Scientists Develop Photocatalytic Systems That Reduce CO2 Using Sunlight

CO₂ photoreduction into a transportable fuel like formic acid (HCOOH) is an excellent strategy to combat growing CO₂ levels in the environment.

To help with this, a Tokyo Tech research team put an easily accessible iron-based mineral onto an alumina substrate to create a catalyst that can efficiently convert CO₂ into HCOOH with ~90% selectivity.

CO₂ levels in the atmosphere are growing, and their connection to global warming is now well-known. One effective option has arisen as researchers experiment with numerous approaches to combat this problem: turning extra atmospheric CO₂ into energy-rich molecules.

The production of fuels such as formic acid (HCOOH) by photoreduction of CO₂ under sunlight has lately received a lot of interest due to the two-fold benefit that this method can provide.

It can reduce excess CO₂ emissions while also helping to alleviate the present energy shortfall. HCOOH can produce energy via burning while only releasing water as a byproduct since it is a great transporter of hydrogen with a high energy density.

To make this profitable option a reality, scientists created photocatalytic devices that use light to reduce CO₂. A light-absorbing substrate (i.e., a photosensitizer) and a catalyst capable of enabling the multi-electron transfers necessary to convert CO₂ into HCOOH make up such a system. Thus began the quest for an appropriate and effective catalyst.

Due to their efficiency and potential recyclability, solid catalysts were deemed the best candidates for this task, and the catalytic abilities of many cobalt, manganese, nickel, and iron-based metal-organic frameworks (MOFs) have been investigated over the years, with the latter having some advantages over other metals.

However, instead of HCOOH, most iron-based catalysts described so far only produce carbon monoxide as the major product.

Nonetheless, a team of Tokyo Institute of Technology (Tokyo Tech) researchers led by Prof. Kazuhiko Maeda quickly addressed the problem.

The researchers presented an alumina (Al₂O₃)-supported iron-based catalyst that utilizes alpha-iron (III) oxyhydroxide (-FeOOH; geothite) in a recent paper published in Angewandte Chemie. The new α-FeOOH/Al2O3 catalyst demonstrated outstanding CO₂ to HCOOH conversion characteristics as well as recyclability.

We wanted to explore more abundant elements as catalysts in a CO₂ photoreduction system. We need a solid catalyst that is active, recyclable, non-toxic, and inexpensive, which is why we chose a widespread soil mineral like goethite for our experiments.

Kazuhiko Maeda, Professor, Tokyo Institute of Technology

To make its catalyst, the researchers used a simple impregnation procedure. They employed the iron-loaded Al₂O₃ material for photocatalytic CO₂ reduction at room temperature in the presence of a ruthenium-based (Ru) photosensitizer, an electron donor and visible light with a wavelength greater than 400 nm.

The results were promising. Their system had a selectivity of 80–90% for the major product, HCOOH, and a quantum yield of 4.3%, which indicates system efficiency.

When combined with an efficient photosensitizer, this study presents a first-of-its-kind iron-based solid catalyst that can create HCOOH. It also looks at the significance of a good support material (Al₂O₃) and how that affects the photochemical reduction process.

The findings of this study might assist in the creation of new catalysts for the photoreduction of CO₂ into other valuable compounds that are devoid of precious metals.

Prof Maeda concluded, “Our study shows that the road to a greener energy economy does not have to be complicated. Great results can be attained even by adopting simple catalyst preparation methods and well known, earth-abundant compounds can be used as selective catalysts for CO₂ reduction, if they are supported by compounds like alumina.”

Journal Reference:

An, D., et al. (2022) Alumina-Supported Alpha-Iron (III) Oxyhydroxide as a Recyclable Solid Catalyst for CO₂ Photoreduction under Visible Light. Angewandte Chemie doi:10.1002/anie.202204948.