Efficient and Selective CO2 Reduction Integrated with Organic Synthesis by Solar Energy

Full text of DOI:10.1016/j.chempr.2019.06.019

Article
Efficient and Selective CO2 Reduction
Integrated with Organic Synthesis by Solar
Energy
Qing Guo, Fei Liang, Xu-Bing
Li, ..., Zhe-Shuai Lin, Chen-Ho
Tung, Li-Zhu Wu
lixubing@mail.ipc.ac.cn (X.-B.L.)
lzwu@mail.ipc.ac.cn (L.-Z.W.)
HIGHLIGHTS
Exceptional activity and selectivity
of CO2-to-CO conversion is
realized by QDs
CO2 reduction is firstly integrated
with oxidative organic synthesis
by solar energy
Full utilization of photogenerated
electrons and holes in redox
reactions is achieved
Solar-driven CO2 photoreduction is attractive to produce usable fuels and value-
added chemicals. However, conventional strategies suffer from low activity and/or
expense of sacrificial reagents. Here, the first example of CO2 reduction to CO
integrated with an oxidative organic synthesis of 1-phenylethanol to pinacols
under solar light is reported. This strategy, making full utilization of
photogenerated electrons and holes in redox reactions, provides the exact answer
toward cost-effective CO2 photoreduction.
Guo et al., Chem 5, 1–12
October 10, 2019 ª 2019 Elsevier Inc.
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Article
Efficient and Selective CO2 Reduction
Integrated with Organic Synthesis
by Solar Energy
Qing Guo,1,3 Fei Liang,2,3 Xu-Bing Li,1,3, Yu-Ji Gao,1,3 Mao-Yong Huang,1,3 Yang Wang,1,3
*
Shu-Guang Xia,1,3 Xiao-Ya Gao,1,3 Qi-Chao Gan,1,3 Zhe-Shuai Lin,2,3 Chen-Ho Tung,1,3
and Li-Zhu Wu1,3,4,
*
SUMMARY
The Bigger Picture
CO2 reduction to fuels driven by
solar energy has great importance
for climate and energy issues.
However, the low activity under
visible light and the waste of
sacrificial reagents of traditional
photocatalysis limit its practical
application. Here, CO2
Solar-driven catalytic CO2 reduction is attractive to produce usable fuels and
value-added chemicals. However, conventional strategies suffer from low activ-
ity and/or expense of sacrificial reagents. Here, we describe the first example of
CO2 reduction integrated with the oxidative organic synthesis by solar energy.
With visible-light irradiation, CdSe/CdS quantum dots (QDs) enable photocata-
lytic conversion of CO2 to CO with an exciting rate of ꢀ412.8 mmol gÀ1 hÀ1 and a
high selectivity of > 96% in the presence of triethylamine. More importantly,
CO2-to-CO conversion proceeds smoothly when integrated with oxidative
coupling of 1-phenylethanol to evolve pinacol. 1-Phenylethanol with electron-
donating groups at the p-position of aryl ring, for example, is converted to cor-
responding pinacols with excellent yields up to 98%. The released protons and
electrons of oxidative half-reaction are used for CO2 reduction. Apparently, this
strategy provides the exact answer toward cost-effective CO2 photoreduction
and opens a new horizon for efficient solar-to-fuel conversion.
photoreduction is firstly
integrated with oxidative organic
synthesis to simultaneously
produce solar fuels and value-
added chemicals by solar energy,
which provides the exact answer
for cost-effective CO2
photoreduction and opens a new
horizon for efficient solar-to-fuel
conversion. Owing to its
INTRODUCTION
Catalytic reduction of CO2 into usable fuels and value-added chemicals by solar en-
ergy has emerged as a crucial energy-storage pathway to address greenhouse effect
and energy crisis.1–6 Tremendous efforts have been devoted to converting CO2 into
corresponding gaseous products (for example, CO, CH4, and C2H4)7–9 and liquid
products (for example, HCOOH, CH3OH, and C2H5OH).10–12 A carbon neutral cycle
of coupling CO2 reduction and water (H2O) oxidation together is regarded as a
dream reaction to store solar energy in chemical bonds of CO2 reduction products
and O2 gas.13 Up to date, various types of semiconductor materials (for example,
CdSe,14 TiO2,15,16 InVO4,17 BiOBr,18 and ZnIn2S419) are capable of driving multielec-
tron CO2 reduction (usually to CO or CH4) by employing H2O as the proton and elec-
extremely high activity and cost-
effective nature, this strategy
shows great potentials in solar-
driven CO2 conversion on a large
scale in the near future.
tron donor. However, the kinetically sluggish oxidation half-reaction (2H2O / 4eÀ
+
4H+ + O2) makes these photosystems limited by low activity and poor durability. On
the contrary, CO2 photoreduction at the expense of sacrificial reagents (for example,
ascorbic acid,20 sodium sulfite (Na2SO3),21 triethanolamine (TEOA)22–24 and
N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD)25
) as the hole scavengers
shows accessibility toward effective CO2 conversion. Unfortunately, such a strategy
is not economically feasible and wastes the oxidizing power of photogenerated
holes. It is highly desirable to utilize these excited holes to drive meaningful chemical
reactions. In recent years, the examples of H2 photogeneration coupled with oxida-
tive organic synthesis have been reported to simultaneously produce solar fuel (H2)
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Scheme 1. Schematic Illustration of Photocatalytic CO2 Reduction
Photocatalytic CO2 reduction coupled with oxidative organic synthesis by semiconductor QDs in
this work.
and value-added organic chemicals.26–29 However, to the best of our knowledge, a
solar-light-driven system that effectively integrates CO2 photoreduction with oxida-
tive organic synthesis has yet to be demonstrated.30–32
Here, we firstly report semiconductor QDs photocatalyzed CO2 reduction inte-
grated with oxidative organic transformation under visible light (Scheme 1), thus
enabling the full utilization of excited electrons and holes to simultaneously produce
gas product of CO and liquid product of value-added chemical (pinacol). Pinacol
emerges as the important structural motifs in natural products and pharmaceutical
intermediates.33 Synthesis of pinacol mainly focuses on reductive dimerization of al-
dehydes and/or ketones in the aid of co-reductants (for example, low-valent metals
of Cu, Zn, and Mg).34,35 Recent interest in visible-light mediated coupling of alcohols
has become an effective pathway to evolve pinacol.36–38 Our initial experiments
indicate that CdSe/CdS QDs can catalyze CO2-to-CO conversion under visible-light
irradiation (l = 450 nm) with an exciting rate of ꢀ412.8 mmol gÀ1 hÀ1 and a high
selectivity of >96% in the presence of triethylamine (TEA). More importantly, photo-
catalytic CO2-to-CO conversion proceeds smoothly and effectively when integrated
with the oxidative C-C coupling of 1-phenylethanol derivatives to corresponding
pinacols, not ketones, with yields as high as >98%. This strategy provides the exact
answer for solar driven catalytic CO2 reduction and opens a new horizon for efficient
solar-to-fuel conversion owing to its extremely high activity and cost-effective
nature.
1Key Laboratory of Photochemical Conversion
and Optoelectronic Materials, Technical Institute
of Physics and Chemistry (TIPC), Chinese
RESULTS
Visible-Light-Driven CO2 Reduction Using Semiconductor QDs
Academy of Sciences, Beijing 100190, P. R. China
Due to their adjustable band positions, excellent light-harvesting properties, and
2Key Laboratory of Functional Crystals and Laser
Technology, Technical Institute of Physics and
as the photocatalysts (see preparation procedures in Supplemental Experimental
Chemistry (TIPC), Chinese Academy of Sciences,
abundant surface sites, semiconductor QDs consisting of II-VI elements were chosen
Procedures and corresponding characterization details in Figures S1 and S2).39
Beijing 100190, P. R. China
3School of Future Technology, University of
Chinese Academy of Sciences, Beijing 100049,
P. R. China
The initial experiments of photocatalytic CO2 reduction using semiconductor QDs
were performed in the presence of sacrificial reagent (triethylamine; TEA) in organic
solvent of dimethylformamide (DMF). DMF was employed as the solvent for two rea-
sons: (1) the excellent solubility of CO2 gas, and (2) the strong coordination of DMF
at surface Cd suppressing proton adsorption and reduction.40 Surprisingly, CdSe/
CdS QDs were able to effectively reduce CO2 to CO with a rate of 318.9 G
5.8 mmol gÀ1 hÀ1, which was much higher than those of CdSe, CdS and the physical
4Lead Contact
*Correspondence:
lixubing@mail.ipc.ac.cn (X.-B.L.),
lzwu@mail.ipc.ac.cn (L.-Z.W.)
https://doi.org/10.1016/j.chempr.2019.06.019
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Figure 1. Photocatalytic CO2 Reduction in the Presence of Sacrificial Reagents
(A) The rate of photocatalytic CO2 reduction reaction using different semiconductor QDs as the photocatalysts.
(B) Photocatalytic CO2 reduction of CdSe QDs modified with different atomic thickness of CdS shell under the same conditions.
(C) Long-time photocatalytic CO2 reduction experiment using CdSe/CdS QDs as photocatalysts and TEA as electron donor under Xe lamp irradiation
(300 mW with 400 nm filter).
(D) The generation of CO2 reduction products as a function of time with solar simulator (AM1.5G) as the light source by using CdSe/CdS QDs as
photocatalysts.
(E) GC-MS chromatogram of gas products obtained by photocatalytic reaction under a 13CO2 atmosphere with different irradiation time.
(F) Growth kinetics of 13CO and the corresponding decay kinetics of 13CO2 along with 450 nm LEDs irradiation obtained from GC quantization.
The error bars represent the relative deviation of three independent experiments.
mixture of CdSe and CdS QDs under the same conditions (room temperature;
450 nm LEDs) (Figure 1A). 1H NMR and ion chromatography revealed that no liquid
products (e.g., CH3OH, HCOOH) were produced. That is to say, H2 gas is the only
by-product of CO2 photoreduction. According to following equation (h = n(CO)/
[n(CO) + n(H2)] 3 100%), the selectivity of CO2-to-CO catalyzed by CdSe/CdS
QDs was above 94%.
As the thickness of CdS shell greatly influences the light harvesting and charge sep-
aration of the heterostructured QDs,41 the influence of CdS shell thickness on CO2
photoreduction is investigated. According to our previous reports,42,43 CdSe QDs
(ꢀ2.0 nm) covered with different shell thickness of CdS were successfully synthesized
in aqueous solution using a successive ion layer adsorption and reaction (SILAR)
approach. Obviously, introduction of CdS shell onto CdSe core significantly
changed the activity of CO2 photoreduction (Figure 1B). Due to the high selectivity
and activity of CO evolution, CdSe/CdS QDs with ꢀ3 atomic layers of CdS were
taken as the photocatalysts afterwards. Under the optimal conditions, the system
produced CO with an exceptional rate of ꢀ412.8 mmol gÀ1 hÀ1 and a selectivity
of >96% under visible light (Figure S3). Taking monochromatic LEDs as the light
source (l = 450 nm, 60 mW cmÀ2, and 1.0 cm2), the apparent quantum yield
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(AQY) of CO evolution was determined as high as 32.7%, see details in Figure S4.
Compared with typical QDs based photocatalytic CO2 reduction systems, the pre-
sent photosystem exhibited much higher activity and/or selectivity in the absence
of any external cocatalysts (Table S1). Furthermore, CdSe/CdS QDs also demon-
strated excellent stability toward CO2 photoreduction. As shown in Figure 1C,
both activity and selectivity of CO production were well preserved after 6 recycling
experiments. Excitingly, this system could effectively convert CO2 to CO with an
average rate of ꢀ63.1 mmol gÀ1 hÀ1 and a selectivity of >97% under the irradiation
of AM1.5 (Figure 1D), indicating its great potential applications in conversion solar
energy into chemical fuels on a large scale.
In order to reveal the origin of evolved H2, deuterium substituted DMF (d7-DMF) was
used as the solvent of CO2 photoreduction. Gas chromatography (GC) analysis
confirmed that H2, instead of D2, was detected. This result indicated that sacrificial
reagent (TEA) here worked as both electron and proton donor. To further validate
the origin of CO from CO2 reduction, an isotopic experiment using 13CO2 as reac-
tant was carried out, and the obtained gaseous products were analyzed by gas
chromatographyÀmass spectrometry (GC-MS). As shown in Figure 1E, the signals
at m/z = 29 and m/z = 45 were observed, which could be assigned to the photogen-
erated 13CO and the residual 13CO2, respectively. This result qualitatively confirmed
the produced CO originated from CO2. As signal at m/z = 17 was not observed,
further confirming that the system did not produce CH4 from CO2 photoreduction
(Figure S5). Furthermore, the growth kinetics of 13CO and the corresponding decay
kinetics of 13CO2 during the photocatalytic process were monitored. It clearly
showed the continuous decrease of 13CO2 along with the increase of corresponding
13CO with the reaction proceeding, and 13CO2 was quantitatively converted to 13CO
after 12 h visible-light irradiation (Figure 1F). These results clearly indicated that CO
was originated from CO2 photoreduction.
Solar-Light-Driven CO2 Reduction Integrated with Oxidative Organic
Synthesis
Compared with photocatalytic CO2 reduction in the aid of sacrificial reagents
(e.g., TEA, ascorbic acid), CO2 photoreduction coupled with desired organic synthe-
sis is more challenging and meaningful. On the basis of above results, here, we real-
ized efficient and selective CO2 photoreduction integrated with an oxidative organic
synthesis of pinacol through oxidative C-C coupling reaction by solar energy for the
first time. Taking colloidal semiconductor QDs as the photocatalysts, the photogen-
erated holes initiated the oxidative coupling of 1-phenylethanol to pinacol (see
details in 1H NMR, 13C NMR and mass spectrum of the oxidative product in Figures
S6 and S7), while the released protons of the oxidative half-reaction participated in
CO2 photoreduction. This protocol shows considerable advantages in full utilization
of solar energy as two useful products, solar fuel (CO) and value-added chemical
(pinacol), are obtained simultaneously. Furthermore, the reduction product (CO)
and oxidation product (pinacol) are spontaneously separated in the gaseous phase
and solution, respectively.
Detailed experiments revealed that CdSe/CdS QDs showed higher activity of CO
production and better pinacol yield than those of CdSe or CdS QDs under the
same conditions (Table 1). The conversion was also greatly influenced by the shell
thickness of CdS from 0 to 7 atomic layers. When the shell thickness of CdS was 3
atomic layers, the system demonstrated the highest activity of CO evolution
(27.63 mmol gÀ1
h
À1) with a selectivity of ꢀ94%. Meanwhile, the production of
oxidation product of pinacol also showed a climax rate (26.5 mmol gÀ1 À1) with a
h
4
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Table 1. Visible-Light-Driven CO2 Reduction Integrated with Pinacol Coupling Reaction
QDs
–R
Shell Thickness (N)a
Gas Products
(mmol gÀ1 hÀ1 b
CO
Liquid Products
Pinacol
eÀ/h+e
)
Selectivity (%)c
(mmol gÀ1 hÀ1
)
Yield (%)d
H2
CO
Acetop-
henone
Pina-
col
CdSe
–H
0
0
1
3
5
7
3
3
3
3
1.59
2.29
1.30
1.55
1.05
1.10
1.03
0.32
0.46
–
22.85
7.46
22.41
27.63
7.57
7.90
6.71
7.13
8.34
–
93.5
76.5
94.5
94.7
87.8
87.8
86.7
95.7
94.8
–
1.77
1.32
1.06
1.18
1.01
0.93
0.34
–
21.2
9.40
20.6
26.5
8.3
60.4
24.6
53.5
74.0
20.9
21.0
18.2
92.2g
98.7h
–
1.06
0.91
1.09
1.05
0.93
0.99
1.15
1.04
1.10
–
CdS
–H
CdSe/CdS
CdSe/CdS
CdSe/CdS
CdSe/CdS
CdSe/CdSf
CdSe/CdS
CdSe/CdS
CdSe/CdS
–H
–H
–H
–H
8.2
–H
6.4
–CH3
–OCH3
–NO2
7.17
8.01
–
–
–
Reaction conditions: 2.9 mg QDs were dispersed in 5.0 mL DMF. Then, 1-phenylethanol (0.5 mmol) and Na2CO3 (0.5 mmol) were added into the solution.
The solution was bubbled with CO2 for 30 min and was irradiated with blue LEDs (l = 450 nm) for 2.0 h at room temperature.
aAtomic layers of CdS on CdSe QDs core, which was calculated on the amounts of Cd and S precursors consumed.
bGeneration rates of CO and H2 were determined by GC with CH4 as internal standard.
cSelectivity of CO was calculated according to equation of n(CO)/[n(CO) + n(H2)] 3 100%.
dThe produced pinacol was determined using 1H NMR analysis with diphenyl carbinol as an internal standard. The yield was calculated using the equation of
2n(pinacol) /n(1-phenylethanol)Consumed 3 100%, d.r. value was determined by 1H NMR analysis of the unpurified reaction mixture to be 1.5:1.
eThe ratio of electrons and holes consumed in redox reactions was calculated by following equation: e–/h+ = [2 3 n(H2) + 2 3 n(CO)]/[2 3 n(acetophenone) + 2 3
n(pinacol)].
fThe system was irradiated under AM1.5 for 4.0 h.
gYield of pinacol after 10.0 h irradiation, d.r.=1.3:1.
hYield of pinacol after 10.0 h irradiation, d.r.=1.2:1.
yield of ꢀ74%. Control experiments implied that, in the absence of visible light,
1-phenylethanol, or CO2, the reaction would not proceed at all (Figure S8). Under
the optimal conditions, the conversion could also occur smoothly to evolve CO
and pinacol simultaneously under the irradiation of AM1.5, indicating its potential
application in solar energy conversion. In addition, the tendency of AQY of pinacol
generation matched well with the UV-vis absorption spectrum of CdSe/CdS QDs
(Figure S9), confirming that QDs worked as the active photocatalysts of this conver-
sion. It should also be noted that the ratio of consumed excited electrons and holes
were quite close to 1.0, indicating the good balance between redox reactions.
To obtain direct evidences on the reaction, the concentration-time curves of reac-
tant and products were monitored every 15 min by gas chromatography with FID
detector (SHIMADZU, GC-2010plus) and 1H NMR. As shown in Figure S10, the con-
centration of 1-phenylethanol gradually decreased while the C-C coupling product
of pinacol apparently increased. Notably, acetophenone always kept at an
extremely low concentration compared with that of pinacol during the whole pro-
cess of light irradiation. Meanwhile, the evolved amount of oxidative product (pina-
col) well matched with that of reductive product (CO), indicating that acetophenone
was only a by-product in our system. In addition, the generation of CO was inhibited
when small amount of ketone was added to reaction system (Figure S11), further
indicating that the produced acetophenone was not intermediate but a by-product.
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The detailed reaction mechanism will be discussed below. Further investigation indi-
cated that the introduction of electron-donating groups on aryl ring significantly
enhanced the yields of corresponding pinacols. For example, 1-phenylethanol
with methoxyl group (-OCH3) at the p-position could give an exceptional yield of
pinacol production (>98%). On the contrary, the electron-withdrawing group (for
example, -NO2) significantly declined the yield (see details in Table 1 and corre-
sponding characterizations in Figures S12–S15).
DISCUSSION
Mechanisms of Photocatalytic CO2 Reduction and C-C Coupling
According to our previous results,43 the conduction bands (ECB) of CdSe QDs and
CdSe/CdS QDs are about À1.60 and À1.40 V versus normal hydrogen electrode
(NHE), the reduction of free CO2 to the one-electron reduction intermediates of
anion radical is thermodynamically unfavorable for the extremely negative reduction
potential (CO2 + eÀ / CO2,À, E0Redox = À1.90 V versus NHE).44 The adsorption and
activation of CO2 molecules on QD surface would be the first step of the whole pro-
cess. Spectroscopic results provided evidences on the binding of CO2 molecules on
QD surface. The emission intensity of CdSe QDs in solution significantly enhanced
while the UV-vis adsorption spectrum showed negligible change under CO2 atmo-
sphere (Figures 2A and S16). The enhanced emission was due to the adsorption of
CO2 on QD surface, thus inhibiting the annihilation of excited electron-hole pairs
at trap states. In order to confirm this point, density functional theory (DFT) calcula-
tions were carried out on the (111) facet of CdSe, see details in Experimental Proced-
ures. It revealed that the perfect (111) facet of CdSe was not beneficial for CO2
adsorption, while CO2 was energetically favorable to adsorb at the Se-vacancy on
CdSe surface (Figure 2B). This result matched well with the enhanced emission inten-
sity of QDs in the presence of CO2. Adsorption configuration of CO2 on CdSe sur-
face was optimized without any symmetry constraint. CO2 molecule adsorbed at
the Se-vacancy via the Cd-O and Cd-C interaction with an adsorption energy
of À0.96 eV. The adsorbed CO2 was greatly distorted from linear to bended config-
uration with an O1ÀCÀO2 bond angle of 118ꢁ. Besides, both CÀO1 and CÀO2
˚
bonds were stretched from 1.171 to 1.248 and 1.312 A, respectively. The reduced
O1ÀCÀO2 angle and elongated CÀO bonds indicated that the adsorbed CO2
was highly activated. We also investigated the adsorption and activation of CO2
on CdS shell. As shown in Figure S17, sulfur vacancy of CdS shell played a very similar
role in CO2 activation to enable the bended molecular configuration and the elon-
gated C-O bonds.
X-ray photoelectron spectroscopy (XPS) surface analysis revealed that the stoichi-
ometry ratio of surface Cd to Se was ꢀ3.6, indicating the Cd-rich surface of CdSe
QDs used here (Figure S18; Table S2). To directly confirm the presence of Se-
vacancy, the powder electron paramagnetic resonance (EPR) was performed. As
shown in Figure S19, the EPR signal centered at g = 2.0040 well matched with the
reported singly charged anion vacancies of semiconductor QDs.45 As it is chal-
lenging to directly measure the exact reductive potential of the absorbed CO2 spe-
cies at anion vacancy, CdSe QDs with different sizes were used for photocatalysis to
gain indirect thermodynamic evidences on CO2 reduction (Figure 2C). Under visible
light, the activity of CO2-to-CO conversion decreased with increasing the size of
CdSe QDs from 2.0 to 4.5 nm, which was a result of the declined ECB potential of
CdSe QDs from À1.60 to À1.05 V versus NHE.46 When the size of QDs was larger
than 2.8 nm, trace amount of CO was produced. Thus, the reductive potential of
the adsorbed CO2 species was estimated to be ca. À1.33 V versus NHE, which
6
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Figure 2. Insights into QDs Photocatalyzed CO2 Reduction
(A) Photoluminescent emission of CdSe QDs solution before and after bubbling with CO2 gas for 20 min.
(B) The interaction between CO2 molecules and CdSe (111) facet with and without Se-vacancy (the orange, yellow, gray, and red balls represent Cd,
Se, C, and O atoms, respectively).
(C) The variation of band-position alignment (upper panel) and activity of CO2 photoreduction (nether panel) of CdSe with different diameter.
(D) Full potential energy for the adsorption and dissociation of CO2 via proton-coupled electron transfer to generate CO on cubic CdSe or CdS (111)
facet (gray, red, white, buff, and yellow atoms represent C, O, H, Cd and Se (S) atoms, respectively).
The error bars represent the relative deviation of three independent experiments.
was more positive than ECB of CdSe and CdSe/CdS QDs used here, thereby indi-
cating the feasibility of photo-driven CO2 reduction on QD surface.
The elementary steps of CO2 reduction on QD surface were further revealed by DFT
calculations. As shown in Figure 2D, CO2 could be converted to CO through the
following steps: (1) firstly, the adsorbed CO2* molecules on surface generate
COOH* intermediates through a proton-coupled electron transfer process; (2) sec-
ondly, the COOH* intermediate was further reduced and dissociated to produce
OH* and CO* intermediates, and (3) finally, OH* combined with a proton to produce
H2O and CO* detached from the surface to generate the product of CO. During this
process, the adsorption energy of COOH* was À2.4 eV, which made it highly difficult
to detach from QD surface. This was the reason why no HCOOH was detected in our
system. Desorption of CO* was endothermic by 0.625 eV with a barrier of 1.389 eV.
However, the adsorption energy of CO was only À0.46 eV, the smallest among CO2,
COOH, and CO. Therefore, CO was the sole carbon-based product here. We found
that the second step of COOH* protonation needed to overcome an energy barrier
of 2.425 eV, which was regarded as the rate-determining step of the whole process.
Interestingly, the reaction showed much lower energy barriers of CO2 adsorption
and COOH* reduction (0.097 and 1.773 eV) on CdS than those obtained on CdSe
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Figure 3. Mechanisms of Photo-Induced Oxidative Coupling of 1-Phenylethanol to Pinacol
(A) Comparison of bond cleavage energy of CaÀH, CbÀH and OÀH bonds of 1-phenylethanol.
(B) EPR spectrum of argon (Ar) saturated acetonitrile solution of DMPO and CdSe/CdS QDs (1) with and (2) without the addition of 1-phenylethanol upon
10 s visible-light irradiation (l = 450 nm).
(C) Full potential energy for 1-phenylethanol deprotonation on CdSe and CdS (111) facet with Se/S-vacancy by DFT calculations, respectively.
(0.470 and 2.425 eV). Furthermore, light response and charge separation efficiencies
of CdSe QDs could also be significantly enhanced by introducing a certain amount of
CdS shell to form the quasi type-II structure (Figures S20 and S21). Based on the
aforementioned advantages, the process of CO2 photoreduction on CdSe/CdS
QDs was thermodynamically and kinetically promoted.
The selective oxidative coupling of 1-phenylethanol to produce pinacol was also
investigated. DFT calculations were firstly performed to explore the bond cleavage
energy of CaÀH, CbÀH, and OÀH bonds to the corresponding radicals. Compared
with CbÀH and OÀH bonds, the CaÀH bond showed the lowest bond cleavage en-
ergy (ꢀ58 kcal molÀ1) to give corresponding carbon-centered radical (Figure 3A),
indicating the better feasibility of CaÀH bond break under the same conditions.
DFT results also indicated that 1-phenylethanol vertically adsorbed atop the anion
vacancies of CdSe or CdS via the substituent group while aryl ring was far away
from the surface (see details in Figure S22). Moreover, it is reported that the oxida-
tion potential of CaÀH bond was about +1.10 V versus NHE,47 which was more nega-
tive than the valence band (EVB) position of CdSe/CdS QDs (+1.15 V versus NHE).
Thus, the photogenerated holes could sufficiently oxidize CaÀH bond to produce
carbon radical. Control experiments indicated that the addition of a small amount
of radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) into the reaction
system would inhibit the production of neither CO nor pinacol, indicating the reac-
tion proceeded via radical intermediates. Thereafter, in situ EPR studies using 5,5-
dimethyl-1-pyrroline N-oxide (DMPO) as radical scavenger revealed the formation
of C radical in the absence of CO2 as the coupling constants (aN = 14.9, aH = 22.1)
of the signal (Figure 3B) were consistent with reported values of C radical,28 and
the molecular weight equaled to the adduct of DMPO and Ca radical (Figure S23).
Based on the above results, a tentative reaction mechanism of the oxidative half-re-
action was proposed. The excited QDs were reductively quenched by the surface
bound 1-phenylethanols, thereby producing carbon-centered radicals, which
coupled to produce corresponding pinacols on the surface of QDs. As pinacol pre-
sented much lower binding strength on QDs than those of 1-phenylethanol and in-
termediates (see details in Table S3), the produced pinacol molecules were released
8
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Please cite this article in press as: Guo et al., Efficient and Selective CO2 Reduction Integrated with Organic Synthesis by Solar Energy, Chem
(2019), https://doi.org/10.1016/j.chempr.2019.06.019
into the solvent and then additional 1-phenylethanols from the solution bound to the
active sites to complete the reaction cycle. The mechanism of surface reaction pre-
sented here is quite different from our previous report of oxidizing alcohols to
carbonyl compounds in solution through a radical relay process.48 Notably, the en-
ergy barrier of Ca-H cleavage at S-vacancy of CdS (2.268 eV) was much lower than
that at Se-vacancy of CdSe (3.470 eV) (Figure 3C). This might be one of the reasons
for the significantly enhanced selectivity of pinacol production after introducing suit-
able thickness of CdS shell on CdSe QDs. The introduction of CdS shell simulta-
neously promoted CO2 reduction and CaÀH activation. As a result, the selectivity
of CO evolution and the yield of pinacol production were both dramatically
enhanced. The detailed mechanism diagram of CO2 reduction coupled with organic
transformation was provided in Figure S24.
EXPERIMENTAL PROCEDURES
Solar-Light-Driven CO2 Reduction with Sacrificial Reagents49
First of all, 1.0 mL synthesized QDs solution was precipitated by adding excessive
amount of isopropanol. Subsequently, it was re-dispersed in 5.0 mL DMF, resulting
QDs DMF solution (c = 1.1 3 10À5 mol/L). Then, 1.0 mL TEA was added to 5 mL QDs
DMF solution. The solution was degassed by bubbling CO2 for 30 min to ensure the
dissolution of enough CO2 and 600 mL ultrapure CH4 was injected into the system to
work as the internal standard for quantitative GC analysis. Finally, the system was
irradiated under 450 nm LEDs (130 mW cmÀ2) at room temperature.
Solar-Light-Driven CO2 Reduction Integrated with Pinacol Synthesis50
First of all, 1.0 mL synthesized QDs solution was precipitated by adding excessive
amount of isopropanol. Subsequently, it was re-dispersed in 5.0 mL DMF, resulting
QDs DMF solution (c = 1.1 3 10À5 mol/L). Then, 60 mL (0.5 mmol) 1-phenylethanol
was added to 5 mL QDs DMF solution. 53 mg (0.5 mmol) Na2CO3 was employed
as the basic additive. The solution was degassed by bubbling CO2 for 30 min to
ensure the dissolution of enough CO2 and 600 mL ultrapure CH4 was injected
into the system to work as the internal standard for quantitative GC analysis.
Finally, the system was irradiated under 450 nm LEDs (130 mW cmÀ2) at room
temperature.
Quantification of Gas and Liquid Products
The generated gas in the reaction headspace for photocatalytic experiments were
quantified by a gas chromatograph (GC, Shimadzu GC2014CAFC/APC) equipped
˚
with a thermal conductivity detector and a 5 A molecular sieves GC column, argon
as a carrier gas. GC analysis revealed the production of CO and H2. Thus, CH4
was chosen as the internal standard for quantitative GC analysis. Under the experi-
mental condition, the response factors of CO/CH4 and H2/CH4 for GC analysis were
about 0.39 and 4.08, respectively, which were established by calibration with known
amounts of CO, H2 and CH4, and determined before and after measurements. On
the other hand, the produced liquid products were monitored by 1H NMR, 13C
NMR analysis (Avance-400) and liquid chromatograph-mass spectra (LC-MS,
Q-Exactive), revealing the major generation of pinacol. For further quantification
analysis, 0.1 mmol benzhydrol was added into CDCl3 solution as the internal stan-
dard. Error bars on CO, H2, and pinacol were calculated from at least three indepen-
dent experiments.
DFT Calculations
Taking CdSe (111) facet as an example, the first-principles calculations were per-
formed by the pseudopotential51 methods implemented in the package52 based
Chem 5, 1–12, October 10, 2019
9

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(2019), https://doi.org/10.1016/j.chempr.2019.06.019
on the DFT. A 400 eV plane wave basis set the cutoff, and the cutoff energy
was used throughout our calculations. The convergence thresholds between
optimization cycles for energy change and maximum force were set as 5.0 3
10À6 eV/atom and 0.03 eV/A, respectively. The Monkhorst-Pack grids53 of
˚
2 3 2 3 1 k-points were used for the (111) surface of cubic CdSe. Based on the
optimized reactant and product structures, the transition state (TS) searches were
performed using complete LST-QST methods54 so as to determine accurate
activation barriers of the reaction. In the work, we constructed stoichiometric
˚
˚
˚
slabs of 12.96 A 3 12.96 A 3 11.46 A (eight atomic layers) with a vacuum thickness
˚
of 15 A to model the (111) surface of CdSe. In all calculations, the atoms in the
bottom layers were fixed, but the atoms in the two topmost layers, as well as C,
O, and
H atoms, were allowed to relax. The geometry structure of the
˚
˚
˚
isolated CO2 molecule was optimized using a supercell of 15 A 3 15 A 3 15 A.
˚
The calculated CÀO bond length and bond angle (OÀCÀO) were 1.171 A and
180.0ꢁ, which were in good agreement with the experimental values.55 After
optimization, the electron density of states (DOS) and Mulliken charge analysis56
were performed.
Taking CO2 as an example, the adsorption energy was defined as:
Eads = E(CO + slab) – [ECO + Eslab],
2
2
where the first term is the total energy of the slab with the adsorbed CO2 molecule
on the surface, the second term is the total energy of isolated CO2 molecule, and the
third term is the total energy of the bare slab of the surface (perfect or Se-vacancy
slab). For CO, the similar calculated method is used. According to the above defini-
tions, a negative Eads value corresponds to an exothermic adsorption, and the more
negative the Eads, the stronger the adsorption.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2019.06.019.
ACKNOWLEDGMENTS
We are grateful for financial support from the Ministry of Science and Technology of
China (2017YFA0206903), the National Science Foundation of China (21861132004
and 21603248), the Strategic Priority Research Program of the Chinese Academy of
Science (XDB17000000), Key Research Program of Frontier Science of the Chinese
Academy of Sciences (QYZDY-SSW-JSC029), the Youth Innovation Promotion Asso-
ciation of Chinese Academy of Sciences (2018031), and K.C. Wong Education Foun-
dation, China. We specially thank Prof. Qing Zhang at Dalian Institute of Chemical
Physics (CAS) for GC-MS measurements.
AUTHOR CONTRIBUTIONS
X.-B.L. and L.-Z.W. conceived and designed the research and wrote the manu-
script. Q.G. carried out all experiments and analyzed the data. C.-H.T. helped to
analyze data and write the paper. F.L. performed the DFT calculations under the
supervision of Z.-S.L., and Y.-J.G., M.-Y.H., Y.W., S.-G.X., X.-Y.G., and Q.-C.G.
helped to synthesize semiconductor QDs and perform some of the photocatalytic
experiments.
10
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(2019), https://doi.org/10.1016/j.chempr.2019.06.019
DECLARATION OF INTERESTS
L.-Z.W., X.-B.L., Y.-J.G., M.-Y.H., and Z.-J.L. have
a patent: CN. Pat,
201510083816.X. L.-Z.W., Q.G., X.-B.L., and S.-G.X. have a patent: CN. Pat,
201910383837.1.
Received: March 15, 2019
Revised: April 6, 2019
Accepted: June 24, 2019
Published: July 23, 2019
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