Catalysts porous catalyst structures

Reaction and Mass Transfer in Porous Catalyst Structures... 

Fig. 7.2. Conversion of a reactant vs. temperature.The concentration of reactants [R] within the porous catalyst structure. Concentration of R is (a) uniform for kinetic control, (b) decreasing within the catalyst for pore diffusion control, and (c) zero immediately at the surface of the catalyst for bulk mass transfer.

Legal limits for the emissions of the main pollutants in the automobile exhaust gases are becoming more and more strict The development of new and advanced catalytic converters demands not only experimental work, but also extensive and detailed modelling and simulation studies. The models become more complex, when all the important physical and chemical phenomena arc considered. Particularly the use of non-stationary kinetic models (microkinetics) with surface deposition of reaction components (Jirtit et al., 1999, e.g.) and the incorporation of diffusion effects in porous catalyst structure lead to a large system of partial differential equations. 

This value, assumed by Zalc and Loffler, was rather low. Recent catalyst development work revealed operation of water-gas shift catalysts at 180 L (h gcat) and thus only 350 g catalyst would be required. However, catalyst coatings need to be applied to achieve such improved performance, which increases the utilisation of the porous catalyst structure. 

In Eqs. (3.9) and (3.10), and Ut are suitable mass and heat transport lumped coefEcients and A is the cross section of the solid wall. In Eqs. (3.9)-(3.11), the parameter B is the ratio of the catalyticaUy active to the geometrical surface area. This parameter cannot account for porous (intraphase) diffusion in the washcoat of technical catalysts but only for geometrical surface area increase. Intraphase transport modehng may be required for technical catalysts with sufficiently thick washcoats and well-dispersed catalyst loading within the washcoat volume. High temperatures, in particular, enhance diffusion limitations through the porous catalyst structure. For intraphase diffusion, various modeling approaches have been proposed (Mladenov et al., 2010). Intraphase diffusion wiU not be elaborated in this chapter. 

To be able to model the processes of diffusion and reaction, one needs to know some parameters describing the porous catalyst structure. One characteristic parameter is the internal surface area, that can be determined with adsorption experiments. This is frequently expressed as units of area per gram of catalyst. In this text the specific internal surface area a. is used, with dimension m/tti or m ... 

The relation between the dusty gas model and the physical structure of a real porous medium is rather obscure. Since the dusty gas model does not even contain any explicit representation of the void fraction, it certainly cannot be adjusted to reflect features of the pore size distributions of different porous media. For example, porous catalysts often show a strongly bimodal pore size distribution, and their flux relations might be expected to reflect this, but the dusty gas model can respond only to changes in the... 

To be specific let us have in mind a picture of a porous catalyst pellet as an assembly of powder particles compacted into a rigid structure which is seamed by a system of pores, comprising the spaces between adjacent particles. Such a pore network would be expected to be thoroughly cross-linked on the scale of the powder particles. It is useful to have some quantitative idea of the sizes of various features of the catalyst structur< so let us take the powder particles to be of the order of 50p, in diameter. Then it is unlikely that the macropore effective diameters are much less than 10,000 X, while the mean free path at atmospheric pressure and ambient temperature, even for small molecules such as nitrogen, does not exceed... 

The pressures involved in porosimetry are so high (e.g. 1000 atm = 6-6 ton in" ) that the question as to whether the pore structure is damaged by mercury intrusion naturally arises. This possibility was recognized by Drake, but as a result of several intrusion-extrusion runs at pressures up to 4000 atm on a number of porous catalysts Drake concluded that any deformation caused by compression was elastic and therefore not permanent. 

Temperature gradients within the porous catalyst could not be very large, due to the low concentration of combustibles in the exhaust gas. Assuming a concentration of 5% CO, a diffusion coefficient in the porous structure of 0.01 cms/sec, and a thermal conductivity of 4 X 10-4 caI/sec°C cm, one can calculate a Prater temperature of 1.0°C—the maximum possible temperature gradient in the porous structure (107). The simultaneous heat and mass diffusion is not likely to lead to multiple steady states and instability, since the value of the 0 parameter in the Weisz and Hicks theory would be much less than 0.02 (108). 

A lot of work is currently carried out to extend this idea to fully dispersed two-dimensional (on a YSZ surface) or three-dimensional (in a porous YSZ structure) metal catalysts. The main problems to be overcome is current bypass and internal mass transfer limitations due to the high catalytic activity of such fully dispersed Pt/YSZ catalyst systems. 

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. 

The search for better catalysts has been facilitated in recent years by molecular modeling. We are seeing here a step change. This is the subject of Chapter 1 (Molecular Catalytic Kinetics Concepts). New types of catalysts appeared to be more selective and active than conventional ones. Tuned mesoporous catalysts, gold catalysts, and metal organic frameworks (MOFs) that are discussed in Chapter 2 (Hierarchical Porous Zeolites by Demetallation, 3 (Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature), and 4 (The Fascinating Structure... 

Quantitative analytical treatments of the effects of mass transfer and reaction within a porous structure were apparently first carried out by Thiele (20) in the United States, Dam-kohler (21) in Germany, and Zeldovitch (22) in Russia, all working independently and reporting their results between 1937 and 1939. Since these early publications, a number of different research groups have extended and further developed the analysis. Of particular note are the efforts of Wheeler (23-24), Weisz (25-28), Wicke (29-32), and Aris (33-36). In recent years, several individuals have also extended the treatment to include enzymes immobilized in porous media or within permselective membranes. The important consequence of these analyses is the development of a technique that can be used to analyze quantitatively the factors that determine the effectiveness with which the surface area of a porous catalyst is used. For this purpose we define an effectiveness factor rj for a catalyst particle as... 

This section is concerned with analyses of simultaneous reaction and mass transfer within porous catalysts under isothermal conditions. Several factors that influence the final equation for the catalyst effectiveness factor are discussed in the various subsections. The factors considered include different mathematical models of the catalyst pore structure, the gross catalyst geometry (i.e., its apparent shape), and the rate expression for the surface reaction. 

Effective Thermal Conductivities of Porous Catalysts. The effective thermal conductivity of a porous catalyst plays a key role in determining whether or not appreciable temperature gradients will exist within a given catalyst pellet. By the term effective thermal conductivity , we imply that it is a parameter characteristic of the porous solid structure that is based on the gross geometric area of the pellet perpendicular to the direction of heat transfer. For example, if one considers the radial heat flux in a spherical pellet one can say that... 

When consecutive reactions take place within a porous catalyst, the concentrations of A and V within the pellet will be significantly different from those prevailing at the external surface. The intermediate V molecules formed within the pore structure have a high probability of reacting further before they can diffuse out of the pore. 

The catalytic layer of the air electrode is made from a mixture of the same hydrophobic material and porous catalyst [2]. It comprises hydrophobic zones through which the oxygen is transported in gas phase and zones containing catalyst where the electrochemical reduction of oxygen is taking place. It must be noted that the overall structure of the electrode is reproducible when various kinds of carbon-based catalysts are used. 

Catalysis by solids depends on the amount of surface exposed to the fluid. Large specific surface is obtained with small particles, but primarily with highly porous structures. For instance, to achieve 1 m2/cc the diameter of a sphere must be reduced to 6(10-4) cm, but porous catalysts may have several hundred m2/cc. Practical limitations exist to the smallness of particles that can be used, such as pressure drop and entrainment. In fixed or moving beds, particle diameters are several millimeters, in fluidized beds they may be less than 0.1 mm. 

Skeletal (Raney ) catalysts are made by a very simple technique. An alloy of two metals in roughly equal proportions, where one metal is the desired catalytic material, and the other is dissolvable in hydroxide, is first made. This alloy is crashed and leached in concentrated hydroxide solution. The soluble metal selectively dissolves, leaving behind a highly porous spongelike structure of the desired catalytic metal. Catalysts formed by this technique show high activity and selectivity, and have found wide use in industry, particularly for hydrogenation and dehydrogenation reactions. 

Researchers at 3M have been able to increase catalytic activity with nanotextured membrane surfaces that employ tiny columns to increase the catalyst area. Other materials include nonprecious metal catalysts such as cobalt and chromium along with particles embedded in porous composite structures. 

Membranes can also be used as a reactor where catalysts are used frequently. The membrane may physically segregate the catalyst in the reactor, or have the catalyst immobilized in the porous/microporous structure or on the membrane surface. The membrane having the catalyst immobilized in/on it acts almost in the same way as a catalyst particle in a reactor does, except that separation of the product(s) takes place, in addition, through the membrane to the permeate side. All such configurations involve the bulk flow of the reaction mixture along the reactor length while diffusion of the reactants/products takes place generally in a perpendicular direction to/from the porous/microporous catalyst. 

Figure 7-14 Structure and concentration profiles for a planar porous catalyst, which could...

Pore diffusion can be increased by choosing a catalyst with the proper geometry, in particular the pellet size and pore structure. Catalyst size is obvious (r if pore diffusion limited for the same total surface area). The diameter of pores can have a marked influence on r) because the diffusion coefficient of the reactant Da witl be a function of dp if molecular flow in the pore dominates. Porous catalysts are frequently designed to have different distributions of pore diameters, sometimes with macropores to promote diffusion into the core of the catalyst and micropores to provide a high total area. 

Among the various oxides that are used as substrates, aliunina [2] and silica [3] have been extensively studied however, the structural characterization of silica is certainly the most complete. Studies carried out on porous catalyst supports showed that the - OH concentration on a M O surface can vary... 

Ceramic honeycomb monoliths are porous macro-structured supports consisting of parallel channels. On the walls a thin layer of active material can be applied (Figure 1). Honeycomb catalyst supports were originally developed for use in automotive... 

One of the excellent characteristics of Raney catalyst is a porous skeletal structure composed of ultra fine particles. 

As mentioned earlier, if the rate of a catalytic reaction is proportional to the surface area, then a catalyst with the highest possible area is most desirable and that is generally achieved by its porous structure. However, the reactants have to diffuse into the pores within the catalyst particle, and as a result a concentration gradient appears between the pore mouth and the interior of the catalyst. Consequently, the concentration at the exterior surface of the catalyst particle does not apply to die whole surface area and the pore diffusion limits the overall rate of reaction. The effectiveness factor tjs is used to account for diffusion and reaction in porous catalysts and is defined as... 

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). 

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