Infrared surface characterization of AlF3

Abstract

Three specimens of crystalline AlF₃, coming from different preparation routes, have been examined: no major differences were found in the spectral region of the (ν₃) fundamental mode, whereas the preparation process was observed to introduce appreciable differences in the background spectra that were ascribed to different bulk and surface impurities. Oxidic islands exist both in the bulk and at the surface of all preparations and when AlF₃ is derived from non-oxygen-containing precursors. In the latter case, species containing NH_x moieties exist, both in the bulk and at the surface. The surface hydrated layer of AlF₃ is made up of an extensive ‘regular’ component (i.e. a large amount of undissociated water molecules, coordinated to unsaturated surface Al^VI cations), and of an ‘anomalous’ component (i.e. some OH groups that are eliminated upon vacuum activation at relatively low temperatures). The adsorption of pyridine on samples activated by vacuum treatment at temperatures in the range 300–673 K indicates that on all specimens there are variable amounts of ‘irregular’ surface cations [i.e. coordinatively unsaturated surface Al ions with (quasi-)tetrahedral coordination], some of which are strong enough as Lewis-acidic centres to adsorb CO at ambient temperature. Coordinatively unsaturated surface Al ions with the ‘regular’ octahedral coordination are very weakly acidic, as revealed by the end-on adsorption of CO₂ at 300 K and by CO uptake at ca. 78 K.