An infrared spectroscopic investigation of the surface properties of magnesium aluminate spinel

Abstract

Magnesium aluminate (spinel) samples were prepared by decomposition of Mg, Al mixed oxalate and by thermal decomposition of a Mg(NO 3) 2-impregnated transition alumina. In the latter preparation, which yields samples more suitable for ir work, transformation to the spinel phase occurs at ~400 °C, as monitored by the XRD spectrum, and by a sudden shift of the surface OH band to ~3740 cm −1. Pyridine (Py), CO, and CO 2 were adsorbed in order to investigate the surface acidity and basicity. Py is most effective in revealing Lewis acidity, which is mostly ascribed to coordinatively unsaturated (cus) Mg IV ions, whereas CO and CO 2 σ-coordinate to only a few such sites. (The Roman numeral in Mg IV symbolizes tetrahedral coordination). CO 2 also adsorbs to give bicarbonates as well as monodentate and bidentate carbonates. Evidence is presented to show the importance of the octahedral coordination (Al IV) and of the degree of surface hydration with respect to surface oxygen basicity. The tetrahedral coordination is shown to be important for the formation of surface bicarbonates, which probably have a bridged structure. The partly inverted cation distribution of some samples was monitored by spectral studies in the surface OH group region and by the adsorption behavior of Py, CO, and CO 2.