Nobel Prize in Chemistry 2007: Gerhard Ertl and Chemical Processes on Solid Surfaces

This note covers the Nobel Prize in Chemistry 2007: who won it, what surface chemistry is and why it matters, how Gerhard Ertl built a reliable way to study chemical reactions on solid surfaces, how he used this method to solve the molecular puzzle behind the Haber-Bosch process and the car exhaust catalyst, how the discovery unfolded over his career, and quick facts for exams.

What was the Nobel Prize in Chemistry 2007 awarded for?

The Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry 2007 to Gerhard Ertl with the citation:

"for his studies of chemical processes on solid surfaces"

In plain words, Ertl worked out, step by step, exactly what happens when gas molecules land on a solid surface such as a metal, and then react there.

This sounds narrow, but an enormous amount of everyday and industrial chemistry happens this way: a catalyst surface helps molecules meet, break apart and recombine into new substances. Ertl did not discover a single new reaction.

Instead he built a trustworthy methodology, a disciplined way of combining several experimental techniques, that let chemists see every individual step of a surface reaction with confidence, rather than guessing from indirect clues.

The committee called this field "modern surface chemistry" and said it underlies fuel cells, artificial fertilisers and the clean exhaust systems of cars, which is why the press release headline linked Ertl's work directly to those three practical areas.

Who are the laureates?

Gerhard Ertl

Gerhard Ertl was born on 10 October 1936 in Bad Cannstatt, Germany. He received his doctorate in physical chemistry in 1965 from the Technische Universität München.

At the time of the award, in 2007, he was Professor Emeritus at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin, Germany, an institute that is itself named after Fritz Haber, whose industrial ammonia process Ertl later explained in molecular detail.

Ertl received the whole prize (prize share 1/1), worth 10,000,000 Swedish kronor in 2007. His contribution, as described by the Academy, was to develop a set of reliable experimental procedures for surface chemistry from the 1960s onward, and then to apply that methodology to two reactions of huge practical importance: the formation of ammonia on an iron surface (the Haber-Bosch process) and the oxidation of carbon monoxide on platinum (car exhaust cleaning).

He also carried out long-running studies of how hydrogen atoms arrange themselves on metal surfaces such as platinum, nickel and palladium.

What problem does surface chemistry solve?

Many of the chemical reactions people depend on do not happen in a test tube full of liquid. They happen on the outer layer of a solid, where gas molecules touch a surface such as a metal.

The Academy's press release pointed out that surface reactions explain why iron rusts, how fuel cells work, how the catalysts in cars clean exhaust gases, how artificial fertilisers are produced, and even how the ozone layer is destroyed, since key ozone-destroying steps happen on the surface of tiny ice crystals high in the atmosphere.

The difficulty is that a surface reaction only involves a single, extremely thin layer of atoms. In an ordinary chemical reaction in solution, many layers of molecules react together, giving a strong and easy-to-measure signal.

On a surface, only one layer of atoms is actually doing the reacting, so the signal from any single measurement is very weak, and the surface is extremely easy to contaminate.

In open air, a clean metal surface is instantly covered with stray gas molecules. This is why surface chemists need high-vacuum equipment, so that they can control precisely which molecules are allowed to touch the surface being studied.

This branch of chemistry only became practical once the semiconductor industry, in the 1960s, developed the vacuum technology and clean-room techniques needed to handle very pure, controlled surfaces.

Ertl was one of the first chemists to recognise that these new tools, originally built for making electronic components, could be turned into instruments for studying how chemical reactions actually occur, atom by atom, on a surface.

How did Ertl build a reliable method for studying surfaces?

No single instrument can show every detail of a surface reaction, because the signal is so weak and surfaces are so easily contaminated.

Ertl's central achievement, according to the Academy, was to show how several different experimental techniques could be combined to build up one complete and trustworthy picture of a reaction.

Some of these techniques detect which atoms are present by bombarding the surface with light particles (photons) or electrons and measuring the energy that comes back; others reveal the arrangement of atoms by scattering electrons off the surface in patterns.

A general version of Ertl's working method, as it emerges from the scientific background on his work, runs like this:

1. Choose a well-defined model surface, usually a single flat face of a pure metal crystal, and clean it under high vacuum so that no stray molecules interfere.
2. Expose the surface to a controlled dose of the gas being studied, and use spectroscopic methods, such as photoelectron spectroscopy, to find out which atoms or fragments are sitting on the surface.
3. Use structure-sensitive methods, such as low energy electron diffraction, to work out exactly how those atoms are arranged on the metal surface.
4. Vary conditions such as temperature, gas pressure and the amount of a promoter added to the surface, and measure how the rate of each individual step changes.
5. Compare the results from the idealised, low-pressure model system with measurements taken closer to real industrial conditions, to check that conclusions from the simplified experiment still hold at higher, more realistic pressures.

This careful, many-technique approach is what the Academy meant when it said Ertl had founded "an experimental school of thought" for surface chemistry: not one clever trick, but a dependable discipline that other scientists, in both universities and industry, could reuse for completely different surface reactions.

Draw a flat row of large circles representing metal atoms at a surface.

Above them, show small circles landing from the gas phase (one labelled gas molecule approaching), with an arrow pointing into a sealed, evacuated chamber to show that the whole set-up is kept under high vacuum so no other gas molecules can contaminate the surface.

How did Ertl unravel the Haber-Bosch process?

The Haber-Bosch process converts nitrogen gas from the air, together with hydrogen gas, into ammonia, using finely divided iron as a catalyst.

This reaction is the first and most important step in making artificial nitrogen fertiliser, because very few natural processes, apart from lightning and certain soil bacteria, can "fix" nitrogen from the air into a form that plants can use.

Fritz Haber himself received the Nobel Prize in Chemistry in 1918 for inventing the process, but by the mid-1970s, when Ertl began his studies, nobody had worked out the exact molecular steps by which it happened.

The central question was whether nitrogen reacted with hydrogen as whole N₂ molecules, or whether the nitrogen molecule first had to split into separate nitrogen atoms on the iron surface.

Ertl measured how the amount of nitrogen sitting on the iron surface changed as he added more hydrogen.

He found that the more hydrogen he added, the less nitrogen remained on the surface, which showed that the nitrogen atoms, not whole nitrogen molecules, were being used up by reacting with hydrogen.

This told him that the reaction went through separate atomic nitrogen, and that splitting the strong nitrogen-nitrogen bond was the slowest, rate-limiting step of the whole process, much like one slow traffic light that may cause a city-wide jam.

Because the splitting step was so slow, everything after it happened too fast to observe directly; by the time nitrogen atoms were free, ammonia formed and left the surface almost at once.

Ertl solved this by exploiting the fact that the Haber-Bosch reaction is reversible: he started instead with ammonia gas and watched it break down step by step on the iron surface, using heavy hydrogen (deuterium) to track individual atoms swapping places.

In this way he filled in the missing middle steps of the reaction and, together with earlier structural work, could also explain why adding potassium to the iron catalyst speeds up the whole process: potassium donates electrons to the iron atoms nearby, which makes nitrogen stick to the surface more strongly, the effect the Academy linked to the faster splitting of the nitrogen molecule.

What did Ertl discover about car exhaust catalysts?

Petrol and diesel engines produce poisonous carbon monoxide as a by-product of incomplete combustion. The catalytic converter fitted in a car's exhaust pipe uses a platinum surface to oxidise this carbon monoxide into harmless carbon dioxide before it leaves the vehicle.

Ertl studied this reaction in great detail and found it to be far more complicated than the steady, one-direction Haber-Bosch process.

He showed that the rate of the different steps in carbon monoxide oxidation on platinum can oscillate, speeding up and slowing down over time rather than proceeding at a constant rate, and that the reaction's behaviour depends on how much of the platinum surface is already covered by carbon monoxide.

In some conditions these oscillations become chaotic. Unlike the Haber-Bosch reaction, this process is not reversible, which made it considerably harder to study using the same "run it backwards" trick.

The Academy's scientific background material notes that this oscillating behaviour arises because bare platinum surfaces can rearrange their own atomic structure, and carbon monoxide and oxygen bind differently to the rearranged versus the original surface, so as coverage changes the surface itself keeps flipping between forms, driving the oscillation.

Ertl's work showed that a reaction that looks simple on paper, carbon monoxide gaining one oxygen atom, can hide very rich and complex surface behaviour.

Draw a square representing a flat platinum surface, divided into shifting patches, some shaded dark to represent areas rich in carbon monoxide and some left light to represent areas rich in oxygen.

Add a small arrow labelled "time" beneath a row of four such squares to show the patches moving and changing shape as the reaction oscillates.

How did the discovery unfold?

Why does it matter?

Ertl's methodology gave chemists, for the first time, a dependable way to see individual molecular steps on a surface, rather than inferring them indirectly from the gas above it.

The Academy stated that his approach is used in both academic research and the industrial development of chemical processes, because it lets engineers understand which single step in a multi-step industrial reaction is the slow, rate-limiting one, the step that must be sped up to improve the whole process.

The press release and the scientific background material together linked this to practical concerns: cleaner car exhaust systems, more efficient production of artificial fertiliser, groundwork for improving fuel cells and semiconductor manufacturing, and a better understanding of corrosion in nuclear power plants and airplanes.

The presentation speech also noted that understanding surface processes is important for safely managing radioactive waste storage, since corrosion over very long timescales is a serious concern there.

Open questions remain in the wider field: oscillating and chaotic surface reactions, of the kind Ertl found in carbon monoxide oxidation, are still studied as examples of non-linear dynamics, and surface scientists continue to try to close the "pressure gap" between idealised low-pressure laboratory experiments and the much higher pressures used in real industrial reactors.

How does this connect to what you study?

School chemistry usually introduces catalysts as substances that speed up a reaction without being used up, and mentions the industrial manufacture of ammonia as an example of a reaction needing a catalyst.

Ertl's work is the molecular detail behind exactly that example: it shows what "the iron catalyst helps the reaction" actually means at the level of individual atoms sticking to, moving across and leaving a metal surface.

It also connects to the idea of reaction rate and the rate-determining step, taught in chemical kinetics: Ertl's finding that splitting the nitrogen molecule is the slowest step of ammonia synthesis is a real-world illustration of why the slowest step in a multi-step reaction controls the overall speed, just as the slowest worker on an assembly line sets the pace for the whole line.

Quick facts for exams

The Nobel Prize in Chemistry 2007 was awarded, in full, to Gerhard Ertl, a German chemist born in Bad Cannstatt in 1936, for his studies of chemical processes on solid surfaces.

The award was announced by the Royal Swedish Academy of Sciences on 10 October 2007. At the time, Ertl was Professor Emeritus at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin, Germany.

His work built a reliable experimental method for surface chemistry, which he used to explain the molecular steps of the Haber-Bosch process (making ammonia for fertiliser on an iron surface) and the oxidation of carbon monoxide on platinum in car exhaust catalysts. The prize carried a value of 10,000,000 Swedish kronor.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Surface chemistry — the branch of chemistry that studies reactions happening on the outer layer of a solid, where gas or liquid molecules meet it.
• Catalyst — a substance that speeds up a chemical reaction, such as iron in ammonia synthesis or platinum in exhaust cleaning.
• Heterogeneous catalysis — catalysis in which the catalyst is in a different physical state from the reacting molecules, for example a solid metal catalysing a gas reaction.
• Adsorption — the sticking of gas or liquid molecules onto the surface of a solid.
• Desorption — the release of molecules that were previously stuck to a surface, back into the gas phase.
• High vacuum — an extremely low-pressure, near-empty chamber used to keep a surface free of unwanted gas molecules during an experiment.
• Haber-Bosch process — the industrial reaction that combines nitrogen and hydrogen gas, using an iron catalyst, to make ammonia for fertiliser.
• Ammonia — the compound of nitrogen and hydrogen (used to make fertilisers) produced by the Haber-Bosch process.
• Rate-limiting step — the slowest step in a multi-step reaction, which controls how fast the overall reaction can proceed.
• Oscillating reaction — a reaction whose rate rises and falls repeatedly over time, rather than staying steady.
• Photoelectron spectroscopy — a technique that identifies atoms on a surface by measuring the energy of electrons knocked out when the surface is hit by light particles.
• Low energy electron diffraction (LEED) — a method that reveals the arrangement of atoms on a surface by scattering electrons off it in a pattern.
• Corrosion — the gradual damage of a material, such as rusting iron, caused by chemical reactions at its surface.
• Fuel cell — a device that generates electricity from a chemical reaction, often involving hydrogen, at catalytic surfaces.
• Catalytic converter — the device in a car's exhaust system that uses a platinum catalyst to convert poisonous carbon monoxide into carbon dioxide.

Common errors and misconceptions

• Misconception: Ertl invented the Haber-Bosch process. Correct: The process was invented decades earlier, and Fritz Haber received the Nobel Prize in Chemistry in 1918 for it; Ertl later worked out its molecular mechanism.
• Misconception: Surface chemistry only matters in a laboratory. Correct: The Academy linked it to everyday processes such as rusting, fertiliser production, exhaust cleaning and ozone layer destruction.
• Misconception: Ertl discovered a brand new reaction that nobody knew about. Correct: He mainly developed a reliable methodology and applied it to already-known, economically important reactions to reveal their exact molecular steps.
• Misconception: The Haber-Bosch reaction and the car exhaust reaction behave the same way. Correct: The Haber-Bosch reaction is reversible and proceeds steadily, while carbon monoxide oxidation on platinum can oscillate and even become chaotic.
• Misconception: A catalyst surface reaction involves many layers of atoms reacting at once. Correct: Only a single outer layer of atoms is actually involved, which is why the experimental signal is weak and contamination is a constant risk.
• Misconception: Potassium reacts chemically with nitrogen in the Haber-Bosch process. Correct: Potassium acts as a promoter by donating electrons to nearby iron atoms, which helps nitrogen bind more strongly to the surface.
• Misconception: The Nobel Prize in Chemistry 2007 was shared between several scientists. Correct: Gerhard Ertl received the whole prize alone.

Exam-style questions with model answers

Q1. In which year was the Nobel Prize in Chemistry 2007 announced, and what was its exact citation? [2 marks]

1. The prize was announced on 10 October 2007, and the citation stated that Gerhard Ertl was honoured "for his studies of chemical processes on solid surfaces".

Q2. Name two everyday or industrial processes that the Nobel committee linked to surface chemistry. [2 marks]

1. The committee linked surface chemistry to the production of artificial fertilisers through the Haber-Bosch process and to the cleaning of car exhaust gases through catalytic converters.

Q3. Explain why studying reactions on solid surfaces is experimentally difficult. [4 marks]

1. Only a single layer of atoms at the surface actually takes part in the reaction, unlike in a solution where many layers of molecules react together, so the measurable signal from any one technique is weak.
2. A clean surface is extremely reactive and gets contaminated almost instantly if exposed to ordinary air, since stray gas molecules stick to it immediately.
3. Because of this, experiments must be carried out in high-vacuum chambers that carefully control exactly which molecules reach the surface.
4. No single measurement gives a complete picture, so several different techniques, each sensitive to a different property, must be combined to confirm what is happening.

Q4. How did Ertl determine that nitrogen had to split into separate atoms before reacting with hydrogen in the Haber-Bosch process? [4 marks]

1. Ertl measured the concentration of nitrogen sitting on an iron surface while gradually adding hydrogen gas to the system.
2. He observed that as more hydrogen was added, the amount of nitrogen on the surface steadily decreased.
3. This showed that the nitrogen present on the surface was being consumed by reacting with hydrogen, which would only happen if that nitrogen was already in atomic form.
4. If the reaction had instead involved whole nitrogen molecules reacting with hydrogen, the atomic nitrogen on the surface would have remained unaffected by how much hydrogen was added, so his result confirmed that nitrogen first splits into atoms, and that this splitting is the slow, rate-limiting step.

Q5. Discuss how Gerhard Ertl's work on the Haber-Bosch process and on carbon monoxide oxidation illustrates his broader contribution to surface chemistry. [5 marks]

1. In the Haber-Bosch process, Ertl built a step-by-step molecular account of how nitrogen and hydrogen react on an iron surface to form ammonia, identifying nitrogen splitting as the slow, rate-limiting step and explaining why adding potassium speeds the process up by donating electrons to nearby iron atoms.
2. Because the reaction is reversible, he was able to study the later steps by starting from ammonia and tracking its breakdown using heavy hydrogen, which let him fill in parts of the mechanism that were too fast to observe directly in the forward direction.
3. In the carbon monoxide oxidation reaction on platinum, used in car exhaust catalysts, he showed that the reaction rate can oscillate and even become chaotic, because the platinum surface itself can rearrange, changing how strongly carbon monoxide and oxygen bind to it as coverage changes.
4. Across both studies, his consistent approach was to combine several different surface-sensitive techniques to build a complete, reliable picture of each reaction, rather than relying on any single measurement, and this combined methodology is what the Academy honoured as the foundation of modern surface chemistry.

Q6. What institute was Gerhard Ertl affiliated with at the time of the award, and in which country is it located? [2 marks]

1. Ertl was Professor Emeritus at the Fritz-Haber-Institut der Max-Planck-Gesellschaft, located in Berlin, Germany, at the time of the award.

Q7. Why was it important that Ertl's model-system results could be shown to apply at the higher pressures used in real industrial reactors? [3 marks]

1. Ertl's detailed measurements were made on clean, idealised metal surfaces under carefully controlled low-pressure, high-vacuum conditions, which are far simpler than conditions inside an actual industrial reactor.
2. By checking that the surface composition and reaction rates measured under these idealised conditions matched the behaviour observed closer to real industrial pressures, Ertl confirmed that his conclusions were not just laboratory artefacts.
3. This connection between simplified model experiments and real industrial processes is what allowed his methodology to be useful for improving actual chemical plants, not only for academic understanding.

Key takeaways

• Gerhard Ertl won the Nobel Prize in Chemistry 2007 alone for studies of chemical processes on solid surfaces.
• He developed a reliable methodology combining several techniques to study reactions happening in a single layer of atoms on a surface.
• His work needed high-vacuum equipment, borrowed from the semiconductor industry, to keep surfaces free from contamination.
• He showed that splitting nitrogen into atoms is the slowest, rate-limiting step of the Haber-Bosch ammonia process.
• He explained why adding potassium to the iron catalyst speeds up ammonia production.
• He showed that carbon monoxide oxidation on platinum, used in car exhaust catalysts, can oscillate and behave chaotically.
• His methodology connected idealised laboratory experiments to real industrial reaction conditions.
• The Academy linked surface chemistry to fertilisers, fuel cells, exhaust cleaning, corrosion and ozone layer chemistry.

Test yourself

When and where was Gerhard Ertl born?

Gerhard Ertl was born on 10 October 1936 in Bad Cannstatt, Germany.

What was the exact citation for the Nobel Prize in Chemistry 2007?

The citation read "for his studies of chemical processes on solid surfaces", awarded entirely to Gerhard Ertl.

Which industrial process did Ertl study to find its rate-limiting step?

He studied the Haber-Bosch process, in which nitrogen and hydrogen react on an iron surface to form ammonia.

What did Ertl find to be the rate-limiting step of ammonia synthesis?

He found that splitting the nitrogen molecule into separate nitrogen atoms on the iron surface was the slowest step.

Why does adding potassium speed up the Haber-Bosch process?

Potassium donates electrons to nearby iron atoms, which makes nitrogen stick more strongly to the surface and lowers the energy needed to split it.

What unusual behaviour did Ertl find in carbon monoxide oxidation on platinum?

He found that the reaction rate can oscillate over time and sometimes become chaotic, because the platinum surface itself rearranges as coverage changes.

Why do surface chemists need high-vacuum equipment?

A clean metal surface is contaminated almost instantly by stray gas molecules in normal air, so high vacuum is needed to control exactly which molecules reach it.

At which institute did Ertl hold his position at the time of the award?

Gerhard Ertl was Professor Emeritus at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin, Germany.