Thèses

Vendredi 27 Juin 2025 à 13h30.

Photo-catalysis based on molybdenum clusters (Mo6) for the environment, sustainable energy and CO2 recycling

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Directeur de thèse / thesis director :
Luke MacAleese

Membres du jury / jury members :
Isabelle Compagnon,
Héloïse Dossmann,
Jean-Yves Salpin,
Karine Costuas,
Luke MacAleese

Résumé / Abstract :
The goal of this work is to explore the reactivity of molybdenum clusters through a new approach that combines UV-visible laser spectroscopy and tandem mass spectrometry by investigating the intermediate species involved. The combination of the two techniques allows us the characterization of reactive intermediate species in the gas phase. The experimental setup, developed in the research group Spectrobio allows for the isolation and photoactivation of molecular ions using light sources coupled to a linear ion trap mass spectrometer. In a secondary configuration, neutral gases such as CO2 and O2 can be introduced in the ion trap where ion-molecule reactivity is studied.

The necessity of this study comes from the urgent need for sustainable photochemical processes, such as photocatalysis, that can convert harmful compounds into useful ones. Molybdenum clusters were reported to photocatalyze the reduction of CO2 to methanol and thus, offer a fertile ground for the study of light-induced reactivity due to their accessible redox states and long-lived excited states.

In the first part of this study, solution-phase modifications of halogenated molybdenum clusters were investigated. It was shown that ligand substitution by hydroxide groups happens under the combination of basic and aqueous conditions and in addition, this process is accelerated under light irradiation. The resulting species were characterized in the gas phase using different activation methods such as Laser-Induced Dissociation (LID) and Collision-Induced Dissociation (CID). Additionally, these species were studied by UV-visible spectroscopy which allowed us to observe their photodissociation and optical properties and to assign specific spectral features to these potentially photoactive species.

A second type of chemical modification of the clusters involved CO2 coordination, likely resulting from the aging process of solutions used. Even though efforts were put to promote these substitutions in solution, its enhancement could not be possible. The photogenerated ions displayed different reactivity with regards to CO2 and O2. Already, the irradiation of the clusters produced different species of varied oxidation states as electron rich, oxidized and reduced species were accessed. The reactivity of these photofragments (in particular [Mo6I9]-) with CO2, only forms species with odd numbers of oxygen atoms, which points to towards the cluster assisted C-O bond activation. This contrasts with the reactivity with O2 that leads to species with only even number of oxygen atoms. The reactivity of completely de-halogenated clusters with O2 led to the highly oxidized species such as Mo6O18H2O and the question of cluster transition from hexanuclear (Mo6) metal core to a POM (polyoxometalate) structure compound, similar to the Lindqvist polyoxometalate clusters ([Mo6O19]2-).

This work, finally, explores different paths of possible photoactivity of the molybdenum clusters but also demonstrates the interest of the coupling between mass spectrometry and laser spectroscopy for the exploration of reaction pathways of intermediate species in photocatalytic processes.

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