Gas Adsorption Measurements

Mercury porosimetry is generally regarded as the best method available for the routine determination of pore size in the macropore and upper mesopore range. The apparatus is relatively simple in principle (though not inexpensive) and the experimental procedure is less demanding than gas adsorption measurements, in either time or skill. Perhaps on account of the simplicity of the method there is some temptation to overlook the assumptions, often tacit, that are involved, and also the potential sources of error. 

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings. Surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. It is emphasized that a powder does not have a unique surface, unless the surface is considered to be absolutely smooth, and the magnitude of the measured surface depends upon the level of scrutiny (e.g., the smaller the gas molecules used for gas adsorption measurement the larger the measured surface). 

The determination of the specific surface area of a zeolite is not trivial. Providers of zeolites typically give surface areas for their products, which were calculated from gas adsorption measurements applying the Brunauer-Emmet-Teller (BET) method. The BET method is based on a model assuming the successive formation of several layers of gas molecules on a given surface (multilayer adsorption). The specific surface area is then calculated from the amount of adsorbed molecules in the first layer. The space occupied by one adsorbed molecule is multiplied by the number of molecules, thus resulting in an area, which is assumed to be the best estimate for the surface area of the solid. The BET method provides a tool to calculate the number of molecules in the first layer. Unfortunately, it is based on a model assuming multilayer formation. Yet, the formation of multilayers is impossible in the narrow pores of zeolites. Specific surface areas of zeolites calculated by the BET method (often termed BET surface area) are therefore erroneous and should not be mistaken as the real surface areas of a material. Such numbers are more related to the pore volume of a zeolite rather than to their surface areas. 

Powder X-ray diffraction (XRD) data were collected via a Siemens D5005 diffractometer with CuKa radiation (A. = 1.5418 A). Routine transmission electron microscopy (TEM) and Z-contrast microscopy were carried out using an HITACH HD-2000 scanning transmission electron microscope (STEM) operated at 200 kV. Nitrogen gas adsorption measurements (Micromeritics Gemini) were used to determine the surface area and porosity of the catalyst supports. Inductively coupled plasma (ICP) analysis was performed via an IRIS Intrepid II XSP spectrometer (Thermo Electron Corporation). 

The gases C02, NH3, H20, H2 and N2 are liberated through the porous walls, and the hollow tubes are generated, which host the solid metallic Pt particles. The porosity of the walls enables gas adsorption measurements on the metal particles. After adsorption of CO at room temperature infrared bands at 2069 cm"1 and 1854 cm 1 have been found, which are... 

With a small loss of accuracy, the straight-line BET plot (Figure 5.13) can be assumed to pass through the origin and Vm can be calculated on the basis of a single gas-adsorption measurement (usually with p/pa between 0.2 and 0.3). This procedure is frequently adopted for routine surface area measurements. 

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. 

Due to the high water solubility of MAA, partitioning of the MAA in the water phase was expected. After polymerization, the obtained miniemulsions (latexes) and the colloidal nanoMIPs were characterized by gravimetric analysis, dynamic light scattering (DLS), gas adsorption measurements (BET), and transmission electron microscopy (TEM) as shown in Fig. 9. 

When the main purpose of the gas adsorption measurements is to characterize the adsorbent surface or its pore structure, the preferred approach must be to follow the change in the thermodynamic quantity (e.g. the adsorption energy) with the highest available resolution. This immediately leads to a preference for the differential option, simply because the integral molar quantity is equivalent to the mean value of the corresponding differential quantity taken up to a recorded amount adsorbed. Their relationship is indicated by the mathematical form of Equation (2.64), which is explained in the following section. 

It should be kept in mind that any change in surface area, surface chemistry, or microporosity will result in a change in the energy of immersion. Because immersion calorimetry is quantitative and sensitive, and because the technique is not too difficult to apply in its simplest form, it can be used for quality testing. The preliminary outgassing requires the same care as for a BET measurement, but, from an operational standpoint, energy of immersion measurements are probably less demanding than gas adsorption measurements. 

There are situations in which crystallites are readily visible, especially on supports which do not offer excessive electron scatter. In these cases, metal content can be quantitatively determined for areas which have highly dispersed metal and agglomerated metal. This information in conjunction with the crystallite size distribution provides the microscopist with the information required to make an estimate of metal dispersion (13). These estimates are valuable especially in situations where conventional gas adsorption measurements cannot be made on the metal, i.e., when the crystallites are contaminated, have multiple oxidation states, or are poisoned. 

C.H. Massen, J.A. Poulis, E. Robens Criticism on Jantti s three point method on curtailing gas adsorption measurements. Adsorption 6 (2000) 229-232. 

J.A. Poulis, C.H. Massen, E. Robens, K.K. Unger A fast two-point method for gas adsorption measurements. In K.K. Unger, G. Kreysa, J.P. Baselt (eds.) Characterisation of Porous Solids V. Studies in Surface Science and Catalysis, Elsevier, Amsterdam 2000, p. 151-154. ISBN 0 444 502 599. 

E. Robens, J.A. Poulis, C.H. Massen Fast gas adsorption measurements for complicated adsorption mechanisms. In V.A. Tertykh, V. Pokrovskiy (eds.) Proceedings of the 28 International Conference on Vacuum Microbalance Techniques, 1999, Kyiv. Journal of Thermal Analysis and Calorimetry 62 (2000) 429-433. 

FIGURE 10.11 lUPAC classification of physisorption isotherms. (Following Schoofs, T., Surface area analysis of finely divided and porous solids by gas adsorption measurements, in Particle and Surface Characterisation Methods, R.H. Muller and W. Mehnert, Eds., Medpharm GmbH Scientific, Stuttgart, 1997.)... 

E. Robens, C.H. Massen, J.A. Poulis On curtailing the time for gas adsorption measurements by extrapolation. IX. POROTEC-Workshop uber die Charakterisierung von feinteiligen und porosen Festkorpern, 11.-12. 11. 1998, Bad Soden. 

The gas adsorption measurements by Zettlemoyer and co-workers (13, 14) appeared to indicate that some precipitated silicas (e.g., HiSil 233 from Pittsburgh Plate Glass Company) behaved as nonporous adsorbents. Thus, reversible Type II isotherms of nitrogen and argon were obtained by Bassett et al. (14), who concluded that unrestricted monolayer-multilayer adsorption had occurred. More recent work (15) showed that this interpretation is probably an oversimplification of the physisorption process. 

BET Adsorption Data. A wealth of information about the size and shape of pores may be obtained from adsorption isotherms where the mols of nitrogen adsorbed on the membrane are measured as a function of pressure. However, the use of this techniques is not widespread due to the tedious regimen required in gas adsorption measurements. Further, the hysteresis effects make conclusions about pore-structure ambiguous. 

Si-MCM-48 and Al-MCM-48 materials have been investigated by means of Xe NMR spectroscopy and gas adsorption measurements. The chemical shift of adsorbed Xe in these samples is found to be in the same range as for MCM-41 materials. Furthermore, Al-MCM-48 shows a higher chemical shift and a broader line width compared to Si-MCM-48. This might be due to specific Xe-Al interactions or to interactions of Xe with a more irregular surface. 

Adsorption capacity related parameters are usually determined from gas adsorption measurements. The specific surface area is calculated by applying the Brunauer-Emmett-Teller (BET) equation [17] to the isotherms generated during the adsorption process. The adsorption of N2 at 77 K or CO2 at 273 K are the most commonly used to produce these isotherms. The BET theory is based upon the assumption that the monolayer is located on... 

Industrial applications of nanoporous carbons are based on both their porosity and surface properties, and consequently, their characterization is of great importance. The results presented here demonsfrate a great usefulness of gas adsorption measurements for the characterization of nanoporous carbons. Low-pressure measurements provide an opportunity to study the microporous structure and surface proptaties of these materials and to monitor changes in these properties that result fiom structure and surface modification. High-pressure adsorption data allow for a detailed characterization of mesoporous structures of carbonaceous porous materials, providing their surface areas and pore size distributions. 

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