Perseverance Required: Surface Science

But this is not the only place where catalysis plays an important role in your life. Without catalysis you wouldn't be living the life you live. Catalysis is responsible not only for cleaning up exhaust gases from cars and industrial plants but also for making possible the production of a vast majority of chemicals. And modern life is highly dependent on chemicals. Think of what you've been doing today?perhaps you put on a polyester jumper in the morning (it's made from chemical substances), and went to work by car or bus (which run on a specific chemical, petrol), and right before lunch you took a headache pill (made from chemicals). All of these are made with the help of catalysts.

There is one thing that characterises most catalysts, and that is the surface of the catalyst material. We ask, what is so special about the surface? Well, it is the surface of the material that comes into contact with the chemical that is to be converted into something else. It is the surface properties and the interaction of the surface with the chemicals that determine what happens in the catalytic process. A subtle change in the catalyst surface might completely alter the chemical process and, for example, result in an unwanted chemical product. The consequences can be devastating. Just imagine if your headache pill contained the wrong chemical?you might be poisoned! And in this context, subtle is really subtle. The precise properties of individual atoms of the surface make all the difference!

Surface atoms made visible: The left-hand image shows a silver surface recorded by a scanning tunnelling microscope, an instrument which images surfaces at the atomic scale. Each bright feature corresponds to a single silver atom. The center image is that of an oxidised silver surface. With patience, the right instrument, an some luck, we can obtain more highly resolved images, as seen on the right-hand side, which shows the exact same surface as the middle image. Comparing the two images shows that the bright features of the middle image do not correspond to single silver atoms. The real distribution of silver atoms (grey spheres), and oxygen atoms (red spheres) is shown as an overlay in the right-hand image.

Thus, surfaces are extremely important. This is true not only for catalysis but also for a large number of other modern technologies. The reason is that every material has a surface, and it is the surface that comes into contact with other materials.

Surfaces are important, but they are also difficult to understand! Understanding surfaces means understanding what the individual surface atoms do, their properties and how we can control these properties. To obtain an understanding, we need proper tools. One such tool, the scanning tunnelling microscope, was invented about 30 years ago by Gerd Binnig and Heinrich Rohrer at IBM Research, Zurich. This fantastic invention allows a researcher to image individual atoms of a surface under certain conditions.

Because things are different at the atomic scale, imaging with a scanning tunnelling microscope is not always easy. For example, oxidised silver surfaces are difficult to image. Silver surfaces in an oxygen atmosphere have long attracted the interest of scientists because they are excellent catalysts for some very important industrial oxidation reactions. But what is the state of the silver surface during the catalytic process?

This question is difficult to answer, because the industrial catalyst is a very complicated device with many components. Therefore, scientists have been asking a much simpler question: what is the atomic-scale structure of an oxidised silver surface? The particular silver surface shown in the left-hand panel of the image has been studied over and over again since the 1960s, and different models of its structure after oxidation have been proposed (see figure: a scanning tunnelling microscopy image of the oxidised surfaces is shown in the middle panel). But there have been signs that these models were all wrong. Therefore, a couple of years ago, two collaborations? in Aarhus and Berlin, and in Vienna, Stuttgart, and Lund?took up the quest to re-investigate this surface. It took more than a year of calculations and experiments with, among others, scanning tunnelling microscopy, to determine the true structure of this surface. One reason it took so long was that the microscope did not show the exact placement of the individual surface atoms?the result of the influence of neighbouring surface atoms on each other. It took one year of our own work, and, in fact, 35 years of the work of many other researchers, just to find the location of the atoms on that surface!

You can imagine how much more complicated it is to really control the structure of a surface at the atomic scale. Control is what is really needed for advanced technologies, and presently, tools for atomic-scale control are being developed by scientists all around the world. But my guess is that it will take us several decades before we really, really know how to control reactions at the surface of oxidised silver.