Hydroxyl groups as IR active surface probes on MgO crystallites

Abstract

Highly dispersed MgO was produced under diverse non-equilibrium conditions, among them those of chemical gasphase deposition. The resulting powders were then degassed in HV at temperatures up to 800°C. These procedures permit the production of MgO samples which strongly differ in both specific surface area and surface topography. Correspondingly an enormous variety of surface sites is made available. Their distribution was studied by FT-IR spectroscopy using OH groups as surface probes. A unique correlation could be established between the characteristic absorption pattern in the OH stretching region on one hand and the experimental conditions applied in the course of the production and of the pretreatment procedure on the other. This required an assignment of the IR spectra in terms of coordination and hydrogen bonding of surface OH groups on MgO. The strategy of assignment involved adsorption experiments with HX (X=OH, H, SH, NH₂), thermal desorption studies, the evaluation of both the isotope effect ∼ν_(OH)/∼ν_(OD) and the first overtone 2∼ν_(oh) as well as ab initio calculations related to hydrogenated and hydrated MgO clusters.