Gas adsorption instruments

A series of silica gels were synthesised from TEOS, selecting the experimental conditions (pH and water/TEOS ratio) in order to obtain samples with different porosity. Porous texture characterisation of these samples was done by gas adsorption (N2 and CO2 adsorption at 77 K and 273 K, respectively) (Quantachrome. Autosorb 6). The samples were degassed at 623 K under vacuum, until 10 torr. Water adsorption studies were carried out at 298 K in an automatic volumetric gas adsorption instrument (Belsorb 18). Experimental data was corrected for adsorption on inner wall of apparatus. Additionally, a blank experiment on all bulbs used showed that water adsorption on the inner surface of glass was negligible. Previous to water adsorption... 

After evacuation (<1x10 Pa) of the samples at 673 K for 4 h, benzene adsorption was conducted at 298 K using automatic volumetric gas adsorption instruments (BELSORP 18 PLUS, Bel Japan Inc.). The pore volume of the... 

This experiment was carried out through a self-designed triaxial gas adsorption instrument. The device can simulate the storage environment of... 

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. 

Much of the pioneering work which led to the discovery of efficient catalysts for modern Industrial catalytic processes was performed at a time when advanced analytical Instrumentation was not available. Insights Into catalytic phenomena were achieved through gas adsorption, molecular reaction probes, and macroscopic kinetic measurements. Although Sabatier postulated the existence of unstable reaction Intermediates at the turn of this century. It was not until the 1950 s that such species were actually observed on solid surfaces by Elschens and co-workers (2.) using Infrared spectroscopy. Today, scientists have the luxury of using a multitude of sophisticated surface analytical techniques to study catalytic phenomena on a molecular level. Nevertheless, kinetic measurements using chemically specific probe molecules are still the... 

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. 

The sizing methods involve both classical and modem instrumentations, based on a broad spectrum of physical principles. The typical measuring systems may be classified according to their operation mechanisms, which include mechanical (sieving), optical and electronic (microscopy, laser Doppler phase shift, Fraunhofer diffraction, transmission electron miscroscopy [TEM], and scanning electron microscopy [SEM]), dynamic (sedimentation), and physical and chemical (gas adsorption) principles. The methods to be introduced later are briefly summarized in Table 1.2. A more complete list of particle sizing methods is given by Svarovsky (1990). 

Most commercial instruments using gas adsorption for surface area and porosity determination are based on the BET isotherm. In Eq. (1.45), the monolayer capacity Vm can be used to calculate the surface area on the basis of the area occupied by each adsorbed gas molecule. According to Eq. (1.45), a plot of p/[Va(p0 - p)] versus p/po is linear. From the slope and the intercept, Vm can be obtained. Thus, the specific surface area S can be obtained as... 

Schoofs, T., Surface area analysis of finely divided and porous solids by gas adsorption measurements, in Particle and Surface Characterisation Methods, R.H. Muller and W. Mehnert, Eds., Medpharm GmbH Scientific, Stuttgart, 1997. Webb, PA., and Orr, C., Analytical Methods in Fine Particle Technology, Micromeritics Instrument Corp., Norcross, GA, 1997. 

Adsorption isotherms were determined in an automated volumetric gas adsorption apparatus (Autosorb 1, Quantachrome Co.). Adsorption of nitrogen was performed at 77 K. Before measurements, samples were outgassed at 672 K for at least 8 hr in vacuum. Where some bumoff of the char was desired, the reactions were performed in an Online Instruments TG-plus thermogravi metric analyzer. The reactions were performed in a mixture of helium and oxygen, flowing at a rate of about 220 cc/min. Samples of 30-50 mg were dispersed on a circular platinum pan with a large flat surface and raised sides, resulting in a particle beds of about 1 mm thickness. Temperatures between 573-748 K... 

Loukopoulos, V. Gavril, D. Karaiskakis, G. An inverse gas chromatographic instrumentation for the study of carhon monoxide s adsorption on Rh/Si02, under hydrogen-rich conditions. Instnim. Sci. Technolog. 2003, 31 (2), 165-181. 

The task of reviewing the thermodynamics of gas adsorption by carbons is complicated by several factors. One of them is the variabihty of carbon materials discussed in Chapter 2. Another is the large amount of published literature, produced over a long period and generated under very different environments and thus potentially very different conditions. This has prompted us to focus our analysis on recent work, assuming that older studies may be outdated by new ones thanks to advances in instrumentation and to the logical progress of science. 

A variety of instruments designed specifically for gas adsorption-desorption analysis are commercially available. Common to all is the sample preparation and measurement methodology... 

As we here are mainly interested in adsorption measurement techniques for industrial purposes, i. e. at elevated pressures (and temperatures), we restrict this chapter to volumetric instruments which on principle can do this for pure sorptive gases (N = 1), Sect. 2. Thermovolumetric measurements, i. e. volumetric/manometric measurements at high temperatures (300 K - 700 K) are considered in Sect. 3. In Section 4 volumetric-chromatographic measurements for multi-component gases (N>1), are considered as mixture gas adsorption is becoming more and more important for a growing number of industrial gas separation processes. In Section 5 we discuss combined volumetric-calorimetric measurements performed in a gas sensor calorimeter (GSC). Finally pros and cons of volumetry/manometry will be discussed in Sect 6, and a hst of symbols. Sect. 7, and references will be given at the end of the chapter. 

An instrument for volumetric measurements of pure gas adsorption basically consists of a gas storage vessel (volume Vsv) and an adsorption chamber (Vac) being connected by a tube bearing a valve. Both vessels should completely be placed within a thermostat (water, oil, air etc.) and provided with tubes for gas supply and evacuation as well as with thermometers and manometers to measure the temperature (T) and pressure (p) inside the vessels, cp. Figure 2.1. 

The sensor gas calorimeter (SGC) basically consists of a classical volumetric gas adsorption device complemented by two gas thermometers (cp. Fig. 2.9). The core of the instrument is an adsorption vessel which is placed within a second vessel, the sensor gas jacket. This jacket vessel is filled with gas at pressure (psc) acting as a sensor via a capillary (1) coimecting the vessel with a difference manometer (P3). Additionally, a reference vessel also filled with the (same) sensor gas at pressure (prg) is placed in the thermostat and cormected via capillary (2) to the manometer (P3). Upon opening the valves (V7, V7A) the pressures (psc, Prg) of the sensor gases in the jacket vessel and the reference vessel are equalized, i. e. we have pso = Prg. Thermal equilibrium at temperature (T ) in the system... 

In conclusion it can be said, that the sensor gas calorimeter (SCjC) is a very useful instrument for simultaneous measurements of adsorption isotherms and (integral and differential) heats of adsorption. Also hints on the kinetics of the gas adsorption process can be gained from the time dependence of the pressure signal curve, cp. Fig. 2.11. However, to achieve high sensitivity and accuracy of measurements, type and amount of the sensor gas have to be chosen very carefully. At low temperatures (77 K) helium is recommended at reference pressures of about (0.1 - 0,2) MPa. At higher temperatures (298 K) nitrogen should be preferred at the same pressures, [2.23, 2.29]. 

Volumetric measurements of gas adsorption equilibria reduce, if the mass of the sorbent sample used has been determined, to measurements of pressures and temperatures in gas phases. For this a variety of high precision measuring instruments operating in a fairly wide range and partly also in corrosive environment are available today. Of course these instmments prior to measurement have to be calibrated with meticulous care which may be laborious and even cumbersome. For pressure measuring devices, calibration with pressure maintaining valves of Desgranges Huot has proved to be successful. 

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