• Title, Summary, Keyword: transition metal

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Spectral and Thermal Studies of Transition Metal PSSA Ionomers

  • Shim, Il-Wun;Risen, William M. Jr.
    • Bulletin of the Korean Chemical Society
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    • v.9 no.6
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    • pp.368-376
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    • 1988
  • Transition metal PSSA ionomers containing Co(II), Ni(II), Cr(III), Ru(III), and Rh(III) are investigated by IR, Far-IR, UV-Vis and DSC. Reliable IR Spectroscopic criteria are established for assessing the degree of ion-exchange of PSSA ionomers and the local structures around metal cations in them. In the hydrated transition metal PSSA ionomers, the ionic groups are solvated by water molecules and there is no significant interactions between sulfonate group and metal cations. The visible spectra indicated that metal cations are present as [M$(H_2O)_6$]$^{n+}$ with Oh symmetry. Their $T_g$ values increase as the extent of ionic site concentration increases, but there is no direct dependence of $T_g$ on the nature of metal cations or their oxidation states. Thus, the water content in PSSA ionomer is found to have dominant influence on $T_g$ of hydrated transition metal PSSA ionomers. Dehydration of the hydrated transition metal PSSA ionomers results in direct interaction between ionic groups and significant color changes of the ionomers due to the changes of the local structures around metal cations. On the base of spectral data, their local structures are discussed. In case of dehydrated 12.8 and 15.8 mol % transition metal PSSA ionomers, no glass transition is observed in 25-$250^{\circ}C$ region and this is believed to arise from the formation of highly crosslinked structures caused by direct coordination of sulfonate groups of metal cations. In the 6.9 mol % transition metal PSSA ionomers, the glass transition is always observed whether they are hydrated or dehydrated and this is though to be caused by the sufficient segmental mobility of the polymer backbone.

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Magnetic Properties of Transition Metal Monolayers on Ta(001) Surfaces

  • Youn, S.J.;Hong, S.C.
    • Journal of Magnetics
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    • v.13 no.4
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    • pp.140-143
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    • 2008
  • The magnetic and structural properties of transition metal (Mn, Fe, Co) monolayers on Ta(001) surfaces are investigated theoretically by using the first principles full-potential linearized augmented plane wave method. Mn and Fe monolayers become ferromagnetic on Ta(001) surfaces while Co monolayers becomes non-magnetic. The paramagnetism of Co monolayers is explained by the Stoner theory of magnetism. The magnetic coupling of a transition metal overlayer with a substrate is ascribed to the orbital hybridization between the s and d orbitals of the transition metal.

Transition Metal-Catalyzed Ortho-Functionalization in Organic Synthesis

  • Park, Young-Jun;Jun, Chul-ho
    • Bulletin of the Korean Chemical Society
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    • v.26 no.6
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    • pp.871-877
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    • 2005
  • Recent progress in the filed of transition-metal mediated C-H bond activation has had a great influence on organic synthesis. Among such transition-metal catalyzed reactions, ortho-functionalization via the chelationassisted strategy has been paid great attentions as one of the powerful methodologies for converting aromatic compounds into ones that are more functionalized at the exclusively ortho-position. In this context, various transition metal-catalyzed ortho-functionalizations such as alkylation, alkenylation, silylation and carbonylation are described briefly and their prospects are suggested.

Transition metal-mediated/catalyzed fluorination methodology developed in the 2000s

  • Bae, Dae Young;Lee, Eunsung
    • Journal of Radiopharmaceuticals and Molecular Probes
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    • v.3 no.2
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    • pp.122-128
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    • 2017
  • In the 2000s, there has been a significant advance on carbon-fluorine (C-F) bond formation reactions via transition metal mediated or catalyzed methods. Of course, for the past 10 years, transition metal catalysis improves C-F bond formation in terms of practicality and even can be applied to C-18F bond formation reaction. In this mini-review, we summarize various transition metal mediated or catalyzed fluorination reactions, which were developed in the mid-2000s.

Metal-insulator Transition in Low Dimensional $La_{0.75}Sr_{0.25}VO_3$ Thin Films

  • Huynh, Sa Hoang;Dao, Tran M.;Mondal, Partha S.;Takamura, Y.;Arenholz, E.;Lee, Jai-Chan
    • Proceedings of the Materials Research Society of Korea Conference
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    • pp.19.1-19.1
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    • 2011
  • We report on the metal-insulator transition that occurs as a function of film thickness in ultrathin $La_{0.75}Sr_{0.25}VO_3$ films. The metal-insulator transition displays a critical thickness of 5 unit cell. Above the critical thickness, metallic films exhibit a temperature driven metal-insulator transition with weak localization behavior. With decreasing film thickness, oxygen octahedron rotation in the films increases, causing enhanced electron-electron correlation. The electron-electron correlations in ultrathin films induce the transition from metal to insulator in addition to Anderson localization.

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Synthesis and Selective Transport Properties of Cleft-Type Ionophores Having Two Convergent Hydroxamic Acid Functions

  • Kim, Deok Hui;Choe, Mi Jeong;Jang, Seok Gyu
    • Bulletin of the Korean Chemical Society
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    • v.22 no.7
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    • pp.699-702
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    • 2001
  • A series of cleft-type ionophores having two convergent hydroxamic acid functions are prepared and their selective ionophoric properties toward heavy metal and transition metal ions have been investigated. Hydroxamic acids 3 exhibited a prominent selectivity toward heavy metal ions of Hg2+ and Pb2+, and transition metal ions of Cu2+ over other transition metal and alkaline earth metal ions from slightly acidic source phase (pH 6) to an acidic receiving phase (pH 1). Selective ionophoric properties toward Pb2+ and Cu2+ ions over other surveyed metal ions are also confirmed by the FAB-MS measurements.

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Hydrogen Production from Ammonia Decomposition over Transition Metal Carbides (전이금속 카바이드를 이용한 암모니아 분해 반응으로부터 수소생산)

  • CHOI, EUI-JI;CHOI, JEONG-GIL
    • Transactions of the Korean hydrogen and new energy society
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    • v.30 no.1
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    • pp.1-7
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    • 2019
  • The preparation and catalytic activities of various transition metal carbide crystallites (VC, MoC, WC) were examined in this study. In particular, the effect of different kinds of transition metal crystallites were scrutinized on the ammonia decomposition reaction. The experimental results showed that BET surface areas ranged from $8.3m^2/g$ to $36.3m^2/g$ and oxygen uptake values varied from $9.1{\mu}mol/g$ to $25.4{\mu}mol/g$. Amongst prepared transition metal carbide crystallites, tungsten compounds (WC) were observed to be most active for ammonia decomposition reaction. The main reason for these results were considered to be related to the extent of electronegativity between these materials. Most of transition metal carbide crystallites were exceeded by Pt/C crystallite. However, the steady state reactivities for some of transition metal carbide crystallites (WC) were comparable to or even higher than that determined for the Pt/C crystallite.

Photodynamic and Antioxidant Activities of Divalent Transition Metal Complexes of Methyl Pheophorbide-a

  • Yoon, Il;Park, Ho-Sung;Cui, Bing Cun;Li, Jia Zhu;Kim, Jung-Hwa;Lkhagvadulam, Byambajav;Shim, Young-Key
    • Bulletin of the Korean Chemical Society
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    • v.32 no.spc8
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    • pp.2981-2987
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    • 2011
  • A comparative study of the photodynamic and antioxidant activities of methyl pheophorbide-a (MPa, 1) and its transition metal(II) complexes (2-5) is described. Four transition metal complexes (palladium(II): 2, zinc(II): 3, cobalt(II): 4 and copper(II): 5) of MPa were prepared by reaction between the corresponding transition metal and 1, respectively, and were characterized by $^1H$-NMR and UV-vis spectroscopic and mass spectrometric analyses. In vitro results show a photodynamic therapy (PDT) efficacy with A549 cells might be attributed to a heavy atom effect of the transition metal complexes of MPa. Among them, 4 and 5 showed higher photodynamic activity than that of 1 at the concentration of 5 ${\mu}M$ at 24 h incubation after photoirradiation. The images of morphological change for 2-5 show evidence for the PDT effect with A549 cells. And the all transition metal complexes of MPa showed higher antioxidant activity than that of MPa, in which 4 showed the highest antioxidant activity.

One-step synthesis of dual-transition metal substitution on ionic liquid based N-doped mesoporous carbon for oxygen reduction reaction

  • Byambasuren, Ulziidelger;Jeon, Yukwon;Altansukh, Dorjgotov;Ji, Yunseong;Shul, Yong-Gun
    • Carbon letters
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    • v.17 no.1
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    • pp.53-64
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    • 2016
  • Nitrogen (N)-doped ordered mesoporous carbons (OMCs) with a dual transition metal system were synthesized as non-Pt catalysts for the ORR. The highly nitrogen doped OMCs were prepared by the precursor of ionic liquid (3-methyl-1-butylpyridine dicyanamide) for N/C species and a mesoporous silica template for the physical structure. Mostly, N-doped carbons are promoted by a single transition metal to improve catalytic activity for ORR in PEMFCs. In this study, our N-doped mesoporous carbons were promoted by the dual transition metals of iron and cobalt (Fe, Co), which were incorporated into the N-doped carbons lattice by subsequently heat treatments. All the prepared carbons were characterized by via transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). To evaluate the activities of synthesized doped carbons, linear sweep was recorded in an acidic solution to compare the ORR catalytic activities values for the use in the PEMFC system. The dual transition metal promotion improved the ORR activity compared with the single transition metal promotion, due to the increase in the quaternary nitrogen species from the structural change by the dual metals. The effect of different ratio of the dual metals into the N doped carbon were examined to evaluate the activities of the oxygen reduction reaction.