Conventional chemisorptions techniques

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... 

We recently reported that a heterogeneous copper catalyst prepared with a non-conventional chemisorption-hydrolysis technique is able to promote a hydrogen transfer reduction using a donor alcohol. In this case, the role of copper is cricial, both for activity and selectivity [20]. 

Table 1 compares our results obtained for the EuroPt-1 catalyst using the CO methanation technique with results reported by other groups [13-17] using x-ray diffraction, conventional chemisorption, and TEM. A dispersion of 45% was calculated using the spherical particle approximation and an assumed adsorption stoichiometry of 1 CO molecule per exposed Pt atom. If the adsorption stoichiometry is instead assiuned to be 0.7, in keeping with previous studies of CO chemisorption out on Pt catalysts [17,18], then the dispersion becomes 65% in close agreement with the studies shown in Table 1 employing other techniques. 

BET and porosity were measured by N2 chemisorption (volumetric technique) on a Coulter SA 3100 equipment. The accessible copper surface area (SCu) is determined by the conventional N2O adsorption technique on reduced catalysts (H2, 270°C, 15 h). XRD measurements were performed on a Siemens D 5000 equipment using the CuKa radiation. 

In view of the known rapid chemisorption by tungsten, it must be concluded that the film is contaminated. Enough gas has been desorbed from the glass so that the film after filament burnout is saturated to such an extent that it cannot reduce the pressure below the initial value. On the other hand, if evaporation is carried out in short bursts, the initial low pressure drops steadily, showing that tungsten acts as a getter under these conditions and that cleaner films may be prepared by slight modification of the conventional techniques. 

Column 2 of Table 2 shows the dispersion values which we measured previously using conventional volumetric adsorption techniques. For the Pt/Rh catalyst, where it was necessary to use H2 as the probe molecule due to a strong and irreversible interaction of CO with one of the washcoat components (not present in the Pd/Rh catalyst), the volumetric chemisorption data agree fairly well with the CO-H2 methanation technique. However, for the Pd/Rh catalyst, the volumetric chemisorption method employing CO gave a nonsensical dispersion of 174% for the fresh catalyst and an implausible dispersion of 96% for the vehicle-aged catalyst. 

Table 2 clearly shows that of the three techniques compared (CO methanation, volumetric chemisorption, and XRD) only the CO methanation technique yielded a complete set of realistic dispersion/particle-size data for all four of the fresh and aged commercial catalyst samples analyzed. Volumetric chemisorption was very difficult to perform on these samples because of interferences from the metal oxide components of the catalyst, while XRD was limited by its inability to detect small particles with our conventional source and detector. The CO methanation technique is relatively fast (less than two hours for... 

Various techniques are used to obtain information on the active centers of catalysts, such as selective poisoning, measurement of the catalyst acidity and its strength, field electron and ion microscopy, infrared spectroscopy, fiash-filament desorption, differential isotopic method, etc. A temperature-programmed desorption method, which will be described and discussed in the present article, is in principle similar to the fiash-filament desorption method, reviewed recently by Ehrlich (1). It differs, however, from it in several respects. Modifications have been necessary in order to make the construction and operation of the apparatus easier and to adapt it to studies of materials other than metals, for example the conventional oxide catalysts. The conditions employed are much more similar to those ordinarily used in catalytic reactions than is the case with the fiash-filament method. An additional important feature of the modified technique is that it permits in some cases simultaneous study of a chemisorption process and the surface reaction which accompanies it. At the same time the modifications made have sacrificed some of the simplicity of the flash-filament method. For example, an obvious complication may arise from the porous structure of the conventional catalytic materials, in contrast to the relatively smooth surfaces of metal filaments. The potential presence of this and other complications requires extension of the relatively simple theoretical treatment of flash-filament desorption to more complicated cases. 

Adsorption on solid electrodes of high hydrogen overpotential such as Bi, Pb, Tl, and Cd, approximates to that on the Hg electrode. In contrast, the adsorption of organic compounds on the metals of the Pt group is irreversible due to chemisorption. However, even in this case the adsorption may be treated using conventional techniques, usually based on the lattice gas approach. Here, we briefly present the latest and interesting work carried out by Vayenas group on the electrochemical promotion. 

Simultaneously, a high spectral resolution is possible, which contrasts conventional infrared or electron energy loss spectroscopies. In this way, for example, different physisorption and chemisorption sites of ethylene on ZnO can be identified via their Raman resonances (Wijekoon et al. 1987). The structure and electronic properties of monomolecular organic films can also be in-vesfigafed sensitively using this technique. 

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