Steam reforming catalyst

Details Steam reforming catalyst Steam reforming catalyst

1999-08-24

2001-10-16

Padmanabhan, Sreeni (Department: 1621)

Chemistry of inorganic compounds

Hydrogen or compound thereof

Elemental hydrogen

C252S373000, C502S326000, C568S383000

Reexamination Certificate

active

06303098

ABSTRACT:

TECHNICAL FIELD
This invention relates to steam reforming of hydrocarbons to produce a hydrogen-containing gas suitable as a fuel for a fuel cell.
BACKGROUND OF THE INVENTION
This is an improvement in the invention described in U.S. Pat. No. 5,248,566 issued Sep. 28, 1993, the disclosure of which is herein incorporated by reference and U.S. application Ser. No. 08/867,556 filed Jun. 2, 1997.
Fuel cells are being developed for use in both stationary form and in automotive propulsion systems as alternatives for the internal combustion engine in buses, vans, passenger cars and other vehicles. The major motivations for developing fuel cell powered vehicles are low pollution emissions, high fuel energy conversion efficiencies, superior acceleration, low noise and vibration and the possible use of coal or biomass derived alcohols rather than petroleum-base fuels. Although petroleum based fuels can also be used, the present invention is directed most specifically to systems for using hydrocarbons such as gasoline, diesel fuel or alcohols such as methanol as fuel.
The two most important operational requirements for a stand-alone fuel cell power system for a vehicle are the ability to start-up quickly and the ability to supply the necessary power and demand for the dynamically fluctuating load. The rapid start-up requirement is obvious.
A variety of hydrocarbons are possible fuels for use in transportation applications but most likely alcohols such as methanol are preferred fuels for use in fuel cells for transportation applications. Methanol is a commodity chemical that is manufactured from coal, natural gas and other feed stocks, while ethanol is often produced from grain. For use in a fuel cell, however, alcohols and hydrocarbons must first be converted (reformed) to a hydrogen rich gas mixture. The desired features for such a fuel reformer include rapid start-up, good dynamic response, fuel conversion, small size and weight, simple construction and operation and low cost.
Methanol has been used in steam reforming for providing a hydrogen rich gas stream for mobile combustion engines, see Koenig et al. U.S. Pat. No. 4,716,859, and water, as a reaction product from a fuel cell, has been recycled for use in steam reforming of methanol, see Baker U.S. Pat. No. 4,365,006. Steam reforming of methanol is endothermic and complicates, by its energy requirements, its use in a vehicle. Supplying the hydrogen rich gas on demand in an intermittent variable demanding environment is also a difficult requirement to meet and has been addressed by Ohsaki et al. U.S. Pat. No. 4,988,580 but this suggestion is not applicable to a small, mobile system. The catalytic exothermic partial oxidation-reforming of fuels to produce hydrogen-rich gas streams is known, see Rao U.S. Pat. No. 4,999,993. The use of a partial oxidation reformer had not been used in a vehicle to accomplish the purposes of this invention prior to the disclosure of the Kumar et al. '566 patent which is satisfactory for its intended purposes, but was based on theoretical considerations.
Converting hydrocarbon fuels to hydrogen can be done by steam reforming (reaction of the hydrocarbon with steam) or by partial oxidation (reaction with a substoichiometric amount of air). Steam reforming reactors are fairly bulky and are heat-transfer limited, while the partial oxidation reaction is more rapid and exothermic but less developed.
The parent of this application provided a catalyst for the exothermic partial oxidation reaction and was desirable since the endothermic steam reforming reaction required temperatures of about 1000° C. At lower temperatures, the reactors can be smaller, can be made from less expensive materials like steels which are easier to fabricate and the product gas contains higher concentrations of hydrogen and less carbon monoxide, which is desirable. However, an appropriate catalyst for the endothermic steam reforming reaction has heretofore not been available, while the parent to this application disclosed a catalyst suitable for exothermic partial oxidation reaction. This invention is based on the surprising discovery that catalysts that are effective for the conversion of a wide range of hydrocarbons, including aliphatic, aromatics and others to hydrogen in the exothermic partial oxidation reaction are also effective in the endothermic steam reforming reaction.
SUMMARY OF THE INVENTION
This invention relates to a steam reforming catalyst. More specifically, this invention relates to a catalyst for steam reforming hydrocarbon fuels such as gasoline or diesel fuel to produce a high percentage yield of hydrogen suitable for supplying a fuel cell. The difficulty of converting hydrocarbons (e.g. methane, iso-octane, hexadecane, toluene, etc.), a main component of natural gas, gasoline, and diesel to hydrogen is the fact that the hydrogen/oxygen bond is thermodynamically stronger than the carbon oxygen bond at moderate temperatures. Under thermal equilibrium conditions, the reaction products will therefore be rich in water and poor in hydrogen. In order to produce a hydrogen-rich gas, a bifunctional catalyst is required which can “dehydrogenate” the hydrocarbon molecule, and then selectively oxidize the carbon chain.
This invention relates to the development of a new class of materials that can be used for the steam reforming of hydrocarbons.
The steam reforming process involves the reaction of a hydrocarbon fuel with water (steam) to produce hydrogen and carbon dioxide as per the equation C
n
H
m
O
p
+(2n−p)H
2
O=nCO
2
+(2n−p+m/2)H
2
. The reaction is strongly endothermic, requiring a large amount of heat to be supplied. Steam reforming of natural gas is traditionally done in the petrochemical industry at temperatures in excess of 900° C. The new catalyst, which uses a combination of a metal and an oxide-ion conductor, can achieve the conversion of methane and other types of hydrocarbons at less than 800° C. The lower temperature operation is beneficial for a number of reasons which include: I) wider choice of materials of construction; ii) higher process efficiency, iii) lower yields of carbon monoxide—which must be subsequently converted to carbon dioxide and hydrogen via the water-gas shift reaction; (iv) faster start-up process—essential for applications involving frequent start-up of process—essential for applications involving frequent start/stop cycles as in transportation and in distributed power generation units; etc.
The catalyst of the invention is a cement containing curia as the oxide ion conduction material to convert the carbon to carbon oxides, and ruthenium or platinum as the hydrogen extracting material. The catalyst can be prepared from a high surface area powder of doped curia (Ce
1−x
Ln
x
O
p
or, Ce
1−x−y−z
Ln
x
Cs
y
Li
z
O
p
, where Ln-lanthanide, e.g. La, Sm, Gd, etc., and x≦0.5) and a second powder which could be either a metal (Pt) or a metal oxide which would get reduced in-situ in the reactor. Alternatively, catalyst materials can also be prepared from self-sustaining combustion syntheses.
Other metals include all noble and transition metals. Other oxide ion conducting materials include zirconia, bismuth oxides or vanadates, lanthanum gallate, perovskite containing manganese, cobalt, or others forming oxygen deficient structures.


REFERENCES:
patent: 3997477 (1976-12-01), Takeuchi
patent: 4088608 (1978-05-01), Tanaka et al.
patent: 5929286 (1999-07-01), Krumpelt et al.

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