Microwave-Synthesized Alloyed Chevrel Phase Sulfides Allow Tunability in the CO2 Reduction Reaction

The exponential increase of atmospheric carbon dioxide (CO₂) in recent years, generated by the continued reliance on fossil fuels as a primary energy source, has resulted in increased worldwide health effects and damages to countless ecosystems. To counteract the continued damage felt from fossil fuel usage, sustainable fuels harnessed from atmospheric CO₂ must be implemented in industrial processes to counteract the impact of fossil fuel usage felt worldwide. The combination of renewable energy, from sources such as solar and fuel, with electrocatalysts capable of reducing CO₂ selectively to liquid fuels, such as methanol, can replace certain fossil fuel usage and potentially regress the levels of atmospheric CO₂. Within the world of advancing electrocatalysts, inorganic crystal frameworks capable of hosting metal cations within structured cavity architectures are viable candidates due to the direct structural and electronic tunability available through the intercalation of differing metal cations. One such material of interest is the Chevrel phase sulfides (CPs). Intercalated CPs with the general formula M_aMo₆S₈ (M = transition, alkali, alkaline, or post transition metals; a = 0-4) unlock new avenues to investigate charge transfer effects from interstitial cations on an extended crystal framework through accessible synthetic pathways with short reaction times. Microwave-assisted solid-state synthesis can be used to successfully insert two or more metal cations into the same CPs cavity, resulting in a quaternary or alloyed CPs framework. Through the co-insertion of multiple cations in the cavities of the extended solid, we gain direct control over the stabilization of CO₂R intermediates towards desired fuel products through the electron density donated from the intercalants to the crystal framework. In this work, we present the microwave-assisted solid-state synthesis of multinary CPs materials (M_xM’_yMo₆S₈, M, M’ = Cr, Mn, Fe, Ni; x, y = 1.5-2) tested for aqueous CO₂ reduction to gaseous and liquid fuels. Charge transfer investigations were conducted on the experimental materials via X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), specifically X-ray Absorption Near-Edge structures (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) studies. The structural properties of the quaternary catalysts are characterized through refinement of the powder X-ray diffraction (PXRD) data, as well as high resolution transmission electron microscopy (HR-TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES).