The control of adhesion at metal/oxide interfaces is of key importance in modern applications, whenever three-dimensional metal clusters or two-dimensional metal overlayers are to be synthesized on an oxide support. By focusing on the zinc/alumina system, we address here one of the long-standing issues in this context, which is the poor wetting of wide bandgap oxides by noble and post-transition metals. It has recently been recognized to have detrimental industrial consequences for the adhesion of anti-corrosive zinc coatings to new high strength steels grades. We have combined photoemission, thermal desorption and plasmonics with atomistic simulation to describe the energetics of zinc deposits on dry and hydroxylated α-Al 2 O 3 (0001) surfaces. Both experimental and computational results show that an activated reaction of the metal with the OHcovered surface, followed by hydrogen desorption, produces dispersed interfacial moieties involving both oxidized Zn species and undercoordinated oxygen ions, that lead to a significant improvement of adsorption/adhesion characteristics on the hydroxylated surface. In particular, the key role of interfacial undercoordinated anions, remnants of the hydroxylation layer, is highlighted for the first time, pointing to a general mechanism by which surface hydroxylation appears as a promising route towards a systematic improvement of wide band gap oxide wetting by metals. 1
The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H 2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10 −8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O 3C) and Mg at inverse corners (Mg IC) were identified to be the most reactive and to split H 2 irreversibly already in the lowest pressure regime (P H 2 < 10 −3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm −1) in the intermediate range of pressures (10 −3 −1 mbar), along with the appearance of bands at 3605/1225 cm −1 , was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At P H 2 > 1 mbar, fully reversible adsorption was shown to occur at O 4C (either on mono-or diatomic steps) and Mg 3C sites. Another OH/MgH couple (3697/1030 cm −1) that became reinforced at high P H 2 but remained stable upon pumping was correlated to O 3C and Mg IC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H 2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H 2 and other hydrogen-containing molecules, such as CH 4 , on oxide surfaces, but also for the advancement of hydrogen-storage technologies.
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