Introduction During the transition from fossil to more sustainable energy resources, the use of methane/biomethane in gas engines for mobile applications or heat and power plants is a widely spread approach for reducing the CO2 emissions [1]. In comparison with conventional liquid fuels, lean-burn gas engines have also the advantage of emitting lower amounts of unburnt hydrocarbons, nitrogen oxides, carbon monoxide and particulate matter. Nonetheless, an important drawback is the high greenhouse gas potential of methane emissions. Among different catalysts tested for reducing the CH4 slip, Pd-based systems are known for their high efficiency. However, the improvement of the low-temperature activity and catalyst stability are still stringent problems of this technology. Previous studies have shown that Pd-based catalysts are particularly sensitive to water deactivation and sulfur poisoning [2,3]. In this study, we systematically investigate the influence of the noble metal interaction with the support on the catalyst performance, activation behavior and stability for a series of Pd/CeO2 catalysts. Materials and Methods Pd-CeO2 based catalysts with a noble metal loading between 1-3 wt. % were supported on ceria with different properties by incipient wetness impregnation. Several pre-treatment procedures under oxidizing and reducing conditions were applied before the catalytic tests in a gas mixture containing 3200 ppm CH4, 10% O2 in N2. To obtain structure-activity correlations, the catalysts were characterized by complementary in situ and ex situ methods including: X-ray diffraction (XRD), temperature programmed reduction, electron microscopy and X-ray absorption spectroscopy (XAS). Additionally, the nature of active species was identified by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements supplemented by density functional theory (DFT) calculations. Results and Discussion The results obtained in this study indicate a strong impact of catalyst pre-treatment conditions on the activity of Pd/CeO2-catalysts. Whereas an oxidizing treatment at high temperatures results in catalyst deactivation, a significant activity gain was obtained upon reduction in CO- or H2-containing gas mixtures. In line with our previous studies [4,5], this positive effect of a reducing treatment could be correlated with the tuning of the interface between the reduced noble metal nanoparticles and ceria support. This aspect is essential also for the long-term catalyst stability. The formation or redispersion of Pd nanoparticles depending on the catalyst treatment and gas atmosphere was monitored during environmental transmission electron microscopy (ETEM) measurements. Further differences in the noble metal structure leading to a higher or lower catalyst activity and durability were uncovered by in situ DRIFTS and operando XAS characterization, and were additionally supported by DFT calculations [6]. Significance To efficiently use noble metals and prevent their sintering, the use of strongly interacting supports is generally applied. However, for reactions occurring at high temperatures such as methane oxidation, not only the growth of the noble metal particles leads to catalyst deactivation but also the formation of highly dispersed and inactive Pd species. Herein, we show that this process can be rationally exploited by targeted catalyst preparation and effective activation procedures. Acknowledgments This work was supported by the German Research Foundation (DFG, grant No. 431423888 - DYCAT project; additionally ID426888090-SFB 1441 “TrackAct”) and the Agence National de la Recherche (grant No. ANR-19-CE05-0038, PRCI DYCAT project). References 1.P. Lott, M., Casapu, J.-D., Grunwaldt, O. Deutschmann, Appl. Catal. B 2024, 12324 2.P. Lott, M. Eck, D. E. Doronkin, A. Zimina, S. Tischer, R. Popescu, S. Belin, V. Briois, M. Casapu, J.-D. Grunwaldt, O. Deutschmann, Appl. Catal. B 2020, 278, 119244. 3.K. Murat, Y. Mahara, J. Ohyama, Y. Yamamoto, S. Arai, A. Satsuma, A., Angew. Chem. Int. Ed. 2017, 56, 50, 15993. 4.F. Maurer, J. Jelic, J. Wang, A. Gänzler, P. Dolcet, C. Wöll, Y. Wang, F. Studt, M. Casapu, J.-D. Grunwaldt, Nature Catal. 2020, 3, 824. 5.A.M. Gänzler, M. Casapu, P. Vernoux, S. Loridant, F. J. Cadete Santos Aires, T. Epicier, B. Betz, R. Hoyer, J.-D. Grunwaldt, Angew. Chem. Int. Ed. 2017, 56, 13078. 6.A. Salcedo, D. Zengel, F. Maurer, M. Casapu, J.-D. Grunwaldt, C. Michel, D. Loffreda, Small 2023, 2300945.