Preparation of a Cu(BTC)-rGO catalyst loaded on a Pt deposited Cu foam cathode to reduce CO2 in a photoelectrochemical cell

Article abstract of DOI:10.1039/c8ra05964k

To increase the reaction productivity and selectivity of the CO₂ photoelectrochemical reduction reaction, a Cu (benzene 1,3,5-tricarboxylic acid [BTC])-reduced graphite oxide (rGO) catalyst was prepared by using a facile hydrothermal method and used in a CO₂ photoelectrochemical cell (PEC) as a cathode catalyst. Characterization of the catalyst proved that successfully bonding of rGO to Cu(BTC) not only facilitated faster transfer of electrons on the surface of the catalyst but also created more active sites. CO₂ photoelectrochemical reduction experimental results showed that the total carbon atom conversion rate was up to 3256 nmol h^-1 cm^-2 which was much higher than when pure Cu(BTC) was used as a cathode catalyst. The liquid product's selectivity to alcohols was up to 95% when-2 V voltage was applied to the system with Cu(BTC)-rGO used as the cathode catalyst.

Full text of DOI:10.1039/c8ra05964k

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Preparation of a Cu(BTC)-rGO catalyst loaded on
a Pt deposited Cu foam cathode to reduce CO in
2
Cite this: RSC Adv., 2018, 8, 32296
a photoelectrochemical cell
Jun Cheng, * Xiaoxu Xuan, Xiao Yang, Junhu Zhou and Kefa Cen
To increase the reaction productivity and selectivity of the CO
a Cu (benzene 1,3,5-tricarboxylic acid [BTC])-reduced graphite oxide (rGO) catalyst was prepared by using
a facile hydrothermal method and used in a CO photoelectrochemical cell (PEC) as a cathode catalyst.
Characterization of the catalyst proved that successfully bonding of rGO to Cu(BTC) not only facilitated
faster transfer of electrons on the surface of the catalyst but also created more active sites. CO
photoelectrochemical reduction experimental results showed that the total carbon atom conversion rate
2
photoelectrochemical reduction reaction,
2
2
Received 13th July 2018
Accepted 9th September 2018
ꢀ
1
ꢀ2
was up to 3256 nmol h cm which was much higher than when pure Cu(BTC) was used as a cathode
catalyst. The liquid product's selectivity to alcohols was up to 95% when ꢀ2 V voltage was applied to the
system with Cu(BTC)-rGO used as the cathode catalyst.
DOI: 10.1039/c8ra05964k
rsc.li/rsc-advances
such as ethanol is low. Cu foam, which has 3D structure, is
a good cathode base in electrochemical reactions. Cheng Jun
Converting CO to clean fuels is considered an efficient way to et al. used Cu foam as a support base and loaded graphene on
1
. Introduction
14
2
solve both the problems of global warming and energy shortage the matrix to reduce CO
2
in a photoelectrochemical cell.
at the same time. Articial photosynthesis is a promising way However, the selectivity toward a specic product is still low.
in which CO is reduced to organic chemicals such as methanol,
Metal organic frameworks (MOFs), as an emerging porous
ethanol and formic acid, by using electricity and light with the material, have been reported in many researches.
1–4
2
1
5–21
Numerous
researchers have reported its outstanding performance in gas
5
–7
help of catalysts.
Given the large specic surface area and extremely high separation and catalytic reduction or oxidation.
2
2–25
Bohan Shan
electron transition rate of graphene, using graphene as a CO2 et al. synthesized a novel Co-based MOF to separate CO /N and
reduction catalyst is a topic of great interest in the research CO /CH . Moreover, the CO separation rates have reached 61.4%
eld. Fan Yu Jia et al. studied the role that graphene played in and 11.7%, respectively. However, this Co-based MOF in the gas
the CO and H
hydrocarbonylation reaction and graphene separation process is only a physical adsorption and desorption
obviously helped increase the absorption of H
as the CO
conversion rate. Moreover, selectivity toward meth- used an electrochemical method, which successfully synthesized
anol was increased to 85%. However, high pressure should be a highly active Cu-based MOF material and used it as a catalyst to
applied to ensure the proceeding of the reaction and this requires reduce CO with an electrolyte consisting of a dimethylformamide
good resistance of the reactor. Geioushy R. A. et al. synthesized solution of tetrabutyl ammonium tetra uoroborate saturated with
a Cu O/graphene catalyst and used it in the CO electrochemical CO . The result showed that this catalyst is an effective catalyst to
reduction reaction. The product analysis result showed that this reduce CO to organics. However, the organic synthesized was
catalyst had good selectivity toward ethanol generation. However, mainly oxalic acid and liquid fuels like ethanol and methanol were
8,9
26
2
2
2
4
2
1
0
11

2
2
27
and CO
2
, as well process but not in a chemical reaction. R. Senthil Kumar et al.
2
2
2
12
2
2
2
2
28
the energy used in this system totally came from electricity; not obtained. Guoqiang Song et al. fabricated a porous Cu-
renewable energies such as solar energy were not used. Cheng benzene 1,3,5-tricarboxylic acid (BTC) silica monolith heteroge-
13
Jun et al. brushed the mixture of Naon and reduced graphite neous catalyst for the selective oxidation of alkylbenzenes.
oxide (rGO) on a Ni foam and used it as the cathode catalyst to However, the usage of Cu(BTC) in CO photoelectrochemical
reduce CO to valuable chemicals. However, the major products reduction reactions has never been reported.
2
2
of Naon and rGO are formic and acetic acids, which cannot be
In this paper, a facile hydrothermal method was implied to
used as fuel directly. Moreover, the selectivity toward liquid fuels synthesize Cu(BTC)-rGO catalyst, characterizations of the cata-
lyst proved faster electron transfer rate and existence of more
reaction active sites on the surface. These features facilitated
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou
the improvement of productivity and selectivity of CO photo-
2
3
8
10027, China. E-mail: juncheng@zju.edu.cn; Fax: +86 571 87951616; Tel: +86 571
7952889
electrochemical reduction reaction.
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Then, the cathode was fabricated using the following
method: 90 mg of as synthesized Cu(BTC)-rGO was mixed with
the Naon solution; aerward, the solution was placed on a Pt-
2
. Experiment
2.1. Cathode preparation
2
.1.1. GO synthesis. The GO used in this work was prepared deposited Cu foam using the drop–dry method.
29
using the Hummers' method. A total of 1 g high purity
2.1.3. Fabrication of Cu(BTC)/Pt–Cu foam cathode. Blue
graphite was added into 25 ml of concentrated sulfuric acid. A mixture of Cu(BTC) and Naon was obtained by mixing 90 mg
total of 3.5 g of potassium permanganate was slowly added into Cu(BTC) with Naon. Then, the mixture was applied on a Pt-
ꢁ
the solution into the 0 C ice bath; aerward, the mixed solution deposited Cu foam using a drop–dry method.
was stirred. The mixture was stirred and heated in an oil bath at
ꢁ
a temperature of 35 C for 2 h. Aerward, the mixture was
2
2
.2. Preparation of Pt–TiO nanotube anode
diluted with 100 ml of deionized water and then oxidized with
ml of 30% H in the ice bath. The resulting suspension was The Pt–TiO nanotubes (Pt-TNTs) were prepared based on the
washed with 5% diluted HCl, ultrasonically stripped, and was anode oxidation method. The titanium sheet was rst rinsed in
placed into a freeze dryer to be dried.
isopropanol and ethanol for 15 min for ultrasonic cleaning
.1.2. Fabrication of Cu(BTC)-rGO/Pt–Cu foam cathode. before oxidation. Aerward, the titanium sheet was oxidized in
8
2 2
O
2
2
Cu(BTC)-rGO was synthesized using a facile hydrothermal the uorine-containing ethylene glycol electrolyte (EG + 0.3%
method. A total of 150 mg of as synthesized GO was dissolved in NH F + 2 vol% H O) for 2 h, with 60 V applied voltage and Pt as
4
2
1
0
5 ml of ethanol; this solution was marked as L1. A total of the cathode. Then, the sheet was washed with deionized water
.93 g of Cu(NO was dissolved in 15 ml of DMF; this solution and was placed into the muffle furnace to be calcined under
3 2
)
ꢁ
ꢁ
ꢀ1
was marked as L2. A total of 0.44 g of BTC was dissolved in 30 ml 450 C for 3 h, with the temperature rising rate of 2 C min
.
of the ethanol and DMF mixture (volume, 1 : 1); this solution Thus, the TiO nanotube crystals can be turned into anatase,
2
was marked as L3. Aerward, L1, L2, and L3 were mixed aer which has better light response. The obtained anatase TiO2
stirring for 30 min. Then, the mixture was poured into a Teon- nanotubes on the titanium sheet was deposited with Pt in 1 g
ꢀ
1
ꢀ2
lined autoclave aer stirring for 30 min, and the autoclave was
L
2 6 2
of H PtCl $6H O (depositing current density: 2.5 mA cm )
ꢁ
kept under 120 C for 6 h. Black-blue precipitates were formed, to improve its photocatalytic performance.
centrifuged, and washed with methanol and deionized water
several times to remove the undesirable impurities. Aerward,
ꢁ
the precipitates were dried in dryer at 80 C for 24 h. In this 2.3. CO
2
photoelectrochemical reduction in a PEC
To test the performance of the as fabricated cathodes and nd
the best reaction condition of CO photoelectrochemical
reduction. CO photoelectrochemical reduction reactions with
process, the added GO was transformed to rGO, given that GO
was unstable under high temperature and can be easily
reduced.
2
2
The Cu foam was deposited with Pt by using an electro-
two different cathodes were conducted, the reactions with
different voltage applied on Cu(BTC)-rGO/Pd–Cu foam cathode
were also conducted.
ꢀ
1
chemical deposition method in a 0.1 g L H PtCl4 with an
2
applied voltage of ꢀ0.2 V and deposition time of 180 s.
2
Fig. 1 Schematic of photoelectrochemical cell for CO reduction.
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The CO
ducted in an H-shaped double chamber reactor. Then, a Naon these electrons, as well as H , reacted and produced organics
17 membrane (Shanghai Hesen Co., Ltd.) was used to separate under the catalytic effect of the cathode catalyst. The schematic
2 2
photoelectrochemical reduction reaction was con- to the cathode through the external circuit. Aerward, CO and
+
1
the two chambers. During the reaction process, light was of the photoelectrochemical cell for CO reduction is shown in
2
applied to the anode Pt-TNT, and voltage was applied between Fig. 1. Aer the reaction, the liquid and gas products were
the anode and cathode. The photocurrent was detected with collected and detected.
CHI660D electrochemical workstation (CH instruments Co.,
Ltd.), and sunlight was simulated using Perfectlight PLS-
2.4. Characterization
SXE300CUV xenon lamp.
2
The working area of cathode was 1 cm. The anolyte was The element analysis of Cu(BTC)-rGO and Cu(BTC) was con-
.5 M H SO , while the catholyte was dimethylformamide ducted using a VG ESCALAB MARK II X-ray photoelectron
0
2
4
(
DMF). A water decomposition process happened on the anode spectroscopy (XPS) with Mg Ka radiation. X-ray diffraction
+
Pt-TNT under light. The H generated by the oxidation of H
under the catalytic effect of Pt-TNT crossed the Naon Netherlands) diffractometer with Cu-Ka radiation. The
membrane and took part in the CO reduction reaction on the morphologies of Cu(BTC)-rGO/Pt–Cu foam cathode and powder
2
O
(XRD) analysis was performed using an X'Pert PRO (PANalytical,
2
cathode. At the same time, the photo-generated electrons went Cu(BTC)-rGO were characterized by eld-emission scanning
Fig. 2 Cu 2p XPS spectra of Cu(BTC)-rGO (a) and Cu(BTC) (b); C 1s XPS spectra of Cu(BTC)-rGO (c) and Cu(BTC) (d); O 1s XPS spectra of
Cu(BTC)-rGO (e) and Cu(BTC) (f).
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produced in the cell were analyzed using a gas chromatography
system (GC; Agilent 7820A, USA) equipped with a thermal
conductivity detector and a ame ionization detector. A column of
DB-FFTP (F50 m ꢂ 0.32 mm ꢂ 0.5 mm) was used to detect
alcohol. Acids were analyzed using an ion chromatography system
(HPIC, Integrion, Dionex, USA) equipped with a conductivity
detector and an AS11-HC-4 mm analytical column (4 ꢂ 250 mm).
3
. Results and discussion
3.1. Cathode catalyst characterization
To investigate the state of copper cluster and quantitatively
analyze the element–atom ratio in the as obtained Cu(BTC)-rGO
and Cu(BTC), we conducted XPS around the characteristic peak
of Cu, C, and O. The XPS spectra of Cu, C, and O in Cu(BTC)-rGO
and Cu(BTC) were shown in Fig. 2. Two characteristic peaks of
2
+
divalent Cu were observed at 934 eV and 954 eV, corre-
3
/2
1/2
Fig. 3 XRD patterns of Cu(BTC) and Cu(BTC)-rGO.
sponding to Cu 2p and Cu 2p , respectively. Meanwhile, the
presence of the shake-up satellites, which are the other peaks
that appeared in the range of 930–965 eV, except for two main
characteristic peaks, found in the Cu spectra was an indica-
electron microscopy (SEM) using SU-8010 (Hitachi, Japan). The
loading amount of Pt on the Cu foam was determined by the
energy-dispersive X-ray spectroscopy (EDX) analysis on the SEM
system equipped with an energy-dispersive X-ray analytical
system. The Brunauer–Emmett–Teller (BET) analysis was con-
ducted to measure the specic surface area and pore size of the
as synthesized material on a Micromeritics ASAP 2020.
30
tion of the presence of Cu(II) species in Cu(BTC)-rGO and
Cu(BTC).
The quantitative analysis can be conducted by tting the XPS
spectra. The element atom number ratio in the samples was
calculated by using the following equation:
n(E )/n(E ) ¼ [A(E )/S(E )]/[A(E )/S(E )]
(1)
1
2
1
1
2
2
2
.5. CO
CO reduction products were analyzed offline. Liquid products, Where n is the atom number, E is the element, S is the corre-
including alcohols and acids, were immediately sampled through sponding sensitive factor, and A is the area of characteristic
the cell septum aer the CO reduction reaction. Alcohols peaks.
2
reduction product analysis
2
2
Fig. 4 SEM patterns of Cu(BTC)-rGO/Pt–Cu foam cathode ꢂ150 (a), Cu(BTC)-rGO catalyst ꢂ2500 (b), Cu(BTC) catalyst ꢂ1100 (c), TEM pattern
of Cu(BTC)-rGO catalyst (d).
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The calculation result showed that in Cu(BTC)-rGO, the and Cu(BTC). According to a previous study, the mixture of
element atom number ratios of Cu, C, and O were 4.9%, 23.7%, Cu and Cu O is an efficient catalyst to selectively reduce CO to
x
2
and 71.5%, respectively. Moreover, in Cu(BTC), the element ethanol; thus, the presence of these particle can promote the
atom ratios of Cu, C, and O were 5.7%, 18.4%, and 75.8%, reaction selectivity to ethanol.
respectively. The C atom ratio in Cu(BTC)-rGO was much higher
The morphology of cathode catalyst was shown in Fig. 4.
than that in Cu(BTC). Further mining of the C 1s XPS spectra Accordingly, Cu foam evidently had a 3D porous structure that
showed that three characteristic peaks were observed at 284 eV, can minimize the diffusion resistance for mass transport. This
2
286 eV, and 289 eV, which correspond to C–C/C]C, C–O and property of Cu foam favored the efficient transfer of CO and
O]C–O respectively. The weight ratios of these three functional fast emission of products, which would accelerate the reaction
groups were calculated. In Cu(BTC)-rGO, the ratios were 81.8%, rate.
7.3%, and 10.9%, respectively. Moreover, in Cu(BTC), the ratios
Fig. 4(b) observed evidently wrinkled morphology, which was
were 57.6%, 22.2%, and 20.2%, respectively. Thus, the one of the typical characteristic of rGO. Many octahedral shaped
successfully bonding of rGO in Cu(BTC)-rGO was proved; rGO Cu(BTC) particles were distributed uniformly on the rGO
bonding in the sample can accelerate the transfer of electron on surface. Fig. 4(c) showed that morphology of Cu(BTC). TEM
the surface of the catalyst, leading to easier adsorption of the pattern of Cu(BTC)-rGO catalyst was given in Fig. 4(d), many
3
1,32
intermediates such as CO
Successfully bonding of rGO into the Cu(BTC) crystal lattice surfaced of rGO.
in Cu(BTC)-rGO was also proven by the XRD pattern. As shown
in Fig. 3, similar characteristic peaks in Cu(BTC)-rGO and area of Cu(BTC)-rGO and Cu(BTC) were 938 m g and 1322 m
2
* in the CO
2
reduction reaction.
octahedral shaped Cu(BTC) can also be observed bonded to the
According to BET measurement results, the specic surface
2
ꢀ1
2
ꢀ1
Cu(BTC) were observed. However, the peaks in the Cu(BTC)-rGO
g
respectively. Though rGO bonding lead to the decrease of
XRD pattern shied to the right due to rGO bonding. Both of the specic surface area of the catalyst, it successfully changed the
two samples consisted of a face-centered cubic crystal lattice of pore structure of the catalysts. Fig. 5 showed the nitrogen
ꢁ
ꢀ
the Fm3m space group and had typical peaks at 2q ¼ 9.5 (220), sorption isotherms of Cu(BTC)-rGO (a) and Cu(BTC) (b).
ꢁ ꢁ ꢁ ꢁ ꢁ
1
3.5 (400), 14.7 (331), 16.5 (422), 17.5 (511), and 19.1 (440). Accordingly, Cu(BTC)-rGO nitrogen adsorption plot showed
33
This result is in agreement with that of the previous reports. In a small hysteresis between adsorption and desorption branches
addition to the characteristic peaks of the Cu(BTC) crystal, suggesting the existence of few mesopores in the catalyst.
several small peaks in the range of 35 –45 were observed, However, Cu(BTC) nitrogen adsorption plot showed there only
ꢁ
ꢁ
which were the characteristic peaks of CuO, Cu O, and Cu. This existed micropores in Cu(BTC). Pore size distribution [Fig. 5(c)
2
x
result indicated the synthesis of Cu and Cu O in Cu(BTC)-rGO and (d)] further conrmed this conclusion. The existence of
Fig. 5 Nitrogen sorption isotherms: Cu(BTC)-rGO (a), Cu(BTC) (b); pore size distribution: Cu(BTC)-rGO (c), Cu(BTC) (d).
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Fig. 6 Electric current densities with different cathodes used: Cu(BTC)-rGO/Pt–Cu foam cathode (a), Cu(BTC)/Pt–Cu foam cathode (b).
mesopores better connected the micropores thus improving reacted and produced organics under the catalytic effect of the
mass transfer rate.
cathode Cu(BTC)-rGO catalyst.
Pt nanoparticles were supposed to be observed on the Cu
Fig. 6 showed the current densities of the system when
foam. However, as the SEM pattern shows, Pt particles on the Cu(BTC)-rGO was used as catalyst (a), and Cu(BTC) was used as
Cu foam matrix were hard to be observed mainly due to the catalyst (b). The distance between TiO anode and light resource
small size. To identify the Pt content on the cathode, EDX was 2 cm and the average incident light intensity provided at the
2
ꢀ
2
analysis was conducted. EDX analysis showed that the Pt electrode surface was measured to be 10 mW cm . Accordingly,
content on the cathode was about 0.56 wt%. The existence of Pt the average current density under CO purging condition with
2
ꢀ
2
nanoparticles also contributed to facilitate charge transfer and Cu(BTC)-rGO used as catalyst was 2.13 mA cm . However, the
34,35
ꢀ2
intermediates adsorption on the surface of the catalyst.
average current density decreased to 1.75 mA cm
with
Cu(BTC). This is a strong evidence to prove the improving of
electron transfer rate on the surface when rGO is bonded to
4
. Cu(BTC)-rGO catalytic effect on
Cu(BTC). Moreover, when CO
the current density showed sharp increase compared to that
when N was bubbled.
The J–V curves of Cu(BTC)-rGO catalyst and Cu(BTC) catalyst
obtained from CO2 electrochemical reduction in DMF (three
electrode conguration, without photoanode) were shown in
Fig. 7 and clearly, the current density of the system using
Cu(BTC)-rGO catalyst was much higher than that when Cu(BTC)
was used as catalyst.
2
was bubbled into the solution,
the selectivity reduction of CO to
ethanol
2
2
The CO
2
photoelectrochemical reaction was performed in
a novel photoelectrochemical reactor consisting of photo anode
and electric cathode. A water decomposition process occurred
on the anode Pt-TNT under light. Moreover, the generated H
passed through the Naon membrane and into the cathode. At
the same time, the photo electrons went to the cathode through
+
2
The results of the CO photoelectrochemical reduction
+
experiment were shown in Fig. 8 Accordingly, the Cu(BTC)-rGO
loaded on the Pt-deposited Cu foam cathode showed the best
the external circuit, and CO and these electrons, as well as H ,
2
catalytic effect to reduce CO when the applied voltage on the
2
system was ꢀ2 V. Moreover, the total carbon atom conversion
Fig. 7 J–V curves of Cu(BTC)-rGO catalyst and Cu(BTC) catalyst Fig. 8 Carbon atom conversion rates and Faraday efficiencies with
obtained from CO
2
electrochemical reduction in DMF (three electrode various applied voltage (ꢀ1.4 V, ꢀ1.7 V, ꢀ2 V, ꢀ2.3 V, ꢀ2.6 V) using
configuration, without photoanode).
Cu(BTC)-rGO/Pt–Cu foam cathode.
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ꢀ
1
ꢀ2
rate reached 3255.87 nmol h cm , and the liquid products
selectivity towards alcohols was up to 95%. The Faraday effi-
ciency was measured to be 34.1% with ꢀ2 V voltage applied.
The catalytic effect of Cu(BTC) under the condition with ꢀ2 V
voltage applied was also studied. The result showed that the
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ꢀ
1
ꢀ2
total carbon atom conversion rate was 2561.76 nmol h cm
,
which was much lower than that when Cu(BTC)-rGO was used
as the cathode catalyst. This result mainly due to the reason that
lacking of rGO in the material will lead to lower transfer rate of
electrons on the surface of the catalyst, resulting in the decrease
of the carbon atom conversion rate.
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. Conclusion
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In this research, Cu(BTC)-rGO was synthesized using a facile
hydrothermal method. A Cu(BTC)-rGO/Pt–Cu foam cathode was
fabricated and used to photoelectrochemically reduce CO . The
cathode showed the best catalytic effect with applied voltage of
1
1
2
ꢀ
2 V. The CO
2
photoelectrochemical reduction result showed
that the carbon atom conversion rate reached up to 3256 nmol
ꢀ
1
ꢀ2
h
cm , and the carbon atom conversion selectivity to liquid
1
1
1
1
2
fuels was up to 94.6% when ꢀ2 V voltage was applied. The CO
2
reduction mechanism on Cu(BTC)-rGO/Pt–Cu foam cathode
was also proposed. In this process, rGO in the catalyst and the
Pt nanoparticles on the Cu foam rst transferred electron to
CO2 to produce the C1 intermediates. As Pt showed good
selectivity to the generation of CO, more CO than other C1
organics were produced. Then, the porous structure of Cu(BTC)
facilitated C–C bonding, forming the enol-like intermediates
that are key intermediates in alcohols generation. In conclu-
sion, the bonding of rGO to Cu(BTC) is proved to be a promising
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There are no conicts to declare.
Acknowledgements
17045.
This study was supported by the National Natural Science
Foundation-China (51676171), National key research and
development program-China (2016YFE0117900).
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