Recent progress in nanomaterials preparation, characterization and theoretical modeling opens the way to tailoring catalysts and understanding their operating mode at the atomic level. Many strategies are possible for improving the performances of metal-based catalysts, e.g., as done in our laboratory, through the use of alloy phases (noble metal nanoalloys1 and non-noble intermetallics2) and/or reducible oxide supports (e.g. TiO2 3 and CeO2 4). It will be shown that the targeted catalytic processes (selective hydrogenations and preferential CO oxidation) all benefit from metal-metal and/or metal-oxide synergies. As these reactions involve hydrogen-rich environments, hydrogen absorption in the metal lattice can have a drastic influence on the catalytic properties.5,6 Moreover, advanced microscopy and spectroscopy techniques reveal additional major effects of the gaseous atmosphere and the support on the catalyst phase, such as changes in nanoparticle morphology, chemical arrangement and oxidation state (Fig. 1).4,7 Finally, as will be shown from operando techniques in the prototypical case of CO oxidation over Pt/-Al2O3, the influence of the thermochemical environment is even more dramatic for the stability of recently emerging “singe-atom catalysts”.8,9 Confronting experimental observations with computer simulations enables us to identify the key structural drivers and elucidate the reaction mechanisms.2,7,9,10 (1) Piccolo, L. Surface Studies of Catalysis by Metals: Nanosize and Alloying Effects. In Nanoalloys: Synthesis, Structure and Properties; Alloyeau, D., Mottet, C., Ricolleau, C., Eds.; Engineering Materials; Springer London, 2012; pp 369–404. (2) Piccolo, L.; Chatelier, C.; Weerd, M.-C. D.; Morfin, F.; Ledieu, J.; Fournée, V.; Gille, P.; Gaudry, E. Sci. Technol. Adv. Mater. 2019, 20, 557–567. (3) Nguyen, T.-S.; Laurenti, D.; Afanasiev, P.; Konuspayeva, Z.; Piccolo, L. J. Catal. 2016, 344, 136–140. (4) Morfin, F.; Nguyen, T.-S.; Rousset, J.-L.; Piccolo, L. Appl. Catal. B 2016, 197, 2–13. (5) Zlotea, C.; Morfin, F.; Nguyen, T.-S.; Nguyen, N.-T.; Nelayah, J.; Ricolleau, C.; Latroche, M.; Piccolo, L. Nanoscale 2014, 6, 9955–9959. (6) Zlotea, C.; Oumellal, Y.; Provost, K.; Morfin, F.; Piccolo, L. Appl. Catal. B 2018, 237, 1059–1065. (7) Piccolo, L.; Li, Z. Y.; Demiroglu, I.; Moyon, F.; Konuspayeva, Z.; Berhault, G.; Afanasiev, P.; Lefebvre, W.; Yuan, J.; Johnston, R. L. Sci. Rep. 2016, 6, 35226. (8) Dessal, C.; Len, T.; Morfin, F.; Rousset, J.-L.; Aouine, M.; Afanasiev, P.; Piccolo, L. ACS Catal. 2019, 9, 5752–5759. (9) Dessal, C.; Sangnier, A.; Chizallet, C.; Dujardin, C.; Morfin, F.; Rousset, J.-L.; Aouine, M.; Bugnet, M.; Afanasiev, P.; Piccolo, L. Nanoscale 2019, 11, 6897–6904. (10) Goyhenex, C.; Piccolo, L. Phys. Chem. Chem. Phys. 2017, 19, 32451–32458.