Defect concentration on rutile surfaces of both monocrystals and powder particles can be determined by infrared spectroscopy using carbon monoxide as a probe molecule. Credit: Graphics: Dr. M. Xu, RUB |
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 renowned 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, technically
the 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 importance to the catalytic activity of the
oxide powder and, hence, to the yield.
