General info

People

Projects

Publications

Our Hardware

Our Software

Information for students

Vacancies

Links

Chrétien Hermse finished his Ph.D with the writing of his thesis, online available from the library.

My research

My main research topic lies in describing the behaviour of small molecules on metal surfaces. The movie above gives an impression of what happens when NO molecules bind to a small rhodium catalyst particle of 10x10 nm. The NO molecules, in light green, first adsorb randomly on the surface and hop around. After a while they start to order into zig-zag lines, this is when about 50% of the surface is covered (see the number on the right side of the picture). Then a second type of NO molecules, in dark green, start appearing on the surface. These molecules bind in a different way than the other molecules, and a new ordered structure is formed with now approximately 70% of the surface covered. Take a look at individual frames.

Why did we look at this system?

Rhodium is used at industrial scale in car exhaust gas catalysis. Every car nowadays runs around with a catalyst composed of platinum, palladium and last but definitely not least rhodium particles. The rhodium particles are used to reduce the NO in the exhaus gasses to nitrogen. The use of rhodium on an industrial scale for exhaust gas catalysis has caused many detailed studies on well-defined model catalysts, giving us many experiments to compare our simulations to.

Why is this movie special?

Sacnning tunneling microscopy (STM) has been a great hype in the last decade, and you can try to image individual molecules like the ones we simulate on surfaces. But... it is incredibly slow. It takes minutes to record such detailed images. These molecules at room temperature perform several thousands hops over the surface, thus it is impossible to image them individually. So what experimentalists usually do, is that they cool their system down, such that the molecules are `frozen', and don't move anymore. The other thing they can do is that they completely fill the surface with molecules, in that case (think of people in a train during rush hour), the molecules can't move either.

STM is therefore nice, but can not tell you anything about partially filled surfaces at reaction temperatures: the only thing you will see is a blur. In our simulations on the other hand, we are fully in control, and so we can study what happens under realistic conditions. In this way we can study the ordering, segragation and reaction of different molecules. And we make fun movies too.

How did we make this movie?

The light green NO molecules in the movie are binding to three surface rhodium atoms. This is the most stable way to bind NO molecules. Since binding of two NO molecules to the same rhodium atom is impossible, at a certain coverage further adsorption in this way is blocked. It is then that NO molecules start binding on top of a single rhodium atom. This is the second type of NO, displayed in dark green. This type of NO binds only weakly, and usually does not react on the surface. The fact that these NO molecules cannot bind to the same rhodium atom comes from electronic structure calculations, which are quite straight-forward. Especially if you have someone else do them for you. Based on these calculations it turns out that depending on the distance between the NO molecules, there is a repulsion. At short distance, this repulsion is large, and effectively blocks adsorption there. At larger distance the repulsion is smaller, and bonding to those sites is possible.

Who else was involved in this work?

• my coach and copromotor Tonek Jansen;
• my promotor Rutger van Santen;
• Sander van Bavel, who does experimental measurements on this sytem;
• Johan Lukkien, who wrote the Monte-Carlo program;
• Frederic Frechard, who did the electronic structure calculations.

These pages are maintained by Bouke Bunnik (theory@www.catalysis.nl). Comments and suggestions are welcome.