Adsorption and spin state properties of Cr, Ni, Mo, and Pt deposited on Li+ and Na+ monovalent cation impurities of MgO (001) surface: DFT calculations

Abstract

We have analyzed, by means of density functional theory calculations and the embedded cluster model, the adsorption and spin-state properties of Cr, Ni, Mo, and Pt deposited on a MgO crystal. We considered deposition at the Mg(2+) site of a defect-free surface and at Li(+) and Na(+) sites of impurity-containing surfaces. To avoid artificial polarization effects, clusters of moderate sizes with no border anions were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The interaction between a transition metal atom and a surface results from a competition between Hund's rule for the adsorbed atom and the formation of a chemical bond at the interface. We found that the adsorption energies of the metal atoms are significantly enhanced by the cation impurities, and the adsorption energies of the low-spin states of spin-quenched complexes are always more favorable than those of the high-spin states. Spin polarization effects tend to preserve the spin states of the adsorbed atoms relative to those of the isolated atoms. The metal-support interactions stabilize the low-spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. Spin quenching occurs for Cr and Mo complexes at the Mg(2+) site of the pure surface and at Li(+) and Na(+) sites of the impurity-containing surfaces. Variations of the spin-state properties of free metals and of the adsorption and spin-state properties of metal complexes are correlated with the energies of the frontier orbitals. The electrostatic potential energy curves provide further understanding of the nature of the examined properties.

Theoretical Approaches to Excited-State-Related Phenomena in Oxide Surfaces

“…Spin polarization was also found to be important for Ni on the Al-terminated α-Al 2 O 3 (0001) surface . Nevertheless, one must be aware that the presence of impurities at the surface may change this trend, enhancing the adsorption energy and the tendency toward the quenching of the spin of the adatom …”

“…433 Nevertheless, one must be aware that the presence of impurities at the surface may change this trend, enhancing the adsorption energy and the tendency toward the quenching of the spin of the adatom. 434 The case of reducible oxides such as TiO 2 or CeO 2 introduces some complexity, because, in this case, the adsorbed adatom or metal cluster can be oxidized by electron transfer to the substrate. This in turn is difficult to describe within the current DFT approaches, as discussed by Pacchioni for the bulk materials.…”

Computational screening of transition metal-doped CdS for photocatalytic hydrogen production

“…Moreover, please notice that the spinstates of TM doping on the support can be different depending on whether it interacts with a pure or impurity site and may affect the reactivity of metal atoms. For a detailed explanation, the reader is referred to the work by Shalabi et al 54 . To ensure the accuracy of the computational results, we first compared the parameters of the bulk system with experimental studies.…”

Spin transition energies of Cr in complexes and CO binding with Cr deposited on S2−and Se2−anion impurities of MgO (001) surface density functional theory calculations

“…In either case the surface reactivity is significantly modified by their presence. While the effect of cation impurities has been investigated in detail [3][4][5][6][7][8], very little is known about anion substitution. In the bulk, Pandey et al [9,10] calculated the excitation energies for S 2− -and Se 2− -doped MgO, and derived interatomic potentials for S 2− .…”