Molecular beam experiments on model catalysts

Heterogeneous catalysts are highly complex materials with respect to both their composition and structure. The reaction kinetics on their surfaces is known to depend sensitively on numerous structural and chemical factors including the particle size and structure, the support or the presence of poisons and promotors. At the microscopic level, however, little is known about the microscopic origin of these effects. The main reason for this lack of knowledge is the vast complexity of typical heterogeneous catalysts and several experimental difficulties related to the application of modern surface science techniques to these systems. In this review, we summarize recent attempts to overcome these difficulties, based on the application of molecular beam methods to well-defined model catalysts. Using supported model catalysts, many aspects of the complex structural properties of real catalysts can be reproduced in a well-controlled manner. At the same time, the molecular beam approach allows quantitative and detailed investigations of the kinetics and dynamics of surface reactions. The combined information on structure and kinetics provides detailed insights into the kinetic phenomena on complex catalyst surfaces at the microscopic level. The processes studied so far include the kinetics of adsorption and desorption involving various adsorbates and the reaction kinetics of several simple surface reactions, such as, e.g. CO oxidation, NO dissociation, NO-CO reaction and methanol oxidation. Several kinetic phenomena have been identified which are specific to supported catalyst systems. These include, e.g. the role of the support in the reaction kinetics, the role of different types of adsorption and reaction sites on small active particles, the interplay of these sites via surface diffusion, the surface mobility of reactants, coverage fluctuations on small particles, local differences in catalytic activity and selectivity on active particles, as well as the role of co-adsorbates under reaction conditions. Special emphasis is put on the development of realistic microkinetics models based on experimentally determined structure and kinetics. (c) 2005 Elsevier B.V. All rights reserved.