Number of Outer Electrons As Descriptor for Adsorption Processes on Transition Metals and Their Oxides
Federico Calle-Vallejo, Nilay G. Inoglu, Hai-Yan Su, José I. Martínez, Isabela C. Man, Marc T. M. Koper, John R. Kitchin and Jan Rossmeisl, “Number of outer electrons as descriptor for adsorption processes on transition metals and their oxides”, Chemical Science 4(3), 1245-1249 (2013). https://doi.org/10.1039/c2sc21601a
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Question
For transition metals and their alloys, the d-band model explains trends in adsorption energies, but no comparable general framework existed for transition metal oxides, where adsorption depends on the metal identity, its oxidation state, coordination and ligand arrangement. Nor was there a link between the adsorption behavior of a metal and that of its oxides. The paper asks whether one simple electronic-structure quantity can capture the trends in adsorption of the oxygen reduction and evolution (ORR and OER) intermediates *O, *OH and *OOH across 3d metals, monoxides and perovskites, and whether it explains the known scaling relations among those intermediates (Calle-Vallejo et al. 2013).
Approach
The descriptor is the number of “outer” electrons, meaning the valence electrons left on the metal atom after oxidation (for example 6 for metallic Cr, 4 for Cr2+ in CrO, 3 for Cr3+ in LaCrO3 and 2 for Cr4+ in SrCrO3). The authors computed adsorption energies with the plane-wave code Dacapo, ultrasoft pseudopotentials and the RPBE functional on four-layer slabs (bottom two layers fixed), with cutoffs of 400 eV for perovskites and 450 eV for monoxides and metals, a 4 x 4 x 1 k-point grid and spin polarization for all oxides and for the magnetic metals. Binding energies changed by less than 0.05 eV between four and six layers for selected perovskites. Two case studies were run: varying the 3d metal (Sc to Cu) within a compound class (metals at oxidation state 0, rock-salt monoxides MO at +2, La/Y perovskites with M at +3, and Ca/Sr/Ba perovskites with M at +4), and varying the oxidation state of a single metal (Ni). A third case in the supporting information, on a Mn2O3 (110) surface with fixed stoichiometry, uses coordination number to capture trends (Calle-Vallejo et al. 2013).
Key findings
Within every compound class, adsorption of *O, *OH and *OOH weakens systematically as the number of outer electrons increases across the 3d series, consistent with the d-band picture for metals and, for octahedrally coordinated oxides, with the nonbonding or antibonding character of the eg and t2g levels. The slope for *O is about twice those for *OH and *OOH, and the latter two run roughly parallel, which accounts for the scaling of *OH and *OOH against *O with slope about 0.5 and of *OOH against *OH with slope about 1. Closed-shell compounds (LaScO3, YScO3, SrTiO3, CaTiO3, BaTiO3 and CaO) are exceptions: they are inert because their ions satisfy the octet or noble-gas configurations (Calle-Vallejo et al. 2013).
For Ni compounds, raising the formal oxidation state weakens adsorption. The authors explain this through surface Ni atoms with five oxygen neighbors on the (100) surfaces: adsorbates complete the octahedron, so forming the bond requires the cation to give up electrons, and a more oxidized cation has fewer to give (Calle-Vallejo et al. 2013).
Combining both trends gives an adsorption-energy grid for *OH as a function of outer electrons, with positive-slope lines along the 3d series and negative-slope lines across oxidation states, both of the same magnitude and crossing at integer electron counts. The mean absolute error between the grid lines and DFT values is 0.20 eV. La and Y perovskites with the same M site adsorb similarly, as do Ca, Sr and Ba perovskites, suggesting that strain from lattice constant differences and the identity of the A cation are secondary. Overlapping two grids yields a single scaling line; the intercept of the *OOH versus *OH relation is 3.2 eV, which the authors connect to an intrinsic ORR-OER overpotential (Calle-Vallejo et al. 2013).
The authors note limitations: large adsorbate-induced reconstructions can break linearity, oxygen vacancies were not included, and DFT+U was not used, although they cite evidence that trends and scaling would persist with +U even if absolute values shift (Calle-Vallejo et al. 2013).
Significance
The paper offers a single, chemically intuitive descriptor that unifies adsorption on transition metals and their oxides and rationalizes ORR-OER scaling relations as a consequence of shared electronic-structure trends. The grid is proposed as a predictive tool for doped oxides and oxide mixtures, and the authors frame catalyst reactivity as driven by the tendency of each component to satisfy simple electron-counting rules (Calle-Vallejo et al. 2013).