Professor Somorjai’s research interest is in the fields of surface science and catalysis. His group is studying the structure, bonding, and reactivity of solid-gas, solid-liquid, and solid-solid interfaces on the molecular scale. This knowledge is then utilized to understand macroscopic surface phenomena, the rate and product selectivity in multipath catalytic reactions. Since most catalysts are nanoparticles, metal and oxide nanoparticles are synthesized using colloidal chemistry in the 1-10 nm range their catalytic activities are studied as it changes as a function of size and shape and composition. High surface area oxides supports are constructed that are mesoporous and the role of oxide-metal interfaces are studied in catalytic reactions. The molecular mechanisms of catalytic reactions are studied using instruments for nanoscale characterization of surfaces under reaction conditions. These include sum frequency generation surface vibrational spectroscopy (SFG), high-pressure scanning tunneling microscopy (high pressure STM) and high-pressure x-ray photoelectron spectroscopy (high pressure XPS). Synchrotron based spectroscopy techniques (microspectroscopy, x-ray absorption techniques of NEXAFS, EXAFS) are also utilized to monitor surfaces under reaction conditions, small metal nanoparticles were found to heterogenize homogeneous solution phase catalytic reactions and studies of correlating heterogeneous and enzyme catalytic reactions are in progress. Professor Somorjai has educated a generation of leading scientists in the field. Out of more that 130 Ph.D. students and 220 postdoctoral fellows, approximately 110 hold faculty positions now.
Gabor A. Somorjai was born in Budapest, Hungary, on May 4, 1935. He was a fourth year student of Chemical Engineering at the Technical University in Budapest in 1956 at the outbreak of the Hungarian Revolution. He left Hungary and emigrated to the United States, where he received his Ph.D. degree in Chemistry from the University of California, Berkeley in 1960. He became a U.S. citizen in 1962.
After graduation, he joined the IBM research staff in Yorktown Heights, New York, where he remained until 1964. At that time, he was appointed Assistant Professor of Chemistry at the University of California, Berkeley. In 1967, he was named Associate Professor, and in 1972 promoted to Professor. Concurrent with his faculty appointment, he is also a Faculty Senior Scientist in the Materials Sciences Division, and Group Leader of the Surface Science and Catalysis Program at the Center for Advanced Materials, at the E.O. Lawrence Berkeley National Laboratory.
Professor Somorjai has educated more than 130 Ph.D. students and 250 postdoctoral fellows. He is the author of more than 1200 scientific papers in the fields of surface chemistry, heterogeneous catalysis, and solid state chemistry. He has written four textbooks, Principles of Surface Chemistry, Prentice Hall, 1972; Chemistry in Two Dimensions: Surfaces, Cornell University Press, 1981; and Introduction to Surface Chemistry and Catalysis, Wiley-Interscience, 1994 and the Second Edition in 2010; as well as a monograph, Adsorbed Monolayers on Solid Surfaces, Springer-Verlag, 1979.
His honors include the following:
Colloidal chemistry is used to control the size, shape, morphology, and composition of metal nanoparticles. Model catalysts as such are applied to catalytic transformations in the three types of catalysts: heterogeneous, homogeneous, and enzymatic. Real-time dynamics of oxidation state, coordination, and bonding of nanoparticle catalysts are put under the microscope using surface techniques such as sumfrequency generation vibrational spectroscopy and ambient pressure X-ray photoelectron spectroscopy under catalytically relevant conditions. It was demonstrated that catalytic behavior and trends are strongly tied to oxidation state, the coordination number and crystallographic orientation of metal sites, and bonding and orientation of surface adsorbates. It was also found that catalytic performance can be tuned by carefully designing and fabricating catalysts from the bottom up. Homogeneous and heterogeneous catalysts, and likely enzymes, behave similarly at the molecular level. Unifying the fields of catalysis is the key to achieving the goal of 100% selectivity in catalysis.
Two major breakthroughs have revolutionized molecular catalysis science over the last 20 y. The first is in the development of nanomaterials science (1–4), which has made it possible to synthesize metallic (5– 7), bimetallic, and core-shell nanoparticles (8, 9), mesoporous metal oxides (10, 11), and enzymes (12–16) in the nanocatalytic range between 0.8 and 10 nm. The second innovation is in the advancement of spectroscopy and microscopy instruments (17–20)—including nonlinear laser optics (21), sum-frequency generation vibrational spectroscopy (22–24), and synchrotronbased instruments, such as ambient pressure X-ray photoelectron spectroscopy (8, 25–27), X-ray absorption near-edge structure, extended X-ray absorption fine structure (28–30), infrared (IR) and X-ray microspectroscopies (31), and high-pressure scanning tunneling microscopies (32, 33)—that characterize catalysts at the atomic and molecular levels under reaction conditions (34). Most of the studies that use these techniques focus on nanoscale technologies, such as catalytic energy conversion and information storage, which have reduced the size of transistors to below 25 nm (35).
Catalysts are classified into three types—heterogeneous, homogeneous, and enzymatic—and, in most cases, range in size from 1 to 10 nm, which is even smaller than the transistors being developed by the latest size-fabrication technologies. Heterogeneous catalysts work in reaction systems with multiple phases (e.g., solid-gas or solid-liquid phase); homogeneous catalysts reside in the same phase as the reactants, almost always in the liquid phase; and enzymatic catalysts, which are most active in an aqueous solution, make use of active sites in proteins. The catalysis of chemical energy conversion provides ever-increasing selectivity in producing combustible hydrocarbons, gasoline, and diesel.
The tenets that direct our catalysis research involve nanoparticle synthesis, characterization under reaction conditions, and reaction studies using these nanoparticles to determine kinetics, selectivity, deactivation, and other catalytic kinetic parameters. These variables are studied in the same research group because they are the underpinning of molecular catalysis. The hypothesis that we are striving to support is that the three fields of catalysis (heterogeneous, homogeneous, and enzyme) behave similarly on the molecular level.
Engineering chemical and physical properties at molecular levels is a challenging task that requires tools and strategies, known as in situ probing, to characterize catalysts in action. This in situ method measures macroscopic and microscopic properties simultaneously or separately under identical or similar conditions in an attempt to correlate function and structure (31). In addition to this in situ approach, surface techniques—with temporal, spatial, and chemical resolutions in their respective scales of subsecond, subnanometer, and vibrational and electronic levels—are prerequisites to gaining molecular insight into catalysts’ operations (42–44). These techniques are often based on detecting outbound electrons, photons, or ions of catalysts of interest upon excitation with high-energy electrons or photons in a broad electromagnetic spectrum, ranging from radio waves to IR and visible light and to UV and X-rays. Monochromated in energies and collimated or focused in space, these probes carry electronic, vibrational, and bonding information, giving away fundamental details of the otherwise hidden components of a catalyst—pieces of a puzzle and snapshots of a bigger picture.
When CO oxidation was studied on rhodium nanoparticle surfaces as a function of size below 2 nm, the CO oxidation rates increased by 30-fold. Ambient pressure XPS studies (46) indicated that the higher turnover rates are due to the oxidation state of rhodium changing from metallic rhodium to rhodium3+ (Fig. 4B). Similar studies on platinum indicated that platinum nanoparticles above 1.5 nm are metallic; however, the studies also found that platinum below 1.5 nm and as low as 0.8 nm are in the 2+ and 4+ oxidation states (29, 47). Because very few bulk atoms are available for these nanoparticles, they become dominated by low coordination surface atoms, and, as a result, their electronic structure changes. Nørskov and coworkers (48) have studied this process and found that the adsorption energy of oxygen on gold nanoparticles changes as a function of a decrease in size. Because there is a decrease in the gold coordination number at the adsorption sites and fewer than 55 atoms, which is slightly more than 1 nm, the gold becomes oxidized to gold 1+ and 3+ instead of metallic gold.
The homogeneous catalysts are usually single transition metal ions, surrounded by ligands. As a result, we tried to use these small nanoclusters, which have controlled high oxidation states, to carry out homogeneous catalysis. We adsorbed the small nanoclusters on dendrimers, tree-like polymers that hold these nanoparticles throughout its branches. We found that these dendrimer-encapsulated nanoparticles are excellent homogeneous catalysts so we managed to heterogenize homogeneous catalysts by using nanoparticles composed of 40 atoms of rhodium, palladium, gold, or platinum for reactions including hydroformylation, decarbonylation, and other commonly known homogeneous catalytic reactions (3, 31, 47, 49–53). We demonstrated that catalytic reactivity and selectivity could be tuned by changing the dendrimer properties, in a similar fashion to ligand modification in an organometallic homogeneous catalyst (51, 53). X-ray absorption spectroscopy studies [X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)] showed (28–30) that the nanoparticles dispersed to small low coordination clusters under oxidizing conditions but reassembled to the original 1-nm particles when under reduction by reactants, products, or the dendrimers (Fig. 4C). This process is also reversible.
Likewise, single site homogeneous catalysts are low coordination systems comprised of ligands that control electronic structure and chemistry at the molecular level (29, 54). By controlling types and binding of ligands, a high level of selectivity (regio- and enantio-selectivity) is obtained in homogenized catalytic reactions. Capping agents used in colloidal synthesis, similar to ligands in single site organometallic complexes, can be used to control selectivity in heterogenized homogeneous reactions, which, as of today, remains a great challenge.
Platinum is an excellent hydrogenation catalyst of many organic molecules, such as crotonaldehyde. When platinum nanoparticles of the same size are placed on two different oxides—silica and titania—one can see that the turnover rates and the selectivities are much higher when titania rather than silica is used as a support. The importance of the oxide support for metal catalysts to change selectivities and product distributions is well-known. This phenomenon of the oxide support effect on catalytic reaction rates, where the oxide alone does not carry out the same or any reaction, is called positive strong metal support interaction (SMSI) in the literature (55). It is the charge transfer ability of reducible oxide supports that acts on the performance of metal catalysts: But how does charge regulate catalytic processes?
An explanation was found by surface physics studies of hot electron emission under light illumination that was carried out by exothermic catalytic reactions on metal surfaces (56), such as CO oxidation or hydrogen oxidation (Fig. 5A). The deposited energy produces high kinetic energy electrons that have a mean free path within the metal in the range of 5–10 nm. The chemical energy deposition in metals to produce hot electrons has been well demonstrated by Huang et al. (57) using highly vibrationally excited NO molecules impinging on gold compared with lithium fluoride surfaces. On gold, the NO molecules in the 15th vibrationally excited state lose 1.5 eV of energy to produce molecules in the eighth vibrational state. On the other hand, the vibrationally excited NO molecules lost no energy when they scattered from lithium fluoride, which has no free electrons. The hot electron generation can be observed by using exothermic catalytic reactions on a Schottky diode on platinum and titanium oxide where the platinum is less than about 5 nm in thickness. The charge flow between the platinum and the titanium oxide allows one to determine the current flow in the battery configuration shown in Fig. 5A. One can detect a so-called chemicurrent, which is correlated with the turnover rate of exothermic CO oxidation or hydrogen oxidation reactions (58). Theoretical calculations showed that the transition state in these processes involves CO2 − or H2O−, which yields the chemicurrent that is proportional to the turnover rate. In this study, acid–base catalysis is correlated with charge concentration, and not with surface area. On the other hand, covalent bond catalysis is known to be surface area-dependent. These two modes of catalysis are the major ways chemistry occurs in most catalytic processes.
When one places metal nanoparticles into a mesoporous oxide support, many oxide–metal interfaces are produced within the mesopores between the metal and the oxide. Studies have found that these oxide–metal interfaces have major effects on catalytic reactions. The isomerization of n-hexane on naked oxide only results in the cracking of the n-hexane molecules whereas mesoporous oxides give rise to 100% selectivity of n-hexane isomers of high octane numbers (59) in the presence of platinum (Fig. 5B). Fig. 5 C–E shows similar effects when platinum is placed on various oxide mesopores. Platinum nanoparticles produce very little CO oxidation within mesoporous silica, but, when they are placed on mesoporous cobalt oxide, more than a 1,000-fold increase in catalytic turnover for CO oxidation kinetics is found.
Fig. 5C, Right shows mesoporous transition metal oxide-supported platinum nanoparticles. This image is the oxide–metal interface, which produces large, strong metal support effects. There is a charge transfer between the metal and the oxide under reaction conditions. If the oxide is alone, as in the case of n-hexane conversion with pure oxides of niobium oxide, titanium oxide, and other oxides, only the cracking of the n-hexane molecules is observed. However, if the platinum is in the mesopores, a 100% selectivity to isomerization is produced, which is very high and an important factor in making high-octane gasoline.
Fig. 5C, Left shows platinum nanoparticles in contact with microporous oxides when the platinum nanoparticle is much larger and cannot fit into the micropores. In this case, the chemistry that occurs—known as bifunctional catalysis—is the sum total of the chemistry of platinum and the microporous oxide, which act in parallel or consecutively. In the other case, when the mesoporous transition metal oxide can accommodate the 3-nm platinum inside its mesopores, there are oxide–metal interfaces, where charge transfer and acid–base catalysis occur, which are uniquely selective in many circumstances. Similar results are seen when CO oxidation is carried out on platinum supported by silica or another transition metal oxide (60). The turnover rate on silica is small, equal to pure platinum turnover; however, when cobalt oxide is the mesoporous support, the turnover rate is amplified by a 1,000-fold (Fig. 5E). This finding is indeed a major increase in catalytic activity.
It is clear that we can heterogenize homogeneous catalysts. However, enzymes (12–16) are also very important catalytic systems, and recent studies have focused on how to synthesize pure enzymes as well as on how to look for similarities between enzymes and all three catalytic systems on the molecular scale. For example, it is known that, when enzymes are immobilized on a surface, they gain the reusability and ease of separation seen in heterogeneous catalysts, as well as in some cases becoming more stable to wider pH and temperature ranges (61–63). It is planned to advance previous work by immobilizing enzymes on a solid surface and studying the enzyme–solid interface at the molecular level, and to develop a molecular understanding of all three types of catalysts under similar conditions of reactions and chemical environments.
In our attempt to focus on the chemical correlations between the three catalysis groups—heterogeneous, homogeneous, and enzymatic—the future looks very promising for molecular catalysis science studies. Catalysis of homogeneous, heterogeneous, or enzymatic origin alike involve nano-sized materials. These nanocatalysts comprise inorganic and/or organic components. Charge, coordination, interatomic distance, bonding, and orientation of catalytically active atoms are molecular factors shared by all three fields of catalysis. By controlling the governing catalytic components and molecular factors, catalytic processes of a multichannel and multiproduct nature could be run in all three catalytic platforms to create unique end products. Fig. 6 illustrates the promise of a molecularly unified catalytic scheme of the future.
Steady-state measurements for catalysis are limited in the types of information which can be extracted. To further our understanding of complex reactions occurring on catalyst surfaces, we turn to non-steady state reactor techniques whereby we can follow the changes in product formation in time. Currently studying the Fischer-Tropsch reaction (CO hydrogenation to longer chain hydrocarbons), the formation of product molecules at the onset of the reaction reveals the dynamic and complex, multi-step processes that are operative on cobalt catalyst surfaces. Other multi-step gas phase and liquid phase catalytic reactions are monitored at short reaction times (seconds) in flow reactors constructed for this purpose.
Newly synthesized mesoporeous metal-organic frameworks (MOFs) are utilized in catalytic reactions on platinum (hydrocarbon conversions) and copper (methanol synthesis) surfaces and compared with the chemistry on more traditional supports (mesoporous silica, alumina, zeolites, and transition metal oxides).
Catalytic conversion of light alkanes (methane, ethane, and propane) are studied at high temperatures (500-800 degrees centigrade) using transition metal oxide catalysts where the oxygen atoms in the oxides play important roles in the oxydehydrogenation process. X-ray photo electron spectroscopy (XPS) is utilized to identify the active oxidation states responsible for the catalytic selectivity, and carbon dioxide and hydrogen addition are utilized to adjust to the desired oxidation states. Studies around the elective temperatures with the addition of alkali (sodium) test the importance of the effects on the molten state on the oxygen atom diffusion in the transition metal oxides on the oxydehydrogenation rates of the alkanes.
One of the much-debated subjects in lithium ion batteries is the solid-electrolyte interphase (SEI). The SEI is a physical barrier that hinders further electrolyte reduction on the anode surface, prevents electron flow to the electrolyte solution while enabling the diffusion of only lithium ions from the anode into the electrolyte solution. These SEI properties greatly affect the Li-ion battery performance.
We are currently studying electrolyte reduction products on crystalline and amorphous silicon electrodes in a lithium half-cell system using sum frequency generation vibrational spectroscopy (SFG-VS) under reaction conditions.
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Since the publication of the first edition of this book (published June 8, 2010), molecular surface chemistry and catalysis science have developed rapidly and expanded into fields where atomic scale and molecular information were previously not available. This revised edition of Introduction to Surface Chemistry and Catalysis reflects this increase of information in virtually every chapter. It emphasizes the modern concepts of surface chemistry and catalysis uncovered by breakthroughs in molecular-level studies of surfaces over the past three decades while serving as a reference source for data and concepts related to properties of surfaces and interfaces.
The book opens with a brief history of the evolution of surface chemistry and reviews the nature of various surfaces and interfaces encountered in everyday life. New research in two crucial areas-nanomaterials and polymer and biopolymer interfaces-is emphasized, while important applications in tribology and catalysis, producing chemicals and fuels with high turnover and selectivity, are addressed. The basic concepts surrounding various properties of surfaces such as structure, thermodynamics, dynamics, electrical properties, and surface chemical bonds are presented. The techniques of atomic and molecular scale studies of surfaces are listed with references to up-to-date review papers. For advanced readers, this book covers recent developments in in-situ surface analysis such as high- pressure scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and sum frequency generation vibrational spectroscopy (SFG). Tables listing surface structures and data summarizing the kinetics of catalytic reactions over metal surfaces are also included.
As a young man, Gabor Somorjai couldnt have known he would one day be forced to flee his native Hungary. But upheaval in Europe during and after World War II led him to the United States where he immersed himself in science and soon began building a research group at one of the powerhouses of scientific discovery. The Sputnik wake-up call that triggered the huge influx of government support for scientific research in the second half of the twentieth century bolstered fundamental research programs like the one Somorjai established at the University of California, Berkeley, and Lawrence Berkeley National Laboratory. Key discoveries in his field, surface science, led the way to advances in catalysis know-how that underpin todays energy storage and transformation technology and safeguard the environment. By revealing the unique ways microscopically thin layers of atoms and molecules control the chemistry and physics of surfaces, modern surface science also spawned rapid development in microelectronics, high-power computing, and communication and information technology.