Volumetric adsorption

Usually, physisorption is carried out using nitrogen as an adsorbate at 77.3 K, the boiling point of liquid nitrogen. The solid material is called the adsorbent. Adsorption can be measured in many different ways. So-called volumetric adsorption, in which volumes of gas are introduced. sequentially while simultaneously measuring the pressure, is a commonly used technique. Fig. 3.41 shows a schematic of the equipment used (referred to as barometric equipment ). 

Krypton Sorption. Volumetric adsorption using gases with low saturated vapor pressure has been found to be an effective technique to gain detailed structural information for small quantities of porous materials, especially using krypton (Kr).27 The substitution of nitrogen by Kr reduces significantly the amount of unadsorbed molecules in the dead volume, allows for the characterization of small surface areas, and is thus ideal for mesoporous... 

Nitrogen adsorption and desorption isotherms were performed at 77 K on a Micromeritics ASAP 2400 volumetric adsorption system. The pore size distribution and surface area were deduced from the adsorption isotherms using the BJH method and the BET equation. 

Nitrogen adsorption - desorption isotherms were obtained from a volumetric adsorption analyzer ASAP 2010 manufactured by Micromeritics. The samples were first degassed for several hours at 350°C. The measurements were then carried out at -196°C over a wide relative pressure range from 0.01 to 0.995. The average pore diameter and the pore size distribution were determined by the B JH method from the adsorption branch of isotherm [18],... 

The specific surface area of the fibers was determined using inert gas adsorption in a commercial volumetric adsorption system (Micromeritics Instrument Corp.). Krypton gas was used because of its sensitivity to the small specific surface areas of the glass fibers ( 0.2 mz/g). The fibers were degassed at 100°C to a pressure of 80mTorr before introducing the adsorbate gas into the sample chamber. Several samples were also outgassed at 80 and 200°C (to 80 mTorr) to confirm that outgassing was sufficiently complete under the standard test conditions. A standard five-point surface area determination was made for each inert gas adsorption experiment. 

It is to be noted that the adsorption isotherm on the freely vibrating group is of the simple Langmuir type and that there is excellent agreement between the volumetric and spectroscopic data showing that the volumetric adsorption isotherm which is commonly measured is indeed the summation of the adsorption on the hydroxyl group plus the adsorption on the remainder of the surface (19). 

For laboratory-scale modification, distinction has to be made between static and dynamic adsorption procedures. In a static procedure, the substrate is contacted with a known volume of gas at a well-defined pressure. The modifying gas may be stationary or circulating in a closed loop. Modification in a static gas adsorption apparatus allows the careful control of all reaction parameters. Temperature and pressure can be controlled and easily measured. Adsorption kinetics may be determined by following the pressure as a function of the reaction time. Figure 8.13 displays a volumetric adsorption apparatus, in which mercury is used, as a means to change the internal volume and for pressure measurement. 

FIGURE 4.11 Cumulative pore volume corresponding to the three samples presented in Figure 4.4. These cumulative pore volumes have been obtained by applying the DFT methods to each N2 adsorption isotherms using the software provided by two different commercial volumetric adsorption analyzers. 

The volumetric adsorption experiment [6] shown in Figure 6.2 consists of a thermostated sample cell of volume Vg at an experimental temperature, T, a container with a precisely determined volume, named the calibrated volume, Vc, a connection to the gas reservoir, and a transducer for pressure measurement. In addition, the volume at stopcock 3, at ambient temperature, Tv and the thermostat is denoted V2, and the volume between stopcocks 1, 2, and 3, also at ambient temperature, Tr, is denoted Vt. 

FIGURE 6.2 Schematic representation of a volumetric adsorption experiment. 

A typical commercial volumetric adsorption apparatus is shown in Figure 6.8 [20], The volumetric sorption equipment is equipped with pressure transducers in the dosing volume compartment of the apparatus and high-precision pressure transducers dedicated to measure the pressure in the sample cell [21,22], Hence, the sample cell is isolated throughout equilibration, which assures a very small void volume, and as a result a highly accurate determination of the adsorbed amount [21],... 

FIGURE 6.8 Representation of a standard volumetric adsorption apparatus. (Taken from Thommes, M., in Nanoporous Materials Science and Engineering, Lu, G.Q. and Zhao, X.S. (eds.), Imperial College Press, London, UK, 2004, 317. With permission.)... 

Accessible metal fractions were determined by hydrogen chemisorption. The volumetric adsorption experiments were performed at room temperature in a conventional vacuum apparatus. Hydrogen uptake was determined using the double isotherm method, as previously reported (3). The platinum dispersion (D ) was calculated by assuming a... 

A classical volumetric adsorption apparatus equipped with absolute capacitance pressures transducers can be used for the estimation of adsorption isotherms in the pressure range 10-3 mbaradsorption measurements the carbon black samples are extracted with toluene and water/methanol (1 1) and after drying degassed overnight at 200 °C at a pressure below 10 4 mbar. The time allowed for equilibrium of each point of the isotherm is 5-90 min depending on the sample and the adsorbed amount. 

The adsorption isotherms for water with five of the samples were obtained at 25° C. by using a volumetric adsorption apparatus described previously (13). Prior to adsorption measurements these samples were also outgassed at 160° and 10-6 mm. Hg. Each adsorption isotherm consisted of 20 to 40 experimental points covering the relative pressure range 0.01 to 0.95. 

Praliaud and Martin (77) proposed the formation of Ni-Si and Ni-Cr alloys on silica and chromia supports, respectively, under H2 at sufficiently high temperatures. They suggested that hydrogen spilt over from Ni to the Cr203 carrier and partially reduced it to Cr°, which was then alloyed with Ni as indicated by magnetic measurements. The same technique in conjunction with IR spectroscopy and volumetric adsorption of H2 was applied to partially reduced Ni-on-alumina and Ni-on-zeolite catalysts by Dalmon et al. (78). These supported Ni systems contained Ni° and Ni+. H2 was found to be activated only when the couple Ni°/Ni+ was present according to... 

In Table 1 manufacturers of carrier gas and volumetric adsorption measuring instruments are compiled. Most of these companies offer also mercury porosimeter and gas pycnometer. Mercury porosimetry extends the measuring range of the sorption method towards larger pores. Table 2 gives a survey on the commercial offer of gravimetric apparatus. 

As determined from volumetric adsorption data. Maximum pressure 12 Torr. 

As determined from volumetric adsorption data recorded al 308 1C As determined fnmi volumetric adsorption data recorded at 233 K. 

BET measurements were performed in a conventional volumetric adsorption apparatus at the temperature of liquid N2 (77.4 K). Prior to measurements the samples were pretreated in vacuum at 573 K for 1 hour (Table 1, fourth column). 

The adsorption calorimetric measurements were carried out at 423 K on a SETARAM microcalorimeter of calvet-type connected with a standard volumetric adsorption apparatus. The pressure measurements were made using a MKS Baratron membrane manometer. Prior to the ammonia adsorption, the samples (900 mg) were carefully calcined in high vacuum at 673 K for 15 h. 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

Nitrogen adsorption-desorption isotherms were measured at 77 3 K using a commercial volumetric adsorption system Autosorb-1 or NOVA-1200 (both from Quantachrome Corp ). The samples were outgassed at 10" Torr and 573 K for 3 h prior to the analysis X-ray powder diffraction (XRD) patterns were obtained on PW 1840 diffractometer (Phillips), with Co Ka radiation, 40 KV, 25 mA. 

G.C. Frye, A.]. Ricco, S.J Martin and C.J. Brinker, Characterization of the surface area and porosity of sol-gel films using SAW devices. Mater. Res. Soc Symp. Proc., 121 (1988) 349. S.L. Hietala, D.M. Smith, V.M. Hietala, G.C. Frye and S.J. Martin, Pore structure characterization of thin films using a surface acoustic wave/volumetric adsorption technique. Langmuir, 9 (1993) 249. 

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