Chemistry of inorganic compounds – Rare earth compound
Reexamination Certificate
2002-07-16
2003-08-12
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Rare earth compound
C423S592100, C423S593100, C423S594800, C423S594120, C423S594160, C423S594170, C423S596000, C423S608000, C423S635000, C423S219000
Reexamination Certificate
active
06605264
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an Oxygen Ion Conducting/Oxygen Storage (OIC/OS) material, and especially relates to an OIC/OS material having niobium (Nb) as part of the crystal structure and which exhibits higher OS capacity and more facile OS properties compared to materials of similar Ce-content. This new material further exhibits a unique property of reversible crystal structure changes upon calcination in oxidizing and reducing environments.
BACKGROUND OF THE INVENTION
It is known that ceria (CeO
2
) plays a number of important roles in automotive three-way conversion (TWC) catalysts for the removal of post-combustion pollutants. Among these are: stabilization of precious metal (PM) dispersion, alumina support stabilization, promotion of the water gas shift (WGS) reaction, promotion of the carbon monoxide (CO)+oxygen (O
2
) reaction to give carbon dioxide (CO
2
); the nitric oxide (NO)+CO reactions to give CO
2
and N
2
, and finally oxygen storage (OS) properties. The oxygen storage ability of CeO
2
arises due to the facile nature of the Ce
4+
/Ce
3+
redox reaction in typical exhaust gas mixtures: the reduction of CeO
2
to cerous oxide (Ce
2
O
3
) provides extra oxygen under fuel rich conditions and oxidation of Ce
2
O
3
to CeO
2
builds up an oxygen reserve under fuel lean conditions. Such a facile oxygen storage—oxygen release ability is important for controlling the ratio of oxidants (air(A)) and reductants (fuel(F)) (A/F ratio) in the exhaust, so that CO and hydrocarbons (HCs) can be oxidized simultaneously with the reduction of nitrogen oxides (NO
x
). The A/F ratio is defined as the weight of air divided by the weight of fuel. For a typical gasoline fuel, an A/F ratio of 14.5-14.7 gives an exhaust composition where there are enough oxidants (O
2
+NO
x
) to completely convert the unburnt HCs and CO in the exhaust to carbon dioxide (CO
2
), water (H
2
O), and nitrogen (N
2
). This is referred to as stoichiometric operation and typically occurs during a cruise or idle operation of the vehicle. During accelerations exhaust compositions with excess HCs and CO are generated (rich mixtures with A/F values less than the stoichiometric value) and during deceleration, compositions with excess oxidants are generated (lean mixtures with A/F values greater than the stoichiometric value). The facile release and consumption of oxygen is important during driving conditions that generate these A/F transients away from stoichiometry so as to prevent the break through of pollutants such as HCs, CO and NO
x
. Catalysts that are used in these applications are referred to as three-way-conversion (TWC) catalysts as they convert the three main pollutants (HCs, CO and NO
x
) to innocuous products.
In older TWC catalysts, pure CeO
2
was used as the oxygen storage component. However, in the older TWC catalysts, because of poor thermal stability, a large loss of oxygen storage capacity occurs above 900-1,000° C. Modem TWC catalysts require more durable and facile OS characteristics. This has resulted in the replacement of pure CeO
2
with solid solutions based on Ce—Zr. Unlike composite metal oxides in which a solid solution is not formed between all the components in the mixture of metallic and oxygen species present, these solid solutions typically refer to a single, substantially homogeneous, metal oxide crystallite or crystallites characterized in that the oxygen atoms in the crystal structure are attached to metal ions of more than one metallic species. These type of materials are further characterized by having a single crystal structure and are referred to as single phase materials of tetragonal or cubic crystal structure. Lower valent rare earth or alkaline earth dopants can also be present in these newer materials. These type materials have the following general properties:
a) They have much higher OS capacity than pure CeO
2
. This arises, as in pure CeO
2
, only Ce
4+
ions at the surface of the crystallites are redox active. However, for Ce—Zr based solid solutions bulk Ce is also redox active in typical exhaust gas compositions and reduction of bulk Ce results in oxygen migration to the crystallite surface where it can be used to oxidize HCs or CO. Thus, these materials are referred to here as OIC/OS type materials as their function involves both oxygen storage and oxygen mobility characteristics. These differences between pure CeO
2
and Ce—Zr based solid solutions are illustrated in
FIGS. 1 and 2
.
b) A further advantage of Ce—Zr based solid solutions is that they are thermally more stable than pure CeO
2
. This results, after aging, in slower sintering rates or particle growth rates and higher aged OS capacity.
c) It has also been found that increasing the Zr content in Ce—Zr solid solutions results in a lowering of the cerium reduction energy in going from Ce
4+
to Ce
3+
and at the same time in a decrease in the activation energy for oxygen ion mobility within the lattice. This is illustrated in
FIGS. 3 and 4
from a theoretical analysis of binary Ce—Zr solid solutions by Balducci et al., J. Phys. Chem., B., Vol. 101, No 10, P. 1750, 1997. In
FIG. 3
it is further observed that the presence of lower valent ions that introduce oxygen vacancies further lower the reduction energies from Ce
4+
to Ce
3+
. (Line A is cerium reduction energy for isolated Ce
3+
and V
0
−
vacancies; B is cerium reduction energy for Ce
3+
−V
0
−
clusters; and C is cerium reduction energy for Ce
3+
−V
0
−
−Ce
3+
clusters.)
d) A further advantage of Zr-rich solid solutions is that after severe aging (greater than 1,000° C.), all the Ce in the solid solution remains accessible for oxygen storage. In contrast, only a fraction of the Ce is available for OS in intermediate Zr-content compositions. This is illustrated in
FIG. 5
, curve
55
, where the “available” OS for aged (greater than 1,000° C.) Ce—Zr solid solutions of varying Zr content are plotted. The OS capacity was measured using Temperature Programmed Reduction (TPR) analysis. For this measurement the aged sample is exposed to a 5% H
2
/95% Ar mixture and the rate of H
2
uptake is measured as a function of temperature. The fraction of Ce reduced is measured based on the following reaction:
2Ce
4+
O
2
+H
2
→Ce
2
3+
O
3
+H
2
O.
The maximum available OS based on Ce content is presented as Curve
51
. It is seen that Curves
51
and
55
(maximum OS based on Ce content and “available” OS) coincide only in a narrow and low range of Ce contents from 0-20 mole % Ce. An increase in Ce content above 20 mole % does not result in a corresponding increase in “available” OS higher than 0.45 millimoles per gram (mmoles/g). The consequence of this limited OS availability is that in severely aged intermediate Zr-content or Ce-rich solid solutions, only part of the Ce is redox active and capable of participating in redox reactions, whereas the rest of the Ce behaves as a structure forming element. This is true for both binary Ce—Zr mixtures and for multi-component mixtures with other rare earth and alkaline earth dopants present.
Thus, the formation of high Zr-content Ce—Zr solid solutions has some disadvantages. One clear disadvantage is the continuous drop in OSC capacity with increased Zr content even though these materials tend to have the best thermal stability and the most facile OS properties.
What is needed in the art are OIC/OS materials having high oxygen storage capacity while maintaining or even improving upon the thermal stability and facile nature of the redox function of Zr-rich compositions.
SUMMARY OF THE INVENTION
The present invention comprises an OIC/OS material, a catalyst comprising the OIC/OS material, and a method for converting hydrocarbons, carbon monoxide and nitrogen oxides using the catalyst. This OIC/OS material comprises: up to about 95 mole percent (mole %) zirconium; about 0.5 to about 40 mole % cerium; about 0.5 to about 15 mole % R, wherein R is a rare earth met
Bortun Anatoly I.
Nunan John Gerard
Bos Steven
Cichosz Vincent A.
Delphi Technologies Inc.
Vanoy Timothy
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