CO adsorption on small Aun (n = 1–4) structures supported on hematite. I. Adsorption on iron terminated α-Fe2O3 (0001) surface

Abstract

This is the first of two papers dealing with the adsorption of Au and formation of Aun nanostructures (n = 1-4) on hematite (0001) surface and adsorption of CO thereon. The stoichiometric Fe-terminated (0001) surface of hematite was investigated using density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) form with Hubbard correction U, accounting for strong electron correlations (PBE+U). The structural, energetic, and electronic properties of the systems studied were examined for vertical and flattened configurations of Aun nanostructures adsorbed on the hematite surfaces. The flattened ones, which can be viewed as bilayer-like structures, were found energetically more favored than vertical ones. For both classes of structures the adsorption binding energy increases with the number of Au atoms in a structure. The adsorption of Aun induces charge rearrangement at the Aun/oxide contact which is reflected in work function changes. In most considered cases Aun adsorption increases the work function. A detailed analysis of the bonding electron charge is presented and the corresponding electron charge rearrangements at the contacts were quantified by a Bader charge analyses. The interaction of a CO molecule with the Aun nanostructures supported on α-Fe2O3 (0001) and the oxide support was studied. It is found that the CO adsorption binding to the hematite supported Aun structures is more than twice as strong as to the bare hematite surface. Analysis of the Bader charges on the atoms showed that in each case CO binds to the most positively charged (cationic) atom of the Aun structure. Changes in the electronic structure of the Aun species and of the oxide support, and their consequences for the interactions with CO, are discussed.

Adsorption of gold subnano-structures on a magnetite(111) surface and their interaction with CO

“…The binding energy of the Au atom in site A is smaller than that determined in our previous work (1.98 eV), 29 which was obtained within PW91+U and a lower value of U eff = 3.61 eV. It is also worth noting that the binding of the Au monomer to iron terminated Fe 3 O 4 (111) is by about 0.6 eV stronger than that to the Fe-rich termination of the α-Fe 2 O 3 (0001) surface, 20 and the binding of Au to the Fe atom of the FeO/Pt(111). While all three E ad values are of similar magnitude, their quantitative comparison is hin- dered due to different GGA functionals and unit cells applied.…”

“…However, anticipating the discussion presented in section 3.2, let us note that the CO binding is strongest to Au atoms adsorbed in sites D. Interestingly, for the Au monomer adsorbed on the clean Fe oct terminated (001) of magnetite, the binding energy is more than doubled, 51 compared to that presented here for the (111) surface. 20 The distance between the adsorbed Au#1 atom and the surface Fe atom is increased compared with that of the Au monomer (2.46 Å), both for the vertical (2.53 Å) and tilted (2.65 Å) dimers. 66 The Au dimer was constructed in two different ways.…”

“…The reason of this discrepancy is not clear. 20 The adsorption bond strength per Au atom of a tilted dimer, where each of the Au atoms binds directly to the oxide atoms, is distinctly higher (2.17 eV) than that of a Au monomer, as well as that of the vertical dimer. It is also worth noting that the binding of the Au monomer to iron terminated Fe 3 O 4 (111) is by about 0.6 eV stronger than that to the Fe-rich termination of the α-Fe 2 O 3 (0001) surface, 20 and the binding of Au to the Fe atom of the FeO/Pt(111).…”

“…To take account of the strong Coulomb interactions between 3d electrons localized on the Fe ions an additional Hubbard term U was included (DFT+U), using the rotationally invariant approach of Dudarev et al. 18,20,59 To represent electron-ion core interactions the projector augmented wave (PAW) potentials 60,61 were employed with Fe 3d 7 4s 1 , O 2s 2 2p 4 , Au 5d 10 6s 1 , and C 2s 2 2p 2 states considered as valence states. 56 Since the Fe 3 O 4 (111) and α-Fe 2 O 3 (0001) phases have been reported to coexist on the reconstructed hematite (0001) surface, 57,58 the value of U eff should be transferable between the differ-ent valence oxides.…”

“…20 This study concentrates on interaction of similar clusters with the Fe 3 O 4 (111) surface, in order to understand how the oxide support affects the structure and charge state of deposited gold adparticles, and their interaction with CO molecules. 20 This study concentrates on interaction of similar clusters with the Fe 3 O 4 (111) surface, in order to understand how the oxide support affects the structure and charge state of deposited gold adparticles, and their interaction with CO molecules.…”