A fundamental knowledge of the interaction of carboxylic acids, such as formic acid, with magnetite surfaces is of prime importance for heterogeneous catalysis and the synthesis of novel materials. Despite this, little is known about the atomic scale adsorption mechanisms. Here we show by in-situ surface X-ray diffraction that the oxygen rich subsurface cation vacancy reconstruction of the clean magnetite (001) surface is lifted by dissociative formic acid adsorption, reestablishing a surface with bulk stoichiometry. Using density functional theory, the bulk terminated, fully formic acid covered surface is calculated to be more stable than the corresponding clean, reconstructed surface. A comparison of calculated and experimental infrared bands supports the bidentate adsorption geometry and a specific adsorption site. Our results pave the way for a fundamental understanding of the bonding mechanism at carboxylic acid/oxide interfaces.

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We investigated structural changes of the magnetite (001) surface upon exposure to water vapor at pressures up to 10 mbar as well as upon exposure to atomic hydrogen using surface X-ray diffraction, scanning tunneling microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy. Following a full structural analysis, we found for both adsorbates that the surface is roughening on an atomic level, indicating significant iron diffusion at the surface and in the near-surface region at room temperature. We found that this process is accompanied for both adsorbates by a lifting of the subsurface cation vacancy reconstruction present after preparation under ultrahigh vacuum conditions. In the case of water vapor, the lifting process arises in a pressure range between 10 −4 and 10 −3 mbar. We also observe an enhanced reactivity of the atomic hydrogen exposed surface with carbon species from the residual gas even at ultrahigh vacuum conditions.