Static gas adsorption

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. 

Surface Energy Distribution from Static Gas Adsorption. 547... 

SURFACE ENERGY DISTRIBUTION FROM STATIC GAS ADSORPTION... 

Molecular diameter of helium is less than 0.2 nm, and it is almost not adsorbed by any sample. Hence, it is an ideal gas to measure the total volmne including the volume of pores in particles and stacking gaps between particles. The testing equipments are conventional static gas adsorption devices. It detects the pressure difference of helium gas during the test, and then to Vsk of the sample is calculated by gas law, getting the value of skeletal density as shown in Eq. (7.42). 

The measurements of external and internal specific surface area have already been discussed in Chapter 1, Section 1.1.3. The principles and the isotherm equation of the BET method to measure external specific surface area, including macro- and mesopores, have been presented in Chapter 1, Section 1.3.4.1.5. The external specific surface area is usually determined by nitrogen gas adsorption at the temperature of liquid nitrogen. Both static (one-point) and dynamic (five-point) methods are applied. The calculations are made by Equation 1.75 (Chapter 1), using one or five different pressure values. The external specific surface area is calculated from the maximum number of surface sites, that is, monolayer and the cross-sectional area of nitrogen molecules. 

The adsorption apparatus, shown in Figure 1, uses a static volumetric method to measure pure gas adsorption, and an open-flow adsorption/desorption method for mixture... 

In terms of catalysis, important equilibrium processes include low-temperature gas adsorption (capillary condensation) and nonwetting fluid invasion, both of which are routinely used to characterize pore size distribution. Static diffusion in a Wicke-Kallenbach cell characterizes effective diffusivity. The simultaneous rate processes of diffusion and reaction determine catalyst effectiveness, which is the single most significant measure of practical catalytic reactor performance. 

Of the established static techniques, which we have considered here, that involving gas adsorption isotherm measurements remains one of the most powerful and widely applicable. It is indeed very accessible with the availability of automated commercial equipment and the variety of data treatment facilities available. Nevertheless, it is still circumscribed by the assumptions implicit in the choice of a pore shape model in the case of mesoporous materials. Its application to microporous structures has recently advanced considerably, although there are here certain reservations which still exist concerning the general application of theories to describe adsorption in such small pores in ill defined structures. 

In another static method of measuring gas adsorption, the amount of gas adsorbed is measured by the increase in the weight of the adsorbent. For this, a very useful balance was developed by McBain and Bakr (Figure 13 2), the essential part of which is a helical spring... 

In static studies of gas adsorption, the carbon usually is degassed or outgassed before the adsorption.1,2 To do this, the carbon is heated at a temperature within the range 300° to 900° C, and then cooled the entire process is conducted under high vacuum. McBain6 found that degassing alters the isotherm to a type in which nearly all the adsorption occurs at very low pressures. Degassing also increases the rate of adsorption, and equilibrium conditions are attained without the delay often encountered otherwise. 

The surface properties of supported Pt particles were examined by gas adsorption and X-ray photoelectron spectroscopy (XPS) with Shimadzu ESCA-750. The uptake of Hj and CO was measur by a static volumetric method at 292 K and an equilibrium pressure of about 25 kPa. In XPS, the catalysts were exposed to air on the introduction into the system, and the spectra for Pt and other species were collected after Ar sputtering. Tlie bulk structure of the particles was examined by extended X-ray absorption fine structure (EXAFS). Pt L -edge EXAFS spectra were measured at room temperature at the BL-lOB station of Photon Factory in the National Laboratory for High Energy Physics (Proposal No.93G147). In addition, the supported Pt was extracted by immersing the catalysts in a mixed acid of HCl and HNO3 at room temperature for 24 h and the amounts of Pt extracted were measured by AAS. 

Table 1 Parameters obtained from gas adsorption-desorption analysis after low- and high-current-density anodization and after static thermal oxidation in air porosity calculated gravimetrically and from total pore volume author s data) ...

Figure 5. Typical diagram of a static volumetric apparatus for gas adsorption.

Figure 2.1. Experimental setup for (static) volumetric measurements of pure gas adsorption equilibria.

Figure 2.6. Experimental set-up for (static) volumetric / manometric measurements of multicomponent gas adsorption equilibria.

Abstract The physical principles and basic experimental techniques of impedance spectroscopy, i. e. static or frequency dependent dielectric permittivity measurements of sorbent/sorbate systems are given. These measurements can be used to characterize the state of a sorbent material in industrial adsorption processes. Combined with either manometric or gravimetric measurements of adsorption equilibria leading to calibration curves, permittivity measurements also allow fairly simple and quick measurements of gas adsorption equilibria. Kinetic processes and catalytic reactions inside a sorbent/sorbate system also can be observed. Pros and cons of dielectric measurements are discussed. List of Symbols. References. 

Figure 6.1. Installation for measurements of the static or frequency dependent dielectric permittivity of gas adsorption systems. IFT University of Siegen, 1988.

A Micromeritics ASAP 2010 gas adsorption analyser (stainless steel version) was used to measure the adsorption isotherms of ethane, ethene, propane, propene, -butane, and isobutane on Kureha carbon in the pressure range from 0.002 to 120 kPa. The instrument was equipped with a turbomolecular vacuum pump and three different pressure transducers (0.13, 1.33, and 133 kPa, respectively) to enhance the sensitivity in different pressure ranges. The static-volumetric technique was used to determine the volume of the gas adsorbed at different... 

Two practical methods are often used for gas adsorption studies, the dynamic flow and static-volumetric method. 

Both the static and continuous flow (quasi equilibrated) techniques may be described as vacuum volumetric methods, whereas DYNAMIC technique (known by some workers as continuous flow) does not use vacuum technology,but instead employs a non-adsorbing carrier gas/adsorptive mixture. 

Single and multiple point instruments are available that operate in static volumetric, continuous flow and gravimetric modes. A brief description of some of these is given below and a listing of commercial gas adsorption instruments is given in Table 2.5. 

Essentially the static, volumetric gas adsorption equipment available commercially is for determining the amount of gas physically or chemically adsorbed on a powder surface. It is available for either single point or multipoint techniques and may be manual or automatic. Surface areas down to 1 m2 can be determined to 0.1 m2 using nitrogen adsorption provided care is taken. With coarser powders the dead space errors makes nitrogen unsuitable. Since the amount of gas in the dead space is proportional to the absolute pressure it is preferable to use gases with low saturation vapor pressures. Krypton with a... 

The chemisorption of carbon monoxide is an established method for determining the surface area of dispersal metals, particularly in supported catalysts. The average area occupied by each molecule depends on whether attachment is on one or two sites, a state that can vary from metal to metal and with surface coverage [85]. The quantity of chemisorbed gases is commonly measured by volumetric methods with apparatus similar to that used for static BET gas adsorption measurements. 

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