Molecular Approaches to the Mechanism of CO2 Reduction to Methanol by Cobalt Phthalocyanine

The electrochemical reduction of CO2 has received significant scientific interest over the past two decades as a key step in the synthesis of CO2 into combustible fuels. Cobalt phthalocyanine (CoPc) adsorbed on carbon nanotubes has risen as a rare electrocatalyst that reduces CO2 beyond two electrons, specifically to methanol. Recent efforts have improved the efficiency of this process and established that methanol production proceeds by reduction of a free CO intermediate, however, many aspects of the catalytic pathway remain unclear, which may underpin persistent issues of stability and selectivity. Here we employ electrochemistry, spectroelectrochemistry, X-ray adsorption spectroscopy (XAS), synthesis of catalytic intermediates, and density functional theory (DFT) calculations to understand the catalytic mechanism and identify distinguishing molecular features. We find that CoPc undergoes three sequential reductions between 0 and -2.5 vs NHE; the first reduction occurring at the Co center, and the second two reductions populating the Pc ring. XAS and theory show the Pc ligand to be noninnocent, interacting with the electronic structure of the Co center and causing Co to adopt a Co(II) state as the complex undergoes the second and third reductions. Consistent with previous studies, [CoPc]2- is found to be active for CO2 reduction, however, we find that the catalyst must be reduced further to [CoPc]3- to bind CO at the Co site. Carbonylation of [CoPc]3- under aprotic conditions leads to fast decomposition, forming [Co(CO)4]-. However, we show that [CoPc]3-, if ring protonated by weak acid to form [CoPc-H]2-, reacts with CO to generate a formyl without decomposition, the first step in methanol synthesis. DFT calculations of the mechanism indicate that the reaction of [CoPc-H]2- with CO to generate the formyl in solution proceeds by an intermolecular proton transfer between the Pc ring of [CoPc-H]2- and the Co-CO of [Co-COPc-H]2-. Overall, this study points to the importance of the Pc ligand in determining the catalytic behavior of CoPc and provides the first examples of several isolated reduced CoPc complexes and catalytic intermediates that we hope will guide future work.