The Parahydrogen Conversion

The chemical mechanism (61, 62) was supposed to involve the dissociative chemisorption of hydrogen molecules, in a loosely bound fashion. In this mechanism, the hydrogen, dissociated into atoms on the surface of the catalyst, recombined to give normal hydi ogen, i.e., a mixture in which the 

A parallelism was drawn with Taylor s notions about activated adsorption. The reaction Hj+Dj— 2HD on nickel (63) was found to possess similar kinetics to the conversion, and the same mechanism was proposed for this reaction. On the other hand, this reaction did not go on charcoal at low temperatures (64, 65), thus confirming the paramagnetic mechanism in this case. The work up to 1935 is admirably summarized in the book by A. Farkas (66), and we restrict ourselves to a consideration of more recent work. 

The original explanation still holds good. It has, however, occurred to the author that the normal method of comparing the conversion with the H2-I-D2 reaction does not allow a completely unequivocal decision as to whether the conversion is paramagnetic or not, at temperatures as low as 77 K. Chromium oxide at this temperature shows a very rapid conversion, but a relatively slow Hj-fDj reaction (64). A difference of activation energy of 1.2 kcal. such as may arise from zero point energy effects (63) would cause the H2-I-D2 reaction to go more than a thousand times more slowly than the conversion at this temperature, if the latter involved only the chemical mechanism. 

While the paramagnetic substances hemin, hematin, and copper phthalocyanine catalyze the conversion at room temperature, the diamagnetic hematoporphyrin and metal-free phthalocyanine are inactive (67). No catalysis was found for the H2-fD2 reaction. Since the surface area of the hemin crystals could be estimated, the collision efficiency of the conversion could be calculated. The value of 10 ° so determined is in reasonable agreement with Wigner s theory (59). 

Turkevich and Selwood (68) have examined the conversion on the solid free radical a, a-diphenyl- -picryl hydrazyl. This substance did not show a very strong van der Waals adsorption of hydrogen, and its efficiency as a catalyst was much increased by mixing it with zinc oxide, which by itself was a powerful adsorbent but a weak catalyst. 

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When studying the kinetics of diffusion of hydrogen through palladium, Farkas (28) noticed the difference in catalytic activity of both sides of the palladium disks or tubes for the parahydrogen conversion the energy of activation was greater on the inlet side than on the outlet side, where due to extensive desorption of the hydrogen its concentration could be lower. 

The poisoning effect of hydrogen when dissolved in palladium was for the first time properly described and interpreted by Couper and Eley (29) in 1950 in their study of the fundamental importance of the development of theories of catalysis on metals. The paper and the main results relate to the catalytic effect of an alloying of gold to palladium, on the parahydrogen conversion. This system was chosen as suitable for attempting to relate catalyst activity to the nature and occupation of the electronic energy... 

We have, then, another example of an alloy and reaction in which the simple d-band theory has to be modified in a rather speculative way in order to explain experimental results. Actually, this is unnecessary for the formic acid reaction if we take the more recent value of about 0.4 for the number of d-band holes per palladium atom. This is not a satisfactory solution, because it is then difficult to explain the low activation energy for the parahydrogen conversion on Pd-Au alloys containing between 40 and 60% Pd. 

The parahydrogen conversion has been studied on Pd-Ag films (47), wires (148), and foils (149). The films were prepared by evaporation from... 

Couper, A. and Eley, D.D. "The parahydrogen conversion on palladium-gold alloys." Disc. Faraday Soc. 8 172-184 1950. 

In considering the properties of the solid surface and its influence on the chemistry of the reactants, I should like to recall to your attention papers by Harrison and McDowell (9) which merit, I believe, a measure of careful consideration. The authors were principally concerned with a detailed and quantitative examination of the phenomenon published in 1941 by Turkevich and Selwood. These authors had found that a mixture of zinc oxide and a,a-diphenyl- 3-picrylhydrazyl was much more powerfully a converter of para- to orthohydrogen than would be concluded on the basis of the mixture law and their separate activities in the conversion process. This phenomenon can be rationalized on the basis of concepts developed by Wigner. The more recent paper of Harrison and McDowell demonstrates, however, that, whereas neither the hydrazyl nor zinc oxide has any marked ability to produce the hydrogen-deuterium exchange reaction at 77° K, the reaction proceeds on the mixture at a rapid and reproducible rate, 2.4 times faster than the parahydrogen conversion on the mixture at the same temperature ( ) and 81 times faster than it would have occurred on the zinc oxide constituent. 

Stage 1. Hydrogen is very rapidly adsorbed to give a decrease in resistance. The process is completely irreversible at 0° and less than 10 mm. pressure. The extent of this stage is not related to the total film resistance, to rapid hydrogen sorption, or to the rate of the parahydrogen conversion. It varies greatly with the conditions of preparation of the film. It is probably dependent on defect structure. 

The value of this approach is to differentiate between the structure sensitive adsorption, of Stage 1, which involves the initial high heats of adsorption extensive adsorption, at pressures above 1(T mm. with an undetermined upper limit and a low heat of adsorption, which is responsible for the parahydrogen conversion and the slow sorption process with an appreciable heat of sorption which causes poisoning of the parahydrogen conversion. The work can give no indication of the physical processes involved in the three types of adsorption. 

This enzyme activates the simplest of all substrates and causes the parahydrogen conversion and hydrogen deuteride reaction. Krasna and Rittenberg (26, 27) have postulated formation of an enzyme hydride EH,... 

Bond Type in the Adsorbed Film Production of Hydrogen Atoms by Hot Tungsten Recombination of Hydrogen Atoms The Parahydrogen Conversion. ... 

A. Farkas (69) found the parahydrogen conversion was catalyzed by a tungsten wire and described his results in terms of the dissociation and recombination of hydrogen in a loosely bound chemisorbed layer. Roberts results on the stability of the chemisorbed 61m showed that this mechanism was unlikely to hold for the specihc catalyst and conditions concerned (70). The rate of combination of chemisorbed atoms at room temperature is immeasurably slow and cannot account for the extremely rapid conversion observed. In reply, Farkas suggested (71) that most of the chemisorbed hydrogen was inactive, the conversion proceeding by his mechanism for a few active spots, not detectable in Roberts work. 

The atomic exchange reaction proceeded with a similar velocity to the parahydrogen conversion, thus establishing the identity of the latter as a surface exchange reaction. 

Farkas and Farkas (81, 82) assumed that the rate of dissociation of hydrogen molecules on the catalyst was measured by the parahydrogen conversion. They further assumed that the hydride was adsorbed by dissociation of a hydrogen atom, and that hydrogen and hydride competed for the chemisorbed layer. 

The new knowledge about the parahydrogen conversion indicates the need for a reconsideration of these views. Much more work is required, but it may be supposed that in the presence of other gases the conversion is proportional to (a) the concentration of chemisorbed hydrogen atoms and (b) the concentration of hydrogen molecules adsorbed in the van der Waals layer, or perhaps more specifically on gaps in the chemisorbed layer. 

The parahydrogen conversion on this view is then, broadly, a test for the presence of chemisorbed hydrogen atoms. The effect of an increased pressure of hydride in decreasing the velocity of parahydrogen conversion is to be attributed in the first place to a displacement of hydrogen by hydride from the van der Waals layer. 

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