Recycling Emissions: Using Photocatalytic Pathways For CO2 Conversion

Evan Bai '29

Since the beginning of the Industrial Revolution, humans have relied on massive amounts of fossil fuels, such as coal and oil, to generate energy to power factories and transportation. Burning fossil fuels releases carbon dioxide (CO2) into the atmosphere, which acts as a blanket trapping infrared radiation that would otherwise leave Earth’s atmosphere and heating the Earth by around 2.6 degrees ºFsince pre-industrial times (Rhode, 2025). If humans continue to pump CO2 into the atmosphere at this rate, global temperatures will rise , increasing the number of heatwaves, severe storms, and droughts and destroying wildlife habitats, accelerating species loss for various reasons including the burning or melting of habitats caused by an increase in global temperature (UN Causes and effects of climate change, 2022). There are two main solutions to address the problem: reducing greenhouse gas emissions or converting them into reusable energy, essentially recycling emissions. The former has led to attempts at switching to more sustainable sources of energy and capturing and storing emitted CO2 underground. Recycling emissions is an old concept made feasible by new technical advances and discoveries made in recent years. Rather than switching to cleaner sources of energy with less emissions that may cost billions of dollars, emissions are instead collected and recycled into new sources of energy, reducing both waste emitted into the atmosphere and the need to extract even more fossil fuels from the ground.

One proposed method for reusing carbon dioxide is the Sabatier process, which converts CO2 and hydrogen gas (H2) into methane (CH4) and water (H2O) (Amez et al., 2021). How could this help the climate? While CH4 is a potent greenhouse gas, it can be burned as a source of fuel, producing CO2 and H2O as byproducts. The CO2 can be recycled into CH4, allowing the process to continue indefinitely with no net carbon emission. There are, however, limitations to the Sabatier process. First, it requires extremely high temperatures, around 300 to 500 ºC, making it impractical, as generating enough energy to heat a catalyst, a substance that speeds up a chemical reaction that isn’t consumed, to the temperature threshold likely requires unsustainable fuel sources. The Sabatier process also requires large amounts of hydrogen gas as a present catalyst while the reaction is underway. Hydrogen gas is mostly derived from burning fossil fuels, which defeats the whole purpose of recycling carbon. Cleaner hydrogen gas production from the separation of water molecules demands huge amounts of water and electricity, which is costly and inefficient. This is where photocatalytic reduction comes in.

Photocatalytic reduction, similar to the Sabatier process, reacts CO2 with H2 to produce CH4and H2O. However, it uses light energy as a catalyst to convert the CO2 into CH4. Photocatalysts, commonly metals on zirconium dioxide (ZrO2), absorb photons from sunlight, which excite electrons in the catalyst’s atoms, allowing them to move more freely and participate in chemical reactions. Their electrons can be transferred to CO2 molecules, making the CO2 molecules unstable and more likely to separate and form CH4. This means that the energy required to reduce CO2 into CH4 comes from photons, not thermal energy, allowing for the process to occur at a much lower temperature compared to the Sabatier process. Hydrogen gas can also be directly obtained from a photocatalyst . The excited photocatalyst uses energy from the sunlight to accelerate a process similar to electrolysis, the process of splitting water into hydrogen and oxygen gas, obtaining H2 (Izumi 2013). In theory, photocatalytic reduction solves the high temperature and cost requirements of the Sabatier process; however, it does not work well in practice. Aside from low efficiency, scientists were previously unsure of how this process worked, “whether it is driven by true photocatalytic processes involving light-induced electron excitation, or by heat generated from light, known as the photothermal effect” (Izumi, 2026). When reactants are heated up, they gain kinetic energy and move at a quicker speed, becoming more likely to collide with one another and undergo a chemical reaction. This speeds up the overall reaction. It is important to note that this photothermal effect only occurs in extremely small, direct regions of a specific material in the reaction, rather than heating the entire system uniformly, such as by putting a catalyst over a flame. This uncertainty made it difficult for researchers to refine and optimize the catalyst, as they couldn’t determine which variables affected efficiency.

Researchers at Chiba University found a way to separate the two effects, photothermal and photocatalytic. First, they controlled the temperature of two different catalysts, ruthenium–nickel–zirconium oxide (Ru–Ni–ZrO₂) and nickel–zirconium oxide (Ni–ZrO₂), the key difference being the presence of ruthenium (Ru), by using a cooling bath to regulate temperature and controlling light intensity. In one case, researchers allowed the two catalysts to perform reactions without the cooling bath, essentially how they would operate in a standard environment. The Ru-containing catalyst showed an enhanced performance with a conversion rate 2.7 times faster than the Ru-free catalyst. When heat was present, the presence of Ru decreased the energy required to split the CO2, allowing more reactions to happen with the same amount of energy. When the catalysts were cooled down, this essentially isolated the photocatalytic effect as there was no heat present to speed up the reaction. At least, in theory. Researchers surprisingly found that tiny regions on the nickel heated up to high temperatures of ~126 ºC. These hot, localized regions allowed the Ni-containing catalyst to produce 1.72 times more CH4 than expected, revealing that the photothermal and photocatalytic effects worked together rather than separately to increase the photocatalyst’s performance (Izumi 2026). This experiment clarified the reaction pathway, or how exactly the reactants changed into the final product. Scientists gained new understandings of how photocatalytic reduction worked, making it much easier to design more efficient catalysts.

Now that scientists know how to improve the efficiency of photocatalysts, they can develop future models to help the photothermal and photocatalytic reactions complement each other in the most efficient way possible. This would significantly increase the activity of future catalysts, allowing them to produce more energy such as methane more quickly. More efficient catalysts could be used more widely as a source of energy, allowing for a closed-carbon cycle to be established as CO2 from burnt fossil fuels is captured and recycled into reusable fuels instead of being released into the atmosphere as waste.


References

Amez, I., Gonzalez, S., Sanchez-Martin, L., Ortega, M. F., & Llamas, B. (2021). Underground methanation, a natural way to transform carbon dioxide into methane. ScienceDirect; Elsevier. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/sabatier-reaction
Chiba University. (2026, April 16). Closing the carbon cycle: Unraveling the roles of light and heat in CO2 photocatalysis. Phys.org. https://phys.org/news/2026-04-carbon-unraveling-roles-photocatalysis.html
IZUMI, Y. (2026, April 16). Closing the Carbon Cycle: Unraveling the Roles of Light and Heat in CO2 Photocatalysis | CHIBADAI NEXT. CHIBADAI NEXT. https://www.cn.chiba-u.jp/en/news/press-release_e260416/
Izumi, Y. (2013). Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond. Coordination Chemistry Reviews, 257(1), 171-186. https://doi.org/10.1016/j.ccr.2012.04.018
Rohde, R. (2026, January 14). Global Temperature Report for 2025 - Berkeley Earth. Berkeley Earth. https://berkeleyearth.org/global-temperature-report-for-2025/
United Nations. (2022). Causes and Effects of Climate Change. United Nations. https://www.un.org/en/climatechange/science/causes-effects-climate-change