Enhancing Catalytic Reactions by Aligning Reactive Intermediates: Phenyl Radicals on Cu{111}

M. M. Kamna, T. M. Graham, and P. S. Weiss,

Submitted for publication.

Abstract

We have imaged the surface reaction intermediates in the prototypical Ullmann coupling reaction forming biphenyl from iodobenzene on Cu{111}. Substantially below the temperature at which products form, the phenyl radical intermediates pair to form complexes. This step along the reaction coordinate indicates that catalytic reaction rates may be significantly enhanced by having substrate-mediated interactions align reactants and intermediates in the correct orientation for reaction to proceed. Thermal dissociation of iodobenzene on Cu{111} results in adsorbed phenyl radicals and iodine atoms. At higher temperature, biphenyl forms by coupling of surface-bound phenyl. We have isolated the phenyl intermediates in the coupling reaction on the Cu{111} surface and studied them with scanning tunneling microscopy. Phenyl radicals are mobile at 77K and form stable oriented pairs as well as larger aggregates of molecules.