Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-05-25
2002-08-06
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S326000, C502S327000, C502S328000, C502S331000, C502S332000
Reexamination Certificate
active
06429167
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ruthenium-on-alumina catalysts. More particularly, the invention relates to ruthenium-on-alumina catalysts—in which ruthenium or similar components are carried by an &agr;-alumina porous material—which are advantageously used in a variety of a hydrogen production processes, inter alia, in processes making use of a steam reforming process applied to light hydrocarbons and oxygen-containing compounds for producing synthetic gas, reduction gas suitable for use in direct-reduction iron making, city gas, and hydrogen gas. The ruthenium-on-alumina catalysts of the present invention are also advantageously used in a reformer (for producing hydrogen) contained in a fuel cell.
2. Background Art
In conventional steam reforming or like processes for light hydrocarbons by use of a catalyst, there have generally been used catalysts formed of a transition metal such as nickel carried by a catalyst carrier such as &ggr;-alumina.
In current steam reforming processes, in order to economize construction costs and operation costs, heat flux tends to be raised, whereas steam/carbon ratio (S/C) tends to be decreased. Under such operation conditions, carbon easily precipitates on the catalyst, to cause an increased pressure difference in piping. As a result, the catalyst tube sometimes clogs to make continuing the reaction difficult. Thus, there is strong demand for a catalyst with which the amount of carbon generating on the catalyst is greatly suppressed as compared with that attained by conventional catalysts, while exhibiting high catalytic activities.
Ruthenium-on-alumina catalysts containing ruthenium as a catalytic component have become of interest as catalysts which permit suppressed carbon precipitation thereon and which have enhanced activities. Since ruthenium-on-alumina catalysts exhibit excellent catalytic performance as proven by their high activities and ability to suppress precipitation of carbon even under conditions of a low steam/carbon ratio during operation, Japanese Patent Application Laid-Open (kokai) No. 5-220397, among others, discloses ruthenium-on-alumina catalysts in which zirconium oxide derived from a precursor zirconia sol and a ruthenium component are carried by aluminum oxide containing alkaline earth metal aluminate.
However, the ruthenium-on-alumina catalysts disclosed in the above reference has the problem that their activities are insufficient under reforming conditions of a low steam/carbon (S/C) ratio of 2 or less and at a high temperature of not less than 680° C. Also, since ruthenium is a noble metal of high price, making ruthenium-containing catalysts industrially useful requires, in addition to securing satisfactory catalytic performance, the suppression of ruthenium content so as to reduce catalyst costs. Moreover, in view that steam reforming reactions are performed at high temperature, there is sought development of catalysts that not only have high activities, but are also resistant to heat.
From the point of prevention of environmental destruction caused by air pollution, hydrogen fuels have become of interest as alternate energy sources in place of gasoline, etc. The hydrogen fuels are converted into electric energy by, for example, a fuel cell. Hydrogen which serves as the starting material is generally produced from hydrocarbons or oxygen-containing compounds through a steam reforming process. In particular, in place of hydrocarbons such as city gas and LPG which have conventionally been used in fuel cells, oxygen-containing compounds such as methanol and dimethyl ether have recently come to be expected to serve as fuel for transportation power sources (electric cars). As a catalyst for reforming oxygen-containing compounds, there has been used ruthenium, nickel, or a similar metal carried by (or impregnated in) a catalyst carrier such as alumina.
A typical fuel cell generally contains a reactor for reformation. In recent years, reformation reactors in the form of a plurality of concentric hollow cylinders have acquired popularity, in which catalyst layers are arranged to form cylindrical shells so as to make the entirety of the fuel cell compact and to improve its performance (Japanese Patent Application Laid-Open (kokai) Nos. 3-122001 and 60-264303).
In reformation reactors of the above type, a burner is placed at the center of the innermost shell and a plurality of catalyst layers are disposed so as to surround the burner, to thereby maximize the area of heat transfer surface and reduce the size of the reactor. Therefore, at the time of starting up and stopping the operation of the reactor, the temperature of the inside of a catalyst layer differ greatly from that of the outside of the catalyst layer. The temperature difference induces strain in the circumferencial direction of each shell due to difference in thermal expansion, applying on catalyst layers a compression force which may crush the catalyst under pressure. When the catalyst is crushed, powder generates and clogs catalyst layers and downstream piping, and as a result, operation may be discontinued due to elevated flow resistance.
As a measure for preventing destruction of catalyst under pressure, “Fuel Association Journal,” Vol. 68, No. 3 (1989) discloses from pages 236 to 243 a ruthenium-on-alumina catalyst in which ruthenium serves as a catalyst and &agr;-alumina serves as a catalyst carrier.
However, since the ruthenium-on-alumina catalyst disclosed in this journal uses &agr;-alumina prepared by firing &ggr;-alumina at 1300° C., the molded &agr;-alumina has an insufficient crushing strength for use in multi-shell-type reformation reactors, as they require high crushing strength. Moreover, since ruthenium is carried by &agr;-alumina obtained by firing &ggr;-alumina, the resultant catalyst has a specific surface area of as small as 6.6 m
2
/g, and therefore, even when ruthenium of high activity is used as an active component, ruthenium cannot be sufficiently dispersed on and within the carrier, and as a result, only insufficient catalyst activity can be obtained. Furthermore, when &agr;-alumina is prepared through firing at a sufficiently high temperature so as to increase the crushing strength, the resultant &agr;-alumina of a closest packing structure is generally not suitable as a catalyst carrier, because it does not have micropores of a submicron or smaller size, and in addition, has only a small specific surface area even when it is molded into a catalyst carrier. In other words, when a catalyst carrier constructed of &agr;-alumina is impregnated with an active component, the specific surface area necessary for satisfactorily dispersing active components on and within the carrier is insufficient, and thus, even though the amount of the catalyst component is increased, sufficient activity cannot be obtained.
Also, as described above, Japanese Patent Application Laid-Open (Kokai) No. 5-220397 discloses a ruthenium-on-alumina catalyst in which zirconium oxide derived from a precursor zirconia sol and a ruthenium component are carried by aluminum oxide containing alkaline earth metal aluminate.
However, in consideration that the zirconia sol used in that publication is present in the form of particles of 100 angstroms or more, the zirconium oxide derived therefrom is considered to grow into large particles. Moreover, since the alkaline earth metal aluminate is present as crystals, particles thereof are also considered to grow into large particles. Thus, the catalyst is predicted to have disadvantages of reduced specific surface area and insufficient catalytic activities.
SUMMARY OF THE INVENTION
The present invention was accomplished in view of the aforementioned problems, and the object of the invention is to provide a ruthenium-on-alumina catalyst—in which a porous alumina material which is useful as a catalyst carrier due to its excellent heat resistance and crushing strength is impregnated with an active component, ruthenium—which has a number of notable features: remarkably excellent activ
Maeno Hironobu
Matsumoto Hiroto
Griffin Steven P.
Idemitsu Kosan Co. Ltd.
Nguyen Cam N.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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