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Kristijan Lorber, 2023, doctoral dissertation

Abstract: Dry reforming of methane (DRM) is an attractive reaction for converting the two major greenhouse gases CH4 and CO2 into the product syngas. H2 and CO as synthesis gas are important chemical feedstocks for the further production of valuable products as well as for the production of long-chain hydrocarbons by the Fisher-Tropsch process. High operating temperatures due to the endothermic nature of the DRM process and the occurrence of several side reactions such as Reverse Water Gas Shift, Methane Cracking and Boudoard reaction make the DRM process unattractive for industrial application. For the catalytic application of DRM in thermal mode (thermal energy drives the reaction), different CeO2 morphologies, namely nanorods, nanocubes, and nanospheres, were synthesized by a hydrothermal method. The best catalyst for DRM was found to be 2 wt. % Ni loaded in CeO2 rods morphology (2Ni-R). Characterization techniques (XRD, N2-physisorption, TEM, in-situ XANES/EXAFS TPR and CO2 TPD) were used to investigate the structural and redox properties of the catalysts. The mechanism of CO2 activation on reduced Ni/CeO2-x during DRM was proposed using DFT calculations and in-situ DRIFTS measurements combined with mass spectrometry. The 2Ni-R catalyst, which performed best in thermal DRM reaction, was studied under photo-thermal conditions where it was stimulated by both visible light and thermal energy. The catalytic activity was observed even at low (140 °C) temperatures, and the obtained CH4 and CO2 conversion, as well as H2/CO ratio exceeded thermodynamic limitations. XRD, TEM, and H2-physisorption techniques were used for structural characterization, while in-situ UV-Vis measurements were performed to study the optical properties of the catalyst. By using suitable long-pass filters and with the help of theoretical calculations, we were able to distinguish two photo mechanisms which contribute to photocatalytic activity under photo-thermal mode of the DRM reaction. Shorter wavelengths (< 450 nm) supported the charge transfer and generation mechanism in reduced CeO2-x, while longer wavelengths (> 450 nm) promoted near-field enhancement. However, under full spectrum of visible light (400 - 800 nm), the charge transfer and generation mechanism was dominant and led to 2-3 times higher CH4 activation rates compared to near-field enhancement.
Keywords: DRM, CeO2 nanoshapes, reaction mechanism, photocatalysis
Published in RUNG: 22.09.2023; Views: 68; Downloads: 1
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Winning combination of Cu and Fe oxide clusters with an alumina support for low-temperature catalytic oxidation of volatile organic compounds
Tadej Žumbar, Iztok Arčon, Petar Djinović, Giuliana Aquilanti, Gregor Žerjav, Albin Pintar, Alenka Ristić, Goran Dražić, Janez Volavšek, Gregor Mali, Margarita Popova, Nataša Zabukovec Logar, Nataša Novak Tušar, 2023, original scientific article

Abstract: A γ-alumina support functionalized with transition metals is one of the most widely used industrial catalysts for the total oxidation of volatile organic compounds (VOCs) as air pollutants at higher temperatures (280−450 °C). By rational design of a bimetal CuFe-γ-alumina catalyst, synthesized from a dawsonite alumina precursor, the activity in total oxidation of toluene as a model VOC at a lower temperature (200−380 °C) is achieved. A fundamental understanding of the catalyst and the reaction mechanism is elucidated by advanced microscopic and spectroscopic characterizations as well as by temperature-programmed surface techniques. The nature of the metal−support bonding and the optimal abundance between Cu−O−Al and Fe−O−Al species in the catalysts leads to synergistic catalytic activity promoted by small amounts of iron (Fe/Al = 0.005). The change in the metal oxide−cluster alumina interface is related to the nature of the surfaces to which the Cu atoms attach. In the most active catalyst, the CuO6 octahedra are attached to 4 Al atoms, while in the less active catalyst, they are attached to only 3 Al atoms. The oxidation of toluene occurs via the Langmuir−Hinshelwood mechanism. The presented material introduces a prospective family of low-cost and scalable oxidation catalysts with superior efficiency at lower temperatures.
Keywords: Iron oxide clusters, copper oxide clusters, alumina support, synergistic effect, low-temperature total catalytic oxidation, toluene, Cu, Fe XANES, EXAFS
Published in RUNG: 06.07.2023; Views: 372; Downloads: 4
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CO[sub]2 activation over nanoshaped CeO[sub]2 decorated with nickel for low-temperature methane dry reforming
Kristijan Lorber, Janez Zavašnik, Iztok Arčon, Matej Huš, Janvit Teržan, Blaž Likozar, Petar Djinović, original scientific article

Abstract: Dry reforming of methane (DRM) is a promising way to convert methane and carbon dioxide into H2 and CO (syngas). CeO2 nanorods, nanocubes, and nanospheres were decorated with 1−4 wt % Ni. The materials were structurally characterized using TEM and in situ XANES/EXAFS. The CO2 activation was analyzed by DFT and temperature-programmed techniques combined with MS-DRIFTS. Synthesized CeO2 morphologies expose {111} and {100} terminating facets, varying the strength of the CO2 interaction and redox properties, which influence the CO2 activation. Temperature-programmed CO2 DRIFTS analysis revealed that under hydrogen-lean conditions mono- and bidentate carbonates are hydrogenated to formate intermediates, which decompose to H2O and CO. In excess hydrogen, methane is the preferred reaction product. The CeO2 cubes favor the formation of a polydentate carbonate species, which is an inert spectator during DRM at 500 °C. Polydentate covers a considerable fraction of ceria’s surface, resulting in less-abundant surface sites for CO2 dissociation
Keywords: surface carbonates, in situ characterization, Ni XANES, Ni EXAFS, spectator species, CeO2 nanoshapes, CO2 activation
Published in RUNG: 13.07.2022; Views: 897; Downloads: 0
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In-situ XAS study of catalytic N[sub]2O decomposition over CuO/CeO[sub]2 catalysts
Maxim Zabilsky, Iztok Arčon, Petar Djinović, Elena Tchernychova, Albin Pintar, 2021, original scientific article

Abstract: We performed in‐situ XAS study of N 2 O decomposition over CuO/CeO 2 catalysts. The Cu K‐edge and Ce L 3 ‐edge XANES and EXAFS analyses revealed the dynamic and crucial role of Cu 2+ /Cu + and Ce 4+ /Ce 3+ ionic pairs during the catalytic reaction. We observed the initial formation of reduced Cu + and Ce 3+ species during activation in helium atmosphere at 400 °C, while concentration of these species decreased significantly during steady‐state nitrous oxide degradation reaction (2500 ppm N 2 O in He at 400 °C). In‐situ EXAFS analysis further revealed a crucial role of copper‐ceria interface in this catalytic reaction. We observed dynamic changes in average number of Cu‐Ce scatters under reaction conditions, indicating an enlarging the interface between both copper and ceria phases, where electron and oxygen transfer occurs.
Keywords: in-situ XAS, Cu EXAFS, CuO/CeO2 nanorod catalys, N2O decomposition
Published in RUNG: 29.01.2021; Views: 1784; Downloads: 0
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Effect of Na, Cs and Ca on propylene epoxidation selectivity over CuOx/SiO2 catalysts studied by catalytic tests, in-situ XAS and DFT
Janvit Teržan, Matej Huš, Iztok Arčon, Blaž Likozar, Petar Djinović, 2020, original scientific article

Abstract: This research focuses on epoxidation of propylene over pristine, Na, Ca and Cs modified CuOx/SiO2 catalysts using O2. The selectivity of the reaction is analyzed using a combination of catalytic tests, in-situ XAS and DFT calculations. The initially present subnanometer CuO clusters are present in all catalysts which re-disperse/flatten during reaction. During catalytic reaction, the Cu1+ becomes the predominant oxidation state. There is no correlation between propylene oxide (PO) selectivity and copper oxidation state. DFT analysis of the propylene reaction pathway revealed that Na, Cs, and Ca addition decreases the bonding strength of propylene to CuO and decreases the O2 activation barrier, while simultaneously increase the exothermicity of O2 dissociation. The Na induced Cu-O bond modification decreases the activation barrier from 0.87 to 0.71 eV for the oxametallacycle (OMC) ring closure (first step in the reaction pathway favoring selectivity towards PO) compared to pristine 5Cu catalyst. At the same time, we observed an increase (from 0.45 to 0.72 eV) of the barrier for the abstraction of allylic hydrogen. The opposite effect is achieved by Ca addition: the activation barrier for OMC ring closure increases to 1.08 eV and that for allylic hydrogen stripping decreases to 0.16 eV.
Keywords: Alkali modification, propylene epoxidation, reaction mechanism, copper oxide, activation barrier.
Published in RUNG: 05.06.2020; Views: 2322; Downloads: 0
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In-situ XAS analysis of nanoshaped CuO/CeO2 catalysts used for N2O decomposition
Iztok 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: 3238; Downloads: 0
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Operando XAS analysis of CuO/SiO2 and CuO/CeO2 catalysts
Iztok 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: 3030; Downloads: 0
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