Kenneth S. W. Sing

Recommendations for the characterization of porous solids (Technical Report)

These recommendations aim to be a tool for the selection and appraisal of the methods of characterization of porous solids, and to also give the warnings and guidelines on which the experts generally agree. For this purpose, they successively consider the description of a porous solid (definitions, terminology), the principal methods available (stereology , radiation scattering, pycnometry, adsorption, intrusion, suction, maximum buble pressure, fluid flow, immersion or adsorption calorimetry, thermoporometry , size exclusion chromatography, Xenon NMR and ultrasonic methods) and finally the general principles which are worth being followed in the selection of the appropriate method.

Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)

Abstract Gas adsorption is an important tool for the characterisation of porous solids and fine powders. Major advances in recent years have made it necessary to update the 1985 IUPAC manual on Reporting Physisorption Data for Gas/Solid Systems. The aims of the present document are to clarify and standardise the presentation, nomenclature and methodology associated with the application of physisorption for surface area assessment and pore size analysis and to draw attention to remaining problems in the interpretation of physisorption data.

Handbook of porous solids

INTRODUCTION. Historical Aspects. Definitions, Terminology and Classification of Pore Structures. GENERIC METHODS FOR THE CHARACTERIZATION OF POROUS MATERIALS. Modelling of Ideal Pore Structure. Fractal Analysis. Microscopy and Stereology. Scattering and Diffraction Methods. Adsorption from Gas Phase. Adsorption from Liquid Phase. Capillarity and Fluid Displacement. Mercury Porosimetry. Fluid Flow. Thermoporometry. Surface Hydrophobicity/Hydrophilicity. Surface Acidity. NMR Techniques. Positron Annihilation Spectroscopy. GENERAL PRINCIPLES FOR SYNTHESIS AND MODIFICATION OF POROURS MATERIALS. CLASSES OF MATERIALS. Clathrates and Inclusion Compounds. Crystalline Microporous Solids. Porous Metal Organic Frameworks. Layered Structures and Pillared Layered Structures. Ordered Mesoporous Oxides. Porous Glasses. Other Oxides. Carbons. Porous Polymers and Resins. Aerogels. Macroporous Solids. Miscellaneous. MASS TRANSFER IN POROUS MATERIALS. Diffusion and Adsorption in Porous Media. Transport Phenomena and Reaction in Porous Media. TECHNOLOGICAL ASPECTS. Technical Adsorption of Gases. Membrane Technology. Drying Processes. Liquid Separation. Gas Liquid Chromatography. Environmental Protection. Water Treatment. Respiratory Protection. Catalysis. Pharmaceutical Application. Cements. Optically and Electronically Functional Materials. Miscellaneous and Novel Applications.

The use of nitrogen adsorption for the characterisation of porous materials

Abstract Problems, which may arise when low-temperature nitrogen adsorption is used for the characterisation of porous materials, are discussed in this review. Continuous or discontinuous manometric techniques can be employed for nitrogen adsorption measurements at 77 K. For pore structure analysis, the nitrogen adsorption–desorption isotherms should be determined over the widest possible range of relative pressure, while allowing for slow equilibration and other operational problems, particularly at very low pressures. In spite of its artificial nature, the Brunauer–Emmett–Teller (BET) method is still used for the determination of surface area. In principle, nitrogen isotherms of Types II and IV are amenable to BET analysis provided that pores of molecular dimensions are absent and that the BET plot is obtained over an appropriate range of the isotherm. An empirical method based on the application of standard adsorption data is useful for checking the validity of the BET-area. All the computational procedures for pore size analysis have limitations of one sort or another. The various assumptions include an ideal pore shape, rigidity of the structure and an oversimplified model (capillary condensation or micropore filling). The derived pore widths and pore volumes should be regarded as effective (or apparent) values with respect to the adsorption of nitrogen at 77 K.

Physisorption Hysteresis Loops and the Characterization of Nanoporous Materials

The classification of adsorption hysteresis loops recommended by the IUPAC in 1984 was based on experimental observations and the application of classical principles of pore filling (notably the use of the Kelvin equation for mesopore analysis). Recent molecular simulation and density functional (DFT) studies of the physisorption of gases by model pore structures have greatly improved our understanding of the mechanisms of hysteresis and it is therefore timely to revisit the IUPAC recommendations. In this review, we conclude that there is no immediate need to change the IUPAC classification of physisorption isotherms and hysteresis loops. However, in the light of recent advances, we are able to offer a revised checklist for the analysis of nitrogen isotherms on nanoporous solids: this includes a carefully regulated application of DFT in place of a classical procedure such as the well-known Barrett-Joyner-Halenda (BJH) method.

Adsorption methods for the characterization of porous materials

Abstract No other type of adsorption method can offer the same scope for the characterization of porous solids as gas adsorption. Adsorption from solution measurements are easy to carry out, but are often difficult to interpret. Although immersion calorimetry is experimentally demanding, the technique can yield useful information provided that the corresponding adsorption isotherm data are also available. Nitrogen (at 77 K) is the most widely used adsorptive for the characterization of porous materials. Although the Brunauer–Emmett–Teller (BET) theory is based on an over-simplified model of multilayer adsorption, the BET method continues to be used as a standard procedure for the determination of surface area. Generally, the derived values of BET-area can be regarded as effective areas unless the material is ultramicroporous (i.e. containing pores of molecular dimensions). It is advisable to check the validity of the BET-area by using an empirical method of isotherm analysis. In favourable cases, this approach can be used to evaluate the internal and external areas. The computation of mesopore size distribution should be undertaken only if the nitrogen isotherm is of Type IV. Because of network–percolation effects, analysis of the desorption branch of the hysteresis loop may give a misleading picture of the pore size distribution; also, a considerable range of delayed condensation is to be expected if the pores are slit-shaped. Recent work on MCM-41, a model mesoporous adsorbent has improved our understanding of the mechanisms of mesopore filling. Adsorptive molecules of different size are required to provide a realistic evaluation of the micropore size distribution.

Physisorption of argon, nitrogen and oxygen by MCM-41, a model mesoporous adsorbent

Adsorption isotherms of argon, nitrogen and oxygen have been determined at 77 K on a sample of MCM-41, a novel form of aluminosilicate. All the isotherms are Type IV in the IUPAC classification. The argon and oxygen isotherms exhibit well defined hysteresis loops, whereas the nitrogen isotherm is completely reversible. This unusual character is attributed to capillary condensation taking place within a narrow range of tubular pores of effective width 3.3–4.3 nm. MCM-41 shows considerable promise as a model mesoporous adsorbent.

Analysis of nitrogen adsorption isotherms on porous and nonporous silicas by the BET and αs methods

Abstract Nitrogen adsorption isotherms were determined on a wide range of porous and nonporous silicas. Isosteric heats of adsorption were calculated from the isotherms (over the temperature range −192° to −178°C) on representative materials. Standard data for nitrogen adsorption at −196°C on nonporous hydroxylated silica are tabulated for the p p o range 0.001–0.90. The results indicate that certain high-area silicas are truly nonporous, but some grades of commercial Aerosil are porous. Surface areas are calculated from the isotherms by means of the BET method and the new αs-method. The latter is a graphical procedure in which the amount adsorbed is plotted against αs for the standard adsorption data, where αs is the ratio of the amount adsorbed (at the given p p 0 ) to the amount adsorbed at p p 0 = 0.4. Deviations of the a,-plots from linearity are explained in terms of micropore filling and capillary condensation. In the absence of micropore filling, the surface areas calculated from the slope of the αs-plots are in excellent agreement with the BET-areas. Enhanced isosteric heats and C values are associated with micropore filling; the isotherm is therefore distorted in the BET range and the BET-area is not valid. In certain cases, when micropore filling and monolayer coverage at low p p 0 are followed by multilayer formation and capillary condensation at higher p p 0 , a nearly linear αs-plot results, but again neither the BET-area nor the αs-area can provide a meaningful value of the internal surface area.

Adsorption of nitrogen by porous and non-porous carbons

Abstract Nitrogen isotherms have been determined at 77 K on a number of carbon blacks and microporous carbons. Application of the Dubinin-Radushkevich (DR) method has shown that linear plots are given by both nonporous and microporous samples. The range of linearity is considerably reduced by increasing the micropore size, while graphitisation of nonporous carbon leads to the formation of two distinct linear regions. Application of the α s method provides strong evidence for two stages of micropore filling: 1. (a) a primary process involving enhanced adsorbate-adsorbent interactions 2. (b) a secondary process which is the result of cooperative effects associated with the filling of wider micropores.

The use of physisorption for the characterization of microporous carbons

Abstract Most active carbons are highly microporous (i.e., their large internal surface is located within pores of width no greater than 2 nm.) Gas adsorption (physisorption) techniques are widely used for the characterization of microporous carbons, but the interpretation of the data is not entirely straight-forward. Because of the proximity of the pore walls, the total micropore volume cannot be readily evaluated and it is recommended that it should be expressed in the form of the effective pore volume available to a particular adsorptive. Micropore filling appears to occur in two stages: 1. (1) a primary process in pores of molecular dimensions and 2. (2) a secondary, or cooperative, process in somewhat wider pores. No single adsorptive can be expected to provide a reliable assessment of the micropore size distribution; although nitrogen (at 77 K) is useful for routine measurements, it is recommended that a number of larger probe molecules should be used to characterize the micropore structure.