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| Home > Publications database > Strontium titanate based materials for use as oxygen transport membranes in membrane reactors |
| Book/Dissertation / PhD Thesis | FZJ-2025-03880 |
2025
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-849-0
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Please use a persistent id in citations: doi:10.34734/FZJ-2025-03880
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.
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