Lean-Burn NOx-Reducing Catalysts, "DENOx”

It has already been reported how stoichiometric operation compromises the efficiency of engines, but that for control of NOx, it is necessary to operate either at stoichiometric or sufficiently weak (say, an equivalence ratio of O.6), such that there is no need for NOx reduction in the catalyst.

If a system can be devised for NOx to be reduced in an oxidizing environment, this gives scope to operate the engine at a higher efficiency. A number of technologies are being developed for "DENOx," some of which are more suitable for diesel engines than spark ignition engines. The different systems are designated active or passive (passive being when nothing must be added to the exhaust gases). The systems are as follows:

Selective Catalytic Reduction (SCR). In this technique, ammonia (NH3) or urea (CO(NH&) is added to the exhaust stream. This is likely to be more suited to stationary engine applications. Conversion efficiencies of up to 80% are quoted, but the NO level must be known, because if too much reductant is added, ammonia would be emitted.

Passive DENOx. These use the hydrocarbons present in the exhaust to chemically reduce the NO. There is a narrow temperature window (in the range 160-220°C [320- 428"FI for platinum catalysts) within which the competition for HC between oxygen and nitric oxide leads to a reduction in the NOx (Joccheim et al., 1996). The temperature range is a limitation and is more suited to diesel engine operation. More recent work with copper-exchanged zeolite catalysts has shown them to be effective at higher temperatures. By modifying the zeolite chemistry, a peak NOx conversion efficiency of 40% has been achieved at 400°C (752°F) (Brogan et al., 1998).

Active DENOx Catalysts. These use the injection of fuel to reduce the NOx, and a reduction in NOx of approximately 20% is achievable with diesel-engined vehicles on typical drive cycles, but with a 1.5% increase in fuel consumption (Pouille et a]., 1998). Current systems inject fuel into the exhaust system, but there is the possibility of late in-cylinder injection with future diesel engines.

NOx Trap Catalysts. In this technology (first developed by Toyota), a three-way catalyst is combined with a NOx-absorbing material to store the NOx when the engine is operating in lean-burn mode. When the engine operates under rich conditions, the NOx is released from the storage media and reduced in the three-way catalyst.

NOx trap catalysts have barium carbonate deposits between the platinum and the alumina base. During lean operation, the nitric oxide and oxygen convert the barium carbonate to barium nitrate. A rich transient (approximately 5 s at an equivalence ratio of 1.4) is needed every five minutes or so, such that the carbon monoxide, unburned hydrocarbons, and hydrogen regenerate the barium nitrate to barium carbonate. The NOx that is released is then reduced by the partial products of combustion over the rhodium in the catalyst. Sulhr in the fuel causes the NOx trap to lose its effectiveness because of the formation of barium sulfate. However, operating the engine at high load to give an inlet temperature of 600°C (1 112"F), with an equivalence ratio of 1.05, for 600 s can be used to remove the sulfate deposits (Brogan et al., 1998)