Surface area porous solids

When the discussion turns to removal of some component from a fluid stream by a high surface area porous solid, such as silica gel, which is found in many consumer products (often in a small packet and sometimes in the product itself), then the term "adsorption" becomes more global and hence ambiguous. The reason for this ironically is that mass transfer may be convoluted with adsorption. In other words the component to be adsorbed must move from the bulk gas phase to the near vicinity of the adsorbent particle, and this is termed external mass transfer. From the near external surface region, the component must now be transported through the pore space of the particles. This is called internal mass transfer because it is within the particle. Finally, from the fluid phase within the pores, the component must be adsorbed by the surface in order to be removed from the gas. Any of these processes, external, internal, or adsorption, can, in principle, be the slowest step and therefore the process that controls the observed rate. Most often it is not the adsorption that is slow in fact, this step usually comes to equilibrium quickly (after all just think of how fast frost forms on a beer mug taken from the freezer on a humid summer afternoon). More typically it is the internal mass transport process that is rate limiting. This, however, is lumped with the true adsorption process and the overall rate is called "adsorption." We will avoid this problem and focus on adsorption alone as if it were the rate-controlling process so that we may understand this fundamentally. 

The chromia-alumina catalysts of present concern are, for the most part, fairly high surface area, porous solids available from several manufacturers, generally in a pelleted form. Surface areas usually range from 50 to 300 m /gm, and the chromia contents vary from 5 to 20wt %. Depending upon the particular application, various promoters may be present in the catalyst, the most common of these being alkali metals or alkaline earths in concentrations up to about 2wt %. The preparation of chromia-alumina catalysts is the subject of an extensive patent and... 

The surface chemistry of commercial grade calcium hydroxide was studied by sorption methods, thermal analysis and spectroscopy. Part of this investigation was the study of the sorption of water vapour by the partially dtennally decomposed calcium hydroxide. It was found that when the decomposition takes place in the absence of air, the result is a high surface-area, porous solid. The pores are slit-shaped. Complete rehydroxylation takes place upon exposure to water vapour. The whole process is reversible, and reproducible. 

FIGURE 13.5 Surface area of solid (A) and porous particle-based (B) packings. Surface area expressed as square meters per gram, particle size as micrometers, and pore size as angstrom units. 

The complex three-dimensional structure of these materials is determined by their carbon-based polymers (such as cellulose and lignin), and it is this backbone that gives the final carbon structure after thermal degradation. These materials, therefore, produce a very porous high-surface-area carbon solid. In addition, the carbon has to be activated so that it will interact with and physisorb (i.e., adsorb physically, without forming a chemical bond) a wide range of compounds. This activation process involves controlled oxidation of the surface to produce polar sites. 

Supported hcteropoly compounds, where heteropoly compounds are dispersed on the surface of porous solids, are important for applications, since the surface area of heteropolyacids is usually low. The structure, stabil-... 

In the INS spectrum of vapour deposited LDA ice, it is the higher energy optic band, at 37 meV, that dominates [51], which is considerably different from the spectrum of recovered LDA. This indicates that porosity in the vapour deposited LDA has produced an increased surface area, where water molecules are not completely hydrogen bonded. These molecules may be able to relax to the lower energy configurations in Fig. 9. A similar phenomenon has also been observed in the INS spectra of water on the surface of porous solids, such as Vycor and silica gel [54]. 

The most common electrode material used in LC-EC is carbon, either as solid glassy carbon disks in thin-layer cells, or as a high-surface-area porous matrix through which the mobile phase can flow. Gold electrodes are useful to support a mercury film and these are primarily used to determine thiols and disulfides, and also for carbohydrates using pulsed electrochemical detection... 

The specific surface area of solid materials is usually determined by applying the Brunauer-Emmett-Teller (BET) equation to nitrogen adsorption data between relative pressures (P/Pq) approximately 0.05 and 0.3 [51]. However, there are many shortcomings of the BET model. For example, it does not consider adsorption in pores. It is well known that the BET method seriously overestimates the specific surface area for many porous materials. For carbons, the theoretically highest possible specific surface area is approximately 2630 m /g... 

By using porous carbon materials with a veiy high surface area in both positive and negative electrodes, a large amount of electrical charge was found to be stored, and electric double-layer capacitors (EDLCs) were developed [81,82], The fundamental concept of this capacitor is the formation of electric double-layers on the surface of electrodes, as illustrated in Fig. 28. The total amount of electeic charges aligned in double layers on both electrodes increases by the application of potential difference and it is easily understood to depend on the area of this interface, i.e. the surface area of solid electrodes. 

Relationship of reduction process and catalytic activity. Prior to reduction, fused iron catalyst is a dense solid and without catalytic activity. The activity of the fused iron catalyst is not only related with the chemical components and preparation method, but also dependent upon the reduction process because all physical properties such as the surface area, porous structure, pore size and distribution, specific volume of pore, especially, size and formation of a-Fe crystallite etc. are produced during the reduction process. Different reduction processes produce different physical properties, and the surface area and porous structure are also different. The high performance catalysts can only be obtained when the reduction process are carefully controlled. Therefore, the reduction of the catalysts is the last step of the catalysts preparation, and also a key step. 

Initially, heat of wetting was used to measure total surface areas of solids including micro-porous materials. Later, this heat of wetting method was used to obtain differential heats of adsorption, as can be done from the isosteric method and by flow calorimetry. 

The BET equation has found widespread use in adsorption studies. For example, the model has been used for the characterization of porous materials, i.e. getting information about the surface area and the nature of the surface. Using Equation 7.3, the specific surface area of solids can be estimated from the V parameter of BET ... 

Adsorption may in principle occur at all surfaces its magnitude is particularly noticeable when porous solids, which have a high surface area, such as silica gel or charcoal are contacted with gases or liquids. Adsorption processes may involve either simple uni-molecular adsorbate layers or multilayers the forces which bind the adsorbate to the surface may be physical or chemical in nature. 

We have considered briefly the important macroscopic description of a solid adsorbent, namely, its speciflc surface area, its possible fractal nature, and if porous, its pore size distribution. In addition, it is important to know as much as possible about the microscopic structure of the surface, and contemporary surface spectroscopic and diffraction techniques, discussed in Chapter VIII, provide a good deal of such information (see also Refs. 55 and 56 for short general reviews, and the monograph by Somoijai [57]). Scanning tunneling microscopy (STM) and atomic force microscopy (AFT) are now widely used to obtain the structure of surfaces and of adsorbed layers on a molecular scale (see Chapter VIII, Section XVIII-2B, and Ref. 58). On a less informative and more statistical basis are site energy distributions (Section XVII-14) there is also the somewhat laige-scale type of structure due to surface imperfections and dislocations (Section VII-4D and Fig. XVIII-14). 

Efficient use of a catalyst requires high rates of reaction per unit volume and, since reaction takes place on the surface of a solid, catalysts have high surface areas per unit volume. Therefore, tlie typical catalyst is porous, witli... 

Porous and nonporous solids of high surface area... 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

A Type II isotherm indicates that the solid is non-porous, whilst the Type IV isotherm is characteristic of a mesoporous solid. From both types of isotherm it is possible, provided certain complications are absent, to calculate the specific surface of the solid, as is explained in Chapter 2. Indeed, the method most widely used at the present time for the determination of the surface area of finely divided solids is based on the adsorption of nitrogen at its boiling point. From the Type IV isotherm the pore size distribution may also be evaluated, using procedures outlined in Chapter 3. 

Isotherms of Type 111 and Type V, which are the subject of Chapter 5, seem to be characteristic of systems where the adsorbent-adsorbate interaction is unusually weak, and are much less common than those of the other three types. Type III isotherms are indicative of a non-porous solid, and some halting steps have been taken towards their use for the estimation of specific surface but Type V isotherms, which betoken the presence of porosity, offer little if any scope at present for the evaluation of either surface area or pore size distribution. 

The physical adsorption of gases by non-porous solids, in the vast majority of cases, gives rise to a Type II isotherm. From the Type II isotherm of a given gas on a particular solid it is possible in principle to derive a value of the monolayer capacity of the solid, which in turn can be used to calculate the specific surface of the solid. The monolayer capacity is defined as the amount of adsorbate which can be accommodated in a completely filled, single molecular layer—a monolayer—on the surface of unit mass (1 g) of the solid. It is related to the specific surface area A, the surface area of 1 g of the solid, by the simple equation... 

The principal aim of the second edition of this book remains the same as that of the first edition to give a critical exposition of the use of the adsorption methods for the assessment of the surface area and pore size distribution of finely divided and porous solids. 

In writing the present book our aim has been to give a critical exposition of the use of adsorption data for the evaluation of the surface area and the pore size distribution of finely divided and porous solids. The major part of the book is devoted to the Brunauer-Emmett-Teller (BET) method for the determination of specific surface, and the use of the Kelvin equation for the calculation of pore size distribution but due attention has also been given to other well known methods for the estimation of surface area from adsorption measurements, viz. those based on adsorption from solution, on heat of immersion, on chemisorption, and on the application of the Gibbs adsorption equation to gaseous adsorption. 

It would be difficult to over-estimate the extent to which the BET method has contributed to the development of those branches of physical chemistry such as heterogeneous catalysis, adsorption or particle size estimation, which involve finely divided or porous solids in all of these fields the BET surface area is a household phrase. But it is perhaps the very breadth of its scope which has led to a somewhat uncritical application of the method as a kind of infallible yardstick, and to a lack of appreciation of the nature of its basic assumptions or of the circumstances under which it may, or may not, be expected to yield a reliable result. This is particularly true of those solids which contain very fine pores and give rise to Langmuir-type isotherms, for the BET procedure may then give quite erroneous values for the surface area. If the pores are rather larger—tens to hundreds of Angstroms in width—the pore size distribution may be calculated from the adsorption isotherm of a vapour with the aid of the Kelvin equation, and within recent years a number of detailed procedures for carrying out the calculation have been put forward but all too often the limitations on the validity of the results, and the difficulty of interpretation in terms of the actual solid, tend to be insufficiently stressed or even entirely overlooked. And in the time-honoured method for the estimation of surface area from measurements of adsorption from solution, the complications introduced by... 

The book is addressed to those workers whether in academic institutions or in industrial laboratories, whose work is concerned, either directly or indirectly, with the surface area or the pore structure of finely divided or porous solids. 

---