Low temperature parahydrogen enrichment on nonstoichiometric rutile

Abstract

The reduction of rutile by vacuum outgassing between 450–825°C, or by hydrogen or carbon monoxide at 350°C, yields active catalysts for low temperature parahydrogen enrichment due to the presence of surface Ti³⁺ ions. Since the kinetics indicate saturation of these active sites, the absolute rate of enrichment at 77 K and 1.3 × 10³ N m^–2 can be taken as a direct measure of the extent of surface non-stoichiometry. Poisoning experiments with oxygen indicate Ti³⁺ ions are not limited to the surface layer alone, but are distributed between surface and sub-surface layers, and are effectively mobile at temperatures considerably below the Tammann temperature. The activity for parahydrogen enrichment arising from oxygen molecules physically adsorbed on fully oxidised rutile has been used to derive an equation, which is used to estimate the concentration of Ti³⁺ ions in reduced surfaces. This did not exceed ∼8.0 × 10¹⁶ ion m^–2 for the most active surface studied.