Irreversible modification of the surface

A well-established stepwise procedure was followed [16, 23, 25, 56]. Small successive doses of the adsorptive were admitted and left in contact with the adsorbent until the thermal equilibrium was attained. The 1st run of adsorption performed on the activated sample (pretreated in high vacuum conditions and/or in controlled atmosphere) will be hereafter referred to as ads. I. At any individual dose of gas introduced in the system, the evolved heat AQ " was measured within the calorimetric cells, while the adsorbed amount Anads was measured by volumetry. Ads. I was followed by a desorption run (des. I), performed by simple evacuation of the cell. In such a way the reversibly adsorbed phase was desorbed and either the pristine surface was restored, in case of an entirely reversible adsorption, or the pristine surface was not recovered, in case of a (partially) irreversible adsorption. Ads. II was subsequently performed in order to assess which fraction (if any) of the pristine surface sites was irreversibly occupied by the adsorbed phase (in the adopted conditions). By subtracting the ads. II curve from the ads. I one, the adsorbed fraction not removed by evacuation is evaluated. The ads. n component will be hereafter referred to as the reversible adsorbed phase, whereas the (ads. I - ads. n) component will be referred to as the irreversible phase (in the adopted conditions). Subsequent runs of adsorption (ads. IE, IV etc.) are performed in some cases, if the irreversible modification of the surface is expected/suspected not to be extinguished during the ads. I [21, 23, 26]. Adsorption measurements are usually performed at least twice on a virgin portion of the same batch of the material, activated in the same conditions, to check the experiments reproducibility. The routinely run protocol of adsorption-desorption-adsorption cycles is schematically illustrated in Fig. 1.8. 

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