71. K-edge absorption spectra of isoelectronic gaseous hydrides: a combination of atomic and molecular channelsRobert Hauko, Jana Padežnik Gomilšek, Alojz Kodre, Iztok Arčon, Giuliana Aquilanti, 2018, published scientific conference contribution abstract Abstract: The fine detail in the x-ray absorption spectra in the energy region of absorption edges
provides the insight into the mechanism of inner-shell photoexcitation: in particular in spectra of
free atoms or simple molecules, the simplest being gaseous hydrides [1-2].
Measured K edge absorption spectra of hydrides of 3p (PH3, H2S in HCl) and 4p (GeH4,
AsH3, H2Se, HBr) elements, and published data of 2p hydrides (CH4, NH3, H2O, HF) as well as
SiH4 [3-6] and the noble gases at the end of the isoelectronic series (Ne, Ar, Kr) are compared to
the respective calculated spectra, obtained by atomic HF86, GRASP codes [7] and molecular
DFT (Density functional theory) ORCA code [8]. For a clearer view of intraatomic processes,
the weak and simple structural (XAFS) signal of the molecule is removed from the spectra.
Among the spectral features below the continuum limit, those with the lowest energy
belong to the transition of the core electron to the lowermost free orbitals with the molecular
character. They are, as a rule, wider than the transitions to the higher orbitals with prevailing
atomic character. The theoretical description with DFT code without specific adaptations is
sufficient for a qualitative picture of the pre-edge structure. The fine structure immediately above
the K edge stems from the coexcitation of valence electrons. We have proved that the
coexcitations can be explained as a two-step process: the inner-shell photoeffect followed by the
shake-up of a valence electron predominantly to a free atomic orbital. This process is markedly
different from coexcitations of more tightly bound electrons [9].
In the collection of consecutive and homologous data, analyzed by a common procedure,
the reaction channels can be identified with better precision and reliability than in analysis of
individual spectra. Our analysis showed that the energies and probabilities of single-electron
transitions into the molecular orbitals are strongly affected by the symmetry of the molecule,
essentially in the same way in 3p and 4p homologues, but not in 2p homologues with a stronger
influence of the core charge. In transitions to atomic orbitals the influence of the molecular field
is negligible. Keywords: hidridi, rentgenska spektroskopija, XAFS Published in RUNG: 12.09.2018; Views: 3788; Downloads: 0 This document has many files! More... |
72. In-situ XAS analysis of nanoshaped CuO/CeO2 catalysts used for N2O decompositionIztok Arčon, Maxim Zabilsky, Petar Djinović, Albin Pintar, 2018, published scientific conference contribution abstract Abstract: The goal of this research is to establish the working state and correlations between atomic structure and catalytic activity of nanoshaped CuO/CeO2 catalysts used in N2O decomposition reaction. The catalysts contained CuO nanoclusters dispersed over different CeO2 morphologies: nano-rods and nano-cubes. N2O is a side product of nitric and adipic acid production and a very potent greenhouse gas that is formed in amounts estimated at about 400 Mt/a of CO2 equivalent. Consequently, the development of robust, active and selective catalysts for N2O decomposition is of a great environmental and economical interest. CeO2-based materials promoted by CuO represent a new class of catalysts that exhibit considerable activity in N2O decomposition reaction between 300 and 500 °C [1-3], and are significantly cheaper and more efficient than Pt, Pd or Rh based catalysts.
In order to maximize the efficiency of the catalyst, the active site in this reaction needs to be identified and the mechanism clarified. In-situ Cu K-edge and Ce L3-edge XANES and EXAFS analysis was done on a set of CuO/CeO2 catalysts with different ceria morphology (nano-cubes, nano-rods) and Cu loadings between 2 to 8 wt. %, during N2O decomposition reaction, under controlled reaction conditions at 400 °C. The XAS spectra were measured in-situ, in a tubular reactor, filled with protective He atmosphere at 1 bar, first at RT, then during heating, and at final temperature of 400 °C, during catalytic reaction, when the catalyst was exposed to a small amount (0.2 vol%) of N2O mixed with He.
The Cu K-edge and Ce L3-edge XANES and EXAFS analysis reveals changes in valence and local structure of Cu and Ce in the CuO/CeO2 catalysts. In the initial state (in He at RT), copper is present in the form of CuO nanoparticles attached to the CeO2 surface. After heating in He to 400 °C, partial (10%) reduction of Ce [Ce(IV)→Ce(III)] is detected, significant part of Cu(II) is reduced to Cu(I) and Cu(0) species, and direct Cu-Cu bonds are formed. During catalytic N2O decomposition at 400°C, all Ce(III) is oxidized back to Ce(VI), and a major part of Cu is oxidized back to Cu(II), with about 5% of Cu(I) remaining in equilibrium state. Observed structural and valence changes of copper strongly depend on its loading and CeO2 morphology.
With systematic In-situ XAS analysis of different nanoshaped CuO/CeO2 catalysts, we identified the structural characteristics and changes of Cu and Ce phases during catalytic N2O decomposition reaction, which could lead to identification of the active catalytic site during the reaction and further improve the performance of these promising catalytic materials. Keywords: EXAFS, CuO/CeO2 catalyst, N2O decomposition Published in RUNG: 12.09.2018; Views: 3678; Downloads: 0 This document has many files! More... |
73. XAS studies of Sn modified TiO2 coatingsKsenija Maver, Iztok Arčon, Urška Lavrenčič Štangar, 2018, published scientific conference contribution abstract Abstract: Coatings based on TiO2 are extensively investigated material for the preparation of surfaces which are referred as self-cleaning. Under solar illumination these coatings catalyze pollutants degradation and enhance their removal from the surface due to photoinduced superhydrophilicity [1]. There are two main drawbacks of using pure TiO2 (anatase) as a photocatalyst; i) its band gap lies in the UVA region so it can exploit only a part of the sunlight spectrum, and ii) it has a high degree of recombination between photo generated electrons and holes on the surface. There are various strategies to improve the photocatalytic efficiency of TiO2, one of them is the modification with transition metals.
Based on our previous experiences with sol-gel synthesis of low-temperature TiO2 thin films [2], we prepared a series of Sn modified TiO2 photocatalysts. As a starting material only organic (Ti and Sn alkoxide) precursors were used. The loadings of Sn cations were varied in the range of 0.05 to 20 mol.%. The coatings, deposited on glass substrates by dip-coating technique, were dried at 150 oC. In addition, another set of photocatalyst coatings was prepared by further calcination at 500 °C in air. The comparison of photocatalytic activities of Sn modified TiO2 to unmodified TiO2 showed that Sn loadings in the range of 1-10 mol.% improved photocatalytic activity up to 8 times. At lower loadings of Sn, the photocatalytic activity was improved only by 30 %. After the coatings are calcined, their photocatalytic activity was significantly reduced.
The objective of the research was to examine the mechanism responsible for photocatalytic properties of Sn modified TiO2 and to to clarify the role of Sn cations in the TiO2 photocatalytic process. For this purpose, Sn and Ti K-edge XANES and EXAFS analysis was used to precisely determine the local structure and the site of incorporation of Sn cations on titania nanoparticles in the coatings. We examined the hypothesis that the solid-solid interface was a crucial structural feature that facilitates charge separation and enhances photocatalytic efficiency of titania. Keywords: Sn, EXAFS, TiO2 fotokatalizator Published in RUNG: 12.09.2018; Views: 3355; Downloads: 0 This document has many files! More... |
74. Cadmium localization and chemical environment in a cadmium bioindicator Gomphrena clausseniiPaula Pongrac, Mina T. Villafort Carvalho, Tânia S. Serra, Hiram Castillo Michel, Katarina Vogel-Mikuš, Iztok Arčon, Luka Jeromel, Mitja Kelemen, Boštjan Jenčič, Anja Kavčič, Primož Pelicon, Mark G. M. Aarts, 2018, published scientific conference contribution abstract Keywords: cadmium Published in RUNG: 12.09.2018; Views: 3510; Downloads: 0 This document has many files! More... |
75. K-edge absorption spectra of gaseous hydridesAlojz Kodre, Robert Hauko, Jana Padežnik Gomilšek, Iztok Arčon, Giuliana Aquilanti, 2018, published scientific conference contribution abstract Abstract: X-ray absorption spectra in the energy region of absorption edges reveal fine details of the
mechanism of inner-shell photoexcitation: in particular in spectra of free atoms or simple
molecules, the simplest being gaseous hydrides [1-2]. In a collection of data from consecutive
and homologous elements, analyzed by a common procedure, the reaction channels can be
identified with better precision and reliability than in analysis of individual spectra.
Absorption spectra of the hydrides of 3p elements (PH3, H2S in HCl) were measured at the
XAFS beamline of the Elettra synchrotron in Trieste: a new type of adjustable absorption cell
for measurement of noxious gases at room temperature and at low photon energies was
developed for the purpose. For the analysis, data from an earlier experiment on 4p hydrides
(GeH4, AsH3, H2Se, HBr), and published data of 2p hydrides (CH4, NH3, H2O, HF) [3-4] as
well as SiH4 and the noble gases concluding the isoelectronic series (Ne, Ar, Kr) were
adopted. The spectra are compared to respective calculated spectra, obtained by atomic HF86,
GRASP codes and molecular DFT (Density functional theory) ORCA code [5].
Our analysis of the pre-edge structures showed that the energies and probabilities of singleelectron
transitions into the lowermost orbitals with the molecular character were strongly
affected by the symmetry of the molecule, essentially in the same way in 3p and 4p
homologues, but not in 2p homologues with a stronger influence of the core charge. In
transitions to higher orbitals with prevailing atomic character the influence of the molecular
field is negligible.
The fine structure immediately above the K edge stems from the coexcitation of valence
electrons. These coexcitations can be explained as a two-step process: the inner-shell
photoeffect followed by the shake-up of a valence electron predominantly to a free atomic
orbital. The process is markedly different from coexcitations of more tightly bound electrons
[3]. The results of relative shake-up probabilities can be compared to results of emission
spectroscopies, the probabilities of double excitation to bound states show a correlation with
the dissociation probability of the molecule. Keywords: večelektronske vzbuditve, hidridi, rentgenska absorpcijska spektroskopija Published in RUNG: 12.09.2018; Views: 4061; Downloads: 0 This document has many files! More... |
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77. Fluorinated ether based electrolyte for high-energy lithium-sulfur batteries : Li [sup] + solvation role behind reduced polysulfide solubilitySara Drvarič Talian, Steffen Jeschke, Alen Vižintin, Klemen Pirnat, Iztok Arčon, Giuliana Aquilanti, Patrik Johansson, Robert Dominko, 2018, published scientific conference contribution abstract Keywords: Li-žveplive baterije, XANES, polisulfidi Published in RUNG: 12.09.2018; Views: 3142; Downloads: 0 This document has many files! More... |
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80. Operando XAS analysis of CuO/SiO2 and CuO/CeO2 catalystsIztok Arčon, Janvit Teržan, Petar Djinović, Maxim Zabilsky, Albin Pintar, 2018, published scientific conference contribution abstract Abstract: The possibilities of the operando XAS analysis of catalysts will be presented on two case
studies of promising new catalytic materials: alkali doped nano-dispersed copper oxide
clusters on ordered mesoporous SiO2, which is highly active and selective towards propylene
epoxidation [1], and nanoshaped CuO/CeO2 catalysts used in N2O decomposition reaction [2].
Operando Cu K-edge and Ce L3-edge XANES and EXAFS analysis was performed during
catalytic reactions under controlled reaction conditions in a tubular reactor filled with
protective He atmosphere at 1 bar. The spectra were measured before the reaction at RT, then
during heating, and during catalytic reaction at 400 °C under controlled atmosphere.
Operando XANES analysis is used to monitor the changes in valence states and local
symmetries of Cu and Ce cations in the catalysts. A partial reduction of Cu2+ to Cu+ and Cu0
and Ce4+ to Ce3+ species was detected during catalyst activation, and re-oxidation during
catalytic reaction. Different dynamics of reaching a quasi-steady oxidation state were revealed
as the tested catalysts approached the quasi-steady state after 300 min of reaction.
Operando EXAFS spectra are used to precisely determine local structure of Cu and Ce
cations, to identify structural characteristics and changes of Cu and Ce species during the
catalytic reactions. In this way, the active site in the catalytic reactions can be identified and
the mechanism of the reaction clarified.
The results of operando XAS analyses are crucial to guide further material modification, to
obtain more effective catalyst, and material which is more resistant to inhibiting effects that
cause catalyst deactivation during catalytic reaction. Keywords: katalizatorji, Cu XANES, EXAFS Published in RUNG: 12.09.2018; Views: 3392; Downloads: 0 This document has many files! More... |