Alan Francis Diwell

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.

Engineered control strategies for improved catalytic control of NOx in lean burn applications

A system combining suitable catalyst technology with a specifically engineered fuelling strategy can lead to improved emission control in lean burn engine applications. The basis of the catalyst design has been to maximize response to rich transients and sustain NO/sub x/ reduction on reverting to lean conditions. The fuelling control strategy is engineered to optimize the degree and duration of enrichment during accelerations. As an example of the level of NO/sub x/ control which can be achieved, a concept catalyst has, when used on conjunction with the control strategy, been shown to provide a 29% reduction in NO/sub x/ emissions over an ECE-15 test, compared with an uncontrolled vehicle.

The Impact of Sulphur Storage on Emissions From Three-Way Catalysts

The relationship between H/sub 2/S emissions from three-way catalysts and the storage of sulphur on the catalyst surface has been investigated. Thermodynamic data predict that sulphur storage primarily will occur on Al/sub 2/O/sub 3/ and CeO/sub 2/ under lean and stoichiometric conditions, at up to 500/sup 0/C. Rich transients could then induce the decomposition of the Ce-S-O and Al-S-O compounds, releasing sulphur into the gas phase. Experimental studies have supported this model. A mechanism has been proposed for the subsequent formation of H/sub 2/S. The mechanism by which catalyst poisons attenuate H/sub 2/S emissions from engine-aged catalysts also has been studied. The effect has been shown to be related to decreased storage of sulphur, caused by stable catalyst-poison species at the catalyst surface.

LEAD SPECIES IN VEHICLE EXHAUST: A THERMODYNAMIC APPROACH TO LEAD TOLERANT CATALYST DESIGN

As part of a programme to develop lead tolerant emission control catalysts for Europe, a fundamental thermodynamic approach has been used to achieve an understanding of the lead species in vehicle exhaust under widely varying conditions and to consider the possible interactions which may occur between the catalyst and lead species and which may give rise to poisoning. A model has been developed which calculates the gas stream equilibrium and identifies the most stable solid phases which precipitate from the gas phase, until a final equilibrium has been reached. Results covering a wide range of conditions are presented, and compared with rig and engine test data--providing a means for determining the proportion and composition of gaseous and solid lead compounds, and the effect of oxygen content and temperature of the exhaust on the lead species formed. The data assist in the design to total vehicle emission systems with improved lead tolerance.

Car exhaust emission control in Europe

Abstract Although Europe is following the example of the USA in progressively lowering permitted emission levels of pollutants from motor vehicles, it seems unlikely that lead-free petrol will be introduced in the near future. Proposed regulations seek a level of emission control which will in all probability, necessitate the use of oxidation catalysts. In the presence of leaded fuel conventional autocatalysts used in the USA are susceptible to poisoning after relatively low mileages. Recent research has led to an improved understanding of the nature and type of the lead species involved and the feasibility of achieving 50 000 km road usage using lead-tolerant catalysts has been demonstrated.