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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: 506; Downloads: 27
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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: 1289; Downloads: 0
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