The surface chemistry of amorphous silica. Zhuravlev model

A review article is presented of the research results obtained by the author on the properties of amorphous silica surface. It has been shown that in any description of the surface silica the hydroxylation of the surface is of critical importance. An analysis was made of the processes of dehydration (the removal of physically adsorbed water), dehydroxylation (the removal of silanol groups from the silica surface), and rehydroxylation (the restoration of the hydroxyl covering). For each of these processes a probable mechanism is suggested. The results of experimental and theoretical studies permitted to construct the original model (Zhuravlev model-1 and model-2) for describing the surface chemistry of amorphous silica. The main advantage of this physico-chemical model lies in the possibility to determine the concentration and the distribution of different types of silanol and siloxane groups and to characterize the energetic heterogeneity of the silica surface as a function of the pretreatment temperature of SiO2 samples. The model makes it possible to determine the kind of the chemisorption of water (rapid, weakly activated or slow, strongly activated) under the restoration of the hydroxyl covering and also to assess of OH groups inside the SiO2 skeleton. The magnitude of the silanol number, that is, the number of OH groups per unit surface area, αOH, when the surface is hydroxylated to the maximum degree, is considered to be a physico-chemical constant. This constant has a numerical value: αOH,AVER=4.6 (least-squares method) and αOH,AVER=4.9 OH nm−2 (arithmetical mean) and is known in literature as the Kiselev–Zhuravlev constant. It has been established that adsorption and other surface properties per unit surface area of silica are identical (except for very fine pores). On the basis of data published in the literature, this model has been found to be useful in solving various applied and theoretical problems in the field of adsorption, catalysis, chromatography, chemical modification, etc. It has been shown that the Brunauer–Emmett–Teller (BET) method is the correct method and gives the opportunity to measure the real physical magnitude of the specific surface area, SKr (by using low temperature adsorption of krypton), for silicas and other oxide dispersed solids.