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, <i>fac</i>-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 <i>i</i>PrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving <i>fac</i>-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 <i>i</i>PrOH. 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/<i>i</i>PrOH to establish an equilibrium solution of isopropyl carbonate and CO₂, while with Et₂NH/<i>i</i>PrOH, 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.