Journal of Colloid and Interface Science 223, 112–125 (2000) doi:10.1006/jcis.1999.6629, available online at http://www.idealibrary.com on Surface Properties of Mesoporous Carbon–Silica Gel Adsorbents R. Leboda, ∗, 1 V. V. Turov,† B. Charmas, ∗ J. Skubiszewska-Zieba, ∗ and V. M. Gun’ko† ∗ Department of Chemical Physics, Faculty of Chemistry, Maria Curie-Sklodowska University, M. C. Sklodowska Sq. 3, 20-031 Lublin, Poland; and †Institute of Surface Chemistry, 252022 Kiev, Ukraine Received July 27, 1999; accepted November 9, 1999 Carbon/silica (carbosil) samples prepared utilizing mesoporous silica gel (Si-60) modified by methylene chloride pyrolysis were stud- ied by nitrogen adsorption, quasi-isothermal thermogravimetry, p-nitrophenol adsorption from aqueous solution, and 1 H NMR methods. The structural characteristics and other properties of car- bosils depend markedly on the synthetic conditions and the amount of carbon deposited. The changes in the pore size distribution with increasing carbon concentration suggest grafting of carbon mainly in pores, leading to diminution of the mesopore radii. However, heat- ing pure silica gel at the pyrolysis temperature of 550 ◦ C leads to an increase in the pore radii. The quasi-isothermal thermogravimetry and 1 H NMR spectroscopy methods used to investigate the water layers on carbosils showed a significant capability of carbosils to adsorb water despite a relatively large content of the hydrophobic carbon deposit, which represents a nonuniform layerincompletely covering the oxide surface. C  2000 Academic Press Key Words: mesoporous silica gel; carbon/silica gel adsorbents; 1 H NMR, water adsorption;nitrogen adsorption; p-nitrophenol ad- sorption; quasi-isothermal thermogravimetry; surface heterogene- ity;pore size distribution;adsorption energy distribution. INTRODUCTION Carbon–mineral adsorbents are new-generation materials (1–3) that can be used as catalyst supports (4, 5) or adsorbents in chromatography (6–8), trace analysis (9), technology of wa- ter and sewage purification (1, 2, 10, 11), and other processes (12, 13). Additionally, they are potential polymer fillers, con- densation agents for various dispersed media, and biocatalyst carriers (14, 15). These materials are also used as intermediates in preparation of the carbon adsorbents and can provide desirable and original properties of finished adsorbents (16–18). The features of carbon–mineral adsorbents arise from the availability of polar inorganic fragments (metal oxides) and a nonpolar carbon phase, which can adsorb both nonpolar organics and polar substances (9, 11, 19), and their adsorption characteris- tics depend considerably on synthetic conditions. This is because pyrolysis is a complex multistage process (3, 20, 21) involving dehydrogenation of organic substances and their chemisorption 1 To whom correspondence should be addressed. Fax: 48 81 533 33 48. on the matrix surface, accompanied by many chemical transfor- mations leading to formation of bonded carbon clusters. With increases in temperature and pyrolysis time, these clusters en- large and overlap, forming a pregraphite lattice composed of condensed benzene rings (22). When T < 1000 K, large graphite planes are not formed, as formation of a turbostratic structure is a final stage of carbonization (23, 24). When the carbonization time is short, a disordered carbon layer forms on the support surface and the size of the condensed aromatic systems does not exceed several nanometers (22, 25). Clearly, interaction of water with an adsorbent surface is of importance in a major portion of applications. Features of these interactions can be investigated using various methods, one of which is 1 H NMR, which is appropriate for studing the structure of adsorbed water and organic molecules, as the chemical shift of adsorbed molecules depends on the structure of formed com- plexes (26–32). If molecules adsorbed onto the polyaromatic fragments of carbosils are inside a contour, through which the rotary currents of π -ring electrons flow, these molecules are in the area of local magnetic anisotropy, being screened from the action of the outer magnetic field. Therefore, their chemical shift can be smaller than that for a pure substance or its solution in an inert solvent. The 1 H signal displacement toward strong magnetic fields is observed for substances adsorbed on graphite (26, 33), graphite carbon adsorbents (28, 29), and some kinds of carbonized silicas (carbosils) (29, 30). If there are a large num- ber of surface groups including oxygen atoms and thay are able to form complexes with adsorbed molecules through hydrogen bonds, then in the case of proton-donor sites on the adsorbents, it is possible to shift the signal of mobile protons toward the weaker magnetic field (34, 35). Such an effect is observed for water molecules adsorbed onto the surfaces of carbon black and nonporous carbosil (29). As seen in the literature (36), the chemical reaction used gives a carbon deposit on silica matrices with a quite uniform chem- ical structure. The use of quasi-isothermal thermogravimetric analysis to characterize various adsorbents has been discussed in detail elsewhere (37, 38). Despite many investigations of carbon–silica adsorbents, their structural features and their influence on adsorption of various compounds are largely unclear. Therefore, the aim of this paper was to elucidate the structural properties of carbosils as related 112 0021-9797/00 $35.00 Copyright C  2000 by Academic Press All rights of reproduction in any form reserved.