How is the activity calculated in photocatalytic CO₂ research?

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The photocatalytic reduction process of CO₂ simulates photosynthesis, utilizing solar energy to convert CO₂ and H₂O into fuels and high-value chemicals. The photocatalytic method, with its characteristics such as being environmentally friendly and mild conditions, is considered one of the most promising solutions to global energy and environmental issues^{[1, 2]}.

Due to the diverse types of products involved in the photocatalytic CO₂ reduction reaction, the different products are generated due to variations in the required number of electrons during the reaction. Therefore, the calculation methods for different product yields in the photocatalytic CO₂ reduction reaction are closely related to the number of electrons transferred during the reaction. The following table summarizes the specific products and their corresponding transferred electron numbers in the photocatalytic CO₂ reduction reaction:

Table 1. Photocatalytic reduction of CO₂ to various products and corresponding electrode reactions^[3].

The activity evaluation indicators for photocatalytic CO₂ reduction mainly include the following six aspects:

 1. Rate of target product formation in photocatalytic CO₂ reduction (R_product)^[4]: The quantity of target product substance generated per unit time and unit mass of catalyst. The calculation formula is as follows:

n_product: Quantity of product substance (μmol); 

R_product: Rate of target product formation (μmol·h^-1·g^-1); 

m: Mass of catalyst (g); 

t: Reaction time (h). 

2. Rate of electron consumption in photocatalytic CO₂ reduction (R_electron)^[5]: Effective rate of photo-generated electrons involved in the reaction. The calculation formula is as follows:

R_electron: Rate of electron consumption (μmol·h^-1·g^-1); 

R_product: Rate of target product formation (μmol·h^-1·g^-1); 

K₁, K₂, K₃: Different numbers of electrons transferred for different products, as shown in Table 1. 

3. Theoretical oxygen production in photocatalytic CO₂ reduction (R_oxygen)^[6]: Derived from the number of effective photo-generated electrons (holes) involved in the reaction, indicating the amount of O₂ generated in the reaction. 

Theoretical oxygen production unit: μmol; 

n_product: Quantity of target product substance (μmol); 

K₁, K₂, K₃: Different numbers of electrons transferred for different products, as shown in Table 1. 

4. Selectivity of photocatalytic CO₂ reduction (S_product)^[7]: Percentage of target product quantity in the total product quantity. 

R_product: Rate of target product formation (μmol·h^-1·g^-1); 

R_electron: Rate of electron consumption (μmol·h^-1·g^-1); 

K₁, K₂, K₃: Different numbers of electrons transferred for different products, as shown in Table 1. 

5. Apparent Quantum Yield (AQY) of photocatalytic CO₂ reduction^[4]: The ratio of the number of transferred electrons to the number of incident photons at a specific monochromatic wavelength in the reaction system. 

Ne: Total number of transferred electrons in the reaction; 

n_product: Quantity of the target product substance (μmol); 

K₁, K₂, K₃: Different numbers of electrons transferred for different products, as shown in Table 1; 

Np: Number of incident photons. For more details, refer to the article “Quantum Yield (AQY) Calculation Tutorial: You Deserve to Have It!” 

6. Solar to Chemical Energy Conversion Efficiency (STC) in photocatalytic CO₂ reduction^[8]: The efficiency of converting solar energy into chemical energy. The calculation formula is as follows: 

R_product: Rate of target product formation (mol·s^-1); ∆Gr: Molar Gibbs free energy of the target reaction (J·mol^-1); 

H₂O(l) → H₂(g) + 1/2O₂(g)           ∆Gr = 237 kJ·mol^-1 [2]; 

CO₂(g) → CO(g) + 1/2O₂(g)         ∆Gr = 257  kJ·mol^-1 [2]; 

CO₂(g) + 2H₂O(l) → CH₄ (g) + 2O₂(g)   ∆Gr = 890.9 kJ·mol^-1 [2]; 

Psun: Optical power density of AM 1.5G standard solar spectrum (1000 W·m^-2); 

S: Illuminated area (m²).

The above sections were translated and summarized by the author based on reference materials. The author's proficiency is limited; if there are any errors, kindly point them out!

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