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

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