Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-03-15
2002-05-14
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C502S302000, C502S304000, C502S340000, C502S349000, C502S355000
Reexamination Certificate
active
06387338
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an OIC/OS material, and especially relates to an OIC/OS material having a stable cubic crystal structure.
BACKGROUND OF THE INVENTION
Solid electrolytes based on zirconia (ZrO
2
), thoria (ThO
2
) and ceria (CeO
2
) doped with lower valent ions have been extensively studied. The introduction of lower valent ions, such as rare earths (Y, La, Nd, Dy, etc.) and alkaline earths (Sr, Ca and Mg), results in the formation of oxygen vacancies in order to preserve electrical neutrality and this in turn gives rise to oxygen ionic conductivity at high temperature (e.g. greater than 800° C.). Typical commercial or potential applications for these solid electrolytes includes their use in solid oxide fuel cells (SOFC) for energy conversion, electrochemical oxygen sensors, oxygen ion pumps, structural ceramics of high toughness, heating elements, electrochemical reactors, steam electrolysis cells, electrochromic materials, magnetohydrodynamic (MHD) generators, hydrogen sensors, catalysts for methanol decomposition, potential hosts for immobilizing nuclear waste and oxygen storage materials in three-way-conversion (TWC) catalysts.
Stabilized ZrO
2
has been studied as the most popular solid electrolyte. In the case of doped ZrO
2
both partially and fully stabilized ZrO
2
have been used in electrolyte applications. Partially stabilized ZrO
2
consists of tetragonal and cubic phases while the fully stabilized form exists in the cubic fluorite structure. Both CeO
2
and ThO
2
solid electrolytes exist in the cubic crystal structure in both doped and undoped forms. The amount of dopant required to filly stabilize the cubic structure for ZrO
2
varies with dopant type. For Ca it is in the range of 12-13 mole %, for Y
2
O
3
and Sc
2
O
3
it is greater than 18 mole % Y or Sc and for other rare earths (Yb
2
O
3
, Dy
2
O
3
, Gd
2
O
3
, Nd
2
O
3
and Sm
2
O
3
) in the range of 16-24 mole % of Yb, Dy, Gd, Nd, and Sm.
Fully or partially stabilized ZrO
2
, as well as other commonly studied solid electrolytes, have a number of drawbacks. In order to achieve sufficiently high conductivity and to minimize electrode polarization the operating temperatures have to be very high, in excess of 800-1,000° C. For solid oxide fuel cells for example, reducing the operating temperatures below 800° C. would result in numerous advantages such as greater flexibility in electrode selection, reduced maintenance costs, reduction in the heat insulating parts needed to maintain the higher temperatures and reductions in carbonaceous deposits (soot) that foul the operation of the fuel cell.
Further, in the automotive industry there is great interest in developing lower temperature and faster response oxygen sensors to control the air to fuel ratio (A/F) in the automotive exhaust. In the case of three-way-conversion (TWC) catalysts solid solutions containing both ZrO
2
and CeO
2
are used as oxygen storage (OS) materials and are found to be more effective that pure CeO
2
both for higher OS capacity and in having faster response characteristics to A/F transients.
Oxygen storage capacities (OSC) in these applications arises due to the facile nature of Ce
4+
⇄Ce
3+
oxidation-reduction in typical exhaust gas mixtures. The reduction of the CeO
2
to Ce
2
O
3
provides extra oxygen for the oxidation of hydrocarbons (HCs) and CO under fuel rich conditions when not enough oxygen is available in the exhaust gas for complete conversion to carbon dioxide (CO
2
) and water (H
2
O). The use of binary CeO
2
/ZrO
2
and ternary CeO
2
/ZrO
2
/M
2
O
3
based catalysts in such applications have advantages over the use of pure CeO
2
containing catalysts. This arises because in pure CeO
2
only surface Ce
4+
ions can be reduced in the exhaust at typical catalyst operating temperatures of 300-600° C. (See FIG.
1
). However, in binary CeO
2
/ZrO
2
or ternary CeO
2
/ZrO
2
/M
x
O
y
solid solutions more oxygen is made available through the reduction of bulk Ce
4+
and the subsequent migration of ‘O’ to the surface of the solid solution crystallites where it reacts with the HCs and CO as is demonstrated in FIG.
2
.
The ‘O’ migration to the surface of the solid solution crystallites is made possible by the formation of the solid solution and is thus an analogous process to that occurring when these same materials are used as solid solution electrolytes. Thus, a more accurate description of these materials for TWC catalyst applications is to view them as oxygen ion conducting/oxygen storage (OIC/OS) materials. These materials have a much higher oxygen storage capacity compared to pure CeO
2
, especially after catalyst aging and the formation of large crystallites. Further, the response of these solid solutions to changes in the exhaust gas enviromnent is more rapid compared to pure CeO
2
with the result that they operate more effectively in preventing CO/HC breakthrough during accelerations and they further provide oxygen at lower temperatures.
Aging of electrolytes is a phenomena usually associated with a decrease in the ionic conductivity at a constant temperature with time. The aging process is a function of composition, operating temperature, time and temperature cycling. The two main causes of aging are: a) ordering of the cation and anion sublattice and b) decomposition of the metastable phases. In single phase cubic systems the major cause of aging is formation and growth of microdomains and disproportionation at high temperatures into different phases. Aging of cubic Y stabilized ZrO
2
oxygen ion conducting electrolytes for example can occur through disproportionation into a Y-rich cubic phase and a Y-poor tetragonal phase. Thus, phase stability at high temperatures is an important property of solid solution electrolytes and maintaining phase stability in an optimized cubic or tetragonal phase after high temperature operation or cycling is a highly desirable property.
For TWC catalyst applications the newest OIC/OS materials consist of a range of CeO
2
/ZrO
2
solid solutions with lower valent dopants added to increase the number of oxygen vacancies and to increase the thermal stability and oxygen ion conductivity of the solid solutions after sintering at high temperatures. Zr-rich compositions have the advantage in that the reduction energies for Ce
4+
→Ce
3+
decrease with increasing Zr content and that the activation energies for mobility of ‘O’ within the lattice decreases. This is demonstrated in
FIGS. 3 and 4
(Balducci et al., J. Phys. Chem. B., Vol. 101, No 10, p.1750, 1997). However, the Zr-rich systems suffer from the disadvantage in that the OSC capacity is decreased due to the lower CeO
2
content. Thus, strategies to optimize the availability (OIC) of the OSC function go counter to those that maximize oxygen storage capacity (OSC).
A further disadvantage of the Zr-rich systems is that the stable crystal structure is tetragonal rather than the more desirable cubic structure. The crossover composition between cubic and tetragonal occurs in the range of 35-45 Mole % ZrO
2
. Compositions having higher ZrO
2
content have the tetragonal crystal structure while compositions of lower Zr content are cubic. Further, composites that increase the facile nature of both the CeO
2
reduction and mobility of ‘O’ (OIC) within the solid solution lattice at a given and high Ce content are advantageous. This is true not only for oxygen storage (OSC) applications in TWC catalysts but also for preparing highly effective electrolytes in solid oxide fuel cells (SOFCs) where high conductivity at low temperatures is a major requirement.
What is needed in the art are OIC/OS materials having stable cubic crystal structures, and high oxygen storage and oxygen ion conductivity properties.
SUMMARY OF THE INVENTION
The present invention comprises an OIC/OS material, a catalyst comprising the OIC/OS material, and a method for converting nitrogen oxides using the catalyst. This OIC/OS material comprises: up to about 95 mole percent (mole %) zirconium; up to about
Anatoly Bortun I.
Nunan John Gerard
Cichosz Vincent A.
Griffin Steven P.
Medina Maribel
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