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.
A detailed investigation concerning the atomic structure of Cr 2 O 3 and Pd/Cr 2 O 3 ultrathin films deposited on a Ag(111) single crystal is presented. The films were prepared by MBE (molecular beam epitaxy) and characterized in situ 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 Cr 2 O 3 /Ag(111) surface were observed, along with significant contractions of the oxide's outermost interlayer distances. The deposition of Pd atoms on the Cr 2 O 3 surface formed a four-monolayer film, fcc 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.
In this work, the reduction properties of ceria (CeO2) used as support of metallic nanoparticles (Au, Pd, Au0.9Pd0.1, and Au0.8Pt0.2) were elucidated. The catalysts were exposed to a reduction treatment in H2 atmosphere at 500 °C. In situ X-ray absorption spectroscopy (in situ XAS) and in situ time-resolved XAS measurements at the Ce L3 edge were used to probe the local atomic order around Ce atoms and the Ce oxidation state. In this way, it was observed that the supported metallic nanoparticles improve the reduction process of the support. Moreover, the reduction of ceria is dependent on the composition of the metallic nanoparticle supported. By means of in situ XAS measurements at the Au and Pt L2,3 edges, it was possible to obtain information concerning the fractional change in the number of 5d-band electron holes relative to a reference material for the Au- and Pt-containing nanoparticles. In this way, it was observed as evidence for the charge transfer effect from the nanoparticles to the support, which is responsible for the improved reduction of the CeO2 support in the presence of nanoparticles. This result is corroborated by the observation of energy shifts on the Au 4f, Pd 3d, and Pt 4f binding energy values of the measured X-ray photoelectron spectroscopy (XPS) spectra. In this way, this work contributes to elucidating the physical mechanisms responsible for the enhanced support reduction effect existing in modern ceria-based catalyst.
Pd nanoparticles (NPs) were successfully obtained by the reduction of PdCl2 with L-ascorbic acid, whose morphology was revealed by HRTEM to be a worm-like system, formed by linked crystallite clusters with an average short-axis diameter of 5.42 nm. In situ UV-vis absorption measurements were used to monitor their formation, while XPS and XRD characterization confirmed the NPs' metallic state. A straightforward way to support the obtained Pd NPs on activated carbon (AC) was used to prepare a catalyst for NO decomposition reaction. The Pd/AC catalysts proved to be highly active in the temperature range of 323 to 673 K, and a redox mechanism is proposed, where the catalyst's active sites are oxidized by NO and reduced by carbon, emitting CO2 and enhancing their capacity to absorb and dissociate NO.
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.
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