Adsorption single-point

The Cr203 content of each catalyst was determined by atomic absorption spectroscopy (Varian/Spectr AA-20 plus) on acid-digested samples. Total surface areas were determined by a single point BET method (nitrogen adsorption-desorption at 77.5 K) using a mixture of 29.7% N2 in helium. Samples were wet-loaded into the flow tube and dried at 423 K in a hydrogen flow for 15 minutes and then for another 30 minutes at 513 K before cooling in helium. 

The surface area of graphite is determined by nitrogen adsorption using a BET single point method (equipment is available from Quatachrome Instruments, Boynton Beach, FL, USA). 

As a result, the plots of o- / vs. E for different bulk concentrations of the solute intersect at the same point (Umax. max). which corresponds to the adsorption maximum. This implies that at the adsorption maximum does not vary with increasing F. Consequently, the surface dipole potential due to the adsorbing molecules exactly matches that of the desorbed solvent molecules, " provided that the solvent and solute molecules assume only one orientation at the surface. In practice, however, a single point of intersection of the vs. E plot for different bulk solute concentrations is observed rather rarely. For example, a gradual inaease in the surface concentration of 2-propanol on mercury in aqueous solution brings about a small positive shift of a. A much bigger shift has been observed for hexafluoro-2-propanol (HFP). " It has been ascribed to the reorientation of the HFP molecules in such a way that both -CF3 groups are directed toward the electrode, which in turn results in... 

Flowever, the focus of the major part of the chapters lies on the couphng chemistry used for DNA immobilization. Successful immobihzation techniques for DNA appear to either involve a multi-site attachment of DNA (preferentially by electrochemical and/or physical adsorption) or a single-point attachment of DNA (mainly by surface activation and covalent immobihzation or (strept)avidin-biotin linkage). Immobilization methods described here comprise physical or electrochemical adsorption, cross-linking or entrapment in polymeric films, (strept)avidin-biotin complexation, a surface activation via self-assembled monolayers using thiol linker chemistry or silanization procedures, and finally covalent coupling strategies. 

These steps describe the determination of a single point on an adsorption isotherm. By adjusting the mercury level, we can increase the pressure of the equilibrium gas with more adsorption occurring. Steps 3 and 4 are thus repeated until the full isotherm is mapped. Example 9.1 illustrates numerically how a point on the isotherm is established. 

Surface areas were determined by single point B.E.T. method of nitrogen adsorption. 

The most successful immobilization techniques for DNA or oligonucleotides appear to be those involving multi-site attachment—either electrochemical or physical adsorption—or single-point attachment— mainly covalent immobilization or strept(avidin)/biotin linkage [26]. As an example, single-point covalent immobilization can be performed on... 

Fig. 21.1. Schematic representation of the manipulation of DNA biosensors using different strategies. (A) DNA biosensors modified with DNA by (Al) dry-adsorption and (A2) wet-adsorption on GEC platform. (B) DNA biosensors based on the single-point immobilization of biotinylated DNA on Av-GEB universal affinity platform. (C) Immobilization of DNA on magnetic beads followed by (Cl) the capture of the modified beads on m-GEC electrode (more details in Pividori and Alegret [58]).

Although the specific antibodies can be directly attached onto a GEC surface by simple dry-adsorption (the simplest immobilization method and the easiest to automate), this procedure implies multisite attachment, and improved immobilization yields have been obtained with single-point attachment strategies. 

The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/AljO in benzene hydrogenation. 

Advantages of this procedure are (a) rapidity (heat exchanges are favoured by the gas mixture flowing at atmospheric pressure) (b) sensitivity (surface areas as low as 0.5 m2 can be measured) and (c) simplicity (it does not require vacuum or dead volume calibration). The main interest of this procedure is to provide one experimental point for the application of the single point BET determination (cf. Chapter 6). Thus, one adsorptive-carrier gas mixture (for example 10% N2, 90% He) can be stored and used as required. Measurement of equilibrium pressure is not required, but the atmospheric pressure, p, should be known. In the above example, the equilibrium pressure is p/9. If possible, the mixture is chosen so that the determination is made within the expected linear range of the BET plot. 

Groszek (1966) early developed a simple flow-through adsorption calorimeter, which is somewhat similar to a differential thermal analysis (DTA) system (because of its single-point temperature detector) and is therefore well suited for the detection of thermal effects and for screening experiments. To obtain meaningful results requires more sophisticated equipment, however. A heat flowmeter microcalorimeter is normally used for this purpose. Such a microcalorimeter, especially designed for liquid-flow adsorption and for the complementary determination of AmitH, is illustrated in Figure 5.18. 

D-4567 Single-point Determination of the Specific Surface Area of Catalysts Using Nitrogen Adsorption by the Continuous Flow Method Single-point surface area using continuous flowing nitrogen-helium gas mixture ... 

Adsorption/desorption isotherms of nitrogen at 77 K were measured with an automated apparatus ASAP 2010 (Micromeritics, USA). The specific surface areas, Sbet, were calculated from the linear form of the BET equation, taking the cross-sectional area of the nitrogen molecule to be 16.210 m. Pore size distributions were calculated in the standard maimer by using BJH method [6]. The total pore volumes, Vp, for the samples under study were determined from a single point adsorption at a relative pressure of 0.98 by converting the value of the adsorbed gas to the volume of the liquid adsorbate. 

N2-adsorption isotherms were evaluated using BET, t-plot and BJH analyses. The specific surface area 5bet was calculated via BET analysis [9] for relative pressures in the range between 0.05 and 0.23, making use of seven data points. With. S bet and the single point pore volume Up measured at maximum relative pressure (> 0.9950), the average pore diameter Dave was calculated under the assumption of cylindrical pores following the equation... 

Surface areas (SBET) were measured with a Micromeritics Flowsorb II using the single point approximation of the BET equation for the adsorption of N2 at 77 K. Carbon deposits on the catalyst were quantified by elemental analysis involving combustion to CO2, absorption in a Ba(C104)2 solution and coulometric titration. 

All chemisorption experiments were single point measurements at 8.10 Pa. By measuring the adsorption isotherms from 10 -10 Pa for a few catalysts, it was checked that a relative comparison of the thus obtained chemisorption values was as justifiable as any other method based on other measuring points, or on extrapolation of measuring points to zero pressure, as advocated by Benson and Boudart (24). No corrections were made for chemisorption on the bare supports as such, because this was found negligible. 

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