How catalysts remove dangerous nitrogen oxides

Davide Ferri (left) and Filippo Buttignol in the laboratory where they used infr
Davide Ferri (left) and Filippo Buttignol in the laboratory where they used infrared spectroscopy to study how zeolite catalysts work. © Paul Scherrer Institute PSI/Mahir Dzambegovic
Catalysts belonging to the zeolite family help to remove toxic nitrogen oxides from industrial emissions. Researchers at the Paul Scherrer Institute PSI have now discovered that their complex nano porous structure is crucial. Specifically, individual iron atoms sitting in certain neighbouring pores communicate with each other, thereby driving the desired reaction.

Industry produces gases that are harmful to both humans and the environment and therefore must be prevented from escaping. These include nitric oxide and nitrous oxide, the latter also known as laughing gas. Both can be produced simultaneously when manufacturing fertilisers, for example. To remove them from the waste gases, companies use zeolite-based catalysts. Researchers at the Paul Scherrer Institute PSI, in collaboration with the Swiss chemical company CASALE SA, have now worked out the details of how these catalysts render the combination of these two nitrogen oxides harmless. The results of their research have been published in the journal Nature Catalysis and provide clues as to how the catalysts could be improved in the future.

An entire zoo of iron species

"The Lugano-based company CASALE contacted us because they wanted to develop a better understanding of how their catalysts used for the abatement of nitrogen oxide actually work," says Davide Ferri, head of the Applied Catalysis and Spectroscopy research group at the PSI Center for Energy and Environmental Sciences. The zeolites used for this are composed of aluminium, oxygen and silicon atoms forming a kind of framework. Zeolites occur naturally - as minerals in rock formations, for example - or they can be manufactured synthetically. Many catalysts used in the chemical industry are based on these compounds, with additional elements added to the basic structure depending on the specific application.

When the zeolite framework also contains iron as an active substance, it enables the conversion of the two nitrogen oxides, nitric oxide (NO) and nitrous oxide(N2O), into harmless molecules. "However, these iron atoms can be located in many different positions of the zeolite framework and can possess various forms," says Filippo Buttignol, a member of Ferri’s group. He is the principal author of the new study, which he conducted as part of his doctoral thesis. "The iron can lodge in the small spaces of the zeolite in the form of single atoms, or else several iron atoms can bound together and with oxygen atoms in slightly larger spaces in the regular lattice as diatomic, multiatomic or polyatomic clusters." In short, the catalyst contains an entire zoo of different iron species. "We wanted to know which of these iron species is actually responsible for the catalysis of nitrogen oxides."

The researchers, who specialise in spectroscopic analyses, knew exactly which three types of experiment they needed to carry out to answer this question. They performed these while the catalytic reaction was taking place in their zeolite sample. First they used the Swiss Light Source SLS at PSI to analyse the process using X-ray absorption spectroscopy. "This allowed us to look at all the iron species simultaneously," explains Buttignol. Next, in collaboration with ETH Zurich, they used electron paramagnetic resonance spectroscopy to identify the contribution of each species. And finally - again at PSI - the scientists used infrared spectroscopy to determine the molecular aspect of the different iron species.

Catalyst: A material that enables a chemical reaction to take place which would otherwise be much more difficult to achieve. Individual atoms or agglomerates of atoms of the catalytic material can move to and from between different chemical states (see redox reaction), but always return to their original state. This means that a catalyst is neither consumed nor permanently altered during the process.

Spectroscopy: Spectroscopic analyses use visible light or other parts of the electromagnetic spectrum (including ultraviolet and infrared radiation, as well as X-rays, microwaves and other spectral ranges, all’of which are invisible to the human eye). Many different techniques exist, which differ in their details. What they all’have in common is that the light interacts with the sample and the result reveals information about certain aspects or properties of the sample.

X-ray absorption spectroscopy (XAS): This particular spectroscopic analysis uses X-rays. The sample absorbs individual parts of the X-ray spectrum, allowing researchers to deduce certain properties of the sample.

Electron paramagnetic resonance (EPR) spectroscopy: This involves placing the sample in a magnetic field and simultaneously irradiating it with microwaves.

Infrared spectroscopy: The infrared range of the spectrum can be used to excite vibrations or rotations of molecules. This means that infrared spectroscopy can be used to quantitatively characterise known substances or to determine the structure of unknown substances.

Tetrahedron: A tetrahedron is a pyramid whose base is a triangle (as are all’its sides).

Redox reaction: The term redox reaction is a portmanteau for "reduction-oxidation" reaction. In a redox reaction, two chemical substances - a reducing agent or reductant and an oxidising agent or oxidant - exchange electrons. The former loses or donates electrons, while the latter gains or accepts them.

Where do the pollutants come from?

To find the sources of the pollution, the researchers measured the air composition on the roof of the Beijing University of Chemical Technology in a project on and compared the sources of smog during the summer and the winter months. A new type of mass spectrometer was used, which can analyse the ambient aerosols’ molecular composition in real time. This molecular information makes it possible to identify the sources of pollution. The scientists distinguish between primary aerosols - i.e. solid and liquid suspended particles that are emitted into the atmosphere - and secondary aerosols - which form as they travel through the atmosphere. The latter are particularly important in Beijing.

Differences between summer and winter

Dällenbach and his colleagues found that the sources of particulate matter extend far beyond the capital city and that these sources differ in chemical and geographical terms between the summer and the winter months. In winter, the secondary organic aerosols are caused by the combustion of wood and coal and originate for the main part in the greater Beijing-Tianjin-Hebei region. In summer, on the other hand, air flows in from the south and pollution is dominated by urban emissions, for example from traffic and industry, probably from the Xi’an-Shanghai-Beijing belt.

"Our work shows that although we are focusing on pollution within Beijing, smog is a large-scale regional phenomenon in which aerosols from different sources are transported over hundreds of kilometres," explains Dällenbach. Reducing air pollution therefore calls for coordinated and large-scale measures across the entire Beijing metropolitan area and beyond. Dällenbach and his research group are also using the methods they have developed to understand smog in Europe, as well as in poorly represented urban centres in the Global South.