Summary, Assessment, and Forecast

The automotive TWC is one of the major achievements of modern research in heterogeneous catalysis. There have been major efforts to elucidate the fundamental reaction pathways and the catalyst characteristics that account for the success of the TWC. The chemistry involved is understood in considerable detail as a result of work with idealized models of the TWC. The mechanisms of the CO oxidation and NO reduction reactions with 

hydrogen, and he have been discussed in detail in this review. In particular, the application of various modern surface science techniques has advanced our understanding of the principles of the reactions on the atomic scale and the relationship of surface composition and structure to catalytic properties. 

Surface science offers many opportunities in catalysis research because a variety of techniques are available to characterize in detail the composition and structure of the catalyst surface and to identify the adsorbed species. A frequent criticism of the surface science approach is that it is far removed from real catalysis since most of the surface science techniques can only be applied at low pressures and with model catalysts, often single-crystal surfaces. The so-called pressure gap has been bridged by combining, in the same apparatus, the techniques needed for surface analysis and characterization with the ability to measure reaction rates at elevated pressures. In addition, many techniques can also be apphed in situ at elevated pressures. 

In this review, hterature data concerning CO oxidation and NO reduction on model catalysts have been reviewed and compared with those reported for supported catalysts. The major differences in behavior of the three noble metals—Pt, Pd, and Rh—used in TWC have been assessed. It is concluded that the major mechanisms are reactions of the L-H type between Oads, COads, and the dissociation products of NO, viz., Nads and Oads, with N2 formed by combination of 2 Nad,. NH3 by hydrogenation of Nads, and N2O by reaction between Nad, and NOads- Although other mechanisms have been proposed and their possible existence cannot be ruled out, the effects of the surface composition and structure, the specific differences in behavior of Pt, Pd, and Rh, the effect of changes in temperature, and variations in partial pressures can be fully understood on the basis of these reaction pathways. 

Bowkor. M., and. Toyncr, R. W., insight Spec, inorg. Chem. 145 (1995). 

---