Seminars Archive

Reaction Kinetics on Nanostructured Model Catalysis

Abstract
Heterogeneous catalysis is a key phenomenon in many fields of 21st century technology, such as environmental chemistry, the production of most chemicals or energy storage and conversion. In spite of this enormous economical impact, our understanding of the underlying chemical processes is surprisingly poor. The reason for this apparent contradiction becomes obvious, if we consider the microscopic properties of technical catalysts: Typical heterogeneous catalysts are highly complex materials. Their chemical and structural complexity is their major advantage, as it allows us to tune their properties in order to maximize the selectivity and activity. The price to pay is that in most cases the complexity of catalyst surfaces precludes an understanding of the underlying reaction kinetics at the molecular level. It is obvious that a truly fundamental understanding of reaction kinetics on a heterogeneous catalyst would require a direct experimental link between the structural features at the microscopic level on the one side and the role of these features in reaction kinetics on the other. Towards this aim, we apply a combined approach: First, we prepare model catalysts, which allow us to introduce certain structural or chemical features of a catalyst surface in an extremely well-controlled fashion, but without having to deal with the full complexity of the real system. Secondly, we utilize molecular beam methods, reactor methods and time-resolved in-situ surface spectroscopies in order to access the reaction kinetics on these model systems. Using this approach, we can identify the microscopic origins of numerous kinetic effects on catalyst surfaces. A series of examples from recent work is discussed, including a number of phenomena on supported model catalysts. These phenomena range from geometric and electronic effects to support effects, communication effects on nanostructured surfaces, phase transformations and structural rearrangements under the influence of reactants. On the basis of detailed and quantitative experiments, microkinetic models are developed, which are able to describe these kinetic phenomena. Catalysts which operate under non-stationary conditions represent a new challenge for such model studies. One example are so-called storage catalysts which can take-up and release larger amounts of specific reactants, e.g. in form of oxygen or nitrogen compounds.