Atomic structure and dehydration mechanism of amorphous silica: Insights from 29Si and 1H solid-state MAS NMR study of SiO2 nanoparticles

Detailed knowledge of the atomic structure of hydrous species on surface of amorphous silica and the effect of temperature and particle size on their atomic configurations are essential to understand the nature of fluids-amorphous silicates interactions and the dehydration processes in the amorphous oxides. Here, we report the 29Si, 1H MAS, and 1H-29Si heteronuclear correlation (HetCor) NMR spectra of 7 nm and 14 nm amorphous silica nanoparticles-a model system for natural amorphous silica-where previously unknown details of changes in their atomic structures with varying dehydration temperature and particle size are revealed. Diverse hydroxyl groups with varying atomic configurations and molecular water apparently show distinct dehydration trends. The dehydration (i.e., removal of water) of amorphous silica nanoparticles mostly results in the increase of isolated silanol by removing water molecules from hydrogen-bonded silanols associated water molecules. With further increase in dehydration temperature, the intensity of isolated silanol peak decreases above ∼873 K, suggesting that the condensation of isolated silanol may occur mainly above ∼873 K. The entire dehydration (and dehydroxylation) process completes at ∼1473 K. Both the water (i.e., physisorbed water and hydrogen-bonded water) and hydrogen-bonded silanol species show a dramatic change in the slope of intensity variation at ∼873 K, indicating that most of silanols is hydrogen-bonded to water rather than to other silanols. The fraction of hydrogen-bonded proton species is also much smaller in 14 nm amorphous silica nanoparticles than in 7 nm amorphous silica nanoparticles mainly due to the presences of larger fractions of water and hydrogen-bonded silanol species. 29Si NMR results show that with increasing dehydration temperature, the fraction of Q4 species apparently increases at the expense of Q2 and Q3 species. The fractions of Q2 and Q3 structures in 7 nm amorphous silica nanoparticles are larger than those in 14 nm amorphous silica nanoparticles. Dehydration of 7 nm amorphous silica nanoparticles occurs at a lower temperature than that of 14 nm amorphous silica nanoparticles. 29Si MAS NMR results show that a possible simultaneous dehydroxylation can also occur with removal of the hydrogen bonded silanol in the 7 nm silica nanoparticles. The energy penalty of dehydroxylation estimated from 29Si MAS NMR spectra varies with Q species and is smaller in 7 nm than in 14 nm amorphous silica nanoparticles. These results demonstrate that the particle size of nanoparticles plays an important role in controlling the hydrogen contents, and thus overall hydrogen bond strength of hydroxyl groups and atomic structure of silanols can control dehydroxylation of amorphous silica nanoparticles. The structural information and mechanistic details obtained from the current study provide insights into the structure of hydrous species and dehydration mechanisms in crystalline and amorphous silicates in diverse geological settings, highlighting usually unknown effects of particle size on the dehydration processes.