Photocatalytic Hydrogen Evolution From Aqueous Solutions of Glycerol Under Visible Light Irradiation
Photocatalytic Hydrogen Evolution From Aqueous Solutions of Glycerol Under Visible Light Irradiation
Photocatalytic Hydrogen Evolution From Aqueous Solutions of Glycerol Under Visible Light Irradiation
article info
abstract
Article history:
30 July 2013
photocatalytic activity was tested in a batch reactor in the reaction of hydrogen evolution
from aqueous solutions of glycerol under visible light irradiation (l > 420 nm). The highest
Keywords:
achieved photocatalytic activity was 449 mmol H2 per gram of photocatalyst per hour; the
highest quantum efficiency was 9.6% (l > 420 nm). The activity of the multiphase catalysts
Semiconductor photocatalysts
was shown to exceed that of the single-phase catalysts by a factor of 2.1, likely because of
Cd1xZnxS
Glycerol
Copyright 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1.
Introduction
* Corresponding author. Pr. Lavrentieva, 5, Novosibirsk 630090, Russia. Tel./fax: 7 (383) 3331617.
E-mail address: kozlova@catalysis.ru (E.A. Kozlova).
1
Tel./fax: 7 (383) 3331617.
0360-3199/$ e see front matter Copyright 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijhydene.2013.08.031
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working under visible light is of great interest for the use of the
solar light energy [17]. A quite narrow-band-gap of about
2.4 eV has the well-known semiconductor CdS. Unfortunately,
CdS is prone to undergo photocorrosion due to reactions with
photogenerated holes [18]. Some attempts have been made to
improve the stability and activity of cadmium sulfide. For
example, mixing of ZnS and CdS to form a Cd1xZnxS solid
solution has been shown to be an effective way to improve the
activity of CdS [19e22]. The band gap of the solid solution can
be continuously adjusted by changing its composition.
Another approach to the improvement of the photocatalytic
activity of sulfide photocatalysts is coupling the sulfide and
oxide nanoparticles [23]. Such multiphase composites can
combine the visible light responsibility of sulfides and stability
of oxides. Besides, the charge injection from the CB of the
narrow-band-gap semiconductor (CdS or CdS/ZnS) to the CB
of TiO2 or ZnO can lead to the efficient charge separation and
decreased electronehole recombination [16,24,25]. Recently
photocatalytic semiconductor materials containing heterostructures are widely used for the hydrogen production from
aqueous solutions of both inorganic [22,26e28] and organic
[12,29] electron donors.
In this research, we synthesized multiphase Pt/Cd1xZnxS/
ZnO/Zn(OH)2 and single-phase Pt/Cd1xZnxS photocatalysts
for the photocatalytic production of hydrogen from aqueous
solutions of glycerol under visible light irradiation. The
photocatalysts were synthesized using a two-step self-templated synthesis with Cd(OH)2 and Zn(OH)2 intermediates,
which has been proposed earlier for the CdS synthesis
[30,31]. Because the molar volume of sulfides is smaller than
that of hydroxides, this technique allows one to obtain materials with high porosity [32]. The photocatalysts were
characterized by the XRD, low-temperature N2 adsorptionedesorption, UVevis spectroscopy and TEM highresolution microscopy. The kinetic dependences of the
hydrogen evolution on the initial concentration of glycerol
and on pH were obtained under irradiation with the wavelength longer than 420 nm. It was shown that the activity of
the multiphase catalysts exceeds that of the single-phase
catalysts by a factor of 2.1.
2.
Experimental
2.1.
Catalyst preparation
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2.2.
Catalyst characterization
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2.3.
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Cd1-xZnxS
(3)
3.
3.1.
Catalyst characterization
3.1.1.
Zn(OH)2
A
45
50
ZnO
55
60
C
B
A
20
40
2 , degrees
60
Phase composition
%, wt
CS,a nm
l, nm
SBET, m2/g
Vp, cm3/g
W0(H2),c mmol/min
F,b %
Cd0.1Zn0.9S
Cd0.2Zn0.8S
ZnO
3-Zn(OH)2
Cd0.4Zn0.6S
ZnO
3-Zn(OH)2
100
52
5
43
28
<1
w71
<2
<2
w35
>100
<2
w35
>100
430
453
228
50
0.24
0.18
0.131
0.273
3.3
7.0
442
44
0.21
0.230
5.9
a Crystallite size.
b Quantum efficiency.
c The rate was measured on platinized photocatalysts (1 wt% of Pt).
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3.1.2.
Diffuse reflectance spectra (Fig. 2a) were measured to determine the light absorption by photocatalysts A, B and C. The
spectrum of specimen A is typical of the single-phase
Cd0.1Zn0.9S solid solution. It is well known that Cd1xZnxS
solid solutions with x from 0.5 to 0.9 have the band gap energy
from 2.5 to 2.9 eV (430e495 nm) [20,21,32]; the absorption edge
of ZnO varies from 370 to 400 nm [39,40], whereas the Zn(OH)2
adsorption edge lies at about 250 nm [40]. The spectra of
composite photocatalysts B and C demonstrate a combination
of three spectra: Cd1xZnxS (range 450e600 nm), ZnO (range
350e450 nm) and Zn(OH)2 (range 350e250 nm). These spectra
confirm that specimen C consists mostly of Cd1xZnxS and
Zn(OH)2, whereas specimen B consists of Cd1xZnxS, ZnO and
Zn(OH)2. The relative intensities of the Cd1xZnxS, Zn(OH)2 and
ZnO spectra indicate that most of the surface is covered with a
sulfide solid solution.
We tried to estimate the band gap energy of Cd1xZnxS
nanoparticles in the composite photocatalysts. Because CdS
and ZnS are direct semiconductors, their absorption edges [41]
can be calculated using the Tauc function as shown in Fig. 2B.
The Tauc functions F(R)2(hv)2 versus hv are plotted in the range
of wavelengths from 420 to 600 nm because ZnO and Zn(OH)2
do not absorb light in this range. The band gap energy was
calculated by constructing a tangent to the curves and by
finding the X axis intercept for this tangent. The determined
adsorption edges are shown in Table 1. For specimen A, the
adsorption edge is 430 nm, which is in a good agreement with
the published data, which report that the adsorption edge of
Cd0.1Zn0.9S varies from 429 to 455 nm [21,32]. The evaluated
adsorption edges for specimens B and C are 452 and 442 nm,
3.1.3.
3.2.
100
a
A
B
C
40
20
A
B
C
80
(F(R)h)
%R
60
Kinetic experiments
We conducted initial experiments on the hydrogen production from glycerol aqueous solutions under visible light irradiation with pure and platinized specimens A, B, and C. It was
shown that the rate of hydrogen evolution was equal to zero
on the non-platinized photocatalysts both in neutral and basic
media. It was also shown that no hydrogen was formed
with the use of platinized photocatalysts in neutral media.
Earlier, it was shown that the addition of NaOH increases
the adsorption of organic substrates on the CdS/ZnS
100
80
60
40
20
0
250 300 350 400 450 500 550 600
, nm
0
2.6
2.8
3.0
E, eV
3.2
3.4
Fig. 2 e Diffuse reflectance spectra of photocatalysts A, B, C (a) and the Tauc plot for absorption edge determination (b).
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Fig. 3 e HRTEM images of specimens A (a, b) and B (c, d) with EDX (specimen A (e), specimen B sulfide part (f), specimen B
oxide part (g)).
25
photocatalysts [42]. The isoelectric points for ZnS and CdS are
in the range of 3.0e8.5, depending on crystalline structure [43],
so under basic conditions sulfide surface is negatively
charged. However, in basic media the substitution of surface
S2 by hydroxyl ions take place. Zinc hydroxide and cadmium
hydroxide have isoelectric points in the range 9.6e11.5 [44].
Thus, under basic conditions, the surface of the solid solution
(specimen A) and composites (specimens B and C) exists
mainly in ^ZneOH or ^ZneO and ^CdeOH or ^CdeO
forms [42]. We supposed that at pH w 13, glycerol, like glucose
[42], can form ions C3H7O that can compete with hydroxyls at
the Zn and Cd sites on the Cd1xZnxS surface, providing a
strong adsorption.
Fig. 4 shows that the rate of the hydrogen production under
basic conditions is quite slow at the beginning of the reaction
for all photocatalysts, but after about one hour, it grows and
becomes linear. The same results have been observed previously: the kinetic curves of the hydrogen production from
glycerol aqueous solutions had an apparent induction period
of 0.5e1.0 h [6,45,46]. According to the mechanism proposed
by Bowker et al. and de Oliveira Melo et al., on the first stage of
A
B
20
C
15
10
5
0
0
20
40
60
80
100
120
time, min
Fig. 4 e Kinetic curves of the photocatalytic hydrogen
evolution from aqueous solutions of glycerol in the
presence of synthesized photocatalysts with 1 wt% of Pt.
C(cat) [ 0.77 g LL1; T [ 20 C; C0 (glycerol) [ 100 mM, C0
(NaOH) [ 100 mM.
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0.18
0.12
0.06
0.00
0.4
0.24
50
K C0
;
1 K C0
(4)
0.36
0.30
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0.3
0.2
L-H
0.1
0.0
Fig. 5 e Initial rate of the hydrogen evolution in dependence on the initial concentrations of NaOH (a) and glycerol (b) (the
curve is the approximation by the LangmuireHinshelwood equation). Photocatalyst B with 1 wt% of Pt was used in all
experiments. C(cat) [ 0.77 g LL1; T [ 20 C; C0 (glycerol) [ 200 mM (a), C0 (NaOH) [ 100 mM (b).
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4.
Conclusions
A way for the synthesis of multiphase platinized photocatalysts Pt/Cd1xZnxS/ZnO/Zn(OH)2 has been proposed. It
was shown that these photocatalysts are more active in the
photocatalytic hydrogen evolution from aqueous solutions of
glycerol under visible light irradiation (l 420 nm) than most
common platinized single-phase Cd1xZnxS photocatalysts.
The highest achieved photocatalytic activity was 449 mmol H2
per gram of the photocatalyst per hour. The highest quantum
efficiency was 9.6% that exceeds the values reported before
[46]. High activity of multiphase systems may be caused by the
formation of heterojunctions between nanoparticles of
Cd1xZnxS, ZnO and Zn(OH)2, which result in the efficient
charge separation by minimizing the electronehole recombination. It is also found that the optimal pH for hydrogen
evolution is 12.5e13.5 while the optimal initial concentration
of glycerol is 2e4 vol%.
Acknowledgements
We gratefully acknowledge the support of Russian Federation
Department of Science and Education (Federal Target Program
Scientific and Educational Personnel, contract 8440; President Grant for the Leading Scientific Schools NSh-524.2012.3;
President Scholarship for Young Scientists SP-85.2012.1); RFBR
(grant #12-03-31104) and SB RAS (joint project #35). We also
acknowledge Drs. S.V. Cherepanova and E. Yu. Gerasimov for
help with obtaining and treating of XRD and TEM data.
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