Heterointercalation in Chevrel-Phase Sulfides: A Model Periodic Solid for the Investigation of Chain Electron Transfer

Modulation of electron density localization on periodic crystal solids through electron transfer from interstitial cations can directly influence the bonding configurations of small-molecule intermediates at the catalyst binding site. This study presents the microwave-assisted solid-state synthesis of four heterointercalant Chevrel-phase (CP) sulfides with varying metal cation intercalants with compositional and electronic structure investigations of the electron density redistribution as a result of intercalation. The heterointercalant CP sulfides, with the general formula Cu_<i>x</i>M_<i>y</i>Mo₆S₈ (where M = Cr, Mn, Fe, Ni; <i>x</i>, <i>y</i> = 1.5-2.5), are presented here for the probe reaction of electrochemical CO₂ reduction. A change in product selectivity is observed toward the production of methanol at low overpotentials of -0.5 V vs reversible hydrogen electrode (RHE), as a result of the intercalant combination present within the CP interstitial cavity. Structural confirmation of all materials was examined through Rietveld refinement of the powder X-ray diffraction (PXRD) data, high-resolution transmission electron microscopy (HR-TEM), and selected-area electron diffraction (SAED). Electron transfer from the intercalated metal cations to the Mo₆S₈ cluster was investigated via X-ray photoelectron spectroscopy (XPS) of the intercalated metal cations and the chalcogenide cluster. Electron transfer was further confirmed through X-ray absorption analysis (XAS) of the K-edges of Mo and intercalants. Intermediate studies of electrochemical reduction of formaldehyde to methanol resulted in a faradaic efficiency of ∼78% methanol production on Cu_<i>x</i>Ni_<i>y</i>Mo₆S₈. The results presented herein identify distinct principles for materials design that can be utilized in other compositional spaces within the broad families of periodic crystal solids.