Washcoated structured catalysts

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. 

The results of the parametric studies (e.g., the influence of noble metal distribution and correlation length) provide a better understanding of the reaction-transport effects in porous, supported heterogeneous catalysts (Bhattacharya et al., 2004). In the combination with semi-deterministic methods of the reconstruction (simulation of the catalyst preparation process), the results can be used for the optimization of the washcoat structure. 

As an alternative to foams, fibrous materials in the form of tissues or filters may be used as structured catalysts [53,62-65]. Of particular interest for MSRs are sintered metal fibers (SMFs) [64,65]. These materials have open and homogeneous structures with porosities of 70-90% and high thermal conductivities, which ensure homogeneous temperatures in the catalytic bed. SMF materials consist of thin metallic fibers of 10-20 pm diameter. The small fiber diameters ensure high external fluid-solid mass transfer [66]. The fibers can be covered with a homogeneous layer of zeolitic material [66] or a washcoat, which can be impregnated with an active material [67,68]. Luther et al. [67] integrated the SMF catalyst in a... 

Aluminas also play an important part in the preparation of structured catalysts. They can be present on or constitute the structured support of catalysts. Common shapes are monohthic honeycombs or open-cell foams. Aluminas can be used as washcoat, a high-surface-area layer that gives the geometrical framework a suitable morphology to support and disperse the catalytically active phase, or they can be used as primer, an intermediate layer between the geometrical support and the washcoat that acts as glue between the two layers (433). Alumina plays a central role for the preparation of efficient automotive three-way-catalysts (434). 

To maintain catalytic performance during long-term vehicle operation, porosity of the cordierite material is important to fix the catalyst layer on the cell wall of the honeycomb substrates. Figure 13.1.7 shows the washcoat layer, which contains y-alumina composite carrier and precious metal catalyst, on the porous cell wall surface. This photograph shows a 6mil/400cpi standard cell structure catalyst made of 35% porosity cordierite. 

Pennemann, H., Hessel, V., Kolb, G Lowe, H. and Zapf, R. (2008) Washcoat-based catalysts for the partial oxidation of propane using a micro structured reactor. Chem. Eng. J. 135, 66-73. 

Since commercially available monolithic supports for environmental catalysts are not suitable for this class of applications, as explained above, in the early studies homemade high conductivity structured catalysts were prepared by assembling washcoated slabs of aluminium or stainless steel to form plate-type catalytic cartridges, which were also equipped with thermowells for sliding thermocouples in order to monitor the temperature distributions. The washcoat consisted of Pd (3%w/w) on y-Al203, and the catalysts were tested in the oxidation of CO, selected as a model exothermic reaction. 

The process has been commercially implemented in Japan since 1977 [1] and a decade later in the U.S., Germany and Austria. The catalysts are based on a support material (titanium oxide in the anatase form), the active components (oxides of vanadium, tungsten and, in some cases, of molybdenum) and modifiers, dopants and additives to improve the performance, especially stability. The catalyst is then deposited over a structured support based on a ceramic or metallic honeycomb and plate-type structure on which a washcoat is then deposited. The honeycomb form usually is an extruded ceramic with the catalyst either incorporated throughout the stmcture (homogeneous) or coated on the substrate. In the plate geometry, the support material is generally coated with the catalyst. 

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). 

The use of proven catalyst recipes would greatly curtail development times, and the absence of extraneous material avoids unwanted catalytic effects and enhances thermal stability. The fixation of catalyst on ceramic substrates such as washcoats is a well-known, reliable, and relatively straightforward procedure. The fabrication of complex small-scale ceramic structures is, however, more awkward than for metals or plastics, and they exhibit relatively poor mechanical strength. Furthermore, the porous ceramic nanostructure must be sealed to prevent contact between the reaction medium and coolant. 

Oxidation catalysts were used until 1979 in both the particulate (bead) form and monolith structure. Road experience demonstrated that the particulate beds were not mechanically stable and were breaking apart. In contrast the washcoated monoliths were found to be highly reliable so they became the structure of choice. 

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). 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

A bare monolithic structure can be coated with a catalyst support layer in several ways. Figure 21 shows a SEM image of a typical commercial cordierite monolith structure. Washcoating can be done by (partly) filling the pores of the macroporous walls with the washcoat material or by depositing a washcoat as a layer on top of the walls. These methods are shown schematically in Figure 22. 

It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 150 - 225 °C and time up to 72 hours. 

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