Direct verification of incipient mechanochemical interaction between two metallic species mediated by OH groups

Incipient chemical interaction between Ca(OH)₂ and SiO₂ at the contact point of dissimilar solid particles under mechanical stress was examined by X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), solid state nuclear magnetic resonance (NMR), as well as conventional analytical tools. Rapid electronical interaction in the presence of active OH groups was detected by XPS as a chemical shift of the binding energy of Si2p and Ca2p electrons. Detailed analysis of FT-IR with H-D exchange and preadsorption treatments by pyridine and NH₃ proved to be a versatile tool to show that acid-base reaction is the main mechanism of Si-O-Ca complex formation. Appearance and survival of strong Brønsted acid site confirmed by IR also demonstrates the formation of Si-O-Ca bonds. ¹H-solid state NMR evidenced the increase in the acidity by polarization of the OH groups during milling. A radical mechanism, examined by EPR, also plays a significant role in the incipient interaction, together with above-mentioned electron pair mechanism.