X-ray Absorption Near Edge Spectroscopy Results Research Articles

Reactivity and Evolution of Ionic Solid-Electrolyte-Interphases in Battery Electrolytes

The instability of the solid electrolyte interphase (SEI) on the lithium (Li) metal anode is a major challenge towards improving the Coulombic efficiency (CE) and cycle life of Li batteries. In classical SEI models with mosaic and layered structures, ionic phases (e.g., Li2O and LiF) are enriched closest to the Li|SEI interface, while the outer SEI are highly dependent on electrolytes and typically assigned to less-reduced species, such as semi-carbonates and organic Li salts.1, 2 The formation of SEI on Li is often interpreted as a consequence of reactivity between Li metal and electrolytes; however, there remains a lack of understanding about the interplay between electrolytes and specific SEI phases once they are formed. Some recent computational studies have shed light on the chemical reactivity between thin (~1 nm) single-phase inorganic SEI (Li2O, LiOH, and Li2CO3) on Li metal and DME electrolytes containing LiTFSI or LiFSI salts, using ab initio molecular dynamics (AIMD) calculations;3, 4 experimental insights are still much-needed.Herein, we study two ionic SEI phases, Li2O (Fig. 1a-b) and LiF (Fig. 1c-d), that are nearly ubiquitously found across many native SEIs, and investigate their stability at the SEI|electrolyte interface. We find, by using a combination of electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and non-destructive X-ray absorption near-edge spectroscopy (XANES), that the ionic SEI|electrolyte interfaces can undergo significant chemical evolution as a function of electrolyte. As shown in Fig. 1a, distinctive lower-frequency semicircles emerged in the Nyquist plots of EIS when 1 M LiTFSI EC/DEC and 1 M LiPF6 EC/DEC were used, indicating significant changes at the interface between the ionic Li2O SEI and carbonate-based electrolytes, especially in the presence of LiPF6 salt. The surface atomic concentration of F also increased significantly from 4 at.% in 1 M LiTFSI DOL/DME to 44 at.% in 1 M LiPF6 EC/DEC, suggesting that a F-rich Li2O|electrolyte interface leads to an additional charge transfer barrier. Non-destructive Li K-edge XANES of the Li2O SEI soaked in 1 M LiPF6 EC/DEC electrolyte displayed a dominant LiF peak at 62.3 eV with an evident blue shift compared with the standard LiF peak at 62.0 eV (Fig. 1b),5 which could be attributed to the solvation of F-rich species at the interface by organic phases from carbonates decomposition, forming an “organic/F-rich” outer SEI layer. Similar organic/F-rich outer SEI was also observed on the LiF SEI soaked in 1 M LiPF6EC/DEC electrolyte, as evidenced by EIS (Fig. 1c), XPS (Fig. 1d) and XANES results. The organic/F-rich outer SEI induced by reactivity between ionic SEI phases and carbonate-based electrolytes can significantly exacerbate the subsequent plating overpotentials. Our experimental results suggest that electrolyte solvent and salt selection is of great importance to optimize transport in ionic-rich Li interfaces, which can have important implications ultimately for SEI stability and thus CE. Figure 1. (a) Nyquist plot of EIS spectra of Li|Li2O SEI in the three different electrolytes, where the thickness L and conductivity σ of the Li2O SEI were acquired by fitting the higher-frequency semi-circle (or the sole semi-circle in the case of DOL/DME).6 (b) Li K-edge XANES fluorescence yield (FLY) of Li|Li2O interface after soaking in EC/DEC solvent or 1 M LiPF6 EC/DEC electrolyte for 20 hours. (c) Nyquist plot of EIS spectra of Li|LiF SEI in the three different electrolytes. (d) C1s and F1s XPS of the surface layer on Li|LiF SEI after soaking in each electrolyte.

Effect of FSP-inserted Cu on Physicochemical Properties of Cu/Al2O3 Catalyst

The copper inserted on Cu/Al2O3 catalysts with various Cu loading (10-40 wt%) were synthesized via flame spray pyrolysis (FSP). These catalysts were characterized using X-ray diffraction (XRD), N2 physisorption, temperature programmed reduction (TPR) and X-ray absorption near edge spectroscopy (XANES). The XRD results confirmed the formation of copper aluminate spinel (CuAl2O4) on the FSP-inserted Cu catalyst. The CuO crystallite size of the Cu/Al2O3 catalysts was increased with increasing Cu loading during the flame spray pyrolysis step. The incorporation of copper and aluminum precursors during the flame spray pyrolysis step can inhibit the growth of Al2O3 particles resulting in higher BET surface area and smaller particle size than pure Al2O3 support. The data from TPR and XANES results can predict the ratio of CuO and CuAl2O4 in the FSP-made support. Less than 20 wt% loading of the FSP-inserted Cu showed high concentration of CuAl2O4 phase in the FSP-made material. The composition of CuO and CuAl2O4 phase can be controlled by varying Cu loading in flame spray pyrolysis step. This is a promising alternative way to synthesize the desired catalyst. An example was the catalytic testing of the selective hydrogenolysis of glycerol. The presence of both CuO and CuAl2O4 phases in the Cu/Al2O3 catalyst enhanced the catalytic activity and promoted the selectivity to acetol product. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

X-ray absorption near edge spectroscopy study of warm dense MgO

We report time-resolved X-ray Absorption Near Edge Spectroscopy (XANES) measurements of warm dense MgO. We used a high power nanosecond pulse to drive a strong uniform shock wave into an MgO sample, and a picosecond pulse to generate a broadband X-ray source near the Mg K-edge. We used this setup to obtain XANES spectra across a large area of the phase diagram, with densities up to 6.8 g/cc and temperatures up to 30 000 K, conditions at which no prior investigations of electronic and ionic structure exist. Our XANES results, together with quantum molecular dynamic simulations, demonstrate that the sample metallizes due to the bandgap closure as it melts, after which it shows typical behavior for a disordered ionic liquid.

Activating abnormal capacity in stoichiometric NaVO3 as cathode material for sodium-ion battery

Traditional sodium-ion battery cathodes utilize the change in oxidation state of the transition metal(s) in the structure to accommodate the electron transfer during charge and discharge. Recent researches on sodium-rich compounds such as Na2RuO3 and Na2IrO3 suggest that anionic reaction with oxygen can be an additional source of electrons during electrochemical reactions. Here we demonstrate for the first time that stoichiometric NaVO3, despite the valence of vanadium of 5+, delivers a reversible capacity by activating it beyond 4.5 V. Elemental analysis confirms Na removal and insertion from/into NaVO3 during charge/discharge. X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy results show that the oxidation state of vanadium remains unchanged, while oxygen is likely to compensate for the charge transfer during charge/discharge. Theoretical calculation on spin density of electrons in the lattice also supports the involvement of oxygen during sodium removal. Our demonstration of the unique behaviors of NaVO3 provides a new exciting direction for research in sodium-ion battery cathodes.

Examination of the site preference in garnet type (X3A2B3O12; X=Y, A/B= Al, Ga, Fe) materials

Although many studies of quaternary garnet type oxides have been performed, few studies have investigated quinary garnet type oxides. In this study, quinary (Y3AlxGayFe5-x-yO12) type garnet materials have been studied along with three quaternary garnet-type materials (Y3AlxGa5-xO12, Y3AlxFe5-xO12, and Y3GaxFe5-xO12) using X-ray absorption near-edge spectroscopy (XANES) and powder X-ray diffraction. Fe K-, Ga K-, and Al L2,3- edge XANES spectra were collected and analyzed to study the change in metal site preference depending on composition. The analysis of Fe K-edge XANES spectra indicated that Fe3+ primarily occupied the octahedral site in the Y3AlxFe5-xO12, Y3GaxFe5-xO12, and Y3AlxGayFe5-x-yO12 materials. The Ga K-edge XANES spectra from Y3AlxGa5-xO12 confirmed that Ga3+ has a strong preference for residing in the tetrahedral site. Ga increasingly occupies the tetrahedral site with increasing Al content in Y3AlxGa5-xO12. Al L2,3-edge XANES spectra from Y3AlxGa5-xO12, Y3AlxFe5-xO12, and Y3AlxGayFe5-x-yO12 confirmed the site preference of Al, Ga, and Fe. Rietveld refinement of powder X-ray diffraction patterns from the Y3AlxGa5-xO12 series was also performed and agree with the XANES results.

Damage and annealing recovery of boron-implanted ultra-shallow junction: The correlation between beam current and surface configuration

Abstract The condition of the beam current in the implantation process is a key issue in the damage rate and structural evolution in the sequent annealing process, especially for ultra-shallow layers. In this work, we develop a compensative optical method combined with UV Raman, X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge spectroscopy (XANES) to inspect the influence of the beam current in the implantation process. The optima condition of the beam current in the implantation process is determined by higher effective Si-B bond portion in UV Raman spectra and less the peak of B–B bond in XPS spectra which is caused by B cluster defects. Results of XANES indicate that the B oxide layer is formed on the surface of the ultra-shallow junction. The defects in the ultra-shallow junction after annealing are analyzed by novel optical analyses, which cannot be inspected by a traditional thermal wave and resistance measurement. This work exhibits the structural variation of the ultra-shallow junction via a variant beam current and provides a valuable metrology in examining the chemical states and the effective activation in the implantation technology.

Polymerized Complex Synthesis and Effect of Ti Dopant on Magnetic Properties of LaFeO3 Nanoparticles.

Structure, characterization, and magnetic properties of Ti-doped LaFeO3 (LaFe(1-x)Ti(x)O3, x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) nanoparticles synthesized by polymerized complex method are investigated. All LaFe(1-x)Ti(x)O3 nanoparticles were successfully obtained from calcination of the precursor at 1050 degrees C in air for 2 h. The calcined LaFe(1-x)Ti(x)O3 samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and vibrating sample magnetometry (VSM). The XRD and TEM results showed that all LaFe(1-x)Ti(x)O3 samples had a single phase nature with the orthorhombic structure. The valence states were mixed in the Fe3+ and Fe4+ state for Fe ions and Ti4+ state for Ti ions, as confirmed by XPS and XANES results. The weak ferromagnetic behavior with the highest magnetization (M) of -0.23 emu/g at 10 kOe was obtained for x = 0.4. The origin of the ferromagnetism (FM) for all LaFe1-x)Ti(x)O3 samples supports the magnetic coupling between Fe3+ and Fe4+ ions via double exchange (DE) interaction.

Polymer pyrolysis synthesis and magnetic properties of LaFeO3 nanoparticles

LaFeO3 nanoparticles were synthesized by a polymer pyrolysis method. LaFeO3 nanoparticles were successfully obtained from calcination of the precursor at different temperatures from 700°C to 1000°C in air for 3h. The calcined LaFeO3 nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption near edge spectroscopy (XANES), and vibrating sample magnetometry (VSM). The XRD and TEM results showed that all LaFeO3 samples had a single phase nature with the orthorhombic structure. The estimated crystallite sizes were in the range of 34.8±1.5-74.0±2.8nm, as increased with increasing calcination temperature. The valence states of Fe ions were in the Fe3+ state, as confirmed by XANES results. The weak ferromagnetic behavior with the highest magnetization (M) of 0.32emu/g measured at 10kOe was obtained for the sample calcined at 700°C. Moreover, this sample exhibited the Curie temperature (TC) above 400K. The uncompensated spins at the surface was discussed as the origin of ferromagnetism of the LaFeO3 nanoparticles.

Electrosynthesis of ZnO nanorods and nanotowers: Morphology and X-ray Absorption Near Edge Spectroscopy studies

Deposition mechanism of nano-structured ZnO films has been investigated in the absence and presence of chloride ions from aqueous solution. The resulting opto-electronic properties were interpreted extensively, using X-ray diffraction (XRD), X-ray Absorption Near Edge Spectroscopy (XANES), field emission scanning electron microscopy (FE-SEM), UV-Visible spectroscopy and four probe techniques. The ZnO deposition is mass transport controlled process and the interaction of chloride ions with the surface has great influence on diffusion kinetics, considering the substantial species (Zn2+ and OH−) involved in the construction of ZnO film. This effect does not change major lattice parameters, as shown with detailed analysis of XRD data. However, the texture coefficient (Tc) (002) value is higher in presence of chloride ions containing synthesis solution which gave vertically aligned, well defined and uniformly dispersed nanorods structure. The calculated Eg values are in the range 3.28–3.41eV and 3.22–3.31eV for ZnO nanorods and nanotowers synthesized at different deposition periods, respectively. Furthermore, the charge mobility values regarding the deposition periods were measured to be in the ranges from 130.4 to 449.2cm2V−1s−1 and 126.2 to 204.7cm2V−1s−1 for nanorods and nanotowers, respectively. From XANES results, it was shown that the Zn K-edge spectrum is dominated by the transition of Zn 1s core electrons into the unoccupied Zn 4p states of the conduction band. Comparing the rod and tower nano-structured ZnO thin films, the excitation behavior of valence band electrons is different. Moreover, the density states of Zn 4p are higher for ZnO nanorods.

X-ray absorption near-edge spectroscopy study of photoluminescent Ca2.955Sc2−xAlxSi3O12:Ce3+ green phosphors

Al3+ co-doped Ca2.955Sc2Si3O12:Ce3+ green phosphors were systematically investigated using X-ray absorption near-edge spectroscopy (XANES) to understand the influence of Al3+ incorporation on photoluminescence behavior. XANES analysis was used to confirm the doping site of Al atoms in the Ca2.955Sc2Si3O12:Ce3+ host, the valence state of Ce atoms, the symmetry around Ca and Ce atoms, and the bonding modes of oxygen atoms with cation atoms in the as-prepared Ca2.955Sc2−xAlxSi3O12:Ce3+ (CSAS:Ce). XANES results indicate that Al3+ occupies the Sc3+ sites and stabilizes the crystal structure of the host material. Furthermore, Al3+ can effectively inhibit the oxidization of Ce3+ to Ce4+, which enhances the photoluminescence of CSAS:Ce green phosphors.

Structure, optical and magnetic properties of LaFeO3 nanoparticles prepared by polymerized complex method

This work reports the study the structure, optical and magnetic properties of LaFeO3 nanoparticles synthesized by the polymerized complex method. The LaFeO3 nanoparticles were successfully obtained from calcination of the precursor at different temperatures from 750 to 1,050 °C in air for 2 h. The calcined LaFeO3 nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Visible spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and vibrating sample magnetometry. The XRD and TEM results showed that all LaFeO3 samples had a single phase nature with the orthorhombic structure. The estimated crystallite sizes were in the range of 44.5 ± 2.4–74.1 ± 4.9 nm. UV–Vis spectra showed strong UV and Vis absorption with small band gap energy. The valence states of Fe ions were in the Fe3+ and Fe4+ state, as confirmed by XPS and XANES results. The weak ferromagnetic behavior with specific saturation magnetization of 0.1 emu/g at 10 kOe was obtained for the small particle of 44.5 ± 2.4 nm. The uncompensated spins at the surface was proposed as playing a part in the magnetic properties of small sized LaFeO3.

Structural, optical and photo catalytic properties of Cu-doped ZnO nanoparticles synthesised by co-precipitation method

In this work, co-precipitation method was utilised to synthesise Cu-doped ZnO nanoparticles using zinc nitrate (Zn(NO 3 ) 2 ·6H 2 O), and copper(II) nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) as starting precursors for Zn and Cu sources, respectively. Structural and physical properties of as-prepared powders were examined by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). The XRD results reveal that Cu-doped ZnO nanopowders are in hexagonal wurtzite structure and their crystallinity deteriorates with increasing Cu doping content. FTIR results additionally show the existence of relevant chemical bonding in the samples. Meanwhile, XPS and XANES results indicate the existence of Cu ion with relevant electronic state in ZOptical properties of the samples were investigated and the corresponding results suggest that Cu additive plays a crucial role in their optical properties that can be adjusted to meet the requirement for practical solar energy harvesting applications such as sun-light photocatalyst. The photocatalytic activity of as-prepared Cu-doped ZnO photocatalyst was investigated by the degradation of Rhodamine B solution. The catalyst with 6% Cu doping content exhibits the enhancement in photocatalytic activity with reaction rate of 0.0267 min –1 . The significant improvement in photocatalytic activity of the doped sample may be associated to the increase of functional hydroxyl radicals due to the incorporation of specific content Cu dopant.

Polarization-dependent x-ray absorption spectroscopy of hexagonal and orthorhombic TbMnO3 thin films

Pure phase TbMnO3 manganite thin films with hexagonal (h-TMO) and orthorhombic (o- TMO) crystal structures were prepared by pulsed laser deposition. The distinctive orientation alignments between film and substrate obtained here have allowed us to perform the x-ray absorption near edge spectroscopy (XANES) measurements with the electric field applied along the three major crystallographic directions. The XANES results, as expected, display significantly different spectral features for the h-TMO and o-TMO films. In addition, the XANES spectra also exhibit strong polarization dependence at O K and Mn L edges for both samples.

Polarized X-ray absorption spectroscopy of single-crystal Mn(V) complexes relevant to the oxygen-evolving complex of photosystem II.

High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3d(z(2))-4p(z) mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3d(xz,yz) transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.

Tribological performance and chemistry of films for di-n-butyl dithiocarbamate derivatives in rapeseed oil

Two di-n-butyl dithiocarbamate derivatives were easily synthesized. Their tribological performances as lubricating oil additives in rapeseed oil were evaluated using a four-ball machine, and their chemistry of films was analyzed with X-ray absorption near-edge spectroscopy (XANES). The results indicate that the two compounds possess excellent anti-wear property and good load-carrying capacity. According to the XANES results, for the thermal films, the outer surfaces are mainly composed of N, S-containing polymer and ferric sulfate, and the near-surface and the bulk are composed of ferrous sulfate, while for the anti-wear films, the outer surfaces are only composed of ferric sulfate, but the near-surface and the bulk are mainly composed of ferrous sulfate.

The distribution, solid-phase speciation, and desorption/dissolution of As in waste iron-based drinking water treatment residuals

Arsenic concentrations and solid-phase speciation were assessed as a function of depth through Fe-media beds for two commercially available products (Granular Ferric Hydroxide ®—GFH and Bayoxide E33 ®—E33) from pilot-scale water treatment field tests. These results were compared with data from solution (de-ionized water—DI-H 2O) concentrations of As equilibrated with Fe-media in an anoxic environment at 4 °C. The materials had a high capacity for As (GFH media 9620 mg kg −1 As, E33 Media 5246 mg kg −1). Arsenic concentrations decreased with bed depth. For E33, X-ray absorption near-edge spectroscopy results showed that As(V) was the dominant solid-phase species. For GFH, As(III) was detected and the proportion (relative to As(V)) of As(III) increased with bed depth. Arsenic concentrations in DI-H 2O equilibrated with the media were low (⩽35 μg l −1) over a period of 50 d. Arsenic concentrations in the equilibrated solutions also decreased with depth. Results from tests on soluble As speciation show that As in solution is in the form of As(V). Kinetic desorption experiments carried out at different pH values (3, 5, 7, 8, and 9) show that the media exhibit some acid/base neutralization capacity and tend to bind As sufficiently. Concentrations of As in the pH desorption experiments were in the same order of magnitude as the toxicity characteristic leaching procedure extractions (tens of μg l −1) except at low pH values. For the GFH media tested at a pH of three, As increases in solution and is mainly associated with colloidal (operationally defined as between 0.1 and 1.0 μm) iron.

Nitrogen incorporation in carbon nitride films produced by direct and dual ion-beam sputtering

Carbon (C) and carbon nitride (CNx) films were grown on Si(100) substrates by direct ion-beam sputtering (IBS) of a carbon target at different substrate temperatures (room temperature-450 °C) and Ar∕N2 sputtering gas mixtures. Additionally, the effect of concurrent nitrogen-ion assistance during the growth of CNx films by IBS was also investigated. The samples were analyzed by elastic recoil detection analysis (ERDA) and x-ray absorption near-edge spectroscopy (XANES). The ERDA results showed that significant nitrogen amount (up to 20 at. %) was incorporated in the films, without any other nitrogen source but the N2-containing sputtering gas. The nitrogen concentration is proportional to the N2 content in the sputtering beam and no saturation limit is reached under the present working conditions. The film areal density derived from ERDA revealed a decrease in the amount of deposited material at increasing growth temperature, with a correlation between the C and N losses. The XANES results indicate that N atoms are efficiently incorporated into the carbon network and can be found in different bonding environments, such as pyridinelike, nitrilelike, graphitelike, and embedded N2 molecules. The contribution of molecular and pyridinelike nitrogen decreases when the temperature increases while the contribution of the nitrilelike nitrogen increases. The concurrent nitrogen ion assistance resulted in the significant increase of the nitrogen content in the film but it induced a further reduction of the deposited material. Additionally, the assisting ions inhibited the formation of the nitrilelike configurations while promoting nitrogen environments in graphitelike positions. The nitrogen incorporation and release mechanisms are discussed in terms of film growth precursors, ion bombardment effects, and chemical sputtering.

A stable monomeric nickel borohydride.

A stable discrete nickel borohydride complex (Tp*NiBH(4) or Tp*NiBD(4)) was prepared using the nitrogen-donor ligand hydrotris(3,5-dimethylpyrazolyl)borate (Tp*-). This complex represents one of the best characterized nickel(II) borohydrides to date. Tp*NiBH(4) and Tp*NiBD(4) are stable toward air, boiling water, and high temperatures (mp > 230 degrees C dec). X-ray crystallographic measurements for Tp*NiBH(4) showed a six-coordinate geometry for the complex, with the nickel(II) center facially coordinated by three bridging hydrogen atoms from borohydride and a tridentate Tp(-) ligand. For Tp*NiBH(4), the empirical formula is C(15)H(26)B(2)N(6)Ni, a = 13.469(9) A, b = 7.740(1) A, c = 18.851(2) A, beta = 107.605(9) degrees, the space group is monoclinic P2(1)/c, and Z = 4. Infrared measurements confirmed the presence of bridging hydrogen atoms; both nu(B[bond]H)(terminal) and nu(B[bond]H)(bridging) are assignable and shifted relative to nu(B-D) of Tp*NiBD(4) by amounts in agreement with theory. Despite their hydrolytic stability, Tp*NiBH(4) and Tp*NiBD(4) readily reduce halocarbon substrates, leading to the complete series of Tp*NiX complexes (X = Cl, Br, I). These reactions showed a pronounced hydrogen/deuterium rate dependence (k(H)/k(D) approximately 3) and sharp isosbestic points in progressive electronic spectra. Nickel K-edge X-ray absorption spectroscopy (XAS) measurements of a hydride-rich nickel center were obtained for Tp*NiBH(4), Tp*NiBD(4), and Tp*NiCl. X-ray absorption near-edge spectroscopy results confirmed the similar six-coordinate geometries for Tp*NiBH(4) and Tp*NiBD(4). These contrasted with XAS results for the crystallographically characterized pseudotetrahedral Tp*NiCl complex. The stability of Tp*Ni-coordinated borohydride is significant given this ion's accelerated decomposition and hydrolysis in the presence of transition metals and simple metal salts.

Interaction of SO 2 with CeO 2 and Cu/CeO 2 catalysts: photoemission, XANES and TPD studies

CeO2 and Cu/CeO2 are effective catalysts/sorbents for the removal or destruction of SO2. Synchrotron‐based high‐resolution photoemission, X‐ray absorption near‐edge spectroscopy (XANES), and temperature‐programmed desorption (TPD) have been employed to study the reaction of SO2 with pure and reduced CeO2 powders, ceria films (CeO2, CeO2−x, Ce2O3+x) and model Cu/CeO2 catalysts. The results of XANES and photoemission provide evidence that SO4 was formed upon the adsorption of SO2 on pure powders or films of CeO2 at 300 K. The sulfate decomposed in the 390–670 K temperature range with mainly SO2 and some SO3 evolving into gas phase. At 670 K, there was still a significant amount of SO4 present on the CeO2 substrates. The introduction of O vacancies in the CeO2 powders or films favored the formation of SO3 instead of SO4. Ceria was able to fully dissociate SO2 to atomic S only if Ce atoms with a low oxidation state were available in the system. When Cu atoms were added to CeO2 new active sites for the destruction of SO2 were created improving the catalytic activity of the system. The surface chemistry of SO2 on the Cu‐promoted CeO2 was much richer than on pure CeO2. The behavior of ceria in several catalytic processes (oxidation of SO2 by O2, reduction of SO2 by CO, automobile exhaust converters) is discussed in light of these results.

Characterization of Mixed-Metal Oxides Using Synchrotron-Based Time-Resolved x-ray Diffraction and x-ray Absorption Spectroscopy

ABSTRACTExperiments are described showing the utility of synchrotron-based time-resolved x-ray diffraction (TR-XRD) and x-ray absorption near-edge spectroscopy (XANES) for characterizing the physical and chemical properties of mixed-metal oxides that contain Mo and a second transition metal (Fe, Co or Ni). TR-XRD was used to study the transformations that occur during the heating of a FeMoO4/Fe2(MoO4)3 mixture and the α⇒β phase transitions in CoMoO4 and NiMoO4. The Mo LII- and O K-edges in XANES are very useful for probing the local symmetry of Mo atoms in mixed-metal oxides. The results of XANES and density-functional calculations (DMo13, DFT-GGA) show large changes in the splitting of the empty Mo 4d levels when going from tetrahedral to octahedral coordinations. XANES is very useful for studying the reaction of H2, H2S and SO2 with the mixed-metal oxides. Measurements at the S K-edge allow a clear identification of S, SO2, SO3 or SO4 on the oxide surfaces. Changes in the oxidation state of molybdenum produce substantial shifts in the position of the Mo LII- and MIII-edges.