In situ X‐ray Absorption Spectroscopy in Homogeneous Conditions Reveals Interactions Between CO₂ and a Doubly and Triply Reduced Iron(III) Porphyrin, then Leading to Catalysis

Abstract

Iron porphyrins are attractive catalysts for the electrochemical reduction of carbon dioxide (CO2), owing to their high activity and selectivity while being tunable through ligand functionalization. Iron tetraphenyl porphyrin (FeTPP) is the simplest of them, and its catalytic behavior toward CO2 has been studied for decades. Although kinetic information is available, spectroscopic signatures are lacking to describe intermediate species along the catalytic cycle. In situ UV‐Visible and X‐ray absorption near edge spectroscopy (XANES) were used to monitor the local and electronic structure of FeTPP homogeneously dissolved in dimethyl formamide (DMF) under reductive potentials. The Fe(III) starting species was identified, together with its one, two and three electron‐reduced counterparts under both argon and CO2 atmospheres. Under argon, the second and third reductions lead to species with electronic density shared between the metal and the porphyrin backbone. In the presence of CO2 and with a low amount of protons, the doubly and triply reduced species interact with CO2 at the metallic site. In light of these results, an electronic structure for a key intermediate along the catalytic cycle of the CO2‐to‐CO reduction reaction is proposed.

Time‐Resolved Mechanistic Depiction of Photoinduced CO2 Reduction Catalysis on a Urea‐Modified Iron Porphyrin

“…These considerations are consistent with CO 2 activation and reduction by the UrFe porphyrin in its formal Fe I state. By employing X‐Ray Absorption Near Edge Spectroscopy (XANES), a recent study has provided clues that an interaction between the doubly‐reduced Fe I TPP and CO 2 is possible [46] . In another study, the formation of a formal [Fe I ‐CO 2 ] adduct has been suggested to be the rate‐determining step in the photocatalytic CO 2 ‐to‐CO conversion when UrFe is used as a catalyst, with BIH as a stronger and irreversible electron donor [28] .…”

Controlled Potential Electrolysis: Transition from Fast to Slow Regimes in Homogeneous Molecular Catalysis. Application to the Electroreduction of CO2 Catalyzed by Iron Porphyrin

“…The active form of the catalyst seems to be Fe(0)TPP. Recently, it was reported that the triply reduced Fe(III)TPP, formally Fe(0)TPP, forms an adduct with CO 2 and that this adduct is the active form of the catalyst [32] . Intriguingly, the UV‐vis spectrum recorded under CO 2 at a potential corresponding to catalysis (−1.72 V vs. SCE) and attributed to this adduct has the same features as the doubly reduced Fe(III)TPP, i. e. formally Fe(I)TPP.…”

“…Recently, it was reported that the triply reduced Fe(III)TPP, formally Fe(0)TPP, forms an adduct with CO 2 and that this adduct is the active form of the catalyst. [32] Intriguingly, the UV-vis spectrum recorded under CO 2 at a potential corresponding to catalysis (À 1.72 V vs. SCE) and attributed to this adduct has the same features as the doubly reduced Fe(III)TPP, i. e. formally Fe(I)TPP. In light of the present analysis, we alternatively propose that the detected species, both in UV-vis and XAS, is actually Fe(I)TPP.…”

Instability of Cobalt-Substituted Polyoxometalates during the Oxygen Evolution Reaction: An Operando X-ray Absorption Spectroscopy Study

“…22 All data were normalized to the intensity of the incident beam. The spectroelectrochemical cell, described elsewhere 23 and shown in Figure S6, is composed of a 0.06 mm × 6 mm × 2 mm GC working electrode, a Pt plate CE, and a silver wire floating RE. Electrodes were carefully washed by successive sonication in acetone, absolute ethanol, and water, 5 min each.…”