Three-way catalytic converters TWCs

Due to the introduetion of the three-way catalytic converter (TWC) in Europe in the mid-eighties, determination of platinum group elements (PGE) in environmental compartments has received elevated interest over die past two decades. While improving air quahty by converting hydrocarbons, carbon monoxide, and nitrogen oxides into the less harmful substances carbon dioxide, nitrogen, and water, the use of these devices constitutes a new source for the widespread emission of PGE into the environment. 

Mathematical model of three-way catalytic converter (TWC) has been developed. It includes mass balances in the bulk gas, mass transfer to the porous catalyst, diffusion in the porous structure and simultaneous reactions described by a complex microkinetic scheme of 31 reaction steps for 8 gas components (CO, O2, C2H4, C2H2, NO, NO2, N2O and CO2) and a number of surface reaction intermediates. Enthalpy balances for the gas and solid phase are also included. The method of lines has been used for the transformation of a set of partial differential equations (PDEs) to a large and stiff system of ordinary differential equations (ODEs . Multiple steady and oscillatory states (simple and doubly-periodic) and complex spatiotemporal patterns have been found for a certain range of operation parameters. The methodology of studies of such systems with complex dynamic patterns is briefly introduced and the undesired behaviour of the used integrator is discussed. 

Three-Way Catalytic Converters (TWCs) An Introduction s61 Reactions products H2+CO2... 

Three-way catalytic converters (TWC) are designed to reduce three types of harmful automobile emissions carbon monoxide (CO), unburned hydrocarbons in the fuel (HC), and nitrogen oxides (NOx). The term nitrogen oxides refers to both NO2 and NO. The TWC accomplishes these tasks using precious metal catalysts (platinum, palladium, and rhodium) for chemical reactions that take place at high temperatures (e.g., 1000-1600°F) and short residence times (-0.05 s). 

Three-way catalytic converter chemical reactions and surface chemistry, 10 46-50 design of, 10 39-46 surface chemistry and reaction mechanisms, 10 49-50 Three-way control valves, 20 686 Three-way conversion (TWC) catalysts, 10 38, 40... 

Pellets and ceramic monolithic substrate structures were initially involved in three-way catalytic converters for washcoat deposition, while metal foil monolithic substrates were also introduced since the late 1970s. TWCs manufactures were soon concentrated on cordierite (2Mg0-2Al203-5Si02) ceramic monoliths or on Fe-based alloys foil monoliths (iron-chromiimi—alimiimmi ferritic steels). Both options are used nowadays, although ceramic monoliths are preferably used, despite the several advantages of the latter [2]. 

Since nitrous oxide, NjO, is a designated "greenhouse" gas, and may contribute to depletion of the ozone layer, its removal from emissions to atmosphere is desirable [1]. However, there are several reports that NjO can be formed at low selectivity as an undesirable by-product of NO+CO conversions during the initial warm-up-from-cold periods in three-way-catalytic (TWC) converters or components thereof [1-3]. TWC s commonly contain Rhodium and Ceria and although N,0 dissociation over RhjO, has been extensively studied [4], the following are among mechanistic possibilities as yet... 

Fig. 7.10. Three-Way Catalytic (TWC) converter profile for conversion vs. air-to-fuel ratio.

The intermittent operation mode of the Otto-cycle engine leads to a pulsed-flow operation of the three-way catalyst (TWC). Because air always enters into the cylinders when the engine is shut off and restarted, an air plug (cylinders filled with air) will precede the exhaust gas pulse (fig. 2). The objective of our study was to gain information about the dynamic behaviour of TWC s under pulsed-flow operation and to find a suitable model, that enables a prediction of the optimal operation mode of the catalytic converter. 

In the case of automotive gas exhaust emissions, the SCR process is Umited by technical problems, so that it has been dealt with only recently, essentially for emission control for trucks. From the general point of view, deNOx for cars is still an unresolved problem and different processes are under investigation. The development of three-way catalysts (TWC) over the last 30 years has been a remarkable technical achievement Current catalytic converters, based on various combinations of Pt, Pd and Rh as the active ingredients, provide a very high level of emission control for the removal of CO, NOx and unburnt hydrocarbons. Under normal working conditions for a stoichiometric gasoUne engine (temperatures from about 400 to 800 °C) they have proved to be efficient and reliable. 

Recently, nonstationary experiments on the microkinetics of typical reactions on Pt/Ce/y-AbOs three-way catalyst were performed (Harmsen et al., 2(XX), 2(X)la,b). Their results were evaluated in the form of detaUed reaction schema. Mukadi and Hayes (2(X)2) used these kinetic expressions in the model of TWC monolith converter. They pointed to the significance of the effects of diffusion in the catalytic washcoat on the performance... 

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