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Reactivity of transition metals - influence of the degree of oxidation of active substrate
Kettner, Miroslav ; Nehasil, Václav (advisor)
The impacts of fluorine doping of ceria are studied by means of surface science experimental methods. Fluorine-doped and fluorine-free ceria layers are epitaxially grown on rhodium single crystals and their properties are compared in regular and inverse catalyst configurations. A procedure for epitaxial growth of CeO2(110) and CeOxFy(110) layers on Rh(110) single crystal is developed and described in detail. Shape alterations of Ce 3d spectrum brought about by fluorine doping are explained and a suitable deconvolution method is proposed. Special attention is focused towards stability of fluorine in ceria. Presented data show that fluorine incorporation in ceria lattice causes stable reduction of ceria, which withstands up to 200řC in near-ambient pressure conditions. Morphological changes are observed due to elongation of surface lattice constant of reduced ceria. Oxygen storage capacities and hydrogen oxidation reaction rates of four different studied systems are compared and discussed. The twofold nature of oxygen exposure of fluorinated ceria is discovered and explained. Oxygen repels fluorine from the surface, while the remaining part of fluorine is expelled to adsorbate positions, where its electronic state is altered. Moreover, such fluorine is prone to interact with atomic hydrogen. This reaction is...
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Reactivity of transition metals - influence of the degree of oxidation of active substrate
Kettner, Miroslav ; Nehasil, Václav (advisor) ; Jiříček, Petr (referee) ; Lykhach, Yaroslava (referee)
The impacts of fluorine doping of ceria are studied by means of surface science experimental methods. Fluorine-doped and fluorine-free ceria layers are epitaxially grown on rhodium single crystals and their properties are compared in regular and inverse catalyst configurations. A procedure for epitaxial growth of CeO2(110) and CeOxFy(110) layers on Rh(110) single crystal is developed and described in detail. Shape alterations of Ce 3d spectrum brought about by fluorine doping are explained and a suitable deconvolution method is proposed. Special attention is focused towards stability of fluorine in ceria. Presented data show that fluorine incorporation in ceria lattice causes stable reduction of ceria, which withstands up to 200řC in near-ambient pressure conditions. Morphological changes are observed due to elongation of surface lattice constant of reduced ceria. Oxygen storage capacities and hydrogen oxidation reaction rates of four different studied systems are compared and discussed. The twofold nature of oxygen exposure of fluorinated ceria is discovered and explained. Oxygen repels fluorine from the surface, while the remaining part of fluorine is expelled to adsorbate positions, where its electronic state is altered. Moreover, such fluorine is prone to interact with atomic hydrogen. This reaction is...
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Biocompatibility of transition metal alloys: physical-chemical background
Rafaj, Zdeněk ; Nehasil, Václav (advisor) ; Kolářová, Tatiana (referee)
Titanium alloys are widely used for manufacturing of bone implants. Recent studies proved superior mechanical and chemical properties of TiNb alloys. The performance of TiNb is analyzed on Ti39Nb alloy and on evaporated layers of Ti, Nb and TiNb. Performance is compared to Ti6Al4V and pure Ti. This work is focused on the early stage of a bone growth process (studied in vitro). An investigation of this early stage has not been found in any available literature. At this early stage, CaHPO4 compound is formed. This compound is accompa- nied by Mg oxide formation. It is a difference to later stage of this process (as observed in many studies) where the layer is created solely by hydroxylapatite, Ca10(PO4)6(OH)2. The thermal oxidation of surface leads to a grain structure and to a rougher surface. The surface with different roughness effects growth rate depending on material. Generally, the best performance is achieved with TiNb (only polished as well as subsequently thermally oxidized), followed by Ti. 1
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Study of cerium oxide-metal interaction with gases - carbon monoxide oxidation and methanol decomposition
Kolářová, Tatiana ; Nehasil, Václav (advisor) ; Čech, Vladimír (referee) ; Jirka, Ivan (referee)
Title: Study of cerium oxide-metal intraction with gases - carbon monoxide oxidation and methanol decomposition Author: Tatiana Kolářová Department: Department of Surface and Plasma Science Supervisor: Doc. RNDr. Václav Nehasil, Dr., Department of Surface and Plasma Science Abstract: Dissertation thesis is focused on the study of two catalytic systems on the basis of cerium oxide-metal. The systems were characterized by various methods of surface analysis. The first part of the work concerns influence of the preparation parameters on the character of gold nanoparticles deposited onto the sputtered cerium oxide layers from a solution. Catalytic activity of prepared Au/CeOx/(C)/Si systems, containing particles with the mean sizes of 4- 10 nm, was tested toward CO oxidation at atmospheric pressure. The results show that with increasing amount of gold present on the samples, also the productivity of CO2 during the reactions grows. The work is further focused on the preparation of an inverse model CeOx/Pt(111) system. On this system, adsorption of methanol at low temeperatures and its decomposition during a heating was studied. It is apparent from presented results that the decomposition of methanol on the inverse CeOx/Pt(111) systems behaves in a different way than on a clean platinum sample or a thick...
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Study of dependence of the metal-oxide electron structure on the reactivity of these systems
Ševčíková, Klára ; Nehasil, Václav (advisor) ; Jiříček, Petr (referee) ; Plšek, Jan (referee)
The presented thesis focuses on studying the interaction between rhodium and cerium oxide and its impact on the reactivity. We investigated two different systems, Rh/CeOx and Rh-CeOx, by means of the photoelectron spectroscopy and the temperature programmed reactions. Rh/CeOx stands for rhodium nanoparticles supported by cerium oxide thin film. We show that there is an electronic metal-substrate interaction between rhodium and cerium oxide. The type of the interaction depends on a degree of cerium oxide reduction and it has a tremendous impact on the reactivity of the system. On the other hand, Rh-CeOx represents cerium oxide thin films doped by rhodium. We characterized the properties of the films with various concentration of rhodium. We show that the morphology, chemical composition and reactivity of the samples strongly depend on the concentration of the rhodium dopant. Powered by TCPDF (www.tcpdf.org)
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Laboratory Investigation of Dust Grain Charging
Beránek, Martin ; Němeček, Zdeněk (advisor) ; Jusko, Pavol (referee) ; Nehasil, Václav (referee)
The present thesis is focused on study of dust grain charging. The experimental part covers interaction between dust grains and high-energy electrons and self-discharging of grains by both field electron emission and field ionization. The second part of the thesis describes construction and evaluation of a linear electrodynamic trap of the novel design. We have observed charging of small dust grains towards high positive electric potentials when bombarded by the high-energy electron beam. We have described an increase of the secondary electron- electron emission yield from negatively charged grains due to the surface field. Further, self-discharging characteristics for both positively and negatively charged grains were measured. The relationship between discharging rate and the rate of the flow of atoms leaving the grain surface due to diffusion was observed for positively charged grains. This suggests significantly lower surface field necessary for ionizing such atoms compared to the ionization of atoms of surrounding gas and compared to the typically published field ionization thresholds. Based on the design published in the master thesis, a linear quadrupole trap of novel design was constructed. Testing measurements confirmed functionality and expected characteristics of the trap. In addition, effects of...
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Studium adsorpčních vlastností oxidovaného povrchu slitiny TiNb optickými metodami
Rafaj, Zdeněk ; Nehasil, Václav (advisor) ; Dvořák, Filip (referee)
Contemporary commercially used bone implants include a Ti-6Al-4V alloy. Due to the toxicity of aluminum and vanadium, a TiNb alloy with nontoxic elements appears to be a promising replacement. However, only limited information about this alloy is available. This study investigates TiNb surface by XPS and by a complementary method FT-RAIRS. Two sets of samples were prepared. One was only polished and spontaneously oxidized in the air (natural oxide), the other one was thermally oxidized (1h at 600ř C) after the polishing. Two samples of each oxide were treated either with H2O or with NaOH. It was found that after the preparation both samples (a natural and a thermal oxide) are in highly oxidized states TiO2 and Nb2O5. In case of the H2O treated natural oxide, these oxidation states are reduced by heating. Only small reduction after NaOH treatment is observed. This behavior is explained by a thin layer of Ti and Nb oxides. Thermal oxide seems to remain without changes during all procedures. IR spectroscopy does not prove an OH- groups appearance which is important for biocompatibility of a bone implant.
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Electronic structure of bimetallic systems - study of gas molecule interaction
Píš, Igor ; Nehasil, Václav (advisor) ; Bartoš, Igor (referee) ; Polčík, Martin (referee)
Bimetallic Rh-V system was studied by means of surface science experimental methods. Properties of ultra-thin Rh-V layers supported by γ-Al2O3 were compared with model systems prepared by vacuum V deposition on Rh(111), Rh(110) and polycrystalline rhodium. Formation of ordered V- Rh(111)-(2×2), V-Rh(110)-(2×1) and V-Rh(110)-(1×2) subsurface alloys and their electronic and atomic structure were investigated and models of the surface reconstructions were proposed. Influence of the subsurface alloy formation on interaction with CO and O2 molecules as well as the influence of the molecule adsorption on this alloy was investigated. The bond between CO molecules and Rh-V alloy surface was weakened due to pronounced changes in surface valence band structure. Oxygen which adsorbed on the alloy surface reacted with the subsurface vanadium at elevated temperature and blocked the interaction of the metal substrate with CO molecules.
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Study of CeOx / Rh inverse catalyst
Kettner, Miroslav ; Nehasil, Václav (advisor) ; Johánek, Viktor (referee)
Title: Study of CeOX / Rh inverse catalyst Author: Bc. Miroslav Kettner Department: Department of Surface and Plasma Science Supervisor: doc. RNDr. Václav Nehasil, Dr., Department of Surface and Plasma Science Abstract: Inverse catalysts of cerium oxide deposited on polycrystalline and monocrystalline (111) rhodium substrates were studied by means of surface science experimental methods. Growth characteristics of cerium oxide were investigated in dependence on deposition conditions and different thickness determination methods were compared. Acording to oxidizing or reducing exposition conditions changes in degree of oxidation of CeOX were observed. Further spectra analysis showed additional Ce-Rh alloy formation. Adsorption positions CO on-top and CO hollow on Rh (111) surface were differentiated by spectra fitting procedures. Oxygen absorption and reverse desorption in CeOX was confirmed. Performed experiments indicate that this process occurs through oxide-metal interface. CO oxidation reaction mechanisms at room temperature were proposed. Cerium oxide presence is necessary for reaction occurrence. Significant influence of deposited CeOX on proposed CO oxidation reaction mechanisms was proved by this way. Keywords: Rhodium, cerium oxide, alloy, inverse catalyst, CO oxidation.
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Low temperature CO oxidation on bimetallic structures
Hanyš, Petr ; Nehasil, Václav (advisor) ; Janovská, Michaela (referee) ; Plšek, Jan (referee)
Title: Low temperature CO oxidation on bimetallic structures Author: Petr Hanyš Department: Department of Surface and Plasma Science Supervisor: Doc. RNDr. Václav Nehasil, Dr., Department of Surface and Plasma Science Abstract: Bimetallic system Sn-Rh in the form of the model Sn/Rh(111) system, supported Rh+Sn/Al2O3 and Rh/SnO2 clusters and Rh/SnO2 and Rh+Sn/Al2O3 powder samples was studied using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), molecular beams (MB), transmission and scanning electron microscopy (TEM, SEM), powder X-ray diffraction (XRD) and high pressure reactor within the framework of the study. It was found that formation of Sn-Rh alloy influences catalytic properties of Rh by lowering CO adsorption energy and capacity. It was also found that presence of Rh in this bimetallic system mediates reversible oxidation/reduction of Sn/SnO2, whereas Sn oxide acts as an oxygen reservoir for low temperature CO oxidation. Keywords: bimetallic systems, rhodium, tin, low temperature CO oxidation
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