Chemisorption nonactivated

For adsorption from the gas phase onto nonporous, impermeable surfaces, phys-isorption and nonactivated chemisorption are governed largely by gas-phase kinetics and are instantaneous on the time scale of chemical sensor measurements. 

The barrier to chemisorption, E, with 0 for kinetically nonreactive systems, >0 for activated chemisorption, and E <0 for nonactivated chemisorption ... 

The dependence of Sq on T follows from Eq. (2.3). For activated chemisorption (i.e., E > 0), Sq increases with increasing T this behavior is not unique since it can also occur for activated reactions in both direct and indirect mechanisms. By contrast, for nonactivated chemisorption (i.e., Ej < 0), So decreases with increasing T this is a unique signature for such a process that is often looked for experimentally. 

Nonactivated chemisorption This occurs quickly and indicates very low energy of activation or zero-activation energy. 

Steric Factor Not every molecule with the necessary activation energy will chemisorb and only those traversing the particular configuration associated with the activated complex will do so. Thus s may be much less than unity for even nonactivated chemisorption. 

A very extreme version of surface corrugation has been found in the nonactivated dissociation reactions of Fl2 on W [, ], Pd and Rli systems. In these cases, the very strong chemisorption bond of the FI atoms gives rise to a very large energy release when the molecule dissociates. In consequence, at certain sites on the surface, the molecule accelerates rapidly downliill into the dissociation state. At the unfavourable sites, there... 

The microcalorimetric measurements of Della Gatta and his co-workers in their investigation of the interaction of water vapour with highly dehydroxylated y-alumina confirm that in this system also, the nondissocia-tive chemisorption of water is nonactivated, whilst the dissociative chemisorption is always activated. Thus the pseudo-equilibrium between the two chemisorbed states is displaced towards dissociative chemisorption as the temperature is increased above 150 C. 

The analysis of the shape of the heat emission peaks in the above studies indicated that peaks typical of fast chemisorption processes became more asymmetric for moderate coverages. This indicated that an activated process started at those coverages superimposed on a nonactivated step. IR spectroscopy verified this conclusion. Della Gatta et at. (112) showed that such a peak can be deconvoluted by subtracting from it a peak of the same height from the second adsorption run for which the adsorption is reversible. 

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... 

Rates of chemisorption of hydrogen on a given nickel catalyst were found to be nonactivated,. but the desorption activation energy was 13.8 kcal/g mole, about the same magnitude as the heat of adsorption (—13.5 kcal/g mole). 

The chemisorption of water (dissociative adsorption) involving the breaking of the siloxane bridges and the formation of new silanol groups is a rapid nonactivated (or weakly activated) process. 

Fast chemisorption followed by slow activated adsorption involving only small amounts of the adsorbent have been reported from studies in the gas phase. Trapnell reports that adsorption may be nonactivated in its initial stages, but becomes activated when more than about half of the surface becomes covered.Experimental evidence for mixed chemisorption (activated adsorption) and diffusion control has been reported by Ward for the adsorption of hydrogen on copper. ... 

Having said this, CO chemisorption is a very valuable probe molecule to count surface metal atoms, not only because of its typically higher heat of adsorption compared to H2, but also because it is non-dissociative and invariably nonactivated. However, it is complicated somewhat by the need for two isotherms and its ability to adsorb both in a linear mode interacting with a single surface metal atom, i.e.,... 

Activation Energy - Chemisorption can be an activated process therefore, only those molecules possessing the required activation energy can be chemisorbed. However, many, if not most, chemisorption processes on clean metal surfaces are nonactivated at temperatures near or above 300 K, especially for nondissodative adsorption. 

Note 1. From the formal point of view, the equations for the adsorption kinetics on a heterogeneous surface with a given adsorption-time distribution may be modified to have the same expression [Eq. (42)], as the desorption kinetics with the same desorption-time distribution (see, for instance. Refs. [35]). The forms (46) and (47), however, may be apphed only to thermally activated adsorption kinetics chemisorption usually runs in this situation, whereas physisorption is generally a nonactivated process. 

Figure 1 Schematic potential energy diagram for (a) nonactivated and (b) activated chemisorption of hydrogen on a metal surface.

Dissociation of H2 and formation of a chemisorption bond between atomic hydrogen and a surface site consisting of a certain number of substrate atoms. H dissociative adsorption is quasi-nonactivated on most transition metals. 

As shown in Fig. 5, the deposition rate is independent of temperature, and the chemisorption is nonactivated hence, the activation energy E = 0. Under the experimental conditions, we suppose that/(0) = 1 and y % 1 hence, the rate of chemisorption is equal to the impingement frequency S. The theoretical deposition rate 0 in this high temperature range is... 

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