Pt/CeO2 as Catalyst for Non-Oxidative Coupling of Methane:Oxidative Regeneration

Direct non-oxidative coupling is a promising route for methane upgrading, yet its commercialization is hindered by the lack of efficient catalysts. Pt/CeO2 catalysts with isolated Pt species have attracted increasing interest in recent years. Herein, we studied the catalytic role and evolution of isolated Pt centers on CeO2 prepared by flame spray pyrolysis under the harsh reaction conditions of non-oxidative methane coupling. During the reaction at 800 °C, the isolated Pt sites sinter leading to a loss of the ethylene and ethane yield. The agglomerated Pt can be redispersed by using an in situ regeneration strategy in oxygen. We found that isolated Pt centers are only able to activate methane at the initial reaction stage, and the CePt5 alloy acts as the active phase in the prolonged reaction

Chemically Resolved Interface Structure of Epitaxial Graphene on SiC(0001)

Atomic-layer 2D crystals have unique properties that can be significantly modified through interaction with an underlying support. For epitaxial graphene on SiC(0001), the interface strongly influences the electronic properties of the overlaying graphene. We demonstrate a novel combination of x-ray scattering and spectroscopy for studying the complexities of such a buried interface structure. This approach employs x-ray standing wave-excited photoelectron spectroscopy in conjunction with x-ray reflectivity to produce a highly resolved chemically sensitive atomic profile for the terminal substrate bilayers, interface, and graphene layers along the SiC[0001] direction. DOI: 10.1103/PhysRevLett.111.215501 PACS numbers: 61.48.Gh, 61.05.cm, 68.49.Uv, 79.60.Ài Epitaxial graphene (EG) grown on the Si-terminated face of silicon carbide [SiC Early studies revealed that EG/SiC(0001) possesses a complex 6 p 3 Â 6 p 3R30 (6R3) reconstructed interfacial layer [10], referred to herein as the interfacial, or EG 0 , layer. This layer has significant influence on the growth, morphology, and electronic behavior of the overlaying graphene Because of the importance of the interfacial layer to the behavior of EG/SiC(0001), there have been numerous efforts to characterize its structure, including low-energy electron diffraction In this Letter we detail the structure of the interface by employing a suite of x-ray characterization techniques, including depth-sensitive XPS, x-ray standing waveenhanced XPS (XSW-XPS), and x-ray reflectivity (XRR). These tools, when employed collectively, provide the chemically specific structural information necessary to clarify previously unknown details of the EG/SiC(0001) interface. This approach ultimately enables the construction of a chemically resolved interfacial map with sub-Å resolution along the SiC[0001] direction. The XSW technique affords conventional photoelectron spectroscopy with high spatial resolution due to the influence of the XSW [here produced by the SiC(0006) Bragg reflection] on the photoabsorption process. A depiction of this phenomenon is shown i

Hard X -Ray Photoelectron Spectroscopy (HAXPES) characterisation of electrochemical passivation oxide layers on Al-Cr-Fe Complex Metallic Alloys (CMA).

A Hard X-ray Photoelectron Spectroscopy (HAXPES) characterisation of the passivation layers formed by electrochemical polarisation of Al–Cr–Fe complex metallic alloys is presented. By employing X-ray excitation energies from 2.3 to 10.0 keV, the depth distributions of Al- and Cr-oxide and hydroxide species in the (Al,Cr)-containing passive layers could be determined. Simultaneous analyses of the shallow Al 2s and deep Al 1s core level lines (respectively, more bulk- and surface-sensitive) provided complementary information to effectively determine the depth-resolved contributions of hydroxide and oxide species within the passivation layer. A Cr threshold concentration of 18 (at.%) was found for effective passivation at pH 1

Peculiarities of the magnetic-history-dependent phase in CePtSn

Due to its peculiar properties and presence of many types of different basic mechanisms determining the magnetic ground state, CePtSn compound is one of the well-studied alloys from the CeTX group with TiNiSi-type structure. In this article we present the results of elastic and transport properties, and we concentrate our efforts on the description of the evolution for field applied along the b-axis, where a magnetic-history-dependent (MHD) phenomenon occurs. Based on this results and the fact that the low-temperature zero-field-cooled state may be described as the presence of two kinds of spin slips in magnetic structure, we conclude that the MHD behavior may be interpreted as the magnetic field annealing of one set of spin stacking faults in the magnetic structure in favor of the other one. Keywords: CePtSn; Thermal expansion; Magnetostriction; AnisotropyJRC.E.6-Actinides researc

Flow Cell for Operando X-Ray Photon-in-Photon-out Studies on Photo-Electrochemical Thin Film Devices

Photo-electro-chemical (PEC) water splitting represents a promising technology towards an artificial photosynthetic device but many fundamental electronic processes, which govern long-term stability and energetics are not well understood. X-ray absorption spectroscopy (XAS), particularly its high energy resolution fluorescence-detected (HERFD) mode, emerges as a powerful tool to study photo-excited charge carrier behavior under operating conditions. The established thin film device architecture of PEC cells provides a well-defined measurement geometry, but it puts many constraints on conducting operando XAS experiments. So far, operando cells have not been developed that enable to concurrently measure highly intense X-ray fluorescence and photo-electro-chemical current without experimental artifacts caused by O2 and H2 bubbles formation. Moreover, we are missing a standardized thin film exchange procedure. Here, we address and overcome the instrumental limitations for operando HERFD-XAS to investigate photo- and electrochemical thin film devices. Our cell establishes a measurement routine that will provide experimental access to a broader scientific community, particularly due to the ease of sample exchange. Our operando photo-electro-chemical cell is optimized for the HERFD-XAS geometry and we demonstrate its operation by collecting high-resolution Fe K-edge spectra of hematite (α-Fe2O3) and ferrite thin film (MFe2O4, M= Zn, Ni) photoelectrodes during water oxidation.<br /

New reflections on hard X-ray photon-in/photon-out spectroscopy.

Analysis of the electronic structure and local coordination of an element is an important aspect in the study of the chemical and physical properties of materials. This is particularly relevant at the nanoscale where new phases of matter may emerge below a critical size. X-ray emission spectroscopy (XES) at synchrotron radiation sources and free electron lasers has enriched the field of X-ray spectroscopy. The spectroscopic techniques derived from the combination of X-ray absorption and emission spectroscopy (XAS-XES), such as resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) XAS, are an ideal tool for the study of nanomaterials. New installations and beamline upgrades now often include wavelength dispersive instruments for the analysis of the emitted X-rays. With the growing use of XAS-XES, scientists are learning about the possibilities and pitfalls. We discuss some experimental aspects, assess the feasibility of measuring weak fluorescence lines in dilute, radiation sensitive samples, and present new experimental approaches for studying magnetic properties of colloidal nanoparticles directly in the liquid phase

New reflections on hard X-ray photon-in/photon-out spectroscopy.

Analysis of the electronic structure and local coordination of an element is an important aspect in the study of the chemical and physical properties of materials. This is particularly relevant at the nanoscale where new phases of matter may emerge below a critical size. X-ray emission spectroscopy (XES) at synchrotron radiation sources and free electron lasers has enriched the field of X-ray spectroscopy. The spectroscopic techniques derived from the combination of X-ray absorption and emission spectroscopy (XAS-XES), such as resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) XAS, are an ideal tool for the study of nanomaterials. New installations and beamline upgrades now often include wavelength dispersive instruments for the analysis of the emitted X-rays. With the growing use of XAS-XES, scientists are learning about the possibilities and pitfalls. We discuss some experimental aspects, assess the feasibility of measuring weak fluorescence lines in dilute, radiation sensitive samples, and present new experimental approaches for studying magnetic properties of colloidal nanoparticles directly in the liquid phase

Reference-free grazing incidence x-ray fluorescence and reflectometry as a methodology for independent validation of x-ray reflectometry on ultrathin layer stacks and a depth-dependent characterization

International audienceNanolayer stacks are technologically very relevant for current and future applications in many fields of research. A nondestructive characterization of such systems is often performed using x-ray reflectometry (XRR). For complex stacks of multiple layers, low electron density contrast materials, or very thin layers without any pronounced angular minima, this requires a full modeling of the XRR data. As such a modeling is using the thicknesses, the densities, and the roughnesses of each layer as parameters, this approach quickly results in a large number of free parameters. In consequence, cross correlation effects or interparameter dependencies can falsify the modeling results. Here, the authors present a route for validation of such modeling results which is based on the reference-free grazing incidence x-ray fluorescence (GIXRF) methodology. In conjunction with the radiometrically calibrated instrumentation of the Physikalisch-Technische Bundesanstalt, the method allows for reference-free quantification of the elemental mass depositions. In addition, a modeling approach of reference-free GIXRF-XRR data is presented, which takes advantage of the quantifiable elemental mass depositions by distributing them depth dependently. This approach allows for a reduction of the free model parameters. Both the validation capabilities and the combined reference-free GIXRF-XRR modeling are demonstrated using several nanoscale layer stacks consisting of HfO 2 and Al 2 O 3 layers

Crystal Chemistry of Thallium in Marine Ferromanganese Deposits

International audienceOur understanding of the up to seven orders of magnitude partitioning of thallium (Tl) between seawater and ferromanganese (FeMn) deposits rests upon two foundations: (1) being able to quantify the Tl(I)/Tl(III) ratio that reflects the extent of the oxidative scavenging of Tl by vernadite (δ-MnO2), the principle manganate mineral in oxic and suboxic environments, and (2) being able to determine the sorption sites and bonding environments of the Tl(I) and Tl(III) complexes on vernadite. We investigated these foundations by determining the oxidation state and chemical form of Tl in FeMn crusts and nodules from the global oceans at a Tl concentration ranging from several hundreds ppm (mg/kg) down to the low ppm level. Seventeen hydrogenetic crusts and eleven nodules from the Pacific, Atlantic, and Indian oceans and Baltic Sea were characterized by chemical analysis, X-ray diffraction, Raman spectroscopy, Mn K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and Tl L3-edge high energy-resolution XANES (HR-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The Tl concentration increases linearly from 1.5 to 319 ppm with the Mn/Fe ratio in Fe-vernadite from hydrogenetic crusts, whereas the percentage of Tl(III) to total Tl varies between 62% and 100% independent of both the Mn/Fe and Mn(III)/Mn(IV) 2 ratios. The data, complemented by molecular modeling of the Tl(III) coordination and by XANES calculations, suggest that the enrichment of Tl in Fe-vernadite is driven by (1) the oxidative uptake of octahedrally coordinated Tl(III) above the vacant Mn(IV) sites and on the layer edges of the vernadite layers, and (2) the sorption of Tl(I) on the crystallographic site of Ba at the surface of the vernadite layers, which is analog to the surface site of K. Thus, Tl has a high affinity for vernadite regardless of its oxidation state, and the lack of correlation between Tl(III) and the Mn/Fe ratio in FeMn crusts is explained by the affinity of Tl(I) for the Ba site. The Tl concentration varies between 2 and 112 ppm in surface and buried nodules independent of the Mn/Fe ratio, and the percentage of Tl(III) varies between 0% and 100%. Nodules subjected to sediment diagenesis with replacement of layered vernadite by tunneled todorokite are depleted in Tl and have more reduced thallium. Knowledge of the complex interplay of mineralogy, surface chemical processes, and crystallographic siting are required to understand the variability of Tl concentrations, redox state, and acquisition processes by marine FeMn deposits