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Review The use of nitrogen adsorption for the ...

Colloids and SurfacesA: Physicochemical and Engineering Aspects 187 188 (2001) 3 9 ReviewThe use of nitrogen adsorption for the characterisation ofporous materialsKenneth Sing *School of Chemistry,Bristol Uni ersity,Bristol,UKAbstractProblems, which may arise when low-temperature nitrogen adsorption is used for the characterisation of porousmaterials, are discussed in this Review . Continuous or discontinuous manometric techniques can be employed fornitrogen adsorption measurements at 77 K. For pore structure analysis, the nitrogen adsorption desorptionisotherms should be determined over the widest possible range of relative pressure, while allowing for slowequilibration and other operational problems, particularly at very low pressures.

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1 Colloids and SurfacesA: Physicochemical and Engineering Aspects 187 188 (2001) 3 9 ReviewThe use of nitrogen adsorption for the characterisation ofporous materialsKenneth Sing *School of Chemistry,Bristol Uni ersity,Bristol,UKAbstractProblems, which may arise when low-temperature nitrogen adsorption is used for the characterisation of porousmaterials, are discussed in this Review . Continuous or discontinuous manometric techniques can be employed fornitrogen adsorption measurements at 77 K. For pore structure analysis, the nitrogen adsorption desorptionisotherms should be determined over the widest possible range of relative pressure, while allowing for slowequilibration and other operational problems, particularly at very low pressures.

2 In spite of its artificial nature, theBrunauer Emmett Teller (BET) method is still used for the determination of surface area. In principle, nitrogenisotherms of Types II and IV are amenable to BET analysis provided that pores of molecular dimensions are absentand that the BET plot is obtained over an appropriate range of the isotherm. An empirical method based on theapplication of standard adsorption data is useful for checking the validity of the BET-area. All the computationalprocedures for pore size analysis have limitations of one sort or another.

3 The various assumptions include an idealpore shape, rigidity of the structure and an oversimplified model (capillary condensation or micropore filling). Thederived pore widths and pore volumes should be regarded as effective (or apparent) values with respect to theadsorption of nitrogen at 77 K. 2001 Elsevier Science All rights :Physisorption; nitrogen adsorption ; BET method; Surface area; Pore size IntroductionGas adsorption is of major importance for thecharacterisation of a wide range of porous materi-als.

4 Of all the many gases and vapours, which arereadily available and could be used as adsorptives, nitrogen has remained universally the aid of user-friendly commercial equip-ment and on-line data processing, it is now possi-ble to use nitrogen adsorption at 77 K for bothroutine quality control and the investigation ofnew materials. In view of the importance of thetechnique, it is of interest to trace its earliest reported studies of the adsorptionnitrogen and other gases at liquid air temperature* Present address: Fairfield, Whitelackington, Ilminster,Somerset TA19 9EF, UK.

5 Tel.:+ (K. Sing).0927-7757/01/$ - see front matter 2001 Elsevier Science All rights : S0927-7757(01) and Surfaces 188 (2001) 3 94( 88 K) appear to have been made by Dewar [1]and Ramsay [2] in the course of their investiga-tions of the composition of the atmosphere andthe separation of the noble gases. There followeda number of other low-temperature studies of theadsorption of nitrogen and other gases by char-coal, including an extensive series of measure-ments over a wide range of temperature by IdaHomfray [3].

6 Langmuir s monumental work on monolayeradsorption [4] resulted in renewed interest in theinterpretation of adsorption data. According tothe Langmuir model (now called ideal localisedmonolayer adsorption ), the amount adsorbed atthe plateau of a Type I isotherm corresponds tocomplete monolayer coverage. In the early 1930s,it was realised that multilayer adsorption of nitro-gen can occur at liquid nitrogen temperature (77K). The work of Benton and White [6] promptedBrunauer and Emmett [7] to adopt gas adsorptionfor the determination of surface area.

7 They foundthat the adsorption isotherms of nitrogen andseveral other gases on an iron synthetic ammoniacatalyst were all of similar sigmoidal shape (laterdesignated, Type II). Empirical evidence indicatedthat the changeover from monolayer to multilayeradsorption occurred at the beginning of the mid-dle, nearly linear, section of the isotherm (termedPoint B). In 1938, the publication of theBrunauer Emmett Teller (BET) theory [8] ap-peared to provide theoretical support for thisproposal since the uptake at Point B was found tobe in good agreement with the BET monolayercapacity, obtain the specific surface area,a, fromnm,Brunauer, Emmett and Teller followed Emmettand Brunauer [9] in assuming the completedmonolayer to be in a close-packed state.

8 In spiteof the perceived theoretical limitations of the BETmodel, the BET- nitrogen method soon becameestablished as a standard procedure for surfacearea use of nitrogen adsorption for pore sizeanalysis dates from the late 1940s. It is based onthe application of the Kelvin equation, with acorrection for the multilayer thickness on the porewalls. One of thefirst computational procedureswas proposed by Shull [10] but the method [11]devised by Barrett, Joyner and Halenda (BJH) in1951 remains the most popular way of derivingthe pore size distribution from an appropriatenitrogen , over a long period many investi-gators overlooked Langmuir s caveat [4] that theequations he derived for plane surfaces are notapplicable to such highly porous adsorbents ascharcoal.

9 It was not until much later that itbecame clear that the plateau of a Type Iisotherm is generally associated with the comple-tion of porefilling [5] rather than monolayercoverage. The importance of Langmuir s com-ments concerning the nature of adsorption innarrow pores was appreciated by Dubinin [12],who argued that very narrow pores (microporesof width 2 nm) arefilled at low relative pres-sure by a volume-filling process. It is now widelyaccepted [13] that this microporefilling mecha-nism is not the same as capillary condensation inmesopores with widths in the range 2 50 nm [5].

10 In the light of recent developments in computersimulation, density functional theory and adsor-bent design [14], it may seem surprising that theBET and BJH methods are still used for theevaluation of surface area and pore size distribu-tion! It is the aim of this short Review to throwsome light on this MethodologyGas adsorption manometry is the method gen-erally used for the determination of adsorptionisotherms of nitrogen at the temperature of liquidnitrogen ( 77 K). This type of approach wasknown as a volumetric determination (or alter-natively as the BET volumetric method ) since itoriginally involved the measurement of gas vol-umes, before and after adsorption .


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