Proposed emission standards present a significant challenge for diesel applications. Shown in Figure 1 are results obtained with several commercially available vehicles (ECE-EUDC with cold start). Even the cleanest applications require 60% NOx conversion to meet the EURO-4 standard.
Figure 2 presents a conversion capability of NOx adsorbers. The NOx conversion levels obtainable under steady state conditions using a Pt-based DeNOx catalyst (HC/NOx=10) and a NOx adsorber catalyst are shown relative to the NOx emission of a IDI diesel (~2 l) sampled over the European driving cycle. Because of its higher conversion capability and broader operating temperature window, the NOx adsorber offers great potential.
Unlike catalysts, which continuously convert NOx to N2, NOx adsorbers are materials which store NOx under lean conditions and release and catalytically reduce the stored NOx under rich conditions. NO and NO2 are acidic oxides and can be trapped on basic oxides.
The majority of NOx emitted from the engine is in the form of NO. NO2 is more easily adsorbed than NO. The following are the chemical reactions representing the oxidation and adsorption of NO:
|Oxidation:||2NO + O2 = 2NO2|
|Adsorption:||2NO + 1.5O2 + MO = M(NO3)2|
However, the oxidation of NO to NO2 is equilibrium limited. The equilibrium conversion of NO is shown in Figure 3 as a function of temperature.
Although the oxidation of NO is limited thermo-dynamically at elevated temperatures, the kinetic rate of oxidation increases with increasing reaction temperature. Formation of stable salts can drive the reaction forward. By trapping the product NO2 as a stable metal salt such as Ba(NO3)2, the adsorption of NOx can proceed rapidly with extremely high efficiency (>95%). The relative free energy levels of the reactants are shown in Figure 4.
The free energy release associated with the combustion reactions drives the desorption process. Like a sponge, which eventually fills with water, a NOx adsorber must periodically be regenerated. This process requires the input of energy. In the case of a NOx adsorber, that energy if provided in the form of reductants, such as CO, or the hydrocarbons in diesel fuel. For a diesel application, the 3 CO required for rich desorption (Figure 5) corresponds to 0.0612 C16H34.
The same reducing conditions which favor NOx release are also suitable for its subsequent reduction to N2. The adsorption and desorption of NOx is insufficient for controlling the emission of NOx - the stored NOx must eventually be reduced to N2. Under the rich conditions used to release NOx from the adsorbent, noble metals are capable of rapidly reducing NOx to N2 with high efficiency ("Rich Reduction" curve in Figure 5). For a diesel application, the overall reaction stoichiometry corresponds to 0.051 moles of C16H34 per mole of NO.
Operating window of fresh NOx adsorber is shown in Figure 6. The quantity of NOx which can be stored by an adsorbent is dependent upon its temperature. That capacity diminishes at elevated temperatures where the Ba(NO3)2 becomes thermally unstable.
The Pt in a typical NOx adsorber can also catalytically reduce NOx over a limited temperature range. The measured adsorption at 200oC has a significant contribution from catalytic DeNOx conversion.
Figure 7 presents Operating Window of NOx Adsorber following aging at 900oC (16 hr: air/10%). Although the catalytic DeNOx function of the NOx adsorber is significantly reduced upon aging at elevated temperatures, its ability to oxidize NO and store NOx shows more than adequate thermal stability for application in typical diesel exhaust.
Theoretical calculations indicate that the HC utilization required for desorption and reduction of NOx using an adsorber system is potentially lower than what is observed with DeNOx catalysts when tested on vehicles over the European driving cycle.
|Pt-Based Lean NOx Catalyst||NOx Adsorber (theory)|
HC continuously available
No HC supplementation (HC/NOx < 1):
HC/NOx = 6:
HC supplemented periodically
0.051 C16H34/NO required for desorption and regeneration of an adsorber
For 250 ppm NOx:
For diesel applications both in-cylinder and post-injection strategies can be considered for supplying the reductant required for adsorber regeneration. The following is a list of issues to be resolved:
The hydrocarbons which are supplemented into the exhaust to regenerate the trap are consumed by the regeneration process, as well as oxidized by exhaust oxygen. The relative quantity of HC's needed for each process can be estimated by calculation. It was assumed that a 50% conversion of NOx (250 ppm) at SV=50,000/hr can be maintained for 120 sec and the trap can be regenerated with a 2 sec rich pulse.
For 1 liter catalyst:
125 ppm NOx in 120 sec = 0.427 g NOx
0.107 g C16H34 required for regeneration
8% O2 in 2 sec
0.916 g C16H34 required for combustion
HC/NOx = 7.79 (C1 basis)
Combustion HC/Regeneration HC = 8.56
The calculation shows that almost 90% of the hydrocarbons are wasted for combustion and only 10% are used for regeneration. In a real system, the combustion of reductant targeted for adsorber regeneration must be minimized.
As the exhaust flow rate through the adsorber decreases, the quantity of oxygen, which must be combusted, decreases proportionately. Lowering the space velocity through the NOx adsorber for a given period of rich regeneration can minimize the fuel economy penalty associated with the operation of a NOx adsorber system. This is illustrated in Figure 8.
Clearly, to be used effectively on diesel applications, strategies must be developed to minimize the oxygen admitted to the adsorber during its regeneration. Possibly, an adsorber could be regenerated periodically when exhaust flow rates are reduced, such as during periods of low speed operation or at idle. Variations of the exhaust flow rate from a 2.1l passenger diesel engine over the European test cycle are shown in Figure 9.
Fuel sulfur can be converted to stable sulfates providing competition with NOx for storage sites. Current sulfur levels in typical diesel fuels present a significant challenge for the application of NOx adsorbers. The stability of BaSO4 makes it difficult to reverse the poisoning effect of sulfur (Figure 10).
Regenerable sulfur traps currently under development for gasoline applications might be effective, however, sulfur levels in diesel fuels will still need to be controlled at lower levels.
With appropriate developmental work and control of fuel sulfur levels, adsorber systems could provide the levels of NOx control needed for attaining future emission levels with diesel powered vehicles.