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

Abstract

Axial coordination of pyridyl moieties to CoPc (either exogenous or within poly-4-vinylpyridine polymer) dramatically increases the complex’s activity for the CO2 reduction reaction (CO2RR). It has been hypothesized that axial coordination to the Co active site leads to an increase in the Co d z 2 orbital energy, which increases the complex’s nucleophilicity and facilitates CO2 coordination compared to the parent CoPc complex. The magnitude of the energy increase in the Co d z 2 orbital should depend on the σ-donor strength of the axial liganda stronger σ-donating ligand (L) will increase the overall CO2RR 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 CO2 with an increased activity as the σ-donor ability of L is increased. These observed electrochemical activity trends are correlated with computationally derived CO2 binding energy and charge transfer terms as a function of σ-donor strength. The findings of this study support our hypothesis that the increased CO2RR activity observed upon axial coordination to CoPc is due to the increased energy of the d z 2 orbital, and highlight an important design consideration for macrocyclic MN4-based electrocatalysts.

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

“…In both mechanisms, (i) CO 2 binding is the selectivity-determining step between the HER and CO 2 RR pathways and is a possible rate-determining step (RDS); (ii) the subsequent proton-transfer step is also a possible RDS. Adapted with permission from ref . Copyright 2021 American Chemical Society.…”

“…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 ). 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 a is likely the operative mechanism under our CO 2 RR conditions …”

“…M.McCroryC. C. L. Rivera Cruz, K. E. Liu, Y. Soucy, T. L. Zimmerman, P. M. McCrory, C. C. L. 2021111320313216 …”

“…CoPc was first reported for the CO 2 RR in the 1970s and has been shown to operate with moderate activity and reaction selectivity with a Faradaic efficiency for CO production (FE CO ) of approximately 35–60% over the competing HER in aqueous phosphate buffer when coated onto a planar carbon electrode. , More recently, it has been reported that CoPc can catalyze the reduction of CO 2 to formic acid and methanol under specific conditions, − 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 or synthetic modification of the Pc ring. , For these reasons, CoPc one of the most promising and well-studied CoN 4 porphyrinoid catalysts for the CO 2 RR.…”

Atomic‐ and Molecular‐Level Modulation of Dispersed Active Sites for Electrocatalytic CO2Reduction

“…This causes the cobalt centres to gain more nucleophilic character, which are more prone to activate and transfer electron to CO 2 (Figure 8e). However, the degree to which the formation energy of *COOH is stabilised depends on the σ‐donor strength of the axial ligands [75] . Wang and coworkers studied the effect of axial coordination groups on the CoPc/CNT system [54f] .…”