Three-way converters

These results are direct measurements of the CO concentrations which are present at the inlet to a three-way converter under realistic operating conditions. The concentrations of other exhaust components at the converter inlet can be estimated using concentration crossplots, for each component versus CO, obtained from time-averaged measurements at constant air-fuel ratio settings. 

Ce02-supported noble-metal catalysts such as Pt, Pd and Rh are of interest because of their importance in the so-called three-way converter catalysts (TWC), designed to reduce emissions of CO, NOx and uncombusted hydrocarbons in the environment and to purify vehicle-exhaust emissions. Such catalysts are also of current interest in steam reforming of methane and other hydrocarbons. Conventional practical catalysts for steam reforming consist of nickel supported on a ceramic carrier with a low surface area and are used at high temperatures of 900 C. This catalyst suffers from coke formation which suppresses the intrinsic catalyst activity. Promoters such as Mo are added to suppress coke formation. Recently, Inui etal(l991) have developed a novel Ni-based composite... 

Hydrocarbons in the exhaust react with oxygen and with NO. Although these reactions have had much less attention than oxidation of CO and reduction of NO by CO, reactions of hydrocarbons play an important role in the overall reaction mechanism of the three-way converter, particularly because the converter is by no means a homogeneously mixed reactor, see Fig. 5.15 [57]. Hence, zones exist where e.g. ethylene and nitrogen atoms are coadsorbed on the noble... 

Other configurations of fuel cell vehicles can be realized combining the advantages of different types of storage systems. As an example, the Fig. 5.24 shows the combination of rechargeable batteries with a super capacitor system. In this case, a three-way converter is required to connect the two storage systems with the fuel cell stack and interface the different voltage versus current characteristics of the devices interconnected [46]. 

Aim of the present contribution is the investigation of the surface chemical role played by the presence of Ce02 in some Pt catalysts supported on alumina, i.e., some model catalysts fairly similar to those actually working in three-way converters. 

Two types of catalytic converters are currently being used for meeting the passenger car emission standards in the U.S. three-way converters and dualbed converters. Both converters contain three-way catalysts, but with the dual-bed converter the three-way catalyst is followed by an air injection/ oxidation catalyst system. As for the earlier oxidation catalysts two forms of catalyst support are used pellets (thermally stable transitional alumina) and monoliths (cordierite honeycombs coated with a thin alumina washcoat). Figure 7 shows four catalytic converters currently being used by General Motors. 

Most of today s motor vehicles contain a catalytic converter to rednce the toxicity of emissions from their internal comhnstion engines. These catalytic converters have been nsed for many years as a means to comply with ERA emission standards. Since 1981 most vehicles have nsed three-way converters. They convert emissions to safer prodncts in three ways. Exhansts, before they reach the catalytic converter, contain nitrogen oxides, carbon monoxide, and some amonnt of noncombnsted hydrocarbons from gasoline. In the catalytic converter they are converted to less toxic prodncts, as shown in the following examples ... 

Early converters were designed to oxidize CO, H2, and unburnt hydrocarbons (oxidizing converters). More recently, "three-way converters have been developed to oxidize hydrocarbons, H2, and CO, and reduce NO emissions simultaneously. This goal is achieved by using a suitably promoted Platinum-Rhodium catalyst supported on alumina and by carefully controling the air/fuel ratio. The catalyst is a thin porous alumina layer "wash-coated" on the wall of the monolith channels. Typically the wash-coat is about a few tens of a micrometer thick, except at the corners of the channel where it is thicker. 

The three-way converter. Since the concentration of NO is very small, the heat generated by the reduction reaction is expected to have a minor effect on temperature and conversion predictions (Chen et al. (1988)). Owing to the absence of accurate data on NO, reduction kinetics, a preliminary approach to three-way catalytic converters consists of simulating only oxidation reactions under stoichiometric conditions. The standard parameters used in the simulations are given in Table 6 except that the feed mole fraction of O2 and N2 are now 0.01225... 

Figure 3.7. Effect of feed temperature on solid temperature profile. Three-way converter.

The non uniform velocity distribution represented by two macrochannels dealt with in section 3.2.2 gives nearly the same light-off time as a uniform velocity distribution when the kinetic rate constants are increased by 10%. This clearly shows that various parameters of different physical origin can compensate each other. This indicates that it is impossible to accurately predict conversions in three-way converters due to the lack of reliable kinetic data. 

In the oxidizing converter, it has been accepted that heat and mass transfer processes are the main controlling steps except at cold start. An analysis of the converter behaviour at stoichiometric conditions shows that this conclusion is not a general one, and that transfer processes and chemical reactions are competing in three-way converters. This suggests that more reliable kinetic rate expressions are required to obtain reliable simulation results. 

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