Reaction Mechanism of 2-Propanol Dehydrogenation with a Carbon-Supported Ru-Pt Composite Catalyst in the Liquid Phase

Article abstract of DOI:10.1246/bcsj.76.2045

The reaction mechanism of dehydrogenation of 2-propanol substrates with and without deuterium-substitution with the Ru-Pt/carbon catalyst was studied to obtain strategies for catalyst designing suitable to direct 2-propanol fuel cells (D2PFC) and the catalyst-assisted chemical heat pump system. The Ru-Pt/carbon composite catalyst gave kinetic isotope effects of 1.54 and 1.96 for dehydrogenation at 82.4°C from (CH₃)₂CHOD and (CH₃)₂CDOH, respectively, which were contrasted to the corresponding magnitudes of 1.69 and 1.57 with a Ru/carbon catalyst and those of 1.13 and 1.81 with a Pt/carbon catalyst. The rate constants of dehydrogenation as a function of the H/D ratio in molecular hydrogen suggested that the step to form molecular hydrogen from surface hydrogen species was slow on the Ru catalyst, whereas the step to split the methine C-H bond was rather difficult for the Pt and Ru-Pt catalysts. Reflecting the facile dissociation at the hydroxy group on the catalyst surface, deuterium transfer from the hydroxy to the methyl groups of both acetone and 2-propanol proceeded tremendously for (CH₃)₂CHOD.

Full text of DOI:10.1246/bcsj.76.2045

Ó 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 2045–2049 (2003) 2045
Reaction Mechanism of 2-Propanol Dehydrogenation with a Carbon-
Supported Ru–Pt Composite Catalyst in the Liquid Phase
ꢀ
y
yy
Yuji Ando, Masaru Yamashita, and Yasukazu Saito
Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST),
6-1, Onogawa, Tsukuba, Ibaraki 305-8569
1
yNew Energy and Industrial Technology Development Organization,
Sunshine 60, 30F, 1-1, 3-Chome, Higashi-Ikebukuro, Toshima-ku, Tokyo 170-6028
yyFaculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601
Received May 22, 2003; E-mail: ando-yuji@aist.go.jp
The liquid-phase dehydrogenation of (CH3)2CHOH, (CH3)2CHOD, and (CH3)2CDOH to yield acetone and molec-
ular hydrogen was performed selectively under boiling and refluxing conditions by heating a suspended solution of a
carbon-supported ruthenium and platinum composite catalyst. The Ru–Pt/carbon composite catalyst gave kinetic iso-
ꢁ
tope effects of 1.54 and 1.96 for dehydrogenation at 82.4 C from (CH3)2CHOD and (CH3)2CDOH, respectively, which
were contrasted to the corresponding magnitudes of 1.69 and 1.57 with a Ru/carbon catalyst and those of 1.13 and 1.81
with a Pt/carbon catalyst. The rate constants of dehydrogenation as a function of the H/D ratio in molecular hydrogen
suggest that the step to form molecular hydrogen from surface hydrogen species was slow on the Ru catalyst, whereas
the step to split the methine C–H bond was rather difficult for the Pt and Ru–Pt catalysts. Reflecting the facile disso-
ciation at the hydroxy group on the catalyst surface, deuterium transfer from the hydroxy to the methyl groups of both
acetone and 2-propanol proceeded tremendously for (CH3)2CHOD.
A large number of studies about fuel cells has been
conducted. With regard to direct methanol fuel cells
DMFCs), decrements of a large overpotential for methanol
der to obtain strategies for catalyst designing suitable to
D2PFCs and the catalyst-assisted chemical heat pump system.
(
Experimental
oxidation and of methanol crossover from the anode side to
the cathode seem to be inevitable for its practical use. Recent-
ly, Qi et al. have studied the electrochemical performance of
direct 2-propanol fuel cells (D2PFCs) using Ru–Pt and Pt
blacks as the anode and cathode, respectively, and reported
that D2PFCs exhibit a higher cell voltage at a current density
Catalyst Preparation for 2-Propanol Dehydrogenation with
Composite Metals. We prepared Ru–Pt/carbon catalysts by re-
ducing an aqueous solution of mixed metal chlorides after adsorp-
tion onto active carbon powders. Commercially available pow-
ders of highly-porous active carbon (1.9 g, Kansai Netsukagaku
Co.), derived from coconut shell and activated with KOH, were
used for overnight impregnation at room temperature with an a-
2
ꢁ
of less than about 100 mA/cm at 61 C and a much smaller
1
crossover rate than those of DMFCs. Umeda et al. have
queous solution (100 mL) of RuCl3 3H2O(N. E. Chemcat Co.)
reported that acetone is the only oxidation product of 2-pro-
2
Á
and K2PtCl4 (Kojima Chemical Co.) mixed in atomic ratio of
Ru/Pt = 1/1. An aqueous solution of NaBH4 (900 mg/20 mL)
was added dropwise (2 mL/min) at room temperature to the ad-
sorbed metal salts (5 wt % as metal) for reduction in a suspended
state. After standing for about 15 min, the carbon-supported cat-
alysts were filtered and washed with a large amount of water
panol around the anode surface of D2PFCs.
Catalytic 2-propanol dehydrogenation, shown in Eq. 1, pro-
ceeds selectively under boiling and refluxing conditions, as
3
proved with suspended copper chromite, Raney nickel, nick-
4
5
el boride, nickel fine particles prepared with a gas-evaporation
6
technique, carbon-supported ruthenium, and ruthenium–plat-
7
ꢁ
8
inum composite metal (Ru–Pt/carbon). The Ru–Pt/carbon
(1000 mL). The catalysts were evacuated at 50 C overnight
and kept under nitrogen.
catalyst gave higher activity and selectivity for 2-propanol de-
hydrogenation than the other, where the activity became the
Reaction Procedure for 2-Propanol Dehydrogenation in the
Liquid Phase. A prescribed amount (200 mg) of the Ru–Pt/car-
bon catalyst was dispersed in 5 mL of unlabeled 2-propanol (here-
inafter described as 2-propnaol-d0) ultrasonically (Kaijo Denki
Co., Sona 50a) for 10 min in a Schlenk flask (20 mL). After sub-
stituting the atmosphere with flowing nitrogen gas, the reactor was
8
largest at the atomic ratio of ruthenium/platinum = 1/1.
(CH3)2CHOH (l) ! (CH3)2CO (l) þ H2 (g):
ð1Þ
The substrate 2-propanol and the reaction products of both
acetone and hydrogen are removed from the liquid-phase reac-
tor and separated by fractional distillation, which is the basis of
ꢁ
heated by an oil bath (100 C) and agitated by a magnetic stirrer
(
500 rpm) in order to boil the suspended solution vigorously at
9
ꢁ
a catalyst-assisted chemical heat pump system.
In this paper, the reaction mechanism of 2-propanol dehy-
drogenation with the Ru–Pt/carbon catalyst is elucidated in or-
82.4 C. The product gas was collected in a gas burette (250
mL) through a refluxing condenser for 1.5 h. The reaction prod-
ucts were identified by gas chromatography using an active carbon
Published on the web October 15, 2003; DOI 10.1246/bcsj.76.2045

2
046 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
2-Propanol Dehydrogenation with Ru–Pt/Carbon
column for gases intermittently and a PEG 20M column for the
liquid-phase component after the reaction.
Deuterium Analyses on Gaseous and Liquid Phase
Components.
2
The dehydrogenation of deuterium-substituted
-propanol, i.e., (CH3)2CHOD (2-propanol-O-d) and (CH3)2-
CDOH (2-propanol-2-d) was also performed for 1.5 h in the same
Schlenk flask (20 mL) and 5 mL of the reactant 2-propanol. The
deuterium distribution in the product dihydrogen was determined
every fifteen minutes using a quadrupole-type mass spectrometer
2
(
NEC Anelva Co., AQA 360), whereas the H NMR spectra of the
solution were taken after appropriate reaction periods in order to
analyze how the substituted deuterium atoms were transferred
from the deuterium-substituted substrate to other liquid-phase
components by using an FT-NMR spetrometer (JEOL Co., GX-
4
00).
Results and Discussion
Dehydrogenation of Deuterium-Substituted 2-Propanol.
-Propanol substrates with and without deuterium-substitution
2
were dehydrogenated with the suspended Ru–Pt/carbon cata-
lyst under boiling and refluxing conditions. It was found that
the hydrogen evolution rate of 2-propanol-d0 was apparently
higher than that of 2-propanol-O-d, which was followed by
Fig. 2. Langmuir-type analysis for the dehydrogenation of
2-propanol-d0 ( ), 2-propanol-O-d ( ), and 2-propanol-
2-d ( ) with Ru–Pt/carbon catalyst (data taken from
Fig. 1).
2
-propanol-2-d, as shown in Fig. 1.
The hydrogen evolution rates were well analyzed for an ap-
propriate amount (200 mg) of the Ru–Pt/carbon powder cata-
lyst suspended in 100 mL of boiling 2-propanol with use of the
following Langmuir-type equation:
Deuterium Distributions among the Product Hydrogen.
The deuterium distributions of newly-evolved hydrogen from
2-propanol-O-d and -2-d were shown to be a function of the
reaction time, Figs. 3a and 3b. At the beginning of the reac-
tion, 2-propanol-O-d and -2-d gave deuterium distributions
widely different from each other. Relatively large amounts
of deuterium atoms were evolved from 2-propanol-O-d as
HD (40%) and D2 (10%). On the contrary, the product hydro-
gen from 2-propanol-2-d was mostly H2, accompanying HD
(5%) and D2 (less than 1%).
8
v ¼ k=ð1 þ K½acetoneꢂÞ:
ð2Þ
Here, v is the reaction rate, k is the rate constant and K is the
equilibrium constant of acetone adsorption, respectively.
Within a limited conversion range of 2-propanol dehydro-
genation, the Ru–Pt/carbon catalyst (200 mg) suspended in a
small amount of 2-propanol (5 mL) was well analyzed using
the same equation as shown in Fig. 2.
The molar fractions of H2, HD, and D2 from 2-propanol-O-
d and -2-d were found to vary as the reaction time elapsed.
With regard to the H/D ratio in the product hydrogen, defined
as
H/D ¼ ð2 ꢃ ½H2ꢂ þ ½HDꢂÞ=ð2 ꢃ ½D2ꢂ þ ½HDꢂÞ;
ð3Þ
a gradual increase in the H/D ratio was observed from 2.4 at
the beginning of the reaction to 6.6 during the reaction period
of 1.5 h for 2-propanol-O-d, whereas the H/D magnitudes de-
creased from 30.6 to 7.5 for 2-propanol-2-d. It is to be noted
that the molar fractions of H2, HD, and D2 after the reaction
period of 1.5 h reached 80%, 20%, and a trace amount, respec-
tively, regardless of the reactant 2-propanol.
The dissociative adsorption of 2-propanol at its hydroxy
group would proceed facile over the catalyst surface. The sur-
face hydrogen atoms from 2-propanol-O-d ought to contain
more D than from -2-d besides 2-propoxide. The methine
C–H bond of adsorbed 2-propoxide must dissociate as a pre-
requisite step for hydrogen evolution, since molecular hydro-
gen is formed from methine and hydroxy hydrogen atoms.
Provided that molecular hydrogen is formed from methine
and hydroxy hydrogen atoms, the H/D ratio should be unity
for 2-propanol-O-d and -2-d. However, the H/D ratios ob-
served for these substrates were much larger than unity. It
is therefore reasonable that the H or D atoms of newly-evolved
Fig. 1. Time course for the dehydrogenation of 2-propanol-
d0 ( ), 2-propanol-O-d ( ), and 2-propanol-2-d ( ) with
Ru–Pt/carbon catalyst under boiling and refluxing condi-
ꢁ
tions (82.4 C).

Y. Ando et al.
Bull. Chem. Soc. Jpn., 76, No. 10 (2003) 2047
Fig. 3. Time course for the deuterium distribution in molecular hydrogen evolved from 2-propanol-O-d (a) and 2-propanol-2-d (b)
ꢁ
with Ru–Pt/carbon catalyst under boiling and refluxing conditions (82.4 C).
: H2, : HD, : D2.
hydrogen are contaminated as a result of time-sequential H–D
exchanges among adsorbed organic compounds.
Deuterium Distribution in the Liquid Phase. According
evolved hydrogen, H–D equilibration on the catalyst surface
would result in a gradual increase in the H/D ratio. With re-
gard to 2-propanol-2-d, the catalyst surface would be covered
mostly with H and 2-propoxide at the beginning stage of the
reaction. As the reaction time elapses, deuterium atoms af-
forded from C–D dissociation of this 2-propoxide are distrib-
uted to the catalyst surface, with the H/D ratio gradually
decreasing. Deuterium atoms of the substrate are included fi-
nally in either gaseous hydrogen or transferred to some other
groups of acetone and 2-propanol as a consequence of the ke-
to–enol tautomerism and the surface hydrogenation of the ad-
sorbed acetone. Therefore, it is quite understandable that the
H/D ratios of newly-evolved hydrogen from both 2-propa-
nol-O-d and 2-propanol-2-d were larger than 1, and gradually
coincided after the establishment of stationary deuterium dis-
tributions.
2
to the H NMR spectra for the reaction solutions of 2-propa-
nol-O-d, the deuterium atoms were transferred from the hy-
droxy group to the methine and methyl groups of 2-propanol
extensively, even after 30 min (Table 1).
The surface deuterium atoms and adsorbed acetone change
into 2-propanol-2-d as follows:
(
CH3)2COads þ Dads ꢀ (CH3)2CDOads;
CH3)2CDOads þ Hads ꢀ (CH3)2CDOH:
ð4Þ
ð5Þ
(
During 2-propanol reproduction, many deuterium atoms were
thus transferred to the methine group. Moreover, chemical
equilibration for adsorbed acetone between its keto and enol
10
forms is rapid on the catalyst surface. During the keto–enol
tautomerism, deuterium transfer would also be possible to the
methyl group of acetone,
Reaction Mechanism of 2-Propanol Dehydrogenation on
the Ru–Pt/Carbon Catalyst. Based on the experimental re-
sults of 2-propanol-O-d and -2-d, the reaction mechanism of 2-
propanol dehydrogenation was reconsidered on the Ru–Pt/car-
bon catalyst by comparing with the component catalysts of
Ru/carbon and Pt/carbon. The kinetic isotope effects for hy-
drogen evolution from 2-propanol-2-d and 2-propanol-O-d at
the initial stage on a Ru/carbon catalyst were obtained previ-
(CH3)2COads ꢀ CH2=CO{CH3 ads þ Hads:
ð6Þ
The mechanism of this deuterium transfer and the surface hy-
drogenation of adsorbed acetone would yield methyl-deuterat-
ed 2-propanol as one of the solution components from the
7
(CH3)2CHOD substrate. Since the H/D ratio of adsorbed hy-
ously as 1.57 and 1.69. Yamashita et al. reported there that
the slowest step on the Pt/carbon catalyst was the splitting
drogen on the catalyst surface is reflected to that of newly-
Table 1. Deuterium Redistribution during Dehydrogenation of (CH3)2CHOD
Deuterium distribution/mmol
Reaction
time/h
Reactant
2-Propanol
Methine
Acetone
methyl
0.0
Water
Hydroxy
Methyl
hydroxy
0
0.5
62.2
13.8
12.0
0.2
20.0
18.1
0.8
28.3
30.8
1.1
1.0
0.4
ꢀ
(
CH3)2CHOD
—
1.1
1
.5
Carbon-supported Ru–Pt (1:1, 5 wt %) 200 mg/5 mL 2-propanol under boiling and refluxing conditions
ꢁ
2
(
82.4 C). ꢀBelow the identification limit of H NMR.

2
048 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
2-Propanol Dehydrogenation with Ru–Pt/Carbon
Table 2. Isotope Effects for Catalytic 2-Propanol Dehydro-
genation
kH=kD
Metal
catalyst
Ref.
O-d
2-d
Ru
Pt
1.69
1.13
1.57
1.81
7
11
Carbon-supported metal (5 wt %) 200 mg/5 mL 2-propanol un-
ꢁ
der boiling and refluxing conditions (82.4 C).
process of the methine C–H bond, because the magnitude of
the kH=kD obtained from 2-propanol-2-d was much larger than
11
that obtained from 2-propanol-O-d. These isotope effects are
summarized in Table 2.
The rate constant of 2-propanol dehydrogenation was de-
rived from the observed reaction rate (v), acetone concentra-
tion, and the equilibrium constant of the acetone adsorption
(
adopted commonly. The rate constants observed on the Ru–
K), to which the magnitude observed for 2-propanol-d0 was
7
Pt/carbon catalyst, together with those on Ru/carbon, were
Fig. 5. Rate constant of 2-propanol dehydrogenation as a
function of H/D ratio in molecular hydrogen obtained
for Ru–Pt/carbon catalyst under boiling and refluxing con-
given as a function of the H/D ratio in molecular hydrogen
(Figs. 4 and 5). The rate constant derived from the two kinds
ꢁ
ditions (82.4 C).
: (CH3)2CHOD, : (CH3)2CDOH.
of deuterium-substituted 2-propanol on the Ru/carbon catalyst
was not constant, but increased remarkably, and finally coin-
cided with the rate constant of the d0 substrate, the H/D ratio
of which should be infinity. From the sensitive dependence of
the rate constant on the H/D ratio in molecular hydrogen for
the Ru/carbon catalyst and the small kinetic isotope effects
for hydrogen evolution from 2-propanol-2-d and 2-propanol-
O-d, the slowest step is now concluded to be hydrogen evolu-
tion from the catalyst surface. On the contrary, the rate con-
stant over the Ru–Pt/carbon catalyst is almost constant regard-
less of the H/D ratio or the 2-propanol substrate. The slowest
step on the Ru–Pt/carbon catalyst should not be considered to
be the formation of molecular hydrogen from the catalyst sur-
face.
Table 3. Reaction Parameters for Catalytic 2-Propanol De-
hydrogenation
ꢄ1 ꢄ1
3
ꢄ1
2
-Propanol
k/mol h
g
kH=kD
K/dm mol
d0
0.927
0.474
0.602
—
1.96
1.54
1.05
1.18
1.87
2-d
O-d
Carbon-supported Ru–Pt (1:1, 5 wt %) 200 mg/5 mL 2-propanol
ꢁ
under boiling and refluxing conditions (82.4 C). Rate equation:
v ¼ k=ð1 þ K½acetoneꢂÞ; k: rate constant, K: retardation con-
stant.
The isotope effects, kH=kD, obtained on the Ru–Pt/carbon
catalyst for 2-propanol-O-d and 2-propanol-2-d below 5% ace-
tone conversion, are shown in Table 3, together with other re-
action parameters. Since the kinetic isotope effect of 2-pro-
panol-O-d was relatively smaller than that of 2-propanol-2-d,
the slowest step on the Ru–Pt/carbon catalyst would be shifted
from the desorption process of hydrogen to the splitting proc-
ess of the methine C–H bond compared with the Ru/carbon
catalyst.
Enthalpy profiles of 2-propanol dehydrogenation with the
Ru/carbon, Pt/carbon, and Ru–Pt/carbon catalysts are depict-
ed for a comparison in Fig. 6. As far as the Ru/carbon catalyst
in concerned, the dissociation process of the methine C–H
bond is facile, whereas a large activation energy is required
for the formation of molecular hydrogen. On the contrary,
the Ru–Pt/carbon catalyst could decrease the activation energy
for the formation of molecular hydrogen, being inherited from
the hydrogen-releasing character of platinum. Nevertheless,
the advantage of the Ru/carbon catalyst with respect to the
C–H bond scissoring seems to be taken over to smooth the bar-
riers of elementary steps.
Fig. 4. Rate constant of 2-propanol dehydrogenation as a
function of H/D ratio in molecular hydrogen obtained
for Ru/carbon catalyst under boiling and refluxing condi-
These characteristics of the Ru–Pt/carbon catalyst would be
the reason why this bimetallic composition exhibits splendid
ꢁ
8
tions (82.4 C).
: (CH3)2CHOD, : (CH3)2CDOH.
catalytic activity for 2-propanol dehydrogenation.

Y. Ando et al.
Bull. Chem. Soc. Jpn., 76, No. 10 (2003) 2049
Fig. 6. Enthalpy profile of catalytic 2-propanol dehydrogenation.
Lett., 1991, 351.
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