1. CO[sub]2 activation over nanoshaped CeO[sub]2 decorated with nickel for low-temperature methane dry reformingKristijan 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: 893; Downloads: 0
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2. Effect of Na, Cs and Ca on propylene epoxidation selectivity over CuOx/SiO2 catalysts studied by catalytic tests, in-situ XAS and DFTJanvit 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: 2318; Downloads: 0
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3. 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: 3025; Downloads: 0
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