Probing possible structure sensitivity in the exchange of isotopic oxygen with the surface of MgO

• Published in: Journal of catalysis Vol. 234; no. 1; pp. 14 - 23
• Main Authors: Mellor, I.M., Burrows, A., Coluccia, S., Hargreaves, J.S.J., Joyner, R.W., Kiely, C.J., Martra, G., Stockenhuber, M., Tang, W.M.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 15.08.2005; Elsevier; Elsevier BV
• Subjects: Catalysis; Chemistry; Exact sciences and technology; FTIR; General and physical chemistry; Isotopic oxygen exchange; Magnesium oxide; Photoluminescence spectroscopy; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; UV–visible diffuse reflectance spectroscopy; UV–visible diffuse reflectance spectroscopy; Isotopic oxygen exchange; Magnesium oxide; FTIR; Photoluminescence spectroscopy; Infrared spectrometry; Heterogeneous catalysis; Oxygen; Isotope exchange; Photoluminescence; Structure; Reflectance; UV-visible diffuse reflectance spectroscopy
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2005.05.015

The possibility of structure sensitivity in the heterolytic exchange of isotopic oxygen with the surface of a series of magnesium oxide samples has been probed. Two general approaches have been adopted: (i) variation in particle size for MgO samples exhibiting the {100} termination plane and (ii) variation in termination plane by alteration of the preparation method. In the former case, the surface area normalised exchange rate is found to vary insignificantly for the samples tested, despite a variation in their surface areas of nearly an order of magnitude. In addition, at the temperatures employed, only a fraction of a monolayer of the available oxygen is found to exchange, and, once exchanged, sites can be re-exchanged back with very similar reaction kinetics. It is proposed that the mechanism of reaction may involve the formation and decomposition of intermediate ozonide species generated on reaction of gas-phase O 2 with surface O −, or is associated with the presence of ensembles of reactive low coordinate Mg 2+O 2− pairs. Studies involving samples prepared via different routes have demonstrated the order of activity to be {111} or {110} mean surface planes > {100} > {111}, suggesting a beneficial role for ensembles of low coordinate Mg 2+O 2− ion pairs. In addition, it has been found that samples prepared from hydroxide-based precursors exhibit activity in the exchange process involving two lattice ions (R 2). This has been rationalised in terms of the exchange of diatomic surface species, such as peroxide ions and/or superoxide ions.