The catalysis of the para -hydrogen conversion by the solid free radical αα -diphenyl- β -picryl hydrazyl

Abstract

A detailed study of the kinetics of the heterogeneous catalytic ortho-para-hydrogen conversion on the solid free radical $\alpha \alpha $-diphenyl-$\beta $-picryl hydrazyl, shows that this conversion follows the usual first-order kinetics and that the rate is independent of pressure. Examination of the rate at 90, 190 and 290 degrees K indicates that it decreases with increasing temperature. This type of behaviour has been observed for other paramagnetic heterogeneous conversions, and the 'negative activation energy' has been assumed to indicate that the catalysis takes place in a physically adsorbed layer of hydrogen molecules. Some evidence that this is the true explanation of the phenomenon is shown by the results of studies of the adsorption of hydrogen on the free radical. Experiments carried out at 77 and 90 degrees K indicate that hydrogen is physically adsorbed by the free radical to form a weakly sorbed layer which obeys the Langmuir adsorption isotherm for the case when the surface is only sparsely covered. A theoretical discussion is given for a heterogeneous conversion occurring during the collision of the hydrogen molecule with the catalyst surface both in the absence, and in the presence, of a physically adsorbed layer. It is shown that only the theory based on the latter model is in agreement with experimental results, and so it is concluded that in the present case the catalysis is due to the interaction between the hydrogen molecules in a physically adsorbed layer, and the inhomogeneous magnetic field at the surface of the catalyst, which enables the otherwise forbidden ortho-para transitions to take place.