HYDROXYL-GROUPS AS IR ACTIVE SURFACE PROBES ON MGO CRYSTALLITES

Highly dispersed MgO was produced under diverse non-equilibrium condit ions, among them those of chemical gasphase deposition. The resulting powders were then degassed in HV at temperatures up to 800-degrees-C. These procedures permit the production of MgO samples which strongly d iffer in both specific surface area and surface topography. Correspond ingly an enormous variety of surface sites is made available. Their di stribution was studied by FT-IR spectroscopy using OH groups as surfac e probes. A unique correlation could be established between the charac teristic absorption pattern in the OH stretching region on one hand an d the experimental conditions applied in the course of the production and of the pretreatment procedure on the other. This required an assig nment of the IR spectra in terms of coordination and hydrogen bonding of surface OH groups on MgO. The strategy of assignment involved adsor ption experiments with HX (X = OH, H, SH, NH 2), thermal desorption st udies, the evaluation of both the isotope effect nu(OH)/nu(OD) and the first overtone 2nu(OH) as well as ab initio calculations related to h ydrogenated and hydrated MgO clusters.