Catalytic washcoat

The following, well-acceptable assumptions are applied in the presented models of automobile exhaust gas converters Ideal gas behavior and constant pressure are considered (system open to ambient atmosphere, very low pressure drop). Relatively low concentration of key reactants enables to approximate diffusion processes by the Fick s law and to assume negligible change in the number of moles caused by the reactions. Axial dispersion and heat conduction effects in the flowing gas can be neglected due to short residence times ( 0.1 s). The description of heat and mass transfer between bulk of flowing gas and catalytic washcoat is approximated by distributed transfer coefficients, calculated from suitable correlations (cf. Section III.C). All physical properties of gas (cp, p, p, X, Z>k) and solid phase heat capacity are evaluated in dependence on temperature. Effective heat conductivity, density and heat capacity are used for the entire solid phase, which consists of catalytic washcoat layer and monolith substrate (wall). 

In the spatially ID model of the monolith channel, no transverse concentration gradients inside the catalytic washcoat layer are considered, i.e. the influence of internal diffusion is neglected or included in the employed reaction-kinetic parameters. It may lead to the over-prediction of the achieved conversions, particularly with the increasing thickness of the washcoat layer (cfi, e.g., Aris, 1975 Kryl et al., 2005 Tronconi and Beretta, 1999 Zygourakis and Aris, 1983). To overcome this limitation, the effectiveness-factor concept can be used in a limited extent (cf. Section III.D). Despite the drawbacks coming from the fact that internal diffusion effects are implicitly included in the reaction kinetics, the ID plug-flow model is extensively used in automotive industry, thanks to the reasonable combination of physical reliability and short computation times. 

C. Heat and Mass Transfer between Bulk Gas and Catalytic Washcoat... 

Mass and heat transfer between flowing gas and catalytic washcoat layer along the monolith channel are in ID gas models approximated by distributed... 

Porous catalytic washcoat exhibits bimodal pore size distribution with larger macropores (rp 100-500 nm) among individual support material particles (e.g. - , zeolites), and small meso-/micropores (rp 3-6nm) inside the particles. Typical pore size distribution and electron microscopy images of y-A C -based washcoat can be found, e.g. in Stary et al. (2006) and Koci et al. 

Then the classical Thiele modulus ( ) and the effectiveness factor (t/), expressing the extent of internal diffusion limitations in the catalytic washcoat layer of thickness 8, can be calculated according to (cf. Aris, 1975 Froment and Bischoff, 1979, 1990)... 

When the internal diffusion effects are considered explicitly, concentration variations in the catalytic washcoat layer are modeled both in the axial (z) and the transverse (radial, r) directions. Simple slab geometry is chosen for the washcoat layer, since the ratio of the washcoat thickness to the channel diameter is low. The layer is characterized by its external surface density a and the mean thickness <5. It can be assumed that there are no temperature gradients in the transverse direction within the washcoat layer and in the wall of the channel because of the sufficiently high heat conductivity, cf., e.g. Wanker et al. 

There exists a relation between the volume of catalytic washcoat layer (represented in the ID model by the volume fraction q>s in the solid phase) and the characteristic thickness of the layer 8 used in the spatially 2D model. This relation depends on the chosen washcoat geometry—for slab geometry used here it is... 

At the cold start of the engine the catalyst is not able to oxidize carbon monoxide and hydrocarbons present in the exhaust. Therefore, zeolites are added into y-Al203-based catalytic washcoat for HC adsorption at low temperatures, resulting in an integrated adsorber-reactor system (Jirat et al., 2001 Kryl et al., 2005). For optimum operation of such a system, the consecutive HC desorption induced by increasing temperature should not occur earlier than the catalyst light-off. 

To model mass and energy transport in monolith systems, several approaches are discussed, leading from a representative channel spatially ID approach to 2D (1D+1D) modeling explicitly including washcoat diffusion. Correlations are given to describe heat and mass transfer between bulk gas phase and catalytic washcoat. For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model of the computer-reconstructed washcoat section can be employed. 

Several length-scales have to be considered in a number of applications. For example, in a typical monolith reactor used as automobile exhaust catalytic converter the reactor length and diameter are on the order of decimeters, the monolith channel dimension is on the order of 1 mm, the thickness of the catalytic washcoat layer is on the order of tens of micrometers, the dimension of the pores in the washcoat is on the order of 1 pm, the diameter of active noble metal catalyst particles can be on the order of nanometers, and the reacting molecules are on the order of angstroms cf. Fig. 1. The modeling of such reactors is a typical multiscale problem (Hoebink and Marin, 1998). Electron microscopy accompanied by other techniques can provide information on particle size, shape, and chemical composition. Local composition and particle size of dispersed nanoparticles in the porous structure of the catalyst affect catalytic activity and selectivity (Bell, 2003). 

Some XPS and SEM-EDX analysis were also realized on the engine bench aged catalysts in order to measure the surface composition of the catalytic washcoat which was modified by the poisons layer deposit. 

Samples under study are commercial cordierite honeycomb catalysts. The catalytic washcoat is conventionally composed of ceria-promoted transition alumina-supported 5Pt/lRli (weight %). Samples are cut from original converters in the form of cylinders of 1 inch (diameter) X 3 inches (length) to meet the geometrical requirements of the experimental set-up (see below). 

Dif actograms are obtained from pure catalytic washcoat (free of cordierite) after immersion of monoliths in an ultrasonic bath in distilled water and subsequent dr5nng of the resulting suspension. For precise measurements of X-ray line positions, ceria (from the catalysts itself) is used as internal standard. 

Running experiments concern mechanical and morphological modifications of catalytic washcoat resulting from simultaneous temperature and atmosphere cyclings. 

Figure 8.1 Representation of the several scales in a cat alytic monolithic reactw. (a) Monolith honeycomb [1] (Source Araki et at [1]. Reproduced with permission of Elsevier), (b) Single washcoated monolith channel (c) Catalytic washcoat.

Catalytic monoliths are structured heterogeneous reactors. These two features require the consideration of the reaction-diffusion problem in the catalytic washcoat (the internal region), which can be designed with much more independence from the external domain, when compared to nonstructured reactors. In general, the operating conditions will be such that... 

The measurement and calculation of the effective diffusivity in a catalytic washcoat has been given in [96] from a ID model for ceramic (e.g., cordierite) monoliths. A quick survey... 

Effectiveness factor calculation methods in catalytic washcoats... 

D effective diffusivity in a catalytic washcoat, m /s dfiim liquid film thickness... 

Modeling of the reactor domain shown in Figure 11.8 requires simultaneous solution of continuity and conservation (of momentum, energy, and species mass) equations in the fluid and catalytic washcoat phases in every channel as well as of the heat flow between each channel in three dimensions. These sets of equations and the necessary boundary conditions, presented in Tables 11.2 and 11.3, respectively, can be simplified as a result of the following assumptions arising fi om the particular positioning of the channels ... 

Catalytic washcoat phase Equation of continuity 3Vxb dVyb 3v ... 

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