UPNA Researcher Converts CO2 to Methane Using Solar Light

UPNA Researcher Converts CO2 to Methane Using Solar Light

A researcher at the INAMAT2 Institute of the Public University of Navarra has transformed carbon dioxide into renewable methane using sunlight as the sole energy source. The reaction itself is not new: it is the Sabatier reaction, proposed in 1902, where CO2 combines with hydrogen to produce methane and water. According to the researcher, the novelty lies in the energy source: sunlight, at least partially, instead of additional heat. The work relies on photo-thermocatalysis, a discipline that uses solar energy to heat the catalyst while simultaneously exciting its electrons.

What is Photo-Thermocatalysis and Why It Matters

Photo-thermocatalysts are characterized by harnessing all solar energy for heating—thermochemical contribution—and inducing electron excitation—photochemical contribution. Both processes facilitate the chemical reaction. Distinguishing these two processes when illuminating the catalyst is crucial for improving design, which was one of the main objectives of the doctoral thesis. The researcher developed a method to differentiate the photochemical and thermochemical contributions of the process.

The final proof of concept was conducted outdoors. Catalysts based on ruthenium and nickel were exposed to concentrated sunlight to trigger the Sabatier reaction. Methane was produced from carbon dioxide without requiring additional energy input to reach the reaction temperature, which is typically achieved through extra heat.

The Ruthenium Problem: 40 Tons Annually and One Million Per Kilo

Ruthenium is the 78th most abundant element in the Earth's crust, found at approximately 100 parts per billion. It occurs in minerals with other platinum group metals in the Ural Mountains and North and South America, and in commercially significant quantities in pentlandite mined from Sudbury, Ontario, and pyroxenite deposits in South Africa. Global production hovers around 40 tons annually—fitting into a single truck—and the price per kilo is roughly one million euros. Most reserves are located in South Africa and possibly Russia.

Some argue that the "sunlight only" headline obscures the dependency on a scarce and expensive metal. Counterarguments suggest that the catalyst is a starting material, not a daily consumable, and that the research is still in the laboratory phase. The most optimistic calculation assumes ruthenium is recovered and reused; the pessimistic scenario posits that its price will skyrocket if industrial-scale use is attempted.

Is It Photosynthesis or Something Else?

It is not photosynthesis in the strict sense. Photosynthesis is 6 CO2 + 6 H2O → C6H12O6 + 6 O2, where C6H12O6 is glucose. The researcher's reaction also uses CO2 and water to obtain a hydrocarbon, methane, but the chemical process differs. Photosynthesis means synthesis via photons, and adhering to that literal definition, it is indeed a form of artificial photosynthesis. The term serves as a popular label rather than a rigorous chemical description.

Part of the analysis notes that the finding is not entirely new: in 2023, a joint team from City University of Hong Kong and collaborators developed a stable artificial photocatalytic system capable of transforming CO2 in water into methane using light, more efficiently than natural photosynthesis. The difference would lie in the method for separating photochemical and thermochemical contributions, and in integration into microrreactors manufactured via 3D printing.

From Lab to Panel: Microrreactors and 3D Printing

The proposed hybrid photocatalyst is based on metal-organic frameworks (MOFs) synthesized from semiconductor oxides to improve reactant adsorption and photochemical capabilities of standard photocatalysts. Various metal nanoparticles are included as co-catalysts to facilitate chemical reactions. This hybrid material is integrated into microrreactors fabricated trinc 3D printing protocols, assembled to create a photocatalytic cell or panel similar to current photovoltaic panels.

The produced methane is intended for combustion, not released into the atmosphere. This closes the cycle: CO2 and water enter, methane and water exit, and the methane is burned to provide energy and regenerate CO2. The problem is that methane produces 72 times the greenhouse effect of CO2, according to industry data. Any leak in the process would have a climate impact far exceeding that of the gas being recycled.

Thermodynamics and Underlying Skepticism

Mr. Planck, building on the works of Messrs. Kelvin and Clausius, disagrees: it is impossible to construct a machine operating in a regular cycle that does nothing else than raise a weight and cause corresponding cooling of a thermal reservoir. The Sabatier reaction does not abusa thermodynamics, but it consumes hydrogen, which must be produced. Hydrogen is more volatile than methane, making it harder to transport and store without losses.

The lingering question concerns efficiency. It remains to be seen what electrical or cogeneration yield this achieves compared to standard photovoltaic cells. Once it offers higher efficiency, greater durability, and a price similar to or lower than photovoltaic panels, and becomes commercialized, we will know if this is a revolution or just a press release. For now, the researcher took their catalysts outside and produced methane with sunlight. Plants have been doing something similar for thousands of years.