New organic photocatalyst achieves highly efficient and selective CO2 conversion

The ongoing global energy crisis is exacerbated by the relentless consumption of fossil fuels during the rapid expansion of modern industries. This relentless consumption is also severely impacting the survival and growth of humanity due to the emission of greenhouse gases.

As we’re aware, natural photosynthesis has the ability to convert solar energy into sustainable power. This has sparked significant interest in a carbon-neutral strategy that harnesses sunlight to convert carbon dioxide into fuels.

The effectiveness of this photosynthesis-inspired approach relies heavily on the design of photocatalysts. Inorganic semiconductors have been extensively studied for the CO2 photoreduction reaction (CO2PRR), but they come with their limitations. Metal oxides and metal chalcogenides, for instance, have fixed band gaps, leading to suboptimal light absorption. Furthermore, they suffer from high carrier recombination rates and a low specific surface area, both of which hinder photocatalytic efficiency and limit photon utilization.

Photocatalysts based on porous organic frameworks (POFs) have garnered immense interest due to their ability to adjust bandgaps and charge separation through designable building blocks. However, many previously reported POFs still depend on metal reactive sites, necessitating the use of photosensitizers and sacrificial reagents to boost their CO2PRR performance.

These approaches are clearly uneconomical. CO2 photoreduction tends to yield mixed products in varying ratios due to differences in the number of electrons and protons reacting with CO2. The quantity and purity of the desired product are generally low due to incomplete conversion, a challenge stemming from CO2’s high chemical inertness. Achieving highly efficient and selective CO2 conversion has proven to be a formidable task.

A team of researchers, led by Prof. Yong Yang from Nanjing University of Science and Technology and Prof. Yong Zhou from Nanjing University in China, has now reported a series of three-dimensional porous aromatic frameworks derived from Tröger’s base (3D-X-TB-PAFs, where X can be TEPE, TEPM, or SPF). These frameworks feature specific reaction sites and unique charge transfer properties, as detailed in the Chinese Journal of Catalysis.

The inclusion of V-shaped Tröger’s base (TB) units and aromatic alkynes imparts permanent porosity to these polymers, enhancing photon scattering cross-sections and improving CO2 adsorption and activation capabilities. Both density functional theory (DFT) calculations and optoelectronic measurements confirm the development of intramolecular built-in polarization and electron-trap sites facilitated by TB. These features modulate charge separation and customize reaction sites, working in synergy with enhanced CO2 accumulation.

Moreover, this framework regulates product distribution in CO2 photoreduction alongside H2O photooxidation. Among the 3D-PAFs, TEPE-TB-PAF constructs the most efficient electron transport channel due to its full conjugated TEPE-T structure.

Remarkably, TEPE-TB-PAF achieves a competitive CO formation rate (194.50 μmol g-1 h-1) with near-complete selectivity (99.74%) without the need for co-catalysts or sacrificial agents. The low energy barrier for CO desorption and high energy barrier for *CHO formation significantly contribute to the efficiency of TEPE-TB-PAF, as confirmed by computational investigations and in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectra.

This breakthrough not only provides efficient building blocks for synthesizing multifunctional organic photocatalysts but also offers a revolutionary perspective on enhancing both photocatalytic reactivity and selectivity simultaneously.

Source: Chinese Academy of Sciences

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