Iron Pairs Research Articles

Breaking the linear correlations for enhanced electrochemical nitrogen reduction by carbon-encapsulated mixed-valence Fe7(PO4)6

Electrochemical nitrogen reduction (NRR) is deemed as a consummate answer for the traditional Haber–Bosch technology. Breaking the linear correlations between adsorption and transition-state energies of intermediates is vital to improve the kinetics of ammonia synthesis and obtain a less energy-intensive process. Herein, carbon-encapsulated mixed-valence Fe7(PO4)6 was prepared and applied as an electrocatalyst for high-efficiency NRR. A dramatic faradaic efficiency (FE) of 36.93% and an NH3 production rate of 13.1 μg h−1 mgcat.−1 were obtained at −0.3 V versus RHE, superior to nearly all Fe-based catalysts. Experiments and DFT calculations revealed that the superior performance was ascribed to the synergistic effect of mixed-valence iron pair, which braked the linear correlations to improve the kinetics of ammonia from collaborative hydrogenation and *NH3 separation. This work proves the feasibility of mixed-valence catalysts for nitrogen reduction and thus opening a new avenue towards artificial nitrogen-fixation catalysts.

Theoretical study of Fe–Fe bonding in a Series of Iron Carbonyl clusters [(μ-H)2Fe3(CO)9(µ3-As)Mn(CO)5], [Et4N][(µ-H)2Fe3(CO)9(µ3-As)Fe(CO)4] and [Et4N][HAs{Fe2(CO)6(µ-CO)(µ-H)}{Fe(CO)4}] by QTAIM Perspective

Abstract The bonding interactions such as Fe-Fe, Fe-H and Fe-CO, existing in the iron carbonyl clusters; [(μ-H)2Fe3(CO)9(µ3-As)Mn(CO)5] (1), [Et4N][(µ-H)2Fe3(CO)9(µ3-As)Fe(CO)4] (2) and [Et4N][HAs{Fe2(CO)6(µ-CO)(µ-H)}{Fe(CO)4}] (3), have been studied using atoms in molecules (AIM) approach. Many topological parameters of the electron density for these clusters have been computed at bond critical points (bcp’s). The conceptual framework of AIM theory indicates the absence of Fe-Fe direct bonding, since no bond critical point is found between Fe metals and therefore, no bond path connecting them in 1, 2 and 3. Also, from these results, a comparison was done for related but different interactions involving; different Fe…Fe interactions and bridged Fe-H bond versus other bridged Fe-ligand such as Fe-CO. An interaction of (Fe1-H1-Fe2-H2-Fe3) plan existing in each of cluster 1 and 2 is 5c-4e type. While in cluster 3, an interaction core (Fe1-H1-Fe2) is 3c-2e. Finally, the existence of hydride bridging ligands has an efficient role to reduce the electron density delocalized between hydride bridged iron pairs oppositely with hydride unbridged ones.

At A Glance On Kothala himbutu

Kothala himbutu (Salacia reticulate; SR) is a very useful medicinal plant in Sri Lanka and the southern region of India. SR contains salacinol and kotalanol neosalacinol and neokotalanol which can isolated from the Salacia species by using iron pair chromatography [1& 2]. It is a woody climber and widely used in Ayurvedic medicine for diabetes and obesity treatment. Previously, the action of SR for diabetes is not described properly. Researchers tried to explore the effect of SR and found that it has an alpha glucosidase inhibitor activity in invitro study. Moreover, SR may increase lipolysis and reduce insulin resistance by enhancing mRNA expression for hormone sensitive lipase (HSL) and adiponectin [3].

Interface energetics and structure of the pearlitic microstructure in steels: An atomistic and continuum investigation

Atomistic modeling is used to investigate the energetics and structures of commonly reported orientation relationships between ferrite and cementite within pearlite: the Bagaryatskii, the Isaichev, and the Pitsch-Petch, as well as their associated near orientations. Dislocation arrays are found to form for all orientation relationships, with their spacing and direction a function of lattice mismatch. Within each orientation relationship, different interfacial chemistries are found to produce identical dislocation spacings and line directions, but differing interfacial energies. This chemistry component to the interfacial energy is characterized and it is determined that in addition to the lattice mismatch, there are two structural factors within the cementite terminating plane that affect the energetics: the presence of like site iron pairs and proximity of carbon atoms to the interface. Additionally, an alternate method for determining the interfacial energy of systems in which there are multiple chemical potentials for a single element is developed and implemented, an approach which is likely valid for other similar systems. The Isaichev orientation relationship is found to be the most favorable, while the “near” orientation relationships are found to be at least as energetically favorable as their parent orientation relationships. A continuum model based on O-lattice theory and anisotropic continuum theory was also applied to the atomistic results, yielding interfacial energy approximations that match well with those from atomistics and allowing for the characterization of the Burgers vectors, which are found to lie in high symmetry directions of the ferrite within the interface plane.

Thermal and electrochemical characterization of a new poly (ethylene oxide) copolymer—gel electrolyte containing polyvalent ion pair of cobalt (CoII/III) or iron (FeII/III)

In this work, gel polymer electrolytes based on the PEO copolymer—poly(ethylene oxide-co-2-(2-methoxyethoxy)ethyl glycidyl ether)), (P(EO-EM))—with a fixed P(EO-EM):GBL (γ-butyrolactone) weight ratio of 30–70% and different concentrations of redox mediators CoII:CoIII or FeII:FeIII were prepared as a possible substitute to the redox couple I−/I3 in dye-sensitized solar cell. The gel polymer electrolytes showed an increase in thermal stability with the addition of iron or cobalt salts. Maximum conductivity of 10−6 and 10−5 S cm−1, at room temperature, was observed for the system containing Co and Fe, respectively. Conductivity variation as a function of temperature showed Arrhenius type thermal activated process. Photoinduced absorption spectroscopy (PAS) tests showed the successful regeneration of the L0 dye by Gel+2 wt% FeII/III and Gel+5 wt% CoII/III. The Gel+2 wt% FeII/III electrolyte was also able to reduce the N719, D35, and Z907 dyes.

Tribochemistry of adaptive integrated interfaces at boundary lubricated contacts

Understanding how an adaptive integrated interface between lubricant additives and solid contacts works will enable improving the wear and friction of moving engine components. This work represents the comprehensive characterization of compositional and structural orientation at the sliding interface from the perspective of surface/interface tribochemistry. The integrated interface of a lubricant additive-solid resulting from the friction testing of Graphite-like carbon (GLC) and PVD-CrN coated rings sliding against cast iron under boundary lubrication was studied. The results indicate that in the case of the CrN/cast iron pair the antiwear and friction behavior were very strongly dependent upon lubricant. In contrast, the tribology of the GLC surface showed a much lower dependence on lubrication. In order to identify the compounds and their distribution across the interface, x-ray microanalysis phase mapping was innovatively applied and the principle of hard and soft acids and bases (HSAB) to understand the behaviour. Phase mapping clearly showed the hierarchical interface of the zinc-iron polyphosphate tribofilm for various sliding pairs and different sliding durations. This interface structure formed between lubricant additives and the sliding surfaces adapts to the sliding conditions – the term adaptive interface. The current results help explain the tribology of these sliding components in engine.

Porosity and shape of airborne wear microparticles generated by sliding contact between a low-metallic friction material and a cast iron

The wear of brakes in transport vehicles is one of the main anthropogenic sources of airborne particulate matter in urban environments. The present study deals with the characterisation of airborne wear microparticles from a low-metallic friction material / cast iron pair used in car brakes. Particles were generated by a pin-on-disc machine in a sealed chamber at sliding velocity of 1.3m/s and contact pressure of 1.5MPa. They were collected on filters in an electrical low pressure impactor, and an investigation was conducted to quantify their shape and porosity. Scanning electron microscopy revealed that most of the 0.1−0.9µm particles are flakes and have a breadth-to-length aspect ratio of 0.7 ± 0.2. Particle porosity was determined by milling particles with a focused ion beam and subsequent analysis of the exposed particle cross-sections. Most of the 0.3–6.2µm particles were revealed to have porosity of 9 ± 6%. Analysis of the relationship between effective particle density, particle material density, dynamic shape factor and porosity showed that the shape factor has a stronger influence on the effective density of airborne wear particles than the porosity factor. The obtained results are useful for accurate prediction of particle behaviour in the atmosphere and in the human respiratory system.

A Study on Emission of Airborne Wear Particles from Car Brake Friction Pairs

The emission of airborne wear particles from friction material / cast iron pairs used in car brakes was investigated, paying special attention to the influence of temperature. Five low-metallic mat ...

The Effect of Alloy Cast Iron Hardness on the Tribological Properties of a Bronze – Cast Iron Pair in Dry and Mixed Friction

The paper reports the results of model testing of dry and mixed friction wear of a worm – worm rack kinematic pair, where the worm is made of CUA19Fe3 bronze, while the rack of alloy cast iron. Such a pair is applied in the feed mechanisms of heavy-duty machine tools. The experiment was conducted with variable cast iron hardness values and pressure levels. The distribution of wear along the friction path for the both elements of the tribological pair has been determined. The results are presented in diagrams and in the form of a relationship of wear intensity as a function of the parameters examined in the experiment.

Predicting critical temperatures of iron(II) spin crossover materials: density functional theory plus U approach.

A first-principles study of critical temperatures (T(c)) of spin crossover (SCO) materials requires accurate description of the strongly correlated 3d electrons as well as much computational effort. This task is still a challenge for the widely used local density or generalized gradient approximations (LDA/GGA) and hybrid functionals. One remedy, termed density functional theory plus U (DFT+U) approach, introduces a Hubbard U term to deal with the localized electrons at marginal computational cost, while treats the delocalized electrons with LDA/GGA. Here, we employ the DFT+U approach to investigate the T(c) of a pair of iron(II) SCO molecular crystals (α and β phase), where identical constituent molecules are packed in different ways. We first calculate the adiabatic high spin-low spin energy splitting ΔE(HL) and molecular vibrational frequencies in both spin states, then obtain the temperature dependent enthalpy and entropy changes (ΔH and ΔS), and finally extract T(c) by exploiting the ΔH/T - T and ΔS - T relationships. The results are in agreement with experiment. Analysis of geometries and electronic structures shows that the local ligand field in the α phase is slightly weakened by the H-bondings involving the ligand atoms and the specific crystal packing style. We find that this effect is largely responsible for the difference in T(c) of the two phases. This study shows the applicability of the DFT+U approach for predicting T(c) of SCO materials, and provides a clear insight into the subtle influence of the crystal packing effects on SCO behavior.

Spin-state-associated ligand distortion and complex solution equilibrium for a pair of Fe(II) pyridyl-imidazoline complexes

Reported herein are the magnetic and structural properties as well as solution equilibrium investigation for a pair of iron (II) pyridyl-imidazoline complexes: [Fe(pizMe)Cl2]2 (1) and [Fe(pizMe)3](FeCl4) (2), where pizMe=2-(2′-pyridinyl)-4,5-dihydro-1-methylimidazole. Dinuclear complex 1 contains high-spin iron (II) centers and a “flat” pizMe ligand (planar at the imidazoline N atom), while the pizMe-containing complex in 2 exhibits characteristics of a low-spin iron (II) center and a pyramidalized pizMe ligand. In solution, the two complexes are present in equilibrium as evinced through NMR and electronic absorption spectral studies. Spin-state switching between 1 and 2 in methanolic solution is a result of ligand lability.

Homochiral iron(II) complexes based on imidazole Schiff-base ligands: Syntheses, structures, and spin-crossover properties

Abstract A pair of mononuclear iron(II) enantiomeric complexes: fac-Λ-[Fe(R-L)3](BF4)2·MeCN (1) and fac-Δ-[Fe(S-L)3](BF4)2·MeCN (2) [L = 1-phenyl-N-(1-methyl-imidazol-2-ylmethylene)ethanamine] have been synthesized and characterized. X-ray crystallography reveals that iron(II) center is surrounded by three bidentate ligands defining a pseudooctahedral [FeN6] coordination environment. R-L ligand induces the fac-Λ isomer, while S-L ligand induces the fac-Δ isomer. Magnetic measurements reveal that both of them display obviously spin-crossover behavior at 365 K. After desolvation, they exhibit a reversible spin transition with a small hysteresis loop of ca. 3 K appearing at about T1/2↑ = 222 K and T1/2↓ = 219 K.

Synthesis, structural and magnetic properties of oxo-, chloroacetato-bridged tetra-nuclear iron(III) complex

Oxo- and chloroacetato-bridged tetra-nuclear iron(III) complex [Fe4O2(ClCH2COO)8(bpy)2]·H2O, where bpy=2,2′-bipyridine, has been synthesized and characterized on the basis of X-ray crystallography, elemental analysis, cyclic voltammetric, UV–vis and IR spectroscopic techniques. X-ray diffraction analysis reveals that the complex crystallizes in the monoclinic space group P2/n with a=9.629(5)Å, b=13.742(5), c=20.437(5)Å, α=γ=90.000(5)°, β=99.792(5)°, V=2664.9(18)Å3 and Z=2. The tetra-nuclear entity consists of a [Fe4(μ3-O)2]8+ unit comprising four FeIII atoms with a “butterfly” arrangement. Each pair of iron(III) atoms occupy the “hinge” or “body” sites, and “wing-tip” sites, respectively. It undergoes two stepwise one electron reductions, one is quasi-reversible at E1/2=+0.061V vs Ag/AgCl (ΔEp=0.082V) and the other is irreversible at EP.C=−0.38V at a scan rates 0.1Vs−1. Variable-temperature magnetic susceptibility data reveals strong antiferromagnetic exchange interactions among the four high-spin FeIII ions. The exchange coupling constant Jbb (body–body interaction) is indeterminate due to prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction (Jwb) was evaluated as −115cm−1, using the spin Hamiltonion model H=−Jwb (S1⋅S2+S2⋅S1+S1′⋅S2′+S2′⋅S1) –Jbb(S2⋅S2′).

Syntheses, crystal structure, spectroscopic, redox and magnetic properties of oxo- and carboxylato-bridged polynuclear iron(III) complexes with phenolate- and pyridine-substituted benzimidazole ligands

Using the same trinuclear [Fe3O]+7 core having oxo and carboxylate bridges, two different types of polynuclear iron(III) complexes have been synthesized. The bidentate ligands, (2-hydroxyphenyl)benzimidazole (HL1) and (2-pyridyl)benzimidazole (L2), produce the dinuclear and tetranuclear complexes [Fe2(L1)4(C6H5COO)]NO3·3H2O (1) and [Fe4O2(C6H5COO)7(L2)2]NO3·H2O·CH3CN (2·CH3CN), respectively. Complex 1 crystallizes in the monoclinic space group P2(1)/c, while 2·CH3CN crystallizes in the monoclinic space group C2/c. In 1, the two hexa-coordinated iron(III) centers are bridged by the two phenolate oxygens of the ligand HL1. The tetranuclear entity 2 consists of a [Fe4(μ3-O)2]8+ unit comprising four FeIII centers with a “butterfly” arrangement. Each pair of iron(III) centers occupy the “body” or “hinge” and “wing-tip” sites, respectively. The Fe2IIIFe2III complex 2 undergoes two stepwise one electron reductions at E1/2=−0.625V and −0.11V, while 1 displays an irreversible reduction wave at EP·C=−0.3V. Variable-temperature magnetic susceptibility measurements have been carried out for 1 and 2 in the temperature range 2–300K. Complex 1 exhibits a very weak Fe⋯Fe exchange interaction, J=−0.24(1)cm−1 (H=−J(S1·S2)). In complex 2, moderate antiferromagnetic exchange interactions occur among the four high-spin FeIII centers. The exchange coupling constant Jbb (body–body interaction) is indeterminate due to the prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction (Jwb) is evaluated as −73(3) cm−1, using the spin Hamiltonion model H=−J1(S1·S2+S2·S3+S3·S4+S4·S1)−J2(S2·S4).

Photomagnetic K0.25Ni1–xCox[Fe(CN)6]·nH2O and K0.25Co[Fe(CN)6]0.75y[Cr(CN)6]0.75(1–y)·nH2O Prussian Blue Analogue Solid Solutions

Magnetically bistable solid solutions of Prussian blue analogues with chemical formulas of K(α)Ni(1-x)Co(x)[Fe(CN)(6)](β)·nH(2)O (Ni(1-x)Co(x)Fe) and K(α)Co(γ)[Fe(CN)(6)](y)[Cr(CN)(6)](1-y)·nH(2)O (CoFe(y)Cr(1-y)) have been synthesized and studied using mass spectrometry, Mössbauer spectroscopy, X-ray diffraction, temperature-dependent infrared spectroscopy, and dc magnetometry. These compounds provide insight into interfaces between the photomagnetic Co-Fe Prussian blue analogue and the high-T(C) Ni-Cr Prussian blue analogue that exist in high-T(C) photomagnetic heterostructures. This investigation shows that the bistability of Co-Fe is strongly modified by metal substitution, with Ni(1-x)Co(x)Fe stabilizing high-spin cobalt-iron pairs and CoFe(y)Cr(1-y) stabilizing low-spin cobalt-iron pairs, while both types of substitution cause a dramatic decrease in the bistability of the material.

Spatially resolved measurement of electrochemical activity and pH distributions in corrosion processes by scanning electrochemical microscopy using antimony microelectrode tips

A new method for the spatially resolved measurement of pH during corrosion processes with the scanning electrochemical microscope (SECM) is presented. Antimony tips are employed because the dual-function characteristics of this material allow the combined amperometric/potentiometric operation of the SECM. The applicability of this technique is illustrated by considering the galvanic corrosion of a model zinc–iron pair immersed in 0.1 M NaCl aqueous solution. Spatially resolved images of pH and oxygen concentration above the metal specimens could be obtained in the same experiment.

Magnetic properties of a new iron lead vanadate Pb 2FeV 3O 11

Temperature dependence of dc/ac magnetization and electron paramagnetic resonance (EPR) spectra of Pb 2FeV 3O 11 iron lead vanadate has been investigated. The dc magnetic measurements have shown the presence of antiferromagnetic interactions with Curie–Weiss temperature, T CW = −15.2 K, in the high temperatures range while the field cooled (FC) magnetization revealed a maximum at T N = 2.5 K which coincides with a long range magnetic ordering. Temperature dependence of χ′ has shown a maximum at the same temperature. EPR spectrum of Pb 2FeV 3O 11 at room temperature is dominated by nearly symmetrical, very intense and broad resonance line centered at g eff ∼ 2.0 that could be attributed to the correlated system of iron ions. The temperature dependence of magnetic resonance parameters (amplitude, g-factor, linewidth, integrated intensity) has been determined in the 4–300 K range and it suggests the existence of short range correlated spin system up to high temperatures. The temperature dependence of the amplitude of the resonance line has shown a pronounced maximum at 12.5 K that indicates on the existence of two subsystems of weakly and strongly coupled iron pairs. Comparison of dc magnetic susceptibility and EPR integrated intensity points to the presence of correlated spin agglomerates that play an important role in determination of the magnetic response of Pb 2FeV 3O 11.

Iron(III), chromium(III) and cobalt(II) complexes with squarate: Synthesis, crystal structure and magnetic properties

The preparation and variable temperature-magnetic investigation of three squarate-containing complexes of formula [Fe 2(OH) 2(C 4O 4) 2(H 2O) 4]·2H 2O ( 1) [Cr 2(OH) 2(C 4O 4) 2(H 2O) 4]·2H 2O ( 2) and [Co(C 4O 4)(H 2O) 4] n ( 3) [H 2C 4O 4 = 3.4-dihydroxycyclobutene-1,2-dione (squaric acid)] together with the crystal structures of 1 and 3 are reported. Complex 1 contains discrete centrosymmetric [Fe 2(OH) 2(C 4O 4) 2(H 2O) 4] diiron(II) units where the iron pairs are joined by a di-μ-hydroxo bridge and two squarate ligands acting as bridging groups through adjacent oxygen atoms. Two coordinated water molecules in cis position complete the octahedral environment at each iron atom in 1. The iron–iron distance with the dinuclear unit is 3.0722(6) Å and the angle at the hydroxo bridge is 99.99(7)°, values which compare well with the corresponding ones in the isostructural compound 2 (2.998 Å and 99.47°) whose structure was reported previously. The crystal structure of 3 contains neutral chains of squarato- O 1, O 3-bridged cobalt(II) ions where four coordinated water molecules complete the six-coordination at each cobalt atom. The cobalt–cobalt separation across the squarate bridge is 8.0595(4) Å. A relatively important intramolecular antiferromagnetic coupling occurs in 1 whereas it is very weak in 2, the exchange pathway being the same [ J = −14.4 ( 1) and −0.07 cm −1 ( 2), the spin Hamiltonian being defined as H ^ = - J S ^ 1 · S ^ 2 ]. A weak intrachain antiferromagnetic interaction between the high-spin cobalt(II) ions occurs in 3 ( J = −0.30 cm −1). The magnitude and nature of these magnetic interactions are discussed in the light of their respective structures and they are compared with those reported for related systems.

Pin-On-Disc Characterization of Brass/Ferritic and Pearlitic Ductile Iron Rubbing Pair

Wear behaviour of special brass produced through two different methods (centrifugal and sand casting) was investigated. The wear tests were carried out at sliding velocities of 0.2 ms–1, 0.3 ms–1, 0.4 ms–1 and 0.5 ms–1 and under 10 N, 20 N, and 40 N variable loads. The sliding distance was 600 m for all the tests. A pin-on-disc device with round specimen inserts was used to conduct friction and wear tests in which the friction coefficient, the contact temperature and the linear wear of the tribo-pairs were continuously recorded against sliding distance. Two different materials were used as the counterparts, namely ferritic ductile iron equivalent to GGG40 and pearlitic ductile iron equivalent to GGG60. The microstructures and wear scars of the brass specimens were examined by optical, scanning electron microscopy (SEM) and X-ray microanalyses by EDAX. A correlation between hardness and wear volume rate was established for the investigated centrifugally cast and sand cast brass specimens. The volume rate of specimens produced by sand casting method was generally found to be higher than those of centrifugally cast specimens. Ferritic ductile counterpart led to higher wear volume rate than pearlitic ductile counterpart for the both specimens. Severe abrasive wear scars were observed for the sand cast specimens/ferritic ductile iron pair. However, severe adhesive wear took place for the centrifugally cast specimen/pearlitic ductile iron pair.

First‐principles modeling of the interactions of iron impurities with graphene and graphite

AbstractResults of first‐principles modeling of interactions of graphene and graphite with iron impurities predict the colossal difference between these two carbon allotropes. Insertion of the iron atoms between the planes of graphite is much more energetically favorable than adsorption of the iron ad atom at the graphite or graphene surface. High mobility of iron ad atom over graphite surface and within bulk graphite is reported. Calculated values of formation energies for the substitutional iron impurities suggest that iron is more destructive for graphite than for graphene. This effect caused formation of a uniform carbon environment of the iron atom inside the multilayer system. In contrast to graphene, segregation of the substitutional iron impurities in graphite under ambient conditions is not energetically favorable. Enhancement of interlayer bonding in contaminated graphite and purity of graphene from iron impurities are also reported.magnified image Interstitial iron ad atom (left) and pair of substitutional iron ad atoms (right) inside bulk graphite.