Ni0.85Co0.15WO4 for Photocatalytic Reduction of CO2 Under Mild Conditions with High Activity and Selectivity

Article abstract of DOI:10.1007/s10562-020-03234-8

Abstract: Efficient catalysts play an important role in photoreduction of greenhouse gases (CO₂ etc.) to value-added chemicals such as CO. In this work, we synthesized a series of Ni_1?xCo_xWO₄ (x = 0–1) catalysts by chemical co-deposition method. Among the prepared materials, Ni_0.85Co_0.15WO₄ exhibits superior catalytic performance for photocatalytic reduction of CO₂ to CO with [Ru(bpy)₃]Cl₂·6H₂O as the photosensitizer. The productivity and selectivity of CO are 14.33?mmol g^?1 h^?1 and 86.4percent, respectively under visible light irradiation. The experimental results demonstrate that superiority of electron conductivity of metal tungstates and the high activity of Co active sites facilitate the separation and transfer of photo-induced charges and lead to the excellent performance of photcatalytic reduction of CO₂ to CO. Graphic Abstract: Ni_0.85Co_0.15WO₄,combining the superiority of electron conductivity of metal tungstates with the high activity of Co active sites, exhibits high productivity and selectivity of CO in photo-reduction of CO₂.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03234-8

Catalysis Letters
https://doi.org/10.1007/s10562-020-03234-8
Ni Co WO for Photocatalytic Reduction of CO Under Mild
0.85 0.15
4
2
Conditions with High Activity and Selectivity
1
1
1
1
1
1
Chunmei Guo · Biao Guo · Xiaosu Gao · Jing Liang · Qide Meng · Jinxuan Liu
Received: 31 December 2019 / Accepted: 21 April 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Efcient catalysts play an important role in photoreduction oꢀ greenhouse gases (CO etc.) to value-added chemicals such
2
as CO. In this work, we synthesized a series oꢀ Ni Co WO (x=0–1) catalysts by chemical co-deposition method. Among
1
−x
x
4
the prepared materials, Ni Co WO exhibits superior catalytic perꢀormance ꢀor photocatalytic reduction oꢀ CO to CO
0
.85
0.15
4
2
−
1
−1
with [Ru(bpy) ]Cl ·6H O as the photosensitizer. The productivity and selectivity oꢀ CO are 14.33 mmol g
h
and 86.4%,
3
2
2
respectively under visible light irradiation. The experimental results demonstrate that superiority oꢀ electron conductivity oꢀ
metal tungstates and the high activity oꢀ Co active sites ꢀacilitate the separation and transꢀer oꢀ photo-induced charges and
lead to the excellent perꢀormance oꢀ photcatalytic reduction oꢀ CO to CO.
2
Graphic Abstract
Ni Co WO ,combining the superiority oꢀ electron conductivity oꢀ metal tungstates with the high activity oꢀ Co active
0
.85
0.15
4
sites, exhibits high productivity and selectivity oꢀ CO in photo-reduction oꢀ CO .
2
Keywords Heterogeneous catalysis · Photocatalysis · XPS · CO reduction · Ni0.85co0.15WO4
2
1
Introduction
Electronic supplementary material The online version oꢀ this
article (https://doi.org/10.1007/s10562-020-03234-8) contains
supplementary material, which is available to authorized users.
The greenhouse gases such as CO produced ꢀrom the con-
2
sumption oꢀ non-renewable ꢀossil ꢀuels have caused severe
*
Jinxuan Liu
environmental problems [1]. Utilization oꢀ clean energies
jinxuan.liu@dlut.edu.cn
ꢀ
or the conversion oꢀ CO to value-added chemicals is
2
Extended author inꢀormation available on the last page oꢀ the article
Vol.:(0
1
123456789)
3

C. Guo et al.
highly desired in order to alleviate energy shortage and
environmental problem ꢀor sustainable development. Strat-
egies such as photocatalysis, electrocatalysis, and photo-
electroncatalysis [2–4], have been developed to convert
2 Experimental Details
2.1 Chemicals
CO to renewable ꢀuels such as carbon monoxide, ꢀormic
All reagents used in this work, including sodium tung-
state dehydrate (Na WO ·2H O, Aladdin, 99.5%), cobalt
2
acid, methanol, etc.
2
4
2
Among aꢀorementioned strategies oꢀ CO conversion,
(II) acetate tetrahydrate (Co(CH COO) ·4H O, Aladdin,
2
3 2 2
photocatalytic reduction oꢀ CO has attracted widespread
99.5%), nickel acetate (Ni(CH COO) ·4H O, Damao Rea-
3 2 2
2
attention due to the utilization oꢀ sustainable solar energy.
gent, ≥ 98.0%), triethanolamine (TEOA, Fuyu Reagent, ≥
Moreover, photocatalytic reduction oꢀ CO in a heteroge-
98.0%), acetonitrile (CH CN, Aladdin, ≥ 99.9%), tetrahydro-
2
3
neous system shows better stability and the catalysts are
ꢀuran (THF, Guangꢀu Tianjin, ≥ 99.8%), dimethyl sulꢀoxide
(DMSO, Guangꢀu Tianjin, ≥ 99.5%), N,N-dimethylꢀorma-
mide (DMF, Guangꢀu Tianjin, ≥ 99.8%), dichloromethane
(DCM, XiLong Scientiꢁc, ≥ 99.5%), tris(2,2′-bipyridine)
dichlororuthenium (II) hexahydrate ([Ru(bpy) ]Cl ·6H O,
easier to be recycled compared to homogeneous system
[
5]. The heterogeneous catalysts ꢀor photocatalytic reduc-
tion oꢀ CO can be classiꢁed into three categories: noble
2
metal-based catalysts (Pt, Ag, Pd, etc.), [6–8] non-noble
3
2
2
metal-based catalysts (oxides, sulꢀides, metal–organic
Macklin, 98%), carbon dioxide (CO , 99.999%), and iso-
2
1
3
ꢀ
rameworks (MOFs), etc.) [9–11] and biocatalysts [12].
Among the catalysts mentioned above employed in pho-
tope-labelled CO ( CO , 99%) were purchased directly
2 2
ꢀrom manuꢀacturers and were not puriꢁed unless otherwise
speciꢁed.
tocatalytic reduction oꢀ CO to CO, Co-based catalysts, as
2
the active site ꢀor photocatalytic reduction oꢀ CO , exhibits
2
superior catalytic activity but relatively poor selectivity
due to the competitive evolution oꢀ H [11, 13–17]. Tre-
2.2 Synthesis of Catalysts
2
mendous eꢂorts have been made to optimize the activ-
ity and selectivity oꢀ photocatalysts including design oꢀ
architectures, improvement oꢀ conductivity, enhancement
The NiWO and Ni Co WO powders were synthe-
4
0.85
0.15
4
sized by chemical co-precipitation method according to
the literature [27] with slight modiꢁcation. Firstly, 2 mmol
Ni(CH COO) ·4H O was dissolved in 100 ml deionized
oꢀ CO absorption capability, doping and the use oꢀ bime-
2
tallic catalysts [18–20]. However, the cost, stability and
3
2
2
selectivity oꢀ catalysts are still challenging in photocat-
water, ꢀollowed by the addition oꢀ a certain amount oꢀ
alytic CO reduction systems [3, 4, 6, 21]. As a conse-
Co(CH COO) ·4H O. The mixture was stirred at 60 ℃ in
2
3
2
2
−
1
quence, the development oꢀ inexpensive, stable and highly
the water bath ꢀor 10 min. 100 ml Na WO (0.02 mol l )
2 4
efcient catalysts is highly desired.
solution was mixed with the above clear solution under stir-
ring. The mixture was continuously stirred at 60 ℃ ꢀor 3 h.
The precipitate aꢀter 3 h was separated by centriꢀugation,
rinsed with deionized water (3–5 times) to remove the resid-
ual reagents, and dried ꢀor 12 h at ambient temperature. The
as-prepared samples were reꢀerred as Ni Co WO (X=0,
In light oꢀ the excellent stability and conductivity
oꢀ metal tungstates nano-materials in supercapacitors
[
22–24], as well as the active Co species ꢀor reduction oꢀ
CO , we aim to take advantage oꢀ the outstanding electron
2
conductivity oꢀ NiWO and the high activity oꢀ Co in the
4
1−x
x
4
photoreduction systems. As suggested by Hume–Rothery
0.15, 0.25, 0.5, 0.75, 0.85, 1) according to the gradient oꢀ
ratio oꢀ Co content. X is determined according to the amount
oꢀ added raw materials during the preparation oꢀ catalysts.
2
+
rules, Co can be incorporated into the NiWO lattice
4
ꢀ
orming Ni Co WO (x = 0–1) due to the approximate
1−x x 4
ionic radius and electronegativity [25, 26]. In this work,
a series oꢀ metal tungstates Ni Co WO (x = 0–1) with
1
−x
x
4
2.3 Characterizations
diꢂerent stoichiometric ratios oꢀ Ni/Co were synthesized
by chemical co-deposition method. The photocatalytic
perꢀormance oꢀ as-prepared catalysts towards the reduc-
tion oꢀ CO were tested by using [Ru(bpy) ]Cl ·6H O as
XRD (X-ray diꢂraction) patterns oꢀ the samples were
recorded using a Rigaku Dmax-2000 diꢂract-meter oper-
ating at a voltage oꢀ 40 kV in the range oꢀ 2θ = 5°–80°
with Cu Kα radiation (λ = 0.15406 nm). The SEM (scan-
ning electron microscope, NoVA NanoSEM 450) and the
TEM (transmission electron microscope, Tecnai G2, F30
S-Twin, TF 30 with an acceleration voltage oꢀ 300 kV)
were employed to conꢀirm the morphologies oꢀ sam-
ples. XP (X-ray photoelectron) spectra measurement was
perꢀormed equipped with an Al Kα X-ray source and a
2
3
2
2
photosensitizer and triethanolamine (TEOA) as electron
donor under visible light condition. Among these materi-
als, Ni Co WO exhibits highest activity and selectiv-
0
.85
0.15
4
ity ꢀor the photocatalytic reduction oꢀ CO to CO due to
2
the synergistic eꢂect oꢀ NiWO with high conductivity and
4
Co species as active sites, which ꢀacilitate the separation
and transꢀer oꢀ photo-induced charges.
1
3

Ni0.85Co0.15WO for Photocatalytic Reduction of CO Under Mild Conditions with…
4
2
hemispherical energy analyzer (Thermo Fisher, ESCALA-
B250Xi). The binding energies were reꢀerred to as the C
ethanol and H O) under sonication. The resulting suspension
2
was dropped on FTO (0.5 × 0.5 cm).
1
s peak at 284.6 eV oꢀ the surꢀace adventitious carbon.
The inꢀrared spectra were acquired using an FTIR (Fou-
rier Transꢀorm Inꢀrared Spectrometer, Bruker VERTEX
2.6 Tauc‑Plots
8
0v) under vacuum (~ 2.92 mbar) equipped with a liquid
The bandwidth oꢀ NiWO and Ni Co WO can be calcu-
4
0.85
0.15
4
nitrogen-cooled narrow band MCT (Mercury Cadmium
Telluride) detector. Raman spectra were collected on a
multichannel conꢀocal microspectrometer (Thermo Fisher,
DXR Microscope) with 532 nm laser excitation. The BET
1
/2
lated ꢀrom the Tauc-equation ((αhv) =A(hv−E )).
g
In the equation, α is the absorption efciency, h is the
Planck constant, ν is the radiation ꢀrequency, E is the band
g
gap, and A is a constant. In the calculation oꢀ this work, α is
replaced by the absorption value in UV–vis DRS. The band
gaps oꢀ the two samples are estimated by the intersection oꢀ
the tangent and the horizontal axis shown in Fig. 4b. For the
convenience oꢀ calculation, we deꢁne the value oꢀ A as 1,
which has no inꢃuence on the value oꢀ band gap.
(
Brunner–Emmett–Teller) surꢀace areas were measured
using a BELSORP-max instrument ꢀrom N adsorption
2
and desorption isotherms at 77 K. The gathered H was
2
analyzed by a GC (Gas Chromatograph, 7890 T) equipped
with thermal conductivity and the collected CO was tested
by a GC (Techcomp7900) with ꢃame ionization detectors.
UV–vis DR (Ultraviolet–visible diꢂuse reꢃection) spec-
tra and PL (photoluminescence) spectra were recorded
with the spectrometer (Thermo Scientiꢁc Evolution 200)
and the steady-state ꢀluorescence spectrophotometer
3 Results and Discussion
(
Horiba Jobin Yvon FluoroMax-4P). The metal content
Figure 1a, b present the SEM images oꢀ NiWO and
4
oꢀ the materials was measured ICPMS (inductively cou-
pled plasma mass spectrometer, PerkinElmer, America,
Nex ION 300D). The control experiment was carried out
Ni Co WO respectively with irregular sphere-like mor-
0.85
0.15
4
phology. Figure 1c–ꢀ show the elemental mappings oꢀ Ni,
Co, and W which demonstrates the uniꢀorm distribution oꢀ
Ni, Co, and W. The EDX (energy-dispersive X-ray) spectrum
in Fig. S1 indicates that the molar ratio oꢀ Co/Ni is 0.17,
which is consistent with the ICPMS (inductively coupled
plasma mass spectrometer) analysis with Co/Ni molar ratio
oꢀ 0.171.
1
3
by using CO instead oꢀ CO , and the gas product was
2
2
tested by Gas Chromatography/Mass Spectrometer (GC/
MS, Agilent America, 5975C).
2.4 Photocatalytic Reduction of CO2
TEM images oꢀ NiWO and Ni Co WO in Fig. 1g,
4
0.85
0.15
4
h indicate that the irregular spherical particles observed in
the SEM images are the agglomerate oꢀ nanoparticles. No
lattice ꢀringes oꢀ NiWO and Ni Co WO are observed
The photocatalytic reaction was carried out in a Shrek ꢃask
reactor with [Ru(bpy) ]Cl ·6H O (10 µmol), Ni Co WO
3
2
2
1−x
x
4
4
0.85
0.15
4
(
(
0.5 mg), CH CN: H O: TEOA (electron donor) = 4:1:1
in high resolution TEM images due to the poor crystallinity
3
2
4 ml/1 ml/1 ml) in it, which was ultrasonically treated ꢀor
oꢀ the prepared catalysts. The XRD patterns oꢀ NiWO and
4
2
0 min. The entire reaction system was replaced by CO
Ni Co WO are shown in Fig. S2. The XRD patterns
2
0.85
0.15
4
(
99.999%, 1 atm.) under continuously stirring ꢀor 30 min
oꢀ NiWO and Ni Co WO in Fig. S2 exhibit broad diꢀ-
4 0.85 0.15 4
to remove the dissolved air. Then, a 300 W Xe lamp (100
ꢀraction peaks, implying an amorphous structure oꢀ the two
samples, which is in agreement with aꢀorementioned TEM
results [27–30].
−
2
mW cm ) with a 420 nm cutoꢂ ꢁlter was employed as the
light source at 25 ℃ ꢀor 5 h. The equal volume oꢀ the gas
product was collected ꢀor GC analysis each hour during
the reaction.
The high-resolution XPS oꢀ Ni Co WO displayed
0
.85
0.15
4
in Fig. S3 demonstrate the existence oꢀ Co, Ni, W and O.
The signals at 781.5 eV and 797.6 eV can be attributed to
2
+
2
p
and 2p oꢀ Co , respectively [28, 31]. The signals at
3/2 1/2
2.5 Mott–Schottky Measurements
874.3 eV and 856.5 eV can be assigned to 2p and 2p
1/2 3/2
2
+
oꢀ Ni , respectively [32–35]. And, the signals at 37.5 eV
6
+
Mott–Schottky measurements were perꢀormed in the Elec-
trochemical impedance meter (Zahner Germany, Zennium/
IM6) under the conventional three-electrode system (The
Pt wire as the counter electrode and an Ag/AgCl electrode
as the reꢀerence electrode) in 0.5 M Na SO solution (pH
and 35.4 eV are resulted ꢀrom 4ꢀ and 4ꢀ oꢀ W [28, 32,
5/2 7/2
34–37]. To understand the inꢃuence oꢀ the incorporation
2
+
Co into NiWO , the high-resolution XPS oꢀ Co 2p, Ni 2p,
4
W 4ꢀ, and O 1s oꢀ all the prepared samples are characterized
as displayed in Fig. S4. No signiꢁcant changes oꢀ binding
energies oꢀ Co 2p, Ni 2p, W 4ꢀ and O 1s are observed, which
2
4
7
.0). To prepare the working electrodes, 1 mg catalysts were
2
+
added to 2 ml ethanol containing 20 μl Naꢁon (5% wt. in
indicates that the introduction oꢀ Co into NiWO does not
4
1
3

C. Guo et al.
Fig. 1 SEM images
oꢀ the a NiWO and b
4
Ni Co WO . c The
0.85
0.15
4
scanned area ꢀor elements
mappings. Elements map-
pings oꢀ d Ni, e Co, and f W
oꢀ Ni Co WO . TEM
0.85
0.15
4
images oꢀ g NiWO and h
4
Ni Co WO
4
0
.85
0.15
change the chemical states oꢀ the materials according to the
Hume–Rothery rule [25, 26].
(0.5 mg) as the catalyst. No liquid products (e.g., HCOOH,
CH OH, CH CH OH) are detected in our system. The high-
3
3
2
−
1
−1
Figure S5(a) displays the FTIR spectra oꢀ
Ni Co WO and NiWO . The vibration absorption
est CO generation rate oꢀ 14.33 mmol g
h
is observed
ꢀor the ꢁrst hour photocatalysis. And, the accumulated ꢀor-
mation oꢀ CO amounts to 15.38 μmol during the ꢁrst ꢀour
hours photocatalysis with a high selectivity oꢀ 86.4%, which
is superior to most oꢀ the reported Co-based heterogeneous
catalysts (Table S1). Aꢀter ꢀour hours photocatalysis, the gas
0
.85
0.15
4
4
−
1
bands at 400–850 cm indicate the presence oꢀ octahedron
−
1
−1
WO . The bands at 810 cm and 570 cm correspond to
6
the vibrations oꢀ O–W–O and W–O bonds, respectively
[
32–35]. The Raman spectra oꢀ Ni Co WO and NiWO
0.85 0.15 4 4
−
1
are displayed in Fig. S5(b). The bands at 950 cm corre-
sponds to the stretching vibration oꢀ the W–O bond and the
production rate oꢀ CO and H declines and reaches a plateau
2
due to the inactivation oꢀ photosensitizer [11, 13, 18, 19].
−
1
−1
bands at 490 cm and 350 cm can be attribute to the
deꢀormation and torsion modes oꢀ W–O [29, 36]. No obvious
diꢂerences can be observed beꢀore and aꢀter the doping oꢀ
To conꢁrm the synergistic eꢂect between the two metals ꢀor
CO reduction during the photoreaction process, a series oꢀ
2
catalysts with varied metal proportions were prepared and
their photocatalytic perꢀormances were evaluated as shown
in Fig. 2c. The Ni Co WO exhibits the highest selectiv-
2
+
Co in NiWO , conducive to the maintenance oꢀ advantages
4
oꢀ NiWO .
4
0.85
0.15
4
2
The speciꢁc surꢀace areas with a value oꢀ 20.68 m /g ꢀor
ity and productivity ꢀor CO among all oꢀ the tested materials.
2
NiWO and 34.46 m /g ꢀor Ni Co WO are obtained
Thus, Ni Co WO was chosen as a model catalyst in
4
0.85
0.15
4
0.85
0.15
4
by analysis oꢀ nitrogen adsorption–desorption isotherms in
Fig. 2a, which may be attributed to the interparticle porosity
oꢀ agglomerated samples as observed in Fig. 1.
this work. The reusability oꢀ Ni Co WO was tested as
0.85 0.15 4
well by addition oꢀ ꢀresh photosensitizers each 4 h during
the reaction. Each test shows high yield and selectivity oꢀ
CO as shown in Fig. 2d, indicating that Ni Co WO
The photocatalytic perꢀormance oꢀ Ni Co WO ꢀor the
1
−x
x
4
0.85
0.15
4
reduction oꢀ CO was evaluated using [Ru(bpy) ]Cl ·6H O
maintains its catalytic perꢀormance ꢀor at least ꢀour times
(16 h). We conclude that the inactivation oꢀ photosensi-
tizer ([Ru(bpy) ]Cl ·6H O) is, thereꢀore, responsible ꢀor the
2
3
2
2
as photosensitizer and TEOA as electron donor. Figure 2b
shows the cumulative gas production (μmol) oꢀ CO (red) and
H (blue) during 5 h photocatalysis using Ni Co WO
3
2
2
decreased production rate oꢀ CO. The Ni Co WO was
2
0.85
0.15
4
0.85
0.15
4
1
3

Ni0.85Co0.15WO for Photocatalytic Reduction of CO Under Mild Conditions with…
4
2
Fig. 2 a Nitrogen adsorption–
desorption isotherms oꢀ the
NiWO and Ni Co WO .
4
0.85
0.15
4
b Production oꢀ CO and H
2
along with reaction time with
Ni Co WO as the catalyst.
0.85
0.15
4
c Selectivity and production oꢀ
CO with diꢂerent element ratios
oꢀ Ni/Co oꢀ catalysts. d Stabil-
ity testing oꢀ Ni Co WO
0.85
0.15
4
within ꢀour recycles. The dos-
age oꢀ catalysts in the reaction
systems is 0.5 mg
Table 1 The photocatalytic reduction oꢀ CO at various reaction con-
Ni Co WO (Entry 4), and without light (Entry 5) were
0.85 0.15 4
2
ditions
perꢀormed and the obtained results are listed in Table 1.
The results demonstrate the necessity oꢀ each component
Entry
CO (µmol)
H (µmol)
CO+H (µmol)
Selectivity
oꢀ CO (%)
2
2
ꢀor photocatalytic reduction oꢀ CO . The eꢂects oꢀ diꢂerent
2
polar solvents like CH CN, tetrahydroꢀuran (THF, Entry 6),
3
1
2
3
4
5
6
7
8
9
15.38
n.d
n.d
0.37
n.d
0.90
0.70
1.80
0.20
n.d
2.42
0.20
0.20
0.30
n.d
15.00
12.00
1.60
1.10
4.01
0.65
17.80
0.20
0.20
0.67
/
15.90
12.70
3.40
1.30
4.01
3.10
86.40
/
/
55.22
/
6.00
5.51
52.94
15.38
/
dimethyl sulꢀoxide (DMSO, Entry 7), N,N-dimethylꢀorma-
mide (DMF, Entry 8), and dichloromethane (DCM, Entry
9
) ꢀor CO photo-reduction were tested, and the catalyst
2
exhibits the best photocatalytic perꢀormance with CH CN
3
as solvent. CO was not detected when using Ar (Entry 10)
instead oꢀ CO , maniꢀesting CO was produced ꢀrom the
2
reduction oꢀ CO . In addition, the control experiment with
2
13
isotope-labelled CO ( CO ) instead oꢀ CO was perꢀormed
2
2
2
and the gas product was measured via GC/MS whose result
1
1
0
1
was shown in Fig. S10(b). The strong signal at m/z = 29
2.45
79.03
13
stemmed ꢀrom CO compared with the normal system (Fig.
13
13
S10(a)), conꢁrming that CO came ꢀrom CO . The signals
2
at m/z=28 and 32 in Fig. S10(b) were ascribed to N and O
2
2
collected aꢀter the reaction and was characterized with XRD,
SEM, XPS and UV–vis as shown in Figs. S6–S9. No sig-
niꢁcant changes in the chemical composition and structure
oꢀ Ni Co WO are observed beꢀore and aꢀter photoreac-
oꢀ air. Signals at m/z=20 and 36 were possibly caused by
the interꢀerence ꢀrom the instrument. The results oꢀ isotope-
labelled CO ꢀurther conꢁrm that CO is produced ꢀrom the
2
photocatalytic conversion oꢀ CO in our reaction system.
0.85
0.15
4
2
tion. This reveals that the Ni Co WO possesses high
Poor gas production was obtained with the physical mixture
0
.85
0.15
4
stability and reusability.
oꢀ NiWO and CoWO as the catalyst with other conditions
4
4
Reꢀerence experiments under normal conditions (Table 1,
consistent (Entry 11), conꢁrming the synergistic eꢂect in
Entry 1, Selectivity = mol (CO)/mol (CO+ H ) × 100%),
Ni Co WO catalyst ꢀor the photocatalytic reduction oꢀ
2
0.85
0.15
4
without TEOA (Entry 2, n.d. represents “Not detect-
able”), without [Ru(bpy) ]Cl ·6H O (Entry 3), without
CO . It is worthy oꢀ noticing that the H evolution rate in Ar
2
2
(Entry 10) with CH CN as solvent is lower than in CO with
3
2
2
3
2
1
3

C. Guo et al.
THF (Entry 6) or DMSO (Entry 7) as solvent. This is due to
the existence oꢀ H O/THF/electron donor (H O/DMSO/elec-
and the horizontal axis shown in Fig. 4b. The band gap oꢀ
Ni Co WO (2.10 eV) is smaller than that oꢀ NiWO
2
2
0.85
0.15
4
4
tron donor) in photocatalytic reaction system, that ꢀacilitates
(2.72 eV), contributing to the capture oꢀ excited elec-
trons to the CB (conduction band). The M-S plots (Mott-
Schottky plots 0.5–1.5 kHz) are perꢀormed to determine
the ꢃat-band potentials oꢀ NiWO and Ni Co WO .
the photocatalytic H evolution [11, 12, 38, 39].
2
To ꢀurther investigate the mechanism oꢀ the superior cata-
lytic activity oꢀ Ni Co WO , the steady-state PL and
0
.85
0.15
4
4
0.85
0.15
4
time-resolved PL were employed to analyze the transꢀer oꢀ
the photo-generated charges. In Fig. 3a, the PL intensity
at 610 nm is decreased with the addition oꢀ trace amount
oꢀ Ni Co WO compared with both the blank system
The NiWO and Ni Co WO possess the character-
4 0.85 0.15 4
istic oꢀ n-type semiconductors due to the positive slopes
in M–S plots [46] as shown in Fig. 4c, d. Generally, the
ꢃat band potential oꢀ n-type semiconductor is close to its
CB potential. Ni Co WO exhibits stronger reducing
0
.85
0.15
4
and the sample containing NiWO (3 µM). The PL inten-
4
0.85
0.15
4
sity quenching implies that recombination oꢀ photo-induced
activity than NiWO due to the more negative potential
4
charges oꢀ Ru(bpy) ·Cl was suppressed efciently. Further,
oꢀ ꢃat band. The ꢃat band potential oꢀ Ni Co WO is
3
2
0.85
0.15
4
the PL liꢀetime oꢀ Ru(bpy) ·Cl was shortened ꢀrom 216 ns
about − 0.62 V (vs NHE, pH 7.0), which is lower than the
3
2
(
blank), 212 ns (with NiWO ) to 197 ns with the presence
redox potential oꢀ CO reduction to CO with the participa-
4
2
+
oꢀ Ni Co WO as shown in Fig. 3b, which discloses
tion oꢀ H (− 0.53 V vs NHE, pH 7.0) [17, 47]. Further-
0
.85
0.15
4
2
+
that the incorporation oꢀ Co into NiWO is beneꢁcial to
more, the smaller semicircle radius oꢀ Ni Co WO
4
0.85
0.15
4
the separation and transꢀer oꢀ photo-induced charge carriers.
The decreased PL intensity as well as the liꢀetime oꢀ photo-
sensitizer Ru(bpy) ·Cl reveals the inhibited recombination
compared with NiWO in the EIS (electrochemical imped-
4
ance spectra) in Fig. S11 indicates its lower transꢀer resist-
ance [30, 31, 36, 46] and enhances the photocatalytic
3
2
and improved transportation oꢀ photo-induced charges can
activity ꢀor the reduction oꢀ CO .
2
ꢀ
acilitate the utilization oꢀ excited electrons, thereby acceler-
Based on aꢀorementioned experimental results a hypo-
thetical reaction mechanism is proposed [15, 16, 48, 49].
Initially, the photosensitizer [Ru(bpy) ]Cl ·6H O is excited
ate the conversion oꢀ CO to CO [18, 40–42].
2
Figure 4a displays the UV–vis DRS oꢀ NiWO and
4
3
2
2
2
+*
Ni Co WO . The ultraviolet absorption bands are
by visible light to ꢀorm the excited state [Ru(bpy) ]
.
0
.85
0.15
4
3
attributed to the excitation oꢀ electrons ꢀrom O to Wt
Aꢀterwards, the excited electrons are transꢀerred ꢀrom
2
p
2g
2
+*
in the WO , and the bands shiꢀts oꢀ Ni Co WO arises
[Ru(bpy) ] to the catalyst Ni Co WO , meanwhile,
6
0.85
0.15
4
3
0.85
0.15
4
2
+
2+
ꢀ
rom the distortion oꢀ Co doping [27, 43]. The band gaps
TEOA donates electrons to [Ru(bpy) ] ꢀollowed by con-
3
+
oꢀ NiWO and Ni Co WO were acquired ꢀrom the
verting itselꢀ to TEOA . CO absorbed on Ni Co WO
4
0.85
0.15
4
2
0.85
0.15
4
1
/2
plots oꢀ (αhυ) vs Energy (eV) calculated according to
the Tauc-Equation [32, 44, 45]. The band gaps oꢀ the two
samples are estimated by the intersection oꢀ the tangent
is then reduced by the electrons coming ꢀrom the excited
2
+
[Ru(bpy) ] , and the photosensitizer returns to the ground
3
state.
Fig. 3 a Steady-state photoluminescence (PL) spectra oꢀ a CH CN/
tra oꢀ a CH CN/H O (V
/VH2O =4) solution containing 300 μM
3
3
3
2
CH3CN
H O (V
/V =4) solution containing 300 µM Ru(bpy) ·Cl
[Ru(bpy) ]Cl (Blank) with Ni Co WO (3 µM) and NiWO
3 2 0.85 0.15 4 4
2
CH3CN H2O
2
4
(
(
Blank) in the presence oꢀ Ni Co WO (3 µM) and NiWO
(3 µM). The samples are excited by the incident light oꢀ 450 nm,
while the PL decay spectra at 610 nm are monitored
0
.85
0.15
4
3 µM) excited with 450 nm light. b Time-resolved PL decay spec-
1
3

Ni0.85Co0.15WO for Photocatalytic Reduction of CO Under Mild Conditions with…
4
2
Fig. 4 a, b UV–Vis DRS
and band gaps oꢀ NiWO
4
and Ni Co WO . Mott–
0
.85
0.15
4
Schottky plots oꢀ c NiWO and
4
d Ni Co WO in neutral
0
.85
0.15
4
aqueous solution
4
Conclusion
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conꢃict oꢀ
In conclusion, we prepared a series oꢀ Ni–Co–W ternary
materials (Ni Co WO , x = 0–1) by co-preposition
interest.
1
−x
x
4
method. These materials were applied to the photocatalytic
reduction oꢀ CO to CO under mild conditions with visible
2
light irradiations. Among these catalysts, Ni Co WO ,
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Aꢀliations
1
1
1
1
1
1
Chunmei Guo · Biao Guo · Xiaosu Gao · Jing Liang · Qide Meng · Jinxuan Liu
1
State Key Laboratory oꢀ Fine Chemicals, Institute
oꢀ Artiꢁcial Photosynthesis, Dalian University
oꢀ Technology, Dalian 116024, China
1
3