Photon Factory Activity Report 2016 #34 (2017)

BL-12C/2015G127

XAFS Characterization of Size-Regulated Carbon-Supported Pt Nanoclusters
with Polymer Matrix Overlayers

Satoshi MURATSUGU*1, Shota MIYAMOTO1, Kana SAKAMOTO1, Kentaro ICHIHASHI1,Nozomu ISHIGURO1, Mizuki TADA1,2
1

Department of Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan.
2
RCMS and IRCCS, Nagoya University, Furo-Cho, Chikusa-ku, Nagoya, 464-8602, Japan.

1 Introduction
We prepared size-regulated Pt nanoclusters fromcarbon-attached Pt tetranuclear (Pt4) complex precursors
and organic polymer matrix overlayers. A newly preparedPt4 complex was attached to the multi-wall carbon
nanotube (MWCNT) and polypyrrole (PPy) matrix
overlayers were successively stacked on the Pt4 complex
attached MWCNT. This composite was transformed to Pt
nanoclusters for catalytic N-alkylation reaction. The
oxidation states and the local coordination structures ofthe Pt4 complex and Pt nanocluster supported on
MWCNT were investigated by Pt LIII-edge XAFS.
2 Experimental
A Pt4 complex (1) with anchoring ligand was attachedon MWCNT (A). Then PPy matrix overlayers werestacked on the surface of A (B). Finally B was reduced toform Pt nanocluster catalyst (C). To see the effect of PPymatrix overlayres, A was reduced in a similar way of B toprepare Pt catalyst (D). Pt LIII-edge XAFS was measured
in a transmission mode at 20 K at the BL-12C station
with a Si(111) double-crystal monochromator. Ionizationchambers filled with pure N2 and pure Ar were used tomonitor incident and transmitted X-rays, respectively.
EXAFS spectra were analysed using ATHENA andARTEMIS programs. k3-Weighted EXAFS oscillations
were Fourier transformed into R-space, and curve-fitting
analysis was performed in R-space with coordination
number (CN), interatomic distance (R), Debye-Waller
factor (σ2), and correction-of-edge energy (∆E0). Phase
shifts and backscattering amplitudes were calculated by
the FEFF8 code.
3 Results and Discussion
The local coordination structures of the MWCNT
supported Pt complex and Pt nanocluster were analyzed
by Pt LIII-edge EXAFS analysis (Table 1). 1 had Pt-Pt
bond at 0.250 ± 0.001 nm (Coordination number (CN):3.9 ± 0.8) and two Pd-O bonds at 0.200 ± 0.001 nm (CN:2.0 ± 0.6) and 0.215 ± 0.003 nm (CN: 1.6 ± 0.6), whichwere similar to the Pt4(CH3COO)8 complex precursor.
The bond distances of the Pt-Pt coordination in A and B
were both fitted as 0.250 ± 0.001 (CN: 1.9 ± 0.2 for A,1.5 ± 0.2 for B), those of Pt-O coordination were bothfitted as 0.200 ± 0.002 nm (CN: 2.7 ± 0.7 for A, 2.7 ± 0.4for B) and 0.216 ± 0.004 nm (CN: 1.5 ± 0.6 for A, 1.3 ±0.4 for B), respectively, indicating that the local
coordination structure of 1 was maintained after it wasattached to the MWCNT surface and successive stackingof PPy matrix overlayers.
The bond distances of the Pt-Pt coordination in C andD after the reduction were fitted as 0.273 ± 0.001 and
0.277 ± 0.001, respectively, which were larger than thoseof A and B, and were close to Pt0 species. The Pt LIII-edge
XANES of C and D indicated that the Pt oxidation stareof was reduced to ca. zero, supporting the EXAFS data.
The CN (Pt-Pt) of C was 3.5 ± 1.1, which was smaller
than that of D (4.8 ± 0.7), indicating that the size of Pt
nanocluster in C was much smaller. In fact, TEM image
analyses of C exhibited small Pt nanoclusters of 1.0–1.3
nm with the narrow size distribution (0.3~0.5 nm), while
broad size distributions with Pt particles with diametersof 3–4 nm were observed on D, suggesting that the PPy
matrix overlayers had a positive effect to control sizeregulation.
Table 1 Structural Parameters Determined by Curve-Fitting Analysis ofPt LIII-edge EXAFS Measured at 20 KShell

CNR /nm∆E0 /eVσ2 /×105 nm21a0.250 ± 0.001
7±32±10.200 ± 0.001
5±22±20.215 ± 0.003
5±22±2A (Pt: 2.0 wt%) b
Pt–Pt 1.9 ± 0.2
0.250 ± 0.001
7±22±1Pt–O 2.7 ± 0.7
0.200 ± 0.002
3±34±2Pt–O 1.5 ± 0.6
0.216 ± 0.004
3±34±2B (Pt: 1.5 wt%) c
Pt–Pt 1.5 ± 0.2
0.250 ± 0.001
7±22±1Pt–O 2.3 ± 0.4
0.200 ± 0.002
4±23±1Pt–O 1.3 ± 0.4
0.216 ± 0.004
4±23±1C (Pt: 1.8 wt%) d
Pt–Pt 3.5 ± 1.1
0.273 ± 0.001
11 ± 37±1Pt–O 0.7 ± 0.3
0.202 ± 0.001
21 ± 33±2Pt–C
1.3 ± 0.6
0.221 ± 0.003
21 ± 3
3±2
D (Pt: 2.0 wt%) e
Pt–Pt 4.8 ± 0.70.277 ± 0.001
14 ± 24±1ak = 30 – 180 nm–1, R = 0.12 – 0.30 nm, Rf = 0.3%.
bk = 30 – 180 nm–1, R = 0.12 – 0.30 nm, Rf = 0.3%.
ck = 30 – 180 nm–1, R = 0.12 – 0.30 nm, Rf = 0.4%.
dk = 30 – 180 nm–1, R = 0.12 – 0.32 nm, Rf = 3.1%.
ek = 30 – 180 nm–1, R = 0.18 – 0.32 nm, Rf = 1.8%.
Pt–Pt
Pt–O
Pt–O

2.2 ± 0.3
2.0 ± 0.6
1.6 ± 0.6

Reference[1] S. Muratsugu, S. Miyamoto, K. Sakamoto, K.
Ichihashi, C.-K. Kim, N. Ishiguro, M. Tada, Submitted.
* smuratsugu@chem.nagoya-u.ac.jp