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Non-oxidative calcination enhances the methane dry reforming performance of ▫$Ni/CeO_{2−x}$▫ catalysts under thermal and photo-thermal conditions
Kristijan Lorber, Vasyl Shvalya, Janez Zavašnik, Damjan Vengust, Iztok Arčon, Matej Huš, Andraž Pavlišič, Janvit Teržan, Uroš Cvelbar, Blaž Likozar, Petar Djinović, 2024, izvirni znanstveni članek

Opis: We analyzed the effect of the calcination atmosphere and visible-light contribution to an accelerated reaction rate and improved H2 selectivity over 2 wt% Ni/CeO2−x nanorod catalysts. Spectroscopic and structural characterization was performed by operando DRIFTS, in situ Raman, UV-vis and XAS techniques, which were complemented by DFT calculations. Calcination in an argon or H2 atmosphere yields 15% more active catalysts in the thermally driven reaction, which are also more susceptible to light-induced rate acceleration compared to the catalyst calcined in air. The most active 2Ni/CeO2 catalyst calcined in hydrogen converts methane with a rate of 7.5 mmol (gcat min)−1 and produces a H2/ CO ratio of 0.6 at 460 °C when stimulated by a combination of visible light and thermal energy. In the absence of visible light illumination and at an identical catalyst temperature, the achieved methane rate was 4.2 mmol (gcat min)−1 and the H2/CO ratio was 0.49. The non-oxidative calcination improves nickel dispersion and the formation of subnanometer sized Ni clusters, together with a higher abundance of surface and bulk oxygen vacancies in ceria nanorods. The Ni–Ov–Ce3+components constitute the catalytically active sites under visible light illumination, which enable the DRM reaction to proceed with an Ea value of 20 kJ mol−1. Visible light also induces the following changes in the 2Ni/CeO2−x catalyst during the DRM reaction: (1) decomposition and desorption of carbonates from the nickel–ceria interface sites, (2) reduced population of nickel surface with carbonyl species and (3) promoted adsorption and dissociation of methane.
Ključne besede: methane dry reforming performance, calcination
Objavljeno v RUNG: 05.07.2024; Ogledov: 262; Prenosov: 6
.pdf Celotno besedilo (4,24 MB)
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Kristijan Lorber, 2023, doktorska disertacija

Opis: 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.
Ključne besede: DRM, CeO2 nanoshapes, reaction mechanism, photocatalysis
Objavljeno v RUNG: 22.09.2023; Ogledov: 1179; Prenosov: 38
.pdf Celotno besedilo (6,64 MB)

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, izvirni znanstveni članek

Opis: 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
Ključne besede: surface carbonates, in situ characterization, Ni XANES, Ni EXAFS, spectator species, CeO2 nanoshapes, CO2 activation
Objavljeno v RUNG: 13.07.2022; Ogledov: 1833; Prenosov: 0
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