Catalytic reaction combined with transport

In summary, this discussion illustrates the general importance of transport processes in many (electro)catalytic reactions. These have to be addressed properly for a detailed (and quantitative) understanding of the molecular-scale mechanism. Because of the problems associated with the direct identification of the reaction intermediates (see above), experiments on nanostructured model electrodes with a well-defined distribution of reaction sites of controlled, variable distance and under equally well-defined transport conditions (first attempts in this direction are described in [Lindstrom et al., submitted Schneider et al., 2008]), in combination with detailed simulations of the ongoing transport processes and theoretical calculations of the... 

It is interesting to note that there is no complete symmetry between the role of substrate diffusion and electron transport in their combination with the catalytic reaction, as can be seen in the structures compared in the equations and also in the fact that linear Koutecky-Levich plots are not obtained in all cases, as noted in Table 4.1. 

To improve the efficiency of combined hydrogen production and C02 capture, several technologies are in development that combine catalytic reactions and the separation of either hydrogen or C02. Major targeted areas of application are the production of bulk hydrogen as a transport fuel and electricity production with pre-combustion C02 capture. 

In a munber of publications we have recently demonstrated that this problem of mass transport hmitation can be circumvented by using catalytic SILP materials [27-32]. Moreover, these catalysts allow the application of fixed-bed reactors for simple continuous processing when applied in combination with gaseous reaction mixtures making the separation and catalyst recychng obsolete. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

We have used CO oxidation on Pt to illustrate the evolution of models applied to interpret critical effects in catalytic oxidation reactions. All the above models use concepts concerning the complex detailed mechanism. But, as has been shown previously, critical. effects in oxidation reactions were studied as early as the 1930s. For their interpretation primary attention is paid to the interaction of kinetic dependences with the heat-and-mass transfer law [146], It is likely that in these cases there is still more variety in dynamic behaviour than when we deal with purely kinetic factors. A theory for the non-isothermal continuous stirred tank reactor for first-order reactions was suggested in refs. 152-155. The dynamics of CO oxidation in non-isothermal, in particular adiabatic, reactors has been studied [77-80, 155]. A sufficiently complex dynamic behaviour is also observed in isothermal reactors for CO oxidation by taking into account the diffusion both in pores [71, 147-149] and on the surfaces of catalyst [201, 202]. The simplest model accounting for the combination of kinetic and transport processes is an isothermal continuously stirred tank reactor (CSTR). It was Matsuura and Kato [157] who first showed that if the kinetic curve has a maximum peak (this curve is also obtained for CO oxidation [158]), then the isothermal CSTR can have several steady states (see also ref. 203). Recently several authors [3, 76, 118, 156, 159, 160] have applied CSTR models corresponding to the detailed mechanism of catalytic reactions. 

The second reaction vessel in a catalytic cracker is called the regenerator. The solid catalyst from the reactor is combined with a compressed air stream from an air blower, and the solid and gas phases flow upward into a bed of fluidized solid catalyst. The early designs used a bubbling bed reactor in which the velocity in the bed is slightly above the minimum fluidization velocity. More recent designs use a transport fluidized-bed reactor. A typical air-to-oil weight ratio is 0.54. 

Combining the kinetics of the catalytic reaction and its interplay with transport phenomena as well as with the hydrodynamics of the chemical reactor to be used as the basis of shaping and operating the final catalyst. 

At present two models are available for description of pore-transport of multicomponent gas mixtures the Mean Transport-Pore Model (MTPM)[4,5] and the Dusty Gas Model (DGM)[6,7]. Both models permit combination of multicomponent transport steps with other rate processes, which proceed simultaneously (catalytic reaction, gas-solid reaction, adsorption, etc). These models are based on the modified Maxwell-Stefan constitutive equation for multicomponent diffusion in pores. One of the experimentally performed transport processes, which can be used for evaluation of transport parameters, is diffusion of simple gases through porous particles packed in a chromatographic column. 

Heterogeneously catalyzed reactions. Macroscopic fluid models are combined with microscopic transport models in the catalyst particles to describe how concentration changes with time and position in a catalytic reactor. Special considerations must be given to the selection of experimental temperature and catalyst particle size to minimize (and hopefully eliminate) internal transport limitations on the catalytic reaction rate. The next requirement is that the flow pattern in the reactor Is accurately represented by the well-mixed or plug-flow assumption. The subsequent discussion applies to gas-phase reactants. 

Heterogeneous catalytic reactions involve by their nature a combination of reaction and transport processes, as the reactant must be first transferred from the bulk of the fluid phase to the catalyst surface. In Figure 11.2, the combined reaction and transport processes are shown schematically for a fast exothermic chemical reaction within a porous catalyst. If the rate of the intrinsic reaction is comparable to the rate of transport processes, significant concentration profiles of the reactants and products will develop. In addition, the temperature of the catalyst particle will be different from that of the bulk fluid. With increasing temperature, the influence of transport phenomena becomes more important and finally limits the overall reaction rate. This has detrimental influences on product yield and selectivity, and may lead to high overtemperatures of the catalyst and its fast deactivation [6]. The influence of transport phenomena is commonly characterized by an effectiveness factor as defined in Eq. (11.2). 

Heterogenous catalytic reactions involve by their nature a combination of reaction and transport processes, as the reactants must be first transferred from the bulk of the fluid phase to the catalyst surface, where the reaction occurs. The combined reaction and transport processes are shown schematically in Figure 2.17. We suppose a porous catalyst particle with a large specific surface area surrounded by liquid or gaseous reaction mixture. 

Coenzyme in the narrow sense, the dissociable, low-molecular-mass active group of an enzyme which transfers chemical groups (see Group transfer) or hydrogen or electrons. C. in this sense couple two otherwise independent reactions, and can thus be regarded as transport metabolites. In a wider sense, a C. can be regarded as any catalytically active, low-molecular-mass component of an enzyme. This definition includes C. that are covalently bound to enzymes as prosthetic groups. A holoenzyme consists of a C. in combination with an apoenzyme (enzyme protein). 

Measurement techniques for the resolution of concentration and temperature profiles in chemical reactors with heterogeneously catalyzed gas-phase reactions are a very useful tool not only for a better understanding of the reaction sequence and derivation of reaction kinetics but also for the elucidation of the coupling between catalytic reaction kinetics and mass and heat transport. The combination of numerical simulations of the reactive flow in catalytic reactors incorporating microkinetic reaction schemes and those recently developed invasive and noninvasive in situ techniques can today support the optimization of reactor design and operating conditions in industrial applications. 

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