3D Structures of Single Metal Sites Formed on a Single-crystal
Oxide Support Studied by PTRF-XAFS Technique
Satoru Takakusagi*1,2 and Kiyotaka Asakura2*Email: takakusa@cat.hokudai.ac.jp
1Research Cluster of Well-defined Surface Nanostructures, Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
2Institute for Catalysis, Hokkaido University, Sapporo 001-0021 Japan

Introduction

Experimental

Dispersion control of metal is highly important in an oxidesupported metal catalyst to optimize its catalyst performance.
Since the metal/oxide interaction is rather weak for mostmetal/oxide systems, the metal atoms are easily aggregatedto form particles. But by premodification of an oxide surface
with an organic molecule having a functional group whichstrongly coordinates to metal, we can expect high dispersionof the metal, ultimately “single metal dispersion.(1)~(6).
Here, 3D structures of Cu, Au, Ni and Pt evaporated on aTiO2(110) surface premodified with ortho-mercaptobenzoic
acid (o-MBA) were studied by polarization-dependent totalreflection fluorescence (PTRF) XAFS technique to determine
the effects of the premodification on the dispersion of themetal atoms. Atomically dispersed metal species wereformed for Ni, Cu, and Au but not for Pt. We discuss theorigin of the different behaviors of these metals by adopting a
new indicator for single metal dispersion.

・Preparation of an atomically flat TiO2(110) surface
A Nb-doped(0.05 wt%) TiO2(110)(Rutile） substrate wasimmersed in10% HF solution and annealed in air. （700℃1h）(Yamamoto et al., Jpn. J. Appl. Phys. 44 (2005) L511、Nakamura et al., J. Phys. Chem. B 109 (2005) 1648)

PTRF-XAFSEXAFS Elements of neighboring atom
 Coordination number Bond length Disorder (Debye-Waller factor)

Polarization dependence of EXAFS
χ (k ) = 3∑ χ i (k ) cos θ i2(K, L1-edge)χ (k ) = ∑ χ i (k )(0.7 + 0.9 cos 2 θ i ) (L2, L3-edge)
EXAFS oscillation derived from i-th scattering atom;χi(k)X-ray・o-mercaptobenzoic acid (o-MBA) on TiO2(110)
The TiO2(110) substrate was then immersed in the ethanol
solution containing1 mM o-MBA for >24 h.

Electricvector

i-th scattering atom

Ri θiAbsorbing atom・Cu , Au, Ni or Pt deposition
Cu , Au, Ni or Pt was vacuum-deposited on
the o-MBA-modified TiO2(110).
Coverage was determined by XPS.
(0.42 ML for Cu, 0.11 ML for Au, 0.27 ML for
Ni and 0.11 ML for Pt. 1 ML= 5.2×1014/cm2).

・PTRF-XAFS measurements
UHV PTRF-XAFS apparatus(< 8×10-8Pa).
BL9A at PF KEK, Japan.
Data analysis: REX2000, FEFF8.4

Three-dimensional structure can be determined.
Fluorescence detection→enable to detect ~1013 atoms / cm2
Total reflection mode
→ X-ray penetration depth decreases to a few nm.

(1) S. Takakusagi et al. The Chem. Rec., in preparation.
(2) S. Takakusagi et al., J. Phys. Chem. C., 120 (2016) 15785.
(3) S. Takakusagi et al., Phys. Chem. Phys. Chem., 15 (2013) 14080.
(4) S. Takakusagi et al., Top. Catal., 15 (2013) 14080.
(5) K. Asakura, S. Takakusagi et al., Faraday Discuss., 162 (2013) 165.
(6) W.-J. Chun et al., Chem. Phys. Lett., 433 (2007) 345.

TiO2(110)

Cu/o-MBA/TiO2(110)“PTRF-EXAFS is a powerful technique to investigate
3D structure of highly dispersed metal species on asingle crystal surface.”

Metal/o-MBA/TiO2(110)o-MBA/TiO2(110)Au/o-MBA/TiO2(110)

Cu K-edge spectra of Cu/o-MBA/TiO2(110)Pt/o-MBA/TiO2(110)Pt L3-edge spectra of Pt/o-MBA/TiO2(110)

Au L3-edge spectra of Au/o-MBA/TiO2(110)

0.10
0.05

0.00

0.00

-0.02

56

7

8

9

E//[110]0.05

3

AFM image-0.02

observed
calculated

-0.04

-0.10
40.10

χ(k)

χ(k)

0.00

E//[110]

0.02

0.02

-0.05

45

6 7 8 9k / 10 nm-1

0.04

-0.04
310 1145

6 7 8 9k / 10 nm-1

10 11

E//[110]

0.02

χ(k)

0.00

0.00

-0.05

-0.02

-0.10
40.10

5678-0.04

93

4

5

E//[110]

0.05

χ(k)

0.04

E//[001]

χ(k)

χ(k)

0.04

E//[001]

0.00

6 7 8 9k / 10 nm-1

Pt atoms were aggregated to form small clusters.

10 11Factors governing single
metal dispersion

Formation of S-Au-O structure.

-0.05

Au-S: 0.232 nm, Au-O: 0.210 nm-0.104567k / 10 nm-1

89Cu-S bond (0.219±0.003 nm) and Cu-O bond
(0.185±0.003 nm). No Cu-Cu bond, suggesting Cu
was atomically dispersed !

Ni K-edge spectra of Ni/o-MBA/TiO2(110)E//[001]

-0.05
4567
k / 10 nm-1

8-0.1090.10
E//[110]χ(k)4567
k / 10 nm-1

89observed
calculated
450.106767

k / 10 nm-10.00-0.05

8989°(𝑀𝑀 − 𝑀𝑀�𝐷𝐷298-0.10

4567

k / 10 nm-189Strengths of metal-metal (M-M), metal-sulfur (M-S) and metal-oxygen (M-O) bonds°) for Ni, Pt, Cu, and Au, and the(bond dissociation energies at 298 K, 𝐷𝐷298
corresponding RS-M-O and RO-M-O values.

E//[110]
0.05

Ti

0.00

0.00

-0.05

-0.05
-0.10

0.00

-0.10

E//[110]

0.05

-0.05

χ(k)

χ(k)

0.05

0.00

-0.05

observed
calculated

0.05

°°
𝐷𝐷298
(𝑀𝑀 − 𝑆𝑆) + 𝐷𝐷298(𝑀𝑀 − 𝑂𝑂�The RS-M-O values for Ni, Pt, Cu, and Au are 3.6, 1.6, 3.1, and 2.8, respectively.
Thus, the RS-M-O value for Pt is the lowest, indicating that this metal generates theleast stable S-metal-O bonding motif and forms small clusters.

0.10

E//[001]

E//[110]

0.05

χ(k)

χ(k)

0.00

-0.10

0.10

0.10

χ(k)

0.10

𝑅𝑅𝑆𝑆−𝑀𝑀−𝑂𝑂 =Ni/o-MBA/TiO2(110)

Structure model of Cu/o-MBA/TiO2(110)(FEFF simulation)
0.05

We defined a term, RS-M-O, that roughly estimates the formation preference andenergetic stability of the S-metal-O bonding motif compared with metal-metalformation, as follows.

-0.10
[110]

4567k / 10 nm-1

89

Formation of a novelsurface S-Cu-O complex .

45k / 10 nm-1°
298 (M-S) and 𝐷𝐷298 (M-O) for Pt are taken from Catal. Today 50 (1999) 629 and J.

Formation of S-Ni-O structure.
Ni-S: 0.219 nm Å, Ni-O: 0.185nm

Cu-S: 0.219 nm, Cu-O: 0.185 nm

a 𝐷𝐷 °

Cu-S bond 2.17Å
Cu-O bond 1.85Å

Therm. Anal. Calorim. 69 (2002) 831, respectively. b Chem. Phys. Lett. 421 (2006)
27, c Chem. Phys. Lett. 470 (2009) 99, d PF activity report Part B, 24 (2006) 26.
○ and × indicate atomic dispersion of the evaporated metal atoms andaggregation of them, respectively.

Summary and Future propects● Cu, Au and Ni were found to be atomically dispersed on the o-MBA/TiO2(110) surface with formation of the S-metal-O bonds (where S is provided by o-MBA and O is present
in the TiO2 lattice). On the other hand, Pt formed small clusters although Pt-S bond was detected and the premodification effectively suppressed significant aggregation.
● The energetic stability of the S-metal-O motif was discussed based on the difference between the S-metal-O and metal-metal bond formation energies. A new indicator,
RS-M-O, has been proposed to explain single metal dispersion on the o-MBA/TiO2(110) surface.

Stepwise synthesis of dimer, trimer, tetramer species etc. might be possible by controlling coverages both of metal atoms and functional organic molecules.
“We are currently investigating 3D structure-activity relationship for various catalytic reactions”
We thank many collaborators for their technical help and the fruitful discussions (Prof. Yasuhiro Iwasawa (The Univ. of Electro-comm.), Prof. Wang-Jae Chun (ICU), Dr. Hiroko ArigaMiwa (Hokkaido Univ.), Dr. Yuki Wakisaka (Hokkaido Univ.), Dr. Yohei Uemura (IMS), Dr. Takahiro Wada (Tokyo medical and dental Univ.), Dr. Hiromitsu Uehara (Hokkaido Univ.)).