Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2000-03-14
2002-05-21
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C501S134000, C502S349000, C502S350000, C502S351000, C502S439000, C252S519120, C252S520200, C252S520210
Reexamination Certificate
active
06391276
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a titania-zirconia or titania-zirconia-alumina powder and a process for producing the same. More particularly, it relates to a novel titania-zirconia or titania-zirconia-alumina powder which is suitable as a catalyst carrier or a co-catalyst, particularly for purifying automotive exhaust gases or as a catalyst carrier for use in high temperature gases containing sulfur, and a process for producing the powder.
BACKGROUND OF THE INVENTION
In order to improve the properties of titania powder used as a catalyst carrier or a co-catalyst, zirconia is frequently used as an additive in combination with an alkaline earth metal, a transition element, and a rare earth element.
For example, JP-A-58-143839 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a catalyst for purifying nitrogen oxides comprising (A) a modified complex oxide obtained by treating a titanium-zirconium binary complex oxide with barium, (B) a vanadium oxide, and (C) an oxide and/or a sulfate of at least one element selected from the group consisting of tungsten, molybdenum, tin, chromium, manganese, cerium, and iron. Thus, it is known that the combined use of zirconia provides a catalyst which exhibits high activity in a broad temperature range and at a high space velocity without being influenced by oxygen, SO
x
, carbonic acid gas, steam, halogen compounds, and hydrocarbons present in exhaust gases or by smog and has a small ability of oxidizing SO
2
to SO
3
.
JP-A-8-192051 discloses a catalyst for purifying exhaust gases which comprises a carrier comprising a titanium-zirconium complex oxide, an NO
x
storage component selected from among alkali metals, alkaline earth metals and rare earth elements and supported on the carrier, and a noble metal supported on the carrier. Accordingly, it is known in the art that a catalyst having a titanium-zirconium complex oxide as a carrier is less susceptible to poisoning by sulfate or sulfite ions than a catalyst having an alumina carrier, that a sulfate of an NO
x
storage component generated by the adsorbed sulfate or sulfite ions easily decomposes at a low temperature, and that the titanium-zirconium complex oxide carrier has improved heat resistance and improved acidity compared with a titanium carrier.
In this way numeral attempts have been made to date to impart heat resistance to titania and to improve the surface acidity of titania by addition of zirconia while retaining the resistance of titania against sulfur poisoning. However, it has been unknown that the contemplated effects can be enhanced by solid dissolving zirconia in an anatase phase of titania or by solid dissolving titania in a tetragonal phase of zirconia and that use of such a solid solution powder as a carrier provides a catalyst with markedly increased performance.
JP-B-7-24774 (The term “JP-B” as used herein means an “examined Japanese patent publication”) proposes an oxidizable substance- and/or nitrogen oxide-containing carrier for a catalyst for treating exhaust gases, which comprise an inorganic refractory oxide obtained by heat-treating a material containing a titanium compound and a zirconium compound at 660 to 900° C., the inorganic refractory oxide comprising 20 to 90 mol % of TiO
2
and 10 to 80 mol % of ZrO
2
and containing at least 20% by weight of a titanium-zirconium complex oxide having a crystal structure of ZrTiO
4
. This proposal contemplates providing a carrier which provides a catalyst having high purifying performance and improved heat resistance by taking advantage of the strong solid acidity and higher heat resistance of ZrTiO
4
than the heat resistance of titania alone or zirconia alone. It is essential, therefore, for the inorganic refractory oxide to contain at least 20% by weight of ZrTiO
4
.
However, JP-B-7-24774 does not mention the importance of preventing sulfur poisoning and of high-temperature non-reactivity with an alkaline salt, which is an NO
x
storage component, in NO
x
-storage and reduction type catalysts; still less suggests that it is rather a key point for this importance that titania-zirconia powder be free from basic sites on its surface.
It is utterly unknown that high performance could be exerted in terms of the above-mentioned sulfur poisoning resistance and non-reactivity with an NO
x
storage component, not in an area mainly comprising ZrTiO
4
, but in a titania-rich solid solution system mainly comprising an anatase phase or in a zirconia-rich solid solution mainly comprising an tetragonal phase.
JP-A-6-304477 describes an amorphous zirconium-titanium complex oxide (partly crystalline but totally amorphous) having a ZrO
2
: TiO
2
weight ratio of 5:95 to 95:5 and a process for producing the same. However, as is taught in JP-B-7-24774 supra, a zirconium-titanium complex oxide obtained by the general co-precipitation method comes to have a composition mainly comprising ZrTiO
4
on being heat-treated at 700° C. or higher temperatures. The crystal structure according to the present invention (a structure having titania solid-dissolved in a tetragonal phase of zirconia, with a small amount of ZrTiO
4
or a monoclinic phase of zirconia being present around the solid solution grains; hereinafter described in detail) is not obtained by the co-precipitation method. Further, JP-A-6-304477 merely describes the amorphous complex oxide of zirconium and titanium, only reciting the general co-precipitation method as a method of production. Accordingly, it is apparently recognized that the amorphous (partially crystalline but totally amorphous) zirconium-titanium complex oxide disclosed in JP-A-6-304477 would be inferior in heat resistance and alkali resistance when used as a carrier for a NO
x
-storage reduction type catalyst, as is with the oxidizable substance comprising the inorganic refractory oxide disclosed in JP-B-7-24774. It is absolutely unknown that it is important for titania-zirconia powder to be not only amorphous but be capable of maintaining a large specific surface area even after heat treatment.
We find no literature on attempts to improve heat resistance of titania powder without changing its characteristic that there are only acidic sites with few basic sites on the surface. It is also unknown that a titanium-zirconium oxide solid solution having zirconia solid-dissolved in an anatase phase of titania or having titania solid-dissolved in a tetragonal phase of zirconia exhibits both excellent heat resistance and sulfur poisoning resistance.
With regard to an oxide solid solution, JP-A-9-221304 discloses a cerium-zirconium oxide solid solution but has no mention of an oxide solid solution having zirconia or titania solid-dissolved in titania or zirconia, respectively.
Commercially available titania powder (titania powder having an anatase crystalline phase) is so inferior in heat resistance that it extremely reduces the specific surface area on heating at high temperature. That is, it is unable to maintain a desired specific surface area after completion of high-temperature treatment. None of available titania powders, except of TiO
2
—SiO
2
system, maintains a specific surface area of 45 m
2
/g or more when heated at 800° C. for 5 hours in the air.
JP-A-9-926 discloses a catalyst for purifying exhaust gases which comprises a carrier of a Ti—Zr—Al complex oxide having supported thereon an NO
x
storage component and a noble metal catalyst. It is hence known as disclosed that a combination of titania, zirconia, and alumina enjoys both the effects of the Ti—Zr complex oxide in preventing sulfur poisoning and improving heat resistance and the effects of an aluminum oxide in further improving the heat resistance. However, it is not known in the art that these effects produced by the Ti—Zr—Al ternary oxide in prevention of asulfur poisoning and heat resistance improvement can be enhanced where the Ti—Zr complex oxide in this ternary oxide system is an oxide solid solution having zirconia or titania solid-dissolved in titania or zirconia, respectively.
Ando Chika
Hatanaka Miho
Kandori Toshio
Suda Akihiko
Takahashi Naoki
Kabushiki Kaisha Toyota Chuo Kenkyusho
Nguyen Cam N.
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