3d Metal Atoms Research Articles

Exohedral Diels-Alder Reactivity of Endohedral Metallofullerene C36.

Understanding the exohedral reactivity of metallofullerenes is crucial for its application in various fields. By systematically controlling the trapped species inside the fullerene its reactivity can be tamed. In this work we report the preferential position of 3d metal atoms inside the C36 cage and their effect on exohedral reactivity in comparison with the neutral and the dianionic cage. The Diels-Alder (DA) reaction between butadiene and all non-equivalent [5-5], [6-5] and [6-6] C-C bonds on the fullerene cage was considered for the analysis, by using density functional theory at the S12g/TZ2P level including COSMO solvation model to elucidate the complete mechanistic pathways. Our results indicate that the preferential position of the metal ion is at the position close to the upper hexagon, and that the general trend in the reactivity of bonds follows the order [5-5]>[6-5]>[6-6]. Moreover, the encapsulation of metal atoms further enhances the reactivity of these bonds, by lowering the LUMOs of the cage, hence maximizing the orbital interactions.

Magnetic properties of 3d metal atoms embedded in a new two-dimensional carbon sheet

A first-principles study is performed to explore the magnetic properties of the 3d transition metal (TM) atoms embedded in a new two-dimensional(2D) nonmagnetic carbon sheet. By the adsorption of 3d metal atoms (M = Sc-Ni) on the single vacancy (SV) site of 2D carbon material, the M@SV complexes are nonmagnetic for M = Ti, Co and Ni. Other metal atoms doping can induce different magnetism. The magnetic moment of Sc@SV system mainly comes from the π electrons of carbon atoms, and these electrons virtual hopping leads to the ferromagnetic coupling interaction with Sc atom. For M = V, Cr, Mn and Fe systems, the more net spin electrons go the separate nonbonding d orbitals, giving the different magnetic moments. Meanwhile, due to the Hund’s rule, the metal atoms and the surrounding carbon atoms have the anti-ferromagnetic coupling interaction. By the simple spd hybridization model, we explore the origin of the various magnetism, and provide some new magnetic single-atom materials for the future spintronics devices and spin-dependent catalysts.

DFT Modeling of Isomerism of Doped Mg24L2 Clusters with Dopants L at the Surface and inside the Magnesium Cage

The isomers of Mg24L2 nanoclusters with dopants L from the firt three periods (L = Li–Zn) in the exo- and endohedral positions of the magnesium cage have been calculated by the density functional theory (DFT) method. The effect of dopants on the structure, stability, and position on the energy scale has been studied. For the systems with atoms L = H, Li, Be N, Na, Co, and Cu, the “open” positions of dopants at the surface are more favorable, unlike their analogs with L = Be and B, for which the “closed” positions in the inner cavity of the cluster are more favorable and their promotion to the surface requires a significant energy input. For clusters with O, Si, P, S, and early 3d metal atoms, the differences in the energies of the exo- and endohedral isomers do not exceed a few kcal/mol. The atoms of the second half of the 3d series are characterized by their association into diatomic dopants L2. The results are compared with the data of similar DFT calculations of isomers of Al42L2 aluminum clusters with the same dopants L = Li–Zn and are of interest for modeling the mechanisms of catalytic hydrogenation of magnesium and aluminum nanoclusters at the molecular level.

Understanding the direct methane conversion to oxygenates on graphene-supported single 3d metal atom catalysts

Understanding the direct methane conversion to oxygenates on graphene-supported single 3d metal atom catalysts

Is the doped MoS2 basal plane an efficient hydrogen evolution catalyst? Calculations of voltage-dependent activation energy.

Transition metal dichalcogenides are cheap and earth-abundant candidates for the replacement of precious metals as catalyst materials. Experimental measurements of the hydrogen evolution reaction (HER), for example, have demonstrated significant electrocatalytic activity of MoS2 but there is large variation depending on the preparation method. In order to gain information about the mechanism and active sites for the HER, we have carried out calculations of the reaction and activation energy for HER at the transition metal doped basal plane of MoS2 under electrochemical conditions, i.e. including applied electrode potential and solvent effects. The calculations are based on identifying the relevant saddle points on the energy surface obtained from density functional theory within the generalized gradient approximation, and the information on energetics is used to construct voltage-dependent volcano plots. Doping with 3d-metal atoms as well as Pt is found to enhance hydrogen adsorption onto the basal plane by introducing electronic states within the band gap, and in some cases (Co, Ni, Cu, Pt) significant local symmetry breaking. The Volmer-Heyrovsky mechanism is found to be most likely and the associated energetics show considerable dopant and voltage-dependence. While the binding free energy of hydrogen can be tuned to be seemingly favorable for HER, the calculated activation energy turns out to be significant, at least 0.7 eV at a voltage of -0.5 V vs. SHE, indicating low catalytic activity of the doped basal plane. This suggests that other sites are responsible for the experimental activity, possibly edges or basal plane defects.

Theoretical insights into the hydrogen oxidation reaction on single metal atoms anchored on the edge of MoS2 nanosheets

The application of alkaline fuel cells (AFCs) is seriously limited by the sluggish kinetics of the anodic hydrogen oxidation reaction (HOR), so it is urgent to develop efficient and stable non-noble metal HOR catalysts. In this work, we explored the HOR performance of single 3d metal atoms anchored at the edge of MoS2 nanosheets (MSA-MoS2, MSA = Fe, Co, and Ni) by the density functional theory calculations. The results suggested that the tetra-coordinated MSA and penta-coordinated meta-Mo atoms at the MoS2 edge could form the dual-active sites for the H* and OH* adsorption. The interaction between MSA and MoS2 supports can efficiently optimize the adsorption free energy of H* (ΔGH*) and OH* (ΔGOH*) by tuning the d band center of the activity sites. The calculated activation energy, rate constant, and exchange current density indicate that among the MSA-MoS2 catalysts, NiSA-MoS2 exhibits the highest activity for HOR, which is attributed to the optimal ΔGH* and ΔGOH*. Moreover, the thermodynamic and electrochemical stability of NiSA-MoS2 is confirmed by the strong formation energy and high oxidation potential. Therefore, NiSA-MoS2 can be considered as a promising candidate for HOR at the anode of AFCs.

Unveiling the active sites of ultrathin Co-Fe layered double hydroxides for the oxygen evolution reaction

Two-dimensional layered double hydroxides (LDHs) have been identified as promising electrocatalysts for the oxygen evolution reaction (OER); however, the simple and effective synthesis of high-quality LDHs remains extremely challenging and the active sites have not been clarified. Herein, we report a facile solution-reaction method for preparing an ultrathin (thickness < 2 nm) nonprecious CoFe-based LDH. Co1Fe0.2 LDH delivers a current density of 10 mA cm−2 and a high turnover frequency of 0.082 s−1 per total 3d metal atoms at a low overpotential of 256 mV. Its mass activity is 277.9 A g−1 at an overpotential of 300 mV for the OER. Kinetic studies reveal the Co site as the main active center for the OER. The doped Fe lowers the reaction barrier by accelerating the charge-transfer process. Theoretical calculations reveal that the surface Co sites adjacent to Fe atoms are the active centers for the OER and the subsurface Fe dopants excessively weaken the OH* adsorption, thus increasing the energy barrier of the rate-determining step. This study can guide the rational design of high-performance CoFe-based LDHs for water splitting.

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co).

7Li NMR shifts and magnetic properties have been determined for three so-called ate complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co; e.g., named lithium-tris(bis(trimethylsilylamide))-manganate(II) in accordance with a formally negative charge assigned to the complex fragment [M{N(SiMe3)2}3]-, which comprises the transition metal). They are formed by addition reactions of LiN(SiMe3)2 and [M{N(SiMe3)2}2] and stabilized by Lewis base/Lewis acid interactions. The results are compared to those of the related "ion-separated" complexes [Li(15-crown-5)][M{N(SiMe3)2}3]. The ate complexes with the lithium atoms connected to the 3d metal atoms manganese, iron, or cobalt via μ2 nitrogen bridges reveal strong 7Li NMR paramagnetic shifts of about -75, 125, and 171 ppm, respectively, whereas the shifts for the lithium ions coordinated by the 15-crown-5 ether are close to zero. The observed trends of the 7Li NMR shifts are confirmed by density-functional theory calculations. The magnetic dc and ac properties display distinct differences for the six compounds under investigation. Both manganese compounds, [LiMn{N(SiMe3)2}3] and [Li(15-crown-5)][Mn{N(SiMe3)2}3], display almost pure and ideal spin-only paramagnetic behavior of a 3d5 high-spin complex. In this respect slightly unexpected, both complexes show slow relaxation behavior at low temperatures under applied dc fields, which is especially pronounced for the ate complex [LiMn{N(SiMe3)2}3]. Dc magnetic properties of the iron complexes reveal moderate g-factor anisotropies with small values of the axial magnetic anisotropy parameter D and a larger E (transversal anisotropy). Both complexes display at low temperatures and, under external dc fields of up to 5000 Oe, only weak ac signals with no maxima in the frequency range from 1 to 1500 s-1. In contrast, the two cobalt complexes display strong g-factor anisotropies with large values of D and E. In addition, in both cases, the ac measurements at low temperatures and applied dc fields reveal two, in terms of their frequency range, well separated relaxation processes with maxima lying for the most part outside of the measurement range between 1 and 1500 s-1.

Spectral studies of bismuth-containing pyrochlores

Thermal stable solid solutions of titanates, niobates, and tantalates of bismuth with a pyrochlore structure doped with 3d-metal atoms were studied using XPS and NEXAFS spectroscopy. Based on spectral studies, it was shown that the manganese, cobalt, nickel and copper atoms in these solid solutions have mainly charge state +2 and iron atoms – charge state +3.

The effect of Pd on hydride formation in Zr(PdxM1−x)2 intermetallics where M is a 3d element

We report how the substitution of 3d metal atoms M by Pd in Zr(PdxM1−x)2 pseudobinary compounds affects the stability of the corresponding hydrides. For the lighter elements in the M=3d metal row, V and Cr, the Pd substitution creates less stable hydrides while it yields almost no change in the stability of the hydrides for Mn, of the middle of the 3d row. We further demonstrate that the two factors determining the stability of the hydrides of these compounds are the heat of formation of the compound, ΔHalloy, and its crystal volume, V. Using a combination of these two parameters we predict that the substitution of the heavier 3d metals Fe and Co by Pd stabilizes the Zr(PdxM1−x)2 hydrides, i.e. they would require lower hydrogenation pressures than those necessary for ZrFe2 and ZrCo2 hydride formations. These findings support other authors’ theoretical prediction for ZrPdFe hydride formation. The empirically fitted constants that link ΔHalloy and V are related to the energies needed to expand or compress the crystal lattice and thus have basic physical meaning. We further evaluate the critical temperatures, Tc, for Zr(PdxMn1−x)2-H2 systems from the experimental pressure hysteresis in the absorption-desorption loops. This enables us to show that the electronic contribution to the H-H interaction in the ZrM2 hydride is unexpectedly neutral for M=Mn, it is positive, i.e. repulsive, for the lighter 3d element M=Cr, and surprisingly negative, i.e. attractive, for the binary PdH0.6 hydride.

Superhalogens Among 3d-Metal Compounds: MF4, MF6, MF12, and MF18 (M = Sc-Zn).

The ground states of the neutral and anionic tetrafluoride and hexafluoride series of 3d-metal atoms from Sc to Zn were assigned by using a double-check approach in which the pure and hybrid density functional methods were interchangeably used. It was confirmed that all these neutral fluorides are superhalogens except for TiF4. The electron affinities of the hexafluorides were shown to be consistently higher than those of the tetrafluorides in accordance with the superhalogen conception of the extra electron delocalization over a larger number of the electronegative ligands. In the search for mononuclear fluorides possessing higher electron affinities, we considered the M(F2)6- and M(F3)6- series where M = Sc-Zn. We found that the optimized geometrical structures in both series may be described as MF6-- k(F2), k = 3 and 6, of which the geometry of the MF6- core mimics that of the corresponding hexafluoride anion and the F2 dimers are kept in a bound state by polarizing forces. In these cases, the electron affinity is decreased by tenths of eV with respect to the electron affinity of the core hexafluorides due to a confinement of the extra electron by the F2 environment.

Isomorphous series of heterometallic complexes Na4[CuxCo(1-x){N(CH2PO3)3}]·13H2O and Na4[CuxZn(1-x){N(CH2PO3)3}]·13H2O (x = 1…0): Synthesis and structural characterization by analysis of 3d-metal coordination polyhedra as compared with isodimorphous series Na4[CuxNi(1-x){N(CH2PO3)3}]·nH2O

The symmetry of the coordination environment of 3d metal atoms in mixed nitrilo-tris(methylenephosphonate) complexes CuxZn(1-x), CuxNi(1-x) and CuxCo(1-x) (x = 1…0) has been studied using the method for quantitative estimation of the degree of similarity of coordination polyhedra (CP) to regular polyhedra. The degree of similarity Φ and the degree of invariance of the electron density η have been proposed and successfully used to estimate the coordination geometry of the analyzed complexes. For mixed Cu–Co–NTP and Cu–Zn–NTP complexes, the degree of similarity of CP to a regular trigonal bipyramid (TBP) has a smooth maximum attributing to changing the CP distortion character. It changes from the distortion characteristic of individual Co- and Zn-based complexes and revealed as longer M–N bond to the distortion characteristic of individual Cu-based complex manifested as lengthening of one of the M–O bonds. In the Cu–Ni–NTP isodimorphous series, the degree of similarity of CP to each of the regular polyhedra used for considering the individual phases (octahedron and TBP for Ni- and Cu-based complexes, respectively) increases with approaching the transition point and has a sharp maximum at this point.

Electrochemical Reduction of CO2 Using Carbon-Based Materials Modified with 3d-Metal Atoms As Electrocatalysts

For the reductive CO2 fixation using solar energy, photo-generated electrons and/or holes must be captured and utilized for chemical reactions that generate products beneficial to humans. To construct the artificial photosynthesis systems, electron transfer catalysts designed to promote electrochemical CO2 reduction reactions (CO2RR) are a critical requirement. Previous studies have shown that precise tuning of adsorption energies of key intermediates is critical for obtaining high activity and reaction selectivity. There are two important factors that determine the adsorption energy of the intermediate on electrocatalysts. First, the metal species in the catalyst, i.e., the transition metals at the active sites with a higher number of d-electrons, tend to reduce the adsorption energy. Previous works have investigated the effects of metal species on the CO2RR using organometallic complexes as platforms to support various metals via coordination bonds. For example, cobalt-modified tetraphenylporphyrin (TPP) is known to exhibit efficient CO2RR activity due to its proper binding strength to COOH*, whereas nickel (Ni) and copper (Cu) modified TPP are known to have low catalytic activity due to the low binding energy of COOH*. The second critical factor is the coordination structure of the metal. First-principles calculations have revealed that a decrease in the coordination number of Pt and Cu in the bulk increases the bond strength of CO* and affects the activity of the CO2RR. Thus, it can be expected that the coordination number can be a control parameter to improve the performance of metal species that were previously thought to have no CO2RR activity. We have investigated the effects of metal species and the coordination structure on CO2RR using graphene and covalent triazine frameworks (CTF)[1,2]. CTF consisting of microporous conjugated polymers with 1,3,5-triazine linker units, are suitable for investigating the above issue due to the high design flexibility. Through the studies, we have revealed that Ni-modified graphene and CTF can work as efficient electrocatalysts for CO2RR due to the unsaturation nature of the Ni atoms on the substrate. In this talk, we report our systematic studies on CO2RR using carbon-based materials with 3d metal atoms, focusing on the effects of metal species and coordination structures.[1] P. Su, K. Iwase, S. Nakanishi, K. Hashimoto, K. Kamiya, Small, 12, 6083-6089 (2016).[2] P. Su, K. Iwase, T. Harada, K. Kamiya, S. Nakanishi, Chem. Sci., 9, 3941-3947 (2018). Figure 1

Ferroelectrics-magnetics with the perovskite structure

Binary () and ternary () ferroelectrics-magnetics with the perovskite structure, containing the 3d-metal atoms, are investigated. Some correlations between their ferro- or antiferroelectric (T C) and magnetic (T N) phase transition temperatures and their interatomic A-O (), and B-O () bonds strains, determined with using of quasi-elastic structure model, have been constructed. The latter structural parameters and phase transition temperatures themselves influence on their T C-T N difference was revealed. Such correlations can help with the search for new solid solution systems between different ferroelectrics-magnets with enough high T C and T N and low T C-T N difference with using of apriori determined and values.

Combined micro X-ray absorption and fluorescence spectroscopy to map phases of complex systems: the case of sphalerite

Combining micro-X-ray absorption spectroscopy (μXAS) and micro-X-ray fluorescence spectroscopy (μXRF) is a promising approach for the investigation of complex multi-phase systems. In this work, we have employed this approach to investigate natural sphalerite, the most common form of Zinc Sulfide. Spatially resolved elemental distribution maps of common 3d metal atoms (Zn, Cu, Ni, Co, and Fe) are superimposed with chemical speciation and structural parameter maps in order to understand the sphaleriteore-formation process and metamorphosis. Chemical speciation and structural parameters have been obtained by analyzing the μXAS spectra collected in several representative points of the sample, after μXRF mapping. In the present case, this X-ray based approach has permitted to determine the spatial distribution of the Zn species in sphalerite. The presence of two main zincite and smithsonite inclusions has been established, with the latter located close to copper impurity center. Since copper is known to remarkably reduce the corrosion resistance of zinc, resulting in the formation of carbonate as the corrosion product, this implies a possible role of Cu in the growth of the carbonate inclusions. The results obtained highlight the efficiency of this method in univocally identifying the spatial distribution of phases in complex systems, thanks to the simultaneous access to complementary information.

Correlation between the M←OH2 Bond Force Constants, Energies, and Lengths and Magnetic Moments of the Metal Atoms in Isostructural Nitrilotris(methylenephosphonic) Complexes [MII(H2O)3μ-NH(CH2PO3H)3] (MII = Cr-Zn)

The isostructural 3d transition metal complexes [MII(H2O)3μ-NH(CH2PO3H)3] (MII = Cr-Zn) were studied by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. In order to gain more in-depth knowledge about the chemical bonds in coordination compounds, correlation analysis was carried out between the force constants and energies of the M←OH2 coordination bonds and the magnetic moments of transition metal atoms. The correlation of the force constants of the M←OH2 bonds with the M-O(w) interatomic distances was in line with the known empirical Badger’s rule. The double linear regression between the bond energy and reciprocal of M-O(w) interatomic distances and atomic magnetic moments shows a close relationship between the M←OH2 bond energy and the spin magnetic moment of 3d-metal atoms. The results account for the stability of Fe-containing heterometallic polymeric NTP complexes and high efficiency of NTP complexes as corrosion inhibitors for iron alloys in aqueous media.

Hydrogenation of 3d-metal oxide clusters: Effects on the structure and magnetic properties.

The geometrical structures and properties of the M8 O12 , M8 O12 H8 , and M8 O12 H12 clusters are explored using density functional theory with the generalized gradient approximation for all 3d-metals M from Sc to Zn. It is found that the geometries and total spin magnetic moments of the clusters depended strongly on the 3d-atom type and the hydrogenation extent. More than the half of all of the 30 clusters had singlet lowest total energy states, which could be described as either nonmagnetic or antiferromagnetic. Hydrogenation increases the total spin magnetic moments of the M8 O12 H12 clusters when MMnNi, which become larger by four Bohr magneton than those of the corresponding unary clusters M8 . Hydrogenation substantially affects such properties as polarizability, forbidden band gaps, and dipole moments. Collective superexchange where the local total spin magnetic moments of two atom squads are coupled antiparallel was observed in antiferromagnetic singlet states of Fe8 O12 H8 and Co8 O12 H8 , whereas the lowest total energy states of their neighbors Mn8 O12 H8 and Ni8 O12 H8 are ferrimagnetic and ferromagnetic, respectively. Hydrogenation leads to a decrease in the average binding energy per atom when moving across the 3d-metal atom series. © 2018 Wiley Periodicals, Inc.

Electronic Properties of 3d-Metal-Doped Zirconia Nanotubes

The electronic structure of hexagonal ZrO2 nanotubes doped by 3d-metal atoms was calculated by the linearized augmented cylindrical wave (LACW) method. The calculated densities of electronic states indicate that partial substitutions of zirconium atoms by transition metals (Sc, Ti, Mn, Co, and Zn) decrease the optical gap of (4,4) ZrO2 nanotubes from 3.7 to 2–2.3 eV. Doping of nanotubes shifts optical absorption from the UV to the visible, and the nanotubes themselves become candidate materials for design of electrodes for electrochemical water photolysis.

Iron for nickel substitution effects on magnetic and magnetocaloric properties of melt-spun Gd75(Ni1-xFex)25 alloys

The melt quenched Gd75(Ni1-xFex)25 alloys are found to exhibit a soft ferrimagnetic behavior in the whole concentration range (0 ≤ × ≤ 1). Substitution of Fe for Ni in the Gd75(Ni1-xFex)25 alloys results in a linear growth of the magnetic ordering temperature from 118 K to 244 K; although, an average value of the magnetic moment per transition metal atom, obtained from saturation magnetization measurements, does not change significantly with increasing Fe content. The variations of the Curie temperature and magnetization in Gd75M25-type amorphous alloys (M = Fe, Ni) are discussed bearing in mind a possible noncollinearity of magnetic moments of 3d metal atoms. The maximal values of the isothermal magnetic entropy change (ΔSm) are observed to exhibit the Fe-poor Gd75(Ni1-xFex)25 alloys. The change in the concentration of 3d-metal of different types in amorphous Gd75M25 alloys with a constant gadolinium content makes it possible to vary both the magnetocaloric effect and the operating temperature range.

Tuning the structure and catalytic activity of Ru nanoparticle catalysts by single 3d transition-metal atoms in Ru12-metalloporphyrin precursors.

Ru nanoparticle catalysts were prepared from Ru12-metalloporphyrin complex precursors containing 3d transition-metal atoms attached to SiO2 surfaces. The single 3d metal atoms at the central position of the Ru12-metalloporphyrin complex precursors exerted a significant influence on the structures and hydrogenation performance of the Ru nanoparticles on the SiO2 surfaces. The Ru12-Cu-porphyrin complex afforded positively charged Ru nanoparticles, which would provide high activity toward aromatic hydrogenation.