Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Sulfur or compound containing same
Reexamination Certificate
2002-02-04
2004-01-06
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Sulfur or compound containing same
C502S217000, C502S326000, C502S339000, C502S350000
Reexamination Certificate
active
06673739
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalyst for CO shift reaction by which hydrogen (H
2
) is formed from carbon monoxide (CO) and water vapor (H
2
O), and more particularly, to a catalyst for CO shift reaction, which exhibits high activity under the conditions where the space velocity is high and the amount of water vapor is small, and can be used for fuel cells and purifying exhaust gases from an internal combustion engines.
2. Description of Related Art
The CO shift reaction has been applied to the synthesis of ammonia, removal of CO from city gas, adjustment of CO/H
2
ratio in the synthesis of methanol and oxosynthesis, or the like. And recently, the CO shift reaction has been also used to remove CO in fuel reforming systems of internal reforming fuel cells. As shown in Reaction equation 1, the CO shift reaction is the reaction of forming H
2
from CO and H
2
O, and is also referred to as water gas shift reaction.
Examples of the catalyst for promoting the CO shift reaction include Cu—Zn catalysts which were reported by Girdler and Dupont Companies in the 1960s, and have been widely used in plants or the like. And in W. Hongli et al, China-Jpn.-U.S. Symp. Hetero. Catal. Relat. Energy Probl., B09C,213 (1982), there is reported that the catalyst obtained by reducing a catalyst in which Pt is supported by an anatase titania carrier at about 500° C. exhibits higher CO shift reaction activity.
In addition, it has been known that catalysts in which &ggr;-Al
2
O
3
supports noble metal such as Pt, Rh, Pd or the like have CO shift reaction activity. Furthermore, it has been also reported that catalysts in which &ggr;-Al
2
O
3
supports Cu have higher CO shift reaction activity, as compared to the catalysts in which &ggr;-Al
2
O
3
supports noble metal such as Pt, Rh, Pd or the like.
In fuel reforming systems of the internal reforming fuel cells for motor vehicles or other moving bodies, or exhaust gas purifying systems adapted to reform CO in exhaust gases of motor vehicles to H
2
, and reduce NO
x
adsorbed on catalysts using the reformed H
2
, the dimensions of catalytic reactors therefore are limited, and consequently, the catalysts for the CO shift reaction need to exhibit high activity even under the reaction conditions where the space velocity is high.
These conventional Cu—Zn catalysts, however, have the problem that the activity thereof is low under the reaction conditions where the space velocity is high. Accordingly, under the reaction conditions where the space velocity is high, such as those in the fuel reforming systems of the internal reforming fuel cells, or the exhaust gas purifying systems, it becomes difficult to convert CO to H
2
efficiently.
And the reaction expressed by the reaction equation 1 is an equilibrium reaction, and accordingly, as the reaction temperature rises, the reaction in the direction of the left-directed arrow mainly occurs to block the conversion of CO and H
2
O to H
2
. Accordingly, if the reaction temperature is increased to improve the activity of the Cu—Zn catalyst under the reaction conditions where the space velocity is high, the conversion of CO and H
2
O to H
2
does not occur efficiently.
Furthermore, where the catalyst for the CO shift reaction is used in the fuel reforming systems of the internal reforming fuel cells, or the exhaust gas purifying systems of motor vehicles, the reactor may become a high temperature atmosphere temporarily due to the use conditions. In such cases, the problem also arises that the particle size of Cu as the active site of the Cu—Zn catalyst, or Cu in the catalyst in which &ggr;-Al
2
O
3
supports Cu, readily becomes larger, and consequently, the activity of the catalyst decreases. The efficient conversion of CO and H
2
O to H
2
becomes further difficult.
Furthermore, where the catalysts for CO shift reaction are used in the fuel reforming systems of the internal reforming fuel cells, as the concentration of H
2
O increases, the reaction of forming H
2
readily proceeds in the reaction of the reaction equation 1. Accordingly, Cu—Zn catalysts or the like have been generally used under the conditions where the H
2
O/CO ratio is 2 or more.
However, in order to carry out this reaction in limited environments such as motor vehicles, water tanks capable of storing a large amount of water and a large-sized evaporator or the like are needed, and consequently, the device becomes undesirably huge. In addition, in order to supply water vapor, a large amount of energy is needed to evaporate water, and consequently, the energy efficiency of the overall system decreases. Accordingly, it is desired to carry out the reaction with water vapor of which the amount is as small as possible. However, when the H
2
O/CO ratio decreases in the conventional catalyst for CO shift reaction, the activity thereof decreases, and consequently, the obtained H
2
is less than an equilibrium value.
Under these circumstances, it has been contemplated to use noble metals which are estimated to exhibit high activity, and to be stable in an elevated temperature atmosphere, as compared to a base metal. As described above, the activity of catalysts in which &ggr;-Al
2
O
3
supports noble metals such as Pt, Rh, Pd or the like is, however, lower than that of the catalysts in which &ggr;-Al
2
O
3
supports Cu. And it has been known that in the catalysts in which anatase titania as a carrier supports Pt, a strong metal support interaction (SMSI) occurs between Pt and anatase titania. Accordingly, where these catalysts are exposed to reaction gases in a low temperature region of 200° C. to 400° C., Pt is covered with one part of a carrier material due to the strong metal support interaction (SMSI, and consequently, active points reduce to lower the activity thereof remarkably.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide practical catalysts which exhibit especially high CO shift reaction activity in a low temperature region where the reaction of CO and H
2
O results in a favorable conversion to H
2
in equilibrium.
The catalyst for CO shift reaction in accordance with the present invention is characterized in that the catalyst includes a carrier composed of titania as a main component, a noble metal supported on the carrier, and a sulfur-containing material adhering to the carrier.
It is preferable that the sulfur-containing material adheres such that the amount of pure sulfur ranges from 0.01 to 2.0 weight % of the carrier. And it is preferable that the noble metal is platinum.
Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification.
REFERENCES:
patent: 4483943 (1984-11-01), Windawi et al.
patent: 4595703 (1986-06-01), Payne et al.
patent: 4920088 (1990-04-01), Kolts
patent: 4956330 (1990-09-01), Elliott et al.
patent: 5066632 (1991-11-01), Baird, Jr. et al.
patent: 5610117 (1997-03-01), Horiuchi et al.
patent: 2002/0107142 (2002-08-01), Yamazaki et al.
Wang Hongli, et al., “The SMSI and Catalytic Activity for Water Gas Shift Reaction of Pt TiO System”, China-Japan-U.S. Symposium Hetero. Catal. Relat. Energy Probl. B09C, pp. 213-217 (no month avail.).
Suda Akihiko
Yamazaki Kiyoshi
Bell Mark L.
Hailey Patricia L.
Kabushiki Kaisha Toyota Chuo Kenkyusho
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