Photoreduction of CO2 in transportable fuel such as formic acid (HCOOH) is a great way to work with CO. to go2rising levels in the atmosphere. To aid in this mission, a Tokyo Tech research team chose a readily available iron-based mineral and loaded it onto an alumina support to develop a catalyst that can efficiently convert CO.2 in HCOOH with ~90% selectivity!
The rising CO2 levels in our atmosphere and their contribution to global warming is now common news. As researchers experiment with different ways to tackle this problem, one efficient solution has emerged: converting excess atmospheric CO2 converted into energy-rich chemicals.
Production of fuels such as formic acid (HCOOH) by photoreduction of CO2 under sunlight has attracted a lot of attention lately because of the twofold benefit that can be gained from this process: it can reduce excess CO2 emissions, and also to minimize the energy shortage we are currently experiencing. HCOOH is an excellent carrier of hydrogen with a high energy density and can provide energy via combustion while only releasing water as a by-product.
To make this lucrative solution a reality, scientists developed photocatalytic systems that contain CO. could reduce2 using sunlight. Such a system consists of a light-absorbing substrate (i.e., a photosensitizer) and a catalyst that enables the multi-electron transfers needed to reduce CO2 in HCOOH. And so the search for a suitable and efficient catalyst began!
Solid catalysts have been considered the best candidates for this task, due to their efficiency and potential recyclability, and over the years, the catalytic capability of many cobalt, manganese, nickel and iron-based metal-organic frameworks (MOFs) has been investigated, with the latter has some advantages over other metals. However, most of the iron-based catalysts reported so far yield only carbon monoxide as the main product, rather than HCOOH.
However, this problem was quickly solved by a team of researchers from the Tokyo Institute of Technology (Tokyo Tech) led by Prof. Kazuhiko Maeda. In a recent study published in Angewandte Chemistrythe team presented an alumina (Al2O3)-supported iron-based catalyst using alpha iron(III) oxyhydroxide (?-FeOOH; geothite). The new ?-FeOOH/Al2O3 catalyst showed superior CO2 to HCOOH conversion properties in addition to excellent recyclability. When asked about their choice of catalyst, Prof. Maeda says, “We wanted more abundant elements as catalysts in a CO2 photo reduction system. We need a solid catalyst that is active, recyclable, non-toxic and inexpensive, so we chose a widely distributed soil mineral such as goethite for our experiments.”
The team adopted a simple impregnation method to synthesize their catalyst. They then used the iron-laden Al2O3 material for photocatalytic reduction of CO2 at room temperature in the presence of a ruthenium-based (Ru) photosensitizer, an electron donor, and visible light with a wavelength greater than 400 nanometers.
The results were quite encouraging; their system showed 80-90% selectivity to the parent product, HCOOH, and a quantum yield of 4.3% (indicating the efficiency of the system).
This study presents a first of its kind, iron-based solid catalyst that can generate HCOOH in combination with an effective photosensitizer. It also explores the importance of good support material (Al2O3) and its effect on the photochemical reduction reaction.
The insights from this research can help in the development of new catalysts – free of precious metals – for the photoreduction of CO2 in other beneficial chemicals. “Our research shows that the path to a greener energy economy does not have to be complicated. Even by using simple catalyst preparation methods, great results can be achieved and known, earth-rich compounds can be used as selective catalysts for CO2 reduction, if they are supported by compounds such as aluminum oxide,” concludes Prof. Maeda.
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