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@PHDTHESIS{Schuster:475676,
author = {Schuster, Ralf},
othercontributors = {Libuda, Joerg and Stierle, Andreas},
title = {{M}odel {S}tudies of {C}atalytic {P}rocesses at the
{S}olid/{L}iquid {I}nterface},
school = {Friedrich-Alexander-University Erlangen-Nuremberg},
type = {Dissertation},
address = {Erlangen},
publisher = {Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)},
reportid = {PUBDB-2022-01419},
pages = {170},
year = {2022},
note = {Publiziert über die Universitätsbibliothek
Erlangen-Nürnberg. Keine Creative Commons Lizenz - es gilt
der Veröffentlichungsvertrag und das deutsche
$Urheberrecht.https://www.katalog.fau.de/TouchPoint/singleHit.do?methodToCall=showHit\&curPos=1\&identifier=2_SOLR_SERVER_1719327390https://opus4.kobv.de/opus4-fau/frontdoor/index/index/docId/18317;$
Dissertation, Friedrich-Alexander-University
Erlangen-Nuremberg, 2021},
abstract = {This thesis focuses on the fundamental understanding of
different catalytic reactions at the interface between a
solid catalyst and a liquid reactant phase. To this end,
both in situ and operando experiments, monitored by
synchrotron X-ray diffraction techniques as well as infrared
spectroscopy based surface science model studies, were
performed. The electrochemical surface oxidation of Pt(111)
and Pt(100) electrodes under operando conditions in a
potential range up to 2.25 VRHE has been investigated in a
combined EC/SXRD study using a rotating disk electrode
setup. The as-prepared Pt(111) surface is described best by
an ideally bulk-terminated model. For Pt(100), a
bulk-terminated surface with 0.17 ML of adatoms, created by
lifting of a pseudohexagonal reconstruction layer, has been
observed. Both surface structures are stable in O2-purged
0.1 M HClO4 for potentials between 0.0 VRHE and 1.0 VRHE, as
no significant changes of the structure factors have been
observed in this potential range. Starting at 1.05 VRHE, the
Pt(111) surface becomes rough due to surface oxidation,
which follows the well-known PE mechanism. Above 1.3 VRHE,
the newly proposed PE+buckling model was used to describe
the extensive surface oxidation. Increasing the potential
beyond 1.55 VRHE leads to a recovery of the structure factor
in the anti-Bragg positions, partially surpassing the
initial SFs of the pristine surface. This SF recovery is,
however, not due to a restoration of the initial surface
structure, but due to a shift of the reference point of the
surface layer to former deeper bulk layers. As the oxidation
proceeds, the measured CTRs arise from a former deeper Pt
layer, which is now the topmost, not fully oxidized platinum
layer. This leads to the formation of a disordered bulk
oxide at high positive potentials. The additional oxygen,
present in the electrolyte, has no influence on the surface
structures of the Pt(111) electrode between 0.05-2.25 VRHE.
Qualitatively similar results were obtained for the surface
oxidation of Pt(100) in an oxygen-purged electrolyte. Here
again, an undulating trend of the structure factor in an
anti-Bragg position as a function of the applied potential
has been observed, but the structure factors remained below
their initial value. This is associated with the formation
of a rougher interface between bulk Pt electrode and
disordered oxide layer, due to lateral movements of the
lifted Pt atoms.A second study addressed the formation and
stability of palladium carbide phases of well-defined
Pd/α-Al2O3(0001) model catalysts under the conditions of
catalytic liquid organic hydrogen carrier dehydrogenation.
The phase composition of supported Pd nanoparticle catalysts
was monitored as a function of particle size and reactant
flow rate and composition in an in situ high-energy grazing
incidence X-ray diffraction study. At 500 K, exposure of
small Pd nanoparticles with an average particle diameter of
6 nm to methylcyclohexane leads to the immediate formation
of a palladium carbide phase. The carbon content in PdxC
gradually increases, until a Pd6C phase is yielded. It was
shown that the stability of these carbide phases critically
depends on the composition of the reactant feed and the
resulting balance between carbon formation and consumption.
Surface carbon species are formed by the thermal
decomposition of hydrocarbons, whereas they are removed from
the surface by reactions with gas phase hydrogen. At a high
gas flow rate, which corresponds to a low H2 partial
pressure over the catalyst, the Pd6C phase immediately
decomposes after its formation. This decomposition is
triggered by the nucleation and growth of graphene deposits
on the surface of the Pd nanoparticles. At a low gas flow
rate, which corresponds to a high H2 partial pressure over
the catalyst, the Pd6C phase is stable under continuous MCH
exposure. Due to the higher H2 concentration in the gas
phase, the surface carbon concentration is not high enough
for the nucleation of a graphene nucleus of critical size.
In the absence of the carbon source from the gas phase, Pd6C
decomposes due to carbon segregation to the surface of the
Pd nanoparticles. Upon exposure of the larger Pd
nanoparticle sample (15 nm average diameter) to MCH at the
low gas flow rate, the coexistence of Pd6C and PdxC carbide
phases was observed. With increasing time on stream, the
PdxC phase is converted to Pd6C. Upon extended exposure to
MCH, the pure Pd phase recovers at the expense of both
carbide phases. Because of slow nucleation, the graphene
deposits are formed only after an induction period, which
triggers the carbide decomposition.Finally, the earliest
stages of the ionothermal synthesis of cobalt oxide
nanoparticles in an IL were scrutinized in an UHV-based
surface IRAS model study. The room temperature synthesis in
an IL recently attracted attention as a promising synthesis
route for the production of taskspecific metal oxide
nanomaterials. Deposition of Co2(CO)8 and [C4C1Pyr][NTf2] at
110 K on Au(111) leads to layered films of molecular cobalt
carbonyl complexes and IL. Several bridged and non-bridged
precursor species (Co2(CO)8 isomers and Co4(CO)12) are
identified based on their IR spectra. These species
dynamically convert between bridged and non-bridged
complexes. In particular, the interaction with the pristine
Au(111) surface facilitates the conversion of CO-bridged
complexes to cobalt carbonyls with exclusively terminal CO
ligands. Such conversions were not observed for the
deposition of Co2(CO)8 on pre-adsorbed [C4C1Pyr][NTf2], as
the precursor/IL interaction is weak as compared to the
precursor/metal interaction. The weak precursor/IL
interaction leads to molecular desorption of precursor
molecules from an IL layer upon heating. In strong contrast,
the formation of cobalt nanoclusters on Au(111) has been
observed at elevated temperatures for precursor layers in
direct contact with the gold support. However, deposition of
Co2(CO)8 on Au(111) or [C4C1Pyr][NTf2]/Au(111) at 225 K
resulted in the formation of Co nanoclusters already during
the deposition process. Precursor-terminated layers are
readily oxidized by ozone at low temperatures, forming a
disordered CoxOy passivation layer. This passivation layer
protects the underlying precursor layers from further
oxidation. A similar protection against oxidation is
achieved by the deposition of an IL layer on top of
pre-adsorbed Co2(CO)8 at low temperatures. At elevated
temperatures, both the cobalt oxide passivation layer and
the IL protection layer get successively permeable for
ozone. This results in the full oxidation of the precursor
multilayer. Notably, the CoxOy passivation layer, formed at
the IL/precursor interface during the exposure of
[C4C1Pyr][NTf2]/Co2(CO)8/Au(111) to ozone at elevated
temperatures, only becomes ozone-permeable at significantly
higher temperatures as compared to the CoxOy passivation
layer formed for a pristine Co2(CO)8/Au(111) film. Thus, it
is concluded that the IL governs the oxide formation, which
leads to a more densely packed cobalt oxide passivation
layer.},
cin = {DOOR ; HAS-User},
cid = {I:(DE-H253)HAS-User-20120731},
pnm = {6G3 - PETRA III (DESY) (POF4-6G3)},
pid = {G:(DE-HGF)POF4-6G3},
experiment = {EXP:(DE-H253)P-P07-20150101 /
EXP:(DE-H253)Nanolab-05-20200101 /
EXP:(DE-H253)Nanolab-01-20150101 /
EXP:(DE-H253)Nanolab-03-20150101},
typ = {PUB:(DE-HGF)11},
url = {https://bib-pubdb1.desy.de/record/475676},
}
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