Probing the surface atomic structure of Au/Cr₂O₃/Pd(111) by photoelectron diffraction

Abstract

A detailed investigation concerning the surface atomic structure of the Au/Cr 2 O 3 model catalyst deposited on a Pd(111) single crystal surface is presented. The system was prepared by molecular beam epitaxy (MBE) and characterized in situ by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). The element-specific short-range order information was obtained from XPD experiments supported by a comprehensive multiple scattering calculation diffraction approach. Based on the experiments, we have strong evidence of Au island formation on the Cr 2 O 3 surface.The experiments indicated that the islands are constructed of two Au monolayers and formed by the important structural relaxations in the three outermost atomic layers of the Au/Cr 2 O 3 surface. Such a surface structure could explain the particular catalytic reactivity displayed by catalysts based on Au nanoparticles dispersed on several oxide matrices.

Evidencing the formation of Pt nano-islands on Cr2O3/Ag(111)

“…44 Results and discussion 40 The non-formation of extra spots or CrystEngComm Paper superstructures in the LEED pattern denotes a pseudomorphic growth and the formation of Pt NIs on the oxide surface. The coherence length in the LEED measurements ranges from 10 to 20 nm, 6 and surface structures with a smaller size than these are not identified instantly by LEED. Therefore, in this work we considered the Pt(111)/Cr 2 O 3 (0001) interface structure during all XPD simulations, where the bulk oxide lateral distance values were used.…”

“…Research on NPs aiming at the enlightenment of their outermost layers, i.e., interlayer distances and packing, is essential for the improvement and development of advanced materials, for instance, more efficient and selective catalysts. [2][3][4][5][6][7] Nevertheless, it is a challenge to extract structural and electronic information straightforwardly via in situ experiments.…”

“…19 TMO supports may act to improve the NPs' catalytic performance, 20 although this factor is strongly linked to the catalyst synthesis route. 2,21,22 Particularly, Cr 2 O 3 is one example of TMO that is often used as a support for real catalysts in alkene dehydrogenation reactions, [23][24][25] or as a support for metallic NPs in model catalysts, 5,6 or investigated due to its magnetic properties when in the form of ultrathin films. 26,27 The Cr 2 O 3 crystalline structure is classified as belonging to the R3c space group.…”

“…For thicknesses up to 5 Å, a p(2 × 2) surface reconstruction structure is observed; however, for thicknesses above 10 Å, the (√3 × √3)R30°surface structure reconstruction is typically achieved in the α-Cr 2 O 3 phase. 5,6 The Pt/Cr 2 O 3 system has received growing attention lately since it was found to be a promising material for both catalytic and magnetic property investigations. [30][31][32][33][34][35] Kasikov et al 30 performed the deposition of Pt islands on Cr 2 O 3 followed by exposure to H 2 and CO diluted in synthetic air and heating at 723 K. The XPS spectra confirmed significant Pt 2+ and Pt 4+ chemical state contributions.…”

“…This information, supported by comprehensive multiple scattering simulation, can provide precise results about bond angles, the orientation of adsorbed molecules, and bond distances, all of these results being selective to the chemical state. [4][5][6] The results extracted from XPS and XPD indicated that Cr 2 O 3 grows on Ag(111) along the Cr 2 O 3 (0001) direction. After Pt deposition with subsequent annealing, XPD results supported by comprehensive computational simulation demonstrated that a fraction of the Cr 2 O 3 film surface (18%) was covered by Pt NIs, with a typical thickness of three atomic layers.…”