Reduction Action of Light-Generated Electrons or Hotspots? —Elucidating the Mechanism of World-Leading CO2 Photofuel Production Activity—

A research group including Masato Sasaki, a master's student, Tomoki Oyumi, a doctoral student, Keisuke Hara (at the time of research) from the Graduate School of Advanced Science and Engineering at Chiba University, Professor Yasuo Izumi from the Graduate School of Science at Chiba University, and Associate Professor Hongwei Zhang from the Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, has succeeded in clearly identifying and distinguishing the roles of "reactions by light-generated electrons (Note 1)" and "reactions at hotspots (Note 2)," which have been long-standing mysteries in the photocatalytic reaction that converts carbon dioxide (CO₂) into fuels such as methane (CH₄), a main component of natural gas and city gas.
Furthermore, they developed a composite catalyst, **Ni–Ru–ZrO2 catalyst**, combining nickel (Ni), ruthenium (Ru), and zirconium dioxide (ZrO2), achieving a world-leading conversion rate of CO2 to CH4 at 10 millimoles per gram of catalyst per hour. This achievement, which elucidated the photocatalytic mechanism in response to catalyst temperature, is expected to serve as a guideline for improving the efficiency of CO2 photoreduction catalysts. These research results were published on March 20, 2026 (US time) in the Journal of the American Chemical Society.
(Paper available here: 10.1021/jacs.5c17533)

Figure 1. CO2 photoreduction reaction test with the catalyst in a water-cooled reactor. Figure 2. Conceptual diagram of CO2 photoreduction by Ni–Ru–ZrO2 catalyst.

■ Research Achievements
If solar energy can be used to convert CO2 into fuels and chemicals in a renewable way, a new carbon-neutral cycle can be created. Conventional photocatalytic technologies faced a major challenge: low energy conversion efficiency. Moreover, the reaction pathway for CO2 reduction under light irradiation was unclear because the light absorbed by the photocatalyst is converted into charge separation and heat (hotspots), which depend complexly on the intensity and temperature of the irradiated ultraviolet-visible light.

1. To investigate the CO2 reduction reaction pathway, the developed Ni–Ru–ZrO2 catalyst was compared with the conventional Ni–ZrO2 catalyst. CO2 photoreduction reaction tests were conducted under a light intensity of 568 milliwatts per cm2 (mW/cm2) (Note 3), with and without water cooling of the reactor (Figure 1, 2).
2. Under a light intensity of 654 mW/cm2, the temperatures of Ni, Ru, and Zr atoms were tracked under conditions with and without cooling using ethylene glycol (Note 4). Under cooling conditions, it was confirmed that light-generated electrons reduce CO2 to COH, which is then accelerated by 7 times compared to thermocatalytic reactions on the Ni surface at approximately 126°C, leading to reduction to methane (Figure 2 left).
3. Under non-cooling conditions, the Ni–Ru–ZrO2 catalyst produced methane at a high rate of over 9 millimoles per gram of catalyst per hour, with the addition of Ru accelerating the reaction rate by 2.7 times. Furthermore, density functional theory (DFT) calculations (Note 5) showed that Ru enabled CO2 adsorption, and methanation proceeded solely due to heating derived from light energy (Figure 2 right).

■ Future Outlook
This research improved photocatalysts that convert CO2 into fuel (methane) using light energy, demonstrated that the Ni–Ru–ZrO2 catalyst promotes CO2 photomethanation at a world-leading rate, and scientifically proved the catalytic role of hotspots in metal nanoparticles. Moving forward, we aim to further improve the efficiency of sustainable CO2 resource utilization technologies using solar energy, such as C2 and C3 compound (Note 6) and alcohol synthesis.

■ Glossary
Note 1) Light-generated electrons: When light or other energy is applied, the negative charges (electrons) and positive charges within a substance separate, a phenomenon called charge separation. Before these recombine, light-generated electrons cause reduction reactions and positive charges cause oxidation reactions in photocatalysts.
Note 2) Hotspots: Localized regions within a substance that become high-temperature when light is absorbed at an atomic level.
Note 3) Milliwatts (mW/cm2): Watt (W) is the energy (joules) per second. 1 joule = 1 newton × meter. Here, it represents the energy irradiated per second per square centimeter.
Note 4) Ethylene glycol: A liquid with the chemical formula HOC2H4OH, used as an antifreeze. It was used in this experiment because it produces fewer bubbles than a water bath when tracking the temperature of each atom with a synchrotron X-ray beam.
Note 5) Density functional theory (DFT) calculations: A theory in quantum mechanics that expresses the wave equation for electrons as a function of electron density. It is used to investigate stable structures, energies, and electron states for systems with many electrons.
Note 6) C2, C3 compounds: Compounds containing two or three carbon atoms in their molecule. Specifically, ethane, propane, ethylene, and propylene.

■ Paper Information
Title: Charge Separation and/or Hot Spots: Clarification of Efficient CO2 Reduction over Ru–Ni Nanoparticles Compared to Photocatalysis on Ru–Ni–ZrO2 Composites
Authors: Masahito Sasaki, Tomoki Oyumi, Keisuke Hara, Hongwei Zhang, and Yasuo Izumi
Journal: Journal of the American Chemical Society
DOI: 10.1021/jacs.5c17533

■ Research Project
This research was supported by the following:
・ Grant-in-Aid for Scientific Research (B) "Flexible and Precise Control of CO2 to Various C2,3 Products by Unsaturated Semiconductor-Metal Nanoparticle Photocatalysts" (24K01522)
・ Grant-in-Aid for Scientific Research (B) "CO2 Multi-electron Photoreduction and Isotope Labeled Species Time-resolved Tracking on Alloy Nanoparticle–Ultrathin Semiconductor Composite Surfaces" (20H02834)

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