Real time imaging of catalytic reactions on surfaces: Past, present and future

Simple surface reactions like the CO-oxidation on single crystal platinum surfaces show a rich scenario of non-linear behavior under specific reaction parameters. Gerhard Ertl started in the late 1980s a new research group focusing on imaging of surface reactions to discover possible spatio-temporal pattern formation. Necessarily we conceived several unique imaging methods, which are discussed in the first part of the paper. An unexpected wealth of adsorbate patterns during the CO-oxidation on Pt was soon discovered and can be modeled by a reconstruction model. But the interaction of a multitude of micrometer scale concentration waves and fronts on the surface complicate our understanding of the underlying mechanisms for such patterns. We tackled those complications by constraining pattern formation within inactive or active boundaries formed by different metals evaporated onto the Pt single crystal, thereby isolating individual features of reaction diffusion systems (for example single pulses). Since 2001 we have been able to dynamically change the surface catalytic activity in real time and space by focusing an addressable laser beam to differentially heat a single crystal surface. Combining fixed microstructures made with different catalytic activities and the local heating of the surface opens new avenues of controlling pattern formation. In contrast to these approaches, varying one of the control parameters in time creates a globally coupled system, with the choice between direct forcing or a feedback experiment. In the last part temperature effects arising from the reaction heat are discussed, which become dominant at higher partial pressures for the reactants or on ultra thin samples where the reaction heat cannot be effectively dissipated into the bulk. (C) 2009 Elsevier B.V. All rights reserved.