Surface area and pore structure

The specific surface area and pore structure are the most basic macroscopic physical properties of solid catalysts. Pore and surface are the reactive rooms of heterogeneous catalytic reactions, and the amount of surface area directly influences the level of catalytic activity. If the surface properties of catalyst are uniform, then their activity is directly proportional to their surface area. Catalytic reactions are generally influenced by the diffusion under industrial conditions, and the activity, selectivity and lifetime and almost all properties of catalyst are related to these two macroscopic physical properties. Although the activity for most catalysts is not proportional to their surface area, the surface area is still a visual physical quantity to evaluate catalyst performance, and sometimes acts as a control index of preparation. 

Therefore, it is not difficult to understand that the characteristic of pore structure and the measurement of surface area have penetrated into the nano-particles as well as molecular channels and holes cage, and its research work also has entered into a new development stage. 

So far, for ordinary industrial catalysts, the main determination methods for pore structure and surface area are still dominated by the vapor physical adsorption technology and pressed mercury method. 

After V and the corresponding p values measured by a series of experiments, then 

If Am is the section area occupied by each of the adsorbate molecule, then 

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The adsorption of a gas by a solid can, in principle, be made to yield valuable information as to the surface area and pore structure of the solid. In practice the range of suitable adsorptives is quite narrow, by far the most commonly used one being nitrogen at its boiling point, 77 K. 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

ADSORPTION-DESORPTION AND THERMAL TECHNIQUES 1.3.1 Surface Area and Pore Structure... 

Table 4 shows the surface properties of skeletal copper catalysts produced by leaching a 50wt% Cu alloy in aqueous sodium hydroxide solution at 293 K. It shows that the surface area decreases with increasing particle size of the alloy. Table 5 shows the effect of temperature of extraction on the surface area and pore structures of completely leached 1000-1180 fim particles of the 50wt% Cu alloy. The results show... 

TableS. The effect of the temperature of extraction on the surface area and pore structure of completely leached 100-1180 pm particles of CuA12 alloy (Ref. 22).

Conventional synthesis methods offer limited control over desirable phase and elemental (bulk and surface) compositions, preferential exposure of active and selective surface planes, surface areas and pore structures of VPO catalysts, which define their catalytic performance in selective oxidation of n-butane. For instance, conventional methods produce VPO catalysts with relatively low surface areas (< 20 m7g) and a limited choice of crystal morphologies. 

Example of application of triangle method. A study was made by Goodwin and Weisz (unpublished) of the intrinsic activity of ZnO when sintered at various temperatures to achieve various amounts of defect structure. In such a series of catalyst samples the surface area and pore structure, and therefore diffusivity, changes over a wide range. Measured activities have to be scrutinized for diffusion effects. Methanol decomposition rates at 270°C. were measured in a Schwab reactor, on particles of two different sizes, Ri = 0.13 cm., and = 0.4 cm. Therefore, = 3. For the various samples the measured activities per gram of... 

While chemical composition is a major factor in determining catalyst properties, catalytic characteristics may vary widely depending on method of catalyst preparation, due to the nature of interaction of catalyst components, their dispersion, crystallographic form, and physical properties such as surface area and pore structure. The composition of the surface is of special importance. Usually only a small fraction of the surface, sometimes called active sites, has a special composition and structure that are responsible for catalyst activity. Hence, small amounts of strategic components situated on the surface can have a profound influence. Catalysts which rely on their acidity can be enhanced during operation by addition of small amounts of H2O or HCl or deactivated by bases such as NH3 or Na20. 

It seemed likely that the amount of carbon needed to produce the sieving properties might depend on the surface area and pore structure of the particular inorganic oxide. To determine if this were true, the carbon content of the IOM-CMS materials was evaluated after each carbon-coating step. The evaluation was done by measuring the specific adsorption of the probe molecules and the nitrogen adsorption isotherms after each carbon coating cycle. 

Ahmed et al. [116] carried ont a detailed stndy with the objective of identifying the properties of activated carbons that are important for the SCR of NO they concluded that chemical properties such as surface oxides and mineral matter play a more important role than their physical properties, such as surface area and pore structure. In effect, they found that the catalyst activity correlated directly with the oxygen content of the carbon samples and inversely with their pH. These results indicate that the NO conversion is favored on more acidic carbons. They also reported that NO reduction by ammonia was negligible in the absence of oxygen. Indeed, it has been shown [117] that oxygen enhances the C-NO reaction through the formation of surface oxygen complexes, which are essential for the C-NO reaction to proceed. 

Ren Gengpo, et al. 2007. Analysis of Surface Area and Pore Structure of Datong Coal. Journal of Combustion Science and Technology, 13(3) 265-268 (in Chinese). 

The BET surface area and pore structures of the RF and carbon aerogels with different R/C ratios and RF mass concentrations are shown in Table 36.3. The surface area of carbon aerogels increases dramatically after pyrolysis, which is due to the creation of micropores as a result of evaporative loss of organic moieties. It is also shown that the micropore area of carbon aerogels decreases with the increase in density [60]. 

BET surface areas and pore structures of Al-CT and Si-CT were determined from isotherms of nitrogen sorption. The samples after calcination at high temperature were used, instead of the samples degassed at high temperature, since the basal spacing of the degassed... 

Surface area and pore structure The BET specific surface areas (m g l) of the nanoparticles left for digestion at various times (0.5h, 8h, 24h and 25d) and dried thereafter at 110 C, 200 C, 300 C, 40QOC, 500°C and 700°C, were determined by the single-point method of N2 adsorption at T=77K using a Carlo Erba 1750 Sorpty apparatus. The results are shown in Figure 3. 

Wilson LD, Mohamed MH, Headley JV. Surface area and pore structure properties of urethane-based copolymers containing /5-cyclodextrin. J Colloid Interface Sci 2011 357(l) 215-22. 

Therefore, the graphitized activated carbons acting as the catalsdic supports are inadvisable. In order to recover the surface area and pore structure of activated carbons, it should be treated via oxidation and so on. 

Due to low water contents, high specific surface areas and pore structures (see the previous section), the water-rock interactions within the Tournemire shales must be characterized by strong short range (nanometre-scale) water (or solute) molecules-mineral interactions. Therefore, the physico-chemical characteristics of the water and its solutes will be different from that of free water which is conventionally considered to take part in water-rock interactions (Horseman et al. 1996). The difference arises from factors such as the very low mobility of water in thin films, the high suction potentials developed owing to water-mineral surface electrostatic interactions and the membrane filtration of anions. The 9% porosity given above must be considered as a maximum value since waters bound chemically to mineral surfaces are included in the estimates in reality free waters are of most importance to the present study. 

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