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@PHDTHESIS{Tang:1046648,
author = {Tang, Yuning},
title = {{S}trontium titanate based materials for use as oxygen
transport membranes in membrane reactors},
volume = {674},
school = {Twente},
type = {Dissertation},
address = {Jülich},
publisher = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
reportid = {FZJ-2025-03880},
isbn = {978-3-95806-849-0},
series = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
Umwelt / Energy $\&$ Environment},
pages = {XIV, 132},
year = {2025},
note = {Dissertation, Twente, 2025},
abstract = {Perovskite-structured oxygen transport membranes hold great
potential for the energy-efficient separation of pure oxygen
from air. While many materials have demonstrated excellent
performance, their industrial application still remains
limited due to the lack of material stability, which hinders
long-term operation under the required conditions.
Preliminary studies suggest that strontium titanate SrTiO3
is a promising candidate for membrane reactors due to its
superior chemical and mechanical stability in harsh
environments. However, its electronic and ionic conductivity
remains negligible over a wide range of temperatures and
oxygen partial pressures. To enhance conductivity, a B-site
doping strategy is employed. The research focuses on
developing SrTiO3-based materials by substituting a portion
of titanium with redox-active transition metals to obtain
both sufficient oxygen permeability and strong chemical
stability. Chapter 1 introduces the foundational principles
of CO2 capture, oxygen separation technology and membrane
reactors incorporating integrated oxygen transport membranes
(OTMs) for efficient oxygen separation. It explores the core
mechanisms governing oxygen transport in these membranes,
with a particular focus on charge carrier dynamics within
the bulk material and at the surface of mixed ionic
electronic conducting (MIEC) materials. The chapter also
discusses conventional single-phase and dual-phase materials
used in OTMs, while presenting the key components of the
single composite system examined in this study. Chapter 2
explores the impact of Fe/Ni co-doping at the B-site of STO3
on its structural and functional properties for application
as an OTM. The findings indicate that Ni doping at the Bsite
enhances both the electronic and ionic conductivities of
SrTi0.65Fe0.35O3-δ (STF35). The oxygen permeance of
SrTi0.65-xFe0.35NixO3-δ (x=0, 0.05, 0.075, 0.1) (STFNx)
slightly increases with Ni concentration and is comparable
to that of the benchmark material La0.6Sr0.4Co0.2Fe0.8O3-δ
(LSCF6428). A key observation is that the even 5 $mol\%$
Ni-doped material STFN005 exhibits a uniformly distributed
Fe/Ni exsolution after annealing in a reducing atmosphere,
which could enhance catalytic performance in membrane
reactors. All samples still possess the main peaks of
perovskite evenly annealed in a very harsh condition, i.e.
syngas with high H2S contamination. However, the stability
of STFNx decreased as Ni content increased Additionally,
two-cycle TGA measurements demonstrated that STF35 and
SrTi0.60Fe0.35Ni0.05O3-δ (STFN005) exhibited greater
stability in reducing environments compared to others.
Therefore, STFN005, with 5 $mol\%.$ Ni doping, emerges as a
promising candidate for partial oxidation of methane (POM)
applications in membrane reactors. Chapter 3 provides a
systematic investigation into the microstructure, functional
properties, and stability of the SrTi0.95-xZr0.05NixO3-δ
(x=0.01, 0.03, 0.05, 0.10, 0.15) (STZNx) series materials
for their potential use as OTMs. XRD analysis confirmed that
the solubility limit of Ni in the perovskite structure is
below 15 $mol\%.$ The results indicate that substituting Ti
with Ni significantly enhances the oxygen permeability and
electrical conductivity of STZNx. All samples maintain their
single-phase structure even after annealing in 2.9 $vol\%$
H2/Ar for 48 hours. Notably, STZN10 exhibits a uniform
distribution of Ni particles on its surface after annealing,
which can serve as active catalytic centers in membrane
reactor applications. Threecycle TGA measurements reveal
that all STZNx materials demonstrate reversible oxygen
exchange, further indicating excellent stability in reducing
atmospheres. Although the oxygen flux of STZN10 is lower
than that of many B-site doped SrTiO3 materials, it presents
unique advantages of Ni exsolution and exceptional
structural stability in reducing conditions for OTM
applications, particularly in high-temperature membrane
reactors involving hydrocarbon processing. Further
optimization strategies can be considered to improve its
oxygen permeability to position STZN10 as a promising
candidate for hightemperature membrane reactor applications
that integrate separation and reaction processes, such as
the partial oxidation of methane, where both catalytic
activity and structural stability are critical. The
microstructures and oxygen permeability of
SrTi0.65-xFe0.35AlxO3-δ (x=0.01, 0.03, 0.05, 0.10, 0.15)
(STFAx) and SrTi0.65-xFe0.35MgxO3-δ (x=0.01, 0.03, 0.05,
0.10) (STFMx) series materials is studied in Chapter 4. XRD
analysis confirmed that the STFAx samples maintain
single-phase structure across the studied composition range
(up to 15 $mol\%).$ In contrast, the solubility limit of Mg
in the STFMx series is below 10 $mol\%.$ Oxygen permeation
measurements indicate that Al doping is unsuitable in the
STFAx system, as increased Al content leads to a decline in
oxygen permeability. Similarly, MgO segregation is observed
on STFMx materials, which negatively impacted the oxygen
permeability of the materials. Consequently, understanding
and mitigating MgO segregation is crucial for optimizing
material performance. further optimization strategies could
be employed to minimize MgO segregation and enhance
permeability, which could still make STFMx as a promising
candidate for oxygen transport membranes (OTMs) in membrane
reactor applications Chapter 5 explores the oxygen flux and
catalytic performance of selected materials, including
STFN005, SrTi0.75Fe0.25O3-δ with STFN005 catalytic layer
$(STF25_cl),$ SrTi0.65Fe0.35O3-δ with STFN005 catalytic
layer $(STF35_cl),$ and STZN10 for use as OTMs in a membrane
reactor. The oxygen flux of STFN005 is evaluated under three
different gas environments: air/Ar, air/10 $vol\%$ CH4 and
15 $vol\%$ CO2/10 $vol\%$ CH4. The results indicate that the
highest oxygen flux of STFN005 (0.55 mL·cm-2·min-1) is
achieved in air/10 $vol\%$ CH4, suggesting that partial
oxidation of methane (POM) reaction promotes oxygen
transport. The remeasured oxygen flux and microstructure
characterization after testing demonstrates the good crystal
stability of STFN005. However, the membrane fractured due to
chemical expansion. Additionally, Ni exsolution is observed
on the membrane surface after the experiments. For
$STF25_cl,$ despite exhibiting lower oxygen permeability
than STFN005 under air/Ar condition, it demonstrates higher
permeability in air/10 $vol\%$ CH4, suggesting the porous
STFN005 catalytic layer significantly enhances catalytic
performance by optimizing surface exchange kinetics and
creating redox-active Fe/Ni sites. Furthermore, $STF25_cl$
remained structurally intact, indicating strong
thermochemical stability in reducing atmospheres. Activation
energy analysis in air/Ar and air/10 $vol\%$ CH4 reveals
that $STF35_cl$ exhibits improved oxygen surface exchange
and bulk diffusion properties compared to STFN005 and
$STF25_cl.$ However, like STFN005, $STF35_cl$ also fractures
after exposure to the reducing atmosphere due to chemical
expansion. STZN10 exhibits the lowest oxygen flux among the
studied materials. The postexperiment membrane photograph
and the microstructure analysis of the sweep side
demonstrates its excellent thermochemical stability in
reducing environments. Product selectivity analysis
identified distinct reaction pathways for each material.
STFN005 excel in syngas production (POM), while
$STF25_cl/STF35_cl$ suit oxy-combustion. STZN10 is promising
for oxidative coupling of methane (OCM). Further
optimization of the membrane architecture, including precise
thickness control and asymmetric structural design and so
on, is expected to enhance membrane performance for target
reactions in membrane reactors. Chapter 6 summarizes all
obtained results, reflects on the overall findings, and
further determines a direction for future research. It shows
that STFN005, with 5 $mol\%$ Ni doping, emerges as a
promising candidate for POM applications in membrane
reactors due to its high oxygen permeance, Ni exsolution,
and good stability in reducing environments. STZN10 is
promising for OCM reaction due Ni exsolution and exceptional
structural stability. However, further optimization
strategies should be considered to improve its oxygen
permeability. MgO segregation is observed on STFMx
materials, which negatively impacted the oxygen permeability
of the materials. Further optimization strategies could be
employed to minimize MgO segregation and enhance
permeability, which could still make STFMx as a promising
candidate for oxygen transport membranes (OTMs) in membrane
reactor applications. The porous STFN005 catalytic layer on
$STE25_cl$ and $STF35_cl$ significantly enhances catalytic
performance by optimizing surface exchange kinetics and
creating redox-active Fe/Ni sites, making them suitable for
oxy-combustion. Further optimization of membrane
architecture, including precise thickness control and
asymmetric structural design and so on, is expected to
enhance membrane performance for target reactions in
membrane reactors.},
cin = {IMD-2},
cid = {I:(DE-Juel1)IMD-2-20101013},
pnm = {1232 - Power-based Fuels and Chemicals (POF4-123)},
pid = {G:(DE-HGF)POF4-1232},
typ = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
doi = {10.34734/FZJ-2025-03880},
url = {https://juser.fz-juelich.de/record/1046648},
}
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