Unusual Stability of a Bacteriochlorin Electrocatalyst under Reductive Conditions. A Case Study on CO₂ Conversion to CO

Abstract

Photosynthetic CO2 fixation is mediated by the enzyme RuBisCo, which employs a nonredox-active metal (Mg2+) to bind CO2 adjacent to an organic ligand that provides reducing equivalents for CO2 fixation. Attempts to use porphyrins as ligands in reductive catalysis have typically encountered severe stability issues owing to ligand reduction. Here, a synthetic zinc–bacteriochlorin is reported as an effective and robust electrocatalyst for CO2 reduction to CO with an overpotential of 330 mV, without undergoing porphyrin-like ligand degradation (or demetalation) even after prolonged bulk electrolysis. The reaction has a CO Faradaic efficiency of 92% and sustains a total current density of 2.3 mA/cm2 at −1.9 V vs Ag/AgCl. DFT calculations highlight the molecular origin of the observed stability and provide insights into catalytic steps. This bioinspired study opens avenues for the application of bacteriochlorin compounds for reductive electrocatalysis with extended life beyond that seen with porphyrin counterparts.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

“…Furthermore, Brudvig and co-workers reported a novel zinc(II)-bacteriochlorin complex to improve the stability of CO 2 RCs by taking advantage of the superior stability of the bioinspired bacteriochlorin ligand. 436 Besides the above main class of CO 2 reduction catalysts, there are many other notable reports of CO 2 RCs. 437 For example, Bouwman and co-workers reported in 2010 a binuclear copper catalyst (complex 88) that could produce oxalate by catalyzing the one-electron reduction of CO 2 at a very low overpotential.…”

Transition metal-based catalysts for the electrochemical CO2reduction: from atoms and molecules to nanostructured materials

“…199 It is worth noting that a synthetic Zn bacteriochlorin has been recently reported as a robust catalyst for selective CO production in DMF/H 2 O mixture without any evidence of catalyst degradation. 202 Moreover, a number of extrinsic factors including the nature of the support material, its pre-treatment or the encapsulation of the molecular catalyst in polymer matrices may affect the CO 2 RR selectivity, reactivity and stability of metalloporphyrins. 203 Fig.…”

Insights on forming N,O-coordinated Cu single-atom catalysts for electrochemical reduction CO2 to methane

“…Conventional molecular catalysts as well-defined metal complexes possess acutely tailorable structures 18 , 19 , which prevail over SACs. However, they are typically involved in homogeneous catalysis 20 – 22 , which limits the catalytic performance by outer sphere electron transfer from the electrode in electrochemical catalysis 23 , 24 . To tackle this problem, researchers tried to immobilize metal phthalocyanines on carbon nanotubes to create heterogeneous catalytic surfaces 24 – 26 .…”