Efficient Visible Light-Driven Splitting of Alcohols into Hydrogen and Corresponding Carbonyl Compounds over a Ni-Modified CdS Photocatalyst

Article abstract of DOI:10.1021/jacs.6b06860

Splitting of alcohols into hydrogen and corresponding carbonyl compounds has potential applications in hydrogen production and chemical industry. Herein, we report that a heterogeneous photocatalyst (Ni-modified CdS nanoparticles) could efficiently split alcohols into hydrogen and corresponding aldehydes or ketones in a stoichiometric manner under visible light irradiation. Optimized apparent quantum yields of 38%, 46%, and 48% were obtained at 447 nm for dehydrogenation of methanol, ethanol, and 2-propanol, respectively. In the case of dehydrogenation of 2-propanol, a turnover number of greater than 44-000 was achieved. To our knowledge, these are unprecedented values for photocatalytic splitting of liquid alcohols under visible light to date. Besides, the current catalyst system functions well with other aliphatic and aromatic alcohols, affording the corresponding carbonyl compounds with good to excellent conversion and outstanding selectivity. Moreover, mechanistic investigations suggest that an interface between Ni nanocrystal and CdS plays a key role in the reaction mechanism of the photocatalytic splitting of alcohol.

Full text of DOI:10.1021/jacs.6b06860

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Communication
Efficient Visible Light-Driven Splitting of Alcohols into Hydrogen and
Corresponding Carbonyl Compounds over a Ni-modified CdS Photocatalyst
Zhigang Chai, Ting-Ting Zeng, Qi Li, Liang-Qiu Lu, Wen-Jing Xiao, and Dongsheng Xu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06860 • Publication Date (Web): 30 Jul 2016
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Efficient Visible Light-Driven Splitting of Alcohols into Hydro-
gen and Corresponding Carbonyl Compounds over a Ni-
modified CdS Photocatalyst
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Zhigang Chai, Ting-Ting Zeng, Qi Li, Liang-Qiu Lu, Wen-Jing Xiao,* and Dongsheng Xu*
†
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and
Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
‡
Key Laboratory of Pesticides and Chemical Biology, Ministry of Education, College of Chemistry, Central China
Normal University, Wuhan 430079, China
Photocatalysis, splitting of alcohol, visible light, semiconductor, hydrogen production
ABSTRACT: Splitting of alcohols into hydrogen and corresponding carbonyl compounds has potential applications in
hydrogen production and chemical industry. Herein, we report that a heterogeneous photocatalyst (Ni-modified CdS na-
noparticles) could efficiently split alcohols into hydrogen and corresponding aldehydes or ketones in a stoichiometric
manner under visible light irradiation. Optimized apparent quantum yields of 38%, 46% and 48% were obtained at 447
nm for dehydrogenation of methanol, ethanol and 2-propanol, respectively. In the case of dehydrogenation of 2-propanol,
a turnover number of greater than 44,000 was achieved. To our knowledge, these are unprecedented values for photo-
catalytic splitting of liquid alcohols under visible light to date. Besides, the current catalyst system functions well with
other aliphatic and aromatic alcohols, affording the corresponding carbonyl compounds with good to excellent conver-
sion and outstanding selectivity. Moreover, mechanistic investigations suggest that an interface between Ni nanocrystal
and CdS plays a key role in the reaction mechanism of the photocatalytic splitting of alcohol.
As a promising form of clean energy, hydrogen energy
has attracted increasing attention, focusing on the highly
efficient conversion of chemical energy into electricity.
The storage and transportation of hydrogen is a key but
challenging technology for mobile applications of proton
1
-4
exchange membrane fuel cells (PEMFCs). Alcohols,
such as methanol and ethanol, are regarded as potential
3
,5,6
carriers for hydrogen production and storage.
There are two ways to produce hydrogen from alco-
hols. The first method is conventional alcohol reforming,
which produces both hydrogen and carbon dioxide under
7,8
Scheme 1. Visible light-driven splitting alcohol over a
Ni-modified CdS photocatalyst.
high temperatures and high pressures. Recently, ruthe-
nium-catalyzed aqueous-phase dehydrogenation of meth-
anol to hydrogen and carbon dioxide at relatively low-
o
A sustainable way to drive the splitting of alcohols is to
utilize solar energy. Photocatalytic hydrogen generation
from water using alcohols as hole scavengers has been
temperatures (65-95 C) and ambient pressure has been
5
reported. The second method of producing hydrogen
from alcohols is the direct catalytic dehydrogenation of
alcohols, which generates hydrogen and corresponding
aldehydes or ketones. The reported direct dehydrogena-
tion reactions of alcohols were conducted at reflux or an
even higher temperature signifying that extra energy is
1
4-17
mostly studied.
Notably, in this system, water is used
as electron acceptor and alcohol is used as a sacrificial
agent and its primary oxidation products, such as alde-
hydes, could be further oxidized to carboxylic acid, car-
bon monoxide (CO) or carbon dioxide because of the
9-13
essential.
1
4,15,17-19
presence of water.
In particular, even a small con-
centration of CO (10 parts per million, p.p.m.) can easily
poison the catalyst in proton exchange membrane fuel
20
cells (PEMFCs). To achieve a stoichiometric splitting of
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Journal of the American Chemical Society
alcohol, the photocatalyst should be able to abstract hy-
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drogen from the alcohol instead of water. Herein, we re-
port a noble-metal-free photocatalyst (Ni nanocrystal-
modified CdS nanoparticles (NPs), Ni/CdS) which could
efficiently split alcohols into hydrogen and corresponding
aldehydes or ketones in a stoichiometric manner using
visible light (Scheme 1). To our knowledge, such an effi-
cient heterogeneous photocatalyst for the visible light-
driven splitting of alcohol has not been reported to date.
Well-crystallized hexagonally structured CdS NPs pre-
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pared according to a previously described method were
employed as photocatalysts (see Supporting Information,
Figure S5). Modification of the surface of CdS NPs with Ni
nanocrystal was performed by in situ photodeposition in
which methanol acts as an electron donor and NiCl2
serves as an electron acceptor (Figure S1). The influence
of solvent on the photodeposition is shown in Figure S2,
which reveals that the photodepositions performed in the
absence of water were more favorable for the hydrogen
evolution. We attribute this to that metallic Ni nanocrys-
tals could be oxidized by water. Therefore, methanol solu-
tion of NiCl was chosen for the photodeposition. Then
2
we studied concentration of NiCl on the rate of hydrogen
2
evolution. As shown in Figure S3, the rate of hydrogen
evolution firstly increased along with increasing amount
of NiCl and then reached to a plateau. The optimized
2
weight ratio of Ni and CdS was 1.6 wt% determined by
inductively coupled plasma-atomic emission spectroscopy
(
ICP) analysis. Transmission electron microscope (TEM)
and high-resolution TEM (HRTEM) images of the Ni/CdS
NPs are provided in Figure 1a and Figure 1b, respectively,
which indicate that a ca. 5 nm Ni nanocrystal was depos-
ited on the CdS surface. The lattice distances measured
for the Ni nanocrystals are 0.20 and 0.18 nm, which corre-
spond well with lattice distances of (111) and (200) of a
metallic Ni (the Powder Diffraction File, PDF#04-0850),
respectively. However, Ni nanocrystal does not show any
obvious peaks in the X-ray diffraction (XRD) pattern
Figure 2. (a) Long-term photocatalytic splitting of 2-
propanol (5 mL) under visible light (λ>420 nm). The bars
denote the rate of hydrogen evolution averaged over 20 h of
illumination. (b) Ultraviolet-visible diffuse reflectance spec-
trum of the CdS NPs and the AQYs of hydrogen evolution
from 2-propanol under different incident lights. The error
bars refer to the wavelength range of the incident light. (c)
Photocatalytic splitting of 2-propanol using different photo-
catalysts (after 6 h).
(
Figure S6), which can be attributed to its low loading
amount and small crystalline size. X-ray photoelectron
spectroscopy (XPS) was performed (Figure S7 and Figure
S10); in the resultant spectrum, peak at 852.1 eV was at-
Photocatalytic splitting of methanol, ethanol and 2-
propanol were performed at room temperature (20 C)
o
1
5
under visible light illumination (λ>420 nm). The genera-
tion of hydrogen gas from these alcohols when Ni/CdS
was used as a photocatalyst was clearly observed (see
Supplementary Video 1). In contrast, no activity was
found using CdS NPs alone. Figure S4 displays quantities
of hydrogen that evolved as a function of irradiation time.
The rate of hydrogen evolution for 2-propanol was 46.6
tributed to metallic Ni; peaks at 853.3 and 856.0 eV were
attributed to be NiO and NiCl , respectively.
2
-
1
-1
mmol h g CdS, which is comparable with reported data
1
5
of a water-alcohol system. Optimized apparent quantum
yields (AQYs) of 38%, 46% and 48% were achieved at
λ=447 nm (intensity of the incident light was 2.60 mW)
for dehydrogenation of methanol, ethanol and 2-
propanol, respectively. To our knowledge, these are un-
precedented AQYs of visible light-driven dehydrogena-
Figure 1. (a) TEM image of Ni/CdS and (b) HRTEM image of
Ni/CdS (the inset image is the Fourier transform of the
marked area).
tion of alcohols, the molar ratio of H and the expected
2
aldehyde or ketone was calculated to be ca. 1.0, indicating
a stoichiometric dehydrogenation reaction. Analysis of
the evolved gas by gas chromatography (GC) reveals that
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CO and CO at concentrations of less than 1 part per mil-
lion (ppm) were produced by all of these reactions. In
addition, over-oxidation products in solution, such as
carboxylic acids, were not detected in GC-MS analysis.
To understand the mechanism of the photocatalytic
splitting of alcohols, a series of experiments was conduct-
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ed. We initially investigated the influence of NiCl and
2
NiS on the photocatalytic splitting of 2-propanol (Figure
2
c). No activity was found when NiCl -modified CdS NPs
To test the stability of the photocatalyst, long-term
2
were employed as photocatalysts. Compared with Ni/CdS,
much lower rate of hydrogen evolution was obtained
when NiS-modified CdS NPs were employed. Platinum
continuous evolution of H from 2-propanol using Ni/CdS
2
(
6 mg) was performed (Figure 2a). After 121 h, 16 mmol of
H (401 mL) had been evolved which corresponded to 25%
2
(
Pt), which is a well-known catalyst for hydrogen evolu-
conversion of 2-propanol to acetone. Therefore, the turn-
over number (TON) based on the amount of metallic Ni
was calculated to be ca. 44,465 after 121 h, clearly indicat-
ing the high stability of the photocatalyst. The turnover
2
7,28
tion reaction (HER),
was also modified on the surface
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of CdS. However, no hydrogen was evolved when it was
used for the photocatalytic splitting of alcohol (Figure 2c),
indicating Pt could not abstract hydrogen from alcohol
-1
frequency (TOF) was ca. 1,222 h for the first 6 h, which is
2
9
-
which is consistent with previous literature. It was de-
termined that the Ni/CdS was deactivated for the pho-
tosplitting of simple alcohols when this catalyst was ex-
posed to air and thoroughly oxidized (similar to Raney
Ni). Moreover, in the absence of Ni, only moderate con-
version (50%) was observed for benzyl alcohol (see entry
2, Table 1), and the selectivity for benzaldehyde was poor
(9%). The main products were hydrobenzoin and benzoin
comparable with the known state-of-the-art data (1,109 h
1
10
)
of a thermal catalytic system.
The dependence of the rate of H evolution from 2-
2
propanol on the wavelength of incident light was investi-
gated. As indicated in Figure 2b, the AQY trend corre-
sponds well to the absorption spectrum of CdS NPs. The
above results reveal that the dehydrogenation of 2-
propanol proceeds through light absorption by CdS and
exclude the possibility of direct thermal catalytic dehy-
30
owing to the coupling of benzyl radicals. However, when
1
2,22-24
Ni/CdS was used, benzyl alcohol was converted to ben-
zaldehyde with high conversion and excellent selectivity
(see entry 1, Table 1). This result indicates that the initial
radical formation may be on the oxygen atom which im-
drogenation by Ni.
From the view point of chemical science and industry,
the conversion of alcohols to aldehydes and ketones is a
1
1,25,26
3
1
fundamental process.
To demonstrate the synthetic
mediately forms the C=O bond through Ni-assisted C-H
activation. The above results strongly suggest that an in-
terface between metallic Ni and CdS plays a key role in
the reaction mechanism. Then we performed kinetic iso-
tope effect (KIE) using a CH OH/CD OH (1:1 molar ratio)
utility of our catalyst system, we examined the feasibility
of visible-light photocatalytic dehydrogenative oxidation
of other alcohols in acetonitrile (MeCN). To our delight,
the current catalyst system functions well with aromatic
and cyclic aliphatic alcohols, affording the corresponding
carbonyl compounds with moderate to excellent conver-
sion and outstanding selectivity (see Table 1). In the case
of long-chain aliphatic alcohols, their conversions were
low but the selectivity of dehydrogenative products was
high (entries 9 and 10, Table 1).
3
3
mixture. The ratio of the reaction rates (k /k ) was esti-
H
D
mated to be ca. 5.1, indicating that the rate-limiting step
involves the activation of an α C-H bond. In addition,
when CD OD was used, D was detected (see Figure S9)
3
2
indicating that the source of hydrogen was indeed from
the alcohol.
[
a]
Table 1. Visible light-driven splitting of alcohols
[b]
[b]
Entry Alcohol
Conv.(%)
Sel.(%)
96
1
Benzyl alcohol
96
50
99
97
94
63
93
30
3
[c]
2
[d]
9
3
4-chlorobenzyl alcohol
1-phenylethanol
Diphenylmethanol
Cinnamyl alcohol
Cyclooctanol
96
97
93
98
99
98
97
99
4
5
6
[e]
7
8
9
Cyclopentanol
1-octanol
10
2-octanol
15
Scheme 2. Proposed potential mechanism of the pho-
tocatalytic splitting of alcohols by Ni/CdS.
[
a] Reaction conditions: alcohol (0.2 mmol), CdS (6 mg),
NiCl ·6H O (5 μmol), MeCN (3 mL), Ar atmosphere, 7 W blue
2
2
LED, 20 h. [b] Determined by GC. [c] Without NiCl ·6H O.
2
2
[d] Hydrobenzoin (38%) and benzoin (37%) were formed. [e]
Alcohol (0.1 mmol).
Based on the above results, we propose a catalytic
mechanism for the photocatalytic dehydrogenation of
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alcohols (Scheme 2). Initially, alcohol can be absorbed on
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the surface of Ni particles, a phenomenon similar to a
step in the mechanism of thermal catalytic dehydrogena-
3
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Beller, M. Angew. Chem. Int. Ed. 2011, 50, 9593.
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1
0
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1
2
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Supporting Information.
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1
(
AUTHOR INFORMATION
Corresponding Authors
(
Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight,
D. W.; Hutchings, G. J. Science 2006, 311, 362.
*
*
wxiao@mail.ccnu.edu.cn
dsxu@pku.edu.cn
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Chem. Soc. 2011, 133, 7296.
(28) Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C. J.
Am. Chem. Soc. 2008, 130, 7176.
Notes
The authors declare no competing financial interest.
(29) Jin, Z.; Li, Q.; Zheng, X.; Xi, C.; Wang, C.; Zhang, H.; Feng,
ACKNOWLEDGMENT
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos. 21303004, 21133001,
L.; Wang, H.; Chen, Z.; Jiang, Z. J. Photochem. Photobiol. A:
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H.; Konig, B. Org. Biomol. Chem. 2012, 10, 3556.
21232003 and 21472057) and the National Key Basic Research
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Blechert, S.; Wang, X. J. Am. Chem. Soc. 2010, 132, 16299.
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J. Adv. Mater. 2012, 24, 229.
Program of China (Grant Nos. 2014CB239303, 2013CB932601).
(
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