Photocatalytic methanol assisted production of hydrogen with simultaneous degradation of methyl orange

Article abstract of DOI:10.1016/j.apcata.2015.10.020

Platinized TiO₂ prepared by photodeposition was evaluated for activity in the simultaneous conversion of methyl orange (MO), and methanol assisted formation of hydrogen. Low concentrations of MO were found ineffective for generation of hydrogen in measurable quantities upon illumination of Pt/TiO₂ in water. On the other hand, methanol induced hydrogen formation was significant. Surprisingly, when methyl orange was added to the methanol/water solution, hydrogen formation was significantly suppressed. The origin of this detrimental effect of methyl orange lies in the strong and preferred adsorption of the dye on the Pt sites of the catalyst, resulting in hydrogenation of the azo bond and suppression of the catalyzed formation of hydrogen. The hydrogenation of the azo bond is corroborated by dis-colorization of the solution and the observation of a mass fragment in LC-MS analysis corresponding to a hydrogenated product of MO (m/z = 172). Similar to hydrogen formation, dye dis-colorization is stimulated by the presence of methanol, without the formation of new chemical compounds, confirming the role of methanol as a hole scavenger in the photocatalytic processes. Finally the presence of oxygen (in lean conditions) delays dye hydrogenation and hydrogen formation, which we discuss is due to oxygen adsorption and formation of superoxide anions over the Pt sites (leading to oxidation of methanol), which is preferred over N=N bond hydrogenation, and proton reduction.

Full text of DOI:10.1016/j.apcata.2015.10.020

Accepted Manuscript
Title: Photocatalytic methanol assisted production of
hydrogen with simultaneous degradation of methyl orange
˜
Author: Joana Romao Rafal Salata Sun-Young Park Guido
Mul
PII:
S0926-860X(15)30197-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.10.020
APCATA 15596
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
26-6-2015
12-10-2015
14-10-2015
˜
Please cite this article as: Joana Romao, Rafal Salata, Sun-Young Park,
Guido Mul, Photocatalytic methanol assisted production of hydrogen with
simultaneous degradation of methyl orange, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2015.10.020
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Photocatalytic methanol assisted production of
hydrogen with simultaneous degradation of
methyl orange
Joana Romão, Rafal Salata, Sun-Young Park, Guido Mul*
Photocatalytic Synthesis Group, MESA+ Institute for Nanotechnology, Faculty of Science and
Technology, University of Twente, Meander 229, P.O. Box 217, 7500 AE Enschede, The
Netherlands. Fax: +31-53-489 2882; Tel:+31-53-489-4230
*Corresponding author. Telephone: +31-53-489 3890, Fax: +31-53-489 2882. E-mail address:
g.mul@utwente.nl
1

Graphical abstract:
2

Highlights
 Pt/TiO2 is effective in photocatalytic methanol reforming to hydrogen
 Methyl Orange (MO) deteriorates photocatalytic methanol reforming activity
 The N=N bond of MO is hydrogenated by photo-excited Pt/TiO2
 O2 inhibits N=N hydrogenation and photocatalytic methanol reforming
 Simultaneous methanol assisted hydrogen formation and dye degradation in not feasible
3

Abstract
Platinized TiO2 prepared by photodeposition was evaluated for activity in the simultaneous conversion of
methyl orange (MO), and methanol assisted formation of hydrogen. Low concentrations of MO were
found ineffective for generation of hydrogen in measurable quantities upon illumination of Pt/TiO2 in
water. On the other hand, methanol induced hydrogen formation was significant. Surprisingly, when
methyl orange was added to the methanol/water solution, hydrogen formation was significantly
suppressed. The origin of this detrimental effect of methyl orange lies in the strong and preferred
adsorption of the dye on the Pt sites of the catalyst, resulting in hydrogenation of the azo bond and
suppression of the catalyzed formation of hydrogen. The hydrogenation of the azo bond is corroborated
by dis-colorization of the solution and the observation of a mass fragment in LC-MS analysis
corresponding to a hydrogenated product of MO (m/z = 172). Similar to hydrogen formation, dye dis-
colorization is stimulated by the presence of methanol, without the formation of new chemical
compounds, confirming the role of methanol as a hole scavenger in the photocatalytic processes. Finally
the presence of oxygen (in lean conditions) delays dye hydrogenation and hydrogen formation, which we
discuss is due to oxygen adsorption and formation of superoxide anions over the Pt sites (leading to
oxidation of methanol), which is preferred over N=N bond hydrogenation, and proton reduction.
Keywords: Photocatalysis, Hydrogen, Platinum, Titanium Dioxide, Methyl Orange, Methanol,
Decontamination, Oxidation, Anaerobic conditions
4

1. Introduction
Potable water in several domestic areas not only contains organic toxins, but also significant
amounts of nitrate, which both have adverse effects on human health.[1, 2] To mitigate these
issues, photocatalysis is a promising technology, in particular if oxidation of organic toxins to
CO2 [3, 4], could be combined with the in situ generation of hydrogen [5, 6], needed for
reductive elimination of nitrate by heterogeneous catalysis. [7] Both reactions (generation of
hydrogen, and reductive elimination of nitrates) could e.g. be performed sequentially in domestic
appliances, designed on the basis of microreactor technology. [7]
The photocatalytic process of oxidative decomposition of organics and hydrogen formation by
reduction of protons, involves photo-excited states, i.e. electrons and holes (see for a schematic
illustration Fig.1). [8, 9] Photogenerated holes when reaching the semiconductor surface induce
the formation of radical species, such as hydroxyl radicals, or organic radicals by reaction with
surface adsorbed organic compounds (Fig.1). Usually conversion of the radical species and
organic contaminants subsequently leads to formation of CO2 and H2O.[4, 8] Several reports
demonstrate that such decomposition of contaminants is feasible in anaerobic conditions, with
simultaneous formation of hydrogen (Fig.1).[10, 11]
The catalyst most commonly used to induce the above illustrated reactions, is TiO2 (and
specifically P25) [12, 13], with a bandgap of 3.2 eV and a conduction band (ECB) energy lower
than the standard redox potential of hydrogen (E (H+/H2) 0 V at pH 0), which makes hydrogen
production thermodynamically feasible.[14] Unfortunately, hydrogen evolution is usually
associated with low apparent quantum efficiency.[8] The reason is a high recombination rate
between the photogenerated holes and electrons, before achieving the desired redox processes on
the semiconductor surface.[8] This problem can be overcome by loading TiO2 with noble metals,
such as Pt, Pd, Au, Cu, or Rh.[8, 9, 14, 15] These metals create effective surface sites for proton
reduction, thus diminishing recombination of electrons and holes.[14]
Decomposition of several organic compounds has been studied in anaerobic conditions, leading
to production of hydrogen [16, 17], such as alcohols [16, 18], sugars [19], organic acids [16, 20],
and others [5, 21]. Alcohols are the organic compounds that induced the highest rates in
5

hydrogen evolution, likely due to their high susceptibility towards adsorption on oxidic surfaces
and subsequent oxidation.[22]
The aim of this paper is to demonstrate the effectivity of Platinized TiO2 (P25) in the
decomposition of methyl orange (MO), a dye that is often used as model compound in studies
involving decontamination of wastewater, and the simultaneous production of hydrogen in the
presence of excess MeOH. MeOH is known to induce hydrogen evolution very efficiently [9].
Both anaerobic, and oxygen lean conditions will be discussed, the latter condition representing
water to be purified in domestic appliances.
2. Experimental
2.1. Materials
All reagents were obtained from Aldrich and used as received. Those include chloroplatinic acid,
methanol and methyl orange. The photocatalyst material, P25 TiO2, was obtained from Evonik
Industries. The solvents (water and acetonitrile) used in the HPLC analysis were purchased from
Biosolve.
2.2. Preparation of Pt-TiO2
Deposition of Pt nanoparticles on TiO2 P25 with an estimated weight loading of 0.5 wt-% was
performed using a photodeposition procedure. [23, 24] To this end, an aqueous suspension of
TiO2 P25 (0.5 g L-1) and chloroplatinic acid (H2PtCl6, 0.5 g L-1) was stirred for 30 minutes in the
dark and then MeOH was added (1 M, 3 ml). The suspension was illuminated (radiation intensity
3.21 mW cm-2 at 375 nm) for 5 h. The powder was collected by centrifugation, and washed three
times consecutively with ethanol (20 ml), and finally with distilled water (20 ml). The powder
was dried overnight at 90°C.
6

2.3. Photocatalytic experiments
The equipment used for photocatalytic reactivity evaluation consists of three parts: a reactor, a
gas chromatograph and a LabView interphase to control gas flow rates induced by mass flow
controllers. A cylindrical batch reactor was used with an internal volume of 50 mL. In the lid of
the reactor a small window (fused silica of 0.5 cm diameter, which allows 90% transmission of
radiation) was incorporated, to allow illumination of the solution from the top. The light source
used was a 120 Watt high pressure mercury lamp from Dr. Gröbel UC-Elektronik GmbH, with a
spectral range from 280 to 650 nm, from which the light was introduced in the reactor through an
optical fiber at a light intensity of 82 mW cm-2 (measured from 300-400 nm, at a distance from
the window of 1 cm). A micro gas chromatograph (GC), equipped with a Parabond Q column
(10m) and a TCD detector was used for the determination of H2, O2 and N2 concentrations.[25]
The working volume of the aqueous photocatalytic suspension was 15 mL, which was
continuously agitated by magnetic stirring at 300 rpm. In this study several aqueous solutions
were prepared to compare photocatalytic activity, including MeOH (2.48 M, or otherwise as
stated) or mixtures of MeOH (2.48 M) and Methyl Orange (MO), (0.06 mM, or otherwise as
stated). All reactions were performed at neutral pH (~7). Two photocatalysts were tested with a
concentration of 1 g L-1, being TiO2 and 0.5%Pt-TiO2. Before initiating the photocatalytic
reaction, the aqueous solution was purged with argon, or argon containing 20% O2 (5 ml min-1,
to saturate the aqueous solution with O2), for 30 minutes in the dark. The total time of
illumination was 60 minutes during which an argon (Ar) flow of 5 mL min-1 was continuously
purged through the reactor.
2.4. Analysis of MO decomposition
From the same reactor used for determination of hydrogen production, samples were taken out
periodically and centrifuged at 7000 rpm for 5 min to remove catalyst particles. Subsequently the
samples were analyzed by UV-vis spectroscopy and liquid HPLC (Agilent 1100 Series). The
HPLC method involved the separation of the organic compounds in the sample using a mixture
of water and acetonitrile (75:25) as eluent, and applying a flow rate of 0.3 ml min-1. The sample
volume injected into the reverse phase column (All Chromolith Merk C18 column 100mm x
3mm i.d.) was 10 µL at room temperature (21ºC), using an autosampler. Detection was
7

performed using mass spectrometry (Bruker MS Esquire 3000+). The separated products were
ionized using positive Electrospray Ionization (+ESI). The MS ionization conditions were: a
flow rate of 0.5 ml min-1, a nebulizer gas pressure of 30 psi, a dry gas flow of 0.3 L min-1 and a
,
dry gas temperature of 325ºC.
3. Results
3.1. Characterization of 0.5% Pt-TiO2
The Pt-TiO2 sample was analyzed by X-Ray Fluorescence (XRF, Bruker S4 Pioneer) in order to
quantify the platinum loading. The loading determined by XRF amounted to 0.44 wt%. The
particle size of Pt was determined by transmission electron microscopy (TEM, Philips
CM300ST-FEG) and the composition of the presumably Pt particles confirmed by Energy
Dispersive X-ray spectroscopy (EDX, Noran System Six). The results of the TEM and EDX
analyses are shown in Fig. 2. From the TEM micrograph a Pt particle size variable between 3
and 5 nm could be determined (Fig.2.a).
3.2. Effect of Pt deposition on hydrogen production activity of TiO2
Fig.3 shows the results obtained from the illumination of aqueous suspensions with and without
methanol in the presence of two photocatalysts (TiO2 and 0.5% Pt/TiO2). In the absence of
methanol, hydrogen production was not observed for both catalyst compositions. From the
aqueous solution with MeOH (2.48 M) hydrogen evolved in measureable quantities significantly
higher for the Pt/TiO2 catalyst than for the TiO2 catalyst (see insert for the trend in activity). The
initial rise in concentration is due to non-steady state behavior (flush out) of the continuously
stirred tank reactor, (CSTR), which requires approximately 10 minutes. Then a steady state in
hydrogen rate of approximately 0.1 µmol H2 g-1cat min-1 is reached for TiO2, although initially the
rate is somewhat higher. The initial rate induced by 0.5 %Pt/TiO2 amounts to 7.5 µmol H2 g-1
cat
min-1, which slowly decays to a steady state rate of 5.5 µmol H2 min-1 g-1cat (Fig.3). The
Apparent Quantum Efficiency (AQE), taking into account a flux of photons of about 3.25*10-5
mol/min (calculated on the basis of 82 mW/cm2 light intensity, 0.5 cm window diameter, and an
average wavelength of 365 nm (The dominant mercury emission line in the UV), and an average
8

rate of hydrogen production of 8.25*10-8 mol/min (based on 15 mg of catalyst), amounts to
approximately 5.5 % (assuming 2 photons are required per mole of H2).
3.3. Effect of process parameters on H2 production in anaerobic conditions.
3.3.1. Effect of the initial MeOH concentration
Different initial concentrations of MeOH were used in order to determine the optimum for
hydrogen production (Fig.4). For all MeOH concentrations tested, an initial maximum in
hydrogen production rate can be observed (after about 10 minutes), followed by a significant
decrease in rate until steady state is reached, variable in quantity for each initial MeOH
concentration. Fig.4b shows that the hydrogen formation rate initially strongly depends on the
MeOH concentration, which dependency exponentially decays to close to zero order at MeOH
concentrations above approximately 1 M. These phenomena are similar to those reported by
Patsoura et al. [17] In view of the constant rates observed in the CSTR, we do not expect
significant deactivation of the catalyst. Fig. 4 also indicates the experiments are fairly
reproducible at high methanol concentrations, since for every different methanol concentration a
fresh amount of catalyst was used.
3.3.2. Effect of MO on MeOH induced hydrogen formation
The effect of MO on the MeOH induced hydrogen production rate was determined for various
MO concentrations, at a fixed concentration of MeOH of 2.48 M. The results of these
experiments are summarized in Fig. 5. First, in the absence of MeOH, hydrogen production was
below the detection limit of the TCD detector. Apparently MO is not able to stimulate hydrogen
formation over Pt/TiO2 to a significant degree at the concentration levels tested (0.09 M).
Further, at a low concentration of 0.06 M, the presence of MO had very little effect on the
MeOH induced hydrogen production. However, increasing MO concentration, some remarkable
changes were observed. First, the hydrogen formation appears to be delayed and the delay time
increases as a function of MO concentration. The maximum hydrogen concentration peak
observed at minute 8 for 0.06 and 0.09 mM, shifts to minute 23 at the highest concentration of
methyl orange (0.31mM). Second, the maximum in amount of hydrogen produced is higher in
the presence of MO, while the transient reaching steady state levels also requires somewhat
9

longer times. The peak in hydrogen production rate obtained in the presence of MO equals to
about 9 µmol H2 g-1 cat min-1.
In order to determine the photocatalytic decomposition of MO, samples were collected after
certain illumination intervals and analyzed by UV-Vis spectroscopy and LC-MS. The results of
MO mineralization in the absence or presence of MeOH are shown in Fig.6.a. After 10 minutes
of illumination full dis-colorization of MO was achieved, while in the presence of MeOH this
required even less time. To identify potential compounds formed from MO in the applied
anaerobic conditions, LC-MS analysis was performed. A species with m/z of 172 (Fig.6.b) was
obtained after 10 minutes of illumination, which we could assign to the formation of a
hydrogenated fragment of MO, to be discussed later.
3.4. Quantification of H2 production in oxygen lean conditions
As stated in the introduction, photocatalytic hydrogen production is likely very much dependent
on the presence or absence of oxygen in the water feed. In purification devices to be applied in
practice, the water feed will likely contain oxygen (air). It is therefore relevant to study the effect
of solved oxygen on the transients described in Fig. 5. To introduce oxygen in the solution, argon
with 20% volume of O2 was purged for 30 minutes prior to starting illumination. During
illumination the purge consisted again of pure Ar. Fig.7 shows the effect of dissolved oxygen on
the MeOH induced hydrogen production transients, in the absence or presence of MO. Clearly,
in the absence of MO, the presence of oxygen (□) induces a delay of about 10 minutes before the
hydrogen concentration maximizes and decays to steady state. In the simultaneous presence of
MO (○) and oxygen, the delay in hydrogen production is significantly longer than with MO
alone (compare Fig. 5), and the attainable steady state hydrogen concentration lower.
In order to further understand the importance of oxygen in MO decomposition, the photocatalytic
decomposition of MO in the absence or presence of MeOH was again followed by UV-Vis
spectroscopy (Fig.8.a) and LC-MS (Fig.8.b) analysis. Fig.8.a, shows that the presence of oxygen
significantly prolongs the illumination time needed to achieve full dis-colorization (90 minutes,
compare Fig. 6), while again the presence of MeOH accelerates this, reducing the time needed
10

for full color elimination to 30 minutes. LC-MS analysis revealed that the intermediate with m/z
of 172 was not formed in significant quantities in aerobic conditions after 10 or 30 minutes of
reaction, 30 minutes being the time at which full dis-colorization was obtained.
4. Discussion
4.1. The necessity of Pt for H2 evolution from MeOH solutions
In the present study, hydrogen production was not observed, when MO was reacted in anaerobic
conditions (in the absence of MeOH), see Fig.3. While the absence of hydrogen formation for
TiO2 P25 is in agreement with literature [26], several studies have reported formation of
hydrogen with simultaneous decomposition of contaminant molecules for Pt containing
catalysts.[5, 15, 21] The absence of hydrogen production with 0.5% Pt-TiO2 in the present study
is probably related to the adsorption and azo-bond cleavage of MO, which likely progresses over
Pt sites preferably as compared to proton reduction, inducing reduction of color intensity and the
formation of the mass fragment 172 m/z (Fig. 6). We assume that oxidation of water to H2O2 is
the hole consuming reaction. Contrary to MO, MeOH was efficient in inducing hydrogen
formation, small over TiO2 (maximum at ~ 0.15 µmol H2 min-1 g-1 cat), but significant (~7.5
µmol H2 g-1 min-1) over 0.5% Pt-TiO2 (Fig.3). The mechanism of MeOH induced hydrogen
cat
formation over 0.5% Pt-TiO2 will be discussed in the following.
4.2. MeOH assisted hydrogen formation
In anaerobic conditions, two reactions of MeOH in aiding the production of hydrogen can be
proposed. In the first reaction (1), MeOH will help to increase the efficiency of proton reduction
over Pt by reacting with photogenerated holes by avoiding electron-hole recombination, and
producing CO.[24]
The four protons react over the Pt sites with photo-excited electrons to produce hydrogen. The
other light induced reaction is oxidation of MeOH to formaldehyde, occurring over the surface of
11

the TiO2 nanoparticles, again followed by formation of hydrogen by reduction of protons over
the Pt sites. The overall process is represented by reaction (2). [9, 15, 22]
Finally, consecutive reactions of formaldehyde to formic acid (3), and formic acid to CO2 (4)
have also been proposed to contribute to hydrogen formation in anaerobic (reductive) conditions.
From the catalytic data reported in the present study, we cannot discriminate which of the
products of methanol conversion is most important. However, we anticipate reaction (1) is
relevant, based on the initial transient in hydrogen production rate. We speculate this initially
decreasing rate is related to poisoning of Pt sites by CO formed in the (partial) oxidation
mechanism by reaction (1). CO is a well-known poison for Pt anodes in direct methanol fuel
cells (DMFC) and in fact is reported to prevent the commercial application thereof.[27, 28] At
some point the extent of poisoning, and competing hydrogenation reactions are apparently in
equilibrium, and the hydrogen formation rate reaches steady state.
4.3. Explaining H2 formation transients from MeOH and MO mixtures in anaerobic conditions
Some remarkable observations regarding the MeOH induced hydrogen formation in the presence
of MO at anaerobic conditions can be made (Fig.5): i) hydrogen formation is delayed by the
presence of MO at concentrations larger than 0.06 mM, ii) the maximum in hydrogen formation
is enhanced in quantity and shows a slower transient to steady state, and iii) MO dis-colorization
is faster in the presence, than in the absence of MeOH. These observations confirm that the Pt
catalyst is substrate selective with favorable affinity for MO as compared to protons, as
previously indicated in paragraph 4.1. The reductive discolorization reaction is illustrated in
Figure 9.
12

Since no other intermediates were observed which could be assigned to possible MeOH-MO
reactions, we assume the explanation for acceleration of N=N hydrogenation induced by MeOH
is similar to the explanation for hydrogen formation, and due to favorable hole scavenging and
occurrence of reaction (1). Once MO has fragmented, the Pt surface is available for proton
reduction to produce hydrogen, explaining the concurrent rise in hydrogen production with the
decreasing color intensity of MO. What is obvious is that MO fragments (Fig. 9) enhance the
maximum attainable hydrogen production in the presence of MeOH. We speculate this might be
related to sulfate formation on the TiO2 surface by consecutive oxidation of e.g. mass fragment
m/z 172. Sulfate formation has been previously reported to promote activity and photonic
efficiency. Sulfates can efficiently react with holes generating sulfate radicals, thus improving
the apparent quantum efficiency by avoiding electron-hole recombination, and likely enhancing
methanol oxidation rates.[29-31]
4.3. Explaining H2 formation transients from MeOH and MO mixtures in oxygen lean conditions
Dissolved oxygen delays MO dis-colorization, as well as hydrogen evolution observed in Figs. 7
and 8. The significant delay by the presence of oxygen in the dis-colorization of MO is in
agreement with the hypothesis that dis-colorization needs photogenerated electrons and occurs
reductively over Pt sites (Fig.10). We anticipate that oxygen competes for these electrons. The
fate of the superoxide and hydroperoxyl radicals formed by reduction of oxygen [13], is likely to
react with MeOH, rather than MO, given the significantly higher concentration of MeOH. After
the delay, oxygen in solution is mostly converted and the reactions follow the processes
discussed for anaerobic conditions. We assume that the acceleration of MO dis-colorization
induced by MeOH is again due to favorable hole scavenging, enhancing apparent quantum
efficiency. This lowers the concentration of MeOH, on which hydrogen formation was
demonstrated to be dependent in Fig. 4, and thus explains the lower steady state hydrogen
production rates.
5. Conclusions
13

The data reported in this study demonstrate that photon-electron stimulated Pt nanoparticles have
multiple catalytic functions. The following order in reactant selective reduction activity was
observed experimentally: O2 > MO > H+ This order in reactivity is based on: i) MeOH induced
.
hydrogen formation is inhibited by the presence of MO, and ii) MO dis-colorization and
hydrogen formation (by H+ reduction) are negatively affected by the presence of oxygen. We
demonstrate that MeOH is oxidatively converted by reaction with holes, diminishing probability
of electron-hole pair recombination, and explaining the promotion in the dye dis-colorization
reaction (both in the absence or presence of oxygen). CO is likely a dominant product of MeOH
oxidation in anaerobic conditions, in agreement with CO poisoning of Pt being at the origin of
the frequently observed maximum in hydrogen production. For practical application of
concomitant methanol induced hydrogen formation and dye degradation, Pt/TiO2 induced
photocatalysis is not practically feasible, due to the strong reactant selective activity of the Pt
nanoparticles. Moderation of the Pt particles, e.g. by alloying, allowing simultaneous, rather than
consecutive processing of oxygen reduction, dye conversion, and production of hydrogen, might
lead to improvements. At the same time, the photonic efficiency of dye degradation might be
improved by combining TiO2 with favorable semiconductor substrates, such as silicon,
commonly used in construction of photocatalytic microreactors.
Acknowledgements
This work is supported by NanoNextNL, a micro and nanotechnology consortium of the
Government of the Netherlands and 130 partners.
14

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17

Fig. 1. Schematic illustration of TiO2 loaded with Pt, and the processes occurring which are relevant for
photocatalytic decomposition of organic compounds with simultaneous H2 production.
18

Fig. 2. Characterization of 0.5% Pt-P25. a) TEM image with several Pt particles indicated by the arrows,
and b) an exemplary EDX spectrum of the particles, identifying the presence of Pt.
a)
b)
19

Fig. 3. Hydrogen concentration as determined in a CSTR for TiO2 and 0.5% Pt/TiO2 photocatalysts from
aqueous suspensions with (2.58 M) or without MeOH, as a function of illumination time. The inset shows
a magnification of the result obtained for MeOH withTiO2.
20

Fig. 4. a)Time dependent hydrogen concentration determined for 0.5% Pt/TiO2 catalyst as a function of
initial MeOH concentration. b) Rate constant values found for hydrogen evolution at increasing initial
MeOH concentrations (M) showing an exponential trend.
a)
b)
21

Fig. 5. MeOH (2.48 M) induced hydrogen concentration transients over 0.5% Pt/TiO2 as a function of the
initial MO concentration (varied between 0.06 and 0.31 mM).
22

Fig. 6. a) Photocatalytic dis-colorization of MO (0.09 mM) in the absence or presence of MeOH (2.48 M)
in anaerobic conditions (Ar). b) Analysis by LC-MS of samples at t=10 min, showing the formation of a
compound with a mass fragment of 172 m/z.
a)
b)
23

Fig. 7. Hydrogen production over 0.5% Pt/TiO2 from aqueous suspensions with MeOH (2.48M) and MO
(0.06 mM), as a function of illumination time, comparing anaerobic (Ar) and aerobic (Ar+O2) conditions.
24

Fig. 8. a) Photocatalytic decolorization of MO (0.09 mM) in the absence or presence of MeOH (2.48 M)
in aerobic conditions (solution saturated with a flow of 5 mL/min of 20% O2 in Ar for 30 min). b)
Analytical analysis by LC-MS from the sample with MeOH after 30 min illumination.
a)
b)
25

Fig. 9. First step in MO photocatalytic degradation, and possible fragments formed by azo-bond cleavage.
Fig. 10. Schematic representation of all the reduction reactions that occur on the Pt surface, and the
(simplified) oxidative MeOH conversion.
26