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 Our research group studies the dynamics of the interactions of molecules with the surfaces of materials. We are interested in surfaces of materials that serve either as catalysts for chemical reactions or as templates for nanodevices or as the devices themselves. However, surfaces of practical heterogeneous catalysts or of nanotechnological devices cannot be studied under ambient conditions such as atmospheric pressure or liquid environments because the presence of the gas or liquid continually modifies the surface. Therefore, in order to probe a surface unambiguously at the molecular level, experiments must be carried out under ultrahigh vacuum (UHV) conditions (<10-10 Torr). The problem arises that the surface chemistry observed under high pressure of gaseous reactants or in liquids is often different from the surface chemistry observed in the low pressure conditions of vacuum. For example, many surface reactions proceed readily under high pressure conditions typical of a commercial, heterogeneous catalytic reaction but appear not to proceed under the low pressure conditions typical of a laboratory experiment, despite favorable thermodynamics. The different chemistry and, in particular, the lack of reactivity at the low pressures where UHV surface science techniques are operable is known loosely as the pressure gap. We have now uncovered many new mechanisms for dissociative chemisorption, desorption and absorption, such as chemistry with a hammer, that are responsible for the different surface chemistries under the different conditions. These mechanisms are the fundamental principles underlying the pressure effect on surface chemistry. Our understanding of them has enabled us to "trick" the surface reactions, which could previously only be observed at ambient conditions, to occur in a UHV environment. In turn, the UHV environment enables the mechanism and intermediates of a surface reaction to be unambiguously identified. For example, the capability of carrying out the ethylene hydrogenation reaction on Ni under UHV conditions has allowed us to demonstrate unequivocally that the hydrogen reactive for hydrogenation is hydrogen buried beneath the surface rather than hydrogen adsorbed on the surface, as depicted in undergraduate chemistry texts. Our group is also interested in the dynamics of a surface chemical reaction.
Dynamics are best studied in a molecular beam-surface scattering experiment,
coupled with optical and electron spectroscopies. This experimental arrangement
enables the angular, energy and mass distributions of product molecules
from a surface chemical reaction to be measured. Because the product molecules
do not undergo collisions before detection, these distributions are directly
related to the detailed dynamics of the last step of the reaction. For
example, we have recently observed a new mechanism for dissociative chemisorption
called atom abstraction. In this process, the dangling bonds of a Si surface
abstract a F atom from an incident F2 molecule
while the complementary F atom is scattered back into the gas phase. The
observation of this surface mechanism is the first of its kind and is
a direct consequence of the power of this experimental arrangement. We
are presently studying the reaction of F2
with a laser-excited Si(100) surface in order to assess the role of electronic
processes in molecule-surface interactions.
The Unique Chemistry of Hydrogen Beneath the Surface: Catalytic Hydrogenation
of Hydrocarbons Fluorine Atom Abstraction by Si(100): II. Model Fluorine Atom Abstraction by Si(100): I. Experimental The Distinctive Reactivities of Surface-Bound H and Bulk H for the Catalytic
Hydrogenation of Acetylene A New Mechanism for Dissociative Chemisorption: Atom Abstraction from
F2 by Si(100) The Chemistry of Bulk Hydrogen: Reaction of H Embedded in Ni with Adsorbed
CH3 Hydrogen Embedded in Ni: Production by Incident Atomic Hydrogen and
Detection by High Resolution Electron Energy Loss New Mechanisms for Chemistry at Surfaces Collision Induced Dissociative Chemisorption of CH4 on Ni(111) by Inert
Gas Atoms: The Mechanism for Chemistry with a Hammer |
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