The Catalytic Layer

Upon addition of the ionomer the total pore volume increases, due to the formation of an additional intergranular porous structure. At the same time the 

The hydrophobizing influence of the ionomer can be explained by the orientation of the sulfogroups located on the external surface of the adsorbed ionomer 

STRUCTURAL AND WETTING PROPERTIES OF FUEL CELL COMPONENTS 

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Catalytic Support Body Monolithic Honeycomb Unit. The terms substrate and brick are also used to describe the high geometric surface area material upon which the active coating material is placed. Monolithic honeycomb catalytic support material comes in both ceramic and metallic form. Both are used in automobile catalysts and each possesses unique properties. A common property is a high geometric surface area which is inert and does not react with the catalytic layer. 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

The concept of a biocatalytic membrane electrode has been extended to the use of a tissue slice as the catalytic layer. An example of this approach is an electrode for AMP which consists of a slice of rabbit muscle adjacent to an ammonia gas electrode. NHj is produced by enzymatic action of rabbit muscle constituents on AMP The electrode exhibits a linear range of 1.4 x 10 to 1.0 x 10 M with a response time varying from 2.5 to 8.5 min, depending on the concentration. Electrode lifetime is about 28 days when stored between use in buffer with sodium azide to prevent bacterial growth. Excellent selectivity enables AMP to be determined in serum. 

The air gas-diffusion electrode developed in this laboratory [2] is a double-layer tablet comprising a porous hydrophobic gas layer and a catalytic layer in which the catalyst is placed. The gas layer of the electrode is prepared from hydrophobic material carbon black modified with PTFE by a special technology [3], The catalytic layer contains a porous catalyst. 

The catalytic layer of the air electrode is made from a mixture of the same hydrophobic material and porous catalyst [2]. It comprises hydrophobic zones through which the oxygen is transported in gas phase and zones containing catalyst where the electrochemical reduction of oxygen is taking place. It must be noted that the overall structure of the electrode is reproducible when various kinds of carbon-based catalysts are used. 

The porous hydrophobic film of previous electrode designs has now been substituted with a new layer based on a mixture of particles of hydrophobic carbon and PTFE binder. This mixture is very similar in composition to the catalytic layer. This particular modification provides several advantages ... 

Fernandez, R., Ferreira-Aparicio, P, and Daza, L. PEMFC electrode preparation Influence of the solvent composition and evaporation rate on the catalytic layer microstructure. Journal of Power Sources 2005 151 18-24. 

Figure 19.5. Process sequence for the lift-off process (the planarized metalhzation process) (a) a resist film is patterned on a dielectric film (b) dielectric patterning (c) a thin catalytic film layer (PVD or CVD Ti, Al) is deposited (d) a lift-off technique removes the excess material, leaving the catalytic layer in the trench only (e) electroless Cu deposition.

The gas response of the field-effect devices is determined by the catalytic properties of the contact material, which includes both the catalytic layer and the underlying material. The temperature plays a dominant role in the detection process because the origin of the gas response is found in the chemical reactions that take place on the sensor surface, and it is furthermore also influenced by the mass transport properties of the molecules in the gas phase. This permits arrays of sensors of a common design to be tailor-made for detection of a range of gases and for use in a range of applications... 

The electrodeposition offer the advantage of a fine control of the thickness of the catalytic layer, thus optimizing the electroactive area and the transparency of the cathode. A cathode that possesses both a remarkable catalytic activity and a partially porous structure with a large active area is important for limiting the concentration... 

For a= 1, soot in the catalytic layer is oxidized fast leaving the soot in the thermal layer unreacted. This has been observed with some early catalytic filters. As a decreases the soot from the top layer replaces more rapidly the soot oxidized in the catalytic layer increasing the global oxidation rate. The corresponding soot layer thickness evolution is shown in Fig. 22. For values of a close to 1 (e.g. 0.9) the catalytic layer is totally depleted from soot at some instances, followed by sudden penetration events from the soot of the thermal layer. These events are clearly shown in the thickness evolution for oc = 0.9 in... 

Fig. 22. For smaller values of a, equilibrium between the soot entering the catalytic layer and the soot oxidized in it is established leading to a constant global oxidation rate.

The evolution of the dimensionless density profile across the soot layer is shown in Fig. 23. The initial gradual replenishment of the soot in the catalytic layer (at t = 140 s) is followed by sudden penetration events (t — 262 and 326 s) before the establishment of a steady state profile (at =531 and 778 s). Regarding the non-catalytic (thermal) layer only a gradual reduction of its thickness, accompanied by a very small reduction of its uniform density is observed. This simple microstructural model exhibits a rich dynamic behavior, however we have also established an experimental program to study the soot cake microstructure under reactive conditions. 

The following reactions occur in presence of an NO oxidation promoting catalyst (such as Pt) in the catalytic layer of the two-layer model (Konstandopoulos et al., 2000) ... 

From the above equations the evolution equation of the catalytic layer can be written in terms of that of a non-catalytic layer multiplied by a recycling factor R given as following ... 

Multiple oxidation reactions and diffusion in the catalytic layer of monolith reactors (with K. Zygourakis). Chem. Eng. Sci. 38, 733-744... 

The thickness of the catalytic layer deposited on channel walls is very small The average varies typically from 10 to 150 p,m. The first approximation concerning the deposit distribution is that the layer is distributed uniformly around the channel periphery. This may be true if circular channels are considered. The typical cross section of the monolith channels is, however, square. In this case a significant nonuniformity in the washcoat thickness is often encountered (Fig. 6). The reason is that the liquid from which the... 

Exemplary results of modeling processes inside the catalytic layer are presented in Fig. 9. The solid lines show the dependency of the overall effectiveness factor on the relative distribution of the catalyst between the comers and the side regions. The two cases represent two levels of the first-order rate constants, with the faster reaction in case (b). As expected, the effectiveness factor of the first reaction drops as more catalyst is deposited in the comers. The effectiveness factor for the second reaction increases in case (a) but decreases in case (b). The latter behavior is caused by depletion of B deep inside the catalytic layer. What might be surprising is the rather modest dependency of the effectiveness factor on the washcoat distribution. The explanation is that internal diffusion is not important for slow reactions, while for fast reactions the available external surface area becomes the key quantity, and this depends only slightly on the washcoat distribution for thin layers. The dependence of the effectiveness factor on the distribution becomes more pronounced for consecutive reactions described by Langmuir-Hinshelwood-Hougen-Watson kinetics [26]. 

As seen from Fig. 13, which gives a model unit of the laboratory cell reactor, this cell contains specially designed nets, to provide turbulence of the liquid flow. From the experimental result it was shown that the effectiveness factor was very high (up to 0.84) in experiments with plates with a very thin catalytic layer located close to the liquid side. In experiments with plates with a thick catalytic layer located in the middle of the plate, the effectiveness factor was very low (down to 0.02), and only a small part of the catalyst layer was utilized, since p-nitrobenzoic acid vanished in the plate after only 10% of the thickness of the catalytic layer had been passed in this particular run. 

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