Atomic Structure of Cr₂O₃/Ag(111) and Pd/Cr₂O₃/Ag(111) Surfaces: A Photoelectron Diffraction Investigation

A detailed investigation concerning the atomic structure of Cr₂O₃ and Pd/Cr₂O₃ ultrathin films deposited on a Ag(111) single crystal is presented. The films were prepared by MBE (molecular beam epitaxy) and characterized <i>in situ</i> by LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy), and XPD (X-ray photoelectron diffraction). Evidences of rotated domains and an oxygen-terminated Cr₂O₃/Ag­(111) surface were observed, along with significant contractions of the oxide’s outermost interlayer distances. The deposition of Pd atoms on the Cr₂O₃ surface formed a four-monolayer film, <i>fcc</i> packed and oriented in the [111] direction, which presented changes in monolayer spacing and lateral atomic distance compared to the expected values for bulk Pd. The observed surface structure may shed light on new physical properties such as the induced magnetic ordering in Pd atoms

Influence of the CeO₂ Support on the Reduction Properties of Cu/CeO₂ and Ni/CeO₂ Nanoparticles

Ceria (CeO₂) is being increasingly used as support of metallic nanoparticles in catalysis due to its unique redox properties. Shedding light into the nature of the strong metal support interaction (SMSI) effect in CeO₂-containing catalysts is important since it has a strong influence on the catalytic properties of the system. In this work, Cu/CeO₂ and Ni/CeO₂ nanoparticles are characterized when submitted to a reduction treatment at 500 °C in H₂ atmosphere with a combination of in situ (XAS – X-ray absorption spectroscopy and time-resolved XAS) and ex situ (TEM – transmission electron microscopy and XPS - X-ray photoelectron spectroscopy) techniques. The existence of a capping layer decorating the Ni/CeO₂ nanoparticles after the reduction treatment is shown, representing evidence for the SMSI effect. The kinetics of the SMSI occurrence is elucidated. It is proposed that the electronic factor of the SMSI effect has a strong influence on the reduction properties of the Ni nanoparticles supported on CeO₂, decreasing its reduction temperature if compared to nonsupported Ni nanoparticles. The same phenomenon is not observed for Cu/CeO₂ nanoparticles, where there is no evidence for the SMSI effect, and no changes on the reduction properties between supported and nonsupported Cu nanoparticles are observed

Core–Shell Fe–Pt Nanoparticles in Ionic Liquids: Magnetic and Catalytic Properties

The reaction of Fe­(CO)₅ and Pt₂(dba)₃ in 1-<i>n</i>-butyl-methylimidazolium tetrafluoroborate (BMIm.BF₄), hexafluorophosphate (BMIm.PF₆), and bis­(trifluoromethanesulfonyl)­imide (BMIm.NTf₂) under hydrogen affords stable magnetic colloidal core–shell nanoparticles (NPs). The thickness of the Pt shell layer has a direct correlation with the water stability of the anion and increases in the order of PF₆ > BF₄ > NTf₂, yielding the metal compositions Pt₄Fe₁, Pt₃Fe₂, and Pt₁Fe₁, respectively. Magnetic measurements give evidence of a strongly enhanced Pauli paramagnetism of the Pt shell and a partially disordered iron-oxide core with diminished saturation magnetization. The obtained Pauli paramagnetism of the Pt shell is 2 orders of magnitude higher than that of bulk Pt, owing to symmetry breaking at the surface and interface, resulting in a strong increase in the density of states at the Fermi level, and thus to enhanced Pauli susceptibility. Moreover, these ultrasmall NPs showed efficient catalytic activity for the direct production of selective short-chain hydrocarbons (C₁–C₆) by the Fischer–Tropsch synthesis with efficient conversion (18–34%) and selectivity (69–90%, C₂–C₄). The selectivity and activity were dependent on the Fe-oxides@Pt particle size. The catalytic activity decreased from 34 to 18% as the NP size increased from 1.7 to 2.5 nm at 15 bar and 300 °C

Pt-Mediated Reversible Reduction and Expansion of CeO₂ in Pt Nanoparticle/Mesoporous CeO₂ Catalyst: In Situ X‑ray Spectroscopy and Diffraction Studies under Redox (H₂ and O₂) Atmospheres

Here, we report the Pt nanoparticle mediated reduction (oxidation) and lattice expansion (contraction) of mesoporous CeO₂ under H₂ (O₂) atmospheres and in the temperature range of 50–350 °C. We found that CeO₂ in the Pt/CeO₂ catalyst was partially reduced in H₂ (and fully oxidized back in O₂) as demonstrated by several in situ techniques: APXPS spectra (4d core levels) for the topmost surface, NEXAFS total electron yield spectra (at the M_5,4 edges) in the near surface regions, and (N)­EXAFS fluorescence spectra (at the L₃ edge) in the bulk. Moreover, XRD and EXAFS showed the reversible expansion and contraction of the CeO₂ unit cell in H₂ and O₂ environments, respectively. The expansion of the CeO₂ cell was mainly associated with the formation of oxygen vacancies as a result of the Pt-mediated reduction of Ce⁴⁺ to Ce³⁺. We also found that pure mesoporous CeO₂ can not be reduced in H₂ under identical conditions but can be partially reduced at above 450 °C as revealed by APXPS. The role of Pt in H₂ was identified as a catalytic one that reduces the activation barrier for the reduction of CeO₂ via hydrogen spillover