Error sources in the evaluation of photocatalytic CO2 reduction activity

At present stage, in the study of photocatalytic CO₂ reduction, the reaction rate of the target product is still at the level of mmol·h^-1·g^-1, and the low reaction rate makes the experimental evaluation results of the photocatalyst activity susceptible to significant interference from external factors. Due to the photoinduced self-decomposition of organic solvent, sacrificant, photocatalyst and seal ring, the experimental results were biased. Based on recent studies, the author summarized the error sources in the evaluation of photocatalytic CO₂ reduction activity at the present stage.

Photoinduced self-decomposition of solvent and sacrificial agent

The optimal photocatalyzed CO₂ reduction reaction is to use water or water vapor as the electron donor ^[1]. However, due to the slow kinetics of water oxidation half reaction (2H₂O → 4e^- + 4H⁺ + O₂), the reaction rate of the whole photocatalyzed CO₂ reduction reaction will be limited ^[2]. In liquid phase photocatalytic CO₂ reduction reaction system, sacrificial agents are often used to replace H₂O as hole consumptive agents, such as triethanolamine (TEOA) ^{[3, 4]}, triethanolamine (TEA) ^[5], isopropyl alcohol (IPA) ^{[6, 7]}, etc.

In addition, in order to improve 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 diagram of photocatalytic CO₂ reduction ^[1]

Studies have shown that under the irradiation of specific wavelengths of light, several common sacrifice agents ACN, TEOA, TEA and EAA may self-decomposition, and the self-decomposition reaction of sacrifice agents will produce a certain amount of CO, CH₄, C₂H₄ and H₂ products. If the target products of photocatalytic CO₂ reduction reaction are CO, CH₄, C₂H₄ and H₂ products, The gas generated by the self-decomposition reaction will seriously affect the accuracy of the detection of target gas products ^[5].

In addition, studies have shown that adding organic solvents or sacrificants into the photocatalytic CO₂ reduction reaction system will not only affect the detection results of gas products, but also liquid products, especially alcohol products ^[10].

FIG. 2. Product formation rates under different conditions when ACN and EAA are used as solvents/sacrificial agents ^[5]

Photoinduced self-decomposition of photocatalysts

Graphite phase carbon nitride (g-C₃N₄) is a common visible light responsive photocatalyst, which has been widely used in photocatalytic CO₂ reduction reaction ^{[11, 12]}. However, recent studies have shown that g-C₃N₄ will undergo photoinduced self-decomposition in gas-phase photocatalytic CO₂ reduction reaction, resulting in CO, CO₂, NO₂ and other products, which seriously affect the experimental results of the evaluation of photocatalytic CO₂ reduction reaction activity ^[13].

Figure 3. g-C3N4 decomposition in gas-solid phase photocatalytic CO₂ reduction reaction and its confirmation ^[13]

Photoinduced self-decomposition of sealing ring

In addition to organic solvents, sacrificants and photocatalysts, photoinduced self-decomposition or thermal decomposition reactions may also occur in the sealing ring near the light window of the reactor or reaction system in the photocatalytic CO₂ reduction reaction system. 

Generally, in the photocatalytic reactor or reaction system, the main material of the sealing ring is NBR, silicone rubber or fluorine rubber. When these sealing rings use xenon lamp as the light source of the photocatalytic reaction, a certain amount of CH₄ or CO will be produced, which will cause a certain degree of interference to the detection of the product.

Raw gas purity

Due to the purity of CO₂ in the feedstock gas, there will be some hydrocarbons in the feedstock gas, which will affect the experimental results. 

It is suggested to use 99.999% CO₂ as the raw gas for photocatalytic CO₂ reduction reaction。

The above part is the author according to the reference literature translation and summary, the author is limited, if there are mistakes, please correct!

References

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