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NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs. NiFe and CoFe layered double hydroxides are among the most active electrocatalysts for the alkaline oxygen evolution reaction. Here, by combining operando experiments and rigorous DFT calculations, the authors unravel their active phase, the reaction center and the catalytic mechanism.

Although Cu/ZnO-based catalysts have been long used for the hydrogenation of CO 2 to methanol, open questions still remain regarding the role and the dynamic nature of the active sites formed at the metal-oxide interface. Here, we apply high-pressure operando spectroscopy methods to well-defined Cu and Cu 0.7 Zn 0.3 nanoparticles supported on ZnO/Al 2 O 3 , γ-Al 2 O 3 and SiO 2 to correlate their structure, composition and catalytic performance. We obtain similar activity and methanol selectivity for Cu/ZnO/Al 2 O 3 and CuZn/SiO 2 , but the methanol yield decreases with time on stream for the latter sample. Operando X-ray absorption spectroscopy data reveal the formation of reduced Zn species coexisting with ZnO on CuZn/SiO 2 . Near-ambient pressure X-ray photoelectron spectroscopy shows Zn surface segregation and the formation of a ZnO-rich shell on CuZn/SiO 2 . In this work we demonstrate the beneficial effect of Zn, even in diluted form, and highlight the influence of the oxide support and the Cu-Zn interface in the reactivity. The nature of the active species over Cu/ZnO catalysts for methanol synthesis remains elusive. Here, the authors shed light on the evolution of the nanoparticle/support interface and correlate its structural and chemical transformations with changes in the catalytic performance.

There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper + species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper + is key for lowering the onset potential and enhancing ethylene selectivity. Carbon dioxide electroreduction is a promising route to hydrocarbon synthesis, but more efficient and selective catalysts are needed. Here the authors show that plasma-activated copper can catalyse the reduction of carbon dioxide to ethylene with high efficiency and reveal cationic copper as the active site.

The occupation of electronic orbitals on the surface and interface of oxide thin films and heterostructures is a key influence over their properties, including magnetism and superconductivity. A new spectroscopy technique now provides the first quantitative, spatially resolved data of orbital occupation in oxide structures. The occupation of d  orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties 1 . Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties 2 , 3 , 4 , 5 , 6 , 7 , 8 , but could not thus far be probed in a quantitative manner 9 , 10 , 11 . Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of ∼3% in the occupation of Ni e g orbitals in adjacent atomic layers of a LaNiO 3 –LaAlO 3 superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.

Nature Communications 7 Article number: 12123 (2016); Published 30 June 2016; Updated 12 September 2016 An incorrect version of the Supplementary Information was inadvertently published with this Article in which the image for Supplementary Fig. 7 was missing. The Article has now been updated to include the correct version of the Supplementary Information.

Heterogeneous processes at solid/gas, liquid/gas and solid/liquid interfaces are ubiquitous in modern devices and technologies but often difficult to study quantitatively. Full characterization requires measuring the depth profiles of chemical composition and state with enhanced sensitivity to narrow interfacial regions of a few to several nm in extent over those originating from the bulk phases on either side of the interface. We show for a model system of NaOH and CsOH in an ~1-nm thick hydrated layer on α-Fe 2 O 3 (haematite) that combining ambient-pressure X-ray photoelectron spectroscopy and standing-wave photoemission spectroscopy provides the spatial arrangement of the bulk and interface chemical species, as well as local potential energy variations, along the direction perpendicular to the interface with sub-nm accuracy. Standing-wave ambient-pressure photoemission spectroscopy is thus a very promising technique for measuring such important interfaces, with relevance to energy research, heterogeneous catalysis, electrochemistry, and atmospheric and environmental science. Heterogeneous chemical processes are vital for many applications, but the crucial interfaces involved are difficult to probe experimentally with elemental and chemical-state specificity. Here, the authors present a photoelectron spectroscopy-based method for studying such interfaces with sub-nanometre accuracy and under realistic pressure conditions

The following sections are included: Introduction Non-Resonant X-Ray Scattering Resonant X-Ray Diffraction Basic Principles and Historical Background Resonant Electric Dipole Scattering Length Orbital Ordering in Ca2RuO4 Introduction Main Properties Orbital Order X-Ray Investigations Spin Reorientation in Ca3Ru2O7 Introduction Main Properties X-Ray Investigations Magnetic Structure Determination in RuSr2GdCu2O8 Introduction Main Properties X-Ray Investigations Spin Orbital Mott State in Sr2IrO4 Introduction Main Properties X-Ray Investigations References