French and UK scientists have developed a spectroscopy technique that has elucidated the reaction mechanism of a silver-alumina catalyst. The researchers say their approach should allow scientists to fine tune both this catalyst and other industrial heterogeneous catalysts to improve performance.

Silver-alumina catalysts are used in lean burn engines, which conserve fossil fuels and limit carbon dioxide emissions but produce nitrogen oxide, a greenhouse gas. Silver-alumina catalysts help remove this nitrogen oxide by reacting it with carbon monoxide. However, the lack of suitable experimental methods to help clarify exactly how supported precious metal catalysts like this work at the molecular level has proved a major obstacle in improving efficiency.

Now, Frédéric Thibault-Starzyk from ENSICAEN, the Université de Caen, France, and colleagues, have combined a high-time resolution Fourier-transform infrared spectrometer (FTIS) with a femtosecond laser and revealed a key intermediate step in the reaction between carbon monoxide and nitric oxide. Using their technique they observed a 2 microsecond flip of a cyanide group from a silver nanoparticle to the alumina support, revealing the importance of the silver-alumina interface.

Thibault-Starzyk and Heike Arnolds, now at the University of Liverpool, UK, triggered the reaction of the silver-alumina catalyst using a femtolaser at David A King's lab at the University of Cambridge, UK. 'Our idea for using a femtosecond laser was that we would avoid the actual heating of the whole surface and thus prevent thermal desorption of the molecules,' explains Thibault-Starzyk. 

The team then tracked the reaction using a FTIS, the time resolution capabilities of which meant they could completely change the timescale at which they were looking at the chemistry. 'People using FTIS tend to look at around ten spectra a second,' says Thibault-Starzyk. 'So going to the nanosecond was a complete change of approach and allowed us to see molecular movement on the surface of the catalyst.' 

The new intermediate the team detected between the silver cluster and the alumina support may lead to more efficient silver-alumina catalysts for use in lean burn engines, or in any metal-supported catalysis, suggests Thibault-Starzyk.

Luca Lietti, a catalysis expert at the Polytechnico de Milano, Italy, agrees. He thinks the technique could be applied to many processes and 'could be very interesting for people working in the field of catalysis and the detection of intermediates in general.' By knowing how a reaction proceeds and knowing the intermediates participating in the reaction, Lietti suggests that more active and more selective catalysts could be developed. 

James Urquhart

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