Highly Selective Photoelectrochemical Conversion of Carbon Dioxide

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Highly selective photoelectrochemical conversion of carbon dioxide to formic acid Mengpei Jiang, Hongjun Wu, Zhida Li, Deqiang Ji, Wei Li, Yue Liu, Dandan Yuan, Baohui Wang, and Zhonghai Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03272 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Sustainable Chemistry & Engineering

Highly selective photoelectrochemical conversion of carbon dioxide to formic acid† Mengpei Jiang,a Hongjun Wu,a* Zhida Li,a Deqiang Ji,a Wei Li,b Yue Liu,a Dandan Yuan,a Baohui Wang,a Zhonghai Zhangc*

a Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Development Road 199, Daqing 163318, China. b College of Petroleum Engineering, Northeast Petroleum University, Development Road 199, Daqing 163318, China. c School of Chemistry and Molecular Engineering, East China Normal University, Dongchuan Road 500, Shanghai 200241, China.

* Corresponding author: Hongjun Wu, [email protected] Zhonghai Zhang, [email protected]

ACS Paragon Plus Environment


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ABSTRACT Harvesting solar energy and converting excess carbon dioxide (CO2) in the atmosphere into energetic products hold promise in addressing both problems of detrimental energy use and serious greenhouse gas effects. Catalytic activity and selectivity were the most important aspects of this investigation. Herein, an N-Fe2O3/TiO2 catalyst was reported as well as the development of a customized method for regulating the catalytic properties and mechanism for CO2 reduction. This method enabled elevated electron transfer, regulated formation of target products (formic acid and ethanol), and control of the specific product proportions. Under optimal photoelectrochemical selective conditions, the maximal rate of formic acid production reached 74896.13 nmol·h-1·cm-2 with a selectivity of 99.89%. Such a catalyst and controlled artificial methods can ensure catalyst selectivity and activity and offer potential applications in the production of useful chemicals from CO2 carbon feedstock.

KEYWORDS: TiO2 NTs, Fe2O3, CO2, photoeletrochemical conversion of CO2, selectivity, HCOOH, CH3CH2OH

Environmental pollution and resource shortages are the world's two major longstanding and unresolved problems. The consumption and output of carbon dioxide (CO2), one of the main gases of the greenhouse effect in nature, should be in a certain balance.1,2 Unfortunately, in recent years, excessive CO2 has been emitted from many industrial processes and has disturbed its steady state.3 Also, it has been predicted that CO2 emissions will further increase to more than 40 Gt by 2035, which


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will cause greenhouse effect caused socio-environmental problems to worsen.4 Although more and more countries are retreating from the situation of being dependent on fossil fuels to reduce CO2 emission, efficient conversion of CO2 into high-value products using renewable energy sources is still urgently desirable.5 Many approaches have been introduced in this field, such as electrochemical reduction,6,7 photocatalytic fixation,8,9 and photoelectrochemical (PEC) conversion.10,11 Among these strategies, PEC conversion of CO2 with solar energy distinguishes itself for having a fast reaction rate, high conversion efficiency, and low energy loss. However, multiple reduction routes of CO2 attaining two, four, or six electrons produce various products, including carbon monoxide (CO) and formic acid (HCOOH), formaldehyde (HCHO), methanol (CH3OH) and ethanol (CH3CH2OH), respectively.12–14 This severely limits the practical applications of the PEC method such that selective PEC conversion of CO2 remains a big challenge. Herein, a composite photoelectrode of a hematite-modified TiO2 nanotube (TiO2 NTs) was designed for efficient highly selective conversion of CO2. Anodized TiO2 NTs were selected as pristine photoelectrodes due to their high surface area and fast electron transfer activity through its one-dimensional nanostructure.15,16 However, the high band gap of 3.2 eV severely restricts its excitation wavelength to the range of ultraviolet (UV) light.17 Therefore, thus far this approach has yielded only low CO2 conversion rates, rarely exceeding several µmol/h.18 To address this concern, in this study, TiO2 NTs were modified with visible light-responsive hematite (Fe2O3/TiO2 NTs) to shrink the band gap to 2.0 eV and shift the absorption edge to ~600 nm. Also,



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the existence of Fe species in Fe2O3/TiO2 NTs was vital for selective conversion of CO2 to HCOOH because of the formation of Fe(II)-COOH as a chemical intermediate.19,20 Therefore, the Fe2O3/TiO2 NTs presented high yield efficiency and satisfactory selectivity, producing > 99% conversion of CO2 into HCOOH. TiO2 NTs were prepared by a previously reported method,21,22 and α-Fe2O3 then loaded to TiO2 NTs using a PEC deposition method. Finally, the Fe2O3/TiO2 NTs were annealed in a nitrogen atmosphere. Detailed procedures can be found in the experimental section and illustrated in Figure S1. A field emission scanning electron microscopy (FESEM) image of TiO2 NTs showed the presence of a double layer morphology with large diameter (150 nm) nanorings in the upper layer and small diameter (50 nm) tubes in the lower layer, indicating a nanostructure that provided a large surface area for loading Fe2O3 (Figure 1a1). After deposition, Fe2O3 nanoparticles were uniformly loaded onto TiO2 NTs (Figure 1a2), as small size particles that were slightly aggregated after annealing (Figure 1a3). In addition, under closer observation, transmission electron microscopy (TEM) images of TiO2 NTs and Fe2O3/TiO2 NTs showed that the nanotubes were >1.5 µm in length (Figure 1b1), with small diameters of 50 nm, which was consistent with the FESEM data. Fe2O3 particles were not only loaded on the outside walls of the TiO2 NTs but also deposited into the NTs as ~5 nm particles (Figure 1b2 and Figure 1b3). Furthermore, high-resolution TEM (HRTEM) images revealed a clear lattice fringe of 0.35 nm, corresponding to anatase TiO2 with (101) plane and a lattice fringe of 0.21 nm, which matched well with the (202) plane of Fe2O3 (Figure 1c).


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The crystal structures of Fe2O3/TiO2 NTs were determined by recording the X-ray diffraction (XRD) patterns of TiO2 NTs and Fe2O3/TiO2 NTs (Figure 1d), where, except for peaks from Ti, both samples showed typical strong TiO2 anatase peaks with dominated diffraction of (110) plane, agreeing well with the HRTEM data. In addition, the Fe2O3/TiO2 NTs sample presented new peaks, which were all ascribed to α-Fe2O3. To better understand the surface compositions and chemical valence state of Fe2O3/TiO2 NTs, X-ray photoelectron spectroscopy (XPS) was performed, which determined the existence of Ti, Fe, O, and N elements (Figure S2) and showed the core-level XPS of the N 1s and Fe 2p (Figure 1e1 and Figure 1e2, respectively). As the Fe2O3/TiO2 NTs were annealed under nitrogen at high temperature, nitrogen traces were doped into the TiO2 lattice, such that the analysis showed binding energy peaks at 399.0 and 400.32 eV, which were attributed to N-Ti-O and Ti-N-O bonds, respectively.23,24 In addition, the core-level XPS of the Fe 2p presented typical binding energy peaks at 711.3 eV and 724.8 eV, which were ascribed to the Fe(III) 2p3/2 and Fe(III) 2p1/2, respectively, along with its satellite binding peaks. The core-level of the O 1s also demonstrated the formation of Ti-O and Fe-O bonds, along with oxygen adsorption. The optical absorption properties of TiO2 NTs (Figure S3) and Fe2O3/TiO2 NTs were estimated through diffuse reflection spectra (DRS) (Figure 1f). TiO2 NTs showed strong absorption in the UV light region with an onset wavelength around 390 nm, corresponding to a band gap of ~3.2 eV.25 After deposition of



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Fe2O3, except for UV absorption, Fe2O3/TiO2 NTs also presented strong optical absorption in the visible light region with an onset wavelength of ~600 nm, which was consistent with the band gap of Fe2O3 of ~2.0 eV and implied that modification of TiO2 with Fe2O3 significantly expanded its optical absorption region. Fe2O3 deposition amounts were controlled and optimized through adjustment of the iron precursor concentration (Table S1).

Figure 1 (a) SEM images of (1) TiO2 NTs, (2) Fe2O3/TiO2 NTs, and (3) Fe2O3/TiO2 NTs after annealing under N2 atmosphere; (b) TEM images of (1) TiO2 NTs and (2,3) Fe2O3/TiO2 NTs; (c) HRTEM images and SEAD patterns of (1,3) TiO2 and (2,4) Fe2O3/TiO2 NTs; (d) XRD patterns of TiO2 NTs and Fe2O3/TiO2 NTs; (e) core-level XPS of (1) N 1s and (2) Fe 2p; and (f) DRS of TiO2 NTs and Fe2O3/TiO2 NTs.


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PEC conversion of CO2 was performed in a CO2-saturated sodium bicarbonate solution with Fe2O3/TiO2 NTs as the photoelectrode and the irradiation wavelengths and bias potentials tuned to adjust its yield and selectivity for CO2 conversion (Figure S4). The reduction of CO2 on Fe2O3/TiO2 NTs under illumination of 380, 420, and 500 nm in a photocatalytic (PC) process generated a variety of organic compounds, including CH2O, CH3OH, C2H5OH, propanol (C3H7OH), and HCOOH, with the selectivity for HCOOH formation at >98% (Figure 2). However, the yield rates were still low at 11000–6000 nmol·h-1·cm-2. For further increases in the CO2 conversion yield rate, a PEC strategy was proposed with negative biased potentials between -0.25 V to -1.0 V (V versus Ag/AgCl). The resulting yield rates were significantly increased to 60000–70000 nmol·h-1·cm-2 in all PEC processes (Figure 2a–2d). In addition, under illumination by different wavelengths, the optimal bias potentials were found to be different. For example, with 420 nm radiation, the bias potential was -0.25 V (-0.50 V) and reached a high yield rate of 73056.34 nmol·h-1·cm-2 (76568.18 nmol·h-1·cm-2) with good selectivity at 99.92% (97.48%); under 380 nm, the bias potential was -0.75 V and reached a yield rate of 74459.48 nmol·h-1·cm-2, with an excellent selectivity of 99.97%; and under 500 nm, the bias potential was -1.00 V and reached yield rate of 74896.13 nmol·h-1·cm-2, with a selectivity of 99.89% (Figure 2a and Figure 2e). All these data were obtained from high-performance liquid chromatography (HPLC) and gas chromatography (GC) measurements (Figure S5 and Figure S6) and summarized in Table S2. Using Equation S1-S3, TOF data (turnover frequency) of PEC conditions were determined, listed in Table S3, and observed to be higher than in



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Table S5 (TOF data of PC conditions, calculated from Table S4). This comparison also signified the vital role of Fe2O3 for CO2 conversion.

Figure 2 Yield rate of (a) HCOOH; (b) C2H5OH; (c) CH3OH; (d) (CH3)2CHOH (e) selectivity of CO2 reduction products under PC (0 V) and PEC conditions on Fe2O3/TiO2 NTs. The mechanism of high yield and good selectivity of CO2 conversion on Fe2O3/TiO2 NTs in PEC processes was elucidated by performing a series of electrochemical (EC) and PEC characterizations. First, cyclic voltammetry (CV) testing on Fe2O3/TiO2 NTs with and without illumination were collected at


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different scan rates (Figure 3a and Figure 3b). From Randles-Sevcik equation (Equation 1)，26

iP = 2.69 × 105 n 3 / 2 AD1/ 2Cv1/ 2

(1)

where A is the active electrode area, D the diffusion constant, C the concentration, and v the scan rate. The function between the peak current density (ip) and the square of the scan rate (v1/2) was plotted (Figure 3c). The slope of ip under EC and PEC conditions was recorded to be 1.76 and 3.99, respectively. According to the Randles-Sevcik equation, the surface of the electrode during the PEC reaction was concluded to be clearly excited and the area greater than the actual electrode area calculated from Equation 2,27

1/ 2

Q=

2nFAD0 t 1/ 2C0

π 1/ 2

*

+ Qdl = kt 1/ 2 + Qdl

(2)

where n is the number of electron transfer, F the faraday constant, A the actual electrode surface, Do the diffusion coefficient of the reaction particles in solution, t the reaction time, C0 the original concentration of the reaction particles, and Qdl the electric charge of the electric double layer. The charge variants versus time on Fe2O3/TiO2 NTs at different bias potentials in EC and PEC conditions were plotted (Figure 3d and Figure 3e) and the observed increasing intercept values signified a notable elevation in electric charge.



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Figure 3 Cyclic voltammetry curves at different scanning rates under (a) EC and (b) PEC conditions; (c) Randles-Sevcik plots; Charge vs time curves under (d) EC and (e) PEC conditions A further study on the PEC performance at different wavelengths on Fe2O3/TiO2 NTs was performed (Figure 4). Linear sweep voltammetry (LSV) testing did not present any reduction peaks when performed in darkness (Figure S7). Under illumination with different wavelengths, LSV tests presented different numbers of reduction peaks. For example, under 420 nm illumination, four reduction peaks were observed and ascribed to hydrogen reduction at -0.25 V, trivalent iron to ferrous iron at -0.5 V, the CO2 reduction peak at -0.75 V, and ferrous iron to elemental iron at -1.0 V. Thus, it was possible, with the proper applied wavelengths to utilize the fact that CO2


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reduction potential fell into the range between the reaction potentials of trivalent iron/ferrous iron and ferrous iron/elemental iron.

Figure 4 LSV curves under different wavelengths of light in CO2-saturated 1 M sodium bicarbonate solution. Based on above analysis and previous reports,28,29 a speculative mechanism was used to explain the reaction pathway of CO2 on the Fe2O3/TiO2 photoelectrode (Figure 5). As shown by the following equations, when receiving illumination, TiO2 was excited to produce photogenerated electrons and holes (Equation 3). Simultaneously, the lower conduction band of the αFe2O3 semiconductor provided an opportunity for electron transfer from TiO2 to αFe2O3, increasing the reactive sites on αFe2O3, and more CO2 was adsorbed to form CO2- ads (Equation 4, 5). After further combination with H2O and electron-rich αFe2O3, O-[FeⅢCOOH]2+ was formed and finally evolved into formic acid (Equation 6-9). The valence of iron was controllable by adjusting the reaction voltage and wavelength, which regulated the generation reactions



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of OH- (Equation 4-6) and ·COOH (Equation 8, 9), transforming the portion of alcohols and acids in the product. Therefore, CO2 reduction to formic acid was facilely achieved via regulating the reaction conditions, with by-product generation readily limited.

hv TiO2 → TiO2 ( H + + e − )

(3)

TiO2 (e − ) + Fe2O3 → TiO2 + Fe2O3 (e − )

(4)

−

CO2 + e − ( Fe2O3 ) → CO2 (ads )

(5)

−

CO 2 + H 2O → HCO3 + OH −

(6)

HCO3 + e − ( Fe2O3 ) → HCOO −

[

O − FeΙΙΙ + HCOO − → O − Fe ΙΙΙ COOH

(7)

]

2+

O − [ Fe ΙΙΙ COOH ]2+ → O − Fe ΙΙ + •COOH

(8) (9)

Figure 5 Proposed reaction pathways for the PEC reduction of CO2.

In summary, Fe2O3/TiO2 NTs arrays were prepared for efficient and


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selective conversion of CO2 into HCOOH. The addition of Fe2O3 expanded the optical absorption of TiO2 and enlarged its active surface. In addition, illumination wavelengths and bias potentials in the PEC processes were optimized, producing a technique that possessed a promising potential for solving the problem of low selectivity for CO2 conversion.

ASSOCIATED CONTENT Supporting Information The supplementary Information is available. Experimental details, XPS survey, core-level XPS of O 1s, photocurrent-time curves, HPLC and GC data, band gap values, yield rate, selectivity and TOF data

(Figure

S1-S7,

Equation

S1-S3,

Table

S1-S5

)(PDF).

See

DOI: 10.1039/c000000x/

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected] (PDF) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21476046, 21306022 and 51490650), Science Fund for Distinguished Young Scholars of Heilongjiang Province of China (No. JC2017002), China



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Postdoctoral Science Foundation (No. 2013M540269), Innovative Team of Science and Technology in Heilongjiang Higher Education Institutes (No. 2013TD004) and Northeast Petroleum University (No. SJQHB201602 and YJSCX2016-018 NEPU). We are grateful to Professor Stuart Licht (The George Washington University) for valuable contributions to the research.

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Highly efficient photoelectrochemical conversion of CO2 to formic acid with nearly 100% selectivity is implemented on Fe2O3/TiO2 nanotube photoelectrode, which demonstrates a new route for designing directional CO2 conversion. 84x47mm (266 x 266 DPI)


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Highly Selective Photoelectrochemical Conversion of Carbon Dioxide

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