The catalysis of the para-hydrogen conversion by zinc oxide

Abstract

A detailed study of the kinetics of the heterogeneous ortho-para -hydrogen conversion on zinc oxide between 77 and 455° K has shown that between the lower temperature and 203° K the rate of the conversion decreases with increase in temperature, whereas at temperatures greater than 203° K the rate of the conversion increases with temperature in accordance with the Arrhenius equation. The process occurring between 77 and 203° K exhibits a ‘negative activation energy’ of 1080 cal mole -1 . This catalysis is most probably due to interaction between the hydrogen molecules and the magnetic dipoles which exist in zinc oxide at low temperatures. In the same temperature range, a mixture of zinc oxide and the solid free radical aa -diphenyl B -picryl hydrazyl is a more efficient catalyst for the £>ara-hydrogen conversion than either of the separate constituents. This mixture is also effective as a catalyst for the exchange reaction between hydrogen and deuterium at 77° K. It is thus shown that the catalytic mixture can chemisorb hydrogen at very low temperatures. Explanations of this phenomenon are suggested in terms of an electron-transfer process between the surfaces of the two solids. Above 273° K, the velocity of the para -hydrogen conversion on zinc oxide increases rapidly with temperature having an activation energy of 13.1 kcal mole -1 from 273 to 361 °K and 10.4 kcalmole -1 from 380 to 455° K. The absolute rate at a given temperature is strongly dependent on the previous treatment of the catalyst. It has been found that the catalyst becomes more active after it has been highly evacuated, while its activity decays in the presence of hydrogen between room temperature and 455° K. The experimental results now presented are compared with those of previous workers and while the available evidence is insufficient to establish conclusively any particular mechanism for the chemisorption of hydrogen, it is shown that the results are consistent with the behaviour of zinc oxide as an excess semi-conductor. It is shown that mechanisms involving adsorption on lattice defects can be devised which account qualitatively for all the experimental facts. A calculation of the absolute rate to be expected for this type of mechanism shows that it is capable of yielding rates of conversion which are of the same magnitude as those determined experimentally.