Exploring the Parameters Controlling Product Selectivity in Electrochemical CO2 Reduction in Competition with Hydrogen Evolution Employing Manganese Bipyridine Complexes.

Selective reduction of CO2 is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental-computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO2 to HCOOH, CO, and H2. In contrast to previous Mn(bpy-R)(CO)3Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpy-based macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H2. Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO)3 moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H2 and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO2 to the metal hydride favoring the formation of H2 over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO2 adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.

Promoting Selective Generation of Formic Acid from CO2 Using Mn(bpy)(CO)3Br as Electrocatalyst and Triethylamine/Isopropanol as Additives.

Urgent solutions are needed to efficiently convert the greenhouse gas CO2 into higher-value products. In this work, fac-Mn(bpy)(CO)3Br (bpy = 2,2'-bipyridine) is employed as electrocatalyst in reductive CO2 conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et3N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn(bpy)(CO)3Br and [Mn(bpy)(CO)3]- and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et3N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et3N is replaced with Et2NH. The origin of this difference lies in the ability of Et3N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO2, while with Et2NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO2 in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO2-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO2 to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

Ligand-Controlled Product Selectivity in Electrochemical Carbon Dioxide Reduction Using Manganese Bipyridine Catalysts.

Electrocatalysis is a promising tool for utilizing carbon dioxide as a feedstock in the chemical industry. However, controlling the selectivity for different CO2 reduction products remains a major challenge. We report a series of manganese carbonyl complexes with elaborated bipyridine or phenanthroline ligands that can reduce CO2 to either formic acid, if the ligand structure contains strategically positioned tertiary amines, or CO, if the amine groups are absent in the ligand or are placed far from the metal center. The amine-modified complexes are benchmarked to be among the most active catalysts for reducing CO2 to formic acid, with a maximum turnover frequency of up to 5500 s-1 at an overpotential of 630 mV. The conversion even works at overpotentials as low as 300 mV, although through an alternative mechanism. Mechanistically, the formation of a Mn-hydride species aided by in situ protonated amine groups was determined to be a key intermediate by cyclic voltammetry, 1H NMR, DFT calculations, and infrared spectroelectrochemistry.