Measurement surface, adsorption methods

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

Figure 2 presents the results of changing properties of sol and gel upon action of syneresis time, and concentration of the salt into water. Using the method of dermic blow, we prepared a series of catalysts at deteimined times of sol or gel aging. The surface area of this series of samples was measured by adsorption methods. The data are presented in Table 1. 

Actually, 286 m g was measured. The difference is explained by the fact that around each point of contact between spherical particles there is an area or annular crevice too small to be penetrated by nitrogen molecules during the measurement of specific surface area, the diameter of the nitrogen molecule being 0.4 nm. For larger particles, the specific surface area, calculated from particle size in electron micrographs, is in better agreement with the area measured by adsorption methods (see Chapter 4, Ref. 142). 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

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... 

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

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). 

Surface Area Measurements. For a discussion of procs used see above under Fisher Sub-Siever and Nitrogen Adsorption. For AP, the Fisher Sub-Siever is most suitable for samples having surface areas from 0.05 to 0.46sqm/g, and the Nitrogen Adsorption method for finer mat, up to 30000 sqcm/cc (Ref 49)... 

Measuring the uptake of a gas by a surface as a function of the dose to which the surface is exposed is the most straightforward way to determine a sticking coefficient. In such experiments, great care should be taken to ensure that gas and surface are in thermal equilibrium. In addition, we need to determine the coverage, either by surface sensitive methods (XPS, AES, IR) or by thermal desorption and ensure that adsorption is not accompanied by desorption. 

Conventional bulk measurements of adsorption are performed by determining the amount of gas adsorbed at equilibrium as a function of pressure, at a constant temperature [23-25], These bulk adsorption isotherms are commonly analyzed using a kinetic theory for multilayer adsorption developed in 1938 by Brunauer, Emmett and Teller (the BET Theory) [23]. BET adsorption isotherms are a common material science technique for surface area analysis of porous solids, and also permit calculation of adsorption energy and fractional surface coverage. While more advanced analysis methods, such as Density Functional Theory, have been developed in recent years, BET remains a mainstay of material science, and is the recommended method for the experimental measurement of pore surface area. This is largely due to the clear physical meaning of its principal assumptions, and its ability to handle the primary effects of adsorbate-adsorbate and adsorbate-substrate interactions. 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

It is often found that the ratio R (measured, for instance, by gas adsorption methods) of actual metal surface area accessible to the gas phase, to the geometric film area, exceeds unity. This arises from nonplanarity of the outermost film surface both on an atomic and a more macroscopic scale, and from porosity of the film due to gaps between the crystals. These gags are typically up to about 20 A wide. However, for film thicknesses >500 A, this gap structure is never such as completely to isolate metal crystals one from the other, and almost all of the substrate is, in fact, covered by metal. In practice, catalytic work mostly uses thick films in the thickness range 500-2000 A, and it is easily shown (7) that intercrystal gaps in these films will not influence catalytic reaction kinetics provided the half-life of the reaction exceeds about 10-20 sec, which will usually be the case. 

In addition to the efflux method, several experiments have been performed by means of an inverse GPC method to assess the capsule permeability. Such measurements are considered to be more accurate and representative for high molecular weight species since they are not sensitive to protein adsorption on the capsule surface. This method, however, cannot be used for a massive screening of capsule permeability due to its time requirements. 

Aside from N2 adsorption, Kr or Ar adsorption can be used at low temperatures to determine low (<1 m2/g) surface areas [46], Chemically sensitive probes such as H2, Oz, or CO can also be employed to selectively measure surface areas of specific components of the catalyst (see below). Finally, mercury-based porosimeters, where the volume of the mercury incorporated into the pores is measured as a function of increasing (well above atmospheric) pressures, are sometimes used to determine the size of meso- and macropores [1]. By and large, the limitations of all of the above methods are that they only provide information on average pore volumes, and that they usually lack chemical sensitivity. 

The discussed problems result in a gradual substitution of traditional BET method for the surface area measurements with CM method. The modem adsorption instruments such as AS AP-2020... 

Measuring the specific surface area, A, related to the mass of PS does not require a textural model (a morpho-independent parameter, i.e., one can apply an approach of partitioning and, correspondingly, the second statement of texturology, as we have already done for volume-related parameters). Let us consider the most widespread adsorption method based on proportionality of adsorption, Q. and the specific surface area in the absence of volumetric effects (capillaiy condensation, micropore filling, etc.) ... 

Table 5.1 Effect of swelling on the surface area of cotton as measured by chemical (thallation) and physical (N2 adsorption) methods. (Source G.A. Roberts, Accessibility of Cellulose , in Paper Chemistry , ed. J.C. Roberts, ch. 2, Blackie, Glasgow, 1991).

Although it is sometimes possible to view pores with an electron microscope and to obtain a measure of their diameter, it is difficult by this means to measure the distribution of sizes and impossible to measure the associated surface area. Adsorptive methods are used instead, employing some of the theories of adsorption explained previously. 

Since the work function is very sensitive to contaminants, the most reliable measurements are done in ultrahigh vacuum conditions. From the determination of the electron work functions of Fe, Co, Ni, Cu, Au, and other metals in the presence of water adsorbed from the gas phase, it follows that water molecules are oriented with oxygen atoms toward the metal surface. The method is very sensitive to the presence of water. For example, upon adsorption of 3 x lO molecules of water per square centimeter of Co film (4% of a monolayer), the work function value is decreased by ca. 0.3 eV. However, these measurements were done at 77 K, meaning that adsorbed water was likely to be in a crystalline or amorphous ice form. Hence, the quoted results are of limited value to understanding the metal-water system in electrolyte solutions. 

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... 

Other techniques for the diffusion of gas through monolayers at the liquid interface have also been investigated (Blank, 1970). In these methods, the differential manometer system was used to measure the adsorption of gases such as C02 and 02 into aqueous solutions with and without the presence of monolayers. The Geiger-Mueller counter with a suitable sorbent and a radioactive tracer gas was used to measure the reduction of evolution of H2S and C02 from the surface solution when a monolayer was present. 

The surface area of the catalysts was measured by the conventional B.E.T. nitrogen adsorption method. 

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