Sorbent Mediated Electrocatalytic Reduction of Dilute CO2 to Methane

Abstract

Efficient CO₂ utilization is a critical component of closing the anthropogenic carbon cycle. Most studies have focused on the use of pure streams of CO₂. However, CO₂ is generally available only in dilute streams, which requires capture by sorbents followed by energy-intensive regeneration to release concentrated CO₂. Direct utilization of sorbed-CO₂ avoids the costly regeneration step, and the sorbent-CO₂ interaction can kinetically activate CO₂ to tune its reactivity toward products that could otherwise be inaccessible with direct CO₂ reduction. We demonstrate that an N-heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (DPIy), quantitatively reacts with CO₂ from dilute streams (0.04 and 10%) to form the sorbent-CO₂ substrate 1,3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate (DPICx). Electrocatalyst iron tetraphenylporphyrin chloride (Fe(TPP)Cl) typically reduces CO₂ to CO; however, with DPICx as the substrate, the eight-electron reduced product methane (CH₄) is produced with a high Faradaic efficiency (>85%) and regeneration of the sorbent DPIy. In addition to the overall energy and capital advantages of integrated CO₂ capture and conversion, this result illustrates how sorbents can serve a dual purpose for both CO₂ capture and chemical auxiliary purposes to access unique products. CO₂ has a spectrum of reactivity with different types of sorbents; thus, these studies demonstrate how sorbent-CO₂ interactions can be leveraged for integrated capture and utilization platforms to access a wider range of CO₂-derived products.

Chart 1. Structures of Fe(TPP)Cl, DPIy, and DPICx

Figure 1

Controlled potential electrolysis (CPE) of 20 mM DPICx (black) with 1 mM Fe(TPP)Cl (blue) and 60 mM H₂O at −2.35 V vs Fe(C₅H₅)₂^+/0. CPE experiments were performed in THF with 0.1 M n-Bu₄NPF₆ on carbon cloth electrodes.

Figure 2

UV-visible spectra of 1 mM Fe(TPP)^2– with 20 mM DPICx and 60 mM H₂O postelectrolysis (−2.35 V vs Fe(C₅H₅)₂^+/0, 4 h) (black) diluted 1:10 with THF and generated spectroelectrochemically (100 μM Fe(TPP)Cl, −2.35 V vs Fe(C₅H₅)₂^+/0) (blue). The features below 300 nm in the black trace correspond to DPIy and unreacted DPICx.

Figure 3

Potential intermediates in the reduction of DPICx to formate catalyzed by Fe(TPP)^2–. The total spin and approximate localization of unpaired electrons are denoted by red arrows. Diisopropylphenyl groups have been omitted for clarity. Formate is the only experimentally observed C1 intermediate prior to the product CH₄ and is only observed by ¹³C NMR when ¹³C-labeled DPICx is used.

Scheme 1. Potential Substrate Intermediates, Assuming That the Sorbent-CO₂ Bond Is Retained Throughout Reduction (Top) or Cleaved Early to Form the Product (Bottom)

The gray CO₂ intermediate is not expected to be present because of the strong binding to NHC and because it is never detected under experimental conditions. All black and green intermediates have been tested as substrates under catalytic conditions except DPI-CHO, which could not be isolated (products and Faradaic efficiencies are shown in Table 1). The green intermediate is also detected under catalytic conditions.