By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Aug 30 2022In a review recently published in the open-access journal Energies, researchers presented a minireview of the photocatalytic evolution of hydrogen peroxide.
Study: Photocatalytic Evolution of Hydrogen Peroxide: A Minireview. Image Credit: Maryna Olyak/Shutterstock.com
Background
The use of photocatalytic reactions has been acknowledged as an alternate method for achieving a number of chemical changes. Oxygen-based photo-adducts that form as aqueous systems in the oxygen atmosphere are frequently used in photocatalytic reactions. The oxidant hydrogen peroxide (H2O2) is regarded as being environmentally friendly for both organic and inorganic materials.
Since hydrogen peroxide is seen as both a viable fuel and an environmentally favorable material with regard to oxidative synthetic techniques, researchers have shown a heightened interest in its synthesis. H2O2 has been used extensively in wastewater treatment or as a bleaching agent in the paper industry because the side products in its reactions are not harmful.
The production of H2O2 has traditionally been carried out using a multi-step, energy-intensive industrial process known as the anthraquinone (AQ) process. This method calls for hazardous organic media extraction procedures, a mix of energy-intensive chemical reactions, and the creation of undesired byproducts. Therefore, new environmentally friendly methods must be developed to produce hydrogen peroxide in huge amounts without endangering the environment.
In the past few decades or so, a different synthetic method has emerged that involves directly combining hydrogen and oxygen gases with a nanostructured catalyst. In the past 20 years or more, alternative strategies utilizing renewable energy sources have been created. The photocatalytic synthesis of hazardous organic media extraction procedures has garnered a lot of interest due to the process's eco-friendliness.
About the Study
In this study, the authors outlined the advancements in research that were made in recent decades toward the creation of practical photocatalytic devices. TiO2-based systems were mainly examined in the initial research, which was conducted in the 1980s. However, because titania had a huge band gap of 3.2 eV, other semiconductors that had strong visible absorption were investigated.
The team studied numerous families of semiconductors, including doped titania systems, metal sulfides, various metal oxides, organic semiconductors, carbon nitride systems, metal-organic frameworks, etc. The creation of functional dopants on the surface of the primary semiconductor simultaneously prevented H2O2 oxidation and electron-hole recombination. The studies of the greater H2O2 generation rates were presented collectively in the current minireview, along with some recommendations for the near future.
The researchers discussed photocatalytic pathways that were proposed in various ground-breaking publications to occur during the synthesis of H2O2. The numerous photocatalytic systems that were employed for the aforementioned synthetic transformation were covered. The most effective photocatalytic systems were listed in a comparative table that also included some comparative statistics. This review showed the literature's current state and suggested new directions for the creation of useful photocatalytic devices.
Observations
The hybrid material, in the presence of an organic electron donor, was able to produce H2O2 at a rate of 361 μmol/g/h at a wavelength of 380 nm. The hybrid that was calcined at 500 °C had the maximum yield after five hours, reaching 730 μmol in an oxygen-rich environment with isopropanol acting as a hole scavenger. The ideal H2O2 concentration of the Cu2(OH)PO4/g-C3N4 heterojunction catalyst, which had a Cu2(OH)PO4 content of 20 wt.%, was 7.2 mM, which was more than 13 and 31.3 times higher than the concentrations of neat g-C3N4 and Cu2(OH)PO4, respectively.
The H2O2 generation rate was approximately 3 mM after 1.5 hours of irradiation. Under visible light, it was discovered that the catalyst produced H2O2 at a rate of 3923 μmol/g/h in acetonitrile (MeCN) solution containing roughly 1.5 vol% water.
The H2O2 yield was predicted to reach up to 15 mM after a three-hour exposure. After six hours of illumination, an ideal photocatalytic system produced up to 1.6 mM of H2O2 without the need for a hole scavenger or pure oxygen gas purging. Aqueous ZnO suspensions were demonstrated to produce H2O2 concentrations up to 2 mM in the presence of organic acids acting as the hole scavengers.
Nanostructures made of C3N4 were thought to be potentially effective photocatalytic devices for the evolution of H2O2. The fact that these structures were created using one-step annealing processes provides strong support for this. By creating functional Z-scheme binary systems with spatial carrier separation, problems with electron-hole recombination processes could be overcome.
Conclusions
In conclusion, this study demonstrated that the most effective photocatalytic systems were those based on metal-organic frameworks, carbon-based conjugated semiconducting nanostructures, or porous carbon nitride assemblies. The creation of functional defect sites and targeted doping with heterostructures appeared to be the most crucial elements. These could reduce the rate of electron-hole recombination and improve the selectivity of the two-electron oxygen reduction reactions. The dopants also served as sites for the adsorption of oxygen, which increased the photocatalytic activity. This raised the issue of high surface area assemblies, which was crucial.
To obtain doped semiconductors with accessible catalytic sites, a variety of methods based on sacrificial template approaches needs to be created. Highly effective photocatalytic systems in aqueous environments could be developed as a result of the combination of porosity increase and chemical modification.
The authors mentioned that some encouraging results were obtained regarding the integration of the proposed hybrids in industrial-scale facilities, but more study needed to be done. The expense of the procedure, including the creation of facilities that use solar light as an excitation source, was the primary factor that needs to be addressed. One-pot techniques for the synthesis of catalytic systems could result in considerable advancement in the sector. As a result, the cost of the synthetic portion of the entire process would be greatly reduced.
The team stated that when comparing the state-of-the-art techniques that were gained a few years ago to the industrial-scale evolution of H2O2 and subsequent usage, the latter is considerably more achievable. They elucidated that both the H2O2 production and the catalysts' sensitivity to visible light significantly improved in recent years. They also emphasized that the creation of highly recyclable photocatalytic devices should be the focus of future studies.
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References
Karamoschos, N., Tasis, D., Photocatalytic Evolution of Hydrogen Peroxide: A Minireview. Energies, 15(17), 6202 (2022). https://www.mdpi.com/1996-1073/15/17/6202
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