Nobel Centennial Symposia
"Frontiers of Molecular Science"

December 4-7, 2001
Friiberghs Manor, Örsundsbro and Stockholm University

Dynamics of Reactions at Solid Surfaces

by Gerhard Ertl
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Berlin, Germany

The interaction of molecules with solid surfaces forms the basis of heterogeneous catalysis. Application of the methods of surface physics to well-defined single crystal surfaces (the so-called "surface science" approach) enables investigation of the underlying elementary processes on atomic scale. This will be exemplified with a rather simple reaction, oxidation of carbon monoxide on platinum. Description of the rate of a chemical reaction in terms of transition state theory requires rapid thermal equilibration of the various degrees of freedom. The breakdown of this concept is demonstrated for CO oxidation on Ru: Irradiation with a femtosecond infrared pulse causes heating of the electron gas which may activate the chemisorbed O atoms leading reaction with coadsorbed CO molecules before equilibration with the vibrations of the surface lattice is achieved.

The "classical" reaction between oxygen and hydrogen on platinum is far more complex: The reaction intermediate OH_ad is not only formed through recombination between Had and Oad, but also (even more efficiently) through interaction with the reaction product: . This introduces an autocatalytic step into the reaction mechanism which accounts for the appearance of propagating reaction fronts and the formation of spatio-temporal concentration patterns on mesoscopic scale.

Generally, phenomena of spatio-temporal self-organization in open chemical systems far from equilibrium may be rationalized in terms nonlinear partial differential equations coupling reaction and diffusion of the species involved (nonlinear dynamics). A large variety of such patterns has, for example, been observed with CO oxidation on a Pt(110) surface, including even chaotic (turbulent) states. Control over such situations leading to the formation of novel patterns may be achieved by a feedback technique in which the overall response (e.g. the reaction rate) regulates the supply of reactant.

Finally, it will be demonstrated how a theoretical concept for the formation of stationary concentration pattern introduced by A. Turing fifty years ago can be experimentally realized with electrochemical systems where migration of ions in an electrical field introduces an additional transport mechanism. It is suggested that in this way a conceptual link between "chemical" and "biological" reacting systems can be established.