Catalysts transport phenomena

Ary given catalytic material can be abstracted based on the same underlying similar architecture — for ease of comparison, we describe the catalytic material as a porous network with the active centers responsible for the conversion of educts to products distributed on the internal surface of the pores and the external surface area. Generally, the conversion of any given educt by the aid of the catalytic material is divided into a number of consecutive steps. Figure 11.13 illustrates these different steps. The governing transport phenomenon outside the catalyst responsible for mass transport is the convective fluid flow. This changes dramatically close to the catalyst surface from a certain boundary onwards, named the hydrodynamic boundary layer, mass transport toward and from the catalyst surface only takes place... 

Adsorption, and chemisorption in particnlar, is closely allied to heterogeneons catalytic reactions both involve similar mass and heat transport constraints, in addition to bond formation at the solid snrface. In fact, adsorption is viewed as a precnrsor to catalytic reaction, and desorption is viewed as the step snbseqnent to the reaction itself. Adsorption of the reactant(s) and prodnct(s) must be strong enongh to deflect the original bonds, bnt not so strong as to poison the catalyst. This phenomenon has been related to the adsorption potential snggested by Polanyi (see Section 14.3.2). 

Equation (7.37) through equation (7.43) form the set of governing equations for the transport phenomenon taking place in the catalyst layer. Before proceeding on to discuss the parameters needed to close this set of equations, it should be pointed out that the transport of electrons and protons respond almost instantaneously to the change in electrical potential compared to the slow process in the transport of gas species. Thus, in equation (7.39) and equation (7.40), the transient terms are neglected. This behavior has been studied by Wu et al. [80]. 

Surface diffusion is yet another mechanism that is often invoked to explain mass transport in porous catalysts. An adsorbed species may be transported either by desorption into the gas phase or by migration to an adjacent site on the surface. It is this latter phenomenon that is referred to as surface diffusion. This phenomenon is poorly understood and the rate of mass... 

Bulk or forced flow of the Hagan-Poiseuille type does not in general contribute significantly to the mass transport process in porous catalysts. For fast reactions where there is a change in the number of moles on reaction, significant pressure differentials can arise between the interior and the exterior of the catalyst pellets. This phenomenon occurs because there is insufficient driving force for effective mass transfer by forced flow. Molecular diffusion occurs much more rapidly than forced flow in most porous catalysts. 

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... 

The phenomenon of metal transport via the creation of volatile metal carbonyls is familiar to workers using carbon monoxide as a reactant. It is often found that carbon monoxide is contaminated with iron pentacarbonyl, formed by interactions between carbon monoxide and the walls of a steel container. Thus, it is common practice to place a hot trap between the source of the CO and the reaction vessel. Iron carbonyl decomposes in the hot trap and never reaches the catalyst that it would otherwise contaminate or poison. Transport of a number of transition metals via volatile metal carbonyls is common. For example, Collman et al. (73) found that rhodium from rhodium particles supported on either a polymeric support or on alumina could be volatilized to form rhodium carbonyls in flowing CO. 

The starting point of a number of theoretical studies of packed catalytic reactors, where an exothermic reaction is carried out, is an analysis of heat and mass transfer in a single porous catalyst since such system is obviously more conductive to reasonable, analytical or numerical treatment. As can be expected the mutual interaction of transport effects and chemical kinetics may give rise to multiple steady states and oscillatory behavior as well. Research on multiplicity in catalysis has been strongly influenced by the classic paper by Weisz and Hicks (5) predicting occurrence of multiple steady states caused by intrapellet heat and mass intrusions alone. The literature abounds with theoretical analysis of various aspects of this phenomenon however, there is a dearth of reported experiments in this area. Later the possiblity of oscillatory activity has been reported (6). 

However, when considering monoliths having comparable fractional catalyst volumes and SA/V ratios as typical catalyst particles in fixed beds, countercurrent flow of gas and liquid is still very problematic. At the small channel diameter of about 1 mm (see Table 2) and at realistic velocities of gas and liquid, the liquid, which should flow downward as a film along the wall, will easily bridge the channel and form a slug, which will be transported upward by the gas. Thus, instead of the desired annular countercurrent flow, a segmented flow, or Taylor flow, in the upward direction will be obtained. This phenomenon is akin to the flooding in packed beds. 

The following discussion shows how the chemical composition, rate of formation, and heat of combustion of the pyrolysis products are affected by the variations in the composition of the substrate, the time and temperature profile, and the presence of inorganic additives or catalysts. The latter aspect, however, is discussed in more detail in Chapter 14. Combustion may be defined as complex interactions among fuel, energy, and the environment. Consequently, the combustion process is controlled not only by the above chemical factors, but also by the physical properties of the substrate and other prevailing conditions affecting the phenomena of heat and mass transport. Discussion of this phenomenon is beyond the scope of this chapter. 

Q phases are already one of the most promising, research-intensive LLC-based drug delivery systems because of the superior diffusion and access characteristics afforded by their 3-D interconnected nanopore systems [129, 151-154]. Initial results have also shown that Q-phase LLC materials also possess superior transport and access properties in membrane applications compared to L and Hu phases [170]. This phenomenon could translate into the design of superior LLC-based heterogeneous catalysts or bulk sorbents, as the interconnected nanochannels may provide more open pathways for better accessibility and selective molecular and/or ion diffusion. However, designing functional amphiphiles that can readily form useful Q-phase materials is not a straightforward task. To date, less than a handful of LLC monomers are known in the literature that can be polymerized in Q phases [172-175]. 

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