ABSTRACT
As demonstrated by Pfefferle and Pfefferle, heterogeneously catalyzed com bustion is a promising method for burning fuel in lean fuel-air mixtures that can significantly reduce pollutants, improve ignition, and enhance the sta bility of flames [1,2]. In addition, heterogeneous catalysts play a key role in various pollutant emission control processes such as the three-way catalyst (TWC) used for the after-treatment of automobile exhaust gases [3]. In the TWC, a catalyst containing Pt/Pd/Rh converts the two reducing pollutants, CO and unburned hydrocarbons (HC), as well as the oxidizing pollutant, NO, to the stable products H20 , C 0 2, and N2 [4]. In this process the global surface reaction
2NO + 2CO -> N2 + 2 C 0 2 (12.1)
has been identified as one of the major reaction pathways for the conversion of NO to N2 [3,5]. However, because heterogeneous reactions sensitively depend on the surface concentrations of reactants and products that are connected with adsorption and desorption equilibria and with gas-phase transport processes depending on operating conditions, different partial processes can become rate-determining. The result can be a completely different behavior for the global surface reaction. As a consequence, the development of appropriate mathematical models for the simulation of sur face reactions and their coupling to the surrounding gas phase is essential to an understanding of heterogeneous catalysis under technically relevant con
INTRODUCTION 337
ditions. Computational tools for the description of different catalytic com bustion systems have recently been developed (see, e.g., Chapter 20). These tools include detailed surface chemistry as well as detailed models for mole cular species transport. However, the surface reaction mechanisms derived so far are based mainly on studies of elementary surface reaction steps carried out under ultrahigh vacuum (UHV) conditions and on well-defined single-crystal surfaces. Use of this kind of surface kinetics data in the mod eling of technical processes that usually take place at high pressure (“pres sure gap”) and, for example, on polycrystalline catalyst material (“materials gap”) emphasizes the importance of developing in-situ diagnostics techni ques that can be applied under practical pressure and temperature condi tions and on realistic catalysts (see Fig. 12.1).
