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

10B, 12C/2003G092

Surfactant-Promoted Formation of Supported Metallic Cu Nanoclusters for
Selective Dehydrogenation of Methanol
1

Mizuki TADA1, Rajaram BAL1, and Yasuhiro IWASAWA*1
Department of Chemistry, Graduate School of Science, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan

Introduction
Metallic nanoparticles dispersed on robust inorganicoxides are widely used in a variety of catalytic reactions,and effective ways to prepare supported nano-sized metalparticles have been extensively explored as a key issue todevelop new efficient catalytic systems. We have foundthat a surfactant CTAB (cetyltrimethylammoniumbromide) promotes the reductive synthesis of metallic Cu
2+nanoclusters on inorganic oxides from Cu ions without
any additional reducing reagent.
Hydrothermal
treatments of a mixture of Cu nitrate and Mo, Zn, Al, andSi cationic precursors in the presence of CTAB produce
the nano-sized metallic Cu clusters supported on
crystalline Mo, Zn, and Al oxides and amorphous Sioxide. The prepared Cu nanoparticles were active andselective for catalytic dehydrogenation of methanol [1].
Experimental
The preparation of Mo-oxide-supported metallic Cu
nanoclusters is typically described as follows. An
aqueous solution of a given amount of Cu(NO3)2 3H2O
was added dropwise with vigorous stirring tocorresponding precursors of Mo, Zn, Al and Si in hotwater. Then an aqueous solution of CTAB was added tothe mixture of two metal precursors. The resultant
mixture was hydrothermally treated at 448 K for 2 h in an
autoclave vessel. The product was washed and dried at373 K, followed by heating at 773 K under He.
The characterization of the supported Cu nanoparticles
was performed by XRF, XRD, XPS, TEM, BET andXAFS. Cu K-edge XAFS spectra were measured at BL10B and 12C stations in a transmission mode at 20 K.
Background subtraction was conducted by the AUTOBK
program, and Fourier-transformed spectra were fitted in R
space by FEFFIT program.
Results and Discussion
XRD spectra of the supported Cu nanoparticles showedthat there were no peaks attributed to Cu species andhighly crystalline structures of Mo or Zn oxides wereformed in the presence of CTAB. TEM and BET results
agreed the crystalline structures of the supports.
The curve-fitting results of Cu K-edge EXAFS spectra
are summarized in Table 1. The coordination number ofCu-Cu of fresh Cu/Mo-CTAB was 4.4 at 0.254 nm, which
is similar to the bond length for Cu metal. The muchsmaller CN of Cu-Cu than that of bulk Cu metal (CN=12)
indicates the formation of nano-sized metallic Cu clusters.

16Users' Report

Table 1 Curve-fitting results of Cu K-edge EXAFS measured at20 KSample

Cu/Mo-CTAB(fresh)a(after reaction)b
Cu/Mo (fresh)cCu/Zn-CTAB(fresh)d(after reaction)e
Cu/Si-CTAB(fresh)f
Cu/Al-CTAB(fresh)gShell

CNDistance/nm

σ2 /10-5nm2

Cu
Cu
O

4.4±0.8
3.9±0.7
2.4±0.3

0.254±0.001
0.254±0.001
0.190±0.002

3±12±18±2Cu
Cu

6.0±1.3
8.8±0.7

0.257±0.001
0.254±0.001

4±14±1Cu

4.4±0.7

0.254±0.001

4±1Cu

1.5±0.2

0.249±0.001

4±1ak=30-120 nm-1, R=0.18-0.28 nm, Rf=1.29%. b k=30-120 nm-1, R=0.180.28 nm, Rf=0.95%. c k=30-114 nm-1, R=0.10-0.20 nm, Rf=1.94%.
dk=30-114 nm-1, R=0.18-0.28 nm, Rf=1.27%. e k=30-114 nm-1,
R=0.18-0.28 nm, Rf=0.17%. f k=30-140 nm-1, R=0.18-0.28 nm,
Rf=1.89%. g k=30-140 nm-1, R=0.16-0.27 nm, Rf=0.93%.

After the selective methanol dehydrogenation, there wasno significant difference in CN and Cu-Cu distance asshown in Table 1. In the absence of CTAB, no interactionof Cu-Cu was observed on Cu/Mo, and Cu oxide wasformed on amorphous Mo oxide. For the Zn-supportedCu nanoparticles, CN of Cu-Cu was initially 6.0 at 0.257
nm but increased to 8.8 at 0.254 nm after the reaction.
The Cu-Cu CNs of fresh Cu/Si-CTAB and Cu/Al-CTAB
were 4.4 at 0.254 nm and 1.5 at 0.249 nm, respectively.
The Cu/Mo-CTAB was the most active catalyst for theselective dehydrogenation of methanol to formaldehyde:
61.2% conv. and 98.5% selectivity. H2 selectivity was100% with no dehydration products. The EXAFS result
after the reaction demonstrates that the Cu/Mo-CTAB isso durable under the reaction conditions. The Cu/ZnCTAB exhibited 26.5% conv. and 99.1% selectivity, but
the increase in the Cu-Cu CN after the reaction indicatessintering the Cu particles. The metallic Cu nanocluster
was created in the organized assembly of Mo and CTAB
under the hydrothermal synthesis conditions, which wasmost active and selective for methanol dehydrogenation.
References[1] R. Bal, M. Tada, and Y. Iwasawa, Chem. Commun., inpress.
* iwasawa@chem.s.u-tokyo.ac.jp