Sara U Erkfeldt

The Effect of Carbon Monoxide and Hydrocarbons on NOx Storage at Low Temperature

One possible way to reduce NOx in lean exhausts is by using NOx trap catalysts. This paper addresses storage of NOx on such catalysts at temperatures below the catalyst light-off. Experiments carried out on commercial samples in synthetic exhausts revealed a large capacity for storage of NOx when NO2 was added at temperatures below 150°C. In contrast, when NO was added instead, no storage took place. CO was found to decrease the storage by reacting with NO2 and forming NO and CO2. Propene inhibited the reaction between NO2 and CO and therefore gave rise to larger NOx storage when CO was present. The paper concludes with a discussion of a possible mechanism for the storage of NOx at low temperatures.

The Development and Performance of the Compact SCR-Trap System: A 4-Way Diesel Emission Control System

The tightening of Heavy Duty Diesel (HDD) emissions legislation throughout the world is leading to the development of emission control devices to enable HDD engines to meet the new standards. NOx and Particulate Matter (PM) are the key pollutants which these emission control systems need to address. Diesel Particulate Filters (DPFs) are already in use in significant numbers to control PM emissions from HDD vehicles, and Selective Catalytic Reduction (SCR) is a very promising technology to control NOx emissions. This paper describes the development and performance of the Compact SCR-Trap system - a pollution control device comprising a DPF-based system (the Continuously Regenerating Trap system) upstream of an SCR system. The system has been designed to be as easy to package as possible, by minimising the total volume of the system and by incorporating the SCR catalysts on annular substrates placed around the outside of the DPF-based system. This novel design gives rise to an easy-to-package emission control device capable of providing very high conversions of all four major pollutants, NOx, PM, CO and HC. The design details are discussed, and the performance of the system over both steady state and transient cycles is presented. NOx conversions of up to 92% have been demonstrated, and the systems emissions of all four pollutants are well inside the Euro V, and probably also the US 2007 limits (subject to verification of PM).

Poisoning and regeneration of NOx adsorbing catalysts for automotive applications

The efficiency and product formation as a function of temperature, time and gas stoichiometry, in the regeneration of an SO2 poisoned NOx storage and reduction (NSR) catalyst has been studied. Using these results a simple model for sulphur poisoning and regeneration of this type of catalyst is proposed. SO2 poisons the catalyst, which results in a decrease of the NOx conversion with increasing SO2 exposure time. The NOx storage capacity can be regained after regeneration of the catalyst. The regeneration is most effective using long regeneration time, high temperature and low lambda value. A measure of the regeneration efficiency, activity based sulphur removal (ASR), is defined. ASR is used to model the regeneration efficiency with the independent variables; regeneration time, temperature and lambda value. The model describes the experimental results well. The amounts of released SO2, H2S and COS, during the regeneration differ depending on regeneration conditions. Low lambda values give high amounts of H2S and low amounts of SO2, whereas the reverse is true for high lambda values. The production of COS is low in all cases. The results can schematically be described with a model where sulphur can migrate between deactivating and non-deactivating positions on the catalyst surface.

Sulphur poisoning and regeneration of NOx trap catalyst for direct-injected gasoline engines

Sulphur poisoning and regeneration of NOx trap catalysts have been studied in synthetic exhausts and in an engine bench. Sulphur gradually poisoned the NO x storage sites in the axial direction of the NO x trap. During sulphur regenerations, hydrogen was found to be more efficient than carbon monoxide in removing the sulphur from the trap. The sulphur regeneration became more efficient the richer the environment (λ<1) and the higher the temperature (at least 600°C). H2S was found to be the main product during the sulphur regeneration. However, it was possible to reduce the H2S formation and instead produce more SO2 by running with lambda close to one or by pulsing lambda. Even if a relatively large amount of sulphur was removed from the NOx trap, these methods gave a much less efficient regeneration per sulphur atom removed than when running relatively rich constantly. Finally, a model that could explain this observation was proposed.

Promoting Effect of Triglyme on Lean NOx Reduction Over Ag/Al2O3

The highly oxygenated hydrocarbon triethylene glycol dimethyl ether or triglyme (CH3O–(C2H4O–)3CH3) was found to efficiently reduce NOx under lean conditions over Ag/Al2O3, but gave a low NOx conversion over Cu-ZSM-5. Furthermore, triglyme showed an extraordinary promoting effect when added together with propene as reducing agent for NOx over Ag/Al2O3 at low temperature. This is most likely due to that triglyme promotes the activation of propene.Graphical Abstract