Catalyst porosity, monolith

A. Nakhjavan, P. Bjombom, M.F.M. Zwinkels, and S.G. Jaras, Numerical analysis of the transient performance of high-temperature monolith catalytic combustors Effect of catalyst porosity, Chem. Eng. Sci. 50 2255 (1995). 

The space time values are calculated by the authors based on the information presented in the source articles. Most of the studies did not report the catalyst porosity and the bulk density, therefore the space time units had to be determined in the gcat.min/cm units. These units make it difficult for using the resulting kinetic information in monoliths where it is more meaningful to use proper time units for the space time due to the open geometry used in the monoliths. The adaptation procedure will be discussed in the forthcoming section. 

The next level is that of shaped catalysts, in the form of extrudates, spheres, or monoliths on length scales varying from millimeters to centimeters, and occasionally even larger. Such matters are to a large extent the province of materials science. Typical issues of interest are porosity, strength, and attrition resistance such that catalysts are able to survive the conditions inside industrial reactors. This area of catalysis is mainly (though not exclusively) dealt with by industry, in particular by catalyst manufacturers. Consequently, much of the knowledge is covered by patents. 

Monoliths comprising cross-linked organic media with a well-defined porosity have emerged as useful supports for immobilizing catalysts. These supports offer advan-... 

Accordingly, in addition to rate parameters and reaction conditions, the model requires the physicochemical, geometric and morphological characteristics (porosity, pore size distribution) of the monolith catalyst as input data. Effective diffusivities, Deffj, are then evaluated from the morphological data according to a modified Wakao-Smith random pore model, as specifically recommended in ref. [63[. 

Heterogeneous catalytic processes can often be intensified by the use of monolithic catalysts (39). These are metallic or nonmetallic bodies forming a multitude of straight, narrow channels of defined uniform cross-sectional shapes (Figure 13). In order to ensure sufficient porosity and to enhance the catalytically active surface, the inner walls of the monolith channels are usually covered with a thin layer of washcoat, which acts as the support for the catalytically active species. 

The macro-porosity emacro and the correlation function corresponding to the macro-pore size distribution of the washcoat were evaluated from the SEM images of a typical three-way catalytic monolith, cf. Fig. 25. The reconstructed medium is represented by a 3D matrix and exhibits the same porosity and correlation function (distribution of macro-pores) as the original porous catalyst. It contains the information about the phase at each discretization point— either gas (macro-pore) or solid (meso-porous Pt/y-Al203 particle). In the first approximation, no difference is made between y-Al203 and Ce02 support, and the catalytic sites of only one type (Pt) are considered with uniform distribution. 

Monoliths are mainly produced by extrusion, although other methods are applied, in particular for the production of metal monoliths from thin corrugated sheets. The size of the channels and the wall thickness can be varied independently, and the optimal values depend on the particular application. Therefore, an optimum can be established between the amount of the solid phase (catalyst loading), the void space in the monolith, and the wall thickness. As a consequence of the extrusion process and the use of plasticizers, the channel walls are not completely dense but possess a macroscopic porosity, t)q)ically 30-40%. Thus, the thermal expansion properties can also be adjusted. 

Silica in the form of thin films as well as oxide monoliths, fibers, and powders can be prepared from sol-gel method. In contrast with the fabrication of conventional inorganic glasses at much higher melting temperature, sol-gel processing is performed at low temperatures to produce oxide materials with desirable hardness, optical transparency, chemical durability, tailored porosity, and thermal resistance. The sol-gel method involves formation of a colloidal suspension (sol) and gelation to form a network in a continuous liquid phase (gel). One starts with an aqueous solution containing oxides or alkoxides, mutual solvent, and catalyst. Usually an external catalyst is added like mineral acids and ammonia as well as acetic acid, KOH, amines, KF, and HF for rapid and... 

These parameters, together with the porosity of the monolith walls, also affect the bulk density of the monolith, and therefore both the weight and the thermal mass of the converter. However, the data reported in the literature indicate that the influence of these parameters on the conversion reached over the catalyst is rather small, especially when considering the performance of aged catalysts. 

Ceramic monoliths have proven themselves effective as substrates for catalyst washcoat and precious metal because they provide a relatively uniform porous surface. In the catalyst application process, the amount of alumina washcoat picked up depends upon the total porosity, as well as, the size distribution and shape of the pores within the wall. Likewise, the amount of precious metal picked up depends largely upon the amount of porous washcoat on the substrate. Catalyst coaters, therefore, have learned to optimize their process around typical properties of the substrate. However, through subtle variances in raw materials and process steps, variances in porosity occur piece to piece and lot to lot. 

On the other hand, the mechanical properties of monolithic carbon gels are of importance when they are to be used as adsorbents and catalyst supports in fixed-bed reactors, since they must resist the weight of the bed and the stress produced by its vibrations or movements. A few smdies have been published on the mechanical properties of resorcinol-formaldehyde carbon gels under compression [7,36,37]. The compressive stress-strain curves of carbon aerogels are typical of brittle materials. The elastic modulus and compressive strength depend largely on the network connectivity and therefore on the bulk density, which in turn depends on the porosity, mainly the meso- and macroporosity. These mechanical properties show a power-law density dependence with an exponent close to 2, which is typical of open-cell foams. 

In an effort to find the optimum loading and performance of the monolith, the temperature and flow non-uniformities must be taken in to account. Therefore, a study was carried out firstly to harmonize the data reported in the literature for comparing the catalyst performances based on similar space times. Selected data from literature on the precious metals for CO oxidation reactions were harmonized and Arrhenius parameters were obtained (1-3,6-9). When the bed porosity data was not available an average estimate of the bed porosity was used to uniformly report the space times in terms of h units. The same analysis was applied to oxidation reactions of hydrocarbons present in the exhaust gas such as benzene, toluene, hexane and octane (3, 9-11). After the performance harmonization of the rate data on selected catalysts, the reaction model was used to estimate conversion profiles in a typical monolithic reactor. 

Fairen-Jimenez D, Carrasco-Marin F, Mraeno-Castilla C (2006) Porosity and surface area of monolithic carbon aerogels prepared using alkaline carbonates and organic acids as polymerization catalysts. Carbtm 44 ... 

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