Angewandte
Chemie
cationic form on LaPO4-H, and thus the lattice structure of
the Au nanoparticles is not very clear in the HRTEM image.
After high-temperature calcination, the Z-contrast image
recorded on Au/LaPO4-H at an identical magnification shows
slight growth of the gold nanoparticles (Figure 4b) as a result
of high-temperature sintering. The average size of the Au
nanoparticles reaches 2–4 nm. The sintering of the gold
nanoparticles of Au/LaPO4-M (Figure 4c and d) is much
more severe, which agrees with the above XRD observation
and further indicates that the interaction of LaPO4-H with
and activity of the gold nanoparticles deposited on the LaPO4
nanoparticles suggest a unique interaction between the gold
and the phosphate support, which highlights new opportuni-
ties in the development of ultrastable gold catalysts for
reactions in oxidative and high-temperature environments.
Received: October 27, 2005
Revised: February 11, 2006
Published online: April 26, 2006
Keywords: gold · lanthanum · nanoparticles · oxidation ·
gold nanoparticles is stronger than that of LaPO -M. The
.
4
supported catalysts
HRTEM image shown in Figure 5b clearly reveals the
crystalline nature of the resulting Au metallic nanoparticles
of calcined Au/LaPO4-H. The distance between two (111)
lattice planes measured from the HRTEM image is approx-
imately 2.35 , which is consistent with the value of 2.36
measured from an Au model structure with the MS modeling
program supplied by Accelrys (see the middle inset of
Figure 5b). The top-right inset of Figure 5b is the electron-
diffraction pattern of this Au nanoparticle, which demon-
strates nicely the crystalline structure of the metallic particles.
The high dispersion and well-developed crystalline struc-
ture revealed by HRTEM for the Au nanoparticles supported
on LaPO4-H provide structural evidence for the unusual
activation of the Au/LaPO4-H system by calcination at 5008C
in 8% O2/He. Our selected-area electron diffraction study
indicates that the gold nanoparticles are mainly attached
epitaxially on LaPO4-H through the most densely packed
(111) plane (Figure 5).[55] This activation process under the
oxidative conditions is in sharp contrast to those required for
the oxide-supported catalyst system.[47,48] Based on the SMSI
model, the latter system requires the reduction of support
oxides to invoke SMSI. The negligible reducibility of LaPO4
implies that Au/LaPO4-H needs to rely on other means of
activation to enhance SMSI. One possible activation mech-
anism during the calcination of Au/LaPO4-H could involve
the surface restructuring of the gold/support interfaces.
Deactivation of Au/LaPO4-M was observed under the same
calcination conditions, and hence the surface restructuring
might be unique to the specific surface configuration of
LaPO4-H. Clearly, the smaller particle size of the LaPO4-H
support may play a key role in this surface-restructuring
process through the supply of a high surface-defect concen-
tration relative to that of the LaPO4-M support. This
observation is very similar to that of Corma and co-workers,
which showed that gold nanoparticles deposited on CeO2
nanocrystals by co-precipitation yielded an activity for CO
oxidation that was two orders of magnitude higher than that
for particles deposited on bulk CeO2.[43] The unique restruc-
turing properties of nanoparticles with ionic lattices have
been recently revealed and utilized in the generation of new
nanoparticle phases.[8,56]
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In conclusion, a new non-oxide support system based on
LaPO4 nanoparticles has been developed for gold catalysts
used in CO oxidation. The key feature of this catalytic system
is that activation can be achieved through calcination under
an oxidative atmosphere. This observation is in sharp contrast
to conventional SMSI catalytic systems, which can only be
activated under a reductive atmosphere. The high stability
Angew. Chem. Int. Ed. 2006, 45, 3614 –3618ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3617