Metal oxide catalysts for nitric oxide reduction

Details Metal oxide catalysts for nitric oxide reduction Metal oxide catalysts for nitric oxide reduction

1998-04-15

2001-04-03

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

C423S239100, C423S402000, C502S302000, C502S303000

Reexamination Certificate

active

06210649

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to compositions of matter that can be used in NO
x
reduction. In particular, the compositions are nanocrystalline metal oxide materials having high surface areas. The invention also provides methods of making these compositions and methods for using these compositions to reduce NO
x
compounds.
BACKGROUND OF THE INVENTION
The reduction of nitrogen oxide (NO
x
) emissions is a key environmental objective for the new millennium. Atmospheric emissions of NO
x
not only have detrimental effects on human health, but also have adverse effects on the ecosystems in which we live. Exposure to high levels of NO
x
produces immediate acute effects and prolonged exposure above ambient levels leads to bronchitis, pneumonia, susceptibility to viral infections and alterations to the immune system. In addition to these direct health effects, NO
x
contributes to urban smog by reacting with volatile organic compounds in the atmosphere to form ozone, and also causes acid precipitation.
The United States anthropogenic NO
x
emissions are estimated at 23 million tons for 1992. Despite improved control strategies, this represents a 5% increase since 1983. The sources of NO
x
pollutants can be classified into two categories: stationary and non-stationary. Non-stationary sources (motorized vehicles) contributed 45% of the total United States NO
x
emissions for 1992. The remaining 55% of the pollutants can be attributed to stationary sources such as power plants (29%), internal combustion engines (11%), industrial boilers (8%), process heaters (3%), gas turbines (1%) and other sources (3%). Since nearly all of these NO
x
emissions arise from the combustion of fossil fuels, the development of improved methods by which NO
x
can be removed from exhaust gases is of critical importance.
Several catalytic approaches for the removal of NO
x
from exhaust gases have been developed in the past few decades. Direct decomposition of NO
x
involves the decomposition of NO
x
into molecular nitrogen and oxygen, but the current generation of catalysts for this reaction are not active nor robust enough to be applied in practice e.g. the zeolite Cu-ZSM-5. Selective catalytic reduction of NO
x
using ammonia as a reducing agent has been industrially applied for several decades, but the use of ammonia leads to high equipment costs due to its corrosive nature, and also gives rise to the phenomenon known as “ammonia slip”, where unreacted ammonia is exhausted to the atmosphere. The use of hydrocarbons as reducing agents is favored over ammonia, and many catalytic systems for NO
x
reduction with hydrocarbons have been developed. These systems typically employ C
2
or higher hydrocarbons.
The large worldwide reserves of natural gas and the availability of methane at gas fired power plants makes methane an attractive reductant for stationary applications. However, over most catalysts methane preferentially reacts with oxygen present in the feed stream before reducing NO
x
. Some of the few catalysts that do promote NO
x
reduction with methane in an oxidizing atmosphere are zeolites such as ZSM-5 or ferrierite zeolites exchanged with Co, Mn, Ni, In, Ga or Pd ions. Near complete conversion of NO
x
to N
2
is achieved when an excess of methane is used at low space velocities.
Currently, catalysts that do not require an ammonia reductant for NO
x
reduction, such as Co-ZSM-5, do not have sufficient catalytic activity for industrial purposes, especially in the presence of water. There exists a need for the development of new materials that do not themselves promote the emission of harmful compounds into the atmosphere.
SUMMARY OF THE INVENTION
The present invention provides new compositions for reducing NO
x
compounds, methods for making these compositions and methods for using compositions to reduce NO
x
compounds.
One aspect of the invention provides a composition comprising a ceramic having at least one element selected from the group consisting of one or more of transition metals, noble metals, Group IA, Group IIA, Group IIIB, lanthanide, and actinide elements. The ceramic has a surface area of at least about 10 m
2
/g and comprises agglomerates of crystallites having a crystallite size of less than about 100 nm.
Another aspect of the invention provides a method for performing X-ray scintillation with Y
2
O
3
particles which includes a dopant selected from the group consisting of Eu, Gd, and a combination thereof.
Another aspect of the invention provides a method for chemically precipitating a ceramic with a water-soluble base having a general formula R
4
N
+
OH
−
.
Another embodiment provides a method for sol-gel processing of an oxide. The oxide is selected from the group consisting of one or more of transition metals, noble metals, Group IA, Group IIA, Group IIIB oxides, lanthanide oxides, actinide oxides, or any combination of these.
Another embodiment provides a method for carrying out an inert gas condensation process. The process involves a doped oxide selected from the group consisting of one or more of transition metals, noble metals, Group IA, Group IIA, Group IIIB oxides, lanthanide oxides, actinide oxides, or any combination of these.
Another aspect of the invention provides a method for synthesizing a ceramic comprising crystallites. The crystallites have a surface area of at least 10 m
2
/g and an average crystallite size of less than about 100 nm.
Another aspect of the invention provides a method for calcining a ceramic comprising crystallites where the calcining step occurs at a temperature of 800° C. The method also includes the step of recovering the ceramic having a surface area of at least 10 m
2
/g and an average crystallite size of less than about 100 nm.
Another aspect of the invention provides a method comprising sintering a ceramic comprising crystallites at a temperature of 800° C. and recovering the ceramic with a theoretical density of at least 90%.
Another aspect of the invention provides a method for performing X-ray scintillation with a sintered ceramic comprising particles having an average particle size of less than about 0.2 &mgr;m.
Another aspect of the invention provides a method which includes the steps of exposing an NO
x
compound to a ceramic where the ceramic has a surface area of at least 1 Om
2
/g. An NO
x
reduction reaction is then allowed to occur.
One embodiment provides a method for NO
x
reduction. The method includes the steps of exposing a ceramic catalyst to NO
x
and allowing NO
x
to undergo a reduction reaction at an elevated rate defined as a rate scaled, for particular reaction conditions, relative to a rate involved in exposing about 100 mg of ceramic catalyst to about 100 ml/min of about 4000 ppm NO, 4000 ppm CH
4
and 4% O
2
, and allowing NO
x
to undergo a reduction reaction at a rate of at least about 2×10
−7
mol N
2
/g sec at 700° C.
Another embodiment provides method comprising exposing NO
2
, NO or N
2
O to a ceramic having a surface area of at least about 10 m
2
/g and allowing NO
x
reduction to occur.


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patent: 5855862 (1999-01-01), Grenier et al.
patent: 5876681 (1999-03-01), Barthe et al.
patent: 5919425 (1999-07-01), Nguyen et al.
patent: 5922295 (1999-07-01), Chattha et al.
Zhang, Xiankuan; Walters, Arden B.; Vannice, M. Albert, “NoxDecomposition and Reduction by Methane Over La2O3”, Applied Catalysis B; Environmental, v. 4, n 2-3 (Sep. 13, 1994), p. 237-256.*
X. Zhang et al., “No Adsorption, Decomposition, and Reduction by Methane over Rare Earth Oxides”, Journal of Catalysis, vol. 155, pp. 290-302, 1995.

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