Catalysts internal structure

The internal structure of the catalyst particle is often of a complex labyrinth-like nature, with interconnected pores of a multiplicity of shapes and sizes, In some cases, the pore size may be less than the mean free path of the molecules, and both molecular and Knudsen diffusion may occur simultaneously. Furthermore, the average length of the diffusion path will be extended as a result of the tortuousity of the channels. In view of the difficulty of precisely defining the pore structure, the particle is assumed to be pseudo-homogeneous in composition, and the diffusion process is characterised by an effective diffusivity D, (equation 10.8). 

Many theoretical embellishments have been made to the basic model of pore diffusion as presented here. Effectiveness factors have been derived for reaction orders other than first and for Hougen and Watson kinetics. These require a numerical solution of Equation (10.3). Shape and tortuosity factors have been introduced to treat pores that have geometries other than the idealized cylinders considered here. The Knudsen diffusivity or a combination of Knudsen and bulk diffusivities has been used for very small pores. While these studies have theoretical importance and may help explain some observations, they are not yet developed well enough for predictive use. Our knowledge of the internal structure of a porous catalyst is still rather rudimentary and imposes a basic limitation on theoretical predictions. We will give a brief account of Knudsen diffusion. 

Another disadvantage of fixed bed reactors is associated with the fact that the minimum pellet size that can be used is restricted by the permissible pressure drop through the bed. Thus if the reaction is potentially subject to diflfusional limitations within the catalyst pore structure, it may not be possible to fully utilize all the catalyst area (see Section 12.3). The smaller the pellet, the more efficiently the internal area is used, but the greater the pressure drop. 

The structure of skeletal catalysts is so fine that electron microscopes are required for sufficient resolution. The use of a focussed ion beam (FIB) miller has enabled a skeletal copper catalyst to be sliced open under vacuum and the internal structure to be imaged directly [61], Slicing the catalyst enabled viewing beyond the obscuring oxide layer on the surface. A uniform, three-dimensional structure of fine copper ligaments was observed [61], which differed from the leading inferred structure at the time of parallel curved rods [54],... 

Figure 5.4 Coarsening of the internal structure differs from sintering or fusing together of catalyst particles.

Surface area is by no means the only physical property which determines the extent of adsorption and catalytic reaction. Equally important is the catalyst pore structure which, although contributing to the total surface area, is more conveniently regarded as a separate factor. This is because the distribution of pore sizes in a given catalyst preparation may be such that some of the internal surface area is completely inaccessible to large reactant molecules and may also restrict the rate of conversion to products by impeding the diffusion of both reactants and products throughout the porous medium. 

Although there still remains some "art" in the production of high activity catalysts, surface area, pore size and other factors relevant to the accessibility of the reactant gases to the catalytic sites are clearly of primary importance. Thus any deposition of product or by-product within the catalyst pore structure is undesirable. In recent years the relationship between impure Claus feed containing hydrocarbons and catalyst lifetime has been well demonstrated (32). Carbon of hydrocarbon polymer deposition on the catalyst usually results in blocking access of the reactant gases to the internal catalytic sites. Product sulfur depos-... 

Traditionally the technique of the medical physicist, magnetic resonance imaging (MRI) has long been used to investigate the internal structure of the human body and the transport processes occurring within it for example, MRI has been used to characterize drug transport within damaged tissue and blood flow within the circulatory system. It is therefore a natural extension of medical MRI to implement these techniques to study flow phenomena and chemical transformations within catalysts and catalytic reactors. 

There is also a large body of literature dealing with additives, such as catalysts, the effect of radiation and mechanical treatment, which affects both the surface and internal structure of crystals, which are not treated in detail in this book. 

The molecular diffusivity D must be replaced by an effective diffusivity De because of the complex internal structure of the catalyst particle which consists of a multiplicity of interconnected pores, and the molecules must take a tortuous path. The effective distance the molecules must travel is consequently increases. Furthermore, because the pores are very small, their dimensions may be less than the mean free path of the molecules and Knudsen diffusion effects may arise Equation 10.170 is solved in Volume 1 to give equation 10.199 for a catalyst particle in the form of a flat platelet... 

A central problem is the selection of the appropriate internals. Structured catalytic packing is mostly employed. The most critical problem at this stage is the availability of correlations for describing the hydrodynamics conditions corresponding to the effective use of the catalyst. 

According to Fig. 10.1, XRD is one of the most frequently applied techniques in catalyst characterization. X-rays have wavelengths in the A range, are sufficiently energetic to penetrate solids and are well suited to probe their internal structure. XRD is used to identify bulk phases and to estimate particle sizes [9]. 

Further exploration of strongly Interconnected pore networks with closed loops as models for the internal structure of particles, were presented by Beeckman [ref. 3D] and Harin et al [ref 31] and Mann et al. [ref. 40] Beeckman derived general analytical solutions for three-dimensional networks which could be extended to account for catalyst deactivation. 

It will become possible to follow in situ the evolution of the internal structure of the catalysts during a reaction by the newly developed environmental HRTEM... 

Liguras, D.K., Goundani, K., and Verykios, X.E. Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ru catalysts. International Journal of Hydrogen Energy, 2004, 29 (4), 419. 

PROBING INTERNAL STRUCTURES OF FCC CATALYST PARTICLES FROM PARALLEL BUNDLES TO FRACTALS... 

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