Photon Factory Activity Report 2018 #36 (2019)

BL-12C/2017G533, 2015G127

XAFS Characterization of Size-controlled Pt Nanoparticles Prepared fromPt4 Complex and Popypyrroles for Enhanced Oxygen Reduction Reaction
Satoshi MURATSUGU*1, Kentaro ICHIHASHI1, Mizuki TADA1,2
1
2

Department of Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan.
Research Center for Materials Science (RCMS) & Integrated Research Consortium on Chemical
Science (IRCCS), Nagoya University, Furo-cho, Chikusa, Aichi 464-8602, Japan.

1 Introduction
We prepared size-controlled Pt nanoparticles on multiwall carbon nanotubes (MWCNTs) decorated with
polypyrrole matrix overlayers from newly prepared Pt4complex and pyrrole monomers. The oxidation states andthe local coordination structures of the prepared Pt
nanoparticles with/without polypyrrole matrix overlayers
were investigated by Pt LIII-edge XAFS. The prepared Pt
nanoparticles with polypyrrole matrix overlayers
exhibited superior oxygen reduction reaction (ORR)
performance as electrocatalysts compared with Pt
nanoparticles without polypyrrole matrix overlayers andcommercial Pt/C catalysts.[1]2 Experimental
Pt nanoparticles with polypyrrole matrix overlayres (A),and Pt nanoparticles without polypyrrole matrix
overlayres (B) and were prepared according to theliterature, respectively.[1]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. Ionization chambers
filled with pure N2 and pure Ar were used to monitor
incident and transmitted X-rays, respectively. EXAFS
spectra were analysed using ATHENA and ARTEMIS
programs. k3-Weighted EXAFS oscillations were Fouriertransformed into R-space, and curve-fitting analysis wasperformed in R-space with coordination number (CN),interatomic distance (R), Debye-Waller factor (σ2), andcorrection-of-edge energy (∆E0). Phase shifts andbackscattering amplitudes were calculated by the FEFF8code.
3 Results and Discussion
The oxidation state of the Pt nanoparticles with/without
polypyrrole matrix overlayres (A and B) were analyzed
by Pt LIII-edge XANES (Figure 1(A)). The white line
heights of A and B were very similar to that of Pt foil,
indicating that the oxidation state of A and B were bothzero.
The local coordination structures of A and B wereanalyzed by Pt LIII-edge EXAFS (Figure 1(B)). The bonddistances of the Pt-Pt coordination in A and B after thereduction were fitted as 0.276 ± 0.001 nm and 0.277 ±0.001 nm, respectively, whose values were identical to Pt0
foil (R = 0.277 ± 0.001). On the contrary, thecoordination number (CN) of Pt-Pt bond in A was 5.4 ±1.0, while the larger CN of 7.3 ± 0.7 was observed in B.
This result supports the formation of smaller Pt
nanoparticles on A, which was confirmed by TEM
analysis. TEM images of A exhibited small Pt
nanoparticles of 1.5 ± 0.5 nm with polypyrrole matrix
overlayres, while the size control of Pt nanoparticles wasnot possible without polypyrrole matrix overlayers withlarge Pt nanoparticles of >3 nm. The Pt-O/N contribution(CN = 0.8 ± 0.4, R = 0.216 ± 0.002 nm) was also
observed in A.
The size-controlled Pt nanoparticles in A exhibited
better activity and durability than non-regulated Pt
nanoparticles in B without polypyrrole matrix overlayers
and a commercial Pt/C catalyst during the ORR at thefuel cell cathode without substantial aggregation of thesize-controlled Pt nanoparticles.

Figure. 1: (A) Pt LIII-edge XANES spectra of A, and B.
(B) k3-Weighted Pt LIII-edge EXAFS Fourier transformsof A, and B.
[1] K. Ichihashi, S. Muratsugu, S. Miyamoto, K.
Sakamoto, N. Ishiguro, M. Tada, Dalton Trans. 2019, 48,7130–7137. (selected as inside front cover)* smuratsugu@chem.nagoya-u.ac.jp