Adsorption nonactivated chemisorption

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. 

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. 

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. 

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. 

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. 

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