Abstract
An attempt has been made to analyze the effect of surface site on the spin state for the interaction of NO with Pd2, Rh2 and PdRh nanoparticles that supported at regular and defective MgO(001) surfaces. The adsorption properties of NO on homonuclear, Pd2, Rh2, and heteronuclear transition metal dimers, PdRh, that deposited on MgO(001) surface have been studied by means of hybrid density functional theory calculations and embedded cluster model. The most stable NO chemisorption geometry is in a bridge position on Pd2 and a top configuration of Rh2 and PdRh with N-down oriented. NO prefers binding to Rh site when both Rh and Pd atoms co-exist in the PdRh. The natural bond orbital analysis (NBO) reveals that the electronic structure of the adsorbed metal represents a qualitative change with respect to that of the free metal. The adsorption properties of NO have been analyzed with reference to the NBO, charge transfer, band gaps, pairwise and non-pairwise additivity. The binding of NO precursor is dominated by the E(i)Mx-NO pairwise additive components and the role of the support was not restricted to supporting the metal. The adsorbed dimers on the MgO surface lose most of the metal-metal interaction due to the relatively strong bond with the substrate. Spin polarized calculations were performed and the results concern the systems in their more stable spin states. Spin quenching occurs for Rh atom, Pd2, Rh2 and PdRh complexes at the terrace and defective surfaces. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support and dimer-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals and dimers. Although the interaction of Pd, Rh, Pd2, Rh2 and PdRh particles with Fs sites is much stronger than the regular sites O2-, the adsorption of NO is stronger when the particular dimers are supported on an anionic site than on an Fs site of the MgO(001). The encountered variations in magnetic properties of the adsorbed species at MgO(001) surface are correlated with the energy gaps of the frontier orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and the role of precursor molecules on the magnetic and binding properties of complexes need to be explicitly taken into account.
Highlights
Fundamental understanding of the electronic structure and activity of transition metal atoms and nanoclusters supported on metal-oxide surfaces is of great interest due to their broad applications in catalysis, coating for thermal applications, corrosion protection, and other technologically important fields [1]-[4]
The interaction of Rh at MgO (001) surface induces a quenching of the magnetic moment, which results in a doublet ground state, separated by 0.267 eV from the lowest quartet
Upon interaction with O2− and Fs surface sites, the high to low spin transition energies ∆EcHo−mLplex of Pd atom is positive indicating that the spin states are preserved and the low spin states are favored
Summary
Fundamental understanding of the electronic structure and activity of transition metal atoms and nanoclusters supported on metal-oxide surfaces is of great interest due to their broad applications in catalysis, coating for thermal applications, corrosion protection, and other technologically important fields [1]-[4]. It has been found that under typical conditions, formation of dimers constitutes the first step in the process of the growth of metal clusters on the oxide surface [7]. Even though in the gas phase there are dimers, trimers, etc., the cluster growth on the surface of the support is dominated by diffusion of adsorbed atoms and not by direct deposition of already existing gas phase clusters. The properties of the deposited nanoclusters depend on the oxide substrate and in particular on the presence of point defects where the cluster can be stabilized. There has been a consensus that defects can act as catalytic centers for chemisorption of small species and as nucleation centers for growing metal clusters and can modify the catalytic activity of these adsorbed metal particles via the metal-support interaction at the interface [9]
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