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Issue 7, 1993
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Adsorption of hydrogen on highly dispersed MgO


Special production techniques enable the preparation of highly dispersed MgO with a specific surface area up to 400 m2 g–1. This results in a significant accumulation of highly reactive, low-coordinated surface centres which are related to two fundamentally different processes of H2 chemisorption on neat MgO: the homolytic and the heterolytic dissociation of H2. The homolytic process is initiated by the presence of O and Mg+ radicals which are created by thermal activation of the oxide sample. This gives rise to an irreversible formation of surface OH groups and to a reversible formation of bridged hydrides (Mg3H). On the other hand the heterolytic dissociation of H2 occurs in the course of a reversible process on the Mg3C2+O3C2– ion pairs (3C indicates three-coordinated). The stabilization of the resulting OH groups necessarily implies the formation of hydrogen bonds with O2– surface anions as proton acceptors. The corresponding hydride groups are highly reactive with respect to physisorbed O2. The related surface redox reaction initiates the formation of O2 ions and of OH groups incorporating oxygen which comes from the MgO surface. An additional type of surface reaction is observed on partially hydroxylated MgO. The admission of D2 initiates H/D isotope exchange reactions involving single and multiple-coordinated surface OH groups. They are observed on time-scales of hours and seconds, respectively.

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Article type: Paper
DOI: 10.1039/FT9938901101
Citation: J. Chem. Soc., Faraday Trans., 1993,89, 1101-1107
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    Adsorption of hydrogen on highly dispersed MgO

    E. Knözinger, K. Jacob and P. Hofmann, J. Chem. Soc., Faraday Trans., 1993, 89, 1101
    DOI: 10.1039/FT9938901101

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