Adsorption on rCr 2 O 3 ( 101 h 2 ) : Probing the Lewis Basicity of Surface Oxygen Anions

Acid/base characterizations of metal oxide surfaces are used often to explain their catalytic behavior. BF 3, a strong Lewis acid, is used in this investigation as a probe molecule to interrogate the basicity of well-defined Cr2O3 (101 h2) surfaces. BF 3 clearly probes differences in oxide ions of different coordination on Cr 2O3 (101 h2). The heat of adsorption of BF 3 shows that terminal chromyl oxygens are stronger Lewis bases than threecoordinate oxygen anions on the nearly stoichiometric surface. The trends in basicity found with BF 3 are the opposite of those found from the heat of adsorption of CO 2, a common probe molecule used to investigate the basicity of oxide surfaces. The use of BF 3 as a probe molecule is complicated by some dissociation and the slow build up of surface boron and fluorine during consecutive thermal desorption runs.