Site-specific hydrogen reactivity of Zn0.05Mg0.95O nanopowders
Résumé
Here we present a combined theoretical and experimental study of hydrogen adsorption on Zn-functionalized MgO surface. We have systematically compared infrared spectra recorded at increasing hydrogen pressure (10−5–100 mbar) on rock salt Zn0.05Mg0.95O and MgO nanocubes. The interpretation of vibrations involving hydrogen was supported by ab initio calculations following the same strategy as for pure MgO. Despite the small Zn concentration, we unambiguously recorded Zn-specific signals. Most interestingly, we find that all chemisorption complexes characteristic of MgO surface reactivity remain highly preserved on Zn0.05Mg0.95O, while the effect of segregated Zn is strongly localized to its first oxygen neighbors. From the intensity variation of the IR couples when passing from spectra of pure to Zn-modified MgO, we (1) confirm the validity of the adsorption models that we previously proposed for MgO and (2) show that Zn segregates preferentially on low-coordination Mg surface sites (88% and 50% substitution at corners and step edges, respectively). This study demonstrates that hydrogen can serve as a specific probe for low-coordinated Zn surface sites on ZnxMg1-xO due to a higher H- affinity of Zn2+ than Mg2+ cations.
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