Selective Metal-Free CO₂ Photoreduction in Water Using Porous Nanostructures with Internal Molecular Free Volume

Abstract

The conversion of CO 2 to a sole carbonaceous product using photocatalysis is a sustainable solution for alleviating the increasing levels of CO 2 emissions and reducing our dependence on nonrenewable resources such as fossil fuels. However, developing a photoactive, metal-free catalyst that is highly selective and efficient in the CO 2 reduction reaction (CO 2 RR) without the need for sacrificial agents, cocatalysts, and photosensitizers is challenging. Furthermore, due to the poor solubility of CO 2 in water and the kinetically and thermodynamically favored hydrogen evolution reaction (HER), designing a highly selective photocatalyst is challenging. Here, we propose a molecular engineering approach to design a photoactive polymer with high CO 2 permeability and low water diffusivity, promoting the mass transfer of CO 2 while suppressing HER. We have incorporated a contorted triptycene scaffold with "internal molecular free volume (IMFV)" to enhance gas permeability to the active site by creating molecular channels through the inefficient packing of polymer chains. Additionally, we introduced a pyrene moiety to promote visible-light harvesting capability and charge separation. By leveraging these qualities, the polymer exhibited a high CO generation rate of 77.8 μmol g −1 h −1 , with a high selectivity of ∼98% and good recyclability. The importance of IMFV was highlighted by replacing the contorted triptycene unit with a planar scaffold, which led to a selectivity reversal favoring HER over CO 2 RR in water. In situ electron paramagnetic resonance (EPR), time-resolved photoluminescence spectroscopy (TRPL), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques, further supported by theoretical calculations, were employed to enlighten the mechanistic insight for metal-free CO 2 reduction to exclusively CO in water.

PtCu Alloy-Modified Triptycene-Based Polymer with Charge Separation for Photocatalytic Hydrogen Evolution from Water/Seawater

“…Figure c,d displays the N 2 adsorption–desorption isotherms of TCP and Pt 7 Cu 1 -TCP. The TCP exhibits a type I isotherm with a Brunauer–Emmett–Teller (BET) surface area of 809 m 2 /g, indicating the presence of abundant microporous structures as evidenced by the sharp N 2 gas absorption observed at low relative pressures . However, Pt 7 Cu 1 -TCP exhibits a lower BET surface area of 668 m 2 /g compared to TCP.…”

Superlative Porous Organic Polymers for Photochemical and Electrochemical CO2 Reduction Applications: From Synthesis to Functionality

“…Adequate conduction and valence band positions are fundamental for the effective realization of the POP-driven photochemical CO 2 conversion endeavors as well depicted in Figure 6. Mohata et al 144 recently examined selective metal-less photochemical CO 2 RR activity in aqueous media by utilizing a POP-based photocatalyst. The group proposed the unique molecular regulation pathway to develop a POP photocatalyst showcasing remarkable CO 2 permeability alongside subdued water diffusion that led to the thermodynamic bias toward CO 2 RR over competing hydrogen formation.…”

“…(a) Designing a unique POP-based photocatalytic that exhibits a symbiotic relationship in accelerating the rate of CO 2 RR. Reproduced from ref . Copyright 2023 with permission from ACS.…”

NAD(P)H-Inspired CO2 Reduction Based on Organohydrides

“…Such metal-based CO 2 R reactions were often limited by the susceptivity, environmental impact, and irreversible hydrogen evolution side reaction. In comparison, metal-free catalysts for CO 2 R are stable, cost-effective, and eco-friendly, thereby providing sustainable and tunable alternatives to traditional metal-based catalysts. − …”