Iron oxide-zirconia composite oxide and method for producing same, and exhaust gas purification catalyst
A composite oxide with a high oxygen storage capacity is provided without using cerium. The composite oxide is an iron oxide-zirconia composite oxide containing iron, zirconium, and a rare-earth element. The total content of Fe 2 O 3 , ZrO 2 , and an oxide of the rare-earth element is not less than 90 mass %, the content of an iron oxide in terms of Fe 2 O 3 is 10 to 90 mass %, and the absolute value of the covariance COV(Fe, Zr+X) of the composite oxide, which has been baked in the atmosphere at a temperature of greater than or equal to 900° C. for 5 hours or more, is not greater than 20.
1. An iron oxide-zirconia composite oxide comprising iron, zirconium, and a rare-earth element, wherein
a total content of Fe 2 O 3 , ZrO 2 , and an oxide of the rare-earth element (X) is not less than 90 mass %,
a content of an iron oxide in terms of Fe 2 O 3 is 10 to 90 mass %, and
an absolute value of covariance COV(Fe, Zr+X) of the composite oxide, which has been baked in the atmosphere at a temperature of greater than or equal to 900° C. for 5 hours or more, determined by the following Formulae (1) to (3), is not greater than 30:
[
Math
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1
]
R
i
(
Fe
)
=
I
i
(
Fe
)
×
100
I
i
(
Fe
)
+
I
i
(
Zr
)
+
I
i
(
X
)
(
1
)
R
i
(
Zr
+
X
)
=
{
I
i
(
Zr
)
+
I
i
(
X
)
}
×
100
I
i
(
Fe
)
+
I
i
(
Zr
)
+
I
i
(
X
)
(
2
)
COV
(
Fe
,
Zr
+
X
)
=
1
n
∑
i
=
1
n
[
{
R
i
(
Fe
)
-
R
av
(
Fe
)
}
×
{
R
i
(
Zr
+
X
)
-
R
av
(
Zr
+
X
)
}
]
(
3
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(in the formula, I i (Fe), I i (Zr), and I i (X) respectively represent ratios of X-ray intensities of iron, zirconium, and the rare-earth element measured at a measurement point i (where i=1 to n) to 100% intensities of the respective elements as measured by subjecting the composite oxide to a ray analysis through EPMA (WDX: wavelength dispersive X-ray spectrometry) under conditions of an accelerating voltage of 15 kV, a sample current of 50 nA, a minimum beam diameter (not greater than 1 μm), and measurement intervals of 1 μm, where R av (Fe) and R av (Zr+X) represent mean values of R i (Fe) and R i (Zr+X), respectively, at all measurement points n).
2. The iron oxide-zirconia composite oxide according to claim 1 , wherein the absolute value of the covariance COV(Fe, Zr+X) of the composite oxide, which has been baked in the atmosphere at a temperature of greater than or equal to 900° C. for 5 hours or more, determined by the Formulae (1) to (3), is not greater than 20.
3. The iron oxide-zirconia composite oxide according to claim 1 , wherein the iron oxide in the composite oxide, which has been baked in the atmosphere at a temperature of greater than or equal to 900° C. for 5 hours or more, contains hematite.
4. The iron oxide-zirconia composite oxide according to claim 1 , wherein the composite oxide contains lanthanum.
5. The iron oxide-zirconia composite oxide according to claim 4 , wherein the composite oxide, which has been baked in the atmosphere at a temperature of greater than or equal to 900° C. for 5 hours or more, contains at least one of a lanthanum-iron composite oxide or a lanthanum-zirconium composite oxide.
6. The iron oxide-zirconia composite oxide according to claim 4 , wherein the absolute value of the covariance COV(Fe, Zr+X) of the composite oxide, which has been baked in the atmosphere at 1100° C. for 10 hours, determined by the Formulae (1) to (3), is not greater than 20.
7. The iron oxide-zirconia composite oxide according to claim 4 , wherein an atomic ratio of lanthanum to iron (La/Fe) is not less than 1.1 and not greater than 2.1.
8. The iron oxide-zirconia composite oxide according to claim 4 wherein an atomic ratio of lanthanum to iron (La/Fe) is not less than 1.1 and not greater than 2.0.
9. The iron oxide-zirconia composite oxide according to claim 1 , further comprising yttrium.
10. An exhaust gas purification catalyst comprising a support and a catalyst layer formed on the support, wherein
the catalyst layer includes a cerium oxide-zirconia composite oxide, Al 2 O 3 , and the iron oxide-zirconia composite oxide according to claim 1 , and
one of the oxides has supported thereon a noble metal catalyst of at least one of Pt or Pd.
11. The exhaust gas purification catalyst according to claim 10 , wherein the noble metal catalyst is supported on the cerium oxide-zirconia composite oxide or Al 2 O 3 .
12. The exhaust gas purification catalyst according to claim 10 , wherein the noble metal catalyst is supported on both the cerium oxide-zirconia composite oxide and Al 2 O 3 .
13. An exhaust gas purification catalyst comprising a support and a catalyst layer formed on the support, wherein
the catalyst layer includes an oxide support containing at least one of CeO 2 , ZrO 2 , or Al 2 O 3 as a main component, the oxide support having supported thereon Pd or Pt that is a noble metal catalyst, a barium compound, and the iron oxide-zirconia composite oxide according to claim 1 .
14. A method for producing an iron oxide-zirconia composite oxide, comprising:
mixing a zirconia sol water suspension containing a rare-earth element with organic acid iron to form a mixed solution to attain a proportion in which a total content of Fe 2 O 3 , ZrO 2 , and an oxide of the rare-earth element is not less than 90 mass %, and a content of an iron oxide in terms of Fe 2 O 3 is 10 to 90 mass %;
thermally condensing the obtained mixed solution to form a gel; and
baking the gel.
15. The method for producing an iron oxide-zirconia composite oxide according to claim 14 , further comprising mixing a lanthanum oxide into the mixed solution of the zirconia sol water suspension containing the rare-earth element and the organic acid iron.