Since their inception in the mid 1990s, many investigations have been performed and considerable progress has been made in understanding the NOX storage and reduction process. As evidence, the NOX storage mechanism has mostly been resolved. However, many aspects of this system remain unsolved and elucidation of results from existing literature is difficult because
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very few investigations employ the same conditions. This is in part due to the large number of adjustable parameters including: choice of support, choice of the storage component, loading of the storage component, impregnation method of the storage component, choice of noble metal, loading of noble metal, impregnation method of the noble metal, impregnation order for the noble metal and storage component, precursors employed for impregnation, choice of promoters (Rh, CeO2 etc.), loading of promoters, lean gas environment (NO, NO+O2, NO2), rich gas environment (H2, CO, C3H6), inclusion of CO2, H2O or both, lean period duration, rich period duration, S.V. employed, pretreatment used, and lastly the inclusion of sulfur. All of these aspects were covered is this review and it emerges that a fundamental understanding of how each parameter affects the observed operation is still lacking. Furthermore, contradictory reports in the literature are really not too surprising given the large number of adjustable parameters, which ultimately leads to significantly different conditions and/or catalysts in many cases that are ultimately used during evaluation of NSR catalysts and comparison between one study to another can be difficult.
In closing, determination of the reduction mechanism will define target parameters for future development of novel NOX storage and reduction catalysts, where synergy between the support, storage component, precious metals and promoters in combination with high thermal and hydrothermal stability and increased sulfur tolerance are desirable attributes for the next generation of NOX storage and reduction catalysts.
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1.4 REAL SYSTEMS
1.4.1 EARLY CATALYTIC CONVERTER SYSTEMS
Fig. 18 shows three different simplified schematics of early catalytic converter systems [14]. In this case, the mixture-formation system is a black box description of the fuel-injection system and the downstream reciprocating piston arrangement represents a simplified example of a single cylinder in an internal combustion engine, where additional details related to fuel- injection, mixing and combustion strategies is out of the scope of this work. Fig. 18A shows an engine exhaust system equipped with a two-way catalytic converter. The catalytic converter in this case is referred to as a two-way catalyst because it only oxidizes CO and HCs. The secondary oxygen added would result in a net lean environment, which favors oxidization of CO and HCs, but any NOX emitted from the engine would not be reduced under these conditions, as discussed at-length previously. The schematic shown in Fig. 18B, is a two-catalyst system, but would still not be considered a three-way catalyst since the upstream catalyst is only responsible for the reduction of NOX and the downstream catalyst is only responsible for the oxidation of CO and HCs. However, this system would be able to effectively remove all three pollutants simultaneously. In this case, the engine would likely be tuned slightly rich to effect NOX reduction over the upstream catalyst, which explains why it is necessary to introduce secondary air over the downstream catalyst to oxidize CO and HCs. The schematic shown in Fig. 18C represents a true three-way (TWC) system and has been the design of choice for automobiles from the late 70s to the present. Notice that a lambda/O2 sensor has now been included and that it is connected to a feedback controller, which tunes the mixture-formation system to ensure that the exhaust gas is as close to the stoichiometric regime as possible. Kaspar et al. [279] and Shelef and McCabe [280] previously discussed the advantages of both on-board diagnostics
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(OBD) and oxygen storage capacity (OSC) on TWC systems, as summarized in Fig. 19. Without OSC, the TWC spends a significant amount of time outside the optimum AFR, which results in lower conversion efficiencies for CO, HCs and NOX. With OBD and OSC, the TWC spends the majority of the time very near the optimum AFR; as a result, conversion efficiencies higher than 95% are achieved [279].
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Figure 18. Schematics showing early catalytic converter systems, Faiz et al. [14].
A.)
B.)
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Figure 19. Effect of the AFR, advanced on-board diagnostics (OBD) and oxygen storage
capacity (OSC) on the operation of TWC systems, Kaspar et al. [279].
1.4.2 MODERN CATALYTIC CONVERTER SYSTEMS
In the 1990s, in California especially, a series of low-emission standards were implemented that started the progression towards a 10-fold reduction in HC emission and a 20- fold reduction in NOX emissions [281]. In the case of HCs, 50-80% of the total HCs emissions are emitted during the cold-start period during FTP testing (i.e., the first 90-180 s of operation) before the TWC reaches the HC-light off temperature (≈300 °C) [15]. Practically, this means that a vehicle could fail an emissions test based on cold-start HCs alone after only 2 min. into a
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23 min. test. As a result, close-coupled TWCs were implemented. These catalysts could achieve light-off within 10 s of operation and are composed of proprietary blends of metal oxides used to stabilize the Al2O3 and prevent Pd sintering [15]. Fig. 20 shows two possible examples of close- coupled catalysts. In one case, a small, close-coupled catalyst is used primarily to convert HCs, while the larger downstream, under-floor TWC is responsible for the majority of the CO oxidation and NOX reduction that takes place. The latter example shows a much larger close- coupled TWC that simultaneously converts all three pollutants, but at higher temperatures in comparison to the under-floor catalyst shown in the first example.
Figure 20. Two examples comparing the location and size of close-coupled TWCs [15].
The combination of TWCs and close-coupled TWCs has allowed auto manufactures to effectively meet emissions standards, but only for gasoline engines operating in the stoichiometric regime. Table 4 shows typical exhaust compositions for a diesel engine, four- stroke spark-ignited gasoline engine and a four-stroke spark-ignited lean burn gasoline engine
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[279]. The conditions were included for the two-stroke engine for comparison, but will not be discussed. Notice that in the case of diesel engines and lean-burn gasoline engines, the O2 concentration is very high (e.g., 4-15%). Comparison of these AFRs (26 and 17, respectively) with the results shown in Fig. 19 clearly demonstrates that poor performance in terms of NOX conversion is expected if a TWC-only system was used. So for diesel engines or lean-burn gasoline engines, different/new catalytic technologies are required. Figs. 21 - 25 summarize the most recent architectures suggested that are capable of removing lean-NOX, but still meeting the CO, HC and PM regulations.
The following chapters focus on the systems shown in Figs. 21 and 22 (Chapters 2 and 3) and in Fig. 23 (Chapter 4). Additional information regarding those systems is presented in the introductions of each corresponding chapter. The systems shown in Figs. 24 and 25 were included for comparison, since they provide corollary examples, but are not the specific focus of this dissertation since they were designed for diesel engines.
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Table 4. Typical Exhaust composition for several common commuter vehicles, Kaspar et al. [279]. Exhaust Components
and conditionsa Diesel Engine
Four-stroke spark- ignited engine Four-stroke lean-burn spark-ignited engine Two-stroke spark- ignited engine NOX 350–1000 ppm 100–4000 ppm ≈1200 ppm 100–200 ppm HC 50–330 ppm C 500–5000 ppm C ≈1300 ppm C 20,000–30,000 ppm C CO 300–1200 ppm 0.1-6 % ≈1300 ppm 1-3 % O2 10–15 % 0.2-2 % 4-12 % 0.2-2 % H2O 1.4-7 % 10-12 % 12 % 10-12 % CO2 7 % 10-13.5 % 11 % 10-13 % SOX 10–100b ppm 15–60 ppm 20 ppm ≈20 ppm PM 65 mg/m3
Temperatures (test cycle) r.t. - 650 °C r.t. – 1100c °C r.t. - 850 °C r.t. - 1000 °C GHSV (hr-1) 30,000-100,000 30,000-100,000 30,000-100,000 30,000-100,000
λ (A/F)d ≈1.8 (26) ≈1 (14.7) ≈1.16 (17) ≈1 (14.7)e
a
N2 is remainder b
For comparison: diesel fuels with 500 ppm of sulfur produce of about 20 ppm of SO2 c
Close-coupled TWC
d
λ defined as ratio of actual A/F to stoichiometric A/F, λ=1 at stoichiometry (AFRSTOICH = 14.7)
e
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Figure 21. Close-coupled TWC, under-floor LNT configuration, as present on the BMW 120i
(Model Year, 2009).
Figure 22. Close-coupled TWC, under-floor LNT + SCR configuration. (Further discussed in
Chapter 3.)
Close-Coupled
TWCs
Under-floor
LNT
Close-Coupled
TWCs
Under-floor
LNT
SCR
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Figure 23. Layout of the passive-NH3, urea-less TWC + SCR approach, Li et al [282]. (Further
discussed in Chapter 4.)
Figure 24. Layout of the emissions system for a light-duty diesel vehicle containing a diesel
oxidation catalyst (DOC) and LNT + SCR configuration and a diesel particulate filter (DPF), McCabe et al. [283].
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Figure 25. Layout of the modern medium- or heavy-duty DPF-SCR system (based on urea),
Johnson et al. [284].
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