Chemisorption low temperature

Let us dwell now on the issue of non-dissociative form of low temperature chemisorption of H2 proposed in [89]. As it has been mentioned above absolutely different behavior of electric conductivity of adsorbent... 

Specifically, Dr. Hauffe s mechanism requires that chemisorption alters the defect structure of the solid, and causes a change in the semi-conductivity. He exemplifies his mechanism by referring to the system ZnO/Ha. This is an unfortunate choice, because recent Japanese work (i) on this system shows that the reactive low temperature chemisorption of hydrogen does not alter the semi-conductivity of ZnO. Thus there is no alteration of defect structure and barrier-layer considerations lose their force. A full statement of our views concerning the adsorption of hydrogen on ZnO has now appeared in print ( ). 

The problem of the nature and the extent of a relationship between catalytic activity and position in the periodic table is important not only in the formulation of a general theory of catalysis but also in respect to the more fundamental problem of interaction between solid surfaces and surrounding phases. There is definite evidence on the important role of unfilled d-bands of transition metals for low-temperature chemisorption of hydrogen ). 

The present curve does not show the maximum found by the previous authors at — 78°C. Private communication from Dr. Paul Emmett indicates that the possible explanation for this is that the reduction procedure here employed may have failed to reduce the iron in those areas where the low temperature chemisorption of hydrogen occurs. According to Emmett these areas are only uncovered by reduction at around 500 C. in a rapid stream of hydrogen freed from oxygen and well dried before passing into the reduction zone. The data in Fig. 7 do, however, show that from room temperature upwards there is alwa s evidence of desorption on raising the temperature followed by readsorption, and the... 

Since both types of adsorption are exothermic, raising die temperature generally decreases the equilibrium quantity of adsorbate. Physisorption is fast, and equilibrium is rapidly reached, even at low temperature. Chemisorption generally requires high activation energies. The rate of adsorption is low at low temperatures, but the process can be rapid at higher temperatures. 

Extensive chemisorption measurements on reduced (Fe, Al) and (Fe, Al, K) samples have been reported by Brunauer and Emmett [163]. From these studies it has been concluded that the Fe-area of the samples can be measured by low temperature chemisorption of CO [163, 193]. 

It was noted in Section XVII-1 that chemisorption may become slow at low temperatures so that even though it is favored thermodynamically, the only process actually observed may be that of physical adsorption. Such slowness implies an activation energy for chemisorption, and the nature of this effect has been much discussed. 

If molecules or atoms form a chemical bond with the surface upon adsorption, we call this chemisorption. To describe the chemisorption bond we need to briefly review a simplified form of molecular orbital theory. This is also necessary to appreciate, at least qualitatively, how a catalyst works. As described in Qiapter 1, the essence of catalytic action is often that it assists in breaking strong intramolecular bonds at low temperatures. We aim to explain how this happens in a simplified, qualitative electronic picture. 

Figure 3. Difference spectra for CO chemisorption on Pd/SiOp at low temperature.

The hydrogen molecule does not chemisorb onto clean sintered gold surfaces at or above 78 K [147] but on unsintered films, a small amount of H2 is chemisorbed if gold surface atoms of low coordination number are present [148]. Stobinski [149] found that H2 can also chemisorb on thin sintered Au films if the surface is covered at low temperatures with a small amount of gold equivalent to 1-3 Au monolayers prior to H2 exposure. This suggests a fundamental role of surface Au atoms of low coordination number in the chemisorption process. Deuterium molecules also chemisorb in a similar fashion on gold films at 78 K and isotope effects were... 

The hydrogenation processes were performed at a relatively low temperature and pressure in the presence of promoted Raney Ni 2400 and Raney Co 2724 catalysts (13) in this study but any common nitrile hydrogenation catalysts (e.g. Fe, Ru, Rh, bulk or supported catalysts) could be used. The advantage of using a low temperature and pressure process is that it lowers the investment cost of an industrial process. Raney Ni 2400 is promoted with Cr and Raney Co 2724 is promoted with Ni and Cr. The particle sizes for both catalysts were in the range 25 - 55 pm. The BET surface area of Raney Ni 2400 and Raney Co 2724 are 140 m2/g and 76 m2/g, respectively, and the active surface area of the Ni and Co catalysts are 52 and 18 m2/g, respectively, based on CO chemisorption (Grace Davison Raney Technical Manual, 4th Edition, 1996). 

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