John William Hayes

Mechanism of the lean NOx reaction over Cu/ZSM-5

Abstract Transient techniques (including TAP, temporal analysis of products) have been used to probe the mechanism of the lean NO x reaction over Cu/ZSM-5. The activation of propene and nitric oxide by Cu/ZSM-5, in the presence and absence of oxygen, have been investigated by TAP to elucidate the nature of the reducing species involved in the formation of nitrogen. Propene is converted to long-lived (carbon-rich)_ species on the zeolite whether oxygen is present or not, and it is these deposited coke species which act as the reductant in this catalyst system. The ability of the coke to reduce nitric oxide is significantly enhanced by the presence of oxygen. The crucial role of the oxygen appears to be to induce the generation of an adsorbed state of NO x on the exchanged-Cu sites; these NO 2 -type species exist at temperatures characteristic of high lean NO x activity. During temperature-programmed reaction under a fuel-lean (oxidising) exhaust-gas, both nitric oxide and propene are retained at lower temperatures; as the temperature rises, so propene retention (as coke) and total oxidation begin to compete. However, there are sufficient reducing species still present on the surface to allow substantial reduction of the -NO 2 species, as the latter approach their limit of thermal stability.

The mechanism of the lean Nox reaction over Pt-based catalysts

Abstract It is now well established that Cu/ZSM-5, the first generation lean NO x catalyst, is not suitable for widespread application on vehicles because it can undergo rapid and irreversible deactivation under real operating conditions. Pt-based catalysts offer an, active and stable alternative to Cu/ZSM-5. Steady state reactor experiments have demonstrated that Pt/Al 2 O 3 catalysts are active for the lean NO x reaction at temperatures as low as 200°C. Both N 2 and N 2 O are generated by such catalysts While there is no simple correlation between NO reduction activity and Pt surface area, there is certainly a very good inverse correlation between the maximum NO reduction activity and the temperature. The most effective catalysts for NO reduction are those which are active at the lowest temperatures. Temporal Analysis of Products (TAP) has been used to obtain an in-depth mechanistic understanding of the lean NO x reaction over a Pt/Al 2 O 3 catalyst. the predominant mechanism for selective NO reduction involves the reduction of oxidised Pt sites by the hydrocarbon, followed by the decomposition of NO on these reduced Pt sites. A detailed model of the lean NO x reaction over Pt/Al 2 O 3 is presented which is capable of explaining all the results obtained in this work, as well as those reported in the literature.