Parahydrogen conversion on the first transition series

Abstract

The effect of pressure on the reaction velocity has revealed a new phenomenon, in which zero, or negative reaction order changes over to a fractional or first order reaction at higher pressures. This was found for Mn, Fe, Co, Ni at 90 °K, and Co at 293 °K. The mechanisms suggested are hydrogen atom recombination in a saturated layer, probably accompanied by hydrogen poisoning at low pressures and hydrogen molecule-atom exchange in a partly filled “second layer” at higher pressures. The reactions on Cr at 90 °K, and Cr, Fe, and Ni at 293 ° K are simple zero order, and Mn, 293 °K, Zn, 368 °K, fractional and first order, respectively. Hydrogen-deuterium equilibration goes at comparable rates with the conversion, for Ti, V, Cr, and Ni; there is evidence for Ni that this holds down to 77 °K. Attempts to estimate the rate of the paramagnetic conversion at 90 °K using the Wigner equation confirm the dominance of the chemical mechanisms for all metals except Zn, where the mechanism is probably paramagnetic. A correlation between reaction velocity and sublimation energy for the range of metals at 293 °K is associated with a mechanism of recombination of H atoms held on sites of minimum bond energy. The low frequency of these sites for body-centered cubic metals gives a frequency factor of 10 16 molecules cm −2 sec −1 compared with 10 19 for close-packed metals. A lower activation energy is found for bcc metals and is associated with a greater interaction energy between adsorbed atoms, in its turn associated with the broad character of the d band in this group of metals.