Photon Factory Activity Report 2014 #32 (2015) B

BL-9C/2013G596

XAFS Analysis of Ni Catalyst Supported on SiO2-covered γ-Al2O3
Hiroaki Sai, Shohei Yamashita, Misaki Katayama, and Yasuhiro Inada*
Graduate School of Life Sciences, Ritsumeikan University,
1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
1 Introduction
The Ni species supported on γ-Al2O3 easily converts toNiAl2O4 and its formation is one of the reasons of thecatalytic deactivation [1]. On the other hand, our previous
chemical state analysis of Ni catalysts has been clarifiedthat the stable NiO species is generated on SiO2 by thetreatment under an oxidative environment at elevatedtemperature [2]. In this study, the XAFS analysis of thesupported Ni species on γ-Al2O3 particles, which surface iscovered by SiO2, has been carried out to evaluate theinfluence of the surface chemical species of the supportingmaterial. Such SiO2-covered γ-Al2O3 (SiO2/γ-Al2O3) wasprepared by the chemical modification technique for thesurface of γ-Al2O3, and the Ni species was supported by
the impregnation method. The purpose of this study is toclarify the chemical state variation of the supported Ni
species of Ni/SiO2/γ-Al2O3 in comparison to those of Ni/γAl2O3 and Ni/SiO2.
2 Experiment
To prepare SiO2/γ-Al2O3, the surface activated γ-Al2O3
by the reflux in hydrochloric acid for 6 h at 115 ºC wasmixed with triethyl octyl orthosilicate, and the suspended
solution in toluene was refluxed for 48 h at 120 ºC. The
obtained powder was filtrated, dried under vacuum for 10 hat 110 ºC, and subjected to the calcination in air for 5 h at
850 ºC. All supported Ni catalysts were prepared using
aqueous solution of nickel nitrate by the standard
impregnation method, and were calcined in air for 3 h at600 ºC.
The X-ray absorption fine structure (XAFS)
measurements were carried out at BL-9C of the Photon
Factory (KEK). The in-situ XAFS measurements wereperformed during the temperature-programmed reduction(TPR) by H2 and oxidation (TPO) by O2 from room
temperature to 900 ºC. The gas atmosphere was switched atroom temperature.
3 Results and Discussion
The XANES spectral change during the TPR process ofNi/SiO2/γ-Al2O3 and Ni/γ-Al2O3 was shown in Fig. 1. The
initial spectrum of both catalysts suggested the mixture ofNiAl2O4 and NiO, indicating that the stable NiAl2O4 species
was generated by the calcination procedure in air. The finalspectrum was in agreement with that of Ni foil, and it wasrevealed that both NiAl2O4 and NiO species were reducedby H2 to form the metallic Ni particles. The value of X-ray
absorbance at the initial white-line peak top was plotted inFig. 1(c) as a function of temperature. It was found that thereduction of NiAl2O4 and NiO started at ca. 700 ºC on both
SiO2/γ-Al2O3 and γ-Al2O3, and that the reduction

Fig. 1: XANES spectral change of Ni/SiO2/γ-Al2O3 (a)and Ni/γ-Al2O3 (b) during the TPR process. The changeof X-ray absorbance is plotted versus temperature for theNi/SiO2/γ-Al2O3, Ni/γ-Al2O3 and Ni/SiO2 in (c).
temperature was remarkably increased by ca. 400 ºC incomparison to NiO supported on SiO2.
The XANES spectral change during the TPO process ofthe Ni/SiO2/γ-Al2O3 and the changes of X-ray absorbanceare given in Fig. 2. A clear spectral change from metallic Ni
to NiO was observed by the TPO procedure. Both thecomplete agreement of the final spectrum with NiO and thesimilar oxidation temperature to Ni/SiO2 indicates that theformation of catalytically inactive NiAl2O4 is suppressed by
the surface coverage of γ-Al2O3 with SiO2.

Fig. 2: XANES spectral change (a) and the X-rayabsorbance change as a function of temperature (b) ofNi/SiO2/γ-Al2O3 during the TPO process. In (b), theabsorbance change is compared with that of Ni/SiO2.
References[1] S. Wang and G. Q. (Max) Lu, Ind. Eng. Chem. Res., 38,2615-2625 (1999).
[2] S. Yamashita, M. Katayama, and Y. Inada, J. Phys.
Conf. Ser., 430, 012051 (2013).
* yinada@fc.ritsumei.ac.jp