Bonding and reactivity at oxide mineral surfaces from model aqueous complexes

The kinetic stability of oxide surfaces affects a broad range of physical phenomena, including mineral dissolution and sorption reactions, stable-isotope fractionation, and catalyst support degradation. Our knowledge of the rates of these processes derives mostly from the rates of net mass transfer between the bulk solid and fluid phases. But from such data it is difficult to determine rates of elementary steps that are needed to test theoretical models. Here we determine the rates of oxygen exchange between an aqueous fluid and specific sites on the 'Al₁₃' polyoxocation- AlO₄Al₁₂(OH)₂₄(H₂O)₁₂/⁷⁺-the structure of which closely resembles the surfaces of some Al-(hydr)oxide minerals in soils and catalyst supports. Extrapolation of these data to 298 K (and near pH 5.3) yields half-lives for oxygen on the complex that range from ~0.6 milliseconds for bound water to 41 seconds and 13 hours for the two distinct, but structurally similar, bridging hydroxyls. This surprisingly large range of labilities (~10⁷) indicates that reactivity is very sensitive to molecular structure. Moreover, these results indicate that well chosen aqueous complexes provide important information to relate bonding to reactivity at mineral surfaces.