Pore size distribution, of catalysts

Gas adsorption methods are often used to determine the surface area and pore size distribution of catalysts. The Brunaucr-Emmctt-Tcllcr (BET) adsorption method is the most widely used standard procedure. (See Pure and Applied Chemistry 57, 603 (1985)). 

With this objective the present work was undertaken to investigate the efrects of some parameters of the impregnation process on the selectivity of promoted Ni/MgO catalysts for the reaction of propane oxidation by air to CO and H2. The properties of catalysts relevant for their selectivity such as the Ni-loading in produced catalyst samples, the nickel surface area and mean Ni-crystallite size as well the pore size distribution of catalyst samples are presented. 

Figure 5. Pore size distributions of catalysts prepared with different amounts of H2SO4.

Figurel4.1 Examples of Hg pore-size distributions of catalysts used fordiesel (red), vacuum gasoil (VGO) (gray), and residue (blackand green) hydrotreatment.

Porosity and pore size distribution of catalyst layers... 

Test Method for Single-Point Determination of the Specific Surface Area of Catalysts Using Nitrogen Adsorption by the Continuous Flou Method Practice for Calculation of Pore Size Distributions of Catalysts from Nitrogen Desorption Isotherms... 

The relation between the dusty gas model and the physical structure of a real porous medium is rather obscure. Since the dusty gas model does not even contain any explicit representation of the void fraction, it certainly cannot be adjusted to reflect features of the pore size distributions of different porous media. For example, porous catalysts often show a strongly bimodal pore size distribution, and their flux relations might be expected to reflect this, but the dusty gas model can respond only to changes in the... 

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. 

The high specific surface area supports (10 to 100 m2/g or more) are natural or man-made materials that normally are handled as fine powders. When processed into the finished catalyst pellet, these materials often give rise to pore size distributions of the macro-micro type mentioned previously. The micropores exist within the powder itself, and the macropores are created between the fine particles when they... 

The structure of the catalysts was characterized by X-ray diffraction, IR-spectroscopy and transmission electron microscopy, their thermal stability was followed by thermal analytical method. The specific surface area and pore size distribution of the samples were determined by nitrogen adsorption isotherms. 

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. 

Fig. 32. Pore size distribution of macroporous epoxies prepared via kinetically controlled CIPS with 1 wt % catalyst...

Accordingly, in addition to rate parameters and reaction conditions, the model requires the physicochemical, geometric and morphological characteristics (porosity, pore size distribution) of the monolith catalyst as input data. Effective diffusivities, Deffj, are then evaluated from the morphological data according to a modified Wakao-Smith random pore model, as specifically recommended in ref. [63[. 

The FCC matrix plays a crncial role in precracking, vaporization, and internal diffusion of heavy feed molecnles on catalyst particles. Therefore, many efforts have been made to optimize the acidity and pore size distribution of the matrix to improve reaction performance. 

Figure 3.66 Pore size distribution of an automotive catalyst.

Another method of estimating the pore size distribution of meso- and macropores is by mercury porosimetry. Here one measures the volume of mercury, a nonwetting liquid, which is forced under pressure into the pores ofa catalyst sample immersed in mercury. The pressure required to intrude mercury into the sample s pores is inversely proportional to the pore size [86]. For cylindrical pores of radius r, this... 

Figure 1 shows the pore size distribution of the two catalysts as determined from the mercury porosimeter data. The most frequent pore radius of the Monolith catalyst is 80°A as compared to 33°A of the Nalcomo 474 catalyst. On the other hand, the surface area of the Monolith catalyst is 92.0 m2/gm as compared to 240 m2/gm of the Nalcomo 474 catalyst. The chemical compositions of the two catalysts also differ as shown in Table II. 

The same equipment as that for measuring surface area can be used to determine the pore size distribution of porous materials with diameters of 20 to 500 A, except that high relative pressures are used to condense N2 in the catalyst pores. The procedure involves measuring the volume adsorbed in either the ascending or the descending branch of the BET plot at relative pressures close to 1. Capillary condensation occurs in the pores in accordance with the Kelvin equation,... 

There are two values of surface area and volume of nitrogen adsorbed (BJH method), obtained with the parent H-Y zeolite and the H-Y/TFA sample (Table 1) the first corresponds to the zeolite-type micropores and the other, to the mesopores. Figure 1 shows the pore size distribution of the H-Y/TFA catalyst there is a sharp peak (not shown here) in the micropore region and another peak at 4nm in the mesopore region. Such a bimodal pore size distribution was also observed with the parent zeolite. 

Table VII. Surface Areas and Pore-Size Distribution of Coked Shell 244 Catalyst...

In the sol-gel preparation of supported metals, a metal precursor is usually added directly to the solution prior to gelling. Regardless of whether the metal precursor participates in hydrolysis and/or condensation, it will become part of the network as the gel forms. Thus, any parameters that are important in solution chemistry (Table 1) could affect the properties of the metal upon activation. An example is the work of Zou and Gonzalez [39] cited in Section 2.I.4.3.A. When these authors used water content as a variable to change the pore size distribution of a series of Pt/Si02 catalysts, they found that the particle size distribution of reduced Pt (in the form of crystallites) is also dependent on the hydrolysis ratio. The average Pt particle size nearly doubles (from about 1.7 to 3nm) as the hydrolysis ratio increases from 10 to 60. As noted earlier, the stability of these catalysts, in terms of the resistance of Pt particles towards sintering, is a function of how well the pore diameter and particle size match. 

Gas adsorption measurements are widely used for determining the surface area and pore size distribution of a variety of different solid materials, such as industrial adsorbents, catalysts, pigments, ceramics and building materials. The measurement of adsorption at the gas/solid interface also forms an essential part of many fundamental and applied investigations of the nature and behaviour of solid surfaces. 

Figure 6.15. Influence of GDL pore-former content on cell performance of a H2/02 single cell (0) 0 mg/cm2, (o) 3 mg/cm2, ( ) 5 mg/cm2, (A) 7 mg/cm2, and (V) 10 mg/cm2 pore-former loading 5 mg/cm2 carbon loading in the GDL and 0.4 mg Pt/cm2 in the catalyst layer [15]. (Reprinted from Journal of Power Sources, 108(1-2), Kong CS, Kim DY, Lee HK, Shul YG, Lee TH. Influence of pore-size distribution of diffusion layer on mass-transport problems of proton exchange membrane fuel cells, 185-91, 2002, with permission from Elsevier and the authors.)...

The use of adsorption methods for the assessment of the surface area and pore size distribution of heterogeneous catalysts... 

The macro-porosity emacro and the correlation function corresponding to the macro-pore size distribution of the washcoat were evaluated from the SEM images of a typical three-way catalytic monolith, cf. Fig. 25. The reconstructed medium is represented by a 3D matrix and exhibits the same porosity and correlation function (distribution of macro-pores) as the original porous catalyst. It contains the information about the phase at each discretization point— either gas (macro-pore) or solid (meso-porous Pt/y-Al203 particle). In the first approximation, no difference is made between y-Al203 and Ce02 support, and the catalytic sites of only one type (Pt) are considered with uniform distribution. 

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