A titanium disilicide derived semiconducting catalyst for water splitting under solar radiation - Reversible storage of oxygen and hydrogen

Article abstract of DOI:10.1002/anie.200701626

(Chemical Equation Presented) Divide and separate: Photocatalytic splitting of water into hydrogen and oxygen is achieved with a catalyst which is formed on the surface of titanium dicilicide (see picture). The two product gases are reversibly physisorbed by the catalyst. Desorption of hydrogen occurs at ambient temperature, but oxygen is entirely stored up to 100°C in light and can be released upon heating at this temperature in the dark, which allows convenient separation of the gases.

Full text of DOI:10.1002/anie.200701626

Communications
DOI: 10.1002/anie.200701626
Water Splitting
A Titanium Disilicide Derived Semiconducting Catalyst for Water
Splitting under Solar Radiation—Reversible Storage of Oxygen and
Hydrogen**
Peter Ritterskamp, Andriy Kuklya, Marc-AndrØ Wüstkamp, Klaus Kerpen, Claudia Weidenthaler,
and Martin Demuth*
Dedicated to Professor Kurt Schaffner on the occasion of his 75th birthday
Hydrogen and oxygen evolution from water using semi-
conductors and light is an important issue in the exploitation
of solar radiation as a sustainable energy.[1,2] However, a
major drawbackof most of the research in this field relates to
the fact that appropriate semiconductors either are not
readily accessible, absorb solar radiation inefficiently,[3–8] or
produce hydrogen in a sacrificial manner only (i.e. the catalyst
is degraded). We present titanium disilicide (TiSi2) as a
prototype for the promising new class of silicide semiconduc-
tors,[9] which have not, to date, been used for water splitting.[1]
These semiconducting materials are inexpensive and abun-
dant. One disadvantage might be their poor stability (in
particular of TiSi2 in water). However, we anticipated that
sufficient passivation of TiSi2 by limited oxide formation
might render this project successful.[10]
Figure 1. Band-gap range of the TiSi2-based semiconducting catalyst
employed in this work. h+ =hole with positive charge.
The light-absorption characteristics of TiSi2 are ideal for
solar applications: broad-band reflectance measurements
show a band-gap range from 3.4 eV (ca. 360 nm) to 1.5 eV
(ca. 800 nm) for TiSi2 (Figure 1). This behavior is atypical of
semiconductors since these materials usually exhibit small
band-gap spreads. Determination of the quasi Fermi level of
electrons at pH 7 of our catalyst showed values of ꢀ0.43 eV
and ꢀ0.41 eV before and during reaction, respectively;[11] the
latter energy level still fulfils the physical requirement for the
reduction of protons to form hydrogen.[12]
RUL 350 or 540 nm lamps (lmax; ꢁ 60 nm emission range).
Comparable water-splitting kinetics were obtained.
Two phases (A and B) of hydrogen evolution are observed
when darkgrey TiSi 2 powder (ca. 325 mesh, Alfa) is allowed
to react at 558C under standard conditions (see the Exper-
imental Section and Figure 2). Phase A starts at t = 0 with
[H2] = 0 and shows a nonlinear dependence. Phase B is
characterized by the nearly linear part of the hydrogen
evolution curve. We interpret the time dependence of hydro-
gen evolution to be a consequence of simultaneously occur-
ring reactions [Eqs. (1)–(3)]. The course of the reaction in
The broad-band water-splitting capacity has been ascer-
tained by experiments run at individual wavelength ranges.
For this purpose, Rayonet photoreactors were equipped with
TiSi2 þ 6 H2O ! TiSi2 oxides þ 6 H2
ð1Þ
ð2Þ
ð3Þ
[*] P. Ritterskamp, Dr. A. Kuklya, M.-A. Wüstkamp, Dr. K. Kerpen,
Prof. M. Demuth
TiSi2 ðcat:Þ, hn
H O
2
1= O2 þ 2 Hþ þ 2 eꢀ
ƒƒƒƒƒƒƒ!
2
Max-Planck-Institut für Bioanorganische Chemie
45413 Mülheim an der Ruhr (Germany)
Fax: (+49)208-306-3951
2 Hþ þ 2e ꢀ ! H2
E-mail: demuthm@mpi-muelheim.mpg.de
Homepage: http://www.mpibac.mpg.de
Dr. C. Weidenthaler[+]
Max-Planck-Institut für Kohlenforschung
45470 Mülheim an der Ruhr (Germany)
Equation (1), which sacrifices the TiSi2 by oxide formation,
has been verified by runs in the darkat 50–85 8C. Initially, it
shows a similar dependence as the reactions in light, but it
levels off and does not show a linear dependnce as in phase B,
which is attributed to water splitting, and hydrogen produc-
tion comes to a stop after approximately 150 h (Figure 6 in the
Supporting Information). The course of hydrogen evolution
in phase A depends strongly on the quality and composition
of the catalyst, its particle size, the reaction temperature, and
the pH value.[13]
[+] XRPD/XP spectroscopy
[**] We thank the Max-Planck-Gesellschaft for generous financial
support of this work and Dr. Helmut Görner for helpful discussions.
The mass spectrometry, performed by Dipl.-Biol. Katrin Beckmann
and Dr. habil. Johannes Messinger, is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
Figure 3. Schematic representation of the proposed in situformation
of the catalytic centers (cc) for water oxidation (cc1) and proton
reduction (cc2) within the initial hours of reaction on the surface of
commercially available TiSi2 that is partially covered by thin TiO2 and
SiO2 layers.
air, reduced activity of the catalyst is also observed. This
finding points to a mechanism for the formation of the centers
in which limited photooxidation could play a role.
A significant increase in water oxidation and hydrogen
formation is found at higher gas pressure (1.1–1.2 bar) while
the reaction temperature (508C) is kept constant (see
Figure 7 in the Supporting Information). In an analogous
experiment, the temperature was varied (Figure 4), which
Figure 2. Representative reaction of TiSi2 (325 mesh) at 558C under
standard conditions. Hydrogen evolution (red) in phase A: formation
of the catalytic centers (cc1 and cc2 in Figure 3) and start of water
oxidation [Reaction (2)] and proton reduction [Reaction (3)] kinetics.
a: nitrogen flush and pressure release (gas concentrations=0).
Phase B (nearly linear kinetics): Over 96% hydrogen evolution from
water splitting as judged by comparison with analogous dark reac-
tions. Liberation of dioxygen (blue) from storage upon heating to
ꢂ1008C in the dark (run 5 in Table 1); thereafter, irradiation was
continued at 558C.
First convincing evidence that hydrogen production is the
result of catalysis was gained by experiments in which
hydrogen evolution exceeds stoichiometry that would
account for the thermal, noncatalytic, full oxidation of TiSi2
[Eq. (1)]. That is, over 12 equivalents of dihydrogen are
formed in the continuing reactions. Notably, the same argu-
ment rules out any type of corrosion, for example, photo-
corrosion. Two further pieces of evidence supporting photo-
catalysis are presented below.
Figure 4. Temperature-dependent efficiency of water splitting as deter-
mined by hydrogen evolution.
An important experiment concerns the catalytic centers
(cc1 and cc2, as referred to in Figure 3). Reactions were run in
the darkuntil hydrogen production ceased; subsequent
exposure to light did not lead to further detectable hydrogen
evolution (Figure 6 in the Supporting Information). This
finding implies that the interaction of light with the catalyst
from the onset of the reactions is mandatory for its reactivity
for efficient splitting of water. Hence, we may conclude that in
phase A of the light-driven reactions, the catalytic centers for
water oxidation (cc1) and proton reduction (cc2) are formed
on the surface of the commercially available TiSi2. So far we
can exclude that these centers are plain TiO2, since reactions
carried out under the same conditions but with TiO2 (anatase)
as catalyst showed only marginal hydrogen evolution. Fur-
ther, the reactions are run under inert gas but without
degassing the water. Residual dissolved oxygen in water
seems to be essential for the reactivity of the catalytic centers,
since in completely degassed reaction mixtures the activity of
the catalyst is low. On the other hand, in reactions run under
resulted in higher production efficiencies with increasing
temperatures. At 868C, the production of hydrogen reaches
35 mL per 24 h at 1.1–1.2 bar, which corresponds to a
photoenergy conversion h of about 4% for the water splitting
process.[14] We attribute this temperature effect to small but
crucial changes of the redox potential of water at elevated
temperatures.[13,16] The efficiency of water splitting achieved
with our catalyst compares favorably with those reported for
other semiconducting materials used with visible light, all of
which have notable drawbacks.[17] Furthermore, measure-
ments for efficiency calculations that are based on the use of
artificial light sources should be evaluated with caution, since
most literature values seem to be overestimated.[18]
Parallel darkreactions (Figure 4), which sacrificially
produce hydrogen via Equation (1), rendered at most 4%
hydrogen within the higher temperature range. This result
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implies that in reactions with light, water splitting accounts for
of more than 15 wt% for our experiments within the temper-
ature range of the standard conditions.[13]
over 96% of hydrogen evolution. It is especially interesting
that, upon aging, the catalyst becomes more stable toward
thermal decomposition and produces less hydrogen according
to Equation (1), as shown in Figure 4.
We favor the idea that in the present reactions with the
TiSi2-derived catalyst, water-solubilized dioxygen leaves the
catalytic centers (cc1) after photolytic water oxidation and is
photoadsorbed at the original thin-layered TiO2 sites (2–5 nm
thick) at the surface of the catalyst (see XRPD and XP
spectroscopy below). In this way, ionic columns of oxygen are
formed, which might be held together by charge transfer, and
from which O2 can be released upon thermal activation in the
dark.
A technically valuable aspect of TiSi2 is its capacity for
reversible physical hydrogen storage. Approximately
20 mLH2 gꢀ1 TiSi2 can be physisorbed at 308C and 5–
7 mLH2 gꢀ1 TiSi2 at 508C. This storage capacity, while lower
than those found for other inorganic materials that form
metal hydrides, is technically less demanding and requires
lower reaction temperatures for desorption of hydrogen.[19]
In routine runs at temperatures below 1008C, only
hydrogen is found in the gas phase, and dioxygen does not
appear. Also, hydrogen peroxide is detected in neither the gas
nor the liquid phase. In fact, oxygen is efficiently photo-
adsorbed by the catalyst in light and at any temperature.
Conveniently, it is desorbed under very different conditions
than hydrogen, thus allowing easy separation of the two gases.
Quantitative (within the experimental error of ꢁ 15%)
release of dioxygen sets in rapidly when reaction slurries
are heated to ꢂ 1008C in the dark(see Figure 2 and the
Supporting Information, Figure 7); these and further results
are summarized in Table 1. An H2/O2 stoichiometric ratio of
2:1 results in phase B of the reactions, while the initial thermal
hydrogen evolution in phase A ceases (max. 12–16 mL, see
footnote [g] runs 2–5 in Table 1). Notably, this result
exemplifies for the first time efficient and controlled desorp-
tion of physisorbed dioxygen from a semiconductor surface at
a temperature as low as 1008C.[20a]
Water splitting was additionally demonstrated by irradi-
ation under standard conditions but with 10% H218O-
enriched water to give fractions of 18O2 and 16O18O besides
16O2 in the gas phase after desorption, as determined by mass
spectrometry. The measurements revealed 18O isotope enrich-
ment in a ratio of 16O2/16O18O = 43 (theoretical value: 4.5 for
10% enrichment) as compared to the theoretical natural ratio
of 249 in air (see Figure 8 in the Supporting Information). A
more precise result is difficult to reach, because air leakage is
hard to avoid.[13] Moreover, a carbon dioxide signal with
varying intensity has been recorded (inset in Figure 8); the
origin of the carbon is, however, unclear at present.[13]
X-ray powder diffraction (XRPD) and X-ray photoelec-
tron spectroscopy (XPS) revealed structural changes of TiSi2
in water and upon irradiation. The two methods are
complementary since XRPD mainly analyzes the crystalline
phases of the bulkmaterial, whereas information about the
chemical composition of the surface is provided by XPS. An
average sample of TiSi2 shows 80% crystalline and 20%
amorphous phases as determined by XRPD. The major
component of the samples is TiSi2. Both composite regions of
the TiSi2 are covered in part by a thin oxide layer, and even
after extended reaction times Ti0 and Si0 domains remain
detectable by XPS. The oxide layers and domains, as
measured for both titanium and silicon by XPS after sputter-
ing, are limited to a few molecular layers in depth.
The XRPD pattern of a sample which has run for 1000 h at
608C under standard conditions shows several crystalline
phases[23] (Figure 9 in Supporting Information) and is
unchanged as compared to a sample analysis of starting
TiSi2. Main crystalline components are TiSi2, followed by TiSi
and metallic Si and Ti. No reflections belonging to SiO2 or
TiO2 phases are visible.
A different view is given by the XPS data. After removing
the oxide layer gradually by sputtering, the contribution of
metallic components to the surface is significantly increased.
After more than 10 min of sputtering with Ar+ ions, the ratio
of oxidized to metallic species stabilizes. The spectra in
Figure 5 represent the Ti 2p photopeaks before sputtering of
the sample and after different times of sputtering (Figure 5a)
and the corresponding Si 2p peaks (Figure 5b). Both spectra
measured before sputtering show that a thin surface layer is
highly oxidized. Only small photopeaks belonging to the
Table 1: Gas evolution under standard conditions (in mL) at various
reaction times and in separate runs. The gas volumes are given in mL.
[a]
[b]
[c]
[d]
Run
H2
H2
H2
O2
a
b=mL O2 ”2
c=aꢀb
1[e]
2
3
6
24
40
42
112
<6
10
24
30
96
<6
<3[h,i]
5[i]
14[g]
16[g]
12[g]
16[g]
12[i]
15[i]
48[i]
4
5[f]
[a] Total in gas phase. [b] From water splitting according to Equations (2)
and (3). [c] Thermal from Equation (1). [d] Evolved from storage. Gas
phase plus less than 3 mL dissolved in water. [e] See the Supporting
Information, Figure 7. [f] See Figure 2. [g] In these reactions no further
measurable thermal hydrogen evolution is found, that is, these values
remain constant within an error limit of ꢁ15%. [h] Below the detection
limit in gas phase. [i] Desorption at ꢂ1008C in the dark.
With this background, it is of interest to compare our
results with a recent study of the adsorption of oxygen in the
course of its assumed photoreduction at a TiO2 surface. The
study employed ab initio calculations based on infrared
data.[21] In that work, it was concluded that desorption of
(ionized) oxygen should be hardly possible from the surface
of such a gas–solid system. Another investigation in line with
our findings found a high storage capacity of TiO2 for oxygen
(ca. 25 wt%);[20b] an equilibrium between TiO2 and a lower
ꢀ
intermetallic Ti Si compounds are visible. The pattern
reveals no noticeable structural changes of the catalyst even
after extended reaction times.
oxide is proposed as an oxygen trap in analogy to CeO2.[22]
A
From the XRPD experiments we know that no crystalline
oxides are present. On the other hand, the XPS data clearly
preliminary determination found an oxygen storage capacity
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Chemie
separation of the product gases hydrogen and oxygen, which
have different physisorption properties. The stability of the
TiSi2-derived catalyst has been tested over 4–5 months, and a
much longer lifetime can be predicted on the basis of the XPS
and XRPD data.
Recently, the water splitting reactions discussed above
have been successfully tested under concentrated solar
radiation.
Experimental Section
Standard conditions: Suspensions of TiSi2 in either tridistilled or tap
water (both give identical results) at pH 7 were irradiated at 50 to
858C in round-bottomed and gas-tight stoppered flasks (rubber
septum) equipped with a gently stirring magnetic bar. The reactions
were generally carried out under nitrogen without degassing the
water; a few experiments were run under argon, which gave
comparable results. Nitrogen is preferred, however, since it serves
conveniently as an internal reference gas for GC analysis. Externally
positioned halogen lamps with wavelength emissions closely mimick-
ing solar radiation served as the light source with an emission range of
310–800 nm (main range at 380–780 nm). The resulting gas evolution
(hydrogen and dioxygen) was monitored by gas chromatography
(thermal conductivity detection; column: Rt-Msieve 13X, 30 m, S-
40); nitrogen served as reference gas. The gas partial pressure of the
closed reactions was normalized at intervals (dashed lines in Figure 2
and in the Supporting Information, Figure 7). The reactions were run
with TiSi2 (2 g, Alfa)[24] in water (150 mL), ensuring 100% light
absorption within 20 cm2 of aperture and giving rise to optimal
storage of oxygen.
Figure 5. XP spectra before and after sputtering the catalyst with Ar+
ions.
show that the surface is coated by oxidic species. The fact that
these species are “invisible” for XRPD could either mean that
the crystallite size of the oxides is smaller than 2–5 nm, that
the oxide layer is too thin resulting in too small of a diffraction
volume, or that the oxide layer is not crystalline at all. In any
event, the oxide layer is not thicker than 2–5 nm. No
noticeable change of relative peakintensities is found for
“TiSi2” samples by XRPD and XPS after about 200 h of
reaction time. This finding documents that the catalyst has
become chemically stable.
In conclusion, we show here for the first time that a
semiconducting material such as the inexpensive and abun-
dant TiSi2, which is mainly nonoxidic, can serve as a basis for
tandem oxidation of water and reduction of protons to form
oxygen and hydrogen, respectively, under solar radiation. The
catalytically active form of the semiconducter, formed in situ,
absorbs a wide range of the solar spectrum.
Catalytic water splitting is documented by the following
results (a–c):
a) Hydrogen production exceeds stoichiometry that would
account for a noncatalytic process such as given in
Equation (1),
b) hydrogen and oxygen are evolved in a 2:1 ratio, and
c) 18O-labeled dioxygen is found in the gas phase during
reactions with 18O-enriched water.
Received: April 13, 2007
Revised: June 15, 2007
Published online: September 5, 2007
Keywords: heterogeneous catalysis · photochemistry ·
.
semiconductors · surface chemistry · water splitting
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hydrogen formation (cc2) are formed on the surface of
commercially available TiSi2 in the initial hours of reaction in
light and in the presence of a small amount of oxygen. This
result could mean that the formation of the catalytic centers
involves photooxidation. Harmful oxidation of the silicide is
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deliver energy by volume expansion.
The important result concerning reversible photoadsorp-
tion of oxygen at low temperatures allows convenient
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Communications
[11] We thankProfessor H. Kisch and Dipl.-Chem. J. Eberl, Dept. of
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[13] Unpublished results.
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h
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(k = loss of light energy
E
out = 1.05 ” 10ꢀ4 kWh
(35 mL H2/24 h);[15] Ein = kElamp
a
between lamp and reactor= 5 ” 10ꢀ2, measured with a Photon
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20 cm2); h = Eout/Ein 100 = 1.05 ” 10ꢀ4/2.7 ” 10ꢀ3 ” 100 = 3.88%.
[15] http://www.airproducts.com/Products/LiquidBulkGases/Hydro-
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to achieve 8% efficiency (reference [27]).
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[23] ICDD (2005–2006). PDF-2. International Centre for Diffraction
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[25] K. Sayama, K. Mukasa, R. Abe, Y. Abe, H. Arakawa, Chem.
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