Low pressure hysteresis in the sorption of carbon tetrachloride vapour on polymer carbons

Abstract

Adsorption—desorption isotherms for carbon tetrachloride vapour at 20.0°C on microporous cellulose and polyacrylonitrile carbons exhibit low-pressure hysteresis which is reduced or eliminated by widening of pores which accompanies steam-activation. Comparison of sorption of carbon tetrachloride on unactivated carbons heat-treated to 900 and 2700°C shows that the adsorbate is largely confined to external surfaces and macropores and excluded from the major part of the micropores by molecular sieve action. The bulk of adsorbate retained by unactivated carbons on desorption to P/P₀= 0.0 was adsorbed at P/P₀ > 0.9 indicating a pressure-threshold effect for low pressure hysteresis; about 5 % of the micropore volume originally inaccessible to carbon tetrachloride is penetrated by the expansion-intercalation process in the case of cellulose (900°C) unactivated carbon. A model for this process is proposed based on localised fracture of the carbons. Application of the theory of adsorption-extension of Flood and Heyding shows that stresses induced in volume elements of the carbons are commensurate with or greater than measured bulk fracture strengths for carbons.