The Role of the Synthesis Method in the Structure Formation of Cobalt Aluminate

Cobalt aluminate (spinel) was prepared via two synthesis routes firstly a co-precipitation method and secondly a nanocasting method. The surface chemistry of these materials was characterized by infrared-spectroscopy of the surface hydroxyl groups and of coordinatively unsaturated (cus) cations (Al and Co) by carbon monoxide CO at low temperature. The goal was to investigate whether or not the preparation of the spinel phase had an effect on this structural characteristics, namely on the inversion degree. The hydroxyl (deuteroxyl) spectra were characterized by six types of O–H stretching bands. While the O–H stretching frequencies were identical for both materials, the relative intensities of the bands were clearly different indicating a distinct distribution of the different types of hydroxyl groups which most likely originates from a distinct distribution of the cations in the two differently synthesized materials. The preparation method obviously yields spinel structures having variable degrees of inversion. Coordinatively unsaturated cus cations are exposed on the surface during the removal of O–H groups by thermal dehydroxylation. Those Lewis acid centers show an acid strength distribution which is again an indication for formation of two distinct degrees of inversion.