Sulfur substitution in Fe-MOF-74: implications for electrocatalytic CO2 and CO reduction from an ab initio perspective

Employing first-principles methods, we investigated the electrocatalytic reduction of CO2 and CO on two Fe-based MOFs: Fe2DOBDC and Fe2DSBDC. Our primary objective was to discern the impact of substituting S atoms into the framework while maintaining the topological structure. We anticipated significant changes in reduction reactions due to differences in chemical properties such as electronegativity, atomic radius, polarizability, and charge density upon replacing O atoms with S atoms. Atomic charge analysis highlights some of these differences by showing the equatorial Fe-O/S bonds of Fe2DSBDC are less polarized and result in smaller positive and negative charges on the Fe and O/S atoms, respectively. Additionally, the larger S atoms are expected to weaken adsorbate binding due to less favorable van der Waals interactions near the open-metal Fe site. Consequently, the less electropositive Fe site and the larger S atoms of Fe2DSBDC impede the adsorption of reduced CO2 and CO products, while the more electropositive Fe site and smaller O atoms of Fe2DOBDC strongly favor product adsorption. Specifically, the weak binding of HCOOH and CH2O intermediates on Fe2DOBDC (Delta G of -0.07 eV and -0.11 eV, respectively) indicates feasible further reduction to CH2O and CH4 for CO2RR and CORR, respectively. Conversely, these adsorbates exhibit unfavorable binding to the Fe site of Fe2DSBDC (Delta G of +0.14 eV and +0.25 eV, respectively), limiting further reduction possibilities. Thus, CO2 and CO reduction on Fe2DSBDC are likely to yield only 2e(-) products (HCOOH and CH2O, respectively), whereas Fe2DOBDC is expected to produce deeper reduction products (CH2O and CH4, respectively). Additionally, significant differences in free energy for the first reduction steps post-sulfur substitution indicate more favorable energetics for both CO2 and CO reductions (Delta G = -0.12 eV and -0.58 eV, respectively). These findings lay the groundwork for designing novel MOFs with tunable reaction behaviors by strategically replacing O atoms with heavier S atoms in the MOF scaffold.