Metal-Free Catalysis: A Redox-Active Donor–Acceptor Conjugated Microporous Polymer for Selective Visible-Light-Driven CO₂ Reduction to CH₄

Abstract

Achieving more than a two-electron photochemical CO2 reduction process using a metal-free system is quite exciting and challenging, as it needs proper channeling of electrons. In the present study, we report the rational design and synthesis of a redox-active conjugated microporous polymer (CMP), TPA-PQ, by assimilating an electron donor, tris­(4-ethynylphenyl)­amine (TPA), with an acceptor, phenanthraquinone (PQ). The TPA-PQ shows intramolecular charge-transfer (ICT)-assisted catalytic activity for visible-light-driven photoreduction of CO2 to CH4 (yield = 32.2 mmol g–1) with an impressive rate (2.15 mmol h–1 g–1) and high selectivity (>97%). Mechanistic analysis based on experimental results, in situ DRIFTS, and computational studies reveals that the potential of TPA-PQ for catalyzing photoreduction of CO2 to CH4 was energetically driven by photoactivated ICT upon surface adsorption of CO2, wherein adjacent keto groups of PQ unit play a pivotal role. The critical role of ICT for stimulating photocatalysis is further illustrated by synthesizing another redox-active CMP (TEB-PQ), bearing triethynylbenzene (TEB) and PQ, that shows 8-fold lesser activity for photoreduction toward CO2 to CH4 (yield = 4.4 mmol g–1) as compared to TPA-PQ. The results demonstrate a novel concept for CO2 photoreduction to CH4 using an efficient, sustainable, and recyclable metal-free robust organic photocatalyst.

Boosted Inner Surface Charge Transfer in Perovskite Nanodots@Mesoporous Titania Frameworks for Efficient and Selective Photocatalytic CO2 Reduction to Methane

“…Moreover, the CO 2 adsorption energies (Eads) of various active sites on CBB and CABB surfaces indicate that Bi has the highest adsorption energy (0.936 eV and 0.771 eV, respectively), favoring adsorption after accumulation in the mesopores (Figures 5c, S40, and S41). The relative Gibbs free energy (Δ G ) diagrams and corresponding models reveal the CO 2 photoreduction pathways over CBB@M‐Ti and CABB@M‐Ti, considering the reported reaction intermediates (Figures 5d, S42, and S43) [36, 37] . CABB@M‐Ti showed a stronger adsorption capacity owing to a more negative Gibbs free energy (Δ G =−0.59 eV) than CBB@M‐Ti (Δ G =−0.38 eV) during the adsorption of CO 2 , which is consistent with the photocatalytic CO 2 reduction results.…”

“…The relative Gibbs free energy (ΔG) diagrams and corresponding models reveal the CO 2 photoreduction pathways over CBB@M-Ti and CABB@M-Ti, considering the reported reaction intermediates (Figures 5d, S42, and S43). [36,37] CABB@M-Ti showed a stronger adsorption capacity owing to a more negative Gibbs free energy (ΔG = À 0.59 eV) than CBB@M-Ti (ΔG = À 0.38 eV) during the adsorption of CO 2 , which is consistent with the photocatalytic CO 2 reduction results. The formation of *COOH from *CO 2 is a shared rate-limiting step for the photoreduction of CO 2 to CO or CH 4 over CBB@M-Ti and CABB@M-Ti, which requires 1.54 and 1.35 eV, respectively.…”

Organic frameworks confined Cu single atoms and nanoclusters for tandem electrocatalytic CO2 reduction to methane

“…Since the electrochemical conversion of CO 2 into various carbon products usually requires multiple proton‐coupled electron transfer steps through different pathways depending on the kinetics of the electron transfer process, the dissociated protons and key intermediates are essential to determine the final products 17,18 . As one of the highly reduced carbon products, CH 4 is the main component of conventional fuels with the cleanest burning emission in modern energetics 19,20 . Nevertheless, the production of CH 4 involves an eight‐electron reduction process together with proton transfers 21 .…”

“…17,18 As one of the highly reduced carbon products, CH 4 is the main component of conventional fuels with the cleanest burning emission in modern energetics. 19,20 Nevertheless, the production of CH 4 involves an eight-electron reduction process together with proton transfers. 21 The activation of H 2 O, the proton source for producing hydrocarbons in CO 2 RR, plays a significant role.…”

Novel Cerium‐Based Sulfide Nano‐Photocatalyst for Highly Efficient CO2Reduction

“…Recently, CO 2 photoreduction into high‐value chemicals has aroused widespread attention as a promising strategy to alleviate the energy shortage and air pollution problems. [ 1–4 ] Therefore, many efforts have been devoted to investigating effective photocatalysts for CO 2 reduction, for which several types of photocatalysts including metal oxides, [ 5–7 ] carbon nitrides, [ 8–10 ] and sulfides [ 11,12 ] have proved suitable. For example, Collado et al.…”