We report on the unusual properties of oxides at the nanoscale, based on two examples. The first is related to H2 dissociation on MgO surfaces and ultrathin films [1]. The H2 molecule dissociates heterolytically on stepped MgO surfaces with formation of protons (OH groups), and hydride ions (MgH groups). Homolytic splitting on MgO, with formation of a pair of OH groups per adsorbed H2 molecule, is only possible on polar MgO(111) surfaces or under irradiation. On the contrary, homolytic splitting, with formation of adsorbed protons and transfer of the H2 electrons to the empty d states of transition metal ions is typical of reducible oxides. We demonstrate, based on DFT calculations, that homolytic splitting of H2 is the thermodynamically most favored dissociation mode if MgO(001) films of few atomic layers are deposited on a metal support and that the choice of the support is crucial.In the second example we will present DFT results on doping MgO and CaO thin films by substituting Mg2+ cations with transition metal (TM) cations like Cr (MgO) or Mo (CaO) [2]. The TM species replacing the di-valent Mg cation can assume higher oxidation states, e.g. Cr3+ or Mo4+ and the charge imbalance is compensated via formation of cation vacancies. The impurity ions introduce states in the gap of the oxide that can deeply modify its electronic properties. In particular, charge transfer from the d states of the TM impurity to gold atoms or nanoparticles deposited on the surface results in a complete change in shape and adsorption properties. 1)  H. T. Chen, L. Giordano, G. Pacchioni 2013  submitted2)  F. Stavale, X. Shao, N. Nilius, H.-J. Freund, S. Prada, L. Giordano, G. Pacchioni, J. Am. Chem. Soc. 2012 , 134, 11380.

Unusual chemistry of oxides at the nanoscale

246th National Meeting (2013)

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