Surface of the matter

Print edition : November 16, 2007

This years Nobel Prize in Chemistry goes to the German scientist Gerhard Ertl for his work on surface chemistry.

YOU should never give up. You should always try to solve the problem as far as possible. And you must be patient. You must be patient. Thats very important. Indeed, it is this intense focus on a given problem and his dedication, perseverance and, above all, patience that brought this years Nobel Prize in Chemistry to the German chemist Gerhard Ertl. An Emeritus Professor at the Fritz-Haber Institute of Berlins Max-Planck Society for the Advancement of Science and its former director, Ertl won the award for his path-breaking and thorough investigations in surface chemistry that have enabled a detailed description of chemical reactions on solid surfaces. The award came as a birthday gift to Ertl on October 10 when he turned 71.

At a press conference after the announcement, Ertl described the award as the highest honour that the heavens of science can bestow and said, It is difficult to grasp it, even now. I was speechless and then came the tears, I have to confess. After 19 years, the chemistry Nobel has come to Germany, and this is the second Nobel award for surface chemistry. The first was given in 1932 to Irving Langmuir, one of the pioneers of the subject. When, immediately after the announcement, the editor-in-chief of the Nobel website rang him up for an interview, Ertl was actually waiting for a congratulatory call from the German Chancellor Angela Merkel, herself an accomplished chemist.

Surface chemistry, as the term implies, is essentially chemistry in two dimensions. Unlike the chemical reactions in bulk, with substances in test tubes, beakers and glass jars that one normally associates a chemistry laboratory with, surface chemistry has to do with the chemical processes that occur in the few atomic layers that constitute the interface between two phases, such as solid-liquid, solid-gas, solid-vacuum and liquid-gas interfaces. And two dimensions are better suited to probing reactions in greater detail at the atomic level than those in three-dimensional solutions because they are confined to the surface, but it is neither straightforward nor cheap to study how atoms and molecules react on solid surfaces. It involves painstaking and high-precision work, with advanced equipment such as high-vacuum systems, electron microscopes and spectroscopes, and clean rooms. And Ertl put these to innovative use in the past three decades and more. His work has chiefly been concerned with gas-solid interfaces. As Mark Peplow, the editor of Chemistry World, said, he gave us the tools to understand why [oxygen] atoms do not bounce off [iron surfaces] but rather stick to them and turn into iron oxide.

The science of surface chemistry has important industrial applications, such as in the manufacture of artificial fertilizers, and the science is also key to understanding such diverse phenomena as the rusting of iron; the working of catalytic converters, which make automobile exhaust less polluting; the functioning of fuel cells; and the depletion of atmospheric ozone, which is owing to reactions on the surface of minute ice crystals in clouds.

The work we were doing was related to heterogeneous catalysis and this is a topic of great industrial importance, but also of environmental importance, Ertl said in his interview to the Nobel website. Heterogeneous catalysis refers to catalytic reactions where the catalyst is in a different phase from the reactants. So, as soon as you understand something better you can also think of improving it. I think thats the main message [in terms of benefit to mankind] you can learn from it, he added.

Surface science also underlies production processes in the semiconductor industry, and in fact, it was processes developed in the semiconductor industry in the 1950s and 1960s that led to the emergence of modern surface chemistry. Ertl was one of the first to see the potential of these new techniques as tools for the study of surface phenomena itself.

Step by step, says the Nobel citation, he has created a methodology for surface chemistry by demonstrating how different experimental procedures can be used to provide a complete picture of a surface reaction.

In fact, his methodology has laid the foundation for an entirely new approach to research in an area that, given its industrial importance, actually began in the early 20th century. Besides studying fresh problems, he has brought to bear his methodology to address some important, previously unanswered, problems concerning molecules and atoms on surfaces. Ertl has, says the background information note of the Nobel Foundation, an outstanding capacity to refine problems. He has systematically searched for the best experimental techniques to investigate each separate question.

Indeed, as the Nobel Committee has pointed out, he is never satisfied with an isolated, interesting observation but takes it to its logical conclusion. This attitude has led him to revisit continually old problems, even those that he himself has worked on. A scientist, he told the Nobel website interviewer, is never, never at the end; when we solve a problem, five other problems develop anew. That is why a scientist will always think about his work and what he can do next.

Since the basic aim of surface science is to understand how atoms and molecules behave on a pure surface, say of a solid, one needs to be able to determine and measure what substance and in what quantity it is being introduced onto the surface. Contamination would, therefore, spoil all measurements, and because surfaces tend to be chemically very active, it is very difficult to maintain them so that they remain clean enough to study a specific reaction or a system. For instance, gases in the air immediately cover a surface that is exposed to air. This is why high-vacuum systems combined with high-precision measurements are necessary to carry out experiments in surface science. With ingenuity, Ertl showed how, using the various techniques available, reliable results could be obtained in this difficult area of research.

One of the first problems that Ertl studied in the 1970s was the behaviour of hydrogen on metal surfaces. A question that remained unanswered since the work of P. Sabatier in the early 20th century was how hydrogen, after adsorption (concentration of gas molecules or atoms on surfaces) on metals such as nickel, palladium and platinum, organised itself. Sabatier won the Nobel Prize in 1912 for his method of hydrogenating organic compounds in the presence of finely divided metal. It is important to understand the phenomenon because hydrogen gas is used or produced at metal electrodes in electrochemical processes, such as electrochemical solar cells, and the reverse reaction can then be used to generate electricity in a fuel cell.

By combining chemical experiments that measured desorption (the reverse of adsorption) with the technique (used in physics) of low energy electron diffraction (LEED) and modelling, Ertl was able to provide a quantitative description of hydrogen reactions on metal surfaces. He demonstrated how one single method was insufficient to answer relevant and unanswered questions. This approach, which he developed early in his career, is characteristic of Ertls habit of revisiting fundamental problems that he had investigated earlier. He continues to be concerned with problems of hydrogen adsorption and desorption. In his recent work on the topic in 2002-03, he used measurements of the vibrational spectrum. His studies on the behaviour of hydrogen on metal surfaces have also had an important bearing on questions of catalytic mechanisms.

In fact, the next long-standing problem that Ertl studied was the molecular mechanism in the catalytic reaction in the Haber-Bosch process, which captures nitrogen from air and is a basic step in the industrial production of nitrogenous fertilizers. The only natural processes that are known to bind or fix nitrogen in the soil in a form that can be taken up by plants an important ingredient for crop productivity include the work of certain bacteria at the roots of leguminous plants and atmospheric electricity such as lightning strikes. For the invention of the Haber-Bosch process, Fritz Haber after whom, incidentally, the institute where Ertl works has been named won the Nobel Prize in Chemistry in 1918.

In the Haber-Bosch process, nitrogen reacts with hydrogen to form ammonia (N{-2} + 3H{-2} <> 2NH{-3}) at high pressure in the presence of a catalyst. The commonly used catalyst is finely dispersed iron particles with added potassium hydroxide on a substrate of alumina and silica. Nitrogen and hydrogen attach themselves to the surface of the iron grains, which enables the nitrogen-hydrogen bonding to take place more easily. Ertls work provided a detailed description of how the process works. When he took up the problem in the mid-1970s, it had already received the attention of numerous investigators given its enormous economic importance, but the underlying mechanism remained unclear. These investigations, which mainly looked at the problem from the perspective of kinetics, failed to throw light even on the form atomic or molecular in which the gases reacted.

A fundamental question that Ertl addressed was which step in the reaction was the slowest. He argued that to improve the process as a whole the slowest step needed to be speeded up. To investigate, Ertl used a clean, smooth iron surface in a vacuum chamber into which he could introduce different gases in a controlled manner. When a nitrogen molecule (containing two atoms) lands on a surface, it attaches itself as a molecule because the bond between two nitrogen atoms is one of the strongest in chemistry. However, in the presence of iron, the molecule prefers to break up, and the atoms bind to iron rather than to each other after some time. Hydrogen, on the other hand, was already known to dissociate immediately and attach in atomic form to the surface. Ertl basically wanted to determine whether nitrogen reacted with hydrogen in molecular or atomic form to form ammonia.

GERHARD ERTL IN his laboratory in Berlin.-TOBIAS SCHWARZ /REUTERS

As hydrogen was being added to the system, Ertl measured the concentration of nitrogen atoms on the surface. He found that the more hydrogen he added, the fewer the nitrogen atoms he could detect, thereby establishing that hydrogen and nitrogen combined in atomic form to produce ammonia. Thus, he demonstrated that the first step in the process was that between hydrogen and atomic nitrogen. If hydrogen had reacted with molecular nitrogen, nitrogen atoms would have still formed on the surface and not diminished in concentration as hydrogen was being introduced.

Ertls ingenuity lay in devising methods to measure the nitrogen concentration because this is not at all simple. He used different spectroscopic methods to distinguish between molecular and atomic nitrogen, which involved looking at scatterings of electrons or photons from the surface. Electrons of atoms in the surface will be displaced when bombarded by electrons. The energetics of the atoms on the surface, which can be spectroscopically measured, essentially provided information about the nature of the scattering atomic species. He employed another method as well. This involved studying the nature of the surface itself, which gets modified when iron binds to nitrogen. The scattering pattern of electrons from the surface revealed the structure of the surface. The point is that the process had to be investigated in several ways to be certain about what was being seen because, for one, the signal from the surface would be weak (because the effect is from a few atomic layers only) and, secondly, he had to ensure what he was seeing was not an artefact of surface contamination.

Similarly, using different ways of determining which molecular species populated the surface during the reaction, Ertl established that the step that limited the rate of the Haber-Bosch process was actually the splitting of molecular nitrogen into its constituent atoms. Once nitrogen molecules split, they quickly combine with hydrogen to form ammonia. It was already known that adding potassium to the catalyst speeded up the process: Ertl was able to show how potassium helped and why.

However, even with potassium, it was found that the splitting of nitrogen was the slowest of all steps, which meant that the subsequent steps happened so quickly that it was impossible to see and measure their rates. But Ertl persevered and devised ingenious methods which exploited the fact all the steps in the Haber-Bosch process were reversible to determine the rate at which the desorption of ammonia took place.

The above is illustrative of Ertls typical experimental approach to a given problem: use a well-controlled model system to measure the rates and energetics of each step in a surface reaction at the atomic level. These then form the basis to determine the values of various parameters in a realistic application. This is why, points out the Nobel Foundations backgrounder, Ertls methodology has great importance not only in basic research but also for modelling processes in industry.

Another important surface reaction that Ertl investigated in detail in the 1980s was the oxidation of carbon monoxide (CO) to carbon dioxide (CO{-2}) on platinum, a reaction that forms the basis for the catalytic converters in the exhaust pipes of cars. It had been observed that the rate of reaction was not constant but oscillated with time. Although oscillating reactions were known in liquids, this was the first instance on a surface, and no one could explain the behaviour until Ertl arrived on the scene.

To simplify the problem, he introduced the reactants in an ultra-high vacuum, which had single crystals of platinum. Using the technique of LEED, Ertl and his associates discovered the actual catalytic mechanism: the platinum rearranged its surface structure to accommodate atoms of CO within. This surface reconstruction also increased the amount of oxygen (O{-2}) that could be adsorbed by platinum, thereby increasing the reaction rates. They also found that as more CO was converted to CO{-2}, CO coverage over the surface diminished, and the surface reverted to its original structure, thus causing a cyclic variation in reaction rates.

This explanation meant that certain areas on the surface would be covered with CO and certain others mostly with oxygen. To confirm this, Ertl used another technique called photoemission electron microscopy (PEEM), but redesigned it to suit the problem at hand, to take pictures of the platinum surface at micrometre resolution.

The images were produced as a result of electrons being stripped from the surface molecules by the impinging ultraviolet (UV) light. The images had bright and dark areas. These corresponded to CO-rich and O{-2}-rich areas respectively because electrons from the latter had less energy than those from CO. The dark and bright areas were organised into wave-like structures and other geometrical patterns or even chaotic patterns indicating the highly non-linear nature of reaction dynamics (see picture).

This kind of detailed study, not possible in 3D-solutions, is yet another example of Ertls approach to studying surface phenomena: using stripped-down models and a judicious combination of advanced experimental techniques to unravel the underlying mechanisms of complex surface processes.

It is such accurate studies that have led to the development of a systematic methodological basis for investigating and elucidating complicated molecular and atomic processes on surfaces.

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