The surface properties of carbon-I the effect of activated diffusion in the determination of surface area

This study forms part of a research programme concerned with structure, reactivity and electron-spin-resonance properties of carbons prepared from eight organic polymers. The possible use of carbon dioxide, on a routine basis, in the determination of surface area of carbons has been investigated. The results show that the rate of diffusion of nitrogen molecules into the carbon at 77°K can be markedly low. This is caused by the temperature dependence of the rate of the diffusion. Surface area values, determined from the nitrogen isotherms, are accordingly too low. The rate of adsorption of carbon dioxide at 195°K is always very much higher than that of nitrogen and appreciable adsorption of carbon dioxide can occur where no adsorption of nitrogen is detectable. When the carbon structure is equally accessible to nitrogen and carbon dioxide, surface area values calculated from the carbon dioxide and nitrogen isotherms are comparable. The adsorption of carbon dioxide at 273°K can be shown to indicate if the adsorption of the carbon dioxide at 195°K is itself restricted.