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Document Type |
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Article In Conference |
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Document Title |
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Size Effects in Catalysis by Metallic Nanoparticles Size Effects in Catalysis by Metallic Nanoparticles |
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Subject |
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كيمياء |
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Document Language |
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English |
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Abstract |
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Application of nanomaterials in catalysis has a long story and started before
extensive usage of the prefix ‘nano’. Supported metal catalysts, where nanosized
noble metal (Pt, Pd, Rh) particles have been supported on oxide matrixes, are the most
evident examples which prove the above statement. The original motivation of
applying the nanosized metals in heterogeneous catalysis was optimization of the
noble metal loading due to enhancement of the surface-to-volume ratio. However then,
it has been shown that specific catalytic activity (or turnover frequency, TOF) for
some reactions can vary when the sizes of metallic particles are reduced to nanometer
scale. This phenomenon was called as size effect and the reactions, which exhibit size
effects – as structure-sensitive reactions.
In this contribution we will present the data of our recent investigations of the
size effects in three heterogeneous catalytic reactions: low temperature CO oxidation
over Au/Al2O3 catalysts, methane oxidation over Pt/Al2O3 catalysts and ethylene
epoxidation over Ag/Al2O3 catalysts.
A series of supported metal catalysts with naroow particle size distribution were
prepared by the deposition precipitation and chemical vapor deposition (Au/Al2O3),
the incipient wetness impregnation (Au/Al2O3, Pt/Al2O3 and Ag/Al2O3) and by
physical vapor deposition (Ag/HOPG = highly oriented pyrolitic graphite). The latter
samples were used to study the reasons of size effects in oxygen adsorption over
silver nanoparticles with in-situ XPS. The high-surface area catalysts were used both
for catalytic testing and for physical-chemical characterization with UV-Vis
spectroscopy, XPS, SAXS, EXAFS, XANES spectroscopy and TEM. The
morphology of the model samples (Ag/HOPG) was tested with STM.
The catalytic activity testing demonstrate that the reduction of sizes of metallic
particles to nanometer scale can increase not only activity (CO and CH4 oxidation),
but also selectivity (ethylene epoxidation) in the target reactions. The maximum of
activity of the gold and platinum catalysts are achieved when the mean particle size is
about 2-3 nm. Both increase and decrease in the particle sizes reduces the TOF values.
Application of physical methods, including in in-situ mode, allows us to show the coexistence
of zero-valent (metallic) and ionic species of gold and platinum in the most
active catalysts. This result proposes a possibility of bifunctional catalysis in these
systems.
In-situ study of oxygen adsorption on silver particles supported on HOPG shows
that two oxygen species (nucleophilic and electrophilic) are produced on the silver
surface, with their population being varied with silver particle sizes. Electrophilic
oxygen is the unique species for small silver particles (d < 10 nm), whereas
nucleophilic oxygen, which appears at dAg > 10 nm, becomes preferable for
continuous silver films. This variation in the nature of oxygen species with silver
particle sizes explains size effects in increase in selectivity towards ethylene oxide
and decrease in epoxidation activity of silver catalysts with particle sizes below 50 nm.
This explanation was based on the mechanisms of the reaction elucidated by us earlier.
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In conclusion of this presentation, conditions providing the success in systematic
investigation of the nanosized effects in catalysis and their application in practice are
formulated.
The authors acknowledge the financial support from RFBR (grants # 07-03-
00931, 07-03-92117-China) and from Russian Science and Innovation Agency
(contract # 02.513.11.3203).
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Conference Name |
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Tenth International Symposium on Heterogeneous Catalysis |
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Duration |
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From : 22/8/1429 AH - To : 26/8/1429 AH
From : 23/8/2008 AD - To : 27/8/2008 AD |
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Publishing Year |
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1429 AH
2008 AD |
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Article Type |
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Article |
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Added Date |
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Tuesday, February 10, 2009 |
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