Supports physical forms

Catalytic properties are dependent on physical form, principally the exposed surface area which is a function of particle size. Industrial PGM catalysts are in the form of finely divided powder, wine, or gauze, or supported on substrates such as carbon or alumina (see Catalysis Catalysts, supported). 

The combination of solid-state ATR-IR and solid-state NMR data supported the conclusion that the presence of crystalline material was responsible for changes in the dissolution profiles of the different lots. The results appear consistent with historical examples of changes in API physical form of solid, high molecular weight, polyethylene glycol dispersion formulations of amorphous indomethacin and griseofulvin (92-95). 

For the overall performance of potential catalysts in practical application additional factors, such as number of active sites, physical form, and porosity must also be taken into account. The classical commercial iron catalyst is an unsupported catalyst. First of all iron is a cheap material and secondly by the incorporation of alumina a surface area similar to that attained in highly dispersed supported catalysts can be obtained. Of course, for an expensive material such as the platinum group metals, the use of a support material is the only viable option. The properties of the supported catalyst will be influenced by several factors [172]... 

Activated carbons are produced with a wide range of properties and physical forms, which leads to their use in numerous applications (Table 1). For example, their high internal surface area and pore volume are pertinent to their being employed as adsorbents, catalysts, or catalyst supports in gas and liquid phase processes for purification and chemical recovery. General information on the manufacture, properties, and applications of conventional activated carbons can be found in Porosity in Carbons, edited by John Patrick [I],... 

In this chapter, the adsorption of catalyst precursors on oxidic surfaces will be discussed in terms of a chemical-interaction model, i.e. allowing the formation of inner-sphere complexes, but it is important to realize that other models are being advocated in the literature as well on the one hand, one has the physical adsorption models, allowing only the formation of outer-sphere complexes, and on the other, it is proposed that during adsorption one really has a reaction between the precursor and the support to form a new phase or phases. We will meet the latter situation in our discussion of deposition-precipitation (Section 10.3.3), but we will disregard it when discussing impregnation chemistry (Section 10.3.2). 

To return to activated carbons, these can occur in very different physical forms granular (or particulate), powdered, fibrous, or even membrane (the latter can be either unsupported, or, more commonly, supported). These basic physical forms can be combined with binders and extruded to form pellets, monoliths, or even paper. All of these materials, which are very frequently used as adsorbents, differ significantly in shape, but not in their intrinsic nanotextural features. All of them are isotropic and have their BSUs randomly oriented. 

The kinetics reported for acetylene hydrogenation are shown in Table XXVIII. Where comparison can be made, they seem to be independent of the physical form of the catalyst and of the support. 

Porous carbons constitnte a fascinating kind of material. Different types with distinctive physical forms and properties (i.e., activated carbons, high-surface-area graphites, carbon blacks, activated carbon cloths and fibers, nanofibers, nanotubes, etc.) find a wide range of indnstrial applications in adsorption and catalysis processes. The main properties of these materials that make them very useful as catalyst supports, as well as some of their applications, have been described. The use of carbon as a catalyst support relies primarily on the relative inertness of its surface, which facilitates the interaction between active phases or between active phases and promoters, thus enhancing the catalytic behavior. This makes porous carbons an excellent choice as catalyst support in a great number of reactions. 

In order to prepare thin-film samples, the desired elements must usually be chemically or physically separated from the host compound. The elements being determined are collected in a physical form suitable for x-ray analysis using such methods as ion exchange, solvent extraction, or precipitation. Metallic ions, for example, may be collected on resin-loaded paper, which also serves as the mechanical support in the x-ray spectrometer. The absolute sensitivity for elements isolated from the host compound is 0.01 to 1 /ug analysis of elements present in trace concentrations is possible with this preconcentration approach, if the total sample size is sufficiently large. 

The earliest material to resemble a supported metal catalyst was made by Dobereiner, who mixed platinum black with clay in order to dilute its catalytic action. This remains a significant objective, since for most purposes it would be quite impractical to employ undiluted metal. The use of supported metals facilitates handling and minimises metal loss, a particularly important consideration with the noble metals by appropriate choice of the physical form of the support, they can be used in various types of catalytic reactor, such as fixed-bed or fluidised-bed configurations. The support has often been regarded as catalytically inert, but in addition to those cases such as bifunctional catalysts, where the acidic support has long been known to play a vital role, there is a growing number of examples of participation by the support in catalytic processes. The support surface also facilitates the incorporation of modifiers, such as promoters or selective poisons. 

For catalyst supports that are to be used in industrial processes, the principal considerations are (i) chemical stability, (ii) mechanical strength and stability, (iii) surface area and porosity, (iv) cost, (v) physical form and (vi) cooperation (if any) with the active phase. These apply equally to reactions in the gas phase, the liquid phase and to three phase systems, but some of them are of small importance in fundamental research (e.g., i, iii, and vi). Nevertheless because of the general desire (for obvious reasons) to perform basic work that has a detectable relevance to industrial problems, those supports that feature most commonly in large-scale operation also appear most often in academic laboratories. 

The physical form of the support has to be chosen with a view to the type of reactor in which its use is intended. Silica and alumina are available as coarse granules or fine powders, and may be formed into various shapes with the aid of a binder (stearic acid, graphite) they can then be used in fixed bed reactors. For fluidised beds, or for use in liquid media, fine powders are required. Ceramic monoliths having structures resembling a honeycomb are used where (as in vehicle exhaust treatment) very high space velocities have to be used, but they are made of a non-porous material (a-alumina, muUite) and have to have a thin wash-coat of high area alumina applied, so that the metal can be firmly affixed. 

Activation energies are very frequently between 30 and 50 kJ mol and for different metals in comparable physical form it is sometimes said that the same value suffices for all. Orders in hydrogen are always positive and very often are unity, while orders in ethene are usually either zero or sometimes negative. So general is this behaviour that attention is naturally drawn to the few exceptions. With platinum, orders of reaction are essentially independent of the physical form and the support, but are temperature-dependent (Table 7.4), the order in... 

The form and quantity of carbon existing on the surface of a metal catalyst depends inter alia upon the following variables (i) the nature of the metal, (ii) its physical form, i.e. single crystal, powder or black , small supported particle etc., (iii) the nature of the support, if any, (iv) the type of hydrocarbon applied, (v) the presence of other molecules, especially hydrogen, and the hydrogen hydrocarbon ratio, (vi) the time and especially the temperature of exposure. We may briefly consider the importance of each of these factors. 

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