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

Abstract

Molecular catalysis of electrochemical reactions is a field of intense activity because of the current interest in electrifying chemical transformations, including both electrosynthesis of organic molecules and production of fuels via small molecule activation. Controlled potential electrolysis (CPE) is often coupled with in situ in operando spectroscopic methods with the aim to gather mechanistic information regarding the catalytic species involved. Herein, considering a simple mechanism for a homogeneous molecular catalysis of an electrochemical reaction, we establish the concentration profile of the catalyst in the electrolysis cell enabling to envision the information that can be obtained from the coupling of this CPE with a spectroscopic probe in the cell compartment. We show how the characteristic parameters of the system (catalytic rate constant, cell dimensions and stirring rate) affect the response with particular emphasis on the transition between two limiting cases, namely a ‘fast’ catalysis regime where catalysis only takes place in a small layer adjacent to the electrode surface and a ‘slow’ catalysis regime where catalysis takes place in the bulk of the solution. These formal concepts are then illustrated with an experimental example, the electroreduction of CO2 in dimethylformamide homogeneously catalyzed by iron tetraphenylporphyrin and followed by UV‐vis spectroscopy.

Redox Behavior and Kinetics of Hydroxo Ligand Exchange on Iron Tetraphenylporphyrin: Comparison with Chloro Exchange and Consequences for Its Role in Self-Modulation of Molecular Catalysis of Electrochemical Reactions

“…Fast catalysis corresponds in CV to the condition λ CV = k cat F v / R T > 1 with k cat = k 1 C substrate 0 and v the scan rate . In CPE, fast catalysis corresponds to the condition λ CPE = k cat D / δ 2 > 1 where D and δ are respectively the diffusion coefficient of TPPFe and the size of the diffusion layer in CPE, set by the stirring rate of the solution . Because the rate constants for hydroxide coordination to and decoordination from TPPFe(II) are relatively slow ( k f II = 25 M –1 s –1 and k b II = 0.04 s –1 ), we restrict our analysis to a situation where catalysis is fast (see above assumptions) and occurs in a thin diffusion-layer close to the electrode surface generating hydroxide ions that interfere with TPPFe(II) in the diffusion layer as well as in the bulk of the solution (in the case of CPE).…”

“…where D and δ are respectively the diffusion coefficient of TPPFe and the size of the diffusion layer in CPE, set by the stirring rate of the solution. 19 Because the rate constants for hydroxide coordination to and decoordination from TPPFe(II) are relatively slow (k f II = 25 M −1 s −1 and k b II = 0.04 s −1 ), we restrict our analysis to a situation where catalysis is fast (see above assumptions) and occurs in a thin diffusion-layer close to the electrode surface generating hydroxide ions that interfere with TPPFe(II) in the diffusion layer as well as in the bulk of the solution (in the case of CPE). In that context, as shown in the SI, considering first CV, the system is controlled by two dimensionless parameters…”