Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

9C/2004G111

XAFS study on local structure of highly-functionalized metal catalysts
immobilized on the surface of inorganic crystals
Kiyotomi Kaneda*
Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Introduction
Apatites and related compounds are of great interest
because of their structural stability and their potential
ability for cationic and anionic isomorphous substitution[1]. The hexagonal hydroxyapatite (HAP) structure
32+comprises Ca sites surrounded by PO4 tetrahedra. The
chemical composition of HAP can be modified from thestoichiometric form, Ca10(PO4)6(OH)2, to the nonstoichiometric form, Ca9(HPO4)(PO4)5(OH). We havefocused on doping the apatite surface with varioustransition metal cations, which can function as
2+
catalytically active centers, via cation exchanged at Ca
sites[2]. Recently, a calcium vanadate apatite (VAp),
3Ca10(VO4)6(OH)2, was developed by substituting VO4 for
3PO4 in the whole apatite matrix as a nano-structured
heterogeneous catalyst for highly efficient carbon-carbon
bond forming reactions under aqueous conditions[3]. In
this report, local structures of V species in the VAp
catalyst were studied by V K-edge XAFS.
Experimental
The VAp was prepared from an aqueous solution ofCaSO4·2H2O and Na3VO4, followed by calcinations at800oC[3]. Elemental analysis showed the amounts of Caand V were 35.0 and 26.6 wt%, respectively. X-ray
absorption spectra were measured in a transmission modeat the EXAFS facilities of KEK-PF, Tsukuba. Details ofdata analysis are referred to the reported procedure[4].
Results and Discussion
In V K-edge X-ray absorption near edge structure
spectra, the VAp showed a strong pre-edge peakcomparable to those seen for K3VO4. The energy
positions of pre-edge peak, of the main absorption edge
and of the peak due to 1s-4d dipole-allowed transition inthe VAp were good agreement with those observed forK3VO4, respectively.
As depicted in Figure, in the Fourier transforms of k3weighted extended X-ray absorption fine structure data,
peaks due to V-O-V bonds in the second coordination
sphere, detectable at 2.8 Å for V2O5, were not observedfor either K3VO4 or VAp. These results imply that thevanadium species in the VAp catalyst exists in an isolated
tetrahedral geometry with an oxidation state of +5. The
above conclusion from the V K-edge XAFS measurement
51
is well supported by the results of V NMR, UV-vis, andRaman techniques[3].
The VAp acted as heterogeneous and recyclable
catalyst in various types of carbon-carbon bond58

Users' Reportformations under aqueous conditions[3]. Correspondingly,
stability of the V structures in the recovered VAp catalyst
was straightforwardly established by XAFS measurement.
The present preparation method using the character ofapatites can provide a powerful protocol to generate acatalytically active and robust metal species within theapatite frameworks.

5(c)(b)
(a)
01

2

3

4

56

Interatomic distance (Å)
Figure: Fourier-transforms of k3-weighted V K-edge
EXAFS of (a) V2O5, (b) K3VO4, and (c) VAp. EXAFS
were transformed in the range of 3.5~11.0 Å. The phaseshift was not corrected. The peak at 1.5 Å corresponds toV=O and V-OH shells.
References[1] J. C. Elliot, Structure and Chemistry of the Apatites
and Other Calcium Orthophosphates, Elsevier, 1994.
[2] For example, K. Kaneda et al., New J. Chem. 30, 44(2006); Chem. Commun. 3331 (2005); Tetrahedron Lett.
46, 4283 (2005); J. Am. Chem. Soc. 126, 10657 (2004);
125, 11460 (2003); 124, 11572 (2003); 122, 7144 (2000).
[3] K. Kaneda et al., J. Am. Chem. Soc. submitted.
[4] T. Tanaka et al., J. Chem. Soc., Farad. Trans. 84, 2987
(1988).
*kaneda@cheng.es.osaka-u.ac.jp