i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 8 7 8 e8 8 8 4
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Effects of general zero-valent metals power of Co/W/Ni/Fe
on hydrogen production with H2S as a reductant under
hydrothermal conditions
Shiping Zhang a, Fangming Jin b,*, Xu Zeng a, Jiajun Hu a, Zhibao Huo a, Yuanqing Wang a,
Noriaki Watanabe b, Nobuo Hirano b, Noriyoshi Tsuchiya b
a
State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University,
Shanghai 200092, China
b
Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan
article info
abstract
Article history:
Hydrogen production from water with S2 as a reductant under hydrothermal conditions is
Received 1 March 2011
an effective new method, in which S2 is oxidized into S2O32, SO32 and SO42. However,
Received in revised form
the reactor wall had great effect on hydrogen production as large amount hydrogen is
28 April 2011
produced with Hastelloy C-22 reactor while almost no hydrogen generated in SUS 316
Accepted 30 April 2011
reactor. Therefore, the influence of main components of Hastelloy C-22 reactor (Co/W/Ni)
Available online 26 May 2011
and SUS 316 reactor (Fe) on hydrogen production with H2S as the reductant was investigated. The results showed that Fe had negative effect, whereas W, Co and Ni had signifi-
Keywords:
cant positive effect on improving hydrogen production. These results provided a possible
Hydrogen production
explanation for no hydrogen generated with SUS 316 reactor，and some suggestions for
H2S
improving hydrogen production. The highest hydrogen production of 199 mL (2 times than
Hydrothermal condition
the control) was obtained with 4.00 mmol Co, 4.00 mmol W, and 1.00 mmol Ni.
Catalyst
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Water
1.
Introduction
The dependence on fossil fuels as a main energy source has
led to a serious energy crisis and environmental problems. In
addition, their combustion products cause global warming.
Therefore, exploring a new clean energy, such as hydrogen
energy, that can replace the existing fossil fuel system is
currently an important research subject. Hydrogen has been
projected as one of the long-term, sustainable and environmentally friendly energy carriers [1,2], emitting only water as
a by-product during the combustion or oxidation process.
Apart from its use as a clean energy resource, hydrogen can
be used for various other purposes in industrial processes.
Not only can it be used to saturate compounds, crack
hydrocarbons or remove sulfur and nitrogen compounds, but
also it is a good oxygen scavenger and, therefore, can be used
to remove traces of oxygen to prevent oxidative corrosion [3].
Hydrogen can be used in petroleum refining [4], ammonia
production [5], and metal refining, such as nickel, tungsten,
molybdenum, copper, zinc, uranium and lead [6].
At present, much of the hydrogen produced, especially for
the petrochemical industry, is obtained from (a) the fossil
fuels, which consumes a large amount of energy and cannot
solve the depletion of fossil fuels; (b) the electrolysis of water,
which requires high energy consumption; and (c) biomass
[7e12] which is still in the nascent stages and requires further
* Corresponding author.
E-mail address: [email protected] (F. Jin).
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2011.04.227
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development to improve the efficiency and reduce the cost of
production [13]. Therefore, a new highly efficient and
environmentally friendly method for producing hydrogen is
necessary and urgent. Water is an environmentally benign
solvent and the most abundant source of hydrogen in the
earth; therefore, extracting hydrogen from water is
a potential and attractive technology.
High-temperature water (HTW) has unique features
compared to ambient liquid water. The ion product (Kw) at
250e300 C is approximately three orders of magnitude higher
than that of ambient liquid water [14] and has been proven to
be an environmentally friendly reaction media for the
conversion of various types of biomasses [15e21].
Our previous researches on the hydrothermal treatment of
bitumen and sulfur-containing compounds, such as benzothiophene, dinemzothiophere and their derivatives which are
the most difficult to be decomposition intermediates in hydrothermal upgrading bitumen, indicated that most of the sulfur
exists in the form of H2S in the water samples, moreover, that
there is hydrogen produced after the reactions [22e24].
Although the mechanism of producing hydrogen in hydrothermal reactions for treating the sulfur-containing rubber is
not yet fully understood, we proposed that the formed H2S acts
as a reductant for producing hydrogen from water under
hydrothermal conditions due to the strong reducibility of H2S.
Then, we investigated hydrogen generation from water using
H2S as the reducant under hydrothermal conditions. As a result,
a large amount of hydrogen was produced with a Hastelloylined reactor. The possible mechanism of hydrogen production
proposed was H2S þ H2O / S2O32 þ SO32 þ SO42 þ H2[ [25].
However, our recent research found that there was no hydrogen
generated when a SUS 316-lined reactor was used, which
indicated that the metal materials of the reactor walls had
a significant impact on the hydrogen production.
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The purpose of this paper, therefore, was to study the
effect of the major components in the reactors of the SUS 316
and Hastelloy C-22 on hydrogen production with H2S from
water under hydrothermal conditions and to find further
foundations for improving hydrogen production.
2.
Materials and methods
2.1.
Materials
The hydrolysis equilibrium of Na2S generally occur as the
reaction of S2 þ H2 O# H2 S þ 2OH at room temperature
indicated that there was H2S produced. The results of
comparison experiments with Na2S9H2O and homemade H2S
as the reductant under the same conditions have showed that
the H2 production was similar. Thus, Na2S$9H2O (99%) was
used to simplify the handling in this study. Na2S9H2O was
purchased from Wako Pure Chemical Industries, Ltd., Japan.
The main components of the SUS 316 and Hastelloy reactor
walls, Co, W, Ni and Fe, were selected to study their effects on
hydrogen production. Co, W, Ni and Fe powders (100-mesh,
99%, Wako Pure Chemical Industries, Ltd., Japan) were used.
2.2.
Experimental procedure
The reactor system used in this study is shown in Fig. 1.
Considering the corrosion of high concentrations of OH, the
reactor was lined by the corrosion and high-temperature
resistant Hastelloy C-22, which is one type of Hastelloy
reactors, and with an internal volume of 212.71 mL. The
different components between Hastelloy C-22 and SUS 316
reactor wall are mainly the concentrations of Ni, Co, W and
Fe. All of the experiments were conducted in the Hastelloy
Fig. 1 e Scheme of Hastelloy C-22 autoclave.
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reactor with 1% (w/w) Na2S$9H2O at a temperature of 300 C
and a reaction time of 60 min.
The general experimental procedure is as follows. The
desired amount of materials and deionized water were added
into the reactor, and then approximately 1 mL of liquid sample
was removed to determine the initial concentration of H2S by
capillary electrophoresis (CE) because some of the H2S dissolved in water evaporated during the addition of the materials. Afterward, the reactor was sealed under a nitrogen
atmosphere. After a reaction time of 60 min, the reactor was
cooled to room temperature. The reaction time was started
when the temperature reached 300 C. The time required for
raising the temperature of the reaction medium from room
temperature to 300 C was approximately 20 min, and thus,
the actual reaction time was longer than 60 min.
2.3.
Product analysis
After the reactions, the gas samples were collected and quantified by GC323-TCD. The hydrogen concentration was determined based on the average value of at least three samples
with relative errors less than 5% for all the experiments. The
liquid samples were collected and quantified by capillary
electrophoresis (CE, Agilent G1600A). The solid samples were
collected and determined by X-ray diffraction (XRD, Rigaku
RINT-2200VL, Japan). The quantitative analysis of XRD was
determined by the Rietveld method, which is currently the
most powerful method existing for quantitative phase analysis
[26]. The Rietveld calculations in this work were performed by
the software TOPAS 4.2 from Bruker AXS, UK. The software is
based on the fundamental parameter approach (fpa), which
considers this geometric and unit specific parameter [27]. The
hydrogen production reported in this paper was defined as
the volume of produced hydrogen (mL). As the oxidation of
S2 during the reaction is very complicated and the hydrogen
production depends on the oxidation products S2O32, SO32
and SO42 during the reaction, thus, the hydrogen yield was
defined as the percent of the produced hydrogen (mmol) and
the initial Na2S$9H2O (mmol).
Table 1 e Central composite design.
Factors
Units
1 level
þ1 level
Co (A)
W (B)
Ni (C)
mmol
mmol
mmol
1.00
1.00
1.00
4.00
4.00
3.00
are favorable to promote hydrogen production from water.
The change in hydrogen production in the presence of W
differs obviously with that in the presence of Co or Ni. The
hydrogen production does not decrease but reaches
a plateau with increasing the amount of W; however, the
hydrogen production decreases significantly with a further
increase in Co or Ni. However, for Fe, the hydrogen
production decreases drastically by increasing the amount
of Fe (see Fig. 2). As with W, Co and Ni are the main
components of Hastelloy; therefore, the hydrogen produced
in the Hastelloy reactor is probably due to the effect of W,
Co and Ni. A possible reason for hydrogen not being
generated in the SUS 316 reactor is due to the effect of Fe.
After the reactions, the solid samples were analyzed by
XRD (Fig. 3). As shown in Fig. 3, only W was detected in the
solid samples after the reaction in the presence of W, which
indicated that W played a catalytic role in the hydrogen
production from water with H2S. It has been reported that W
can promote the oxidation of sulfide under hydrothermal
conditions [28].
From Fig. 3(E), a large amount of Ni3S2 was detected in the
case of adding Ni. The result of the CE analysis for the
solution samples after the reactions showed that there was
no S2 remaining in the samples (see Fig. 4). These results
suggested that all of the Ni was sulfated into Ni3S2 during the
hydrothermal reactions. Thus, one possible reason for the
improvement in the hydrogen production at first may be due
to the catalytic effect of Ni3S2. To examine the catalytic effect
of Ni3S2, experiments with 2 mmol of Ni3S2 and Na2S$9H2O at
300 C and a reaction time of 60 min were conducted. The
results showed that the hydrogen production was improved
by approximately 15% in the presence of Ni3S2, which
2.4.
Experimental design and statistical analysis by
Design-Expert
The statistical software package Design-Expert 7.1.3 (StatEase, Inc., Minneapolis, MN) was used to design the experiments and analyze the results. The response surface approach
involving a central composite design (CCD) was adopted to
find the optimal level of the factors for the hydrogen production. A set of 16 experiments including four center points was
conducted. The minimum and maximum range of variables
investigated and the full experimental plan with respect to
their actual and coded forms are listed in Table 1.
3.
Results and discussion
3.1.
Effect of Co, W, Ni and Fe on hydrogen production
Fig. 2 shows the effects of W, Co and Ni on hydrogen
production. As shown in Fig. 2, W, Co and Ni, particularly W,
Fig. 2 e Effect of W, Co, Ni and Fe on hydrogen production
from water with 1% Na2S$9H2O (300 C for 60 min).
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Fig. 3 e XRD patterns of solid samples after the reaction
with the addition of Ni, W, Co, Fe and W/Co/Ni (Ni: 2 mmol,
W: 1 mmol, Co: 1 mmol, Fe: 4 mmol, and W/Co/Ni: 1 mmol/
4 mmol/4 mmol, 300 C, 60 min).
indicated that Ni3S2 was a catalyst for improving hydrogen
production from water. However, the hydrogen production
decreased significantly when the amount of Ni increased
further, which may be because the formation of Ni3S2
consumed some S2, which is the reductant for hydrogen
production. Although hydrogen was also produced when Ni
was oxidized to Ni2þ, the loss of electrons from Ni to Ni2þ was
less than that from S2 to S4þ or S6þ. Thus, the formation of a
large quantity of Ni3S2 had both negative and positive effects
Fig. 4 e Capillary electrophoresis chromatogram of the
liquid samples after the reaction without metals and with
the addition of Ni, W, Co and W/Co/Ni (W/Co/Ni: 4 mmol,
4 mmol and 1 mmol, Ni: 1 mmol, W: 1 mmol, Co: 1 mmol,
300 C, 60 min).
8881
on hydrogen production. That is, there is a competition
between the oxidation of Ni to Ni2þ and the catalytic role of
Ni3S2. When the amount of Ni was low, the catalytic role of
Ni3S2 was dominant, which only consumed a small amount
of S2. However, when a large amount of Ni was added, the
main reaction was the oxidation of Ni to Ni2þ, consuming
a large amount of S2 and resulting in a drastic decrease in
hydrogen production. Although the presence of a large
amount of Ni can lead to the decrease in the hydrogen
production due to the consumption of S2 in the case of
hydrogen production with H2S, Ni3S2 is a catalyst for
hydrogen production, and the addition of Ni3S2 is one
method for improving hydrogen production from water with
H2S as the reductant.
From Fig. 3B, a small amount of Co9S8 is formed in the
presence of Co. The amount of Co9S8 is less than that of
Ni3S2. Similar to Ni, although there was some S2 consumed
by Co, the hydrogen production from water still increased by
approximately 22% compared to the absence of Co. This
result is probably due to the catalytic role of Co9S8. Compared
to the addition of Ni, the highest hydrogen production in the
presence of Co reached the peak value first (see Fig. 2)
possibly because the catalytic activity of Co9S8 is higher than
that of Ni3S2. Because Co9S8 is commercially unavailable, the
catalytic role of Co9S8 was not tested. However, a related
study is still in process.
As shown in Fig. 3A, Fe and Fe3O4 are found in the solid
samples after the reactions in the presence of Fe. This finding
suggests that some Fe was oxidized to Fe2þ and Fe3þ. Our
previous research showed that Fe cannot be oxidized to Fe3O4
under hydrothermal conditions at 300 C [29]. The formation of
Fe3O4 may be because Fe reacted with H2S to form FeS, which
is oxidized into Fe3O4 by water under hydrothermal conditions
due to the instability of FeS. To test this assumption, the
experiments with FeS and water were conducted at 300 C for
60 min. The analytical results by XRD for the solid samples
after the reactions showed that Fe3O4 was detected. The Fe3þ
and Fe2þ formed during the hydrothermal reactions were
reduced to Fe2þ and even Fe by S2. Then, some of the
reducing power of S2 was consumed for reducing Fe3þ and
resulted in the inhibition of the production of hydrogen from
water using S2 as the reductant. Some researchers have
reported that Fe3þ can be reduced by S2 during the catalytic
wet oxidation of H2S because Fe3þ has the potential for
decomposition of H2S and it can be regenerated using S2 as
a reductant [30,31]. Therefore, the formation of Fe3þ or Fe2þ
was the reason for the remarkable decrease in hydrogen
production in the presence of Fe.
Because W acts as a catalyst, and its reuse is important. To
examine whether W could be reused, texperiments with
Na2S9H2O and 1 mmol of W obtained after the reactions were
conducted at 300 C for 60 min. The results indicated that the
hydrogen production with the cycled W was similar to that
with fresh W, suggesting that W can be reused.
3.2.
W/Ni
Modeling of hydrogen yield with the addition of Co/
Because the hydrogen yield may be improved further by
adding Co, W and Ni simultaneously, the central composite
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Response Surface Methodology experiment was used to find
the optimal concentrations of Co, W and Ni. The significance
of the three metals and the possible interactions between the
three factors were analyzed by establishing a model. The
results of the hydrogen yield from water are presented in
Table 2.
According to the analysis of the BoxeCox plot, which is
a tool used to determine the most appropriate transformation
to apply to the response data, the data must be transformed to
ensure that the model meets the assumptions required for
analysis of variance (ANOVA). Then, the coefficients of the
regression equation were calculated according to the results
shown in Table 2, using Design-Expert. Based on the
transformation coefficient obtained from the BoxeCox plot
(1.63) and the calculated coefficients, the model of the
hydrogen yield from water can be expressed by the following
regression equation:
H2 yield ð%Þ ¼ 3432:55 þ 151:25A þ 179:18B 425:26C
þ 177:89AB 0:12AC 22:12BC 29:65A2
1:63
240:52B2 þ 606:26C2 þ 78:67ABC
(1)
where A, B and C refer to the levels of Co, W and Ni,
respectively.
The reliability of the model was evaluated by the key
elements obtained from ANOVA of the coefficients in Eq. (1).
The R2 value (a coefficient of determination) of the model was
0.9998. The adjusted R2 and predicted R2 were 0.9995 and
0.9983, respectively. For a good statistical model, the R2 value
should be close to 1.0, and all three factors should be positive
and close to each other. The results of the R2, adjusted R2 and
predicted R2 are coincident with this standard. In addition,
the model has an adequate precision value of 222.772, which
is higher than 4, suggesting that the model could be used to
navigate the design space. The F-value of the model (3036.21)
is high, which indicates that the factors have a significant
effect on the response. The lack of fit p-value of the model is
Table 2 e Experimental design and results of hydrogen
yield.
No
Factors
Response
W (mmol) Co (mmol) Ni (mmol) Hydrogen yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
4.00
1.00
1.00
1.00
2.50
2.50
4.00
2.50
4.00
5.02
2.50
4.00
2.50
0.00
1.00
2.50
1.00
1.00
4.00
4.00
2.50
2.50
4.00
5.02
1.00
2.50
2.50
4.00
0.00
2.50
1.00
2.50
3.00
1.00
3.00
1.00
2.00
2.00
3.00
2.00
1.00
2.00
2.00
1.00
2.00
2.00
3.00
2.00
138
161
138
163
147
148
160
152
166
137
147
178
139
120
144
148
0.7227, which is higher than 0.05, suggesting that the lack of
fit is not significant and that the model fit all the design
points well. Therefore, the proposed model is acceptable and
fits the experimental data well on the basis of ANOVA.
3.3.
Effect of the interactions of Co/W/Ni on the
hydrogen yield
The interactions among Co, W and Ni were examined by
p-values (the tail probability of the distribution of a test
statistic) obtained from the Design-Expert analysis. The pvalues for AB, AC, BC and ABC (where A is Co, B is W and C is
Ni) were less than 0.0001, 0.9775, 0.0032 and less than 0.0001,
respectively. Generally, the term is significant when the pvalue is less than 0.05. The p-value of AB, BC and ABC were less
than 0.05, indicating that there was a positive interaction
between Co and W, W and Ni, and Co, W and Ni.
For the significant positive interaction between Ni and W,
a possible explanation is due to the inhibition effect of W on
the formation of Ni3S2. This effect results in the obvious
improvement in hydrogen production because Ni was
completely sulfated to Ni3S2 and there was no S2 remaining
in the solution in the absence of W, while only a slight amount
of Ni3S2 in the solid samples were found in the presence of W
(see Fig. 3D and E). For the significant positive interaction
between Co and W, a further study is in progress because W
did not inhibit the formation of Co9S8 (see Fig. 3B and D).
When W, Co and Ni were added simultaneously, hydrogen
production from water was increased approximately 2 times
more than that without metals. This increase may be due to
the formation of Ni3S2 being inhibited by the addition of W
under hydrothermal conditions. As shown in Fig. 4, there was
a larger amount of SO32 produced in the liquid sample after
the reaction in the presence of W, Co and Ni. These
observations are further evidence that W inhibited the
formation Ni3S2 and that the hydrogen production from
water was mainly due to the oxidation of H2S to SO32.
Therefore, W is not only a good catalyst for hydrogen
production from water using H2S as the reductant but also
can inhibit the formation of Ni3S2.
To further examine the interactions of the three factors
and determine the optimum levels of each factor required for
the highest hydrogen production, 3-D response surface curves
were plotted according to Eq. (1), including the design point
values. Fig. 5a shows that the interaction between Co and W
was stronger with an increase in the amount of Co and W.
From the contour line on the bottom, the hydrogen
production obviously increased first and then decreased
with an increase in the amount of W and a constant amount
of Co. When W was less than 1.75 mmol, the influence of Co
on hydrogen production was not significant. The hydrogen
production increased remarkably by increasing the amount
of Co when W was higher than 1.75 mmol. The interaction
between Co and W was the strongest for 4.00 mmol of Co
and 4.00 mmol of W.
Fig. 5b shows that the hydrogen production was obviously
improved when Ni was less than 1.00 mmol or higher than
3.00 mmol. The interaction between W and Ni was the
strongest for 1.00 mmol of Ni and 2.50 mmol of W.
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8883
Fig. 5 e Response surface plots for hydrogen yield showing (a) effect of W and Co, (b) effect of W and Ni.
3.4.
Optimization of hydrogen production with the
addition of Co/W/Ni
The optimal values of each factor for the highest hydrogen
yield were predicted by Design-Expert based on Numerical
Optimization, which is a tool used to identify the optimal
settings of the factors used in the design space by the established model. As shown in Fig. 6, the highest area (hydrogen
yield > 176%, which was higher than 100% because the
hydrogen yield is the ratio of the produced hydrogen (mmol)
and the added S2 (mmol), indicating that the hydrogen was
from the water not from H2S) was accessed with the
condition of higher Co and W concentrations. The optimal
level given by Numerical Optimization was 4.00 mmol of Co,
4.00 mmol of W and 1.00 mmol of Ni. The predicted value of
the highest hydrogen yield from water was 178%.
To examine the predictive results by the model, a confirmatory experiment was conducted with 4.00 mmol of Co,
4.00 mmol of W, and 1.00 mmol of Ni at 300 C for 60 min. The
results showed that hydrogen yield was 179%, which was
consistent with the prediction value of the hydrogen yield.
The confirmatory experiment indicated that the proposed
model could predict the hydrogen yield accurately.
4.
Conclusions
W, Co and Ni can obviously enhance hydrogen production
using H2S as the reductant under hydrothermal conditions.
The hydrogen production increased with increasing W,
whereas it increased first and then decreased with increasing
Co or Ni. W not only can catalyze the oxidation of H2S to SO32
but also can inhibit the sulfation of Ni. Although the formation
of Ni3S2 and Co9S8 consumed some of the S2, Ni3S2 and Co9S8
were the dominant catalysts for producing hydrogen. Fe has
an obvious negative effect on hydrogen production from
water using sulfide as the reductant under hydrothermal
conditions. One possible reason for hydrogen not being
generated by SUS 316, while hydrogen was produced by the
Hastelloy reactor, may be due to the high amount of Fe in the
SUS 316 reactor wall and the presence of W, Co and Ni in
the Hastelloy reactor wall. The highest hydrogen yield of
178%, which was 2 times than that without metal power, was
obtained with 4 mmol W, 4 mmol Co, and 1 mmol Ni under
hydrothermal conditions.
Acknowledgments
The authors gratefully acknowledge the financial support
of the National Natural Science Foundation of China
(No: 21077078), the National High Technology Research
and Development Program of China (863 Program) (No:
2009AA05Z405 and 2009AA063903), and International Cooperation Foundation Project of Shanghai Science and Technology
Commission (No: 09160708100).
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Fig. 6 e Response contour plots for the predicted the
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