Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

12C/2004G333

EXAFS studies on the Ni/SiO2 catalysts prepared from t-BuONa-Ni colloid
Tetsuya Shinozaki, Nobuyuki ICHIKUNI*, Shogo SHIMAZU
Chiba University, Inage-ku, Chiba 263-8522, Japan
Introduction
Colloidal metal particles have been expected as asuperior catalyst precursor because of their small particle
size and the narrow size distribution. But the stabilizer,
which is needed to synthesize metal colloid, remained on
the metal surfaces and lead to lower the metal surfacearea as well as the catalytic activity. Thus, the effective
removal of the stabilizer from the colloidal metal particles
on the catalysts is expected.
Ni catalysts are very useful for several catalytic
reactions. But the size of Ni particles on catalyst is
typically 20 - 100 nm. It is difficult to prepare small Ni
particle on the support because of magnetic attractions ofNi metal. The study on the nano-sized Ni catalysts isvery few.
In this study, we prepared Ni/SiO2 catalysts from tBuONa stabilized Ni colloid by impregnation method.
Size of Ni particle and chemical state of Ni/SiO2 catalyst
was characterized by EXAFS analysis.

could be attributed Ni-Ni coordination in the Ni metal.
NiO on Ni/SiO2 catalysts was reduced into Ni metal by H2
treatment at 573 K.
Coordination number of Ni-Ni in Ni/SiO2 (H2)catalyst was 8.0 as shown in Table 1. If Ni particle on thecatalyst was in the cuboctahedron structure, the size of Ni
particles can be estimated at less than 2 nm.
It can be said that impregnation method by using tBuONa-Ni colloid can provide the small Ni particles.
Since Ni species on Ni/SiO2 catalysts is in oxidized form,hydrogen treatment over 573 K is needed to apply it tothe catalytic reaction. Although the hydrogen treatment
temperature is high, such as 573 K, Ni particle still
remained small.

(a)
Experimental
The overall preparation process of Ni colloid was
carried out under N2 atmosphere. Ni colloid wassynthesized by reduction of Ni(OAc)2 using NaH-tBuONa in refluxing THF [1]. 3 wt% Ni/SiO2 catalyst wasprepared by impregnation of SiO2 (Aerosil#200) with Ni
colloid solution. In order to remove the residual stabilizer,hydrogen treatment was carried out at 573 K. Catalysts
were denoted as Ni/SiO2 (as synt) and Ni/SiO2 (H2) for
before and after the hydrogen treatment, respectively.
Ni K-edge EXAFS spectra were measured at BL-12Cof the Photon Factory with Si(111) double crystal
monochromator. The powder samples were transferred
into 3 mm length of Al cells with Kapton windows.
3Curve-fitting analyses of k -weighted EXAFS oscillationsin the k-space were performed by the EXAFS analysis
program REX2000 (Rigaku Co.). Model parameters forcurve-fitting analysis (back scattering amplitude andphase shift) were extracted from an EXAFS oscillationcollected for bulk Ni (N =12, r =0.2492 nm).
Results and discussion
FT of Ni K-edge EXAFS spectra for Ni/SiO2 catalyst
was shown in Fig. The FT profile of Ni/SiO2 (as synt)
was similar to that of bulk NiO. Thus, Ni species onNi/SiO2 catalyst was in oxidized form. It is suggested
that the colloidal Ni particle was easily oxidized in air
atmosphere during the impregnation method due to itssmallness.
To remove the stabilizer, Ni/SiO2 catalyst was treatedin hydrogen at 573 K. The main peak of Ni/SiO2 (H2)122

Users' Report(b)

(c)
(d)
0.0

0.1

0.2

0.3
0.4
0.5
0.6
r / nm
3Figure FT of k -weighted Ni K-edge EXAFS; (a) NiO(x0.5), (b) Ni/SiO2 (as synt), (c) Ni foil (x0.5), (d)Ni/SiO2 (H2).
Table 1: Curve fitting result for Ni-Ni coordination
sampleNr / nm
dE / eV DW / nm
Ni/SiO2 (H2)
8.00.247-7.522
0.008
bulk Ni
120.2492
0.00.006
Reference[1] Pierre Gallezot, Christiane Leclercq, Yves Fort andPaul Caubere, J. Mol. Catal., 93, 79-83 (1994)* ichikuni@faculty.chiba-u.jp