Photon Factory Activity Report 2017 #35 (2018)

BL-9C/2015G025

Characterization of PdCuCr Ternary Nanoparticles Constructed within a Basic Resin
Kohsuke MORI,1,2,3* Kohei NAKA,1 Shinya MASUDA,1 and Hiromi YAMASHITA1,3
1Graduate School of Engineering, Osaka University, Osaka, 565-0871, Japan
2JST, PRESTO, Saitama, 332-0012, Japan
3ESICB Kyoto University, Kyoto, 615-8245, Japan

1 Introduction
Hydrogen has received increasing attention as a candidate
for clean energy, and efficient energy conversion can berealized by combining with fuel cell technology. However,
the widespread application of hydrogen as atransportation fuel is limited, mainly because ofcontrollable storage and delivery problems. A
considerable amount of research effort is currently
devoted to exploring new storage methods.1-5 Among
investigated, formic acid (HCOOH), which is a liquid atroom temperature and contains 4.4 wt% hydrogen, is oneof the major products formed during biomass processing
and is widely recognized as a convenient hydrogen carrier
for fuel cells designed for portable use.
In the course of our ongoing studies to explore
practical catalysts for targeted reactions, we found that
PdCuCr ternary NPs within a macroreticular basic resin
exhibit significant catalytic enhancement over that ofbimetallic PdCu, PdCr, and monometallic Pd NPs. Here,
we report the detail charactyerization of Cr species by insutu XAFS measurements.
2 Experiment
Synthesis of PdCuCr/resin catalyst: Prior to thedeposition, resins were crushed by a ball mill (600 rpmfor 10 min). The resin (1.0 g) was mixed with 80 mL ofaqueous solution containing 10 mL of PdCl2 (29.7 mM), 5
mL of CuCl2·2H2O (29.7 mM), and 5 mL ofCr(NO3)3·9H2O (29.7 mM) and stirred at room
temperature for 1 h. The suspension was evaporated
under vacuum and the obtained powder was driedovernight. Subsequently, the samples were pre-reducedwith NaBH4, giving PdCuCr/resin (Pd 3wt%; molar ratioof Pd : Cu : Cr = 1 : 0.5 : 0.5).
3 Results and Discussion
The Cr K-edge XANES spectrum for CrO3 had an
intense pre-edge peak at 5992 eV, which is characteristicof terminal Cr6+=O species with a tetrahedral
coordination (Figure 1A). The shape of the XANES
spectrum for PdCuCr/resin is quite different from that ofCrO3 due to the absence of the characteristic pre-edge
peak, but is similar to those of Cr3+-type spectra, such as
Cr2O3 and Cr(NO3)3. The edge energy for the PdCuCr
sample is higher than that for Cr foil, which suggests thatthe Cr species is in a +3 oxidation state. In the FT-EXAFS
spectra, the bond length of the first Cr−O bond in PdCuCr
resembles those in Cr2O3 and Cr(NO3)3, but is longer thanthat in CrO3, which reveals the lack of a Cr=O doublebond in the PdCuCr/resin sample (Figure 1B). In
comparison with the reference Cr2O3, the intensity of thesecond peak for PdCuCr/resin is significantly weakened.
This peak in PdCuCr is attributable to the contiguous
Cr−O−M (M: Cr, Cu, Pd) bonds. The reduced intensity inthis region can be explained by the formation ofstructurally disordered and extremely small NPs. No
metallic bonds, detectable at around 2.3 Å for Cr foil,
were observed. Since the reduction potentials of Cr3+ ions(E◦(Cr3+/Cr2+) = −0.42 V, E◦(Cr2+/Cr0) = −0.90 V vs.
NHE) are more negative than Pd2+ and Cu2+ ions
(E◦(Pd2+/Pt0) = +0.99 V, E◦(Cu2+/Cu0) = +0.34 V vs. NHE),
the reduction of Cr3+ ions is retarded more than those of
Pd2+ and Cu2+ ions, and preferentially constituteextremely small clusters on the surface of NPs byinteraction with Pd0 and Cu0.

Figure 1. Cr K-edge (A) XANES and (B) FT-EXAFS spectra.

References[1] M. Wen, K. Mori, Y. Kuwahara, H. Yamashita, ACSEnergy Lett., 2, 1-7 (2017).
[2] K. Mori, T. Taga, and H. Yamashita, ACS Catal., 7,3147 (2017)[3] K. Mori, S. Masuda, H. Tanaka, K. Yoshizawa, M.
Che, H. Yamashita, Chem. Commun., 53, 4677 (2017).
[4] K. Mori, M. Dojo, H. Yamashita, ACS Catal., 3,1114 (2013).
[5] K. Mori, H. Tanaka, M. Dojo, K. Yoshizawa, H.
Yamashita, Chem. Eur. J., 21, 12085 (2015).
[6] K. Mori, K. Naka, S. Masuda, H. Yamashita,
ChemCatChem, in press.
* mori@mat.eng.osaka-u.ac.jp