Exchange of oxygen between carbon dioxide and alumina

Abstract

Oxygen exchange between C 18O 2 and an η-alumina has been studied over a wide range of temperature between 27 and 700 °C mainly by mass spectrometry. The amount of exchangeable oxygen increased with increasing temperature, and finally exceeded 10 15 atoms/cm 2. However, the temperature dependence had an inflection point at about 300 °C, and it became much steeper at higher temperatures. It was concluded that two different sorts of processes take place: unstable (reactive) oxygen atoms on alumina are exchanged readily at relatively low temperatures, while at higher temperatures all oxygen atoms on the surface and also those in the deeper layers are involved in exchange. Strongly chemisorbed carbon dioxide which neither exchanged nor communicated with the gaseous molecules was detected by infrared spectroscopy, and the amounts of this irreversible adsorption were measured under the reaction conditions by using 13CO 2 as a tracer. The maximum surface density of the reactive oxygen atoms exchangeable at lower temperatures was also obtained as 1.32 × 10 14 and 1.03 × 10 14 atoms/cm 2 for the catalyst dehydrated at 650 and 750 °C, respectively. A likely intermediate of the exchange reaction is bicarbonate ion which, in turn, blocks two to three adjacent oxygen atoms for the exchange when adsorbed irreversibly. Below 300 °C, the increase in the exchangeable oxygen with temperature seems to result from the decreasing amount of the irreversible adsorption.