Plasmons shine a light on catalysis
Oct 27, 2009
Catalysts are materials that accelerate chemical reactions without being consumed themselves. They play a vital role in the chemical industry and are also used to remove harmful substances from vehicle exhausts. Now, physicists in Sweden have devised a new way of monitoring catalytic processes in "real-life" situations. The technique uses collective electron oscillations called "surface plasmons" and is said to be better than current analytical methods, which often rely on ultrahigh vacuum (UHV) techniques and single-crystal samples.
Many catalytic systems, including those used in cars, consist of surfaces covered with tiny pieces of catalyst over which gases flow. Although such systems operate at (or above) atmospheric pressure, they are usually studied in very different environments – namely in ultra-clean UHV chambers using large, single-crystal samples. Disparities between what occurs in real systems and in such experiments are known as the "pressure" and "materials" gaps.
Mind the gap
The new technique, developed by Bengt Kasemo and colleagues at Chalmers University, involves studying catalytic processes at realistic pressures and particle sizes. What the team has done is to deposit about 30 nm of gold onto a glass slide, which is then dipped into a plastic colloid – tiny particles suspended in a liquid – that then dries to form a pattern of circles on the surface of the gold. Etching away the exposed gold leaves gold disks about 100 nm in diameter. The sample is then coated with a thin insulating film – about 10 nm deep – and then with nanometre-sized pieces of the catalyst platinum, which cover 10–20 % or less of the surface.
When light from an ordinary lamp is shone through the slide, radiation at certain wavelengths is absorbed to create surface plasmons – collective oscillations of electrons on the surfaces of the gold disks. The transmitted-light spectrum therefore has a dip in absorption at these wavelengths. Although physicists already knew that the position of this minimum shifts in the presence of platinum particles, Kasemo and colleagues discovered that the position also changes when certain molecules such as oxygen or carbon dioxide are adsorbed onto the surface of the platinum.
The team used the technique to study several common catalytic reactions, including the oxidation of carbon monoxide to create carbon dioxide. For example, by passing pure oxygen across the sample and then introducing carbon monoxide (CO), they could monitor how the CO turns into carbon dioxide simply by monitoring the position of the absorption minimum. In particular, the team found that when the CO concentration reached about 7% of the total gas, the minimum shifted suddenly.
This occurs because the platinum surface goes from being covered by oxygen – which supports catalysis – to being covered in carbon monoxide. The latter is referred to as the "poisoning" of the catalyst because it brings the reaction to a halt. Understanding exactly when this transition occurs is crucial in designing and operating catalytic systems. Kasemo and colleagues saw similar effects while using their sample to catalyze two other reactions – the oxidation of hydrogen and the conversion of nitrogen oxides to molecular nitrogen.
Sensitive to immediate environment
This new method relies on a well established technique of plasmonics, according to Bill Barnes of the University of Exeter in the UK. "The optical response is dominated by the localized surface-plasmon resonances supported by the gold disks, and such modes are well known to be very sensitive to their immediate optical environment," he explained.
Niek van Hulst of the Institute of Photonic Sciences in Barcelona is also impressed by the technique. "The elegance of the method is that only a transparent glass with nanoparticles needs to be mounted in the reaction chamber – it's simple and effective," he said.
The work is reported in Science.
About the author
Hamish Johnston is editor of physicsworld.com