Somorjai

In the field of Surface Science, Somorjai contributed greatly to the development of low-energy electron diffraction (LEED) surface crystallography. In 1965, he reported that atomic sites on the (100) surface of platinum crystals were different from those in the bulk. In a subsequent series of crucial experiments, he explored the properties of reconstructed metal surfaces and went on to demonstrate that surface irregularities, such as steps and kinks, were largely responsible for the chemical activity of transition metals. His recent studies reported the reconstruction of oxide surfaces and the surface structure of ice. In parallel with his characterization of clean metal surfaces, Somorjai elucidated the structures of adsorbed organic molecules and the chemical rearrangements of adsorbed ethylene and other alkenes and alkynes, and reported his discovery of alkylidyne bonding. It was his fundamental work using LEED, high resolution electron energy loss spectroscopy (HREELS), and sum frequency generation (SFG), which have made it possible to identify the bonding of hydrocarbons as being similar to that in organo-metallic clusters. His vibrational spectroscopy studies at high pressures (SFG) revealed the presence of weakly adsorbed-bonded olefins and carbonyl clusters that are present only at elevated pressures (atmosphere). His studies of adsorbed ordered and disordered monolayer surface structure by LEED surface crystallography revealed coadsorption-induced ordering. The application of Tensor-LEED calculations led to the finding that most chemisorbed atoms and molecules restructure the metal substrate upon adsorption. This phenomenon is called adsorbate-induced restructuring. The new concepts of surface chemistry uncovered as a result of these studies include the following:

the importance of surface irregularities-steps and kinks-in strong chemisorption and bond dissociation
bonding of chemisorbed hydrocarbons is similar to that in organo-metallic clusters
coadsorption-induced ordering
adsorbate-induced restructuring of metal surfaces.

In the field of Heterogeneous Catalysis, the use of model catalysts for uncovering the structure, bonding reactivity (rate, selectivity) relationships for catalyzed surface reactions was invented and developed by Somorjai. At first, he mainly used small area metal single-crystal surfaces. Later, evaporated thin films, foils, and nanoclusters deposited by electron beam lithography were also utilized. He invented and developed the techniques for combined ultrahigh vacuum and high pressure studies, and molecular surface characterization during catalytic reactions (using the scanning tunneling microscope and SFG). He studied the conversion of hydrocarbons over platinum, demonstrating the surface structure sensitivity of reaction rates and product distribution, along with the key role that atomically rough surfaces, steps and kinks play in controlling reactivity. The focus of these studies included ammonia synthesis over iron and rhenium single crystals, along with carbon monoxide and ethylene hydrogenation, and thiophene hydrodesulfurization over various transition metal crystal surfaces (Pt, Rh, Mo, Re, Fe). The new concepts of heterogeneous catalysis uncovered as a result of these studies include:

The importance of atomically rough surfaces for carrying out high rate surface reactions
The presence of a strongly bound overlayer on the active catalyst surface (carbonaceous or sulfur containing)
Weakly bound olefins are the dominant reaction intermediates during hydrogenation
The unique high activity of bimetallic systems and certain oxide-metal interfaces
The crucial role that coadsorbed "promoters" play (such as potassium, alumina, sulfur) as either structure or bonding modifiers

Adsorbate-induced restructuring of catalyst surfaces, occurring on different time scales (adsorption, catalytic turnover, or longer times), has been suggested as an important event that controls catalytic site activity.

Somorjai has also been a major contributor in developing New Instrumentation for studies in structural surface chemistry and the surface science of heterogeneous catalysis; in particular, he is responsible for several innovations in LEED instrumentation. An apparatus was designed in his laboratory that combined the high vacuum technology required for surface analysis with the high pressure cell necessary for studies of catalytic reactions on small area crystal surfaces. In addition, the use of SFG by nonlinear laser spectroscopy has permitted in situ detection of short lived reaction intermediates by vibrational spectroscopy at low and high pressures.

This low pressure-high pressure technology was extended to incorporate a scanning tunneling microscope (STM) operating at high temperatures and pressures, which permitted in situ monitoring of reaction-induced restructuring of metal crystal surfaces. Thus, the surface technology developed by the Somorjai group evolved from molecular studies of surfaces before and after catalytic reactions, to studies during the reaction. He was the first to develop a molecular beam-surface scattering instrument to determine energy and angular distributions in single crystal surface reactions. The combination of technical development and its application to significant problems of surface chemistry and heterogeneous catalysis is a main characteristic of Somorjai's research.

Research on Polymer Surfaces in the Somorjai group at first focused on polyethylene and polypropylene. Atomic force microscopy (AFM) and sum frequency generation (SFG)-surface vibrational spectroscopy used in combination were found to provide, in an optimal way, correlation between mechanical properties (friction, hardness) and molecular surface structure. Changes of polyethylene surface structure with increasing molecular weight have been detected. The orientations of the surface methyl groups in atactic and isotactic polypropyelene were determined along with the tribological properties as a function of load over a nine order of magnitude (10^-9 to 1 newton) range. Changes of polypropylene suface structure as a function of temperature through the glass transition have been monitored by SFG. The surface of polymer blends are covered with mostly hydrophobic groups at the polymer-air interface. The hydrophylic groups migrate to the surface at the solid-water interface giving rise to large changes of surface tension. The surface composition of polyurethene blends as a function of bulk composition were monitored for several systems to establish the surface phase diagram. The adsorption of amino acids, peptides and proteins on polymer blend surfaces are being studied to attempt to correlate the surface structure of the adsorbed protein monolayer to polymer biocompatibility.