Theoretical Investigation on the Au-Anchor Site in Phosphate-Doped Au/Al₂O₃Catalysts

Abstract

In order to clarify the Au-anchor site in phosphate-doped Al2O3 and investigate the mechanism of Au-stabilization by phosphate, density functional theory (DFT) calculations were carried out. We calculated Au atom adsorption onto three model surfaces, such as sAl2O3 (a stoichiometric Al2O3 surface), fPO-Al2O3 (an Al2O3 surface fully covered by phosphate), and pPO-Al2O3 (an Al2O3 surface partially covered by phosphate). The calculated results showed that the order of stability for Au atom adsorption onto these three model surfaces is Au/fPO-Al2O3 > Au/pPO-Al2O3 > Au/sAl2O3. When Au atom adsorbs to a phosphate-phosphate bridge site of fPO-Al2O3 surface, the stabilization of Au adsorption onto the surface is especially large (the adsorption energy of Au is −3.16 eV). This high stability of Au at the phosphate-phosphate bridge site is due to negative charge transfer from Au to Al2O3 surface, which makes Au-O bond more stable. Hence, we have concluded that "phosphate-phosphate bridge sites" anchor Au.

Extent of Spin Contamination Errors in DFT/Plane-wave Calculation of Surfaces: A Case of Au Atom Aggregation on a MgO Surface

“…The highest occupied molecular orbital (HOMO) is a bonding orbital, and the lowest unoccupied molecular orbital (LUMO) is an anti-bonding orbital. The frontier orbitals do not change when Au 2 is adsorbed onto metal oxides such as MgO [35,43], Al 2 O 3 [18,57], and TiO 2 [58,59,60,61]. Hence, the HOMO and LUMO of Au 2 can be described by using the 6s orbital of 1 Au (φ1) and 6s orbital of 2 Au (φ2):φHOMO=normalA(φ1+φ2) φLUMO=normalB(φ1−φ2)…”

Au Atom Diffusions on Reduced and Cl-Adsorbed Rutile TiO₂(110) Surfaces: A DFT + <i>U</i> Study

“…To clarify the mechanism, we recently investigated the effects of Cl on interfaces between nanosized Au and stoichiometric, reduced, and Cl-adsorbed rutile TiO 2 (110) surfaces [22][23][24]. Recent studies showed that (1) the oxygen defect sites in the reduced TiO 2 surface adsorbs Cl atoms more strongly than Au atoms [22,23], (2) the presence of Cl on the oxygen defect site inhibits the charge transfer from the TiO 2 surface to the Au atom [22][23][24] (the charge transfer is important for Au anchoring [22,25]), (3) a threeor two-dimensional Au cluster interacts with the unsaturated oxygen (O [UC] ) and avoids the Cl in the Cl-adsorbed TiO 2 surface [24], (4) the Au aggregation reactions on the reduced TiO 2 surface are endothermic owing to the destabilization of the Au-Au bond and the strong interaction between Au and the oxygen defect site caused by the charge transfer [24], whereas (5) the Au aggregation reactions on the stoichiometric and the Cl-adsorbed TiO 2 surfaces are exothermic [24]. However, the effect of Cl on Au atom diffusion on the TiO 2 surface remains unclear.…”