Adsorption of carbon dioxide on hydroxylated and dehydroxylated silicas

Abstract

Adsorption isotherms of carbon dioxide were determined at −78.5°C on a standard, nonporous form of silica (TK 800) and on two porous silica gels (one microporous and the other mesoporous), which had been used in previous investigations. To study the effect of surface dehydroxylation, each silica was outgassed at temperatures of 25 and 1000°C. The isotherms are analyzed by the BET and FHH methods and also by means of a new procedure, devised by Gregg, which allows a simple comparison of the shapes of two isotherms to be made by plotting the ratio, f, of their ordinates against the relative pressure, p p 0 . The character of the carbon dioxide isotherm is found to be highly dependent on the temperature of outgassing: With each silica, the change in outgassing temperature from 25 to 1000°C has resulted in a marked decrease in the BET c value and a movement in the location of the BET monolayer to higher p p 0 . These effects are attributed to the removal of surface OH groups by outgassing at 1000°C. The magnitude of the change in the FHH r value (from 1.62 to 1.20) indicates that the effect of dehydroxylation extends to the carbon dioxide multilayer, to a considerably greater extent than has been found with nitrogen. The f plots for gel J (outgassed at 25 and 1000°C) are practically identical at p p 0 < 0.1 and deviate only slightly from the horizontal. On the other hand, the two f plots for gel E are very different and also deviate considerably from the horizontal. It appears that the specific, field gradient-quadrupole interactions are enhanced when the surface OH groups are located within micropores, but that the micropore filling process has distorted the shape of the carbon dioxide isotherm on both the hydroxylated and dehydroxylated forms of gel E.