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>

Enhancing the Electrochemical CO2 Reduction Activity of Polymer-Encapsulated Cobalt Phthalocyanine Films by Modulating the Loading of Catalysts, Polymers, and Carbon Supports

“…In our prior work in this area, we immobilized CoPc−P4VP and related systems onto edge-plane graphite (EPG) electrodes to form CoPc−P4VP/EPG materials. 13,36,44,47 In these previous studies, we sought to understand the microenvironment surrounding the polymer-encapsulated CoPc site. EPGs were an optimal choice of electrode because the high surface area of the electrode surface and intercalation of the CoPc catalyst between the graphitic sheets facilitated charge transport to the CoPc active sites.…”

“…CoPc(pyNMe 2 )−P2VP/GP suspensions were prepared via sonication of 0.0052 g of CoPc and 1.0995 g of 4-(dimethylamino)pyridine in 45 mL of DMF for 1 h in an aluminum foil-jacketed polypropylene conical centrifuge tube (Basix) to form a 0.2 mM CoPc/0.2 M pyNMe 2 /DMF solution, ensuring 5coordinate character using a 1000:1 pyNMe 2 :CoPc ratio, as we have done previously. 44 This solution was then diluted to 1 mL of the desired CoPc and pyNMe 2 concentration via the addition of the stock solution to DMF in a 20 mL scintillation vial. A given mass of P2VP was then added to the 20 mL scintillation vial that contained 1 mL of CoPc/DMF in order to form the desired CoPc(pyNMe 2 )−P2VP solution, which was then sonicated for 30 min to ensure the dispersion of P2VP.…”

Considering the Influence of Polymer–Catalyst Interactions on the Chemical Microenvironment of Electrocatalysts for the CO2 Reduction Reaction

“…In a combined experimental and computational study, we showed that increasing the σ-donor ability of the axial ligand (L) results in an increase in the CO 2 RR activity by CoPc(L) (Figure 5). 4 Computational studies confirmed that the strength of the σ-donor ability was correlated with the relative energy of the d z 2 orbitals, that the HOMO of the pertinent reduced [CoPc(L)−CO 2 ] − involves an interaction between the d z 2 orbital and the CO 2 adduct, and that mechanism 1 in Figure 4a is likely the operative mechanism under our CO 2 RR conditions. 4 In our studies of the polymer-encapsulated CoPc systems, we have been unable to isolate the secondary coordination sphere effects from other polymer effects.…”

“…4 Computational studies confirmed that the strength of the σ-donor ability was correlated with the relative energy of the d z 2 orbitals, that the HOMO of the pertinent reduced [CoPc(L)−CO 2 ] − involves an interaction between the d z 2 orbital and the CO 2 adduct, and that mechanism 1 in Figure 4a is likely the operative mechanism under our CO 2 RR conditions. 4 In our studies of the polymer-encapsulated CoPc systems, we have been unable to isolate the secondary coordination sphere effects from other polymer effects. However, the increased activity of CoPc−P4VP and CoPc(py)−P2VP compared to CoPc(py) provide circumstantial support for the role of secondary-coordination sphere effects in enhancing catalytic activity.…”

“…16,17 More recently, it has been reported that CoPc can catalyze the reduction of CO 2 to formic acid and methanol under specific conditions, 18−20 making CoPc one of the only molecular catalysts capable of reducing CO 2 to highly reduced products. In addition, the activity and stability of CoPc-based system can be tuned by modulating the electronic structure of the system through axial coordination 4 or synthetic modification of the Pc ring. 20,21 For these reasons, CoPc one of the most promising and well-studied CoN 4 porphyrinoid catalysts for the CO 2 RR.…”