Sol-gel alumina membrane for lean NOx catalysts and method...

Details Sol-gel alumina membrane for lean NOx catalysts and method... Sol-gel alumina membrane for lean NOx catalysts and method...

1994-09-23

2001-05-15

Griffin, Walter D. (Department: 1764)

Catalyst, solid sorbent, or support therefor: product or process

In form of a membrane

C502S337000, C502S338000, C502S439000

Reexamination Certificate

active

06232253

ABSTRACT:

TECHNICAL FIELD
This invention relates to lean-NO
x
catalysts, particularly those providing a high surface area catalyst membrane of controlled pore size to remove nitrogen oxide (NO
x
) emissions from lean-burn automobile engines and industrial plants emission systems. The present invention also discloses a method of making such catalysts.
BACKGROUND OF THE INVENTION
The Environmental Protection Agency has instituted stringent automobile-related environmental regulations. A primary focus of the regulation relates to the Corporate Average Fuel Economy (CAFE) standards, which mandate a specified, gradual increase of a corporate fleet's overall fuel economy by the established target dates. CAFE standards have spurred industry wide research and development of “lean-burn engines”. The term “lean-burn engine” used herein is defined as an engine utilizing air/fuel mixtures having an oxygen content in excess of the stoichiometric air/fuel ratio (lean mixtures). The use of such lean air/fuel mixtures, reduces the consumption of fuel and thus enhances an automobile's fuel economy. The effort to increase automobile fuel economy has become paramount in the wake of the Environmental Protection Agency's tightening CAFE standards.
In addition to the CAFE standards, the Environmental Protection Agency has set a schedule for the continual reduction of specific automotive emissions. Thus, the Environmental Protection Agency's regulatory measures have required the automobile industry to simultaneously increase fuel economy while decreasing harmful exhaust emissions. Accordingly, there exists a new found interest in the development of a catalyst to operate efficiently under lean-burn conditions.
Many of the prior art catalysts were designed to optimally operate at or about stoichiometric conditions. These prior art catalysts when combined with a lean fuel mixture result in O
2
being adsorbed by the catalyst, preventing NO
x
from being reduced to nitrogen (N
2
) by the active metal on the catalyst. So while the current three-way catalyst, for example, may effectively reduce NO, hydrocarbons (HC) and CO emissions at stoichiometric conditions, the efficiencies of a three-way catalyst for NO
x
reduction diminish significantly in an oxygen rich environment.
The effectiveness of a catalyst is largely dependent on the air/fuel ratio of the fuel mixture which is employed in an engine. If a lean mixture is utilized, a catalyst exhibits a high activity of oxidation but a low activity of reduction, while the combustion product contains a large amount of oxygen. In contrast, in the presence of a fuel rich mixture, a catalyst exhibits a high activity of reduction but a low activity of oxidation. At stoichiometric air/fuel ratios, oxidation and reduction are balanced.
Accordingly, there is a need for a lean-NO
x
catalyst which efficiently reduces NO
x
emissions in the presence of a lean fuel mixture. In addition to automotive applications, lean-NO
x
catalysts are also valuable in lowering the emissions of stationary power plants that burn fossil fuels. A lean-NO
x
catalyst is defined for purposes of this application as a catalyst that can reduce NO
x
under lean-burn conditions.
In answer to this problem, transition metal-containing zeolite catalysts have been developed to reduce NO
x
emissions for lean mixtures. In particular, copper-zeolite catalysts have been preferred due to the effectiveness of copper as an ion exchange metal at lower temperatures such as that present in lean-burn engine exhaust.
Zeolites are crystalline aluminosilicates. Zeolites are commercially available and are characterized by crystal structures having different channels or pore diameters. Zeolites have fine pores (sized at several angstroms), comparable with the size of a molecule, hence they are termed molecular sieves. It is believed that the hydrocarbons are selectively trapped in those pores or sieves. As the transition metal, introduced by ion exchange, forms active sites in the pores, the hydrocarbons are adsorbed therein and react with nitrogen oxides in the presence of excess O
2
. Accordingly, zeolite catalysts have potential application in the removal of NO
x
from lean fuel mixtures.
The basic principle behind removing NO
x
in a lean fuel mixture with a transition metal, e.g., copper, copper-containing zeolite catalyst relies on adsorbing NO
x
and effectuating a catalytic reaction of the adsorbed NO
x
with the hydrocarbons contained in the exhausted gas, to reduce the NO
x
to N
2
. Although copper-zeolite catalysts have good catalytic activity initially, due to copper's high NO
x
adsorption ability, copper-zeolite catalysts have a series of limitations which are described below.
First, transition metal containing zeolites degrade at high temperatures usually found in automotive exhaust systems. Furthermore, if the zeolite catalyst is exposed to steam-containing air, e.g., automotive exhaust, the activity of the catalyst decreases rapidly. The decrease in activity is also accompanied by dealumination. Steam deactivation of zeolite-based catalysts is the primary reason why such catalysts are not practical in application.
To compensate for the problems associated with zeolites, alumina (Al
2
O
3
) has been used in three-way catalysts as support for active ingredients. The use of Al
2
O
3
and metal containing Al
2
O
3
as lean-NO
x
catalysts has been reported by H. Hamada, Y. Kintaichi, M. Sasaki and T. Ito in “Transition metal-promoted silica and alumina catalysts for the selective reduction of nitrogen monoxide with propane”,
Applied Catalysts
, L1-L8 (1991). Alumina, however, is inefficient in NO
x
reduction such that it is currently impractical for commercial use.
Thus, a need exists for a commercially practical, efficient, lean-NO
x
catalyst as provided by the present invention.
SUMMARY OF THE INVENTION
The present invention teaches a catalyst for reducing emissions from an oxygen-rich environment which includes: a sol-gel processed inorganic membrane and a transition metal loaded on the sol-gel processed membrane.
The present invention also discloses methods for preparing a sol-gel processed alumina membrane and sol-gel processed alumina/silica membrane for use with lean-NO
x
catalysts.
The invention additionally discloses a method of loading a transition metal onto a sol-gel processed membrane for catalytic NO
x
reduction for lean-burn engines.
Further, the present invention teaches a NO
x
reduction system for catalytic reduction under lean-burn conditions in an automobile exhaust stream which combines a lean-NO
x
catalyst, particularly the aforesaid alumina/silica sol-gel catalyst, with a three-way catalyst to provide an emission system which can operate over a broad range of operating conditions.
The instant invention has many advantages. First, the sol-gel process provides inorganic membranes which have customized pore sizes. This flexibility allows the inorganic membranes to in effect mimic the effectiveness of the zeolite pore structure. Second, alumina/silica membranes retain high surface area even after high temperature treatment. Third, these alumina/silica membranes are thoroughly stable high surface area membranes. The resulting catalyst membrane can thereby have significantly increased hydrothermal stability and consequent improved performance.
These and other features and advantages of the present invention are readily apparent from the detailed description of the best mode for carrying out the invention when taken in conjunction with the accompanying drawings.


REFERENCES:
patent: 4699892 (1987-10-01), Suzuki
patent: 4865630 (1989-09-01), Abe
patent: 5017538 (1991-05-01), Takeshima
patent: 5100596 (1992-03-01), Haag et al.
patent: 5155077 (1992-10-01), Montreuil et al.
patent: 5183788 (1993-02-01), Jacobson et al.
patent: 5210062 (1993-05-01), Narula et al.
patent: 5250184 (1993-10-01), Maier
patent: 5258339 (1993-11-01), Ma et al.
patent: 5275795 (1994-01-01), Montreuil et al.
patent: 5342431 (1994-08-01), Anderson et al.
patent: 3642018 (1987-06-01), Non

Affiliated with

Also associated with

Rating

Sol-gel alumina membrane for lean NOx catalysts and method... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Sol-gel alumina membrane for lean NOx catalysts and method..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Sol-gel alumina membrane for lean NOx catalysts and method... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2464516