Investigation of the Ring Size Dependence of Amorphous Silica Hydrolysis

Amorphous silica has attracted great interest as a support for transition metal catalysts. The qualities of these surfaces as catalyst supports can depend strongly on the structure and chemical functionality of the interface. However, our knowledge of the atomic-level heterogeneity of the surface, e.g., surface roughness, short-range structural disorder, and distribution of silanol groups, is limited, making it difficult to rationalize the catalytic activity of amorphous silica-supported catalysts. Here, the reaction energies for hydrolysis reactions that create surface silanols through functionalization of strained siloxane rings are investigated using density functional theory (DFT) calculations with periodic boundary conditions. The hydrolysis energies display broad distributions that depend on ring size. On average, reducing the ring strain by enlarging the ring increases its stability, leading to a shift in the most probable hydrolysis energy from around −190 kJ/mol for two-membered rings to ∼20 kJ/mol for four-membered rings. These results provide molecular-level insight into how structural disorder influences silanol formation on amorphous silica surfaces, with implications for understanding and tuning silica-supported catalysis.