IR STUDIES OF CO AND NO ADSORBED ON WELL CHARACTERIZED OXIDE SINGLE MICROCRYSTALS

A systematic investigation of the surface morphology and of the vibrat ional properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, alpha-Cr2O3, alpha-Al2 O3, MgAl2O4 and other spinels, TiO2, ZrO2 and other oxides of a simila r structure) with regular crystalline habit and exposing thermodynamic ally 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 transit ion metal ion). When non transition metal ions are involved, the molec ule-cation interaction is predominantly electrostatic. This leads to a n increase of the CO stretching frequency, which is roughly proportion al to the polarizing field. On the contrary, when transition metal ion s are involved, beside the predominant electrostatic interactions, a s mall contribution to the bond stability comes also from d-pi overlap f orces, which, although not very important from the energetic point of view, greatly influence the static and dynamic dipoles localized on th e adsorbed molecules. Consequently, the strength of the dipole-dipole interactions occurring in the ordered adlayers of CO and NO adsorbed o n transition and non transition metal oxide surfaces are resulted rema rkably different. On these well defined surfaces, the effects influenc ing 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 sensit ive parameter of the surface perfection. In a few cases (ZnO, alpha-Cr 2O3, etc.) FWHM values comprised in the 1.5-3.7 range have been obtain ed, which are indicative of a single-crystal quality of the exposed fa ces. These spectroscopic results were compared with those obtained wit h quantum calculations. Finally the activity towards CO and NO of perf ect, low index faces and of more defective situations (like those asso ciated with edges, seeps and corners) are compared, in order to have a better insight on the role of surface defectivity in catalytic reacti ons.