Automobile exhaust, function

Catalyst Function. Automobile exhaust catalysts are perfect examples of materials that accelerate a chemical reaction but are not consumed. Reactions are completed on the catalyst surface and the products leave. Thus the catalyst performs its function over and over again. The catalyst also permits reactions to occur at considerably lower temperatures. For instance, CO reacts with oxygen above 700°C at a substantial rate. An automobile exhaust catalyst enables the reaction to occur at a temperature of about 250°C and at a much faster rate and in a smaller reactor volume. This is also the case for the combustion of hydrocarbons. 

Emission Control Technologies. The California low emission vehicle (LEV) standards has spawned iavestigations iato new technologies and methods for further reducing automobile exhaust emissions. The target is to reduce emissions, especially HC emissions, which occur during the two minutes after a vehicle has been started (53). It is estimated that 70 to 80% of nonmethane HCs that escape conversion by the catalytic converter do so during this time before the catalyst is fully functional. 

Figure 7. Effects of Rh location on the performance of fresh automobile exhaust catalyst beads as a function of engine air-fuel ratio (A/F). Adapted from Hegedus et ai [33] and reprinted from Hegedus and McCabe [25], p. 94, by courtesy of Marcel Dekker, Inc.

Automobile exhaust catalysts typically contain noble metals such as Pt, Pd and Rh with a ceria promoter supported on alumina. Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NO ) by reaction with carbon monoxide, although the increasing use of Pd has been proposed. For example, recent X-ray absorption spectroscopy studies of Holies and Davis show that the average oxidation state of Pd was affected by gaseous environment with an average oxidation slate between 0 and +2 for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer. 

One of the early problems with catalytic control of automobile exhaust emissions was during the few minutes immediately after starting the engine when the cold catalytic systems did not function. This was solved by developing a porous zeolite which traps the unburned hydrocarbons while the catalysts are still cold [15]. Once the catalysts have warmed up, the zeolite canister also warms, releasing the trapped hydrocarbons to the catalytic systems to perform their important control reactions. 

Supports, or carriers, perform many functions, but most imponant is maintenance of high surface area for the active component. This is best illustrated with platinum, an important catalytic metal widely used for catalytic reforming " and automobile exhaust clean-up. For high activity, platinum crystallites must have the highest surface area possible. Figure 2.3 shows the relationship between dispersion, defined as the fraction of platinum atoms on the surface of the spherical crystallite, and diameter of the sphere. Dispersion decreases very rapidly between 1 and 10 nm. Ideally, platinum crystallites should be as smalt as possible, but certainly... 

Based on the above discussion, the function of the wet-oxidation catalysts should be confined to (i) activation of oxygen and (ii) direct electron transfer with the reactants (redox reaction) in the first step of the reaction. CeO seems to effectively contribute to both factors. CeOj behaves quite differently from other oxides of lanthanide and is always a constituent of automobile-exhaust purification catalysts. It stabilizes supports and keeps high surface area [64,65], prevents the sintering of precious metals and, thus, stabilizes their dispersed state [66,67], and acts as an oxygen reservoir [68,72]. When combined with precious metals, it works in various reactions other than the purification of vehicle exhausts e.g., detoxification of NjO, methanol decomposition, methanol synthesis, combustion of formaldehyde, etc [47,73-75]. Precious metals are remarkably activated and behave quite differently on CeO compared with their action on other supports. 

In confined spaces it is necessary to keep the concentration of CO from automobile exhaust down to acceptable limits. Therefore, CO infrared gas analyzers have been installed on a large scale in such places (e.g., tunnels, parking areas, etc.). In medicine, respiratory units for lung-function investigations have used infrared analyzers. The use of infrared methods for qualitative and quantitative information on a variety of gases and vapors has been discussed earlier in this chapter in connection with anesthesiology and toxicology applications. 

Catalyst poisons are materials that significantly alter, reduce, or completely destroy the activity of a given catalyst. Such materials generally function by binding strongly and (effectively) irreversibly to the specific surface sites necessary for the functioning of the desired process. Particularly troublesome materials in that sense are sulfur-containing compounds, especially thiols and thioethers. For example, the catalytic converters used to oxidize hydrocarbon residues in automobile exhausts will rapidly lose their effectiveness if exposed to such materials. 

The oxygen sensors, which are central function systems for the automobile exhaust gas purification system, are only used at the stoichiometric point for the three-way catalyst. In order to improve the fuel economy, it needs the combustion control in the lean burn region. Therefore, the sensor is required to have output controlling at the air/fuel ratio in the lean burn region. 

Furthermore, as clearly demonstrated by the three-way catalytic system for gasoline vehicles and DPNR explained in this chapter, the automobile exhaust gas treatment catalysts do not function alone, but rather require a highly controlled engine system. It is therefore necessary to further develop technologies to functionally integrate engine and after-treatment devices in which the soot oxidation catalyst is a component. 

To date there has been limited commercialization of devices that use the ER effect. However, ER fluids worthy of commercialization have been developed and this will accelerate further development of devices. For example, an ER cutting machine has been developed [75, 76]. This machine incorporates a variable rodless cylinder that functions by using an ER fluid with a sulfonated polymer. This machine cuts brittle ceramics, using ER fluid to control the cutting speed very accurately. It is used to manufacture a catalyst for automobile exhaust gas. 

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