Automatic and low-cost determination of BET surface areas

Abstract

where Pr is the relative pressure of the adsorbate, equal to absolute pressurejsaturation pressure at the temperature of the experiment, Va is the amount adsorbed (cm3 STP g-l), and C is the constant whose value depends on the heat of adsorption.Plotting the left-hand side of equation (2) as a function of P, should result in a straight line. In this case Vm can be calculated from the slope and the intercept. Vm is related to the specific surface area SBET through equation (1). Sometimes SBET is calculated from only one point of the adsorption isotherm Morrison 1952, 1953). From equation (2) it can be deduced that this method is valid only if C% 1 + l/'f'r, but a check on the assumption and applicability of the BET method is not possible.A discussion of the basic assumptions in the BET model is beyond the scope of this paper (see for instance Sing 1964). Two cases which lead to wrong results by using a one-point method should, however, be noted. In the case that an appreciable amount of micropores is present, capillary condensation will occur. The amount adsorbed will be larger than predicted by the BET equation which concerns multilayer adsorption only. Calculation of SBET from only one point of the adsorption isotherm results in a value which is too high. A plot according to equation (2) will show clearly the presence of micropores by a deviation from a straight line which is convex with respect to the abscissa. On the other hand, the adsorption might be limited by the pore diameter to only a few adsorbed layers, which results in a concave deviation indicating that equation (2) is not valid in that particular case. These possibilities might occur, notwithstanding that the condition C&l/Pr is satisfied (compare table 2).Thus for routine determinations one needs a cheap, automatic adsorption device, estimating about five points of the adsorption isotherm in a relatively short time, to check whether or not the BET equation is applicable. In this paper such a low-budget device is described. In fact it is a simplification of the apparatus developed by Schlosser (1959); its conception was already outlined by Scholten (1965). The construction and its application will be discussed. Abstract A simple adsorption apparatus for a quick and reproducible determination of Brunauer-Emmett-Teller (BET) surface areas is described. The justification of proposing this device lies particularly in its flexibility, simplicity of construction and low price (Dfl. 1200, $400 including liquid nitrogen level control). The dimensions and the influence of external variables are discussed quantitatively. An adsorption measurement proceeds as follows: the adsorbate (argon) passes through a capillary tube into the adsorption vessel, the flow being constant. The increase of pressure with time is recorded and compared with the blank (empty adsorption vessel). The blank does not depend on the adsorbent. It takes about one hour to determine a surface area of 100 m2. The reproducibility and accuracy are 3 % or better for surface areas larger than ...

Equilibrium adsorption data from temperature-programmed desorption experiments

“…Under certain experimental conditions equilibrium adsorption data can be obtained faster from dynamic experiments, thus reducing the number of experiments. Bosch and Peppelenbos (1977) showed that BET surface areas calculated from a continuous pressure increase at a constant adsorbate feed rate agree excellent with results from equilibrium methods. Foeth et al (1994) calculated equilibrium adsorption data from breakthrough curves, taking into account the change in both velocity and pressure during breakthrough if necessary (Foeth et al, 1996).…”

Preparation of Polycrystalline TiO2 Photocatalysts Impregnated with Various Transition Metal Ions: Characterization and Photocatalytic Activity for the Degradation of 4-Nitrophenol

“…Specific Surface Area Measurements. The specific surface areas were determined by the single-point BET method using a Flow Sorb 2300 apparatus (Micromeritics).…”

Preparation Characterization and Photocatalytic Activity of Polycrystalline ZnO/TiO2 Systems. 1. Surface and Bulk Characterization

“…Specific Surface Areas (BET) and Porosity Measurements. The specific surface areas were measured by the single-point BET method 28 using a Flow Sorb 2300 apparatus (Micromeritics International Corp.). Porosity was monitored from the adsorption-desorption isotherms of nitrogen recorded at 77 K by using a conventional, high-vacuum system (residual pressure ca.…”