Particle washcoat

Figure 10.5. Monolith, washcoat and noble metal particles in an automotive exhaust catalyst.

Platinum serves as the catalyst for the oxidation of CO and hydrocarbons. It is relatively insensitive to contamination by lead or sulfur. At high temperatures it is not known to dissolve in the washcoat, but sintering into larger particles may lead to a substantial loss of platinum surface area with dramatic consequences for the overall oxidation activity. 

Generally, the preparation of washcoated structured catalysts is governed by several parameters, such as the nature and particle size of the precursor powder, loading of powder, nature and concentration of dispersants, temperature of the slurry, use of binders in the slurry and deposition of a primer layer on the monolith. 

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. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

Washcoat thickness is analogous to particle size in that reactants must penetrate its pore structure and interact with the dispersed active sites. The products produced must diffuse through the structure and out into the bulk gas. This phenomenon differs from that involving a particle in that only the gas-solid washcoat surface is available since the other side is bonded to the wall of the monolith. 

In diagnostic tests crushing of the particles will not always be conclusive. Egg-shell catalysts or other types, zeolites and washcoated monoliths are exceptions. In washcoated monoliths the layer thickness is generally already quite low (<50 (im) and crushing will not yield smaller sizes. Cracking catalysts consist often of zeo-litic crystals of /im dimensions and a binder yielding particles of about 30 /im. If diffusion limitations exist in the zeolitic crystals, crushing will not eliminate these. 

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). 

The macro-porosity emacro and the correlation function corresponding to the macro-pore size distribution of the washcoat were evaluated from the SEM images of a typical three-way catalytic monolith, cf. Fig. 25. The reconstructed medium is represented by a 3D matrix and exhibits the same porosity and correlation function (distribution of macro-pores) as the original porous catalyst. It contains the information about the phase at each discretization point— either gas (macro-pore) or solid (meso-porous Pt/y-Al203 particle). In the first approximation, no difference is made between y-Al203 and Ce02 support, and the catalytic sites of only one type (Pt) are considered with uniform distribution. 

The characteristic length of the washcoat section to be simulated is 10 pm thus, we may consider constant temperature profile on this scale. Since the volume diffusion in the macro-pores is much faster than the Knudsen diffusion in the meso-pores of the y-Al203 particles, we may further assume that the CO and 02 gas concentrations in the macro-pores are constant within the simulated washcoat section. For the surface-deposited components CO and O, a zero diffusivity is used, i.e., Df1 — 0. For gaseous CO and 02, the effective diffusivi-ties are based on the Knudsen diffusivity in the meso-pores (with diameter T0nm) of the y-Al203. 

Most of the current converters consist of a flow-through ceramic monolith with its channel walls covered with a high-surface-area 7-AI2O3 layer (the washcoat) which contains the active catalyst particles. The monolith is composed of cordicrite, a mineral with the composition 2MgO 2AI2O3 5Si02. The chemical composition of a modern TWC is quite complex. In addition to alumina, the washcoat contains up to 30 wt% base metal oxide additives, added for many purposes. The most common additives are ceria and lanthana in many formulations BaO and Zr02 are used, and in some converters NiO is present. The major active constituents of the washcoat are the noble metis Pt, Pd, and Rh (typically 1-3 g). Most of the TWC systems in use today are still based on Pt and Rh in a ratio of about 10 1. 

For the TBR, spherical catalyst particles of uniform size with the catalytically active material either uniformly distributed throughout the catalyst or present in a shell were considered. For the MR, channels of square cross section were assumed to have walls covered by the washcoat distributed in such a way that the comers are approximated by the circle-in-square geometry, while the sides are approximated by a planar slab geometry. The volumetric load of catalytic material was a function of the washcoat thickness... 

Figure 16 is constructed for = 10 m and = 0.50. The highest STY is obtained for the TBR using the smallest particles possible. The pressure drop is approximately 5 bar, which is close to what can be accepted. Both pressure drop and STYy decrease fast with increasing particle size, and for the 2-mm particles the STYy has dropped below the highest STYy that can be reached with the washcoated MR. For both reactors the selectivity decreases with increasing catalyst thickness. Selectivity is higher for the MR. The diffusion... 

Possibility to use standard catalyst particles. When hollow extnidates or ring-shaped pellets are used as the catalyst material for the BSR, the catalyst can be manufactured according to standard procedures. Consequently, no additional catalyst development is necessary to apply an existing catalyst in a BSR. This is an advantage over the monolithic reactors, because with those reactors one has to deal with the peculiarities of the washcoat or the ceramic carrier body serving as the support of the active sites. 

Various methods are possible to incorporate a catalytically active phase to the monolith [48-59,85-95]. Figure 3 shows the general scheme for preparing a monolithic catalyst structure from a washcoated monolith. In fact, no fundamental differences exist between incorporation of an active phase in a conventional support (beads, extrudates, spheres) and in monoliths. In practice, precautions are needed because, besides concentration profile on a particle scale, such profile over the length of the monolith also can easily arise. 

Combined with the use of precious metal catalyzed washcoats deposited on the walls, microchannel reactors can realize nearly 10 times reduction in reactor size compared with that of a process that utilizes catalyst particles. The washcoat thickness is usually less than lOOjit and provides greater structural stability. This stability arises from smaller thermal expansion ratios and lower temperature gradients. 

Fig. 5.7. The three-way catalyst consists of platinum and rhodium (or palladium) metal particles on a porous oxidic washcoat, applied on a ceramic monolith.

To avoid high-pressure drop and clogging problems in randomly packed micro-structured reactors, multichannel reactors with catalytically active walls were proposed. The main problem is how to deposit a uniform catalyst layer in the microchannels. The thickness and porosity of the catalyst layer should also be enough to guarantee an adequate surface area. It is also possible to use methods of in situ growth of an oxide layer (e.g., by anodic oxidation of a metal substrate [169]) to form a washcoat of sufficient thickness to deposit an active component (metal particles). Suzuki et al. [170] have used this method to prepare Pt supported on nanoporous alumina obtained by anodic oxidation and integrate it into a microcatalytic combustor. Zeolite-coated microchannel reactors could be also prepared and they demonstrate higher productivity per mass of catalyst than conventional packed beds [171]. Also, a MSR where the microchannels are coated by a carbon layer, could be prepared [172]. 

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