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(EDX) analysis. The measured composition is consistent with Pt-Ru stoi-
chiometry in starting cluster. Microdiffraction indicated intriguingly an fcc
structure, while bulk alloys of this composition adopt an hcp structure. Bond
distances in the final particles for Pt-Pt, Pt-Ru and Ru-Ru were estimated
from EXAFS data, as well as metal coordination numbers. A compositional
asymmetry was quantified by comparing the statistically predicted ratio of
Ru-Pt (or Pt-Ru) bonds to Ru-Ru (or Pt-Pt) bonds in the first (and higher)
shell to the measured EXAFS data. It was found that Pt-Pt and Ru-Ru
bonding is preferred over heterometallic bonding in the nanoparticles. The
proposed microstructure of the nanoparticle is an fcc hemisphere
(Figure 3.6, top) where the Pt atoms (dark gray) segregate to the surface
of the nanoparticle and away from the support, maximizing the number of
Pt-Pt bonds. Even if segregation occurred within each particle (but not into
separated monometallic ones), it should be noted that heterometallic
bonding in all shells was strongly evidenced by EXAFS data, providing direct
evidence for the formation of bimetallic nanoparticles.
Following this founding study, other works have concerned carbon-
supported nanoparticles prepared from clusters, given their importance as
electrocatalysts in fuel cells.
59
The oxygen reduction reaction (ORR) in par-
ticular is important in fuel cell technology and was studied by many authors
with Ru-based systems. Several carbon-supported bimetallic systems were
prepared by immobilizing a monometallic carbonyl cluster on a carbon
support, followed by reaction with a compound of a second metal before
reduction into nano-alloy (or preceding the immobilization by the reaction).
Recently, the complexes [Pd
3
(m
3
-PhPz)
6
], [NH
4
]
2
[CoPd
2
(Me
2
Ipz)
4
Cl
4
] and
[NH
4
]
2
[Co
2
Pd(Me
2
Ipz)
4
Cl
4
] bearing pyrazolate ligands were immobilized on
Vulcan carbon and reduced either chemically (with NaBH
4
) or thermally
(with H
2
).
60
The reduction process was followed by high resolution XPS: the
Pd peak was shown to shift to lower binding energy upon reduction. The
thermal treatment gave more reduced metals but larger nanoparticles on the
surface than chemical reduction, and with compositional discrepancies
with respect to nominal Co-Pd ratios. XRD characterization proved alloy
formation in the two bimetallic cases. The obtained Pd/C and PdCo/C
catalysts were selective in ORR reaction and presented better methanol
tolerance than commercial catalysts. In another study devoted to
fuel cells, the clusters py
2
Pt[MoCp(CO)
3
]
2
, (Me)(cod)PtMoCp(CO)
3
and
{Pt
3
(dppm)
3
CO[Mo(Cp)]}[BPh
4
] were used for the preparation of PtMo
2
,
PtMo and Pt
3
Mo catalysts, respectively, supported on Vulcan carbon.
61
The
activation step was carried out thermally by a succession of treatments in air,
nitrogen and getter gas (90/10 N
2
/H
2
mixture), to give final metal contents of
58 wt%, 43 wt% and 29 wt%. Detailed in situ XRD analyses permitted to
follow the activation step and to find the best experimental conditions to
avoid for example formation of phosphides phases. TEM micrographs re-
vealed small (1-10 nm) nanoparticles whose composition was confirmed by
EDXS to be that of the starting cluster precursor. Notably, the second catalyst
was air-stable and available in gram-scale quantities. It was tested as
hydrogen oxidation electrocatalysts in a working H
2
/O
2
proton exchange
d
n
9
r
4
n
g
|
5
.
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