Surface Area and Pore Size Distribution

Determinations of Surface Area and Pore Size Distribution 

Assuming that the condensation rate of the gas molecules onto an already adsorbed layer is equal to the evaporation rate of the molecules from the adsorbed layer, the BET equation can be obtained in a linear form as follows 

The total BET surface area A B T is calculated from the volume of monolayer coverage Vmoao to 

Now let us overview the theoretical adsorption models for characterization of the pore structures according to the pore size range. For physical adsorption of the gas molecules on such microporous solids as activated carbons and zeolites, Dubinin and Radushkevich95 developed an empirical equation, which describes the volume filling process in the micropores. Their theory incorporates earlier work by Polanyi96 in regard to the adsorption potential Ad defined as 

AG is Gibbs free energy. For carbon materials being comprised of slit-shaped pores, the Dubinin-Radushkevich (D-R) equation is given as 

---

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. 

The Use of Gas Adsorption for the Determination of Surface Area and Pore Size Distribution... 

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. 

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. 

Rehydration Bonded Alumina. Rehydration bonded aluminas are agglomerates of activated alumina, which derive their strength from the rehydration bonding mechanism. Because more processing steps are involved in the manufacture, they are generally more expensive than activated aluminum hydroxides. On the other hand, rehydration bonded aluminas can be produced in a wider range of particle shape, surface area, and pore size distribution. 

Supports. The principal component of a typical catalyst is the porous support (49,50). Most supports are robust soHds that can be made with wide ranges of surface areas and pore size distributions. The most widely appHed supports are metal oxides others are carbon, kieselguhr, organic polymers, and zeoHtes. 

Various analytical tests determine zeolite properties. These tests supply information about the strength, type, number, and distribution of acid sites. Additional tests can also provide information about surface area and pore size distribution. The three most common parameters governing zeolite behavior are as follows ... 

Uses of adsorption studies Determination of catalytically active surface area and elucidation of reaction kinetics Determination of specific surface areas and pore size distributions... 

The structure of the catalysts was characterized by X-ray diffraction, IR-spectroscopy and transmission electron microscopy, their thermal stability was followed by thermal analytical method. The specific surface area and pore size distribution of the samples were determined by nitrogen adsorption isotherms. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

Nitrogen physisorpllon CQfiHe/SA 3100 BET surface area and pore size distribution Trained Free... 

Unfortunately there are no routine methodologies for evaluating the morphology of solvent wet resins at least in terms of generating quantitative data on surface area and pore size distribution. It is possible to use the adsorption of a suitable molecule from the solvent, assume monolayer coverage and an molecular area for the molecule, and hence compute a surface area. This technique has been used in assessing the surface area of e.g. sihca and alumina but has not proved valuable in the case of resins. 

Characterization measurements. Surface areas and pore size distributions were obtained from adsorption isotherms, using BET and BJH methods. The pore size distribution was computed from the... 

SBY, yet the HDN activities of the catalysts are almost the same, especially for Mo-Ni / Zr-Si-Al catalyst. It is well known that not only surface chemistry of the support but also geometrical factors, like the surface area and pore-size distribution, are of major importance for performance of HDN catalyst. The pores are not only paths for reactants and products but also influence the deposition of the active metals during preparation. Mo-Ni/Zr-Si-Al catalyst has bigger surface area (over 600 M2/g) than SBY(240 M2/g), from the point of effective diffusivity, Zr-Si-Al is better than SBY. If the acidity of Zr-Si-Al support was increased properly by some modification methods, the synthesis samples would be a good HDN catalytic materials. 

The most versatile surface characterisation methods are based on gas or vapour sorption and these techniques can provide physico-chemical information such as enthalpies, surface energies and diffusion constants but also surface area and pore size distributions. 

Silica MCM-41 was synthesized hydrothermally at 373 K for 7 days by using water glass and n-hexadecyltrimethylammonium bromide in a manner similar to that reported by Beck et al. [2]. The quality of MCM-41 prepared here was examined by the measurements of XRD, specific surface area and pore size distribution (calculated from N2 adsorption isotherm), and TEM. 

X-ray diffraction patterns of powdered catalysts were recorded with a Rigaku RINT 1200 diffractometer using a radiation of Ni-filtered Cu-Ka. BET surface area and pore size distribution were calculated from the adsorption isotherm of N2 at 77 K. The BJH method was used for the latter. Aluminum content was determined by ICP spectrometer. FTIR spectra of adsorbed NH3 were recorded with a JASCO FT/IR-300 spectrometer. The self-supporting wafer was evacuated at prescribed temperatures, and 25 Torr of NH3 was loaded at 473 K. After NH3 was allowed to equilibrate with the wafer for 30 min, non-adsorbed NH3 was evacuated and a spectrum was collected at 473 K. The differential heat of adsorption of NH3 was measured with a Tokyo-riko HTC-450. The catalyst was pretreated in the presence of 100 Torr oxygen and evacuated at 873 K. The measurements were run at 473 K. 

The most common method used for the determination of surface area and pore size distribution is physical gas adsorption (also see 1.4.1). Nitrogen, krypton, and argon are some of the typically used adsorptives. The amount of gas adsorbed is generally determined by a volumetric technique. A gravimetric technique may be used if changes in the mass of the adsorbent itself need to be measured at the same time. The nature of the adsorption process and the shape of the equilibrium adsorption isotherm depend on the nature of the solid and its internal structure. The Brunauer-Emmett-Teller (BET) method is generally used for the analysis of the surface area based on monolayer coverage, and the Kelvin equation is used for calculation of pore size distribution. 

Analysis of Fractions. Surface areas and pore size distributions for both coked and regenerated catalyst fractions were determined by low temperature (Digisorb) N2 adsorption isotherms. Relative zeolite (micropore volume) and matrix (external surface area) contributions to the BET surface area were determined by t-plot analyses (3). Carbon and hydrogen on catalyst were determined using a Perkin Elmer 240 C instrument. Unit cell size and crystallinity for the molecular zeolite component were determined for coked and for regenerated catalyst fractions by x-ray diffraction. Elemental compositions for Ni, Fe, and V on each fraction were determined by ICP. Regeneration of coked catalyst fractions was accomplished in an air muffle furnace heated to 538°C at 2.8°C/min and held at that temperature for 6 hr. 

Table VII. Surface Areas and Pore-Size Distribution of Coked Shell 244 Catalyst...

Two Co-Mo-alumina catalysts obtained from a commercial vendor as either marketed or special research samples were used in this study. The surface area and pore-size distributions (using the mercury penetration technique) were determined by an independent commercial laboratory. The catalyst properties are given in Table II. Note that the monodispersed (MD) and bidispersed (BD) catalysts have the same metallic composition and are chemically similar. 

Gas adsorption measurements are widely used for determining the surface area and pore size distribution of a variety of different solid materials, such as industrial adsorbents, catalysts, pigments, ceramics and building materials. The measurement of adsorption at the gas/solid interface also forms an essential part of many fundamental and applied investigations of the nature and behaviour of solid surfaces. 

Gas adsorption methods are often used to determine the surface area and pore size distribution of catalysts. The Brunaucr-Emmctt-Tcllcr (BET) adsorption method is the most widely used standard procedure. (See Pure and Applied Chemistry 57, 603 (1985)). 

In this review we will confine ourselves to the application of types II and IV isotherms in the determination of surface areas and pore size distributions. In practice the range of suitable adsorptives in quite narrow, by far the most commonly used one being nitrogen at its boiling point (about 77 K). 

Surface areas and pore size distributions of mesoporous materials are most easily studied by nitrogen adsorption and nitrogen capillary condensation. The most appropriate method for the study of macroporosity is mercury porosimetry [6,7], a technique which will not be treated here. 

The applied pressure is related to the desired pore size via the Washburn Equation [1] which implies a cylindrical pore shape assumption. Mercury porosimetry is widely applied for catalyst characterization in both QC and research applications for several reasons including rapid reproducible analysis, a wide pore size range ( 2 nm to >100 / m, depending on the pressure range of the instrument), and the ability to obtain specific surface area and pore size distribution information from the same measurement. Accuracy of the method suffers from several factors including contact angle and surface tension uncertainty, pore shape effects, and sample compression. However, the largest discrepancy between a mercury porosimetry-derived pore size distribution (PSD) and the actual PSD usually... 

Other workers [10,11] matched the NMR and mercury porosimetry derived pore size distributions to estimate p. More recently, Davis and co-workers [12] have shown that p can be found via a series of Ti experiments, varying the quantity of fluid sorbed on the solid surface. In that work it was shown that a plot of inverse average T, versus the surface area (as determined via conventional methods) times solid concentration (SA C) will give a line with slope (p/2) and intercept a. This value of p can then be applied to find unknown surface areas and pore size distributions using experimentally determined T, s for similar material at the same fluid, frequency and temperature. 

Nitrogen adsorption/condensation measurements were performed using an Autosorb-1 analyzer to calculate sample surface area and pore size distribution. BET analysis at 77 K was applied for extracting the monolayer capacity from the adsorption isotherm and a N molecular cross-sectional area of 0.162 nm2 was used to relate tne monolayer capacity to surface area. PSD s were calculated from the desorption branches of the isotherms using a modified form of the BJH method [18]. Mercury intrusion measurements were performed using an Autoscan-33 continuous scanning mercury porosimeter (12-33000 psia) and a contact angle of 140°. 

---