Photon Factory Activity Report 2019 #37 (2020)

BL-9C/2019G649

Reduction Property of NiO Species on Metallic Ni Core under CH4 Atmosphere

1 Introduction
The supported Ni particles are widely used as catalysts
for dry and steam reforming reaction of methane. Under
catalytic reaction conditions, the Ni species is exposed by
the oxidative and reductive molecules including in thereactants and products gases. The chemical state
conversion of the Ni species may be caused by the reaction
with those molecules, and it affects the catalytic activity.
The redox property of the Ni species located at the particle
surface is important for catalyst design, because thecatalytic reaction is proceeded at the particle surface. The
purpose of this research is to clarify the reduction propertyof the NiO species located at the surface of metallic Ni
particles under the CH4 atmosphere.
2 Experiment
The SiO2-supported Ni catalyst (5 wt%) was preparedby the impregnation method using aqueous solution ofNi(NO3)2. The stoichiometric amount of citric acid relative
to Ni2+ was added in the precursor solution to control theNi particle size. The average Ni particle size wasdetermined to be 3 nm by the TEM observation.
The in-situ XAFS measurements using the flow-type
cell made of SiO2 glass was performed at BL-9C station ofPF (KEK). The surface of the Ni(0) particle was oxidized
by the O2 exposure at room temperature [1,2]. After that,the chemical state analysis on the Ni species wasperformed during the temperature-programmed reduction(TPR) process under the 10 vol% CH4/He gas flow (100cm3/min).
3 Results and Discussion
The metallic Ni particle generated by the reduction withH2 was exposed to the O2 environment at room temperature,and the fraction of NiO was estimated to be 45% by thelinear combination fitting analysis based on the XANESspectrum. The thickness of the surface NiO layer wascalculated as ca. 3 Å based on the fraction of NiO and theparticle size. It was suggested that the single surface layerof the Ni particle was locally oxidized [1,2].
Figure 1 shows the composition changes of the Ni
species during the TPR process. The surface NiO specieson the metallic Ni core was reduced at 300 °C by CH4. Itis ca. 200 °C lower than the reduction temperature of NiO
particles under the CH4 atmosphere. Under the H2atmosphere, the NiO species located on the metallic Ni
core is reduced at much lower temperature of around
120 °C. These results indicate that the NiO species locatedon the metallic Ni core has an excellent ability to release

Mole Fraction of NiO Mole Fraction of NiO

Yusaku YAMAMOTO*, Hitoe NONOBE, Shunsuke OSAKA,
Shinji OKAMOTO, and Yasuhiro INADA
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University,
1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan

1.0(A)

0.8
0.6
0.4
0.2
0.0
1.0(B)

0.8
0.6
0.4
0.2
0.00100

200300400500600

700

T / °CFig. 1: The temperature changes of NiO fraction during
the TPR process under CH4 (A) and H2 (B)atmosphere. The solid lines and dashed lines show thereduction of NiO species on metallic Ni core and NiOparticle, respectively.
the oxide ion. In addition, the Ni particle surface is easily
oxidized under the mild condition such as roomtemperature. It is expected that these unique characteristics
promote the catalytic reactions driven by the redox
reactions of the active Ni particles.
The reduction of NiO by CH4 was proceeded at highertemperature (ca. 520 °C) than by H2 indicating the lower
reducibility of CH4. It was remarkable that the reduction ofNiO under CH4 was rapidly completed at that temperature,
which was contrast to the corresponding process under H2.
At the beginning of the reduction of NiO by CH4, thegenerated Ni(0) species drives the catalytic CH4
decomposition to generate the H2 molecules, which canenhance the reduction of remaining NiO.
References[1] Y. Yamamoto, S. Yamashita, N. Afiza, M. Katayama,
and Y. Inada, J. Phys. Conf. Ser. 712, 012075 (2016).
[2] Y. Yamamoto, A. Suzuki, N. Tsutsumi, M. Katagiri, S.
Yamashita, Y. Niwa, M. Katayama, and Y. Inada, J.
Solid State Chem. 258, 264 (2018).
* y-yusaku@fc.ritsumei.ac.jp