Electronic Properties and Reactivity of Simulated Fe³⁺ and Cr³⁺ Substituted α‑Al₂O₃ (0001) Surface

Metal oxide-based minerals naturally contain transition metal impurities isomorphically substituted into the structure that can alter the structural and electronic properties as well as the reactivity of these metal oxides. Natural α-Al₂O₃ (corundum) can contain up to 9.17% (w/w) Fe₂O₃ and 1.81% (w/w) of Cr₂O_3. Here we report on changes in the structural and electronic properties of undoped and doped α-Al₂O₃ (0001) surfaces using periodic density functional theory (DFT) methods with spin unrestricted B3LYP functional and a local atomic basis set. Both structural and electronic properties are altered upon doping. Implications for doping effects on photochemical processes are discussed. As metal oxides are major components of the environment, including atmospheric mineral aerosol, DFT was also used to study the effect of transition metal impurities on gas/surface interactions of a model acidic atmospheric gas molecule, carbon monoxide (CO). The theoretical results indicated that the presence of Fe³⁺ and Cr³⁺ impurities substituted on the outer layer of natural corundum surfaces reduces the propensity toward CO adsorption relative to the undoped surface. However, CO–surface interactions resemble that of bulk α-Al₂O₃ when the impurity is substituted below the first surface layer. The presence and location of the mineral dopant were found to significantly alter the structural and electronic properties and gas/surface interactions studied here