732
Bull. Chem. Soc. Jpn. Vol. 83, No. 6, 732–734 (2010)
Short Articles
Experimental
Influence of Calcination
Catalyst Preparation. The Ir/SiO2 (w = 2%) catalyst was
prepared by impregnating a SiO2 support with a surface area
of 346 m2 g (Silbead, Mizusawa Industrial Chem.) with an
Temperatures on Iridium Dispersion
and Hydrogenation Activity in
Silica-Supported Iridium Catalysts
¹1
aqueous solution of H2[IrCl6]¢nH2O (Aldrich). The slurry was
stirred for 1 h, vaporized under vacuum, and dried at 403 K in
an oven overnight. The residue was heated in an oven at a rate
of 4.5 K min¹1 and calcined at 403-773 K for 3 h. The resulting
powder was then placed in a reduction tube and reduced under
flowing hydrogen at 673 K for 5 h. The catalysts were sieved
and powders of less than 100 mesh (<149 ¯m) were used for
the liquid phase hydrogenation reaction.
Ryoji Shibuya, Masa-aki Ohshima,
Hideki Kurokawa, and Hiroshi Miura*
Graduate School of Science and Engineering, Saitama
Characterization. Ir dispersion, defined as the ratio of
Irsurface over Irtotal, was estimated by CO adsorption at 300 K
using a dynamic pulse adsorption apparatus (Ohkura Riken,
BP-1). The catalyst was pre-reduced at 673 K for 1 h and
purged with He for 0.5 h. Oxygen adsorption was also used in a
similar manner to confirm the reliability of the dispersion
measurements. X-ray diffraction (XRD) patterns were obtained
before and after reduction using RINT-Ultima III diffractometer
(RIGAKU) with a Cu K¡ radiation under a voltage of 40 kV,
current of 40 mA, and scan speed of 4.0° min¹1. X-ray
fluorescence (XRF) measurements were performed under
vacuum using a Philips PW2400 apparatus.
University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570
Received February 15, 2010
E-mail: hmiura@mail.saitama-u.ac.jp
The influence of calcination temperatures on the
dispersion of Ir/SiO2 catalysts has been studied. At low
calcination temperatures, the chloride precursor was dis-
persed on the silica surface and formed highly dispersed
Ir/SiO2 catalysts upon reduction. At high calcination
temperatures, the formation of IrO2 was confirmed, which
resulted in low dispersion after reduction.
Crotonaldehyde Hydrogenation.
The reaction rate of
crotonaldehyde hydrogenation was evaluated in a stainless
steel autoclave (100 mL) equipped with a mechanical stirrer, a
thermowell, and a pressure gauge. Catalysts were pre-reduced
at 673 K for 1 h. Crotonaldehyde (36.5 mmol) in ethanol
(50 mL) was used as a reactant. The reaction rate was estimated
at 323 K using the catalyst (0.3 g) under an initial H2 pressure
of 1.0 MPa at a 1000 rpm agitation. The products were analyzed
by gas chromatography using the Shimadzu GC-18A apparatus
equipped with a DB-WAX capillary column (30 m) at 343 K.
The turnover frequency (TOF) was calculated from the reaction
rate and the amount of CO adsorption.
Supported noble metal catalysts are widely used for a variety
of catalytic reactions. For example, supported Ru, Rh, and Pd
catalysts have shown excellent activity for the hydrogenation
of aromatic hydrocarbons, while Pd and Pt catalysts have
demonstrated excellent activity for the hydrogenation of
unsaturated bonds like C=C and C¸C.1 In the platinum metal
group, Ir is known to be active for NO reduction,2-4 partial
methane oxidation,5 hydrazine decomposition,6-8 and selective
hydrogenation of ¡,¢-unsaturated aldehydes.1,9-13 However, no
detailed research has been reported on hydrogenation reactions
over Ir catalysts compared to other noble metals. Especially,
the effects of catalyst preparation variables are still unknown.
Because NO reduction and hydrazine decomposition are high
temperature reactions, the catalysts are prepared using proce-
dures involving high temperature calcination. Very low Ir
dispersions have been found in some cases,2,3 suggesting that
the effects of calcination temperatures may be important. In this
paper, we report the effect of calcination temperatures on Ir
dispersion and activity of Ir/SiO2 catalysts in hydrogenation
reactions. We found a drastic change in Ir dispersion by
changing the calcination temperature in a narrow range.
Because varieties of Ir catalysts with different Ir dispersions
could be prepared by controlling the calcination temperature
alone, dispersion effects on the catalytic activity during
hydrogenation were readily identified using catalysts prepared
through the same procedure, eliminating the effects of
preparation variables. We studied crotonaldehyde hydrogena-
tion, which contains C=C and C=O double bonds. Selective
hydrogenation of the C=O bond yields unsaturated crotyl
alcohol, a useful raw material in pharmaceuticals and fragran-
ces.9,10 Ir is reported to be a selective catalyst for this reaction.10
Results and Discussion
Dispersion of Iridium.
The dispersion of iridium in
Ir/SiO2 catalysts prepared by calcination at 403-773 K and
reduction at 673 K was measured by CO and O2 adsorption
experiments. The dispersion was calculated by assuming that
the adsorption stoichiometries corresponded to CO/Ir = 1 and
O/Ir = 1.14
Dispersion values determined by CO adsorption agreed very
well with values determined by O2 adsorption (Table 1).
Figure 1 clearly shows that the dispersion was independent
of the calcination temperature between 403 and 673 K but
decreased drastically between 673 and 723 K. A difference of
only 50 K in the calcination temperature caused the dispersion
to decrease from above 50 to below 10%.
The influence of reduction temperatures was also studied. A
series of catalysts were calcined at 403 K and then reduced at
temperatures ranging between 473 and 873 K. Another series of
catalysts were calcined at 773 K and then reduced at the same
temperature range. Both series of catalysts displayed an almost
constant dispersion upon reduction at 473-673 K but dispersion
decreased gradually because of sintering when the reduction