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.
Technical terms explained
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.
Catalysis happens at individual but communicating atoms
Each of these three methods contributed a piece of the puzzle, eventually leading to the following overall picture: Catalysis takes place at single iron atoms which are located in two very specific, neighbouring sites of the zeolite lattice. During the process, these two iron atoms act in concert with each other. One of them, sitting at the centre of four oxygen atoms in the zeolite arranged in the form of a square and responsible specifically to convert nitrous oxide, communicates with a different iron atom, which is surrounded by oxygen atoms arranged in the form of a tetrahedron and at which the nitric oxide reacts.
“Only where this precise arrangement is found do we see iron contributing to the catalysis of the simultaneous abatement of the two gases,” says Buttignol. Each of these iron atoms gave up an electron and took it back again, in other words the typical redox reaction of catalysis took place there over and over again.
Removing hazardous nitrogen oxides more efficiently
Ferri sums up the significance of the new study: “If you know exactly where the chemical reaction takes place, you can start adjusting the manufacture of catalysts accordingly.”
The catalysis of nitric oxide and nitrous oxide and thus their removal from industrial waste gases is important because both are toxic to humans. Beyond this, both gases are also harmful to the environment: nitric oxide is one of the causes of acid rain, while nitrous oxide has such a strong impact on the climate that one molecule of it contributes almost 300 times more to the greenhouse effect than a molecule of carbon dioxide.
Text: Paul Scherrer Institute PSI/Laura Hennemann