Catalysts average pore radius

The reference Pt-Ba/y-Al203 (1/20/100 w/w) catalyst shows surface area values in the range 140-160 m2/g, a pore volume of 0.7-0.8cc/g and an average pore radius close to 100 A (measured by N2 adsorption-desorption at 77 K by using a Micromeritics TriStar 3000 instrument). Slight differences in the characterization data are associated to various batches of the ternary catalyst [24,25],... 

To develop analytical models for processes employing porous catalysts it is necessary to make certain assumptions about the geometry of the catalyst pores. A variety of assumptions are possible, and Thomas and Thomas (15) have discussed some of these. The simplest model assumes that the pores are cylindrical and are not interconnected. Develop expressions for the average pore radius (r), the average pore length (L), and the number of pores per particle (np) in terms of parameters that can be measured in the laboratory [i.e., the apparent particle dimensions, the void volume per gram (Vg), and the surface area per gram (Sg). ... 

We will start by developing an expression for the average pore radius T. If we denote the mass of an individual catalyst particle by raP, simple geometric considerations indicate that the void volume per particle is given by... 

The Knudsen diffusion coefficient may be evaluated from equation 12.2.4 if the catalyst property values are used to estimate the average pore radius. From equation C of Illustration 6.2,... 

When the catalyst is a porous solid, most of the surface area of the catalyst is the surface area of the inner surface of the pores. Therefore, most of the reaction proceeds in the pore. Gas molecules are transferred to the outer surface of the catalyst by diffusion. Generally speaking, the diffusion is faster than the diffusion inside the pores. Gas molecules collide with the inner wall of the pore before they collide with another molecule for the porous catalyst having an average pore radius rp of a few nm. Such diffusion is called Knudsen diffusion and its diffusion constant D is given by ... 

The average pore radius of a catalyst particle is the average of the dominant pore radii as observed from the frequency plot of the catalyst. 

Equation (5) holds for the catalyst used, as diffusion in the porous catalyst particle is of the Knudsen type (pore radius mean free path of the gas molecules). For this catalyst the average pore radius, f, calculated from the equation, for cylindrical pores... 

The catalyst used in this study was triply-promoted 1 0, CaO and AlzO, iron supplied by United Catalyst Inc. Louisville, Kentucky, U.S.A. under the trade number "C 73-1-01 . Porosity is about 0.45 and average pore radius about 24 nm. To activate the catalyst, a 60-h reduction at 440°C in pure hydrogen at a space velocity of 1500 1/h was employed. By the end of this period, the activity had become constant. Long-term catalyst activity was checked frequently and, over the course of the study, remained constant. The percentage variation among runs made under identical conditions was within 4%. 

Table 3.1 Values of internal surface area, pore volume, and average pore radius for typical catalysts (from Perry [1])...

It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 150 - 225 °C and time up to 72 hours. 

The adsorption-desorption isotherms of nitrogen at -196 °C obtained on all the catalysts under investigation were mainly of Type IV of Brunauer s classification [16], exhibiting hysteresis loops closed at P/Po ranging between 0.25 and 0.55. The adsorption data are summarized in Table 1, including BET-C constant, specific surface area(SBEj), total pore volume (Vp), estimated from the saturation values of the adsorption isotherms and average pore radius (r P), assuming cylindrical pore model for which superscript (cp) was used. 

The starting alumina was SAS-1/16 suppRed by La Roche. After grinding to adequate particle size and calcination in air at 700°C for 4 hours, its properties resulted in catalyst size, 0.5-1.0 mm surface area BET, 200 m g l pore volume, 1.0 cm g- average pore radius, 5.3 nm mode pore radius, 6.1 nm isoelectric point, 7.6. 

The present discussion is limited for the most part to low temperature nitrogen adsorption studies and sintering experiments. An adsorption-desorption isotherm yields at once surface area, pore volume, average pore radius and an approximate pore size distribution. Such an isotherm is thus an excellent fingerprint of the physical structure of the catalyst. Sintering curves, or temperature-area plots, obviously demonstrate relative thermal stabilities of these structures under the conditions... 

Area, Pore Volume and Average Pore Radius for Representative Cracking Catalysts in the Virgin, Steam-Treated, Vacuum-Sintered,... 

The synthetic silica-alumina catalysts, TCC Beads, Aerocat, and Diakel, are composed of pores appreciably larger but with average pore radius values almost exclusively in the small pore range of 15 to 25 A. (Ries, Johnson, Melik, and Kreger, 48 Shull, Elkin, and Roess, 57). The complete absence of large pores is indicated for the TCC Beads and the Aerocat Microspheres. In the case of silica-alumina catalysts, the hysteresis loops have considerable breadth and area in contrast to the silica-magnesia isotherms. 

The adsorption-desorption isotherms for synthetic silica-alumina catalysts are somewhat similar to those of silica-magnesia in that there is very little adsorption in the higher relative pressure region. However, the average pore radius of the silica-alumina preparations, generally in... 

Commercial use of a Fluid Filtrol catalyst effected more than a twofold decrease in area, 339 to 141 sq. m./g., in the case of the Equilibrium E sample and an approximately corresponding increase in average pore radius, 24.2 to 47.1 A. The isotherm for the steam-treated sample... 

The DHC deactivation is accompanied by coke production. The conversion degree depends more strongly on the coke content of the catalyst than BET surface area and pore volume do (Fig.2). The average pore radius even increases at low coke values (which could be interpreted by coke filling the smallest pores). It should be mentioned that these results could hardly be explained assuming that the decease of conversion is caused by increasing mass transport limitations. 

If the total pressure p is 1 atm and absolute temperature T is in the vicinity of 298 K, then kinetic theory predicts ordinary molecular diffusivities on the order of 0.1 cm /s, which are comparable to Knudsen diffusivities if the average pore radius is 0.1 xm (i.e., 10 A) and molecular weights are about 50 Da. When pore radii are larger than 1 xm, ordinary molecular diffusion provides the dominant resistance to mass transfer in porous catalysts at standard temperature... 

Most of the previous discussion was based on taking the >Iid catalyst pellet to have a simple pore structure—one average pore radius. For the case of a micromacro pore size distnbution, Mingle and Smith [137] and Carberry [92] have derived expressions for the overall effectiveness fiictor for both micro and macro diffusioo. As a simple example, consider a first-order reaction, with intrinsic rate constantil[,per surfacearea of catalyst Then the massbalancefor the micropores is ... 

The main use of the pore volume is in eq. (2) below. If the pores of a catalyst were assumed to be a single continuous cylinder of uniform size with smooth walls then the average pore radius would be given by ... 

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