Ammonia and water as probes for the surface reactivity of covalent solids: cristobalite and silicon carbide

Abstract

Reactive sites on α-cristobalite and α-silicon carbide outgassed over a wide range of temperature (423–1573 K) have been studied by adsorption microcalorimetry. By successive adsorption–desorption cycles the fraction of each adsorbate reversibly adsorbed at room temperature and the effect of preadsorption of one adsorptive on the adsorption of the other have been evaluated. Both molecules are dissociated on strained bridges and adsorbed via H-bonds on surface hydroxyls, but they do not react strictly with the same sites. H-bonding of H₂O takes place on two silanols located at a defined distance, with an enthalpy in the range 50–100 kJ mol^–1, and on isolated silanols, with an enthalpy lower than the latent enthalpy of liquefaction (44 kJ mol^–1). The two processes discriminate hydrophilic and hydrophobic patches at the surface. Ammonia interacts with single silanols, either isolated or terminal in a cluster: in the latter case electronic effects due to adsorption involve all SiOHs in mutual interaction. Strained siloxane bridges dissociate both molecules with formation either of two new silanols or of one silanol and one silylammine group. This latter surface functionality interacts with H₂O, although with a lower enthalpy than SiOH, but does not interact with NH₃. Silicon carbide exhibits similar behaviour to silicas, when outgassed up to 773 K, but after heating at 1073 K develops a peculiar reactivity, whereby large amounts of both H₂O and NH₃ are dissociated, in contrast to silica whose reactivity is progressively depressed upon thermal treatments. The above data are discussed in the context of systematic surface characterization of covalent solids, aiming to relate the surface properties of the particles to their lung toxicity when inhaled.