Structural Properties,Mössbauer Spectra,and Magnetism of Perovskite‐Type Oxides SrFe1–xTixO3–y

Perovskite‐type phases SrFe_1–xTi_xO_3–y with 0.1 ≤ x ≤ 0.7 have been prepared from the oxides, and, in order to reach high oxygen contents and Fe^IV fractions, annealed at oxygen pressures of 60 MPa. The materials were characterised by powder x‐ray and neutron diffraction, ⁵⁷Fe Mössbauer spectroscopy, and magnetic susceptibility measurements. All samples of the series crystallise in a cubic perovskite structure and reveal considerable oxygen deficiency. The Mössbauer parameters suggest that for x = 0.1, where the Fe^IV fraction is about 90%, the itinerant electronic state of SrFeO₃ is essentially retained. In materials with larger x increasing amounts of Ti^IV and Fe^III ions lead to a stronger localisation of the σ* (Fe 3 d – O 2 p) electrons. There is no evidence for a charge disproportionation of Fe^IV in any of the materials. Magnetic susceptibility measurements show a divergence of zero‐field cooled and field‐cooled data below a temperature T_m and deviations from Curie‐Weiss behaviour above T_m. The data are indicative of spin‐glass behaviour due to disorder and competing exchange interactions.     

Thermolysis of Noble Metal Nanoparticles into Electron‐Rich Phosphorus‐Coordinated Noble Metal Single Atoms at Low Temperature

Noble metal single atoms coordinated with highly electronegative atoms, especially N and O, often suffer from an electron‐deficient state or poor stability, greatly limiting their wide application in the field of catalysis. Herein we demonstrate a new PH₃‐promoted strategy for the effective transformation of noble metal nanoparticles (MNPs, M=Ru, Rh, Pd) at a low temperature (400 °C) into a class of thermally stabilized phosphorus‐coordinated metal single atoms (MPSAs) on g‐C₃N₄ nanosheets via the strong Lewis acid–base interaction between PH₃ and the noble metal. Experimental work along with theoretical simulations confirm that the obtained Pd single atoms supported on g‐C₃N₄ nanosheets exist in the form of PdP₂ with a novel electron‐rich feature, conceptionally different from the well‐known single atoms with an electron‐deficient state. As a result of this new electronic property, PdP₂‐loaded g‐C₃N₄ nanosheets exhibit 4 times higher photocatalytic H₂ production activity than the state‐of‐art N‐coordinated PdSAs supported on g‐C₃N₄ nanosheets. This enhanced photocatalytic activity of phosphorus‐coordinated metal single atoms with an electron‐rich state was quite general, and also observed for other active noble metal single atom catalysts, such as Ru and Rh.     

Mechanism of formation of the complex oxide Gd<Subscript>2</Subscript>SrFe<Subscript>2</Subscript>O<Subscript>7</Subscript>

The mechanism of formation of the perovskite-like layered structure of the oxide Gd₂SrFe₂O₇ was studied. The limiting stages are those of formation of phases with perovskite (GdFeO₃, SrFeO_3?x ) and K₂NiF₄ (GdSrFeO₄) structures. The Mössbauer study has shown that iron atoms exist in a heterovalent state (Fe³⁺ and Fe⁴⁺) only in the structure of SrFeO_3?x ).     

Reaction Mechanisms of Strontium Ferrites Synthesis

The reactions between strontium and iron nitrates have been studied in an open atmosphere system using three different molar ratios, 1:1 (I), 1:2 (II) and 2:1 (III) at different temperatures as pointed out from the DTA data. The reaction mechanism was discussed based on the chemical composition characterized by means of thermal analysis, X‐ray diffraction patterns, infrared spectra and magnetic susceptibility. It was found that the reaction products depend on both temperature of reaction and the ratio between reactants. The reaction products were found to be composed of a variety of iron compounds that possess different valences: SrFeO_2.86, SrFeO_2.97, SrFe₂O₄, SrFe₁₂O₁₉, Sr₂Fe₂O₅ and Sr₇Fe₁₀O₂₂ in addition to some accessory reaction products namely α‐Fe₂O₃ and FeO(OH).     

Understanding LiOH Chemistry in a Ruthenium‐Catalyzed Li–O2 Battery

Non‐aqueous Li–O₂ batteries are promising for next‐generation energy storage. New battery chemistries based on LiOH, rather than Li₂O₂, have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru‐catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e⁻ oxygen reduction reaction, the H in LiOH coming solely from added H₂O and the O from both O₂ and H₂O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li₂O₂, LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long‐lived battery. An optimized metal‐catalyst–electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.     

Perovskites as Precursors for Ni/La2O3 Catalysts in the Dry Reforming of Methane: Synthesis by Constant pH Co‐Precipitation,Reduction Mechanism and Effect of Ru‐Doping

LaNiO₃ perovskite is an interesting precursor for Ni/La₂O₃ catalysts for the dry reforming of methane at high temperatures. Precursors have been synthesized by co‐precipitation without, with 2.5 at %, and with 5 at % Ru doping. The presence of Ru leads to a stabilization of the perovskite structure and hinders the decomposition into NiO and Ruddlesden‐Popper mixed oxides La_n+1Ni_nO_3n+1, which was observed for the Ru‐free sample upon calcination at 1000 °C (n = 3). Upon reduction in hydrogen, a mechanism involving at least two steps was observed and the first major step was identified as the partial reduction of the precursor leading to a LaNiO_2.5‐like intermediate. The second major step is the reduction to Ni metal supported on La₂O₃ independent of the Ru content of the catalyst. In the presence of Ru, indications for Ni‐Ru alloy formation and for a higher dispersion of the metallic phase were found. The catalytic activity in DRM of the catalyst containing 2.5 % Ru was superior to the catalysts with more or without Ru. Furthermore, the propensity of coke formation was reduced by the presence of Ru.     

Redox‐Inert Fe3+ Ions in Octahedral Sites of Co‐Fe Spinel Oxides with Enhanced Oxygen Catalytic Activity for Rechargeable Zinc–Air Batteries

Bimetallic cobalt‐based spinel is sparking much interest, most notably for its excellent bifunctional performance. However, the effect of Fe³⁺ doping in Co₃O₄ spinel remains poorly understood, mainly because the surface state of a catalyst is difficult to characterize. Herein, a bifunctional oxygen electrode composed of spinel Co₂FeO₄/(Co_0.72Fe_0.28)_Td(Co_1.28Fe_0.72)_OctO₄ nanoparticles grown on N‐doped carbon nanotubes (NCNTs) is designed, which exhibits superior performance to state‐of‐the‐art noble metal catalysts. Theoretical calculations and magnetic measurements reveal that the introduction of Fe³⁺ ions into the Co₃O₄ network causes delocalization of the Co 3d electrons and spin‐state transition. Fe³⁺ ions can effectively activate adjacent Co³⁺ ions under the action of both spin and charge effect, resulting in the enhanced intrinsic oxygen catalytic activity of the hybrid spinel Co₂FeO₄. This work provides not only a promising bifunctional electrode for zinc–air batteries, but also offers a new insight to understand the Co‐Fe spinel oxides for oxygen electrocatalysis.     

Phase‐Controlled Synthesis of Transition‐Metal Phosphide Nanowires by Ullmann‐Type Reactions

Transition‐metal phosphide nanowires were facilely synthesized by Ullmann‐type reactions between transition metals and triphenylphosphine in vacuum‐sealed tubes at 350–400 °C. The phase (stoichiometry) of the phosphide products is controllable by tuning the metal/PPh₃ molar ratio and concentration, reaction temperature and time, and heating rate. Six classes of iron, cobalt, and nickel phosphide (Fe₂P, FeP, Co₂P, CoP, Ni₂P, and NiP₂) nanostructures were prepared to demonstrate the general applicability of this new method. The resulting phosphide nanostructures exhibit interesting phase‐ and composition‐dependent magnetic properties, and magnetic measurements suggested that the Co₂P nanowires with anti‐PbCl₂ structure show a ferromagnetic–paramagnetic transition at 6 K, while the MnP‐structured CoP nanowires are paramagnetic with Curie–Weiss behavior. Moreover, GC‐MS analyses of organic byproducts of the reaction revealed that thermally generated phenyl radicals promoted the formation of transition‐metal phosphides under synthetic conditions. Our work offers a general method for preparing one‐dimensional nanoscale transition‐metal phosphides that are promising for magnetic and electronic applications.     

Direct Observation of Oxygen Dissociation on Non‐Stoichiometric Metal Oxide Catalysts

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi‐step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ and (La_0.8Sr_0.2)_0.95MnO_3±δ, using gas‐phase isotope‐exchange with a 1:1 ¹⁶O₂:¹⁸O₂ ratio. Oxygen transport mechanisms between gas–surface reactions and surface–bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ is better at oxygen dissociation than (La_0.8Sr_0.2)_0.95MnO_3±δ. Heteroexchange, involving lattice oxygen, dominates on La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ. In contrast, (La_0.8Sr_0.2)_0.95MnO_3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.     

Reductive Coupling of Diynes at Rhodium Gives Fluorescent Rhodacyclopentadienes or Phosphorescent Rhodium 2,2’‐Biphenyl Complexes

Reactions of [Rh(κ²‐O,O‐acac)(PMe₃)₂] (acac=acetylacetonato) and α,ω‐bis(arylbutadiynyl)alkanes afford two isomeric types of MC₄ metallacycles with very different photophysical properties. As a result of a [2+2] reductive coupling at Rh, 2,5‐bis(arylethynyl)rhodacyclopentadienes ( A ) are formed, which display intense fluorescence (Φ=0.07–0.54, τ=0.2–2.5 ns) despite the presence of the heavy metal atom. Rhodium biphenyl complexes ( B ), which show exceptionally long‐lived (hundreds of μs) phosphorescence (Φ=0.01–0.33) at room temperature in solution, have been isolated as a second isomer originating from an unusual [4+2] cycloaddition reaction and a subsequent β‐H‐shift. We attribute the different photophysical properties of isomers A and B to a higher excited state density and a less stabilized T₁ state in the biphenyl complexes B , allowing for more efficient intersystem crossing S₁→T_n and T₁→S₀. Control of the isomer distribution is achieved by modification of the bis‐ (diyne) linker length, providing a fundamentally new route to access photoactive metal biphenyl compounds.     

Time‐Resolved,In Situ DRIFTS/EDE/MS Studies on Alumina‐Supported Rhodium Catalysts: Effects of Ceriation and Zirconiation on Rhodium–CO Interactions

The effects of ceria and zirconia on the structure–function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ‐Al₂O₃) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ‐Al₂O₃ from [Ce(acac)₃] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)_x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ‐Al₂O₃. The structure–function correlations of ceria and/or zirconia‐doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier‐transform spectroscopy/energy‐dispersive extended X‐ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time‐resolved, in situ conditions. CeO_x and ZrO₂ facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ‐Al₂O₃ samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.     

First‐principles calculations on the energetics,electronic structures and magnetism of SrFeO2

The electronic and magnetic properties of SrFeO₂ with different magnetic configurations have been calculated via the plane‐wave pseudopotential density functional theory method, using the experimental lattice parameters. The results give an antiferromagnetic ground state for SrFeO₂ with an absolute magnetic moment agreeing very well with the experimental report. In comparison with the counterparts whose magnetic moments are parallel to the c axis, the structures with spin moments parallel to the a (or b) axis exhibit no observable preference in total energy, but show different density distributions of the Fe 3d and Fe 3d states. The square‐planar crystal field splits the Fe 3d orbitals into a high‐level d, a low d, and intermediate d_xy and d_xz or d_yz components. The exchange splitting is larger than the crystal‐field splitting, resulting in the high‐spin Fe 3d states. Referred to the triplet O₂, the O‐vacancy formation energy from SrFeO₃ to SrFeO₂ has been deduced as well, along with its dependence on the temperature and O₂ partial pressure. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009     

Formation of Co-Ru/MgO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for the Fischer-Tropsch synthesis: A magnetic method for the estimation of cobalt particle size

The physicochemical properties of Co-supported catalysts were studied, and the particle size of Co in Co/MgO/Al₂O₃ and Co-0.2% Ru/MgO/Al₂O₃ catalysts for the Fischer-Tropsch synthesis (FTS) was estimated using a magnetic method. It was found that a considerable fraction of superparamagnetic Co particles, which increase selectivity for C₅₊ hydrocarbons and decrease the yield of methane in the FTS, was present in a ruthenium-containing catalyst along with single-domain ferromagnetic particles. In this case, the catalyst activity increased by a factor of 10.     

Photo‐ and Electronically Switchable Spin‐Crossover Iron(II) Metal–Organic Frameworks Based on a Tetrathiafulvalene Ligand

A major challenge is the development of multifunctional metal–organic frameworks (MOFs), wherein magnetic and electronic functionality can be controlled simultaneously. Herein, we rationally construct two 3D MOFs by introducing the redox active ligand tetra(4‐pyridyl)tetrathiafulvalene (TTF(py)₄) and spin‐crossover Fe^II centers. The materials exhibit redox activity, in addition to thermally and photo‐induced spin crossover (SCO). A crystal‐to‐crystal transformation induced by I₂ doping has also been observed and the resulting intercalated structure determined. The conductivity could be significantly enhanced (up to 3 orders of magnitude) by modulating the electronic state of the framework via oxidative doping; SCO behavior was also modified and the photo‐magnetic behavior was switched off. This work provides a new strategy to tune the spin state and conductivity of framework materials through guest‐induced redox‐state switching.     

Synthesis of functional magnetic porous SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/P(St-DVB-MAA) microspheres by a novel suspension polymerization

In this study, a novel and effective suspension polymerization has been employed to prepare functional magnetic porous SrFe₁₂O₁₉/P(St-DVB-MAA) microspheres in the presence of bilayer surfactants (sodium dodecyl benzene sulfonate (SDBS) and oleic acid (OA)) coated on micro-size magnetic SrFe₁₂O₁₉. This was achieved by pre-polymerizing the organic phase, which contained co-monomers, porogens and treated magnetic particles, at 65°C for 0.5 h under ultrasound conditions. Aqueous solutions containing a dispersion agent were then added to effect suspension polymerization. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and magnetic property measurement system (MPMS) were used to characterize the functional magnetic porous microspheres. The results show that the microparticles are well shaped with a uniform size distribution of about 0.5 ∼ 0.7 mm and the surfaces of the microspheres have many micro-pores with an average diameter of 0.533 μm. There are carboxyl groups (−COOH) on the surface of the microspheres to the extent of 0.65 mmol g⁻¹, as determined by conductometric titration. According to the XRD spectra, iron oxide consists mainly of SrFe₁₂O₁₉ which reveals hexahedral structure. The content of magnetic SrFe₁₂O₁₉ reaches 17.81% (by mass), and the microspheres have good heat resistance. The magnetic porous microspheres are ferromagnetic with high residual magnetization and coercivity, 21.59 emu g⁻¹ and 4.13 kOe, respectively. The saturation magnetisation is around 42.85 emu g⁻¹.      

A Redox‐Mediator‐Free Solar‐Driven Z‐Scheme Water‐Splitting System Consisting of Modified Ta3N5 as an Oxygen‐Evolution Photocatalyst

Tantalum nitride (Ta₃N₅) modified with various O₂‐evolution cocatalysts was employed as a photocatalyst for water oxidation under visible light (λ>420 nm) in an attempt to construct a redox‐mediator‐free Z‐scheme water‐splitting system. Ta₃N₅ was prepared by nitriding Ta₂O₅ powder under a flow of NH₃ at 1023–1223 K. The activity of Ta₃N₅ for water oxidation from an aqueous AgNO₃ solution as an electron acceptor without cocatalyst was dependent on the generation of a well‐crystallized Ta₃N₅ phase with a low density of anionic defects. Modification of Ta₃N₅ with nanoparticulate metal oxides as cocatalysts for O₂ evolution improved water‐oxidation activity. Of the cocatalysts examined, cobalt oxide (CoO_x) was found to be the most effective, improving the water‐oxidation efficiency of Ta₃N₅ by six to seven times. Further modification of CoO_x/Ta₃N₅ with metallic Ir as an electron sink allowed one to achieve Z‐scheme water splitting under simulated sunlight through interparticle electron transfer without the need for a shuttle redox mediator in combination with Ru‐loaded SrTiO₃ doped with Rh as a H₂‐evolution photocatalyst.     

Mononuclear Complexes of Platinum Group Metals Containing η6‐ and η5‐Cyclic Π‐Perimeter Hydrocarbon and Pyridylpyrazolyl Derivatives: Syntheses and Structural Studies

Piano‐stool‐shaped platinum group metal compounds, stable in the solid state and in solution, which are based on 2‐(5‐phenyl‐1H‐pyrazol‐3‐yl)pyridine ( L ) with the formulas [(η⁶‐arene)Ru( L )Cl]PF₆ {arene = C₆H₆ ( 1 ), p‐cymene ( 2 ), and C₆Me₆, ( 3 )}, [(η⁶‐C₅Me₅)M( L )Cl]PF₆ {M = Rh ( 4 ), Ir ( 5 )}, and [(η⁵‐C₅H₅)Ru(PPh₃)( L )]PF₆ ( 6 ), [(η⁵‐C₅H₅)Os(PPh₃)( L )]PF₆ ( 7 ), [(η⁵‐C₅Me₅)Ru(PPh₃)( L )]PF₆ ( 8 ), and [(η⁵‐C₉H₇)Ru(PPh₃)( L )]PF₆ ( 9 ) were prepared by a general method and characterized by NMR and IR spectroscopy and mass spectrometry. The molecular structures of compounds 4 and 5 were established by single‐crystal X‐ray diffraction. In each compound the metal is connected to N1 and N11 in a k² manner.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

Synthesis and Characterization of Ba0.5Sr0.5NixCo0.8‐xFe0.2O3‐δ (x=0 and 0.2) Perovskites as Electro‐catalysts for Methanol Oxidation in Alkaline Media

In this study, the perovskite‐type oxides Ba_0.5Sr_0.5Ni_0.2Co_0.6Fe_0.2O_3‐δ (BSNCF) with different B‐site transition metal elements were investigated as a new catalyst for methanol oxidation. They were prepared by the partial substitution of cobalt by nickel in Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) sample. Both materials prepared using a combined citrate/EDTA complexing method and their catalytic activitiesfor methanol oxidation were comparativelystudied using cyclic‐voltammetry (CV) and chronoamperometry (CA) in alkaline media at room temperature. Structural, textural, morphological and redox properties of the synthesized solids were investigated by several physicochemical methods such as XRD, FTIR, LRS, SEM‐EDX, BET and H₂‐TPR. The as‐prepared materials were obtained with high purity and high crystallinity. The partial substitution of cobalt by nickel in BSCF perovskite changes the structure of the solid which pass from cubic structure to a mixture of cubic and hexagonal forms. This change is accompanied by an increase in BET surface area and a decrease in crystalline size. For the electro‐catalysis measurements, results demonstrated that partially substituting of Co by Ni in BSCF structure enhance the electro‐catalytical activity towards the oxidation of methanol under alkaline conditions, making it an electro‐catalyst candidate for practical utilization in DMFCs.