Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-03-17
2002-04-23
Tran, Hien (Department: 1764)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C422S171000, C422S177000, C422S180000, C423S213200, C423S213500, C423S213700, C502S304000, C502S339000, C502S340000, C502S439000, C060S297000
Reexamination Certificate
active
06375910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-zoned catalytic trap for treating exhaust gas streams, especially those emanating from lean-burn engines, and to methods of making and using the same. More specifically, the present invention provides a multizoned catalytic trap which abates NO
x
, CO and hydrocarbons in the exhaust streams being treated at initial engine start-up conditions as well as at subsequent high engine temperature conditions and exhibits enhanced durability after aging at high temperature and lean operation conditions.
2. Related Art
Emission of nitrogen oxides (“NO
x
”) from lean-burn engines (described below) must be reduced in order to meet emission regulation standards. Conventional three-way conversion (“TWC”) automotive catalysts are suitable for abating NO
x
, carbon monoxide (“CO”) and hydrocarbon (“HC”) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel that results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (“A/F”) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH
1.88
. The symbol X is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that &lgr;=1 is a stoichiometric mixture, &lgr;>1 is a fuel-lean mixture and &lgr; is a fuel-rich mixture.
Engines, especially gasoline-fueled engines to be used for passenger automobiles and the like, are being designed to operate under lean conditions as a fuel economy measure. Such future engines are referred to as “lean-burn engines.” That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are “lean,” i.e., the exhaust gases are relatively high in oxygen content. Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NO
x
emissions from such engines because of excessive oxygen in the exhaust. The prior art discloses attempts to overcome this problem by operating lean-burn engines with brief periods of fuel-rich operation (engines which operate in this fashion are sometimes referred to as “partial lean-burn engines.”) It is known to treat the exhaust of such engines with a catalyst/NO
x
sorbent which stores NO
x
during periods of lean (oxygen-rich) operation, and releases the stored NO
x
during the rich (fuel-rich) periods of operation. During periods of rich operation, the catalyst component of the catalyst/NO
x
sorbent promotes the reduction of NO
x
to nitrogen by reaction of NO
x
(including NO
x
released from the NO
x
sorbent) with HC, CO and/or hydrogen present in the exhaust.
The use of NO
x
storage (sorbent) components including alkaline earth metal oxides, such as oxides of Ca, Sr and Ba, alkali metal oxides such as oxides of K, Na, Li and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr and Nd in combination with precious metal catalysts such as platinum dispersed on an alumina support, is known, as shown for example, at column 4, lines 19-25, of U.S. Pat. No. 5,473,887 of S. Takeshima et al., issued on Dec. 12, 1995. At column 4, lines 53-57, an exemplary composition is described as containing barium (an alkaline earth metal) and a platinum catalyst. The publication Environmental Catalysts For A Better World And Life, Proceedings of the 1
st
World Congress at Pisa, Italy, May 1-5, 1995, published by the Societa Chimica Italiana of Rome, Italy has, at pages 4548 of the publication, an article entitled “The New Concept 3-Way Catalyst For Automotive Lean-Burn Engine Storage and Reduction Catalyst,” by Takahashi et al. (below referred to as “the Takahashi et al. Paper”). This article discloses the preparation of catalysts of the type described in the aforementioned Takeshima et al. U.S. Pat. No. 5,473,887 and using these catalysts for NO
x
purification of actual and simulated exhaust gases alternately under oxidizing (lean) and reducing (rich or stoichiometric) conditions. The conclusion is drawn in the last sentence on page 46 that NO
x
was stored in the catalyst under oxidizing conditions and that the stored NO
x
was then reduced to nitrogen under stoichiometric and reducing conditions. A similar but more detailed discussion is contained in SAE Paper 950809 published by the Society of Automotive Engineers, Inc., Warrendale, Pa., and entitled Development of new Concept Three- Way Catalyst for Automotive Lean-Burn Engines, by Naoto Miyoshi et al., was delivered at the International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995.
U.S. Pat. No. 4,742,038, “Monolithic Catalyst Support and Catalyst Deposited on the Support,” issued May 3, 1988 to S. Matsumoto, discloses a metal substrate for carrying a catalytic material useful for the treatment of exhaust gases from internal combustion engines. U.S. Pat. No. 5,874,057, “Lean NO
x
Catalyst/Trap Method,” issued on Feb. 23, 1999 to M. Deeba et al., discloses a method of NO
x
abatement utilizing a composition comprising a NO
x
abatement catalyst comprising platinum and, optionally, at least one other platinum group metal catalyst which is kept segregated from a NO
x
sorbent material. The NO
x
sorbent material may be one or more of oxides, carbonates, hydroxides and mixed oxides of one or more of lithium, sodium, potassium, rubidium, osmium, magnesium, calcium, strontium and barium.
Prior art catalysts as described above have a problem in practical application, particularly when the catalysts are aged by exposure to high temperatures and lean operating conditions, because after such exposure, such catalysts show a marked decrease in catalytic activity for NO
x
reduction, particularly at low temperature (250 to 350C) and high temperature (450 to 600C) operating conditions.
U.S. Pat. No. 5,451,558, “Process For the Reaction and Absorption of Gaseous Air Pollutants, Apparatus Therefor and Method of Making the Same,” issued on Sep. 19, 1995 to L. Campbell et al., (“the Campbell et al.. Patent”) discloses a catalytic material for the reduction of NO
x
from a turbine in a power generating stack, although the patent also refers at column 1, lines 13-14, generally to a process and apparatus for reducing pollutants “which are produced by combustion of hydrocarbons or hydrogen in an engine or boiler, and primarily in a gas turbine.” As disclosed at column 2, lines 23-37, the turbine exhaust gases are cooled to the range of 250 to 500F (about 121 to 260C) before contacting the catalytic/adsorbent material (column 2, lines 23-37) and the oxidation is stated (column 2, lines 45-48) to occur at temperatures in the range of 150 to about 425F (66 to 218C), most preferably in the range of 175 to 400F (about 79 to 204C). The catalytic species comprises an oxidation catalyst species which may comprise various metals including platinum group metals (see column 3, line 67 through column 4, line 3) deposited on a high surface area support which may be “made of alumina, zirconia, titania, silica or a combination of two or more of these oxides.” The catalyst-containing high surface area support is coated with an adsorbent species which may comprise “at least one alkali or alkaline earth compound, which can be a hydroxide compound, bicarbonate compound, or carbonate compound, or mixtures thereof.” At column 3, lines 16-22, the “carbonate coating” is said to be a “lithium, sodium, potassium or calcium carbonate, and presently the preferred coating is a potassium carbonate.” At column 4, lines 28-31, however, it is stated that the absorber comprises “most preferably sodium carbonate, potassium carbonate or calcium carbonate.” The high surface area support containing the oxidatio
Brandt Stefan
Dahle Uwe
Deeba Michel
Engelhard Corporation
Negin Richard A.
Tran Hien
LandOfFree
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