184.108.40.206.1 Three-Way Catalysts
Two general options can be considered for automotive pollution control: prevention and cure. End-of-pipe technologies can be viewed as the cure option. Environmental legislation has largely been responsible for the introduction and subsequent development of end-of-pipe technologies. The first legislative controls of auto emissions were introduced in California as early as 1947. The chronic local pollution episodes experienced there have led to the most severe emission standards in the world and, as such, have shaped legislative policy elsewhere. The first national (federal) exhaust emission standards in the United States were introduced in 1968. Since then, environmental legislation has been adopted in many countries worldwide . Most countries base their automotive environmental legislation on legislation and test procedures introduced in the United States, Europe, or Japan.
Initial strategies to reduce emissions were based on prevention: engine modification (crankcase controls) allowed CO and HCs emissions to be reduced without tackling exhaust gases themselves. After the introduction of mandatory exhaust controls in the United States, Canada, and Japan and prior to the 1975 model year, emission limits were also met by engine modification without the use of catalytic control. However, increasingly stringent limits meant that end-of-pipe catalytic control eventually had to be introduced. The significant attenuation of CO and HCs required in that year led to the introduction of the so-called conventional oxidation catalysts (COCs — Pd, Pt/Al2O3) together with the use of unleaded gasoline. Engines were operated lean to provide the O2 necessary for the oxidation catalyst to function. Some degree of NO, control was achieved by exhaust-gas recirculation. As NO-, limits became stricter, COCs were superseded by a combination of COC and three-way catalysts (TWCs) and, finally, by TWCs only.
TWCs represent the current state-of-the-art technology for end-of-pipe emission control for gasoline automobiles. Their function is the simultaneous attenuation of the emission levels of three primary classes of pollutants — CO, NO,, and HCs, and their name derives from the ability to simultaneously convert the three:
TWCs represent one the technological success stories of the past 30 years. Pushed by ever more stringent legislation, their formulations have evolved significantly since they were first introduced. However, their improvement is a result of a combination of factors, not just better formulations: materials advances (better mechanical stability), improved engine characteristics (onboard diagnostics, improved carburation), improved fuel characteristics (reformulated fuels), etc., have all contributed to their success.
The physical arrangement of a modern automotive catalyst consists of a thin layer of the porous catalytic material (wash-coat) coated on the channel walls of a ceramic (cordierite, 2MgO.2Al2O3.5SiO2) or sometimes metal monolith. The channels of the monolith are axially orientated in the direction of the exhaust gas flow to ensure efficient flow-through and prevent a pressure buildup in the exhaust system. As would be expected, the exact composition and manufacturing processes used vary with the manufacturer and are subject to confidentiality, but a number of general observations can be made (see [14, 15] and references therein). They all contain highly dispersed noble metals (NMs) particles supported on doped and stabilized (multi-component) high surface area alumina support. Noble metals represent the key component of the TWCs, as the catalytic activity occurs at the metal center. Specifically, Rh is added to promote NO dissociation, while Pt and Pd are the metals of choice to promote the oxidation reactions. Interaction with the various components of the wash-coat critically affects the activity of the supported NMs. Notably, Pd has extensively been added to TWC formulations starting from the mid-1990s in an effort to produce less expensive, Rh-free materials. Better A/F control and modification of the support provided high NO, conversion, comparable with the traditional Rh/Pt catalyst. With respect to the support, the replacement of CeO2 with better-performing CeO2-based materials, specifically CeO2-ZrO2 mixed oxides, in the formulations of the TWCs has significantly increased their performance. The following positive effects have been attributed to the ceria-based components:
Promotion of the noble metal dispersion Enhancement of the thermal stability of the Al2O3 support Promotion of the water-gas-shift (WGS) and steam-reforming reactions Promotion of catalytic activity at the interfacial metal-support sites Promotion of CO removal through oxidation, employing lattice oxygen Storage and release of oxygen under respectively lean and rich conditions; the quantity of oxygen stored or released is known as the oxygen storage capacity (OSC)
The last ability is particularly important to minimize the effect of inevitable oscillations in the A/F ratio. In fact, the A/F ratio must be kept within a narrow window near the stoichiometric composition, as indicated in Figure 11.1. This is achieved by monitoring the oxygen content with a ^-sensor to adjust the A/F ratio. A second ^-sensor placed after the converter is used as part of an onboard diagnostics (OBD) system to check the correct functioning of the TWC.
Once operational, TWCs convert more than 98% of the pollutants. The outstanding issue in relation to TWCs is emissions just after start-up, before the catalyst has reached operating temperature. In addition to the development of new TWCs that are active at low temperature, other possibilities being considered include the use of upstream HC adsorbers to trap initial HC emissions, which are subsequently converted after heat-up; electrical heating of the TWC; and the development of close-coupled catalysts (CCCs): TWCs with high thermal stability to be placed closer to the engine. In this way, the catalyst warm-up time is reduced and limits emissions immediately after start-up .
Lean engine operation (diesel or lean-burn gasoline) has the advantage of producing less NO, as the temperature of combustion is lower, and less CO and HCs, as the combustion is more complete due to the excess of oxygen. The improved fuel economy also means that overall CO2 emissions are reduced. However, even though less primary NO, is formed, the oxidizing conditions (see Table 11.1) mean that TWCs are ineffective for NO, reduction. Extensive research has been conducted into possible catalysts to reduce NO, under the oxidizing conditions of the exhaust, using the HCs present (see Table 11.1). This research has been reviewed [15, 16]. The materials investigated can be grouped into the following categories: Pt/Al2O3 and related systems, Cu-ZSM5 and related systems, metal oxide catalysts, and Ag-based systems. Despite all of the research, it is true to say that all suffer from problems such as insufficient activity or low hydrothermal stability, which make them unsuitable for widespread application in transport.
A different approach to the problem of lean DeNO,, that of the storage-reduction catalyst (SRC), has been developed by Toyota [17, 18]. Here, the NO, produced in lean operation is trapped or stored on a Pt-Ba catalyst. During short switches to rich operation, the stored NO, species pollutants are reduced. A disadvantage is the sulfur sensibility of the storage material, which adsorbs SO, species more strongly than NO, species. This means that it must be used only with low sulfur content fuels. However, the Toyota process is by far the most effective NO, removal method available, and its commercialization has probably largely contributed to a decrease in interest in direct catalytic solutions.
Particulate matter (PM) remains a particular problem for diesel engines, and two technologies are commonly used to control emissions: diesel oxidation catalysts for the liquid fraction of the PM  and particulate filters for the dry carbon (soot) fraction . As indicated above, DeNO, is also an unresolved problem in the case of diesel engines, with the added complication that the amount of gaseous HCs to perform the reduction is small. In principle, the liquid PM could be used to reduce NO,, but this is even more difficult to achieve. The idea of adding a reducing agent has also been tested. For example, urea is a potential solution that has been demonstrated for trucks . However, there are a number of general problems associated with this approach that makes its application to personal automobiles doubtful. These include the space considerations of including an additional (urea) tank onboard, the risk of ammonia slip, and the absence of a urea distribution network.
Pollution prevention in many ways appears to be a more obvious solution, and it is indeed an approach that is actively pursued. As may be inferred from the above, a whole range of measures aimed at pollution prevention are in fact adopted in conjunction with end-of-pipe technologies. These include engine modification (e.g., exhaust-gas recirculation), and the use of reformulated (with low aromatic content) and low-sulfur fuels pollution. For example, the problem of Pb was tackled by removing lead from fuels. The same approach is underway for SO, emissions with the use of low-sulfur-content fuels. A more preventive approach, for example, is the use of alternative fuels such as liquefied petroleum gas (LPG) and natural gas (NG), which are intrinsically cleaner than diesel or gasoline, resulting in much lower values for the pollutants than those shown in Table 11.1 . These could also be combined with postcombustion catalytic control. However, as will be discussed below, the transformation to a hydrogen economy would represent a switch in strategy from the combined approach currently used with end-of-pipe technologies to a purely preventive approach; indeed, it would eliminate the need for these technologies. In many ways, a transfer to a renewable hydrogen economy is the ultimate preventive measure.
11.1.3 Advantages/Disadvantages of H2
While the advantages of H2 as a fuel source are considerable and have led to much enthusiasm in some quarters, there are of course also drawbacks. Although there have been reports on the potentially negative environmental consequences of the release of large amounts of H2 into the atmosphere [23, 24], there seems to be wider general agreement that when it is possible to use H2 it would be a good thing. In most cases, therefore, the drawbacks highlighted relate not to the use of hydrogen but on how to arrive at the hydrogen economy, and these are thus more technical in nature . The challenge of establishing a hydrogen economy can be divided into four categories: production, distribution, storage, and end use. Here, we will limit the comments mainly to the transport sector. These initial considerations will be discussed in more detail in Sections 11.2 to 11.4.
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