Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1996-05-13
2002-10-29
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C060S286000, C060S299000, C422S171000, C422S172000, C422S177000, C423S213700, C423S239100, C423S239200
Reexamination Certificate
active
06471924
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention is broadly concerned with lean NO
x
abatement in gaseous streams such as the exhaust of lean-burn internal combustion engines, e.g., diesel engines. More specifically, the present invention is concerned with a novel method and an apparatus for carrying out the method, comprising use of a trap member having thereon an intimate combination of a sorbent material and an oxidation catalyst.
The art has devoted some effort to developing catalysts specifically designed to abate NO
x
in lean exhaust streams. One such effort is described at pages 45-48 of
Environmental Catalysis 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 Italian of Rome, Italy, in 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 by impregnating precious metals, mainly platinum, and various alkaline and alkaline earth metal oxides, mainly barium oxide and rare earth metal oxides, on supports, mainly alumina. At page 47 of the article, there is, shown both graphically in FIG.
5
and discussed in paragraph 3.1.1., “NO
x
Storage Mechanism”, the concept of employing NO
x
storage compounds and three-way conversion (“TWC”) catalytic components dispersed on a common support material. TWC catalysts are well known in the art and are capable of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. However, TWC catalysts require stoichiometric or rich gas conditions in order to convert NO
x
desorbed from the sorbent material.
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. This paper, referred to below as “the SAE paper”, deals with gaseous streams at rich or stoichiometric conditions and discloses at page 123 a correlation between basicity of the elements used as NO
x
storage material and NO
x
storage capacity. Also discussed is NO
x
adsorption by complex oxides such as YBa
2
Cu
3
Oy and CuO-BaO. The SAE paper also references “Uptake of NO gas by YBa
2
Cu
3
Oy” by K. Tabata et al,
Chem. Lett
., 1988, pp. 799-802 and “NO removal by adsorption into BaO-CuO binary oxides” by M. Machida et al,
J. Chem. Soc. Chem. Commum
. 1990, pp. 1165-1166.
U.S. Pat. No. 5,202,300, “Catalyst For Purification of Exhaust Gas”, issued on Apr. 13, 1993, to M. Funabiki et al, discloses a catalyst composition comprising a refractory support having deposited thereon an active layer containing a palladium and rhodium catalytic metal component dispersed on alumina, a cerium compound, a strontium compound and a zirconium compound.
SUMMARY OF THE INVENTION
Generally, the present invention provides a method for treating a lean, NO
x
-containing gaseous stream, the temperature of which fluctuates and may remain for periods of time within a temperature range which is unsuitable for catalytic abatement of NO
x
. The method provides for using a trap member containing an intimate combination of a NO
x
sorbent and an oxidation catalyst and sorbing NO
x
onto the trap member during those selected periods of time, e.g., when the temperature of the gaseous stream is not suited for catalytic lean NO
x
abatement. During other periods of time, e.g., when the temperature of the gaseous stream being treated is suitable for catalytic lean NO
x
abatement, the method provides for introducing a combustible component, e.g., hydrocarbons, into the trap member in amounts which are limited in order to maintain the bulk or overall composition of the gaseous stream at lean conditions, and oxidizing the combustible component thereon to thermally desorb the NO
x
from the trap member without significantly raising the bulk temperature of the gaseous stream. The resulting NO
x
-enriched gaseous stream can be more efficiently treated to abate NO
x
for two reasons. One is because of its increased NO
x
concentration relative to the source gaseous stream and the other is because its bulk temperature need not be heated to NO
x
thermal desorption temperatures which are higher than the temperature window for most or many lean NO
x
abatement methods. The source of the gaseous stream, e.g., a diesel engine or a lean-burn gasoline engine, need not be subjected to periods of rich or stoichiometric operation to desorb the NO
x
and reduce it over a TWC catalyst, but may operate continuously under lean conditions. The amount of combustible component, e.g., hydrocarbon, introduced to effectuate the thermal desorption need not be so much as to convert the lean gaseous stream to a hydrocarbon (fuel)-enriched gaseous stream.
In accordance with the present invention there is provided a method for the reduction of NO
x
in a lean NO
x
-containing gaseous stream, e.g., the exhaust of a diesel engine. The method comprises the following steps. The lean gaseous stream is passed through a catalyzed NO
x
trap member comprising an intimate combination of a catalyst material effective for promoting the oxidation of a combustible component in the gaseous stream, and a NO
x
sorbent material. The gaseous stream is passed through the trap member within a sorbing temperature range to sorb NO
x
in the trap member whereby to establish a trapping period and provide a NO
x
-depleted gaseous stream exiting the trap member during the trapping period. The gaseous stream exiting the trap member is then passed through a lean NO
x
reduction catalyst member. The combustible component is introduced into the gaseous stream at a location upstream of the trap member under conditions to combust the combustible component on the trap member, the combustible component being introduced in an amount which is limited in order to maintain the bulk composition of the gaseous stream lean but which is sufficient to heat at least a portion of the trap member to a release temperature sufficient to release trapped NO
x
from the trap member into the gaseous stream exiting the trap member, thence through the catalyst member. This serves to establish a desorbing period. Lean NO
x
-reduction conditions are maintained in the catalyst member at least during the desorbing period, e.g., by supplying a NO
x
reductant to the catalyst member, and the establishment of the trapping period and the desorbing period is alternated.
One aspect of the present invention comprises introducing a NO
x
reductant into the lean gas stream between the trap member and the catalyst member.
Another aspect of the present invention provides for maintaining a molar ratio of reductant (on a C
1
basis) to NO
x
of from about 0.5 to 8, preferably from 1 to 4, in the gaseous stream entering the catalyst member.
Other aspects of the present invention include one or more of the following features, alone or in combination: introducing the combustible component upstream of the trap member in an amount sufficient to supply from the combustible component at least part of the NO
x
reductant, and maintaining lean NO
x
reduction conditions in the catalyst member even while passing the NO
x
-depleted gaseous stream therethrough.
The present invention also provides for carrying out the method wherein the NO
x
sorbent material comprises at least one basic oxygenated compound and the catalytic metal component of the catalyst material intimately combined with the sorbent material may comprise one or more catalytic metal components, e.g., platinum group metal components, and preferably platinum. As for the catalyst member positioned downstream of the trap member, i.e., to receive the gaseous stream discharged therefrom, the catalyst member comprises a lean NO
x
abatement catalyst selected fro
Deeba Michel
Farrauto Robert J.
Feeley Jennifer S.
Engelhard Corporation
Langel Wayne A.
Negin Richard A.
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