Lattice Resistance to Hydrolysis of Si−O−Si Bonds of Silicate Minerals:  Ab Initio Calculations of a Single Water Attack onto the (001) and (111) β-Cristobalite Surfaces

• Published in: The journal of physical chemistry. B 2000-06, Vol.104 (24), p.5779-5783
• Main Authors: Pelmenschikov, Alexander, Strandh, Helene, Pettersson, Lars G. M, Leszczynski, Jerzy
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp994097r

Hydrolysis of Si−O−Si linkages of β-cristobalite by a single H2O molecule is studied within the cluster approach at the DFT (B3LYP) and MP2 levels of theory. The 6-31G(d) and 6-311G(d) basis sets are used. Cluster models, including from 6 up to 14 Si atoms, of the (001) and (111) surface planes are considered. These models are specially designed to take into account the steric constraints imposed by the solid matrix on the Si−O−Si linkages and their nearest surroundings. For comparison, the hydrolysis of the Si−O−Si bridge of the free (HO)3Si−O−Si(OH)3 molecule is also calculated. The computed activation energy of the reaction (ΔE a) for the (001) and (111) planes of β-cristobalite is larger by 5 and 16 kcal/mol, respectively, than for (HO)3Si−O−Si(OH)3 (17 kcal/mol). The higher energy barrier for the surface is due to the resistance of the lattice to the relaxation of the activated complex of the reaction. The difference in ΔE a between the (001) and (111) planes suggests that the larger the number of Si−O−Si bridges for a Si atom (2 for the (001) plane and 3 for the (111) plane), the stronger the resistance of the solid matrix to the hydrolysis of a Si−O−Si bridge. This finding allows for the atomic-level substantiation of the earlier hypotheses that (i) the hydrolysis of the first Si−O−Si linkage of a Si atom should be the rate-limiting step for the release of Si(OH)4 and (ii) the dissolution should preferentially take place for the low-linked Si species of the surface. The OH groups produced by the reaction form H-bonds with the nearby Si−OH and Si−O−Si surface species. For both planes, the energy of the reaction (ΔE r) is within the 1−2 kcal/mol range.