Hydrogen-deuterium exchange catalysed by chromia-alumina solid solutions

Abstract

$\alpha $-Alumina, $\alpha $-chromia and ten corundum-phase chromia-alumina solid solutions $\alpha $-Cr$_{x}$Al$_{2-x}$O$_{3}$ with $x$ ranging from 0.002 to 1.4 have been studied as catalysts for hydrogen-deuterium exchange over the temperature range from 100 to 700 K. The adsorption of hydrogen has also been investigated. $\alpha $-Alumina is an active catalyst only above 500 K. When chromium ions are introduced, activity at 500 K and above is increased. In addition, a new zone of catalytic activity is generated at 110-350 K. This low-temperature activity, specific to the presence of chromium, exhibits a maximum at 250-300 K. For both low- and high-temperature activity zones, the catalytic activity is not a monotonic function of chromium content. The activity per chromium ion exhibits a periodic variation as $x$ is increased, with a high value at $x\approx 0.1$ and a second peak as $x$ approaches 2.0. A general analysis of the coordinative unsaturation to be expected on the principal surface planes of the corundum structure is presented and local charge variations at multiplet sites are identified. The implications of introducing transition metal ions are then indicated. The catalytic results are shown to be consistent with this model and with the electronic interactions known to occur between chromium ions in chromia-alumina as their concentration is increased.