Bimetallic effect of silica-supported pt-ru catalyst for hydrogenation of aromatic hydrocarbons

Full text of DOI:10.1246/bcsj.82.627

Short Articles
Bull. Chem. Soc. Jpn. Vol. 82, No. 5, 627–629 (2009)
627
Ru, the amount of total metal loading (Pt + Ru) was kept constant;
¹1
i.e., 0.2 mmol (g-SiO2)
.
Bimetallic Effect of Silica-Supported
Pt­Ru Catalyst for Hydrogenation
of Aromatic Hydrocarbons
The number of metal atoms exposed to the surface was
determined by O ­CO titration at room temperature using a pulse
2
adsorption apparatus. The average surface composition of the
metal particles was obtained by means of this technique. The
sample was prereduced in H2 at 300 °C for 1 h, and then evacuated
at 300 °C for 1 h. The details of the O2­CO titration have been
Tsuyoshi Arakawa, Haruka Seki,
Masa-aki Ohshima, Hideki Kurokawa,
and Hiroshi Miura*
8­10
previously reported in the literature.
The internal composition
was calculated by the difference of total composition and surface
composition.
The liquid-phase hydrogenation of naphthalene and tetralin was
carried out using a stainless steel autoclave equipped with a
mechanical stirrer and thermocouple. Because naphthalene (Merck
Ltd.) contained benzothiophene, it was purified before use by
hydrogen treatment using a Ni catalyst. 7.8 mmol of naphthalene
was dissolved in 40 mL (164 mmol) of tridecane. Tetralin (7.2­
Graduate School of Science and Engineering, Saitama
University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570
Received November 25, 2008; E-mail: hmiura@mail.saitama-u.
ac.jp
2
1.6 mmol) was dissolved in 40 mL of tridecane. Then, 0.05­0.5 g
of the catalyst was prereduced in flowing H2 at 300 °C for 1 h.
The reaction was carried out at 0 or 30 °C for 30­180 min at
A drastic synergistic effect has been found for the
liquid-phase hydrogenation of aromatic hydrocarbons by the
combination of Pt and Ru. The surface composition of Pt­
Ru/SiO2 was estimated using O2­CO titration, and it was
found that Pt was enriched on the surface of the bimetallic
particles. The surface composition of the most active catalyst
was nearly 50% Pt, suggesting that the Pt­Ru ensemble of a
the initial H pressure of 0.5­1.5 MPa with agitation at 1000 rpm.
The reaction products were analyzed by gas chromatography
2
(Shimadzu GC-18A, DB-17 capillary column). In the case of the
naphthalene hydrogenation, only tetralin and decalin were found as
the products, with the tetralin selectivity higher than 95%. In the
case of tetralin, the hydrogenation produced only decalin. The rate
of reaction was obtained by initial rate, keeping the conversion low
enough to use this approximation.
1-to-1 ratio was active for the hydrogenation reaction.
Results and Discussion
The organic chemical hydride method is expected to be an
effective method for the storage and transportation of hydrogen
using the hydrogenation­dehydrogenation reactions of cyclic
hydrocarbons (Scheme 1).1 This method is advantageous
because of its high storage density as well as safety in handling.
The surface composition of 0.2Pt­Ru/SiO was studied by
2
O2­CO titration. The results are shown in Table 1. The catalyst
of the composition Pt/Ru = 20/80 suggested a nearly equal
surface composition to the total ratio, indicating neither Pt nor
Ru was enriched on the surface of the bimetallic particles. On
the other hand, catalysts containing a 40% or higher amount of
Pt suggested a higher concentration of Pt on the surface,
indicating the surface enrichment of Pt.
­6
1
Okada et al. reported a Pt-based dehydrogenation catalyst
having high and stable activity.
The hydrogenation of aromatic hydrocarbons has been
studied for a long time. It is reported that Rh is the most
active metallic catalyst for the hydrogenation.5 However, Rh
is one of the most expensive noble metals and highly active
catalysts using different elements are required. In this paper, we
report supported Pt­Ru bimetallic catalysts which show a much
higher activity than Rh, and discuss the catalyst structure and
mechanism of the synergistic effects.
Such results are in good agreement with the literature
­7
8­10
data.
Because Pt has a larger atomic size and lower surface
energy than Ru, it is natural that Pt is enriched on the surface of
Pt­Ru particles.
The hydrogenation of naphthalene was examined over Pt­
Ru/SiO catalysts. The results are shown in Figure 1. At the
2
stated reaction conditions, Ru/SiO was almost inactive and
2
Pt/SiO was mildly active. The mechanical mixture of Pt/SiO
2
2
Experimental
The Pt­Ru/SiO2 catalyst was prepared by the co-impregnation
Table 1. The Surface Composition and Internal Composi-
tion of Pt­Ru/SiO2 Catalysts
2
¹1
of silica (Silbead, Mizusawa Chem. Co., S(BET) = 330 m g ) with
a mixed solution of metal precursors. H2PtCl6¢6H2O (Kanto
Chem. Co.) was used as the Pt precursor, and RuCl3¢nH2O (Wako
Pure Chem. Co.) was used as the Ru precursor. The slurry was
dried and kept at 130 °C overnight. It was then reduced in flowing
hydrogen at 573 K for 5 h. While changing the atomic ratio of Pt/
Number of surface Surface
Internal
Catalyst
comp.
¹1
atoms/µmol g
comp.
Pt/Ru
(%/%)
comp.
Pt/Ru
(%/%)
Dispersion
/%
Pt/Ru
Pt
Ru Total
100/0
80/20
60/40
40/60
20/80
0/100
35.0
39.5
34.7
31.9
21.6
16.1
67.4
0
67.4 100/0
100/0
+
2H2
+3H2
-3H2
73.5 2.7 76.3 96.4/3.6 69.3/30.7
56.4 10.8 67.2 84.0/16.0 47.3/52.7
36.0 26.1 62.1 57.9/42.1 31.6/68.4
7.5 34.6 42.1 17.8/82.2 20.6/79.4
-
2H2
naphthalene
tetralin
decalin
0
31.6 31.6 0/100
0/100
Scheme 1.
Published on the web May 13, 2009; doi:10.1246/bcsj.82.627

6
28
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
© 2009 The Chemical Society of Japan
and Ru/SiO produced intermediate activity. In clear contrast,
all the bimetallic catalysts suggested a much higher activity
concluded that Pt and Ru had a clear synergistic effect by
forming an ensemble of Pt­Ru.
2
than Pt/SiO . The optimal surface composition was nearly 50%
The experimental results of the tetralin hydrogenation are
shown in Table 2. The synergistic effect of Pt and Ru was also
found in the case of the tetralin hydrogenation. The most active
catalyst was 0.2Pt ­Ru /SiO . Because the surface compo-
2
Pt, and this catalyst suggested a 25-fold higher activity than the
mechanically mixed catalyst. The TOF (based on Pt(s) + Ru(s))
of the hydrogenation was also significantly improved. Thus we
4
0
60
2
sition of this catalyst is 58% Pt, the ensemble of Pt­Ru has a
high hydrogenation activity. The reaction product, decalin,
contained cis and trans isomers. Although the trans isomer is
thermodynamically advantageous, the cis isomer prevailed in
16
14
12
10
8
6
4
2
0
(a)
the reaction products. Ru/SiO had a higher selectivity toward
2
6
the cis-decalin than Pt/SiO , as reported in the literature.
2
The bimetallic Pt­Ru catalysts suggested a selectivity similar to
Pt/SiO .
2
It is reported5,6,11,12 that cis-decalin is formed when ¦9,10
-
octalin is formed as an intermediate and the intermediate is
strongly adsorbed on the catalyst surface. On the other hand,
1
,9
trans-decalin is formed when ¦ -octalin is formed as an
intermediate and the intermediate rolls over on the surface due
to weak adsorption. In case of our Pt­Ru bimetallic catalysts,
1
,9
¦
-octalin is formed and it is weakly adsorbed on the surface,
0
20
40
60
80
100
similar to Pt/SiO2.
Catalyst Composition/Pt %
The reaction rate of tetralin hydrogenation was measured
using the initial rate, and the reaction orders of tetralin and
hydrogen were determined. As shown in Figure 2, the reaction
rate was almost independent of the tetralin concentration over
Pt, Ru, and the Pt­Ru bimetallic catalyst. Figure 3 shows that
the reaction was positively dependent on the H2 pressure.
16
14
12
10
8
6
4
2
0
(
b)
However, the reaction order with respect to H of the bimetallic
2
Pt­Ru catalyst was an intermediate value between those of Pt
and Ru. Thus we could not find any drastic changes in the
parameters of the Langmuir­Hinshelwood mechanism caused
by the formation of the Pt­Ru pair. It seems that the presence of
Ru significantly accelerated the activity of Pt sites.
The activity of Rh/SiO was measured and compared to Pt­
2
Ru/SiO2 using naphthalene and tetralin as the reactants,
because Rh was the most active supported metal catalyst for
7
the naphthalene hydrogenation. As shown in Figure 4, the Pt­
0
20
40
60
80
100
Ru/SiO2 catalyst suggested a 5-fold higher activity than Rh/
Surface Composition/Pt %
SiO for the hydrogenation of naphthalene. The activity of the
2
Pt­Ru/SiO catalyst was 2.5-times higher than that of the Rh/
Figure 1. Hydrogenation of naphthalene over Pt­Ru/SiO2
catalysts. (a) TOF vs. catalyst composition, (b) TOF vs.
2
SiO catalyst for the tetralin hydrogenation.
2
In this way, the combination of Pt and Ru suggested an
excellent hydrogenation activity, superior to Rh which is an
expensive metal. The surface ensemble of Pt­Ru seems to
remarkably increase the activity of Pt. In the field of fuel cell
surface composition.
mixture of Pt/SiO2 and Ru/SiO2. Reaction temperature:
: Pt­Ru/SiO2,
: mechanical
0
0
°C, reaction time: 1­3 h, PH = 1.0 MPa, catalyst weight:
.05­0.4 g.
2
Table 2. Hydrogenation Rate of Tetralin (rtetralin) and Selectivity of Decalin over Pt­Ru/SiO2 Catalystsa)
rtetralin
TOF¢102
/s¹1
Selectivity/%
Catalyst
cis/trans
mmol h g¹1
¹
1
cis-Decalin
trans-Decalin
/
0
0
0
0
0
0
.2Pt/SiO2
3.4
12.7
19.3
21.1
8.2
1.31
4.63
7.96
9.83
5.45
0.91
79.9
77.6
79.1
81.2
85.7
96.5
20.1
22.4
20.9
18.8
14.3
3.5
4.0
3.5
3.8
4.3
6.0
.2Pt80­Ru20/SiO2
.2Pt60­Ru40/SiO2
.2Pt40­Ru60/SiO2
.2Pt20­Ru80/SiO2
.2Ru/SiO2
1.1
27.6
a) Reaction conditions: temperature, 30 °C; reaction time, 30­60 min; catalyst weight, 0.05­0.4 g;
PH = 1.0 MPa.
2

T. Arakawa et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
629
3
2
1
0
.0
.0
.0
.0
0
.2Rh/SiO
2
0
.2Pt40Ru60/SiO
2
0.2Pt/SiO
2
naphthalene
tetralin
0.2Ru/SiO
2
0
5
10
15
20
25
30
35
-
1
-1
Hydrogenation rate /mmol h g-cat
Figure 4. Reaction rate for hydrogenation of naphthalene
and tetralin over Rh/SiO2, Pt­Ru/SiO2, Pt/SiO2, and Ru/
SiO2 catalysts. Reaction temperature: 30 °C, reaction time:
1
h, PH = 1.0 MPa, catalyst weight: 0.05­0.1 g.
2
-
1.0
-
2
-1.5
-1
-0.5
0
not expected. Therefore, our results suggest another kind of
bimetallic effect between Pt and Ru.
-
1
ln (ctetralin /mol L )
Figure 2. Effect of the initial reactant concentration (ctetralin)
on hydrogenation of tetralin over Pt, Ru, and Pt­Ru/SiO2
catalysts. Reaction temperature: 30 °C, reaction time: 1 h,
Conclusion
Silica-supported Pt­Ru catalysts formed bimetallic particles
in which Pt was enriched on the surface. A remarkable
improvement in the activity was found for the hydrogenation of
aromatic hydrocarbons as a result of a bimetallic effect.
Because the surface composition of the most active catalyst
was 50% Pt, the 1/1 ratio ensemble of Pt­Ru seemed effective
PH = 1.0 MPa, catalyst weight: 0.1­0.4 g. : 0.1Pt40­
2
Ru60/SiO2; y = ¹0.03x ¹ 1.95,
:
0.2Pt/SiO2; y =
0.00x + 1.29, : 0.2Ru/SiO2; y = ¹0.14x ¹ 0.11.
3
2
1
0
.0
.0
.0
.0
for the reaction. The activity of Pt­Ru/SiO was much higher
2
than Rh/SiO which has been believed to be the most active
2
metal. We are expecting this catalyst to be available for the
organic hydride method.
References
1
Y. Okada, E. Sasaki, E. Watanabe, S. Hyodo, H. Nishijima,
Int. J. Hydrogen Energy 2006, 31, 1348.
N. Kariya, A. Fukuoka, T. Utagawa, M. Sakuramoto, Y.
Goto, M. Ichikawa, Appl. Catal., A 2003, 247, 247.
S. Hodoshima, S. Takaiwa, A. Shono, K. Satoh, Y. Saito,
Appl. Catal., A 2005, 283, 235.
V. Ya. Davydov, N. Sheppard, E. Osawa, Int. J. Hydrogen
Energy 2004, 29, 1157.
2
3
4
-
1.0
-1.0
-0.5
0.0
ln (PH2/MPa)
0.5
1.0
5
N. Hiyoshi, E. Mine, C. V. Rode, O. Sato, M. Shirai, Appl.
Catal., A 2006, 310, 194.
6
N. Hiyoshi, M. Osada, C. V. Rode, O. Sato, M. Shirai,
Figure 3. Effect of initial hydrogen pressure (PH ) on
2
Appl. Catal., A 2007, 331, 1.
hydrogenation of tetralin over Pt, Ru, and Pt­Ru/SiO2
catalysts. Reaction temperature: 30 °C, reaction time: 1 h,
catalyst weight: 0.1­0.4 g, initial tetralin concentration:
7
T. Suzuki, H. Sekine, M. Ohshima, H. Kurokawa, H.
Miura, Kagaku Kougaku Ronbunshu 2007, 33, 593.
8
9
H. Miura, R. D. Gonzalez, J. Catal. 1982, 74, 216.
H. Miura, T. Suzuki, Y. Ushikubo, K. Sugiyama, T.
¹1
0
.18 mol L , PH = 0.5, 1.0, and 1.5 MPa. : 0.1Pt40­
2
Ru60/SiO2; y = 0.88x + 2.02,
:
0.2Pt/SiO2; y =
Matsuda, R. D. Gonzalez, J. Catal. 1984, 85, 331.
H. Miura, M. Osawa, T. Suzuki, K. Sugiyama, T. Matsuda,
Chem. Lett. 1982, 1803.
0.53x + 1.30, : 0.2Ru/SiO2; y = 1.27x + 0.19.
1
0
1
1
A. W. Weitkamp, Adv. Catal. 1968, 18, 1.
M. F. Williams, B. Fonfé, C. Woltz, A. Jentys, J. A. R. van
electrodes, Watanabe and Motoo13 found that the bifunctional
mechanism improved the activity of the Pt­Ru bimetallic
electrode. In our case however, the reaction circumstance is
quite different from the fuel cell electrode and a similar effect is
1
2
Veen, J. A. Lercher, J. Catal. 2007, 251, 497.
13 M. Watanabe, S. Motoo, J. Electroanal. Chem. 1975, 60,
267.