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

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.


Introduction
Converting CO 2 to clean fuels is considered an efficient way to solve both the problems of global warming and energy shortage at the same time. [1][2][3][4] Articial photosynthesis is a promising way in which CO 2 is reduced to organic chemicals such as methanol, ethanol and formic acid, by using electricity and light with the help of catalysts. [5][6][7] Given the large specic surface area and extremely high electron transition rate of graphene, 8,9 using graphene as a CO 2 reduction catalyst is a topic of great interest in the research eld. 10 Fan Yu Jia et al. 11 studied the role that graphene played in the CO 2 and H 2 hydrocarbonylation reaction and graphene obviously helped increase the absorption of H 2 and CO 2 , as well as the CO 2 conversion rate. Moreover, selectivity toward methanol was increased to 85%. However, high pressure should be applied to ensure the proceeding of the reaction and this requires good resistance of the reactor. Geioushy R. A. et al. 12 synthesized a Cu 2 O/graphene catalyst and used it in the CO 2 electrochemical reduction reaction. The product analysis result showed that this catalyst had good selectivity toward ethanol generation. However, the energy used in this system totally came from electricity; renewable energies such as solar energy were not used. Cheng Jun et al. 13 brushed the mixture of Naon and reduced graphite oxide (rGO) on a Ni foam and used it as the cathode catalyst to reduce CO 2 to valuable chemicals. However, the major products of Naon and rGO are formic and acetic acids, which cannot be used as fuel directly. Moreover, the selectivity toward liquid fuels such as ethanol is low. Cu foam, which has 3D structure, is a good cathode base in electrochemical reactions. Cheng Jun et al. 14 used Cu foam as a support base and loaded graphene on the matrix to reduce CO 2 in a photoelectrochemical cell. However, the selectivity toward a specic product is still low.
Metal organic frameworks (MOFs), as an emerging porous material, have been reported in many researches. [15][16][17][18][19][20][21] Numerous researchers have reported its outstanding performance in gas separation and catalytic reduction or oxidation. 22-25 Bohan Shan et al. 26 synthesized a novel Co-based MOF to separate CO 2 /N 2 and CO 2 /CH 4 . Moreover, the CO 2 separation rates have reached 61.4% and 11.7%, respectively. However, this Co-based MOF in the gas separation process is only a physical adsorption and desorption process but not in a chemical reaction. R. Senthil Kumar et al. 27 used an electrochemical method, which successfully synthesized a highly active Cu-based MOF material and used it as a catalyst to reduce CO 2 with an electrolyte consisting of a dimethylformamide solution of tetrabutyl ammonium tetra uoroborate saturated with CO 2 . The result showed that this catalyst is an effective catalyst to reduce CO 2 to organics. However, the organic synthesized was mainly oxalic acid and liquid fuels like ethanol and methanol were not obtained. Guoqiang Song et al. 28 fabricated a porous Cubenzene 1,3,5-tricarboxylic acid (BTC) silica monolith heterogeneous catalyst for the selective oxidation of alkylbenzenes. However, the usage of Cu(BTC) in CO 2 photoelectrochemical reduction reactions has never been reported.
In this paper, a facile hydrothermal method was implied to synthesize Cu(BTC)-rGO catalyst, characterizations of the catalyst proved faster electron transfer rate and existence of more reaction active sites on the surface. These features facilitated the improvement of productivity and selectivity of CO 2 photoelectrochemical reduction reaction.

Experiment
2.1. Cathode preparation 2.1.1. GO synthesis. The GO used in this work was prepared using the Hummers' method. 29 A total of 1 g high purity graphite was added into 25 ml of concentrated sulfuric acid. A total of 3.5 g of potassium permanganate was slowly added into the solution into the 0 C ice bath; aerward, the mixed solution 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 diluted with 100 ml of deionized water and then oxidized with 8 ml of 30% H 2 O 2 in the ice bath. The resulting suspension was washed with 5% diluted HCl, ultrasonically stripped, and was placed into a freeze dryer to be dried.
2.1.2. Fabrication of Cu(BTC)-rGO/Pt-Cu foam cathode. Cu(BTC)-rGO was synthesized using a facile hydrothermal method. A total of 150 mg of as synthesized GO was dissolved in 15 ml of ethanol; this solution was marked as L1. A total of 0.93 g of Cu(NO 3 ) 2 was dissolved in 15 ml of DMF; this solution was marked as L2. A total of 0.44 g of BTC was dissolved in 30 ml of the ethanol and DMF mixture (volume, 1 : 1); this solution was marked as L3. Aerward, L1, L2, and L3 were mixed aer stirring for 30 min. Then, the mixture was poured into a Teonlined autoclave aer stirring for 30 min, and the autoclave was kept under 120 C for 6 h. Black-blue precipitates were formed, 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 process, the added GO was transformed to rGO, given that GO was unstable under high temperature and can be easily reduced.
The Cu foam was deposited with Pt by using an electrochemical deposition method in a 0.1 g L À1 H 2 PtCl 4 with an applied voltage of À0.2 V and deposition time of 180 s. 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 Ptdeposited Cu foam using the drop-dry method.
2.1.3. Fabrication of Cu(BTC)/Pt-Cu foam cathode. Blue mixture of Cu(BTC) and Naon was obtained by mixing 90 mg Cu(BTC) with Naon. Then, the mixture was applied on a Ptdeposited Cu foam using a drop-dry method.

Preparation of Pt-TiO 2 nanotube anode
The Pt-TiO 2 nanotubes (Pt-TNTs) were prepared based on the anode oxidation method. The titanium sheet was rst rinsed in isopropanol and ethanol for 15 min for ultrasonic cleaning before oxidation. Aerward, the titanium sheet was oxidized in the uorine-containing ethylene glycol electrolyte (EG + 0.3% NH 4 F + 2 vol% H 2 O) for 2 h, with 60 V applied voltage and Pt as the cathode. Then, the sheet was washed with deionized water and was placed into the muffle furnace to be calcined under 450 C for 3 h, with the temperature rising rate of 2 C min À1 . Thus, the TiO 2 nanotube crystals can be turned into anatase, which has better light response. The obtained anatase TiO 2 nanotubes on the titanium sheet was deposited with Pt in 1 g L À1 of H 2 PtCl 6 $6H 2 O (depositing current density: 2.5 mA cm À2 ) to improve its photocatalytic performance.

CO 2 photoelectrochemical reduction in a PEC
To test the performance of the as fabricated cathodes and nd the best reaction condition of CO 2 photoelectrochemical reduction. CO 2 photoelectrochemical reduction reactions with two different cathodes were conducted, the reactions with different voltage applied on Cu(BTC)-rGO/Pd-Cu foam cathode were also conducted. The CO 2 photoelectrochemical reduction reaction was conducted in an H-shaped double chamber reactor. Then, a Naon 117 membrane (Shanghai Hesen Co., Ltd.) was used to separate the two chambers. During the reaction process, light was applied to the anode Pt-TNT, and voltage was applied between the anode and cathode. The photocurrent was detected with CHI660D electrochemical workstation (CH instruments Co., Ltd.), and sunlight was simulated using Perfectlight PLS-SXE300CUV xenon lamp.
The working area of cathode was 1 cm. 2 The anolyte was 0.5 M H 2 SO 4 , while the catholyte was dimethylformamide (DMF). A water decomposition process happened on the anode Pt-TNT under light. The H + generated by the oxidation of H 2 O under the catalytic effect of Pt-TNT crossed the Naon membrane and took part in the CO 2 reduction reaction on the cathode. At the same time, the photo-generated electrons went to the cathode through the external circuit. Aerward, CO 2 and these electrons, as well as H + , reacted and produced organics under the catalytic effect of the cathode catalyst. The schematic of the photoelectrochemical cell for CO 2 reduction is shown in Fig. 1. Aer the reaction, the liquid and gas products were collected and detected.

Characterization
The element analysis of Cu(BTC)-rGO and Cu(BTC) was conducted using a VG ESCALAB MARK II X-ray photoelectron spectroscopy (XPS) with Mg Ka radiation. X-ray diffraction (XRD) analysis was performed using an X'Pert PRO (PANalytical, Netherlands) diffractometer with Cu-Ka radiation. The morphologies of Cu(BTC)-rGO/Pt-Cu foam cathode and powder Cu(BTC)-rGO were characterized by eld-emission scanning 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 conducted to measure the specic surface area and pore size of the as synthesized material on a Micromeritics ASAP 2020.
2.5. CO 2 reduction product analysis CO 2 reduction products were analyzed offline. Liquid products, including alcohols and acids, were immediately sampled through the cell septum aer the CO 2 reduction reaction. Alcohols 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 Â 0.32 mm Â 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 Â 250 mm).

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 divalent Cu 2+ were observed at 934 eV and 954 eV, corresponding to Cu 2p 3/2 and Cu 2p 1/2 , 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 indication 30 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: Where n is the atom number, E is the element, S is the corresponding sensitive factor, and A is the area of characteristic peaks.  This journal is © The Royal Society of Chemistry 2018 The calculation result showed that in Cu(BTC)-rGO, the element atom number ratios of Cu, C, and O were 4.9%, 23.7%, and 71.5%, respectively. Moreover, in Cu(BTC), the element atom ratios of Cu, C, and O were 5.7%, 18.4%, and 75.8%, respectively. The C atom ratio in Cu(BTC)-rGO was much higher than that in Cu(BTC). Further mining of the C 1s XPS spectra showed that three characteristic peaks were observed at 284 eV, 286 eV, and 289 eV, which correspond to C-C/C]C, C-O and O]C-O respectively. The weight ratios of these three functional groups were calculated. In Cu(BTC)-rGO, the ratios were 81.8%, 7.3%, and 10.9%, respectively. Moreover, in Cu(BTC), the ratios were 57.6%, 22.2%, and 20.2%, respectively. Thus, the successfully bonding of rGO in Cu(BTC)-rGO was proved; rGO bonding in the sample can accelerate the transfer of electron on the surface of the catalyst, leading to easier adsorption of the intermediates such as CO 2 * in the CO 2 reduction reaction. 31,32 Successfully bonding of rGO into the Cu(BTC) crystal lattice in Cu(BTC)-rGO was also proven by the XRD pattern. As shown in Fig. 3, similar characteristic peaks in Cu(BTC)-rGO and Cu(BTC) were observed. However, the peaks in the Cu(BTC)-rGO XRD pattern shied to the right due to rGO bonding. Both of the two samples consisted of a face-centered cubic crystal lattice of the Fm 3m space group and had typical peaks at 2q ¼ 9.5 (220), 13.5 (400), 14.7 (331), 16.5 (422), 17.5 (511), and 19.1 (440). This result is in agreement with that of the previous reports. 33 In addition to the characteristic peaks of the Cu(BTC) crystal, several small peaks in the range of 35 -45 were observed, which were the characteristic peaks of CuO, Cu 2 O, and Cu. This result indicated the synthesis of Cu and Cu x O in Cu(BTC)-rGO and Cu(BTC). According to a previous study, 12 the mixture of Cu and Cu x O is an efficient catalyst to selectively reduce CO 2 to ethanol; thus, the presence of these particle can promote the reaction selectivity to ethanol.
The morphology of cathode catalyst was shown in Fig. 4. Accordingly, Cu foam evidently had a 3D porous structure that can minimize the diffusion resistance for mass transport. This property of Cu foam favored the efficient transfer of CO 2 and fast emission of products, which would accelerate the reaction rate. Fig. 4(b) observed evidently wrinkled morphology, which was one of the typical characteristic of rGO. Many octahedral shaped Cu(BTC) particles were distributed uniformly on the rGO surface. Fig. 4(c) showed that morphology of Cu(BTC). TEM pattern of Cu(BTC)-rGO catalyst was given in Fig. 4(d), many octahedral shaped Cu(BTC) can also be observed bonded to the surfaced of rGO.
According to BET measurement results, the specic surface area of Cu(BTC)-rGO and Cu(BTC) were 938 m 2 g À1 and 1322 m 2 g À1 respectively. Though rGO bonding lead to the decrease of specic surface area of the catalyst, it successfully changed the pore structure of the catalysts. mesopores better connected the micropores thus improving mass transfer rate.
Pt nanoparticles were supposed to be observed on the Cu foam. However, as the SEM pattern shows, Pt particles on the Cu foam matrix were hard to be observed mainly due to the small size. To identify the Pt content on the cathode, EDX analysis was conducted. EDX analysis showed that the Pt content on the cathode was about 0.56 wt%. The existence of Pt nanoparticles also contributed to facilitate charge transfer and intermediates adsorption on the surface of the catalyst. 34,35 4. Cu(BTC)-rGO catalytic effect on the selectivity reduction of CO 2 to ethanol 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 the external circuit, and CO 2 and these electrons, as well as H + , reacted and produced organics under the catalytic effect of the cathode Cu(BTC)-rGO catalyst. Fig. 6 showed the current densities of the system when Cu(BTC)-rGO was used as catalyst (a), and Cu(BTC) was used as catalyst (b). The distance between TiO 2 anode and light resource was 2 cm and the average incident light intensity provided at the electrode surface was measured to be 10 mW cm À2 . Accordingly, the average current density under CO 2 purging condition with Cu(BTC)-rGO used as catalyst was 2.13 mA cm À2 . However, the average current density decreased to 1.75 mA cm À2 with Cu(BTC). This is a strong evidence to prove the improving of electron transfer rate on the surface when rGO is bonded to Cu(BTC). Moreover, when CO 2 was bubbled into the solution, the current density showed sharp increase compared to that when N 2 was bubbled.
The J-V curves of Cu(BTC)-rGO catalyst and Cu(BTC) catalyst obtained from CO 2 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.
The results of the CO 2 photoelectrochemical reduction experiment were shown in Fig. 8 Accordingly, the Cu(BTC)-rGO loaded on the Pt-deposited Cu foam cathode showed the best catalytic effect to reduce CO 2 when the applied voltage on the system was À2 V. Moreover, the total carbon atom conversion   rate reached 3255.87 nmol h À1 cm À2 , and the liquid products selectivity towards alcohols was up to 95%. The Faraday efficiency was measured to be 34.1% with À2 V voltage applied.
The catalytic effect of Cu(BTC) under the condition with À2 V voltage applied was also studied. The result showed that the total carbon atom conversion rate was 2561.76 nmol h À1 cm À2 , 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.

Conclusion
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 2 . The cathode showed the best catalytic effect with applied voltage of À2 V. The CO 2 photoelectrochemical reduction result showed that the carbon atom conversion rate reached up to 3256 nmol h À1 cm À2 , and the carbon atom conversion selectivity to liquid fuels was up to 94.6% when À2 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 CO 2 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 conclusion, the bonding of rGO to Cu(BTC) is proved to be a promising way to improve the productivity and selectivity of CO 2 photoelectreochemical reduction reaction. We believe that this would offer a new strategy for the synthesis of efficient CO 2 photoelectrochemical reduction MOF based catalyst.

Conflicts of interest
There are no conicts to declare.