Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2000-05-09
2003-04-29
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S437200
Reexamination Certificate
active
06555088
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a method of catalytic conversion of carbon monoxide with water to carbon dioxide and hydrogen in a gas mixture that contains hydrogen and other oxidizable components. In a further aspect, the present invention also relates to a catalyst suitable for the catalytic conversion.
The conversion of carbon monoxide with water to carbon dioxide and hydrogen in the presence of catalysts is a known method of producing hydrogen-rich gas mixtures, which is based on the following exothermic reaction:
CO+H
2
O⇄H
2
O+CO
2
&Dgr;H>0 (1)
In the above system the following side reactions can occur:
CO methanation: CO+3H
2
⇄CH
4
+H
2
O &Dgr;H>0 (2)
and
CO
2
methanation: CO
2
+4H
2
⇄CH
4
+H
2
O &Dgr;H>0 (3)
The reaction in accordance with reaction equation (1) is called carbon monoxide conversion or CO conversion herein. The term “water gas shift reaction” is commonly used for this.
To production of hydrogen-rich gas mixtures from hydrocarbons or alcohols by steam reforming, partial oxidation or autothermal reforming is a known process. These gas mixtures (reformates) contain 1 to 40 vol % carbon monoxide, depending on the method that is used.
To use the reformate as fuel in fuel cells, it is necessary to reduce the carbon monoxide contained in them as far as possible, in order to avoid poisoning of the platinum-containing anode catalyst of the fuel cell in the oxidation of the hydrogen. In addition, the conversion of carbon monoxide in accordance with reaction equation (1) leads to an increase of the hydrogen content of the reformate and thus to an improvement of the efficiency of the overall process.
For reasons of size and weight, catalysts for conversion of carbon monoxide with very high activity and selectivity are required for use in motor vehicles. The high space-time yields that can be achieved by this allow the volume of the reactors that are required to be kept small.
The known catalysts for the conversion of carbon monoxide have chiefly been developed for stationary industrial applications. The emphasis lay in the production of pure hydrogen, ammonia and other large scale products that are based on the use of synthesis gas mixtures (CO/H
2
). Catalysts for the conversion of carbon monoxide in accordance with reaction equation (1) are also called shift catalysts herein.
These known catalysts are full body catalysts that contain non-noble metals. They are used in two stage processes. In the first process stage, a so called high-temperature CO conversion (high-temperature water gas shift, HTS) is carried out on Fe/Cr catalysts at temperatures between 360 and 450° C. In the subsequent second stage, a low-temperature CO conversion (low-temperature water gas shift, LTS) is undertaken on Cu/ZnO catalysts at temperatures between 200 and 270° C. After the low temperature process stage, carbon monoxide concentrations of less than 1 vol % in correspondence with the thermal equilibrium are obtained.
The conventional catalysts for the conversion of carbon monoxide have crucial disadvantages:
The described two-stage conduct of the process is necessary because of the properties of these catalysts. While Cu/ZnO-containing catalysts become deactivated about 270° C. because of recrystallization, or sintering, of the copper, the Fe/Cr-containing catalysts that are used in the high temperature range cannot be used at low temperatures because of insufficient activity. If the indicated temperature range of the high temperature catalysts is exceeded, methanation reactions (reaction equations (2) and (3)) can occur, which reduce the selectivity of the high temperature catalysts and, because of this, lower the overall efficiency of the hydrogen generation system.
Both the known high-temperature and the low-temperature catalysts are bulk catalysts in which the catalyst material is pressed to form pellets or other molded bodies. Accordingly, they consist entirely of catalytically active mass and are also called complete catalysts. As a rule, they have a very high bulk weight.
The known industrial methods for conversion of carbon monoxide on catalysts according to reaction equation (1) operate at space velocities of the gas mixture between 300 and 3000 h
−1
. These low velocities are not sufficient for use in motor vehicles.
High bulk weights and low space velocities lead to low specific conversion rates R
CO
for the carbon monoxide, which is understood within the scope of this invention to mean the amount of carbon monoxide N
CO
converted per weight of the catalyst m
cat
and reaction time &Dgr;t. The weight of the catalyst here is given in grams, the reaction time in seconds and the amount of carbon monoxide in moles:
R
CO
=
n
c0
m
Cat
Δ
t
[
mol
g
·
s
]
(
4
)
The known Cu/ZnO and Fe/Cr catalysts have to be activated by reduction before they are used. The activated catalysts are sensitive to oxygen. Upon contact with atmospheric oxygen, they are reoxidized and deactivated in an exothermic reaction.
In comparison with the just described industrial high temperature and low temperature catalysts based on Fe/Cr or Cu/ZnO, noble metal catalysts for these uses are also known, mainly from the scientific literature.
In “The Chemistry and Catalysis of the Water Gas Shift Reaction. 1. The Kinetics over Supported Metal Catalysts,” J. Catal. 67 (1980) 90-102, D. C. Grenoble et al. describe powdered catalysts that contain Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh or Ir as active components and that are deposited on aluminum oxide (Al
2
O
3
) as a support material. The kinetic tests gave a reaction order of about 0.2 for carbon monoxide and about 0.5 for the water that was used.
In “Methanization and Water Gas Shift Reactions over Pt/CeO
2
,” J. Catal. 96 (1985), 285-287, Steinberg et al. observed poor selectivities in view of the carbon monoxide conversion according to reaction (1). Accordingly, the product gas mixture contains high proportions of methane.
In “Water gas shift conversion using a feed with a low steam to carbon monoxide ratio and containing sulfur,” Catal. Today 30 (1996) 107-118, J. Ross et al. investigate a Pt/ZrO
2
catalyst, in addition to Fe/Cr, Cu/ZnO and Co/Cr catalysts. This catalyst shows a carbon monoxide conversion of 50% at 320° C. The Pt/ZrO
2
catalyst shows the highest tolerance for sulfur-containing compounds among the tested compounds. It shows a conversion of 25% at 300° C. and a conversion of 70% at 350° C. This corresponds to a specific carbon monoxide conversion rate R
CO
(300° C.)=7.00 ×10
−6
mol/g
cat
·sec), or R
CO
(350° C.)=1.95×10
−5
mol/g
cat
·sec).
FR 2567866 A describes a copper- and/or palladium-containing catalyst on a carrier of ZnAl
2
O
4
spinel, which is obtained by impregnating the spinel formed into particles with diameters between 0.4 and 0.6 mm with solutions of copper and/or palladium and calcining it. A conversion of 86% is achieved with this catalyst at pressures of 40 bar and a temperature of 250° C. at a very high excess of water (H
2
O/CO=10).
The powdered catalyst systems that have been investigated in the scientific literature are not suitable for industrial use.
The known complete catalysts in the form of tablets, pellets or irregularly shaped particles are used as so-called bulk catalysts. In addition, the achievable specific conversion rates with these catalysts are low.
It is thus an object of this invention to devise a method for conversion of carbon monoxide in a hydrogen-containing gas that enables, under the conditions of mobile use in motor vehicles with their rapidly changing performance requirements, a high specific conversion rate for carbon monoxide with good selectivity, a high temperature stability and an insensitivity to oxygen in the educt gas mixture.
In particular, another object of the present invention is to achieve operational readiness as fast as possible after a cold start.
Another ob
Baumann Frank
Wieland Stefan
DMC2 Degussa Metal Catalysts Cerdec AG
Kalow & Springut LLP
Langel Wayne A.
Medina Maribel
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