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April 7, 2008
Volume 86, Number 14
pp. 21-30

Molecular Chemistry And Catalysis By Surfaces

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IT IS WITH UTMOST GRATITUDE that I accept the Priestley Medal, the highest honor of the American Chemical Society. My predecessors at the University of California, Berkeley, who have received the Priestley Medal are many, and I am exceedingly proud to join them in receiving this honor.

It is said that we use a profession to take us where we want to go with our life. My accomplishments in my life as a researcher and teacher are in chemistry in the U.S. I am fortunate to be living in times when the U.S. is the leader in science and technology and when spectacular advances in chemistry are occurring with increasing frequency.

I have been blessed with superb graduate students and postdoctoral fellows—about 350 altogether—over the four decades of my research and teaching at Berkeley. Of these, more than 100 are in academia, and the others are in industry. I am very proud of their accomplishments, both at Berkeley and after they struck out on their own. Many of them are in this room.

They have put their stamp on modern surface chemistry and its applications in many new fields of science and technology, ranging from catalysis, biointerfaces, nanoscience and nanotechnologies, magnetic storage, and tribology to integrated circuit technologies. Their many accomplishments have enriched chemistry and are their monument for future generations of chemists to admire. The Priestley Medal is a tribute to their research at Berkeley.

Mitch Jacoby/C&EN
Priestley Medal 2008 Gabor A. Somorjai

My laboratory has also hosted a large number of senior visiting scientists from all over the world, mostly from academia and research institutes. They came to learn the use of instruments and to participate in our research, which reflects the scientific challenges of modern surface chemistry. And then they returned to their home bases on all five continents to spread the gospel. The graduate students, postdoctoral fellows, and senior visiting scientists have formed a core of researchers in modern surface science, and their camaraderie and interactions have created the international and vanguard character of the field.

I was born in 1935 in Budapest, Hungary, just in time to live through the worst ravages of the Second World War, as the front between the battling armies moved through Hungary. The lives of my immediate family (my mother and sister) and my own were saved by the Swedish diplomat Raoul Wallenberg in one of the houses under Swedish protection.

In the early 1950s, during my high school years, the Russian occupation brought the communist takeover of Hungary's government, and my "bourgeois" family background (my family owned a shoe store) classified us as "class enemies," which disqualified me from attending the university. However, because I was a good basketball player on my high school team, I was exempted and accepted into the Technical University in 1953, during the darkest period of Stalinist rule.

I was among 200 first-year university students in chemical engineering at the Technical University in Budapest. It was well-known that only 50 would be allowed to graduate after the four-year program, because the planned socialist economy had only 50 positions for chemical engineers.

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In my senior year, in the fall of 1956, a few months before my graduation, the Hungarian Revolution broke out with the active participation of the students in the Technical University. After 10 days of freedom, the Russian troops reoccupied Budapest, and after two weeks, by the end of November, I escaped into Austria, taking with me my girlfriend, Judy, now my wife for more than 50 years.

We were lucky enough to be placed on a first-preference quota for university students, and we arrived in the U.S. in early January 1957. Our incredible luck held up when we were admitted as students in the College of Chemistry at the University of California, Berkeley, by Kenneth Pitzer, who was the dean at that time.

Having taken a graduate course in thermodynamics from Leo Brewer, one of the most exciting and creative teachers of physical chemistry, I decided that chemistry was where I wanted to be. My plan was to do research in either catalysis or polymers, two subjects that I found intriguing—mostly because I did not know much about them—and they seemed to be important. This trait to move into new territories of chemistry from areas that I understood more comfortably has stayed with me all through my professional life.

None of the faculty members at Berkeley did research in either field, but Richard Powell, an inorganic kineticist, offered me a research topic in catalysis. It involved small-angle X-ray scattering studies of platinum particles deposited on porous alumina, a typical catalyst used in the petroleum refining industry. So I joined his group! The security of a teaching assistantship that paid me $169.42 per month now provided a firm foundation for Judy and me to get married, which we did on the Labor Day weekend of 1957.

In keeping with the culture of physical chemistry at Berkeley, I had to build my own apparatus. Learning how to use the machine shop was essential, and the ability to pick the locked machine shop door on the weekends was a necessary requirement for the Ph.D.

THOSE WERE THE DAYS of Sputnik, and federal funds in support of research were increasing by more than 25% per year, and many Berkeley faculty had ample funding for research. However, a good number of them refused to accept money from the federal government, convinced that the influx of government money would destroy the autonomy of the university. My research director was one of those professors who shunned federal money. As a result, I had to work as a teaching assistant every semester in order to make a living.

I received my Ph.D. in three years, and having taught nonstop during all this time, I did not find a faculty job that appealing, so I joined IBM Research in New York after my graduation in January of 1960.

Surfaces were an area of chemistry I had come to like very much because important catalytic reactions took place on surfaces in ways that I wanted to understand. At Berkeley, I learned that modern physical chemistry requires molecular-level studies of systems, but techniques to study surfaces in that way were not available then.

At IBM, I learned solid-state physics and semiconductor materials science; the early 1960s were times of rapid development of the transistor, the injection laser, and other solid-state devices. I learned that for these solid-state devices to work faster, they had to be smaller to minimize the electron transit time through them, and consequently, their surfaces become an even larger fraction of the whole device. As devices become all surface, all of their electronic properties are controlled by their surface chemistry and contact interfaces.

The dire needs of the solid-state-device technologies for atomic-scale analysis to reveal the structure and composition of surfaces and interfaces became the driver for developing instrumentation. Among them, low-energy electron diffraction (LEED) was perhaps the first that permitted studies of surface structures on the atomic scale. Thanks to the needs of space sciences, high vacuum–to keep surfaces clean–became available at a reasonable cost. And modern surface science was born.

By 1963, it was clear that I liked research very much and that I was good at it, so I decided to leave IBM and look for a faculty position. After an intense job search, I joined the chemistry faculty at Berkeley in July of 1964, and Judy and I moved back to the West Coast with our one-month-old baby girl, Nicole, our first-born.

At Berkeley I started working with platinum, the granddaddy of all catalysts, in the hope of learning how and why it is so good at carrying out so many chemical surface reactions. It was first used in 1823 to produce flames, instantly aiding the combustion of hydrogen in air. Before the discovery of the match, platinum was used as a lighter. By the end of the 19th century, Paul Sabatier had compiled a book on organic reactions accelerated by platinum. Today, platinum is at the heart of the catalytic converter that cleans the exhaust gases from automobiles, and the metal produces high-octane gasoline from naphtha.

From the perspective of over 40 years of research, I see three phases in my research that helped bring surface chemistry to its present level of molecular sophistication. The first phase used metal single-crystal surfaces in ultrahigh vacuum. My aim was to determine the structure of clean surfaces and adsorbed molecules by using LEED-surface crystallography.

Parallel with these studies of surface structures, we developed molecular beam-surface scattering for studies of reactions and energy transfer between incident molecules and metal surface atoms. Studies combining molecular beam-surface scattering and LEED-surface crystallography revealed the unique activity of defects, atomic steps, and kinks on metal surfaces to dissociate H–H, C–H, C–C, C≡O, and O=O bonds.

The LEED-surface crystallography studies resulted in the discovery of reconstructed clean surfaces, such as those of metals (platinum, gold, and iridium), ice at temperatures as low as 100 K, and alkali halides (NaCl and LiF).

We solved the first surface structure of any organic molecule on a metal surface [ethylene on Pt(111) and Rh(111)] with precise bond distances and bond angles. This was followed by surface structures of many other small organic molecules. The outcome of these studies was the discovery of adsorbate-induced restructuring of metal surfaces to form structures similar to metal-organic complexes.

The second phase of my research led to studies of metal single-crystal surfaces at high pressures. I was uneasy with studies of surfaces only in vacuum, often expressing my frustration to my students by telling them that life is not an ultra-high-vacuum system. In addition, I could not study catalysis in that circumstance because the reaction probability of most catalytic reactions occurring on a surface is very low. For example, for the dissociation of methane on a platinum surface, it is one per 108 molecules incident on the surface.

So we designed a combined high-pressure/ultra-high-vacuum system that permitted studies of reactions at high pressures, as well as surface analysis, which requires vacuum before and after reactions. Using these hybrid systems, we investigated ammonia synthesis on iron and rhenium crystal surfaces and hydrocarbon conversions over platinum, molybdenum, and rhodium crystal surfaces. We discovered that both reaction rate and product composition depend on the catalyst's surface structure.

We then converted the nonlinear optics technique of second harmonic generation into a surface-sensitive tool-sum frequency generation vibrational spectroscopy. This tool proved to be ideal for examining adsorbates—that is, catalytic reaction intermediates and polymers at buried interfaces, such as between a solid and a high-pressure gas, a solid and a liquid, or a solid and another solid.

Using our high-pressure cell technology, we constructed a new high-pressure scanning tunneling microscope for the determination of surface structures in equilibrium with a gas. We found that the surface is not at all static but is highly dynamic. In fact, for heterogeneous catalysis to occur, as in the hydrogenation of ethene to ethane or in hydrogenation and dehydrogenation of cyclohexene to cyclohexane and benzene, respectively, everything on the surface must move. This finding was underscored by our discovery that immobile adsorbed layers result when surface reactions are poisoned.

These new techniques allowed us to monitor surfaces and molecules on surfaces at high pressures, as well as in the presence of liquids at the buried interfaces. They opened many other applications of surfaces to molecular scrutiny. This was the surface chemistry revolution we had been waiting for!

The applications include biointerfaces, electrochemistry, and corrosion, just to mention a few. The properties of interfaces, and the technologies based on these properties, could now be revisited, studied, and understood on the molecular scale. For example, tribology—the science of friction, lubrication, and wear—has experienced a renaissance with the development of new technologies for superior lubricants and wear-resistant coatings. The technology of bioimplants to replace heart valves, joints, and arteries extends and improves the quality of life for many, and it is a field of molecular surface science application that is developing explosively.

IN THE THIRD PHASE of our research, we moved from single-crystal surfaces to surfaces of monodispersed metal nanoparticles in the 1–10-nm range and with well-controlled shape. All catalysts-heterogeneous, enzyme, and homogeneous-are nanoparticles, and nature is telling us to use the unique size and structure of nanoparticles to improve catalytic turnover.

We tried different ways to make metal nanoparticles. Electron beam lithography was slow, and photolithography gave us particles bigger than 20 nm, whereas metal catalysts are much smaller, in the 1–10-nm range. Eventually, we found that synthesis of metal nanoparticles by colloid chemistry was the best way to proceed.

From studies of two-dimensional monolayer film deposits of metal nanoparticles and 3-D deposits of metal nanoparticles in mesoporous supports, we discovered that both the size and the shape of nanoparticles are important ingredients in controlling reaction selectivity. We also discovered that the oxide-metal interface at the metal nanoparticle periphery is an important catalytic site that controls both activity and selectivity.

Both nature and technology have produced nanosized catalysts for good reason. Rearranging the catalyst surface requires breaking metal-metal bonds, which requires energy. When a metal atom has many neighbors, many nearest-neighbor bonds have to break for rearrangement to occur. When a metal atom has fewer neighbors, as would be the case in a nanoparticle, less energy is required for rearrangement to occur.

Similarly, the reacting molecules, reaction intermediates, and products must also readily alter their bond distances to rearrange rapidly. So reaction is favored when the reacting molecules also require that a relatively small number of bonds be broken and re-formed as the chemistry occurs.

So there is a punch line to this story: The platinum surface restructures during catalytic reactions, and it has different structures when it carries out oxidation reactions and when it rearranges organic molecules. This chameleon-like behavior makes platinum versatile in so many catalytic reactions. It also rearranges more easily in a nanoparticle form, where fewer atoms and molecules participate in the restructuring during the catalytic turnover.

OUT OF MY tumultuous life history have come several lessons that I have tried to instill in my students. First, always look into the future, and if you fail, do not feel sorry for yourself. Instead, have the perseverance to try again, and sooner or later, you will succeed. Success is the result of perseverance. Second, everybody needs mentoring, and if you can give a helping hand to the next generation of students or chemists, do it!

Third, have a dream or vision of what you want to accomplish in science that is worth spending your entire life on, not just a strategy for doing the next experiment or raising the money to do it. And fourth, know every detail of your experiment and the instruments you use, to the point of being able to build it on your own. This is the best way to become a leader no matter where life and chemistry take you.

I am pleased to see the growth of surface chemistry in the American Chemical Society. When I was the chair of the Division of Colloid & Surface Chemistry in 1976, only 130 surface chemistry papers were presented at national meetings. At the last meeting in Boston, 630 papers were presented.

There is a megatrend of integration of many subfields in chemistry, and joint symposia among the divisions are the rule rather than the exception. ACS and its national meetings reflect the ever-changing directions and breadth of the discipline, as well as the world-class science of the U.S. and of the international chemistry community.

In the spirit of looking into the future, I believe that for decades to come, research in energy conversion that will take us gradually from the petroleum era to the use of sustainable-energy technologies will be at the center of chemistry. Along with sustainable energy, protection of the environment–air, water, and soil–is a societal challenge that will occupy much of the chemistry community. These challenges will also provide fantastic opportunities for development of new chemistry and chemistry-based technologies.

Solar energy conversion—through biomass, through splitting of water or carbon dioxide, or through direct conversion to electricity—is a major challenge. Clean coal conversion processes and utilization of nuclear energy through hybrid chemical systems that provide transportable energy on demand are yet to be developed. Efficient energy storage will always be part of the energy-conversion problems and their solutions.

Invention of green chemical processes by selective catalysts that make the desired product molecule only, thereby eliminating waste by-products, is the challenge faced by all existing industrial chemical processes. Because all catalysts are nanoscale materials, rapid development of selective catalysis is becoming a reality through the synthesis of nanomaterials and the phenomenally fast development of nanosciences.

ACS could become a leader in replacing petroleum as a chemical energy source by using the Petroleum Research Fund for research focused for this purpose.

As for me, I married Judy in 1957 on $169 a month. We now have two children and four grandchildren. I have managed to create a lot of good chemistry, and there is more to come. In light of the energy crisis, we need much more surface chemistry and catalysis to help the U.S. achieve energy independence. I have trained a new generation of chemists who are employed in the U.S. and all over the world—and it is still a work in progress. I cannot ask for anything more.


Gabor Somorjai
Pioneering Technique
In low-energy electron diffraction (LEED, left), an electron gun shoots a beam of electrons at a crystal surface. Atoms there backscatter the electrons, forming a diffracted beam that produces a spot on a fluorescent screen. The diffraction spots??? positions enable determination of the ordered arrangement of the atoms on the crystal surface. Studies combining LEED and surface crystallography revealed the structures of four crystal faces of platinum (right): Pt(111), hexagonal surface; Pt(100), square surface; Pt(755), a surface with regularly spaced steps; and Pt(13,11,9), a surface with kinks in the steps of one atom in height.

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Gabor Somorjai
Instrumentation Advance
In a molecular beam-surface scattering experiment (left), an incident beam of molecules scatters back after impinging on a crystal surface. The detector (mass spectrometer) indicates what comes back, and the chopper measures its velocity. Such experiments can be used to follow the dissociation of molecules on a surface. They have revealed that dissociation of H–H, C–H, C≡C, C–O, and O–O bonds occur preferentially at atomic steps and kinks. The platinum atoms on terraces are much less active.
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Gabor Somorjai
Cosmetic Surgery
Shown are surface reconstructions of a metallic solid (platinum), an ionic solid (NaCl), and a hydrogen-bonded solid (ice). Ideal structures for platinum and NaCl are shown for comparison. In ideal surface structures, the atoms occupy the sites expected from the projection of the bulk atom to the crystal surface. When surface atoms occupy a different site, the surface is said to be ???reconstructed.??? The absence of atoms above the surface lowers the coordination number (the number of nearest neighbors) and changes the optimal configuration for bonding in the surface layer. For platinum, surface atoms pack more closely in a reconstructed hexagonal arrangement instead of an ideal square lattice. For NaCl, the sodium ions at the reconstructed surface are below the surface chloride ions instead of being in the same surface plane as they would be in the ideal arrangement. For ice, the top surface layer is disordered and waterlike at a temperature as low as 100 K. The layer's thickness increases as temperature rises toward the melting point of ice (273 K) and causes the slipperiness of ice. The disordered waterlike layer also helps dissolve impurities in the surface layer. Results were obtained through LEED, which has not yet been carried out at temperatures lower than 100 K.
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Gabor Somorjai
Adsorption Effects
Molecules adsorbed on a metal change the surface and are themselves changed. Shown at left is restructuring of the surface of Pt(111) induced by ethylene (C2H4) adsorbed in the form of ethylidyne (C2H3). The three platinum atoms that bind the organic molecule are pulled out of the hexagonal arrangement of the Pt(111) crystal surface and form a cluster similar to a metalloorganic molecule, Pt3CC H3. Shown at right is the effect of adsorption on benzene. Whereas benzene in the gas phase has equal C–C bond distances, benzene adsorbed on Pt(111) has unequal bond lengths.
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Gabor Somorjai
Hybrid Hardware
Analysis of catalyst structure and composition of metal catalysts using electrons and ions must be carried out in vacuum, whereas analysis of catalytic reactions requires a high-pressure system. A hybrid system enables both. The chamber at top left shows an approximately 1-cm2 platinum sample in vacuum for analysis of structure and composition. The chamber at bottom left shows a tube reactor, which encloses the metal catalyst during reaction. As the catalyst reacts, rates of reaction and product distribution are measured. To study the effect of surface structure on catalysis, the catalyst is shuttled between the vacuum chamber, where surface analysis is carried out, and the reactor enclosure, where reaction studies at high pressures are performed. Experiments enabled by this system reveal that reactions of hydrocarbons are very sensitive to the surface structure of platinum. Conversion of n-hexane to benzene, and of n-heptane to toluene, is better on a hexagonal surface (top right). Isobutane is converted to n???butane by square and stepped surfaces and to a mixture of methane, ethane, and propane by a kinked surface (bottom right).
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Gabor Somorjai
Surface-Sensitive Tool
Surface-Sensitive Tool. Sum frequency generation (SFG) vibrational spectroscopy identifies adsorbed molecules by their vibrational frequencies. In an SFG apparatus (left), the frequency ??vis of the visible laser beam (green) is fixed, and the IRbeam (red) frequency ??IR is varied. When ??IR coincides with a vibrational transition from |??0> to |??1> of an adsorbed molecule, the molecule is excited to the virtual state |n> and emits the sum frequency beam (purple). Selection rules dictate that the twophoton transition to the virtual state is allowed only in a medium that lacks inversion symmetry. Thus, the technique probes only those molecules adsorbed on the surface. SFG vibrational spectroscopy revealed three reaction intermediates in cyclohexene hydrogenation/dehydrogenation on a platinum surface (right). Peaks in the SFG spectra correspond to intermediates adsorbed on the catalyst. The y-axis is in arbitrary units of intensity.
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Gabor Somorjai
Disorder Catalyzes
In high-pressure scanning tunneling microscopy (HP-ST M) (scheme at center), a metal tip scans the platinum catalyst at atomic distances and measures the tunneling current, which sensitively depends on the tip-to-atom distance. Tripod legs guide the tip across the surface at a speed of about 10 nm per millisecond. The technique allows the simultaneous monitoring of reactants and products of cyclohexene hydrogenation/dehydrogenation over a catalytically active, disordered platinum surface (ST M image at left) and a CO - poisoned, ordered platinum surface (ST M image at right). Conversion to mostly cyclohexane and some benzene occurs on the disordered surface (graph at left) but not on the ordered surface (graph at right). In both graphs, the y-axis indicates the pressures of reactants and products, which are proportional to their amounts.
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Gabor Somorjai
Metal Nanoparticles
To probe real catalysts more fully, studies advanced from work on singlecrystal surfaces of metals to work on nanoparticles. In the Somorjai lab, nanoparticles were prepared in two forms by colloid chemistry under controlled conditions: as 2-D nanoparticle arrays and as nanoparticles dispersed in 3-D mesoporous oxide channels.
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Gabor Somorjai
Size Dependence
Pt nanoparticles of different sizes (transmission electron microscopy images at left) direct the course of cyclohexene hydrogenation/dehydrogenation differently (graph at right). Large particles favor hydrogenation to cyclohexane (blue), whereas small particles favor dehydrogenation to benzene (red). The y-axes indicate the fraction of each product in the mixture.
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Gabor Somorjai
Shape Dependence
The shape of Pt nanoparticles (transmission electron microscopy images at left) affects the course of benzene hydrogenation. Cuboctahedral Pt, or Pt(111), favors hydrogenation to cyclohexane (red) and cyclohexene (orange). Cube Pt, or Pt(100), favors exclusive hydrogenation to cyclohexane (green). Cuboctahedral Pt favors hydrogenation to cyclohexane (red) over hydrogenation to cyclohexene (orange) by as much as 10 times. The amount of product formed increases with temperature to a maximum, after which further heating slows the reaction.
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Chemical & Engineering News
ISSN 0009-2347
Copyright © 2009 American Chemical Society


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