Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1998-11-02
2001-04-17
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S212000, C423S213200, C423S239100
Reexamination Certificate
active
06217837
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to the use of sol-gel manufactured alumina-based metal oxides materials with precious metal for absorbing nitrogen oxides from an oxidizing engine exhaust gas as during lean-burn engine operation. More particularly, the alumina-based material includes at least one of alkali metal or alkaline earth metal, and optionally a lanthanide metal. Preferably, the alumina-based metal oxide material is a single phase metal-alumina material made from particular hetero-metallic alkoxides.
BACKGROUND OF THE INVENTION
A recent approach for gasoline engines is to operate them in a lean-burn mode, where the A/F ratio is higher than stoichiometry, in order to improve fuel economy. Traditional three-way catalysts for treating engine exhaust gases operate efficiently to convert nitrogen oxides (NOx), carbon monoxide and hydrocarbons under stoichiometric conditions but are not efficient to convert the NOx in lean-burn situations. Lean-NOx catalysts formulated to convert NOx in an oxidizing exhaust are generally based on zeolites and convert the NOx by using reducing agents like hydrocarbons and carbon monoxide in the exhaust gas. They suffer from lack of good high temperature hydrothermal durability, that is, at high temperatures the structure of the zeolite collapses and dealumination occurs. Another approach for lean-burn exhaust gas treatment has focused on the use of lean-burn NOx traps, i.e., materials which are able to absorb nitrogen oxides from the exhaust gas during lean-burn operation and then later release them when the oxygen concentration in the exhaust gas is reduced. For example, when the A/F ratio is made rich or stoichiometric. Conventional NOx absorbents are alkaline earth metals like barium with a precious metal catalyst like platinum carried on alumina. The widely held mechanism for this absorption phenomena is that during lean-burn operation the platinum first oxidizes NO to NO
2
and the NO
2
subsequently forms a nitrate complex with the trapping material, e.g., the barium. In the regeneration mode, under a stoichiometric or rich environment, the nitrate decomposes and the NOx released is reduced catalytically over the platinum with reducing species like HC or CO in the exhaust gas.
One serious deficiency of such conventional absorbent materials is that the barium reacts with sulfur oxides generally present in exhaust gas to form barium sulfate. This inactivates the barium for NOx absorption. It is suggested that to decompose the barium sulfate it should be subjected to elevated temperatures of at least 600° C. or more in reducing gas conditions. One negative aspect of this regeneration process is that it leads to detrimental changes in the NOx absorbent such as reduced surface area and crystallization of the aluminate phases thereby reducing the efficiency of the NOx absorbent. Alkali metals like potassium have also been suggested as NOx absorbents, however, they are even more easily deactivated by sulfur than alkaline earth metals like barium. Repeated regeneration of the absorbent by heating, as discussed above, contributes to a loss of surface area in the alumina support material and contributes toward further sintering in the platinum precious metal responsible for the conversion of NOx to NO
2
. Precious metal sintering results in a decrease in the active sites that convert NOx to NO
2
, and hence a decrease in the total amount of NOx trapped on the available absorbent.
It would be desirable if a NOx absorbent could be found which would be more resistant to sulfur poisoning while being a good NOx absorbent. It would also be very desirable if the improved material could withstand repeated heating to de-sulfur, i.e., reactivate the absorbent, without significantly reducing the amount of surface area.
Sol-gel processing allows for the low temperature preparation of aluminum oxide materials of high purity and controlled microstructure. These materials can be in such forms as gels, aerogels and xerogels. Sol-gel processes have been found useful in preparing supported and unsupported inorganic membranes for chemical sensor and separation applications, for automobile catalyst washcoats, and for optical coatings for automotive glass. For example, in U.S. Pat. No. 5,403,807 by Narula, a method is disclosed for making a single phase metal-alumina sol-gel material which includes alkaline earth metal and optionally lanthanides. These materials are disclosed as being stable at high temperatures making them ideally useful as catalyst supports.
We have now unexpectedly found that alumina matrix materials including alkali metals and/or alkaline earths and optionally lanthanides when made by sol-gel techniques are excellent materials for use as NOx absorbents. These materials overcome the deficiencies of prior art NOx absorbents.
In U.S. application Ser. No. 09/134992 filed Aug. 17, 1998 U.S. Pat. No. 6,129,898 and entitled “NOx Trap Catalyst For Lean Burn Engines” commonly assigned with the present invention, a sol-gel oxide material is disclosed useful for NOx absorption. It comprises oxides of aluminum, magnesium and zirconium.
SUMMARY OF THE INVENTION
A method for treating exhaust gas containing carbon monoxide, hydrocarbons, and nitrogen oxides generated by a lean-burn internal combustion engine, the method comprising the step of: bringing the exhaust gas from said lean-burn engine in contact with an alumina-based metal oxide material made by sol-gel techniques preferably from alkoxides of the metals, and further includes at least 0.1 wt. % precious metal selected from the group consisting of platinum, palladium, rhodium, and a mixture of any of them. The alumina-based metal oxide material is an oxide based on metals consisting essentially of: (I) aluminum, (II) at least one of: (a) alkali metals, (b)alkaline earth metals, or (c) a mixture of them, and, optionally, (III) a lanthanide metal. Under lean-burn conditions, where the exhaust gas contains more oxygen than is required for oxidizing components to be oxidized in the exhaust gas, nitrogen oxides are absorbed on the metal oxide material and when the oxygen concentration in said gas is lowered the absorbed nitrogen oxides are desorbed from the metal oxide and reduced over the precious metal.
The preferred alumina-based metal oxide for use in the present invention method are (a) single phase alkali metal-aluminum-oxide materials, (b) single phase alkaline earth-metal aluminum oxide materials, or (c) single phase alkali metal-alkaline earth metal-aluminum oxide materials, any of which optionally may also include another metal, a lanthanide, al of these materials will herein be collectively referred to as single phase metal-alumina materials. These materials are made from alkoxides including certain heterometallic alkoxides which will be discussed in detail below. The ratio of alkali metals, alkaline earth metals, and/or lanthanide to alumina in these single phase metal-alumina materials can be changed by including varying amounts of, e.g., aluminum alkoxide, alkaline earth alkoxides, alkali metal alkoxides, or lanthanide alkoxides, with the heterometallic alkoxide during sol-gel processing thereof. The materials are useful as catalyst supports, e.g., loaded with precious metals like platinum.
REFERENCES:
patent: 5242883 (1993-09-01), Ichikawa et al.
patent: 5403807 (1995-04-01), Narula
patent: 5851501 (1998-12-01), Krutzsch et al.
patent: 5874057 (1999-02-01), Deeba et al.
patent: 5922293 (1999-07-01), Miyoshi et al.
patent: 6001319 (1999-12-01), Yokoi et al.
patent: 6010673 (2000-01-01), Kanazawa et al.
patent: 93/03840 (1993-03-01), None
patent: 0 645 173 (1995-03-01), None
patent: 0-645-173-A2 (1995-03-01), None
patent: 0 754 494 (1997-01-01), None
patent: 0-778-072-A2 (1997-06-01), None
patent: 0-787-522-A2 (1997-08-01), None
Nakouzi-Phillips Sabine Rita
Narula Chaitanya Kumar
Dunn Tom
Ford Global Technologies Inc.
Melotik Lorraine S.
LandOfFree
Use of sol-gel processed alumina-based metal oxides for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Use of sol-gel processed alumina-based metal oxides for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Use of sol-gel processed alumina-based metal oxides for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2538588