Hydrogen production from ethanol steam reforming over noble metal catalysts supported on SiO2: Mechanism of methane production and reaction conditions for suppression of methane production

Article abstract of DOI:10.1246/bcsj.20110306

We investigated ethanol steam reforming over Rh/SiO₂, Pt/SiO₂, Pd/SiO₂, and Ru/SiO₂ at 573-723K using a fluidized-bed quartz reactor to determine which reaction conditions suppressed CH₄ production and increased H₂ and CO ₂ production. At temperatures under 623K over Rh/SiO₂ and Ru/SiO₂, CH₄ was produced not only by ethanol decomposition but also by the reduction of CO₂ and CO to CH ₄. At 723 K, the product yields over all four catalysts were strongly influenced by the partial pressure of ethanol: CH₄ production was suppressed, and H₂ and CO₂ production was increased, at low partial pressure of ethanol. These experimental results can be explained in terms of the partial pressures of CO₂ and H₂.

Full text of DOI:10.1246/bcsj.20110306

©
2012 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 85, No. 4, 517­521 (2012)
517
Hydrogen Production from Ethanol Steam Reforming over Noble Metal
Catalysts Supported on SiO : Mechanism of Methane Production
2
and Reaction Conditions for Suppression of Methane Production
Yuji Ando,* Koichi Matsuoka, Hideyuki Takagi, and Koji Kuramoto
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),
16-1 Onogawa, Tsukuba, Ibaraki 305-8569
Received October 13, 2011; E-mail: ando-yuji@aist.go.jp
We investigated ethanol steam reforming over Rh/SiO2, Pt/SiO2, Pd/SiO2, and Ru/SiO2 at 573­723 K using
a fluidized-bed quartz reactor to determine which reaction conditions suppressed CH4 production and increased H2 and
CO2 production. At temperatures under 623 K over Rh/SiO2 and Ru/SiO2, CH4 was produced not only by ethanol
decomposition but also by the reduction of CO2 and CO to CH4. At 723 K, the product yields over all four catalysts were
strongly influenced by the partial pressure of ethanol: CH4 production was suppressed, and H2 and CO2 production was
increased, at low partial pressure of ethanol. These experimental results can be explained in terms of the partial pressures
of CO2 and H2.
Considerable research has been focused on the development
of fuel cells as clean and efficient power sources. Fuel cells
reactor (i.d. 20 mm, height 300 mm) at 573­723 K. The reactor
temperature was controlled by means of a heater with a thermo-
couple in the catalyst bed. The static bed height was 115 mm,
and 20 g of catalyst was placed in the quartz reactor. The
reagent feed consisted of a gaseous mixture of ethanol, water,
and N2, which was generated as follows: a 1:2 (v/v) mixture
of ethanol and water was injected with a syringe pump into a
vaporizer at 573 K, and before entering the reactor, the vapor
require a supply of H to generate electricity, and renewable
2
sources for that H2 are desirable from the standpoint of
sustainability. Thus, ethanol, which can easily be obtained from
biomass, has held the spotlight as a H2 source. Steam reforming
1
­6
of ethanol (eq 1) over various supported noble
and non-
7
­12
noble
reviewed.
metal catalysts has been extensively investigated and
1
3­16
Noble metal catalysts, especially Rh, show high
catalytic performance at low temperatures. One disadvantage
of using noble metal catalysts is that a substantial amount of
was mixed with a N flow supplied by a mass flow controller.
2
15
The outlet products were analyzed online by gas chromatog-
raphy (GL Sciences Inc., Micro GC CP4900). They were
separated on a packed column (molecular sieves 5A or Porapak
Q) and detected with a thermal conductivity detector.
Measurement of Deposited Carbon. Thermogravimetric
analyses (TGA) were performed after the ethanol steam
reforming reaction to determine the extent of carbon deposition
by using TGA-50 (Shimadzu), heating from room temperature
to 1273 K.
CH is produced as a by-product at reaction temperatures under
4
1
,5
7
73 K. Therefore, methods to suppress CH production and
4
increase H2 and CO2 production is necessary.
CH3CH2OH þ 3H2O ! 2CO2 þ 6H2
ð1Þ
In this study, we investigated the steam reforming of ethanol
over Rh/SiO , Pt/SiO , Pd/SiO , and Ru/SiO at 573­723 K,
2
2
2
2
focusing on the mechanism of CH production and the iden-
4
Results and Discussion
tification of reaction conditions that suppressed CH production
4
and increased H and CO production.
Effect of Reaction Temperature. We tracked the effect of
reaction temperature on ethanol steam reforming over Rh/SiO2,
Pt/SiO , Pd/SiO , and Ru/SiO (Figure 1). Product yield in
2
2
Experimental
2
2
2
Catalyst Preparation. We prepared noble metal catalysts
supported on SiO as follows. SiO (10 g, Wako Pure Chemical
each figure describes the amount of reaction products produced
from 100 mol of ethanol under each reaction condition. The
carbon deposition was not detected by TGA method on all the
catalysts after the ethanol steam reforming reaction.
2
2
Industries) was impregnated at room temperature with an aque-
ous solution (20 mL) containing a metal precursor (1.5 mmol of
RhCl ¢3H O, K PtCl , RuCl ¢3H O, or PdCl ) and then dried
Over Rh/SiO , the main products at 573 K were CH and CO,
3
2
2
4
3
2
2
2
4
at 313 K for 12 h in air. The impregnated, dried SiO was
and the H and CO production was relatively low (Figure 1a).
2 2
2
reduced with gaseous H2 at 573 K for 2 h. After filtration and
washing with a large amount of water (500 mL), the obtained
solid was dried at 313 K for 12 h.
Ethanol Steam Reforming. Ethanol steam reforming was
conducted at atmospheric pressure in a fluidized-bed quartz
The H2 and CO2 production increased with increasing reaction
temperature, whereas the CH production decreased at higher
4
temperatures. The CO production fell dramatically as the tem-
perature was increased from 573 to 623 K, and then increased
gradually as the reaction temperature was increased further. We
Published on the web March 28, 2012; doi:10.1246/bcsj.20110306

5
18
Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
Ethanol Steam Reforming on Rh, Pt, Pd, and Ru
3
00
50
00
50
00
3.0
2.5
2.0
1.5
1.0
0.5
300
250
200
150
100
50
3.0
H2
CO
CO2
CH4
H2
CO
a
b
CO2
CH4
(CO +CO)/CH
2
2
1
1
2.5
2.0
1.5
1.0
0.5
(CO +CO)/CH
2
4
2
4
5
0
0
0.0
3.0
0
0.0
3.0
5
73K
623K
673K
698K
723K
573K
623K
673K
698K
723K
3
2
2
1
1
00
300
H2
CO
CO2
CH4
H2
CO
c
d
CO2
CH4
(CO +CO)/CH
50
00
50
00
2.5
2.0
1.5
1.0
0.5
0.0
250
200
150
100
50
2.5
2.0
1.5
1.0
0.5
0.0
(CO +CO)/CH
2
4
2
4
5
0
0
0
5
73K
623K
673K
723K
573K
623K
673K
698K
723K
Figure 1. Effect of reaction temperature on product yields in ethanol steam reforming over (a) Rh/SiO2, (b) Pt/SiO2, (c) Pd/SiO2,
¹1
¹1
and (d) Ru/SiO2 at flow rates of 20 ¯L min aqueous ethanol and 2.23 mmol min N2.
suggest that at 573 K ethanol decomposition (eq 2) was the
Desorption from the catalyst surface
Ethanol steam reforming
main reaction and some of the CO reacted with the H to
2
produce CH (the reason will be described later). We believe
CH CH OH
CO + H2
4
3
2
2
that at 623 K most of the CO produced by ethanol decom-
position was converted to CO2 by the water­gas shift reaction
Water–gas shift reaction
Methanation of
CO and CO2
CH4
Ethanol decomposition
(eq 3). Since the water­gas shift reaction is thermodynami-
2
CO + H + CH4
cally favored at low temperatures and kinetically favored at
high temperatures, the reaction rate is thought to be slow at
Desorption from the catalyst surface
Red: Main reaction path at higher temperatures
Blue: Main reaction path at lower temperatures
5
73 K. Ethanol decomposition was likely to have been sup-
pressed at higher temperatures, and ethanol steam reforming,
which is thermodynamically favored at high temperatures,
predominated at 723 K. The increase in CO production at higher
temperatures must have resulted from the reverse water­gas
shift reaction (eq 4).
Figure 2. Mechanism of CH4 production during ethanol
steam reforming over Rh/SiO2.
gators have reported that the reduction of CO and CO to CH
2
4
1
7­20
proceeds over a Rh catalyst.
In our experiments, CO and
2
CH3CH2OH ! CH4 þ CO þ H2
CO þ H2O ! CO2 þ H2
ð2Þ
ð3Þ
ð4Þ
CO®the products of ethanol steam reforming and ethanol
decomposition, respectively®must have reacted with H to
produce CH4 before desorbing from the catalyst surface
(Figure 2).
Over Pt/SiO , the main products at reaction temperatures
2
CO2 þ H2 ! CO þ H2O
If only these four reactions take place in the reactor, the
CO + CO)/CH ratio should not have been less than one,
(
2
4
2
even if the water­gas shift reaction of CO produced by ethanol
decomposition was taken into account. However, the observed
between 573 and 673 K were CH4, CO2, and H2, and the molar
CH :CO :H ratio was 1:1:2 (Figure 1b). As the temperature
4
2
2
(
CO + CO)/CH ratio was less than one below 623 K
was increased from 673 to 723 K, the CO and H production
2 2
2
4
(
Figure 1a), indicating that CH was produced by some other
increased and the CH production decreased. The CO produc-
4
4
process in addition to ethanol decomposition. Other investi-
tion increased gradually as the reaction temperature was

Y. Ando et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
519
3
3
2
2
1
1
50
00
50
00
50
00
2.5
2.0
350
300
250
200
150
100
50
2.5
H2
CO
CO2
CH4
H2
CO
a
b
CO2
CH4
(CO +CO)/CH
2.0
1.5
1.0
0.5
0.0
(CO +CO)/CH
2
4
2
4
1
1
0
.5
.0
.5
5
0
0
0.0
0
A
B
C
D
E
A
B
C
D
E
Reaction conditions of A, B, C, D and E are summarized in Table 1.
Figure 3. Effect of partial pressure of ethanol on product yields in ethanol steam reforming over (a) Rh/SiO2 and (b) Pt/SiO2
at 623 K.
Table 1. Flow Ratio of Ethanol, H2O, and N2 on Ethanol Steam Reforming
EtOHaq
¯L min
EtOH
/mmol min
H2O
/mmol min
N2
Partial pressure GHSV at
GHSV at
723 K/h
Run
¹1
¹1
¹1
¹1
¹1
¹1
/
/mmol min
of EtOH/Pa
623 K/h
A
B
C
D
E
20
20
10
5
0.114
0.114
0.057
0.029
0.014
0.741
0.741
0.370
0.185
0.093
2.23
4.47
4.47
4.47
4.47
3756
2180
1185
620
262
452
415
397
388
304
524
482
461
450
2.5
317
increased further. We suggest that at reaction temperatures
between 573 and 673 K, the overall reaction can be described
by eq 5. The reduction of CO to CH was negligible in this
573 to 723 K, the CO2 and H2 production increased and the
CH production decreased. The (CO + CO)/CH ratio was
4
2
4
0.54 at 573 K, and the ratio increased gradually as the reaction
temperature was increased further. We suggest that at 573 to
673 K, the main reaction was described by eq 5 and that some
of the CO was converted to CH below 623 K. From the mole
2
4
temperature range because the (CO + CO)/CH4 ratio was
2
unity. The reaction described by eq 5 was likely to have been
suppressed at temperatures higher than 673 K, and ethanol
steam reforming predominated at 723 K.
2
4
ratio of the products, we estimate that the percentages of CO2
converted to CH at 573 and 623 K were about 25% and 15%,
4
CH3CH2OH þ H2O ! CH4 þ CO2 þ 2H2
ð5Þ
respectively. The reaction described by eq 5 was likely to have
been suppressed at temperatures higher than 673 K, and ethanol
steam reforming predominated at 723 K.
Over Pd/SiO , the main products at 573 K were CH , CO,
2
4
and H , and the molar CH :CO:H ratio was 1:1:1 (Figure 1c).
2
4
2
As the temperature was increased from 573 to 673 K, the CO2
Effect of Partial Pressure of Ethanol. We evaluated the
effect of variation of the partial pressure of ethanol, which was
controlled by varying the relative flow ratio of aqueous ethanol
and H production increased and the CO production decreased.
2
In contrast, the CH production remained constant in the same
4
temperature range. As the temperature was increased from
and N , on the product yields of the reaction at 623 K over
2
6
73 to 723 K, the CO and H production increased and the
Rh/SiO (Figure 3a). The reaction conditions are summarized
2
2
2
CH4 production decreased. We suggest that at 573 K, ethanol
decomposition (eq 2) was the main reaction and that the extent
of ethanol steam reforming was negligible. The reduction of
in Table 1. As the partial pressure of ethanol was decreased,
the H and CO production increased and the CH production
2
2
4
decreased. The (CO + CO)/CH ratio gradually increased
2
4
CO to CH was also negligible in this temperature range, as
with decreasing partial pressure of ethanol and exceeded unity
when the partial pressure of ethanol was less than 1185 Pa,
indicating that the reduction of CO to CH was suppressed
4
indicated by the fact that the (CO + CO)/CH ratio was unity.
2
4
We suggest that some of the CO produced by ethanol decom-
2
4
position was converted to CO by the water­gas shift reaction
and that ethanol steam reforming predominated at low partial
pressures of ethanol. The partial pressures of H , CO , and CO
2
(eq 3) at 623 K and that most of the CO was converted to
2
2
CO2 at 673 K. The overall reaction at 673 K is described by
eq 5. Ethanol steam reforming was likely to have predominated
at 723 K.
were probably low at the lower partial pressures of ethanol. The
CH4 formation rate on a Rh catalyst has been reported to
1
7
increase with the partial pressures of CO and H . Thus, the
2
2
Over Ru/SiO , the main products at 573 K were CH , CO ,
suppression of CH production at lower partial pressures of
2
4
2
4
and H (Figure 1d). As the temperature was increased from
ethanol most likely resulted from the low partial pressures of
2

5
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Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
Ethanol Steam Reforming on Rh, Pt, Pd, and Ru
6
00
00
00
00
00
00
40
35
600
500
400
300
200
100
40
H2
CO
CO2
CH4
H2
CO
a
b
CO2
CH4
(CO +CO)/CH
35
30
25
20
15
10
5
5
4
3
2
1
(CO +CO)/CH
2
4
2
4
3
2
0
5
20
1
1
5
0
5
0
0
0
0
A
B
C
D
E
A
B
C
D
E
6
5
4
3
2
1
00
40
600
40
35
30
25
20
15
10
5
H2
CO
CO2
CH4
H2
CO
c
d
3
5
CO2
CH4
(CO +CO)/CH
00
00
00
00
00
0
500
400
300
200
100
0
(CO +CO)/CH
30
2
4
2
4
2
5
20
1
1
5
0
5
0
0
A
B
C
D
E
A
B
C
D
E
Reaction conditions of A, B, C, D and E are summarized in Table 1.
Figure 4. Effect of partial pressure of ethanol on product yields in ethanol steam reforming over (a) Rh/SiO2, (b) Pt/SiO2,
c) Pd/SiO2, and (d) Ru/SiO2 at 723 K.
(
CO and H . The increase in the H and CO production at
2
2
2
2
Conclusion
lower partial pressures of ethanol probably resulted from the
suppression of H and CO consumption due to the reduction of
We investigated the effect of reaction temperature and
aqueous ethanol and N2 flow ratio on the distribution of
products obtained from ethanol steam reforming over Rh/SiO2,
Pt/SiO , Pd/SiO , and Ru/SiO . The product yields of the
2
2
CO to CH .
2
4
In contrast, the product yields for H2, CO2, and CH4 at 623 K
over Pt/SiO were hardly affected by the partial pressure of
2
2
2
2
ethanol (Figure 3b). The (CO + CO)/CH4 ratio remained
catalysts differed at temperatures under 623 K, whereas at
723 K ethanol steam reforming predominated over all the
2
close to unity regardless of the partial pressure of ethanol,
confirming the result, shown in Figure 1, that the extent of the
reduction of CO to CH was negligible at 623 K.
catalysts. At temperatures under 623 K over Rh/SiO and Ru/
2
SiO , CH was produced not only by ethanol decomposition
2
4
2
4
We also evaluated the effect of the partial pressure of ethanol
on the reactions over all four catalysts at 723 K. At all the
partial pressures of ethanol investigated, the (CO2 + CO)/CH4
but also by the reduction of CO and CO to CH . At 723 K,
2 4
the product yields of all four catalysts were strongly influenced
by the partial pressure of ethanol: as the partial pressure of
ratio for Rh/SiO was higher at 723 K than at 623 K (compare
ethanol was decreased, CH production was suppressed, and
2
4
Figures 3a and 4a). At high temperatures, the reduction of
CO to CH is not favored thermodynamically, and CO and
H easily desorb from the catalyst surface, thereby eliminating
2
the H2 and CO2 production increased. These experimental
results can be explained in terms of the partial pressures of
CO and H . We clarified the mechanism of CH production
2
4
2
2
2
4
the possibility of subsequent reduction of CO to CH . We
during ethanol steam reforming, and identified reaction con-
ditions that suppressed CH production and increased H and
2
4
observed a large (CO + CO)/CH ratio (31.15) at the lowest
2
4
4
2
partial pressure of ethanol (317 Pa). The product yields over
Pt/SiO2, Pd/SiO2, and Ru/SiO2 at the various partial pres-
sures of ethanol at 723 K were similar to those of Rh/SiO2
CO production.
2
The goal of future studies will be the development of ethanol
steam reforming catalysts that suppress CH production even at
4
(
Figure 4), which agrees with the result, shown in Figure 1,
higher partial pressures of ethanol. The strategy for achieving
this goal could involve either incorporation of functionality
that ethanol steam reforming predominated at 723 K over all
four catalysts.
that suppresses CH production into noble metal catalysts or
4

Y. Ando et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 4 (2012)
521
incorporation of functionality that promotes ethanol steam
reforming into non-noble metal catalysts, which usually show
low activity for the reduction of CO and CO to CH .
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2
4
1
0
Supporting Information
11
Figure S1 shows the stability of these catalysts by compar-
ing with the second run activities. This material is available
free of charge on the Web at http://www.csj.jp/journals/bcsj/.
2
1
2
1
3
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