Adsorption of Benzene on Hematite Surfaces: A van der Waals corrected DFT Study

FUENTE SILVIA A; CAROLINA ZUBIETA; FERULLO, RICARDO M.; AQUINO LINAREZ, LUIS G.; PATRICIA G. BELELLI

Santa Fe

Congreso; VI San Luis Conference; 2018

UNlitorlal

Iron oxides exist as several distinct phases in the environment. Corundum-type hematite (α-Fe2O3) is the most common and often the final form of transformations of other iron oxides. Understanding the interaction of pollutants with mineral surfaces is important because this adsorption process influences the transport and activity of contaminants on soils, sediments and water. In this work, we have studied the interaction of benzene with clean and hydroxylated surfaces of hematite using a quantum-chemical method.It is known that the standard density functional theory (DFT) fails in describing van der Waals (vdW) or dispersion interactions. Since vdW forces govern the interaction of benzene with surfaces, a DFT functional that includes self-consistently vdW contributions was employed. We have chosen the optB86b functional which has been used previously for the adsorption of benzene on metal surfaces1. This approximation is combined with a Hubbard-Hamiltonian correction (U = 4 eV) to improve the description of the Coulomb repulsion of the Fe-3d electrons. The optimized unit cell parameters of the bulk were a = b = 5.041 and c = 13.765 Å, in excellent agreement with the most widely accepted experimental values, a = 5.04 Å and c = 13.75 Å. In bulk hematite, anion and cation layers are ordered following the ???FeFeO3FeFeO3??? sequence in the c direction, i.e., along [0001]. Hematite exhibits an antiferromagnetic ground state with alternating FeFe double layers of spin-up and spin-down electrons along this direction.The (0001) surface is the dominant face in natural and synthetic hematite. There are different ways of finishing this surface. The single Fe-terminated (Fe-O3-Fe-Fe-O3.....) is the most stable under UHV conditions. Under water exposure, hydroxylated surfaces are formed, being the most stable that one composed by a layer of hydroxyl groups on an Fe bilayer ((OH)3-Fe-Fe-O3-....)2. We have modeled both surfaces with symmetric slabs. The corresponding 2 × 2 cells contain 80 and 116 atoms for the clean and hydroxylated surfaces, respectively. A vacuum gap in z-direction of approximately 15 Å was employed. Reciprocal space was described by using a (2 × 2 × 1) Monkhorst?Pack k-point grid.On the clean Fe-terminated (0001) surface, benzene adsorbs strongly in two different modes with the same adsorption energy (-1.05 eV). In both, the molecule is almost parallel to the surface and they only differ on the way they are oriented. In both cases, one C atom of benzene is located on top over a protruding Fe ion (with C-Fe distances of 2.37 and 2.40 Å, respectively) and with the molecule slightly tilted (around 6) with respect to the surface. These geometries are quite different to those ones obtained using empirical vdW corrections3. In comparison, the value of the adsorption energy calculated here using optB86b for benzene/hematite is larger in magnitude than those computed for benzene adsorption on Cu(111), Ag(111) and Au(111) using the same functional1. On the hydroxylated hematite surface, the benzene molecule adsorbs more weakly, with an adsorption energy of -0.53 eV. In this case, the aromatic ring is oriented parallel to the surface with a closest C-H (of OH) distance of 2.49 Å. The results are analyzed by calculating atomic charges, magnetizations and projected density of states.