Deposition of the Active Component

By a careful control of the preparation parameters, it is possible to obtain the TMI successively in solution (Fig. 5. model I), on the surface in extraframework position (models II-V), then in the surface in framework position (model VI). to finally reach the bulk of the oxide (model VII). Models II IV are pertinent to the deposition of the active component precursor. 

As seen (Fig. 2) the metal (Co, Zn)-phase is much better spread over the support surface in the inactive catalyst 5. The (Co+Zn) Si ratio characterizing the covering of the support is 1.25 in the sample 3 and 1.64 in the sample 5. This effect proves that the deposit of the active component covers and eliminates the catalytic active centres at the bare smface of the support. Thus, the support calcination at temperature higher than 950 °C leads to the decrease of the surface area and content of Si02 on the surface due to the interphase difhision and consequently to the decrease of SiOj accessibihty to a reactant. 

For deposition of the active component, palladium, onto the carbon fibrils a precipitation method is utUized . The method consists of four steps, viz., pretreatment of the carbon surface in boiling nitric acid, suspension of the treated fibrils in an aqueous solution and addition of the solvated palladium precursor, injection of a formaldehyde solution to reduce the palladium ions, and, finally, filtering and drying. 

In some cases (e g., textural studies of supported catalysts), it is convenient to usc/lv instead of A since the external volume of a granule remains constant, while introduction of an active component results in an increase in mass of the resulting material. In these cases, one can directly compare Av after deposition of an active component with A vo—the volumetric surface area of initial support. For this purpose it is convenient to use the parameter... 

Consider a nonporous catalyst having an active component of a complex (unknown) composition, deposited over a known support (known parameters the true density of a support and its partial mass, ms). We have no information on the type of active component, only its partial mass, mc= 1 —ms. Also known is the density of solid phase of a catalyst as a whole. Allocate the partitions for this PS, and determine (a) a partial volume of the active component (b) the hue density of the active component. 

Heterogeneous catalysts are solid materials that sometimes consist of the bulk material itself, for example, acid zeolite catalysts [10] or fused catalysts [11], Or in other cases of an active component or components deposited, as a rule, on a highly developed area support, for example, silica, alumina, carbon or in some cases a zeolite. The function of the support is to enhance the catalyst properties, for example, the stability of the active component or components, or in some cases to be even included in the catalytic reaction, for example, by providing acidic sites in bifunctional zeolite catalysts [10],... 

A second problem of catalyst regeneration is often the modification of the dispersion of the active component. Several studies [24, 230] clarify that carbon deposition originating from hydrocarbons not only covers an active particle but may remove it from its support. This mode of carbonization occurs effectively with metals catalyzing the formation of carbon filaments (see above). Figure 35 summarizes this effect. A metal... 

After a general discussion of the production of supported catalysts, the theory of nuclcation and growth of solids is surveyed Next the interaction between supports and precipitating precursors of the active components, which is dominating the nuclcation with precipitation onto suspended supports, is discussed This is followed by a review of the loading of powdered supports suspended in an aqueous solution of the active precursor(s) by deposition-precipitation Highly prom-... 

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. 

For a correct idea of the physicochemical conditions of accumulation of iron-ore sediments, an analysis of the forms of transport and conditions of deposition of iron and silica in recent active volcanic regions is necessary. Such an analysis should include establishing possible sources of the ore material (vents of active volcanoes, fumaroles, hydrothermal volcanic waters), the character and intensity of the process of migration and forms of transport of the ore components, and the conditions of deposition of the ore components in the course of their migration to the sea basin and when the river waters mingle with sea. waters. 

At high temperature and in the presence of Mo complexes, THF is polymerized to yield crown ethers (scheme 3) [4], Probably, this reaction is responsible for deactivation of the bimetallic catalysts supported on Slbunit. The monometallic catalysts are not deactivated even at high yields of resin complexes and high temperature. It is likely that the polymers do not deposit on the active component and the support surface. 

After isolation the supported precipitate is washed, dried and usually calcined to produce a supported oxide which is then reduced, commonly in a hydrogen stream. Reduction of these supported oxides generally proceeds more readily than the mixed oxides produced by coprecipitation since there is only a monolayer in which there is a direct interaction of the active component with the support. This monolayer can be considered to be a silicate or aluminate which is more difficult to reduce than the oxide or hydroxide found in the outer metal-containing layers.33 Precipitation-deposition gives catalysts having compositions similar to those produced by sequential precipitation as shown in Fig. 13.2. 

Quantitative XPS can yield the topology of coke deposits In spent heavy-oil-processlng catalysts. The layer thickness of the coke, its fractional coverage of the catalyst surface and its location with respect to the active components can be derived. Provided that independent information on the dispersion of the active component is available it can be inferred whether the coke is randomly distributed or whether it is deposited preferentially on either active component or the bare carrier surface. 

In the case of carbons traditionally used as the supports, adsorption of H2PdCl4 is comparatively rapid [16], and the concentration of the strong adsorption sites A2 + Af is higher than that of the supported metal. Hence, most of palladium transfers from the solution onto the support prior to the introduction of the alkali agent and forms there the former Pd particles (9) and Pd chloride complexes (11). These surface compounds are chief precursors of the active component in the catalysts containing no more than 5%wt. Pd. A remarkable contribution of the deposition of PNHC particles from the solution is only observed for catalysts with a very high Pd loading or when supports with a very low specific surface area are used. 

Deposition. - Deposition, as used in preparing supported catalysts, is the laying down or placing of the active components on the exterior surface of a support. One means by which this may be achieved is the preparation of catalysts by sputtering, which involves condensing the metal vapour onto an agitated finely dispersed support. However, as this process is performed under a high vacuum, the technique is probably only useful for the preparation of model catalysts. Alternatively, the process may be performed in the liquid phase by the deposition of a metal sol onto a suspended support. 

There are several techniques for preparation of high-area catalysts. Some involve formation of a support or carrier, especially alumina, silica or carbon, onto whose surface is deposited an active catalyst ingredient. However, seldom is the support inactive in the sense that it functions only to spread out the active component. The support usually influences the added ingredient through epitaxial or chemical interaction which alters the behavior of the active component. In Si02— Al203 catalysts, it is the combination of both oxides that provides for the essential acidity. In dual-function catalysts, the support can serve catalytically as the essential acid function. 

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