Surface appearances: IR reveals catalytic details

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  • Published: May 1, 2012
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Surface appearances: IR reveals catalytic details

Surface appearances

German scientists have developed a new infrared technique that lets them study in detail the processes taking place at the surfaces of oxide catalysts. 
Credit: M. Xu, RUB

German scientists have developed a new infrared technique that lets them study in detail the processes taking place at the surfaces of oxide catalysts.

Heterogeneous catalysis is perhaps one of the most important areas of the chemical industry as it allows the effective manufacture of basic and fine chemicals from simple starting materials. In contrast to homogeneous, solution phase catalysts solid catalysts over which reactant solutions pass are more ready to be freshened up and re-used without having to extract them from the homogeneous system. However, there is no finer division of a catalyst than in solution and achieving equivalent efficiencies and turnovers with solid catalysts remains a challenge in the industry and with newer technologies such as catalytic converters for exhaust gases and for chemical storage of solar energy.

Any insights into how heterogeneous catalysts might be improved and so stave off the need to resort to the less convenient homogeneous analogue for a given reaction are always welcome. Now, scientists of Karlsruhe Institute of Technology (KIT) and Ruhr-Universität Bochum (RUB), Germany, report in Angewandte Chemie how such insights might come easier thanks to their development of a novel hybrid IR device that homes in on the vibrations of carbon monoxide, which are very sensitive to defects.

Active cats

The surface of catalytically active solids is key to their function and decades of chemical research have tried to focus on the highly complex processes that take place there. For metals, many of the processes are well understood. For metal oxide catalysts much less so and for non-metal oxide catalysts the picture is even less straightforward and barely touched.

Christof Woell from KIT and Martin Muhler from RUB have investigated mono-crystals and powders of metal oxides, which are technologically the most important group of catalytic materials. By taking these initial steps, the team was able to bridge the often vast gulf between fundamental research on reference systems and the world of real catalysts.

The real-world catalytic defects that Wöll, Muhler and their colleagues can see are down to the loss of individual oxygen atoms from the materials. These defects are actually beneficial rather than a problem. "Oxygen defects act as active centres and give the material a high catalytic activity, explains Wöll. The team was able to calibrate their IR system for reference systems and then for the first time measure defect densities in real catalyst powders using high-performance ultra-high vacuum (UHV) Fourier transform infrared spectroscopy (FTIR spectroscopy).

In a proof of principle experiment funded by the German Research Foundation (DFG) , the team investigated rutile, one of the most important modifications of titanium dioxide. "This material normally is chemically highly inert and rendered catalytically active by the oxygen defects only," explains Wöll. Such defects have only been observed by proxy in such powder materials previously.

Working with Mingchun Xu, Heshmat Noei and Yuemin Wang at RUB and Karin Fink from the Institute of Nanotechnology (INT) of KIT, the team was able to demonstrate the potential of their new method by studying the carbon-carbon coupling reaction of formaldehyde to ethylene. They showed that the density of oxygen defects at the surface of rutile-titanium dioxide is critical to catalytic activity.

Catalytic breakthrough

"Our real breakthrough is not the work on the powders, which has been done for decades, but the work on the macroscopic, centimetre-sized, single crystal," Woell told spectroscopyNOW. "Only when IRRAS-data for such well-defined model systems becomes available, can theory be used to put hypotheses on a solid basis. Due to technical limitations, intensity problems, IRRAS data could not be recorded for carbon monoxide adsorbed on titanium dioxide. To overcome these problems is the main step forward."

The next step will be to use the procedure to quantitatively determinine the active sites on oxide powder particles for other oxides, including zinc oxide and cerium oxide, Woell told us. "Ultimately, we want to understand chemistry and photochemisry on oxide particles in detail, using the so-called Surface Science Approach established by Gerhard Ertl," he adds.

Related Links

Angew Chem, 2012, online: "The Surface Science Approach for Understanding Reactions on Oxide Powders: The Importance of IR Spectroscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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