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

NW2A/2005G231

Time-resolved structure change of Pt/MCM-41 by the adsorption of Hydrogenand Oxygen
Akane Suzuki1,Yasuhiro Inada1, Kiyotaka Asakura2, Masaharu Nomura1*
1: Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan.
2: Catalysis Research Center Hokkaido University, Sapporo 001-0021, Japan

Experimental
3.5 wt % Pt/MCM-41 catalyst was prepared byion-exchange method using an aqueous solution (pH 9)of [Pt(NH3)4]Cl2 (Soekawa Chemical Co.).
Theobtained sample was dried in an oven at 353 K
overnight and calcined at 573 K in a flow of O2(200ml/min).
The DXAFS measurements were performed atNW2A. A Si(111) crystal was bent into the radius of 3m. The DXANES spectra were recorded every 60ms.
Data analysis was performed by using UWXAFS
package and the backscattering amplitudes, phase shift
and mean free paths were calculated with the FEFF8code.
Results and Discussion
The DXAFS spectra can be analyzed up to k = 140-1
nm . Fig.1 shows the variation of XANES at the PtLIII
edge during the reduction process of Pt/MCM-41 under
20 Torr of H2 at 298 K. With the increase of H2
exposure time, the XANES spectra were dramaticallychanged between 11564 and 11584 eV. The intensityof white line (marked A) decreased and a shoulder
appeared in the region of 11574-11584 eV (marked B).
This shoulder increases as the increase of adsorbedhydrogen and was ascribed to the multiple scatteringfrom Pt-H. The DXAFS study revealed that thestructural transformation during H2 adsorption anddesorption proceeds through several steps. After theadsorption of hydrogen, the icosahedral Pt13 clusterwith an average Pt-Pt distance of 0.273 nm is formed.
When the hydrated cluster is exposed to O2, hydrogen isreleased at first (900 ms) and the cluster becomes
smaller (1.8s), then N(Pt-O) increases from 0.3 to 1.1
rapidly (4s). It is suggested that the formation of small
Pt cluster is driven not by the adsorbed oxygen but byhydrogen desorption.
After that, N(Pt-O) slowly

Acknowledgement
MCM-41 was prepared by Dr. Kazu Okumura.

AB1.0µtIntroduction
Platinum catalysts are easily reduced by hydrogenand also oxidized by oxygen at RT. Although theoxidized and reduced species were studied intensively,
the reaction mechanism was not clarified yet. We haveclarified the adsorption and release mechanism ofhydrogen to/from Pt clusters on an MCM-41 surface by
the dispersive XAFS (DXAFS) technique.

increases (120s). On the other hand, when H2 isintroduced to the oxidized cluster, N(Pt-O) decreases
and Pt-H interaction increases within 1s and thisreaction is the driving force of reaction. Then N(Pt-O)
decreases from 1.6 to 0.7 (2s) and N(Pt-Pt) increases,suggesting the formation of Pt cluster frame (4s). Pt-Pt
interaction increases and its distance become longer bythe adsorbed hydrogen and finally forms Pt13 cluster(160s).
The present study clarified the dynamic changeof Pt cluster framework by DXAFS.

0.9
0.8
0.7
11560

11600

E/ eVFigure 1.
A series of Pt LIII -DXANES spectra at Pt LI-edge of 3.5 wt% Pt/MCM-41 under the reduction process.

[1] K. Asakura, T. Kubota, N. Ichikuni and Y. Iwasawa
Proc. 11th Int. Congres on Catalysis, Baltimore, Stud. Surf
Sci. Catal, Vol. 101, 911 (1996).

*masaharu.nomura@kek.jp

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