Bonhoeffer-Farkas mechanisms

It is further referred to as the Bonhoeffer-Farkas mechanism, or mechanism III. 

H2/D2 exchange involves type (/) species, but their exact role is still under debate. A kinetic isotope effect operates such that HD is produced — 1.5 times faster from a 2H2 D2 mixture than from a H2 2D2 mixture. Naito et al favour an Eley-Rideal mechanism at 200 K involving a gas-phase molecule and a type (/) chemisorbed atom. Kokes et a/. initially favoured a similar mechanism involving a type Hi) chemisorbed molecule instead of one in the gas phase. However, in a later paper they show that type Hi) species are only important in allotropic ortho-para) conversion and that the exchange reaction involves a Bonhoeffer-Farkas mechanism, using type (/) adatoms, at all temperatures. Richard et al., on the basis of the variation of the first-order rate constant with pressure, deduced that the mechanism must involve atomic H on surface pair sites that most probably were adjacent ion pairs thus, a Rideal type of mechanism was favoured. 

On the metals the balance of evidence appears to support the Bonhoeffer-Farkas mechanism, and on one oxide, Cr203, we have obtained data on the pressure dependence of k which seems best explicable in such terms. Now... 

This is a Bonhoeffer-Farkas mechanism between two pairs of adsorbed atoms. 

The latter mechanism became known as the Bonhoeffer-Farkas mechanism and seemed to enjoy general acceptance for a few years. 

In the room next to ours. Dr. J. K. Roberts was studying the adsorption of hydrogen on very pure tungsten wires using original techniques developed by him. Little did we know that a few years later he would question the validity of the Bonhoeffer Farkas mechanism of the catalytic para-hydrogen conversion. 

At that time, it seemed to us that the publication of this paper (I had not attended the Faraday Meeting) set off a veritable storm of criticism of the Bonhoeffer-Farkas mechanism of the para-hydrogen conversion and the dissociative mechanism of the deuterium exchange ... 

Then Roberts reasoned that since para-hydrogen conversion can be observed on W at -100 C such a reaction can not be explained by the Bonhoeffer-Farkas mechanism which involves rapid adsorption-desorption processes. 

It took over ten years to show that Roberts experimental results were faulty, the Rideal-Eley mechanism was unnecessary and that Bonhoeffer-Farkas mechanism was correct. And this was done by none other than Professor Rideal (60-62) and his student H. M. W. Trapnell (65,64-67),... 

Although the equality of the three experimental criteria might be purely incidental, it appears more probable that the same molecular mechanism that prevails on platinum only at T < 110 K remains predominant on Pt-Au alloys up to the temperature where rates become immeasurable with the apparatus used. This reaction appears to include an appreciable contribution of a Bonhoeffer-Farkas exchange (77). 

Process 2, the adsorption of the reactant(s), is often quite rapid for nonporous adsorbents, but not necessarily so it appears to be the rate-limiting step for the water-gas reaction, CO + HjO = CO2 + H2, on Cu(lll) [200]. On the other hand, process 4, the desorption of products, must always be activated at least by Q, the heat of adsorption, and is much more apt to be slow. In fact, because of this expectation, certain seemingly paradoxical situations have arisen. For example, the catalyzed exchange between hydrogen and deuterium on metal surfaces may be quite rapid at temperatures well below room temperature and under circumstances such that the rate of desorption of the product HD appeared to be so slow that the observed reaction should not have been able to occur To be more specific, the originally proposed mechanism, due to Bonhoeffer and Farkas [201], was that of Eq. XVIII-32. That is. 

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