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.
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:
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.