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

10B/2004G110

Mo K-Edge XAFS Study of Active Mo-Carbide Species on H-MFIfor Dehydroaromatization
Hirofumi ARITANI,*1 Satoru SHINOHARA,1 Atsushi NAKAHIRA2 and Tsunehiro TANAKA3
1Saitama Institute of Technology, Okabe, Saitama 369-0293, Japan
2
Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
3
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
Introduction
Many workers have focused on this catalyst system as
a novel GTL (Gas-to-Liquid) process from natural gas topetroleum products. They have been revealed that
reduction of Mo6+ ions is brought about in contact withmethane in the initial step, and then Mo ions react
methane to form carbide species, Mo2C, in the next step.1
It is thus accepted that active Mo species are consist of
reduced ions, Mo2+, with Mo-carbide and/or oxycarbide
species on H-MFI. However, deactivation cannot beavoided by carbon deposition on the catalyst surface atthe same time. As a responsible Mo species, α-Mo2C(fcc), β-Mo2C (hcp), and/or oxycarbide-type species havebeen proposed. However, detail of the active Mo species
on both H-MFI and other silica-alumina supports has
been still unclear because of difficulty of reduced Moions (Mo2+). By XRD, crystallized species can only beseen. XPS analysis is difficult because of reduction
during the measurement. Mo K-edge XAFS is very
sensitive to reflect the redox behavior of Mo ions between
before and after the reaction with methane. In particular,
it is important to evaluate the formation of active Mo sites
with reduced ions in Mo-carbide and/or oxycarbide
species. We characterized Mo species on H-MFI withseveral SiO2/Al2O3 ratios before/after the dehydroaromatization of methane (at 973 K for 3 hours) by Mo
K-edge XAFS.
Experimental
Supported molybdena catalysts were prepared byimpregnation of each support with an aqueous solution ofammonium heptamolybdate. The supports described inthis report were H-MFI with various SiO2/Al2O3 ratios
synthesized by hydrothermal synthesis (at 413 K for 7days) and followed by calcination (at 873 K) and ionexchanging by using 0.5 N NH4Cl. The impregnate
solution of ammonium heptamolybdate was stilled atroom temperature and evaporated at 343 K for 6 hours,and then, the paste was dried for overnight and calcined at773 K for 6 hours. Mo K-edge XAFS spectra weremeasured at BL-10B of KEK-PF in a transmission modeat room temperature. FT-EXAFS results were obtained by
FT of normalized EXAFS (k3-weighted) spectra in therange of ∆k = 3.5 - 14.5 A-1.
Results and discussion
Mo K-edge XANES (Fig. 1) and FT-EXAFS (Fig. 2)of MoO3/H-MFI before/after the dehydroaromatization

110Users' Report

reactionwithmethane are shown.
Maximum activity
forbenzene
production is shownin Mo/H-MFI in
The
Si/Al2=90.
XANES shows the
reduced state of Mo
ions;Mo-carbide(Mo2+) species areformed on H-MFI.
Theenergyofabsorption edge (inFig. 1) of Mo/MFI in
Si/Al2=90 is quite
similar than that ofMo2C, indicating aformation of carbide
species dominantly.
It is suggested that
H-MFI support inhigh Si/Al2 ratioshows
thecoexistence of carbide(in major) and oxide
species. In Fig. 2, theFT peak at ca. 2.7 Å
due to Mo-C scattering is clearly be seen in all the Mo/HMFI samples. In case of Si/Al2=90, the Mo-C peak withlarge intensity is appeared while it is sammer than bulk
Mo2C. In adition, a peak at 1.2 Å with small intensity(due to Mo-O) can also be seen. These results strongly
suggest a formation of carbide micromarticles which ispartly oxidized. Thus, so-called oxycarbide species ispresent onto well-despersed Mo2C on H-MFI. Thissuggestion is supported by the results obtained by Mo
LIII-edge XANES.3 Detail of the characterization of active
Mo species is now in progress.
References[1] D. Ma, Y. Shu, M. Cheng, Y. Xu, and X. Bao, J.
Catal. 194, 105 (2000).
[2] W. L. George, D. Meitzner, R. W. Borry III, and E.
Iglesia, J. Catal. 191, 373 (2000).
[3] H. Aritani, S. Shinohara, S. Koyama, K. Otsuki, T.
Kubo, A. Nakahira, Chem. Lett., 35 (2006) 416-417.
*aritani@sit.ac.jp