Figure 5. Hydrogen and carbon dioxide mole fraction contour maps in the
micro-structured heat-exchanger reactor for hydrogen production by steam
methanol reforming.
The mesh and methanol mole fraction contour maps in the micro-structured
heat-exchanger reactor are illustrated in Figure 6 for hydrogen
production by steam methanol reforming. The alcohol steam reforming
catalyst converts a lower alcohol such as methanol into hydrogen and
carbon oxides. In the specific case where the lower alcohol is methanol,
the alcohol steam reforming catalyst may be called a methanol steam
reforming catalyst. The generation of carbon monoxide is undesired as
carbon monoxide is a poison to the fuel cell electrode [49, 50].
Even generating small amounts of carbon monoxide can cause significant
degradation [51, 52]. Consequently, any decrease in generating
carbon monoxide is a positive advance in the steam reforming catalyst
field. The alcohol steam reforming catalysts containing yttrium,
palladium, a metal oxide and cerium, and optionally zinc generally
display high carbon dioxide selectivity and thus have reduced or
mitigated generation of carbon monoxide. It is also difficult to improve
both good methanol activity and good carbon dioxide selectivity in
alcohol steam reforming catalysts [53, 54]. Often, improvement in
one of these properties results in a decrease in the other property
[55, 56]. Consequently, merely combining two catalyst components,
one that improves methanol activity and the other that improves carbon
dioxide selectivity, does not typically result in a material that has
both good methanol activity and good carbon dioxide selectivity. Even
small increases in carbon monoxide generation are disfavored because of
the large resultant poisoning effect [57, 58]. The fuel combustion
and steam reforming processes can be stably and efficiently operated at
lower temperatures, without the need for energy input to sustain or even
to start the micro-combustion process. In some instances, the
micro-combustor is started with hydrogen or vapors such as methanol.
Heat losses can be effectively controlled and reduced. Another advantage
is that the simplicity of the design and the materials used enable mass
production at competitive costs. Another advantage is an extremely fast
response time. When a suitable amount of yttrium is included in a steam
methanol reforming catalyst based on palladium, a metal oxide and
cerium, and optionally zinc, a steam reforming catalyst that departs
from the undesired trade-off relationship results. Specifically, steam
reforming catalysts containing yttrium, palladium, a metal oxide and
cerium, and optionally zinc display both desirable properties of a high
methanol conversion rate and high carbon dioxide selectivity. The steam
reforming catalyst contains a suitable amount of at least one metal
oxide and cerium to contribute to high methanol conversion properties.
The cerium may or may not act as a support. In instances where the steam
methanol reforming catalyst is subjected to high temperatures, such as
during firing of a green ceramic substrate coated with the steam
reforming catalyst, two or more metals of the steam reforming catalyst
may form an alloy. If the steam methanol reforming catalyst does not
contain cerium, then the steam reforming catalyst contains at least one
metal oxide.