Hydrogen-Bond Bifurcation as the Key Mechanism of Pyridine Surface Diffusion on MCM-41

Loss of linearity significantly weakens hydrogen bonds. However, this energy penalty can be partially compensated by the formation of a second hydrogen bond. Theoretical calculations presented in this work demonstrate that such bifurcated hydrogen bonds play a key role in the surface diffusion of adsorbed molecules. In particular, for a model MCM-41 silica surface with characteristics consistent with experimental observations, the activation barrier for pyridine diffusion via the bifurcated hydrogen-bond mechanism is approximately 20 kJ/mol, which is significantly lower than the desorption energy of pyridine from the silica surface (>50 kJ/mol). Owing to the low density of surface silanol groups, MCM-41 approximates a limiting case in which the diffusion of pyridine on the silica surface requires overcoming an exceptionally high energy barrier. For most amorphous silica materials, the density of surface silanol groups is higher, and surface diffusion is therefore expected to dominate completely over desorption at room temperature when the surface coverage of pyridine is significantly below monolayer saturation.