ScienceDaily (Apr. 10, 2012) —
In chemical industry, heterogeneous catalysis is of crucial importance
to the manufacture of basic or fine chemicals, in catalytic converters
of exhaust gas, or for the chemical storage of solar energy. Scientists
of Karlsruhe Institute of Technology (KIT) and Ruhr-Universität Bochum
(RUB) have developed a new infrared spectroscopy method in order to
study processes at surfaces of oxides used as catalysts.
Their results are published in the Angewandte Chemie journal.
Catalysts support many chemical reactions. In heterogeneous catalysis,
the substance used as a catalyst and the reacting substances exist in
various phases. Usually, the catalyst is a solid, while the reacting
substances are gaseous. At the surface of catalytically active solids,
highly complex chemical processes take place. They have to be understood
in detail in order to further improve products and reduce costs. The
processes are known well for metals. However, conversions at the surface
of oxides – compounds of metals or nonmetals with oxygen – have hardly
been studied so far.
The research team of Professor Christof Wöll from KIT and Professor
Martin Muhler from RUB first studied processes at surfaces of oxide
monocrystals and then transferred the findings to powders, the
technically most important form of oxide materials. Doing this, they
were the first to bridge the gap between fundamental research into
reference systems and applied research into real catalysts. A newly
developed combination device for infrared spectroscopy (IR) allows for
highly precise measurements of the vibration frequency of carbon
monoxide. The exact value of this vibration frequency is highly
sensitive to defects.
Such defects result from the removal of individual oxygen atoms from
oxide materials. “Oxygen defects act as active centers and give the
material a high catalytic activity,” explains Professor Christof Wöll,
Director of the Institute of Functional Interfaces (IFG) of KIT. With
the new combination device for infrared spectroscopy, the researchers
from Karlsruhe and Bochum developed a method that was first calibrated
for reference systems. For the first time, they then measured defect
densities of real catalyst powders using a high-performance FTIR
spectrometer made by Bruker Optics (VERTEX series).
To demonstrate their new method, the researchers used rutile, the most important modification of titanium dioxide (TiO2).
“This material used as white pigment and in photocatalysis normally is
chemically highly inert and rendered catalytically active by the
oxygen defects only,” explains Professor Christof Wöll. Professor
Martin Muhler from RUB points out that such defects in powder materials
have only been detected indirectly so far.
With their method, the researchers, including Dr. Mingchun Xu, Dr.
Heshmat Noei, and Dr. Yuemin Wang from RUB as well as Dr. Karin Fink
from the Institute of Nanotechnology (INT) of KIT, followed the “Surface
Science” approach developed by the Noble Prize laureate Gerhard Ertl.
They demonstrated the potential of their method by studying the
carbon-carbon coupling reaction of formaldehyde to ethylene. Doing this,
it was confirmed that the density of oxygen defects at the surface of
r-TiO2 nanoparticles is of decisive im-portance to the catalytic activity of the oxide powder and, hence, to the yield.
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- Mingchun Xu, Heshmat Noei, Karin Fink, Martin Muhler, Yuemin Wang, Christof Wöll. Anwendung des oberflächenwissenschaftlichen Ansatzes auf Reaktionen an Oxidpulvern: die Bedeutung der IR-Spektroskopie. Angewandte Chemie, 2012; DOI: 10.1002/ange.201200585