1.  Field of the Invention
The present invention is directed to a catalyst system for use with internal combustion engines to oxidize hydrocarbons, carbon monoxide and reduce nitrogen oxides in an exhaust gas when the engine is operated at both lean and stoichiometric air/fuel ratios.  More particularly, the catalyst system of this invention includes two catalysts.  The first catalyst is designed specifically to optimize the reduction of noxious emissions under lean conditions.  This first catalyst includes a new Perovskite-based formulation designed to achieve close proximity between precious metal and NOx binding elements. 
The second catalyst is designed specifically to maximize the reduction of HC, CO and NOx under stoichiometric operations and to treat any breakthrough NOx emissions from the first catalyst.  This second catalyst contains precious metals and may optionally include BaO.
2.  Background Art
Catalysts have long been used in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into nonpolluting gases including carbon dioxide, water and nitrogen.  When a gasoline-powered engine is operated at a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.6 and 14.4, catalysts containing precious metals like platinum, palladium and rhodium are able to efficiently convert all three gases simultaneously.  Typically, such catalysts use a relatively high loading of precious metal to achieve the high conversion efficiency required to meet the stringent emission standards of many countries.  Because of the high cost of the precious metals, these catalysts are expensive to manufacture. 
To improve vehicle fuel efficiency and lower CO2 emissions, it is preferable to operate an engine under lean conditions.  Lean conditions are air/fuel mixtures greater than the stoichiometric mixture (an air/fuel mixture of 14.6), typically air/fuel mixtures greater than 15.  While lean operation improves fuel economy, operating under lean conditions increases the difficulty in treating some polluting gases, especially NOx. 
For some catalysts, if the air/fuel ratio is lean even by a small amount, NOx conversion is significantly reduced.  One way to provide air/fuel control is through the use of a HEGO (Heated Exhaust Gas Oxygen) sensor to provide feedback to the control systems.  HEGO sensors, however, over time develop a lean bias as a result of poisoning.  Accordingly, even with a HEGO sensor it is important to have a catalyst that can maximize the reduction of NOx emission under lean conditions. 
To maximize NOx reduction under lean operating conditions, a lean NOx trap is often used.  The inclusion of a NOx trap enhances the reduction of NOx while the engine is operated under lean conditions.  The NOx trap functions in a cyclic manner.  When the NOx trap reaches the effective storage limit, the engine is operated under normal or rich conditions to purge the NOx trap.  After the NOx trap has been purged, the engine can return to lean operation. 
However, in addition to problems associated with thermal stability and sulfur tolerance, lean NOx traps (LNT) have the following two known problems: (1) a problem referred to as “NOx breakthrough”, the breakthrough of NOx during the NOx trap transition from the lean to the rich cycle; and (2) a reduction in fuel economy that results from frequent purges during the rich cycle.  Test results depicted in FIG.  2 shows this NOx breakthrough for LNTs with different oxygen storage capacity (OSC).  This total NOx breakthrough has been found to be greater than 73% of the total NOx emitted during the operation of a lean NOx trap. 
FIG.  2 also shows the effects of oxygen storage capacity on NOx breakthrough of a lean NOx trap during the lean-to-rich transition.  LNT L, which has the highest OSC, results in the largest amount of NOx breakthrough, while the lower the OSC (from LNT M down to LNT N), the lower the amount of NOx breakthrough.  It is believed that NOx breakthrough during the lean-rich transition occurs due to the exothermic heat generated from the oxidation of reductants, CO, HC and H2, by the oxygen released from the oxygen storage material (see FIG.  2)—the temperature rise can be as high as 80–100° C.
With regard to the fuel economy penalty, this is believed to be the result of high oxygen storage capacity of the lean NOx trap, low NOx trapping capacity, and/or high exhaust flow rate.  The OSC requires additional reductants (i.e., fuel) to reduce the oxygen storage materials during each lean-to-rich transition, while the low NOx trapping capacity requires that the frequency of purges be increased. 
The present invention avoids the cost and complexity of the NOx trap and the reduced fuel economy from frequent NOx trap purging by systematically reducing the amount of NOx during engine operation, even under lean conditions. 
To solve the above problems, the present invention provides a new catalyst system comprising two catalysts that can treat all exhaust emissions, CO, HC and NOx under both stoichiometric and lean conditions.  In particular, the forward catalyst uses a newly developed Perovskite-based formulation which achieves the requisite close proximity between precious metal and NOx binding elements. 
The closest known prior art includes modified three-way catalysts.  For example, U.S. Pat.  No.  4,024,706, incorporated herein by reference, teaches a method of enlarging the air/fuel ratio over which a catalyst operates by including an oxygen storage material.  The method involves controlling the air/fuel ratio of the fuel mixture being burned by the engine such that the ratio is transferred into equal amounts going to the rich and lean side of a stoichiometric condition as previously described.  The use of an oxygen storage material, however, is believed to result in NOx breakthrough, which increases NOx emissions rather than reducing them. 
U.S. Pat.  No.  5,977,017 teaches a Perovskite-type catalyst that consists mainly of a metal oxide composition.  The metal oxide composition is represented by the general formula:Aa-xBxMOb,where
A is a mixture of elements originally in the form of a single phase mixed lanthanides collected from bastnasite;
B is a divalent or monovalent cation;
M is at least one element selected from the group consisting of elements of an atomic number from 22–30, 40–51, and 73–80;
a is 1 or 2;
b is 3 when a is 1 or b is 4 when a is 2; and
x is between 0 and 0.7. 
This general Perovskite structure, however, is not designed to maximize NOx storing and releasing functions—by providing the requisite close proximity between the precious metal and the NOx-binding element.  In contrast, the newly developed Perovskite structure of this invention is specifically designed to maximize NOx storage and release. 
U.S. Pat.  No.  4,321,250 also teaches a Perovskite-type catalyst having a ABO3 crystal structure with about 1 to 20 percent of the B cation sites occupied by Rh ions and the remainder of the B cation sites occupied by ions consisting essentially of cobalt and the A cation sites occupied by lanthanide ions of atomic number 57 to 71 and ions of at least one metal of groups IA, IIA or IVA of the period table having an ionic radii of about 0.9 A to 1.65 A and proportioned so that no more than 50 percent of the cobalt ions are tetravalent and the remaining cobalt ions are trivalent.  This composition generally represents Perovskite catalysts that were useful to produce hydrogen in steam reformers. 
The use of such types of Perovskite catalysts as an automotive catalyst or their use in combination with other catalysts to produce a NOx tolerant catalyst was not known prior to this invention.