Method of using catalyst for steam reforming of alcohols

Details Method of using catalyst for steam reforming of alcohols Method of using catalyst for steam reforming of alcohols

1999-10-07

2002-07-02

Griffin, Steven P. (Department: 1754)

Compositions

Gaseous compositions

Carbon-oxide and hydrogen containing

C423S648100, C423S652000

Reexamination Certificate

active

06413449

ABSTRACT:

The present invention relates to a catalyst for the steam reforming of alcohols, which catalyst contains a palladium/zinc alloy and zinc oxide as catalytically active components. The catalyst is used in particular for the steam reforming of methanol to produce a hydrogen-rich gas that can be used as a fuel for vehicles powered by fuel cells.
The steam reforming of methanol in the presence of catalysts is a known process for producing hydrogen-rich gas mixtures, and is based on the following endothermic reaction:
Steam reforming of methanol:
CH
3
OH+H
2
O→3H
2
+CO
2
&Dgr;H>0  (1)
The following secondary reactions may occur:
Reforming of methanol by methanol cleavage:
CH
3
OH→CO+2H
2
&Dgr;H>0  (2)
and
CO conversion:
CO+H
2
O
CO
2
+3H
2
&Dgr;H<0  (3)
In the steam reforming according to reaction equation (1) the steam is used in excess. The so-called “steam to carbon ratio” (S/C) is used to characterize the excess water that is used. Normally a value for S/C of between 1.2 and 2.0 is chosen. In the case of the reforming of methanol S/C is identical to the molar ratio of water to methanol.
For use in fuel cells gas mixtures are required that have a low carbon monoxide content with a high hydrogen content, since carbon monoxide deactivates the anode catalyst at which the oxidation of the fuel takes place. Normally amounts of carbon monoxide in the fuel of below 100 ppm, preferably less than 10 ppm, are required.
If the fuel is obtained by reforming methanol, this requirement can at the present time only be met by a subsequent purification of the reformate gas. The effort and expenditure involved are less the lower the carbon monoxide content in the reformate gas.
For use in vehicles, for reasons of space and weight reforming catalysts are required that have a very high specific hydrogen productivity and a high selectivity, the selectivity of the formation of carbon dioxide being used to characterize the selectivity of the steam reforming.
The specific hydrogen productivity P
Cat
of the catalyst is understood within the scope of the present invention to denote the volume V
H2
of hydrogen produced per unit mass M
Cat
of the catalyst and reaction time t, wherein the catalyst mass is expressed in kilograms, the reaction time is expressed in hours, and the volume is expressed in standard cubic metres:
P
Cat
=
V
H2
M
Cat
·
t
⁡
[
Nm
3
kg
Cat
·
h
]
(
4
)
The carbon dioxide selectivity S
CO2
of the steam reforming is calculated with the aid of the partial pressures of the carbon dioxide P
CO2
and carbon monoxide P
CO
that are formed
S
CO2
=
P
CO2
P
CO2
+
P
CO
⁢
 
[
%
]
(
5
)
A high specific activity is the precondition for achieving a high space-time yield, which enables the volume of the reactor used in the steam reforming to be kept small. The space requirement for the gas purification can also be reduced by a high selectivity.
EP 0687648 A1 describes a two-stage process for carrying out the methanol reforming, in which the methanol is incompletely converted in the first stage in a heat transmission-optimized process at a high specific catalyst loading, followed by reaction in a conversion-optimized second stage at a lower specific catalyst loading that completes the methanol conversion. In the first stage the catalyst is charged as high as possible, preferably to produce more than 10 Nm
3
/h H
2
per kilogram of catalyst. Pellet catalysts and also catalyst-coated metal sheets are proposed as catalyst forms.
Catalysts comprising the base metals copper, zinc, chromium, iron; cobalt and nickel are predominantly used for the methanol reforming. Catalysts based on CuO/ZnO, with which selectivities of more than 95% can be achieved, are particularly advantageous. Catalysts are known that consist completely of CuO and Zno and that can be obtained for example by co-precipitation from a solution of copper nitrate and zinc nitrate. After the co-precipitation the metal obtained is normally calcined in air in order to decompose and convert the precipitated compounds of the metals into the corresponding oxides. Finally the catalyst is reduced, for example, in the gaseous phase.
Alternatively so-called supported catalysts may also be used, in which a porous support or a finely divided, porous support material is impregnated with solutions of copper nitrate and zinc nitrate, and then calcined and reduced. In these cases aluminum oxide is mainly used as a support or support material, although zirconium oxide, titanium oxide, zinc oxide and zeoliths may also be used.
The finely divided catalyst materials thus obtained are as a rule processed into spherical shaped bodies, so-called pellets, or applied in the form of a coating to carrier bodies. These catalysts are hereinafter termed coated catalysts in order to distinguish them from the pellet catalysts. The processes known in the production of monolithic vehicle exhaust gas catalysts, may for example, be used to coat the carrier bodies. To this end the finely divided catalyst material is, for example, dispersed in water, optionally with the addition of suitable binders. The carrier bodies are then coating with the catalyst material by immersion in the coating dispersion. In order to fix the coating to the carrier body, the coated carrier body is dried and then calcined.
The carrier bodies for the coated catalysts serve only as a substrate for the catalytically active coatings. These carrier bodies are macroscopic bodies that must not be confused with the support material for the catalytically active components. Heat exchange metal sheeting or honeycomb bodies of ceramic materials or metal foils are suitable as carrier bodies. For example, the honeycomb bodies made of cordierite that are also used for purifying exhaust gases from combustion engines may be used for this purpose. These bodies comprise axially parallel flow channels for the reactants arranged in a narrow grid over the cross-section. The number of the flow channels per unit cross-sectional area is termed the cell density. The wall surfaces of these flow channels carry the catalyst coating. From DE 19721751 C1 and EP 0884273 a1 it is known that catalysts based on CuO/ZnO shrink by up to 40% and suffer a loss of specific activity during operation. DE 19721751 C1 solves the problem of shrinkage of catalyst layers on a metal sheet by introducing expansion gaps in the layers. According to EP 0884273 A1 the decreasing activity of a pellet packing of a Cu/ZnO catalyst on an aluminum oxide support can be at least partially reversed by periodic regeneration.
In JP 57007255 A2 (according to CA 96:145940) catalysts are described that are obtained by a two-stage impregnation of zirconium oxide-coated aluminum oxide pellets with one or two metals and/or metal oxides of copper, zinc, chromium, iron, cobalt and nickel, and with platinum or palladium. A typical catalyst contains 10 wt. % of copper oxide, 0.3 wt. % of palladium and 20 wt. % of zirconium oxide on the aluminum oxide pellets.
In addition to the catalysts based on base metals the noble metals of the platinum group, in particular platinum, palladium and rhodium on oxidic support materials such as aluminum oxide, titanium oxide and zirconium oxide, are also used for the reforming of methanol. These catalysts lead to the cleavage of methanol according to reaction equation (2) with a content of carbon monoxide in the product gas of up to 33 vol. %. Such catalysts are less suitable for the steam reforming of methanol. EP 0201070 A2, JP 60137434 A2 (according to CA 104:185977), JP 04362001 A (according to WPI 93-033201) and JP 03196839 A (according to WPI 91-298480) are examples thereof.
JP 60082137 describes a catalyst for the methanol cleavage that contains at least one of the noble metals platinum and palladium on an aluminum oxide support, the support having been coating with zinc oxide and/or chromium oxide in a preliminary treatment. For the preliminary coating the aluminum oxide support is impregnated with an aqueous solution of zin

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