Porosity catalyst supports

For benzene oxidation, V + Mo oxides are usually catalysts. Inert supports such as silica, alumina, alumdum, quartz, pumice, metallicaluminum, etc. have been tried. High-surface-area supports are observed to have a deleterious effect on the benzene oxidation to MA. A low-porosity-catalyst support may peel off, particularly so in a fluidized bed. Catalyst supports of medium porosity are employed. 

Control of porosity and siirface-to-voliime ratio, as with catalyst supports. 

Textural mesoporosity is a feature that is quite frequently found in materials consisting of particles with sizes on the nanometer scale. For such materials, the voids in between the particles form a quasi-pore system. The dimensions of the voids are in the nanometer range. However, the particles themselves are typically dense bodies without an intrinsic porosity. This type of material is quite frequently found in catalysis, e.g., oxidic catalyst supports, but will not be dealt with in the present chapter. Here, we will learn that some materials possess a structural porosity with pore sizes in the mesopore range (2 to 50 nm). The pore sizes of these materials are tunable and the pore size distribution of a given material is typically uniform and very narrow. The dimensions of the pores and the easy control of their pore sizes make these materials very promising candidates for catalytic applications. The present chapter will describe these rather novel classes of mesoporous silica and carbon materials, and discuss their structural and catalytic properties. 

Powder X-ray diffraction (XRD) data were collected via a Siemens D5005 diffractometer with CuKa radiation (A. = 1.5418 A). Routine transmission electron microscopy (TEM) and Z-contrast microscopy were carried out using an HITACH HD-2000 scanning transmission electron microscope (STEM) operated at 200 kV. Nitrogen gas adsorption measurements (Micromeritics Gemini) were used to determine the surface area and porosity of the catalyst supports. Inductively coupled plasma (ICP) analysis was performed via an IRIS Intrepid II XSP spectrometer (Thermo Electron Corporation). 

In this chapter, we reviewed the structure-controlled syntheses of CNFs in an attempt to offer better catalyst supports for fuel cell applications. Also, selected carbon nanofibers are used as supports for anode metal catalysts in DMFCs. The catalytic activity and the efficiency of transferring protons to ion-exchange membranes have been examined in half cells and single cells. The effects of the fiber diameter, graphene alignment and porosity on the activity of the CNF-supported catalysts have been examined in detail. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

Activated carbon is often used as a catalyst support because of its high surface area and porosity. For example, Uchida et al. (1993) utilized activated carbon adsorbent as support for TiOz photocatalysts. It is reported that the adsorptive action of the activated carbon support enables the organic pollutants to concentrate around the loaded Ti02, resulting in a high photocatalytic degradation rate. 

Yokota, T., Takahata, Y., Katsuyama, T., Matsuda, Y. A new technique for preparing ceramics for catalyst support exhibiting high porosity and high heat resistance. Catal. Today 69 (1 4), 11-15, 2001... 

Catalyst particles generally consist of a metal deposited onto the surface of a support and are denoted by metal/support, e.g. Pd/C indicates palladium metal on a carbon support. Among the metals used for catalysis, Pd is often found to be the most active metal. (Augustine 1965) For example, in the aqueous hydrodechlorination of 1,1,2-trichloroethane, Pd catalysts achieved significantly more conversion than Pt or Rh catalysts. (Kovenklioglu et al. 1992) Catalyst supports can vary in shape, size, porosity and surface area typical materials include carbon, alumina, silica and zeolites. 

Control of porosity and surface-to-volume ratio, as in the pelleting of catalyst supports. 

The preceding discussion has concentrated on the selection of catalytically active components. Although this is an essential task, this is just one aspect of the whole catalytic process, which also includes selection of catalyst support and the design of the overall catalyst in relation to reaction engineering requirements, so that not only activity and selectivity but also mechanical and chemical stability are ensured. For catalyst supports, the design variables are the degree and the form of the dispersion of the catalytic active components, and the porosity of the support. 

Since natural sunlight can only penetrate a few microns depth, the use of thin films of titania applied to ceramic or metallic supports as maintenance free decontamination catalysts for the photocatalytic oxidation of volatile organic compounds is of interest for the abatement or control of these emissions. The sol-gel technology can be readily incorporated as a washcoating step of the catalyst supports that may be subsequently heat-treated to fix the titania to the support. The surface area, porosity and crystalline phases present in these gels is important in controlling their catalytic activity. Furthermore, the thermal stability and development of porosity with heat-treatment was important if the sol-gel route is to be used as a washcoating step to produce thin films. 

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],... 

This paper describes a novel process for the preparation of spherical mesoporous silica spheres in the submicrometer and micrometer size range. Tetra-n-alkoxysilanes are hydrolysed and condensed in the presence of n-alkylamine as nonionic template and ammonia as catalyst. The porosity and the morphology parameters can be independently adjusted in wide ranges. These materials are promising adsorbents in separations techniques and valuable catalyst supports. 

D.M. Smith, R. Deshpande, C.J. Brinker, W.L. Earl, B. Ewing and P.J. Davis, In-situ pore structure characterisation during sol-gel synthesis of controlled porosity materials. Proc. Syrnp. on Catalyst Supports Chemistry, Forming and Characterization. American Chemical Soc., New York City meeting, August 25-30,1991, pp. 489-495. 

Nanostructured microporous catalysts or catalyst supports offer intensified catalysis as they provide enhanced surface area accessible to the reactants and products. In nonstructured catalysts, although the surface area may be large, they are often inaccessible as a result of surface fouling and diffusion resistance can slow down the rate of reaction. In a recent development, microporous materials were used as templates for the solution deposition of metals, which were subsequently heat treated to obtain porous metallic structures, where the size of the pores ranged from 10 pm to lOnm. " The relative phase volume of these two regions can be controlled and the overall porosity can be in excess of 50%. Fig. 7 illustrates the size scale of structures ranging from 10 pm to 10 nm. 

In the present work, porous boron nitride samples have been prepared from different molecular precursors. The nature and the pretreatment of the precursors have been investigated in order to determine accurately the optimal conditions leading to high surface area porous boron nitride materials suitable to be used as catalysts supports. The best results led to BN powders presenting surface areas of about 250 to 300 m. g. The porosity resulted from the presence of two kind of pores mesopores down to 2 nm, and a microporosity corresponding to pores from 0.5 to 1 nm. Attempts were made to incorporate directly a noble metal precursor into the BN precursor. Up to 1100°C, Pd or Pt were kept in the porous structure, but the very high temperature required to stabilise BN powders lowered drastically the metal concentration on the catalyst As a consequence, only the platinum sample could be obtained and characterized this catalyst is currently being tested. 

Although titania is very useful as a promoter for Cr/silica catalysts, it is a poor catalyst support itself. It does not have the porosity necessary for polymerization, but, more significantly, it also does not seem to provide an adequate chemical environment for the chromium. In one experiment, a high-surface-area (100 m2 g 1) TiC>2 carrier was impregnated with the usual 0.4 Cr atoms nm 2, followed by calcination at 400 and at 500 °C. A small amount of Cr(VI) was stabilized, and when tested for polymerization activity the catalyst provided very low yields of polyethylene. Whereas Cr/silica-titania produces polymers of lower MW than those made by Cr/silica, Cr/titania yielded polymers of higher MW. Indeed, it produced ultrahigh-MW polymer (UHMW PE). This finding is consistent with the view that it is the acidity created when titania is combined with silica that is important, and not the titania itself. 

Pure zirconia itself can also serve as a catalyst support, although it yields catalysts with very low activity, in part because of low porosity. Amorphous zirconia can stabilize a small amount of Cr(VI) during calcination, which produces polymer when exposed to ethylene. Figure 130 shows the MW distributions of polymers obtained with Cr/zirconia activated at 500 °C, and tested under the same reaction conditions as the Cr/Zr-silica examples described above. Cr/zirconia produces very high-MW polymer, quite different from Cr/silica-zirconia. This... 

Aluminum phosphates are the most commonly used phosphates for polymerization catalyst supports because they can be made with the high porosity that is necessary for fragmentation. However, many other metal phosphates are also known and are used in other areas of catalysis. These materials are often quite acidic and can also serve as supports for chromium oxide. 

The impurities stick to the periphery of particle pores making the gas flow into the catalyst difficult or impossible. This in turn leads to a considerable increase in the diffusion resistances during the catalytic process. One way of fighting this phenomenon is to use double-porosity alumina. Micropores of about 20 nm are always useful to develop the specific surface area necessary for a good dispersion and stability of the catalytic phase. Macropores over 100 nm in diameter help to diffuse the reagents within the particles. However, the proportion of macropores must not be too great, as that would diminish the mechanical properties of the support correspondingly. For this reason, Rhone Poulenc has, since 1974, developed and marketed various exhaust, gas catalyst supports with specific surface areas of 2... 

Porous carbon materials are used for many applications in various industrial or domestic domains adsorption (air and water purification, filters manufacture, solvents recovery), electrochemistry (electrodes for batteries, supercapacitors, fuel cells), catalyst support (industrial chemistry, organic synthesis, pollutants elimination),. .. Porous carbons used at the present time are generally activated carbons, i.e. materials prepared by pyrolysis of natural sources, like fhiit pits, wood or charcoal. Pyrolysis is followed by a partial oxidation, under steam or CO2 for instance, leading to the development of the inner porosity. 

Reticulated ceramic foams are sponge-like structures with open accessible pores in the range 10 to 100 per inch and an interconnecting porosity in the range 75-90%. They have low resistance to fluid flow, and the tortuous flow path generates considerable turbulence [1]. Consequently, they have attracted considerable attention as potential catalyst supports [2,3]. 

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