Heterogeneous Adsorption and Local Ordering of Formate on a Magnetite Surface

We report a novel heterogeneous adsorption mechanism of formic acid onthe magnetite (111) surface. Our experimental results and density functional theory (DFT)calculations give evidence for dissociative adsorption of formic acid in quasibidentate andchelating geometries. The latter is induced by the presence of iron vacancies at the surface,making oxygen atoms accessible for hydrogen atoms from dissociated formic acid. DFTcalculations predict that both adsorption geometries are energetically favorable under ourexperimental conditions. The calculations prove that the locally observed (√3 × √3)R 30°superstructure consists of three formate molecules in a triangular arrangement, adsorbedpredominantly in a chelating geometry. The results show how defects can stabilizealternative adsorption geometries, which is a crucial ingredient for a detailed atomisticunderstanding of reaction barriers on magnetite and other oxide surfaces, as well as for thestability of carboxylic acid based nanocomposite materials