An Iron Quaterpyridine Complex as Precursor for the Electrocatalytic Reduction of CO₂ to Methane

Abstract

A Fe quaterpyridine complex was used as a molecular precursor for the electrochemical reduction of CO2 to CH4 in acetonitrile in the presence of triethanolamine. CH4 was produced with a faradaic yield of approximately 2.1 % at 25 °C and 1 atm pressure of CO2 as reactant. Controlled potential electrolysis coupled to ex situ X‐ray photoelectron spectroscopy and X‐ray absorption spectroscopy of the electrode surface revealed the formation of metallic iron covered by iron oxides as species responsible for catalysis.

Electrochemical Conversion of CO2 to CO by a Competent FeI Intermediate Bearing a Schiff Base Ligand

“…Interestingly, methane was also observed as electrolysis product, [66] and confirmed to be electrogenerated from carbon dioxide from a labelling experiment with 13 CO 2 ( Figure S21), with a faradaic efficiency of 1-2 %; this value corresponds to a partial current density for methane generation in the range 6-12 μA cm À 2 , slightly inferior to the one obtained in the case of Fe nanoparticles electrodeposited from Fe(qpy), μA cm À 2 . [66] The yield and selectivity of methane generation likely depend on nanoparticles size, shape and composition. [66]…”

“…Ex-situ X-Ray Photoemission Spectroscopy (XPS) showed peaks at binding energies of 726 and 711 eV associated to Fe 2p 3/2 and Fe 2p 1/2 transitions, typical of iron oxide formed upon air exposure of the electrode ( Figure S20 in Supporting Information). [66] The electrodeposition of the nanoparticles could likely originate from ligand demetalation upon a further reduction of Fe I to Fe 0 intermediates, as observed in the case of the Fe(qpy) catalyst. [38,66] The direct utilization of the unpolished glassy carbon electrode in a CPE experiment, conducted in the absence of Fe complexes in solution, led to immediate, continuous and stable production of H 2 (current density = 0.6 mA cm À 2 , faradaic efficiency in the range 65-80 %), while no CO was detected.…”

“…[38,66] The direct utilization of the unpolished glassy carbon electrode in a CPE experiment, conducted in the absence of Fe complexes in solution, led to immediate, continuous and stable production of H 2 (current density = 0.6 mA cm À 2 , faradaic efficiency in the range 65-80 %), while no CO was detected. Interestingly, methane was also observed as electrolysis product, [66] and confirmed to be electrogenerated from carbon dioxide from a labelling experiment with 13 CO 2 ( Figure S21), with a faradaic efficiency of 1-2 %; this value corresponds to a partial current density for methane generation in the range 6-12 μA cm À 2 , slightly inferior to the one obtained in the case of Fe nanoparticles electrodeposited from Fe(qpy), μA cm À 2 . [66] The yield and selectivity of methane generation likely depend on nanoparticles size, shape and composition.…”

“…[66] The yield and selectivity of methane generation likely depend on nanoparticles size, shape and composition. [66]…”

“…The former is carried out by the molecular Fe I salophen intermediate and leads to selective CO 2 reduction to CO (see proposed pathway in Scheme 3). The latter is driven by the heterogeneous iron nanoparticles formed upon electrodeposition from the molecular precursor, [66] and is mainly oriented towards proton reduction to H 2 with formation of CH 4 as a minor product (Scheme 1).…”