Adsorption solid surface areas from

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... 

In the literature, many different approaches have been proposed for estimating the surface area of a solid. Surface areas may be estimated from the exclusion of like charged ions from a charged interface. This method is intriguing in that no estimation of either site or molecular area is needed. In general, however, surface area determination by means of solution adsorption studies, while convenient experimentally, may not provide the most correct information. Nonetheless, if a solution adsorption procedure has been standardized for a given system by means of independent checks, it can be very useful determining relative areas of a series of similar materials. In all cases, it is also more real as it is what happens in real life. 

It is of interest to determine the reasons for the different behavior of NiO(200°) and NiO(250°). The water content of the solids differs NiO(200°), which is prepared at a lower temperature, retains more water (0.16 H20/mole) than NiO(250°) (0.11 H20/mole). Although it has been shown that the decomposition of nickel hydroxide is a topo-chemical reaction (23) and although the residual hydroxide should be located in the interior of the particles, water molecules may remain adsorbed on the surface of the newly formed oxide phase. Moreover, since dehydration produces a large increase of surface area (from 34 to 130 m /gm), fragmentation of oxide particles is likely and, thence, hydroxyl groups may also remain on the exposed surface. For these reasons, participation of adsorbed water or surface hydroxyl groups in adsorptions and interactions is not, a priori, precluded. 

Lead transport was dominated by the solution phase (> 50% ) but had significant contributions from the oxide, organic, and crystalline phases. Hem (74) has suggested that lead levels are held below equilibrium solubility levels by adsorption processes, altered in this case by low solid surface areas and high TOC levels. 

Both the BET and the Duhinin models are widely thought to adequately describe the physical adsorption of gases on solid carbons. BET surface areas from many microporous carbons range from 500 to 1500 m g . However, values of up to 4000 m g" are found for some super-activated carbons and these are unrealistically high. 

The Harkins and Jura (36) absolute method of calculating specific surface area from adsorption data apparently gives more consistent results than the BET method when different adsorbates are used on dinerent kinds of solids. It is based on an empirical equation ... 

The Langmuir equation and the method of solid surface area determination based on it can be applied for the systems in which the adsorption process is not complicated by formation of a multilayer as well as by adsorption in micropores and capillary condensation. Adsorption of gases at the temperatures higher than the critical temperature on nonporous or wideporous adsorbents is an example of such cases. Despite this limitation, the Langmuir equation is used in technical adsorption for calculations of kinetics and dynamics of impurities uptake from the gas medium or diluted solutions. 

At very low reactant concentrations, as may characterize pollutant removal from water, an additional effect of solid s loading is expected. Here, increased solid surface area results in decreased bulk concentration of reactant, and at corresponding diminution of the rate when strong photon flux gradients exist. In other words, increased catalyst in the darker region of the reactor removes (by adsorption) reactant availability in the illuminated zone. Again, a maximum is expected this influence has not yet been combined with the predicted scattering phenomena to provide a reactor description for low concentration systems. 

It is at once evident that there is a remarkable degree of similarity between the shapes of L-type solute isotherms and Type I physisorption isotherms. However, this similarity is misleading since the adsorption mechanisms involved are likely to be quite different. We have seen already that Type I physisorption isotherms for gas-solid systems are normally associated with micropore filling. In contrast, the plateau of an L-type solute isotherm usually corresponds to monolayer completion. In this respect, solute adsorption appears to correspond more closely to the classical Langmuir mechanism. If this is indeed the case it would seem to be possible to calculate the surface area from nj by the application of a simple equation of the same form as Eq. (2). 

According to the assumptions made, adsorption on the first layer involves a certain heat of adsorption Q different from that for the second and subsequent layers, which is taken to be the same at Qi.. The total solid surface area 5 (== So) is given by ... 

Many adsorption phenomena especially of surfactants, polymers, proteins and the chemical adsorption of gases on solids can be well represented by the Langmuir adsorption isotherm. This equation can be expressed in a suitable linear form and we can obtain the two parameters of the model, of which one is the concentration or volume at maximum (full) coverage or the so-called monolayer coverage. Knowledge of this monolayer coverage and of the specific surface area of the solid can help us estimate the surface area occupied by a molecule at the interface and thus the amount needed for stabilization. The specific solid surface area can be obtained from gas adsorption measurements on the same solid. 

The calculation of the specific surface area should be taken in three steps (1) determination of the gas (vapor) adsorption isotherm on the solid materials to be investigated, (2) calculation of the function v /(p) from the measured isotherm, and (3) based on the function y(p) selection of the appropriate isotherm equation and the calculation of the specific surface area from this equation which has to include the value of the total monolayer capacity ( ). 

Dye adsorption from solution may be used to estimate the surface area of a powdered solid. Suppose that if 3.0 g of a bone charcoal is equilibrated with 100 ml of initially 10 Af methylene blue, the final dye concentration is 0.3 x 10 Af, while if 6.0 g of bone charcoal had been used, the final concentration would have been 0.1 x Qr M. Assuming that the dye adsorption obeys the Langmuir equation, calculate the specific surface area of the bone charcoal in square meters per gram. Assume that the molecular area of methylene blue is 197 A. ... 

A variety of experimental data has been found to fit the Langmuir equation reasonably well. Data are generally plotted according to the linear form, Eq. XVn-9, to obtain the constants b and n from the best fitting straight line. The specific surface area, E, can then be obtained from Eq. XVII-10. A widely used practice is to take to be the molecular area of the adsorbate, estimated from liquid or solid adsorbate densities. On the other hand, the Langmuir model is cast around the concept of adsorption sites, whose spacing one would suppose to be characteristic of the adsorbent. See Section XVII-5B for an additional discussion of the problem. 

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. 

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... 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

We therefore felt it timely to attempt a critical exposition and assessment of the common methods for the evaluation of the surface area and pore size distribution of solids from adsorption measurements. Our main concern has therefore been with the use of adsorption data for these purposes rather than with adsorption per se and it is for this reason that our treatment of theoretical matters, whilst sufficiently detailed to bring out the nature of the assumptions underlying the various methods, is not exhaustive we have not set out to write a text-book or a treatise on adsorption, and our choice of material from the literature has been dictated solely by its seeming suitability for the explanation or illustration of the topic under discussion. 

Characterization. When siHca gel is used as an adsorbent, the pore stmcture determines the gel adsorption capacity. Pores are characterized by specific surface area, specific pore volume (total volume of pores per gram of solid), average pore diameter, pore size distribution, and the degree to which entrance to larger pores is restricted by smaller pores. These parameters are derived from measuring vapor adsorption isotherms, mercury intmsion, low angle x-ray scattering, electron microscopy, gas permeabiHty, ion or molecule exclusion, or the volume of imbibed Hquid (1). 

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