Chemistry Reference
In-Depth Information
ALD-synthesized catalyst converted propane at a 50 1C lower temperature
than a conventional alumina-supported catalyst with comparable Pt loading,
and showed better resistance to deactivation. The improved catalytic be-
havior was attributed to the strong epitaxy between the crystalline STO cubes
and the Pt particles, preventing full oxidation of the Pt.
Feng et al. synthesized catalysts for methanol decomposition using ALD
of Pd into mesoporous silica gel.
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,80
Prior to Pd ALD, the support was
covered with a ca. 0.5 nm thick Al
2
O
3
or ZnO ALD coating. Pd nanoparticles
(1 to 2 nm in diameter) were obtained by one or two cycles of the Pd(hfac)
2
/
formalin process.
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While the Pd/Al
2
O
3
catalysts showed quite high and
stable activity over time, the Pd/ZnO catalysts deactivated quickly at tem-
peratures higher than 230 1C. The poor performance of the Pd/ZnO catalysts
was related to the facilitated formation of Pd-Zn alloys under the conditions
required for the decomposition of methanol. The authors showed that an
ultra-thin Al
2
O
3
ALD over-layer could improve the stability and the catalytic
performance of the Pd particles on the ZnO surface, while keeping the active
Pd species accessible for the methanol reactant.
An Al
2
O
3
ALD overcoat was also reported to protect Pd catalysts from
coking
55
and/or sintering
53,54
in high-temperature reactions. Feng et al.
showed the effectiveness of a sub-nanometer thick Al
2
O
3
over-layer to pre-
vent sintering of supported Pd nanoparticles up to 500 1C.
53
The protective
layer was found to preferentially nucleate at corners, steps and edges of the
Pd nanoparticles, while keeping the more active Pd(111) facets accessible for
methanol conversion. Lu et al. used thicker Al
2
O
3
ALD overcoats (
d
n
9
r
4
n
g
|
7
8nm)to
reduce coke formation and improve the thermal stability of Pd nanoparticle
catalysts at temperatures as high as 675 1C.
55
Accessibility for reactant gases
to the surface of the active nanoparticles was achieved by high-temperature
treatments inducing microporosity in the Al
2
O
3
ALD layer. It was further-
more shown that the overcoat had a positive effect on the selectivity of Pd
catalysts in the oxidative dehydrogenation of ethane.
Christensen et al. demonstrated the potential of ALD for synthesizing
bimetallic nanoparticles.
81
Ru-Pt particles with an average size of 1.2 nm
were synthesized on nonporous alumina powder
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using two Ru-ALD
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cycles, one Pt-ALD cycle, and another Ru-ALD cycle. TEM images showed
that the particles were crystalline and had a crystal structure similar to bulk
Pt. At temperatures above 210 1C, the bimetallic catalyst showed higher
methanol conversion than a physical mixture of ALD-synthesized Pt/Al
2
O
3
and Ru/Al
2
O
3
catalysts with a similar loading.
B
.
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Silicycle S10040M: particle size 75-200 mm, surface area 99.6 m
2
g
1
, and pore diameter
30 nm.
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hfac
ΒΌ
hexafluoroacetylacetonate.
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Al
2
O
3
nanospheres (NanoDur, Alfa Aesar): particle size 40-60 nm and surface area 35 m
2
g
1
.
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2,4-(Dimethylpentadienyl)(ethylcyclopentadienyl)Ru was used in combination with O
2
to
grow Ru, while Pt was deposited from MeCpPtMe
3
and O
2
.
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