Chemisorption rate

It might be thought that since chemisorption equilibrium was discussed in Section XVIII-3 and chemisorption rates in Section XVIII-4B, the matter of desorption rates is determined by the principle of microscopic reversibility (or, detailed balancing) and, indeed, this principle is used (see Ref. 127 for... 

Further investigations of the above discussed effects show that, at fixed temperature of the oxide film (catalyst), the jump in the electric conductivity first increases in amplitude, as the portion of alcohol vapor admitted into the vessel increases. On further increase of the admitted portion of alcohol, the jump amplitude reduces (starting with the pressure of 3.6-10 2 Torr). At the pressure of 3.2-10 Torr, the jump in the electric conductivity of the zinc oxide film is less pronounced. Finally, at still higher pressures, it disappears (Fig.4.9). This effect is not unexpected. On our mind, it is associated with the fact that, as the concentration of alcohol vapor increases, the sum of the rate of interaction of the vapor with adsorbed hydrogen atoms and the rate of surface recombination of hydrogen atoms at the time instant of production becomes higher than the chemisorption rate of these atoms. The latter is responsible for the increase of the electric conductivity of the semiconductor oxide film via the reaction... 

Figure 1. Initial chemisorption rates (140°C) and pyrolysis weight loss against maximum charring temperature. Pyrolysis at 5°/min in nitrogen. (Reproduced with permission from Ref. 19. Copyright 1989 Elsevier Scientific Publishing Company, Inc.)...

Heat transfer may also play an important role in an adsorption system. The adsorption process is exothermic. Physical adsorption equilibria behave in a manner similar to vapor-liquid equilibria transfer out of the vapor phase is favored by decreasing temperature. Therefore, rapid dissipation of heat away from the adsorbing surface improves adsorption performance. Chemisorption rates, on the other hand, generally increase with increased temperature. 

Davis obtained a value of Ae of 12.68 kcal./mole from the observed chemisorption rate of nitrogen on tungsten and compared it with a value of Ae of 10 kcal./mole derived according to equation (3). A calculation from the relation that Ae is 10 kcal. - - %RT of the rate by means of equation (6) leads to the result that T is 7.1 X 10 . as shown in Table I. The initial rate of chemisorption calculated on the basis of an activation energy deduced from the observed rate is not reliable because the rates... 

Iron clusters exhibit facile chemisorption toward methanol, the reaction proceeding with little or no cluster-size selectivity. An interesting feature of this system is that the chemisorption rate constants are nearly identical toward various isotopic sjjecies (CH3OH, CH30D,CD3 0H). If dissociation of a C—H or O—H bond was the initial step, then this should be manifested in an observable kinetic isotope effect. Thus the initial chemisorption step most likely involves the lone-pair orbital localized on the oxygen atom. More extensive studies of the chemistry of the Fe methanol system have been explored using infrared multiple-photon dissociation spectroscopy. These results are discussed in detail in Section Vlll. 

Next consider the observation that above a certain cluster size the chemisorption rate constant levels off at some maximum value. This is precisely what is expected for a nonactivated process. No (or a negligibly small) barrier exists and the chemisorption is facile. This alleviation of the activation barrier... 

The POCI3 chemisorption rate in replacement reactions is found out to be above, that is in the interaction of POCI3 with OH-groups of silica [64,66]. 

Chemisorption rate constants and activation energies are the basis for the following quantitative analysis of the reaction mechanism on the oxides surface. The examples of... 

L2) chemisorption is responsible for surface reconstruction via generation of new sites at a rate proportional to the chemisorption rate and (L3) each newly exposed zone has the same topography as the surface from which it was generated. 

For example, see Taylor and Thon, Low, and Hayward and Trapnell for summaries of chemisorption rate measurements. Chromatographic techniques have been used to measure rates of adsorption by analyzing the moments of the peak leaving the colximn of adsorbent particles. For example, see G. Padberg and J. M. Smith, J. Catalysis, 12, 111 (1968). 

Figure 7. Dynamic response of the gas coverage on the metal oxides to the modulated concentration of a target gas chemisorhed on the surfaces characterised hy different chemisorption rates (lO" ) - 0.5 2 - 1 3 - 2 4 - 4 5 - 10.

In Figure 7, the component of the oscillations that mainly depends on the chemisorption rate is illustrated. The dependencies in this figure represent the response of the gas coverage to the modulated exposure of metal oxide... 

In the adsorbed phase, in which the particles are mobile because of surface diffusion, the chemical reaction of the metal-oxide formation takes place via dissociative chemisorption of the oxygen. This means that, since the reaction takes place in the adsorption phase and the various reaction partners can have different condensation coefficients, the ratio of the rates of incidence is a necessary but not adequate criterion. According to Ritter [293, 298], the chemisorption rate of oxygen is the critical step for the completion of the reaction ... 

We may also include a term analogous to the steric factor to be used as a measure of the deviation of chemisorption rates from this ideal limit. 

The shapes of the interatomic potential curves are approximations chosen for mathematical convenience. Such potential functions are generally used in discussions on a variety of properties of molecules and lattices optical absorption and luminescence, laser action, infrared spectroscopy, melting, thermal expansion coefficients, surface chemistry, shock wave processes, compressibility, hardness, physisorption and chemisorption rates, electrostriction, and piezoelectricity. The lattice energies and the vibration frequencies of ionic solids are well accounted for by such potentials. On heating, the atoms acquire a higher vibrational energy and an increasing vibrational amplitude until their amplitude is 10-15% of the interatomic distance, at which point the solid melts. 

---