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NOBEL PRIZE - CHEMISTRY

Ertl was one of the first to understand the potential of the new technology

His methodology served to shed light on highly complicated surface reactions

— PHOTO: AP

Ertl’s research laid the foundation of modern surface chemistry,which has helped explain how catalysts operate in cars, among other things.

One specific branch of chemistry concerned with reactions on solid surfaces instead requires advanced technical equipment like vacuum chambers, electron microscopes, and clean rooms.

Surface reactions play such an important role in both the chemical industry and natural systems. Knowledge of surface chemistry can help explain such diverse processes as why iron rusts, how artificial fertilizers are produced, and how the catalyst in a car’s exhaust pipe works.

Knowledge about chemical reactions on surfaces will also help us produce renewable fuels more efficiently and create new materials for electronics.

Modern surface chemistry started to emerge in the 1960s.

This year’s Nobel Laureate in Chemistry, Gerhard Ertl, was one of the first to understand the potential of the new technology. He is awarded the Nobel Prize for having laid the methodological foundations for an entire field of research.

The great reliability of Ertl’s results is due to the meticulous precision in his work combined with an outstanding capacity to refine problems.

He has painstakingly and systematically searched for the best experimental techniques to investigate each separate question.

Ertl has displayed a unique understanding of how to make use of different experimental technologies, and whenever possible he has been quick to incorporate new technologies in his palette — always in order to produce as complete a picture as possible of the reaction he has decided to investigate. Besides producing important knowledge about specific reactions, he has above all constructed a methodology that other researchers have been able to apply to completely different surface reactions.

Gerhard Ertl first studied the behaviour of hydrogen on metal surfaces. Hydrogen gas can be produced at one of the electrodes in an electrochemical solar cell, and can then be used in the reverse reaction to generate electricity in a fuel cell — just to mention a couple of cases where the behaviour of hydrogen on solid surfaces is instrumental.

There exist many more possible examples. Catalysis is another area where this knowledge is important.

Next Ertl decided to study the Haber-Bosch-process, which is the basic step in the production of artificial fertilizer as it is used to capture nitrogen from air.

This reaction is of enormous commercial importance since lack of nitrogen is often what limits crop yields.

Lightning strikes and the activity of certain earth bacteria found at the roots of leguminous plants are two of the very few natural mechanisms that can bind nitrogen. For the invention of the Haber-Bosch process as such, Fritz Haber was awarded the Nobel Prize in Chemistry in 1918. Ertl’s contribution has been to provide detailed knowledge about how the process works.

In the Haber-Bosch process, nitrogen — which is an important constituent of ordinary air — reacts with hydrogen to form ammonia. This is the first and most challenging step in the production of artificial fertilizer. It is necessary to use a catalyst for this reaction to take place, and this is where surface chemistry plays a role.

The catalyst used in the Haber-Bosch process is finely dispersed iron: the reaction takes place using the surface of the grains of iron as support. Nitrogen and hydrogen both attach to the iron surface: in this manner they react more easily with one another.

One of the crucial questions which Ertl addressed is which step in the reaction is the slowest.

In order to investigate the Haber-Bosch process, Ertl used an idealised system — a clean and smooth iron surface in a vacuum chamber into which he could introduce well controlled amounts of the different gases.

When nitrogen lands on the iron surface, it first attaches as a molecule consisting of two nitrogen atoms. The bond between the two nitrogen atoms is among the strongest in chemistry.

When the molecule has finally attached itself to the iron surface, the two nitrogen atoms may however release one another and bind to iron instead, although this takes some time.

One of the first questions Gerhard Ertl posed was whether nitrogen reacts in its molecular or in its atomic form with hydrogen to finally form ammonia.

From earlier work Ertl already knew that the hydrogen molecule immediately dissociates and attaches in atomic form on the surface.

Ertl measured the concentration of nitrogen atoms on the iron surface while simultaneously adding hydrogen to the system. He saw that the more hydrogen he added, the more the concentration of nitrogen atoms on the surface diminished. Ertl’s conclusion was that nitrogen atoms on the surface disappear as they react with hydrogen.

This shows that the first step in the Haber-Bosch-reaction takes place between hydrogen and atomic nitrogen. If instead the reaction had taken place between hydrogen and molecular nitrogen, atomic nitrogen would still form on the surface but remain unperturbed by the amount of hydrogen added.

Measuring the concentration of nitrogen on the iron surface is however by no means simple. To distinguish atomic nitrogen from molecular nitrogen Ertl used different spectroscopic methods.

The basis for all these methods is that the surface is bombarded by particles (either light-particles, that is photons, or free electrons). Electrons in the atoms at the surface will be forced to move when they are hit by this stream of particles, much as a billiard ball is set in motion when it is struck by another ball.

Using all these different methods together is warranted as it is very difficult to be really sure of what one sees in surface chemistry. Any minute impurity in the system will immediately attach itself to the surface; it will not be diluted as in a solution. The signal from each experimental technique will also be very weak because only a single atomic layer at the surface is being observed.

In the process of conducting this observation, Ertl discovered that the step that limits the rate of the whole Haber-Bosch process is actually the splitting of the nitrogen molecule into its constituent atoms. Once the nitrogen atoms are free from each other they quickly collect enough hydrogen atoms to form ammonia. Speeding up nitrogen splitting was therefore essential.

It was already known that adding potassium to the catalyst is a way of improving the Haber-Bosch process. Ertl could show not only that the addition of potassium did in fact hasten the reaction but also why.

Whatever one does, however, the splitting of the nitrogen molecules will always be much slower than the other steps of the reaction.

This means it is difficult to find any way to study the subsequent steps. Once nitrogen has split up, everything else happens at such a speed that it is impossible to ‘see’ anything at all until ammonia has been formed and leaves the surface.

Not one to give up, he realised that he could study the reaction backwards instead. The Haber-Bosch process is a reversible reaction, which means each step looks the same in both directions.

The direction of the reaction is governed solely by which gases are pumped into the system, either ammonia or hydrogen plus nitrogen.

So Ertl started to study how ammonia attaches to the iron surface, and how it subsequently dissociates step-wise into its building blocks of nitrogen and hydrogen. In this manner he managed to observe the two missing intermediate steps.

Adding heavy hydrogen (which in some measurements will give a different signal from ordinary hydrogen) he could measure the speed at which the ammonia molecule releases one of its (ordinary) hydrogen atoms and subsequently collects a new (heavy) hydrogen.

In this manner he found a way to study the rate of the final step in the reaction.

Yet another surface reaction which has great practical importance is the oxidation of carbon monoxide on platinum.

One important role of the catalyst in the exhaust pipes of cars is to ensure the efficiency of this reaction. Carbon monoxide is toxic and must be converted to carbon dioxide before leaving the exhaust pipe.

Ertl has also studied this reaction in great detail. He has shown that the rates of different steps in the reaction will vary over time.

Some of the steps oscillate between different rates, and the reaction proceeds differently depending on the coverage of the platinum surface.

Sometimes these variations lead to a chaotic course of events: the reaction is therefore not reversible and much more difficult to study.

Ertl has shown the great complexity of a seemingly simple reaction, where carbon monoxide collects an extra oxygen atom to become carbon dioxide. This illustrates how his methodology also serves to shed light on highly complicated surface reactions.

Recently, Ertl has chosen to study hydrogen on metal surfaces in order to make use of the new experimental technologies which are constantly emerging. In this way new pieces are gradually incorporated into his experimental methodology to provide an increasingly complete picture of surface reactions. — Excerpted from public information available on http://nobelprize.org

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