Relating the electronic structure and reactivity of the 3d transition metal monoxide surfaces
Zhongnan Xu and John R. Kitchin, “Relating the electronic structure and reactivity of the 3d transition metal monoxide surfaces”, Catalysis Communications 52, 60-64 (2014). https://doi.org/10.1016/j.catcom.2013.10.028
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Question
For metals, simple electronic descriptors such as the d-band center and width explain and predict adsorption trends, which makes coarse computational screening of alloys possible. Oxides lack descriptors of comparable transparency, even though linear scaling relations, electron counting rules and correlations with oxygen p-band or metal eg and d-band properties hint that such descriptors exist. This short communication asks which features of the electronic structure of a rock-salt transition metal monoxide control how strongly it binds atomic oxygen, and how those features arise from the structural and chemical changes that turn a metal into its oxide (Xu 2014).
Approach
Inspired by the Gelatt, Williams and Moruzzi analysis of transition metal compounds, which split formation into an energy cost of lattice expansion and an energy gain from bonding to the non-metal, the authors break the path from an fcc metal to its rock-salt monoxide into two perturbations. First the fcc metal is isotropically expanded to the oxide’s volume per metal atom; then an interpenetrating fcc oxygen lattice is inserted. Six 3d metals were studied (Ti, V, Mn, Co, Ni, Cu), all constrained to fcc, together with TiO, VO, MnO, CoO, NiO and CuO in the rock-salt structure.
Calculations used VASP with PAW potentials, the PBE functional, a 520 eV cutoff and spin polarization in ferromagnetic and antiferromagnetic orderings (the paper does not mention a Hubbard U). Surfaces were symmetric six-layer slabs with 10 Å of vacuum, and dissociative oxygen adsorption energies per O atom were computed at top, fcc and hcp sites, with the Wang, Maxisch and Ceder correction of 1.36 eV for O2 overbinding. On the expanded metals only the adsorbate was relaxed. The oxide adsorption energy is then the metal adsorption energy plus an “expansion energy” (the change on expanding the lattice) plus an “oxidation energy” (the change on adding lattice oxygen). Surface d-band and adsorbate p-band centers were taken as the first moment of the projected density of states relative to the Fermi level (Xu 2014).
Key findings
Adsorption trends on the metals and on the monoxides are similar across sites: binding becomes weaker as the number of d electrons increases. The expansion energy is generally positive (it weakens the surface-oxygen bond) and the oxidation energy generally negative (it strengthens it), with consistent trends over the three sites. Both contributions grow in magnitude from the start to the middle of the 3d row and shrink again toward the end.
To link electronic structure to these energies, the authors used the average energy of the adsorbate oxygen p states. On metals this quantity correlates with the surface d-band center, consistent with the d-band model, and a single correlation between adsorbate p-state energy and adsorption energy covers metals, expanded metals and oxides alike, suggesting similar bonding on both kinds of surface. The outliers are expanded Mn and Co, whose lattices are stretched so much that the adsorbate bond resembles an isolated molecular bond.
Expansion affects the d band differently across the row. On early metals such as Ti it narrows the band and shifts its center down, weakening adsorption, consistent with earlier strain studies. On mid metals the tensile strain is so large (26% for Mn) that the d band splits into bonding and antibonding parts, with little hybridization with the oxygen p states and a large loss of binding. On late metals the band narrows; Ni binds more strongly, as earlier strain work predicts, but Cu binds more weakly, which the authors attribute to strains of 19% (Ni) and 17% (Cu), up to four times larger than in earlier Cu studies, that push adsorbate bonding states up and reduce hybridization.
Oxidation creates low-lying metal-oxygen hybridized states from lattice O p and metal d states. When these states are degenerate with the adsorbate bonding states of the expanded lattice, extra hybridization strengthens the surface-adsorbate bond; this is most pronounced for the mid and late monoxides, and the benefit decreases from mid to late as the hybridized states move up in energy. On TiO the hybridized states lie well below the adsorbate states, so little hybridization occurs and, combined with an upshift of the adsorbate states, adsorption is weakened (Xu 2014).
Significance
The decomposition gives a physical route from metal to oxide reactivity: lattice expansion and oxidation have separable, interpretable effects on the d band, and the adsorbate p-band energy works as a common descriptor across metals and oxides. The work points to the energy alignment between bulk metal-oxygen bonding states and adsorbate states as a key electronic feature for oxide reactivity, and was intended as a step toward d-band-like descriptors for oxide catalyst design. The analysis is limited to atomic O on hypothetical fcc metals and rock-salt monoxides computed with plain PBE, and details of the bulk calculations are in supporting information not included in the extracted text (Xu 2014).