Graham Paul Ansell

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 development of a model capable of predicting diesel lean NOx catalyst performance under transient conditions

Abstract Steady state kinetics data from a commercial Pt-based lean NOx catalyst have been used to formulate a kinetic model to describe the performance of the catalyst. It is clear from this analysis that steady state kinetics in isolation are not sufficient to provide a full picture of the operational performance of such a catalyst. However, when this kinetic analysis is combined with mechanistic information obtained over the catalyst, the resulting model is extremely powerful. Within this paper, the development of the kinetic model is described, and the requirement for both accurate mechanistic information and detailed kinetic measurements is clearly demonstrated. The use of the model to predict the performance of a light-duty diesel vehicle under light-off conditions is described, and the power and flexibility of the model within the lean NOx area are emphasised.

Effects of SO2 on the alkane activity of three-way catalysts

Over current Pt-Rh/CeO2-Al2O3 catalysts, the conversion of alkanes occurs by two principal mechanisms: direct oxidation (HC + O2) and steam reforming (HC+H2O). Sulphur dioxide can influence both these mechanisms. Direct oxidation, which predominates when the exhaust-gas is fuel-lean, ispromoted by the adsorption of SOx species by the support. Under fuel-rich atmospheres, the presence of SO2 severelyinhibits steam reforming. The poisoning is associated with the formation of S2− on the platinum and of SO42− on the support, but there is no indication of S-species being retained by the rhodium. It is proposed that each of the two mechanisms is sensitive to a different type of interaction at the metal-support interface. Direct oxidation is enhanced by the transfer of electrons from the precious metal to the support; steam reforming occurs at interfacial sites, which can be blocked by adsorbed SOx species.

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.

The Development of a Model Capable of Predicting Diesel Lean NOx Catalyst Performance Under Transient Conditions

Steady state kinetics data from a commercial Pt-based lean NOx catalyst have been used to formulate a kinetic model to describe the performance of the catalyst. It is clear from this analysis that steady state kinetics in isolation are not sufficient to provide a full picture of the operational performance of such a catalyst. However, when this kinetic analysis is combined with mechanistic information obtained over the catalyst, the resulting model is extremely powerful. Within this paper, the development of the kinetic model is described, and the requirement for both accurate mechanistic information and detailed kinetic measurements is clearly demonstrated. The use of the model to predict the performance of a light-duty diesel vehicle under light-off conditions is described, and the power and flexibility of the model within the lean NOx area are emphasised.