Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-01-12
2003-01-28
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S626000, C423S627000, C423S628000, C502S304000, C502S325000, C502S336000, C502S340000, C502S349000, C502S350000, C502S406000, C502S415000
Reexamination Certificate
active
06511642
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a porous material of an oxide and/or a complex oxide mainly comprising alumina, zirconia, titania, magnesia, iron oxide or ceria, a process of producing the porous material, a catalyst for purifying exhaust gases comprising the porous material, and a method of purifying exhaust gases.
More specifically, it relates to a porous material having a spongy structure which is suitable for use as a catalyst, a carrier for catalysts, various fillers, a carrier for enzymes, an adsorbent, a filler, and so forth and which is characterized in that (1) the mean pore diameter is in a meso-pore region, (2) the pores have a sharp size distribution, (3) at least a part of the pores form a three-dimensional network structure, and (4) the porous material has substantially no fibrous structure, and a porous material having the above characteristics (1) to (4) which is made up of particles having an aspect ratio of 3 or smaller aggregated together while leaving pores among them; and a process for producing these porous materials.
The present invention also relates to a catalyst and a method for purifying exhaust gases from internal combustion engines of automobiles and the like. More specifically, it relates to a three-way catalyst used for engines run around a stoichiometric air/fuel ratio and a catalyst used for so-called lean-burn engines operated in an oxygen-excess atmosphere. Still more specifically, the invention relates to a three-way catalyst for purifying exhaust gases from conventional engines through simultaneous reduction/oxidation of carbon monoxide (CO), hydrogen (H
2
), hydrocarbons (HC), and nitrogen oxides (NO
x
), a catalyst for efficiently reducing nitrogen oxides (NO
x
) in oxygen-excess exhaust gases which contain oxygen in excess of the amount required to completely oxidize the reducing components, such as carbon monoxide (CO), hydrogen (H
2
), and hydrocarbons (HC), and a method for purifying exhaust gases.
2. Description of Related Art
The present invention covers the field of a porous material and the field of exhaust gas purification. Disadvantages or drawbacks of related arts are described below separately.
With respect to an alumina porous material having an appropriate pore structure, JP-A-58-190823 and JP-A-60-54917 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) disclose alumina carriers which have a large pore size with a narrow pore size distribution and yet exhibit excellent mechanical strength.
JP-A-55-27830 teaches a process for producing an alumina porous material having the above-described pore structure, which comprises repeating the steps of adjusting the pH of an aluminum hydroxide slurry to 5 or lower, or 11 or higher and then adjusting the pH to 6 to 10 by addition of a neutralizing agent (a pH swing method). Analogous methods are disclosed in JP-A-58-190823 and JP-A-60-54917.
In regards to a silica porous material having an appropriate pore structure, JP-B-7-64543 (the term “JP-B” as used herein means an “examined Japanese patent publication”) discloses spherical silica particles having a pore volume of 0.8 to 1.8 ml/g, a surface area of 20 to 500 m
2
/g, and an average pore size of 80 to 1000 Å. It also teaches a process for preparing the silica porous material comprising drying silica hydrogel, obtained by neutralization of an aqueous alkali silicate solution, with superheated steam at 100 to 1000° C. to give silica xerogel. According to the disclosure, it is preferred that the silica hydrogen be previously aged under 0.5 to 5 kg/cm
2
of steam for 0.5 to 24 hours.
As for a zirconia porous material, JP-A-8-66631 discloses porous zirconia particles having a sharp pore size distribution that is an important character for use in liquid chromatography, which are obtained by incorporating 0.1 to 10% by weight of silica into zirconium oxide so that the crystal form of zirconium oxide may be prevented from changing during calcination.
With reference to a titania porous material, JP-A-6-340421 proposes needle-like, porous, and fine titanium oxide particles having an average breadth of 80 to 120 Å, an average length of 240 to 500 Å, and an aspect ratio of 2.4 to 6.4, which is produced by a process comprising the steps of (a) allowing a hydrolyzable titanium oxide compound to react with a base to precipitate hydrated titanium oxide, (b) adding a polybasic carboxylic acid to the reaction system to dissolve the hydrated titanium oxide, (c) adding an alkali to the reaction system to hydrolyze the chelated titanium compound, (d) adding an inorganic acid to the precipitate and stirring the system to deflocculate, and (e) dehydrating the resulting fine particles and calcining at 200 to 400° C.
Concerning a magnesia porous material, JP-A-59-232915 discloses a process for producing spinel comprising adjusting the pH of a mixed aqueous solution of a water-soluble magnesium salt and a water-soluble aluminum salt with an alkali in the presence of an alcohol to form a precipitate and drying and calcining the precipitate.
As regards an iron oxide porous material, JP-A-61-268358 describes an iron oxide porous material comprising iron oxide and chromium oxide, having a large pore size with a narrow pore size distribution, and exhibiting excellent durability against oxidation and reduction. Similar prior arts are found with respect to a ceria porous material.
According to the above-mentioned pH swing method, which is substantially a method of producing alumina, the pH of boehmite (AlOOH), a precursor, is swung by use of an acidic material and an alkaline material to cause crystals to dissolve and to precipitate alternately thereby letting the crystals grow in a porous fibrous shape with a narrow pore size distribution. However, because the pH should be swung many times, the process is time-consuming and meets difficulty in controlling the conditions for product consistency. Further, when a second component is to be incorporated, it once settles but is then solubilized because of the pH variations, failing to be uniformly dispersed. Or, where a desired second component is such that forms a precipitate at a pH out of a range of from 6 to 11, it is impossible to incorporate the second component into the precursor. Furthermore, the conventional pH swing method does not provide an alumina porous material having a spongy structure nor a porous material comprising an aggregate of particles having an aspect ratio of 3 or smaller.
In particular, the porous materials described in JP-A-58-190823 and JP-A-60-54917 are composed of fibrous particles. When used as a catalyst carrier, a porous material comprising an aggregate of fibrous particles might be capable of supporting a noble metal in a high disperse state. However, as will be explained later in more detail, there will be a certain crystal plane along the fiber length direction so that the catalyst component tends to be supported on that plane in an increased proportion. This helps the catalyst component agglomerate in high temperature.
The spherical silica proposed in JP-B-7-64543 supra is composed of amorphous particles. Where used as a catalyst carrier, it provides no crystal plane to support a noble metal in a high disperse state. It follows that the noble metal particles easily move on the catalyst surface to undergo sintering, resulting in reduction of activity. Silica has lower affinity to noble metal than, for example, alumina, which also contributes to sintering of the supported noble metal particles and reduction of activity. Additionally, where the silica porous material is used in a three-way catalyst, coking occurs to deactivate the catalyst.
None of the aforementioned other prior arts relating to zirconia, titania, magnesia, iron oxide or ceria porous materials proposes a porous material having a spongy structure characterized in that (1) the mean pore diameter is in a meso-pore region, (2) the pores have a sharp size distribution, (3) at least a part of the pores have a three-di
Akimoto Yusuke
Hatanaka Miho
Suda Akihiko
Tanaka Toshiyuki
Terao Naohiro
Kabushiki Kaisha Toyota Chuo Kenkyusho
Medina Maribel
Silverman Stanley S.
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