Chemisorption absolute rates

Models based on chemisorption and kinetic parameters determined in surface science studies have been successful at predicting most of the observed high pressure behavior. Recently Oh et al. have modeled CO oxidation by O2 or NO on Rh using mathematical models which correctly predict the absolute rates, activation energy, and partial pressure dependence. Similarly, studies by Schmidt and coworkers on CO + 62 on Rh(l 11) and CO + NO on polycrystalline Pt have demonstrated the applicability of steady-state measurements in UHV and relatively high (1 torr) pressures in determining reaction mechanisms and kinetic parameters. 

The question still in doubt concerns the nature of the activation barrier. In the classical treatment, the only one described in Trapnell s monograph (11), a gas molecule diving to an adsorption site must surmount an activation barrier. As pointed out previously by Taylor in his 1932 paper introducing the concept of activated adsorption, this simple picture immediately raises the question of a very small probability factor for adsorption, of the order of 10"6. Of course this small probability factor may be explained away if it is identified with the small fraction of active sites available at the surface. Another possibility is the relatively large negative value of the activation entropy that can be obtained for certain models of the activated complex. A treatment of chemisorption by absolute rate theory was first given in 1940 (16), but the use of the... 

Attempts have been made by Eyring and Sherman (29) and by Okamoto, Horiuti, and Hirota (30) to evaluate the activation energy for the chemisorption of hydrogen on carbon or nickel on the assumption that the surface atoms behave as isolated atoms. The calculated values, although too high, vary markedly with the spacing of surface atoms. Quantitatively, however, we must at present rely upon experimental data. The absolute rates of chemisorption will be calculated here using the observed temperature coefficient. 

The absolute rate of chemisorption of gaseous molecules 5, V, on the bare and plane surface of a catalyst can be given, assuming that the transmission coefficient equals unity (31), by... 

As for the above examples, the adsorption of hydrogen on reduced copper can also be regarded as occurring almost equally over the whole available surface inasmuch as Kwan and Kujirai (32) showed the active sites of the copper specimen to be identical with lattice points in studying the adsorption rate on the basis of the absolute rate theory. Consequently, the author has reached the conclusion that the surface of a number of reduced metallic catalysts is of a homogeneous nature for chemisorption as long as they are prepared by a very careful reduction and are kept free from any poisoning materials. 

The answer to this question can be extracted from an experiment performed by Bootsma and co-workers (ref. 2). They measured the amount of methane exposure necessary to produce a monolayer of carbon on several nickel single crystal catalysts when the methane gas was present above the catalyst at high pressures (>10"2 torr). From that measurement, they calculated a value for the probability for dissociative chemisorption of 10-9. This extremely small value for the dissociation probability immediately suggests why high pressures of methane are necessary for the observation of dissociation. Since the dissociation probability is so low, the absolute flux of incident molecules must be large in order for the dissociation rate to be high enough for carbon deposition to be observable in a reasonable amount of time. The next question is why is the dissociation probability so low ... 

Chemisorption rate constant k = A.expi-E/RT), where A = frequency factor, E = activation energy of adsorption of a substance, R = gas constant and T = absolute temperature. 

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