Magnetically induced catalysis using magnetic nanoparticles (MagNPs) as heating agents is a new efficient method to perform reactions at high temperatures. However, the main limitation is the lack of stability of the catalysts operating in such harsh conditions. Normally, above 500 ºC, significant sintering of MagNPs takes place. Here we present encapsulated magnetic FeCo and Co NPs in carbon (Co@C and FeCo@C) as an ultra-stable heating material suitable for high temperature magnetic catalysis. Indeed, FeCo@C or a mixture of FeCo@C:Co@C (2:1) decorated with Ni or Pt-Sn showed good stability in terms of temperature and catalytic performances. In addition, consistent conversions and selectivities regarding conventional heating were observed for CO2 methanation (Sabatier Reaction), propane dehydrogenation (PDH) and propane dry reforming (PDR). Thus, the encapsulation of MagNPs in carbon constitutes a major advance in the development of stable catalysts for high temperature magnetically induced catalysis.

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A main goal of molecular electronics is to relate the performance of devices to the structure and electronic state of molecules. Among the variety of possibilities that organic, organometallic and coordination chemistries offer to tune the energy levels of molecular components, spin crossover phenomenon is a perfect candidate for elaboration of molecular switches. The reorganization of the electronic state population of the molecules associated to the spin crossover can indeed lead to a significant change in conductivity. However, molecular spin crossover is very sensitive to the environment and can disappear once the molecules are integrated into devices. Here, we show that the association of ultra-small 1.2 nm platinum nanoparticles with FeII triazole-based spin crossover coordination polymers leads to self-assemblies, extremely well organized at the sub-3 nm scale. The quasi-perfect alignment of nanoparticles observed by transmission electron microscopy, in addition to specific signature in infrared spectroscopy, demonstrates the coordination of the long-chain molecules with the nanoparticles. Spin crossover is confirmed in such assemblies by X-ray absorption spectroscopic measurements and shows unambiguous characteristics both in magnetic and charge transport measurements. Coordinating polymers are therefore ideal candidates for the elaboration of robust, well-organized, hybrid self-assemblies with metallic nanoparticles, while maintaining sensitive functional properties, such as spin crossover.

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The fragmentation upon electron impact ionization of Ar4He1000 is investigated by means of mixed quantum-classical dynamics simulations. The Ar4+ dopant dynamics is described by a surface hopping method coupled with a diatomics-in-molecules model to properly take into account the multiple Ar4+ electronic surfaces and possible transitions between them. Helium atoms are treated individually using the zero-point averaged dynamics (ZPAD), a method based on the building of an effective He-He potential. Fast electronic relaxation is observed, from less than 2 ps to ∼ 30 ps depending on initial conditions. The main fragments observed are Ar2+Heq and Ar3+Heq (q ≤ 1000), with a strong contribution of the bare Ar2+ ion, and neither Ar+ nor Ar+Heq fragments are found. The smaller fragments (q ≤ 50) are found to mostly come from ion ejection whereas larger fragments (q > 500) originate from long-term ion trapping. Although the structure of the trapped Ar2+ ions is the same as in the gas phase, trapped Ar3+ and Ar4+ are rather slightly bound Ar2+ · · · Ar and Ar2+ · · · Ar · · · Ar structures (i.e., an Ar2+ core with one or two argon atoms roaming within the droplet). These loose structures can undergo geminate recombination and release Ar3+Heq or Ar4+Heq (q ≤ 50) in the gas phase and/or induce strong helium droplet evaporation. Finally, the translational energy of the fragment center of mass was found suitable to provide a clear signature of the broad variety of processes at play in our simulations.

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La déplétion des énergies fossiles et l’augmentation des émissions de gaz à effet de serre nous incitent à étudier des voies alternatives de production d’énergie mais aussi de valorisation du CO2. L’activation de réactions catalytiques par chauffage magnétique permet, dans ce contexte, de répondre à ces objectifs en offrant la possibilité de stocker de manière dynamique et adaptable les énergies renouvelables intermittentes.Dans ce travail de thèse, nous avons étudié de manière multi-échelle l’activation de réactions catalytiques en travaillant d’abord sur la synthèse et la caractérisation de nano-agents chauffants afin, notamment, d’opérer le procédé à hautes températures. Dans un second temps, nous avons étudié la composition du lit catalytique afin d’optimiser les performances catalytiques du procédé de méthanation du CO2 et de maximiser son efficacité énergétique tout en évaluant son impact environnemental.

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Transition Metal Dichalcogenides (TMDs) are a family of layered materials with potential applications in optics and electronics. Following the discovery of graphene, TMDs were characterized and extraordinary physical properties were discovered: when thinned down to a monolayer, TMDs become direct band gap materials, therefore strongly facilitating light emission. The direct bandgap of these semiconductors is situated on the edge of the Brillouin zone, at the K-point. This is different from standard semiconductors for optoelectronics like GaAs where the bandgap is in the centre of the Brillouin zone. The optical properties are dominated by excitons, and light-matter interaction is extremely strong with up to 20% of light absorption per monolayer. In addition to a bandgap, TMDs present strong spin-orbit coupling and broken inversion symmetry. As a result, the optical transitions across the bandgap have chiral selection rules. The spin states in the valence and conduction bands are well separated in energy by the spin-orbit interaction. This makes it possible to optically address specific spin and valley states in momentum space and monitor their dynamics. As a result monolayer TMDs are exciting model systems for spin and valley physics: these research fields are termed spintronics and valleytronics. This motivated our work on the exact understanding of the optical transitions, their polarization selections rules and the different exciton states.

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