Researchers Determine Key Intermediate Step in NOx Reduction on Alumina-Supported Silver Catalysts

22 May 2009

Reaction mechanisms for the deNOx reaction on an alumina-supported silver catalyst. Source: Thibault-Starzyk et al. Click to enlarge.

Using a new experimental method, researchers in France and the UK have identified the key intermediate step in the reaction between carbon monoxide and nitric oxide on a silver-alumina catalyst for reduction of NO_x in the exhaust from fuel-efficient lean-burn automotive engines.

Using femtosecond laser excitation followed by nanosecond time-resolved in situ Fourier-transform infrared spectroscopy to initiate a catalytic reaction on alumina-supported silver catalysts, they found that a cyanide group flips from a silver nanoparticle to the alumina support (with a lifetime of 2 microseconds).

This, they said, indicates the central role played by the interface between the metal particle and the oxide support. A paper on their work is published in the 22 May issue of the journal Science.

While lean-burn engines help to conserve fossil fuels and limit the rate of CO₂ emissions from motor vehicles, standard three-way catalysts cannot reduce NO_x under the excess of oxygen in the lean-burn engine exhaust. One approach for NO_x reduction is the use of NO_x storage and reduction catalysts. Silver-alumina catalysts are particularly adaptable to this task, the authors write, with their excellent properties for hydrocarbon activation, NO_x reduction, and sulfur tolerance.

One of the central reactions is that between NO and CO, but there is still much speculation as to the mechanistic details of NO_x reduction on silver-alumina catalysts.

Some of us succeeded in establishing the importance of isocyanate species in the pathway from C₂H₅OH + NO + O₂ reaction to N₂ formation on a Ag/Al₂O₃ sample, especially concerning the coordination and transformation sites, via the use of isotopically labeled species in infrared (IR) spectroscopy. We had proposed a tentative reaction mechanism involving intermediates between –CN and –NCO species on the basis of acid-base considerations.

Nonetheless, no clear evidence for these intermediates has been reported, presumably because they are too short-lived to be detected with conventional methods, which only allow millisecond time resolution. Thus, we have developed a spectroscopic technique with higher time resolution, and we show that the key intermediate in the reduction of NO by CO is a CN group in a bridge formation between a silver particle and the alumina support with a lifetime of only 2 µs.

—Thibault-Starzyk et al.

Understanding the reaction mechanism, the authors said, can enable developing improved catalytic activity for NO_x conversion. One example would be tuning the ratio of Al^VI versus AL^IV with the calcination temperature of alumina.

The alumina support has been shown to have a profound influence on the dispersion of silver clusters on the catalyst, and catalytically active Ag_n⁺ clusters form on acid sites on the alumina support. The bridged intermediate reported here can only be formed at the interface between the silver cluster and the support, and the corresponding reaction mechanism explains the key role of the size of silver clusters in catalytic activity.

—Thibault-Starzyk et al.

Resources

• Frédéric Thibault-Starzyk, Etienne Seguin, Sébastien Thomas, Marco Daturi, Heike Arnolds, David A. King (2009) Real-Time Infrared Detection of Cyanide Flip on Silver-Alumina NO_x Removal Catalyst. Science Vol. 324. no. 5930, pp. 1048 - 1051 doi: 10.1126/science.1169041