A new material converts NOx environmental pollutants into harmless byproducts without the use of heat.
Scientists have discovered a new sustainable material that can convert nitrogen oxide – a smog-causing gas produced by the combustion of gasoline and diesel – into oxygen and nitrogen at room temperature.
While the exhaust systems of modern petrol vehicles are equipped with three-way catalysts that help to remove the pollutants carbon monoxide, hydrocarbons and nitrogen oxides (NOX) into less harmful gases, the problem is more complex with diesel engines, says Marco Daturi, a chemistry professor at the University of Caen Basse-Normandie in France.
“The strategy for many years has been to change the combustion parameters to reduce nitrogen oxide emissions at the expense of particle formation,” he said.
The standard technologies for NO removalXsuch as nitrogen oxide and nitrogen dioxide, from diesel exhaust are “lean NOX traps and catalysts. Although these systems are effective in decomposing NOXThey are expensive, complex and sensitive to temperature and poisoning by other gases present in the atmosphere, including oxygen and water.
Daturi said that the development of an efficient catalyst that can handle the latest NOX Compliance with emission limits is a priority.
To find a catalyst that could potentially outperform existing technologies while remaining sustainable, the researchers looked to nature for inspiration. “Millions of years of evolution are much more efficient than a few scientists,” commented Daturi.
Mimicking bacterial enzymes
Their catalyst design was inspired by the bacterial enzyme nitric oxide reductase, which converts nitric oxide (NO) into nitrous oxide (NO).2). The bacteria that produce this enzyme, found in soil, help keep the Earth’s nitrogen cycle in balance. But using living organisms as a source of the enzyme is not practical for industry, Daturi said.
“We tried to see if it was possible to develop a material that had the same properties as these enzymes but was strong and resilient,” he explained.
Daturi’s research focuses on metal-organic frameworks, a class of three-dimensional, porous polymers with a wealth of possible structures and compositions. In addition to catalysis, these diverse materials have applications in dye removal, drug delivery, carbon dioxide capture, and more.
Daturi, his colleague Christian Serre, research director at the CNRS in Paris, and their collaborators recognized the potential of metal-organic frameworks to mimic the active structure of nitric oxide reductase, which contains two iron ions coordinated with amino acids.
Through computer simulations and spectroscopic studies, they found that a particular iron-based metal-organic framework functioned very similarly to the natural process. Serre emphasized that the preparation of the iron-based metal-organic framework catalyst is straightforward and inexpensive and can be done in a variety of ways.
The simplest way, he said, is to mix an iron-containing salt with an organic compound in water below 100°C for two or three days. The product is then filtered, washed and dried overnight at room temperature.
MIL-100(Fe) catalyst takes shape
The catalyst, called MIL-100(Fe), has two different sites where the reaction takes place: a ferrous ion and a ferric ion bridged by an oxygen atom. Both active sites adsorb nitric oxide, but only ferric converts nitric oxide to nitrogen dioxide and is reduced to ferrous iron.
“At low temperatures, when nitric oxide and nitrogen dioxide are close to each other, they easily form nitrous oxide, which is unstable in the presence of water and decomposes into nitrous oxide and molecular oxygen,” Daturi explained.
Finally, nitrous oxide oxidizes the newly formed iron(II) site to iron(III), releasing nitrogen gas. The active sites are then ready for another catalytic cycle.
“The presence of oxygen and water in the atmosphere no longer poses a problem for nitrogen oxide decomposition – on the contrary, they are necessary for the process,” said Daturi.
He added that due to the technical and economic aspects of their approach, their discovery is not intended to replace established nitrogen oxide removal technologies, but can complement them.
In addition to the removal of nitrogen oxides, the MIL-100(Fe) catalyst may also be well suited for dehumidification and energy-efficient cooling systems due to its large pore volume and affinity for water.
“We have already developed environmentally friendly protocols to scale MIL-100(Fe) for industrial-scale production,” said Serre. “We have also contacted potential industrial partners and decision makers to facilitate the development of this technological solution.”
Reference: Marco Daturi, et al., Reduction of nitrogen oxides on iron-metal-organic frameworks at room temperatureAdvanced Materials (2024). DOI: 10.1002/adma.202403053
