Thesis presented October 08, 2021

Abstract:
The scientific community is actively seeking to develop innovative catalysts for the electrochemical nitrogen reduction reaction (ENRR) to produce ammonia (NH₃), which has and will play a pivotal role in our industrialized societies. Herein we systematically investigated different types of solid-state molybdenum sulfides (MoS_x) as ENRR catalysts, including amorphous molybdenum sulfide (a-MoS_x), nanocrystalline MoS₂ (c-MoS_x), electrochemically deposited MoS_x (e-MoS_x), or Fe doped MoS_x (Fe-MoS_x). Using a rigorous protocol for testing the materials in standardized conditions, we establish that none of them showed any catalytic activity towards ENRR, in great contrast with the materials of this family previously discussed in the literature. However, they all exihibited a clear activity towards the electrocatalytic reduction of inorganic azides (N₃⁻) to ammonia (EN₃RR). Here we show that the various forms of MoS_x do bear different type of catalytic sites, which in turn have noticeable impact on their EN₃RR rate and selectivity.
Interestingly we demonstrate that the well-defined {Mo₃S₇} clusters, sharing the structure of the units forming e/a-MoS_x, do exhibited comparable EN₃RR activity to a-MoS_x. It offers a unique model to study EN₃RR under homogeneous conditions with the help of classical analytical methods. Through these studies, we demonstrated that the {Mo₃S₇} core firstly undergoes an electro-driven reorganization to {Mo₃S₄}, which provide two coordination modes for the azido ligand, one of which leading to the fast evolution of ammonia in presence of protons. Noteworthy we show that a similar evolution may be at play for a-MoS_x during the EN₃RR justifying the use of molecular models to understand catalytic activity of the solid-state MoS_x.
In order to stabilize the {Mo₃S₇} clusters, we next consider the design of cavity-like tridentate porphyrin ligands (H₂P) using a computational-guided strategy to select a few potential ligands. After the preparation of the latter we demonstrate that this family of ligands is indeed capable of binding the cluster, and as suggested by the computational screening we observe that the complexation is favored as the overall flexibility of the ligand increases. This work lies down the foundations of a systematic investigation of the potential of molecular {Mo₃S₇} species towards the activation of nitrogenase substrates.

Keywords:
nitrogen reduction, azide reduction, electrocatalysis, molybdenum sulfide clusters

On-line thesis.