Catalyst Coating Techniques

To coat metallic surfaces with a catalyst, a pretreatment to improve the adherence is required [99]. In addition to mechanical roughening, chemical and thermal pretreatment are also frequently applied. Fecralloy, the construction material for metallic monohths, is usually pretreated at temperatures between 900 and 1000 °C. An alumina layer of about 1 pm thickness is formed on the Fecralloy surface under these conditions, which is an ideal basis for catalyst coatings. However, metal oxide layers are formed on stainless steel and may also serve as an adhesion layer. 

Aluminum substrates are frequently pretreated by anodic oxidation to generate a porous surface, which may serve as a catalyst support itself or as an adhesion layer for a catalyst support [99]. 

Once the surface has been pretreated, the coating slurry needs to be prepared. The most common method is to prepare a dispersion of finished catalyst, sometimes including gelation steps. Ceramic monoliths are usually wash-coated by these means. The catalyst carrier or the catalyst itself [100] is mixed with a binder such as 

It has been demonstrated that the slurry viscosity determines the thickness of the coating. The viscosity itself is determined by the concentration of particles, pH value, and surfactant addition [104]. 

Alternative but less commonly applied techniques are spray coating, which requires a decrease in the viscosity of the slurry or sol [99], flame spray deposition [106], and electrophoretic deposition [107]. 

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Catalyst Coating Techniques for Micro Structures and Their Application in Fuel Processing... 

Catalyst coating techniques and hydrogen production in microreactors... 

Bravo et al. described a catalyst coating technique, which was applied for coating of commercial copper/zinc oxide catalyst onto quartz and fused silica capillaries [138]. The catalyst was milled with boehmite alumina and deionised water in a mass ratio of 44 11 100. The thickness of the coating, which was only 1 pm for this gel formulation, could be increased to some 25 pm by addition of hydrochloric acid. The catalyst was still active after the coating procedure. The development work was moving towards coating of ready-made microreactors in the future [138]. 

Figure 4.2 Catalyst coating technique as developed by Kim and Kwon [144].

Catalyst Coating in Micro Channels Techniques and Analytical Characterization... 

Conventional welding may well be applied for prototypes and small series production especially for bonding of reactor/device periphery such as inlet diffusers and fluidic connections. In the case of chemical reactors, overheating needs to be avoided if precoating techniques are applied to avoid damage to the catalyst coating. 

Coating on micro structures is still dominated by manual coating techniques. For future automated production of catalysts and even complete reactors, automated coating procedures will become crucial. Some new approaches in coating technology especially suited for micro structures are described in Section 4.12.4, Online Reactor Manufacturing. 

Due to its high photocatalytic activity towards the complete mineralisation of VOCs [7,8] titania in its anatase form is normally used. Using ceramic monoliths with high titania content (50%) the total oxidation of chlorinated organic compounds at low temperature has been demonstrated [9]. However, since the photons from natural light may only penetrate a few microns into the catalyst surface the use of a wash-coating technique, where only a thin active film of titania is applied to the ceramic or metallic support can be considered as an ideal technique to produce maintenance free photocatalytic reactors. 

The use of the catalyst coating of porous electrodes is one of the main features of fuel cells. Platinum exhibits the best catalytic reactivity. However, only economically reasonable methods for the Pt deposition are preferred because of platinum high cost. So, an electrochemical deposition that allows a selective coating of desired surfaces with precise control of Pt thickness and quality is seen to be one of the most efficient techniques for the fuel cell production. 

The spin coating technique has attracted interest, since it maintains many aspects of technical catalysts prepared by pore volume or incipient wetness impregnation, and simultaneously allows the interpretation and analysis in a similar way as the more well-defined model systems discussed above [30]. Here, a solution of the desired catalyst precursor is dropped onto a wafer covered with an oxide film, which is spun on a rotor to create a liquid layer of uniform thickness in order to mimic traditional wet impregnation preparation of catalysts. Control of the catalyst loading and particle size is to some degree achieved by varying the rotation speed, concentration, and vapor pressure of the solute. Still the method suffers, however, from many of the drawbacks associated with wet-impregnated model catalysts, which imparts detailed mechanistic studies. 

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

The potential capabilities of microemulsion synthesis of colloidal silica and related materials are yet to be realized. For example, the compartmen-talization of reagents at the molecular level in these media may allow close control of chemical homogeneity in the synthesis of glass (e.g., Si-Al) particles. In situ synthesis of silica-supported catalysts and synthesis protocols involving seeding techniques, coating techniques, or infiltration are also possible. 

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