Increasing the CO2 Reduction Activity of Cobalt Phthalocyanine by Modulating the σ-donor Strength of Axially Coordinating Ligands

Abstract

<p>Axial coordination of a pyridyl moieties to CoPc (either exogenous or within poly-4-vinylpyridine polymer) dramatically increases the complex’s activity for CO<sub>2</sub>RR. It has been hypothesized that axial coordination to the Co active site leads to an increase in the Co dz<sup>2</sup> orbital energy, which increases the complex’s nucleophilicity and facilitates CO<sub>2</sub> coordination compared to the parent CoPc complex. The magnitude of the energy increase in the Co dz<sup>2</sup> orbital should depend on the σ-donor strength of the axial ligand—a stronger σ-donating ligand (L) will increase the overall CO<sub>2</sub>RR activity of axially coordinated CoPc(L) and vice versa. To test this, we have studied a series of CoPc(L) complexes where the σ-donor strength of L is varied. We show that CoPc(L) reduces CO<sub>2</sub> with an increased activity as the σ-donor ability of L is increased. These observed electrochemical activity trends are correlated with computationally-derived CO<sub>2</sub> binding energy and charge transfer terms as a function of σ-donor strength. The findings of this study supports our hypothesis that the increased CO<sub>2</sub>RR activity observed upon axial coordination to CoPc is due to the increased energy of the dz<sup>2</sup> orbital, and highlight an important design consideration for macrocyclic MN<sub>4</sub>-based electrocatalysts.</p><p> </p><p> </p>

The Influence of pH and Electrolyte Concentration on Fractional Protonation and CO2 Reduction Activity in Polymer-Encapsulated Cobalt Phthalocyanine

“…Porphyrins and phthalocyanines have been studied as molecular electrocatalysts for the CO 2 RR, with cobalt phthalocyanine (CoPc) being a particularly well-studied catalyst due to its unique electron configuration and favorable binding energy to CO 2 . − CoPc easily adsorbs to carbon electrode surfaces and has been an important model system to study the mechanism of the CO 2 RR , as well as one of the rare molecular catalysts able to reduce CO 2 to methanol under specific reaction conditions. , Our particular interest in CoPc has focused on improving the selectivity of CoPc-catalyzed CO 2 RR over the competing HER by encapsulating the parent complex within coordinating polymers to control the local microenvironment. Our group and others , have shown that encapsulating CoPc within poly­(4-vinylpyridine) (P4VP) results in increased reaction activity and selectivity for the CO 2 RR over the HER, leading to an increase in Faradaic efficiency for CO production from 60% for CoPc to >90% for CoPc-P4VP and a four-fold increase in turnover frequency for CO production. ,, …”