Heterogeneous electrochemical CO₂ reduction has potential advantages with respect to the homogeneous counterpart due to the easier recovery of products and catalysts, the relatively small amounts of catalyst necessary for efficient electrolysis, the longer lifetime of the catalysts, and the elimination of solubility problems. Unfortunately, several disadvantages are also present, including the difficulty of designing the optimized and best-performing catalysts by the appropriate choice of the ligands as well as a larger heterogeneity in the nature of the catalytic site that introduces differences in the mechanistic pathway and in electrogenerated products. The advantages of homogeneous and heterogeneous systems can be preserved by anchoring intact organometallic molecules on the electrode surface with the aim of increasing the dispersion of active components at a molecular level and facilitating the electron transfer to the electrocatalyst. Electrode functionalization can be obtained by non-covalent or covalent interactions and by direct electropolymerization on the electrode surface. A critical overview covering the very recent literature on CO₂ electroreduction by intact organometallic complexes attached to the electrode is summarized herein, and particular attention is given to their catalytic performances. We hope this mini review can provide new insights into the development of more efficient CO₂ electrocatalysts for real-life applications.

This study aimed to demonstrate the behavior of different complexes using IR spectroelectrochemistry (SEC), a technique that combines IR spectroscopy with electrochemistry. Four different Mn and Re catalysts for electrochemical CO₂ reduction were studied in dry acetonitrile. In the case of Mn(apbpy)(CO)₃Br (apbpy = 4(4-aminophenyl)-2,2′-bipyridine), SEC suggested that a very slow catalytic reduction of CO₂ also occurs in acetonitrile in the absence of proton donors, but at rather negative potentials. In contrast, the corresponding Re(apbpy)(CO)₃Br clearly demonstrated slow catalytic conversion at the first reduction potential. Switching to saturated CO₂ solutions in a mixture of acetonitrile and 5% water as a proton donor, the SEC of Mn(apbpy)(CO)₃Br displayed a faster catalytic behavior.

Four different Mn and Re catalysts for electrochemical CO2 reduction have been studied by IR spectroelectrochemistry (SEC) in dry acetonitrile. In the case of Mn(apbpy)(CO)3Br (apbpy= 4(4-aminophenyl)-2,2’-bipyridine) SEC suggests that a very slow catalytic reduction of CO2 occurs in acetonitrile also in the absence of proton donors, but at rather negative potentials. On the contrary, the corresponding Re(apbpy)(CO)3Br clearly displays a slow catalytic conversion already at the first reduction potential. Switching to saturated CO2 solutions in a mixture of acetonitrile and 5% of water as proton donor, the SEC of Mn(apbpy)(CO)3Br displays faster catalytic behavior.

We developed a new electrocatalysts for hybrid homogeneous / heterogeneous conversion of CO2 in water. A Molybdenum-based organometallic compound has been linked by covalent C-C bond to a carbon-cloth electrode surface. This technique allows using the Mo catalyst in water, a solvent in which the catalyst is not soluble, for the direct electrocatalytic conversion of CO2 to methanol. Although the system displays moderate current density (1÷4 mA/cm2), the very high faradaic efficiency of the CO2-to-methanol conversion (65%), the good selectivity (H2 is the only other reduction product), the simply experimental setup and the potential industrial scale-up of the technique makes this approach of great interest. Chronoamperometric measurement carried in water for 21 hours show a stable and constant catalytic performance with no other carbon-reduced products. Spectroelectrochemistry and detailed DFT calculations shed light on the catalytic process, which consists of six protons coupled electron transfer (PCET) steps. We propose the catalytic cycle that converts CO to methanol in which the first step is a PCET localized on the CO coordinated to the metal.