Method for low temperature catalytic production of hydrogen

Details Method for low temperature catalytic production of hydrogen Method for low temperature catalytic production of hydrogen

2001-03-26

2003-07-22

Langel, Wayne A. (Department: 1754)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C423S648100, C423S655000

Reexamination Certificate

active

06596423

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for the production of hydrogen. More specifically, this invention relates to a catalytic process for the production of hydrogen at low temperatures for use in methanol or proton exchange membrane fuel cells.
2. Description of the Related Art
Fuel cells combine hydrogen and oxygen without combustion to form water and to produce direct current electric power. The process can be described as electrolysis in reverse. Fuel cells have been pursued as a source of power for transportation because of their high energy efficiency, their potential for fuel flexibility, and their extremely low emissions. Fuel cells have potential for stationary and vehicular power applications; however, the commercial viability of fuel cells for power generation in stationary and transportation applications depends upon solving a number of manufacturing, cost, and durability problems.
The most promising fuel cells for widespread transportation use are Proton Exchange Membrane (PEM) fuel cells. PEM fuel cells operate at low temperatures, produce fast transient response, and have relatively high energy density compared to other fuel cell technologies. Any fuel cell design must: (a) allow for supply of the reactants (typically hydrogen and oxygen); (b) allow for mass transport of product (water) and inert gases (nitrogen and carbon dioxide from air), and (c) provide electrodes to support catalyst, collect electrical charge, and dissipate heat.
Proton exchange membranes (PEM) fuel cells that typically utilize Pt on carbon support (Pt/C) as anode electrocatalyst operate at a lower temperature of 80° C. hold commercial promise. For methanol fuel cells, H
2
feed can be produced via one of the following reactions:
CH
3
OH+H
2
O→3H
2
+CO
2
&Dgr;H=+49.4
kJ.mol
−1
  (1)
CH
3
OH+½O
2
→2H
2
+CO
2
&Dgr;H=−192.2
kJ.mol
−1
  (2)
Steam reforming of methanol in Reaction 1 is carried out at temperatures greater than 280° C. over supported Cu/Zn catalysts as described by Velu, Suzuki and Osaki in Chem. Communications, No. 23, 2341-2342 (1999). Partial oxidation of methanol in Reaction 2 is also feasible and the reaction is exothermic. See Cubeiro and Fierro in Journal of Catalysts, 179, 150-162 (1998). However, a shortcoming of the above process is that the hydrogen feed produced in this manner has a high content of carbon monoxide (CO). It is known that Pt is readily poisoned by CO. Therefore, a major challenge to the commercializing of the PEM fuel cell technology is to produce H
2
that is essentially free of CO. Several catalysts of the type Pt—Ru/C or Pt—Mo/C, have been formulated to increase CO tolerance of the Pt catalyst as discussed in a review article by Mukerjee, et al., Electrochemical and Solid-State Letters. 2(1) 12-15 (1999). But even at a CO content of 100 ppm in the H
2
feed, severe catalyst poisoning is observed.
H
2
produced via Reaction 1 or 2 contains more than 100 ppm CO. Currently, a catalytic water-gas-shift (WGS) step as illustrated by Reaction 3 is added to remove CO to acceptable levels (<20 ppm) prior to feeding H
2
to the fuel cell.
 CO
(g)
+H
2
O
(g)
<=>H
2(g)
+CO
2(g)
&Dgr;H=−39.4
kJ.mol
−1
  (3)
Reaction 3 is typically catalyzed by promoted iron oxides at temperatures greater than 300° C. as discussed by C. L. Thomas, in “Catalytic Processes and Proven Catalysts”, Academic Press, New York, 1970. As a result, such high temperature pretreatment unnecessarily adds cost to the process. Moreover, in the gas phase, Reaction 3 is in an equilibrium that invariably leaves some CO in the product H
2
stream.
Accordingly, there is still a need in the art of PEM fuel cells to utilize hydrogen that is essentially free of carbon monoxide. Additionally, there is also a need to provide the hydrogen gas in a process that is conducted at low temperature by using inexpensive and simple methods.
OBJECTS OF THE INVENTION
It is, therefore, an object of the present invention to provide an improved process for the production of hydrogen gas.
It is a further object of the invention to provide a catalytic process for the production of hydrogen gas which contains reduced carbon monoxide content.
SUMMARY OF THE INVENTION
The present invention, which addresses the needs of the prior art, provides a process for the catalytic production of a hydrogen feed by exposing a hydrogen feed to a catalyst which promotes a water-gas-shift reaction in a liquid phase. The hydrogen feed can be provided by any process known in the art of making hydrogen gas. It is preferably provided by steam reforming or oxidation of methanol or by any other process that can produce a hydrogen feed for use in proton exchange membrane fuel cells. The step of exposing the hydrogen feed takes place preferably from about 80° C. to about 150° C. Formate is formed when the water-gas-shift reaction is base catalyzed.
The catalyst used in the process of the present invention can be selected from homogenous transition metal complexes. The transition metal of the complex is preferably a metal selected from Group VIII A of the periodic table, including, for example, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt and Cu. The transition metal can be coupled to at least one N donor ligand such as 2,2′-dipyridyl (BIPY), sodium salt of ethylenediamine tetraacetic acid, ethylenediamine, 1,10-phenanthroline, 4,4′-dipyridyl, 1,4,8,11-tetraazacyclotetradecane (CYCLAM), N,N-Bis(2-hydroxybenzyl)ethylenediamine H
4
(SALEN), or mixtures thereof. The catalytic process of the invention is carried out preferably in a highly basic liquid phase such as provided by water, methanol, glyme, polyglycol, other alcohols from C
2
to C
10
or ethers from C
2
to C
10
and mixtures thereof. The liquid phase is made basic by adding bases in an amount sufficient to promote formate formation. The pH of the liquid phase is preferably greater than 8.
As a result of the process of the present invention, a new integrated system that operates at low temperatures is provided. The system consists of two steps: 1) catalyzed methanol decomposition at a temperature of less than 150° C. to produce 1 mol CO and 2 mol H
2
followed by, 2) fast and complete CO conversion to CO
2
with concomitant production of 1 mol of H
2
via the present invention. The present integrated system thus produces 3 mol H
2
/mol methanol at low temperature of less than 150° C. compared to schemes for methanol fuel cell systems that are under development.
Other improvements which the present invention provides over the prior art will be identified as a result of the following description which set forth the preferred embodiments of the present invention. The description is not in any way intended to limit the scope of the present invention, but rather only to provide the working example of the present preferred embodiments. The scope of the present invention will be pointed out in the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a process for the catalytic production of hydrogen feed at low temperatures for use in proton exchange membrane fuel cells. More specifically, the gaseous feed formed by the process of the present invention is hydrogen rich and contains very low levels of carbon monoxide.
In the process of the present invention a hydrogen feed can be formed by any process known in the art. A hydrogen feed is preferably formed by steam reforming or oxidation of methanol, methane or biomass. Hydrogen feed can also be obtained from gasification of coal and other carbonaceous materials including, without limitations, wastes of organic materials, plastics, farm, wood chips and other industrial wastes. Once formed, the hydrogen feed is exposed to a catalytic liquid phase homogeneous systems to achieve a water-gas-shift reaction for CO removal to levels less than 50 ppm. In the reaction known as water-gas-shift, water is reacted with carbon

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