Monolayer analysis specific surface

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

Because a monolayer-dispersed oxide or salt on a support with a highly specific surface usually is present in considerable quantity, the sensitivity of extended X-ray absorption fine structure (EXAFS) analysis is good... 

Analysis of Heterogeneity. The monolayer analysis consists of three elements an adsorption isotherm equation, a model for heterogeneous surfaces, and an algorithm such as CAEDMON, which uses the first two elements to extract the adsorptive energy distribution and the specific surface from isotherm data. Morrison and Ross developed a virial isotherm equation for a mobile film of adsorbed gas at submonolayer coverage (6) ... 

The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuKa radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of p/Ta 10.6 and 9.9, and L ,ax 226 and 271.6 nm, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa= 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford). 

Table 5 contains also the corresponding values of the relative pressure p/pa.x(A), which were used to find the monolayer capacities for the samples studied. Thus, the minimum or inflection point on APDs for nitrogm/active carbon systems at 77 K around 4 kJ mof gives A X(a) and consequently, p/p ojc(A), which marks on the corresponding adsorption isotherm the value of the monolayer capacity. The latter can be easily converted to the total specific surface area S rfA) if the area occupied by single adsorbate molecule is known. A comparison of the total specific surface area (see St,x(A) in Table 4) and those evaluated by the BET method (see Smst in Table 1) and by the a>-plot method (see St in Table 4) shows that the APD method gives much smaller values. The values of seem to be more realistic than those obtained by the BET method and Us-plot analysis, which are based on the BET model, because in contrast to the BET model the APD method allows for a more accurate estimation of the amount adsorbed in the monolayer. Tire methods based on the BET model do not take into account the correction for molecules adsorbed inside micropores and therefore, they overestimate the total specific surface area of microporous solids. It should be mentioned that the evaluation of the total specific surface area on the basis of the Kj-plot is usually done by... 

For the NP5 microporous active carbon there is anotlier peak located between 0.5 and 3 kJ mof, which represents tlie completion of the micropore filling process. A distinct minimum before this peak marks the value of the adsorption potential at the point at which the micropores are filled. The value of this minimum, a x(A), is given in Table 5 for the NP5 active carbon. This table gives also p/p o.x(Aj corresponding to A x(ap which marks on the adsorption isotlierm the value for the micropore volume VmxxA)- For instance, the APD method gave the micropore volume of 0.84 cm g for the NP5 carbon, which is close to that obtained by the tts-plot analysis (0.81 cm g ). This comparison shows that the APD-based analysis provides valuable information about surface and structure of active carbons. The minimum and inflection points that appear on the APD curves provide the values of the adsorption potentials, which correspond to the completion of the monolayer formation as well as the completion of the micropore filling. Thus, these values can be used to evaluate the total specific surface area and the micropore volume. 

The specific surface area of a powder is conveniently determined through a Brunauer-Emmett-Teller (BET) analysis. Here, the adsorption of nitrogen is determined and then analysed. Since the cross-sectional surface area of a nitrogen molecule is known, the total area of a powder sample can therefore be determined. BET analysis is conveniently carried out by using commercial instruments build solely for this purpose. In some cases, the solid sample cannot be treated (dried) to give a dry powder. In such cases, the specific surface area can be determined by the adsorption of a surfactant, with a known cross-sectional surface area, that adsorbs as a monolayer at that specific surface. This method is, of course, rather uncertain since it requires the assumption of monolayer adsorption. 

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