Photon Factory Activity Report 2014 #32 (2015) B

NW2A/2013G596

Dynamic Chemical State Change of SiO2-Supported Ni Species under CO–NOReaction Conditions As Studied by Time-Resolved DXAFS Technique
Tetsuya Matsuoka, Shohei Yamashita, Yusaku Yamamoto, 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 characterization of SiO2-supported Ni species has
been systematically performed under the reductive (H2 andCO) and oxidative (O2 and NO) reaction gas atmosphereby means of in-situ XAFS method at elevated temperature[1]. The XAFS analysis revealed that the NiO species onSiO2 was quantitatively reduced to metallic Ni(0) and thatthe reduction reaction completed at ca. 750 K. It was also
evaluated that the oxidation of the supported Ni(0) species
to NiO occurred at ca. 800 K under the dilute NO gas flow.
The chemical conversion of the Ni species on SiO2 offers
an idea that the oxidation of CO and the reduction of NO(the CO–NO reaction) simultaneously progress under theexistence of the SiO2-supported Ni catalyst at the elevatedtemperature higher than 800 K. The CO–NO reaction andits achievement without any rare metal catalysts areimportant to solve the environmental problems due to thetoxic emissions produced in the modern society.
The dynamic and cyclic changeover of the chemical
state is expected for the SiO2-supported Ni species, andthus the static observation is not adequate to evaluate thecontributions of the Ni species to the CO–NO reaction. In
this study, the time-resolved DXAFS experiments havebeen applied to detect the dynamic chemical state changeof the Ni species after a rapid injection of the reaction gas
in order to clarify the catalytically active Ni species.
2 Experiment
The SiO2-supported Ni catalyst was prepared by theimpregnation method using an aqueous solution of nitrate
salt of Ni. The obtained sample was calcined at 600 ºC in
air. An adequate amount of sample powder was mounted ina SUS tube, and it was installed in an observation chamber.
The time-resolved DXAFS measurements were performed
at NW2A (PF-AR, KEK) using a Si(111) bent crystal as
the polychromator and a photodiode array detector with aCsI(Tl) phosphor. The reaction gas (CO, NO, and theirmixture) was injected to the evacuated sample chamber tostart the CO–NO reaction. The successive acquisition ofthe transmitted intensity with the typical exposure time of
30 ms was synchronized by the gas injection. The timeresolved DXAFS measurements were carried out at 750 ºC.
3 Results and Discussion
The XANES spectral change for the reduction of NiO
by CO and the oxidation of metallic Ni(0) by NO wasshown in Fig. 1. Both of the reactions were clearlyproceeded at 750 ºC, and the apparent reaction rate wasdependent on the injected gas pressure. The typicalobservation time was ca. 120 s for the reduction and ca.

Fig. 1: Dynamic XANES change for the reduction (a) ofNiO by CO and the oxidation (b) of metallic Ni(0) by NO
after the rapid injection of reaction gas measured at 750ºC.
100 s for the oxidation. The appearance of some isosbestic
points indicates no existence of other metastable Ni species
between NiO and Ni(0).
The time-course change of the observed X-ray
absorbance at the white line peak energy of NiO was givenin Fig. 2 for the reduction of NiO by CO, the oxidation ofNi(0) by NO, and the reaction of Ni(0) with the CO and NO
gas mixture. The single step reaction was observed for theone-way reactions between NiO and Ni(0), whereas theinitial change from Ni(0) to NiO by the oxidation due toNO was returned by the existence of CO with the reducingability. The two-phase change suggests that the CO–NO
reaction is actually undertaken and catalyzed by the SiO2supported Ni catalyst.

Fig. 2: X-Ray absorbance change for the reduction ofNiO by CO, the oxidation of Ni(0) by NO, and the
CO-NO reaction of Ni(0).
Reference[1] T. Matsuoka, et al., PF Activity Report 2013, 31B, 262(2014).
* yinada@fc.ritsumei.ac.jp