During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the ...active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst’s interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.
Mercury is in some sense an enigmatic element. The element and some of its compounds are a natural part of the biogeochemical cycle; while many of these can be deadly poisons at higher levels, ...environmental levels in the absence of anthropogenic contributions would generally be below the threshold for concern. However, mercury pollution, particularly from burning fossil fuels such as coal, is providing dramatic and increasing emissions into the environment. Because of this, the environmental chemistry and toxicology of mercury are of growing importance, with the fate of mercury being vitally dependent upon its speciation. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation, but is severely limited by poor spectroscopic energy resolution. Here, we provide a systematic examination of mercury Lα1 high energy resolution fluorescence detected XAS (HERFD–XAS) as an approach for chemical speciation of mercury, in quantitative comparison with conventional Hg LIII-edge XAS. We show that, unlike some lighter elements, chemical shifts in the Lα1 X-ray fluorescence energy can be safely neglected, so that mercury Lα1 HERFD–XAS can be treated simply as a high-resolution version of conventional XAS. We present spectra of a range of mercury compounds that may be relevant to the environmental and life science research and show that density functional theory can produce adequate simulations of the spectra. We discuss strengths and limitations of the method and quantitatively demonstrate improvements both in speciation for complex mixtures and in background rejection for low concentrations.
TbPc2 single‐molecule magnets adsorbed on a magnesium oxide tunnel barrier exhibit record magnetic remanence, record hysteresis opening, perfect out‐of‐plane alignment of the magnetic easy axes, and ...self‐assembly into a well‐ordered layer.
Developing efficient electrocatalysts for alkaline water electrolysis is central to substantial progress of alkaline hydrogen production. Herein, a Ni5P4 electrocatalyst incorporating single‐atom Ru ...(Ni5P4‐Ru) is synthesized through the filling of Ru3+ species into the metal vacancies of nickel hydroxides and subsequent phosphorization treatment. Electron paramagnetic resonance spectroscopy, X‐ray‐based measurements, and electron microscopy observations confirm the strong interaction between the nickel‐vacancy defect and Ru cation, resulting in more than 3.83 wt% single‐atom Ru incorporation in the obtained Ni5P4‐Ru. The Ni5P4‐Ru as an alkaline hydrogen evolution reaction catalyst achieves low onset potential of 17 mV and an overpotential of 54 mV at a current density of 10 mA cm‐2 together with a small Tafel slope of 52.0 mV decade‐1 and long‐term stability. Further spectroscopy analyses combined with density functional theory calculations reveal that the doped Ru sites can cause localized structure polarization, which brings the low energy barrier for water dissociation on Ru site and the optimized hydrogen adsorption free energy on the interstitial site, well rationalizing the experimental reactivity.
Ni5P4 nanoparticles incorporating single‐atomic Ru are synthesized by a nickel‐vacancy‐assisted method, and meticulous experimental analyses combined with density functional theory calculations confirm that the incorporation of Ru induces localized structural polarization to optimize the catalytic energetics.
The electrochemical conversion of carbon dioxide (CO2) into value‐added chemicals is regarded as one of the promising routes to mitigate CO2 emission. A nitrogen‐doped carbon‐supported palladium (Pd) ...single‐atom catalyst that can catalyze CO2 into CO with far higher mass activity than its Pd nanoparticle counterpart, for example, 373.0 and 28.5 mA mg−1Pd, respectively, at −0.8 V versus reversible hydrogen electrode, is reported. A combination of in situ X‐ray characterization and density functional theory (DFT) calculation reveals that the PdN4 site is the most likely active center for CO production without the formation of palladium hydride (PdH), which is essential for typical Pd nanoparticle catalysts. Furthermore, the well‐dispersed PdN4 single‐atom site facilitates the stabilization of the adsorbed CO2 intermediate, thereby enhancing electrocatalytic CO2 reduction capability at low overpotentials. This work provides important insights into the structure‐activity relationship for single‐atom based electrocatalysts.
A palladium (Pd)‐based single‐atom catalyst with PdN4 sites is identified as an efficient electrochemical CO2 reduction reaction (CO2RR) catalyst. It is capable of converting CO2 into CO without the Pd hydride formation essential for typical bulk Pd‐based catalysts. This study reveals the comprehensive structure–activity relationship for the isolated and bulk Pd catalysts in electrochemical CO2RR.
Unravelling the intrinsic mechanism of electrocatalytic oxygen evolution reaction (OER) by use of heterogeneous catalysts is highly desirable to develop related energy conversion technologies. Albeit ...dynamic self‐reconstruction of the catalysts during OER is extensively observed, it is still highly challenging to operando probe the reconstruction and precisely identify the true catalytically active components. Here, a new class of OER precatalyst, cobalt oxychloride (Co2(OH)3Cl) with unique features that allow a gradual phase reconstruction during OER due to the etching of lattice anion is demonstrated. The reconstruction continuously boosts OER activities. The reconstruction‐derived component delivers remarkable performance in both alkaline and neutral electrolytes. Operando synchrotron radiation‐based X‐ray spectroscopic characterization together with density functional theory calculations discloses that the etching of lattice Cl− serves as the key to trigger the reconstruction and the boosted catalytic performance roots in the atomic‐level coordinatively unsaturated sites (CUS). This work establishes fundamental understanding on the OER mechanism associated with self‐reconstruction of heterogeneous catalysts.
A new class of precatalyst for the oxygen evolution reaction (OER), cobalt oxychloride (Co2(OH)3Cl), displays unique features that allow a gradual phase reconstruction due to electrochemistry‐induced etching. The reconstruction continuously boosts the OER activity. Operando synchrotronradiation‐based X‐ray spectroscopic characterizations together with density functional theory calculations provide atomic‐level insight that the boosted catalytic performance results from coordinatively unsaturated sites of the cobalt.
Developing highly efficient, low-cost oxygen reduction catalysts, especially in acidic medium, is of significance toward fuel cell commercialization. Although pyrolyzed Fe-N-C catalysts have been ...regarded as alternatives to platinum-based catalytic materials, further improvement requires precise control of the Fe-N x structure at the molecular level and a comprehensive understanding of catalytic site structure and the ORR mechanism on these materials. In this report, we present a microporous metal–organic-framework-confined strategy toward the preferable formation of single-atom dispersed catalysts. The onset potential for Fe-N-C is 0.92 V, comparable to that of Pt/C and outperforming most noble-metal-free catalysts ever reported. A high-spin Fe3+-N4 configuration is revealed by the 57Fe Mössbauer spectrum and X-ray absorption spectroscopy for Fe L-edge, which will convert to Fe2+-N4 at low potential. The in situ reduced Fe2+-N4 moiety from high-spin O x -Fe3+-N4 contributes to most of the ORR activity due to its high turnover frequency (TOF) of ca. 1.71 e s–1 sites–1.
A prototype in situ X‐ray absorption near‐edge structure (XANES) system was developed to explore its sensitivity for ultra‐thin films of iron‐nitride (Fe‐N), cobalt‐nitride (Co‐N) and nickel‐nitride ...(Ni‐N). They were grown using DC‐magnetron sputtering in the presence of an N2 plasma atmosphere at the experimental station of the soft XAS beamline BL01 (Indus‐2, RRCAT, India). XANES measurements were performed at the N K‐edge in all three cases. It was found that the N K‐edge spectral shape and intensity are greatly affected by increasing thickness and appear to be highly sensitive, especially in low‐thickness regions. From a certain thickness of ∼1000 Å, however, samples exhibit a bulk‐like behavior. On the basis of the obtained results, different growth stages were identified. Furthermore, the presence of a molecular N2 component in the ultra‐thin regime (<100 Å) was also obtained in all three cases studied in this work. In essence, this prototype in situ system reveals that N K‐edge XANES is a powerful technique for studying ultra‐thin films, and the development of a dedicated in situ system can be effective in probing several phenomena that remain hitherto unexplored in such types of transition metal nitride thin films.
Sputter deposition is combined with in situ N K‐edge absorption measurements to study thin films of magnetic nitrides (Fe‐N, Co‐N, Ni‐N) ranging from sub‐monolayer to bulk. Ultra‐thin films of these nitrides were found to demonstrate a different behavior that can be coupled with structural changes taking place with the increasing film thickness.
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•Phosphate enhances the sorption of cadmium(II) to ferrihydrite.•Without phosphate, cadmium(II) forms bidentate complexes with ferrihydrite.•Enhanced sorption with phosphate can be ...explained by formation of a ternary surface complex.
Phosphate influences the sorption of metals to iron (hydr)oxides. An enhanced formation of inner-sphere complexes on the (hydr)oxide surface can be attributed to electrostatic interactions and/or to changes in metal coordination on the iron (hydr)oxide surface. Phosphate was expected to increase cadmium(II) sorption on ferrihydrite. It should be possible to identify changes in cadmium(II) coordination upon phosphate addition by Extended X-ray absorption fine structure (EXAFS) spectroscopy and implement the identified complexes in a surface complexation model (SCM).
The effect of phosphate addition on cadmium(II) sorption to ferrihydrite was studied by a series of batch experiments covering the pH range from 4 to 8. EXAFS spectroscopy was performed on ferrihydrite from the batch experiments at the cadmium K edge. The identified surface complexes were incorporated in the Charge distribution multisite complexation (CD-MUSIC) model, and new surface complexation constants were optimized.
Without phosphate addition cadmium(II) formed inner-sphere bidentate complexes on the ferrihydrite surface. With phosphate there was an increased cadmium(II) sorption that could not be explained by electrostatic interactions alone. The enhancement was best explained by the formation of a ternary complex including cadmium(II), phosphate and ferrihydrite surface groups.
Seemingly simple yet surprisingly difficult to probe, excess protons in water constitute complex quantum objects with strong interactions with the extended and dynamically changing hydrogen‐bonding ...network of the liquid. Proton hydration plays pivotal roles in energy transport in hydrogen fuel cells and signal transduction in transmembrane proteins. While geometries and stoichiometry have been widely addressed in both experiment and theory, the electronic structure of these specific hydrated proton complexes has remained elusive. Here we show, layer by layer, how utilizing novel flatjet technology for accurate x‐ray spectroscopic measurements and combining infrared spectral analysis and calculations, we find orbital‐specific markers that distinguish two main electronic‐structure effects: Local orbital interactions determine covalent bonding between the proton and neigbouring water molecules, while orbital‐energy shifts measure the strength of the extended electric field of the proton.
Oxygen K‐edge spectroscopy of hydrated proton complexes in solution provides compelling evidence of the pronounced impact on the electronic structure of the water molecules participating in the hydration, with a hierarchy in coupling strengths of the proton with profoundly strong orbital interactions for the inner three water molecules, and Coulombic field effects of the proton positive charge with the first hydration shell water molecules.
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