Transport with catalysts

As our first approach to the model, we considered the controlling step to be the mass transfer from gas to liquid, the mass transfer from liquid to catalyst, or the catalytic surface reaction step. The other steps were eliminated since convective transport with small catalyst particles and high local mixing should offer virtually no resistance to the overall reaction scheme. Mathematical models were constructed for each of these three steps. 

Agitated reactor (possibly with catalyst particles) Catalytic and noncatalytic Reactions, polymerizations (special agitator required) High transport rates, convenient to operate, easy variation of parameters, most versatile Catalyst erosion... 

Catalysts from active carbon additionally activated with cobalt- or iron- phthalocyanines are also studied [7], The results show that at current densities up to 50 mA/cm2, the polarization of the air electrodes with catalyst from active carbon promoted with FePc is lower than that of the electrode with catalyst from active carbon promoted with CoPc. At higher current density the polarization of the electrode with catalyst from active carbon promoted with CoPc is lower, which is probably connected to the lower transport hindrances, due to the more favorable structure of this catalyst. 

The described method for the diagnostic of the activity and the transport hindrances in air gas-diffusion electrodes is very useful in the research of porous catalysts for air electrodes. The comparison of the activity and the transport hindrances of air electrodes with catalysts from various types of active carbon allow a proper selection to be accomplished. 

In Figure 11, we have presented the AE - I curves of air electrodes with catalysts from active carbon and active carbon promoted with different amounts of silver (from Figure 5). It is seen that the transport hindrances in the electrodes with catalysts from pure active carbon and with active carbon promoted with 5% of silver are near to each other. The transport hindrances in the air electrodes with catalyst containing 30% of silver are much higher. That s why catalysts containing large amount of silver are suitable to be used in air electrodes operating at comparatively low current densities. 

Finally, an interesting concept, recently advanced, is the implementation of active materials as nanotube arrays. These systems have high surface area to optimize contact between semiconductor and electrolyte, and good light trapping properties. Their inner space could also be filled with catalysts or sensitizers and/ or pn junctions to obtain charge separation and facilitate electron transport [136]. 

In electrochemical systems, metal meshes have been widely used as the backing layers for catalyst layers (or electrodes) [26-29] and as separators [30]. In fuel cells where an aqueous electrolyte is employed, metal screens or sheets have been used as the diffusion layers with catalyst layers coated on them [31]. In direct liquid fuel cells, such as the direct methanol fuel cell (DMFC), there has been research with metal meshes as DLs in order to replace the typical CFPs and CCs because they are considered unsuitable for the transport and release of carbon dioxide gas from the anode side of the cell [32]. 

Methanol can be used directly as a transportation fuel, or it can be converted into gasoline with catalysts such as the ZSM-5 zeolite catalyst. 

High current density performance of PEFCs is known to be limited by transport of reactants and products. In addition, at high current densities, excess water is generated and condenses, filling the pores of electrodes with liquid water and hence limiting the reactant transport to catalyst sites. This phenomenon known as flooding is an important limiting factor of PEFC performance. A fundamental... 

Continuous flow reactors allow a new way of catalyst preparation. The fluidic pathways can be used for the transport of impregnation liquid or solid particles if means are supplied to localize the catalysts at defined positions inside the reactor. One possibility is the flow impregnation of wash-coated micro structures with catalyst solutions [38],... 

On a small scale, cracking ammonia can produce hydrogen for reduction. Transport and storage of hydrogen as ammonia is compact, and the cracking procedure involves only a hot pipe packed with catalyst and... 

Since base-stabilized NaBH solutions not in contact with catalyst are stable and produce virtually no H2, in the event of a leak or a spill, no H2 will be generated. Because little free H2 is actually stored in the system, concerns about onboard bulk H storage or distribution are reduced. As NaBH solutions are easier to store onboard a vehicle than H2 gas, these solutions offer a practical alternative to direct H2 fueling. NaBH solutions are also easier and safer to distribute to consumers than bottled H2 gas and can be easily transported from terminal locations to service stations via truck. NaBH solutions, with a viscosity and density close to that of water, can be dispensed in the same manner as gasoline with minor modification to the dispensing equipment. 

For a fixed bed reactor it is very important that the pressure drop over the bed is as low as possible. This condition is usually fulfilled by using pellets, extru-dates or spheres with a diameter greater than 3 mm. A fixed bed can have a height of ten meters or more. For this reason the catalyst particles in a fixed bed must have a high mechanical strength, otherwise the particles in the lower part of the bed will break under the weight of the upper half of the catalyst bed. In the riser reactor there is a continuous transport of catalyst pellets. Here it is necessary that... 

Let us first consider the catalyst/polyolefin particle in the early stage of its evolution. The particle consists of the solid catalyst carrier with catalyst sites immobilized on its surface, polymer phase, and pores. The first-principle-based meso-scopic model of particle evolution has to be capable of describing the formation of polymer at catalyst sites, transport of monomer(s) and other re-actants/diluents through the polymer and pore space, and deformation of the polymer and catalyst carrier (including its fragmentation). Similar discrete element modeling techniques have been applied previously to different problems (Heyes et al., 2004 Mikami et al., 1998 Tsuji et al., 1993). 

The basic equations for an unsteady-state process of one-dimensional (in the -direction) heat and mass transport with a simultaneous chemical reaction in a porous catalyst pellet are... 

Madon and Boudart propose a simple experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions [R. J. Madon and M. Boudart, Ind. Eng. Chem. Fundam., 21 (1982) 438]. The experiment involves making rate measurements on catalysts in which the concentration of active material has been purposely changed. In the absence of artifacts from transport limitations, the reaction rate is directly proportional to the concentration of active material. In other words, the intrinsic turnover frequency should be independent of the concentration of active material in a catalyst. One way of varying the concentration of active material in a catalyst pellet is to mix inert particles together with active catalyst particles and then pelletize the mixture. Of course, the diffusional characteristics of the inert particles must be the same as the catalyst particles, and the initial particles in the mixture must be much smaller than the final pellet size. If the diluted catalyst pellets contain 50 percent inert powder, then the observed reaction rate should be 50 percent of the rate observed over the undiluted pellets. An intriguing aspect of this experiment is that measurement of the number of active catalytic sites is not involved with this test. However, care should be exercised when the dilution method is used with catalysts having a bimodal pore size distribution. Internal diffusion in the micropores may be important for both the diluted and undiluted catalysts. 

Reactive depletion of CO from catalyst sites leads to much higher rates of secondary olefin hydrogenation reactions as pellets and reactors become limited by the rate of arrival of fresh reactants at catalytic sites. We have simulated intrapellet CO depletion experimentally by continuously decreasing the space velocity of mixtures with a H2/CO ratio (3 1) higher than the stoichiometric consumption value (—2.1 1). This reactant ratio was chosen because it corresponds to the relative rates of H2 and CO transport in stoichiometric mixtures through FT liquids at 473 K. As a result, the resulting axial gradients that occur in the catalyst bed as H2 and CO reactants are consumed resemble those that develop within transport-limited catalyst pellets. 

Materials commonly used for the gas diffusion layers are carbon paper or woven carbon mats (examples of which are shown in Fig. 3.41). They combine the cormectivity allowing electron transport with a pore structure suitable for hydrogen or oxygen gas access to the catalyst layer. In cell manufacture, the catalysts may be deposited either on the gas diffusion layer or on the membrane. 

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