Surface properties of amorphous silicas

The surface properties of amorphous silicas are largely influenced by the nature of the surface silanol (SiOH) groups (1-3). Lewis acid-base sites are absent unless the silica has been activated at very high temperatures, Bronsted acidity at the gas-solid interface is low or nonexistent, and the siloxane bridges are relatively unreactive toward most molecules. This chapter discusses some methods that employ chemical modification and H-D exchange to probe the nature of the surface hydroxyl groups on... 

Lowen, W. K., and Broge, E. C. 1961. Effects of dehydration and chemisorbed materials on the surface properties of amorphous silica. J. Phys. Chem. 65 16. 

Reprinted from Chemical Engineering Science, 56(14), Stallons JM, Inglesia E, Simulations of the structure and properties of amorphous silica surface, 4205 1216, 2001, with permission from Elsevier. 

Currently, the definitive book on the chemistry of silica is the monograph by Ralph K. Iler (I). This book contains 350 references to publications by Soviet researchers. The books by A. V. Kiselev and coworkers (2,3), also published in the United States, are noteworthy for their description of Soviet research on silica surface chemistry and the adsorption properties of oxide adsorbents such as silica. Somewhat more attention is devoted in this chapter to the surface chemistry of amorphous silica, because this field is of special interest to me (4, 5). 

The properties of amorphous silicas of high specific surface area, from the smallest colloidal particles to macroscopic gels, depend largely on the chemistry of the surface of the solid phase [1]. 

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous silica particles in a liquid, almost always water. Commercial products contain silica particles having diameters of about 3—100 nm, specific surface areas of 50—270 m2/g, and silica contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

Due to its high surface area, surface chemistry and physics dominate the properties of fumed silica. The O—Si-O being 0.3 to 0.4 nm let estimate about only 20 silicon dioxide units spanning the diameter of a primary particle of amorphous silica. Fumed silica therefore has an extremely high surface to bulk ratio up to about 10 %. This is why even bulk methods of chemical analysis are suitable to follow chemical reactions on its surface elemental analysis, IR or NMR methods, etc. 

The overall performance of a catalyst is known to depend not only on the inherent catalytic activity of the active phase but also on the textural properties of the solid. The ability to control the specific surface area and the pore size distribution during the synthesis of amorphous silica-aluminas has been described for both surfactant micelle templated syntheses (M41-S (1), FSM-16 (2), HMS (3), SBA (4), MSU (5), KIT-1 (6)) and cluster templated sol-gel syntheses (MSA (7), ERS-8 (8)). 

Most real surfaces are heterogeneous in adsorption properties. Even the regular structure of a particular crystal face never extends over a macroscopic area. There are numerous factors, primarily the roughness, which give rise to imperfections. For the surface of amorphous silica, heterogeneity is an inherent fundamental property. 

The surface atomic structure of silica gel was also simulated in Refs. [17, 18]. Silica gel is another form of amorphous silica which is formed not in the process of cooling of the liquid but as a result of coagulation at room temperature. Its surface is the surface of small microspheres which together form an irregular porous structure. We consider here only the simulation of the surface atomic structure presented in Refs. [17, 18]. The pore structure of such a material clearly depends on the arrangement of the microspheres in space. Together with the surface atomic structure, the pore structure determines the adsorption properties of silica gel and we consider it in this context in the next section. 

In the 1950s, A. Kiselev, Zhdanov, and co-workers (12, 84, 155-159) showed that when the adsorption isotherms of water are expressed as absolute isotherms (referred to as the unit surface of the SiC>2 sample), widely different forms of amorphous silica having a completely hydroxyl-ated state adsorb the same amount of water at the same relative pressure (p/po <0.3). Thus the plots of absolute adsorption isotherms for different samples showed that the surfaces of these samples are of a similar nature. The adsorption properties of nonporous silica and silica having large pores (i.e., an absence of micropores) depend above all on the presence of OH groups and on the degree of hydroxylation of the surface. 

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