Gravimetric helium adsorption

Most adsorption data were collected by volumetric method until microbalance of high sensitivity appeared few years ago. It can hardly say which method is superior to the other, and both methods need the value of the skeleton volume of sample adsorbent. This volume has to be subtracted from the whole volume of the sample container to obtain the volume of void space, which is used for the calculation of the amount adsorbed. The skeleton volume of sample adsorbent was directly used in the calculation of buoyancy correction in gravimetric method. This volume was usually determined by helium assuming the amount of helium adsorbed was negligible. If, however, helium adsorption cannot be omitted, error would yield in the skeleton volume and, finally, in the calculated amount adsorbed. However, the effect of helium adsorption would be much less for volumetric method if the skeleton volume is considerably less than the volume of void space, but the volume of void space cannot affect buoyancy correction. In this respect, helium adsorption would result in less consequence on volumetric method especially when the skeleton volume was determined at room temperature and pressures less than IS MPa. The skeleton volume (or density) was taken for a parameter in modeling process in some gravimetric measurements. However, the true value of skeleton volume (or density) can hardly be more reliable basing on a fitted parameter than on a measured value. Therefore, one method of measurement cannot expel the other up to now, and the consequence of helium adsorption in the measured amount adsorbed should be estimated appropriately. 

Gravimetric and Volumetric Measurements of Helium Adsorption Equilibria on Different Porous SoMds,... 

Microbalances with alphanumerical display and electronic data recording systems allow one to observe the approach to equilibrium for gas adsorption processes in porous sorbent samples. Typical relaxation times can be one or several seconds, minutes, hours, and -sometimes - even days, cp. helium adsorption data Sect. 2 of Chap. 1. Hence gravimetric measurements do allow one to check whether an adsorption system actually has reached its equilibrium state, i. e. these measurements deliver in principle also information concerning the kinetics of the adsorption process, represented for example by (phenomenological) diffusion coefficients, cp. Sect. 2.3 and Sect. 4.4 and [3.27, 3.48). 

Gravimetric and Volumetric Measurements of Helium Adsorption Equilibria on Different Porous Sohds, p. 261-266, Proceedings of IV Conference on Porous Sohds, COPS rv, Bath UK, 1996, B. Me. Enany et al., Eds., The Royal Society of Chemistry, Special Puhl. No. 213, London, 1997. 

To what extent helium is adsorbed has been of major concern in adsorption studies for both volumetric and gravimetric methods. Until recently, the experimental error was often attributed to the finite adsorption of helium at high pressures, and different remedial methods were suggested [38-40]. The effect of helium adsorption on the gravimetric technique is clearly shown in Eq. (8). The volume difference, AF, will be overestimated if the adsorption of heUum is not negligible. Its effect on the volumetric technique ean be explained in terms of Fig. 1. The volume of the solid phase of adsorbent, F, is experimentally determined by heUum. This volume is sometimes called dead space or hehum volume of the adsorption cell, which is, indeed, the volume of adsorbent inaccessible to the hehum molecules. However, this value is usually taken for the volume of adsorbent inaccessible to the adsorbate molecules. The difference in molecular dynamic size and shape between helium and adsorbate is logically a source of error. The irregular solid surface and/or the complex strueture of micropores inevitably render uncertainty in the determination of P. As a eonsequenee of helium adsorption, the dead volume is underestimated. 

Adsorption equilibria and uptakes were gravimetrically measured with two balances, i.e., Cahn 1100 microbalance and Magnetic Suspension Balance (Rubotherm Co.). Adsorbents were loaded and regenerated in the balance under the flow of ultra high purity Helium gas at ISO 3S0 C, then adjusted to the measuring temperature and evacuated to 10 mmHg by using a turbo molecular pump. Adsorbates used were as follows propane (min.99.5), isobutane (min.99.5%), n-butane (min.99.S%), 1-butene (min.99.5%), isobutene (min.99.5%) and t-2-butene (min.99.8%). 

Once V is known, m = m (p, T, m ) can be calculated from Eq. (1.5). Such a situation is very common for gravimetric measurements of helium gas adsorption equilibria. An example is sketched in Figure 1.8 showing gravimetric adsorption data of activated carbon Norit R1 Extra exerted to He (5.0). As can be seen, for high pressures the reduced mass data ( ) easily can be linearly correlated, i. e. adsorption of helium has reached a state of saturation. Hence, Vhc can be determined via Eq. (1.6) and also the mass of helium adsorbed initially at low gas pressures can be calculated from Eq. (1.5) as ... 

Figure 1.10. Adsorption process of helium (He(5.0)) on AC Norit R1 Extra at T = 298.17 K during 58 hours measured gravimetrically (magnetic suspension balance, Rubotherm). The interruption of measurement data at about 24 h is due to limitations in data storage capacity, i. e. an overflow of data, which made a change of the data storage device necessary.

If preliminary saturation is reached, increase of the helium gas pressure does not lead to gravimetrically measurable increase of adsorption for several hours. 

To measure binary coadsorption equilibria by tbe volumetric-gravimetric method one proceeds as follows A sorbent sample of 1 g - 3 g and appropriate counterweights, typically lead or silver balls, are placed to the buckets of the microbalance. Then the sorbent is activated by exposing it to helium gas at higher temperatures, i. e. 433 K for activated carbons, 673 K for zeolites and inorganic molecular sieves. After cooling down and evacuation (< 10 Pa) the adsorption chamber is prepared for an adsorption experiment. 

For rigid sorbent materials, combined oscillometric-gravimetric or oscillometric-manometric measurements in a saturation state of an adsorption system also allow one to determine both the total mass (m m ) and the Volume (V ) of the system without using the so-called helium volume hypothesis, cp. Fig. 5.8 and Chap. 1. 

Methanol chemisorption experiments have also been carried out at atmospheric pressure in a specially adapted thermal gravimetric analyzer (TGA) microbalance coupled with a PC for temperature and weight monitoring [38]. The system allowed a controlled flow of high purity gases air for pretreatment, helium and a mixture of 2000 ppm methanol in helium for adsorption experiments. 

Dynamic vapor sorption kinetics and isotherms. Different vapor concentrations were generated with gravimetrically calibrated vapor diffusion tubes at 25 C under constant flow of carrier gas (15 ml/min helium). Vapor concentrations were adjusted by dilution with a second computer controlled helium flow (0-200 ml/min). Sorption measurements were carried out similar to those described above for nitrogen. Equilibrium was usually assumed and the next partial pressure was adjusted when the frequency change of the QCM was less than 1 Hz in 90 seconds. The close coincidence of adsorption and desorption branches of many isotherms shows that the measurements are ofen close to true equilibrium. 

---