Density Functional Theory Study on the Morphology Evolution of Hydroxylated β-Cristobalite Silica and Desilication in the Presence of Methanol

Abstract

In the present work, the hydroxylation process of three low-index β-cristobalite silica surfaces, that is, C(100), C(101), and C(011) surfaces under different hydroxyl coverages has been investigated using density functional theory (DFT) calculations. The surface hydroxylation process is described as consecutively dissociative adsorption of water molecules on the hypothetically pristine crystalline silica surfaces, resulting in Q2 terminal silanols and Q3 H-bonded silanols. Based on the calculated Gibbs surface free energies at different hydroxylation conditions, the evolution of the β-cristobalite nanoparticle morphology and the exposed hydroxylated surface structure ratios are predicted using the Wulff construction principle. The fully hydroxylated C(100) and C(101) surfaces are two exposed structures of the rodlike β-cristobalite silica until 1000 K, while the fully dehydroxylated C(101) surface becomes the only exposed surface structure on the octahedral β-cristobalite silica when the temperature reaches over 1700 K. The surface desilication of three fully hydroxylated β-cristobalite structures in the presence of methanol via the formation of tetramethyl orthosilicate was also studied. Compared with the fully hydroxylated C(100) and C(101) surfaces in which the surface desilication is thermodynamically and kinetically hindered, the fully hydroxylated C(011) surface is unstable with the plausible desilication process.