Lean NOX catalysts

In addition, DOCs are also capable of reducing NOx, under certain conditions. Early work on these lean NOx catalysts concentrated on Cu/ZSM-5 catalysts (Amiridis et al., 1996 Walker, 1995), but platinum (Amiridis et al., 1996 Burch and Millington, 1995) and silver (Breen and Burch, 2006) -based catalysts, with better hydrothermal resistance than the zeolite systems, are also available. Unfortunately, NOx reduction under lean conditions only occurs over a narrow temperature range and therefore modelling can aid in optimisation of the catalyst and emissions system. 

There has been a growing demand for a lean NO catalyst in order to decrease the relatively low NO emission of the lean bum engine sufficiendy to meet the future standards. Lean NO catalysts have been developed based on zeolites (see Molecular sieves). Cu-promoted ZSM-5 zeolite has shown ability to reduce NO in an exhaust having excess oxygen at an efficiency of 30 to 50% (153). Durability is not proven. Research has revealed that certain hydrocarbons are preferred for the reduction of NOx, and that CO and H2 apparendy do not reduce NO, over such lean NOx catalysts (154). 

M. J. Heim rich and M. L. DeViney, Lean NOx Catalyst Evaluation and Characterisation, SAE 930736, Society of Automotive Engineers, Warrendale, Pa., 1993. 

Diesel engines, which are used in the larger vehicles, are important sources of particles and NOx, but emit relatively low amounts of CO and HCs. Diesel particulate emissions can, over time, be controlled. The control of NOx is problematic, and an appropriate technology is not available. Lean NOx catalysts are being pursued but conversion efficiencies remain low. 

A great effort is underway to develop reliable aftertreatment systems for lowering NOx emissions from diesel and LB engines. A variety of approaches have been proposed for NOx aftertreatment of advanced vehicles including lean NOx catalysts (LNC), NOx storage and reduction (NSR) catalysts, selective catalytic reduction with urea (urea-SCR), and plasma-assisted catalysis (PAC). Lean NOx catalysts are mainly designed to reduce NOx with unburned hydrocarbons already included in the exhaust stream in the presence of O2 but result in... 

Diesel/Lean NOx Catalyst Technologies , SAE Fuels Lubricants, San Antonio, October 1996, SP-1211. 

The current paper offers an alternative systems approach that broadens the temperature window for managing NOx in a full lean environment. The system has a trap component which adsorbs NOx over a temperature range where current lean NOx catalysts are not active. The trapped NOx is periodically desorbed and presented to a downstream lean NOx catalyst when conditions are optimal for its reduction. The predominant species present in the exhaust is NO. The principle is to oxidize NO to NOx above 150 C to enhance its adsorption. The trapped or stored NOx is desorbed by an exotherm generated within the washcoat by oxidation of a small amount of injected hydrocarbon, i.e. diesel fuel while maintaining the environment lean and not significantly modifying the bulk gas temperature. The injection temperature is controlled to allow for efficient downstream reduction of the NOx over a lean NOx catalyst i.e. 200-250 C for Pt or above 400°C for Cu/ZSM-5. 

The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOx adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft- ) deposited on y-Al203 on a 400 cpsi cordierite monolith. 

Experiments were also carried out on the trap + lean NOx catalyst system to demonstrate proof of concept. The trap was maintained at 300 C (maximum adsorption) and the downstream lean NOx catalyst maintained at 210 C where its activity is a maximum. The space velocity was 25,000 h and the inlet gas was 250 vppm NO in background gas [10% O2,10% H2O, 50 vppm SO2, with the balance N2] and 7000 vppm Cj (as propylene) as the injected hydrocarbon. The injected hydrocarbon was cycled in an on/off manner to allow the trap to experience NO adsorption and desorption. The feed to the downstream lean NOx catalyst was a steady addition of propylene at a 4 1 Ci/NO ratio based on the feed NO. Thus the actual Ci/NO ratio is less because of the contribution from the desorbed NOx. 

A system designed to manage NOx between 150 and 500 C in a lean bum environment has been explored as an alternative to current lean NOx catalyst limitations. 

The trap inlet injection temperature must be controlled to be compatible with the maximum activity of the downstream lean-NOx catalyst. 

It is clear, therefore, that we need to investigate alternative lean NOx catalysts. One such family of catalysts comprises the platinum group metals supported on metal oxides. Promising results from such systems have already appeared in the literature [5]. Here we restrict our attention to a series of Pt/MOs catalysts of various loadings and prepared from various R precursors. 

It is evident from this table that although a natural gas vehicle could meet current standards. Phase 1 standards pose a challenge for this lean CNG catalyst due to the lack of NOx control. Lean NOx catalysts, which generally use HC as the reductant for the NOx [23], may provide a solution in this regard as long as enough natural gas HC will participate in the HC-NOx reaction under automotive conditions. 

U sing plasma to change the composition of NOx from NO to NO2 permits a new class of catalysts to be used that are potentially more durable and more active than conventional lean-NOx catalysts. More details can be foimd in the hterature (Hoard Servati, 1998 Penetrante et al., 1998). 

Luders H, Backes R, Huthwohl G, Ketcher DA, Horrocks RW, Hurley RG, Hammerie RH (1995) An urea lean NOx catalyst system for light duty diesel vehicles. SAE Technical Paper 952493... 

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