Surface heterogeneity of carbon–silica adsorbents studied on the basis of the complex adsorption investigations

Abstract

Silica-gel (SG) Si-60 was coated by pyrolytic coke using CVD from methylene chloride. The porous structure of carbon–silica adsorbents (carbosils) was analyzed on the basis of nitrogen adsorption isotherms at 77 K. It has been shown that carbosils are mesoporous adsorbents with complete absence of micropores. Total pore volume and surface area decrease during the carbon deposition on silica surface. Heterogeneity of surface of carbosil samples was estimated by adsorption energy distribution (AEDs) calculated from adsorption isotherms of nitrogen at 77 K and hexane, cyclohexane, benzene and chloroform adsorption isotherms at 473 and 483 K. AEDs were obtained by solving Adsorption Integral Equation with Hill-de Boer kernel function. N2 AEDs show that distribution of silica active surface sites responsible for N2 adsorption at 77 K is not altered during the carbon deposition. However, coke deposition leads to improving of polar–apolar surface composition, resulting in increase of adsorption of apolar (hexane) and decrease of adsorption of polar (chlorophorm) substances. The main changes observed in AEDs are connected with decreasing of silica accessible area at stable carbon structure during CVD of carbon.