The electrocatalytic reduction of CO₂ to energy-rich syngas, methanol, or even hydrocarbons offers an attractive route for converting greenhouse gas into value-added chemicals and storage of energy. Layered transition metal dichalcogenide nanomaterials, such as MoS_2, WSe₂, have emerged as electrocatalysts towards CO₂ reduction with high efficiency and low overpotentials. However, the underlying chemical and electronic states defining the catalytic activity have not yet been completely defined. These key states can be probed for both metals and ligands in selected MX₂ (M=W, Mo, W; X=S, Se) using synchrotron-based X-ray absorption spectroscopy (XAS). Metal d-states are said to be the active states for catalysis, these states are not accessible with metal K-edge XAS. On the other hand, studying ligand K- pre-edge gives insight into active metal states as pre-edge reflects a transition to metal-ligand (4d/5d-3p/4p) hybridized states. In-situ/operando XAS characterization technique offers an opportunity to probe changes in the electronic and chemical states during active conditions.
In this work, in-situ/operando XAS was performed at Se K-edge for MSe₂ (M= W, Mo) nanosheets using high energy resolution fluorescence detection (HERFD). HERFD gives more spectral resolution than conventional partial fluorescence yield by reducing the lifetime broadening, which helps track small electronic and chemical structures changes. Measurements were carried out in the presence and absence of CO₂ in the electrolyte to identify the key states. Se K-edge showed changes at the pre-edge and edge, indicating changes in the number of available occupied states and Z_eff.

Alireza Ahmadiparidari

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