Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2001-01-26
2003-07-01
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S213200, C423S213500, C423S244010, C423S244020, C423S244040, C423S244070, C423S244090, C423S244100
Reexamination Certificate
active
06585945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to sulfur tolerant catalyst composites useful for reducing contaminants in exhaust gas streams, especially gaseous streams containing sulfur oxide contaminants. More specifically, the present invention is concerned with improved NOx trap catalysts for use in diesel engines as well as lean burn gasoline engines. The sulfur tolerant NOx trap catalyst composites comprise a platinum component, a support, and a NOx sorbent component prepared by hydrothermal synthesis. The NOx sorbent component comprises a first metal oxide and a second metal oxide. The metal in the first metal oxide is selected from the group consisting of aluminum, titanium, zirconium, silicon, and composites thereof, and the metal in the second metal oxide is selected from the group consisting of Group IIA metals, Group III metals, Group IV metals, rare earth metals, and transition metals. The metal in the first metal oxide is different from the metal in the second metal oxide. The sulfur tolerant NOx trap catalyst composites are highly effective with a sulfur containing fuel by trapping sulfur oxide contaminants which tend to poison conventional NOx trap catalysts. The sulfur tolerant NOx trap catalyst composites are particularly suitable for diesel engines because the composites can be regenerated at moderate temperatures with rich pulses, rather than at high temperatures.
2. Related Art
Diesel powered vehicles represent a significant portion of the vehicle market worldwide. In Europe, the market share of diesel passenger cars is about one third and is expected to grow even higher in the years ahead. Compared to gasoline powered vehicles, diesel vehicles offer better fuel economy and engine durability. As diesel passenger cars become more popular both in Europe and elsewhere, emissions reduction is an increasingly urgent issue. In fact, Euro Stage IV regulations (year 2005) are calling for a 50% reduction of NOx emissions (0.25 g/km) compared to the Stage III (year 2000) level (0.5 g/km). For some vehicles, it would be difficult to meet the Euro IV NOx emissions target by engine improvement alone. It may be impossible to meet Euro V NOx regulations (0.125 g/km) without highly efficient after-treatment technologies.
Reducing NOx from diesel exhaust is very challenging. The 3-way catalyst technology, which is widely used in the gasoline cars, is not operational in diesel vehicles. A 3-way catalyst requires the exhaust emissions near a stoichiometric point, neither fuel rich (reducing) nor lean (oxidizing), while diesel emissions are always lean. In the early 90's, the concept of NOx trap catalyst was explored for gasoline partial lean burn (PLB) engines where the NOx catalyst would trap NOx in a lean environment and reduce it in a rich environment.
To apply the NOx trap concept to diesel passenger cars, some special issues related to diesel emission characteristics needed to be addressed. The exhaust temperature for light-duty diesel vehicles is typically in the range of 100-400 ° C., which is much lower than the gasoline exhaust. Therefore, low temperature oxidation activity and reduction during a rich spike is critical. One of the most difficult challenges in applying this concept is the issue of sulfur poisoning. The exhaust sulfur forms a very strong sulfate on any basic metal site, which prevents the formation of nitrate, rendering the catalyst ineffective for trapping NOx. As with other catalytic converters, thermal stability is another important issue for practical application.
The operation of a NOx trap catalyst is a collection of a series of elementary steps, and these steps are depicted below in Equations 1-5. In general, a NOx trap catalyst should exhibit both oxidation and reduction functions. In an oxidizing environment, NO is oxidized to NO
2
(Equation 1), which is an important step for NOx storage. At low temperatures, this reaction is typically catalyzed by precious metals, e.g., Pt. The oxidation process does not stop here. Further oxidation of NO
2
to nitrate, with incorporation of an atomic oxygen, is also a catalyzed reaction (Equation 2). There is little nitrate formation in absence of precious metal even when NO
2
is used as the NOx source. The precious metal has the dual functions of oxidation and reduction. For its reduction role, Pt first catalyzes the release of NOx upon introduction of a reductant, e.g., CO (carbon monoxide) or HC (hydrocarbon) (Equation 3). This may recover some NOx storage sites but does not contribute to any reduction of NOx species. The released NOx is then further reduced to gaseous N
2
in a rich environment (Equations 4 and 5). NOx release can be induced by fuel injection even in a net oxidizing environment. However, the efficient reduction of released NOx by CO requires rich conditions. A temperature surge can also trigger NOx release because metal nitrate is less stable at higher temperatures. NOx trap catalysis is a cyclic operation. Metal compounds are believed to undergo a carbonate
itrate conversion, as a dominant path, during lean/rich operations. The sulfur poisoning of a NOx trap catalyst is depicted below in Equations 6-7. In Equation 6, S occupies a site for NOx and in Equation 7, SOx replaces CO
3
or NOx.
Oxidation of NO to NO
2
NO+½O
2
→NO
2
(1)
NOx Storage as Nitrate
2NO
2
+MCO
3
+½O
2
→M(NO
3
)
2
+CO
2
(2)
NOx Release
M(NO
3
)
2
+2CO→MCO
3
+NO
2
+NO+CO
2
(3)
NOx Reduction to N
2
NO
2
+CO→NO+CO
2
(4)
2NO+2CO→N
2
+2CO
2
(5)
SOx Poisoning Process
SO
2
+½O
2
→SO
3
(6)
SO
3
+MCO
3
→MSO
4
+CO
2
(7)
In Equations 2, 3, and 7, M represents a divalent metal cation. M can also be a monovalent or trivalent metal compound in which case the equations need to be rebalanced.
Comparative investigations on the currently most discussed lean burn DeNOx technologies comprising the continuously operating selectively catalytic reduction (SCR) of V-, Pt-, Ir-technologies as well as the discontinuously operating NOx adsorption technology suggest that the latter technology shows the most promising overall performance in terms of NOx, HC and CO removal in view of the proposed EURO III/IV legislation. The sulfur concentration yields decisive influence on the long-term activity of the NOx adsorption catalysts and it is shown by a worst case study, that even the use of low-sulfur fuel does not prevent the accumulation of sulfur on the NOx adsorption catalyst. The accumulation of sulfur on the catalyst has to be counteracted by an engine induced desulfation strategy, by which the sulfur is driven out of the NOx adsorption catalyst. This requires the provision of reducing exhaust gas at elevated temperature for a short period of time. An optimization of the desulfation parameters is mandatory in order to suppress the formation of H
2
S. It is conjectured that the thermal degradation of the NOx adsorption catalyst proceeds via two different deactivation modes. The first one is based upon the loss of Pt dispersion and is accelerated by the presence of oxygen while the second one can be traced back to the reaction between NOx storage components and the porous support material. Wolfgang Strehlau et al., Conference “Engine and Environment” 97.
Direct injection technology for diesel engines as well as for gasoline engines are the most favored ways to reduce the CO
2
emissions in the future. NOx adsorber technology for gasoline DI engines as well as for HSDI diesel engines is the favored technology to meet future emission limits. Adsorber catalysts have demonstrated their potential to meet future emission legislation levels on prototype basis for gasoline and diesel engines. Improving the NOx adsorber technology and the integration of the adsorber system into the powertrain for the introduction into the European market is the challenge for the near future.
Using a catalyst cannot
Dettling Joseph C.
Wu Joseph H-Z.
Engelhard Corporation
Negin Richard A.
Silverman Stanley S.
Vanoy Timothy C.
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