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1.
Operando XAS analysis of CuO/SiO2 and CuO/CeO2 catalysts
Albin Pintar, Maxim Zabilsky, Petar Djinović, Janvit Teržan, Iztok Arčon, 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.
Found in: osebi
Keywords: katalizatorji, Cu XANES, EXAFS
Published: 12.09.2018; Views: 1848; Downloads: 0
.pdf Fulltext (62,04 KB)

2.
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.
Found in: osebi
Keywords: Alkali modification, propylene epoxidation, reaction mechanism, copper oxide, activation barrier.
Published: 05.06.2020; Views: 1152; Downloads: 0
.pdf Fulltext (22,78 MB)

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