Highly Efficient Photocatalytic System Constructed from CoP/Carbon Nanotubes or Graphene for Visible-Light-Driven CO2 Reduction

Article abstract of DOI:10.1002/chem.201800335

Visible-light-driven conversion of CO₂ to CO and high-value-added carbon products is a promising strategy for mitigating CO₂ emissions and reserving solar energy in chemical form. We report an efficient system for CO₂ transformation to CO catalyzed by bare CoP, hybrid CoP/carbon nanotubes (CNTs), and CoP/reduced graphene oxide (rGO) in mixed aqueous solutions containing a Ru-based photosensitizer, under visible-light irradiation. The in situ prepared hybrid catalysts CoP/CNT and CoP/rGO show excellent catalytic activities in CO₂ reduction to CO, with a catalytic rates of up to 39 510 and 47 330 μmol h^?1 g^?1 in the first 2 h of reaction, respectively; a high CO selectivity of 73.1 % for the former was achieved in parallel competing reactions in the photoreduction of CO₂ and H₂O. A combination of experimental and computational studies clearly shows that strong interactions between CoP and carbon-supported materials and partially adsorbed H₂O molecules on the catalyst surface significantly improve CO-generating rates.

Full text of DOI:10.1002/chem.201800335

A Journal of
Accepted Article
Title: Highly Efficient Photocatalytic System Constructed from CoP/
Carbon Nanotubes or Graphene for Visible-Light-Driven CO2
Reduction
Authors: Wen Gan Fu
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To be cited as: Chem. Eur. J. 10.1002/chem.201800335
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Highly Efficient Photocatalytic System Constructed from
CoP/Carbon Nanotubes or Graphene for Visible-Light-Driven CO2
Reduction
Zi-Cheng Fu,[a,b] Rong-Chen Xu,[c] Joshua T. Moore,*[b] Fei Liang,[d] Xiao-Cun Nie,[c] Chen Mi,[c] Jiang
Mo,[c] Yong Xu,[a] Quan-Qing Xu,[c] Zhi Yang,[c] Zhe-Shuai Lin,[d] Wen-Fu Fu*[a,c]
Dedication ((optional))
Abstract: Visible-light-driven conversion of CO2 to CO and high-
value-added carbon products is a promising strategy for mitigating
CO2 emissions and reserving solar energy in chemical form. We
report an efficient system for CO2 transformation to CO catalyzed by
bare CoP, hybrid CoP/carbon nanotubes (CNTs), and CoP/reduced
graphene oxide (rGO) in mixed aqueous solutions containing a Ru-
based photosensitizer, under visible-light irradiation. The in situ
prepared hybrid catalysts, i.e., CoP/CNT and CoP/rGO, show
excellent catalytic activities in CO2 reduction to CO, with a catalytic
rates of up to 39,510 and 47,330 μmol h−1 g−1 in the first 2 h of
reaction, respectively; a high CO selectivity of 73.1% for the former
was achieved in parallel competing reactions in the photoreduction
of CO2 and H2O. A combination of experimental and computational
studies clearly shows that strong interactions between CoP and
carbon-supported materials and partially adsorbed H2O molecules
on the catalyst surface significantly improve CO-generating rates.
primarily on two types of catalytic material: homogeneous metal
complexes[4] semiconductors.[5]
and inorganic The
heterogeneous semiconductor catalysts reported to date are
inefficient, largely because of inadequate use of solar light, and
poor catalytic activity and product selectivity in competitive
reduction between CO2 and H2O because of the sluggish
kinetics involving multiple electrons and protons.[6] Molecular
catalysts such as Fe,[7] Co,[8] Ni,[9] and Mn[10] complexes are
robust and give effective reduction of CO2, but their instability
and recycling issues severely restrict their practical applications
in photocatalysis. The development of new photocatalytic
systems for CO2 reduction with good catalytic efficiencies and
high quantum yields remains
a grand challenge in both
approaches. Among various candidates, Co compounds are one
of the most promising catalysts.[11] Wang and coworkers used a
Ru-based photosensitizer (PS) and a Co-containing zeolitic
imidazolate framework to construct a visible-light-driven system
for CO2 reduction,[12] but the catalytic activity and selectivities for
CO and H2 were a significant challenge. This approach was
developed by using Co3O4 hexagonal platelets, and gave highly
efficient CO2 conversion with a CO yield of 2,003 μmol h−1 g−1 in
the first 3 h of reaction and high selectivity of 77.1%.[13]
The heavy reliance on fossil fuels for the production of energy
has increased atmospheric CO2 emissions, which is considered
to be an important factor in anthropogenic climate change.[1]
Artificial photosynthesis has therefore attracted wide attention,
with specific importance attached to the development of efficient
and solar-light-driven methods that reduce CO2 and convert it
into energy-rich carbon-based products such as CO, methanol,
methane, and aliphatic hydrocarbons.[2] To combat the chemical
inertness (fG = 396 kJ mol−1) of CO2 and the associated limits
of the reaction’s efficiency, most efforts have focused on
exploration of promising new highly active catalytic systems.[3]
Current research on catalysts for CO2 reduction focuses
Metal phosphide nanoparticles (NPs) have high stability and
characteristics similar to those of zero-valent metals. The
metallic-state compounds have been extensively studied as
efficient electrocatalysts in water splitting,[14] and are highly
efficient cocatalysts for photodriven H2 evolution and robust
catalysts for hydrolytic dehydrogenation of ammonia borane, as
reported in our previous papers.[15,16] However, the use of metal
phosphide NPs as catalysts for the photoreduction of CO2 is
challenging because of significant interference of water
reduction during photocatalysis. Generally, the transformation of
CO2 is conducted in nonaqueous solvents to suppress
competitive reduction of water itself to H2. In aqueous media, H2
evolution by H2O reduction is kinetically and thermodynamically
more favorable than CO2 reduction.[17] Therefore, enhancement
of the catalytic activity and product selectivity for CO2 reduction
by changing the catalyst morphology and size, the structure of
the hybrid catalyst interface and surface-modification of catalyst
NPs to promote adsorption and transformation of CO2 are
effective strategies for overcoming the above drawbacks,
especially in electrochemical CO2 reduction.[18] Bao and
coworkers found that the presence of OH groups on the catalyst
surface enhanced the adsorption and activity of CO2.[19] Our
previous work combining experimental studies and first-
principles calculations also showed that the competing
adsorption of OH groups and H2O molecules on CoP surfaces
changes the chemical environments of catalyst surface, and
remarkably causes the changes of reaction rate and initiation
[a)
Z. –C. Fu, Y. Xu, Prof. Dr. W. –F. Fu
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials and HKU-CAS Joint Laboratory on New Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing 100190, P.R. China
E-mail: fuwf@mail.ipc.ac.cn
[b]
[c]
[d]
Z. –C. Fu, Prof. Dr. J. T. Moore
Department of Chemistry, Tennessee State University, 3500 John
A. Merritt Blvd., Nashville, TN 37209, USA
E-mail: jmoore17@tnstate.edu
R. –C. Xu, X. –C. Nie, C. Mi, J. Mo, Prof. Dr. Q. –Q. Xu, Prof. Dr.
Z. Yang, Prof. Dr. W. –F. Fu
College of Chemistry and Engineering, Yunnan Normal University
Kunming 650092, P.R. China
F. Liang, Prof. Dr. Z. –S. Lin
Center for Crystal R&D, Key Lab of Functional Crystals and Laser
Technology, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing 100190, P.R. China
Supporting information for this article is given via a link at the end
of the document.
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period for catalytic H2 generation from ammonia borane
hydrolysis.[16b] We therefore expect that limiting the adsorption of
H2O molecules on the catalyst surface could improve the
catalytic performance in CO2 photoreduction via tuning surface
interactions between H2O and CO2 to provide a high local CO2
concentration and facilitate proton-coupled transformation of the
initial Co–CO2 adduct.
Here, we present for the first time
a highly efficient
photocatalytic system consisting of CoP modified with carbon
nanotubes (CoP/CNT) or reduced graphene oxide (CoP/rGO) as
the catalyst for visible-light-driven CO2 to CO conversion in a
mixed water/acetonitrile medium with Ru(bpy)3Cl2 (bpy = 2,2'-
bipyridine) as a PS and triethanolamine (TEOA) as an electron
donor. The catalytic activities and CO selectivities of CoP/CNT
and CoP/rGO are better than those of bare CoP. The
dependence of the photocatalytic CO2 to CO transformation on
the amount of H2O was studied. A mechanism of CO2-to-CO
conversion based on experimental results and density functional
theory (DFT) calculations is proposed.
CoP NPs were prepared using a method previously
reported by our group.[16b] The hybrid Co3O4/CNT and
Co3O4/rGO were synthesized by a facile scalable approach
involving a hydrothermal reaction in an autoclave (SI)[20] and
Co3O4 phosphorized with NaH2PO2 to form the CoP-embedded
carbonaceous materials (Scheme 1).[21]
Figure 1. (a) XRD patterns of CoP, CoP/rGO, and CoP/CNT; (b) TEM image
of in situ prepared CoP/CNT; (c) TEM image and corresponding EDX
elemental mapping images of C, Co, and P for CoP/CNT; (d) HRTEM image of
CoP/CNT; (e) TEM and (f) HRTEM images of in situ prepared CoP/rGO.
Figure 1a); the peak at about 26 confirms the presence of the
carbon support.
High-resolution transmission electron microscopy (HRTEM)
images (Figures 1d and 1f) show strong interactions between
CoP and the carbonaceous supports. The well-resolved lattice
fringes with interplanar distances of 0.19 and 0.25 nm for the
CoP NPs incorporated into carbonaceous materials are
consistent with the (211) and (111) planes, respectively, of the
CoP crystal structure. Energy-dispersive X-ray (EDX) elemental
mapping analysis in selected field (Figure 1c) suggests that Co
and P in CoP/CNT are uniformly distributed on the CNT surfaces
via oxide–CNT anchoring. The chemical states of Co, P, and C
in CoP/CNT and CoP/rGO were further identified by X-ray
photoelectron spectroscopy (XPS). Figure S2 shows that the
low-energy peaks with binding energies of 779.2 and 782.2 eV
for CoP/CNT (778.9 and 781.8 eV for CoP/rGO, Figure S3) are
slightly positively shifted from those for metallic Co (778.1778.2
eV), suggesting weak charge transfer from Co to P; this is
consistent with the Co 2p3/2 state.[22] The high-energy peaks
centered at 794.1, 798.2, and 803.1 eV for the former two
correspond to Co 2p1/2 electrons in CoP, whereas the latter was
associated with the excitation of high-spin Co2+ ions, originating
from superficial air oxidation of CoP.[23] For the P(2p) region, the
Scheme 1. Schematic diagram of in situ preparation of hybrid CoP/CNT.
The CoP NP loadings on the CNTs or rGO were controlled to
43 and 48 wt%, respectively. The amounts were determined by
elemental analysis using inductively coupled plasma-atomic
emission spectrometry (ICP-AES). The Co:P atomic ratios were
0.93:1 and 1:0.96 for the former and latter, respectively,
confirming successful formation of the desired CoP NPs.
Scanning transmission electron microscopy (STEM) images
(Figure 1b–f) show that the average size of the CoP NPs
embedded in the carbonaceous materials was 6–10 nm; bare
CoP NPs are about 15 nm in diameter (Figure S1). The amounts
of C, H, and N elements, determined using a Vario EL III
instrument, were 34.20%, 2.59%, and 1.48%, respectively, for
CoP/CNT, and 27.61%, 2.58%, and 3.17%, for CoP/rGO; this
indicates the presence of N species. The lower N percentage
probably originated from the ammonium hydroxide used in the
initial preparation process.[20] In the X-ray diffraction (XRD)
patterns of pure CoP and the hybrid CoP/CNT and CoP/rGO all
the diffraction peaks from CoP can be indexed to a phosphide
with an orthorhombic structure (JCPDS Card No. 29˗0497;
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peaks located at 130.7 and 129.9 eV correspond to P 2p1/2 and
P 2p3/2 states, respectively (Figure S2), and indicate that P is
nearly charge neutral. The broad peaks centered at 134.2 and
133.3 eV can be ascribed to oxidized P species.[24] The C 1s
spectrum was deconvoluted into three peaks centered at 284.8,
286.2, and 288.5 eV, corresponding to C=C, C−C, and carbon
bonded to N- or O-containing species, respectively.[25] The
Raman spectra of CoP/CNT and CoP/rGO show the
characteristic D band (1348 cm−1) and G band (1587 cm−1) of
the carbon species, and the ratios of ID to IG for the former and
latter are 1.50 and 1.42, respectively, implying destructive
coalescence of the hybrid materials (Figure S4). The high ID:IG
suggests that the introduction of CoP leads to more structural
CoP/rGO (in the range used, i.e., 1 to 8 mg), a negative
dependence of generated CO2 and H2 on the hybrid catalyst
content appeared, because of both adsorption of PS molecules
on rGO and a light-shielding effect. The selectivity for CO varied
from 62.3% to 68.3% (66.7%, 0.5 g CoP/rGO) when CoP/rGO
amounts from 1 to 8 mg were used. As shown in Figure 2d, a
comparison of the yields for photocatalytic conversion of CO2 to
CO indicates that the as-prepared hybrid CoP/rGO is an
excellent catalyst; this is because of synergistic coupling
between CoP and the carbon support, and the catalytic rate
reached 47,330 μmol h−1 g−1 in the first 2 h of the reaction.
These observations are in good agreement with the particle size
of CoP and interfacial interactions between CoP and
defects and disordered structures in the carbonaceous materials, carbonaceous materials. The order of the selectivity for CO over
boosting the catalytic performance.[26]
competing H2 generation is CoP < CoP/rGO < CoP/CNT;
CoP/CNT gave the highest CO selectivity, i.e., 73.1% (a catalytic
rates of up to 39,510 μmol h−1 g−1), implying that other factors
such as changes in the chemical environment around CoP
significantly affect the competitive reaction pathways.
The photocatalytic systems for CO2 reduction were assembled
using as-prepared CoP, CoP/CNT, and CoP/rGO as catalysts,
and Ru(bpy)3Cl2 as a PS in a CO2-saturated TEOA/CH3CN/H2O
(v/v, 1/2/1) mixed solvent. To verify that the CO originated from
CO2, isotopic labeling experiments were conducted using 13CO2
in a gas chromatography-mass spectrometry (GC-MS) system
under identical catalysis conditions. Figure S5 proves that CO
was formed by reduction of CO2, and precluded possible
degradation of organics used. The photoreduction gas products
were identified using GC, with Ar as the carrier gas. Further
control experiments showed that no CO was generated if any of
the following components were absent: light, the PS, or the
catalyst. In Ar, the system generated a large quantity of
hydrogen without any CO evolution. In addition, the solution
after the photoreaction was analysed by ion chromatography,
only trace amounts of HCOOH were detected, CO was therefore
the sole carbon-containing product. The generation rates of CO
and H2 were evaluated as a function of catalyst concentration at
fixed amounts of PS at ambient temperature (Figure 2a and 2c,
Figure S6). In the case of CoP, the photoreduction yields of CO2
increased with increasing catalyst concentration, and reached a
limiting value, with a CO selectivity of 61.1% (Figure 2a). When
the amount of CoP was further increased, the total CO2-to-CO
conversion and H2 liberation decreased because of a light-
shielding effect and aggregation of catalyst during
photoreduction. Under the same reaction conditions, the optimal
CO and H2 evolution rates on CoP/CNT were significantly better
than the CO2 reduction rate on CoP (Figure S6). The enhanced
performance is attributed to CNT introduction improving the
electronic conduction paths and preventing NPs aggregation.[27]
The maximum CO generation rate, 122.6 µmol, and an average
selectivity of 70.5% were achieved after 2 h of visible-light
irradiation over the applied catalyst range (Figure S6). At higher
catalyst concentrations, decreases in CO and H2 production
analogous to those for CoP as the cocatalyst were observed.
The PS dependences of CO2 and H2O photoreduction were also
studied under identical conditions; the obtained data showed a
curve-shape change with increasing PS concentration (Figure
2b). A comparison with the results for the dependence of
photoreduction rates on concentration of CoP/CNT (Figure S6)
and PS shows that the selectivity decreased slightly (Figure 2b,
average value of 63.8%) with increasing PS concentrations
because of the increasing contribution of PS itself to H2 evolution.
The results in Figure 2c clearly show that in the presence of
Figure 2. Effect of (a) CoP and (c) CoP/rGO catalyst content on catalytic
performance in CO2 reduction in 20 mL of CO2-saturated TEOA/CH3CN/H2O
(v/v, 1/2/1) solvent containing [Ru(bpy)3]Cl2 (5 mg) under irradiation with
stirring for 2 h at 298 K. The total volume of the quartz reaction tube was 60
mL (300 W Xe lamp with 400 nm filter); (b) PS dependence of photoreduction
of CO2 at fixed amount (2 mg) of CoP/CNT under similar conditions as in (a)
and (c); (d) yields of CO and H2 after irradiation of suspension containing
[Ru(bpy)3]Cl2 (5 mg) and catalyst (1 mg) for 2 h under identical conditions to
those in (a), (c), and Figure S6.
To get an in-depth understanding of the improved
photoreduction performance, the yields of CO and H2 were
measured in this study as a function of irradiation time. The
results, shown in Figure 3, clearly show changes in the relative
amounts of the products from both competing pathways over
time. Initially, a higher H2-generating rate was observed. The
photocatalytic reaction for the conversion of CO2 to CO became
more crucial with increasing reaction time, and rapid
acceleration of the CO-generating rate occurred after irradiation
for 30 min. The crossing points occurred at about 1.4, 1.0, and
1.2 h during irradiation for the CoP, CoP/CNT, and CoP/rGO
catalysts, respectively; this is consistent with the selectivity for
CO seen in Figure 2d. This means that in the initial period
species formed by the interaction between molecules in reaction
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system and surface on catalyst tunes the competing catalytic
pathways, and changes the catalytic properties of the catalyst
surface.[16b] If this speculation is correct, the interactions
between H2O and atoms on the catalyst surface may be
important. The dependence of the CO-generating rate on the
H2O content in CO2 photoreduction on CoP/CNT was therefore
studied as a function of various H2O contents. The results clearly
demonstrate that the amount of H2O on the catalyst surface
plays a significant role in improving the catalytic efficiency and
selectivity in CO2 transformation. In the absence of H2O, the
catalyst shows a lower CO-generating activity, but the rate of
CO production increases sharply with addition of H2O (Figure
3d), and reaches the limiting values after adding 5 mL of H2O.
Subsequent addition leads to low catalytic efficiency (inset in
Figure 3d), and H2 generation shows a similar trend (Figure S7),
which indicates that the interactions between H2O molecules
and atoms on the catalyst surface accelerate the CO- and H2-
generating reactions and excess H2O decreases the catalytic
activity. The former effect is beneficial for proton transfer and
increasing the local CO2 concentration, but the latter results in a
decrease in the number of active catalytic sites because the
catalyst surface is covered by H2O.
atom of the CoP surface (6.682 Å) approaches a P•••H distance
of 1.410 Å, the interaction between the H atom and the O atom
of CO2 is accompanied by a H•••O distance of 3.705 Å, and
energy of 3.73 eV is released. The transition state TS1, in which
the H atom is closer to O, is generated by overcoming an energy
barrier of 2.42 eV. For TS1, the C–O bond length of 1.34 Å is
close to that of a typical single bond (ca. 1.34 Å), and it
undergoes an energetically downhill transformation to the
intermediate OCOH*. Afterwards, the change in the C•••OH
distance from 2.316 in TS2 to 3.681 Å is also energetically
downhill. This shows that the HO–CO bond was split to give an
OH group and a stable intermediate with a Co•••CO distance of
1.786 Å; the H–O and CO bond lengths are 0.972 and 1.158 Å,
respectively. It was noted that generation of CO as the final
product from Co•••CO requires overcoming of a slightly lower
energy barrier (1.50 eV) than the activation energy of TS1,
suggesting that the key kinetic step in photoreduction CO2 is the
proton transfer step. Figures S9 and S10 show that the
activation energy for TS1 in the presence of H2O is 0.99 eV
lower than that in the presence of H+. The lower reaction barrier
for the rate-determining step reveals that CO2 photoreduction is
favored in the presence of H2O; this is in good agreement with
the experimental results. In addition, it can be seen from Figure
S11 that the CoP/rGO catalyst retained about 97% of its initial
activity after five cycles, and XPS analysis shows that the peak
profiles of the recovered catalysts were almost unchanged
(Figures S12 and S13). The slight drop in activity could be
ascribed to the breaking away of CoP from the carbon materials
because of violent stir during the catalytic reaction.
Figure 3. CO and H2 evolution as function of irradiation time for visible-light
catalytic performance in 20 mL of CO2-saturated TEOA/CH3CN/H2O (v/v,
1/2/1) solvent containing [Ru(bpy)3]Cl2 (10 mg) and (a) CoP (2 mg), (b)
CoP/CNT (2 mg), and (c) CoP/rGO (2 mg) at 298 K. (d) Dependence of CO-
generating rate in CO2 photoreduction on H2O content in 15 mL of CO2-
saturated TEOA/CH3CN (v/v, 1/2) solvent at fixed amounts of CoP/CNT (2 mg)
and [Ru(bpy)3]Cl2 (10 mg); inset: CO evolution weakened with increasing H2O
amount from 5 to 7 mL, and then to 9 mL.
Figure 4. Relative energy change diagram for CO2 reduction to CO catalyzed
by CoP in system containing proton donor.
DFT calculations show that the adsorption energies for CO2
on the CoP (010) surface without and with P defects are −0.012
and −0.223 eV, respectively. During CO2 photoreduction these
surface defects facilitate the approach of carbon atoms of CO2
molecules to Co atoms, resulting in the configuration of the CO2
molecule changing from linear to V-shaped, and C–Co and O–
Co distances of 1.955 and 1.968 Å, respectively (Figure 4). The
C=O bond lengths of 1.211 and 1.280 Å in the V-configuration
are considerably longer than those of 1.172 and 1.169 Å in the
linear molecule (Figure S8). The calculation results indicate that
high catalytic activity arises from P defects on the CoP
surface.[28,29] When the H atom of a proton donor far from the P
In summary, coupled CoP/CNT and CoP/rGO catalysts show
higher catalytic activities and CO selectivity for CO2-to-CO in the
presence of
a Ru-based PS under visible-light irradiation
compared with those for CoP alone. A catalytic rate of 47,330
μmol h−1 g−1 (CoP/rGO) and 73.1% (CoP/CNT) selectivity for
CO2 conversion to CO were achieved in parallel with water
reduction. The effects of interactions between CoP and
carbonaceous materials, P defects on the CoP surface, and
partially adsorbed H2O molecules, which lowers the activation
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Acknowledgements
This work was financially supported by the National Key Basic
Research Program of China (973 Program 2013CB834804) and
the Ministry of Science and Technology (2012DFH40090). We
thank the National Natural Science Foundation of China
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Keywords: cobalt phosphide/carbon nanotube • cobalt
phosphide/graphene • CO2-to-CO conversion• photocatalysis •
Ru-based photosensitizer
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10.1002/chem.201800335
Chemistry - A European Journal
COMMUNICATION
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Photocatalytic Reduction of CO2 to CO
COMMUNICATION
Z. –C. Fu, R. –C. Xu, J. T. Moore,* F.
Liang, X. –C. Nie, C. Mi, J. Mo, Y. Xu, Q.
–Q. Xu, Z. Yang, Z. –S. Lin, W. –F. Fu*
New systems for visible-light-driven
reduction of CO2 to CO catalyzed by
hybrid CoP/CNT or CoP/rGO with a
Ru-based photosensitizer in a mixed
aqueous medium were constructed. A
catalytic rate of 47,330 μmol h−1 g−1
and 73.4% CO selectivity were
achieved. Strong interactions between
CoP and carbonaceous materials and
partially adsorbed H2O molecules on
the catalyst surface greatly improved
the CO-generating rates.
Page No. – Page No.
Highly Efficient Photocatalytic
System Constructed from
CoP/Carbon Nanotubes or Graphene
for Visible-Light-Driven CO2
Reduction
This article is protected by copyright. All rights reserved.