Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

NW2A/2003G294

Temperature Dependence of Reduction Processes of Cu Species
Supported in Zeolite Measured by DXAFS
Yasuhiro INADA1*, Akane SUZUKI1, Shigenobu FUNAHASHI2, Masaharu NOMURA1
1
KEK-PF, Tsukuba 305-0801, Japan
2
Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
Introduction
The Cu species supported in MFI zeolite (Cu–ZSM-5)
have been characterized by the XAFS spectroscopy and it
has been clarified that the CuO-like state existed initiallyis reduced by CO to the metal-like Cu(0) state. The
dynamic processes have been directly monitored by using
the time-resolved dispersive XAFS (DXAFS) instrument
at 773 K, and the transient Cu(I) states are demonstrated
to exist during the reduction process. The CO pressure
dependence of the conditional rate constants derived by
the kinetic analyses reveals the reaction mechanism in theatomic scale. In this study, the time-resolved DXAFS
measurements have been carried out at various
temperatures to determine the activation parameters of thecomponent processes. Although the determination ofparameters has not been finished yet, an interesting
reduction behavior is observed at lower temperature.
Experiments
The Cu–ZSM-5 powder was prepared by an ionexchange method at ambient temperature and pH 5.2–5.8
using an aqueous solution of Cu(NO3)2 suspended by
ZSM-5 (Tosoh, Si/Al = 11.9). The ZSM-5 powder
adsorbed Cu(II) was filtered, washed by distilled water,
dried at 393 K for 12 h in air, and calcined at 773 K in airfor 1–2 h before the use.
The time-resolved DXAFS measurements were carriedout at the NW2A beamline using an Si(111) bent crystal(Bragg-type) and a self-scanning photodiode array
coupled by a CsI(Tl)-embrocated fiber optical plate.
The reduction processes of Cu–ZSM-5 by CO were
followed at 523–773 K under various pressures of CO.
The reaction was initiated by an injection of CO gas witha known pressure. The XAFS spectrum was recordedwith the acquisition time of 4–8 ms depending on the ring
current.
Results and Discussion
The observed time-resolved XANES spectra are shownin Figure 1. At 773 K, the reduction proceeds in twophases, i.e., the initial Cu(II) state is finally reduced toCu(0) via the Cu(I) states as the transient species, asreported previously. While at lower temperature at 523 K,
the final state is not Cu(0) and the reduction is found to beterminated at the Cu(I) state. No further reduction fromCu(I) to Cu(0) was checked by the static XAFS
measurement at 1 h after the initiation of the reaction,
indicating that the reduction from Cu(I) to Cu(0) is44

Users' Report

Figure 1: Time-resolved XANES spectra during thereduction of Cu–ZSM-5 by CO at 523 and 773 K.

Figure 2: X-ray absorbance change at 8.998 keVand various temperatures. The absorbance of 2.060,2.005, and 1.965 corresponds to the Cu(II), Cu(I),and Cu(0) state, respectively.
thermodynamically unfavorable. In Figure 2 are shown
the X-ray absorbance changes followed at 8.998 keV forthe reactions with CO of 6.2–6.7 kPa except for at 523 K,
for which the CO pressure is 9.2 kPa. The kinetic
analyses of the data at high temperature range reveal thatthe reaction model with two or three Cu(I) intermediates
reproduces well the observed kinetic trances. In contrast,
at 523 K, the reaction seems to be proceeded by a single
step as suggested by the existence of isosbestic points
seen in Figure 1. This is not contradicted with the results
at higher temperatures, since the conversion rate between
the Cu(I) species is faster than the first reduction process.
* yasuhiro.inada@kek.jp