The article below is my short commentary on the field of photocatalysis with Titanium dioxide, TiO2, as it was a topic I read a lot about during my Master’s studies.

Titanium dioxide (TiO2) is a well-known photocatalyst and it’s widely used in photocatalytic reactions. Photocatalysis is a process in which a material, known as a photocatalyst, absorbs light energy and uses it to catalyze chemical reactions. TiO2 is particularly attractive as a photocatalyst due to its high stability, low cost, environmental friendliness, and non-toxicity. However, like any technology, TiO2 also has its negative sides, which I’ll write about.

One of the main advantages of TiO2 as a photocatalyst is its ability to degrade organic pollutants in water and air, and this is where most of the uses for photocatalysis currently lay. TiO2 photocatalysis can be used to remove pollutants such as volatile organic compounds (VOCs), pesticides, and dyes. The process works by absorbing light energy, which excites the electrons in the TiO2 and causes them to move to a higher energy level. These excited electrons then react with water or oxygen molecules to produce hydroxyl radicals or superoxide ions, which are highly reactive and can degrade the pollutants. This process can also be used to weaken the bond inside cellulosic or starch biomass, and thus be included as the pretreatment step in the production of bioethanol or other products that this biomass can yield.

Another advantage of TiO2 is its ability to produce hydrogen gas (H2) through the process of water splitting. When TiO2 is exposed to light, it can generate electrons and holes, which can be used to split water molecules into H2 and oxygen (O2). This process, known as photocatalytic water splitting, has great potential for producing clean and renewable energy.

My Master’s thesis was on other photocatalysis uses – the use of photocatalysis in the production of bioethanol from a starchy material (we used cornmeal). There have been some papers done on this technology, but in most of the available research, the material used is of lignocellulosic nature. To my knowledge, only one paper (López-Vásquez et al., 2019) has dealt with using photocatalysis in the production of bioethanol out of starch biomass (they used potatoes). We did a great number of experiments, and one of those experiments was reported in a paper we published in a conference journal (Savić et al., 2022), and some of the experiments were analyzed by me and became my Master’s thesis.

As one of the questions in my Master’s defense was to talk a bit about TiO2 drawbacks, I’ll write a concise summary of this topic below.

One of the main limitations of TiO2 photocatalysis is its low efficiency in the visible light region. TiO2 is mainly active in the ultraviolet (UV) region of the electromagnetic spectrum, and its efficiency decreases significantly in the visible light region. This means that most of the solar energy is not utilized (only 5% of solar light is UV light), and more energy is needed to produce the same amount of photocatalytic activity.

There’s also the problem of recombination of the photogenerated electron/hole pairs that are produced – the recombination process halts the reactions needed to degrade organic material.

Another limitation of TiO2 is its poor selectivity. TiO2 photocatalysis is known to degrade not only the target pollutants but also other organic compounds present in the environment. This can lead to the formation of unwanted by-products, which can be harmful to the environment and human health.

One of the negative sides of using TiO2 is, because it’s used in the form of nanoparticles (to maximize surface area), the post-recovery of TiO2 after treatment, as simply “dumping” the nanoparticles down the drain is not environmentally reasonable.

Finally, TiO2 photocatalysis has a limited application range, primarily because it is not stable in acidic or alkaline environments. This means that TiO2 photocatalysis is not suitable for treating acidic or alkaline waste streams.

The high stability, low cost, and non-toxicity of TiO2 make it a popular photocatalyst. However, it also has its limitations such as low efficiency in the visible light region, poor selectivity, and limited application range. Researchers are continually working to improve the performance of TiO2 photocatalysis by developing new materials and methods. For example, by combining TiO2 with other semiconductors, such as ZnO, or by modifying the surface of TiO2 with different types of catalysts. With these new developments, the use of TiO2 as a photocatalyst will become more efficient, selective, and applicable to a wider range of processes.

References

López-Vásquez, A. F., Cobo-Angel, M. I., & Convers-Sánchez, J. D. (2019). Effect of photocatalytic pretreatment of potato starch for bioethanol production using Saccharomyces cerevisiae during simultaneous saccharification-fermentation (SSF). Dyna, 86(208), 251-256.

Savić, A., Škipina, B., Jokanović, V., Marinković, Đ., Velemir, A. & Stojković M. (June, 2022). Influence of photocatalytic pretreatment of cornmeal on bioethanol production. XI International Conference of Social and Technological Development, 508-513. http://conf.univerzitetpim.com/wp-content/uploads/2022/11/STED-2022-PROCEEDINGS.pdf