Chemisorption system

Since in chemisorption systems it is reasonable to suppose that the strong adsorbent-adsorbate interaction is associated with specific adsorption sites, a situation that may arise is that the adsorbate molecule occupies or blocks the occupancy of a second adjacent site. This means that each molecule effectively requires two adjacent sites. An analysis [106] suggests that in terms of the kinetic derivation of the Langmuir equation, the rate of adsorption should now be... 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

Metal Oxide - Since metals are less electrophilic than silicon, metal oxide adsorbents show even stronger selectivity for polar molecules than do siliceous materials. The most commonly used metal oxide adsorbent is activated alumina, used primarily for gas drying. Occasionally, metal oxides find applications in specific chemisorption systems. For example, several processes are under development utilizing lime or limestone for removal of sulfur oxides from flue gases. Activated aluminas have surface areas in the range of 200 to 1,000 ftVft Average pore diameters range from about 30 to 80 A. 

Adsorbent regeneration is normally accomplished by reversing the adsorption process, either by decreasing the system pressure or, more commonly, by increasing the system temperature. In some cases, particularly in chemisorption systems, the adsorbent activity can be restored by reaction with a suitable reagent. 

Nalewajski, R. F. and A. Michalak. 1998. Charge sensitivity/bond-order analysis of reactivity trends in allyl-[Mo03] chemisorption systems A comparison between (010)- and (100)-surfaces. J. Phys. Chem. A 102 636-640. 

The total acidity of different samples was measured by NHj-TPD with Micromeritics Autochem II (ASAP 2920) chemisorption system. The pyridine FT-IR spectra of the catalysts were recorded on a BIO-RAD FT3000 FT-IR spectrometer after desorption at 250 and 450°C, respectively. Then the total and strong acidity of Eewis and Bronsted acid was obtained from the integrated absorbance of the respective bands. 

At the time that this survey of chemisorption bondlengths was undertaken, the database of molecular adsorption structures [139] was even more sparse, inadequate for any clear pattern to be established. More recently, we have returned to this problem through a detailed study of one specific chemisorption system, namely CO on Ni surfaces [142,143]. The objective was two-fold. First, by conducting PhD experimental structure determinations for different phases of CO on Ni(100) and Ni(lll) it was possible to obtain Ni—CO chemisorption bondlengths for 1-,... 

It is commonly accepted that chemisorption of CO on transition metals takes place in a way that is quite similar to bond formation in metal carbonyls (4). First experimental evidence for this assumption was obtained from a comparison of the C—O stretching frequencies (5) and was later confirmed by data on the bond strength (6) as well as by valence and core level ionization potentials obtained by photoelectron spectroscopy (7). Recent investigations have in fact shown that polynuclear carbonyl compounds with more than about 3-4 metal atoms exhibit electronic properties that are practically identical to those of corresponding CO chemisorption systems (8, 9), thus supporting the idea that the bond is relatively strongly localized to a small number of metal atoms forming the chemisorption site. 

Chemisorption systems are sometimes used for removing trace concentrations of contaminants, but the difficulty of regeneration makes such systems unsuitable for most process applications so most adsorption processes depend on physical adsorption. The forces of physical adsorption are weaker than the forces of chemisorption so the heats of physical adsorption are lower and the adsorbent is more easily regenerated. Several different types of force are involved. For nonpolar systems the major contribution is generally from dispersion-repulsion (van der Waals) forces, which are a fundamental property of all matter. When the surface is polar, depending on the nature of the sorbate molecule, there may also be important contributions from polarization, dipole, and quadmpole interactions. Selective adsorption of a polar species such as water or a quadrupolar species such as CO2 from a mixture with other nonpolar species can therefore be accomplished by using a polar adsorbent. Indeed, adjustment of surface polarity is one of the main ways of tailoring adsorbent selectivity. 

Fig. 25. A1 ternative toluene chemisorption systems involving the bipyramidal surface cluster of vanadium pentoxide. The structures (a, c, e, g, i) represent molecularly adsorbed toluene, while the remaining systems (b, d, f, h) model the dissociative adsorption, with two methyl hydrogens chemically bonded to the surface oxygens at the pyramid bases. Asterisks indicate positions which give rise to unstable MEC. The three atoms of the parallel complex (i), marked with an arrow, denote the extra instabilities appearing when the closed-system constraint (d N = 0) is imposed. The results are taken from Ref. 8. At each diagram the (I, E) stability diagnosis is also indicated (see Fig. 24)...

The remaining panels of Fig. 30 b, c, and d, correspond to closed chemisorption systems, representing the diagonal, off-diagonal, and total CT FF indices, respectively, for water - rutile electron transfer. 

Fig. 32. (Continued). The reactive IRM (see Fig. 31) contributions to f( w IRM), Panels a) and /CT( cof"M), Panels b) of the molecularly adsorbed (Part A) and the transition-state (Part B) chemisorption systems of Fig. 30...

These illustrative results clearly demonstrate the great utility of the IRM reference frame in diagnosing charge transfer effects in chemisorption systems. They provide truly two-reactant reactive channels, the shapes of which resemble the interaction region of the corresponding FF indices. They have been found to be much more selective indicators than the corresponding external FF indices. 

One of the primary aims of the research program described in this review has been to formulate adequate two-reactant reactivity concepts, and the underlying coordinate systems, which can be used to diagnose reactivity and selectivity trends in systems of very large donor/acceptor reactants, e.g., chemisorption systems. The CSA approach [52], which provides the basis for the present work, is both relevant and attractive from the chemist s point of view, since many branches of chemistry—the theory of chemical reactivity in particular—consider responses of chemical species to perturbations of the external potential and... 

Dynamics of precursors in activated dissociative chemisorption systems... 

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