Parahydrogen conversion catalysts

EFFECT OF ADSORBED NITROGEN ON CATALYTIC ACTIVITY OF ORTHO-PARAHYDROGEN CONVERSION CATALYSTS... 

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... 

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. 

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

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. 

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. 

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. 

Ortho-parahydrogen conversion in the presence of various catalysts has occupied the attention of investigators from the early thirties to the present time, " It has been fairly well established that heterogeneous catalysis is due to interaction between adsorbed molecules and the inhomogeneous magnetic field caused by more or less isolated magnetic centers on the surface of the catalyst. Most of the... 

---