�Fuel cell reactions are very demanding processes that require very
pure hydrogen,� said Brookhaven chemist Jose Rodriguez. �You need to
find some way to eliminate the impurities, and that�s where the
water-gas shift reaction comes into play.�
The �water-gas shift� (WGS) reaction combines CO with water to produce
additional hydrogen gas and carbon dioxide. With the assistance of
proper catalysts, this process can convert nearly 100 percent of the
CO into carbon dioxide. Rodriguez�s group, which includes researchers
from Brookhaven�s chemistry department, the Center for Functional
Nanomaterials (CFN), and the Central University of Venezuela, studied
two �next-generation� WGS nanoscale catalysts: gold-cerium oxide and
gold-titanium oxide.
�These nanomaterials have recently been reported as very efficient
catalysts for the WGS reaction,� said Brookhaven chemist Jan Hrbek.
�This was a surprising finding because neither bulk gold nor bulk
ceria and titania are active as catalysts.�
To determine how these nanocatalysts work, the research team developed
so-called �inverse model catalysts.� The WGS catalysts usually consist
of gold nanoparticles dispersed on a ceria or titania surface � a
small amount of the expensive metal placed on the inexpensive oxide.
But to get a better look at the surface interactions, the researchers
placed ceria or titania nanoparticles on a pure gold surface.
�For the first time, we established that although pure gold is inert
for the WGS reaction, if you put a small amount of ceria or titanium
on it, it becomes extremely active,� Rodriguez said. �So although
these inverse catalysts are just models, they have catalytic activity
comparable to, and sometimes better than, the real deal.�
Using a technique called x-ray photoelectron spectroscopy at
Brookhaven�s National Synchrotron Light Source, as well as scanning
tunneling microscopy and calculations, the researchers discovered that
the catalysts� oxides are the reason for their high activity.
�The oxides have unique properties on the nanoscale and are able to
break apart water molecules, which is the most difficult part of the
WGS reaction,� Hrbek said. Added Brookhaven physicist Ping Liu: �After
you dissociate the water, the reaction continues on to eliminate CO.
But if you don�t have nanosized oxide particles, none of this will
work.�
The researchers plan to continue their study of these catalysts at the
NSLS and CFN in order to further explore the reaction mechanism and
optimize its performance.
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