Rising carbon dioxide (CO2) emissions from human activities are the largest contributor to global warming. According to the International Energy Agency (IEA), global CO₂ emissions reached an all-time high of 37.8 gigatons in 2024. While some of this CO2 is absorbed by soil, forests, and the oceans, a large fraction remains in the atmosphere, where it can persist for hundreds to thousands of years, leading to long-term impacts on the global climate.
To address this challenge, scientists are exploring ways to convert CO2 into useful fuels, creating a closed carbon cycle. One promising approach is photocatalytic reduction, in which CO2 is converted into methane using a catalyst powered by sunlight. However, the efficiency of this process is still too low for practical use. A key difficulty lies in understanding how the reaction occurs—whether it is driven by true photocatalytic processes involving light-induced electron excitation, or by heat generated from light, known as the photothermal effect.
Now, a team led by Professor Yasuo Izumi at the Graduate School of Science at Chiba University, Japan, has elucidated these pathways. Their study, available online on March 20, 2026, and published in Volume 148, Issue 13 of the Journal of the American Chemical Society on April 8, 2026, achieved one of the highest reported rates of CO2-to-methane conversion to date, reaching up to 10 millimoles per gram of catalyst per hour. By clarifying the underlying reaction mechanisms, their work provides important insights that could guide the design of more efficient catalysts for CO2 conversion.
The team included first author Masahito Sasaki, along with Tomoki Oyumi and Dr. Keisuke Hara from the Graduate School of Science and Engineering, Chiba University, and Associate Professor Hongwei Zhang from the Biogas Institute of the Ministry of Agriculture and Rural Affairs, China (and a former Ph.D. student at Chiba University).
Prof. Izumi explains the current challenge: "The true reaction pathway and the catalytic role responsible for it remain uncertain in photocatalysis, where charge separation, hot spots, and energetic modulation of ground and excited states are involved."
To separate photothermal and photocatalytic effects in CO2 reduction, the researchers irradiated Ru–Ni–ZrO2 and Ni–ZrO2 catalysts with ultraviolet (UV)–visible light at varying intensities from 90 to 900 milliwatts per centimeter square (mW cm−2) while carefully controlling the temperature of the system, either maintaining it at 295 K (22 °C) using a cooling bath or allowing it to increase under irradiation.
Without the cooling bath, the Ru–Ni–ZrO2 catalyst converted CO2 to methane more than 2.7 times faster than the Ni–ZrO2 catalyst, reaching over 7.9 millimoles per gram of catalyst per hour. Under these conditions, the photothermal effects became increasingly dominant. CO2 is directly adsorbed onto Ru–Ni active sites, where it is more easily activated and dissociated into CO and oxygen atom with a low activation energy of 0.45 eV—much lower than the 0.79 eV required on pure nickel.
In contrast, when the cooling bath was applied, the reaction was primarily driven by photocatalytic processes, with some contribution from local heating. Light generates separated electrical charges on the ZrO2 surface, forming intermediate species via OCOH intermediates at oxygen vacancy sites. These intermediates are then transferred to nickel sites, where they undergo multiple hydrogenation steps to form methane. Under these conditions, localized 'hotspots' can form on nickel, where temperatures can reach 126 °C under strong irradiation (654 mW cm−2). At these sites, the methane formation rate is 1.72 times higher than expected from simple thermal reactions, showing that both charge separation and local heating work together.
Together, these findings show that CO2 reduction depends on a balance between photocatalytic and photothermal processes, with their relative contributions determined by temperature and light intensity. By clearly identifying how these mechanisms interact, the study provides a deeper understanding of light-driven CO2 conversion and offers a pathway toward designing more efficient catalysts.
The researchers aim to further expand this approach to produce more complex and valuable chemicals. "Going forward, we aim to further enhance the efficiency of sustainable CO2 utilization technologies using sunlight, such as the synthesis of C2 and C3 compounds and alcohols," says Prof. Izumi.
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