Hydrogen is considered the energy source of the future. But production is often expensive and dependent on rare precious metals. Researchers from Kyushu University and Osaka University have now presented a process that only requires iron ions and UV light to split methanol into hydrogen.
The production of hydrogen from fossil sources is reaching its limits because it contradicts the goal of climate neutrality. Alternatives such as the splitting of alcohols are considered promising, but often fail due to the high cost of the catalysts required.
These are usually based on rare precious metals such as ruthenium or iridium and require expensive organic ligand structures. A team from Kyushu University and Osaka University has now found a way that relies on the most common transition metal on Earth: iron.
Takahiro Matsumoto, an associate professor in the Faculty of Engineering at Kyushu University, who led the study, said:
Our research group has long been interested in developing catalysts from abundant and inexpensive elements. This time we focused on sustainability and examined the suitability of common metals as catalysts for the production of hydrogen.
Accidental discovery in the laboratory leads to a new hydrogen process
A coincidence in the laboratory put the researchers on the trail. While they were actually studying iron complexes, they discovered that just one free iron ion combined with UV light was enough to break down methanol. The process completely dispenses with ligands, which are otherwise necessary to stabilize the metal centers.
The process takes place in a clear solution without any measurable particles forming – so it is a homogeneous system. In addition to hydrogen, two liquid by-products are created: formaldehyde and formic acid. The analysis showed that both compounds are formed in almost identical amounts.
This is how the process works at room temperature
A significant practical advantage of the new process lies in the operating conditions. While comparable systems with precious metal complexes often require temperatures of over 90 degrees Celsius, the iron-based system already works at room temperature.
UV light in the range of 250 to 385 nanometers drives the reaction. If you turn off the light source, hydrogen production stops immediately. The light not only provides the starting impulse, but also continuously supplies the process with energy.
The addition of caustic soda is absolutely necessary for efficient implementation. It acts as a chemical door opener: the lye prepares the methanol so that the iron can bind it better. Oxygen from the air also plays a crucial role.
Hydrogen from methanol
In an oxygen-free environment, hydrogen formation does not occur. The kinetics of the reaction follow an equilibrium in which the lye optimizes the combination of the methanol with the iron ion. However, the exact mechanism of how oxygen is involved in the reaction still needs to be further researched.
The performance of the system is mixed compared to other methods. The quantum yield is 5.9 percent, measured under a specific 365-nanometer LED light source. This is well below the 90.8 percent achieved with platinum-titanium dioxide catalysts.
The drop in performance is severe in aqueous solutions. In a mixture of methanol and water, the system only achieves a hundredth of the yield possible in pure methanol. The process is currently practically meaningless for use in water-containing biomass.
What the process means for the hydrogen industry
Despite the limitations in quantum yield and in water, the process offers new perspectives. Using ferric chloride as a catalyst is extremely cost-effective and avoids dependence on rare raw materials.
The stability of the system allows continuous operation of at least 72 hours and reuse of the catalyst without significant loss of activity. A first attempt to enlarge it shows that the approach also works outside the test tube: after 15 hours, the system produced over 1.5 milliliters of hydrogen gas in a 50 milliliter flask.
The authors of the study, published in the journal Communications Chemistry, still see a need for clarification. The exact reaction mechanism is not yet fully understood. In addition, the efficiency in aqueous environments must increase to make the process practical for the use of biomass. According to the researchers, further development of the system could help reduce the costs of hydrogen production in the long term.
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