Hydrogen peroxide (H₂O₂), as a renewable energy carrier and a clean green oxidant, is widely applied in fine chemicals, biopharmaceuticals, environmental remediation, and other fields[1-3]. The reaction of H₂O₂ only produces O₂ and H₂O as by-products, causing no environmental pollution[4, 5].
The most common industrial method for H₂O₂ production is the anthraquinone process, which involves four main steps: hydrogenation, oxidation, substitution, and recycling. Although the anthraquinone process can produce H₂O₂ on a large scale, it consumes a huge amount of energy and requires the use of toxic organic raw materials and solvents, leading to severe environmental pollution, which does not meet the development requirements of green chemistry[6-8]. Therefore, developing green methods for H₂O₂ production is of great significance[9-11].
In recent years, more and more studies have shown that H₂O₂ can be synthesized via photocatalysis, with only water and O₂ as raw materials[12, 13]. Generally, photocatalytic H₂O₂ synthesis involves several steps, including light absorption, charge carrier generation and separation, and surface redox reactions[14-18]. Photocatalytic H₂O₂ production has multiple advantages, such as environmental friendliness, high efficiency, flexibility, sustainability, and innovation, making it a new environmental technology with broad application prospects and sustainable development potential.
Under light conditions, photocatalysts generate many photogenerated electron-hole pairs. The photogenerated electrons in the conduction band of the photocatalyst have strong reductive properties, while the photogenerated holes in the valence band have strong oxidative properties[19]. Photogenerated electrons drive the oxygen reduction reaction (ORR) and photogenerated holes induce the water oxidation reaction (WOR), both important surface redox reactions, and are the main causes of photocatalytic hydrogen peroxide evolution (Figure 1).
Figure 1. Schematic of photocatalytic H₂O₂ synthesis on semiconductor
Specifically, the two modes of photocatalytic H₂O₂ synthesis, ORR and WOR, involve the photocatalyst generating photogenerated electrons and holes under light. The photogenerated electrons can reduce O₂ to H₂O₂, while the photogenerated holes can oxidize H₂O to H₂O₂[20-22]. As shown in Figure 2, the photogenerated electrons and holes should have appropriate redox potentials to satisfy the conditions for ORR and WOR reactions[8, 23].
Figure 2. Energy level diagram
It can be seen that the ORR for H₂O₂ synthesis is a two-electron process, which can be considered either a direct two-electron process or two single-electron processes (an indirect two-electron process)[8, 24]. For the two single-electron processes, O₂˙⁻ is an important intermediate product[25]. The redox potential of O₂/H₂O₂ (0.68 V vs. NHE) is higher than that of O₂/O₂˙⁻ (-0.33 V vs. NHE), making the direct two-electron process thermodynamically more favorable[8].
In the photocatalytic O₂ reduction process, there exists a competing reaction in which O₂ can be further reduced by photogenerated electrons to form H₂O (four-electron ORR)[26-27]. The redox potential of O₂/H₂O (1.23 V vs. NHE) is more positive than that of O₂/H₂O₂, making the four-electron ORR more favorable from a thermodynamic perspective[28]. Typically, photocatalysts have low adsorption energy for oxygen, which facilitates product desorption. Overall, the two single-electron processes are more likely to occur kinetically. Additionally, O₂ easily reacts with photogenerated holes to produce singlet oxygen (O₂˙⁻/¹O₂, 0.34 V vs. NHE), which leads to a decrease in H₂O₂ yield.
In conclusion, the direct two-electron process for photocatalytic H₂O₂ synthesis is more favorable thermodynamically, but from a kinetic viewpoint, the two single-electron processes are more advantageous. Whether the direct two-electron process or the two single-electron processes, there are some competing reactions. Therefore, inhibiting the four-electron oxygen reduction reaction and singlet oxygen formation, and selectively producing H₂O₂, is one of the key challenges in photocatalytic H₂O₂ synthesis. The two-electron WOR is similar to the two-electron ORR in photocatalytic H₂O₂ synthesis.
Photocatalysts play a central role in photocatalytic H₂O₂ oxidation reactions. Various functional materials have been developed as photocatalysts for H₂O₂ production, achieving some promising results[29-31]. To improve the efficiency of photocatalytic H₂O₂ synthesis, two main methods are generally used: optimization of reaction conditions and modification of photocatalysts[32].
No.1 Optimization of Reaction Conditions
• Solvent Type
Generally, appropriate solvents can not only act as electron donors to capture holes and provide sufficient protons but also help to separate photogenerated carriers. The most commonly used solvents are alcohols, such as ethanol and isopropanol. Alcohols can be oxidized to aldehydes, generating protons to reduce O₂. Yamashita[33] and others prepared a hydrophobic titanium-doped zirconium-based MOF using a sample method, which exhibited high H₂O₂ production rate (9700.00 μmol L⁻¹ h⁻¹) under visible light irradiation in a benzyl alcohol aqueous solution. However, the use of alcohols also brings high costs and complex purification processes. Lan[34] and others constructed stable cobalt-based metal-organic cages with synergistic effects of metal-nonmetal active sites, where the reaction substrates could contact active sites through the coordination chemistry of the cage. The H₂O₂ production rate in pure water was as high as 146.60 μmol L⁻¹ h⁻¹. In addition, photocatalytic H₂O₂ production using seawater is also a feasible method. Das[35] synthesized a g-C₃N₄ catalyst, which exhibited excellent activity in photocatalytic H₂O₂ production. Using pure water or seawater for photocatalytic H₂O₂ production shows good application prospects, but low efficiency remains the biggest obstacle to practical applications.
• pH Value
The pH value of the reaction system is also a crucial factor influencing the performance of photocatalysts in H₂O₂ production, though many studies have overlooked this aspect. Wang [36] and others prepared organic polymer dots (PFB-PCBM Pdots), which can only produce photocatalytic H₂O₂ under alkaline conditions (Figure 3a). Mao [37] and his team developed a cyclodextrin-pyrimidine polymer that shows higher photocatalytic H₂O₂ production under acidic conditions (Figure 3b). Compared to alkaline environments, acidic conditions provide more protons, and the active sites of photocatalysts may be affected by the pH, thus altering the production pathway.
Figure 3. (a) Effect of pH on photocatalytic H₂O₂ production [38]; (b) Effect of pH on photocatalytic H₂O₂ production [39]; (c) Schematic diagram of photocatalytic H₂O₂ production [40]; (d) Schematic diagram of the reaction pre-fixation strategy for ultrafine BiOI nanoparticles on TiO₂ nanorods, including SEM, TEM images, and UV-Vis spectrum of the sample [41]; (e) "Boat in bottle" and "Bottle in boat" schematic diagrams; (f) TEM images of photocatalyst and schematic diagram of photocatalytic H₂O₂ synthesis [42]
• O₂ Content
For the two-electron ORR process, O₂ is the key raw material. As the dissolved oxygen in water is limited, injecting oxygen into the reaction system is an effective way to enhance the photocatalytic H₂O₂ production rate. The four-electron oxidation reaction triggered by holes can provide oxygen for the two-electron reduction reaction. Domen [43] and others found that the holes generated in CoOx/Mo:BiVO₄/Pd can oxidize water to form oxygen, which is further reduced to H₂O₂ through the two-electron reduction reaction (Figure 3c).
No.2 Photocatalyst Modification
• Enhancement of Light Absorption
Strong light absorption is a necessary property for photocatalysts. Generally, the light absorption characteristics of a photocatalyst are determined by its band structure. To improve the light absorption ability, several strategies have been proposed, including surface modification engineering, loading quantum dots, doping, and localized surface plasmon resonance (LSPR) effects. The introduction of photosensitizers is one of the most common surface modification techniques. Feng [44] decorated BiOI nanodots uniformly on TiO₂ nanorods assembled into microflowers, creating a representative nanoregion photocatalytic heterostructure (Figure 3d), where the photosensitive nanodots were well dispersed on the TiO₂ surface, effectively enhancing TiO₂'s visible light capture capability. Quantum dots, which have unique optical properties at the nanoscale, are considered a feasible strategy to improve the light absorption of catalysts. Two typical methods for loading quantum dots onto catalysts, the "boat in bottle" and "bottle in boat" methods, are shown in Figure 3e. Cao [45] and others used a simple approach to construct phosphorus-doped porous g-C₃N₄ (Figure 3f), which has a typical pore structure and improved light absorption capability. The highest H₂O₂ production rate reached 1968 μmol g⁻¹ h⁻¹, with the photocatalytic H₂O₂ production pathways including two-electron ORR and two-electron WOR.
When the frequency of incident photons matches the oscillation frequency of the metal's internal plasmons, resonance occurs, leading to strong absorption of the incident light. This phenomenon is known as the LSPR effect (Figure 4a). Li [46] and others encapsulated Au nanoparticles in NH₂-UiO-66 nanocages to create the Au@NH₂-UiO-66/CdS composite material. The introduced gold nanoparticles showed a clear LSPR peak at 523 nm. Due to the LSPR effect of the gold nanoparticles, the Au@NH₂-UiO-66/CdS composite exhibited excellent catalytic performance in hydrogen evolution reactions, with the gold nanoparticles showing a clear LSPR peak at 523 nm (Figure 4b).
Figure 4. (a) Schematic of surface plasmon resonance effect; (b) TEM image and UV-Vis spectrum of the sample [47]; (c) Sample spectrum and simulated mechanism [48]; (d) Comparison of photocatalytic H₂O₂ evolution for different catalysts [49]
• Improvement in Charge Separation
The recombination of photogenerated electrons and holes is an inevitable issue that severely limits the practical application of photocatalysts.
Due to the multi-valent states of certain metal ions, these ions can play a role in electron transfer through redox coupling. Therefore, doping with metal ions is considered an effective way to suppress light-induced electron and hole recombination. Xue[50] and others used a simple method to decorate gold nanoparticles with cobalt porphyrin. Due to the role of cobalt ions in electron transfer, the charge separation efficiency on the gold nanoparticles significantly improved, and the photocatalytic H₂O₂ yield reached 235.93 μmol L⁻¹ (Figure 4c).
The construction of heterojunctions can also improve charge separation efficiency. The combination of two materials forms a heterojunction at the interface, allowing for the transfer of photogenerated charge carriers through the heterojunction between different components. In the study by Wang[51] and others, a nitrogen-doped carbon (TCN)/ZnIn₂S₄ (ZIS) heterojunction was prepared using an in situ growth method. The formation of a type II heterojunction improved the charge separation ability, achieving a H₂O₂ yield of 2.77 mmol g⁻¹ h⁻¹ on the TCN/ZIS (Figure 4d). Ye[52] and others prepared a novel Z-scheme heterojunction by coupling g-C₃N₄ with Zn-phthalocyanine, achieving a H₂O₂ yield of 114 μmol g⁻¹ h⁻¹ in pure water.
• Enhancement of Surface Photocatalytic Reactions
It is widely believed that the main pathways for photocatalytic H₂O₂ synthesis involve two-electron ORR and two-electron WOR, and these reactions typically occur on the surface of the catalyst. Therefore, enhancing surface photocatalytic reactions has a positive impact on H₂O₂ production on photocatalysts.
• Inhibition of H₂O₂ Decomposition
In alkaline environments or at high temperatures, H₂O₂ easily decomposes into H₂O and O₂. Additionally, H₂O₂ formed on the surface of photocatalysts can further react with photogenerated electrons or holes. Therefore, it is crucial to desorb H₂O₂ from the surface of the photocatalyst in a timely manner. Yamashita[53] and others modified titanium-doped Zr-based MOFs with octadecylphosphonic acid to make them hydrophobic, improving photocatalysis and H₂O₂ dispersion in phenylethanol and water, thus inhibiting H₂O₂ decomposition.
Solar-driven photocatalytic processes offer a promising green method for producing H₂O₂. Despite many years of research aimed at exploring the feasibility of photocatalytic H₂O₂ production for engineering applications, significant progress has been made, yet there is still a long way to go. It is well known that the core of photocatalytic H₂O₂ synthesis lies in the photocatalyst; however, most photocatalysts still lack strong light absorption and high electron-hole pair separation efficiency. Future research could continue to focus on these issues. Additionally, improving surface photocatalytic reactions positively affects H₂O₂ synthesis on photocatalysts. Aside from these issues, the lack of suitable reactors makes photocatalyst recovery and regeneration difficult, limiting the practical engineering application of photocatalytic H₂O₂ synthesis. Despite the many challenges and issues hindering further development of photocatalytic H₂O₂ synthesis, it remains an attractive field.
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