Chemisorption on oxides

To conclude this brief introduction, it appears that there may be some relation between semiconductivity and chemisorption on oxide semiconductors. In catalysis a much more complex situation is expected and generalizations will be hard to come by. Indeed, it is well known that chemisorption is a necessary condition in heterogeneous catalysis but it is by no means a sufficient one. Furthermore, as was illustrated above, a given molecule can be chemisorbed in different ways on an oxide surface, and it will prove necessary to decide in each particular case which mode of adsorption is important in a catalytic process. 

The work described in the previous section was essentially concerned with the physical rather than chemical adsorption of some highly polarizable molecules on zeolites. With pyridine, such information can sometimes be obtained as easily using infrared spectroscopy. However, transmission IR spectroscopy cannot so easily be used to study chemisorption on oxides if it is essential to obtain low frequency spectral data (e.g., adsorbent-adsorbate stretching modes) because of the opacity of most oxides over much of the low frequency spectral region. Recent work has shown that the Raman technique can be extremely useful in this context (4). 

The studies of Garner and his co-workers in the years 1928-1939, which had established the existence of two types of carbon monoxide and hydrogen chemisorption on oxides and which identified irreversible chemisorption with incipient reduction, were followed in the immediate postwar period by an intensive study of the properties of copper oxide (12-15). The work was later extended to nickel oxide (16) and cobalt oxide (17,18). With each of these oxides it was established that carbon monoxide was capable of reacting not only with lattice oxygen, but also with adsorbed oxygen. The concept of irreversible chemisorption involving a carbonate ion and ulti-... 

Table 3 gives results which have been obtained with evaporated metal films, indicating the type of adsorption found for various gas-metal combinations. Chemisorption on oxides also shows fast and slow effects. The situation is more complicated than for metals because both the metal ions and the oxide ions are potential chemisorption sites. Usually, oxides are prepared as powders and the concept of a clean surface is necessarily different from that for metals. It is difficult to obtain a stoichiometric oxide surface and diffusion effects are often important. The data of chemisorption on oxides is given in Table 4. 

KINETIC MODELS USED FOR DESCRIPTION OF CHEMISORPTION ON OXIDES SURFACE... 

Cobalt occurs in two oxidation states in soil, +2 and H-3, but Co is the dominant form in soil solution. This metal associates preferentially with Fe and Mn oxides because of chemisorption and co-precipitation. There is evidence that, on Mn oxides, Co is oxidized and strongly bound as Co. Consequently, strongly oxidizing conditions in the soil are likely to favor the adsorption of cobalt. As the soil pH is raised, Co solubility decreases because of increased chemisorption on oxides and silicate clays, complexation by organic matter, and possibly precipitation of Co(OH)2. Organic matter complexes with Co are fairly labile, so that organically bound Co ... 

Undoubtedly with many chemisorptions on oxides more than one mechanism is possible (3). The adsorption of hydrogen on ZnO is an example where there may be two mechanisms. The work of Beebe and Dowden... 

In the calculations of methane interaction with nickel or titanium surfaces, five different modes of coordination have been considered (Figure III.2) [6c], It has been concluded that acts of methane activation by a separate molecule of a metal complex on the one hand and the metal surface on the another hand are very similar. In both processes the C-H bonds of methane are ruptured and new M-H bonds are formed. Intermediate species formed during alkane chemisorption on oxide catalysts have been investigated both by MO calculations and spectroscopic methods [7]. 

These two gases are also strategic for pollution and energy problems. In particular, the current elimination of CO2 uses chilling, pressure, contact with amine solutions [181], chemisorption on oxide surfaces or adsorption within zeolites, carbons, or membranes [182], but MOFs present a valuable alternative for this removal. 

---