1. 1? Catalyst preparation
Copper exchange LTA(Cu/Al=0.49, Si/Al= 16) was prepared by continuous wet ion exchange in copper acetate solution at room temperature. Then put Cu/LTA? 90 filtration washing? Dry overnight at℃, and then dry in air at 550℃. Calcined at 100℃ for 8? H. in order to play Cu/LTA? Powder deposition on cordierite monolith (width 2.54? Cm, 5.08 long? Cm), using the conventional dip coating method. Monolithic catalyst in 1 10? Dry overnight at 20℃ and dry at 550℃. Calcined at 100℃ for 5? H. As a contrast, the whole coated Cu/SSZ- 13 sample was prepared by impregnation method, in which the ratio of Cu/Al and Si/Al was higher than that of Cu/LTA? The corresponding ones are similar. In addition, the prior art commercial SCR catalyst mainly containing Cu/SSZ- 13 was provided by the catalyst manufacturer and was denoted as "COM" in this study. It is worth noting that the pore unit density of all monolithic catalysts is the same (400? Cpsi), and the number of coatings is similar.
In order to test the hydrothermal stability, the core diameter of the whole sample is 680 under the condition of wet airflow containing 10% moisture. Aging at 25℃? h，750？ Aging at 25℃? H and 900? 12℃ aging? H. In order to simulate the desulfurization process of LNT-SDPF system, 800? 16℃ hydrothermal aging? H's entire sample was exposed to 620? Simulated hypoxia (λ is 2.00,20? S)- oxygen enrichment (λ? 0.9 1, 12? S) 4 consecutive years? H. it should be noted that there should be no LNT? Direct evaluation of SCR by catalyst under oxygen-rich and oxygen-poor treatment? Tolerance of catalyst to oil-rich conditions.
1.2? Catalyst characterization
In order to study the state of Cu on each catalyst, H2-TPR was analyzed by BELCATII (BEL-JapanInc). Will it be 0. 15? Put10g catalyst sample into a quartz tube at 500℃. Pretreatment with 10%O2/Ar gas flow at℃? 1? H, and cool to room temperature. Then, at the same temperature, the feed gas is converted into 10% H2/ argon. After the signal of thermal conductivity detector (TCD) is stable, the sample is mixed in the H2/ argon flow of 10%. Raise the temperature to 900℃/ min? While recording the consumption of H2.
1.3? Reactor system
The nitrogen oxide reduction activity of the whole sample was measured in a laboratory reactor system. Core size of the whole sample (width 2.54? Cm, 5.08 long? Cm) At 500? Pretreatment at 30℃? Min, the background gas is composed of 9.5%O2, 5%H2O and N2, and cooled to 100? ℃. Then at 50? 000? The gas space velocity of h- 1 is changed from 500? Mg/L? No (or 250? Mg/L? No and 250? Mg/L? NO2)，500？ Mg/L? NH3、9.5%O2、5%？ H2O, 5%CO2 and N2 are balanced to form a simulated diesel feed gas flow. The catalyst sample was completely exposed to the feed gas flow and then mixed with 10. Raise the temperature to 600℃/ min? Conduct transient test at℃. For oxygen-rich and oxygen-poor samples, the temperature is 600? ℃ and 640? Special inspection of SCR at℃? Activities. Inject 300 during steady-state test. Mg/L? No and 300~ 1? 200? Mg/L NH3, while keeping the concentration of other gases the same as that of the transient test. Under steady-state conditions, NH3 oxidation test was conducted by removing NO from SCR feed gas stream.
Other catalytic properties, such as NH3 storage capacity and NH3 coverage-dependent nitrogen oxide conversion, were measured by a four-step test scheme. During the test, the whole sample with core size is always exposed to 9.5% O2, 5% CO2 and 5% H2O? Turn 500 on and off at the same time? Mg/L? No? Another 500? Mg/L? NH3。 The steps are as follows: (1)500? Mg/L? NO+ background gas; (2)500? Mg/L? No +500? Mg/L, NH3+ background gas; (3)500? Mg/L? NH3+ background gas; (4)500? Mg/L? NO+ background gas.
To simulate WLTC? The model test adopts the laboratory-scale reactor system developed by Hyundai Motor Group (HMG) (Figure 1). Through the WLTC test process of chassis dynamometer, the original emission and catalyst temperature data are directly obtained from the vehicle measurement to simulate WLTC? Program. Then, the accurate engine output curve including temperature and emission is reproduced by a laboratory-scale reactor, and repeated tests have excellent reproducibility (1%), as shown in figure 1. Part of the mass flow controller is used to control the flow at 2? S is the unit to simulate the dynamic gas composition and flow rate, and the catalyst temperature depending on the vehicle speed is simulated by the fast response heater. Before simulating WLTC test, a certain amount of NH3 was at 250? 1 single sample with core size simulates carrier sample at℃. Then cool the sample to room temperature and start the test cycle. Simulated WLTC? The test procedure includes low speed (t? & lt780? S), medium speed (780? s 1？ 170? S) the third stage. Considering the LNT-SDPF system, the feed gas flow rate of the test cycle is based on the output emission of LNT. In this study, the inlet and outlet concentrations of all gas components (such as NO, NO2 and NH3) were determined by on-line FT-IR spectrometer (MKS instrument, 2000 series multi-gas analyzer) equipped with gas unit.
2? Results and discussion
2. 1? Cu/LTA？ Hydrothermal stability of the whole coating sample
In fig. 2, it is shown that in Cu/LTA? Based on Cu/SSZ- 13? The conversion rate of nitrogen oxides is 900? 12℃ hydrothermal aging? H contrast. Cu/LTA？ It shows excellent catalytic activity for SCR catalyst in the prior art at the whole reaction temperature, and shows particularly high hydrothermal stability. Like at 250? At℃, copper/tantalum? The conversion rate of nitrogen oxides reached 80%, while the conversion rate of industrial Cu/SSZ- 13 catalyst was 30%. This result is consistent with that of using powder catalyst before. In the activity test of the two catalysts containing a small amount of N2O, the N2 selectivity was always above 95%. As mentioned earlier, SDPF contains copper /SSZ- 13? The carbon deposition on the system is limited (about 5? G/L) to keep the SCR catalyst temperature at 800? It's below centigrade. Considering Cu/LTA? Can keep its SCR performance at 900? Based on the copper/tantalum state? SDPF system can allow the target smoke load to increase to 8? General ledger. In this case, the filter regeneration interval can be changed from 250? A mile extends to 400 miles? Miles, it can definitely improve fuel economy.
Under actual driving conditions, a certain amount of NH3 should always be stored in the SCR catalyst, so as to immediately respond to the ever-changing NOx emission in diesel engine exhaust gas. Therefore, NH3 storage capacity is considered as SCR? One of the important characteristics of catalyst. Figure 3? The changes of dynamic ammonia storage capacity of Cu/LTA and Cu/SSZ- 13 catalysts with hydrothermal aging temperature are described. At 680? After aging at 100℃, copper/tantalum? The NH3 storage capacity of is equivalent to that of Cu/SSZ- 13 in the temperature range covered. However, when the aging temperature rises to 750? ℃ and 900? At℃, copper/tantalum? It shows a higher value than Cu/SSZ- 13, indicating that its NH3 storage capacity is insensitive to hydrothermal aging. Consider SCR? The NH3 loading target on the catalyst depends on the NH3 storage capacity, which is very beneficial to the urea injection strategy controlled by vehicles. In fact, it is well known that the NH3 storage capacity of SCR catalyst is closely related to its acidic characteristics. The storage of NH3 at low temperature may come from Cu2+ ions exchanged on zeolite, and the Bronsted acid site produced by [CuOH]+ substance or zeolite itself may be the cause of high temperature counterpart. It is reported that the crystal structure of Cu/SSZ- 13 is 850? It collapses with the transformation of copper ions into CuOx clusters, leading to the deterioration of acid centers. In contrast, even at 900? /kloc-heavy water heat treatment at 0/2℃? After h, Cu/LTA? Zeolite skeleton also shows its stability, which may be one of the main reasons for the decrease of NH3 storage after hydrothermal aging.
2.2? Tolerance to high temperature, oxygen-enriched and anoxic conditions
As mentioned above, in LNT-SDPF? In the sulfation process of the system, the SCR coated on the particle filter? The catalyst can also be exposed to oxygen-rich and oxygen-poor conditions at high temperature. In order to study the effects of oxygen-rich and oxygen-poor treatments on the removal activity of nitrogen oxides at high temperature, at 800? 16℃ hydrothermal aging? H, at 620? , single sample of each core size. Exposed to oxygen-rich and oxygen-poor cycles at 4℃? H. then, 600? ℃ and 640? The steady-state SCR activity test is carried out at℃, and under normal circumstances, the SDPF temperature at this temperature can be reached when the filter is regenerated. As shown in figure 4(a), Cu/LTA? For Cu/SSZ- 13 catalyst, it always has high nitrogen oxide removal activity. 600? ℃? When NH3/ NOx emission ratio is 4, the NOx conversion rate on Cu/SSZ- 13 (3 1%) is even lower than that on Cu/LTA (53%) when NH3/ NOx feed ratio is 1. This shows that in the process of filter regeneration, the required urea injection amount may be higher than that based on Cu/LTA SDPF system has low cost and is ideal for reducing the greenhouse gas CO2 emission generated by urea decomposition. As shown in fig. 4(b), this test also detected the NH3 oxidation activity of aged samples with oxygen enrichment and oxygen depletion. In fact, NH3 oxidation is a useless reactant of side reaction, which leads to the decrease of nitrogen oxide removal activity at high temperature. At 620? After Cu/LTA aging at℃ with rich oxygen and poor oxygen? The oxidation activity of NH3 on the catalyst is much lower than that of Cu/SSZ- 13 catalyst. Especially 640? At℃, the unit is copper/tantalum? 73% NH3 conversion, while Cu/SSZ- 13 is at 600? At lower temperature, the conversion rate of NH3 reaches 100%. Compared with Cu/SSZ- 13, this shows that more NH3 can be used for Cu/LTA? It reacts with NH3/SCR, so that it can exert excellent high-temperature SCR performance.
With Cu/LTA? In contrast, copper /SSZ- 13 produces a lot of NO during NH3 oxidation, as shown in Figure 4(c). This non-selective oxidation of NH3 to NO leads to the oxidation of NH3 to NO at 640? It has unusual SCR performance at 100℃. When the reaction temperature rises to 640? At℃, copper/tantalum? As shown in Figure 4(a), the conversion rate of nitrogen oxides is still higher than 70%, indicating that it has strong durability to oxygen-rich and oxygen-poor conditions. For the model Cu/SSZ- 13 catalyst, it was observed that the conversion of nitrogen oxides was negative, indicating that the outlet concentration of nitrogen oxides was higher than the inlet concentration, which may be caused by NO formed by NH3 oxidation. In fact, it is known that the NO selectivity of NH3 oxidation reaction on Cu/ zeolite is related to the CuOx content on the catalyst surface. Therefore, it is necessary to understand the formation of CuOx catalysts, so as to explain the different NH3 oxidation behaviors scientifically.
Fig. 5 depicts Cu/LTA? And H2-TPR curves. Researchers generally believe that less than 400? The peak value of Cu2+ at 20℃ is reduced to Cu+, while the peak value of Cu+ at high temperature is reduced to Cu .Cu/SSZ- 13, which is shown at 220? The reduction peak near℃ is related to Cu2+ or [CuOH]+, adjacent to the eight-membered ring (8MR), and the peak-shoulder peak is at 300? It is caused by Cu2+ on the double six-membered ring (D6R). For copper/LTA, less than 400? In the low temperature region of 100℃, at 270? Only 1 symmetrical peak appears near℃, which indicates that the peak in LTA? There may be only 1 species of Cu2+ in the zeolite skeleton. What does this have to do with Ron? The results of XRD/ Rietveld refinement are consistent, that is, all Cu2+ ions seem to coordinate with three oxygen atoms out of the plane of a single six-membered ring. 600? The reduction peak of Cu+ on Cu/SSZ- 13 appears near℃, which is very close to the temperature of oxygen-rich and oxygen-poor treatment. Conversely, Cu/LTA? In this temperature range, the process of reducing copper ions to copper metal is not fully activated. Compared with Cu/SSZ- 13, Cu/LTA? This delayed reduction can be explained by the different electronegativity of zeolite, which depends on the framework type and affects the reducibility of cations. When copper ions are reduced to metallic copper under oil-rich conditions, there is no electrostatic interaction between copper and zeolite, which may lead to copper agglomeration and transformation into CuOx clusters under oil-poor conditions. Therefore, the reduction of Cu ions in zeolite skeleton by LTA is low, which can reduce the formation of CuOx (non-selective NH3 oxidation source) during oxygen-enriched and oxygen-deficient aging, which may be Cu/LTA? The main reason for its strong durability at high temperature.
2.3? Overcome Cu/LTA? Low temperature activity strategy
As mentioned above, a certain amount of NH3 is first stored in SCR. Catalyst, and then react with nitrogen oxides emitted by diesel engine under actual driving conditions. Nitrogen oxide conversion depends on SCR? NH3 coating on the catalyst. Fig. 6 comparing Cu/LTA? And the nitrogen oxide conversion rate of Cu/SSZ- 13 is 250? The function of NH3 coverage at 100℃ is obtained through four steps. Is this an evaluation of SCR? One of the important standards of catalyst in industrial application. At 900? In hydrothermal aging at 100℃, copper/tantalum? The reduction efficiency of nitrogen oxides of Cu/SSZ- 13 catalyst is usually much higher than that of Cu/LTA. It has extraordinary thermal stability, as shown in Figure 6(a). When two samples are at 680. When it is moderately aged at 100℃, copper/tantalum? At the initial NH3 coverage level, it is different from Cu/SSZ- 13? The conversion efficiency of equivalent nitrogen oxides is shown in Figure 6(b). But with the further improvement of NH3 coverage, Cu/LTA? Can't reach the same conversion level of nitrogen oxides as Cu/SSZ- 13, which shows that it is necessary to improve Cu/LTA? Low temperature activity. The low temperature SCR performance of Cu/ zeolite may be related to the local environment of Cu, which affects the redox performance and NO? Oxidation capacity and reactant adsorption. Right, Cu/LTA? Low temperature SCR? Further research on its properties and improving its inherent reactivity is under way.
In diesel aftertreatment system, DOC? Or LNT? Isooxidation catalyst is always placed in front of SCR, which means NO2 can be obtained upstream of SDPF system through NO oxidation on DOC/LNT. In this case, the SCR catalyst will have a "fast SCR" reaction to increase its low temperature activity. As shown in fig. 7, when the NO2/ NOx ratio is 0.5, Cu/LTA? The SCR performance of the whole coating sample is different from that of COM based on Cu/SSZ- 13. By comparison, the "fast SCR" reaction is ensured to be carried out effectively. Due to the aggregation of NH4NO3 in the pore structure of zeolite, the catalyst temperature changes from 175? The temperature began to rise. At 680? After mild hydrothermal aging at 30℃, the low temperature activity of Cu/LTA is equivalent to that of COM, while that of Cu/LTA? The display scale is 500? COM above℃ has higher SCR performance, as shown in Figure 7(a). At 900? When severely aged at 100℃, copper/tantalum? As shown in fig. 7(b), it shows excellent SCR activity for COM in the whole reaction temperature range. These results show that Cu/LTA? Low temperature activity is not the main problem.
However, according to the vehicle driving conditions and the layout of the aftertreatment system, NO2 upstream of SDPF system cannot always be more than enough, so another method is needed to overcome Cu/LTA? The strategy of nitrogen oxide removal activity at low temperature is based on. In order to solve the concern of low temperature activity, Cu/LTA? And the volume ratio is 1: 1, while maintaining the same total volume as the single brick sample. As shown in fig. 8(a), in the absence of NO2 feed, at 680? The conversion rate of nitrogen oxides in double-layer brick aged at℃ is equivalent to that in single-layer brick. Comparing the configuration sequence of single brick and double brick, at high temperature (400? Above℃), Cu/LTA was observed. In the former, the nitrogen oxide conversion rate of commercial catalyst is higher than that of reverse structure, and the two samples show similar low temperature SCR? Activities. Compared with Cu/SSZ- 13, Cu/LTA? The low NH3 oxidation ability of the catalyst may lead to the difference of SCR activity at high temperature. At 900? After hydrothermal aging at℃, the double-layer brick samples show better SCR performance than commercial catalysts, as shown in Figure 8(b), which indicates that Cu/LTA? The strong hydrothermal stability of has been fully reflected in the integral sample of double bricks. The results show that the double brick system has the advantages of both Cu/LTA (thermal stability) and Cu/SSZ- 13 (low temperature activity).
2.4? Cu/LTA with double brick system? Transient performance of
The advantages of double brick structure are further verified by simulating WLTC model test in a laboratory-scale reactor system using the whole sample of core size. In fact, this kind of core size test requires much less time and energy than the full-scale test on the chassis dynamometer, while always maintaining excellent reproducibility (1%). Fig. 9(a) shows the cumulative NOx emission and vehicle speed with time during the WLTC mode test without NO2. It is worth noting that considering the LNTSDPF system, the dynamic feed gas flow is based on LNT? Was discharged from the hospital. Blank test results show that at 780? s、 1？ 170? s、 1？ 540? In the United States, NOx emissions suddenly increased because the engine was operated with rich oil to reduce NOx stored on the LNT catalyst. At low speed (600? S below), SCR of all catalysts? Performance is negligible because they hardly heat up (200? Below℃). Once the reactor system reaches the medium-speed state (780? S), all the catalysts are activated, and the reduction activity of nitrogen oxides is at a high speed (1? 170? S above) further increase.
During the simulated WLTC test, Cu/LTA? Double brick samples at 680? When aging at 100℃, the emission level of accumulated nitrogen oxides is similar to that of a single brick .900? After hydrothermal aging at℃, the nitrogen oxide emissions of the two catalysts increased, but the nitrogen oxide level of the double brick sample was still lower than that of the single brick sample. In fact, even at medium speed (1? 170? S) below, at 900? As shown in fig. 9(a), the single brick aged at℃ also shows a very small reduction efficiency of nitrogen oxides. In addition, the model was tested at 900? The samples of double-layer brick aged at℃ show lower NH3 escape phenomenon than the samples of single-layer brick, although 2? This sample is at 680? After mild aging at 20℃, it shows similar behavior, as shown in Figure 9(b).
At 900? The difference of NH3 escape between the two samples after aging at 100℃ may be due to the influence of Cu/LTA. NH3 storage (Figure 3). So, even after 900? After severe aging at 100℃, copper/tantalum alloy? Combined with the most advanced COM, it is also considered to meet the engineering target in tail pipe NOx concentration, which indicates that it is possible to improve the current soot load target of SDPF and improve fuel economy.
3? conclusion
In the industrial SDPF system, the catalytic performance and durability of Cu/LTA coating samples were systematically evaluated under various conditions. At 900? ℃? After hydrothermal aging, Cu/LTA? The SCR performance of the catalyst is higher than that of the commercial catalyst based on Cu/SSZ- 13, indicating that its good thermal stability is beneficial to SDPF system. At 620? Lean/rich cyclic aging was carried out at 60℃ to simulate LNT-SDPF? After the desulfurization process, even at 640? At℃, Cu/LTA still maintains its nitrogen oxide reduction activity at high temperature, while the commercial catalyst based on Cu/SSZ- 13 oxidizes NH3 into nitrogen oxide, and shows the reverse nitrogen oxide conversion. H2-TPR results show that LTA? What is the proportion of copper ions in the skeleton? Cu ions in Cu have poor reducibility, which may reduce the formation of CuOx under the condition of high temperature enrichment. CuOx is not the first choice for NH3 oxidation. Although the low temperature SCR activity of Cu/LTA is 680? After mild aging at℃, it is not as good as COM, but it can be overcome in the presence of NO2, which mainly exists in DOC? Or LNT? Downstream. Through Cu/LTA? And COM? The composition of the double brick sample is 680? After aging at 100℃, its performance is equivalent to that of a single brick, and the conversion rate of nitrogen oxides at low temperature is equivalent, while Cu/LTA? Good thermal stability, keeping up to 900? The simulated WLTC test results show that Cu/LTA? It may be a good candidate for the next generation SCR technology, especially for applications requiring high heat resistance.
Note: This article was published in the second issue of Automobile and New Power magazine in 2020.
Author: [Han]? Jin et al.
Finishing: Yan Hongmei?
Editor: Yu Zhan
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.