TY - JOUR
T1 - Resolving deactivation pathways of Co porphyrin-based electrocatalysts for CO2 reduction in aqueous medium
AU - Marianov, Aleksei N.
AU - Kochubei, Alena
AU - Roman, Tanglaw
AU - Conquest, Oliver J.
AU - Stampfl, Catherine
AU - Jiang, Yijiao
PY - 2021/3/19
Y1 - 2021/3/19
N2 - Carbon-supported first-row transition metal complexes drive electroreduction of CO2 to CO in aqueous medium with remarkable activity and selectivity. However, their durability under negative potentials is quite low and the deactivation mechanisms are still not clear. Herein, we present an in-depth mechanistic study on the stability of Co porphyrin-based catalysts during CO2 reduction in an aqueous electrolyte. The mechanisms of the degradation reactions were evaluated for Co tetraphenylporphyrin (CoTPP) using a combination of spectral, electrochemical, and theoretical methods. Our evidence shows that two major pathways contribute to the gradual activity loss. The first route is oxidative and yields the catalytically inactive complex [CoIIITPP]OH. The second pathway is based on reductive carboxylation of the porphyrin ring via transient formation of [Co0TPP]2– and [Co0TPP-CO]2–. The latter reaction disrupts the π-system of the porphyrin structure and leads to the complete disintegration of the macrocyclic core. In contrast to the earlier reports, we found that the direct poisoning by CO, demetallation, and reduction to chlorins play no significant role in the deactivation process. It was further determined that the bulky donating functional groups disfavor the formation of dianionic species and restrict access of CO2 to the vulnerable meso-position of the porphyrin ligand, thus improving the catalyst stability. The effect was found to be especially strong for the −OMe-substituted complex CoTPP-(OMe)8 that shows excellent reusability under overpotentials below 500 mV. In turn, electronegative substituents such as fluorine suppress the activity of the catalyst and provide no advantages in terms of durability.
AB - Carbon-supported first-row transition metal complexes drive electroreduction of CO2 to CO in aqueous medium with remarkable activity and selectivity. However, their durability under negative potentials is quite low and the deactivation mechanisms are still not clear. Herein, we present an in-depth mechanistic study on the stability of Co porphyrin-based catalysts during CO2 reduction in an aqueous electrolyte. The mechanisms of the degradation reactions were evaluated for Co tetraphenylporphyrin (CoTPP) using a combination of spectral, electrochemical, and theoretical methods. Our evidence shows that two major pathways contribute to the gradual activity loss. The first route is oxidative and yields the catalytically inactive complex [CoIIITPP]OH. The second pathway is based on reductive carboxylation of the porphyrin ring via transient formation of [Co0TPP]2– and [Co0TPP-CO]2–. The latter reaction disrupts the π-system of the porphyrin structure and leads to the complete disintegration of the macrocyclic core. In contrast to the earlier reports, we found that the direct poisoning by CO, demetallation, and reduction to chlorins play no significant role in the deactivation process. It was further determined that the bulky donating functional groups disfavor the formation of dianionic species and restrict access of CO2 to the vulnerable meso-position of the porphyrin ligand, thus improving the catalyst stability. The effect was found to be especially strong for the −OMe-substituted complex CoTPP-(OMe)8 that shows excellent reusability under overpotentials below 500 mV. In turn, electronegative substituents such as fluorine suppress the activity of the catalyst and provide no advantages in terms of durability.
KW - porphyrin electrocatalyst
KW - CO₂ reduction
KW - catalyst stability
KW - deactivation pathways
KW - kinetic stabilization
KW - thermodynamic stabilization
KW - CO reduction
UR - http://purl.org/au-research/grants/arc/DP1901013720
UR - http://www.scopus.com/inward/record.url?scp=85103513794&partnerID=8YFLogxK
U2 - 10.1021/acscatal.0c05092
DO - 10.1021/acscatal.0c05092
M3 - Article
SN - 2155-5435
VL - 11
SP - 3715
EP - 3729
JO - ACS Catalysis
JF - ACS Catalysis
IS - 6
ER -