Sources of Error in the Evaluation of Photocatalytic CO₂ Reduction Activity

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In the current stage of research on photocatalytic CO₂ reduction, the reaction rates for target products are still at the level of mmol·h^-1·g^-1. The lower reaction rates make the experimental assessment of photocatalyst activity susceptible to significant interference from external factors. This is due to the light-induced self-decomposition of organic solvents, sacrificial agents, photocatalysts, and sealing materials, leading to deviations in experimental results. The author, based on recent studies, summarizes the sources of error in the evaluation of photocatalytic CO₂ reduction activity in the current stage of research.

Self-Decomposition of Solvents and Sacrificial Agents

The ideal photocatalytic CO₂ reduction reaction involves using water or water vapor as the electron donor^[1]. However, due to the slow kinetics of the water oxidation half-reaction (2H₂O → 4e^- + 4H⁺ + O₂), it limits the reaction rate of the entire photocatalytic CO₂ reduction reaction^[2]. In liquid-phase photocatalytic CO₂ reduction reaction systems, sacrificial agents like triethanolamine (TEOA)^{[3, 4]}, triethylamine (TEA)^[5], isopropanol (IPA)^{[6, 7]}, etc., are often used as hole scavengers in place of H₂O.

Additionally, to enhance the solubility of CO₂ in water, a certain amount of acetonitrile (ACN)^{[8, 9]} or ethyl acetate (EAA)^[6] is often added to the reaction system.

Figure 1. Schematic of Photocatalytic CO₂ Reduction Reaction^[1]

Research has shown that under specific wavelength light irradiation, common sacrificial agents such as ACN, TEOA, TEA, and EAA may undergo self-decomposition. The self-decomposition reactions of sacrificial agents can produce a certain amount of gases like CO, CH₄, C₂H₄, and H₂. If the target gas products of photocatalytic CO₂ reduction include CO, CH₄, C₂H₄, and H₂, the gas produced from self-decomposition can significantly affect the accuracy of the detection of target gas products^[5].

Furthermore, studies have shown that the addition of organic solvents or sacrificial agents in photocatalytic CO₂ reduction reaction systems not only affects the detection of gas products but also interferes with the detection of liquid products, especially alcohol products^[10].

Figure 2. Formation Rate of Products under Different Conditions with ACN, EAA, etc., as Solvents/Sacrificial Agents^[5]

Self-Decomposition of Photocatalysts

Graphitic carbon nitride (g-C₃N₄) is a common visible-light-responsive photocatalyst widely used in photocatalytic CO₂ reduction reactions^{[11, 12]}. However, recent studies have shown that g-C₃N₄ undergoes light-induced self-decomposition reactions in gas-phase photocatalytic CO₂ reduction reactions, producing CO, CO₂, and NO₂, severely affecting the experimental results of photocatalytic CO₂ reduction reaction activity assessment^[13].

Figure 3. Decomposition of g-C₃N₄ in Gas-Solid Phase Photocatalytic CO₂ Reduction Reaction and Confirmation^[13]

Self-Decomposition of Sealing Materials

In addition to organic solvents, sacrificial agents, and photocatalysts, sealing materials near the windows of reactors or reaction systems in photocatalytic CO₂ reduction reaction systems may also undergo light-induced self-decomposition or thermal decomposition.

Typically, in photocatalytic reactors or systems, the main materials for sealing are nitrile rubber, silicone rubber, or fluorine rubber. When using a xenon lamp as the light source for photocatalytic reactions, these sealing materials may produce a certain amount of CH₄ or CO, causing a certain degree of interference in the detection of products.

Purity of Raw Gases

Due to the purity of raw gases, there may be some hydrocarbon compounds present in the raw CO₂ gas, affecting the experimental results.

It is recommended to use CO₂ with a purity of 99.999% as the raw gas for photocatalytic CO₂ reduction reactions.

The above content is a translation and summary based on reference materials. If there are any errors, please feel free to correct them, as my knowledge is limited!

References