IR spectroscopy detects that oxygen defects act as active centres

In the chemical industry, heterogeneous catalysis is vital for the manufacture of basic or fine chemicals, in catalytic converters of exhaust gas and for the chemical storage of solar energy. Scientists from the Karlsruhe Institute of Technology (KIT) and Ruhr-Universität Bochum (RUB) in Germany have developed a new infrared spectroscopy method in order to study processes at surfaces of oxides used as catalysts; their results have been published in Angewandte Chemie (doi: 10.1002/ange.201200585).

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 improve further products and reduce costs. The processes are understood well for metals. However, conversions at the surface of oxides 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 IR spectroscopy allowed them to take 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 centres and give the material a high catalytic activity,” explains Professor Christof Wöll. With the new device, they 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 FT-IR spectrometer.

Chemistry of defects: Defect concentration on rutile surfaces of both monocrystals and powder particles can be determined by infrared spectroscopy using carbon monoxide as a probe molecule. (Graphics: Dr M. Xu, RUB.)

To demonstrate their new method, the researchers used rutile, the most important modification of TiO2. “This material used as white pigment and in photocatalysis normally is chemically highly inert and rendered catalytically active by the oxygen defects only,” explained Professor Christof Wöll. Professor Martin Muhler from RUB pointed 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 critical importance to the catalytic activity of the oxide powder and, hence, to the yield.

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