Catalyst eggshell

Naturally, the number of surface sites limits the amount of Pt complexes that can be adsorbed. Typically, the.se catalysts contain up to 1 wt% Pt. Adsorption is a fast process and in general an eggshell catalyst will be formed. Figure 3.29 shows the processes that occur. 

Effective medium theory, 37 154 Eggshell catalysts, 39 231 EH method, 37 153 EHT, see Extended Hiickel treatment Eigenberger model, oscillatory reactions, 39 80-81, 83... 

Kellner and Bell 80) have also reported a decrease in turnover rate with increasing metal dispersion in Ru/AhOs catalysts at both 0.1 and 1 MPa reactant pressures (H2/CO = 3, 498 K). Methane turnover rates at 498 K and 1 MPa decreased from 0.003 to 0.0004 s as the Ru dispersion increased from 25 to 80% 80). Smith and Everson 84) also reported a 10-fold decrease in turnover rate as the dispersion of 0.5% Ru/AhOs eggshell catalysts increased from 16 to 78%. C5+ selectivity and chain growth probability, however, were not strongly affected by Ru dispersion. 

Similar improvements in synthesis rates and C5+ selectivities are observed when Co sites are selectively placed within 0.1-0.2 mm of the outer surface of large Si02 pellets. Eggshell catalysts are more active and selective for Cs+ synthesis than evenly impregnated large pellets (Table VII), but both... 

Fig. 28. Site density, pellet size, and eggshell thickness effects on FT synthesis selectivity (Co catalysts Si02, TiOi, and AI2O3 supports 473 K, H2/CO = 2.1, 2000 kPa, 50-62% CO conversion)., Dispersion/support effects , pellet size effects O, eggshell catalysts (Table VII) A, from Refs. 47 and 48.

The data in Fig. 28 clearly show that intermediate values of x, which limit olefin removal and enhance secondary readsorption reactions but still permit unrestricted and rapid access of CO and H2 to reaction sites, lead to maximum C5+ selectivity. They also show that eggshell catalysts allow access to these intermediate values of x for any pellet size. The design of eggshell pellets with values of x between 0.2 and 2.0 x 10 m leads to high C5+ selectivity (Fig. 28a) and maintains catalytic rates and activation energies near their intrinsic kinetic values (Table VII). 

In a fixed-bed the reactor vessel is filled with catalyst particles having sizes in the range 1-3 mm. The catalyst particles may be spherical, cylindrical, or of more sophisticated forms, including eggshell catalysts, with active species concentrated near the outer surface of the particle. The gas and liquid phases are passed through the bed. 

In contrast to traditional energy sources, microwave, laser, and sonic energy can be delivered to specific solid sites or small areas by fine-tuning frequencies and energy levels. It is especially suitable for preparation of nano-structured catalyst. For example, inherent eggshell catalysts can be produced with sonochem-ical preparation by which nano-dispersed metal particles are formed with instantaneous decomposition of metal solution by high-intensity irradiation of ultrasound in local area. In the preparation of M02C on ZSM-5 with ultrasound at 20 kHz, narrowly distributed particles of about 2nm in diameter are uniformly dispersed on the outer surface of the ZSM-5 support. ... 

The ANOF technique proved to be a promising method for obtaining eggshell catalysts with a very good mechanical and chemical resistance. By appropriate choice of the metallic substrate, electrolyte composition and anodization conditions, catalysts with tailor-made pore structure, pore density, pore length, and compositions can be controlled. The nickel catalysts supported on alumina, magnesia or titania were found to be efficient for the selective oxydehydrogenation of cyclohexane to cyclohexene. 

If the catalyst is deposited as a thin layer on the inside or outside of a tube, the slab model can be used if the thickness of the catalyst layer is much less than the tube radius. The slab model is also used to analyze the performance of eggshell catalysts, which have a layer of active catalyst near the outer surface of the pellet, and of catalyst monoliths, which have a thin layer of catalyst on the inside of square, triangular, or hexagonal passages. Flowever, when the catalyst layer is very thin, pore diffusion effects are... 

The sample in Fig. 3(a) is a common eggshell catalyst all the active metal is concentrated on the upper shell of the support grain. As a large grain was used for such a test, it is possible to observe a white heart due to "uncontaminated" alumina. On the contrary, the sonicated sample in Fig. 3(b) is characterized by a black halo due to the ruthenium atoms that migrated inside the support. This kind of catalyst is commonly called egg-white catalyst and is industrially prepared in the presence of citric or tartaric acid. Sonochemically the same result is obtained without using any chemical which could poison the active sites. 

Extensive catalytic tests on small and large uniformly impregnated pellets and on the eggshell catalysts of this study show that maximum 5+ selectivities are obtained at intermediate levds of transport restrictions (Figure 4). The parameter %, and is proportional to ... 

Fig. 4. C5+ Selectivity as a function of increasing transport limitations at 473 K, H2/CCH2.1, 2000 KPa and 50-60% CO conversion. O represent eggshell catalysts while are powder catalysts and A are even pellets of different diameter.

Yang, M., Sun, Y, Xu, A., etal. (2007). Catalytic Wet Air Oxidation of Coke-plant Wastewater on Ruthenium-based Eggshell Catalysts in a Bubbhng Bed Reactor, Bull. Environ Contam.Toxicol., 79, pp. 66-70. 

The solid catalysts are mostly applied as bulk materials, mixtures of active constituents and inert diluent and eggshell catalysts using more or less inert cores of alumina or silica These compositions are available as tablets, spheres, extrudates or spr -dried finer particles depending on the reactors to be used. Fixed bed multitubular and fluidized bed reactors are used. Outstanding industrially operated examples are the conversions of propene to acrylonitrile or 3-picoline to nicotinonitrile, for example. Scheme 7.4 depicts various examples on reactants, nitrile products and their subsequent uses. 

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