Coatomer Research Articles

Carbon solubility in face centred cubic Co 1− yNi y alloy lattice analysed by statistical thermodynamics

Carbon solubility in face centred cubic (fcc) Co 1− y Ni y alloy lattice was analysed on the basis of statistical thermodynamics under the a priori assumption of negligible C-C interaction in the fcc Co 1− y Ni y lattice. The minimum C solubility observed around the composition y between 0.6 and 0.8 was concluded to be ascribable to the minimised number of available interstitial sites for occupation by C atoms in the fcc Co 1− y Ni y lattice. The observed patterns of variations for the C solubility and the extent of stabilisation of the C atom in the fcc Co 1− y Ni y lattice with respect to the composition y appeared to be comparable to those observed for fcc Fe 1− y Ni y C x when compared in terms of the average number of 3 d electrons per metal atom.

Real Time Observation of Decomposition of Methanol on Ru(001)-p(2 × 2)-O by a Temperature Programmed ESDIAD/TOF System

Abstract For the purpose of utilizing ESDIAD (Electron Stimulated Desorption Ion Angular Distribution) as a real time probe for surface chemical processes, we have developed a new instrument which can measure ESDIAD images and time of flight (TOF) spectra of desorbed ions in temperature programmed surface processes. TOF measurements are carried out to identify the mass of desorbed ions and to determine their kinetic energy distribution. This temperature-programmed (TP)-ESDIAD/TOF system was used to observe the decomposition processes of methanol on Ru(001)-p(2 × 2)-O. A halo pattern from the methyl group of methoxy species was observed at 100-200 K by the use of CH3OD, followed by a broad pattern from the methyl group at 230-250 K, and by a near-center pattern from the oxygen atom of CO above 300 K.

Spirocyclic Peptidomimetics Featuring 2,3-Methanoamino Acids

The spirocyclic peptidomimetics cyclo-[{(2S,3S)-cyclo-R}GD{(2S,3S)-cyclo-R}GD] (3) and cyclo-[{(2R,3S)-cyclo-R}GD{(2R,3S)-cyclo-R}GD] (4), containing 2,3-methanoarginine (cyclo-Arg‘) residues, were prepared. Conformations of these molecules were studied via a combination of CD and 1D and 2D NMR, and these data were interfaced with extensive molecular simulations. Conformational biases for molecules 3 and 4 were thereby deduced and were compared with conclusions previously obtained (Burgess, K.; Lim, D.; Mousa, S. A. J. Med. Chem. 1996, 39, 4520) for cyclo-[RGDRGD] (2). Molecule 3 has a relatively clear conformational bias toward a structure with a type II β-turn between the Gly CO and Gly NH atoms, but 4 is more flexible, existing in conformers wherein the Asp and Arg side chains tend to be on opposite faces of the backbone cyclic structure. These conclusions were used to interpret the relative affinities of 2−4 with respect to the vitronectin receptor.

Mössbauer spectroscopy investigations and behaviour of impurity atoms in irradiated metallic targets

The properties of hot atomic microimpurities ( 57.58Co, 54Mn) in cyclotron metallic targets were investigated using the reaction 55Mn + α with emission Mossbauer spectroscopy employed to estimate the electronic state of the hot atoms. γ-Radiation self-absorption and sectioning methods were used to define the diffusion coefficients at 670–1200 K. The position of the 57Co atoms in the manganese lattice after irradiation and subsequent heating at the temperature of polymorphic α → β manganese (1000 K) transformations was established. Rapid diffusion of cobalt atoms at the polymorphic α → β manganese transformation was revealed. The radiochemical behaviour of 57,58Co, 54Mn after target dissolution was studied. The conclusion arrived at is that hot impurity atoms are a very effective tool in investigations of metallic cyclotron targets.

Core Electron Energies, Infrared Intensities, and Atomic Charges

Carbon 1s electron binding energies determined by X-ray photoelectron spectroscopy and mean dipole moment derivatives obtained from experimental infrared intensities are shown to be related through the simple potential model proposed by Siegbahn and collaborators. The sp3 carbon atoms in 13 halomethanes, 2 ethanes, 3 methylacetylenes, cyclopropane, and ethylene oxide have 1s energies, which, after correction for electrostatic potentials from neighboring atoms, are linearly related to the carbon mean dipole moment derivatives, presenting a slope of 15.50 ± 0.29 eV/e. The sp2 carbons of ethylene, three haloethylenes, and three carbonyl compounds also exhibit a linear relationship having a significantly different slope of 17.37 ± 0.87 eV/e. The sp carbon atoms in acetylenes, cyanides, CO, CS2, CO2, and OCS show a third linear relationship, with a slope of 18.90 ± 0.75 eV/e. These slopes are proportional to the inverse atomic radii of sp3, sp2, and sp carbon atoms and according to the simple potential equation can be interpreted as estimates of Coulomb repulsion integrals involving these hybridized orbitals and the 1s core electron orbitals. Two basic assumptions of the potential model are investigated. The effect of relaxation energies on the 1s electron ionization processes is estimated as the difference between ΔSCF ionization energies and Koopmans' frozen orbital estimates obtained from 6-31G(d,p) wave functions. These results are compared with values obtained previously from the equivalent cores estimating procedure. Also the conceptual validity of identifying the carbon mean dipole moment derivatives as atomic charges is discussed within the framework of the charge−charge flux-overlap model.

Local structure of as-prepared and partially reduced Co,Ti,Sn-substituted Ba-hexaferrite powder

In this study, the local structure around Fe and Co atoms in as-prepared and partially reduced Co,Ti,Sn-substituted Ba-hexaferrite powders are examined using extended x-ray absorption fine structure (EXAFS). It appears that the substitutional atoms in general and Co in particular show a preference for the Fe(IV) and Fe(III) sites. After partial reduction in H2 at 300 °C, both the Fe and Co EXAFS show a mixture of two phases: substituted Ba-hexaferrite and bcc metal. There does not appear to be any large preference in reduction among the five different types of Fe sites. However, a much larger fraction of the Co is reduced than is the Fe. Thus the partially reduced sample is not merely a collection of unreduced and totally reduced particles. The after-reduction spectra for both Fe and Co EXAFS were fit using a linear combination of spectra from as-prepared material and bcc metal, which enabled quantitative calculation of the relative fraction of each metal reduced.

Catalytic addition of CH bonds to multiple bonds

Abstract

Calcination Temperature Effect on Supported Ruthenium Catalysts for Fischer-Tropsch Synthesis Reaction.

Adjustment of air calcination temperature can significantly change the metal dispersion of Ru/Al2O3 catalyst and control the activity and selectivity of the Fischer-Tropsch synthesis. The reaction characteristics of the low-temperature-calcined catalysts were different from those of the high-temperature-calcined catalysts. The change in CO conversion by the catalysts calcined at different temperature took place in the range 200-250°C. n-Hexane in supercritical state was used to effectively extract high-molecular-weight products in the catalyst bed and reactor, to attain a complete distribution of the products. The chain-growth probability, as well as the olefin-to-paraffin ratio of the products was remarkably influenced by Ru dispersion. The water added to the synthesis gas promoted the chain-growth probability and enhanced the olefin contents in the products. The strength of the bond between adsorbed CO and surface Ru atoms was greatly controlled by calcination temperature. Oxygenates were formed on low-temperature-calcined catalyst but hydrocarbons were formed only on high-temperature-calcined catalyst in the ethylene-added Fischer-Tropsch reaction.

Identification of a surface reaction product by detection of a vibrational combination band

NCO molecules formed by the reaction of adsorbed CO and nitrogen atoms are unambiguously identified by means of infrared absorption spectroscopy. This identification of a surface reaction product is achieved by the detection of the combination band of two intramolecular modes. Isotopic substitution experiments using 12C 18O and 13C 16O are used to confirm the mode assignments through the unequal shifts of the various vibrational modes. The intermode anharmonicity is accurately determined to be 25.5–26 cm −1 which establishes the first reliable number of this type for an adsorbate species.

Study for the stability of CO 2 clathrate-hydrate using molecular dynamics simulation

Investigation using molecular dynamics simulation has been conducted in order to clarify the stability and to obtain the fundamental understanding of CO 2 clathrate-hydrate. It was revealed from the MD simulation that the CO 2 clathrate-hydrate is unstable as compared with both empty and Argon clathrate-hydrates. The reason for the unstableness is discussed based on the database obtained from the simulation. The repulsive force acting between the O atoms of CO 2 and O atom in H 2O consisting the cage have a destabilizing effect on the CO 2 clathrate-hydrate lattice structure.

Grain boundary diffusion effects on the sputter depth profiles of Co–Ag bilayers

We investigated the effects of surface roughness and grain boundary diffusion (GBD) at elevated temperatures on the Auger-electron-spectroscopy sputter depth profiles of Co–Ag bilayers and the GBD process of Ag atoms in Co. The Ag layer in the Ag/Co bilayer is transformed from a uniform layer to discrete islands by heat treatment. Enhanced mobility during sputtering at elevated temperatures makes Ag atoms migrate continually from islands to cover neighboring exposed Co, which reduces the size of Ag islands. On the other hand, the surface morphological modification of Co/Ag bilayer does not occur by heat treatment, and the depth profile at 340 °C resembles that from a uniformly intermixed film, which shows a drastic difference from that of Ag/Co. This is explained with a very thin and stable accumulation of Ag on the Co layer by GBD. A model of structural changes in Co/Ag subjected to ion sputtering at elevated temperatures is proposed on the basis of the results. The activation energy and pre-exponential factor for GBD of Ag in Co are found to be 0.46±0.06 eV and ∼1×10−8 cm2/s, respectively.

Magnetic properties of sputtered CoSm multilayers

Several CoSm multilayers have been obtained using rf magnetron sputtering. The variation of their magnetic moment with the temperature has been studied between 5–200 K. We have observed anomalous behaviours which indicate the existence of complicated magnetic interactions between the atoms of Co and Sm.

Atomic disorder and amorphization of B2-structure CoZr by ball milling

For a considerable number of intermetallic compounds it has been found that ball milling introduces atomic (chemical) disorder. Disorder due to milling was demonstrated by x-ray diffraction in AlRu, crystallizing in the B2 structure (ordered b.c.c.) by a decrease of the intensity of superlattice reflections relative to fundamental reflections. The same technique was used to investigate disordering by milling in Ni{sub 3}Al, crystallizing in the L1{sub 2} structure (ordered f.c.c.). In both cases the disorder is anti-site disorder of both components, i.e. both atomic species substitute on the wrong sublattices. Besides x-ray diffraction measurements of magnetic properties turned out to be useful in monitoring structural changes due to milling. The change in the superconducting transition temperature, measured by magnetic a.c. susceptibility, was used to demonstrate atomic disordering by milling in Nb{sub 3}Sn and Nb{sub 3}Au. The type of disorder turned out to be anti-site disorder. Such a type of disorder occurs in the same materials also at high temperatures or after irradiation by neutrons. The disordering was accompanied by an increase of the lattice parameter. An increase in high-field magnetization accompanied by a decrease of the lattice parameter during milling was found in B2 CoGa and B2 CoAl. In principlemore » in the completely ordered state both compounds are non-magnetic, because the CO atoms are shielded from one another by Ga and Al atoms, respectively. However, when a Co atom is transferred to the wrong sublattice, it is surrounded by Co atoms as nearest neighbors and bears a magnetic moment. This explains the strong increase of the magnetization due to milling.« less

Hyperfine interaction study of the decoration of the internal wall of nanosized voids by impurity probes

We observe the trapping of implanted radioactive 57Co atoms at empty nano-cavities formed in {111} Si by the desorption of He from bubbles. The absorption of 14.4 keV γ-rays roughly monitors the displacement of the activity, while anodic stripping and counting the remaining 122 keV γ-ray intensity is employed to obtain an accurate depth profile and to isolate the wanted fraction of the sample. Mössbauer spectroscopy shows that only damage and/or CoSi 2-nuclei related sites are present in samples implanted at 300 and 400°C. In a sample implanted with Co at RT at the same side as the cavity-generating He atoms, the Mössbauer spectra reveal two new quadrupole split components after 30 min annealing at 700°C. We present arguments for the tentative identification of these to a cavity edge site (79%, strong bonding) and to a cavity plane site (21%, weak bonding).

States of cobalt and iron in catalysts supported on TiO2 from data of diffuse reflectance IR spectra of adsorbed carbon monoxide

The variations in the oxidation states of cobalt and iron atoms in pure and mixed Co- and Fe-containing catalysts supported on TiO2 as a function of the conditions in which the catalysts were prepared and preliminarily treated were studied by analysis of the IR spectra of carbon monoxide adsorbed on these catalysts. A mutual influence of the components was discovered; iron was found to promote reduction of cobalt.

Magnetic hyperfine field on 111Cd in metastable bcc cobalt.

By means of perturbed angular correlation spectroscopy on molecular-beam-epitaxy-grown Fe/Co multilayers, we succeeded in measuring the magnetic hyperfine field at the $^{111}\mathrm{Cd}$ nuclear probe in metastable bcc Co, giving a value of -16.4(2) T. Hereby the systematics for the hyperfine field at Cd atoms in hcp, fcc, and bcc Co is completed. We demonstrate how the specific coordination influences the hyperfine fields for the different Co structures.

Superparamagnetism in melt-spun CuCo granular samples

We have studied the magnetic properties of a melt-spun granular sample of Cu 87Co 13. The Cu matrix contains very fine particles of Co, having a bimodal distribution of sizes: ‘larger’ particles ( ∼ 300 A ̊ ) and very much smaller particles ( < 15 A ̊ ) with the latter exhibiting superparamagnetism. The magnetisation data indicate a spread of sizes for the smaller superparamagnetic particles, ranging from individual Co atoms up to clusters of more than 100 atoms of Co. Assuming a simple distribution of particle sizes, we calculated the magnetic field dependence of the magnetisation of the superparamagnetic particles and find excellent agreement with experiment.

Optical absorption of fcc Co particles: Properties of conduction electrons of fcc Co.

Optical plasma-resonance absorption of fcc Co island films consisting of fcc Co particles smaller than about 300 \AA{} in diameter was measured in the photon energy region from 0.5 to 6.5 eV. Comparison of the broadening and the location of this absorption with those of Ni island films shows that the d character of conduction electrons of fcc Co is stronger than that of Ni, and that the number of conduction electrons per atom of fcc Co is large compared to that of Ni. The difference in the character can qualitatively be attributed to the difference in the s-d hybridization, which is larger in fcc Co than in Ni. The difference in the number may be due to the difference in the s-d hybridization.

The Microwave Spectrum and Structure of the CH 3OHCO Dimer

The rotational spectrum of CH 3OHCO has been observed in the region 7-18 GHz with a pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometer. In order to obtain detailed structural information the spectra of CH 3OH, CH 3OD, CD 3OH, and CD 3OD combined with CO and 13CO were examined. Each of the isotopic species studied exhibits two states, which are interpreted as A and E symmetry states arising from internal rotation of the methyl group. The E-state assignments were verified by observing their first-order Stark effect. The structure of the complex is a bent hydrogen bond of the carbon atom of CO at a distance of 2.41 Å from the hydroxyl hydrogen of methanol and planar heavy atoms. The effective barrier to internal rotation for CH 3OHCO, V 3 = 183.0 cm −1, is one-half of the value for the methanol monomer.

X-ray photoelectron study of the state of Ba atoms in YBa2Cu3O7−δ doped with Co and Zn atoms

Investigating changes in the charged state of atoms upon superconductivity-suppressing substitutions is one of the methods to examine a relationship between the electronic structure of HTSC materials and the transition temperature to the superconducting state. In this paper, we measured Ebnd of the inner levels of Ba atoms in Co- and Zn-doped YBa2Cu3O7−δ by X-ray photoelectron spectroscopy. Upon copper substitution by Co and Zn atoms, Ebnd of the inner levels diminishes for Ba atoms but remains the same for Cu and O atoms. The change of Ebnd of the inner levels of Ba atoms is attributed to the change of the Fermi level position in the “anionic” sublattice of (Cu3O7−δ)7−.