Oxygen adsorption onto pure and doped Al surfaces – the role of surface dopants

Abstract

Using density functional theory (DFT) with the PBE0 density functional we investigated the role of surface dopants in the molecular and dissociative adsorption of O2 onto Al clusters of types Al50, Al50Alad, Al50X and Al49X, where X represents a dopant atom of the following elements Si, Mg, Cu, Sc, Zr, and Ti. Each dopant atom was placed on the Al(111) surface as an adatom or as a substitutional atom, in the last case replacing a surface Al atom. We found that for the same dopant geometry, the closer is the ionization energy of the dopant element to that of elemental Al, the more exothermic is the dissociative adsorption of O2 and the stronger are the bonds between the resulting O atoms and the surface. Additionally we show that the Mulliken concept of electronegativity can be applied in the prediction of the dissociative adsorption energy of O2 on the doped surfaces. The Mulliken modified second-stage electronegativity of the dopant atom is proportional to the exothermicity of the dissociative adsorption of O2. For the same dopant element in an adatom position the dissociation of O2 is more exothermic when compared to the case where the dopant occupies a substitutional position. These observations are discussed in view of the overlap population densities of states (OPDOS) computed as the overlap between the electronic states of the adsorbate O atoms and the clusters. It is shown that a more covalent character in the bonding between the Al surface and the dopant atom causes a more exothermic dissociation of O2 and stronger bonding with the O atoms when compared to a more ionic character in the bonding between the dopant and the Al surface. The extent of the adsorption site reconstruction is dopant atom dependent and is an important parameter for determining the mode of adsorption, adsorption energy and electronic structure of the product of O2 adsorption. The PBE0 functional could predict the existence of the O2 molecular adsorption product for many of the cases investigated here.

Adsorption of Hydrogen Sulfide, Hydrosulfide and Sulfide at Cu(110) ‐ Polarizability and Cooperativity Effects. First Stages of Formation of a Sulfide Layer.

“…In general, bonding accompanied by larger charge transfer implies less geometric degrees of freedom in the overlap between orbitals of the adsorbate with surface states when compared to smaller transfers of charge. [41,42] For larger transfers of charge, less surface sites obey the symmetry criteria that are required for a favorable overlap with the orbitals of the adsorbate. This makes the reaction become more surface site sensitive when compared with adsorption that is accompanied with smaller charge transfers.…”

Surface chemistry of oxygen on aluminum-Performance of the density functionals: PBE, PBE0, M06, and M06-L

“…The levels of theory here employed could not predict the molecular adsorption product for some of the dopants. This issue has been discussed in our previous work that employed these models together with the PBE0 functional and the same arguments valid for this functional there can be extended to the other functionals here used . In Table can be seen that for the same dopant and same geometry, the larger difference between the least and the most exergonic adsorption is of 0.519 eV for the surface with the Ti adatom dopant.…”

“…The details on the construction and benchmark of the cluster models used here can be found in one of our previous works . Overall, the choice of cluster models was based on the fact that these can produce adsorption data with good accuracy, provided that only low coverages of adsorbate are modeled .…”

“…Care should be taken when issuing these statements. Additionally, the presence of other chemical elements at the surface of Al is known to affect the interactions of O 2 with these surfaces and change the dissociation barriers for O 2 …”

“…Additionally, the presence of other chemical elements at the surface of Al is known to affect the interactions of O 2 with these surfaces and change the dissociation barriers for O 2 . [20,21] Even though computing the energy barrier for molecular adsorption of O 2 is challenging for electronic structure methods that rely on single-determinant theory, after the O 2 molecule is adsorbed-and the (S 3 ) ! (S 1 ) spin state transition in O 2 already occurred-the chemistry of the system O 2 -metalsurface can be described with good accuracy using density functional theory (DFT).…”