Coatomer Research Articles

Diffusion of solutes in fcc Cobalt investigated by diffusion couples and first principles kinetic Monte Carlo

The interdiffusivity of Al and the transition metal solutes Ti, V, Cr, Mn, Fe, Nb, Mo, Ru, Ta, W, and Re in fcc Co is characterized at 1373 K, 1473 K and 1573 K by binary diffusion couples. The experimental results are complemented by first-principles calculations in combination with kinetic Monte Carlo simulations to investigate the diffusion of Re, W, Mo and Ta in fcc Co. The interdiffusion coefficients of alloying elements in fcc Co are generally smaller than in fcc Ni, but the correlation between interdiffusion coefficients and the atomic number of metal solutes is comparable in Co and Ni. With increasing atomic number and decreasing atomic radii the interdiffusion coefficients of the investigated elements, except for Mn and Fe, decrease strongly. The trends in the diffusivity determined by experiment and simulation are in excellent agreement. Re is the slowest diffusing element in fcc Co among the investigated elements. The electronic structure calculations indicate that this is caused by strong directional bonds between Re and neighboring Co atoms. The overall lower diffusivity of solute atoms in Co as compared to Ni suggests that diffusion controlled processes could be slower in Co-base superalloys.

Matrix isolation model studies on the radiation-induced transformations of small molecules of astrochemical and atmospheric interest

The radiation-induced transformations of small molecules at low temperatures play an important role in the interstellar, planetary and atmospheric chemistry. This work presents a review of our recent model studies on the radiation chemistry of relevant molecules in solid noble gas (Ng) matrices, including some preliminary new results. Among the triatomic molecules, water and carbon dioxide were studied in detail. The radiation-induced degradation of isolated H2O yields hydrogen atoms and OH radicals, while oxygen atoms are produced at higher doses. Isolated CO2 molecules are decomposed to yield CO and trapped oxygen atoms. Upon annealing the trapped O and H atoms are mobilized selectively at different temperatures and react with other trapped species. The formation of HCO and HOCO radicals was observed in the mixed H2O/CO2/Ng systems. Other studies were concerned with the radiation-induced degradation of simple organic molecules (methanol, formic acid) and chlorofluorocarbons (CFCl3, CF2Cl2). Preliminary results for methanol revealed deep dehydrogenation yielding HCO and CO, whereas CO2, CO and HOCO were detected as primary products for formic acid. In the case of chlorofluorocarbons, significance of ionic channels was demonstrated. The implications of the results for modeling the processes in astrochemical ices and atmosphere are discussed.

Enhanced CO oxidation over potassium-promoted Pt/Al2O3 catalysts: Kinetic and infrared spectroscopic study

A series of K-promoted Pt/Al2O3 catalysts were tested for CO oxidation. It was found that the addition of K significantly enhanced the activity. A detailed kinetic study showed that the activation energies of the K-containing catalysts were lower than those of the K-free ones, particularly for catalysts with high Pt contents (51.6 kJ/mol for 0.42K-2.0Pt/Al2O3 and 63.6 kJ/mol for 2.0Pt/Al2O3). The CO reaction orders were higher for the K-containing catalysts (about −0.2) than for the K-free ones (about −0.5), with the former having much lower equilibrium constants for CO adsorption than the latter. In situ Fourier-transform infrared spectroscopy showed that surface CO desorption from the 0.42K-2.0Pt/Al2O3 catalyst was easier than from 2.0Pt/Al2O3. The promoting effect of K was therefore caused by weakening of the interactions between CO and surface Pt atoms. This decreased coverage of the catalyst with CO and facilitated competitive O2 chemisorption on the Pt surface, and significantly lowered the reaction barrier between chemisorbed CO and O2 species.

Influence of Co and Dy Doping on the Optical and Magnetic Properties of ZnO Nanoparticles for DMS Application

Dysprosium-(Dy) and cobalt (Co)-doped zinc oxide (ZnO) nanoparticles with various concentrations have been synthesized by simple chemical precipitation method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDAX) measurements were conducted respectively for the structural, morphological, and compositional investigation of the sample. XRD analysis indicated that diffraction peaks of all the samples can be indexed to the hexagonal wurtzite structure of ZnO. No secondary phases including Co and Dy ions were detected in the samples even for the highest doping concentrations of Dy. Average particle size is found to be 25–34 nm. Scanning electron microscope and transmission electron microscope analysis revealed that the dopant atoms of Co and Dy are homogeneously distributed in ZnO wurtzite structure. The samples were characterized by EDAX to confirm the expected stoichiometry. In the photoluminescence spectra, Dy co-doped samples show a violet emission along with a broad yellow emission due to the4F9/2–6H15/2 inner shell transitions of Dy3+ ion. The photoluminescence (PL) enhancement process in Dy co-doped ZnO:Co is mainly due to Dy as a sensibilizer which could effectively enhance the luminescence intensity. Absorption spectra of dysprosium- (Dy) and cobalt (Co)-doped ZnO nanoparticles exhibited enhanced optical absorption in visible region. Magnetization measurements have shown that the particles have room temperature ferromagnetic behavior with relatively high coercive fields which are decreasing with the increase of doping concentration. The analysis of optical and magnetic properties shows that Co- and Dy-doped ZnO nanoparticles are a promising material for DMS application and have potential applications in optoelectronic devices.

Cyclization of a 1,4-diborabutadiene ligand with both atoms of CO.

A 1,4-dibora-1,3-butadiene iron complex was successfully synthesized through the stoichiometric reaction of an iron bis(borylene) complex with diphenylacetylene. This complex was treated with CO and PMe3 , which led to the formation of an unusual six-membered B2 C3 O ylidic ring bound to both the PMe3 group and zerovalent iron center. The reaction is a very rare example of the incorporation of both atoms of CO into a ring system.

Synthesis and first use of pyridine-2,6-diylbis(pyrazine-2-ylmethanone) in metal cluster chemistry: a {Mn(III)3Na2} complex with an ideal trigonal bipyramidal geometry.

The successful organic synthesis of a new dipyrazole/pyridine-dicarbonyl organic molecule, namely pyridine-2,6-diylbis(pyrazine-2-ylmethanone) [(pz)CO(py)CO(pz)], followed by its employment in Mn coordination chemistry has yielded the neutral cluster compound [Mn3Na2O(N3)3(L)3] (1), where L(2-) is the (pz)C(CH2COCH3)(O(-))(py)C(CH2COCH3)(O(-))(pz) dianion. The latter group was formed in situ, presumably by the nucleophilic attack of the carbanion (-)CH2COCH3 to the carbonyl carbon atoms of (pz)CO(py)CO(pz), in the presence of Mn(n+) ions under basic conditions and in solvent Me2CO. Complex 1 possesses an almost ideal trigonal bipyramidal topology, with the two Na(I) ions occupying the apical positions and the three Mn(III) ions residing in the equatorial trigonal plane. The bridging ligation about the metal ions is provided by a μ3-O(2-) ion and six μ-OR(-) groups from the L(2-) ligand, while peripheral ligation is completed by three terminal azido groups and the pyridine N and carbonyl O atoms of L(2-). Magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the paramagnetic Mn(III) centres; the use of an anisotropic, equilateral Mn(III)3 triangle model allowed us to fit the magnetic data and obtain the best-fit parameters: J = -10.8 cm(-1), D = -5.3 cm(-1), and g = 1.99. The combined results demonstrate the rich chemical reactivity of carbonyl groups and the ability of poly-ketone ligands to stabilize cluster compounds with unprecedented structural motifs and interesting architectures.

Fine Spin Filtering Effect in Co-Phthalocyanine Molecule Induced by the Spin Polarization of Co Atom

Spintronic devices will play a very important role in future information technology. In this study, By spin-polarized density-functional theory calculations combined with the Keldysh nonequilibrium Green’s method, the effect of the spin direction of Co atom in Co- phthalocyanine molecule in modulating spin filtering effects under external biases are investigated. Here, an individual single molecule Co-phthalocyanine is sandwiched between two infinite 8-zigzag-graphene nanoribbon electrodes. we find that the spin direction of the Co atom relative to the magnetic polarization of the left and right electrodes can improve the spin filtering effect greatly. when the polarization direction of the two electrodes is antiparallel and the polarization of Co atom in the Co-phthalocyanine molecule upward, the configuration posesses almost perfectly spin-filter effect. The underlying mechanism of the perfect spin filtering action is applied.

Activation of Catalytic Properties of Metal Nanoclusters

Within the methods of electron density functional and ab initio pseudopotential, we have obtained the spatial distributions of the density of valence electrons and the electron energy spectra for metallic and oxidized small clusters from the atoms of Cu, Ni, and Co with the aim to determine the mechanisms of their high catalytic activity. We have determined a certain organization of the electronic structure of the isolated metallic (Cu, Ni, Co) atoms in the ground and excited states, which is conserved at the association of these atoms in two atomic clusters. This has allowed us to draw conclusion about the easier excitation of d-electrons of copper as compared with the electrons of nickel and cobalt. This provides a possible freeing of d-orbitals that is fundamentally important for the catalytic activity of copper centers.

Bulk and surface properties of Co–Fe and Fe–Pd liquid alloys

We have employed quasi-chemical approximation to study the concentration dependence of bulk thermodynamic properties of two liquid alloys, Co–Fe and Fe–Pd at temperatures of 1863K and 1873K, respectively, at fixed atmospheric pressure. In order to investigate the surface structure in the alloys we have also performed a comparative study of surface tension using two statistical models. The study of the alloys indicates a positive deviation from the Raoultian behaviour with a higher degree of segregation in Fe–Pd system than that in the Co–Fe system. The surface tension is found to increase, both in Co–Fe and Fe–Pd alloys, with the addition of Fe-component. Furthermore, the surface segregation of the atoms of the metal with lower surface tension (Pd atoms in Fe–Pd alloy and Co atoms in Co–Fe alloy) is pronounced. Maximum surface segregation is expected to occur in the Co–Fe and Fe–Pd alloys around the compositions xCo=0.5 and xFe=0.8, respectively.

Unexpected magnetic properties in carbon-doped SnO2 from first-principles calculation

We employ spin-polarized density functional theory (DFT) calculations within generalized gradient approximation (GGA) to study the electronic structure and magnetic properties of C-doped SnO2 (SnO2:C) bulk and thin films. Our results indicate that a singly substitutional C (CO) does not induce magnetism, while the CO–CO pairs can unexpectedly activates short-range ferromagnetism in SnO2 bulk. The intrinsic defect O vacancy (VO) triggers local moment on the isolated CO atom but do not enhance the ferromagnetic (FM) coupling between CO atoms. When the substitutional CO atoms located at the surface of SnO2 thin films, system exhibits anti-ferromagnetic (AFM) feature, which is inconsistent with experimental observation. This diversity of magnetic behavior in SnO2:C system highlights the delicate interplay between electron correlations and localization. The magnetic properties are closely related to the intrinsic defect VO and the reduction of some Sn+4 ions to Sn+2 as a possible charge compensation mechanism.

Effect of the physical state on the scavenging of quasi-free electrons by a solute

The reactivity of carbon tetrachloride toward quasi-free electrons generated during the decay of 57Co atoms in its alcohol solutions frozen at 80 K has been studied by Mossbauer emission spectroscopy. It has been found that an increase in CCl4 concentration to 10 mol/L slightly affects the yield of 57Fe2+ and 57Fe3+ ions as 57Co decay products. The inertness of CCl4 has been interpreted as a consequence of a change of the energy state of quasi-free track electrons results from freezing the alcohol solution. This effect inhibits the dissociative attachment reaction of scavenging of track electrons and increases the ground-state energy of quasi-free electrons (on passing to the frozen matrix), thereby fundamentally altering the energy balance of the electron scavenging reaction in comparison with the liquid medium and, hence, decreasing the rate constant of electron scavenging by CCl4.

Physicochemical impact studies of gamma rays on “aspirin” analgesics drug and its metal complexes in solid form: Synthesis, spectroscopic and biological assessment of Ca(II), Mg(II), Sr(II) and Ba(II) aspirinate complexes

Metal aspirinate complexes, M2(Asp)4, where M is Mg(II), Ca(II), Sr(II) or Ba(II) are formed by refluxed of aspirin (Asp) with divalent non-transition metal ions of group (II) and characterized by elemental analysis and spectroscopic measurements (infrared, electronic, 1H NMR, Raman, X-ray powder diffraction and scanning electron microscopy). Elemental analysis of the chelates suggests the stoichiometry is 1:2 (metal:ligand). Infrared spectra of the complexes agree with the coordination to the central metal atom through three donation sites of two oxygen atoms of bridge bidentate carboxylate group and oxygen atom of CO of acetyl group. Infrared spectra coupled with the results of elemental analyzes suggested a distorted octahedral structure for the M(II) aspirinate complexes. Gamma irradiation was tested as a method for stabilization of aspirin as well as their complexes. The effect of gamma irradiation, with dose of 80Gy, on the properties of aspirinate complexes was studied. The aspirinate chelates have been screened for their in vitro antibacterial activity against four bacteria, gram-positive (Bacillus subtilis and Staphylococcus aureus) and gram-negative (Escherichia coli and Pseudomonas aeruginosa) and two strains of fungus (Aspergillus flavus and Candida albicans). The metal chelates were shown to possess more antibacterial activity than the free aspirin chelate.

An Introduction of CO2 Conversion by Dry Reforming with Methane and New Route of Low-Temperature Methanol Synthesis

Carbon dioxide is one of the highest contributors to the greenhouse effect, as well as a cheap and nontoxic building block for single carbon source chemistry. As such, CO₂ conversion is one of most important research areas in energy and environment sciences, as well as in catalysis technology. For chemical conversion of CO₂, natural gas (mainly CH₄) is a promising counterpart molecule to the CO₂-related reaction, due to its high availability and low price. More importantly, being able to convert CH₄ to useful fuels and molecules is advantageous, because it is also a kind of "greenhouse effect" gas, and can be an energy alternative to petroleum oil. In this Account, we discuss our development of efficient catalysts with precisely designed nanostructure for CO₂ reforming of CH₄ to produce syngas (mixture of CO and H₂), which can then be converted to many chemicals and energy products. This new production flow can establish a GTL (gas-to-liquid) industry, being currently pushed by the shale gas revolution. From the viewpoint of GTL industry, developing a catalyst for CO₂ reforming of CH₄ is a challenge, because they need a very high production rate to make the huge GTL methane reformer as small as possible. In addition, since both CO₂ and CH₄ give off carbon deposits that deactivate non-precious metallic catalysts very quickly, the total design of catalyst support and supported metallic nanoparticles is necessary. We present a simple but useful method to prepare bimodal catalyst support, where small pores are formed inside large ones during the self-organization of nanoparticles from solution. Large pores enhance the mass transfer rate, while small pores provide large surface areas to disperse active metallic nanoparticles. More importantly, building materials for small pores can also be used as promoters or cocatalysts to further enhance the total activity and stability. Produced syngas from methane reforming is generally catalytically converted in situ via one of two main routes. The first is to use Fischer-Tropsch synthesis (FTS), a process that catalytically converts syngas to hydrocarbons of varying molecular weights. The second is methanol synthesis. The latter has better atomic economy, since the oxygen atom in CO is included in the product and CO₂ can be blended into syngas as a reactant. However, production of methanol is very inefficient in this reaction: only 10-15% one-pass conversion typically at 5.0-10.0 MPa and 523-573 K, due to the severe thermodynamic limitations of this exothermal reaction (CO + 2H₂ = CH₃OH). In this Account, we propose and develop a new route of low-temperature methanol synthesis from CO₂-containing syngas only by adding alcohols, including methanol itself. These alcohols act as homogeneous cocatalysts and the solvent, realizing 70-100% one-pass conversion at only 5.0 MPa and 443 K. The key step is the reaction of the adsorbed formate species with alcohols to yield ester species at low temperatures, followed by the hydrogenation of ester by hydrogen atoms on metallic Cu. This changes the normal reaction path of conventional, high-temperature methanol synthesis from formate via methoxy to methanol.

Effect of adsorbed magnetic and non‐magnetic atoms on electronic transport through surfaces with strong spin‐orbit coupling

AbstractAdsorption‐induced reduction of the surface state conductivity in epitaxial Bi(111) films, a prototype system with a large Rashba‐induced surface state splitting by adsorbed atoms of Bi, Fe and Co was investigated by macroscopic surface magneto‐transport measurements at a temperature of 10 K. A detailed analysis of magneto‐transport, DC‐transport and Hall data reveals that the scattering efficiencies for Co and Fe are by a factor of two larger than for Bi. While for the latter, charge transfer and change of band filling near the Fermi level is negligible, we found an increase of hole concentration upon Co and Fe adsorption. These atoms act as acceptors and retract roughly 0.5 electrons from the surface per adsorbed atom. Besides the dominant classical magneto‐conductance signal the films show signatures of weak anti‐localization (WAL) reflecting the strong spin‐orbit coupling in Bi(111) surface states. Our measurements show that the control of hybridization is important in order to make use of local spin‐moments and to increase the backscattering rate in strongly spin‐orbit coupled systems, e. g., topological insulators.

Effect of CeO2 on CO removal over CeO2-modified Ni catalyst in CO-rich syngas

A CeO2-modified Ni catalyst has been studied as a substitute for Ni bulk catalyst in a CO removal reaction using various characterization methods. CO removal was enhanced slightly and presented at lower reaction temperatures following promotion of CeO2 on Ni. The enhanced ability to reduce CO was mainly a result of methanation rather than WGS during a CO removal reaction. Based on X-ray diffraction and temperature-programmed reduction, CeO2 appeared to change the Ni surface properties. Because the bond strength between C and O atoms in CO was weakened by the surface oxygen of CeO2 on Ni, the CeO2-promoted Ni catalyst showed higher CO conversion and lower selectivity to WGS than Ni bulk catalyst.

Theoretical studies of the CO2–N2O van der Waals complex: Ab initio potential energy surface, intermolecular vibrations, and rotational transition frequencies

Theoretical studies of the potential energy surface and bound states were performed for the CO(2)-N(2)O van der Waals complex. A four-dimensional intermolecular potential energy surface (PES) was constructed from 11,466 ab initio data points which were calculated at the coupled-cluster single double (triple) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to two equivalent isomers with a slipped parallel structure in which the O atom in N(2)O is near the C atom in CO(2) and a T-shaped isomer in which the terminal N atom in N(2)O is closest to the C atom in CO(2). The two slipped parallel isomers are energetically more stable than the T-shaped isomer by 178 cm(-1). Four fundamental vibrational excited states for the slipped parallel isomers and two fundamental vibrational excited states (torsion and disrotation) for the T-shaped isomer were assigned via bound states calculations based on this PES. The theoretical vibrational frequencies are in good agreement with the available experimental values for the slipped parallel isomers. Rotational excitations (J = 0-6) for the ground vibrational state of the slipped parallel structure were calculated and the accuracy of the PES in the vicinity of minima is validated by the good agreement between the theoretical and experimental transition frequencies and spectroscopic parameters.

W(110)上の Co/Ni 多重層における磁気異方性の理論的研究

We investigated the magnetic property of Co/Ni multi-layers on W(110), using first principles calculation based on density functional theory. We found that one atomic monolayer of Ni (Ni 1ML) and two atomic monolayers of Ni (Ni 2ML) on W(110) exhibited no magnetic moment. These results occur because the minority spin states of Ni atoms are occupied by the electrons transferred from W atoms. These results also agree with the experimental results where the 1ML and 2ML of Ni layers exhibited no magnetic anisotropy. We calculated the magnetic anisotropy energy in each of the three directions, viz., the two surface in-plane directions and one surface out-of-plane direction, in Ni 1ML, Ni 2ML and Co 1ML/Ni 2ML on W(110). We found that the Co surface atoms of Co 1ML/Ni 2ML exhibit surface in-plane magnetic anisotropy. We also estimated the effect of magnetic dipole-dipole interaction on the magnetic anisotropy energy in a Co 1ML/Ni 2ML multi-layer on W(110). We found that the effect is not dominant.

Direct, in situ determination of pH and solute concentrations in formic acid dehydrogenation and CO2 hydrogenation in pressurised aqueous solutions using 1H and 13C NMR spectroscopy

In water, spin-lattice relaxation times (T(1)) and calibration curves for chemical shifts have been determined for the (13)C and the (1)H(C) atoms in HCOOH, HCOONa, CO(2), Na(2)CO(3) and NaHCO(3) by NMR spectroscopy. These data facilitate kinetic and mechanistic studies for H(2) storage/delivery in the carbon dioxide-formic acid systems under H(2) and CO(2) pressures.

Scattering at magnetic and nonmagnetic impurities on surfaces with strong spin-orbit coupling

Adsorption-induced reduction of surface-state conductivity in epitaxial Bi(111) films, a prototype system with large Rashba-induced surface-state splitting, by adsorbed atoms of Bi, Fe, and Co has been investigated by macroscopic surface magnetotransport measurements at a temperature of 10 K. A detailed analysis of magnetotransport, dc transport, and Hall data reveals that the scattering efficiencies for Co and Fe are larger by a factor of 2 than that for Bi. While for the latter charge transfer and change of band filling near the Fermi level are negligible, we find an increase of hole concentration upon Co and Fe adsorption. These atoms act as acceptors and immobilize on average about 0.5 electrons per adsorbed atom. Besides the dominant classical magnetoconductance signal the films show signatures of weak antilocalization, reflecting the strong spin-orbit coupling in Bi(111) surface states. This behavior can be changed to weak localization by the adsorption of high concentrations (0.1 monolayers) of magnetic impurities (Fe,Co), similarly to results found on the topological insulator Bi${}_{2}$Se${}_{3}$. Our results demonstrate that details of chemical bond formation for impurities are crucial for local spin moments and electronic scattering properties.

Tungsten Oxide Clathrates for Water Oxidation: A First Principles Study

Tungsten oxide (WO3) is a good photoanode material for oxidizing water, but it is not an efficient absorber of sunlight because of its large band gap (2.6 eV). Recently, stable clathrates of WO3 with interstitial N2 molecules (xN2·WO3, x = 0.034–0.039) were synthesized, with a band gap of 1.8 eV. We studied the structural and electronic properties of these clathrates using ab initio calculations, and we analyzed the physical origin of the gap reduction reported experimentally. We found that both structural changes caused by the insertion of N2, and a small charge overlap between N2 and WO3, are responsible for the gap decrease. We compared the effect of N2 intercalation to that of other closed shell species, in particular CO and rare gas atoms. Our calculations predicted that CO insertion lowers the band gap by about the same amount as N2 but it leads to a change of both the oxide valence and conduction band positions, while the presence of N2 only affects the conduction band minimum. We also predicted th...