Heterometallic Carbonyl Cluster PrecursorsHeterometallic 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

Carbon Black Carbon Black

H2

673K, 1h

[PtRu5]

CO + CH4 ?

ca. 200 m2/g

• 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 X-ray absorption spectroscopy. Includes in situ catalyst cell, gas supply manifold, x-ray detectors, andx-y-z translator.

Experimental DetailsExperimental DetailsAfter 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).

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.

EeV

1977

sin

X-rays

I0

FY

TY

SampleSee Fig. 2

See Fig. 3

20.00 nm

20 nm

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.

Electron MicroscopyElectron Microscopy

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.

1

2

9 nm

1

2

Cu

Ru Cu

PtRu1

2

Pt

At. % Ru: 83 %At. % Pt: 17%

Energy Dispersive Energy Dispersive X-ray AnalysisX-ray Analysis

a

z=[112]

111

C

A1.64

D

A1.91

131

220

CAD

b

c

B

CA

200

z=[011]

111

022

C

A1.61

B

A1.14d

Using microdiffraction, the structure of individual nanoparticles can be studied. Here, we show 2 sample diffraction patterns showing an fcc structure.

ElectronElectronMicrodiffractionMicrodiffraction

0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8

673 K

573 K

473 K

423 K

Pt L3 edge

673 K

573 K

473 K

423 K

Ru K edge

r (Å) r (Å)

(r

)(Å

-4)

673 KH2

473 KH2

CO

CO

CO

CO

CO

CO

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.

Nor

mal

ized

Abs

orpt

ion

0

1

Photon Energy (eV)

11550 11560 11570 11580 11590

300 K

423 K473 K523 K573 K

Temperature (K)E

ne

rgy

Sh

ift (

eV

)

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.

X-ray Absorption Near Edge Spectroscopy (XANES)X-ray Absorption Near Edge Spectroscopy (XANES)

|(r

)|(Å

-3)

|(r

)|(Å

-3)

r (Å)

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.

Multiple Shell FitMultiple Shell Fit

ConclusionsConclusions

• 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.