Transcript No Slide Title

Heterometallic Carbonyl Cluster Precursors
• Heterometallic molecular cluster precursor
- mediate transport and growth of nanoscale bimetallic particles
• Use PtRu5C(CO)16 as a precursor for carbon-supported [PtRu5]
nanoparticles
H2
CO + CH4
?
[PtRu5]
Carbon Black
ca. 200 m2/g
673K, 1h
Carbon Black
• Characterize: Microstructure of the resulting nanometer sized alloy
phases
- X-ray spectroscopy
- electron microscopy
Figure 1. Si(111) cyrstal bender (D.
Adler)
Figure 2. Catalyst cell for
in-situ EXAFS data
collection
Figure 3. Experimental set-up in
the X16C “hutch” for in situ Xray absorption spectroscopy.
Includes in situ catalyst cell, gas
supply manifold, x-ray detectors,
andx-y-z translator.
Experimental Details
After depositing and activating the cluster precursor (1h under H2), in situ extended
X-ray absorption fine structure (EXAFS) data was collected on the X16C beamline
(Scheme 2) at the National Synchrotron Light Source at Brookhaven National
Laboratory, Upton, NY (Fig. 1). The beamline utilized a state-of-the-art focusing
crystal and catalyst cell (Fig. 2-3).
See Fig. 2
r
de
en
b
)
11
(1
Si
X-rays

E
Sample
1977eV
sin 
I0
)
11
(1
Si
TY
FY
See Fig. 3
Scanning transmission electron microscopy (STEM) experiments were carried out
on a Vacuum Generators HB501 located at the Center for Microanalysis of Materials
at the Materials Research Laboratory, Urbana, IL.
Electron Microscopy
1
2
3
5
0
20.00 nm
3
0
0
1
0
2
5
0
8
2
0
0
6
NumberofNanoclusters
AverageFirst-ShelCordination
1
5
0
4
1
0
0
2
20 nm
5
0
0
0
0369
1
2
1
5
1
8
2
1
2
4
2
7
3
0
3
3
3
6
3
9
N
a
n
o
c
l
u
s
t
e
r
D
i
a
m
e
t
e
r
(
A
)
On the right is a sample dark field micrograph of [PtRu5]/C. From these
micrographs, a particle size distribution can be obtained (shown on the left).
The size distribution is also compared with an average first-shell coordination
derived from a model (cuboctahedron) nanocluster.
9 nm
2
2
1
Energy Dispersive
X-ray Analysis
1
Ru
Cu
At. % Ru: 83 %
At. % Pt: 17%
Pt
Cu
Pt
1
2
Ru
The upper images are sample
bright (right) and dark (left)
field micrographs of supported
[PtRu5] nanoclusters. Below,
are sample energy dispersive
X-ray Analysis (EDAX) spectra
taken on the carbon support (2)
and sample nanocluster (1).
Since EDAX is a sensitive to
individual elements as well as
the amount of these elements
present, the composition of
individual nanoparticles can be
obtained.
C
1.64
A
b
131
a
111
220
D
A
D
 1.91
A
Electron
Microdiffraction
C
z=[112]
B
1.14
A
d
c
200
C
 1.61
A
111
B
A
022
C
z=[011]
Using microdiffraction,
the structure of individual
nanoparticles can be studied.
Here, we show 2 sample
diffraction patterns showing
an fcc structure.
Temperature programmed reduction of a Pt-Ru nanoparticle with structure determined
from Extended X-ray Absorption Fine Structure (EXAFS). On the left are the EXAFS
spectra during the temperature evolution. On the left is a schematic representation of the
nucleation and growth of the nanoclusters.
Pt L3 edge
Ru K edge
(r)(Å-4)
H2 673 K
673 K
673 K
573 K
573 K
H2 473 K
0
1
2
3
4
5
r (Å)
6
473 K
473 K
423 K
423 K
7
8 0
1
2
3
4
5
r (Å)
6
7
8
X-ray Absorption Near Edge Spectroscopy (XANES)
0)
300 K
423 K
473 K
523 K
573 K
1
1
ChangeinEnergy(E-
Normalized Absorption
2
0
0
11550
11560
11570
11580
Photon Energy (eV)
11590
2
5
0
3
0
0
3
5
0
4
0
0
4
5
0
5
0
0
5
5
0
6
0
0
6
5
0
7
0
0
Temperature
T
e
m
p
e
r
a
t
u
r
e
(
K
)(K)
This technique shows the nucleation and growth of metallic particles from the molecular
precursors. On the left are sample spectra taken at increasing temperature. We see a
decrease in the white line intensity as well as a shift of peak position to a more metallic
state. This shift is better seen in the plot on the left which shows the energy shift towards
the metallic state (0 eV) as temperature increases.
Multiple Shell Fit
Table 1. Metal bond distances obtained by simultaneously fitting the P
2
edge EXAFS data measured from the carbon-supported [PtRu5]/C nanop
Pt L3-edge, [PtRu5]/C
Bond Distance (Å)
|(r)|(Å-3)
MS Fit
1
Bond
1st shell
2nd shell
3rd shell
4th shell
Pt-Pt
2.69(3)
3.78(3)
4.66(4)
5.38(3)
Pt-Ru
2.70(1)
3.79(2)
4.70(2)
5.40(1)
Ru-Pt
2.70(1)
3.79(2)
4.70(2)
5.40(1)
Ru-Ru
2.67(1)
3.78(1)
4.68(1)
5.42(2)
0
0
1
2
3
4
5
6
7
8
9
|(r)|(Å-3)
2
Bond
1st shella
Ru K-edge, [PtRu5]/C
Pt-Pt
2.5(1.6)
1.4(6)
1.9(1.4)
1.4(8)
MS Fit
Pt-Ru
4.0(1.0)
1.0(5)
3.0(1.0)
1.5(5)
Ru-Pt
0.7(2)
0.2(1)
0.6(2)
0.3(1)
Ru-Ru
5.4(5)
1.3(3)
2.8(4)
0.8(3)
1
0
0
1
2
3
4
5
r (Å)
Coordination Number
10
6
7
8
9
10
2nd shell
3rd shell
4th shell
Multiple shell fit of the Pt L3 and Ru K- edge
EXAFS data for [PtRu5]/C. The tables show
the coordination number and bond distances
derived from this fit procedure.
Conclusions
•
Supported bimetallic nanoclusters with exceptionally
narrow size (ca 1.5 nm) and compositional (1:5) distributions
were prepared using a Pt-Ru molecular cluster precursor.
The structure of the resulting nanoclusters was characterized
with in situ EXAFS, high-resolution transmission electron
microscopy, and electron microprobe methods.
• The local environment of the Pt, as evidenced by EXAFS,
indicates the formation of a close-packed structure in which
the Pt resides preferentially in more ordered Ru metal lattice
sites. In support of the EXAFS, microdiffraction results
indicate the formation of fcc microstructure which is different
from the structure extrapolated from the solid state, i.e, hcp.
•Future work is aimed at probing the nanocluster
microstructure with in situ EXAFS in an operational fuel cell.