Adsorption of carbon dioxide on mesoporous silicas near the critical temperature

The adsorption of carbon dioxide on mesoporous silicas was studied near the critical temperature (T-c) of CO2, i.e. 304.2 K. The critical temperature in the mesopores at which the first-order phase transition (capillary condensation) was observed (T-cp) was estimated from the inverse slope of the adsorption isotherms and the behaviour of the adsorption isotherms per unit surface area. The values of T-cp for CO2 in mesopores whose radii, r(p), were 3.02 and 2.14 nm were higher than those estimated using a slab model in which the adsorbing parts of layer were clearly separated from the non-adsorbing regions. The differential heat of adsorption decreased with increasing pore size of the mesoporous adsorbent. This result also suggested a deviation from the slab model.