Isotherms and energetics of carbon dioxide adsorption on .gamma.-alumina at 100-300.deg.
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Adsorption isotherms and isosteric heats of adsorption were measured for the interaction of carbon dioxide with γ-alumina at five temperatures in the range 100-300°. A decrease in heat of adsorption with increasing surface coverage was observed at all temperatures, and the isotherms conformed well at low coverages to the Freundlich equation, indicating an energetically heterogeneous alumina surface. Entropy calculations indicate that at >150° the surface species are extremely mobile, even at low coverages. Consideration of previous infrared and catalytic site poisoning studies of CO2 on alumina suggests that at >150°, the adsorbed layer consists almost entirely of relatively free carbonate-type structures, formed by interaction of CO2 molecules with surface oxide ions. At <150°, particularly at low coverages, the surface species appear much less mobile. Bicarbonate-type entities are probably formed only at high coverages (>6%) and at low temperatures (<150°) after the most energetic sites have interacted with CO2 to produce carbonate species.
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