Adsorption gravimetry

Most experiments described in Section 3.2.1 (with the exception of adsorption gravimetry) rely on the use of a calibrated volume. The basic calibration can be carried out in two ways directly or indirectly. Direct calibration implies that this part of the equipment can be isolated, removed and weighed (either evacuated or filled with dry air), filled with an outgassed liquid of known density and then weighed again. 

The buoyancy correction needed in adsorption gravimetry has the same origin as the dead space correction in adsorption manometry it is due to the volume of the sample... 

A pertinent question is as the volume of the adsorbed phase increases, do we have to take into account the corresponding increase of buoyancy (e.g. the buoyancy doubles after saturation of an adsorbent with 50% porosity). The answer is no, provided we want to assess the surface excess mass m°. As illustrated in Figure 3.22, because of the buoyancy effect, we do not measure the total mass of the adsorbed layer (shaded+hatched areas) but simply a surface excess mass (hatched area only). Thus, adsorption gravimetry and the Gibbs representation are highly compatible (Findenegg, 1997). 

Figure 3.23. Gas adsorption gravimetry percentage enor in surface area as a result of neglecting totally the buoyancy effect on the adsorbent (assumptions nitrogen BET, monolayer at 100 mbar, bulk density of adsorbent from 1 to 8).

Figure 3.24. Gas adsorption gravimetry temperature difference between sample and cryostat (at 77 K) as the vacuum around the sample is progressively reduced (after Rouquerol and Davy, 1978).

Now, these measurements are somewhat difficult to carry out, not only because of the question of the pressure but also because of the problems of accuracy, due to the corrections -and errors - due to the void volume (in adsorption manometry) [3-5] or to the buoyancy (in adsorption gravimetry) [6], For this reason, it may be found advisable to cross-check the experimental results by making use of a variety of different techniques. 

The aim of this paper is therefore to check the consistency of three basic techniques (adsorption manometry, adsorption gravimetry and adsorption microcalorimetry) in the 0-50 bar pressure range, using a standard NaX zeolite adsorbent and selecting three adsorbable gases, namely ... 

Adsorption gravimetry lends itself to in-situ outgassing (up to 500°C with our equipment) but can only accommodate one sample at a time. A major advantage only obtained with the special assembly making use of a sinker is the permanent measurement of the density of the gas phase it allows not only to make at any time a correct buoyancy correction but it also allows to provide the users of adsorption manometry equipment with the data they need to make a safe void volume correction. 

Chevrot et al. [164] presented a new experimental technique to study low temperature constant rate thermodesorption of different adsorbates. In the cited paper they analyze water thermodesorption from mesoporous silica MCM-41 and the results are compared with data obtained with adsorption gravimetry. According to the authors this technique has better sensitivity and resolution than the traditional temperature programmed desorption technique. Other advantages of this technique are that a small quantity of sample is needed, solids with lower smface area can be studied, and the lower temperature at which the equipment can operate (163 K). 

Evolution of the external surface area and the two types of microporosity of atiapulgite (structural and inter-fiber) were examined as a function of a vacuum thermal treatment upt to 500°C. The methods used include controlled transformation rate thermal analysis, N2 and Ar low temperature adsorption calorimetry, water vapor adsorption gravimetry and quasi equilibrium gas adsorption procedure of N2 at 77K and CO2 at 273 and 293K. Depending on the outgassing conditions,i.e. the residual pressure, the structure folds 150 to 70 C. For lower temperature, only a part (18%) of the structural microp< osity is available to N2,13% to argon and 100% to CC>2.With water, the structure can rehydrate after the structure is folded up to an outgassing temperature of 225°C. 

Adsorption gravimetry of water vapor was carried out with the experimental apparatus described in ref. 10. Prior to each experiment 100 mg samples were outgassed with a residual pressure of 0.1 Pa during 18 h and a temperature of 25, 70,100, 130, 225, 300, 380 and 500°C. 

In situ infrared (IR) spectroscopy and calorimetry were used by the Lercher group during alkane adsorption measurements by gravimetry for various zeoHtes [5, 14]. IR spectra of the OH-streching vibrations of Bronsted acid sites in Ferrierite during adsorption of n-butane at 333°K are illustrated in Figure 13.7. 

X-ray diffraction powder patterns were recorded on a CGR Theta 60 instrument, using monochromated CuKa radiation. The adsorption capacities for several adsorbates were measured at room temperature by gravimetry, using a Cahn RH microbalance as proposed by Vaughan and Lussier (3 ). The samples were first treated in air for 5 hours at 480°C. The experiment was performed by passing, over the sample, a stream of nitrogen saturated by the vapor pressure of the sorbate at room temperature, the relative pressure P/Po was then equal to 1. 

Strictly speaking gravimetry belongs to the relative methods. The only difference is in the position of the RM in the measurement process the weighed product itself serves as an RM - usually with the a priori assumption, that its purity is 100% and stoichiometry is correct. The mass fraction of the weighed substance should be taken into account in the equation used. Problems associated with the formation of solid phase [4], e.g. surface adsorption effects (ionic species and water [5]) are significant in analyses aiming at relative uncertainties at about 10 4. 

Physicochemical methods, i.e. adsorption of probe molecules followed by varied analytical techniques (gravimetry, chromatography, calorimetry, spectroscopic techniques, etc.) are currently used for estimating more precisely the concentration of the potential active sites.[34 36] However, very few methods are well adapted for this purpose most of the methods employed for the characterization of the acidity of solid catalysts lead to values of the total concentrations of the acid sites (Brpnsted + Lewis) and to relative data on their strength, whereas few of them discriminate between Lewis and Brpnsted acid sites. It is however the case for base adsorption (often pyridine) followed by IR spectroscopy, from which the concentrations of Brpnsted and Lewis sites can be estimated from the absorbance of IR bands specific for adsorbed molecules on Brpnsted or Lewis sites. 

Gravimetry can be associated with adsorption calorimetry, either by using two samples (one in the microcalorimeter, the other in the microbalance) in contact with the same atmosphere of adsorptive (Gravelle, 1972) or using a single sample, located in the cylindrical pan of a microbalance and surrounded by a Tian-Calvet thermopile (LeParlouer, 1985). 

Gravimetry can also be associated with adsorption manometry, which is a simple and safe way to study co-adsorption of two gases, provided their molar masses are sufficiently different (Keller et al, 1992). The manometric experiment provides a total amount adsorbed n°a = n° + n%, whereas the gravimetric experiment provides the total mass adsorbed m t=m° + m. Since n°Mx = m and n M1 = m" we have two unknowns, n° and n, and two equations. From these we can obtain, for instance ... 

Figure 3 Circles GCMC adsorption isotherm (right) and isosteric heat of adsorption (left) obtained in the numerical mesoporous pseudo-vycor shown in Fig. 1. Experimental curves solid line T. Takei etal. [25] (gravimetry in vycor pore size 4.72 nm). dashed line N. Maikova et al. [26] (micro-calorimetry in vycor, pore size 4 to 7 nm). The vertical arrow on the left corresponds to bulk water (44 kJ/mol).

The adsorption isotherm calculated is of type IV in lUPAC classification, showing a rapid increase at low pressure as expected for hydrophilic surfaces. The steep rise in adsorption arround P/P°=0.7 is due to capillary condensation in the mesoporous solid. The result is comparable to two available experimental adsorption isotherm of water measured by very different techniques (gravimetry and calorimetry). This result, and the good agreement of the simulated isosteric heat of adsorption at very low coverage (75 kJ/mol) with experimental data, show that the model presented is able to describe quantitatively the hydrophilicity of the vycor surface with no adjustable parameters. 

Complementarity of microcalorimetry, manometry and gravimetry in the study of gas adsorption by microporous solids up to 50 bar... 

This work compares and contrasts the experimental results using three different experimental techniques manometry, gravimetry and microcalorimetry. The system studies was the adsorption of argon, nitrogen and carhon dioxide on a NaLSX zeolite between 20 and 60°C and up to 50 bars. 

We report in Figures 1 and 2, with the same presentation and scale, the experimental results obtained by adsorption manometry and gravimetry, for the three gases and the three temperatures. Incidentally, this is a good example where the formerly used representation in adsorbed volume or adsorbed mass is clearly less convenient than the more universal representation in adsorbed amount (or still more precisely in the present case, in surface excess amount ). The shapes of the adsorption isotherms, with very different curvatures from one gas to the other, are a clear indication of the increased gas/solid interaction as one passes from argon to nitrogen and then to carbon dioxide, for which the isotherms are practically of type I. 

Fig 1 Adsorption isotherms on NaLSX Fig 2 Adsorption isotherms on NaLSX zeolite determined by manometry zeolite determined by gravimetry... 

Adsorption and diffusion of alkanes in zeolites and in well-structured porous materials like MCM-41 materials are studied widely [1,5,8,9,11], The reported difusivities however differ sometimes by orders of magnitude. These differences are sometimes attributed to the use of microscopic techniques in stead of macroscopic techniques [12]. We, however, think that a major part of the found differences must be imputed to the use of a carrier gas. Adsorption is often studied in diluted systems with methods as ZLC [3], gaschromatography [4], inverse gas chromatography [10], gravimetry [12], > ile others are not using carriers gasses at all. 

Water adsorption on MCM-41 was studied by NMR [94,106-108] and by gravimetry and FTIR [109]. Schmidt et al. [106,108] measured the freezing point of adsorbed water by... 

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