E†ects of rhodium dispersion on catalytic behavior of
Rh/active-carbon catalysts for H/D exchange reaction between
CH and D
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Hirotoshi Sakagami, Shingo Ogata, Nobuo Takahashi* and Takeshi Matsuda
Department of Materials Science, Kitami Institute of T echnology, 165 Koencho, Kitami,
Hokkaido 090-8507, Japan
Received 18th January 2001, Accepted 8th March 2001
First published as an Advance Article on the web 5th April 2001
The H/D exchange reaction between CH and D was carried out over Rh/active-carbon catalysts, which were
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prepared from RhCl and Rh(NO ) . In the case of the catalysts prepared from RhCl , Rh species were
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3 3
3
homogeneously dispersed on the support from external surface to the inside of pores. Metallic particles of Rh
were found to be the predominant species on the catalysts prepared from Rh(NO ) in the low Rh-loading
3 3
region of \2 wt.%, whereas the number of highly dispersed Rh species remarkably increased with an increase
in the amount of Rh-loading in the region of [2 wt.%. The reaction rate per unit gram of catalyst and the
product distribution in methane reÑected well the Rh-dispersion on the catalysts. The catalysts which
contained the highly dispersed Rh species as predominant species were found to be more active for the H/D
exchange reaction than the catalysts with relatively large metal particles of Rh. On the former, the ratio of
CH D/CD was observed to be much higher than that on the latter.
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X-ray photoelectron spectroscopy (XPS). In addition, the H/D
exchange reaction between CH and D was studied on the
two kinds of catalyst, Rh(C)/AC and Rh(N)/AC.
Introduction
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For the evaluation of noble metal dispersion on the support,
the chemisorption of carbon monoxide (CO) has frequently
been used, where adsorption of one CO molecule on each
surface metal atom was assumed. Although such 1 : 1 stoichi-
ometry has been pointed out to be questionable in the case of
a highly dispersed phase of noble metal,1h3 the amount of CO
adsorbed would strongly reÑect the dispersion. We have
already reported that Rh supported on active-carbon pre-
pared from rhodium trichloride (denoted by Rh(C)/AC)
adsorbed a very large amount of CO compared with that from
rhodium trinitrate (denoted by Rh(N)/AC).4h6 The transmis-
sion electron microscope (TEM) measurements showed that
the dispersion of Rh species on Rh(C)/AC was very high com-
pared with Rh(N)/AC.7 On the other hand, metallic Rh
species were found to be the main species on Rh(N)/AC. Thus,
the Rh compound used in the preparation of the Rh/AC cata-
lyst strongly a†ected the dispersion of Rh. However, the e†ects
of the amount of Rh-loading on the dispersion with respect to
the two kinds of Rh compounds, RhCl and Rh(NO ) , have
Experimental
The active-carbon support (obtained from Wako Pure Chemi-
cal Ind., Ltd, speciÐc surface area \ 930 m2 g~1, average pore
diameter (d ) \ 4 nm) was immersed in 0.1 mol dm~3 of
av
hydrochloric acid solution and then thoroughly washed with
de-ionized water. Rh/AC catalysts with Rh loading from 1.0
wt.% (97 ] 10~6 mol g(cat)~1) to 4.0 wt.% (389 ] 10~6 mol
g(cat)~1) were prepared by a conventional impregnation
method, where rhodium trichloride trihydrate (RhCl É 3H O)
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or rhodium trinitrate (Rh(NO ) ) was deposited on the
3 3
support from the appropriate aqueous solution. The solid was
dried at 383 K for 12 h in an oven.
The samples for the CO chemisorption measurements were
prepared as follows. The solid pretreated with H at 573 K for
2
3 h in a conventional Ñow system was transferred into a glass
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3 3
not been clariÐed.
Kemball and co-workers have studied extensively the H/D
exchange reaction between methane (CH ) and D over
various kinds of single-component metal catalysts and
bimetallic alloy catalysts.8 In the literature, CD was found to
be the main product at the early stage of the reaction on Rh-
cell, and it was re-treated with H at 573 K for 1 h before the
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chemisorption measurement. The amounts of CO adsorbed
on the catalysts were measured at 296 K by a conventional
static method using a glass-made apparatus connected to a
vacuum line. The Ðrst CO adsorption run was followed by the
second run after the evacuation of the sample for 10 min at
room temperature. The sum of physical and chemical adsorp-
tion was estimated by extrapolating the Ñat part of the Ðrst
adsorption isotherm to the zero of the equilibrium pressure
for adsorption. The amount of physically adsorbed CO was
estimated from the second isotherm in the same way for the
estimation of the total adsorption. The amount of chemically
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foil. On the other hand, CH D was the main product on
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Rh/SiO catalysts.9,10 In the case of highly dispersed sup-
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ported noble metal catalysts, e.g., Pt/Al O , CH D was found
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to be the predominant product at the early stage of the reac-
tion.3 Multiple and stepwise exchange processes were pro-
posed to be the reaction routes for CD and CH D formation,
respectively.8 Thus, the catalytic behavior in the exchange
reaction between CH and D may reÑect the di†erence in the
Rh dispersion on the Rh/AC catalysts.
In the present work we have studied the dispersion of Rh
on active-carbon support by means of CO chemisorption and
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adsorbed CO (denoted by q ) was estimated by subtracting
CO
the physical adsorption from the total adsorption. The value
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for q on the active carbon support itself (\1 ] 10~6 mol
CO
g(cat)~1) was found to be much less than those on the Rh/AC
catalysts.
1930
Phys. Chem. Chem. Phys., 2001 3, 1930È1934
This journal is ( The Owner Societies 2001
DOI: 10.1039/b100687h