Chemistry Reference
In-Depth Information
catalysts are based on gold or copper. Gold is widely known as a catalytically
inert material due its highly stable structure, however, Haruta et al. have
shown that gold is extremely active at sizes below 15 nm and in the presence
of 3d transition metal oxides.
192
It has been demonstrated that there exists a
1/d
3
relationship between particle size and CO oxidation.
193
Metal oxide
supports with high oxygen mobility such as TiO
2
,Fe
2
O
3
,NiO
x
, and CoO
x
enhance the catalytic activity of Au.
Conventional Au-based catalysts exhibit reduced catalytic activity in the
presence of CO
2
and H
2
O, typically attributed to a blockage of catalytic sites
by these species. In realistic fuel reforming scenarios, significant amounts of
CO
2
and H
2
O are part of the reformate gas. For methanol steam reforming,
as an example, approximately 25% of the gas is CO
2
and a substantial part of
the water steam remains unreacted. A major challenge of CO-PROX is the
creation of novel catalysts enabling close to complete CO conversion at
temperatures below 100 1C with potentially large amounts of CO
2
and H
2
Oin
the gas mixture. Furthermore, this conversion must be achieved without the
addition of excess O
2
to minimize competitive oxidation of H
2
fuel.
Au/Fe
2
O
3
catalyst nanoparticles calcined at 400 1C achieved 99.5% CO
conversion at 100 1C reaction temperature and with excess O
2
, in the pres-
ence of CO
2
and H
2
O. These nanoparticles were synthesized by wet im-
pregnation, co-precipitation, and sol-gelation.
194
Au/Fe
2
O
3
nanoparticles
have been shown to reach higher catalytic activity after calcination than Au/
TiO
2
and Au/Al
2
O
3
in the presence of H
2
Oat801C.
195,196
Traces of water were
found to increase the catalytic activity of Au/Fe
2
O
3
nanostructures, but larger
amounts of water required significantly higher reaction temperatures.
197
Gold nanostructures can be catalytically active at low temperatures, even
down to 0 1C.
198,199
The size and dispersion of these nanocrystalline gold
particles are essential parameters determining the catalytic activity towards
CO oxidation.
200
A recent study demonstrated that the negative effect of CO
2
and H
2
O on the catalytic activity can be compensated by creating an inverse
catalyst with smaller Fe
2
O
3
support particles (5-7 nm) and relatively large
active Au particles (15-25 nm), achieving 99.85% CO conversion at 80 1C and
in the presence of up to 10% H
2
O and 25% CO
2
.
158
The increased Fe
2
O
3
surface area compared to conventional larger support structures signifi-
cantly enhances O
2
adsorption and, therefore, CO oxidation.
d
n
3
r
4
n
g
|
4
.
References
1. S. M. Haile, Acta Mater., 2003, 51, 5981.
2. N. Hotz, J. Sol. Energy Eng., 2012, 134, 041010.
3. N. Hotz, M. T. Lee, C. P. Grigoropoulos, S. M. Senn and D. Poulikakos,
Int. J. Heat Mass Transfer, 2006, 49, 2397.
4. N. Hotz, S. M. Senn and D. Poulikakos, J. Power Sources, 2006, 158,
333.
5. N. Hotz, R. Zimmerman, C. Weinmueller, M.-T. Lee, C. P. Grigoropoulos,
G. Rosengarten and D. Poulikakos, J. Power Sources, 2010, 195, 1676.
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