Parahydrogen conversion

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... 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

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... 

Fig. 23. Activation energy for parahydrogen conversion over Pd-Ag alloy films 47).

The points for Ag and Pd-Ag alloys lie on the same straight line, a compensation effect, but the pure Pd point lies above the Pd-Ag line. In fact, the point for pure Pd lies on the line for Pd-Rh alloys, whereas the other pure metal in this series, i.e., rhodium is anomalous, falling well below the Pd-Rh line. Examination of the many compensation effect plots given in Bond s Catalysis by Metals (155) shows that often one or other of the pure metals in a series of catalysts consisting of two metals and their alloys falls off the plot. Examples include CO oxidation and formic acid decomposition over Pd-Au catalysts, parahydrogen conversion (Pt-Cu) and the hydrogenation of acetylene (Cu-Ni, Co-Ni), ethylene (Pt-Cu), and benzene (Cu-Ni). In some cases, where alloy catalysts containing only a small addition of the second component have been studied, then such catalysts are also found to be anomalous, like the pure metal which they approximate in composition. 

Paracrystallinity, Cu/ZnO/AIjOj, 31 295 2"-Paracyclophanes, 32 453 Paracyclophanes, macrocycles, 29 206-208 Paraffin, see also Alkanes alkylation, 10 165, 27 98 carbon selectivity, bed residence time effects, 39 249-250 cracking, 39 283 cyclization, 28 295 rates, 28 301, 302 double cyclization, 28 312-314 in exhaust gases, 24 66, 67 hydrogenolysis, 30 43-44 hydroisomerization, 39 183 oxidation, 32 118-121 solubility enhanced hydrogeolysis, 39 285 Parahydrogen conversion rate correlations, 27 48-50... 

Electronics of Supported Catalysts Georg-Maria Schwab The Effect of a Magnetic Field on the Catalyzed Nondissocitive Parahydrogen Conversion Rate P. W. Selwood... 

EPR has been observed and studied in porous carbons by numerous authors 178-182). The carbons studied have been prepared by pyrolysis of organic material such as dextrose 180), coal 181), and natural gas or oils 181,182). Porous carbons are of considerable technological importance and show catalytic activity for the ortho-parahydrogen conversion, the hydrogen-deuterium reaction, and many reactions of inorganic complex ions 156). Relationships between the characteristics of the EPR absorption and the catalytic activity of porous carbons for the o-p Hj and Hj-D reactions have been demonstrated by Turkevich and Laroche 183). 

Analogous parahydrogen conversion and deuterium exchange reactions, catalyzed by NH2, have been observed in liquid ammonia (Wilmarth and Dayton, 61). The kinetics are of the same form as those of the OH -cat-alyzed reaction in water and the mechanism is open to similar interpretations. The NH2 -catalyzed reaction is much faster, its rate constant at —50° being 10 times that of the OH -catalyzed reaction at 100°. The assumption of equal frequency factors for the two reactions leads to a calculated activation energy for the NH2 -catalyzed reaction of about 10 kcal. This low value has been attributed to the much greater base strength of NH2 relative to OH . The results provide some support for the hydride ion mechanism. 

Fig. 17. Activation energy for parahydrogen conversion on palladium-gold alloys. The broken line denotes the paramagnetic susceptibility in arbitrary units. [Couper, A., and Eley, D. D., Discusaions Faraday Soc. 8, 172 (1950).]...

Bonhoeffer, Farkas, and Rummel (ISOa) found that parahydrogen conversion proceeds measurably on sodium chloride between 20 and 340° with an apparent activation energy of about 8 kcal./mole. De Boer (74) has suggested that this reaction may involve endothermic adsorption of hydrogen, the observed activation energy being that of the adsorption process. 

Where the rate of reduction of a substrate is lower than the rate of parahydrogen conversion or deuterium-water exchange in the absence of substrate (Table VI), contributing factors may be (1) inhibition of hy-... 

Few of the many known ferromagnetic solids are suitable as catalysts for the nondissociative ortho-parahydrogen conversion. This is especially true if measurements are needed in the neighborhood of the magnetic phase transition, Tc. The reasons for this are threefold the solid may decompose at the temperature necessary to free the surface from contaminants, the Curie point may be so low that the experimental difficulties are formidable, and many such solids show strong dissociative conversion activity near Tc. Of the three solids named above none is very satisfactory. 

Similar mechanisms can be provided for other reactions, such as parahydrogen conversion, hydrogen exchange reactions, dehydrations, etc. Reactions of the exchange type, where the energy levels in the two species are close, if not identical, can proceed in the van der Waals layer at low temperatures by virtue of the quantum mechanical exchange interaction. However, other classes of reactions are unlikely to proceed exclusively in other than the chemisorbed state. 

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