Photoreduction of CO2 into transportable fuel like formic acid (HCOOH) is a great way to deal with CO2increasing levels in the atmosphere. To aid in this mission, a Tokyo Tech research team took a readily available iron-based mineral and loaded it onto an alumina carrier to develop a catalyst capable of efficiently converting CO2 in HCOOH with a selectivity of about 90%!
CO on the rise2 levels in our atmosphere and their contribution to global warming are now mainstream news. As researchers experiment with different ways to combat this problem, an effective solution has emerged: converting excess atmospheric CO2 energy-rich chemicals.
Production of fuels like formic acid (HCOOH) by photoreduction of CO2 under sunlight has attracted a lot of attention recently because of the dual benefit that can be gained from this process: it can reduce excess CO2 emissions, and also help to minimize the energy shortage we currently face. Being an excellent carrier of high energy density hydrogen, HCOOH can provide energy through combustion while releasing only water as a by-product.
To turn this lucrative solution into reality, scientists have developed photocatalytic systems capable of reducing CO2 using sunlight. Such a system consists of a light-absorbing substrate (i.e., a photosensitizer) and a catalyst that can enable the multi-electron transfers needed to reduce CO2 in HCOOH. And so began the search for a suitable and effective catalyst!
Solid catalysts have been considered the best candidates for this task, due to their efficiency and potential recyclability, and over the years the catalytic capabilities of many metal-organic frameworks (MOFs) based on cobalt, manganese , nickel and iron have been explored, with the latter having certain advantages over other metals. However, most iron-based catalysts reported so far only produce carbon monoxide as the main product, instead of HCOOH.
However, this problem was quickly solved by a team of researchers from the Tokyo Institute of Technology (Tokyo Tech) led by Professor Kazuhiko Maeda. In a recent study published in Angewandte Chemiethe team presented an alumina (Al2O3)-supported iron catalyst that uses iron(III) oxyhydroxide alpha (?-FeOOH; geothite). The new ?-FeOOH/Al2O3 the catalyst showed higher CO2 with HCOOH conversion properties as well as excellent recyclability. Asked about their choice of catalyst, Professor Maeda says: “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, so we chose a common soil mineral like goethite for our experiments.”
The team adopted a simple impregnation method to synthesize their catalyst. They then used the iron-loaded Al2O3 material for photocatalytic CO reduction2 at room temperature in the presence of a photosensitizer based on ruthenium (Ru), an electron donor and visible light with a wavelength greater than 400 nanometers.
The results were quite encouraging; their system showed 80-90% selectivity towards the main product, HCOOH, and a quantum yield of 4.3% (indicating the efficiency of the system).
This study presents a solid iron-based catalyst, the first of its kind, which can generate HCOOH when accompanied by an effective photosensitizer. It also explores the importance of a suitable support material (Al2O3) and its effect on the photochemical reduction reaction.
The results of this research could help in the development of new catalysts – without precious metals – for the photoreduction of CO2 in other useful chemicals. “Our study shows that the road to a greener energy economy need not be complicated. Great results can be achieved even by adopting simple methods of preparing catalysts and well-known and earth-abundant compounds can be used as selective catalysts for CO2 reduction, if they are supported by compounds like alumina,” concludes Professor Maeda.
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