Parahydrogen enrichment and hydrogen–deuterium equilibration at low temperatures on oxides of the first transition series

Parahydrogen enrichment on eleven first transition series metal oxides has been invostigated at 77 and 90 $^\circ$K over the pressure range 1 to 200 Torr (1 Torr = 133$\cdot$3 N m$^{-2}$). The order of activity at 77 $^\circ$K and 10 Torr was found to be: $\mathrm{Fe}_2\mathrm{O}_3 > \mathrm{Co}_3\mathrm{O}_4 > \mathrm{CoO > MnO > NiO} > \mathrm{V}_2\mathrm{O}_3 > \mathrm{Cr}_2\mathrm{O}_3 > \mathrm{CuO > ZnO} > \mathrm{V}_2\mathrm{O}_5 > \mathrm{TiO}_2.$ This activity series is interpreted in terms of the paramagnetic mechanism in which the rate is dependent upon the magnetic properties of the metal ions arising from their 3d electronic configuration. Comparison of the pressure dependencies for enrichment with those of hydrogen adsorption isotherms at 77 and 90 $^\circ$K suggests that enrichment occurs on a small fraction of the oxide surface. The rates of hydrogen-deuterium equilibration at 10 Torr have been measured on the eleven oxides at 297 $^\circ$K, and at 77 and 90 $^\circ$K both before and after exposure to hydrogen at 297 $^\circ$K. The order of activity at 297 $^\circ$K was found to be: $\mathrm{Co}_3\mathrm{O}_4 > \mathrm{CoO > NiO} > \mathrm{Cr}_2\mathrm{O}_3 > \mathrm{V}_2\mathrm{O}_3 > \mathrm{ZnO > CuO > Fe}_2\mathrm{O}_3 > \mathrm{MnO > TiO}_2 > \mathrm{V}_2\mathrm{O}_5.$ This activity series confirms, at a single reaction temperature on well-sintered oxides, the twin-peaked activity pattern previously found for this reaction (Dowden, Mackenzie & Trapnell 1956). At 77 and 90 $^\circ$K the simple twin-peaked activity pattern was not observed and high activity was confined to V$_2$O$_3$, Co$_3$O$_4$ and NiO, and to CoO after exposure to hydrogen at 297 $^\circ$K.