Dispersion of catalysts

Sufficiently porous to permit dispersion of catalyst on its interior surfaces. 

Carbon support plays a vital role in the preparation and performance of catalysts since it influences the shape, size and dispersion of catalyst particles as well as the electronic interactions between catalyst and support [154,155]. 

As will be discussed in Sect. 8, the actual density number of active sites on the electrode for a particular reaction may also depend on the degree of dispersion of catalyst or coverage by adsorbate. The geometric area of the electrode may differ from the true electroactive area, which may also change with electrode potential. 

Substances which increase the rate of a chemical reaction without themselves being used up or incorporated into the finished product are called catalysts. In heterogeneous catalysis the reaction takes place on the surface of a solid support. The activity of the catalyst in this case is determined by the structure and size of the surface area as well as the way the catalyst is produced. Catalysts are not limited to immobilization on solids, they can also be introduced as homogeneous catalysts in solution. Between heterogeneous and homogeneous catalysts there is the possibility to evenly distribute small particles (dispersions) of catalyst in a liquid phase. 

Given the relatively limited surface area available for catalyst on the surface of the membrane or its pores, a high catalyst surface area can be achieved with minimum catalyst loading by applying a very fine dispersion of catalyst particles. Dispersion of the catalyst particles affects the reaction kinetics and selectivity. 

Table 2 represents the dispersity index estimated from XPS data, from which the information about relative dispersity of catalysts is given. The results support the view that using y-alumina leads higher dispersion of Fe phase compared to the other supports. 

Conversion of CH dehydrogenation and CP hydrogenolysis and total metal dispersion of catalysts... 

The fillers are normally added after thorough dispersion of catalyst in the resin and other additives such as internal mold release, pigments, wetting agents. But the addition may precede the dispersion of catalyst if it is a shrink control filler. Table 3 gives some examples of the fillers used. 

Another drawback was a criterion of convergence by GA. It is intrinsically difficult or impossible to verify the goal of optimization by GA. In fact fluctuation of maximum value and that of dispersion of catalyst composition are main indexes for the convergence. Instead of GA, therefore, more straightforward method was applied. It is so-called "grid search" (GS) all the activities of all possible combination of catalyst component were predicted by ANN. The global optimum on the ANN was found rapidly with the assistance of GS (13. 23. 39. 40. 42- 44). 

CO pulse chemisorption measurements were carried out on a Micromeritics AutoChem 2910. Pd metal dispersion of catalyst was calculated from CO pulse chemisorption results. Before introducing CO, the catalyst samples were reduced at 75°C using 4% hydrogen in argon for 45 minutes. Catalyst samples were cooled to room temperature for CO chemisorption. Helium carrier gas flow rate was about 30 ml/min. 

Figure 5.2 Relationship between Pt dispersion of catalysts and amount of oxygen surface complexes (measured as (xmol/g of CO2 and CO evolved) of the supports. The furnace carbon black was used as received, after a 12-hour H2 treatment at 1223 K, after a 12-hour H2 treatment at 1223 K followed by a 48-hour treatment in 8 N H2O, and after a 12-hour H2 treatment at 1223 K followed by 48 hours in 12 N H2O2. (Adapted from ref. 15.)...

The pore size distribution in the carbon support is an important factor for a well performance of the catalyst. Pores in the nanometric scale are classified by lUPAC in three groups the micropores are those with diameters lower than 2 nm, the mesopores with diameters between 2 and 50 nm and the macropores with diameters larger than 50 run. Each pores size offers different benefits, the micropores produce materials with high surfaces area but could be inaccessible to liquid solutions or have slow mass transport. A material with mesopores has a lower surface area but better accessibility than those with micropores. FinaUy, materials with macropores show the lowest surface area, but they are easily accessible to liquid fuel. For this reason, the structured carbons, principally mesoporous carbon, have attracted considerable attention due to their potential application in the catalyst area, where the challenging is to favour the dispersion of catalyst and allow the accessibility of liquids that feed the anode side of a DMFC. In the following sections a description of different carbons support wdl be discussed stressing on the effect of the porous structure. 

For similar reasons an inert liquid phase is often used if the solid is a catalyst, heterogeneously catalyzing a desired gas phase reaction. Apart from enhancing the heat transport rates, the fine dispersion of catalyst and the control of the reactants (re)distri-bution over the available catalyst can be important factors. 

Wang et al. (2004) and Zhang (2008). Systematic modifications at the fabrication stage include (i) the selection of materials (e.g., carbon or metal-oxide-based support materials, Pt or Pt-alloy catalyst materials, perfluorinated or alternative ionomer materials), (ii) size dispersion of catalyst and support particles, (iii) gravimetric composition of the ink (amounts of carbon, ionomer, and Pt) and solvent properties, (iv) thickness of the CL, and (v) fabrication conditions (temperature, solvent evaporation rate, pressure, and tempering procedures). 

One of the important application area of ultrasound (US) is catalytic reactions with the participation of low molecular mass compounds and heterogeneous catalysts. The effect of ultrasound on catalytic reactions in the presence of platinum and rhodium catalysts of various dispersities was investigated in Ref [1]. It was demonstrated that ultrasound can provide for the occmrence of chemical processes that cannot be performed even in the presence of catalysts. It is assumed that the main mechanism of its action on catalytic processes consists in the dispersion of catalyst particles however, as was shown in Ref [1], the adhesion of particles can occur during the action of the so-called Bjerknes forces, that is, forces that promote the attraction of particles (primarily small particles) to a deformed bubble followed by their sticking together. As a consequence, the diffusion of reagents to the surface of a particle becomes more pronoimced and the rate of the process inereases. 

Diffusion Continued) effect of, on activation energy, 104 effect of, on reaction order, 105 effect of, oh reaction rate, 95 importance of, 115 Knudsen, 484 molecular, 484 on surface, 98, 208 Diffusivity. See Effective diffusivity Dispersion (of catalyst), 19 Dispersion coefficients (axial and radial), 287, 493, 497... 

The preparation of a novel catalytic membrane system to be used in multiphase H2O2 production has also been discussed in detail by Tennison et al. in 2007. In their review, it was shown that it is possible to produce a membrane system that is potentially suitable for use in both multiphase and gas phase membrane reactor systems based on a 2-layer ceramic substrate. Moreover, the performance is sensitive to the degree of perfection of the support. The carbon membrane deposited within the nanoporous layer of the substrate has the structure and surface area to enable high dispersions of catalyst metals to be achieved when oxidized in carbon dioxide that have shown good performance in the direct synthesis of H2O2. When prepared under nitrogen, despite the simple production route, the carbon membrane shows excellent gas separation characteristics. 

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