Conversion and equilibration rates of hydrogen on nickel

Abstract

The pH₂ conversion rate has been compared with that for oD₂ and H₂+ D₂→ 2HD on nickel wires rigorously cleaned in vacuo of 10^–10 torr. The rates are significantly higher than on evaporated films, prepared under similar conditions but still possibly contaminated. The results at 1 torr fall into three temperature ranges (a) 330–400°K, a fractional order (ca. 0.6) reaction, activation energy 5–7.5 kcal mole^–1. There is evidence for a zero order H₂+ D₂ reaction setting in as the pressure is lowered to 0.3 torr; (b) 200–300°K, a fractional order reaction, 0.05–0.40, with a lower activation energy, 1.5–2.7 Kcal mole^–1. In (a) and (b), pH₂, oD₂ and H₂+ D₂ rates are very similar to each other. On decreasing the temperature to (c) 77–200°K, the activation energies for the pH₂ and oD₂ rates decrease to zero, so that at 77°K these two reactions go 10³ times faster than H₂+ D₂. The mechanisms advanced are for (a) Bonhoeffer-Farkas, (b) Eley-Rideal, and (c) a paramagnetic conversion induced probably by unpaired d electrons in the Ni and proceeding through a Harrison-MacDowell mechanism.