Automotive catalysis studied by surface science

Abstract

In this tutorial review I discuss the significant impact that surface science has had on our understanding of the catalytic phenomena associated with automobile exhaust depollution catalysis. For oxidation reactions it has generally been found that reactions are self-poisoned at low temperatures by the presence of strongly adsorbed reactants (such as molecular CO and NO), and that the rapid acceleration in rate at elevated temperatures (often called ‘light-off’) is due to the desorption of such adsorbates, which then frees up sites for dissociation and hence for oxidation reactions. In some circumstances such autocatalytic phenomena can then manifest themselves as oscillatory reactions which can vary in rate in both space and time. For NO reduction, the efficiency of depollution (by production of molecular nitrogen) is strongly affected by the nature of the metal used. Rh is extremely effective because it can dissociate NO much more readily than metals such as Pd and Pt, enabling oxygen removal (by reaction with CO to CO₂) even at room temperature. Rh is also very selective in producing predominantly N₂, rather than N₂O. NO_x storage and reduction (NSR) is an important recent development for removal of NO_x under the highly oxidising conditions of a lean-burn engine exhaust, and the strategy involves storing NO_x on BaO under oxidising conditions followed by the creation of reducing conditions to de-store and reduce it to nitrogen. By the use of STM it has been shown that this storage process is extremely facile, occurring fast even under UHV conditions, and that the storage occurs on BaO in the vicinity of Pt, with most of the oxide being converted to nitrate.