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

Abstract

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.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO2 Reduction by Manganese Bipyridyl Complexes

“…For example, acquiring an advanced understanding of proton-coupled electron transfer in the CO 2 reduction catalytic cycle will allow us to manipulate reactive intermediates under nonequilibrium conditions to promote a desired outcome and develop new processes and materials to secure a clean energy future. − Although CO 2 is a strong electrophile, its conversion via nucleophilic attack is often inhibited by a lack of thermodynamic driving force due to the often large reorganization and activation energies required. The utilization of proton-coupled electron transfer reduces the thermodynamic energy requirement for the reduction of CO 2 significantly (eqs and ) relative to the highly endergonic one-electron reduction to the CO 2 •– radical anion described by eq . Of course, taking advantage of proton-coupled reduction in the conversion of CO 2 raises the additional challenge of catalyst selectivity due to competitive metal–hydride formation which can lead to hydrogen gas and/or formate production. − Mn electrocatalysts have risen as promising alternatives to heavy metal catalysts due to their ability to catalyze CO 2 reduction at low overpotentials while maintaining a high selectivity for CO evolution or even selective formate production in some cases. − Previously, we were able to demonstrate a high selectivity for CO production at the low overpotential protonation-first pathway with the electrochemically activated [ fac -Mn I {[(MeO) 2 Ph] 2 bpy}­(CO) 3 (CH 3 CN)]­(OTf) precatalyst, saving 0.55 V in overpotential relative to the more commonly observed reduction-first pathway . The mechanistic differences between the protonation-first and the reduction-first pathways are illustrated in Scheme .…”

Origin of the Boosting Effect of Polyoxometalates in Photocatalysis: The Case of CO2 Reduction by a Rh-Containing Metal–Organic Framework

“…This yields the Rh­(III)-hydride species 4a , which lies slightly above 3 in terms of free energy. The formation of metal hydrides assisted by amines has not only been proposed before for the RhCp* catalyst in homogeneous conditions but also for other catalysts in the context of CO 2 RR. − Also of note, this process is faster than the putative coordination of CO 2 to 3 , which involves a higher, overall free-energy barrier of 14.8 kcal mol –1 , in line with the product selectivity toward formate. As shown in Figure b, the experimental evolution of the product concentration over time follows a logarithmic growth, reaching a plateau after ca.…”

Efficient Electrochemical Reduction of CO2 to Formate in Methanol Solutions by Mn‐Functionalized Electrodes in the Presence of Amines**

“…Amine moieties were proposed not only to provide binding sites, hence a reservoir of CO 2 under the form of carbamates, but also to stabilize and promote the formation of the hydride catalytic intermediate ( HMn ), thereby favoring formate production instead of CO [19c,20] . Contemporarily to our current work a third paper from Daasbjerg group [21] appeared, regarding the use of amines with Mn catalysts, but in homogeneous solutions, different solvent, and different amine concentrations, which represent a nice complement to our heterogeneous approach. These amines, which are supposed to work as proton shuttle, were either introduced in large excess with respect the catalyst [19a] or upon an elaborated synthetic procedure as a side‐arm of the Mn catalyst (Figure 1).…”