Mechanistic Insights into Hydrogen Evolution by Photocatalytic Reforming of Naphthalene

Article abstract of DOI:10.1021/acscatal.0c01713

Heterogeneous photocatalysis has been widely considered, among other applications, for environmental remediation and hydrogen production. While these applications have been traditionally seen as well-separated areas, recent examples have highlighted the possibility of coupling them. Here, we demonstrate the simultaneous production of H2 and naphthalene removal from aqueous solutions with (unoptimized) photonic efficiencies of 0.97 and 0.33percent, respectively, over Pt-TiO2 under simulated sunlight. Photocatalytic and spin-trapping experiments in the presence of a hydroxyl radical and hole scavengers evinced that only the photogenerated holes play a significant role in the oxidation of naphthalene. Isotopic labeling analyses showed that the evolved H2 isotopologues match those of the solvent and that deuterated water (but not deuterated naphthalene) decreases the reaction rate, suggesting its involvement in the rate-determining step. Moreover, the use of Ti18O2 does not lead to the significant formation of 18O-enriched CO2, suggesting that water is the source of the oxygen atoms. Ultimately, by considering the stable and transient reaction intermediates, we propose a plausible reaction pathway. Our work illustrates that environmental remediation can be effectively coupled to solar fuel production, providing a double purpose to photocatalytic reactions, while the mechanistic insights will be of use for the further development of this strategy.

Full text of DOI:10.1021/acscatal.0c01713

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Article
Mechanistic Insights into Hydrogen Evolution
by Photocatalytic Reforming of Naphthalene
Osama Al-Madanat, Yamen AlSalka, Mariano Curti, Ralf Dillert, and Detlef W. Bahnemann
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c01713 • Publication Date (Web): 01 Jun 2020
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Mechanistic Insights into Hydrogen Evolution by Photocatalytic Reforming of Naphthalene
Osama Al-Madanata,b,*, Yamen AlSalka , Mariano Curti , Ralf Dillert , Detlef W. Bahnemann
a,c
a
a,c
a,c,d*
a
Gottfried Wilhelm Leibniz Universität Hannover, Institut für Technische Chemie, Callinstr. 3, D-30167 Hannover,
Germany
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Mutah University, Chemistry Department, Mutah, 61710 Al-Karak, Jordan.
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Gottfried Wilhelm Leibniz Universität Hannover, Laboratorium für Nano- und Quantenengineering, Schneiderberg
9, D-30167 Hannover, Germany
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d
Saint-Petersburg State University, Laboratory “Photoactive Nanocomposite Materials”, Ulyanovskaya str. 1,
Peterhof, Saint-Petersburg, 198504 Russia
* Corresponding authors:
Osama Al-Madanat: Tel.: +49-511 762 2773; E-mail: al-madanat@iftc.uni-hannover.de
Detlef W. Bahnemann: Tel.: +49 511 762 5560; E-mail: bahnemann@iftc.uni-hannover.de
Abstract
Heterogeneous photocatalysis has been widely considered, amongst other applications, for environmental
remediation and hydrogen production. While these applications have been traditionally seen as well separated areas,
recent examples have highlighted the possibility of coupling them. Here we demonstrate the simultaneous production
of H and naphthalene removal from aqueous solutions with (unoptimized) photonic efficiencies of 0.97% and 0.33%,
2
respectively, over Pt-TiO under simulated sunlight. Photocatalytic and spin trapping experiments in the presence of
2
hydroxyl radical and hole scavengers evinced that only the photogenerated holes play a significant role in the
oxidation of naphthalene. Isotopic labeling analyses showed that the evolved H isotopologues match those of the
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solvent, and that deuterated water (but not deuterated naphthalene) decreases the reaction rate, suggesting its
1
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involvement in the rate-determining step. Moreover, the use of Ti O does not lead to the significant formation of
2
18
O-enriched CO , suggesting that water is the source of the oxygen atoms. Ultimately, by considering the stable and
2
transient reaction intermediates, we propose a plausible reaction pathway. Our work illustrates that environmental
remediation can be effectively coupled to solar fuel production, providing a double purpose to photocatalytic
reactions, while the mechanistic insights will be of utility for the further development of this strategy.
Keywords:
Polycyclic aromatic hydrocarbons, persistent organic pollutants, photoreforming, titania, H production, EPR,
2
isotopic study, reaction mechanism
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Introduction
Achieving a fully sustainable energy production system based on sunlight involves the development of multiple
and diverse technologies. Although solar light represents a huge untapped potential, it is intermittent in nature,
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making the use of energy vectors an unavoidable requirement . In this regard, the photocatalytic production of
molecular hydrogen (H ) in the solar-driven process is a promising strategy to store sunlight as chemical energy via
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photoinduced reduction of water 2, 3. As an energy vector, H possesses the highest gravimetric gross calorific
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value among all fuels (142ꢀMJ kg ), and burns cleanly yielding only pure water . Nonetheless, its photocatalytic
production commonly requires the use of a sacrificial electron donor (SED) to push the oxidation half-reaction and
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achieve acceptable efficiencies , in a process commonly called photocatalytic reforming of the SED. This entails,
however, the consumption of compounds that may have a significant cost with respect to that of the produced H2.
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Lanterna and Scaiano have recently illustrated an interesting alternative . These authors utilized water samples
from Canadian rivers to perform a photocatalytic reaction where both the reduction and oxidation half-reactions were
purposeful: while water was reduced to form H , the accompanying half-reaction was exploited to oxidize dissolved
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pollutants, thus simultaneously improving the water quality. This strategy increases the H production efficiency with
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respect to pure water, avoids the use of potentially costly SEDs, and achieves the above-mentioned two-fold purpose.
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Another attractive dual-purpose system was described by Horiuchi et al., combining solar H production with a
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plant factory. The use of plant biomass as SEDs aids the photocatalytic H production, while the emitted CO is
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useful for plant growth.
Industrial effluents are an attractive target to apply this approach. Although they commonly carry a broad range
of pollutants, a particularly troublesome group is that of persistent organic pollutants such as polycyclic aromatic
hydrocarbons (PAHs). These compounds, mainly distributed by the incomplete combustion of fossil fuels and by
accidental oil spills, are ubiquitously found in the environment, have a great tendency to bioaccumulate, and,
importantly, present a broad range of deleterious health effects 7, 8. Among the different PAHs, naphthalene is a
particularly noxious member due to its relatively large solubility in water, which leads to increased bioavailability
and ultimately to adverse effects in the environment and human health.
As for many other pollutants, oxidative processes based on photocatalysis have often been considered for the
removal of PAHs. Nevertheless, while several studies have explored their photocatalytic oxidation in aerobic
conditions (where the reduction half-reaction is the conversion of molecular oxygen to the superoxide radical anion)
9-13,
there are no reports on the remediation of PAHs with the simultaneous production of H . At the same time,
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reports on simpler aromatic compounds such as benzene and its derivatives have indeed shown that coupling their
photocatalytic degradation to H production could be a viable strategy 14-16. In comparison to relatively simpler SEDs
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like methanol, however, the involvement of the oxidation intermediates may play an important and undesired role 17;
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to unravel these, mechanistic investigations on both the reductive and oxidative half-reactions are desired .
In the present study, we report the investigation of the removal of naphthalene concurrently with the H2
production reaction, using a platinized TiO photocatalyst under anaerobic irradiation with simulated sunlight.
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Moreover, we delve into the underlying mechanism and possible degradation pathways by monitoring the formation
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of organic intermediates, performing isotopic substitution experiments, and studying the reaction in the presence of
active species scavengers and spin traps.
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.1 Raw materials
The commercial titanium dioxide (TiO ) Hombikat UV100 was supplied by Sachtleben Chemie GmbH.
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Naphthalene (≥ 99%), naphthalen-1-ol (97%), naphthalen-2-ol (99%), 1,1'-binaphthalene (97%), naphthalene-1,4-
dione (97%), 1,2,3,4-tetrahydro-1-naphthalenone (97%), 3,4-dihydronaphthalen-1(2H)-one (97%), 5-
hydroxynaphthalene-1,4-dione (97%), dimethyl phthalate (99%), diethyl phthalate (99%), potassium iodide (99.5%),
deuterium oxide (D O, 99.9 atom % D), 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 97%), and chloroplatinic acid
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hexahydrate (H PtCl ·6H O, 99.9%) were purchased from Sigma-Aldrich. Methanol (HPLC grade), acetone (GC
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grade), dichloromethane (GC grade), and ethyl acetate (GC grade) were purchased from Carl-Roth. All reagents were
used as received without any further purification. All solutions were prepared with deionized water obtained from a
Millipore Mill-Q system (18.2 MΩ cm, 25 ⁰C).
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.2 Preparation and characterization of the photocatalysts
Platinum-loaded TiO samples were prepared by a photodeposition method following the procedure of Melvin
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et al. . The TiO powder (1.5 g) was dispersed in 150 mL of a 10% aqueous methanol solution with 3.86 mL of
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.01M platinum precursor (H PtCl ) and stirred for 1 h prior to its purging for 1 h with argon. The obtained suspension
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was then illuminated for 4 h with UV(A) light employing a Philips CLEO 15W lamp inside a climatic chamber with
an adjusted temperature of 18°C. The solid was then collected by centrifugation, washed once with methanol and
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four times with distilled water, and finally dried at 100 °C for 24 h. Ti O was kindly provided by Dr. Juan Felipe
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Montoya and synthesized according to a previously reported procedure. The synthesized Ti O was mixed with
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the proper volume of a platinum nanoparticles suspension prepared by laser ablation and kindly provided by
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Particular GmbH, Germany. Then it was stirred for 1h, and finally dried overnight to prepare 0.5 wt% Pt-Ti O .
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The synthesized platinized TiO and pristine UV100 photocatalysts were characterized with the Brunauer–
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Emmett–Teller technique (Table S1), X-ray diffraction (XRD) (Figure S1), transmission electron microscopy
(TEM) (Figure S2), and electron paramagnetic resonance spectroscopy (EPR) (Figure S3). A detailed discussion of
the physicochemical characterization can be found in the supplementary information file.
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.3 Sample preparation and photocatalytic experiments
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Due to the low solubility of naphthalene in water (2.44 × 10 mol L at 25 °C ) and its high vapor pressure,
special care was taken to prepare deaerated solutions of known concentrations, as summarized in Scheme S1. As a
first step, a stock saturated and the deaerated solution was prepared in deionized water. In brief, 0.100 g of
naphthalene was introduced in a double-jacket two-necked container with airtight screw caps, containing 200 mL of
Milli-Q water. The mixture was purged with a gentle stream of argon for 1 h at room temperature. The temperature
was then maintained at 40 °C by using a thermostatic water bath with continuous magnetic stirring for 2 h to increase
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the solubility of naphthalene (4.30 × 10 mol L at 40 °C ). Subsequently, the temperature was decreased and
maintained at 25 °C with continuous stirring for 4 h. Afterward, the stirring was stopped, and the solution was left
for 2 h to settle the undissolved naphthalene crystals. This procedure results in a solution with an estimated
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concentration of 2.44 × 10 mol L (31ppm); the exact value was determined via HPLC-UV.
The photocatalytic experiments were carried out in batch mode. The photocatalytic H evolution experiments
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were performed in 20 mL glass vials tightly closed with crimp caps and silicone septa. In a typical experiment, 15
mg of the photocatalyst powder was suspended in ca. ~9.5 mL of deionized water inside a vial. The vial was then
closed with the crimp septum and purged with argon for 30 min. Afterward, the necessary volume from the stock
naphthalene solution was injected inside the vial by using a pre-purged (with argon) needle and syringe, in order to
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reach a final 1.56 × 10 mol L (20 ppm) naphthalene concentration. The total reaction volume was 15.0 mL, with a
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photocatalyst mass concentration of 1.0 g L . Finally, the vials were horizontally fixed inside an orbital shaker and
left in the dark for 1 h to establish the adsorption equilibrium. Afterward, the orbital shaker was placed at a 30 cm
distance below a solar simulator consisting of a 1000 W xenon lamp (Hönle UV Technology, Sol 1200) equipped
with a filter to mimic the emission profile of sunlight. The temperature of the vials was maintained at 25 ± 2 °C by
using a water bath. From the measurement of the lamp irradiance in the wavelength range between 320 nm and 380
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nm at the irradiated area, the photon flux density was found to be I = 3.31 × 10 mol m s . Ignoring light losses
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due to reflection and scattering, 2.18 × 10 mole of photons entered the irradiated suspension per second.
The photocatalytic experiments in the presence of scavengers were performed under the same conditions, using
-1
a 20 mmol L concentration of either potassium iodide (KI) as the hole scavenger or tert-butyl alcohol (TBA) as the
hydroxyl radical scavenger. For the radical trapping experiments, the necessary amount of the spin trapping agent
-1
5,5-dimethyl-1-pyrroline N-oxide (DMPO, Aldrich) to achieve a 20 mmol L final concentration was also added.
The suspensions were then transferred under an argon stream to a small quartz flat cell cuvette (FZK 160-7x0.3,
Magnettech GmbH, Germany) designed for EPR analysis. The samples were irradiated at room temperature (~ 25
°
C) directly in the EPR spectrometer microwave cavity, and the spectra were recorded before and during irradiation.
Identical experimental conditions were used for intermediates identification. However, a 250 mL borosilicate
reactor was chosen to perform these experiments in a higher volume of the reaction suspension, to guarantee a
detectable amount of these intermediates. All the photocatalytic tests were done by triplicate, yielding relative
standard deviations (RSD) between 4% and 8%.
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.4 Mineralization measurements
The extent of naphthalene mineralization was monitored by total organic carbon (TOC) measurements, using
a Shimadzu TOC-5000A analyzer. Different concentrations of standard potassium hydrogen phthalate and sodium
bicarbonate solutions were used for the calibration of the instruments before the analyses. The initial concentration
was measured after the equilibrium adsorption of the naphthalene on the Pt/TiO surface for 1h, and afterward, the
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TOC was recorded at regular intervals. All samples were filtrated with syringe filters (0.2 µm) and immediately
analyzed, to minimize the volatilization of naphthalene.
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A HIDEN HPR-20 QIC gas analyzer equipped with a quadrupole mass spectrometer (QMS) was used to identify the
evolved gases during the photocatalytic tests. The experiments were carried out in a 65 mL cylindrical air-tight reactor
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with a quartz window. A continuous flow of 5 mL.min of argon carried the headspace into the QMS system. In all
experiments, the aqueous suspension volume was 50 mL, while the temperature was kept constant at 25 ± 1 °C. Blank
experiments showed no detectable gases evolution in the absence of either light, the photocatalyst or naphthalene.
The same setup was employed for the isotopic substitution experiments. In this case, however, irradiation was
performed in the vials in an identical way to the photocatalytic experiments, and then the headspace was swept into
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the QMS using a 0.75 mL min argon flow. The quantitative of the evolved gases was done by calibrating the QMS
with standard argon-diluted H (Linde Gas, Germany) and D (Sigma-Aldrich).
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2.5 Extraction procedures
To ensure the complete extraction of stable intermediates, two complementary approaches were employed. In
the liquid-liquid procedure, the irradiated suspension (250 mL) was transferred to a 500 mL separatory funnel and
extracted with three 25 mL aliquots of dichloromethane. The organic layer was passed through a 0.45 mm filter paper
containing anhydrous sodium sulfate to remove the TiO particles and water. Finally, the organic layer was
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concentrated to 1 mL using a gentle stream of nitrogen.
In the solid-phase extraction procedure, the irradiated suspension was immediately filtered through 0.2 µm
PTFE syringe filters to remove the TiO particles, then transferred to a brown glass bottle. The extraction was
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performed within 24 hours to avoid any losses or degradation of the by-products, using packed cartridges (OASIS
®
HLB, 6 mL/500 mg, from Waters Ireland). The cartridges were connected to a solid-phase extraction apparatus
(Supelco, USA) and preconditioned with 6 mL of methanol, 6 mL of ethyl acetate and 6 mL of acidified deionized
-1
water (pH 2.0) at a flow rate of 1 mL min . Then a sample’s aliquot of 200 mL (pH adjusted to 2.0 with 1 M HCl)
-1
was loaded into the cartridge and passed through it at a 3-4 mL min rate. Subsequently, the cartridge was pre-dried
under low pressure for 5 min to remove the excess of water. Finally, the retained analytes were eluted using three 4
-1
mL aliquots of ethyl acetate at a flow rate of 1 mL min . The collected extract was passed through anhydrous sodium
sulfate to remove any remaining water and then concentrated to around 0.5 mL using a gentle stream of nitrogen.
2.6. Analytical methods
High-Performance Liquid Chromatography (HPLC) analyses were performed using an Ecom system equipped
with a UV-vis detector operating at 276 nm and a Knauer Vertex Plus column packed with Eurospher II 100-5 C18
A material (length × inner diameter 150 mm × 4 mm with precolumn). The oven temperature was 30 °C and the
mobile phase was a mixture of methanol and water. Different elution programs were applied for naphthalene and
intermediates identification as described in Table S2.
The formation of short-chain organic acids was monitored via High-Performance Ion Chromatography (HPIC),
using a Dionex ICS-1000 equipped with a conductivity detector and an electro-regenerator suppressor. The column
was an anion exchange resin (IonPac AS9-HC, L × I.D. 250 mm × 2 mm) and the eluent was a mixture of alkaline
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solutions of 8 × 10 mol L Na CO and 1.5 × 10 mol L NaHCO with a flow rate of 0.3 mL min . The temperature
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of the conductivity cell was kept constant at 35 C during the analysis period (90 min).
Gas chromatography with thermal conductivity detection (GC-TCD) was used to quantify the
photocatalytically evolved H . For this purpose, a 50 μL gas sample was periodically taken from the headspace over
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the suspension using a Valco gas-tight sampling syringe equipped with a push-button valve. The gas sample was then
injected into the injection port of a Shimadzu GC-8A, equipped with a stainless-steel molecular sieve 5A GC column
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Sigma-Aldrich, USA). The temperature of the injection port and the TCD detector was maintained at 120 °C, while
the column temperature was fixed at 80 °C.
The extracted reaction intermediates were analyzed by GC-MS using a GCMS-QP5050 (Shimadzu) coupled
with AOC-5000 Plus autosampler and a capillary Agilent DB-5ms column (L × I.D. × film thickness 30 m × 0.1 µm
-1
× 0.32 mm). The carrier gas was helium at a constant flow of 1.25 ml min . The oven temperature was programmed
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at 70 °C for 3 min, followed by a raise to 180 °C at a rate of 10 °C min , held for 2 min, and then a final raise at a
-1
rate of 20 °C min to 315 °C and held for 10 min. The injection port and the interface temperatures were set at 250
°
C and 280 °C, respectively. The mass spectrometer was operated in the positive electron impact mode with
ionization energy of 70 eV; the detection was performed in scan mode and peaks were identified according to the
NIST library.
2
.7 EPR in situ experiments
The electron paramagnetic resonance (EPR) spin-trapping technique was applied to detect and monitor the
formation of paramagnetic intermediates. Such intermediates were detected upon in situ irradiation using an X-band
EPR MiniScope MS400 (Magnettech GmbH, Germany) spectrometer operating at a 9.51 GHz field and equipped
with a UV spot-light (LC8, Hamamatsu, 200 W super-quiet mercury-xenon lamp). The acquisition parameters were
as follows: center field: 335.4086 mT, sweep time 15 s, number of points: 4096, number of scans: 1, modulation
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amplitude: 0.2 mT, power: 10 mW, gain: 5. The EPR spectra simulations were carried out using EasySpin .
2
.8. Transient Absorption Spectroscopy
The transient absorption spectroscopy (TAS) measurements were carried out in diffuse reflectance mode by
means of an Applied Photophysics equipment, employing an LKS80 nanosecond Laser Flash Photolysis
Spectrometer with a pulsed Nd:YAG laser (Quantel, Brilliant B). Samples were measured as aqueous slurries inside
flat quartz cells. Prior to the measurements, the samples were purged with argon for 30 min. The excitation
-2
wavelength was 355 nm, with an average energy per pulse of 6 mJ cm . The obtained transient signal is detected
upon measuring the change in reflectance (∆퐽) before (J ) and after the laser excitation (J), according to Equation 1.
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∆
J =
(1)
J0
3.
Results and discussion
.1 Naphthalene degradation and photoreforming
3
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We start our analysis by considering the requirements for the photocatalytic production of H over bare and
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platinized Hombikat UV100 TiO (from here on, UV100 or Pt-UV100, respectively). First, as expected from the
2
known lack of water splitting activity of bare TiO2 23, no H is evolved in the absence of naphthalene. Similarly, there
2
is no H production in the absence of either light or a photocatalyst. However, the naphthalene concentration
2
moderately decreases under irradiation in the absence of the photocatalyst (32% after 4 h), possibly due to the
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photolysis of the probe compound . The direct photolysis of PAHs has been reported previously 13, 24, and it is
13
2
5
considered as one possible method for eliminating these compounds from the environment. For the photolysis to
occur the absorption spectra of the organic molecules must overlap with the emission spectra of the source. In the
present case only a limited overlap occurs in the range 300-315nm (Figure S4), which results in a relatively slow
photolysis of naphthalene. It has been reported that, upon irradiation, naphthalene undergoes oxidation to its radical
cation,24, 25 which reacts with water or O to mainly yield naphthol, quinone, and aldehyde. The detection of by-
26
2
products in our system proved unsuccessful, most likely due to the low concentration and the sequential
27
photooxidation of these products in our system. The estimated photonic efficiency for the direct photolysis of
-3
28
naphthalene after 4 h irradiation is 2.3 × 10 %. This value is low compared to that reported by Vialaton et al. of
-1
2
.5 × 10 % for the photolysis of naphthalene under aerobic conditions at 313 nm irradiation. On the other hand, the
absorption coefficient of TiO is much higher in the range of the light source emission, and thus we expect that, when
2
TiO and naphthalene are both present, most light is absorbed by TiO , significantly decreasing the photolysis rate.
2
2
The degradation of naphthalene is greatly enhanced in the presence of UV100 and Pt-UV100 (Figure 1). After
irradiating the aqueous suspension for 4 h, the naphthalene concentration decreases by 71% and 86%, respectively.
However, we only observe H evolution in the presence of Pt-UV100. The lack of H evolution over bare TiO is a
2
2
2
2
9
known issue, ascribed to the high overpotential for this reaction on the TiO surface . In the present case, whilst
2
photogenerated holes in TiO may be consumed by naphthalene, the absence of molecular oxygen as the electron
2
acceptor necessitates that the accompanying electrons either reduce one of the organic species present in the system
3
+
30
or accumulate in the form of trivalent titanium (Ti ) inside the photocatalyst.
Regarding the choice of the photocatalyst, we exclusively employ here the pure-anatase Hombikat UV100,
either bare or loaded with 0.5 wt% of platinum (prepared by photodeposition). Although the photocatalyst
optimization is out of the scope of this study, we mention that this selection is not arbitrary but is based on
experimental results showing that i) UV100 is more active than Aeroxide P25; ii) the photodeposition method of Pt
on TiO surface results in a higher activity than other deposition methods such as physical mixing of titania with Pt
2
nanoparticles, and iii) a 0.5 wt% Pt loading seems to be optimal for the reaction under investigation. We show the
detailed characterization of UV100 and Pt-UV100 in the Supporting Information.
The analysis of the kinetic profiles reveals some interesting features. As shown in Figure 1, during the prior
dark period around 14% of the initial naphthalene amount adsorbs on the surface of the photocatalysts. Although the
compound is not expected to strongly interact with the TiO surface, it is apparent that the high specific surface area
2
3
1
of UV100 results in significant adsorption . The adsorption kinetics of naphthalene on the surface of 0.5 w% Pt-
UV100 are shown in Figure S5. We observe that the adsorption equilibrium is attained within 1h. The adsorbed
-2
amount at the equilibrium corresponds to a coverage of around 0.05 molecule nm . A similar value for naphthalene
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on the surface TiO (anatase) has been reported by Mahmoodi and Sargolzaei, with the difference that the dynamic
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equilibrium was established after 5 min in that case. . The adsorption of aromatic compounds on the TiO surface
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+4
was studied by Nagao and Suda , who suggested that the adsorption involves the formation of Ti ‧‧‧π-electron
and/or OH(surface)‧‧‧π-electron type complexes.
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Figure 1. Kinetic profiles for the photocatalytic degradation of naphthalene and molecular hydrogen evolution over
-1
UV100 and Pt-UV100. Conditions: batch reactor, reaction volume = 15 mL; [Naphthalene] = 156 µmol L ;
-1
[
catalyst] = 1 g L ; T = 25 °C; illumination under simulated solar light. Lines are guides for the eye.
Upon irradiation, we observe a rapid decrease of the naphthalene amount in the presence of both photocatalysts
during the first 30 min of illumination. This decrease is found to be accompanied by rapid H evolution only in the
2
presence of Pt-UV100. After this initial 30 min period, the changes in the amounts of naphthalene as well as evolved
H became significantly slower, suggesting the depredation of accumulated intermediates at the photocatalyst surface
2
and the competition of naphthalene and intermediates for the holes during the photocatalytic process. This hypothesis
of a poisoning of the photocatalyst surface is supported by the observation of a color change of the photocatalyst
from light gray to brown after 1 h of illumination (Figure S6). This observation is in agreement with previous reports
which have shown that the photocatalytic degradation of naphthalene and other aromatic compounds may yield
intermediates, likely of polymeric nature, that deactivate the catalyst surface 34, 35. Additionally, we performed
recycling experiments by collecting the photocatalyst and irradiating it in the presence of molecular oxygen for 1 h
to remove the adsorbed organic compounds (Figure S6) for overcoming possible poisoning problems.
As illustrated in Figure 1, the loading of platinum nanoparticles on TiO not only enables the evolution of H possibly
2
2
3
6
through the reduction of the overpotential for the electron transfer to the proton , but also promotes the degradation
of naphthalene. We ascribe this observation to the charge separation effect caused by the Schottky barrier between
TiO and platinum, which results in a lower recombination rate of the photogenerated electron-hole pairs 37, 38. On
2
another hand, the reduction of the adsorbed protons on the surface of the TiO by depletion of the photogenerated
2
electrons could be another factor for enhancing the photocatalytic activity of the Pt/TiO and lessening the charge
2
carrier recombination. Since the reduced protons (i.e., hydrogen atoms) migrate to the Pt islands to combine yielding
3
9
H molecules, the degradation of the naphthalene is enhanced on the positively charged TiO surface . Similarly,
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Sun et al. have reported that the rates of light-induced phenol decomposition and total carbon removal increased by
a factor of 1.5 when UV100 was loaded with 1 wt% Pt.
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From the photocatalytic naphthalene reforming experiments over Pt-UV100, we determined the initial (< 30
min) photonic efficiencies, ξ, to be (0.97 ± 0.06) % for H formation (3.8 µmol / 30 min) and (0.33 ± 0.01) % for
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naphthalene degradation (1.3 µmol / 30 min) after correction by subtracting the quantity of adsorbed naphthalene.
4
1
The employed equations are detailed in the SI file. These are typical values for photocatalytic processes , although
much lower than those reported, for instance, for the photocatalytic reforming of methanol over Pt-UV100 (between
4
2
20 and 30 %) . Methanol, however, gives rise to current-doubling effects, that in conjunction with its well-known
hole scavenging properties results in a highly efficient H evolution.
2
We observe a considerably higher ξ for H production than for naphthalene degradation. This suggests that H
2
2
is not only produced in association with the direct oxidation of naphthalene but also with the oxidation of organic
intermediates, as will be discussed below. The 30 min period in which the ξ was determined is long enough to reach
a significant conversion for naphthalene accompanied by the formation of reaction intermediates.
Besides the disappearance of naphthalene, it is of high importance for practical applications to evaluate the
extent of complete mineralization of the reactant yielding CO and H O. Therefore, we monitored the concentration
2
2
of the total organic carbon (TOC) being present in the aqueous solution after the removal of the photocatalyst Pt-
UV100. The TOC data points in Figure 2 showed a gradual decline to around 55 % after 4 h of irradiation. As
mentioned above, the concentration of naphthalene decreases by 86 % in the same period of time, pointing to the
formation of stable intermediates. By converting the amount of naphthalene to organic carbon, TOC , during the
N
photocatalytic process in presence of the Pt-UV100 and subtracting its curve from that of measured TOC, we estimate
the kinetic profile of all organic intermediates (Figure 2). This curve follows the expected profile for reaction
intermediates, reaching a maximum at around 30 min and then progressively decreasing. We have identified these
intermediates by various analytical techniques and describe the results in Section 3.2, together with a plausible
reaction pathway. We note, however, that a decrease in the TOC amount could possibly reflect the adsorption of the
by-products on the TiO surface, instead of mineralization.
2
To rule out this possibility, we performed in-line monitoring of the reaction atmosphere by means of the
quadrupole mass spectrometry (QMS) technique. Blank experiments showed no detectable evolution of gases in the
absence of either irradiation, the photocatalyst, or naphthalene. Additionally, no significant signals were detected for
the formation of CO, CH , and other small alkanes and alkenes. We show the time course of the QMS signals for H
4
2
and CO which are proportional to the amount of produced gases in Figure S7. We note here that experimental
2
limitations (namely, a significant evaporation of naphthalene) prevent us from quantifying the absolute amounts of
generated CO and H . The following description is thus only qualitative. The observation of a CO signal indeed
2
2
2
indicates that complete mineralization of the reactant occurs. After a very short initial delay, CO formation shows
2
an increasing rate over time during the analyzed period. Contrarily, after a fast-initial growth, the H QMS signal
2
shows a marked decrease in its formation rate, in accordance with the results of Figure 1. On the other hand, the loss
of naphthalene due to its large vapor pressure during these experimental runs, which were performed in an open
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system, prevents us to perform a direct comparison between the evolved amount of CO and the decrease in the TOC.
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However, the brownish color of the recovered photocatalyst and its disappearance during the irradiation in the
presence of O could indicate that at least part of the TOC decrease is related to the adsorption of by-products on the
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Figure 2. Kinetic profile for the relative amount of total organic carbon (TOC) during the photocatalytic reforming
of naphthalene over Pt-UV100.
3.2 Identification of intermediates
We investigated the intermediates formed during the photocatalytic reforming of naphthalene over Pt-UV100
using a combination of chromatographic techniques. Firstly, we employed liquid-liquid and solid-liquid extraction
procedures followed by gas chromatography-mass spectrometry (GC-MS) to obtain an overview of the stable
intermediates. The identified by-products are shown in Chart 1, while the details of their detection (retention times,
molecular ion peaks, and mass fragmentation patterns) are listed in Table S3. Among these compounds, we found
that naphthalen-1-ol and naphthalen-2-ol are the main constituents after 4 h of irradiation (0.25 ± 0.01 and 0.13 ±
0
.01 µmol, respectively). In contrast, we detected naphthalene-1,4-dione in a lower amount (0.030 ± 0.003 µmol),
and only traces of the remaining compounds. Complementarily, we also analyzed the samples for short-chain organic
acids using high-performance ion chromatography (HPIC). From these analyses, we determined the presence of
formic acid and acetic acid in low concentrations (0.051 ± 0.008 and 0.029 ± 0.005 µmol, respectively).
Although these are, to the best of our knowledge, the first results concerning the photoreforming of
naphthalene, we can perform a meaningful comparison with the intermediates previously observed during its aerobic
photocatalytic oxidation, summarized in Table S4. Although there is some degree of coincidence, it is revealing that
in the present case most detected intermediates correspond to earlier stages of the degradation process. This is
consistent with the poisoning effect discussed above, that may limit the extent of mineralization under anaerobic
conditions. Similarly, we find short-chain acids, produced from ring cleavage reactions,10, 12, 43 in very low
concentrations.
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OH
OH
OH
OH
naphthalen-1-ol (A)
naphthalen-2-ol (B)
naphthalene-1,4-diol (C)
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O
O
OH
O
OR
OR
O
O
O
naphthalene-1,4-dione (D)
dialkyl phthalate (E)
5-hydroxynaphthalene-1,4-dione (F)
OH
O
1
,2,3,4-tetrahydro-1-naphthalenol (G)
1,1'-binaphthalene (H)
3,4-dihydronaphthalen-1(2H)-one (I)
Chart 1. Detectable intermediates during the photocatalytic reforming of naphthalene using Pt-UV100.
Considering that naphthalen-1-ol and naphthalen-2-ol are the main detected intermediates, we now focus on
the time dependence of their concentrations, as monitored by high-performance liquid chromatography (HPLC). The
kinetic profiles, shown in Figure 3, closely match the degradation of naphthalene (Figure 1) with a high formation
rate during the first 30 min, and a significant retarding at longer times. The detected amounts, however, are
considerably smaller than those expected from the complete conversion of naphthalene. While, around 1.5 μmol of
naphthalene was degraded after 4 h, only a combined amount of 0.39 μmol of naphthalen-1-ol and naphthalen-2-ol
was formed. In the first stage of the reaction (Figure 3), the formation rate of these naphthalenols is larger than the
rate of their consumption, thus, increased their amount in the medium. Subsequently, in the second stage, the formed
amount becomes almost constant as both rates nearly equivalent. This may suggest the formation of a relatively large
quantity of unidentified intermediates, consistent with the hypothesis of polymeric species poisoning the TiO2
1
4
particles , or alternatively strong adsorption of the naphthalenols over Pt-UV100. Regarding the latter, Mills et al.
have suggested that the structurally similar phenol and 4-chlorophenol may indeed show strong binding to TiO2 44.
A further interesting observation is that the concentration ratio of naphthalen-1-ol to naphthalen-2-ol is not
unity but instead ranges from 1.7 to 2 throughout the reaction. We find a plausible explanation for this behavior on
the basis of Dewar’s molecular orbital theory 45 and the adsorption of naphthalene on the surface of the TiO . Briefly,
2
this theory predicts that the difference in the activation energy of two related aromatic substitution reactions is mainly
determined by the difference in the so-called localization energies of the extended π-systems. Of relevance here, the
localization energies have been quantified in the form of Dewar’s reactivity numbers; a lower reactivity number
corresponds to a faster aromatic substitution reaction, and vice versa. The reported values for the 1- and 2- positions
1
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in naphthalene are 1.81 and 2.12, respectively. Moreover, as determined from near edge x-ray absorption
spectroscopy (NEXAFS), naphthalene molecules adsorb on the surface of rutile (110) in a flat-lying geometry with
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tilt angles about 24°, at relatively high altitudes that reach 3–4 Å from the substrate surface . Matsuura et al,
demonstrated that position 1 on the naphthalene molecule is more reactive for removing an electron than that of
position 2 due to the higher charge density in this position. Thus, hydroxylation at position 1 results in a more stable
product than hydroxylation at position 2, which has been confirmed by Hückel-McLachlan molecular orbital
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calculations by Steenken 48 and experimentally by Hykrdova et al.,34 who reported the preferred formation of
+3
naphthalen-1-ol in the TiO (heterogeneous) and Fe (homogenous) systems. This becomes congruent with Dewar’s
2
reactivity numbers hypothesis and thus explains the preferential formation of naphthalen-1-ol. Considering these
results, it can be concluded that the interaction of the naphthalene molecules with the TiO surface does not
2
significantly change its electronic distribution in naphthalene. Hence, the electronic density at the carbon atoms (C1
3
4
>
C > C ) of the naphthalene molecule governs the position of hydroxylation .
2 9,10
Figure 3. Kinetic profile for the formation of naphthalen-1-ol and naphthalen-2-ol during the photocatalytic
reforming of naphthalene over Pt-UV100.
Although the mechanism for the formation of naphthalen-1-ol and naphthalen-2-ol will be discussed in section
3
.4, we mention here that the overall reaction is given by Equation 2. Consequently, the expected H to naphthalenols
2
ratio is 1:1, thus being at odds with our observation of a >10:1 ratio, as deduced from Figure 1 and Figure 3. This
discrepancy can be explained from two perspectives. On the one hand, there may be parallel channels by which
naphthalene is degraded. One such example is given by Equation 3, that results in one of the detected intermediates,
1,1'-binaphthalene, and the formation of hydrogen. Additionally, the kinetic profile of naphthalenols in Figure 3 not
only reflects their formation rate but also their simultaneous consumption rate by a photoreforming reaction, skewing
the ratio towards higher H values. This is illustrated by Equations 4 and 5, that result in the formation of two
2
equivalents of H via the successive transformation of naphthalene-1-ol into two of the detected intermediates,
2
napthalen-1,4-diol, and naphthalene-1,4-dione. Moreover, the strong binding of the naphthalenols to the TiO surface
2
could also explain their low observed amounts.
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(3)
hv
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+
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+
H ( )
(4)
(5)
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OH
O
O
hv
Pt/TiO2
+
H ( )
2
3.3 Mechanistic study
.3.1 Participation of the TiO surface as a source of oxygen atoms
3
2
Recently, besides photogenerated holes and hydroxyl radicals as active species in photocatalytic processes, the
role of bridging oxygen radicals at the TiO surface (O ) has been discussed14, 49, 50. Holes could be trapped by bridging
2
s
•
oxygen atoms on the TiO surface (≡Ti-O -Ti≡) to produce the active species oxygen radical (≡Ti-O Ti≡), which
2
s
s
ultimately can be incorporated into the reaction products, creating oxygen vacancies that are subsequently healed by
water. Therefore, isotope labeling is a useful strategy to evaluate such mechanism, e.g. by using mass spectroscopy
to monitor the isotopic composition of the evolved species. As we demonstrated in section 3.1, CO was one of the
2
final products during the photocatalytic reforming of naphthalene. Thus, if the bridging oxygens are the oxygen
source for hydroxylation of naphthalene, in this case, we expect that the isotopic composition of the evolved CO2
should (partially) reflect that of TiO . To investigate the participation of bridging oxygens, we performed the reaction
2
18
over isotopically labeled Pt-Ti O and monitored different CO isotopologues before and during the illumination by
2
2
means of QMS.
Figure 4 illustrates the time evolution of the QMS signals of different CO isotopologues and their fragments
2
1
8
during the photocatalytic reforming of naphthalene over Pt-Ti O . Before illumination, no changes in the background
2
1
8
signals were observed for one hour; i.e., no catalytic reaction occurred between naphthalene and Pt-Ti O in the
2
1
6
absence of light. Upon irradiation, only the signal of C O (m/z = 44) significantly increased, indicating the
2
photocatalytic mineralization of naphthalene without the incorporation of oxygen atoms from the lattice. However,
it should also be mentioned that oxides readily undergo lattice oxygen exchange with gaseous O , not only under UV
2
5
1
18
irradiation but in dark conditions as well . The absence of O-labelled CO was also observed in batch reactor
2
experiments where we increased the catalyst mass concentration 5-fold, collected the evolved gases in the headspace
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of the reaction vial and analyzed them via injection to the QMS to increase sensitivity. Civis and co-workers studied
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the photocatalytic decomposition of formic acid on the surface of Ti O by the means of Fourier-transform infrared
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absorption spectroscopy, and found that the process does not lead to the evolution of either C O O nor C O . On
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the contrary, Montoya and co-workers20, 50 did observe C O O evolution during the photocatalytic oxidation of
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benzene over Ti O in anaerobic conditions. Although we investigated the same kind of organic compounds, namely
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aromatic hydrocarbons, we believe that naphthalene could be photocatalytically oxidized by a different mechanism.
This can be argued from several angles, first, we observe a significant production of 1,1'-binaphthalene, which cannot
be accounted for by the bridging oxygen mechanism. Second, the one-electron standard redox potentials for the
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oxidation of benzene and naphthalene in acetonitrile are 2.65 and 1.85 V vs. NHE respectively . Thus, neglecting
the slight variation in aqueous solution, the one-electron oxidation of naphthalene by its direct reaction with holes
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(valence band potential: 2.68 V vs. NHE at pH 7) is thermodynamically favored with respect to the same reaction
for benzene. Hence, as long as the reaction does not fall into the Marcus inverted region, it will be significantly faster.
The observation by Fox et al. of increasing photocatalytic oxidation rates for substituted naphthalenes as the
exergonicity increased supports this notion 55. Accordingly, abstraction of an electron from naphthalene by the
valance band hole, which has a more positive potential, is likely to take place. And third, as shown in Figure 1,
naphthalene adsorbed significantly on the TiO surface, unlike benzene 56. Indeed, temperature programmed
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desorption measurements over rutile single crystals have shown a significantly stronger binding by naphthalene .
57
Notably, the direct-indirect hole transfer model (DT-IT) of Salvador et al , proposes that the specific pathway
depends on the degree of electronic interaction of the dissolved molecules with the semiconductor surface. The
relatively strong interaction of naphthalene with TiO leads us to conjecture that, although both mechanisms may be
2
operative, in contrast with benzene, the direct transfer of holes to naphthalene is favored with respect to the indirect,
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bridging oxygen mediated transfer. . Therefore, according to what we presented above; we propose that
naphthalenols are the product of naphthyl radical cation formation followed by its reaction with water. More
experimental evidences supporting this argument will be presented in the following sections.
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Figure 4. QMS signals for isotopologues of CO, CO , and O from the reforming of naphthalene over Pt-Ti O .
2
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Experimental conditions: reaction volume: 5 ml; [Naphthalene] = 230 µmol L , light intensity I -I =30 mWcm-
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[catalyst] = 2 g L , T = 25 °C, and irradiation with an Osram XBO 1000-Watt xenon arc lamp as a solar simulator
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.3.2 Scavengers and electron paramagnetic resonance (EPR) spin-trapping technique
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To determine the involvement of different active species in the photoreforming of naphthalene, we performed
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the reaction in the presence of two scavengers: potassium iodide (KI), which acts as a hole scavenger , and 2-
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methylpropan-2-ol ((CH ) COH, TBA), a known hydroxyl radical (˙푂퐻) scavenger . As shown in Figure 5 (a), the
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photocatalytic conversion of naphthalene is completely inhibited in the presence of KI, with a degradation rate
identical to that of the photolytic process. In contrast, the addition of TBA does not appreciably affect the
photocatalytic process, yielding a kinetic profile virtually identical to that in the absence of scavengers. These results
suggest that free hydroxyl radicals (˙푂퐻 ) present, have a minor contribution in the process. In contrast, the use of
free
a hole scavenger leads to a halt in the photocatalytic process, confirming that naphthalene’s disappearance is initiated
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by an oxidative half-reaction. Miller and Olejnik , as well as Beltran et al. , showed that neither TBA nor
bicarbonate ions, respectively, affect the degradation rate of different PAH compounds in the presence of hydroxyl
radicals in homogeneous systems. Therefore, they concluded that the photooxidation of these compounds was
initiated via the formation of a radical cation, without involving free hydroxyl radicals. Accordingly, our results are
in good agreement with their findings, despite the difference between both types of systems.
Complementarily, we monitored the formation of naphthalen-1-ol and naphthalen-2-ol during these
experiments, as shown in Figure 5 (b) and (c). Their kinetic profiles are consistent with those of naphthalene
degradation: while the addition of TBA does not significantly affect the formation of naphthalenols, the addition of
KI hinders naphthalene degradation almost completely due to the consumption of holes by this scavenger,
highlighting their role in the photoreforming process. This result is not only important from the mechanistic point of
view, but also for potential applications, since the simultaneous presence of hole-scavenging species in industrial
effluents may completely inhibit naphthalene reforming.
The involvement of ˙푂퐻free in photocatalytic reactions is the subject of a long-standing debate. Salvador has
provided strong arguments against the oxidative formation of ˙푂퐻free on the grounds of thermodynamic and kinetic
arguments, stating that their observation in photocatalytic systems must respond to the reduction of molecular oxygen
61.
However, recent works have challenged this idea, suggesting that the irradiation of anatase TiO does lead to the
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formation of ˙푂퐻 , although this is not the case for the rutile polymorph . We thus employed electron paramagnetic
free
resonance (EPR) spectroscopy to obtain additional insights regarding the role of ˙푂퐻free in our system. To this end,
we performed in-situ naphthalene photoreforming experiments in the presence of either TBA or KI, and the spin trap
5
,5-dimethyl-1-pyrroline N-oxide (DMPO). As expected, under dark conditions no radicals are detected. Upon
irradiation, however, the tested samples under all conditions lead to the appearance of a signal attributable to the
DMPO-OH adduct (four lines with relative intensities of 1:2:2:1), Figure 6. The simulation of the EPR spectrum
(
Figure S8) yields the spin Hamiltonian parameters a = 1.453 mT, a = 1.494 mT and g = 2.0059, in accordance
N H
with previously reported values for this adduct 63, 64. Notably, we observe the production of the ˙푂퐻 adduct in the
absence of molecular oxygen, providing evidence in favor of their formation from the oxidation of water 65.
Additionally, we could detect neither organic radicals nor their DMPO adducts, likely due to a relatively low
concentration of naphthalene and its intermediate radicals inside the EPR cell, together with a large multiplicity for
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the hyperfine splitting of the adducts. A similar conclusion was reached by Steenken et al., who could not detect
any signal from organic radicals during the reaction of naphthalene (and some of its derivatives) with both ˙푂퐻 and
푆푂 ˙ in aqueous solution, both in the presence and the absence of spin trapping agents.
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By performing the same experiment (i.e. aqueous DMPO with Pt-UV100) either in the absence or presence of
naphthalene, we obtained similar signals (Figure S9), showing that, under these conditions, naphthalene is not strong
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˙
푂퐻 scavenger. In the presence of TBA, however, the adduct signal shows a decrease of 35 % (Figure 6) and vanished
within 90 sec comparing to more than 240 sec in its absence (Figure S10), confirming TBA’s ˙푂퐻 scavenging
properties. Nevertheless, this lower quantity of available ˙푂퐻 radicals does not impact on naphthalene’s degradation
(
Figure 5), providing evidence against a degradation mechanism initiated by the attack of ˙푂퐻free.
The addition of KI to the system, on the other hand, practically nullifies the adduct formation, as could be
expected from the efficient hole consumption by this scavenger. Regarding the role of holes, although we can confirm
+
their importance, we cannot distinguish between a direct hole (h ) transfer to naphthalene or its indirect oxidation via
∙
surface-trapped holes ( ―푂퐻 ).
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Figure 5. Kinetic profiles for the disappearance of naphthalene (a), the formation of naphthalen-1-ol (b), and the
formation of naphthalen-2-ol (c) during the photocatalytic reforming of naphthalene in the presence of different
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scavengers over Pt-UV100. [catalyst] = 1 g L ; 156 µmol L aqueous solution of naphthalene; 20 mmol/L of KI and
TBA; UV illumination. The initial amount of naphthalene for all reactions was considered after 1 h dark adsorption.
Lines are guides for the eye.
Moreover, by means of an electrochemical degradation process, Li and Goel 66 showed that naphthalene
degradation was not affected by the presence of acetone and methanol as an ˙푂퐻 competitor, suggesting that
naphthalene degradation occurred as a result of direct electron transfer. Thus, they excluded the indirect oxidation
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via the ˙푂퐻. We applied a simple model by Luo et al. to determine the Gibbs free energy (ΔG°) for the single
electron transfer reaction from benzene, naphthalene, and anthracene to ˙푂퐻 to produce the corresponding radical
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cation and hydroxide ions. The calculated values are +24.6, +1.7, and -21.5 kcal mol , respectively. It is thus apparent
that by increasing the number of aromatic rings in the organic compound the reaction becomes more spontaneous.
Hence, naphthalene has a higher tendency to react with ˙푂퐻 forming a radical cation, rather than an adduct, as is the
case for benzene.
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Figure 6. EPR spectra recorded during the photocatalytic reforming of naphthalene after 15 seconds in the presence
of DMPO, DMPO- TBA, and DMPO-KI. [catalyst] = 1 g/L; 156 µmol/L aqueous solution of naphthalene; 20 mmol/L
of KI and TBA; irradiation using UV(A); microwave frequency: 9.51 GHz.
3
.3.3 Diffuse reflectance transient absorption spectroscopy
To evaluate the formation of short-lived species during the reaction, we turned to diffuse reflectance transient
absorption spectroscopy. Figure 7 shows the spectra obtained 300 ns after the laser excitation (355 nm, 6 mJ pulse-
1)
of anaerobic Pt-TiO slurries in the presence of (a) a (1:1) water/acetonitrile mixture and (b) naphthalene in the
2
same solvent. In the absence of naphthalene, excitation of Pt-TiO leads to a relatively featureless transient spectrum,
2
slightly increasing at short wavelengths. Since Pt acts as an electron sink, this signal can be readily attributed to
68
trapped holes, with an absorption maximum reported at around 430 nm. . On the contrary, in the presence of
naphthalene, we could identify at least two distinct bands: a very broad one between 540 and 670 nm, and a narrow
band at 685 nm. Since in this case, where both a hole scavenger (naphthalene) and an electron scavenger (Pt
nanoparticles) are present, these bands can be ascribed to transient species related to naphthalene. Indeed, an
absorption band at 685 nm has been attributed to the naphthalene radical cation in homogeneous photolysis
experiments.48, 69 Moreover, due to its high reactivity, the radical cation can readily react with a second naphthalene
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molecule, giving rise to a naphthalene dimer radical cation which shows an absorption centered at 580 nm. In
addition, we also note that electronic interactions between polyaromatic radical cations and the TiO surface have
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been attributed to the occurrence of new absorption bands, red-shifted with respect to the free species. 70 Taking this
into consideration, we attribute the broad transient spectrum of Figure 7 to a convolution of the signals of the free
naphthalene radical cation, the radical cation of its dimer, and the same ions interacting with the TiO surface.
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Figure 7. Diffuse reflectance transient absorption spectroscopy spectra obtained 300 ns after the laser excitation
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355 nm, 6 mJ pulse ) of anaerobic Pt-TiO slurries in the presence of (a) a (1:1) water/acetonitrile mixture and (b)
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naphthalene (NP) in the same solvent.
3
.3.4 Origin of the evolved molecular hydrogen.
In order to identify the origin of the evolved H , we performed isotopic substitution studies on the
2
photoreforming of naphthalene, using in-line monitoring of H , HD, and D via quadrupole mass spectrometry
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(QMS), as shown in Figure S11.
The first evident effect of replacing H O by D O is a progressive decrease in the total amount of evolved gases
2
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as the D O fraction increases Figure 8. Considering the two extreme cases, pure H O and pure D O, the ratio between
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the amounts of evolved gases after 4h is ~1.3. Although we observed a smaller effect, similar behavior has been
observed for the photocatalytic reforming of formaldehyde 71 and benzene 14 over platinized TiO and interpreted on
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the basis of a hindered production of ˙푂퐻 radicals when using D O, which limited the rate of the reaction. In the
2
present case, however, we rule out that these radicals play an important role in naphthalene’s decomposition (Figure
6
). In our interpretation, therefore, the observed kinetic isotope effect responds to a rate-determining step related to
proton reduction 72, 73, although further experiments are necessary to ascertain this hypothesis.
With respect to the identity of the evolved gases, it is remarkable that neither H nor HD are detected when
2
using 100% D O. We note that similar results have been previously interpreted as proof that water (and not the SED)
2
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is the source of H in the photocatalytic reforming of methanol . However, proton exchange reactions in aqueous
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media are extremely fast, and thus it is also possible that the hydrogen atoms originate from the sacrificial donor.
Since the solvent’s isotopic species are present in an overwhelming majority, the exchange reaction could mask the
real origin of the atoms. On the other hand, recent reports have shown that in some photocatalytic systems the
production of H from SEDs can be sufficiently fast and spatially localized to avoid proton exchange, leading to
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isotopic compositions of the gases matching those of the sacrificial donor . From our analysis, we can conclude that
such effects are not operative in the present system.
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naphthalene and deuterated naphthalene over Pt-UV100 employing D O and H O. Conditions: reaction volume, 15
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mL; [Naphthalene] = 235 µmol L ; [catalyst] = 1 g L ; T = 25 °C; illumination with simulated solar light.
In the experiments performed with intermediate fractions of H O and D O (Figure S12), we observe, as
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expected from these arguments, a mixture of gases that approximately follows the isotopic composition of the solvent.
The distributions, however, are skewed towards protium-rich gases (i.e. primarily H , and in second term HD), due
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to the above-mentioned preferential protium reduction. A clear illustration of this effect is given by the 50% H O –
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0% D O composition, which yields H and HD in almost a similar amount, but no D .
2 2 2
Additionally, we studied the effect of using fully deuterated naphthalene, Figure 8. In agreement with the
previous experiments, the ratio between the amounts of evolved gases after 4h is 1.4. The photoreforming of
deuterated naphthalene in H O leads to the exclusive formation of H . In D O, obviously, only D is formed.
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Interestingly, the total amount of evolved gases in both solvents coincides, within experimental error, with the
amounts observed using non-deuterated naphthalene. This suggests that, in contrast with water, hydrogen abstraction
from naphthalene is not involved in the rate-determining step of the reaction. Similar conclusions have been drawn
for the photocatalytic reforming of aqueous benzene over platinized TiO2 14.
3
.4 Mechanism of the photocatalytic reforming of naphthalene
We now unify the acquired knowledge into a plausible mechanism for the photocatalytic reforming of
naphthalene, summarized in Scheme 1. Considering that naphthalene only absorbs light of wavelengths shorter than
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15 nm (Figure S4) , the use of simulated sunlight leads to the exclusive excitation of TiO , with the concomitant
2
generation of electron-hole pairs (Equation 6). From the EPR experiments and the associated photocatalytic runs in
the presence of scavengers, we postulate that ˙푂퐻free radicals do not play an important role in the initial reaction of
naphthalene. Naphthalene’s degradation, thus, is initiated either by a direct hole transfer or indirectly via its reaction
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with surface trapped holes ( ―푂퐻 ). This reaction can in principle follow two pathways: either a single electron
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transfer (SET) leading to the production of a radical cation (Equation 7), or a hydrogen abstraction reaction, leading
to a naphthyl radical (Equation 8). If the latter were to occur, however, we would expect a strong effect from the
presence of ˙푂퐻 , since they would promptly react with the naphthyl radicals to yield naphthalenol (Equation 9).
free
Instead, we propose the formation of the naphthalene radical cation (Equation 7), as we supported by the transient
absorption spectroscopy experiments. This carbocation radical swiftly reacts with water to form an OH adduct
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(Equation 10) 75, 76, in coincidence with the reported reactions of benzene with both persulfate and Pt-TiO in aqueous
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media 77, 78. The adduct itself reacts in either a further oxidation step (Equation 11), or a hydrogen abstraction by a
hydrogen atom via a special type of “current doubling” (Equation 12) to form (as exemplified) naphthalen-1-ol (A),
or naphthalene-2-ol (B). In this stage, we excluded the current doubling mechanism from either the naphthalene
radical cation or the naphthalene radical for several reasons. The large difference between the oxidation potential of
single electron transfer (1.85 V vs NHE) for naphthalene comparing with the conduction band ( -0.5 V vs NHE) 79
of the TiO does not allow the naphthyl radical cation to inject its unpaired electron in the conduction band of the
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TiO . Moreover, the ionization energy of the naphthalene radical cation is much larger than that for naphthalene
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81
(
around 8.0 eV ), which requires a much higher potential to transfer the unpaired electron . This is in line with the
observations of Hykaway et al., 82 who reported that naphthalene did not show any current doubling effect during its
oxidation to a carbocation radical on the surface of the TiO electrode.
2
Moreover, by using isotopically labeled TiO we conclude that the surface oxygen atoms are not incorporated
2
in the naphthalenols, thus leaving water as the source of oxygen in the products. We note that this mechanism is
conceptually equivalent to that proposed by Yoshida et al. 14 for the photocatalytic reforming of benzene, although
we consider here the TiO surface only implicitly.
2
The only non-oxygenated intermediated we observed during the photooxidation of naphthalene over Pt-UV100
is 1,1'-binaphthalene (H). Its formation can be explained by the coupling of two naphthyl radical cations accompanied
by proton release (Equation 13), or of one radical cation with the mother compound (Equation 14) The equivalent
compound biphenyl has previously been observed in the anaerobic photocatalytic oxidation of benzene by Hashimoto
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et al, . Thus, the formation of this compound supports the SET mechanism.
The naphthalenols are susceptible to be further oxidized, by a similar mechanism, to different diols, of which
we only observed naphthalene-1,4-diol (C) in significant amounts. In turn, the diols can be readily oxidized to form
the respective naphthoquinones, such as the detected naphthalene-1,4-dione (D). A new hydroxylation step yields
12
another of the detected intermediates, 5-hydroxynaphthalene-1,4-dione (F) . Alternatively, D can after successive
oxidation steps, undergo a ring cleavage reaction to produce phthalic acid (K), which upon alkylation with alkyl
radicals (•R) can produce the detected dialkylphthalates (E). The formation of the esters could be aided by the
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production of carboxylate radicals in the first step of the photo-Kolbe reaction, and their subsequent coupling with
8
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11
alkyl radicals (Equations 15 and 16). In fact, Zhang et al. found that phthalate esters are the main intermediates
during the photooxidation of phenanthrene over TiO suspensions. Moreover, this reaction has been reported in the
2
literature during the degradation of different PAHs in homogeneous systems 85, 86. The ring-opening reactions and
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further oxidations eventually lead to short-chain carboxylic acids, of which we observed formic and acetic acids.
Their decarboxylation via a photo-Kolbe reaction 87 can thus be the source of the alkyl radicals.
As noted above, we detected the 1,4-diol (C), but not the 1,2- isomer. The reactivity of such diols is however
very high. In the case of the anaerobic degradation of benzene, it has been reported that the equivalent diol undergoes
a rapid ring cleavage reaction to yield muconic acid, which upon successive oxidation yields CO2 77. We thus interpret
the absence of the 1,2-diol as proof of its high reactivity, instead of its lack of formation. A similar argument applies
to naphthalene-1,2-dione (J).
0
1
2
3
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9
0
1
2
3
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8
9
0
1
2
3
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8
9
0
1
2
3
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9
0
1
2
3
4
5
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7
8
9
0
Pt/TiO + hv
Pt/TiO2 h+ + e-
(6)
2
h+ / –OHS
(+ OHS )
(7)
h+ / –OHS
H+
(8)
+
OH
(9)
+
+
OHfree
H
OH2+
H
OH
(10)
H+
H O
2
+
OH
H
H
OH
h+ / –OH
(11)
(12)
S
H+
OH
+
OH
+
H(ads)
+ H ( )
2
2
+
2H+
(13)
(14)
+
2H+
+
Looking at the reductive side, the photogenerated electrons have a strong tendency to accumulate in the
platinum particles. Once there, they can reduce protons to hydrogen atoms (Equation 17), which, upon dimerization,
form H (Equation 18). Notably, although the successive oxidation of naphthalene may yield protons (e.g. Equations
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Error! Reference source not found. and Error! Reference source not found.), the fast exchange reaction with
water (Equation 19) implies that the origin of the protons that eventually are reduced may be indistinguishable.
In the same way, as the organic intermediates can compete with naphthalene for holes, they can also compete
for the photogenerated electrons. This explains the observation of partially reduced compounds, such as 1,2,3,4-
tetrahydro-1-naphthalenol (G) and 1,2,3,4-tetrahydro-1-naphthalenone (I). Additionally, the reduction may not be
direct, but rather related to the hydrogen atoms produced by Equation 12. For instance, Yoshida et al. have shown
4
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
4
their direct involvement in the photoreforming of benzene .
(
15)
16)
-
+
R'CO2 + h
[ R'CO2
]
'R + CO2
(
RCO2 + 'R
RCO2'R
-
Pt/TiO e + H
Pt/TiO + H
(ads)
(17)
(18)
(19)
2
(ads)
2
2H(ads)
H ( )
2
H O + 'H
HO'H + H(aq)
2
(aq)
4.
Conclusions
We have studied the photocatalytic degradation of naphthalene, the most abundant polycyclic aromatic
hydrocarbon in water, coupled with the production of molecular hydrogen, a potential energy vector for a sunlight-
based economy. Although the photocatalyst TiO initiates the degradation of naphthalene in anaerobic conditions
2
under simulated sunlight irradiation, the evolution of molecular hydrogen requires the presence of a platinum co-
catalyst. By applying a combination of chromatographic and mass spectroscopic techniques, we have performed
close monitoring of the stable reaction intermediates, finding that 1- and 2-naphthalenol, acetic acid, and formic acid
are the ones present in largest concentrations in the solution, while H and CO are the only gaseous products. Many
2
2
possible pathways for photoinduced reforming of naphthalene involving different reactive species on the surface of
18
18
photoexcited Pt/TiO have been tested. The use of Ti O did not lead to a significant evolution of O-enriched CO ,
2
2
2
suggesting that lattice oxygen atoms are not incorporated into this product. In addition, experiments using solvents
with variable H O - D O compositions and deuterated naphthalene led to an isotopic composition of the evolved
2
2
hydrogen matching those of the solvent, and not that of naphthalene. By analyzing the reaction rates, we further
determined that water (but not naphthalene) appears to be involved in the rate-determining step of the reaction.
Furthermore, by performing photocatalytic experiments in the presence of a hole and hydroxyl radical scavengers,
we determined that only the former plays a significant role in the oxidation of naphthalene. Complementary spin
trapping experiments using electron paramagnetic resonance spectroscopy confirm this notion. Moreover, transient
absorption spectroscopy allowed us to detect the radical cations of naphthalene and its dimer as short-lived species
in the nanosecond time scale, in addition to their interaction with the Pt/TiO surface. On the basis of the results, the
2
most probable reaction route is the formation of naphthalene radical cation via SET from naphthalene, scavenging
the photogenerated hole in the Pt/TiO surface. This carbocation radical swiftly reacts with water to form an OH
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adduct which furtherly oxidized by another hole to produce the hydroxylated naphthalene. Concomitantly, the
photogenerated electrons reduced the proton originated mainly from water to molecular hydrogen.
0
1
2
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9
0
1
2
3
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0
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9
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h+
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
4
4
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4
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5
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5
5
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6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
1
2
3
4
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7
8
9
0
H
O
H 2
2O
H +
+
-2H
H+
-
-
H
H
H
OH
H
OH
+3H
+
OH
3e-
h+
-
H+
-H+
h+
(H)
OH
+
(G)
+
4H
4e
OH
+ 2h+
2H
-
(A)
(B)
+
2h /H O
O
2
-
2H+
+
+
-
4H 4h /H O
2
OH
(
I)
O
O
(C)
+
OH
6H+
6h /H O
2
-
(
J)
2h+
d e c a r b
2H+
r i n g
-
O
O
O
o p e n
o x y l a t i o n
h+
2
i n g
H O/-2H+
ring
decarb
2
open
O
(
F)
OH
oxylation
ing
(D)
O
O
OH
OH
O
O
+
R
OR
OR
(K)
ti
on
)
(E)
oxida
reaction
further lbe
Long chain carboxylic acid
and alcohol
(
photo-ko
O
O
+
further oxidation
further oxidation
H C
3
OH
H
OH
CO2 + H2
Scheme 1 Proposed mechanism for the photocatalytic reforming of naphthalene over Pt-UV100 under simulated
sunlight. Note: Compounds inside dashed square were not detected during the by-products identification.
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Supporting information
The Supporting Information is available free of charge at https://pubs.acs.org.
Material characterization (BET surface area, XRD, TEM and EPR), scheme of samples preparation,
photonic efficiency determination, naphthalene dark adsorption, photocatalyst reusability, conditions
for the HPLC separations, QMS signal of Photocatalytic CO and H evolution, retention time and
0
1
2
3
4
5
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9
0
1
2
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0
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0
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9
0
1
2
3
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0
2
2
ions peaks for the detected compounds by GC-MS, identified intermediates in aerobic conditions from
previous study, EPR spectra, QMS signals for the photocatalytic H , HD, and D evolution, and
2
2
UV−Vis absorption spectra for naphthalene solution.
Acknowledgments
Financial support from the Katholischer Akademischer Ausländer-Dienst (KAAD) is gratefully acknowledged
for providing a scholarship for Osama Al-Madanat to perform his Ph.D. Yamen Alsalka gratefully acknowledges the
financial support from the Deutscher Akademischer Austauschdienst (DAAD) and the Federal Foreign Office.
Mariano Curti is grateful to the DAAD together with the Ministerio de Educación, Cultura, Ciencia y Tecnología
(Argentina) for his ALEARG scholarship. Osama Al-Madanat wishes to thank Dr. Samia Ben Hammouda for the
helpful discussion while preparing the manuscript. The authors thank Mr. Carsten Günnemann for helping with the
transient absorption spectroscopy analysis. This work was supported by Saint-Petersburg State University via a
research grant ID 32706707. Financial support from the Global Research Laboratory Program (2014 K1 A1
A2041044), Korea Government (MSIP) through NFR is gratefully acknowledged.
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