Highly efficient visible-light photocatalytic ethane oxidation into ethyl hydroperoxide as a radical reservoir

Article abstract of DOI:10.1039/d1sc00694k

Photocatalytic ethane conversion into value-added chemicals is a great challenge especially under visible light irradiation. The production of ethyl hydroperoxide (CH₃CH₂OOH), which is a promising radical reservoir for regulating the oxidative stress in cells, is even more challenging due to its facile decomposition. Here, we demonstrated a design of a highly efficient visible-light-responsive photocatalyst, Au/WO₃, for ethane oxidation into CH₃CH₂OOH, achieving an impressive yield of 1887 μmol g_cat^?1in two hours under visible light irradiation at room temperature for the first time. Furthermore, thermal energy was introduced into the photocatalytic system to increase the driving force for ethane oxidation, enhancing CH₃CH₂OOH production by six times to 11?233 μmol g_cat^?1at 100 °C and achieving a significant apparent quantum efficiency of 17.9% at 450 nm. In addition, trapping active species and isotope-labeling reactants revealed the reaction pathway. These findings pave the way for scalable ethane conversion into CH₃CH₂OOH as a potential anticancer drug.

Full text of DOI:10.1039/d1sc00694k

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Highly efficient visible-light photocatalytic ethane
oxidation into ethyl hydroperoxide as a radical
reservoir†
Cite this: Chem. Sci., 2021, 12, 5825
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Yao Zhu,‡a Siyuan Fang,‡b Shaoqin Chen,a Youjie Tong,a Chunling Wanga
ab
*
and Yun Hang Hu
Photocatalytic ethane conversion into value-added chemicals is a great challenge especially under visible
light irradiation. The production of ethyl hydroperoxide (CH3CH2OOH), which is a promising radical
reservoir for regulating the oxidative stress in cells, is even more challenging due to its facile
decomposition. Here, we demonstrated
a design of a highly efficient visible-light-responsive
photocatalyst, Au/WO3, for ethane oxidation into CH3CH2OOH, achieving an impressive yield of 1887
ꢀ1
mmol gcat
in two hours under visible light irradiation at room temperature for the first time.
Furthermore, thermal energy was introduced into the photocatalytic system to increase the driving force
Received 4th February 2021
Accepted 17th March 2021
ꢀ1
for ethane oxidation, enhancing CH3CH2OOH production by six times to 11 233 mmol gcat at 100 ꢁC
and achieving a significant apparent quantum efficiency of 17.9% at 450 nm. In addition, trapping active
species and isotope-labeling reactants revealed the reaction pathway. These findings pave the way for
scalable ethane conversion into CH3CH2OOH as a potential anticancer drug.
DOI: 10.1039/d1sc00694k
rsc.li/chemical-science
the stable conversion of ethane into CH3CH2OOH is highly
desirable.
Introduction
Compared with traditional catalytic processes, photo-
catalytic ethane conversion is much more intriguing because it
can take place under a milder condition with a lower cost.1,16–25
So far, TiO2, ZnO, CeO2, MoO3, and V2O5 have been explored as
the photocatalysts for ethane oxidation,22–26 but only UV light
can be absorbed due to their large band gaps. However, in the
solar irradiation spectrum, UV light accounts for only 3%, while
visible light constitutes 44%.27,28 Thus, it is highly expected to
develop visible-light-responsive photocatalysts for ethane
oxidation. Here, WO3 is a promising candidate with a narrow
band gap (2.4–2.8 eV) and a high resistance to photo-corro-
sion.29,30 Nevertheless, pure WO3 generally shows a high charge
carrier recombination rate, requiring modications such as
loading cocatalysts, introducing dopants, and adjusting
morphologies.31–33 Modied WO3 has been successfully applied
in water splitting, pollutant degradation, and solar energy
conversion,34,35 and is desirable to achieve efficient ethane
oxidation under visible light irradiation. This is the rst
hypothesis of the present work.
Ethane, as the second abundant constituent of natural gas
(ꢂ10%) and one of the major pollutants emitted from both
biogenic and anthropogenic sources (ꢂ9.57 Tg per year), can be
utilized to produce value-added chemical substances such as
ethanol, acetaldehyde, acetic acid, ethylene, and hydrochloric
ether.2–4 Although ethyl hydroperoxide (CH3CH2OOH) could be
generated in ethane oxidation, it is generally regarded as an
intermediate species due to its facile decomposition.5,6
However, the synthesis of CH3CH2OOH is very attractive
because it is a reservoir for both reactive oxygen species (ROS)
such as HOx radicals and 1O2, and some carbon-containing free
radicals such as alkyl, alkyloxy, and alkylperoxyl radicals.6–9 In
the presence of a trace amount of enzyme, hematin, or some
metal ions, these active species can be released by CH3CH2OOH
to regulate the oxidative stress in the cell, as a promising
approach for cancer therapy.7–13 This will even outperform the
present photodynamic therapy techniques that are highly
dependent on the oxygen and external light.13–15 Thus, achieving
Being able to absorb visible light cannot ensure the effective
utilization of visible light to drive corresponding redox reac-
tions. For example, Pt-loaded black TiO2 could absorb visible
light, but it exhibited a negligible activity for water splitting
under visible light irradiation.36 Here, as revealed in our
previous work, though a narrow band gap of the photocatalyst
allows visible light absorption, it meanwhile causes the insuf-
cient driving force for the redox reactions, resulting in a poor
aSchool of Environmental Science and Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
bDepartment of Materials Science and Engineering, Michigan Technological University,
Houghton, Michigan 49931, USA. E-mail: yunhangh@mtu.edu
† Electronic supplementary information (ESI) available: Characterizations of
Au/WO3, schematic reactor, 1H-NMR and GC-MS spectra of the products. See
DOI: 10.1039/d1sc00694k
‡ These authors contributed equally.
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catalytic performance.37 The introduction of thermal energy
into the photocatalytic system, as a novel thermo-photo cata-
lytic approach, can effectively address this issue. This is because
the kinetic energy of the reactants increased by the temperature
rise can compensate the potential driving force for corre-
sponding redox reactions. As a result, the visible-light driven
photocatalytic activities were signicantly improved in a series
of chemical processes including water splitting,37–39 CO2
reforming of CH4,40 steam reforming of CH4,41 CO hydrogena-
tion,42 and CO2 hydrogenation.43 Based on our previous
successes, introducing thermal energy is expected to enhance
the efficiency of ethane oxidation under visible light irradiation,
which constitutes the second hypothesis of this work.
On top of these two hypotheses, revealing the reaction
pathway of photocatalytic ethane oxidation is even more
essential. On the one hand, multiple products with various
oxidation degrees might be generated in ethane oxidation.
Elucidating the reaction pathway will help to exploit the strat-
egies to enhance the selectivity for the desirable product. On the
other hand, identifying the roles of each reactants (e.g., oxygen
and water) will contribute to the exploration of approaches to
accelerate the reaction. So far, only a few efforts have been
made, mainly based on the results of nuclear magnetic reso-
nance spectroscopy,44 in situ infrared spectroscopy and trapping
active species,25,45 whereas the solid and comprehensive inves-
tigation into the reaction mechanism has not been conducted
yet.
Fig. 1 TEM images of Au/WO3. (a) TEM image. (b) and (c) HRTEM
images. (d) HAADF-STEM image. (e) Au element EDX map. (f) W
element EDX map. (g) O element EDX map.
Au/WO3 was prepared by photo-depositing Au onto mono-
clinic WO3 nanoparticles. The Au loading amount was 0.33 wt%
as determined by ICP-MS. In spite of the small loading amount,
a weak peak corresponding to metallic Au (at 38.337ꢁ) was
observed in the XRD pattern (Fig. S1†).47 The morphological
structure of Au/WO3 is shown in Fig. 1a, where Au nanoparticles
with an average size of ꢂ16 nm dispersed on the surface of WO3
(see Fig. S2† for the size distribution of Au). Moreover, the d-
spacings of lattice fringes of 0.260, 0.230, and 0.202 nm were
recognized (Fig. 1b and c), attributed to WO3(202), Au(111), and
Au(200), respectively.47,48 Furthermore, elemental mapping
(Fig. 1d–g) revealed the uniform distribution of Au, W, and O.
The BET surface area and the BJH average pore diameter and
In this work, Au/WO3 was designed and synthesized as
a visible-light-responsive photocatalyst for visible-light driven
photocatalytic ethane oxidation in the presence of oxygen aꢀn1d
volume of this catalyst were determined as 4.51 m2 gꢀ1, 262 A,
ꢀ
water. An impressive yield of CH3CH2OOH (1887 mmol gcat
)
and 0.0368 cm3 gꢀ1, respectively (Fig. S3†). In addition, both
lattice oxygen (O2ꢀ) and adsorbed oxygen (Oꢀ and O2ꢀ) were
observed in the O 1s XPS spectrum (Fig. S4a†),48 while W existed
in the +6 state with the peaks for W 4f7/2 and W 4f5/2 shied to
higher values compared with those for pure WO3, indicating the
strong interaction between Au and WO3 (Fig. S4b†). The Au 4f
XPS spectrum of the catalyst was just the same as the standard
one of metallic Au (Fig. S4c†).47
was obtained in the two-hour visible-light driven photocatalytic
process at room temperature for the rst time, which was
further enhanced by six times via elevating the reaction
ꢁ
temperature to 100 C, with a high selectivity of 63.6% and an
apparent quantum efficiency up to 17.9% at 450 nm. More
importantly, the synergistic roles of the Au/WO3 catalyst, visible
light irradiation, and thermal energy in efficient ethane oxida-
tion into CH3CH2OOH, as well as the comprehensive reaction
mechanism of ethane conversion, were uncovered.
The UV-vis absorption spectra of Au/WO3 and WO3 are
shown in Fig. 2a. Compared with WO3, Au/WO3 showed an
enhanced light absorption from 450 to 700 nm with an
absorption peak at 550 nm, owing to the SPR effect of Au
nanoparticles.46 The band gap of Au/WO3 was then calculated to
be 2.43 eV, enabling a visible-light driven photocatalytic
process. To ensure that the thermodynamic requirements are
satised, the band structure of Au/WO3 was analysed as
following. The Mott–Schottky plots (Fig. S5†) indicated that the
Fermi level locates at 0.71 V (vs. NHE) while the valence-band
XPS spectrum (Fig. S4d†) showed a main absorption onset at
1.51 eV, by which the valence band maximum was estimated to
be 2.22 V (vs. NHE). Considering the band gap of 2.43 eV, the
conduction band minimum was then obtained as ꢀ0.21 V (vs.
NHE). Thereby, the band structure of Au/WO3 was depicted in
Fig. 2b, consistent with reported ones.46,49
Results and discussion
Design and synthesis of Au/WO3 as a visible-light-responsive
catalyst
There are three requirements for designing an efficient photo-
catalyst. First, the band structure of the catalyst must match the
redox potentials of the desirable reaction. Second, it can absorb
as much light as possible. Third, photo-generated electrons and
holes can be effectively separated to drive corresponding redox
reactions. In this light, to create an ideal catalyst for ethane
oxidation, we selected WO3 as an active component due to its
narrow band gap for visible light response and Au as a promoter
since it can not only assist charge transfer but also enhance
light harvest due to the surface plasmon resonance (SPR)
effect.30,46
Here, a series of reduction reactions would take place with
photo-generated electrons, forming cO2ꢀ, H2O2, and cOH. The
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reduction of gas-phase O2 cannot happen since its redox
potential (ꢀ0.33 V vs. NHE) is above the conduction band
minimum, but dissolved O2 can be reduced with a favorable
redox potential of ꢀ0.16 V vs. NHE.50–52 This implies the vital
importance of carrying out ethane oxidation in an aqueous
condition. Moreover, owing to this unique band structure, the
reduction of H+ into H2 is inhibited, effectively enhancing the
production of above oxidizing agents. Meanwhile, photo-
generated holes could oxidize water with cOH as the byprod-
uct. The formation of these ROS would make a great contribu-
tion to ethane oxidation.
Moreover, Au/WO3 showed a high efficiency for separating
photo-generated electrons and holes as demonstrated by its
stable photocurrent of 1.74 mA cmꢀ2 under visible light irradi-
ation, which was much higher than that of WO3 (1.23 mA cmꢀ2
)
(Fig. 2c). This agreed well with the result of linear sweep vol-
tammetry (Fig. 2d), namely, enhanced current density was
acquired on Au/WO3 compared with that of WO3. Therefore, the
aforementioned three requirements for designing a high-
performance photocatalyst are met, highlighting the great
Fig. 2 Optical and photoelectric properties of the catalyst. (a) UV-
visible absorption spectra of Au/WO3 and WO3. (b) Band structure of
Au/WO3 and redox potentials of possible reactions. (c) Photocurrent–
time curves of Au/WO3 and WO3. (d) Linear sweep voltammetry curves
of Au/WO3 and WO3.
Table 1 Reported photocatalytic activities of ethane oxidation
ꢀ1
Catalyst
ZnO
Oxidant
O2
Light
UV
Pressure
Temp.
Yield (mmol gcat hꢀ1
)
Ref.
1
Gas ow with C2H6 : O2 : He of 3 : 1 : 10
220 ꢁ
C
336 (CH3CHO)
280 (CO2)
28 (CO)
24 (HCHO)
11 (CH3CH2OH)
2400 (CH3CHO)
880 (HCHO)
200 (CO2)
MoO3/SiO2
O2
UV
Gas ow with C2H6 : O2 : He of 3 : 1 : 10
220 ꢁ
C
19
92 (CH3CH2OH)
72 (CH3CHO)
BiOX-POM
V2O5/SiO2
O2
O2
UV + visible
(cold white
LED lamp)
UV
Gas ow with C2H6 : O2 of 1 : 1
RT
20
21
Gas ow with C2H6 : O2 : He of 3 : 1 : 10
220 ꢁ
C
C
C
2480 (CH3CHO)
280 (HCHO)
80 (CO2)
40 (CO)
0.07 (CO2)
28 (CH3CHO)
5 (C2H5CHO)
3 (CH3CH2OH)
4 (CH3CHO)
3 (CO)
Au/TiO2
Pd–MoO3/SiO2
H2O
CO2
UV, 59 mW cmꢀ2
UV, 40 mW cmꢀ2
Gas ow with C2H6 : H2O : Ar of 1 : 1 : 8
Gas ow with C2H6 : CO2 of 1 : 1
RT
22
23
150 ꢁ
ZnO–TiO2/SiO2
CO2
UV, 40 mW cmꢀ2
Gas ow with C2H6 and CO2
200 ꢁ
25 (C2H5CHO)
24
8 (C4H10
)
4 (CH3CHO)
3 (CO)
Pd/TiO2
Au/WO3
CO2
O2
UV
0.1 MPa C2H6 and 0.1 MPa CO2/Ar
RT
231 (C2H4)
118 (CO)
22 (CH4)
944 (CH3CH2OOH)
311 (CH3CHO)
293 (CH3COOH)
5617 (CH3CH2OOH)
1799 (CH3CHO)
665 (CH3COOH)
449 (CH3CH2OH)
25
Visible (l >420 nm),
1.0 MPa 4 : 1 C2H6/N2 and 1.5 MPa O2
RT
This work
This work
100 mW cmꢀ2
100 ꢁ
C
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promise of Au/WO3 as a highly efficient photocatalyst for ethane driving force is highly expected to lead to highly efficient ethane
oxidation under visible light irradiation.
oxidation under visible light irradiation.
To demonstrate above hypothesis, photocatalytic ethane
ꢁ
oxidation was carried out at 50, 100, 150, 180, and 200 C. The
Visible-light driven photocatalytic ethane oxidation over Au/
WO3
results are shown in Fig.ꢀ31a, where the yield of CH3CH2OOH
reached 11 233 mmol gcat at 100 ꢁC, which was six times of
that obtained at room temperature. The production of CH3CHO
and CH3COOH was also enhanced, with a new high-value
product, CH3CH2OH, being obtained. These products were
As shown in Fig. S6,† photocatalytic ethane oxidation was
carried out in a sealed reactor under visible light irradiation (l
>420 nm, 100 mW cmꢀ2). Besides ethane and oxygen, water was
also introduced into the reactor to dissolve oxygen to enable
a thermodynamically favorable photocatalytic process. As ex-
pected, the two-hour room-temperature photocatalytic reaction
over Au/WO3 catalyst (35 ꢁC as measured by the thermocouple)
resulted in an impressive yield of CH3CH2OOH (1887 mmol
gcatꢀ1), together with some CH3CHO (621 mmol gcatꢀ1) and
CH3COOH (585 mmol gcatꢀ1), surpassing current reports (at
room temperature) even though their yields were obtained
under UV light or simulated sunlight irradiation (Table 1).
Here, considering the intrinsic issue of visible-light photo-
catalysis, namely, the contradiction between visible light
absorption and sufficient driving force, we propose that the
introduction of thermal energy, which can help to break the
kinetic limitation of the reaction, will greatly promote ethane
oxidation under visible light irradiation. By analysing the band
structure of Au/WO3, the driving forces for O2 reduction (i.e.,
eneꢀrgy difference between the redox potential of dissolved O2/
cO2 and the conduction band minimum) and H2O oxidation
(i.e., energy difference between the redox potential of cOH/H2O
and the valence band maximum) were found as small as 0.05
and 0.25 V, respectively. The introduction of thermal energy by
simply increasing the reaction temperature will enhance the
kinetic energy of the reactants, thus providing additional
kinetic driving force for the semi-reactions. The compensation
effect of the kinetic driving force with regards to the potential
1
further evidenced by H NMR (Fig. S7†) and GC-MS (Fig. S8–
S11†). The signicant production efficiencies outperform not
only all the reported values (Table 1), but also those obtained
with other catalysts under the same test condition (Fig. S12†),
indicating the superiority of the Au/WO3 catalyst. Herein, Au
contributed to both charge separation and light harvest due to
SPR as demonstrated by the fact that loading other metals
without evident SPR effect showed much poorer efficiencies.
However, although the increase of the Au loading amount from
0.08 to 0.33 wt% promoted ethane oxidation, excessive Au
loading of 0.69 wt% resulted in the decreased yield (Fig. S13†)
because Au nanoparticles could also act as the recombination
centers of electrons and holes.53,54 Furthermore, different from
other reports with CH3CHO as the major product, the selectivity
of CH3CH2OOH we obtained was as high as 63.6%, marking an
effective way to produce such an easy-to-decompose substance
which is promising for cancer therapy. As-produced liquid
could be further puried by column chromatography, etc., for
further application. The apparent quantum efficiency over Au/
WO3 at 100 ꢁC was measured as high as 17.9% at 450 nm, again
demonstrating the excellent ethane oxidation performance
achieved by thermo-photo catalysis. The decrease of the yield in
the temperature range from 150 to 200 ꢁC was due to the
overoxidation of ethane into CO2 and the decreased solubility of
feed gases, which were supported by the enhanced CO2 yield
and lowered total yield, respectively.
In contrast, as shown in Fig. 3b, in the process at 100 ꢁC
without illumination, negligible oxidation products were
detected, evidencing the little contribution of thermal catalysis
to ethane oxidation at this temperature. In addition, the activity
under simulated sunlight irradiation (AM1.5G) was just slightly
higher than that obtained under visible light irradiation, indi-
cating that the activity of ethane oxidation was directly corre-
lated to photo energy (as distributed by light wavelengths).
Furthermore, light irradiation with wavelengths above 510 nm
resulted in very low yields of ethane oxidation products because
these lights cannot be absorbed by Au/WO3 (with a band gap of
2.43 eV) to generate electrons and holes. This also implied the
very limited contribution of hot electrons of Au derived from the
plasmonic effect to ethane oxidation. These observations
demonstrated that ethane oxidation under illumination at
100 ꢁC still followed a photocatalytic pathway. Therefore, our
Fig. 3 Photocatalytic ethane oxidation over Au/WO3. (a) Visible-light hypothesis (that the introduction of thermal energy can accel-
driven ethane oxidation at various temperatures (reacting for 2 h in
erate the photocatalytic process of ethane oxidation under
visible light irradiation by enhancing the driving force) was
exactly conrmed.
Water plays a key role in photocatalytic ethane oxidation
since negligible product was detected in the reaction without
20 mL water). (b) Ethane oxidation under various lights (reacting for 2 h
in 20 mL water at 100 ꢁC). (c) Visible-light driven ethane oxidation with
various water volumes (reacting for 2 h at 100 ꢁC). (d) Visible-light
driven ethane oxidation with various reaction times (in 20 mL water at
100 ꢁC).
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intermediate to CH3CH2OOH. Besides, the conversion effi-
ciency of CH3CHO was just 60.0%, with CH3COOH as the
dominant product with a high selectivity of 91.3%, revealing
that CH3CHO was a key intermediate to CH3COOH. Only minor
amount of CH3COOH could be further converted. Above results
implied CH3CH2OOH was not mainly produced from CH3-
CH2OH, CH3CHO, and CH3COOH, but, as a primary product
directly generated from ethane.3,44
Fig. 4 Identification of the reaction pathway and active species in
visible-light driven photocatalytic ethane oxidation over Au/WO3. (a)
Conversion efficiency using ethanol, acetaldehyde, or acetic acid as
the reactant. (b) Normalized yield obtained with the employment of
trapping agents.
Moreover, to reveal the role of active species in photocatalytic
ethane oxidation, trapping agents, including p-benzoquinone
(p-BQ), tertiary butyl alcohol (TBA), and ethylenediaminetetra-
acetic acid disodium salt dihydrate (EDTA–2Na), were employed
to trap cO2ꢀ, cOH, and holes, respectively. A small addition of p-
BQ (10 mM) led to a 94.4% decrease in the total yield (Fig. 4b),
indicating the vital importanꢀce of cO2ꢀ. In the photocatalytic
reduction semi-reaction, cO2 was rst produced, by which
H2O2 and cOH could be formed successively. Therefore, trap-
water (Fig. 3c). Besides enabling a process with dissolved O2 as
the reduced species to meet the thermodynamic requirement of
photocatalysis, water also accelerates ethane oxidation via the
following two aspects. On the one hand, water is the source of
some ROS such as cOH and H2O2. On the other hand, water
could serve as an effective solvent for the oxidation products,
cleaning the catalyst surface and regenerating the active sites.
Both aspects could be promoted at an appropriately elevated
temperature due to the enhanced water dissociation (to form
OHꢀ which can be oxidized into cOH more easily) and the
accelerated desorption of the products. However, excessive
amount of water led to the decrease of the yield, because in such
a three-phase system, the contact between the feed gases and
catalyst could be suppressed by water. The prolongation of the
reaction from 2 to 10 hours showed a nearly linear enhance-
ment in ethane oxidation (Fig. 3d), indicating there was no
evident photo-corrosion. However, the time extension mainly
promoted the overoxidation into CH3COOH and CO2 while
lowered the selectivity of CH3CH2OOH. Thus, considering the
time effectiveness, activity, and selectivity, two-hour reaction
was the best.
ꢀ
ping cO2 would result in an evideꢀnt decrease of ROS and
thereby a low yield. Moreover, cO2 played a key role for
generating CH3CH2OOH, which would be revealed in the
following part. In contrast, the effect induced by trapping holes
was smaller, causing a 45.6% decrease of the total yield. As
proposed in Fig. 2b, holes would react with H2O to form cOH,
which could then contribute to ethane oxidation. Even though
holes were trapped, cOH could still be produced from the
reduction semi-reaction, thus the inuence on the yield was not
very much noticeable. Moreover, by trapping cOH, the produc-
tion of CH3CH2OOH was slightly affected while the yields of
CH3CHO and CH3COOH signicantly decreased, indicating
that cOH is directly correlated to the formation of CH3CHO and
CH3COOH but not CH3CH2OOH.
Furthermore, isotope labeling experiments with the use of
O182, H2O18, or D2O, were conducted to demonstrate the origin
of each product. The products detected by GC-MS are shown in
Table 2 (see Fig. S8–S11† for the GC-MS spectra). Employing
O18 resulted in the formation of CH3CH2O18O18H, CH3CH2-
2
O18H, CH3CHO18
,
CH3CO18O18H, CH3CO18OH, and CH3-
COO18H, while the use of H2O18 generated CH3CHO18
,
Reaction pathway
Since multiple oxidation products with various oxidation CH3CO18O18H, CH3CO18OH, and CH3COO18H. Moreover,
degrees were detected, visible-light driven ethane oxidation over replacing general water by D2O made the hydroperoxyl group of
Au/WO3 is highly possible to undergo a stepwise process. When CH3CH2OOH be fully deuterated and the carboxy group of
CH3COOH be partially deuterated.
According to the above results, the reaction pathway was
proposed as following. First of all, visible light was absorbed by
using CH3CH2OH as the reactant, its conversion efficiency was
as high as 96.9%, generating the same types of products as
those obtained from ethane oxidation (Fig. 4a), indicating that
CH3CH2OH was an important intermediate species. The major Au/WO3 to generate active electrons and holes (eqn (1)).
product was CH3COOH, while the generation of CH3CH2OOH, Considering the thermodynamic requirements of photo-
as the most abundant and desirable product, showed a poor catalysis (Fig. 2b), photo-generated electrons would transfer
selectivity of 13.2% (much below that of 63.6% in ethane from the conduction baꢀnd of WO3 to Au nanoparticle to reduce
oxidation), suggesting that CH3CH2OH might not be the main dissolved O2 into cO2 (eqn (2)), which could be further
Table 2 Products obtained in isotope labeling experiments
Reactant
CH3CH2OOH
CH3CH2OH
CH3CHO
CH3COOH
O18
CH3CH2O18O18
CH3CH2OOH
CH3CH2OOD
H
CH3CH2O18
CH3CH2OH
CH3CH2OH
H
CH3CHO18
CH3CHO18
CH3CHO
CH3CO18O18H, CH3CO18OH, CH3COO18H and CH3COOH
CH3CO18O18H, CH3CO18OH, CH3COO18H and CH3COOH
CH3COOD and CH3COOH
2
H2O18
D2O
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converted into H2O2 (eqn (3)) and cOH (eqn (4)), while photo- O atom(s) in the hydroxyl groups of CH3CH(OH)2 should be
generated holes remaining on WO3 would oxidize H2O into derived from H2O.
cOH (eqn (5)).
CH3CH2OH + 2cOH / CH3CHO + 2H2O
CH3CH2OOH / CH3CHO + H2O
CH3CHO + H2O # CH3CH(OH)2
(12)
(13)
(14)
Au/WO3 + hv / eꢀ + h+
(1)
(2)
(3)
ꢀ
O2 + eꢀ / cO2
cO2 + 2H+ + eꢀ / H2O2
ꢀ
CH3CH(OH)2 could be further oxidized into CH3COOH by
(4) cOH (eqn (15)), in which the H atom and at least one O atom(s)
H2O2 + H+ + eꢀ / cOH + H2O
H2O + h+ / cOH + H+
in the carboxy group originated from H2O, partially agreeing
with the result of isotope labeling experiments (Fig. S11d†).
(5)
Thus, there should be another pathway leading to the formation
of CH3COOH in which both O atoms came from O2 and the H
atom in the carboxy group was not derived from H2O. This
might be achieved via eqn (16) and (17).
CH3CH3 could react with either h+ (eqn (6)) or cOH (eqn (7))
to form an unstable intermediate radical CH3CH2c.25 The
participation of both h+ and cOH was supported by the result of
trapping experiments, namely, trapping h+ or cOH did not
completely inhibit the reaction. cO2ꢀ could be protonated by H+
in water to generate cOOH (eqn (8)),55,56 which then reacted with
CH3CH2c to form CH3CH2OOH (eqn (9)). Since cOOH and
CH3CH2c were generated on Au and WO3 respectively, the most
favourable site for CH3CH2OOH production was the interface of
Au and WO3. Such a pathway for CH3CH2OOH production
agreed well with the result of isotope labeling experiments,
namely, the O atoms in CH3CH2OOH were from O2 while the H
atom in the hydroperoxyl group originated from water. The
insertion of cO2ꢀ was the preliminary step for the generation of
CH3CH(OH)2 + 2cOH / CH3COOH + 2H2O
CH3CH2OH + 2cOOH / CH3CH(OOH)(OH) + H2O2 (16)
CH3CH(OH)(OOH) / CH3COOH + H2O (17)
(15)
The cleavage of the C–C bond occurred in CH3CH2OH and
CH3CHO because HCOOH and CH3OH were detected aer
photocatalytic oxidation of CH3CH2OH and CH3CHO (Fig. 4a).
The production of HCOOH from CH3CH2OH was proposed via
eqn (18) and (19), with the assistance of cOH. HCOOH could
also be resulted by the reaction between CH3CHO and cOH (eqn
(20)). cCH3 generated in eqn (18) and (20) could combine with
cOH into CH3OH (eqn (21)).
ꢀ
subsequent oxidation products, thus trapping cO2 led to
a sharp drop of the yields (Fig. 4b).
CH3CH3 + h+ / CH3CH2c + H+
(6)
(7)
(8)
(9)
CH3CH2OH + cOH / cCH3 + CH2(OH)2
CH2(OH)2 + 2cOH / HCOOH + 2H2O
CH3CHO + cOH / cCH3 + HCOOH
cCH3 + cOH / CH3OH
(18)
(19)
(20)
(21)
CH3CH3 + cOH / CH3CH2c + H2O
cO2 + H+ / cOOH
ꢀ
CH3CH2c + cOOH / CH3CH2OOH
Since the O atom in CH3CH2OH came from O2 rather than
H2O, it was most likely to be resulted with CH3CH2OOH as the
intermediate as shown in eqn (10) and (11). Here, eqn (11)
ensured that H2O was not the source of the H atom in the
hydroxy group. Eqn (10) and (11) could proceed reversibly
because CH3CH2OOH was detected as one of the products in
photocatalytic conversion of CH3CH2OH (Fig. 4a).
As-produced HCOOH could be decomposed into CO (eqn
(22)) or further oxidized into CO2 by cOH (eqn (23)).
HCOOH / CO + H2O
(22)
CH3CH2OOH # CH3CH2Oc + cOH
(10)
(11)
CH3CH2Oc + CH3CH3 # CH3CH2OH + CH3CH2c
CH3CHO could be produced either from CH3CH2OH (eqn
(12)) or directly from CH3CH2OOH (eqn (13)), by which the O
atom in CH3CHO originated from O2. Moreover, the detected
CH3CHO18 with H2O18 as the reactant was ascribed to the
reversible reaction between CH3CHO and CH3CH(OH)2 (eqn
1
(14)). The existence of CH3CH(OH)2 was evidenced by H NMR
spectrum (Fig. S7†). In this case, both H atoms and at least one
Fig. 5 Schematic reaction pathway for visible-light driven photo-
catalytic ethane oxidation over Au/WO3.
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HCOOH + 2cOH / CO2 + 2H2O
Chemical Science
mLꢀ1 HAuCl4 solution during vigorous stirring at 500 rpm (the
theoretical Au loading amount: 1 wt%). The mixture was
continuously stirred at 500 rpm for 20 min and then ultra-
sonicated for 20 min. Aerwards, the suspension was irradiated
by an ultraviolet lamp (100 W, l <400 nm) for 10 min while
being stirred at 600 rpm. In this process, the color of the
suspension was found to change from yellow to purple,
demonstrating the reduction of Au3+. The as-obtained Au/WO3
suspension was washed and then collected via suction ltration
with a 45 mm lter membrane. Lastly, the collected powder was
dried at 60 ꢁC for 12 h. Au/WO3 catalysts with 0.1, 0.5, and
1.5 wt% Au were also prepared, with 10, 50, and 150 mL HAuCl4
solutions, respectively. The real loading amount was conrmed
by inductively coupled plasma mass spectrometry (ICP-MS).
In addition, Au was also loaded on commercial TiO2, ZnO,
Bi2O3, MoO3, and CeO2 as control samples, using the same
method as above-described. Moreover, to investigate the inu-
ence of different metal cocatalysts, Pt, Pd, Ag, and Rh were
photo-deposited on WO3 with H2PtCl6, PdCl2, AgNO3, and
RhCl3 as the precursors, while Fe and Cu were loaded on WO3
via impregnation and subsequent calcination at 400 ꢁC for 6 h.
(23)
The reaction pathway was depicted in Fig. 5 based on above
discussions. It should be noted that the reactions took place at
the interface of Au and WO3 for relieving the limitation of mass
transfer. Moreover, it could be easily observed that cOH was
mainly responsible for further conversion of CH3CH2OOH. The
high selectivity for CH3CH2OOH here was owed to its fast
desorption from the catalyst surface due to the solvation effect
of water and the elevated temperature to 100 ꢁC, which
supressed its further oxidation. This provides a guidance to
develop more efficient photocatalytic processes for the
production of CH3CH2OOH.
Conclusions
In summary, highly efficient visible-light driven photocatalytic
ethane conversion into a radical reservoir (namely, CH3CH2-
OOH) was achieved over our created Au/WO3 catalyst. The yield
of CH3CH2OOH obtained at room temperature in two hours was
as high as 1887 mmol gcatꢀ1, which was further enhanced by six
times by simply elevating the temperature to 100 ꢁC. Moreover,
a signicant apparent quantum efficiency of 17.9% was ob-
tained at 450 nm. Such a promotion was owed to the compen-
sation effect of the kinetic energy of the reactants with regards
to the insufficient potential energy in photocatalytic ethane
oxidation over the narrow-band-gap Au/WO3 catalyst. Further-
more, the reaction pathway was revealed by converting inter-
mediate substances, trapping active species, and labeling
reactants with isotopes, implying that cOH plays a key role to
determine the selectivity and activity of CH3CH2OOH produc-
tion. Here, the excellent catalytic performance and the thorough
analysis of the reaction mechanism will pave the way for scal-
able ethane conversion into CH3CH2OOH, which shows a great
promise for cancer therapy by regulating the oxidative stress.
Catalyst characterization
X-ray diffraction (XRD) patterns were obtained from Shimadzu
X-ray diffractometer (XRD-6100). The structures and morphol-
ogies of the catalysts were characterized using a FEI Tecnai G2
F20 eld-emission transmission electron microscope (TEM) at
an accelerating voltage of 200 kV. Attached energy dispersive X-
ray spectroscopy (EDS) was employed for elemental analysis of
the as-synthesized samples. The surface areas of the samples
were measured and calculated using the multipoint Brunauer–
Emmett–Teller (BET) technique with a BET Sorptometer (ASAP
2020 PLUS HD88) based on the adsorption isotherms. The
chemical state and composition of the catalyst surface were
determined via an X-ray photoelectron spectroscopy (XPS)
instrument (Thermo EscaLab 250Xi). The element contents
were measured by inductively coupled plasma mass spectrom-
etry (ICP-MS) (Thermo iCAP Q). Ultraviolet-visible (UV-vis)
absorption spectra of the catalysts were obtained using an UV-
vis spectrometer (Shimadzu UV-2600). 1H nuclear magnetic
resonance (NMR) was conducted on a Bruker AVANCE III HD
400 NMR spectrometer using the liquid mixture obtained from
ethane oxidation with deuterated water (D2O).
Experimental
Materials
Monoclinic WO3 (99.8%, Macklin), P25 (99.5%, Sigma-Aldrich),
ZnO (99.9%, Macklin), Bi2O3 (99.99%, Macklin), MoO3 (99.50%,
Alfa Aesar), and CeO2 (99.99%, Macklin) were acquired to
synthesize the catalysts. Freshly prepared 2 mg mLꢀ1 HAuCl4
(Sigma-Aldrich) aqueous solution was used as Au precursor.
Other metal precursors included H2PtCl6 (Sigma-Aldrich), PdCl2
(Sigma-Aldrich), RhCl3 (Alfa Aesar), AgNO3 (Sinopharm),
Cu(NO3)2 (Sinopharm), and Fe(NO3)3 (Sinopharm). The free
radical scavengers, p-benzoquinone (97%), EDTA–2Na (AR), and
tertiary butyl alcohol (AR), were obtained from Macklin Co., Ltd.
Feed gases, ethane (80% ethane + 20% nitrogen) and oxygen
(99.99%), were supplied by Air Liquid Co., Ltd.
DFT calculations
DFT calculations were conducted using Gaussian 16 package.57
The geometric structure of the complex which contains one
organic molecule (i.e., CH3CH2OOH or CH3CH2OH) and two
water molecules was optimized using the B3LYP method58
combined with the 6-311G(d,p) basis set.59 The optimized
structures were conrmed to be stable by frequency calcula-
Catalyst preparation
1
tions. Aerwards, H NMR chemical shis were computed by
Au was loaded on WO3 via photo-deposition. Namely, 0.2000 g the gauge-independent atomic orbital (GIAO) model60 at the
WO3 was added into 40 mL deionized water to form a 5 g Lꢀ1 B3LYP/6-311++G(d,p) level,58,61 using the self-consistent reac-
suspension, followed by adding 100 mL freshly prepared 20 mg tion eld (SCRF)62 technique in simulated water continuum.
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The geometric structure and NMR shis of the reference tetra- 2Na), was added into the catalyst suspension for photocatalytic
methylsilane (TMS) were calculated using the same method.
activity test at 50 ꢁC to avoid the decomposition of scavengers at
high temperatures.
Photocatalytic activity test
Isotope labeling experiment
As shown in Fig. S6,† photocatalytic ethane oxidation was
carried out in a 100 mL sealed stainless steel reactor with
a concentric quartz window on the top. A 300 W xenon lamp
(CEL-HXF300, AULIGHT) equipped with an UVcut420 lter, as
the light source (l >420 nm), was placed right above the reactor
with the light intensity kept at 100 mW cmꢀ2. AM1.5G lter was
employed to simulate the sunlight irradiation in some cases. In
a typical test, 20 mg Au/WO3 and 20 mL deionized water were
added into the Teon vessel, aer which the reactor was vac-
uumed and purged with the C2H6/N2 mixture gas for three times
to remove any impurities in the system. Subsequently, 1.0 MPa
C2H6/N2 mixture (4 : 1) and 1.5 MPa O2 were injected into the
reactor. The photocatalytic reaction then started under visible
light irradiation (l >420 nm) at a certain temperature in the
range of 35–200 ꢁC during continuous stirring at 900 rpm. Aer
reacting for 2 h, the reactor was placed in an ice bath for 30 min
to ensure the termination of the reaction, and then as-produced
CO and CO2 were measured by a gas chromatograph (GC) (Fuli-
9800) equipped with a TDX-01 packed column and a thermal
conductive detector (TCD) working at 80 and 100 ꢁC, respec-
tively. Aerwards, the suspension was ltered, and the liquid
was collected for product detection. CH3CH2OH, CH3CHO, and
CH3OH were analyzed by GC with a Porapak Q column and
a ame ionization detector (FID) by injecting 1 mL liquid. The
temperatures of the injector, column, and FID detector were
150, 120, and 150 ꢁC, respectively. CH3CH2OOH, CH3COOH,
and HCOOH were quantied by a high-performance liquid
chromatograph (HPLC) (Shimadzu EssentiaꢁLC-16) with a C18
column (F 4.6 mm ꢃ 250 m) working at 35 C.
Isotope labeling experiments were conducted simply by
replacing general O2 and H2O with O182, H2O18, or D2O. The
reaction temperature was controlled at 100 ꢁC. The products
were analyzed by gas chromatography-mass spectrometry
(GC-MS) (Shimadzu QP 2020 NX) equipped with an RTX-WAX
column and an electron ionization source working at 70 eV.
The temperatures of the split injector, column, connector, and
ionization source were set as 120, 40, 120, and 200 ꢁC,
respectively.
Electrochemical measurement
The photocurrent–time (i–t) curve and linear sweep voltamme-
try (LSV) curve were recorded for Au/WO3 and WO3 in a three-
electrode cell with a 0.5 M Na2SO4 electrolyte under visible
light irradiation. A uorine-doped tin oxide glass coated with
catalyst, Pt, and saturated Ag/AgCl performed as the working,
counter, and reference electrodes, respectively. LSV measure-
ment was preformed from 1.2 to 0.3 V (vs. SHE) at a sweep rate
of 0.5 mV sꢀ1. Mott–Schottky analysis was carried out at 500,
1000, 1500, and 2000 Hz in the same cell in dark.
Author contributions
Conceptualization, YHH; methodology, YHH; investigation, YZ,
SF, SC, YT, CW and YHH; formal analysis, YZ, SF, and YHH;
writing – original dra, SF; writing – review & editing, YHH;
supervision, YHH.
The apparent quantum efficiency (AQE) was measured using
a band-pass lter of monochromatic light of 450 nm (l1/2 ¼ ꢂ20
nm) at 100 ꢁC following the same procedure as above described.
Conflicts of interest
All the products aer two-hour reaction were quantied, and There are no conicts to declare.
then the AQE was calculated via the following equations:
Nðphoto generated electronsÞ
Nðincident photonsÞ
Notes and references
AQE ¼
(24)
(25)
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