Oxygen Electrocatalysis at MnIII-O x-C Hybrid Heterojunction: An Electronic Synergy or Cooperative Catalysis?

The interface at the metal oxide-carbon hybrid heterojunction is the source to the well-known "synergistic effect" in catalysis. Understanding the structure-function properties is key for designing more advanced catalyst-support systems. Using a model Mn^III-O _x single-layer catalyst on carbon, we herein report a full elucidation to the catalytic synergism at the hybrid heterojunction in the oxygen reduction reaction (ORR). The successful fabrication of the single-layer catalyst from bottom-up is fully characterized by the X-ray absorption fine structure and high-resolution transmission electron microscopy. For oxygen electrocatalysis over this model hybrid heterostructure, our results, from both theory and experiment, show that the synergistic ORR truly undergoes a cooperated two-step electrocatalysis with catalytic promotion (Δ E_onset = 60 mV) near the heterojunction and over the single-layer catalyst through an interfacial electronic interplay, rather than an abstruse transition towards a one-step dissociative pathway. Finally, we report a superior peroxide-reducing activity of 432.5 mA cm^-2 mg_(M)^-1 over the Mn^III-O _x single-layer.

[NiIII(OMe)]-mediated reductive activation of CO2 affording a Ni(κ1-OCO) complex

We report a novel pathway for the reductive activation of CO₂ by the [Ni^III(OMe)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex, yielding the [Ni^III(κ¹-OCO˙^–)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex.

Carbon dioxide is expected to be employed as an inexpensive and potential feedstock of C₁ sources for the mass production of valuable chemicals and fuel. Versatile chemical transformations of CO₂, i.e. insertion of CO₂ producing bicarbonate/acetate/formate, cleavage of CO₂ yielding μ-CO/μ-oxo transition-metal complexes, and electrocatalytic reduction of CO₂ affording CO/HCOOH/CH₃OH/CH₄/C₂H₄/oxalate were well documented. Herein, we report a novel pathway for the reductive activation of CO₂ by the [Ni^III(OMe)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex, yielding the [Ni^III(κ¹-OCO˙^–)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex. The formation of this unusual Ni^III(κ¹-OCO˙^–) complex was characterized by single-crystal X-ray diffraction, EPR, IR, SQUID, Ni/S K-edge X-ray absorption spectroscopy, and Ni valence-to-core X-ray emission spectroscopy. The inertness of the analogous complexes [Ni^III(SPh)], [Ni^II(CO)], and [Ni^II(N₂H₄)] toward CO₂, in contrast, demonstrates that the ionic [Ni^III(OMe)] core attracts the binding of weak σ-donor CO₂ and triggers the subsequent reduction of CO₂ by the nucleophilic [OMe]^– in the immediate vicinity. This metal–ligand cooperative activation of CO₂ may open a novel pathway promoting the subsequent incorporation of CO₂ in the buildup of functionalized products.

Effect of the grafting agent on the structure and catalytic performance of Ti-MCM-22

Ti-MCM-22 prepared by grafting with Ti(OEt)₄ has more Ti(iv) at Td sites than those prepared with Ti(OPr)₄ and Ti(OBu)₄, and gives higher catalytic activities in cyclohexene and cyclooctene epoxidation using t-butylhydrogen peroxide as the oxidant.

Domination of second-sphere shrinkage effect to improve photoluminescence of red nitride phosphors

Red Ca0.99Al(1-4δ/3-x)Si(1+δ+x)N(3-x)C(x):Eu(2+)0.01 (δ = 0.345; x = 0-0.2) nitride phosphors exhibit a blue-shifted emission with increased eye sensitivity function and excellent thermal stability. The variations in the photoluminescence in the Ca0.99Al(1-4δ/3-x)Si(1+δ+x)N(3-x)C(x):Eu(2+)0.01 (δ = 0.345; x = 0-0.2) system are thoroughly investigated. The enhanced emission energy and the improved thermal stability with increasing x are dominated by the second-sphere shrinkage effect via the substitution of small Si(4+) for large Al(3+) with simultaneous charge compensation. Related proofs of the second-sphere shrinkage effect control for photoluminescence are confirmed via high-resolution neutron powder diffraction, EXAFS, and (29)Si solid-state NMR techniques.

A complete high-to-low spin state transition of trivalent cobalt ion in octahedral symmetry in SrCo0.5Ru0.5O(3-δ)

The complex metal oxide SrCo0.5Ru0.5O(3-δ) possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution, and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy. We have discovered that Co(3+) ion is purely high-spin at room temperature, which is unique for a Co(3+) in an octahedral oxygen surrounding. We attribute this to the crystal field interaction being weaker than the Hund's-rule exchange due to a relatively large mean Co-O distances of 1.98(2) Å, as obtained by EXAFS and X-ray diffraction experiments. A gradual high-to-low spin state transition is completed by applying high hydrostatic pressure of up to 40 GPa. Across this spin state transition, the Co Kβ emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. Thereby is the much debated intermediate spin state of Co(3+) absent in this material. These results allow us to draw an energy diagram depicting relative stabilities of the high-, intermediate-, and low-spin states as functions of the metal-oxygen bond length for a Co(3+) ion in an octahedral coordination.

Soft ferromagnetism in mixed valence Sr1−xLaxTi0.5Mn0.5O3 perovskites

The structural, magnetic and electrical properties of the mixed Ti–Mn oxides Sr_1−xLa_xTi_0.5Mn_0.5O₃ (0 ≤ x ≤ 0.5) are reported.

Studying the effects of Zr-doping in (Bi0.5Na0.5)TiO3via diffraction and spectroscopy

The phase transformation in the (Bi_0.5Na_0.5)Ti_1−xZr_xO₃ are characterized using XRD and XANES.

Biogeochemical reductive release of soil embedded arsenate around a crater area (Guandu) in northern Taiwan using X-ray absorption near-edge spectroscopy

This study investigates biogeochemical reductive release of arsenate from beudantite into solution in a crater area in northern Taiwan, using a combination of X-ray absorption near-edge structure (XANES) and atomic absorption spectrometry. Total arsenic (As) concentrations in the soil were more than 200 mg/kg. Over four months of laboratory experiments, less than 0.8% As was released into solution after reduction experiments. The 71% to 83% As was chemically reduced into arsenite (As(III)) and partially weathering into the soluble phase. The kinetic dissolution and re-precipitation of As, Fe, Pb and sulfate in this area of paddy soils merits further study.

X-ray absorption near edge structure and crystallographic studies of the mixed valence oxide SrRu0.8Ni0.2O3

Neutron diffraction methods are used to refine the structure of SrRu(0.8)Ni(0.2)O(3) at room temperature and 4 K. X-ray absorption measurements at the Ru L-edge demonstrate that partial oxidation of the Ru(4+) to Ru(5+) occurs upon introducing Ni into the SrRuO(3) structure to form SrRu(0.8)Ni(0.2)O(3). Whilst the diffraction measurements show that SrRu(0.8)Ni(0.2)O(3) is isostructural with SrRuO(3), the Ni doped compound exhibits an unusual first order orthorhombic-cubic phase transition near 670 K as revealed by x-ray diffraction. The Curie temperature in SrRu(0.8)Ni(0.2)O(3) is lower than that found in SrRuO(3) as a consequence of local distortions reducing the p-d hybridization.

X-ray absorption and neutron diffraction studies of (Sr(1 - x)Ce(x))MnO3: transition from coherent to incoherent static Jahn-Teller distortions

We have carried out Ce L- and Mn K-edge x-ray absorption near edge structure (XANES) measurements to experimentally determine the oxidation states of both Ce and Mn in (Sr(1 - x)Ce(x))MnO(3) (x = 0.1-0.4). It was found that although Ce is predominantly 4 + at low doping levels (x = 0.1 and 0.15), the Ce valency decreases with increasing Ce doping (reaching a value of around 3.5 + at x = 0.4). The average Mn oxidation state decreases with the increase of Ce content, with the percentage of Jahn-Teller active Mn(3+) ions increasing from 26% (x = 0.1) to 57% (x = 0.4). Precise structural parameters were also obtained from high resolution neutron diffraction studies for samples with x = 0.1-0.3. The crystal structure remains tetragonal in I4/mcm for x ≤ 0.3. The octahedral tilt angle increases with increasing Ce content, but the distortion of the MnO(6) octahedra is reduced significantly at x ≥ 0.2 due to a transition from long-range ordered Jahn-Teller distortions to incoherent static distortions.

Iron-monosulfide oxidation in natural sediments: resolving microbially mediated S transformations using XANES, electron microscopy, and selective extractions

Iron-monosulfide oxidation and associated S transformations in a natural sediment were examined by combining selective extractions, electron microscopy and S K-edge X-ray absorption near-edge structure (XANES) spectroscopy, The sediment examined in this study was collected from a waterway receiving acid-sulfate soil drainage. It contained a high acid-volatile sulfide content (1031 micromol g(-1)), reflecting an abundance of iron-monosulfide. The iron-monosulfide speciation in the initial sediment sample was dominated by nanocrystalline mackinawite (tetragonal FeS). At near-neutral pH and an 02 partial pressure of approximately 0.2 atm, the mackinawite was found to oxidize rapidly, with a half-time of 29 +/- 2 min. This oxidation rate did not differ significantly (P < 0.05) between abiotic versus biotic conditions, demonstrating that oxidation of nanocrystalline mackinawite was not microbially mediated. The extraction results suggested that elemental S (S8(0)) was a key intermediate S oxidation product Transmission electron microscopy showed the S8(0) to be amorphous nanoglobules, 100-200 nm in diameter. The quantitative importance of S8(0) was confirmed by linear combination XANES spectroscopy, after accounting for the inherent effect of the nanoscale S8(0) particle-size on the corresponding XANES spectrum. Both the selective extraction and XANES data showed that oxidation of S8(0) to SO4(2-) was mediated by microbial activity. In addition to directly revealing important S transformations, the XANES results support the accuracy of the selective extraction scheme employed here.

Correlation between magnetic properties and the electronic structures of soft magnetic ternary Fe(78-x)Y(x)B(22) (x = 4-9) bulk metallic glasses

Fe and Y K-edge extended x-ray absorption fine structure, Fe(Y) L(3,2)-edge (L(3)-edge) x-ray absorption near-edge structure (XANES) and valence-band photoemission spectroscopy (VB-PES) measurements have been carried out to study soft magnetic ternary Fe(78-x)Y(x)B(22) bulk metallic glasses (BMGs). The combined XANES and VB-PES results do not show broadening of the Fe 3d band to support the previous interpretation of the reduction of the magnetic moment in BMGs by Y-induced decrease of exchange splitting of Fe 3d orbitals. Instead, the density of delocalized/itinerant Fe 3d states in the vicinity of the Fermi level is found to be reduced by Y substitution, which reduces the strength of itinerant-states-mediated ferromagnetic coupling between local spins on the Fe ions and the total magnetic moment of the Fe-based BMGs.

A solution study on the local and global structure changes of cytochrome c: an unfolding process induced by urea

The local and global structural changes of cytochrome c induced by urea in aqueous solution have been studied using X-ray absorption spectroscopy (XAS) and small-angle X-ray scattering (SAXS). According to the XAS result, both the native (folded) protein and the unfolded protein exhibit the same preedge features taken at Fe K-edge, indicating that the Fe(III) in the heme group of the protein maintains a six-coordinated local structure in both the folded and unfolded states. Furthermore, the discernible differences in the X-ray absorption near-edge structure (XANES) of these two states are attributed to a possible spin transition of the Fe(III) from a low-spin state to a high-spin state during the unfolding process. The perseverance of six-coordination and the spin transition of the iron are reconciled by a proposed ligand exchange, with urea and water molecules replacing the methionine-80 and histidine-18 axial ligands, respectively. The SAXS result reveals a significant morphology change of cytochrome c from a globular shape of a radius of gyration R(g) = 12.8 A of the native protein to an elongated ellipsoid shape of R(g) = 29.7 A for the unfolded protein in the presence of concentrated urea. The extended X-ray absorption fine structure (EXAFS) data unveil the coordination geometries of Fe(III) in both the folded and unfolded state of cytochrome c. An initial spin transition of Fe(III) followed by an axial ligand exchange, accompanied by the change in the global envelope, is proposed for what happened in the protein unfolding process of cytochrome c.

Synthesis of thermally stable zirconia-based mesoporous materials via a facile post-treatment

A novel method of preparing thermally stable zirconia-based mesoporous materials was developed. The zirconia-based mesoporous materials of 2D-hexagonal structure were prepared using zirconium sulfate as the zirconium precursor and cetyltrimethylammonium (CTMA) as the pore-directing agent with the aid of salt in the synthesis solution to reduce the sulfate content in the final product and significantly improve the crystallographic ordering. Post-treatment of the mesoporous material with NaCl solution and lowering the ramping rate to less than 0.2 degrees C/min during the calcination process, however, were the key steps to hinder the growth of the dense zirconia phase and to retain the ordered mesostructure up to 600 degrees C. It was found that a portion of the surfactant (8.9-17.4 wt %) and sulfate ions (0.5-1.2 wt %) were removed during the post-treatment, which prevented the remaining sulfate groups from being reduced by the hydrogen-rich surfactant during the calcination process as confirmed by sulfur K-edge X-ray absorption near edge structure (XANES) and infrared spectroscopy. The maintenance of sulfur in the sulfate state seemed to be important in stabilizing the mesoporous structure of zirconia materials. The mesoporous zirconia materials after extraction with NaCl solution three times and calcination at 550-600 degrees C had the composition ZrO(2-x)(SO4)x with x = 0.10-0.27. The material possesses high surface area (approximately 200 m2/g), large pore volume (approximately 0.10 cm3/g), and wormlike mesopores. In comparison with the mesoporous zirconia materials stabilized by chemical treatment, the present route was simpler and more environmentally friendly and resulted in mesoporous zirconia materials of better thermal stability.

New Members of a Class of Iron−Thiolate−Nitrosyl Compounds: Trinuclear Iron−Thiolate−Nitrosyl Complexes Containing Fe3S6 Core

The neutral trinuclear iron-thiolate-nitrosyl, [(ON)Fe(mu-S,S-C(6)H(4))](3) (1), and its oxidation product, [(ON)Fe(mu-S,S-C(6)H(4))](3)[PF(6)] (2), were synthesized and characterized by IR, X-ray diffraction, X-ray absorption, electron paramagnetic resonance (EPR), and magnetic measurement. The five-coordinated, square pyramidal geometry around each iron atom in complex 1 remains intact when complex 1 is oxidized to yield complex 2. Magnetic measurements and EPR results show that there is only one unpaired electron in complex 1 (S(total) = 1/2) and no unpaired electron (S(total) = 0) in 2. The detailed geometric comparisons between complexes 1 and 2 provide understanding of the role that the unpaired electron plays in the chemical bonding of this trinuclear complex. Significant shortening of the Fe-Fe, Fe-N, and Fe-S distances around Fe(1) is observed when complex 1 is oxidized to 2. This result implicates that the removal of the unpaired electron does induce the strengthening of the Fe-Fe, Fe-N, and Fe-S bonds in the Fe(1) fragment. A significant shift of the nuNO stretching frequency from 1751 cm(-1) (1) to 1821, 1857 cm(-1) (2) (KBr) also indicates the strengthening of the N-O bonds in complex 2. The EPR, X-ray absorption, magnetic measurements, and molecular orbital calculations lead to the conclusion that the unpaired electron in complex 1 is mainly allocated in the Fe(1) fragment and is best described as {Fe(1)NO}7, so that the unpaired electron is delocalized between Fe and NO via d-pi* orbital interaction; some contributions from [Fe(2)NO] and [Fe(3)NO] as well as the thiolates associated with Fe (1) are also realized. According to MO calculations, the spin density of complex 1 is predominantly located at the Fe atoms with 0.60, -0.15, and 0.25 at Fe(1), Fe(2), and Fe(3), respectively.

Fabrication of Nanorattles with Passive Shell

This investigation describes the formation of a metal nanorattle with a pure metal shell by varying experimental parameters. The galvanic replacement reaction between silver and chloroauric acid was adopted to prepare hollow metal nanoparticles. This approach is extended to produce nanorattles of Au cores and Au shells by starting with Au(core)Ag(shell) nanoparticles as templates. The effect of temperature on the nanostructure of the final product is also considered. The composition of the shell in nanorattles can be controlled by varying the reaction temperature (to form pure gold or gold-silver alloy, for example). X-ray absorption fine structure spectroscopy is conducted to elucidate the fine structure of these nanoparticles. Partial alloying between the Au core and the Ag shell is observed by extended X-ray absorption fine structure (EXAFS).