Photocatalytic CO₂ reduction reactions are mostly based on the ultraviolet and visible light regions of the solar spectrum, which constitute only about 48% of the total solar spectrum energy. The infrared light region accounts for approximately 52% of the total energy. While the energy in the infrared region is significant, its utilization is challenging, leading to low catalytic efficiency and low solar energy utilization. These are two major limiting factors restricting the further development of photocatalytic CO₂ reduction reactions.
To fully utilize the photothermal effect in the infrared region, raising the temperature of the catalyst's surface, it is necessary to perform in-situ coupling of photochemical reactions and photothermal effects to achieve full-spectrum photothermal catalysis and make optimal use of solar energy.
Light in the solar spectrum can address the issues of pollution and high energy consumption in thermal catalysis, lowering the activation energy barrier for reactions and overcoming thermodynamic limitations.
Heat in the solar spectrum can address the bottleneck issues of low catalytic efficiency and limited energy-mass transfer in photocatalysis. It provides energy for the activation and cleavage of CO₂, breaking through kinetic bottlenecks. Additionally, the synergistic effect of light and heat can significantly promote catalytic activity.
Currently, the bottleneck issues in photothermal catalysis include:
1. Insufficient absorption of the full spectrum energy of sunlight; most photothermal catalysts have a low absorption rate for the full spectrum.
2. Low efficiency of photothermal conversion, limiting the increase in photothermal temperature.
3. Insufficient exposure of active sites on photothermal catalysts.
4. Poor carrier migration and conductivity of photothermal catalysts.
Well-performing photothermal catalysts exhibit excellent absorption properties across the full solar spectrum. The heat generated by photothermal materials, known as the "thermal island effect," can rapidly transfer to the surface-interface and active sites of the photothermal catalyst, enhancing charge transfer and increasing the reaction rate, facilitating material diffusion at reaction sites, and energy-mass transfer.
The photothermal CO₂ reduction reaction is a complex chemical process. The reaction principles vary in practical reactions due to differences in catalyst performance, reaction temperature, and pressure.
This article mainly introduces three basic reaction principles:
1. Photo-Driven Thermal Catalysis
Photo-driven thermal catalysis uses sunlight as the sole heat source. Photothermal catalysts are illuminated to achieve the reaction temperature for reducing CO₂. Essentially, it is still thermally driven, but the advantage lies in using solar energy instead of high-energy-consuming external heat sources. It localizes the temperature increase to the photothermal catalyst's specific area without raising the temperature of the entire reaction system. This photothermal catalysis process involves the coupling and conversion of photons, electrons, and phonons, and can be divided into photothermal effects and localized surface plasmon resonance (LSPR) effects.
Figure 1: LSPR effect achieving photothermal catalytic CO₂ reduction[1]
2. Photothermal Chemical Cycle
The photothermal chemical cycle for CO₂ reduction utilizes ultraviolet light in sunlight to generate photoinduced oxygen vacancies (Vo) on the surface of the photothermal catalyst. In another half-cycle, visible-infrared light is used to raise the reaction temperature, achieving the thermal activation and cracking of CO₂ through Vo. The advantage of the photothermal chemical cycle is the realization of cascaded utilization* of high-quality and low-quality spectra in sunlight.
Figure 2: Principle of photothermal chemical cycle catalyzing CO₂ reduction[2]
3. Photothermal In-Situ Cooperative Catalysis
Photothermal in-situ cooperative CO₂ reduction involves simultaneous reactions of light and heat at catalytic sites. The electron and reaction groups between light reactions and thermal effects are coupled, facilitating in-situ cooperative actions of photocatalysis and thermal catalysis within the same reaction time and space. It can be divided into two types: light-assisted thermal catalysis and thermal-assisted photocatalysis. In light-assisted thermal catalysis, light can induce dynamic vacancies to become reaction sites, while in thermal-assisted photocatalysis, heat promotes the transition and rapid transfer of photogenerated electrons. Heat provides energy for the activation and cleavage of CO₂ and enhances heat and mass transfer processes, promoting reaction rates.
The advantages of photothermal in-situ cooperative catalysis are overcoming the bottleneck issues of low catalytic efficiency and high energy consumption in photocatalysis and thermal catalysis.
The heat source for photothermal in-situ cooperative catalysis can be divided into external heat source heating and solar light heating. External heat source heating has the advantages of precise and flexible temperature control and can utilize the waste heat from treating industrial exhaust gases as an external heat source. Solar light heating can achieve stepped utilization of the solar spectrum, utilizing ultraviolet-visible light for photocatalytic reactions and using infrared light for in-situ photothermal effects to raise the surface temperature of photothermal catalysts, achieving in-situ coupling of light reactions and thermal effects.
Figure 3: Schematic diagram of the basic principles of photothermal CO₂ reduction
perfectlight's PLR-RP series photothermal catalytic reaction evaluation device is a system designed to study photothermal catalytic reactions. Its innovative quartz column light-guiding method and reactor design significantly improve the irradiation efficiency of the light source and the absorbing surface area of the catalyst, meeting the requirements of gas-solid phase reactions under in-situ photothermal cooperation.
The unique innovative ambient irradiation reactor fills the catalyst around the light source, effectively increasing the light-receiving area of the catalyst from the flat irradiation of 0.3 cm² to approximately 20 cm². The catalyst load capacity increases from 0.9 mL to 9 mL, effectively improving light utilization efficiency and substrate adsorption, enhancing conversion rates while maintaining effective light penetration. This provides new ideas for the realization of industrial photothermal reaction systems.
It is believed that with the help of advanced equipment and the exploration of researchers, photothermal CO₂ reduction technology will become one of the key technologies for mitigating climate change in the future, contributing to environmental protection and sustainable development. At the same time, it will play an important role in achieving global carbon neutrality goals.
*Cascaded utilization: Reusing waste or by-products generated in one system or process in another system or process to reduce resource waste and improve efficiency.
Regarding the interpretation of the literature, it is based on the translation and summarization by the author from the referenced literature. The author's proficiency is limited, and corrections are welcome if there are any errors!
Article Information
Wang, Z.; Yang, Z.*, Kadirova, Z. C.; Guo, M.; Fang, R.; He, J.; Yan, Y.; Ran, J., Photothermal functional material and structure for photothermal catalytic CO₂ reduction: Recent advance, application and prospect. Coordination Chemistry Reviews 2022, 473, 214794.
References
[1] Ghoussoub, M.; Xia, M.; Duchesne, P. N.; Segal, D.; Ozin, G., Principles of photothermal gas-phase heterogeneous CO₂ catalysis. Energy & Environmental Science 2019, 12 (4), 1122-1142.
[2] Chenyu X., Wenhui H., Zheng L., Bowen D., Yanwei Z., Mingjiang N., and Kefa C.; Photothermal Coupling Factor Achieving CO₂ Reduction Based on Palladium-Nanoparticle-Loaded TiO₂. ACS Catalysis 2018 8 (7), 6582-6593.
