2007 Nobel Prize, Chemistry: Chemical Processes on Solid Surfaces
Gerhard Ertl, German scientist and Professor Emeritus at the Fritz-Haber Institute of the Max-Planck-Gesellschaft, celebrated his 71st birthday on October 10, 2007. On that day, he received the very best of birthday presents: a Nobel Prize. The Royal Swedish Academy of Sciences in Stockholm announced that he had won the Nobel Prize in Chemistry that year for his studies of chemical processes on solid surfaces (1). More specifically, he was recognized for the methodological approach he used. His technique has become a model for both academic research and industrial process development. He showed how reliable results can be obtained in this area of research.
Chemical reactions often occur in liquids or gases, but many important reactions also take place on surfaces (2). Adam Smith, Editor-in-Chief of the Nobel Prize Foundation’s website, provided an analogy to describe the mechanism of chemical reactions: “Like a successful dinner party, productive chemical reactions depend upon getting the right components to mingle in the right surroundings, and often the best environment for chemistry turns out to be a solid surface” (2).
In order to effectively study surface chemistry, one “must master not only basic theories of solid state physics and chemical reactions, but must hold [a] range of very advanced measuring techniques,” said Professor Gunnar von Heijne, chairman of the Nobel Committee for Chemistry, at the prize announcement (3). Processes for analyzing surface chemistry emerged in the semiconductor industry in the 1960s, and Ertl was one of the first to see their potential. It was Ertl who laid the foundation for this field of science by demonstrating how different experimental procedures could be used to provide a complete picture of a surface reaction. A distinct aspect of Ertl’s approach was his examination of fundamental problems that he had analyzed previously. His field also requires advanced high-vacuum experimental equipment because the individual layers of atoms and molecules must be observed on various surfaces in order to gauge reactions.
The questions Ertl sought to answer have intrigued scientists for years. His work to answer such questions has led to greater understanding and technological improvements in a variety of fields, from catalytic converters to fuel cells to the industrial production of ammonia.
Studying surface chemistry is a painstaking process which requires incredible precision. First, Ertl delved into the intricacies of hydrogen’s interaction with metal surfaces. One can use the production of hydrogen gas from one of the electrodes in an electrochemical solar cell to make energy in a fuel cell instead. These processes involve surface chemistry, and Ertl devoted his life’s studies to developing ways to better understand the detailed dynamics of such reactions. His goals were bold: he decided to investigate the inner workings of the Haber-Bosch process, a means of synthesizing ammonia.
The Haber-Bosch process is the reaction of nitrogen and hydrogen over an iron substrate to produce ammonia. The net reaction is N2 + 3H2 –> 2NH3, with iron particles as the catalyst and occassionally also potassium hydroxide. The reaction takes place using the surface of the iron grains as support, and the use of catalysts in the process produces a high reaction rate. The process is now used to produce 100 million tons of nitrogen fertilizer per year (4).
The Haber-Bosch process is a reaction of huge industrial importance that has been used since the First World War without anyone knowing precisely how it worked. Due to the high demand in Germany of ammonia for explosives during the First World War, the Haber process was essential, since it made ammonia production easier.
What intrigued Ertl was the surface reaction in the Haber process. For 60 years, the details of the Haber-Bosch process were unclear because a clear methodology for studying surface chemistry had not been established. One problem involved the adsorption of hydrogen and nitrogen on the iron: Ertl wanted to know what the energy levels were in each step and to determine which was the slowest, or rate-determining, step. Another challenge was to find a way to study this reaction in a closed, non-contaminated environment, since the reaction takes place on a solid and cannot be performed in a test tube.
The method Ertl used was an idealized system involving a clean and smooth iron surface within a vacuum into which he introduced the different gases. By measuring various concentrations of nitrogen on the iron surface as he introduced hydrogen, Ertl was able to gauge how rapidly the reaction took place between the hydrogen and nitrogen, and whether or not it was on an atomic or molecular level.
Measuring the concentration of nitrogen, however, is not an easy process, like most aspects of surface chemistry. To distinguish atomic nitrogen from molecular nitrogen, Ertl used different spectroscopic methods. He investigated the surface structure of iron, since iron changes its shape when it binds with nitrogen. His method for examining the surface involved photoelectron spectroscopy–bombarding the surface with electrons that are scattered in a specific pattern.
Ertl had previously shown precisely how the hydrogen atoms would be exposed on the metal, demonstrating that the hydrogen molecules were broken down to individual atoms. There was considerable debate, however, over whether the surface interaction between iron and nitrogen was strong enough to dissociate nitrogen molecules, given the high dissociation energy of the nitrogen triple bond. Using an Atomic Emission Spectrometer (AES), Ertl detected the presence of nitrogen atoms on the surface. Following his deductions using the AES, he then mapped out each step of the Haber-Bosch process: hydrogen and nitrogen are adsorbed onto iron, then hydrogen atoms bond one by one to a nitrogen atom, and finally the newly formed ammonia (NH3) is released from the iron catalyst. Using AES, he demonstrated how iron acts as a catalyst.
Ertl also successfully discovered that the rate-determining step was the splitting of the nitrogen molecule. Unfortunately, this was the first step in the reaction, so the subsequent steps happened at rates that were difficult to observe. Ertl decided that instead of looking at the forward reaction, he would observe the process in reverse. He then found all of the energy levels in the Haber process by first introducing ammonia into the vacuum and then looking at how the molecule split apart.
Ertl’s use of a well-controlled modeling system in his elucidation of the Haber-Bosch process is typical of his methodology in measuring rates and activation energies. According to the Royal Swedish Academy of Sciences, “these values can then be used as a basis for calculating how the reaction proceeds in more realistic applications, using much higher pressures” (5). Ertl provided detailed information about how the process works by using a systematic methodology to study the surface reaction.
The other question Ertl wanted to explore was the oxidation of carbon monoxide on platinum, a reaction of great importance for catalytic converters in cars. Since carbon monoxide is toxic, it must be converted into carbon dioxide before it can leave the exhaust pipe. Most exhaust pipes are thus coated with alumina and a platinum group metal for catalytic converting. By studying the reaction rates for each step and their variation over time, Ertl was able to detect that some of the steps oscillate between different rates and that the reaction rate varies depending on the coverage of the platinum surface. Through the use of low-energy electron diffraction imaging, Ertl was able to show that the oscillations occurred because the surface of the platinum was actually restructuring itself. As the concentration of carbon monoxide surrounding the catalyst increased, the surface would take on a new configuration that facilitated carbon monoxide bonding. Then, as the amount of carbon dioxide around the catalyst diminished, it would revert back to its old configuration, favoring the bonding of oxygen.
These reconfigurations had another effect as well: across the surface of the platinum, transient regions would form, some binding oxygen, others binding carbon monoxide. Ertl used an electron microscope to take images of the reaction as it progressed over time – because of the difference in the energy between the electrons each molecule emitted, oxygen-rich areas would appear light while carbon monoxide-rich areas would look dark. What the images showed was astounding: some of the surfaces were spotted, some had random arrays of light and dark areas, but others had spirals, waves, concentric circles, and combinations of these striking patterns. This surprising discovery, along with Ertl’s ensuing explorations into the formation of the patterns, has given us a better understanding of other complex systems.
The reaction, due to its variability, was much more difficult to study than the Haber-Bosch process. Ertl’s methodology revealed the complexity of a seemingly simple process.
The questions concerning surface chemistry required the design of a new method for observing reactions. Because surfaces are so chemically active, precision is necessary. For this reason, a high vacuum system is essential because it prevents surfaces from adsorbing random gases from the atmosphere. “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,” the Royal Swedish Academy of Sciences said (4). He always wanted to paint as clear a picture of a reaction as possible. From the minutiae of chemical reactions to catalytic converters, Ertl’s investigations have greatly furthered scientific knowledge. The intense focus of Gerhard Ertl has paved the road for future discoveries in the field of surface chemistry. His precise methodology has set up techniques for exploring old questions in innovative ways. “His insights have provided the scientific basis of modern surface chemistry,” read the Nobel Foundation’s announcement (3). Ertl’s methodology may be used to study other subjects, such as the formation of rust, or the thinning of the ozone layer, which is caused by chemical reactions on the surfaces of ice crystals in the stratosphere. With ever-improving technology, Ertl’s methods will likely be made even more efficient.
The German scientist was certainly pleased this October to receive such an award for his perseverance and patience. The prize money was $1.5 million, but what he appreciates even more is the recognition he has earned for such hard work. Ertl stated, “this is the best birthday present that you can give to somebody” (6).
Acknowledgments
I would like to thank Eric Ross ’11 and Shreoshi Majumdar ’10 for their helpful advice and discussions.
References:
1. The Nobel Prize in Chemistry 2007. Available at http://nobelprize.org/nobel_prizes/chemistry/laureates/2007/press.html (November 20, 2007).
2. The Nobel Prize in Chemistry 2007. Available at
http://nobelprize.org/nobel_prizes/chemistry/laureates/2007/speedread.html.
3. The Nobel Prize in Chemistry 2007 Announcement. Available at
http://nobelprize.org/nobel_prizes/chemistry/laureates/2007/announcement.html.
4. The Nobel Prize in Chemistry 2007. Available at http://nobelprize.org/nobel_prizes/chemistry/laureates/2007/info.pdf (November 20, 2007)
5. The Nobel Prize in Chemistry 2007. Available at http://nobelprize.org/nobel_prizes/chemistry/laureates/2007/sci.html (November 18, 2007).
6. “Surface chemistry awarded Nobel” BBC News, 10 October 2007. (November 12, 2007). http://news.bbc.co.uk/2/hi/science/nature/7037210.stm.