IR studies of CO and NO adsorbed on well characterized oxide single microcrystals

A systematic investigation of the surface morphology and of the vibrational properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, alpha-Cr2O3, alpha-Al2O3, MgAl2O4 and other spinels, TiO2, ZrO2 and other oxides of a similar structure) with regular crystalline habit and exposing thermodynamically stable and neutral faces, is presented with the aim to elucidate the spectroscopic manifestations of CO and NO adsorbed on well defined crystallographic positions. In particular the structure of CO and NO adsorbed on the cationic sites of extended faces of these model solids is presented and discussed with the aim of elucidating the nature of the Me(x+)... CO/NO bond (Me(x+) = non transition metal ion or transition metal ion). When non transition metal ions are involved, the molecule-cation interaction is predominantly electrostatic. This leads to an increase of the CO stretching frequency, which is roughly proportional to the polarizing field. On the contrary, when transition metal ions are involved, beside the predominant electrostatic interactions, a small contribution to the bond stability comes also from d-pi overlap forces, which, although not very important from the energetic point of view, greatly influence the static and dynamic dipoles localized on the adsorbed molecules. Consequently, the strength of the dipole-dipole interactions occurring in the ordered adlayers of CO and NO adsorbed on transition and non transition metal oxide surfaces are resulted remarkably different. On these well defined surfaces, the effects influencing the half-width (FWHM) of the CO and NO stretching peaks have also been considered. It has been calculated that the FWHM is a very sensitive parameter of the surface perfection. In a few cases (ZnO, alpha-Cr2O3, etc.) FWHM values comprised in the 1.5-3.7 range have been obtained, which are indicative of a single-crystal quality of the exposed faces. These spectroscopic results were compared with those obtained with quantum calculations. Finally the activity towards CO and NO of perfect, low index faces and of more defective situations (like those associated with edges, seeps and corners) are compared, in order to have a better insight on the role of surface defectivity in catalytic reactions.