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.
Jan-Christian Schober (Thu,) studied this question.