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@PHDTHESIS{Schober:646520,
      author       = {Schober, Jan-Christian},
      othercontributors = {Stierle, Andreas},
      title        = {{O}perando {I}nvestigations of {S}tructure-{A}ctivity
                      {R}elationships in {P}d-based {M}odel {C}atalysts for
                      {M}ethane {O}xidation},
      school       = {University of Hamburg},
      type         = {Dissertation},
      address      = {Hamburg},
      reportid     = {PUBDB-2026-00894},
      pages        = {399},
      year         = {2026},
      note         = {Dissertation, University of Hamburg, 2026},
      abstract     = {The high global warming potential of CH4 makes the
                      catalytic conversion of residual CH4 in exhaust gases vital
                      for applications such as CH4 combus tion engines and
                      turbines, power-to-gas or biomass plants. The most active
                      heterogeneous catalyst system for complete CH4 oxidation at
                      low temper atures (< 650K) in lean reaction gas mixtures
                      (overstoichiometric oxygen content) is the class of Pd-based
                      catalysts supported by (mixed) metal oxide supports. The
                      activity of these catalysts is closely linked to PdO content
                      at low temperatures, as well as structure and morphology of
                      the nanoparticles. The catalytic conversion proceeds with
                      the Mars-van-Krevelen mechanism through numerous elementary
                      steps which consumes lattice oxygen of PdO and produces H2O.
                      A persistent challenge in heterogeneous catalysis in gen
                      eral is catalyst deactivation, which can occur through
                      sintering or poisoning, and these processes also affect
                      Pd-based catalysts for CH4 oxidation. Specif ically, the
                      inhibition by H2O, deactivation by PdO reduction, and
                      sintering are three of the core causes for deactivation.
                      Strategies to address these issues are the use of oxide
                      supports with different redox properties and the addi tion
                      of other noble metals to the catalyst nanoparticles, most
                      prominently Pt, which can alter the redox properties of the
                      catalyst, inhibit sintering, and manage catalyst passivation
                      by H2O. This work aims to elucidate the structural and
                      morphological properties of Pd based catalysts under
                      transient conditions during light-off. The objective is to
                      improve the understanding of the mechanisms behind the
                      enhance ment of catalyst performance by nanoparticle support
                      interactions and the addition of Pt. Model catalysts that
                      combine geometric simplicity and mor phological complexity
                      were used to address these questions. This approach enables
                      direct correlation of structural and morphological
                      properties on the atomic scale to the catalyst’s activity.
                      The experiments were carried out in industrially relevant
                      temperature and pressure regimes, thereby bridging the
                      pressure and material gap between single crystal studies and
                      conventional packed-bed or monolith reactor experiments.
                      α-Al2O3(0001) was selected as an inert representative and
                      CeO2 as a redox active support with high oxygen mobility.
                      The CeO2(001) model catalyst support surface was prepared by
                      reactive physical vapor deposition of Ce in atomic oxygen
                      atmosphere on YSZ(001), as commercially available CeO2
                      substrates are unsuitable for grazing incidence X-ray
                      scattering. The re sulting CeO2 films were thoroughly
                      characterized by a comprehensive set of complementary
                      techniques to ensure tight control over its properties. The
                      CeO2 thin films used as catalyst supports exhibited a
                      dislocation lattice at the CeO2/YSZ interface which enabled
                      full coverage of the film despite the iii considerable
                      lattice mismatch. The bulk of the film was fully oxidized
                      with a bulk-like lattice, while the surface was fully
                      hydroxylated and covered with a molecular water level, even
                      after annealing in ultra high vacuum under oxygen
                      atmosphere. Two aspects of the structure and morphology were
                      investigated: (i) the evolution under transient light-off
                      conditions and (ii) the temporal evolution after each
                      increment in a step-wise heating experi ment. For both
                      studies, epitaxial Pd and PdPt nanoparticles were grown by
                      physical vapor deposition, and catalytic testing was
                      conducted in a custom, X-ray compatible operando flow cell
                      equipped with inline mass spectrometry. The light-off
                      experiments conducted with Pd/Al2O3 and Pd/CeO2 showed
                      strong dependence of catalytic activity, structure and
                      morphology from the support material. Notably, the reaction
                      intermediates CO and CH2O, asso ciated with the
                      Mars-van-Krevelen mechanism on PdO(101), were observed in
                      the exhaust gas by mass spectrometry for the first time.
                      These observa tions suggest that the migration and
                      adsorption/desorption of surface species such as OH and CH2O
                      are slower than desorption of gas-phase intermedi ates.
                      Consequently, complete oxidation under conventional
                      conditions may proceed through multiple
                      adsorption–desorption cycles. Structural and mor
                      phological data recorded in parallel by high-energy X-ray
                      diffraction (75keV) revealed distinct oxidation mechanisms
                      depending on the support. The com parative analysis showed
                      that CeO2 inhibited sintering and stabilized the PdO phase
                      more effectively than Al2O3. Furthermore, the detection of
                      ther modynamically unstable phases under reaction conditions
                      provided evidence for strong variations in local chemical
                      potential at the catalyst surface. The CeO2 thin films used
                      in the light-off experiments contained a small amount of
                      rectangular holes, which are associated with oxygen vacancy
                      condensation. A statistical evaluation of SEM images
                      revealed a pronounced size difference between NPs located on
                      the CeO2 film and those located in the holes of the f ilm.
                      These results provide evidence that Ostwald ripening is the
                      dominant sintering mechanism on CeO2(001) supports and that
                      the holes act as dif fusion traps, locally enhancing
                      sintering. In the kinetic investigation, the influence of Pt
                      on catalytic behavior was studied by comparing the activity,
                      structure, and morphological dynamics of Pd/Al2O3 and
                      PdPt/Al2O3. For the first time, HEGISAXS and HEGIXRD were
                      combined to characterize catalysts under operando
                      conditions. Regardless of Pt content, the epitax ial
                      relationship between Al2O3 and the nanoparticles
                      significantly inhibited their oxidation compared to the
                      larger NPs studied in the light-off experi ments. Pronounced
                      morphological changes upon initial H2O desorption were
                      observed only for Pd/Al2O3, accompanied by significant
                      changes in the lat tice constant, ultimately leading to an
                      overall relaxation of the lattice. In contrast, the
                      morphology of PdPt/Al2O3 remained largely unaffected by the
                      iv initial H2O. HEGISAXS indicated significant vertical
                      material transport in Pd/Al2O3, pointing to strong
                      reaction-induced reshaping which is consistent with the
                      trends seen in HEGIXRD. In contrast, this was not observed
                      for PdPt/Al2O3. Changes in the NP morphology in this system
                      are instead at tributed to the formation of PdO bulk and
                      surface phases and slow, thermally driven sintering over the
                      course of the experiment. While the correlation be tween PdO
                      content and catalytic activity was weak, a clear
                      relationship was observed between activity and the strain
                      state of the metal phase. The lattice of Pd/Al2O3 gradually
                      relaxed over the course of the experiment, coinciding with
                      declining activity, whereas PdPt/Al2O3 retained a highly
                      strained lat tice, which correlated with improved
                      performance. In summary, these findings provide direct
                      insight into the mechanisms of cat alyst deactivation and
                      the effects of the nanoparticle-support interactions, Pt
                      alloying, and strain in enhancing resistance to H2O
                      inhibition, PdO reduc tion, and sintering. The use of model
                      catalysts, combinatory X-ray tech niques, and relevant
                      pressure and temperature regimes established detailed
                      structure-activity correlations and highlighted the role of
                      surface diffusion, and desorption of products and
                      intermediates of CH4 oxidation. Overall, these results
                      bridge the material, pressure, and complexity gap between
                      ide alized single crystal investigations and reactor
                      studies.},
      cin          = {FS-NL},
      cid          = {I:(DE-H253)FS-NL-20120731},
      pnm          = {632 - Materials – Quantum, Complex and Functional
                      Materials (POF4-632) / SFB 1441 A05 - Struktur und
                      Zusammensetzung von PtPd Modellkatalysatoren unter operando
                      Bedingungen: Experiment und Theorie (A05) (446698573)},
      pid          = {G:(DE-HGF)POF4-632 / G:(GEPRIS)446698573},
      experiment   = {EXP:(DE-H253)P-P23-20150101 / EXP:(DE-MLZ)External-20140101
                      / EXP:(DE-H253)Nanolab-04-20150101 /
                      EXP:(DE-H253)Nanolab-01-20150101 /
                      EXP:(DE-H253)Nanolab-02-20150101 /
                      EXP:(DE-H253)Nanolab-03-20150101},
      typ          = {PUB:(DE-HGF)11},
      url          = {https://bib-pubdb1.desy.de/record/646520},
}