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

10B/2003G256

Local structure analysis for nitride clusters capsulated in zeoliteby means of XAFS
Takeshi KUBOTA*1, Takeshi KADONO1, Yasuaki OKAMOTO1
1Department of Material Science, Shimane University,Matsue, Nishikawatsu, Shimane 690-8504, Japan

121.4
101.2

Fourier Transform / a.u.

Experimental
Catalyst preparation-3
After evacuation at 673 K for 1h (<1×10 Pa),azeolite sample (0.1g) was exposed to a vapour ofMo(CO)6 for 16 h at room temperature (RT), followed byevacuation for 10 min at RT to remove physisorbed
Mo(CO)6 on the external surface of the zeolite. The
amount of immobilized Mo(CO)6 was two molecules persurparcage (SC) of the zeolite according XRF analysis.
Mo(CO)6/NaY was nitrided with ammonia of 15 kPa(99.999%). This treatment of ammonia was performed
using a closed circulation system. Before the introductionof NH3, Mo(CO)6/NaY sample was evacuate as thetemperature was increased from RT to 523 K at a rate of-1ca. 8.0 K min and kept isothermal at 523 K for 15 min.
15 kPa of ammonia was introduced at 523 K andsubsequently, the temperature was ramped to 673 K at
-1
rate of 10 K min and kept at 673 K for 24 h. After thenitridation, the sample was evacuated at 673 K for 1 h.
The Mo nitride catalyst (10wt% Mo) thus prepared isdenoted MoNx/NaY. The amount of H2 and N2 produced
during the nitrization were analysed by an on-line TCD
gas chromatograph (column: active carbon).
XAFS measurement
The Mo K-edge XAFS spectra of the catalysts
before and after nitrization were measured at BL10B of
KEK-PF in a transmission mode a RT. The synchrotron
radiation was monochromatized by a Si(311)monochromator. The EXAFS data were analyzed using aprogram “Rigaku EXAFS 2000 (REX2000)”. The
analysis involves pre-edge extrapolation and backgroundremoval by a cubic spline method to obtain EXAFS
oscillations, which were Fourier transformed from k-1
space 2.98 - 15 nm to R-space.

Result and Discussion
The N/Mo atomic ratio for the sample treated at
673 K was ca. 0.5 after 24 h suggesting the formation ofMo2N clusters irrespective the initial NH3 pressure.
Figure 1 shows the Fourier transforms (FT) of the k3weighted EXAFS oscillations for the MoNx/NaY (67324) and Mo(CO)6/NaY. The Fourier transform of thenitrided catalyst showed the first shell around 0.2 nm,indicating the presence of light atoms such as C, N and/or
O attached to Mo. Assuming a Mo-Mo shell, thecoordination number of the second shell forMoNx/NaY(673-24) was calculated by EXAFS analysis
to be about unity (Table 2), indicating the formation ofhighly dispersed Mo nitride clusters, possibly dimerclusters in the supercage of NaY. The formation of dimer
clusters of Mo oxide, Mo sulfide [1] and Mo oxycarbide[2] have been reported for the oxidation, sulfidation anddecomposition of Mo(CO)6 encaged in NaY, respectively.

1.0µt / a. u.

Introduction
The catalysts have to be treated at high6+temperatures to reduce Mo oxide to lower oxidation
state. Aggregation of the Mo species is resulted from thehigh temperature treatment to form large clusters.
However, to metal nitrides will be easily synthesized at alow temperature when Mo(CO)6 is used as a precursor.
Mo sulfide clusters and Mo oxycarbide species have beensynthesized by use of Mo(CO)6 encaged in zeolite pores
as a precursor as reported previously. These methods will
be applicable to be preparation of molybdenum nitrides
catalysts. We investigated the preparation and local
structure of Mo nitrides encapsulated in an NaY zeolite.

0.8
0.6

864

0.4
20.2
0.0
19980

20000

20020

20040

20060

20080 20100

00

Photon Energy / eV

12345r / 0.1nm

Figure 1 Mo K-edge XANES spectra (right) and Fourier transformsof Mo K-edge EXAFS oscillations (left) for Mo(CO)6/NaY(dashedline), MoNx/NaY nitrided at 673 K for 24 h (thick line), and Mo
metal (foil) (thin line)

Conclusion
In summary, it was found that Mo nitride
clusters, possibly Mo2N dimers, encaged in micropores ofNaY zeolite were prepared by NH3 treatments ofMo(CO)6 at 673 K. The Mo nitride clusters were highly
dispersed due to a low temperature nitridation reaction.
References[1] Y. Okamoto et al., Catal. Surv. Jpn., 5, 3 (2003).
[2] K. Asakura et al., J. Phys. Chem. B, 103, 1059 (1999).
* yokamoto@riko.shimane-u.ac.jp

Users' Report

7