Preparation and characterization of MgO powders having Ca 2+ or Ba 2+ as surface dopants

Abstract

The tendency of large dopant cations towards surface segregation has here been utilised to prepare high surface area MgO powders carrying Ca 2+ or Ba 2+ as surface impurities in amounts equivalent to 0.2 to 5 monolayers of CaO or BaO. Computational approaches had pointed to possibilities for reconstruction of such impurity monolayers into “rumpled” arrangements as a means of minimizing their energies: the rumpled arrangement favoured for Ca 2+/MgO would have alternate oxygen anions displaced outward from the original (100) plane, whereas that favoured for Ba 2+/MgO would involve large outward displacement of alternate Ba 2+ ions. Ions displaced in that way would necessarily feature lower coordination to counter-ions than species on non-rumpled surfaces, and hence could be expected to exhibit higher surface reactivity and/or catalytic activity. This paper presents results of experiments aimed at: (i) preparation of powdered MgO materials having monolayer amounts of CaO or BaO segregated upon their surfaces; and (ii) testing whether such monolayer-doped Ca 2+/MgO and Ba 2+/MgO materials exhibit physical and/or chemical properties indicative of the “rumpling” favoured by computations. Chemical comparisons made between surface reactivities of Ca 2+/MgO and Ba 2+/MgO versus the pure MgO support include: lability of surface oxide ions, as manifested in ease of heterophase oxygen isotope exchange at 623–873 K, and relative activities for oxidative coupling of methane at 923–958 K. Physical comparisons include: extent and spectral distribution of surface-sensitive luminescence; BET surface areas; X-ray diffraction and scanning electron microscopy.