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OF CATALYSIS
Exchange
Division
48, 120-128
(1977)
of Oxygen
of Chemistry,
between
Carbon
Y. MORIKAWA~
AND
Y.
National Research
Council
Dioxide
and Alumina1
AMENOMIYA
of Canada, Ottawa, Canada,
KlA
OR.9
Received Oct,oher 22, 1976; revised January 31, 1977 Oxygen exchange between 0802 and an v-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 temperat,ure, and finally exceeded 1Ol5 atomsjcmz. However, the temperature dependence had an inflection point at about 3OO”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 detect,ed by infrared spectroscopy, and the amounts of this irreversible adsorption were measured under the reaction conditions by using WO2 as a tracer. The maximum surface densit,y of the reactive oxygen atoms exchangeable at lower temperatures wi&s also obtained as 1.32 X 1Ol4 and 1.03 X 1Ol4 atoms/cm2 for the catalyst dehydrated at 650 and 75O”C, respectively. A likely intermediate of the exchange reaction is bicarbonate ion which, in t,urn, blocks two t,o three adjacent oxygen atoms for the exchange when adsorbed irreversibly. Below 3OO”C, the increase in the exchangeable oxygen wit,h temperature seems to result from the decreasing amount, of the irreversible adsorption. INTRODUCTION
Carbon dioxide is an interesting poison for various reactions catalyzed by alumina. Generally it poisons alumina for reactions involving hydrogen but has little effect on the isomerization reaction of olefins (1-6). The adsorption of carbon dioxide on alumina has been studied extensively mainly by infrared spectroscopy (1, Y-11), but the assignment of some absorption bands is still controversial. Consequently, the mechanism of CO2 poisoning is not yet clear. In general, three kinds of ions, AP+, OH- and 02-, are exposed on the surface 1 Contribution No. 15994 from the National Research Council of Canada, Ottawa, Canada. z NRC Research Associate 1975-1977.
of alumina. The ions themselves, or more likely, various combinations of them constitute active sites as suggested by Peri (12) by a model of alumina surface. The oxide ions, however, have not been studied extensively compared to the other ions. In view of this, we started an investigation of the oxygen atoms on alumina surface by the exchange reaction of oxygen between C1*02 and an q-alumina. Recently, Peri (23) published interesting results of similar exchange reaction between CL802 and oxide catalysts including alumina. He detected very reactive oxide ions on the catalysts which were exchangeable at relatively low temperatures. Nevertheles
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