Now showing 1 - 10 of 18
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    Ion-induced carbide formation of TiFe: evidence from XPS and AES studies
    (01-01-1990) ;
    Viswanathan, B.
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    Srinivasan, V.
    X-ray photoelectron and Auger electron spectroscopic studies on TiFe demonstrate that ion bombardment of the alloy not only results in sputter cleaning or even destruction of the surface but also leads to the formation of compact carbide layers. Such carbide species are of considerable importance in elucidating the reaction mechanism of hydrocarbon synthesis. They are also expected to play a significant role in the hydrogen sorption properties of materials. A better understanding of the nature of sputtered carbon is needed for its use as protective (against corrosion) thin films in magnetic recording applications. A comparison of photoelectron and Auger electron spectra of the surface carbides of TiFe would also be of interest since it is unusual for both types of data to be produced on the same samples where chemical shifts are involved. © 1990.
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    Specific role of polymorphs of supporting titania in catalytic CO oxidation on gold
    (30-04-2011)
    Beck, A.
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    Magesh, G.
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    Kuppan, B.
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    Schay, Z.
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    Geszti, O.
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    Benkó, T.
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    Viswanath, R. P.
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    Viswanathan, B.
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    Guczi, L.
    Dependence of catalytic activity in CO oxidation on the structure of gold nanoparticles supported on various TiO2 polymorphs has been investigated. Pure brookite, brookite-anatase (45:55) and anatase-rutile (85:15) mixtures were compared as supports of Au/TiO2. Au nanoparticles were deposited on different TiO2 supports by adsorption of Au colloids of about 6 and 15 nm mean diameter and by deposition precipitation (DP) with urea providing Au particles smaller than 5 nm in size. Sol derived gold particles were more stable against sintering on brookite than on anatase containing supports. Taking into account the particle sizes of gold and titania for all types of gold deposition methods, the Au-anatase perimeter seems to be significantly more active in CO oxidation than the Au-brookite perimeter. The difference in activity should be originated from the different electronic, physical and surface properties of anatase and brookite. © 2010 Elsevier B.V. All rights reserved.
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    Surface properties and their consequences on the hydrogen sorption characteristics of certain materials
    (01-01-1990) ;
    Viswanathan, B.
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    Swamy, C. S.
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    Srinivasan, V.
    Surface analyses (X-ray photoelectron spectroscopy, X-ray-induced Auger electron spectroscopy and Auger electron spectroscopy) results of some hydrogen storage intermetallic compounds of the type AB, AB5, A2B and the hydride, A2BH4 (A ≡ Ti, La, Ca or Mg; B ≡ Fe, Ni or Cu) show the presence of numerous oxidation (aerial) products, such as oxides, hydroxides and carbonates. In general, the hydroxide and/or carbonate species are present in small amounts and persist only on the outer passivated surface. However, there are cases where they seem to be predominant, at least in the top few layers. Therefore such species are also expected to play an important role in the surface chemistry of the alloys and hydrides similar to that observed with surface oxides. Accordingly, the activation procedure varies for the different materials. Upon activation the surface may be described as a supported metal system which in turn increases the activity thus explaining the rapid kinetics of the hydriding/dehydriding reactions upon cycling processes. The easy activation of certain alloys could also be explained on the basis of the reducibility and the thickness of the surface oxide layers. In other words, it could be demonstrated based on their oxidation resistance and the segregation behaviour of the metals. The specific reactivity of different systems are related to the segregation, oxidation, and reduction behaviours. Surface enrichment of iron in the TiFe system is demonstrated for the first time. © 1990.
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    Magnesium and magnesium alloy hydrides
    (01-01-1986) ;
    Viswanathan, B.
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    Swamy, C. S.
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    Srinivasan, V.
    This review addresses the question of using magnesium and magnesium-based alloys as "rechargeable hydrogen storage" media. The effect of addition of organic compounds as well as other metals (both transition and non-transition elements) on the sorption characteristics of magnesium is considered in detail. The current status of information in this field is reviewed with a view to evaluating the potential of these materials for use as hydrogen storage media in vehicular applications. © 1986.
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    XPS studies of the surface properties of CaNi5
    (01-01-1989) ;
    Viswanathan, B.
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    Srinivasan, V.
    The "air-exposed" sample of CaNi5 contains various oxygenated species on the surface. The surface properties of CaNi5 are analogous to those of LaNi5 with surface enrichment of calcium similar to lanthanum in LaNi5. © 1989.
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    Studies on the thermal characteristics of hydrides of Mg, Mg2Ni, Mg2Cu and Mg2Ni1-xMx (M = Fe, Co, Cu or Zn; 0 < × < 1) alloys
    (01-01-1988) ;
    Viswanathan, B.
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    Swamy, C. S.
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    Srinivasan, V.
    X-ray results on the alloys of the composition Mg2Ni1-xMx (M = Fe, Co, Cu or Zn; 0 < × < 1) suggest a Mg2Ni-type structure. The alloys, Mg2Ni0.75M0.25(M = Fe, Co, Cu or Zn) upon hydriding lead to the formation of quarternary hydrides while on dehydriding yield the starting ternary alloys except for copper containing alloys which show multi-phase regions and follow a different path way for the hydriding-dehydriding process. Thermal studies (TG-DTA) on the hydrides indicate the amount of hydrogen evolved as well as the desorption temperatures. The thermodynamic quantities, namely the enthalpies and entropies of formation of the hydrides were deduced from the DTA peak maximum temperature data. The kinetic parameters such as activation energies, reaction rates and orders of the reaction for the decomposition of the hydrides formed from Mg, Mg2Ni and Mg2Cu alloys were evaluated from the DTA data. As to the modified Mg2Ni system, the copper substituted alloys show lower thermal stability and also presents some interesting properties. Hence, it is considered as one of the promising ternary combinations (Mg-Ni-Cu) for hydrogen storage purposes. © 1988.
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    Thermal studies on Mg2NiH4: Existence of additional hydride phase in the Mg2Ni-Hydrogen system
    (15-03-1988) ;
    Viswanathan, B.
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    Swamy, C. S.
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    Srinivasan, V.
    (TG-DTA) thermal analyses of the Mg2NiH4 system revealed the presence of another hydride phase with composition Mg2NiH. The kinetic parameters for the phase transition as well as for the decomposition of the two hydride phases have been evaluated from DTA data using temperature programmed 'Arrhenius plots'. The thermodynamic quantities, namely the enthalphy and entropy changes for the two phases, have been evaluated and compared with the data reported in the literature. © 1988.
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    X-ray photoelectron spectroscopic investigations of the activation of FeTi for hydrogen uptake
    (01-01-1987) ;
    Viswanathan, B.
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    Swamy, C. S.
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    Srinivasan, V.
    Activation of FeTi alloy could either give rise to surface segregation and subsequent oxidation of titanium and the formation of iron clusters or the formation of metastable TiFeO, suboxide. The validity of either of the postulates could be gauged by monitoring the nature and concentration of each of the oxidation states of iron and titanium by depth profile analysis by XPS. These studies showed that fresh samples contained multiple oxidation states of iron and titanium though the concentrations of lower valent oxides are usually small. Sputtering studies showed that this oxide layer is of considerable depth and during activation, iron oxides are easily reduced to the metallic state as compared to the oxides of titanium. The chemical reaction induced, segregation may be the contributing factor for the enhanced concentration of titanium on the surface of these alloys. © 1987.
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    Evidence for the formation of surface carbonates on some hydrogen storage intermetallic compounds: an XPS study
    (01-01-1990) ;
    Viswanathan, B.
    ;
    Srinivasan, V.
    X-Ray photoelectron spectroscopic (XPS) studies of air exposed samples of LaNi5, CaNi5, Mg2Ni, Mg2Cu and FeTi indicate that the interaction of atmospheric carbon dioxide results in the formation of surface carbonates in addition to the usual oxides and hydroxides. The reaction of carbon dioxide, in fact, is favoured by the oxidized surface which in turn produces the carbonate species on the top few layers, hitherto not reported. In particular, the carbonates seem to be predominant in the case of CaNi5. The adsorption of carbon dioxide also gives an idea of the segregated elements of the different systems. Even though such carbonate species on the alloy surfaces seem to be expected rather than surprising, there is no evidence available in the literature. To our knowledge, this forms the first report on such a problem. The results also provide valuable information concerning the understanding of various surface reactions and the nature of activation processes which often differ between materials involved in hydrogen uptake. © 1990.
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    Surface Studies of Some Hydrogen Storage Materials
    (01-01-1989) ;
    Viswanathan, B.
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    Swamy, C. S.
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    Srinivasan, V.
    XPS, XAES and AES techniques have been applied to study the surface properties of certain hydrogen storage intermetallic compounds, namely, LaNi5, CaNi5, Mg2Ni, Mg2Cu and FeTi. From these studies, it has been derived that the binary alloys undergo surface decomposition and subsequent oxidation upon exposure to air, resulting in a variety of oxygen containing species of the component elements. In addition, the surface compositions differ considerably for the various systems presented in this investigation, mainly because of their different segregation behaviours. © by R. Oldenbourg Verlag, München 1989