Bifunctional property of Pt nanoparticles deposited on TiO2 for the photocatalytic sp3C-sp3C cross-coupling reactions between THF and alkanes

Article abstract of DOI:10.1039/c7cy00535k

The photocatalytic sp³C-sp³C cross-coupling between tetrahydrofuran (THF) and various alkanes was accomplished with Pt loaded titanium oxide (Pt/TiO₂) photocatalysts. The cross-coupling between THF and cyclohexane was systematically studied, which revealed that the reaction followed two routes: the main course was the photooxidation of both substrates on a Pt/TiO₂ photocatalyst to generate radical species followed by their successive coupling; meanwhile, the minor one was a hybrid of photocatalysis by Pt/TiO₂ and thermocatalysis by Pt metal nanoparticles. The activity of the Pt catalysis was suggested to consist in the activation of an sp³C-H bond in THF or alkane molecules adsorbed on its surface and promote the reaction between the activated molecules and photogenerated radical species. Thus, the Pt nanoparticles on TiO₂ were believed to play a bifunctional role of an electron receiver as well as a metal catalyst.

Full text of DOI:10.1039/c7cy00535k

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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
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Bifunctional property of Pt nanoparticles deposited on titanium
oxide for the photocatalytic sp3C–sp3C cross-coupling reactions
between tetrahydrofuran and alkanes†
Received 00th January 20xx,
Accepted 00th January 20xx
Akanksha Tyagi,a Akira Yamamoto,a,b Tatsuhisa Kato,a,c and Hisao Yoshidaa,b
*
DOI: 10.1039/x0xx00000x
The photocatalytic sp3C–sp3C cross-coupling between tetrahydrofuran (THF) and various alkanes was accomplished with Pt
loaded titanium oxide (Pt/TiO2) photocatalysts. The cross-coupling between THF and cyclohexane was systematically
studied, which revealed that the reaction followed two routes: the main course was the photooxidation of both substrates
on the Pt/TiO2 photocatalyst to generate radical species followed by their successive coupling; while a minor one was a
hybrid of photocatalysis by Pt/TiO2 and thermal catalysis by Pt metal nanoparticles. The nature of the Pt catalysis was
suggested to be the activation of an sp3C–H bond in THF or alkane molecule adsorbed on its surface and promote the
reaction between the activated molecule and photogenerated radical species. Thus, the Pt nanoparticles on TiO2 were
believed to have a bifunctional role of an electron receiver as well as a metal catalyst.
www.rsc.org/
recombination with holes.36–38 Thus, the primary role of the
metal particles deposited on TiO2 is to act as an electron
receiver.
1. Introduction
Numerous issues of the current century like sustainable
energy sources,1–3 carbon recycling,4–7 alternative carbon
source,8–12 clean air and water,13–17 green chemical
synthesis,18–24 etc. have been successfully addressed by
Additionally, the metals can also participate catalytically in a
photocatalytic reaction thus acting as co-catalysts.39-46 For
example, in our previous work on the direct photocatalytic
cross-coupling between various ethers and benzene with a
Pd/TiO2 photocatalyst, the Pd nanoparticles were found to
have a bifunctional role.39 Apart from acting as an electron
receiver on TiO2, they catalyzed a dark reaction between a
photogenerated ether radical and benzene. Thus the reaction
between ether and benzene can be recognized as a hybrid of
TiO2 photocatalysis and Pd metal catalysis. Intrigued by this
bifunctional role of metal nanoparticles on TiO2 in that
reaction, we have explored its scope in other cross-couplings.
In the present work, we report the sp3C–sp3C cross-coupling
between tetrahydrofuran (THF) and various alkanes, like
cyclohexane, with Pt/TiO2 photocatalyst (Scheme 1). The direct
activation of an sp3C–H bond to make new C–C bonds is one of
the most desired reactions in synthetic organic chemistry.47-49
There are several reports on the cross-coupling reactions
between cyclic ethers like THF and alkanes or aromatics.50-55
However, all of them have some drawbacks e.g. use of a strong
base,50,51 a metal complex as catalyst,50,52,53 a large amount of
metal co-catalyst,53,55 high reaction temperature52,53,55 and so
on. Usage of such additives makes the techniques less green,
hence, a more sustainable methodology has to be developed
to carry out such reactions. In the present work, we have
employed a heterogeneous metal loaded titanium oxide
photocatalyst and found that the photocatalyst can promote
the cross-coupling reactions through C–C bond formation. We
are interested in elucidating the role of the deposited metal
particles which are crucial for the reaction. Different
heterogeneous
photocatalysis.
Among
different
photocatalysts, TiO2 remains most alluring due to its high
catalytic activity, low cost, non-toxicity, and long-term
stability.25,26 However, it is pointed out that the rapid
recombination of the photogenerated electrons and holes in
TiO2, results in low quantum efficiency. Several methods have
been adopted to overcome this limitation;27 like introducing
surface or bulk irregularities,28,29 constructing heterojunction
with other semiconductors or carbon materials,30–32 adding a
metal or non-metal dopant,33,34 deposition of noble metal
particles,35 and so on. Among these, the most common
strategy is the deposition of noble metal particles on TiO2,
which aligns the Fermi level of the metal with the conduction
band of TiO2 and causes the transfer of the photoexcited
electrons from TiO2 to metal, thereby decreasing their
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techniques like XAFS analysis, ESR measurement, isotopic cross-coupling product from THF and cyclohexane, 1a in Table
experiments, and temperature controlled reactions were used 2, was synthesized in the manner reported in the literature†.57
to elucidate the reaction mechanism. The combined
observations were used to propose a bifunctional role of Pt 2.3.2. Procedure for the reaction tests
nanoparticles on TiO2 for the cross-coupling reactions. These
For the photocatalytic reaction tests, a Pyrex test tube and
findings would be helpful to design efficient catalytic systems.
the xenon lamp served as the reactor and light source,
respectively. An optical filter was used to control the
wavelength of light to
≥ 350 nm. A photocatalytic reaction test
O
O
Pt(0.1)/TiO2, Ar
350 nm
consisted of the following steps; pre-treatment of the
M(x)/TiO2 sample in the reactor by irradiation with the xenon
lamp light for 1 h, sealing of the reactor with silicon septum
before 10 min purging with argon, the introduction of the
reactants by syringe, and photoirradiation with stirring for the
desired time. After the reaction, the analysis of the gaseous
products was done by taking a small portion of the gaseous
phase in an air-tight syringe and injecting into a GC-TCD
(Shimadzu GC-8A). The liquid phase was diluted by ethanol,
filtrated by a syringe equipped with a PTFE filter, and analyzed
by a GC-MS (Shimadzu, GC-MS-QP5050A). The amount of all
products, except 1a and 1e, were determined from calibration
H2
THF
Cyclohexane
2-cyclohexyltetrahydrofuran
Scheme 1 Direct cross-coupling reaction between THF and
cyclohexane with Pt/TiO2 photocatalyst
2. Experimental
2.1. Preparation of catalysts
A TiO2 sample obtained from Catalysis Society of Japan as
JRC-TIO-8 (anatase, specific surface area = 338 m2/g), was
utilized to prepare the various metal loaded TiO2
photocatalysts. The samples were prepared by a photo-
deposition method as follows. 4 g of the TiO2 sample was
dispersed in ion-exchanged water and stirred under
photoirradiation for 30 min from a xenon lamp (PE300 BUV),
followed by the addition of methanol and different metal
precursor solutions like H2PtCl6 for Pt, HAuCl4.4H2O for Au,
RhCl3.3H2O for Rh and PdCl2 for Pd. Then the resultant
suspension was stirred for 15 min in the dark, and 1 h in the
light. Finally, the suspension was filtered, thoroughly washed
with water and dried in the oven at 325 K for 12 h. The
samples so obtained were labelled as M(x)/TiO2, where x
denotes the loading amount of the metal (M) in weight%.
curves for reference compounds which have
a similar
molecular structure to the products. So there are errors to
some extent in the amounts due to the difference in the
sensitivity of actual compounds and the reference compounds
used here. The details are shown in each Table.
2.3.3. Mechanistic studies
The reaction between THF and cyclohexane was employed to
elucidate the mechanism of the sp3C–sp3C cross-coupling
reactions.
An ESR study validated the formation of radical species from
cyclohexane in the presence of UV light and the Pt(0.1)/TiO2
sample. The ESR measurements were performed at room
temperature with an X-band spectrometer (JEOL-RE2X) by
using JEOL's TE011 cavity and by the aid of a spin trapping
2.2. Characterization of catalysts
agent,
PBN
(N-tert-butyl-α-phenylnitrone).
For
the
A CO pulse adsorption analysis was used to determine the
dispersion and the size of the Pt and Pd particles on TiO2. Pt
L3-edge XAFS (X-ray absorption fine structure) analysis
revealed the state of Pt nanoparticles on the Pt/TiO2 sample
recovered after the reaction. The spectra were recorded at
BL9C of Photon Factory at the Institute of Materials Structure
Science, High Energy Accelerator Research Organization (KEK-
PF, Tsukuba Japan). A Pt foil was measured in the transmission
mode, whereas the fluorescence mode was used for the
Pt/TiO2 sample. The Athena software was used to analyze the
obtained spectra.56
measurements, a portion of the suspension of the Pt(0.1)/TiO2
sample (0.02 g) in a cyclohexane solution (1 ml) with PBN (0.09
g) was introduced into the ESR cavity, and then, the cavity was
irradiated with the light of wavelength >400 nm from another
xenon lamp (Asahi Spectra MAX-150) for about 20 min with
simultaneous measurement at room temperature†.
Additionally, kinetic experiments, performed with isotopically
labeled compounds such as deuterated THF and deuterated
cyclohexane, and temperature controlled photocatalytic
reactions gave further insight into the reaction mechanism.
2.3. Reaction tests
2.3.1. Materials
3. Results and Discussion
3.1. Characterization of catalysts
All of the chemicals were of analytical grade and were used
without further purification (cyclohexane, cyclopentane, n-
hexane and 1,1’-bicyclohexyl (Nacalai Tesque, 99.7%)), THF
(Wako Pure Chemicals, 99%), cyclohexane-d12 (Aldrich, 99.6
atom%) and THF-d8 (Euriso-Top)). An authentic sample of the
3.1.1. Metal dispersion and particle size
As mentioned before, CO pulse adsorption analysis was
employed to determine the dispersion and the size of Pt and
Pd particles on the M(x)/TiO2 samples (Table 1). Well dispersed
Pt nanoparticles were obtained on the Pt(0.1)/TiO2 sample
(Table 1, entry 1). On the other hand, for the same loading
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amount, poorer dispersion and a larger particle size were phase: octahydro-2,2’-bifuran (1b, 2.4 μmol) and octahydro-
obtained for Pd particles on the Pd(0.1)/TiO2 (Table 1, entry 2). 2,3’-bifuran (1c, 1.9 μmol) (Table 2, entry 1). Hydrogen was the
sole product observed in the gaseous phase. The other by-
product observed in the liquid phase was 2-
Table 1 Dispersion and average particle size of Pt and Pd particles
butyltetrahydrofuran (1d, 0.7 μmol), which would be formed
by the deoxygenation of 1b by the produced hydrogen. A
on TiO2 estimated by the CO adsorption method
similar reaction with cyclohexane gave 1,1’-bicyclohexyl (1e
)
Metal dispersion
(%)
Particle size
(nm)
Entry
Sample
and hydrogen (Table 2, entry 2). The absence of light or the
Pt(0.1)/TiO2 sample did not result in any product formation,
proving that the homo-coupling reactions are photocatalytic.
Thus, it is suggested that the photogenerated holes on TiO2
can oxidize both THF and cyclohexane to generate the radical
species and proton (eq. 1 and 2) and the radical species can
combine to give the homo-coupling products.
1
2
Pt(0.1)/TiO2 42.6
Pd(0.1)/TiO2 19.8
2.7
5.6
3.1.2. State of Pt nanoparticles on TiO2
Pt LIII-edge XAFS analysis was used to determine the state of
Pt nanoparticles on the Pt/TiO2 sample recovered after the
reaction. Figure 1 shows the normalized Pt LIII-edge XANES of
the Pt(0.1)/TiO2 sample along with the reference samples of Pt
foil and PtO2. The results indicate that the state of Pt in the
Pt/TiO2 sample (Fig. 1b) was closer to that in Pt foil (Fig. 1a)
and very different from PtO2 (Fig. 1c). The photoexcitation of
the Pt/TiO2 sample generates electrons in the conduction band
of TiO2, which can be transferred to the deposited Pt
nanoparticles, making them electron rich. Thus, the state of Pt
nanoparticles in the Pt/TiO2 sample should be metallic. This
result also confirms that the Pt nanoparticles in Pt/TiO2 sample
can efficiently work as an electron receiver and reduce the
recombination of photoelectron and holes.
Then, with the same procedure as before, the reaction was
performed with a mixture of THF and cyclohexane, where the
volumes of them were even at first. This reaction gave 2-
cyclohexyltetrahydrofuran (1a) as the only cross-coupling
product along with 1b, 1c, and 1d, which decreased the
selectivity for 1a (Table 2, entry 3). It should be noted that 1e
was not detected in these conditions.
The amount of the two reactants were varied to improve the
selectivity for 1a. Reaction with a small amount of cyclohexane
did not give any 1a (Table 2, entry 4). On the other hand, the
reaction with a small amount of THF in cyclohexane gave 1a
with high selectivity (Table 2, entries 5 and 6). As entry 6 gave
the highest selectivity for 1a based on THF (STHF), this condition
was employed for further experiments.
(a)
(b)
(c)
The products were not observed in the reactions performed
without the light or the Pt/TiO2 sample (Table 2, entries 7 and
8), confirming that it is
photostability of the cross-coupling product 1a was also
studied and it was found that 1a exhibited high
a photocatalytic reaction. The
11550
11600
11650
11700
a
photochemical stability (Table S1)†. On the other hand, a 24 h
reaction between THF and cyclohexane with the Pt(0.1)/TiO2
sample revealed that the photocatalytic stability of 1a was not
high (Table S2)†. These results are discussed in detail in the
ESI†.
X-ray energy (eV)
Fig.1 Normalized Pt-LIII edge XANES spectra of (a) Pt foil (b)
Pt(0.1)/TiO2 sample after the reaction test and (c) PtO2
3.2. Reaction between THF and Cyclohexane
3.2.1. Reaction with Pt(0.1)/TiO2 sample
3.2.2. Reaction with M(x)/TiO2 samples
Table 3 shows the results of the reaction tests between THF
and cyclohexane with various M(x)/TiO2 samples under the
optimized conditions. Although all the samples gave 1e, the
formation of 1a proceeded only on the Pt and Au loaded TiO2
samples (Table 3, entries 1–4). The Pt(0.1)/TiO2 sample gave
the maximum amount of 1a with high selectivity STHF (Table 3,
Before the examination of the photocatalytic cross-coupling
between THF and cyclohexane with the M(x)/TiO2 samples, the
homo-coupling of the two reactants was tested. Under the UV
light irradiation and with the Pt(0.1)/TiO2 sample, the homo-
coupling of THF proceeded to give two isomers in the liquid
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Table 2 Photocatalytic reaction tests between THF and cyclohexane with the Pt(0.1)/TiO2 sample a
Journal Name
Entry
Reactants (ml)
Cyclohexane
Products (
μ
mol) b
1d
Selectivity c
THF
2.00
0.00
2.00
3.00
0.10
0.01
0.01
0.01
1a
-
1b+1c
1e
-
STHF
SCyH
1
2
0.00
2.00
2.00
0.01
3.00
3.00
3.00
3.00
2.7
-
1.3
-
-
-
-
5.2
0.0
0.0
0.3
3.9
0.0
0.0
-
-
> 99
-
3
0.4
0.0
1.9
1.4
0.0
0.0
19.2
6.7
1.5
0.0
0.0
0.0
0.7
0.0
0.5
0.0
0.0
0.0
2
4
-
51
> 99
-
5
80
26
-
6
7d
8e
-
-
a Reaction conditions: various amount of the reactants with 50 mg of the Pt(0.1)/TiO2 sample was used, reaction time was 1 h, wavelength of
incident light was ≥ 350 nm and the light intensity was 21 mW cm-2, reactor’s temperature was ca. 298 K. b Amount of 1a
, 1b, 1c and 1d was
determined from the calibration curve of 1a that was synthesized as per a literature†. Amount of 1e was determined from the calibration
curve of authentic sample of 1e purchased from the company. c Selectivity for 1a. STHF: Selectivity for 1a based on the converted THF was
calculated as: [100
calculated as: [100
without the Pt/TiO2 sample.
×
amount of 1a (μmol)]/ [(1a
+1b+1c+1d) (μmol)]; SCyH: Selectivity for 1a based on the converted cyclohexane was
×amount of 1a (μmol)]/ [(1a
+
1e) (μmol)]. d Reaction was performed in the dark. e Reaction was performed under irradiation but
entry 1). Pt loading was necessary for the cross-coupling gave only a small amount of 1a (Table 3, entry 5).
reaction, as the reactions performed with the bare TiO2 sample
Table 3 Photocatalytic reaction tests between THF and
cyclohexane with the M(x)/TiO2 samples a
Table 4 Effect of water on the reaction between THF and
cyclohexane with the Pt(0.1)/TiO2 photocatalyst a
Entry
Sample
Products (μmol)
Selectivity
Entry
Water (μl)
Products (μmol)
Selectivity
STHF SCyH
1a
1e
1a
1b+1c
1e
STHF
SCyH
26
1
2
3
4
5
0
2
1.41
3.91
> 99
> 99
> 99
> 99
-
27
41
51
74
-
1
2
Pt(0.1)/TiO2 1.41
Pd(0.1)/TiO2 0.00
Au(0.1)/TiO2 0.09
Rh(0.1)/TiO2 0.00
0.00
3.91
0.09
0.16
0.27
0.03
> 99
2.06
1.12
0.35
0
2.92
1.08
0.12
0
0.00
0.00
0.00
0.00
-
> 99
-
-
36
-
10
20
50
3
4
5 b
TiO2
0.02
> 99
40
a Reaction conditions: 0.01 ml (123
μmol) THF, 3 ml (27.7 mmol)
6 b
7 c
2
2
5.72
4.74
7.37
7.45
> 99
> 99
44
39
cyclohexane and 50 mg of the M(x)/TiO2 sample were used, for other
details see Table 2. b Reaction was done with the pristine TiO2.
a Reaction conditions: Various amount of water was added, 50 mg
of Pt/TiO2 photocatalyst was used, reaction temperature was ca.
308 K, for other details see Table 3. No by-product from THF was
b
c
3.2.3. Effect of water
obtained in these conditions. Reaction time was 2 h. Reaction
time was 4 h.
Effect of the addition of water to the reaction mixture was
also studied (Table 4). The introduction of a small amount of
water significantly improved the amount of 1a, but a further
increase was unproductive (Table 4, entries 1–5). On the other
hand, the formation of 1e gradually decreased with water
3.3. Scope of the reaction
The reaction of THF proceeded well with other cyclic and
acyclic alkanes (Scheme 2)†. The reaction between THF and
cyclopentane gave 2-cyclpentyltetrahydrofuran (2a) as the
only cross-coupling product (Table S3). On the other hand, in
the reaction between THF and n-hexane, only 2-
hexyltetrahydrofuran (3a) was detected (Table S4). Based on
the amount of the cross-coupling product formed in the three
reactions it can be proposed that the cyclic alkanes were more
reactive than acyclic one for these cross-coupling reactions.
which enhanced the selectivity of 1a
, SCyH based on
cyclohexane (Table 4, entries 2–5). For a fixed amount of
water, the amount of 1a increased with time and 4% yield
based on THF was achieved, albeit with low overall selectivity
(Table 4, entry 6). A further increase in the reaction time
decreased the amount of 1a (Table 4, entry 7).
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Signals from the sample
Mn2+
Mn2+
332
334
336
338
340
342
344
Magnetic Field/ mT
Scheme 2 Reaction of THF with different alkanes with Pt/TiO2
photocatalyst
Fig. 2 ESR spectrum of cyclohexyl radical with PBN as a spin trap in the
presence of the Pt/TiO2 photocatalyst under photoirradiation
3.4. Mechanistic Studies
3.3.2. Kinetic Experiments
The mechanism of the cross-coupling reactions was
elucidated by examining the reaction between THF and
cyclohexane with various experimental techniques as follows.
The next point to be clarified was which radical formation
process determines the rate of reaction. Table 5 shows the
results of the kinetic experiments between THF and
cyclohexane with the Pt(0.1)/TiO2 photocatalyst by using
deuterated compounds. When compared to the amount of 1a
3.4.1. ESR measurements
ESR measurements were carried out to evidence the obtained without any isotopically labeled molecule, it
generation of the radical species in the cross-coupling decreased in the case with deuterated cyclohexane (Table 5,
reaction†. Figure 2 shows the ESR spectrum of the cyclohexane entries 1 and 2). The value of kH/kD, a measure of the isotopic
solution with PBN under the UV light in the presence of the effect, was more than unity, which indicates a primary kinetic
Pt(0.1)/TiO2 photocatalyst. The spectrum consists of three isotopic effect (KIE), originating from the higher dissociation
broad signals between the two signals of Mn marker (Mn2+). energy of a C–D bond than that of a C–H bond.
Computer simulations unveiled that each line consisted of a
doublet. Thus, together, they represent triple-doublet reaction performed with deuterated THF (Table 5, entry 3).
generated by nitrogen and the α-hydrogen of PBN in its spin The value of kH/kD obtained for this reaction was less than
adduct with the cyclohexyl radical (species
in eq. S1)†. unity which symbolizes an inverse KIE.
On the other hand, a larger amount of 1a was obtained in the
a
a
Although the wavelength limitation did not allow the
observation of a hyperfine structure, the g-value and the
coupling constants of nitrogen and the α-hydrogen could be Table 5 Kinetic experiments for the reaction between THF and
calculated to be 2.0011 and AN= 1.43 mT, AH= 0.206 mT, cyclohexane with the Pt/TiO2 photocatalyst a
b
respectively.
Entry
Deuterated reactant
Amount of 1a
kH/kD
The measurements performed without the light or the
Pt/TiO2 sample did not give any signals in the ESR spectrum.
Thus, the formation of the cyclohexyl radical (eq. 2) with the
Pt/TiO2 sample under photoirradiation was evidenced.
On the other hand, efforts to trap the THF radicals by PBN
were in vain, because PBN rapidly undergoes
photodissociation in THF.58 However, in our previous study, we
found that the photogenerated holes on the Pd/TiO2
photocatalyst could oxidize diethyl ether (DEE, (C2H5)2O) to the
radical species.39 The radicals were successfully trapped and
identified by the ESR measurement. The reported result was
available to propose the photooxidation of THF to the radical
species (eq. 1).
(μmol)
1.2
1
2
3
None
-
Cyclohexane-d12
0.6
2.0
0.5
THF-d8
2.4
a Reaction conditions: 0.5 ml of THF, 1.5 ml of cyclohexane and 50 mg
of the Pt(0.1)/TiO2 photocatalyst were used for the reaction.
b
Remaining conditions were same as those in Table 3. kH/kD was
determined from the amount of 1a produced in entries 1 and 2 for
entry 2, and those in entries 1 and 3 for entry 3, respectively.
The difference of the dissociation energy can be used to
explain the observations in the kinetic experiments. The C–H
bond in cyclohexane is stronger than that in THF, making its
oxidation difficult than THF (C–H bond dissociation enthalpy in
cyclohexane is 416.3 kJ/mol and that in THF is 385.3 kJ/mol).59
So, if the reaction rate depends on the step involving the
cleavage of C–H bond, then the reaction of THF and
deuterated cyclohexane will give a normal KIE, kH/kD > 1 (Table
5, entry 2).
Thus, it was revealed that the cross-coupling reaction
between THF and cyclohexane with the Pt/TiO2 photocatalyst
involves the photooxidation of both substrates to
corresponding radicals by the photogenerated holes (eqn. 1
and 2).
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Here, it is proposed that the oxidation of THF and
cyclohexane would be a competitive process. The weak C–H
bond in THF would make its photooxidation faster than that of
cyclohexane. However, on replacement with the deuterated
THF, this process would slow down, due to the increased bond
strength of the C–D bond. At the same time, due to slightly
increased availability of holes, the competitive reaction, i.e.,
the oxidation of cyclohexane to the radical species, should be
accelerated in this condition. It is important that the reaction
rate of this rate-determining step would increase when the
rate of the competitive reaction decreases. The amount of 1a
thus increased would give an inverse KIE (Table 5, entry 3).
Therefore, it is suggested that the cross-coupling proceeds in
the radical coupling mechanism, and the photooxidation of
cyclohexane to generate cyclohexyl radical is the rate
determining step in the present reaction condition.
Table 6 Summary of Ea for reaction between THF and
cyclohexane with different photocatalysts
Entry Photocatalyst Ea,cross (kJ mol-1) Ea,homo (kJ mol-1)
1
2
Pt/TiO2
TiO2
65.5
21.3
37.0
33.8
There have been several reports about the activation of
various molecules on the surface of Pt metal nanoparticles.60-62
Based on those reports, it can be proposed that the nature of
Pt catalysis is the thermal activation of the C–H bond in THF or
cyclohexane molecules adsorbed on its surface. Such activated
molecules would more readily react with the photogenerated
radicals to give 1a, hence making the reaction a hybrid of
Pt/TiO2 photocatalysis and Pt metal catalysis.
When the pristine TiO2 was used, the photocatalytic activity
was low and the rate determining step using the thermal
energy should be a certain step during the dark processes after
the photoexcitation, as mentioned above. However, when the
Pt nanoparticles, deposited on the TiO2 surface, function as an
electron receiver in the photocatalytic reaction, the observable
rate determining step using the thermal energy would not be
any dark process mentioned above but the other step, i.e., the
catalytic process on the metal surface.
3.3.3. Temperature controlled experiments
It is generally acceptable that the high activity of the Pt/TiO2
photocatalyst would be due to the efficient function of the Pt
nanoparticles as an electron receiver. In the present study, we
propose that the high activity would be contributed also by the
function of the Pt nanoparticles as metal catalyst. To elucidate
the possibility of the metal catalysis by the Pt nanoparticles,
we studied the effect of temperature on the photocatalytic
reaction between THF and cyclohexane†.
3.3.5. Proposed mechanism
Scheme 3 shows the reaction mechanism proposed for the
reaction between THF and cyclohexane with the Pt/TiO2
photocatalyst, which is described as follows.
As shown in Table S5, the photocatalytic reaction with the
The excitation of TiO2 generates photoexcited electrons and
holes (1). The electrons would move to Pt nanoparticles and
the holes would reach to the TiO2 surface. The holes on the
surface can oxidize the adsorbed THF, C4H8O, and cyclohexane,
C6H12, molecules to generate the corresponding radicals
Pt(0.1)/TiO2 sample in the present condition gave 1a, 1b, and
1c as the main products. The produced amount of 1a increased
with the reaction temperature. Figure S1 shows the pseudo-
Arrhenius plot of these data, from which the apparent
activation energies for the products formation were
calculated. The apparent activation energy for the formation
of 1a (Ea,cross) was 65.5 kJ mol−1 (Table 6). This high value
suggests that acceleration of a thermal catalysis at high
temperatures increases the amount of 1a. The amount of the
homo-coupling product of THF also increased with
temperature, for which the value of Ea,homo was 37.0 kJ mol−1.
Similar experiments were performed with the pristine TiO2.
The value for 1a (Ea,cross) was found to be 21.3 kJ mol−1 while
that for homo-coupling products of THF (Ea,homo) was 33.8 kJ
mol−1. The similar values of Ea,homo for the homo-coupling of
THF on both the Pt/TiO2 and TiO2 samples indicate that this
process will be a purely photocatalytic process. The thermal
energy would be used for a certain dark process such as
migration of electrons and holes, surface reactions, and
desorption of products. On the other hand, the different
values of Ea,cross obtained for 1a suggest that formation of 1a
will involve the metal catalysis by the Pt nanoparticles.
Further, the reaction did not give any product at high
temperatures and without light, which clarifies that the
thermal catalysis by the Pt nanoparticles cannot proceed
without the Pt/TiO2 photocatalysis.
(•C4H7O and •C6H11
) and protons (2 and 3). The two radicals can
combine to give 1a (4), and the two protons can be reduced by
the photoexcited electron to form hydrogen on the surface of
the Pt nanoparticles (5). This is a radical coupling process
driven by two photons.
Simultaneously, the photogenerated radicals of THF and
cyclohexane can move to the Pt nanoparticles and react with
the adsorbed molecule of cyclohexane or THF, respectively.
Although the reaction between the radical species and
molecule may not take place naturally, the Pt metal catalyst
would assist the reaction between them by the activation of
C–H bond of the molecules. This provides additional pathway
for the formation of 1a (6 and 7), which is the hybrid route
consisting of the photocatalytic and catalytic steps. This is a
one photon process.
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ARTICLE
for Challenging Exploratory Research. A. Tyagi would like to
thank JICA for providing the scholarship under IITH-JICA
Friendship project to pursue Ph.D. in Japan.
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Graphical abstract
Pt loaded titanium oxide photocatalysts were utilized for the direct cross-coupling between
tetrahydrofuran and various alkanes with hydrogen evolution. A detailed investigation revealed that
the reaction was a hybrid of Pt/TiO2 photocatalysis and Pt metal catalysis. Thus, the Pt nanoparticles
were found to have a bifunctional role as an electron receiver and a metal catalyst.