Vibrational and structural properties of the RFe4Sb12 (R=Na, K, Ca, Sr, Ba) filled skutterudites

The local atomic structures around the Zr atom of pure (undoped) ZrO2nanopowders with different average crystallite sizes, ranging from 7 to 40 nm, have been investigated. The nanopowders were synthesized by different wet-chemical routes, but all exhibit the high-temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X-ray diffraction. The extended X-ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X-ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia-based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to thezdirection; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X-ray diffraction experiments.

Temperature dependent X-ray and neutron diffraction study of the liquid–solid and solid–solid equilibria in the Al 29.2Ga 27Zn 43.8 ternary alloy

The structure of amorphous alloys has been studied by using various techniques, such as x-ray, neutron, and electron diffraction experiments. X-ray diffraction is the most conventional of all the techniques and is based on EXAFS (extended x-ray absorption fine structure) [1-3] and AXS (anomalous x-ray scattering) [4-5] experiments, which are used for the investigation of the local environment around a species of atom. Recently, x-ray diffraction has been applied to the structural analysis of some quasicrystals [5,7]. It is, of course, hard to determine the local structure of a noncrystalline material only from ordinary x-ray diffraction experiments. However, the role of x-ray diffraction in structural analysis is still important from the viewpoint that this supplies a relatively convenient method, and the appropriate selection of a Sample makes it possible to extract the dominant atomic correlation in the material. In the present study, a conventional θ-2θ diffractometer was used with a monochrometor in the diffracted beam. First, the theoretical background is discussed, the data analysis procedure is given in detail, and error estimation in the data analysis is presented last.

Theoretical Elucidation of Geometrical Structures of the CaMn4O5 Cluster in Oxygen Evolving Complex of Photosystem II Scope and Applicability of Estimation Formulae of Structural Deformations via the Mixed-Valence and Jahn–Teller Effects

ABSTRACTAtmospheric oxygenation and evolution of aerobic life on our earth are a result of water oxidation by oxygenic photosynthesis in photosystem II (PSII) of plants, algae and cyanobacteria. The water oxidation in the oxygen-evolving complex (OEC) in PSII is expected to proceed through five oxidation states, known as the Si (i = 0, 1, 2, 3 and 4) states in the Kok cycle, with the S1 being the most stable state in the dark. The OEC in PSII involves the active catalytic site made of four Mn ions and one Ca ion, namely the CaMn4O5 cluster. Past decades, molecular structures of the CaMn4O5 cluster in OEC in PSII have been investigated by the extended X-ray absorption fine structure (EXAFS). The magneto-structural correlations were extensively investigated by electron paramagnetic resonance (EPR) spectroscopy. Recently, Kamiya and Shen groups made great breakthrough for determination of the S1 structure of OEC of PSII by the X-ray diffraction (XRD) and X-ray free-electron laser (XFEL) experiments, providing structural foundations that are crucial for theoretical investigations of the CaMn4O5 cluster. Large-scale quantum mechanics/molecular mechanics calculations starting from the XRD structures elucidated geometrical, electronic and spin structures of the CaMn4O5 cluster, indicating an important role of the Jahn–Teller (JT) effect of Mn(III) ions. This paper presents theoretical formulas for estimation of the JT deformations of the CaMn4O5 cluster in OEC of PSII. Scope and applicability of the formulas are examined in relation to several different structures of the CaMn4O5 cluster proposed by XRD, XFEL, EXAFS and other experiments. Implications of the computational results are discussed for further refinements of geometrical parameters of the CaMn4O5 cluster.

The structure and ligand environment of Co(salen) nanoparticles and unprocessed Co(salen) have been determined by the combined application of infrared, Raman, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies, and X-ray diffraction (XRD) experiments before and during interaction with O2. The Co(salen) nanoparticles were prepared by the precipitation with compressed antisolvent (PCA) technique using commercially obtained Co(salen) [denoted as unprocessed Co(salen)] as the parent compound. The unprocessed Co(salen) particles exist as dimer species with a square-pyramidal coordination geometry that display no measurable O2 binding at room temperature. In sharp contrast, the Co(salen) nanoparticles show near-stoichiometric O2 adsorption, as demonstrated by microbalance gas binding experiments. The spectroscopy results indicate the presence of CoII centers with distorted tetrahedral geometry in the Co(salen) nanoparticles with no evidence of metallic Co clusters, confirmed by the lack of Co−Co contributions at bonding distances in the EXAFS spectra and the presence of characteristic features of CoII in the XANES spectra. The EXAFS data also indicate that there are on average two Co−N and two Co−O bonds with a distance of 1.81 ± 0.02 and 1.90 ± 0.02 Å, respectively, consistent with typical metal salen structures. Upon O2 binding on the Co(salen) nanoparticles, the XANES results indicate oxidation of the CoII to CoIII, consistent with the vibrational data showing new bands associated with oxygen species bonded to Co centers and the increase in the oxygen coordination number from 1.8 to 2.9 in the EXAFS data. The results indicate that the enhanced O2 binding properties of Co(salen) nanoparticles are related to the unique distorted tetrahedral geometry, which is not observed in the unprocessed samples that contain mainly dimers with square planar geometry. The results presented here provide a fundamental relationship between active center structure and properties of novel molecule-based nanomaterials.

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2022Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures Yanliang Zhou†, Congying Wang†, Xuanbei Peng†, Tianhua Zhang, Xiuyun Wang, Yafei Jiang, Haifeng Qi, Lirong Zheng, Jianxin Lin and Lilong Jiang Yanliang Zhou† National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Congying Wang† National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Xuanbei Peng† National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Tianhua Zhang National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Xiuyun Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Yafei Jiang Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055 , Haifeng Qi Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Lirong Zheng Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 , Jianxin Lin National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 and Lilong Jiang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 https://doi.org/10.31635/ccschem.021.202100912 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Ammonia (NH3) synthesis at mild conditions is of great significance, while the significant bottleneck of this process is the activation of N2 to realize the desired NH3 synthesis performance, which requires deep insight and rational design of active sites at the atomic level. Here, were synthesized atomically dispersed Co-based catalysts with different Co-N coordination numbers (CNs) to explore the coordination-sensitive NH3 synthesis reaction for the first time. Our studies showed that Co-based catalysts increased the NH3 synthesis rate gradually with a decrease in CN. The Co-N2 catalyst exhibited the highest NH3 synthesis rate of 85.3 mmol gCo−1 h−1 at 300 °C and 1 MPa, which outperformed most of the previously reported Co-based catalysts. Various characterizations and theoretical calculations demonstrated that atomically dispersed Co catalyst with low CN could generate more unoccupied Co 3d charges and tetrahedral cobalt(II) sites. The unoccupied Co 3d charge, in turn, promoted the electron donation from the Co active center to the antibonding π-orbital (π*) of N2 and expedites N2 hydrogenation. Furthermore, the Co-N2 catalyst with more tetrahedral cobalt(II) sites could effectively facilitate the desorption of N-containing intermediate species (such as *NH3 and *N2H4) to obtain a high NH3 synthesis rate. Download figure Download PowerPoint Introduction Ammonia (NH3) is not only an essential raw material for the synthesis of nitric acid, fertilizer, ammonium salt, and so on but also a promising renewable energy carrier.1,2 The state-of-the-art manufacture of NH3 comes from the Haber–Bosch (HB) process using iron-based catalysts that require harsh reaction conditions (400–500 °C, 10–30 MPa), thereby suffering from high energy consumption and environmental pollution problems.3–5 Therefore, it is urgent to develop effective routes that are both energy-saving and environment-friendly. Most recently, electric power generated from renewable energy sources is widely used for hydrogen (H2) production via water electrolysis. It becomes economically acceptable to NH3 synthesis from renewable H2 and N2 via an electrolysis-driven HB (eHB) process.6 The eHB route is attractive for realizing efficient utilization of renewable energy and a "carbon-free" society. However, the major challenge of NH3 synthesis lies in the activation of N2 (945 kJ mol−1) at mild conditions to realize desired NH3 synthesis rate,7 which would require a deep insight into the reaction mechanism; therefore, the development of advanced, efficient catalysts that can produce NH3 at mild conditions is beneficial. Currently, industrial applied Fe- or Ru-based catalysts that exist as nanoparticles usually obey the dissociative mechanism,8,9 requiring harsh conditions to dissociate the stable N≡N triple bond. In contrast to the N2 dissociative mechanism, most of the enzyme-catalyzed N2 fixation processes followed the associative mechanism, wherein the N≡N triple bond breaks stepwise after partial hydrogenation of the N2 molecule.10,11 Notably, the dissociation energy of the N–N bond (297 kJ/mol) from *N2H4 intermediates is less than a third of that from the N≡N triple bond. Consequently, the exploitation of advanced catalysts that obey the N2 hydrogenation process instead of direct N2 dissociation is promising for realizing the desired NH3 synthesis performance at mild conditions. Further, NH3 synthesis is a typical structure sensitivity reaction, where the pathway of N2 activation is closely related to the geometric and electronic structures of active sites.8,12 For instance, theoretical studies have demonstrated that N2 could preferentially undergo hydrogenation via an associative pathway other than direct dissociation on Ru(0001) surface,13 RhCo3 clusters,12 and Co3Mo3N catalysts.14 In addition, very recently, a limited number of researches have provided us with experimental evidence that the N2 hydrogenation to N2Hx species is preferred over atomically dispersed Ru and Co, as well as Li-promoted Ru catalysts.6,15,16 Among them, atomically dispersed catalysts present well, having the potential for NH3 synthesis at mild conditions, as the direct N2 dissociation is difficult while the N2 hydrogenation is feasible on single-atom sites.15 Meanwhile, atomically dispersed catalysts with definite structures offer the possibility to investigate the geometric and electronic effects on NH3 synthesis at the atomic level. Differing from nanoparticle catalysts, in which the particle size and crystal face usually play an important part in the reactivity, catalytic activity of atomically dispersed catalysts is mainly determined by the coordination environment of single-atom sites.17–19 However, up to now, the modulation of the coordination structure of single-atom active sites to optimize the catalytic activity of NH3 synthesis has not yet been researched. Moreover, the effects of geometric and electronic structures in these single-atom sites on the dynamic transformation of intermediates remain unclear. Herein, we synthesized a series of atomically dispersed Co-Nx (x = 2∼4) catalysts through the pyrolysis of nitrogen-anchored Co-based precursors. The coordination numbers (CNs) of Co-N over the as-prepared catalysts were modulated by varying the calcination temperature. The coordination-sensitive NH3 synthesis reaction in the case of Co-based catalyst is disclosed through a combination of a suite of elaborated experimental characterizations and theoretical calculations. The geometric and electronic structures of Co active sites could be simultaneously tuned via the modulation of Co-N CNs, which are closely related to the N2 activation and desorption of N-containing intermediate species (such as *N2H4 and *NH3), thus resulting in different catalytic behaviors in the NH3 synthesis. This finding is beneficial to rationally design advanced Co-based catalysts by regulating the geometric and electronic structures to realize efficient NH3 synthesis at mild conditions. Experimental Methods Catalyst preparation Synthesis of hollow N-doped porous carbon spheres support The synthetic method of hollow N-doped porous carbon spheres (HNPCSs) support was referenced in previous work.20 Briefly, NH3 solution (10 mL) and ethanol (240 mL) were dissolved in deionized water (80 mL) and stirred evenly at room temperature (RT). Then, tetraethyl orthosilicate (11.2 mL) was added to this solution and stirred further for 1 h. Subsequently, poloxamer, resorcinol, and formaldehyde (2.24 mL) were added to the above solution and continually stirred for 0.5 h. After the addition of melamine (1.26 g) and formaldehyde (1.68 mL), the mixture was treated at 100 °C for 24 h. After filtration and washing, the sample obtained was calcined under an Ar atmosphere at 700 °C for 2 h. Finally, the sample was etched by hydrofluoric acid (HF) solution to remove the SiO2 substrate. After filtration, washing, and drying, the resulting substance was herein referred to as HNPCSs support. Synthesis of Co-Nx and Co-NPs catalysts In detail, HNPCSs (120 mg) and cobalt phthalocyanine (CoPc; 80 mg) was dispersed in dimethylformamide (DMF; 120 mL), respectively, and the mixtures were stirred for 2 h. Then the CoPc and DMF mixed solution was added slowly to the HNPCSs suspension. After stirring for 24 h, the sample was dried in a vacuum dryer at 60 °C for 12 h. Finally, the samples were pyrolyzed under Ar atmosphere at different temperatures of 450, 550, and 650 °C for 2 h, denoted as Co-N4, Co-N3, and Co-N2, respectively. The synthetic procedure used for the Co-NPs catalyst was similar to that of Co-N2, except that the calcination was performed at 700 °C for 10 h. Synthesis of catalysts as reference The Cs-Co/MgO catalyst was synthesized by an impregnation method. In detail, a certain amount of cobalt nitrate hexahydrate and cesium nitrate was dissolved in deionized water. MgO support was added to this solution, and the mixture was stirred evenly. After impregnation at RT for 24 h, the mixture was dried at 80 °C for 24 h. The acquired catalyst was denoted as Cs-Co/MgO, in which the content of Co was the same as that over Co-N2, and the molar ratio of Cs to Co was ∼1.0. A Fe-based catalyst was purchased commercially from Fujian SJ. FD. Co. Ltd. (Fuzhou, Fujian, China) for comparative studies. NH3 synthesis performance test The NH3 synthesis activities over the as-synthesized catalysts were tested in a pressurized fixed bed reactor. In detail, the catalyst (0.2 g, 20–30 mesh) was loaded into the stainless steel reactor (inner diameter 10 mm). Before measurement, the catalysts were treated in a 25%N2–75%H2 feed gas at 300 °C for 2 h. Subsequently, the catalyst performance was evaluated in 300 °C at a weight hourly space velocity (WHSV) of 60,000 mL g−1 h−1 and 1 MPa. The outlet NH3 was trapped by diluted H2SO4 solution and then detected by ion chromatography (DIONEX, ICS-600; Thermo Scientific, Waltham, MA, USA). The turnover frequency (TOFCo) was calculated based on the total molar of Co metal using the following equation: TOF Co = r n M (1)where, r represents the NH3 synthesis rate (mol g−1 s−1), nM is the molar amounts of Co obtained from inductively coupled plasma (ICP) analysis. Catalyst characterization X-ray diffraction (XRD) measurement was recorded on a PANalytical X'Pert Pro diffractometer (PANalytical, Almelo, The Netherlands) using a Cu Kα radiation source. N2 adsorption/desorption isotherms were collected on an ASAP 2020 apparatus (MicroMetric, Lincolnshire, United Kingdom). Elemental analysis (EA) was conducted with a Vario EL-Cube instrument (Elementar, Jiangsu, China) to measure the N contents of the samples. The Co contents of the catalysts were measured by ICP atomic emission spectroscopy (ICP-AES) analysis equipped with an Ultima 2 spectrometer (Perkin-Elmer, Waltham, MA, USA). Catalyst morphologies were detected on scanning electron microscopy (SEM; a Hitachi Model S-4800; Hitachi, Tokyo, Japan). Transmission electron microscopy (TEM) images of the catalysts were obtained on a JEM-2010 instrument (JEOL Ltd., Tokyo, Japan). Aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) analysis was performed on a JEOL JEM-ARM 200 F instrument (JEOL Ltd., Tokyo, Japan). Raman spectroscopy was measured on a multichannel modular triple Raman system (Renishaw Co., London, United Kingdom). Ultraviolet photoelectron spectroscopy (UPS) was measured using a helium resonance lamp offering He I (hν = 21.2 eV) photons. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific, Shanghai, China) with a monochromatic Al Kα source (hν = 1486.6 eV) as the X-ray source. The XPS binding energy was referenced to the C 1s peak at 284.6 eV of adventitious carbon. Electron-paramagnetic resonance (EPR) test was performed on an E500 spectrometer (Bruker-BioSpin, Shanghai, China) with a 100 KHz magnetic field modulation at RT. UV–vis diffuse reflection spectrum (DRS) data were obtained from a Perkin Elmer Lambda 750s UV–vis spectrometer (Perkin-Elmer, Waltham, MA, USA). The productions of NH3 synthesis were trapped at the aqueous solution containing sulfuric acid and para-(dimethylamino) benzaldehyde for the UV–vis DRS measurement. X-ray absorption spectroscopy (XAS), containing X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses were performed at the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF), China. Spectra were collected at the Co K-edge in transmission mode with a Si (111) double crystal monochromator. The Co foil and CoPc samples were used as references. Density functional theory (DFT) calculations First-principle calculations were performed using the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE),21 as implemented in the Vienna ab initio Simulation Package (VASP 5.4.4).22 The valence electronic states of all atoms were expanded in a plane-wave basis set with a cutoff energy of 400 eV, and gamma points were used for Brillouin zone integration. All atoms were allowed to relax until the forces fell below 0.02 eV Å−1. The energy convergence criterion was set to 10−5 eV. Grimme D3 method with the zero-damping function was used to describe van der Waals interactions.23 The graphene supercell with a surface periodicity of 10 × 10, including 200 atoms as a basis, was employed to construct the Co-Nx (x = 4, 3, and 2) moieties. A vacuum space of 20 Å was used in the normal direction of the graphene plane to ensure negligible interaction between the mirror images. Results and Discussion Structure characterization The process for the synthesis of Co-Nx catalysts with different Co-N CNs is depicted in Figure 1a. The atomically dispersed Co-Nx catalysts were prepared by CoPc pyrolysis supported on HNPCSs precursor. The CN of Co-Nx catalysts was modulated by varying the pyrolysis temperature (as shown in the "Catalyst preparation" section). To highlight the coordination structure of atomically dispersed Co center, Co nanoparticle loaded on the same support (Co-NPs) was prepared for comparison. EXAFS analyses of the Co K-edge were conducted to demonstrate the coordination structure of Co-Nx catalysts. The Co K-edge EXAFS spectra of as-synthesized catalysts and reference Co foil and CoPc, are illustrated in Figure 1b. An apparent peak located at 1.5 Å for the as-synthesized Co-Nx samples was attributed to Co-N coordination, compared with CoPc reference.24 Meanwhile, no typical peaks for Co-Co coordination at 2.2 Å were visible, indicating the atomically dispersed Co atoms on these Co-Nx catalysts. Besides, the peak intensity of Co-N coordination decreased with increased pyrolysis temperature, suggesting a decrease in Co-N CN. Based on fitting results, the average CNs of Co-N of the three samples were 3.9, 3.0, and 2.1 (Table 1), and thus, the samples were denoted as Co-N4, Co-N3, and Co-N2, respectively. Additionally, the fitting result of Co K-edge EXAFS spectra of Co-N2 based on our hypothetical model was consistent with the experimental result (Figure 1c), demonstrating the validity of the depicted models. In the case of Co-NPs, the Co-Co coordination existed predominant beside the weak Co-N coordination, indicating that Co entities were distributed mainly in the form of nanoparticles. Figure 1 | (a) Schematic diagram of the preparation process of Co-Nx with different CNs. (b) Co K-edge EXAFS spectra of as-prepared samples. (c) EXAFS spectra and the corresponding curve-fitting results of Co-N2 catalyst. Download figure Download PowerPoint Table 1 | EXAFS Data Fitting Results of the As-Synthesized Catalysts Samples Shell CN R (Å)a σ2 × 102 (Å2)b ΔE0 (eV)c R-Factor (%) Co foil Co-Co 12.0 2.49 0.7 6.3 0.9 Co-N4 Co-N 3.9 1.92 0.4 8.1 0.07 Co-N3 Co-N 3.0 1.91 0.3 7.2 0.09 Co-N2 Co-N 2.1 1.89 0.3 −1.1 0.8 Co-NPs Co-N 2.0 2.03 0.7 −1.8 0.7 Co-Co 2.1 2.49 0.7 −1.8 0.7 Note: The accuracy of parameters: CN, ±20%; R, ±1%; σ2, ±20%; ΔE0, ±10%. The fitting range in k- and R-space is 3.0–12.3 Å−1 and 1.0–2.7 Å, respectively. aR, bonding distance. bσ2, the Debye–Waller factor. cΔE0, inner potential shift. SEM ( Supporting Information Figure S1) and TEM images (Figures 2a and 2b) simultaneously displayed uniform microspheres with the hollow structure of Co-Nx and Co-NPs catalysts. The average diameter of the microspheres was ∼150 nm of these samples. The high-resolution TEM (HR-TEM) images (Figure 2c and Supporting Information Figure S2) did not show any Co nanoparticles sights, implying a high dispersion of Co species as clusters or single atoms over Co-Nx catalysts. We further used the AC-HAADF-STEM characterization to inspect the Co species over Co-Nx catalysts. We found that plenty of highly dispersed Co single atoms were visible over Co-N2 (Figure 2d), Co-N3 (Figure 2e), and Co-N4 (Figure 2f) catalysts. In comparison with Co-Nx catalysts, distinct nanoparticles were observed over the Co-NPs catalyst ( Supporting Information Figure S3). Combined with the EXAFS results, we deduced that atomically dispersed Co-Nx catalysts with different Co-N CNs were synthesized successfully. Figure 2 | (a and b) TEM and (c) HR-TEM images of Co-N2 catalyst. AC-HAADF-STEM images of (d) Co-N2, (e) Co-N3, and (f) Co-N4 catalysts. Download figure Download PowerPoint The XRD patterns ( Supporting Information Figure S4) showed that all Co-Nx displayed a broad diffraction peak at ∼26.4°, to the crystal plane of carbon The highly was by Raman measurement. The Raman spectra ( Supporting Information Figure displayed that samples peaks at and corresponding to the and of or carbon and Furthermore, the intensity of the to of these samples were ( Supporting Information Table that the coordination structure has a on carbon Notably, the diffraction peaks on the Co-N4 sample are attributed to the CoPc suggesting an of CoPc species and the of Co-N4 which with the fitting results of XRD diffraction peaks were related to Co or CoPc species in the Co-N3 and Co-N2 samples. to the above EXAFS and our demonstrated that the pyrolysis of CoPc would form atomically dispersed Co sites over Co-Nx catalysts after the pyrolysis of the at 700 °C for 10 h, the of diffraction peaks at and in Co-NPs catalyst were to the (111) and crystal of Co respectively. The average size of Co nanoparticles over Co-NPs was to the of (111) crystal plane via which with the particle observed in the HR-TEM ( Supporting Information Figure S3). showed that the Co contents in these samples were distributed in the range of ( Supporting Information Table to the results, the N contents of the synthesized Co catalysts ( Supporting Information Table S1) decreased with increased pyrolysis temperature. N2 ( Supporting Information Table S1) results showed that the surface of Co-Nx catalysts increased from to g−1 with increased pyrolysis temperature. However, at a high pyrolysis temperature, the surface of Co-NPs catalyst decreased to of the geometric and electronic structure was performed to insight into the of different CNs on the geometric and electronic structure of Co sites. to the results (Figure the peak intensity in the range of eV was of the geometric of Co that tetrahedral or The from the tetrahedral of Co sites were to to this The intensity of this peak over Co-N2 was than that of Co-N3 or Co-N4, indicating that more Co were at the tetrahedral sites in the Co-N2 Besides, the of the absorption in the images (Figure could be used to the states of Co A of absorption from Co-N4 to Co-N2 was indicating that the valence of Co species decreased with decreased CN of the Co-Nx catalysts. Figure | Co K-edge spectra and the images in the range of (a) eV and (b) eV. Download figure Download PowerPoint Further, XPS were performed to obtain the valence of Co and N species over the as-synthesized catalysts. shown in Figure the Co spectra were fitting into the peaks with binding of and eV, to and peaks of surface Co were observed on these catalysts, except for Co-NPs, which with the results of XRD and The atomic ratio of over the Co-N4, Co-N3, and Co-N2 catalysts determined from XPS results were and (Table This finding was supported by the of Co entities over Co-N2 as shown in the (Figure results, that the decreased CN have been promoted to generate more unoccupied Co 3d over the Co-N2 results were further by The spectra ( Supporting Information Figure showed that the of and over these catalysts were attributed to an electron in the of and carbon We found that the intensity of increased with the of CN, indicating that more on the Co-N2 catalyst. The N 1s spectra (Figure three of N and N over Co-Nx the comparison of the content of different N species from the peak (Table we observed that the N content increased from to while the N content decreased from to with of CN over Co-Nx catalysts. This result that the atomically dispersed Co mainly with the these demonstrated that the geometric and electronic structures of the atomically dispersed Co catalyst were modulated through different CNs of the in turn, the catalytic activity the NH3 synthesis. Figure | (a) Co and (b) N 1s XPS spectra of the as-synthesized catalysts. Download figure Download PowerPoint Table 2 | The XPS Spectra Results of the As-Synthesized Catalysts Samples N (%) N (%) N (%) (%) (%) (%) Co-N4 24 60 Co-N3 12 Co-N2 20 Co Catalyst activity and Figure the NH3 synthesis rate over the as-synthesized catalysts with a 25%N2–75%H2 gas and a of mL The NH3 synthesis rate in the case of Co-N2 was 85.3 gCo−1 h−1 at mild conditions of 300 °C and 1 MPa, than over Co-N3 gCo−1 and Co-N4 gCo−1 An NH3 synthesis rate of gCo−1 h−1 was acquired over Co-NPs, which is more to over the atomically dispersed Co-Nx catalysts. Meanwhile, the NH3 synthesis rate over the carbon as support was mmol h−1 at 300 °C and 1 MPa, as shown in Supporting Information Table suggesting that the of support to the catalytic activity of Co-Nx could be results that NH3 synthesis is a coordination-sensitive reaction, and the Co-N2 catalyst with the CN the highest NH3 synthesis rate at mild conditions. It has been reported that the addition of a electronic to Co-based catalysts could NH3 synthesis performance, referred to as an electronic Herein, we the of to the highly active Co-N2 catalyst on NH3 synthesis It showed that the NH3 synthesis rate over was that of the (Figure The typical Cs-Co/MgO and the Fe-based catalysts were also performed and evaluated for comparison ( Supporting Information Table 300 °C and 1 MPa, the NH3 synthesis rate over Co-N2 mmol gCo−1 was than of Cs-Co/MgO mmol and Fe-based mmol catalysts, indicating the performance of NH3 synthesis activity over Co-N2 catalyst. For based on the total number of Co active sites on these catalysts, were calculated and depicted in Figure Notably, the of Co-N2 and were and respectively, which outperformed most of the previously reported Co-based catalysts in Supporting Information Table Figure | (a) NH3 synthesis rate based on Co content over as-synthesized catalysts at 1 and 300 (b) TOF was calculated on the total number of Co atoms basis over the catalysts. (c) N2 and (d) H2 reaction over Co-N2 and Co-NPs catalysts. Download figure Download PowerPoint A test of the Co-N2 catalyst was conducted at 300 °C and 1 MPa. shown in Supporting Information Figure the NH3 synthesis rate a stable of mmol gCo−1 h−1 a The used catalyst was further tested using HR-TEM and EXAFS shown in the Supporting Information Figure no or at different were observed over the used Co-N2 catalyst. Moreover, the Co K-edge EXAFS spectrum of the used Co-N2 catalyst ( Supporting Information Figure showed that existed the Co-N coordination, while no Co-Co coordination was indicating the of a highly stable Co-N2 active in the process of NH3 synthesis. the of these demonstrated the of Co-N2 catalyst in NH3 synthesis at mild conditions. studies studies were conducted to demonstrate the of N2 and H2 activation over the Co-Nx catalysts. The reaction of N2 and H2 usually as related to the and the of hydrogen For most catalysts, the reaction of N2 is to suggesting that the could be the direct dissociation of the N≡N triple bond. shown in Figure the reaction of N2 over Co-N2 was than that of over low reaction of N2 has been reported over Ru-based catalysts supported by or where the of bond than direct N≡N triple bond dissociation was Additionally, the reaction of H2 on Co-NPs was (Figure The demonstrated that the of H2 on Co to hydrogen

Accurate Au $L$-edge EXAFS (extended x-ray absorption fine structure) spectra of elemental Au have been collected and analyzed in a wide range of temperatures from 80 to 1400 K in the solid and liquid phases. Suitable samples for high-temperature measurements in the liquid phase were obtained by in situ reduction of an organic precursor. The data analysis was carried out using reliable multiple-scattering calculations including relativistic corrections (GnXAS), especially important for this heavy element. Simultaneous ${L}_{1}$-, ${L}_{2}$-, and ${L}_{3}$-edge refinements were performed and local structural results were compared with those obtained in previous studies based mainly on x-ray diffraction experiments. In the solid phase, an increased asymmetry of the first-neighbor distribution is observed for increasing temperatures while the average distance is found to be compatible with present thermal expansion data. The structure refinement of liquid Au was carried out considering the short-range contribution to the pair distribution function $g(r)$, using available x-ray diffraction data as a constraint for long-range order. We have shown that EXAFS is extremely sensitive to the local structure and that the $g(r)$ of liquid Au is characterized by a clear shift of the foot of the $g(r)$ and a slight shortening of the position of the maximum as compared with the previous data. The present data put to a test advanced computer simulations based on embedded atom models.

The existing forms of Fe are of great interest since they have a profound effect on the biological availability of Fe. In this work, aerosol samples collected in different seasons and at different locations in the Qingdao region were examined by means of extended X-ray absorption fine structure (EXAFS) K-edge analysis of Fe, X-ray diffraction (XRD) and Fe content analysis. The results showed that an iron ion in aerosol particles is surrounded on average by 5.8 (coordinated) O ions. For the six samples examined, the coordination number of the first Fe-O coordination subshell is always 3 with a coordination distance (with O) in the range of 1.952~1.966±0.002 Å, while the coordination number of the second subshell varies from 2.2 to 3.0 with a coordination distance of 2.108±0.002 Å. The coordination is approximately consistent with that of α-Fe2O3, suggesting that iron in aerosol samples is mainly present in the form of α-Fe2O3. The fact that the coordination number in the second subshell is smaller than that of α-Fe2O3 might be an indication that there is a small amount of FeO mixed with α-Fe2O3 in aerosol particles. Existence of FeO is confirmed by a later XRD experiment.

기계적 합금화 방법을 통하여 Fe-Si-C계 합금을 제조하였으며, 합금화 과정에 따른 구조 및 자기적 특성을 분석하였다 구조적 특성의 분석은 X-선 회절 실험과 EXAFS(extended X-ray absorption fine structure)실험을 통하여 수행하였고, 자기적 특성은 시료진동형자력계와 M ssbauer 분광법으로 연구하였다. 합금화 시간이 증가함에 따라 Si, C 원자들은 Fe 구조 속으로 확산되었으며, 12시간 이후로는 과포화된 bcc 고용체를 형성하였다. 합금화 시간에 따른 포화 자화값 및 초미세 자기장은 4시간까지 급격한 감소를 나타내었으며, 그 이후로는 완만한 감소를 나타내었고, 12시간 이후로는 변화가 적음을 나타내었다. 이러한 변화는 구조 특성 분석 결과와 일치하였다. Fe-Si-C alloy system has been made by mechancial alloying process. The structural and magnetic properties were analysed as a function of processing times. The structural properties were investigated by X-ray diffraction and EXAFS experiments. The magnetic properties were measured by vibrating sample magnetometer and M ssbauer spectroscopy. As processing time increases, Si and C atoms diffuse into a <TEX>${\alpha}$</TEX>-Fe structure and then bcc solid solution is formed after 12 hours of the processing time. The magnetization and the hyper fine field decreased steeply up to 4 hours and then it changed slowly. After 12 hours of the processing time, it almost saturated. These results were agreed with the structural variation.

The amount of data collected during synchrotron X-ray diffraction (XRD) experiments is constantly increasing. Most of the time, the data are collected with image detectors, which necessitates the use of image reduction/integration routines to extract structural information from measured XRD patterns. This step turns out to be a bottleneck in the data processing procedure due to a lack of suitable software packages. In particular, fast-running synchrotron experiments require online data reduction and analysis in real time so that experimental parameters can be adjusted interactively. Dioptas is a Python-based program for on-the-fly data processing and exploration of two-dimensional X-ray diffraction area detector data, specifically designed for the large amount of data collected at XRD beamlines at synchrotrons. Its fast data reduction algorithm and graphical data exploration capabilities make it ideal for online data processing during XRD experiments and batch post-processing of large numbers of images.

In most studies related to milled powders, the grain size1 is analyzed via X-ray diffraction (XRD) experiments, and a transmission electron microscopy (TEM) image with high magnification, if provided, is used primarily to confirm the results obtained by XRD experiments. This widely used approach is reasonable in light of the difficulties associated with TEM sample preparation. The present study, however, addresses the hypothesis that such an approach may not be valid when there is an inhomogeneous distribution of grains present. TEM examination, carried out in carefully prepared Al-7.5 wt% Mg samples, in which a global region is observable by TEM, provided the opportunity for quantitative analysis of grain size in cryomilled powders having an inhomogeneous distribution of grain sizes. The cryomilled Al-7.5 wt% Mg had a bimodal grain microstructure of 77% (area fraction) fine grains in the range of 10 to 60 nm and 23% coarse grains of approximately 1 μm. The results show that the XRD analysis yields a grain size that is close to that present in the fine-grained regions (i.e., 10–60 nm). The present study also systematically investigated the influence of the nine possible combinations of the Cauchy (C) and the Gaussian (G) approximations on the calculated grain size value, and the results show that the CC-CC approximation resulted in the largest calculated grain size, the GG-GG generated the smallest one, and the CG-CG, the approximation recommended by Klug and Alexander [1], led to a calculated grain size that is approximately equal to the average one from the CC-CC and GG-GG approximations. The maximum possible fluctuation of grain size values stemming from the various approximations is 38%.

In this work, the binary III-V semiconductors GaP, GaAs, GaSb, InP, InAs, und InSb, and the ternary alloys (In,Ga)P and (In,Ga)As were studied using temperature dependent extended x-ray absorption fine structure spectroscopy (EXAFS) measurements. In the alloy systems, the element-specific effective bond-stretching force constants were determined as a function of composition. To broaden the fundament of the conclusions, literature values of the ternary II-VI alloy Zn(Se,Te) were incorporated in the discussion. Different trends with composition are visible for (In,Ga)P on the one hand and (In,Ga)As and Zn(Se,Te) on the other hand. Strikingly, most of the six bond species under study (Ga-P, In-P, Ga-As, In-As, Zn-Se, and Zn-Te) exhibit the same relative change in bond-stretching force constant as a function of the relative change in bond length. Additionally, exactly the same relation is known from the literature as describing pressure-dependent EXAFS measurements of CdTe. The composition-dependent change of bond-stretching force constants in ternary zinc-blende semiconductor alloys is therefore caused mainly by the forced bond length change occurring in these materials. In addition to the bond-stretching force constants, effective bond-bending force constants were determined for the binary materials. Either type of force constants can be described as a function of ionicity and reduced mass of the interatomic bond. In the analysis great care has been taken to properly assess the uncertainties of the results. The comprehensive testing scenarios required to do this, also enable a general evaluation of the potential of the determination of bond force constants from temperature-dependent EXAFS measurements.