Recent progress in nanomaterials preparation, characterization and theoretical modeling opens the way to tailoring catalysts and understanding their operating mode at the atomic level. In this seminar, I will present several of our strategies for improving the performance of metal-based catalysts through the use of alloy phases (noble metal nanoalloys and non-noble intermetallics) and/or reducible oxide supports (TiO2, CeO2). 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. Advanced microscopic and spectroscopic techniques reveal additional major effects of the gaseous atmosphere on the catalyst phase, such as changes in nanoparticle morphology, chemical arrangement and oxidation state. Confronting experimental observations with computer-simulation results enable us to identify the key structural drivers and elucidate the reaction mechanisms.