Catalyst Preparation, Fabrication, and Activation

The task of developing a suitable catalyst for commercial applications involves many considerations, ranging from obvious factors such as catalyst activity and selectivity to variables such as the catalyst shape and the composition of the binder used in a pelletizing process. This section is devoted to a discussion of these considerations and of the techniques involved in manufactnring industrial catalysts. 

The lifetime of a catalyst is another factor that plays a key role in determining process economics. This lifetime is the period during which the catalyst produces the desired product in yields in excess of or equal to a designated value. The life of a catalyst may terminate because of 

Natural products and common industrial chemicals in massive form are seldom useful as catalysts because they have low specific surface areas, may contain various amounts of impurities that have deleterious effects on catalyst performance, do not usually have the exact chemical composition desired, or are too expensive to use in bulk form. The preparation of an industrial catalyst generally involves a series of operations designed to overcome such problems. Many catalysts can be produced by several routes. The actual choice of technique for the manufacture of a given catalyst is based on ease of preparation, homogeneity of the final catalyst, stability of the catalyst, reproducibility of quality, and cost of manufacture. The two most commonly used techniques for catalyst preparation are the only ones we shall consider  

Precipitation or gel formation of the active component or components through the interaction of aqueous solutions of two or more chemical compounds. 

Impregnation of a carrier using a solution containing a compound of the catalyst component desired. 

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A series of innovative catalyst preparation techniques has been identified. Each technique is surveyed, and the resulting catalyst is subjected to electrochemical and spectroscopic evaluation. Spectroscopic data are reintegrated in a sophisticated computer model that allows detailed bulk structural information to be derived. For this modeling, the methods are ranked according to the likelihood of selectively fabricating highly active crystal faces. Thus, the model provides a link to design catalysts based on a structure-function approach. 

Catalyst coating is another important process in fabricating micro fuel cells. Uniform and reliable catalyst coating is extremely crucial for mini type fuel cells. The preparation method and compatibility between catalyst and substrate affect the activity and durability of the catalyst layer. 

Liu et al. synthesized fully aromatic poly(ether ketone)s based on the monomer p-biphenyl-hydroquinone. Then they prepared site-selective sulfonated biphenyl-ated poly(ether ether ketone)s (BiPh-SPEEKDKs) by mild and rapid sulfonation reactions. MEA fabrication and fuel cell operating conditions were conducted according to Los Alamos National Laboratory protocols. The cell was preconditioned under H2/air fuel cell operation at 0.7 V for 2 h. Pt-Ru black was used for anode and Pt black for cathode and 5% commercially available Nafion dispersion. The catalyst ink was painted onto the membrane until the catalyst loading reached 8 mg cm for anode and 6 mg cm for cathode. Active area is 5 cm. The cell was held at 80°C 1 and 2 M aqueous methanol solution was fed to the anode with a flow rate of 1.8 mL min- 90°C humidified air was fed at 500 seem without back pressure. The current density of BiPh-SPEEKDK at 0.5 V at 80°C in 1 and 2 M methanol reached... 

Laser electrodispersion (LED) method makes possible to fabricate dense nanostmctured catalysts with unique catalytic properties. In contrast to earlier laser ablation techniques, where nanoparticles were synthesized from vaporized matter, LED is based on the cascade fission of hquid metalhc drops. Fabricated catalysts consist of ensembles of nanoparticles that are uniform in size and shape, amorphous and stable to coagulatioa The catalytic activity of these self-assembled Pt, Ni, Pd, Au and Cu catalysts with extremely low metal content (<10 mass.%) in hydrogenation and hydrodechlorination is several orders of magnitude higher compared to that for separated metal clusters, highly loaded metal films and supported catalysts prepared by usual methods. 

There are, however, continuing difficulties for catalytic appHcations of ion implantation. One is possible corrosion of the substrate of the implanted or sputtered active layer this is the main factor in the long-term stabiHty of the catalyst. Ion implanted metals may be buried below the surface layer of the substrate and hence show no activity. Preparation of catalysts with high surface areas present problems for ion beam techniques. Although it is apparent that ion implantation is not suitable for the production of catalysts in a porous form, the results indicate its strong potential for the production and study of catalytic surfaces that caimot be fabricated by more conventional methods. 

Further projects dealt with the fabrication of heterogeneous, basic or acidic solid-state catalysts or adsorbents carrying, for instance, amino or sulfonic acid groups. Amino-functionalized silicas were prepared and analyzed for the catalytic activity in Knoevenagel condensation reactions of aldehydes or ketones with ethyl cyanoace-tate ions by Macquarrie et al.154 155 Recently, Zhang et al.156,157 reported on the successful preparation of amino-functionalized silica thin films by means of the EISA approach. 

Unlike the RDE technique, which is quite popular for characterizing catalyst activities, the gas diffusion electrode (GDE) technique is not commonly used by fuel cell researchers in an electrochemical half-cell configuration. The fabrication of a house-made GDE is similar to the preparation of a membrane electrode assembly (MEA). In this fabrication, Nation membrane disks are first hot-washed successively in nitric acid, sulphuric acid, hydrogen peroxide, and ultra-pure water. The membranes are then coated with a very thin active layer and hot-pressed onto the gas diffusion layer (GDL) to obtain a Nation membrane assembly. The GDL (e.g., Toray paper) is very thin and porous, and thus the associated diffusion limitation is small enough to be ignored, which makes it possible to study the specific kinetic behaviour of the active layer [6],... 

EBL was used to fabricate uniform platinum nanoparticle arrays on Si02 (mean platinum particle diameter 30-1000 nm 52,53,106,107,398)), and evaporation techniques were used to prepare smaller particles and a continuous platinum film. The EBL microfabrication technique allows the production of model catalysts consisting of supported metal nanoparticles of uniform size, shape, and interparticle distance. Apart from allowing investigations of the effects of particle size, morphology, and surface structure (roughness) on catalytic activity and selectivity, these model catalysts are particularly well suited to examination of diffusion effects by systematic variations of the particle separation (interparticle distance) or particle size. The preparation process (see Fig. 1 in Reference 106)) is described only briefly here, and detailed descriptions can be found in References 53,106,399). 

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