Role of Hydroxyl Groups in the Basic Reactivity of MgO: a Theoretical and Experimental Study

Clean (i.e., carbonate- and hydroxyl-free) and hydroxylated MgO surfaces of various morphologies were used to study the correlation between thermodynamic Brønsted basicity (evaluated by the deprotonation of methanol and propyne followed by FTIR) and basic reactivity (evaluated by the conversion of 2-methyl-but-3-yn-2-ol (MBOH)). A correlation is evidenced for clean surfaces, but for hydroxylated surfaces, OH groups, though weak Brønsted bases, are much more active than oxide O2- ions. Some of the hydroxyl groups whose VOH infra-red stretching band is included in the narrow high-frequency OH band were shown to be responsible for such activity. In order to identify them, the irregularities of surfaces involving low coordination (LC) ions were modeled by DFT calculations (mono- and di-atomic steps for 4C ions, corners, kinks and divacancies for 3C ions). All of them dissociate water: except for kinks and divancancies where the OH formed can be considered as isolated, in all the other cases, OH groups are H-bonded. The í OH infra-red frequencies obtained by calculations are consistent with the experimental spectrum which exhibits two domains: the broad low-frequency (${\rm V}_{\rm OH} 3700$ cm-1) corresponds to isolated OH (O2CH, O3CH and O4CH in kinks and divacancies) and H-bond acceptor OH groups (O1CH and O2CH). The latter are inferred to be implied in basic reactivity of hydroxylated surfaces toward MBOH