Photochemical hydrogen evolution using Sn-porphyrin as photosensitizer and a series of Cobaloximes as catalysts

Article abstract of DOI:10.1142/S1088424616500243

Herein, we report a photochemical hydrogen evolution system consisting of various cobalt based catalysts, a metallated Sn porphyrin as photosensitizer and a triethanolamine as a sacrificial electron donor in acetonitrile/H₂O (1:1) solution. Since a tin metallated porphyrin is used for the first time as photosensitizer in this type of systems, a systematic study was performed in order to elucidate the best conditions for H2production. Upon visible irradiation hydrogen production was detected with the best result obtained at pH 7 with a TON of 150, after 100 h in the presence of catalyst 1. Moreover, when TiO₂ nanoparticles were added at the catalytic system 2 the hydrogen production was increased from 53 TON to 131 TON, under the same experimental conditions.

Full text of DOI:10.1142/S1088424616500243

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2016; 20: 1–8
DOI: 10.1142/S1088424616500243
Published at http://www.worldscinet.com/jpp/
Photochemical hydrogen evolution using Sn-porphyrin
as photosensitizer and a series of Cobaloximes as catalysts
Georgios Landrou, Athanassios A. Panagiotopoulos, Kalliopi Ladomenou
and Athanassios G. Coutsolelos*◊
Laboratory of Bioinorganic Chemistry, Department of Chemistry, University of Crete, Voutes Campus, 70013
Heraklion, Crete, Greece
Dedicated to Professor Kevin M. Smith on the occasion of his 70th birthday
Received 3 November 2015
Accepted 16 February 2016
ABSTRACT: Herein, we report a photochemical hydrogen evolution system consisting of various
cobalt based catalysts, a metallated Sn porphyrin as photosensitizer and a triethanolamine as a sacrificial
electron donor in acetonitrile/H2O (1:1) solution. Since a tin metallated porphyrin is used for the first
time as photosensitizer in this type of systems, a systematic study was performed in order to elucidate the
best conditions for H2 production. Upon visible irradiation hydrogen production was detected with the
best result obtained at pH 7 with a TON of 150, after 100 h in the presence of catalyst 1. Moreover, when
TiO2 nanoparticles were added at the catalytic system 2 the hydrogen production was increased from 53
TON to 131 TON, under the same experimental conditions.
KEYWORDS: porphyrins, metallo-porphyrins, hydrogen production, photocatalysis, cobaloxime.
reported first the homogeneous photogeneration of
INTRODUCTION
hydrogen using [Co(dmgH)2(OH2)2] (where dmgH =
The energy resources that are mostly used nowadays
are fossil fuels, however they appear to have many
environmental drawbacks. Therefore, there is the need
dimethylglyoximate) as a catalyst with [Ru(bpy)3]2+ (bpy =
2,2′-bipyridine) as photosensitizer and triethanolamine
(TEOA) as a sacrificial electron donor in a DMF solution
to produce alternative fuels in order to be pollution-
[12]. Since then many cobaloxime-based systems
free, storable and economical. A promising solution that
have been used for hydrogen production and many
fulfills these requirements is hydrogen production via
photocatalytic water splitting [1–3].
photosensitizers including metal-based complexes [13–
22] and metal free organic dyes [23–26]. The organic
Photocatalysis is defined as the chemical reaction
dyes have the advantage of being relatively inexpensive,
induced by photoirradiation in the presence of a catalyst
but they may be less stable than the inorganic ones at
(photocatalyst). This procedure is inspired by nature
the hydrogen generating conditions. Porphyrins have
where a well-known photocatalyst is chlorophyll in
found prominent use as sensitizers in hydrogen [27] and
photosynthesis by plants. Therefore, various photo-
oxygen production [28], since nature use them as energy
catalytic hydrogen evolution catalysts and different
harvesters and electron transfer agents. More specifically,
photosensitizers (dyes) have been extensively studied
various metalloporphyrins (Zn, Al, Sn) have been exten-
[4–8]. Among, the catalysts that have been used so far,
sively used as photosensitizers for H2 production. The
cobaloximes are particularly attractive due to their high
redox center in metalloporphyrins is the porphyrin
activity and easy preparation [9–11]. Lehn and co-workers
ring where the electrochemical potential is sensitively
influenced by the kind of central metal ions. Among a
series of metalloporphyrins tin(IV)-porphyrin derivatives
◊ SPP full member in good standing
have already been used as mimics for the photocatalysis
of light-driven water-oxidation and proton-reduction [29].
*Correspondence to: Athanassios G. Coutsolelos, email:
acoutsol@uoc.gr
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G. LANDROU ET AL.
Tin(IV)-porphyrins are known to have suitable
redox potentials for water splitting [30–33]. Also, these
porphyrins are very stable as the large, highly-charged
Sn4+ ion fits perfectly into the porphyrin ring system
[30, 32]. In addition, it has already been demonstrated
that tin(IV) porphyrins can be used in combination
with heterogeneous platinum-based hydrogen evolution
reaction catalysts for photocatalytic H2 production [29,
34, 35]. In a recent example, a water-soluble tin porphyrin
([SnIVCl2TPPC],wasusedasphotosensitizerwithacobalt-
based proton-reduction catalyst ([CoIIICl(dmgH)2(py)]).
The system, under visible light illumination and in
the presence of triethanolamine (TEOA) as electron
source, evolves H2 for h [36]. Inspired by the above
findings and in order to extend our previous study [37],
this report presents the use of a Sn(IV)-porphyrin as a
photosensitizer in a photocatalytic hydrogen production
system in the presence of various cobalt-based catalysts.
In our previous work we reported an homogeneous
photocatalytic system that uses a Zn porphyrin sensitizer,
a cobaloxime catalyst and TEOA as sacrificial donor,
for hydrogen evolution [37]. Upon visible irradiation
hydrogen production was detected with the best result
obtained at pH 7 with a TON of 280, after 35 h in a
50/50 acetonitrile/H2O solution. Therefore, in order
to understand if a different metal on the sensitizer can
influence the hydrogen production of the system we
present a bioinspired photocatalytic system that uses tin-
meso-tetrakis(pyridine)porphyrin [SnTPyP(OH)2] along
with its methylated derivative [SnTMPyP4+]Cl4 as photo-
sensitizers and a series of cobalt bis-dimethylglyoximato
complexes [CoIII(dmgH)2(L)Cl] (1–3) with different
pyridine ligands, as a catalyst (Fig. 1).
EXPERIMENTAL
Chemicals
All chemicals were purchased from the usual
commercial sources and used as received.
Instrumentation
1H NMR spectra were recorded on Bruker AMX-
500 MHz and Bruker DPX-300 MHz spectrometers as
solutions in deuterated solvents by using the solvent
peak as the internal standard. High-resolution mass
spectra were performed on a Bruker ultrafleXtreme
MALDI-TOF/TOF spectrometer. Cyclic voltammetry
experiments were carried out at room temperature using
Fig. 1. Chemical structures of porphyrin SnTPyP(OH)2 and [SnTMPyP4+]Cl4 as photosensitizers and a series of cobaloxime
catalysts with different axial ligands (1–3)
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PHOTOCHEMICAL HYDROGEN EVOLUTION USING Sn-PORPHYRIN AS PHOTOSENSITIZER AND A SERIES
3
an SP-300 Bio-Logic potentiostat. All measurements
were carried out in DMF with a solute concentration
of ca. 1.0 mM in the presence of tetrabutylammonium
tetrafluoroborate (0.1 M) as supporting electrolyte.
A three-electrode cell setup was used with a glassy
carbon working electrode, a silver/silver chloride
reference electrode, and a platinum wire as counter
electrode. All potentials are reported vs. the ferrocene/
ferricinium couple added in the electrolyte at the end
of each measurement. UV-vis absorption spectra were
measured on a Shimadzu UV-1700 spectrophotometer
using 10 mm path-length cuvettes. The emission spectra
were measured on a JASCO FP-6500 fluorescence
spectrophotometer equipped with a red-sensitive WRE-
343photomultipliertube(wavelengthrange200–850nm).
Quantum yields were determined from corrected
emission spectra following the standard methods using
meso-tetraphenylporphyrin (TPP) (F = 0.11 in toluene
[38]) or zinc meso-tetraphenylporphyrin (ZnTPP) (F =
0.03 in toluene [38]) as standards. Emission lifetimes
were determined by the time-correlated single-photon
counting technique using an Edinburgh Instruments
mini-tau lifetime spectrophotometer equipped with an
EPL 405 pulsed diode laser at 406.0 nm with a pulse
width of 71.52 ps and pulse periods of 200 ns and 100 ns
and a high-speed red sensitive photomultiplier tube
(H5773-04) as detector.
2H), 2.40 (s, 12H). 13C NMR (75 MHz; CDCl3): d, ppm
152.73, 151.21, 139.03, 125.78, 13.24. HRMS (MALDI-
TOF): m/z 367.9340 (calcd. for [M – Cl]+ 368.0769).
Synthesis of [Co(dmgH)2(4-COOH-py)Cl] (2).
Yield 58%. Anal. calcd. for C14H19ClCoN5O6: C, 37.56;
H, 4.28; N, 15.64%. Found C, 38.34; H, 4.75; N, 15.32.
1H NMR (300 MHz; DMSO): d, ppm 18.44 (s, 2H,
OH), 14.14 (s, 1H, OH), 8.21 (d, J = 6.1 Hz, 2H), 7.85
13
(d, J = 6.0 Hz, 2H), 2.31 (s, 12H). C NMR (75 MHz;
DMSO): d, ppm 164.61, 152.73, 151.37, 141.17, 125.17,
12.70. HRMS (MALDI-TOF): m/z 412.0126 (calcd. for
[M – Cl]+ 412.0667).
Synthesis of [Co(dmgH)2(4-OH-py)Cl] (3). Yield
25.5%. Anal. calcd. for C13H19ClCoN5O5: C, 37.20; H,
4.56; N, 16.69%. Found C, 37.70; H, 4.96; N, 16.65. 1H
NMR (300 MHz; DMSO): d, ppm 18.49 (s, 2H, OH),
11.80 (s, 1H, OH), 7.65 (s, 2H), 6.79 (s, 2H), 2.32 (s,
12H). 13C NMR (75 MHz; DMSO): d, ppm 166.60,
152.18, 150.68, 114.17, 12.55. HRMS (MALDI-TOF):
m/z 384.1298 (calcd. for [M – Cl]+ 384.0718).
Hydrogen evolution measurements
For photoinduced hydrogen evolution, each sample
was prepared in a 30 mL Schlenk flask. Prior to sample
preparation, an aqueous solution of TEOA (10% vol) was
adjusted to pH 6, 7, 8 or 9 using conc. HCl. The components
were then dissolved in 10 mL of a 1:1 (v/v) mixture of
acetonitrile and the aqueous TEOA (10%) solution. The
sample flask was sealed with a Suba-seal septum and
degassed by bubbling nitrogen through the solution for
15 min at room temperature. The samples were irradiated
with a 500 W Xenon lamp using a cutoff filter designed to
remove all light with l < 440 nm. The amounts of hydrogen
evolved were determined by gas chromatography (external
standard technique) using a Shimadzu GC-2010 plus
chromatograph with a TCD detector and a molecular sieve
5 Å column (30 m-0.53 mm). Control experiments were
run under the same conditions as the hydrogen evolution
experiments with the removal of one of the components of
the hydrogen generating samples.
For some control experiments a filter assembly was
used consisting of a narrow bandpass filter at 572 nm
(bandwidth about 10 nm FWHM) and a color-glass filter
KG5. The color-glass filter transmits almost everything
between 330 and 700 nm but blocks the deep UV and
IR radiation of the lamp. Extra care was taken to prevent
stray UV light from reaching the sample by covering the
area around the detector and a molecular space between
filter assembly with a large black carton.
Synthesis
Synthesisofthephotosensitizer[SnTPyP(OH)2]. The
tin 5,10,15,20-tetra(4-pyridyl)porphyrin [SnTPyP(OH)2]
was synthesized according to the literature [39].
Synthesis of water soluble [SnTMPyP4+]Cl4. Tin
meso-tetrakis(1-methyl-pyridinium-4-yl)porphyrin
chloride [SnTMPyP4+]Cl4 was prepared according to the
literature [33].
General synthesis of catalysts 1–3. The general
synthesis of cobalt complexes with dimethylglyoxime,
[Co(dmgH)2(L)(L′)] is described in the literature [40].
CoCl2∙6H2O (500 mg, 2.15 mmol), dimethylglyoxime
(551 mg, 4.70 mmol), and NaOH (86.0 mg, 2.15 mmol)
were dissolved in 95% ethanol (20 mL) and heated to
70°C. Pyridine (L) (2.15 mmol) was then added and the
resulting solution cooled to room temperature. A stream
of air was then passed through the solution for 30 min,
which caused precipitation of a brown solid. The
suspension was stirred for 1 h and filtered. The precipitate
was successively washed with water (5 mL), ethanol (2 ×
5 mL), and diethyl ether (3 × 5 mL). The product was
then extracted with acetone. Removal of the solvent from
the extracts yielded pure complex.
An excess of mercury (ca. 40 eq) was added to our in
order to investigate the effect of mercury during hydrogen
production. More particularly, hydrogen production was
not affected by the presence of mercury in the reaction
mixture indicating that metallic cobalt particles are not
formed during the experiment. Metallic mercury remains
shiny without forming an amalgam.
Synthesisof[Co(dmgH)2(py)Cl](1).Yield52%.Anal.
calcd. for C13H19ClCoN5O4: C, 38.68; H, 4.74; N, 17.35%.
Found C, 39.32; H, 5.54; N, 16.99. 1H NMR (300 MHz;
CDCl3): d, ppm 18.20 (broad, OH), 8.27 (d, J =
5.4 Hz, 2H), 7.70 (t, J = 7.5 Hz, 1H), 7.22 (d, J = 7.0 Hz,
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G. LANDROU ET AL.
For the hydrogen evolution experiments the conditions
were as follows: Hydrogen production upon irradiation
(l > 440 nm) of solution (acetonitrile-TEOAaq (10%, pH
6, 7, 8 or 9) (1:1 v/v) containing PS and the respective
catalyst 1–3. In the case of the experiments containing
TiO2 the procedure was the following. A solution of 2
(0.1 mmol in 0.5 mL of TEOA, 10% v/v, pH 7) was added
to a stirred dispersion of TiO2 (20 mg in 1 mL TEOA).
The mixture was left stirring for 1 h, was then centrifuged,
and the filtered, clear supernatant was then analysed by
UV-vis spectrophotometry. The amount of 2 adsorbed to
TiO2 was quantified by the absorbance difference at lmax
(244 nm) before and after exposure to TiO2 nanoparticles.
The amount adsorbed to TiO2 particles after 1 h was 89
2% [41].
catalyst were used in order to compare their hydrogen
performance. All the experiments that were done in
order to assess the photocatalytic activity of catalysts
1–3 towards hydrogen production were performed in
deoxygenated acetonitrile/H2O (1:1) solutions with
TEOA as sacrificial donor and [SnTPyP(OH)2] as a
photosensitizer. Various concentrations were used of the
various components as discussed later, in order to obtain
the best hydrogen production. The concentrations of the
photosensitizer and the catalysts were selected based on
the observations from our previous work where a relative
photocatalytic system was used [37]. It must be noted that
in the case of catalyst 3 no hydrogen could be observed
under any conditions. The absence of catalytic activity
observed when 4-hydroxypyridine (3) was used, may be
due to the formation of a 4-pyridone tautomer [42], and
this in accordance to other unpublished results from our
group [43].
RESULTS AND DISCUSSION
A series of cobalt bis dimethylglyoximato complexes
[CoIII(dmgH)2(L)Cl] (1–3) with different pyridine axial
ligands(Fig.1)weresynthesizedaccordingtotheliterature
procedure by dissolving CoCl2∙6H2O, 2 equivalents of
dimethylglyoxime and 1 equivalent of NaOH in 95%
ethanol at 70°C [40].Then, 1 equivalent of the appropriate
ligand was added and the final products were obtained.
Moreover, metallated tin meso-tetrakis(pyridyl)porphyrin
[SnTPyP(OH)2] [39] and tin meso-tetrakis(1-methyl-
pyridinium-4-yl)porphyrin chloride [SnTMPyP4+]Cl4
[33]were prepared according to the literature and used as
photosensitizers in hydrogen evolution system (Fig. 1).
Firstly, we wanted to compare the influence in hydrogen
production by using water soluble porphyrin-based
photosensitizers with different metals. More specifically,
in our previous studies [37] combination a Zn water
soluble porphyrin ([ZnTMPyP4+]Cl4) sensitizer and
cobaloxime catalyst [CoIII(dmgH)2(py)Cl]) (1) produced
H2 with 280 TON after 25 h at pH = 7, upon irradiation
(l > 440 nm) of solutions (1:1 acetonitrile–water)
containing ([ZnTMPyP4+]Cl4) (4.0 × 10-5 M), catalyst 1
(4.9 × 10-4 M) and TEOA [5% (v/v)]. In the present study,
instead of Zn porphyrin, [SnTMPyP4+]Cl4 was used as a
photosensitizer and cobaloxime 1 as catalyst. After 24 h
of irradiation, under the same experimental conditions no
H2 production was obtained. Cobaloximes 2 and 3 were
also used, but once again no hydrogen was produced. For
that reason, we considered to use the non-water soluble
tin porphyrin ([SnTPyP(OH)2]) as a photosensitizer.
Following the aforementioned experimental conditions
using ([SnTPyP(OH)2]) and 1, after 140 h hydrogen
production was observed with 50 TON. Since Sn-pyridyl
porphyrin along with cobaloximes was used for the first
time for hydrogen production, a systematic study of
this system was necessary. Therefore, different pH, dye
and catalyst concentrations were examined in order to
obtain the best hydrogen production of the photocatalytic
system. Additionally, different ligands on the cobaloxime
Effect of catalyst’s concentration
In order to study the effect of catalyst concentration in
hydrogenproductionallparameterswerekeptconstantsuch
as pH (pH = 7), sacrificial electron donor (SED) (5% v/v)
and photosensitizer (PS) (CPS = 4.0 × 10-5 M). Therefore,
two different concentrations (C = 4.9 × 10-4 M and C= 9.8 ×
10-4 M) were used for catalysts 1and 2.When concentration
C = 4.9 × 10-4 M was used both catalysts (1 and 2) produced
trace amount of H2 and after the lapse of several hours of
continuous irradiation. In a subsequent experiment, when
theconcentrationofthecatalysts(1and2)wasdoubled(C=
9.8 × 10-4 M) a significant quantity of H2 was observed.
More specifically, the photocatalytic system comprising
catalyst 1, found to held 110 catalytic cycles at 100 h
(Fig. 2, red curve) while the system containing the catalyst
2,itwasfoundtoproduce37catalyticcyclesat100h(Fig.2,
blue curve). This was also observed in other systems
described in the literature where the gradual reduction
of the cobaloxime’s concentration decreases the number
of catalytic cycles. For this reason in the systems using
cobaloximes as catalysts the amount of the catalyst is much
higher than that of the photosensitizer [23, 37, 44, 45].
Effect of photosensitizer’s concentration
In order to study the effect of the concentration of the
photosensitizer all other parameters were kept constant
and used the ones that worked best, at pH 7, concentration
of SED at 5% v/v, concentration of catalyst at C = 9.8 ×
10-4 M. We noticed that changes of the photosensitizer’s
concentration occurred significant variations in the
number of catalytic cycles. It was found that the best
result was obtained using 8.0 × 10-5 M of photosensitizer
giving 150 TON after 100 h when catalyst 1 was used
(Fig. 3) and 53 TON after 100 h in the case of catalyst 2
(Fig. 3). This is in accordance with the literature where
in the case of catalyst 2 the electron withdrawing-group
reduces the hydrogen production due ti the instability of
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PHOTOCHEMICAL HYDROGEN EVOLUTION USING Sn-PORPHYRIN AS PHOTOSENSITIZER AND A SERIES
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Fig. 2. Plot of hydrogen production upon irradiation (l > 440 nm) of solutions (1:1 acetonitrile/water) containing PS (CPS = 4.0 ×
10-5 M), 1 and 2 (4.9 × 10-4 M and 9.8 × 10-4 M) and TEOA 5% v/v at pH 7
Fig. 3. Plot of hydrogen production upon irradiation (l > 440 nm) of solutions (1:1 acetonitrile/water) containing PS (CPS = 4.0 ×
10-5 M and CPS = 8.0 × 10-5 M), 1 and 2 (9.8 ×10-4 M) and TEOA 10% v/v at pH 7
the photocatalytic system due to the decreased basicity
of the pyridyl nitrogen [46]. Additionally, by decreasing
or increasing this concentration the hydrogen production
was reduced. According to the literature [37, 47, 48]
the concentration of the photosensitizer must have an
optimum value, since with little change of this value the
performance of the system is significantly reduced.
both photocatalytic systems containing catalysts 1 and
2. The pH value is an important factor in hydrogen
production since the efficiency of the system depends
on the concentration of the protonated and the non
protonated form of the sacrificial donor. It is known that
the optimum pH value is often close to the pKa value
of the sacrificial electron donor, in our case the pKa of
TEOA is 7.76. Therefore, at pH < pKa there is a low
amount of SD that makes the quenching and regeneration
of the photosensitizer less efficient. This can explain why
the H2 production at pH = 6 is negligible [49].
Effect of proton’s concentration (pH)
At this stage the effect of pH was studied in our
photocatalytic hydrogen production systems. More
specifically the TON numbers were measured at pH 7,
8 and 9. At lower pH 6 no hydrogen was released from
On the other hand, at pH value 7, both systems show
the maximum rate of hydrogen production. In particular,
catalyst 1 gave 150 TON after 100 h of radiation while
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G. LANDROU ET AL.
Fig. 4. Plot of hydrogen production upon irradiation (l > 440 nm) of solutions (1:1 acetonitrile/water) containing PS (8.0 × 10-5 M),
1 (9.8 × 10-4 M) and TEOA 5% v/v at pH 7, 8, 9
Fig. 5. Plot of hydrogen production upon irradiation (l > 440 nm) of solutions (1:1 acetonitrile/water) containing PS (8.0 × 10-5 M),
2 (9.8 × 10-4 M) and TEOA 5% v/v at pH 7, 8, 9
catalyst 2 gave 53 TON at 100 h as shown in Figs 4 and
5, respectively. By increasing the pH at 8, a decrease
in total hydrogen production was observed as well as a
reduction in the hydrogen production rate. These data
are consistent with previous work of our group [37]. At
higher pH 9 less hydrogen production was observed,
compared to pH 8. Moreover, the initial hydrogen
production rate at pH 7 is considerably slower than that
at pH 8 and pH 9.
Effect of TiO2 nanoparticles in H2 evolution
In order to further improve H2 production TiO2
nanoparticles were added to the solution of catalysts 1
and 2. The presence of TiO2 will allow the attachment
of only catalyst 2 at the surface of the particles through
the formation of ester bonds due to the presence
of the carboxylic acid anchoring group [41]. There-
fore, 2-modified TiO2 catalyst increased its catalytic
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PHOTOCHEMICAL HYDROGEN EVOLUTION USING Sn-PORPHYRIN AS PHOTOSENSITIZER AND A SERIES
7
Fig. 6. Plot of hydrogen production upon irradiation (l > 440 nm) of solutions (1:1 acetonitrile/water) containing PS (8.0 × 10-5 M),
1 (9.8 × 10-4 M) or 2 (9.8 × 10-4 M) and TEOA 5% v/v at pH 7, with and without TiO2
performance by a three-fold (Fig. 6). On the other
hand, the photocatalytic system containing 1 and TiO2
nanoparticles, upon irradiation produced no changes on
H2 production, since the absence of an anchoring group
(Fig. 6).
The COST Action CM1202 PERSPECT-H2O is also
acknowledged.
Supporting information
1
13
A full list of FT-IR, UV-vis, H NMR and C NMR,
spectra of selected compounds (Figs S1–S12) are given
in the supplementary material. This material is available
free of charge via the Internet at http://www.worldscinet.
com/jpp/jpp.shtml.
CONCLUSIONS
In summary, a photocatalytic system consisting
of different cobaloxime catalysts, a Sn-porphyrin
based dye as photosensitizer, a TEOA as sacrificial
donor in acetonitrile/H2O solution, is reported. When
4-hydroxypyridine ligand was present in cobaloxime
catalyst 3 no hydrogen production was observed under
various experimental conditions. In the case of catalysts
1 and 2, by increasing their concentration at 9.8 × 10-4 M
the H2 production was increased. Moreover, when the
concentration of the photosensitizer was doubled from 4 ×
10-5 M to 8 × 10-5 M and with the catalyst’s concentration
that worked best, systems 1 and 2 gave 150 TON and
53 TON, respectively. Additionally, variations of the pH
showed that at pH 7 the maximum H2 production rate was
observed. Finally, when TiO2 nanoparticles were added
at the catalytic system 2 the hydrogen production was
increased from 53 TON to 131 TON.
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