Physical properties of catalysts

The surface area of a solid has a pronounced effect on the amount of gas adsorbed and on its activity as a catalyst. For example, if a sample of fresh Raney nickel, which is highly porous and has a large surface, is held in the hand, the heat due to adsorption of oxygen can be felt immediately. No heat is apparent in the same mass of nonporous nickel. This relationship between surface area and extent of adsorption has led to the development of highly porous materials with areas as high as 1,500-m /g. Sometimes the catalytic material itself can be prepared in a form with large surface area. When this is not possible, materials which can be so prepared may 

The dependence of rates of adsorption and catalytic reactions on surface makes it imperative to have a reliable method of measuring surface area. Otherwise it would not be possible to compare different catalysts (whose areas are different) to ascertain the intrinsic activity per unit surface. For surface areas in the range of hundreds of square meters per gram-- a-porous materiahwith equivalent-cylindrical-pore-r-adii-(see SecT -8--5)Tn-the range of 10 to 100 A is needed. The following example shows that such areas are not possible with nonporous particles of the size which can be economically manufactured. 

Example 8-1 Spray drying and other procedures for manufacturing small particles can produce particles as small as 2 to 5 microns. Calculate the external surface area of nonporous spherical particles of 2 microns diameter. What size particles would be necessary if the external surface is to be 100 m /g The density of the particles is 2.0 g/cm.  

Solution The external surface area per unit volume of a spherical particle of diameter dp is 

If the particle density is pp, the surface area, per gram of particles, would be 6 

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Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Physical properties of catalysts also may need to be checked periodically, includiug pellet size, specific surface, porosity, pore size and size distribution, and effective diffusivity. The effectiveness of a porous catalyst is found by measuring conversions with successively smaller pellets until no further change occurs. These topics are touched on by Satterfield (Heterogeneous Cataly.sls in Jndustiial Practice, McGraw-Hill, 1991). 

On the other hand, a catalyst in which the CrV04 was one of major constituents had little catalytic activity for the ammoxidation of xylene. These observations indicate that the nature and the distribution of metal ions and oxygen ion on the catalyst surface affect the catalytic activity and selectivity. It is difficult to predict the relationship between the adsorptivity of reactants and the physical properties of catalyst, but it may be assumed that adding more electronegative metal ions affects the electronic properties of the vanadium ion, which functions as an adsorption center. Further details on the physical properties of catalysts for the ammoxidation of xylenes will be reported later. 

Table 3 Performance Enhancement Through the Modification of the Physical Properties of Catalyst Supports... 

Since the cell wall structure of the wood is not swollen by the vinyl monomer, there is little opportunity for the monomer to reach the free radical sites generated by the gamma radiation on the cellulose to form a vinyl polymer branch. From this short discussion, it is reasonable to conjecture that there should be little if any difference in the physical properties of catalyst-heat initiated or gamma radiation initiated in situ polymerization of vinyl monomers in wood. 

Adsorption studies, chiefly from the author s laboratory, will be described at the outset (Sections II and III). The physical properties of catalyst surfaces will then be discussed (Sections IV-VI). Finally, the structure theory of heterogeneous catalysis proposed by Horiuti will be reviewed (Section VII). 

Summary In concluding the treatment of physical properties of catalysts, let us review the purpose for studying properties and structure of porous solids. Heterogeneous reactions with solid catalysts occur on parts of the surface active for chemisorption. The number of these active sites and the rate of reaction is, in general, proportional to the extent of the surface. Hence it is necessary to know the surface area. This is evaluated by low-temperature-adsorption experiments in the pressure range where a mono-molecular layer of gas (usually nitrogen) is physically adsorbed on the catalyst surface. The effectiveness of the interior surface of a particle (and essentially all of the surface is in the interior) depends on the volume and size of the void spaces. The pore volume (and porosity) can be obtained by simple pycnometer-type measurements (see Examples 8-4 and 8-5). The average size (pore radius) can be estimated by Eq. (8-26) from the... 

Ahlborn Wheeler is widely recognized in the catalytic community for his pioneering contributions to catalysis and particularly his development of an understanding of the quantitative relationship between the physical properties of catalysts and their activity and selectivity. The fundamental relationships which he developed have stood the test of time. They are recognized as his personal intellectual contributions. At the same time, he worked closely with others and published the results of outstanding research with some fifteen different collaborators while at Princeton University, Shell Oil Co., the DuPont Co., Houdry Process Co., Aerojet-General Corp., and Lockheed Missile and Space Co. 

The remainder of this section is devoted to a discussion of the experimental techniques used to determine the other physical properties of catalysts that are of primary interest for reactor design purposes the void volume and the pore size distribution. 

The measurable physical properties of catalyst particles commonly used in geometric models are the total surface area Sg (m /g), pore volume Vg (cm /g), solid density ps (g/cm ), void fraction or porosity Gp, and occasionally pore-volume distribution. 

The accomplishments of this group were responsible for changing the study of enzymes from an obscure art concerned with biological mysteries to a branch of science investigating chemical and physical properties of catalysts of biologic origin. 

Catalyst selection should be based on catalyst reactivity, reaction selectivity, and physical properties such as particle size, density, and resistance to attrition. For process development, heat and mass transfer phenomena together with reactivity and physical properties of catalysts must be taken into account. The catalytic process begins with gas reactant transferring to the catalyst outer surface and subsequent intraparticle diffusion of the reactant through the pores of the catalyst. Reactants then absorb onto the catalyst surface and react to form product. These products desorb from the surface, and, through intraparticle diffusion, the products exit from the pores and outer catalyst surface. Consider the example of the ammoxidation of propylene to produce acrylonotrile over multicomponent molybdenum/bismuth catalysts ... 

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