Silanol Functionalization of Amorphous Silica Surface from Density Functional Theory Simulations

Amorphous silicates are among the most widely used supports in heterogeneous catalysis, yet the development of accurate atomistically detailed models remains a major challenge. Their structure is composed of SiO 4 tetrahedral units arranged into siloxane rings of varying sizes, which gives rise to substantial surface heterogeneity. Although this heterogeneity plays a central role in determining surface reactivity, it also makes the experimental characterization of amorphous silica particularly difficult. Density functional theory (DFT) simulations offer a route to probe the properties of individual sites within this disordered network and to establish structure–reactivity relationships. In this work, we use large cluster models of amorphous silica treated at DFT level to investigate the thermodynamics of the surface silanol functionalization reaction. We find a broad distribution of functionalization energies across clusters, with significant variation even among sites with the same ring size, showing that reactivity cannot be described by ring size alone. To rationalize this, we construct smaller models of silica sites and analyze the functionalization energy as a function of effective model volume. Our results show that large-scale reorganization accompanies surface functionalization, which needs to be properly accounted for to obtain converged reaction energies.