Photon Factory Activity Report 2016 #34 (2017)
BL-9A, AR-NW10A/2015G586

EXAFS Characterization of an Interface Model Nanocatalyst: Binary metal (Ti, Cu)
Oxyhydroxy–Organic Framework
1Yusuke Yoshida1 and Yasuo Izumi1,*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku,
Chiba 263-8522, Japan

1 Introduction
For heterogeneous catalysis, metal–support interfaces
have frequently been considered as active sites. Theinterface between the nanoparticles and the metal oxide
support is believed to play a crucial role in the catalysis.
However, in nature, the interfaces have been difficult tocharacterize by spectroscopic or physicochemical
methods. In this study, exact model composite consistingof [Ti8O8(OH)4]12+ clusters and [Cu2(OH)6]2− linkers wasprepared as a molecular model of supported catalysts.
[Ti8O8(OH)4]12+ cluster is tetragonally linked by bdc (1,4benzene dicarboxylate) ligands (MIL125). The interface
between [Ti8O8(OH)4]12+ and added Cu species duringcatalyst synthesis was characterized by XAFS.
2 Experimental Section
0.4 mL of Cu nitrate (0.5 mmol) aqueous solution wasprepared and 200 mg of MIL125 was added. The mixture
was magnetically stirred at 300 rpm and 290 K for 20 min
and the water was evaporated at 353 K. The atomic ratioof Cu/Ti was 1/2 by incorporating 0.4 mL of Cu nitrate.
The powder was calcined in air at 573 K for 2 h. The
obtained green sample was denoted as Cu/MIL125.
Ti K and Cu K-edge XAFS spectra were measured at290 K in a transmission mode on a beamline 9C in thePhoton Factory and NW10A in the Photon Factory
Advanced Ring at KEK. The storage ring energy was 2.5
GeV and the ring current was 450 mA for Ti K-edge. Thevalues were 6.5 GeV and 37–54 mA for Cu K-edge. ASi(1 1 1) and Si(3 1 1) double-crystal monochromator
was inserted in the X-ray beam path. The X-ray intensity
was maintained at 65% of the maximum flux using aPiezo translator set on the crystal. The slit opening size infront of the I0 ionization chamber was 1.0 mm (vertical) ×2.0 mm (horizontal). The Ti K and Cu K-edge absorptionenergy values were calibrated to 4964.5 and 8980.3 eV,
respectively, for the spectra of Ti and Cu metal foils.
The XAFS data were analyzed using an XDAP
package. Multiple shell curve fit (CF) analyses were
performed for the Fourier-filtered k3-weighted EXAFS
data in k- and R-space using empirical amplitude andphase-shift parameters extracted from the EXAFS data ofTiO2 powder (JRC-TIO7) and CuO powder.
3 Results and Discussion
Based on the Cu–O and Ti–O interatomic distances
determined by EXAFS for Cu/MIL125 (both 0.195 nm;
Table 1) and assuming complete tetrahedral (Td) oroctahedral (Oh) coordination around Cu, three plausible
Cu linker models were drawn (Chart 1).
The first model is [Cu2(OH)6]2−, in which the Cu sites
are Td coordinated (Chart 1A). The second and third are[Cu2(OH)6(OH2)4]2− and [Cu4(OH)10(OH2)6]2−, in which
the Cu sites are Oh coordinated (Chart 1B and C,respectively).
In models A, B, and C, the nearest distances between
Ti atoms in the neighboring [Ti8O8(OH)4(bdc)2]8+ sheet
were calculated as 0.838, 0.936, and 1.100 nm,
respectively (Chart 1). Thus, model C was rejected as thelinker in comparison to the experimental value: 0.874 nm
based on XRD result [1]. If the O–Cu–O angles forcentral Cu2O2 core in model A were slightly distorted
from 109.5° (Td) to 96.1° (model A’, Chart 1) or if thedihedral angle of two CuO4 planes was distorted from180° to 132° (model B’), the Cu linker length L became
0.874 nm, in agreement with XRD.

Chart 1. Plausible Cu hydroxide linker models. The Cu–O and Ti–O interatomic distances were fixed at 0.195 nm
based on EXAFS. (A, A’, B’) [Cu2(OH)6]2−, (B)[Cu2(OH)6(OH2)4]2−, and (C) [Cu4(OH)10(OH2)6]2−. The
Cu site symmetry was Td (A, A’), planar (B’), and Oh (B,C).

Photon Factory Activity Report 2016 #34 (2017)

Table 1Best-fit results of Ti and Cu K-edge EXAFS analyses for Cu/MIL125*1
Ti–O
Ti(–O–)Ti
Ti(–O–)Cu
Cu–ON
5.2(±1.2)*1R(nm)0.1945(±0.0004)N
3.8(±0.3)

R(nm)0.276(±0.002)N
2.6(±0.6)R(nm)0.3797(±0.0008)

The values in parenthesis are fit errors.

Based on the Ti K-edge EXAFS CF analyses forCu/MIL125 (Table 1), the Ti(–O–)Ti interatomic distancechanged from 0.305 nm (mean) for MIL125 to 0.276 nm.
Its associated N value increased from 2.0 to 3.8. In
addition, a new farther peak appeared at 0.33 nm (phaseshift uncorrected) [1], that can be ascribed to a Ti(–O–)Cu interatomic pair fitted with the R value 0.380 nm andN value 2.6 (Table 1), strongly suggesting that new Ti(–O–)Cu interatomic links formed around the[Ti8O8(OH)4]12+ clusters. The N value was consistent withmodels A’ or B’ for Cu/MIL125 (Chart 1) withinvariation of ±0.6 (Table 1), because the models assumed
that two-thirds of terephthalates coordinated to the Ti
atoms in MIL125 were replaced by [Cu2(OH)6]2− linkers;
thus, the N[Ti(–O–)Cu] value was two. The distance(0.380 nm; Table 1) was in agreement with the Ti···Cuinteratomic distance in model B’ (0.373 nm; Chart 1)
rather than 0.355 nm in model A’. The formula would be
Ti8O8(OH)4·(bdc)2·[Cu2(OH)6]4 (Compound 1) forCu/MIL125.
The connection between [Cu2(OH)6]2− linkers and[Ti8O8(OH)4]12+ clusters was investigated by Cu K-edge
EXAFS. The N value for Cu–O pairs (3.6; Table 1)
supported the coordination of four O atoms both inmodels A’ and B’ (Chart 1) within the variation of ±0.6
(Table 1). The N value for Cu(–O–)M (M = Cu or Ti) was
shown to be 2.8 (Table 1), consistent with coordination toone Cu and two Ti (Chart 1A’, B’). The interatomic
distance for Cu(–O–)M (0.310 nm; Table 1) was2
1× 0.355 +× 0.261 =332
10.324 nm and model B’: × 0.373 +× 0.259 = 0.335
33consistent with model A’:nm.
The theoretical Ti and Cu K-edge XANES patterngenerated using ab initio multiple scattering calculation
code, FEFF8.4 and the greater Cu 1s-3d pre-edge peakintensity directly supported model B’ as linkers forCu/MIL125 [1]. However, model A’ may be also
acceptable.
AcknowledgementsThe authors are grateful for the financial supports fromthe Grant-in-Aid for Scientific Research C (22550117,26410204) from Japan Society for the Promotion ofScience.
Reference[1] Y. Yoshida and Y. Izumi, J. Catal. 332, 1–12 (2015).
* yizumi@faculty.chiba-u.jp

N
3.6(±0.6)

R(nm)0.195(±0.001)

Cu(–O–)M(M = Ti, Cu)
N
R(nm)
2.8
0.310
(±2.2) (±0.006)