Dispersion surface measurement

There are numerous techniques which provide information related to the surface energy of solids. A large array of high-vacuum, destructive and non-destructive techniques is available, and most of them yield information on the atomic and chemical composition of the surface and layers just beneath it. These are reviewed elsewhere [83,84] and are beyond the scope of the present chapter. From the standpoint of their effect on wettability and adhesion, the property of greatest importance appears to be the Lifshitz-van der Waals ( dispersion) surface energy, ys. This may be measured by the simple but elegant technique of... 

The analysis demonstrates the elegant use of a very specific type of column packing. As a result, there is no sample preparation, so after the serum has been filtered or centrifuged, which is a precautionary measure to protect the apparatus, 10 p.1 of serum is injected directly on to the column. The separation obtained is shown in figure 13. The stationary phase, as described by Supelco, was a silica based material with a polymeric surface containing dispersive areas surrounded by a polar network. Small molecules can penetrate the polar network and interact with the dispersive areas and be retained, whereas the larger molecules, such as proteins, cannot reach the interactive surface and are thus rapidly eluted from the column. The chemical nature of the material is not clear, but it can be assumed that the dispersive surface where interaction with the small molecules can take place probably contains hydrocarbon chains like a reversed phase. 

The fresh and spent catalysts were characterized with the physisorption/chemisorption instrument Sorptometer 1900 (Carlo Erba instruments) in order to detect loss of surface area and pore volume. The specific surface area was calculated based on Dubinin-Radushkevich equation. Furthermore thermogravimetric analysis (TGA) of the fresh and used catalysts were performed with a Mettler Toledo TGA/SDTA 851e instrument in synthetic air. The mean particle size and the metal dispersion was measured with a Malvern 2600 particle size analyzer and Autochem 2910 apparatus (by a CO chemisorption technique), respectively. 

There are also instruments with dispersion surfaces compatible with two-dimensional sensors. Their sensitivity and spectral response allow simultaneous measurement of thousands of lines (Fig. 15.7). 

Figure 9 shows the dispersion surface of a film derived from the BCB-1 monomer during heating from room temperature to 200°C. The dielectric constant was found to be relatively flat over this surface with a value of er 2.65 + 0.2. The uncertainty in th measurement was due to the error in measuring the ratio of the sample area to the thickness and is systematic over the entire surface. The slight rise at 10 MHz is due to losses in the experimental rig which could not be properly subtracted out of the measurement. This flat response over such a wide frequency range is characteristic of non-polar polymers. 

The measured adsorption effect at the electrode is influenced by all dissolved and/or dispersed surface-active substances according to their concentration in the solution, adsorbability at the electrode, kinetics of adsorption, structure of the adsorbed layer, and some other factors. Adsorption of organic molecules on electrodes causes a change of the electrode double-layer capacitance. It is the result of an exchange between the counterions and water molecules from solution, followed by changes in the dielectric properties and the thickness of the double layer on the electrode surface, that is, parameters that determine the electrode capacitance (Bockris et al., 1963 Damaskin and Petrii, 1971). 

Dispersions were measured using a modified Mettler BE22 balance. Pressed samples of approximately 30 mg were oxidised at 200°C under 100 mbar of 02 for 12 hours followed by reduction under 100 mbar of H2 at 400°C for 12 hours before exposure at room temperature to CO. In calculating dispersions it was assumed that one atom of CO associated with one surface metal atom. 

The amount of metal available on the catalyst surface following C03O4 reduction rather than its dispersion determines the extent of CO chemisorption, and so the presence or absence of FT activity. Dissociative chemisorption, carbide formation and coke formation appear therefore to have been dynamic processes with migration of Co° atoms occurring as progressive carburization of metal clusters occurred. Thus metal dispersion, as measured by CO absorption at room temperature, is of questionable utility in terms of a description of FT activity since the Co° atoms are static. 

The metallic surface area and dispersion are measured by CO pulse chemisorption, performed on a Micromeritics Pulse ChemiSorb 2700. 

Platinum and chlorine (samples made with chloride precursors) contents of the catalyst samples were determined with X-ray fluorescence spectroscopy (XRF) (Phillips PW 1480 spectrometer). BET surface areas of catalysts were within 5% that of the silica support material. Platinum dispersion was measured with hydrogen chemisorption in a volumetric set-up, using a procedure described elsewhere [3]. Stoichiometry of H/Pt = 1 was assumed for calculating the platinum dispersion [4]. Transmission electron microscopy (TEM) (Phillips CM 30, 300kV) was used to check the platinum particle size in some of the catalysts. Average platinum particle size was determined based on analysis of about 100 platinum crystallites. 

Four supported nickel catalysts NifCr203, NijSi02, NifMgO and NilZr02 were investigated by Isotopic Transient Kinetics Method [2]. The catalyst samples were prepared by coprecipitation method. B.E.T. surface area and metal dispersion were measured by krypton adsorption and hydrogen chemisorption, respectively [3, 4]. The main measured values of the metallic surface area and the total surface area of the catalysts are reported in Table 1. 

The CO-methanation technique, like conventional chemisorption techniques, is subject to imcertainty over CO/metal-atom stoichiometries. However, for our objective of characterizing aging effects in large numbers of automotive catalysts, we are interested in measuring relative changes in dispersion/surface area rather... 

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