under
close-to-reality conditions. (Photo: KIT-ITCP)
X-ray Methods Reveal Interactions of Active Metals with Gas
Molecules – Publication in Chemical Communications
(July 17, 2015) How
do catalytic converters work? Scientists of Karlsruhe Institute of Technology
(KIT) studied the reactions under close-to-reality conditions: With the help of
X-rays, they observed the interactions of the nitrogen monoxide pollutant
molecule and of the reduction agent ammonia with iron and copper centers, i.e.
transition metal ions in Fe-ZSM-5 and Cu-SSZ-13, where the reaction takes
place. Their results can now be used to further improve the exhaust gas
aftertreatment. The researchers present their approach in the journal Chemical
Communications. (DOI: 10.1039/C5CC01758K)
Modern catalytic converters for the treatment of exhaust
gases in vehicles with a combustion engine have largely contributed to reducing
of pollutant emissions. By oxidation or reduction, i.e. the donation or
acceptance of electrons, the catalysts convert combustion pollutants, such as
carbon monoxide, nitrogen oxides and hydrocarbons, into carbon dioxide, water,
and nitrogen. Increasingly strict emission regulations call for a further
reduction of fuel consumption and for an efficient use of the exhaust gas
aftertreatment system. When adding the reduction agent ammonia formed by e.g.
decomposition of urea, nitrogen oxides are converted into harmless nitrogen and
water vapor over the catalytic converter. For this purpose, typically an urea
solution (AdBlue®) is injected into the exhaust gas section upstream of the
catalytic converter.
To improve catalytic converters, it is required to precisely
understand their function and the individual reaction steps. “Reliable findings
relating to the reactions taking place may only be obtained under
close-to-reality conditions,” Professor Jan-Dierk Grunwaldt, Holder of the
Chair for Chemical Technology and Catalysis of KIT, says. “This means that we
have to watch the catalytic converters at work. Synchrotron radiation sources
are perfectly suited for this purpose.” Synchrotron radiation is
electromagnetic radiation (from infrared to hard X-rays) of several hundreds or
even a million electron-volts in energy. Using hard X-rays, the properties of
the active metal centers in the catalytic converter and their interactions with
the gas molecules can be observed. Two methods may be applied: (i) X-ray
absorption spectroscopy (XAS) allows the determination of oxidation state and
coordination number, i.e. the number of nearest neighbors of an atom; (ii)
X-ray emission spectroscopy (XES) which can be used to distinguish between
different molecules adsorbed on the catalytic converter. On this basis, it can
be concluded which molecules cause reduction, when competing adsorption takes
place, i.e. if several substances compete for adsorption on catalytic
converters, and how individual molecules coordinate on the metal atom.