Photoelectrocatalytic Reduction of CO2 into Chemicals Using Pt

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Photoelectrocatalytic Reduction of CO2 into Chemicals Using PtModified Reduced Graphene Oxide Combined with Pt-Modified TiO2 Nanotubes Jun Cheng,* Meng Zhang, Gai Wu, Xin Wang, Junhu Zhou, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang 310027, China ABSTRACT: The photoelectrocatalytic (PEC) reduction of CO2 into high-value chemicals is beneficial in alleviating global warming and advancing a low-carbon economy. In this work, Pt-modified reduced graphene oxide (Pt-RGO) and Ptmodified TiO2 nanotubes (Pt-TNT) were combined as cathode and photoanode catalysts, respectively, to form a PEC reactor for converting CO2 into valuable chemicals. XRD, XPS, TEM, AFM, and SEM were employed to characterize the microstructures of the Pt-RGO and Pt-TNT catalysts. Reduction products, such as C2H5OH and CH3COOH, were obtained from CO2 under band gap illumination and biased voltage. A combined liquid product generation rate (CH3OH, C2H5OH, HCOOH, and CH3COOH) of approximately 600 nmol/(h·cm2) was observed. Carbon atom conversion rate reached 1,130 nmol/(h·cm2), which were much higher than those achieved using Pt-modified carbon nanotubes and platinum carbon as cathode catalysts. materials17−20 have been performed targeting fuel cells, supercapacitors, organic pollutant degradation, water decomposition, and CO2 reduction. Encouraging results have been reported in the latest literature on the application of graphenebased material in photoelectrocatalytic reduction of CO2 as the following: (1) Graphene-based material itself was used as a photocatalyst in the photocatalytic reduction of CO2 to mainly produce methanol.21−23 However, the photocatalytic activity of graphene-based material markedly decreased after the reaction was conducted for 2 h, because a part of the oxygenated functional groups on graphene-based material was reduced under illumination. (2) Graphene was combined with a semiconductor photocatalyst, such as TiO2 and WO3, for photocatalytic reduction of CO2.23−30 Although the photocatalytic activity of a graphene-semiconductor photocatalyst improved significantly, the CO2 reduction product was mainly gas product (such as methane) and liquid product was very little. (3) Graphene-based material was used as an electrocatalyst in CO2 reduction.31 However, the preference to bind hydrogen became a major obstacle in this reaction path. Therefore, it is necessary to develop an efficient reaction system to keep stable catalytic activity, improve CO2 reduction efficiency (preferably liquid products), and reduce energy consumption.

1. INTRODUCTION The greenhouse effect and energy shortage are two major issues in today’s society.1 The conversion of CO2 into high-value chemicals to achieve an energy-saving and low-carbon economy is highly significant.2,3 Photoelectrocatalysis4−9 is one of the most promising ways to reduce CO2 because it is regarded as an artificial model for photosynthesis due to its direct use of solar energy. However, the quantum efficiency of current photocatalysts is extremely low, and the energy barrier of CO2 reduction is still high.10 Therefore, the conversion efficiency of CO2 photoelectrocatalytic (PEC) reactions is far from being industrially applicable. Enhancing reduction rate, controlling product selectivity, and increasing solar energy utilization are the major challenges for the PEC reduction of CO2. Ideal catalysts, which can efficiently reduce CO2 with low energy consumption, play an important role in solving these problems.11,12 Indeed, more effective catalytic materials and reaction systems for CO2 reduction must be developed. The conversion of CO2 into liquid products, which can be stored, transported, and used easily, is favorable. Previous studies8,13 found that nanostructured carbon, such as carbon nanotubes (CNTs), can create a virtually high CO2 pressure at the surface of catalysts, thus favoring chain growth to produce liquid products. This phenomenon is mainly based on the concept of nanoconfinement.3,14,15 Recently, graphene as a one-atom-thick sheet of carbon has attracted the attention of researchers all over the world. This material exhibits excellent thermal, mechanical, photosensitive, and electrical properties, which make it promising for potential applications in many fields.16 Several studies on the applications of graphene-based © 2014 American Chemical Society

Received: Revised: Accepted: Published: 7076

August 9, 2013 May 13, 2014 May 20, 2014 May 20, 2014 dx.doi.org/10.1021/es500364g | Environ. Sci. Technol. 2014, 48, 7076−7084

Environmental Science & Technology

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configuration with two compartments for water oxidation and CO2 reduction.8,13 The photoanode and electrocathode (with projected surface area of 7 cm2) were joined together by a Nafion membrane through hot pressing at 120 °C under a pressure of 20 kg cm−2. The average incident light intensity provided at the surface of the anode was 10 mW/cm2 with a 300 W Xe-arc lamp (SOLAREDGE700, Beijing Philae Technology Co., Ltd., China). The electrolytes used in the anodic and cathodic compartments were 1 M NaCl (pH = 6.9) and 1 M NaHCO3 solutions (CO2 saturated, pH = 8.9), respectively. A constant potential of 2 V provided by a potentiostat (SK1731SL3A, Hangzhou Sanke Electrical Co., Ltd., China) was applied through the cell. Figure 1 shows the schematic diagram of the PEC system.

In the current study, a novel light-driven and electrically biased PEC cell, which can improve product selectivity over electrocatalyst and reduce energy input over photocatalyst, was developed to convert CO2 into valuable chemicals. Nanomaterial Pt-modified reduced graphene oxide (Pt-RGO) was used as the cathode electrocatalyst for CO2 reduction. Pt-modified TiO2 nanotubes (Pt-TNTs) were used as the anode photocatalyst for water decomposition for its excellent performance.32−35 The efficient dual-chamber PEC reactor, which combines the electrical cathode and photoanode, was designed to separate the photoinduced process into two physically distinct areas related to water oxidation (to form oxygen, protons, and electrons) and CO2 reduction. This reaction system was able to limit the charge recombination, reduce the energy input, and increase the efficiency of CO2 reduction reactions.

2. EXPERIMENTAL SECTION 2.1. Preparation of Cathode. Graphene oxide (GO) was prepared by oxidation of pristine graphite powder (G) (Nanjing Pioneer Nanomaterials Technology Co., Ltd., China) according to the Hummers’ method.36 Pt-RGO was prepared by the following procedure.37 Approximately 0.8 g of GO was dissolved in a solution containing 200 mL of ethylene glycol, 55 mL of distilled water, and 0.014 g of H2PtCl6·6H2O. After having been stirred for 2 h and sonicated for 1 h, the mixture was placed into a reactor with a Teflon inner layer. The reduction reaction was performed at 120 °C for 24 h under constant stirring, and the formed catalyst was separated by filtration and washed with ethanol. Finally, the catalyst was dried in a freeze-dryer. Commercial multiwalled CNTs were directly purchased from Shenzhen Nanotech Port Co., Ltd. (China). The samples were first treated in 65% nitric acid under reflux at 100 °C for 15 h to create oxygen functional groups on the surfaces of the CNTs, which are necessary to anchor Pt nanoparticles.38 Pt-modified CNT was prepared using the same method as Pt-RGO. Platinum carbon catalyst (Pt−C) with a high Pt loading amount (20 wt %) was purchased from Shanghai He Sen Electric Co., Ltd. (China) and used as received. The cathode electrode was composed of a nickel foam matrix (110 mesh, Inco Hi-tech Material Co., Ltd., Dalian) and a catalyst layer. A mixture of 8 mg/cm2 catalyst, 25 μL/cm2 distilled water, and 30 μL/cm2 Nafion solution was applied onto one side of the nickel foam, and drying was performed for 24 h at room temperature. Finally, the formed cathode was rolled to a thickness of 1 mm using a roller (DYG-703, Dali Electric Co., Ltd., China). 2.2. Preparation of Anode. TNTs were prepared by anodization of Ti foil (99.7%, 0.025 mm thick, Baoji Titanium Co., Ltd., China) under an applied voltage of 50 V for 3 h.8 An ethylene glycol solution containing 2 vol % H2O and 0.3 wt % NH4F was used as the electrolyte. To increase the activity of TNTs for water photoelectrolysis,39 the samples were loaded with Pt through electrodeposition. The electrodeposition was carried out at a constant current of 2.5 mA/cm2 for 5 min in an electrolytic cell, using the TNTs as the working electrode, a nickel sheet as the counter electrode, and an aqueous solution of 1 g/L H2PtCl6·6H2O as the electrolyte. After electrodeposition, the titanium samples were annealed at 450 °C in air for 3 h to transform amorphous TiO2 to anatase phase. 2.3. PEC Reactor. The PEC reactor equipped with a quartz window was made of Plexiglas. The reactor had a two-electrode

Figure 1. Schematic diagram of the photoelectrocatalytic (PEC) system.

2.4. Analytical Methods. The morphology of the catalysts was characterized by field emission scanning electron microscopy (SEM) using an SU-70 (Hitachi, Japan) operated at an energy beam of 30.0 kV. Transmission electron microscopy (TEM) was conducted using Tecnai G2 F20 STWIN (FEI, USA) with an accelerating voltage of 200 kV. Atomic force microscopy (AFM) images were obtained using a Nanoscope Multimode and Explore atomic force microscope (Multimode, Veeco Instruments, USA). The crystal structures of the catalysts were determined using X-ray diffraction (XRD) using X’Pert PRO (PANalytical, Netherlands) with Cu Kα radiation ranging from 5° to 90°. The composition of the nanocomposites was determined by energy dispersive X-ray spectroscopy (EDS) analyses on the SEM and TEM system equipped with an energy dispersive X-ray analytical system. The CO2 reduction gaseous products were analyzed by gas chromatography (GC) using a thermal conductivity detector (TCD) and a flame ionization detector (FID). The liquid product analysis was performed off-line and sampled from the catholyte after the reaction for 8 h. The acids in the liquid products were analyzed on an ion chromatograph (ICS 2000, Dionex, USA) equipped with a conductivity detector and a IonPac AS11-HC analytical column (Φ4 × 250 mm) by direct injection. The alcohols in the liquid products were detected on a gas chromatograph (GC6890N, Agilent, USA) equipped with a flame ionization detector (FID) and a DB-624 column (Φ30 m × 0.53 mm × 3 μm) and injected with a 7694 E headspace autosampler. 7077

dx.doi.org/10.1021/es500364g | Environ. Sci. Technol. 2014, 48, 7076−7084


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3. RESULTS AND DISCUSSION 3.1. Characterization of Cathode Catalysts. 3.1.1. Characterization of Pt-RGO. The XRD patterns of G, GO, and PtRGO are shown in Figure 2 (a). A sharp and strong diffraction

was relatively low, in accordance with the XPS characterization of C 1s in Pt-RGO in Figure 3 (b). The interlayer distance of RGO was smaller than that of GO due to the reduction of oxygen functional groups. Significantly sharp diffraction peaks of Pt were detected for Pt-RGO at 2θ = 39.7°, 46.4°, 69.5°, and 79.3°, which corresponded to the (111), (200), (220), and (311) planes of Pt, respectively. The mean crystallite size of Pt on RGO was 3.6 nm, calculated from the broadening of Pt (111) plane using Scherrer’s equation.46 Figure 2 (b) shows the TEM image of Pt-RGO. Pt nanoparticles with a uniform size of about 3.7 nm were homogeneously dispersed on the surface of RGO, which is consistent with the XRD results. The loading amount of Pt on Pt-RGO was approximately 1 wt % as confirmed by the EDS analyses. XPS spectra of C 1s in GO and Pt-RGO are shown in Figure 3 (a) and (b), respectively. The asymmetric C 1s spectrum could be deconvoluted into four peaks at 284.5, 286.6, 287.7, and 289.0 eV, which were assigned to the sp2-hybridized C, the C in C−O bonds, the carbonyl C (CO), and the carboxylate C (OC−O), respectively.47 The contents of oxygen in the form of C−O was the most common (40.1%) in GO, and the CO (10.1%) and OC−O (4.7%) were presented with lower quantity. The C−O and CO functional groups in PtRGO decreased obviously (from 40.1% to 12.1% and 10.1% to 4.8%, respectively). However, the OC−O (4.1%) kept almost constant. It was confirmed that GO was mainly reduced through the removal of C−O and CO bonds in this work. Deconvolution of Pt 4f in Pt-RGO showed the presence of two pairs of doublets [Figure 3 (c)]. Peaks at 71.4 (Pt 4f7/2) and 74.5 (Pt 4f5/2) were attributed to metallic Pt, and peaks at 72.3 (Pt 4f7/2) and 75.6 (Pt 4f5/2) could be assigned to Ptδ+ which anchored with the C−O group in Pt-RGO.48 Metallic Pt and Ptδ+ account for 73.6% and 26.4% in Pt-RGO, respectively. Figure 4 shows the AFM images of GO and Pt-RGO. The single layer thickness and double layer thickness of GO were 1.2 and 2.4 nm, respectively. The single layer thickness of GO was significantly larger than that of G due to the presence of oxygen functional groups. The reduction of the oxygen functional groups led to a smaller single layer thickness of RGO (0.7 nm) compared with that of GO. However, the single layer thickness of RGO was still larger than that of G. This result was attributed to the incomplete reduction of the oxygen functional groups [Figure 3 (b)]. The size of the agglomerated Pt nanoparticles was about 3 nm to 5 nm, which was consistent with the XRD and TEM results. 3.1.2. Characterization of Pt-modified CNT (Pt-CNT). The XRD patterns of the CNTs and Pt-CNT in Figure 5 (a) clearly show a sharp and strong diffraction peak at 26°, which is attributed to the (002) reflection of graphite. This result indicates that the graphitization degree of the CNTs was not influenced by the use of Pt. A faint Pt diffraction peak was detected for Pt-CNT at 39°, which corresponded to the (111) plane of Pt. The mean crystallite size of Pt on CNT was 2.8 nm, calculated from the broadening of Pt (111) plane using Scherrer’s equation.46 The TEM image of Pt-CNT in Figure 5 (b) indicates cincinal and long CNTs with diameters between 10 and 20 nm. Pt nanoparticles with an average diameter of 2.8 ± 0.6 nm were dispersed on the wall of CNTs [Figure 5 (c)], which agreed with the XRD results. The loading amount of Pt on Pt-CNT was approximately 0.9 wt % as determined by EDS. The deconvolution results of the XPS spectra of Pt-CNT showed that 66.9% of Pt was present as metal and 33.1% was in an oxidized state [Figure 5 (d)].

Figure 2. XRD pattern and TEM image of Pt-RGO: (a) XRD patterns of pristine graphite (G), graphene oxide (GO), and Pt-modified reduced graphene oxide (Pt-RGO) with 2θ between 5° and 80° and (b) TEM image of Pt-RGO.

peak at 26.6° was observed for G, indicating the high crystallinity of the graphite. The X-ray (002) peak of GO became relatively wide and shifted from 26.6° to about 9° compared with that of G, indicating an increase in interlayer distance according to the Bragg formula.40−42 The higher interlayer distance of GO was due to the presence of a large number of oxygen functional groups (such as CO, C−OH, and −COOH) that led to a certain degree of bending and folding of the GO crystal layer, as can be confirmed by the XPS spectra of C 1s in GO [Figure 3 (a)].43,44 The diffraction peak of GO (002) disappeared completely in the XRD pattern of PtRGO. The suppression of GO peak indicated that the conjugated graphene network (sp2 carbon) has been reestablished because of the reduction process.44,45 A broad peak between 17° and 30° was detected for Pt-RGO, which was attributed to the (002) peak of RGO. The crystallinity of RGO 7078



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Figure 3. XPS spectra of (a) C 1s in GO; (b) C 1s in Pt-RGO; and (c) Pt 4f in Pt-RGO.

3.2. Characterization of Anode Catalyst. TNTs were used as the anode catalyst for water decomposition to produce intermediates for CO2 reduction. The samples were loaded with Pt through electrodeposition to increase the activity for water decomposition. Figure 6 (a) and (b) present the SEM images of the TNTs before and after Pt loading. Raw TNTs had an average tube length of approximately 6.2 μm, outer diameter of 90 nm, and wall thickness of about 10 nm. In the Pt-TNTs, Pt nanoparticles with a uniform size of about 5 nm were uniformly dispersed on the wall of TNTs. The loading amount of Pt was approximately 5 wt % for the Pt-TNT/Ti composite electrode, as confirmed by EDS. The XRD patterns of TNT and Pt-TNT are shown in Figure 6 (c). TNTs were mainly presented as anatase with a lower quantity of rutile. Faint diffraction peaks of Pt were detected at 39°, which corresponded to the (111) plane of Pt [Figure 6 (d)]. The mean crystallite size of Pt on TNT was 5.2 nm, as calculated from the broadening of Pt (111) plane using Scherrer’s equation.46 3.3. Reduction of CO2 into Valuable Chemicals. CO2 reduction using Pt-RGO catalysts was carried out in a novel, light-driven, and electrically biased PEC cell with a Pt-TNT photoanode. To evaluate the catalytic activity of Pt-RGO, studies comparing the use of Pt-CNT and Pt−C as cathode catalysts were carried out. Figure 7 (a) compares the product generation rates of the PEC cell using different cathode electrodes. Hydrogen was the only product for the blank experiment (bare nickel foam cathode without catalyst layer), indicating that CO2 could not be reduced on a Ni electrode under the experimental conditions. Nevertheless, cathodes using the Pt-RGO, Pt-CNT, and Pt−C catalysts converted CO2 into C2H5OH or other hydrocarbons. Finally the performance of this PEC reactor in the absence of CO2 was studied. Pt-RGO was used as the cathode catalyst, and Pt-TNTs were used as the

anode photocatalyst for water decomposition. The electrolytes used in the anodic and cathodic compartments were 1 M NaCl and 1 M Na2SO4 solutions (pH = 8.9), respectively. The Na2SO4 solution was used to take place of the NaHCO3 solution and adjusted with NaOH to keep the same pH value. A constant potential of 2 V provided by a potentiostat was applied through the PEC cell. It turned out that hydrogen was the only product with a generation rate of 10.2 μmol/(h· cm2), and no carbon containing compounds were produced. Therefore, it was confirmed that CO2 was actually reduced, and the products were not the result of hydrogenation of the acidand alcoholic groups of the oxidized carbon. The highest carbon atom conversion rate of 1,130 nmol/(h· cm2) was obtained with the Pt-RGO catalyst (Figure 8). A combined acid and alcohol generation rate of 600 nmol/(h· cm2) was obtained with the Pt-RGO catalyst, which was significantly higher than those with Pt-CNT [82 nmol/(h· cm2)] and Pt−C [220 nmol/(h·cm2)]. The outstanding catalytic activity of Pt-RGO was mainly attributed to its high reactant adsorptivity and efficient charge transportation. The theoretical specific surface area of RGO (2630 m2/g),49,50 which is significantly higher than that of CNT and C, provides a large number of adsorption sites for the reactants (CO2 and H+). Furthermore, the electron transport mobility of RGO is extremely high [15000 cm2/(V·s)] at room temperature,51 which facilitated the fast reactions of electrons, protons, and CO2. In addition, the large surface area and porous structure of nickel foam were beneficial for the adhesion of catalysts and the reduction of CO2. The product yields with Pt−C were much higher than those with Pt-CNT. However, the theoretical specific surface area of CNT was higher than that of C, and the sizes of Pt nanoparticles on these catalysts were quite close. This result is probably attributed to the much higher Pt loading amount of Pt−C (20 wt %) than Pt-CNT (0.85 wt %). 7079



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Figure 4. AFM images and section analysis of graphene oxide (GO) and Pt-modified reduced graphene oxide (Pt-RGO): (a) GO: single layer; (b) GO: double layers; (c) Pt-RGO: single layer; and (d) Pt-RGO: double layers.

Proton interference was a main and common problem during CO2 reduction reactions due to the low hydrogen overpotential of the Ni electrode. Further research is needed to reduce proton interference and to elucidate CO2 reduction mechanisms. However, liquid products such as methanol, ethanol, formic acid, and acetic acid offered a prospective blueprint for CO2 reduction using Pt-RGO cathode catalysts. In this work, the photoinduced process was separated into two physically distinct areas related to water oxidation (to form oxygen, protons, and electrons) and CO2 reduction. At the

The selectivity of the CO2 reduction products for the Ptmodified cathode catalysts is summarized in Figure 7 (b). The main products of CO2 reduction were methane and acetic acid when Pt-CNT was used as the cathode catalyst. However, the main products were ethanol and acetic acid when Pt−C and PtRGO were used. The current efficiency of the acids and alcohols generated by Pt-RGO (78% in total) was lower than those generated by Pt−C (92% in total) due to the lower Pt loading amount of Pt-RGO. Single-product selectivity of CO2 reduction by Pt-RGO was low and needed improvement. 7080



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Figure 5. (a) XRD patterns of CNT and Pt-CNT with 2θ between 10° and 90° (inset was peak separation result of Pt-CNT with 2θ between 35° and 50°); (b) TEM image of Pt-CNT; (c) Pt nanoparticles size distribution on Pt-CNT; and (d) XPS spectra of Pt 4f in Pt-CNT.

anode, holes (h+) and electrons (e−) were generated when TiO2 was under illumination. The light-induced electron holes resulted in the splitting of water molecules into gaseous oxygen and hydrogen ions: 4h+ + 2H 2O → O2 + 4H+

(1)

As the bias potential was applied, the electrolysis of water also leads to formation of O2 at the anode. The proton ions generated at the anode passed through the Nafion membrane to the cathodic chamber, while the electrons were driven to the cathode by a potentiostat, for CO2 reduction. The reduction of CO2 to hydrocarbons, which were highly sensitive to material properties, reaction conditions, and applied potential, was a complex multistep reaction involving shared intermediates and multiple pathways.52,53 However, it was often described as a reaction between CO2 and hydrogen proton (H+), such as

Figure 6. SEM image and XRD pattern of Pt-TiO2 nanotubes (PtTNTs) used as photoanode: (a) SEM image of TiO2 nanotubes (TNTs); (b) SEM image of Pt-TNT; (c) XRD patterns of TNT and Pt-TNT with 2θ between 10° and 90°; and (d) XRD patterns of TNT and Pt-TNT with 2θ between 36° and 42°.

CO2 + 2H+ + 2e− → CO + H 2O

(2)

CO2 + 2H+ + 2e− → HCOOH

(3)

CO2 + 6H+ + 6e− → CH3OH + H 2O

(4)

CO2 + 8H+ + 8e− → CH4 + 2H 2O

(5)

Acetic acid and ethanol could be described as a further interaction between HCOOH, CH3OH, and the surface adsorbed methylene groups (:CH2).10,13 Both the production rate and compositions of the hydrocarbons obtained in the photoelectrocatalytic CO2 reduction experiment were highly 7081



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adsorptivity and efficient charge transportation. These values were much higher than those obtained with Pt-CNT and Pt−C cathode catalysts. Liquid products such as methanol, ethanol, formic acid, and acetic acid offer a prospective blueprint for CO2 reduction using Pt-RGO cathode catalysts. The product selectivity of CO2 reduction with proton directional reactions should be improved in future studies. The combination of graphene and Cu (or its compounds) deserves more attention, for copper electrocatalysts have demonstrated particular selectivity toward CO2 conversion to higher order hydrocarbons.53−55 In addition, some Cu compounds such as cuprous oxide can work as P-type semiconductor under illumination. Combining graphene with semiconductors to form composite photocatalysts can extend its light absorption edge, improve the migration rate of charge carriers, and enhance the adsorption capacity of contaminants.56 It can reduce the energy input for CO2 reduction under double-side illumination to build a photoelectrochemical cell with P-type semiconductor as cathode and N-type semiconductor as anode. More detailed studies on surface catalytic activity based on some electrochemical methods such as CV, LSV, and EIS through an electrochemical workstation are required to elucidate the CO2 reduction mechanisms.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 571 87952889. Fax: +86 571 87951616. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 7. Chemical generation rates and current efficiency under varying cathodes in a photoelectrocatalytic reactor with Pt-TiO2 nanotubes photoanode: (a) chemical generation rates and (b) current efficiency.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (51176163), Special Funds for Major State Basic Research Projects of China (2010CB227001), National High Technology R&D Program of China (2012AA050101), International Sci. & Tech. Cooperation Program of China (2012DFG61770 and 2010DFA72730), National Key Technology R&D Program of China (2011BAD14B02), Zhejiang Provincial Natural Science Foundation of China (LR14E060002), Program for New Century Excellent Talents in University (NCET-11-0446), and Specialized Research Fund for the Doctoral Program of Higher Education (20110101110021).

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REFERENCES

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Photoelectrocatalytic Reduction of CO2 into Chemicals Using Pt

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