Дисертації: "Molybdenum"

1

Ellis, S. R. "Characterisation of molybdenum compounds relevant to the molybdenum cofactor of the oxo-molybdenum enzymes." Thesis, University of Manchester, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376593.

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2

Rickman, Sarah. "Growth and characterization of molybdenum disulfide, molybdenum diselenide, and molybdenum(sulfide, selenide) formed between molybdenum and copper indium(sulfide, selenide) during growth." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file 0.94 Mb., 85 p, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:1435848.

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3

Womiloju, Olusola Olumuyiwa. "(Pentamethylcyclopentadienyl)molybdenum oxides." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape7/PQDD_0017/NQ54601.pdf.

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4

Bancroft, Matthew N. "Molybdenum-sulfur chemistry." Thesis, University of Sheffield, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.301264.

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5

Paudel, Yadab Kumar. "Tribological properties of adaptive [molybdenum nitride/molybdenum sulfide/silver] nanocomposite coatings /." Available to subscribers only, 2008. http://proquest.umi.com/pqdweb?did=1597633321&sid=7&Fmt=2&clientId=1509&RQT=309&VName=PQD.

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6

Murugappan, Karthick. "Molybdenum trioxide and molybdenum carbide as promising hydrodeoxygenation catalysts for biomass conversion." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/111404.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 171-176).
Growing concerns due to rising CO2 emissions have made biomass an indispensable source of renewable fuels and chemicals. However, biomass inherently has high oxygen content, which translates to low energy density, thereby necessitating a deoxygenation step before being utilized as fuels. Recently, MoO3 and Mo2C have emerged as promising earth-abundant cheap catalysts that perform hydrodeoxygenation (HDO) at relatively low temperatures ( 673 K) and ambient H2 pressures wherein oxygen is selectively removed as water. However, there exists a significant knowledge gap in understanding the stability and the active phases responsible for HDO of these catalysts. Furthermore, their applicability for real biomass conversions has not been largely demonstrated. In this thesis, first, HDO of m-cresol, a biomass-derived model compound, is investigated over bulk and supported MoO 3 catalysts. Detailed reactivity and characterization studies reveal that Mo5+ species plays a critical role during HDO. Specifically, TiO2 and ZrO2 are identified as ideal supports as they feature superior HDO reactivity and stability over bulk MoO3 by stabilizing intermediate Mo oxidation states (i.e. Mo5+) while bulk MoO 3 over-reduces to inactive metallic Mo. Translating from model compound studies, supported MoO3 catalysts are demonstrated to be effective in converting biomass (pine) pyrolysis vapors to hydrocarbons (ca. 30 % yield). In comparison with MoO 3, Mo2C is significantly more stable and selective for HDO of 4-methylanisole to toluene under identical reaction conditions. Mo2C predominantly breaks the stronger phenolic C-0 bond while MoO 3 also breaks the weaker aliphatic C-0 bond, likely due to the presence of Bronsted acid sites. To gain insights into the surface active sites, operando near-ambient XPS is employed during HDO and this technique revealed that HDO seems to operate via distinct active sites over both these materials. Finally, Mo2C is shown to be effective in upgrading real lignin streams to a single product propylbenzene, a precursor for renewable polymer. Overall, this thesis demonstrates the applicability of MoO3 and Mo2C in real biomass conversions and provides insights on the working nature of these catalysts, which will enable the design of more effective HDO catalysts.
by Karthick Murugappan.
Ph. D.

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7

Zhang, Liping. "Hydrodenitrogenation of organonitrogen compounds over nickel-molybdenum sulfide and Molybdenum nitride catalysts /." The Ohio State University, 1996. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487942739805674.

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8

Khanfar, Mohammad F. "Molybdenum-modified platinum electrodes /." Internet access available to MUN users only, 2003. http://collections.mun.ca/u?/theses,160874.

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9

Campbell, Selina. "New organo-molybdenum complexes." Thesis, University of Cambridge, 1993. https://www.repository.cam.ac.uk/handle/1810/272559.

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10

TRUJILLO, REBOLLO ANDRES. "SOLVENT EXTRACTION OF MOLYBDENUM." Diss., The University of Arizona, 1987. http://hdl.handle.net/10150/184009.

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The equilibrium and the kinetics of the reaction of Mo (VI) with 8-hydroxyquinoline; 8-hydroxyquinaldine; KELEX 100; LIX63; and LIX65N were studied by solvent extraction. From the equilibrium studies it was concluded that in weakly acidic solution (pH 5 to 6) the overall extraction reaction is (UNFORMATTED TABLE FOLLOWS) MoO₄²⁻ + 2H⁺ + 2HL(o) ↔ (K(ex)) MoO₂L ₂(o) + 2H₂O (TABLE ENDS) where HL is the monoprotic bidentate ligand, "(o)" refers to the organic phase, and K(,ex) is the extraction constant. It was concluded that the complexation reaction requires four protons to convert molybdate into molybdenyl. The extractions constants for LIX63 and 8-hydroxyquinaldine, corrected for the side reaction of the ligand and metal, are 10¹⁶·⁴³ and 10¹⁴·⁴⁰, respectively. In the case of LIX65N, the plot of log(D) vs pH has a maximum at pH 1.0, which was explained qualitatively in terms of protonation of the ligand and molybdic acid at low pH. The extraction constant for the reaction of molybdic acid and the neutral ligand was estimated to be 100,000. The kinetics of extraction Mo (VI) with LIX63, 8-hydroxyquinoline, 8-hydroxyquinaldine, and Kelex 100 were studied in this work. In all cases, except 8-hydroxyquinoline, the rate-determining step of the reaction involves the formation of a 1:1 complex between the neutral ligand and several Mo(VI) species differing in the degree of protonation. The rate-determining step for the reaction of Mo(VI) with 8-hydroxyquinoline involves the formation of a 1:2 complex. The rate constant for the reaction of HMoO₄ with 8-hydroxyquinaldine is four orders of magnitude smaller than the corresponding value reported in the literature for 8-hydroxyquinoline. The slower reaction with 8-hydroxyquinaldine was attributed to the presence of the methyl group next to the nitrogen atom of the ligand which hinders its binding with molybdenum in the rate determining step of the reaction.

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11

Yu, Jenwei Roscoe. "Methane activation over molybdenum disulfide, molybdenum carbide, and silver(110). Molecular orbital theory." Case Western Reserve University School of Graduate Studies / OhioLINK, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=case1059138320.

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12

Kjornrattanawanich, Benjawan. "Reflectance, optical properties, and stability of molybdenum/strontium and molybdenum/yttrium multilayer mirrors /." For electronic version search Digital dissertations database. Restricted to UC campuses. Access is free to UC campus dissertations, 2002. http://uclibs.org/PID/11984.

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13

Şahin, Engin Sözüer Topaloğlu Işıl. "Synthesis And Characterization Oxo-İmido [Tris (3,5-Dimethyl-Pyrazol) Borate] Molybdenum Complexes/." [s.l.]: [s.n.], 2004. http://library.iyte.edu.tr/tezler/master/kimya/T000444.rar.

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14

Sundaram, S. K. "Corrosion and electrochemical protection of molybdenum and molybdenum disilicide in molten silicate glass environments." Diss., Georgia Institute of Technology, 1994. http://hdl.handle.net/1853/18906.

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15

Walker, Gary Lawrence Patrick. "Studies in imido molybdenum chemistry." Thesis, Imperial College London, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.266639.

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16

Page, T. S. "Molybdenum independent nitrogenases in cyanobacteria." Thesis, Swansea University, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638392.

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Anabaena variabilis ATCC 29413, Anabaena cylindrica CCAP 1403/2A, Anabaena sp. PCC 7120, Anabaena sp. UCSB26, Microcoleus chthonoplastes and Calothrix sp. PCC 7601 appear to possess Mo-independent nitrogenases. All of these cyanobacteria grew diazotrophically in the presence of 1 mM tungstate, as well as in medium from which Mo had been carefully removed, whether or not V had been added. In growth media lacking Mo they reduced C₂H₂ to C₂H₆ as well as C₂H₄. Nevertheless, growth and C₂H₂ reduction were greatest in medium containing Mo. Nostoc sp. UCSB7, Nostoc gun UCSB31, Mastigocladus laminosus CCAP 1447/1 and Scytonema javanicum CCAP 1473/1 grew and fixed N₂ in the presence of 1 mM tungstate, but did not reduce C₂H₂ to C₂H₆ in medium from which Mo had been removed. These strains are good scavengers of small traces of Mo. In cultures of Anabaena variabilis ATCC 29413, Anabaena cylindrica CCAP 1403/2A and Anabaena sp. PCC 7120, C₂H₂ activity recovered at a faster rate in V-medium than in Mo-medium, following incubation at low temperatures (5°C - 20°C). No evidence was provided to suggest that any of the cynobacteria studied possess a Mo-independent nitrate reductase. Anabaena variabilis ATCC 29413 and Anabaena sp. PCC 7120 grew in NO₃^- containing medium supplemented with V, but only at the expense of N₂-fixation: C₂H₂ reduced to C₂H₆ under these conditions. Anabaena variabilis ATCC 29413, Anabaena cylindrica CCAP 1403/2A, Anabaena sp. PCC 7120 and Nostoc sp. UCSB7 grew in medium containing NO₂^- or NH₄⁺ when supplemented with Mo, V or W. No C₂H₂ reduction was observed under any of these conditions. Growth rates were significantly slower in the presence of NH₄⁺ than on N₂, NO₃^- or NO₂^-. Isolation of the V-nitrogenase from Clostridium pasteurianum was performed using an S-200 gel filtration column. EPR studies have shown that the component 1 protein of this enzyme possesses both FeMoco and FeVaco. The C₂H₂ reduction pattern is indicative of a Mo-independent system.

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17

Forster, Glyn Daniel. "Dithiocarbamate stabilised molybdenum imido complexes." Thesis, University College London (University of London), 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261809.

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18

Tuit, Caroline Beth 1973. "The marine biogeochemistry of molybdenum." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/58369.

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Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, and the Woods Hole Oceanographic Institution), 2003.
Includes bibliographical references.
Prevailing wisdom holds that the vertical distribution of molybdenum (Mo) in the open ocean is conservative, despite Mo's important biological role and association with Mn oxides and anoxic sediments. Mo is used in both nitrogenase, the enzyme responsible for N2 fixation, and nitrate reductase, which catalyzes assimilatory and dissimilatory nitrate reduction. Laboratory culture work on two N2 fixing marine cyanobacteria, Trichodesmium and Crocosphaera, and a marine facultative denitrifier, Marinobacter hydrocarbanoclasticus, showed that Mo cell quotas in these organisms were positively correlated with Mo-containing enzyme activity. Mo concentrations in Crocosphaera dropped almost to blank levels when not fixing N2 suggesting daily synthesis and destruction of the entire nitrogenase enzyme and release of Mo. Trichodesmium cultures, however, retained a pool of cellular Mo even when not fixing N2. Colonies of Trichodesmium collected in the field have Mo:C tenfold higher than seen in culture, these Mo:C ratios were reflected in SPM samples from the same region. Fe:C ratios for Trichodesmium were between 12-160 pmol:mol in field and culured samples. The Fe:C ratio of Crocosphaera was established to be 15.8 =/+ 11.3 under N2 fixing conditions. Mo cellular concentrations in cultured organisms were too small to significantly influence dissolved Mo distributions, but may slightly affect Suspended Particulate Matter (SPM) distributions. Mean SPM Mo:C ratios were slightly elevated in regions of N2 fixation and denitrification.. A high precision (=/+ 0.5%) isotope dilution ICP-MS method for measuring Mo was developed to re-evaluate the marine distribution of Mo in the dissolved and particulate phase.
(cont.) Mn oxides were not found to significantly influence either the dissolved or SPM Mo distribution. Dissolved Mo profiles from the Sargasso and Arabian Sea were conservative. However, dissolved Mo profiles from the Eastern Tropical Pacific showed both depletion and enrichment of dissolved Mo possibly associated with interaction of Mo with coastal sediments. Dissolved Mo profiles in several California Borderland Basins showed 1-2 nM Mo depletions below sill depth. A more focused study of water column response to sediment fluxes using the high precision Mo analyses is necessary to determine whether these phenomena are related.
by Caroline Beth Tuit.
Ph.D.

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19

Rockett, Chris H. "Flexural Testing of Molybdenum-Silicon-Boron Alloys Reacted from Molybdenum, Silicon Nitride, and Boron Nitride." Thesis, Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/16293.

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MoSiB alloys show promise as the next-generation turbine blade material due to their high-temperature strength and oxidation resistance afforded by a protective borosilicate surface layer. Powder processing and reactive synthesis of these alloys has proven to be a viable method and offers several advantages over conventional melt processing routes. Microstructures obtained have well-dispersed intermetallics in a continuous matrix of molybdenum solid-solution (Mo-ss). However, bend testing of pure Mo and Mo-ss samples has shown that, while the powder processing route can produce ductile Mo metal, the hardening effect of Si and B in solid-solution renders the matrix brittle. Testing at elevated temperatures (200°C) was performed in order to determine the ductile-to-brittle transition temperature of the metal as an indication of ductility. Methods of ductilizing the Mo-ss matrix such as annealing and alloying additions have been investigated.

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20

Ray, Charles David 1967. "Photoelectron spectroscopy of dimolybdenum tetracarboxylates: Probing the electronic nature of the molybdenum-molybdenum quadruple bond." Thesis, The University of Arizona, 1992. http://hdl.handle.net/10150/278093.

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Photoelectron spectroscopy is used to investigate the electronic structure of molybdenum-molybdenum quadruple bonds in dimolybdenum tetracarboxylates. The variable energy photoelectron spectra of the valence region of dimolybdenum tetraacetate are reported for a range of incident photon energies. The pi components of the metal-metal bond contain the most molybdenum 4d character. The sigma component has contribution from both the ligand and the molybdenum 4p orbitals on the adjoining molybdenum. The delta component has significant overlap with the ligand orbitals. A comparative gas phase and surface ultraviolet photoelectron study of dimolybdenum tetrabenzoate is also reported. This is the first dimolybdenum tetracarboxylate where the ligand is capable of large resonance overlap with the metal center. There is significant orbital overlap between the ligand and the metal-metal bond, especially with the delta component. Both of these studies show that there is electronic communication between the metal-metal bond and the carboxylate ligand.

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21

Lee, Chee Kiat. "Microstructure evaluations and thermomechanical properties of spinel (MgAl₂O₄) dispersed molybdenum alloys." Morgantown, W. Va. : [West Virginia University Libraries], 2005. https://eidr.wvu.edu/etd/documentdata.eTD?documentid=4213.

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Thesis (M.S.)--West Virginia University, 2005.
Title from document title page. Document formatted into pages; contains vii, 42 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 40-42).

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22

Mitra, Subhasis. "Studies on the chemistry of molybdenum." Thesis, University of North Bengal, 1993. http://hdl.handle.net/123456789/741.

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23

Boldt, Paul Henry. "Room temperature indentation of molybdenum disilicide." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0003/NQ42836.pdf.

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24

Fulstow, A. N. "Some adsorption properties of molybdenum disulphide." Thesis, University of Hull, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.375608.

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25

McKay, David. "Catalysis over molybdenum containing nitride materials." Thesis, University of Glasgow, 2008. http://theses.gla.ac.uk/174/.

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Reactions involving nitrogen transfer/ fixation are of great industrial interest, as novel processes could help industries meet their increasingly challenging economic and environmental targets. An example of this could be the direct synthesis of aniline from benzene, which would avoid the lengthy, uneconomic, and environmentally unfriendly process currently employed. This work explores the reactivity of bulk and supported transition metal nitride catalysts, with particular emphasis on the possible reactivity of lattice nitrogen within bulk nitride catalysts. The experimental work has focussed on two main objectives: determining the most active transition metal nitride catalyst for ammonia synthesis, and hydrogenating these materials to in the absence of N2 to produce reactive NHx species. These materials were then tested for the possible direct synthesis of aniline from benzene by entrapment of reactive NHX species. The second objective was to achieve nitrogen reactivity with nitride catalysts akin to the Marsvan Krevelen mechanism observed in oxidation catalysis. It has been shown that the binary nitrides, γMo2Nand βMo2N0.78, have comparable ammonia synthesis activities although measurements indicate that βMo2N0.78 may have a much greater activity on a surface area normalised basis. Meanwhile the δMoNphase has an intermediate surface area, but very low activity. The influence of morphology in the ammonia synthesis reaction was investigated by testing nanorod forms of βand γphase molybdenum nitrides and comparing their ammonia synthesis activities with molybdenum nitride powders. Morphology was found to have little effect on the reaction and the influence of structure sensitivity is thought to be limited in this case. What was apparent, was that the highly specific temperature programmed reaction synthesis required to prepare γMo2Nproduced an ammonia synthesis catalyst with no catalytic advantage over one that is prepared in mixtures of H2/N2 (βMo2N0.78). The influence of preparation on the ammonia synthesis activity of ternary nitride catalysts was also investigated by preparing materials in NH3 or H2/N2 atmospheres. Treatment of iron and cobalt molybdenum oxide under H2/N2 was not sufficient to yield a pure phase nitride, however NiMoO4 was fully reduced to Ni2Mo3N. Co3Mo3N, prepared using NH3, was the most active of the ternary nitride catalysts tested, and preparing the materials in H2/N2 failed to increase the activity, with the exception of Ni2Mo3N. Reaction of Co3Mo3N with H2/Ar significantly decreased the nitrogen content of the material, and it is believed that a previously unknown eta12 Co6Mo6N phase has been formed as a result of the nitrogen removal. Hydrogen was shown to be essential to induce this change, despite the fact that most of the eliminated N ended up in the form of N2. Prolonged treatment with H2/Ar at elevated temperature did not remove any additional nitrogen. It is believed that the incomplete loss of nitrogen is a direct consequence of the migration of nitrogen between crystallographic sites as the stoichiometry is reduced. In the case of iron and nickelmolybdenum nitrides the loss of nitrogen was evidenced by combustion analysis, however no new phases of material were formed. Similar experiments, with conducted with different molybdenum nitride polymorphs have shown the removal of nitrogen with a mixed phase of constituent metal and nitrided species, with only βMo2N0.78 fully decomposing to the pure metal. The loss of nitrogen, and hence its potential for reaction, is evident. However, in all cases the predominant form of lost nitrogen is N2, which is believed to be a consequence of the thermodynamics of ammonia decomposition at higher temperatures. Restoration of stoichiometry by treatment with H2/N2 has been observed for a number of materials, i.e. Co3Mo3N, γMo2N. In the case of Co6Mo6N, the NH3 synthesis activity has been found to be comparable with Co3Mo3N. HZSM5 supported nitride catalysts were also tested for ammonia synthesis and it was observed that the introduction of iron as a dopant has significant promotional effects. XPS evidence confirmed the presence of Fe0 in the material, in addition to the molybdenum (oxy)nitride species observed for MoO3/HZSM5. FTIR spectroscopy was used to conduct isotopic nitrogen exchange experiments over nitrided HZSM5 and MoO3/HZSM5. Under the conditions of the experiment, it was shown that the presence of “molybdenum nitride” facilitated the exchange of 15N2 with zeolite framework NHx species. This shows that supported γMo2Nspecies can be a source of reactive and mobile Nspecies, potentially opening up possibilities for its application as a source of spillover nitrogen. A potential route for the direct synthesis of aniline from benzene by hydrogenating ternary nitrides with benzene in the feed, trapping the possible reactive nitrogen species, was investigated. GCMS data showed the no reaction occurred, as only benzene was found in the David Mckay Abstract iv product condensate. In all cases a significant amount of carbon was incorporated/ deposited on the catalysts. In the case of the cobalt molybdenum sample, the XRD data confirmed the conversion of nitride to the carbide, however postreaction XRD of the iron and nickel samples did not indicate carbide formation.

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26

Hunter, Stuart Michael. "Molybdenum nitrides : structural and reactivity studies." Thesis, University of Glasgow, 2012. http://theses.gla.ac.uk/3221/.

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This thesis describes the preparation, structure and activity of a range of binary, ternary and quaternary molybdenum nitrides. It has been shown that all of the samples analysed can be formed through the reaction of the respective molybdate precursor with either ammonia or 3:1 H2/N2 gas mixture. The structures of the nitrides have been studied in detail. These structural findings were then linked to the activity potential of the materials to act as stores of activated nitrogen. The main body of work focused on the ternary molybdenum nitrides of cobalt, iron and nickel with a view to understanding their differences and similarities. Full structural analysis was performed using powder X-ray diffraction (PXRD) and neutron diffraction (PND). The activity of the nitrides was examined by reaction with 3:1 H2/N2 and with 3:1 H2/Ar at various temperatures. Particular attention was paid to the reactivity of lattice nitrogen. The cobalt molybdenum nitride was shown to be special case in this regard where the nitrogen is mobile and relocates within the lattice to a different crystallographic site. This mobility and relocation is concomitant with the loss of 50% of the lattice nitrogen from the system resulting in a phase change from Co3Mo3N to the unprecedented Co6Mo6N phase. The physical and chemical properties of this novel phase have been fully characterised and studied. Interestingly, the isostructual Fe3Mo3N behaves differently and the nitrogen remains fixed and the structure and stoichiometry constant throughout the testing procedure. Further studies of the ternary molybdenum nitrides extended to nickel molybdenum nitride, which was shown to be the least active when tested under both gas mixtures, and analogously to the iron molybdenum nitride the nitrogen is fixed within the β-Mn structured nitride. Further investigations were undertaken, resulting in the successful formation of a series of quaternary nitrides (Fe3-xCoxMo3N). These materials show properties similar to the Co-Mo-N system when the material is cobalt rich and behave similarly to the Fe-Mo-N system when iron rich.

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27

Wawata, Ibrahim. "Methanol oxidation on molybdenum oxide catalysts." Thesis, Cardiff University, 2015. http://orca.cf.ac.uk/74613/.

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28

Günyar, Alev Topaloğlu Sözüer Işıl. "Oxo-Imido molybdenum tripyrazolyl borate complexes/." [s.l.]: [s.n.], 2004. http://library.iyte.edu.tr/tezler/master/kimya/T000464.pdf.

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29

Varadarajan, Ashok. "Thermomechanical property evaluation of molybdenum alloys." Morgantown, W. Va. : [West Virginia University Libraries], 2004. https://etd.wvu.edu/etd/controller.jsp?moduleName=documentdata&jsp%5FetdId=3479.

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Thesis (M.S.)--West Virginia University, 2004.
Title from document title page. Document formatted into pages; contains ix, 76 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 73-75).

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30

Howarth, Christopher. "Enyl and dienyl complexes of molybdenum." Thesis, University of Bath, 1995. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.307069.

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31

Willis, Colin R. "Molybdenum mediated carbon-carbon bond formation." Thesis, University of Bath, 1988. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380889.

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32

Oloye, Femi Francis. "Synthesis and characterisation of zirconia supported molybdenum oxide and molybdenum carbide catalysts for hydroconversion of n-heptane." Thesis, University of Aberdeen, 2016. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=230933.

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The current upgrading catalysts are mainly based on the use of expensive noble metals, which are subject to deactivation due to sintering and coking. An alternative would be to introduce a non-noble metal-based catalyst. In this work, supported molybdenum carbide based systems have been assessed for this purpose. These catalysts were formed by impregnation of zirconia (and zirconium hydroxide) and sulfated zirconia (and zirconium hydroxide) with different loadings of MoO3, with an aim of finding a balance between acid sites and metal-like sites (a site capable of performing dehydrogenation and hydrogenation function without necessarily being a metal). The synthesised catalysts were carburised between 823 and 1123 K using a mixture of methane and hydrogen (4:1) in an attempt to obtain β-Mo2C/ZrO2 or β-Mo2C/S ZrO2. Carburisation at 923 K and above resulted in molybdenum carbide with minimal or no oxygen contents. The conversion and specific rate increased with temperature. Conversion was inversely proportional to space velocity. Analysis of the products distribution as a function of conversion implies that the reaction did not simply follow a consecutive reaction pathway, but that other parallel routes were involved. Conversion increased the research octane number (RON) to ca. 66 due to the increased fraction of pentane isomers. Catalyst carburised at 823 K was approximately four times more active compared to those carburised at 923 K and above, but were of similar activity with Pt/sulfated zirconia. The non-noble metal based catalysts were stable at the reaction temperature while the Pt/sulfated zirconia catalyst deactivates.

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33

Rempel, Kirsten U. "The solubility and speciation of molybdenum in aqueous liquid and vapour : an experimental study." Thesis, McGill University, 2008. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=115874.

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We have conducted an experimental investigation of the solubility and speciation of Mo in HCl-, NaCl-, and NaOH-bearing water vapour, and of the partitioning of Mo between coexisting aqueous liquid and vapour at 300 to 370°C and up to saturated pressure. Our results indicate that Mo concentration is enhanced in HCl-bearing water vapour at fHCl > 0.1 bar, and in NaOH-saturated water vapour, but is unaffected by the presence of NaCl. This suggests that Mo speciates as MoO3·nH2O in water vapour at equilibrium with NaCl or fHCl < 0.1 bar. The dependence of SigmafMo on fHCl at higher acidity points to the formation of Mo oxychloride (MoO2Cl 2). For the system MoO3-NaOH-H2O, log Sigma fMo increases with increasing fH2O, and with log SigmafNa in a ratio of 0.28+/-0.4, but Sigma fNa does not change with increasing fH2O. This suggests the formation of a small proportion of sodium molybdate (Na 2MoO4) in addition to MoO3·nH2O. Our partitioning experiments show that at lower temperature and fluid density, Mo partitions more strongly into the liquid than the vapour, but the Mo concentration in the vapour increases as the temperature-pressure conditions approach those of the critical point of water (374°C and 221 bar), surpassing that in the liquid at ∼360°C. The results of our experiments indicate that both the liquid and vapour phases may be important for the transport of Mo in porphyry ore-forming systems, and that vapour-phase solubility is enhanced in high fHCl magmatic gases.

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34

Recatalá, Ferrandis David. "Photoactive Materials Based on Molybdenum Cluster Sulfides." Doctoral thesis, Universitat Jaume I, 2015. http://hdl.handle.net/10803/296561.

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Coordination Chemistry deals with the synthesis and study of the physicochemical properties of metal complexes. Cluster Chemistry is a subfield of Coordination Chemistry, which focuses on the functionalization of complexes in which two or more metal atoms are directly bonded. Over the past few years, Cluster Chemistry has attracted a growing interest among scientists from diverse areas, mainly due to the fascinating properties of these compounds. A historical evolution of the term cluster, as well as an outline of the role of coordinated ligands and structural types in the final properties of metal clusters are provided in Chapter 1.

This PhD Thesis is devoted to the synthesis, characterization and applications of two families of group VI metal clusters containing dithiolene or diimine ligands, as detailed in Chapter 2. The synthetic approaches employed for the preparation of a series of dinuclear M2Q2S2 cluster chalcogenides (M = Mo, W; Q = O, or S) bearing bifunctional dithiolene ligands are described in Chapter 3. These metal clusters present great potential for the design of heterometallic systems.

Chapter 4 is concerned with the preparation of an extensive family of mixed-ligand diimine-halide (or diimine-dithiolene) trinuclear molybdenum sulfides based on the Mo3S7 core. A great number of bipyridine and phenanthroline derivatives have been coordinated to these Mo3S7 units. The most important feature of the resulting cluster complexes of formula Mo3S7X4(diimine), where X = Cl, or Br, is their crystallization as [Mo3S7X4(diimine)·X]- aggregates, in which the sulfur axial atoms participate in non-bonding interactions with halide anions.

The physicochemical properties of both series of metal clusters mentioned above are explored in Chapters 5 and 6. The luminescence properties of bis(dithiolene) M2Q2S2 clusters (M = Mo, W; Q = O, or S), together with those of Mo3S7 clusters functionalized with imidazophenanthroline ligands are detailed in Chapter 5. These diimine Mo3S7 complexes exhibit luminescent anion sensing behavior. The optical limiting capabilities of both series of compounds, namely M2Q2S2- and Mo3S7-based clusters, are also described in Chapter 5 with the aim of finding correlations between molecular structures and third-order nonlinear optical functions.

Chapter 6 examines the electro- and photocatalytic activity of diimine Mo3S7 clusters immobilized on TiO2 nanoparticles toward the hydrogen evolution reaction. This study has been stimulated by the analogy between the structure of Mo3S7 and the catalytic active sites of MoS2 nanoparticles. The electrochemical properties of these TiO2 electrodes are assessed in two different media, that is, aqueous perchloric acid and sulfide-sulfite mixtures. The role of the diimine ligands in the adsorption process is also described in this Chapter.

All experimental procedures employed in this work, together with the characterization of all compounds are presented in Chapter 7. Finally, the general conclusions of this PhD Thesis are provided in Chapter 8.

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35

Choudhary, Habib Ullah. "Anomalous behaviour of molybdenum in steel welds." Thesis, University of Cambridge, 1993. https://www.repository.cam.ac.uk/handle/1810/219195.

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The addition of molybdenum to steel welds in quite small concentrations leads to a variety of anomalous microstructural and mechanical property effects. In some cases, the effects manifest even when there are no obvious changes in microstructure at the resolution of a transmission electron microscope. There are two particular discrepancies. A quantitative analysis of molybdenum-containing steel welds indicates that there is a degree of strengthening which cannot be explained by the known solid solution or microstructural effects of molybdenum in steels. Secondly, in multirun welds, the addition of molybdenum appears to make the microstructure which evolves during solidification (the primary microstruet1lre) extremely stable. These and other associated phenomena are examined in this thesis. The molybdenum effects mentioned above have been reproduced in detail, using a series of 'high-purity' multirun welds. Having confirmed that molybdenum increases the fraction of primary microstructure in such welds, an attempt was made to see whether the effect is attributable to a change in the austenitisation characteristics with alloying additions. Extensive work using dilatometric techniques backed by microscopy analysis has demonstrated that molybdenum does not lead to any substantial or unexpected changes in the ability to form austenite. The second hypothesis, that the primary microstructure is stabilised as molybdenum increases the tempering resistance, is proven and provides a good explanation of the observations. A series of tempering experiments have established that the anomalously high strength of the molybdenum containing welds cannot be attributed to solid solution strengthening or microstructural effects. Indeed, it appears that there is a submicroscopic secondary hardening type effect which enhances the strength. Even the thermal treatment that occurs as the weld cools from the solidification temperature is shown to be sufficient to induce molybdenum based secondary hardening type effects. Some preliminary atomic resolution effects also lend support to this concept. Titanium as a trace element is important in steel welds, as an element which promotes the intragranular nucleation of acicular ferrite on titanium-rich phases. It is demonstrated that the titanium effect is not intrinsically different for molybdenumcontaining welds. However, the extra hardenability associated with molybdenum certainly helps to suppress the formation of other grain boundary nucleated phases which might swamp events that occur on the inclusions within the grains. An interesting observation is that titanium has a positive effect in limiting the grain boundary nucleated phases, because the intragranularly nucleated acicular ferrite to some extent stifles the formation of other phases.

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36

Yang, Xiaofan. "Corrosion and passivation of molybdenum-bearing alloys." Thesis, University of Surrey, 1995. http://epubs.surrey.ac.uk/843256/.

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Molybdenum-bearing alloys are widely used in industry because of their excellent corrosion resistance. However, the role of molybdenum in passivation is a subject which has been a matter for discussion and controversy for many years. In the previous work carried out in this laboratory, Professor Castle and Dr Qiu suggested that molybdenum oxide might provide the nuclei for formation of the passive film. This hypothesis is the basis of the present work. In order to find out the evidence for the existence of the molybdenum oxide nuclei, furthermore, to establish a model of passivation for the molybdenum-bearing alloys, the passivation of molybdenum-bearing alloys are investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), especially in-situ AFM, in conjunction with SEM and TEM. During the passivation study, it was found that the chemical composition on anodically polarised surfaces varied with potential. For 516 alloy, the peak value of molybdenum in the surface is at the low potential of the passive region close to the Flade potential. Therefore, molybdenum exerts its greatest function in this potential region. A further AFM study at this potential revealed, surprisingly, that a platelet layer formed on the passivation surface. The dendritic structure on the surface of the platelets presented under the high resolution of STM, which shows the crystallization property of the platelets. These platelets are mainly composed of chromium and molybdenum oxides and they are only found in the passivation of the molybdenum-bearing alloy, so the formation of the platelets may be associated with molybdenum nucleation in passivation. The in-situ AFM studies provide the evidence for the formation and disappearance of the platelets during the passivation of molybdenum-bearing alloys, i.e. the platelets form at the early stage of passivation and they gradually merge into the passive film if they are not disturbed by the environment. Based on the above finding, the role of molybdenum in passivation is proposed as following: molybdenum oxide precipitated on the surface seeds chromium oxide to form platelets at the early stage of passivation and the formed platelet layer prohibits the dissolution of the passive species from the metal. By this way, molybdenum facilitates the formation of the passive film on alloys. Using Fe-Cr-Mo duplex stainless steels, the corrosion of molybdenum-bearing alloys are studied in HCl and the kinetics of the corrosion are traced by in-situ AFM. During corrosion, it was observed that the ferritic phase in duplex stainless steel dissolves more rapidly than the austenitic phase and the dissolution occurs on the austenite preferentially along grain boundaries, sub-grain boundaries and the planes with high energy. By switching the electrochemical condition from active dissolution to passivation and then changing it back, it is found that under the passivation condition after the active dissolution, the corrosion changes from selective dissolution of the crystallographic feature to general corrosion. This situation persists even though the electrochemical condition changes back to the active condition from the passivation condition. Since AFM is a new technique and this is one of the first attempts at applying it to a corrosion study, an in-depth explanation of the images obtained from a corroded rough surface is an important topic on which so far little work has been reported. In this study, therefore, the artifacts and the reality of the structure in AFM images obtained in the corrosion study are discussed.

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37

Walker, Daniel James. "Fluorinations of tungsten and molybdenum carbonyl complexes." Thesis, University of Leicester, 1997. http://hdl.handle.net/2381/30011.

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The dimeric complexes (n5-C5H5)M(CO)3 2 (where M = W or Mo) and (H3C{ti5-C5H4})Mo(CO)3 2 have been treated with XeF2 in organic solvents. All three materials react to form air sensitive products which solid-phase IR studies show to contain highly oxidised metal carbonyl components. The product from the fluoridation of (H3C{T 5-C5H5})Mo(CO)3 2 is insufficiently stable in solution for it to be characterised further. 19F NMR studies show that (n5-C5H5)W(CO)3 2 reacts to form the new complex (T 5-C5H5)WF5, but that the molybdenum analogue reacts to form an unidentified product which, although it produces an AB4 pattern in the 19F NMR, does not appear structurally related to (n5-C5H5)WFs. (n5-C5H5)W(CO)3 2 was found to react with AgF in anhydrous HF solution to give (t 5-C5H5)WF5. Solid-phase IR studies showed that a highly oxidised metal carbonyl, identical to that obtained in the XeF2 reaction, was also formed. Reaction of (T 5-C5H5)M(CO)3(CH3) (where M = W or Mo) with anhydrous HF was found to yield the complexes (n5-C5H5)M(CO)3 2 (where M = W or Mo). The molybdenum product was characterised by X-ray crystallography, whilst the tungsten dimer was characterised using mass spectroscopy. The mechanism for this redox process is unknown, but is believed to involve formation of the complexes (T 5-C5H5)M(CO)3F, as intermediates. Although solid-phase IR shows that both Mo(CO)6 and W(CO)6 react with XeF2 to give oxidised metal carbonyls, the products appear very different. W(CO)6 yields a variety of salts of the W202F9 " anion, none of which dissociate fully in solution, and which thus give separate 19F NMR spectra, for the anion. One of these salts may contain the W2(CO)8 2+ cation, which has been characterised in the product mixture by +EI MS. Mo(CO)6 reacts to form a molybdenum fluoride whose 19F NMR spectrum has been observed, but which has not been identified.

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38

Matei, L., R. Galea, K. Moore, D. Niculae, W. Gelbart, B. Abeysekera, G. McRae, and R. R. Johnson. "Thermal separation of 99mTc from Molybdenum targets." Helmholtz-Zentrum Dresden - Rossendorf, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-164746.

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Thermal separation is defined as a mass transfer process driven by molecular forces. The process involves the heat transfer between two phases with different composition. In general, thermal separation occurs when heat is generated in the system additionally to the already existing phases. In a second phase the mass is transferred in the system (adsorption) and at the end of this step the separation is completed. The thermal separation can be achieved in temperature or concentration gradient function of system configuration [1]. Thermo-chromatography is a process in which the separation occurs in gase-ous phase. By passing a heated gas through a column a thermal gradient is created with a continuously decreasing temperature along the column. The separation occurs based on the different volatilization temperatures, the less volatile species will condense on the column walls at the higher temperatures and the highly volatile compounds will condense at lower temperatures. Parameters like temperature, carrier flow rate, column geometry and length have impact on the absorption of the compound on the column material affecting the separation efficiency. The thermal separation has been used for separation of Molybdenum (Mo) and Technetium (Tc) by either sublimation in the case of 94mTc {2,3,4] or dry distillation in the case of 99mTc from neutron irradiated MoO3 [5]. The thermal separation process has been used in the development of a new type of Mo/Tc generators starting from the MoO3 as target material for production of 99mTc in linear accelerators [6]. Dry distillation has become a standard procedure for separation of radioiodine from tellurium targets [7]. The present paper describes the thermal separation of a three component system (Cu/Mo/Tc) used as a target in the production of 99mTc through the 100Mo(p,2n) reaction. Material and Methods The separation method involves the use of oxygen as a carrier gas and oxidation agent. The method is based on the different volatilization temperatures of Tc formed oxides and the MoO3 formed in the system during the oxidation. In the presence of oxygen the existing Tc is oxidized to its anhydride as Tc2O7 (b.p. 319 ⁰C; m.p. 110.9 ⁰C) following the reaction: 4Tc + 7O2 →2Tc2O7 The T2O7 has a saturated vapor pressure of 310 ⁰C whilst Mo is completely oxidized to MoO3 having a sublimation temperature at 750 ⁰C. The initial experimental setup comprised a quartz tube (6 mm internal diameter, 40 cm long) which is introduced into a horizontal tube furnace (model 55035A, Lindberg). The left end of the quartz tube is connected to a pure oxygen supply which flows through the separation tube at a rate of 10 mL/min. The other end of the tube is opened to the atmosphere and protected with quartz wool. The quartz tube is heated over a length of 23 cm at a temperature of 850 ⁰C. The heated carrier gas is flowing on the tube length and the temperature gradient is created along the tube from 850 ⁰C to room temperature. During the process, the oxygen carries out the Tc oxides to a lower temperature and Tc2O7 is deposited in the cooler region of the tube in a similar manner as described by Tachimory [5]. The temperature gradient is calibrated by meas-uring the temperature inside the tube at each centimeter along its length (FIG. 1). The radioactivity counting is performed by scan-ning the tube along its length every 2 centimeters by using a detection system shown in figure 2. The system comprises a GM tube coupled to a computer controlled linear actuator (Velmex Unislide). The tube is placed at a distance of approximately 25 mm from the collimator of GM. Preliminary testing using Mo powder Prior to testing the three component separation, a reference test was performed by using 120 mg of natural Mo powder (Alpha Aesar, 99.9 %) soaked with 50 MBq NaTcO4 (Cardinal Health, radiochemical purity >95 %). After evaporation the dried powder was introduced into a quartz tube (6 mm ID, 40 mm long) and heated up to 850 ⁰C in the presence of oxygen flowing at a rate of 10 mL/min. Three component separation The targets prepared for the production of 99mTc by a cyclotron were comprised of copper (Cu) (C101, oxygen free) support having a Mo layer deposited on the surface in an elliptical form as described in literature [8,9]. About 60 to 250 mg of Mo (99.9%, Alpha Aesar) was deposited on the target surface. 70 MBq of Tc (Cardinal Health) as NaTcO4 (> 99 % radiochemical purity) was deposited on the Mo insert to mimic the conditions created during proton irradiation. The Tc spike was evaporated to dryness and the Cu/Mo/Tc target was then introduced into the experimental setup. The process was allowed to continue for 20 min. The experiment was carried out by inserting the target plates in a quartz tube (CanSci, Canada) of similar design to those described by Fonslet for the separation of radio-iodine from TeO2 targets [7]. The quartz tube can be seen in FIG. 2 and illustrated with dimensions indicated in FIG. 3. Separation of in-situ cyclotron produced Tc by irradiation of Mo targets with a proton beam. A third set of experiments have been performed for in-situ generated Tc by irradiation of circular targets containing approximately 60 mg Mo deposited on a copper support. The targets were irradiated for 30 min with a proton beam with the energy of 15 MeV and a current of 50 µA. The separation was performed using similar experimental conditions as previously described. The quartz tube was scanned in length by using a RadioTLC scanning system calibrated for 99mTc and 99Mo isotopes. After the thermal separation was completed 99mTc was recovered as NaTcO4 by selectively washing the quartz tube with 1 M NaOH (Fisher) solution. The presence of Mo in the NaTcO4 solution was verified by a colorimetric strip test (EM-Quant Mo test kit, Millipore). The presence of copper was qualitatively analyzed by adding a few drops of concentrated NH4OH (Fisher) solution and checking the formation of Schweitzer reagent. Results Thermal separation of Tc-Mo powder After 20 min the deposition of MoO3 was ob-served as yellow crystals in the region of tem-perature of 770 ⁰C, which is in accordance with the results reported in the literature [5]. The activity of 99mTc was detected at about 5 cm from the exit of the tube furnace in a temperature range starting with 310 ⁰C and ending at 46 ⁰C (FIG. 4).

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39

Bayerköhler, Frieder [Verfasser], and Philipp [Akademischer Betreuer] Kurz. "Iron-molybdenum sulphides as proton reduction catalysts." Freiburg : Universität, 2017. http://d-nb.info/1144148901/34.

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40

Short, Rick. "Incorporation of molybdenum in nuclear waste glasses." Thesis, University of Sheffield, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.444469.

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41

Critchley, David J. P. "Diazanaphthalenes as probes of molybdenum hydroxylase activity." Thesis, University of Bradford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.277185.

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42

Kaye, Simon Peter. "Ion-assisted deposition of molybdenum disulphide films." Thesis, University of Salford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.238835.

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43

Macpherson, I. "Some metallaborane chemistry of molybdenum and tungsten." Thesis, University of Leeds, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376799.

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44

Shonfield, Peter K. A. "Molybdenum and tungsten complexes for moleular electronics." Thesis, University of Bristol, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.264635.

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45

Kassim, Mohammad B. "Studies of nitrosyl and oxo molybdenum complexes." Thesis, University of Bristol, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.274612.

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46

Liu, WenDi. "Optimization of Molybdenum Electrodes for Glass Melting." Digital WPI, 2015. https://digitalcommons.wpi.edu/etd-dissertations/208.

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The U.S. glass industry is a $28 billion enterprise and millions of tons of glasses are melted each day by different heating techniques, such as conventional oil fired furnaces or via electrical heating. The share of electrical heating is bound to rise steadily because it is cleaner and more energy efficient. Due to this situation molybdenum will play a significant role in electrical glass melting, since it is the most frequently used electrode material to deliver the electricity into the glass melts. Although it has a high melting point, high electrical and thermal conductivity and a low coefficient of expansion, molybdenum electrodes fail because of lack of sustainability during the glass melting process. Melt reaction with electrodes is the fundamental barrier to higher melting temperatures. Glass manufacturers have suggested that the need for better performance of molybdenum electrodes will see a rapid advancement in the use of electric heating system in the U.S. This work first focused on post-mortem analysis on used molybdenum electrodes with and without the current load in order to establish failure mechanisms for molybdenum during glass melting. It was determined that service life of molybdenum electrodes are limited by poor oxidation and corrosion resistance of molybdenum with redox reactions. Various studies have shown that the failure mode for molybdenum electrodes is a complex phenomenon. It depends on chemical composition of the electrode, current density and frequency, and chemical composition of the glass melt, specifically polyvalent ions that may be present in the melt. In this work, the MoSiB coating was validated as a promising protection for molybdenum from oxidation attack. Several molybdenum and molybdenum based-alloy electrodes were tested in different molten glasses in the remelter furnace to optimize the structural characteristics that are needed in Mo electrodes. Moreover, the quantitative data and fundamental knowledge gained in this work is being applied for molybdenum electrode production to extend its service life and also improve its quality.

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47

Octon, T. "Optoelectronic properties of two-dimensional molybdenum ditelluride." Thesis, University of Exeter, 2019. http://hdl.handle.net/10871/35713.

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In this thesis the layered, two-dimensional material MoTe2 is examined experimentally for its optoelectronic properties, using a field effect transistor device configuration. MoTe2 experiences a strong light matter interaction, which is highly dependent on the conditions of the measurement, and the wavelength of light used. Light is able to: produce a photocurrent in MoTe2, desorb adsorbates from the surface, and even controllably thin by a single layer at a time. A theoretical study on MoTe2 also provides insights on the source of some of these interesting light matter interactions. MoTe2 is found to be a fast and responsive photodetector when illuminated with red laser light in ambient conditions, with increases in current stemming from the photovoltaic effect. Due to the generated charge carriers from the photovoltaic effect, conductivity can increase by increasing the Fermi energy of the material, or by a photogating effect where excited charges are trapped and behave as an artificial gate for the field effect transistor. The mechanisms of charge trapping are experimentally investigated due to their prevalence in the photodetection mechanisms. A theoretical study points towards the existence of two types of trap states, in not just MoTe2 but all transition metal dichalcogenides, with shallow traps closer to the valence band edge (τ ~ 500 s) and deeper traps (τ ~ 1000 s), further away from the valence band edge. MoTe2, under the effects of higher energy photons from blue and green lasers, showed different photocurrent mechanisms to red light. From the increased energy of the photons, photo-desorption of adsorbates on the surface of MoTe2 occurred causing a decrease in the overall current, in a rarely seen photocurrent mechanism. Again, both shallow and deep traps are evident from the experimental measurements, with the shallow traps being removed when illuminated by higher energy photons. Finally, a humidity assisted photochemical layer-by-layer etching process was developed with an in-situ Raman spectroscopy system, able to thin MoTe2 by a single layer at a time with 200 nm spatial resolution. MoTe2 FETs were created with thinned channels to examine the effect of the thinning technique on optoelectronic properties. Some improvement in optoelectronic performance (higher responsivity, higher mobility) was seen for the thinned channel devices, with great improvement observed for monolayer MoTe2.

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48

Suwardiyanto. "Novel molybdenum/zeolite catalysts for methane dehydroaromatization." Thesis, University of Aberdeen, 2015. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=228007.

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49

Turner, Nigel Arthur. "Studies on the molybdenum centre in enzymes." Thesis, University of Sussex, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.284106.

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50

Wang, Shanshan. "Synthesis and characterization of monolayer molybdenum disulphide." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:182cdf51-c6e2-40bc-abf8-8057130bc2f5.

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Monolayer molybdenum disulphide (MoS₂) is a semiconductor with a direct bandgap of ∼1.9 eV. Much research attention has been attracted to its potential applications in nanoelectronics and optoelectronics recently, all of which are based upon a scalable production of high-quality monolayer MoS₂ and the in-depth comprehension of its fundamental properties. The PhD project focused on developing chemical vapour deposition (CVD) methods to grow monolayer MoS₂ and its related two-dimensional (2D) vertical heterostructures with an understanding of the growth mechanism. Subsequently, a range of characterizations were conducted to investigate the structural, vibrational, optical and mechanical properties of as-grown samples. An atmospheric-pressure CVD (APCVD) approach was first developed to grow monolayer MoS2 crystals on silicon substrates with a 300 nm oxide layer (SiO₂/Si) by using molybdenum trioxide (MoO₃) and sulphur (S) as precursors. A sharp gradient of MoO3 was intentionally created, which induced a location-dependent shape evolution of MoS₂ domains. A qualitative explanation was proposed, attributing this phenomenon to local changes in the Mo:S ratio of precursors and its influence on the kinetic growth dynamics of edges. Subsequently, the CVD setup was improved in both the precursor loading manner and the substrate orientation to achieve centimetre-scale monolayer MoS₂ films with large domain sizes of 10-20μm. Finally, 2D MoS₂/hexagonal boron nitride (h-BN) vertical heterostructures were fabricated via an all-CVD method. Raman and photoluminescence (PL) spectroscopy were applied to show its merits of smaller lattice strain, lower doping level and better interlayer contact compared with CVD-grown MoS₂ on SiO₂/Si substrates and mechanically stacked samples. Aberration-corrected transmission electron microscopy (AC-TEM) was carried out to study the defect structure and fracture mechanics of monolayer MoS₂ at a single atom level. Subtle lattice reconstructions of line defects in various lengths and widths were resolved. Density functional theory (DFT) predicted a reduction of the bandgap as the line defects broaden, which eventually makes them behave as metallic channels embedded in the semiconducting MoS₂. Another joint experimental and theoretical study was conducted to track the real-time crack propagation in monolayer MoS₂. The atomically sharp crack tip went through the preferential lattice direction with least energy release. Increasing defect density was found to induce an enhanced fracture toughness and a fracture mechanism transition from brittle to ductile.

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