(49d) Elucidating Adsorption Mechanisms On Oxides By Comparing Adsorption On FCC Metals and Reduced B1 Monoxide Surfaces
In the past decade, we have been able to use density functional theory to connect adsorption on metal surfaces with their electronic structures. This has allowed us to use calculated electronic structures to both explain adsorption phenomenon and create predictive models to accelerate materials design. However, our understanding of oxides pales in comparison. This is in large part due to both their diverse atomic and complex electronic structures. In an effort to understand the relationship between oxides' electronic structure and their adsorption properties, we compare side by side the bulk thermodynamics and surface adsorption properties of 3d transition metals in fcc structure with their corresponding B1 transition metal monoxides. Understanding that the B1 monoxide structure constitutes two interpenetrating face centered cubic lattices of cations (metals) and anions (oxygen), we construct a thermodynamic path from fcc metals to B1 monoxides through the expansion of the metal lattice and the insertion of an oxygen fcc lattice. We then studied how these structural transformations affected both the adsorption energies and electronic structure. We found that, in general, the expansion of the lattice corresponds to a weakening of the bond, which maximizes for metals with half filled d-orbitals. Interestingly, the introduction of oxygen into the expanded lattice strengthens the bond, which also maximizes for metals with half filled d-orbitals. The net effect is that the trends for the adsorption on the (111) surface of fcc metals mirrors that of their corresponding (111) B1 monoxide surfaces. We also concluded that irrespective of the material, the key descriptor of the oxygen adsorption energy is the center of the adsorbate oxygen p-band, which we then used to interpret the relationship between adsorption energies and bulk electronic structures. In the case of the metal, the oxygen p-band center is inherently tied to the d-band center, but for the oxide, the oxygen p-band center is tied to the properties of the d-band bonding, non-bonding, and bonding orbitals that are created when oxygen is inserted into the expanded metal lattice. We discuss possible structural and electronic structure descriptors of reactivities on oxide surfaces.