Abstract
Electrocatalytic conversion of carbon dioxide has gained much interest for the synthesis of value-added chemicals and solar fuels. Important issues such as high overpotentials and competition of hydrogen evolution still need to be overcome for deeper insight into the reaction mechanism in order to steer the selectivity towards specific products. Herein we report on several metalloprotoporphyrins immobilized on a pyrolytic graphite electrode for the selective reduction of carbon dioxide to formic acid. No formic acid is detected on Cr-, Mn-, Co- and Fe-protoporphyrins in perchloric acid of pH 3, while Ni-, Pd-, Cu- and Ga-protoporphyrins show only a little formic acid. Rh, In and Sn metal centers produce significant amounts of formic acid. However, the faradaic efficiency varies from 1% to 70% depending on the metal center, the pH of the electrolyte and the applied potential. The differentiation of the faradaic efficiency for formic acid on these metalloprotoporphyrins is strongly related to the activity of the porphyrin for the hydrogen evolution reaction. CO2 reduction on Rh-protoporphyrin is shown to be coupled strongly to the hydrogen evolution reaction, whilst on Sn- and In-protoporphyrin such strong coupling between the two reactions is absent. The activity for the hydrogen evolution increases in the order In<Sn<Rh metal centers, leading to faradaic efficiency for formic acid increasing in the order Rh<Sn<In metal centers. In-protoporphyrin is the most stable and shows a high faradaic efficiency of ca. 70%, at a pH of 9.6 and a potential of −1.9V vs RHE. Experiments in bicarbonate electrolyte were performed in an attempt to qualitatively study the role of bicarbonate in formic acid formation.
Highlights
In the past few decades the consequences of anthropogenic carbon dioxide accumulation in the atmosphere have been addressed repeatedly
With online HPLC during voltammetry it was confirmed that no formic acid or other liquid products are produced at any potential on pristine pyrolytic graphite (PG) nor on PP-PG
The other MPPs show higher currents, indicating that the metalloprotoporphyrins are active for the hydrogen evolution reaction (HER) or CO2 reduction, while PP presumably blocks active sites for HER and CO2 reduction on PG
Summary
In the past few decades the consequences of anthropogenic carbon dioxide accumulation in the atmosphere have been addressed repeatedly. Molecular catalysts have been gaining more attention lately as they are relatively inexpensive and more abundantly available compared to (noble) metal catalysts They usually show high activities and good selectivities for various reactions which can be tuned by modifying the catalyst with additional ligands, using electron-donating or electron-withdrawing groups [10,11,12,13]. Savéant and Robert and coworkers have conducted extensive research on molecular catalysts for CO2 reduction and how to influence the selectivity [30,31] They recently showed that CO or HCOOH is produced by changing the metal center of the complex [32]. The selectivity towards formic acid is investigated experimentally where the role of the metal center is scrutinized by studying the pH effect, the concomitant hydrogen evolution and the nature of the electroactive species (i.e., CO2 or HCO−3 ) on different metalloprotoporphyrins immobilized on pyrolytic graphite
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