Supported catalysts active species dispersion

In bulk catalysts prepared by the methods described previously, the active sites on the surface are formed by termination of the crystal or the amporphous solid. In contrast, supported catalysts consist of catalytically active species dispersed over a... 

Of all the reaction parameters involved in a heterogeneously catalyzed process, the selection of the catalyst is probably the most critical in determining the outcome of a particular reaction. The chapters in this section provide the reader with the information needed to make the selection of the catalyst more efficient. Most of the catalysts used for synthetic reactions are composed of the catalytically active species dispersed on a supposedly inert support material. The nature of the support can influence both catalyst stability and activity. It can also define the procedure best applicable for the preparation of a specific catalyst. Because of this, the commonly used supports are discussed in Chapter 9 to provide the background needed in the discussion of supported catalysts presented in the following chapters. 

Among the families of solid bases, we have particularly studied three of them. The first two ones (MgLa mixed oxides and supported alkali fluorides) were applied to fine chemistry while in the third case (CuO), the role of the basic strength of this oxidant on the selective adsorption of NO in the NOx trap technology has been studied. In each case, a fundamental effeet of the active species dispersion on the catalyst basic strength and reactivity has been found. 

CatalyticaHy Active Species. The most common catalyticaHy active materials are metals, metal oxides, and metal sulfides. OccasionaHy, these are used in pure form examples are Raney nickel, used for fat hydrogenation, and y-Al O, used for ethanol dehydration. More often the catalyticaHy active component is highly dispersed on the surface of a support and may constitute no more than about 1% of the total catalyst. The main reason for dispersing the catalytic species is the expense. The expensive material must be accessible to reactants, and this requires that most of the catalytic material be present at a surface. This is possible only if the material is dispersed as minute particles, as smaH as 1 nm in diameter and even less. It is not practical to use minute... 

It is noteworthy that CoSx/NaY showed a considerably high HDS activity, being comparable with that of MoSx/NaY. In contrast to relatively low HDS activities of the Co sulfide catalysts supported on Al Oj, the Co sulfide species supported on activated carbon have been repotted to show even higher HDS activities Aan Mo sulfide catalysts [14,15]. This is attributed to an extremely high dispersion of the Co sulfide species on activated carbon. The high HDS activity of CoSx/NaY suggests a high dispersion of the Co sulfide species. With the HYD of butadiene, CoSx/NaY showed a much lower activity than MoSx/NaY. 

MoSx-CoSx/NaY catalysts, which were prepared by introducing Mo(CO) into CoSx/NaY (l.lCo/SC), showed the identical HDS activities with those of CoSx-MoSx/NaY at the same compositions, as illustrated in Fig.4. Figure-4 suggests that the dispersions of Mo and Co sulfides are not mutually affected by the presence of the other sulfide species or that the formation of catalytically active species, e.g. Co-Mo mixed sulfide species, is independent of the accommodation order. As shown telow, FTIR of NO adsorption, EXAFS, and XPS results supported the latter pwssibility. 

The IR spectra in Fig.7 indicate the preferential adsorption of NO on the Co sites. It may be conjectured that the Mo sulfide species are physically covered by the Co sulfide species or that Co-Mo mixed sulfide species are formed and the chemical natures of the Co and Mo sulfides are mutually modified. The Mo K-edge EXAFS spectra were measured to examine the formation of mixed sulfide species between Co and Mo sulfides. The Fourier transforms are presented in Fig.8 for MoSx/NaY and CoSx-MoSx/NaY. The structural parameters derived from EXAFS analysis are summarized in Table 1. The structure and dispersion of the Mo sulfides in MoSx/NaY are discussed above. With the Co-Mo binary sulfide catalyst, the Mo-Co bondings are clearly observed at 0.283 nm in addition to the Mo-S and Mo-Mo bondings. The Mo-Co distance is close to that reported by Bouwens et al. [7] for a CoMoS phase supported on activated carbon. Detailed analysis of the EXAFS results for CoSx-MoSx/NaY will be presented elsewhere. It is concluded that the Co-Mo mixed sulfides possessing Co-S-Mo chemical bondings are formed in CoSx-MoSx/NaY. 

The typical solid catalyst used in technology consists of small catalytically active species, such as particles of metal, metal oxide, or metal sulfide, dispersed on a low-cost, high-area, nearly inert porous support such as a metal oxide or zeolite. The catalytic species are typically difficult to characterize in-... 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

Catalysts for coal liquefaction require specific properties. Catalysts of higher hydrogenation activity, supported on nonpolar supports, such as tita-nia, carbon, and Ca-modified alumina, are reasonable for the second stage of upgrading, because crude coal liquids contain heavy polar and/or basic polyaromatics, which tend to adsorb strongly on the catalyst surface, leading to coke formation and catalyst deactivation. High dispersion of the catalytic species on the support is very essential in this instance. The catalyst/support interactions need to be better understood. It has been reported that such interactions lead to chemical activation of the substrate 127). This is discussed in more detail in Section XIII. 

In the preparation and stablization of small, supported-catalyst particles, the consideration of surface mobility is essential. If the active component is in a high state of dispersion, conditions under which high mobility is attained must be avoided, since these conditions lead to particle size growth. On the other hand, a poorly dispersed component may be partially redispersed by treatment in a more highly mobile state. In supported catalyst systems, the interaction between the dispersed species (the active component) and the support is always of important concern, and a measure of the mobility of the active component is an indirect measure of this important interaction. 

Colloidal palladium or platinum supported on chelate resin beads were employed for the stereoselective hydrogenation of olefins 86). Colloidal palladium supported on iminodiacetic acid type chelate resin beads was prepared by refluxing the palladium chloride and the chelate resin beads in methanol-water. Using the resin-supported colloidal palladium as a catalyst, cyclopentadiene is hydrogenated to cyclopentene with 97.1% selectivity at 100 % conversion of cyclopentadiene under 1 atm of hydrogen in methanol at 30 °C. Finely dispersed metal particles ranging from 1 to 6 nm in diameter are the active species in the catalyst. 

The principles involved in the investigations described in the foregoing may be important in future studies of the nature of catalysts. Weak interactions between a support and the catalytically active species may lead to sintering and a poorly dispersed catalyst, whereas... 

Catalysts are manufactured by various methods (such as precipitation, extrusion and spray drying) in the form of cylinders, rings, multi-lobed extru-dates and other shapes. They range in size from a few millimetres to several centimetres small spheres are used in fluidized bed reactors. Active phases can be dispersed on the pre-shaped support by several methods such as by impregnation of a solution of the active components. Alternatively the catalysts can be made by the extrusion of mixtures of solid components the support, active phase, and binder. For some reactions that are diffusion limited, the catalyt-ically active species are not uniformly distributed instead they are deposited on the outer shell of the catalyst particle (egg-shell catalysts), since those inside the particle cannot be involved in the reaction. 

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