Producing Hydrogen Peroxide Using Only Water, Oxygen, and Sunlight

KAIST Develops Low-Cost, High-Efficiency Metal Oxide Catalysts

Schematic diagram of hydrogen peroxide production using water and sunlight from oxygen with metal oxides. Provided by KAIST

[Asia Economy Reporter Kim Bong-su] Domestic researchers have developed a high-efficiency catalyst that produces hydrogen peroxide at low cost and high efficiency using only water, oxygen, and sunlight.

The Korea Advanced Institute of Science and Technology (KAIST) announced on the 31st that Professor Kang Jung-gu's research team in the Department of Materials Science and Engineering developed a high-efficiency catalyst that produces hydrogen peroxide using sunlight with only water and oxygen. Hydrogen peroxide is a useful resource widely used in various industries such as disinfection, dyeing, oxidizing agents, pharmaceuticals, semiconductors, displays, and rocket propellants.

The nanostructured catalyst developed by the research team absorbs light and selectively reduces oxygen molecules to hydrogen peroxide molecules. Since it uses abundant and eco-friendly water on Earth as the oxidizing agent, it is an environmentally friendly and economical core technology. This technology uses cobalt, titanium, and iron oxides, which are 1,500 times, 4,500 times, and 115,000 times cheaper, respectively, than the expensive palladium catalysts currently used in the process. Not only is it economical, but it also has eco-friendly characteristics because it produces hydrogen peroxide using only water, oxygen, and sunlight without organic compounds that cause environmental problems.

Currently, most hydrogen peroxide production is done through the "anthraquinone process." This process uses high-pressure hydrogen gas and expensive palladium-based hydrogenation catalysts, which pose economic and safety issues, and releases organic pollutants during the reaction, causing environmental problems.

On the other hand, photocatalysts that use sunlight as an energy source to reduce oxygen to hydrogen peroxide can use transition metal oxides with semiconductor properties physically, making them thousands of times cheaper than existing palladium catalysts. Additionally, they have safe and eco-friendly characteristics because they can produce hydrogen peroxide from abundant oxygen on Earth using solar energy. However, existing photocatalysts for hydrogen peroxide production required the addition of more expensive alcohol oxidants than hydrogen peroxide to transfer electrons in the oxidation reaction from oxygen to hydrogen peroxide. Also, the produced hydrogen peroxide decomposed rapidly on the photocatalyst surface, reducing catalytic efficiency.

In response, the research team nanostructured cobalt, titanium, and iron oxides, which are much cheaper than expensive palladium catalysts, using the urea-hydrothermal synthesis method. For metal oxides with two or more metal combinations, different metals generally mix to form a single-phase structure. However, the team increased the ratio of cobalt precursors to separate iron and cobalt oxides, then used the chemical instability of divalent iron oxide to separate titanium oxide again, synthesizing a triphasic metal oxide where three different metal oxides are separated into their respective oxide phases.

Tri-metal oxide electron microscope image (left) and schematic diagram of the catalytic reaction (right). Provided by KAIST.

The triphasic oxide photocatalyst has a unique structure where two-dimensional wide nanosheets of cobalt oxide exist, and iron oxide-titanium oxide nanoparticles with a core-shell structure are arranged on top. The research team also successfully demonstrated through computational science that the core-shell structured nanoparticles efficiently absorb visible and ultraviolet light and transfer electrons.

Cobalt oxide is well known as a catalyst for water oxidation reactions, capable of adsorbing water molecules, reducing them to oxygen, and providing electrons. In other words, since water is used as the oxidizing agent, it can smoothly transfer electrons to the reduction reaction site without using alcohols as in conventional photocatalysts. Meanwhile, the iron oxide-titanium oxide core-shell nanoparticles can absorb visible and ultraviolet light, efficiently capturing solar energy, and have excellent oxygen adsorption ability, selectively adsorbing oxygen molecules as reactants. Structurally arranged on cobalt oxide nanosheets, electrons generated from water oxidation are received by iron oxide and efficiently transferred to titanium oxide, producing hydrogen peroxide through oxygen reduction reactions. The hydrogen peroxide produced in this way is not decomposed but stably concentrated due to the structural characteristic of the photocatalyst where reduction and oxidation sites are separated.

Professor Kang said, "This eco-friendly technology using renewable energy is highly safe because it does not use hydrogen molecules or organic substances, and it is economical because it uses relatively inexpensive transition metal oxides. Since oxygen reduction reactions, electron-hole transport, and water oxidation reactions occur in each of the three phases, it overcomes the problems of hydrogen peroxide decomposition and alcohol oxidant use in photocatalysts. The high catalytic efficiency achieved is thousands of times cheaper than the most efficient precious metal catalysts known so far and has about 30 times higher productivity, contributing to the commercialization of hydrogen peroxide production through photocatalysts."

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The research results were published online on the 25th of last month in the international materials science journal 'Advanced Energy Materials' (IF 29.37).