Photoelectrochemical Homocoupling of Methane under Blue Light

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Photoelectrochemical Homocoupling of Methane under Blue Light Irradiation Fumiaki Amano,*,†,‡ Ayami Shintani,† Kenyou Tsurui,† Hyosuke Mukohara,† Teruhisa Ohno,§ and Sakae Takenaka∥ †

Department of Chemical and Environmental Engineering, The University of Kitakyushu, Kitakyushu, Fukuoka 808-0135, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan § Department of Applied Chemistry, Kyushu Institute of Technology, Kitakyushu, Fukuoka 804-8550, Japan ∥ Department of Applied Chemistry, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan

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‡

S Supporting Information *

ABSTRACT: Direct conversion of methane (CH4) into valuable chemicals with low-energy input is an important goal in the sustainable chemical industry. Herein, we report a photoelectrochemical activation of CH4 in the gas phase under visible light irradiation at room temperature. The proof-of-concept study revealed that homocoupling of CH4 to form ethane (C2H6) with high selectivity of 54% was induced by photogenerated holes over a tungsten trioxide (WO3) gas-diffusion photoanode coated with a proton-conducting ionomer in the presence of water vapor. The gas−electrolyte−solid triple-phase boundary enables the oxidation of the inert carbon−hydrogen bond of CH4, and the formation of carbon oxides and ethane with a carbon−carbon bond. The gasphase photoelectrochemical system shows incident photon-to-current conversion efficiency of 11% under blue light at an applied voltage of 1.2 V. This work is also the first demonstration of a visible-light-driven hydrogen evolution from CH4. The hydrogen is separated from CH4 and oxidized products by a solid polymer electrolyte membrane. CH4 in the gas phase.5,7 The kinetics at the gas−solid interface is easily controlled by optimizing partial pressure, contact time, and reaction temperature. The promising route for photocatalytic CH4 conversion is nonoxidative coupling of CH4 into C2H6 and H2 under ultraviolet (UV) irradiation at room temperature.7−14 However, the photocatalytic systems require high-energy UV light and show negligible visible-light activity as summarized in Table S1. The quantum efficiencies were very low even under UV light irradiation of λ < 400 nm.12−15 Deep UV light (wavelength λ < 300 nm) with high energy was necessary to obtain a relatively high quantum efficiency of 2%− 5%.7−11 Here we report a new photoelectrochemical (PEC) system for activation of CH4 with high quantum efficiency under visible light irradiation. Applying an electric field enables us to use narrow-bandgap semiconductors that absorb visible light and suppress the recombination of photoexcited electrons (e−) and positive holes (h+) in a space charge layer. Therefore, the PEC system can realize high quantum efficiency under lowenergy visible light.

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sing methane (CH4) as a chemical feedstock is important to realize sustainable development with minimized environmental impact.1−5 Over the last 50 years, extensive research has focused on the direct conversion of CH4 to alcohols or higher hydrocarbons. Currently, an energy-consuming multistep process via syngas, which is a mixture of carbon monoxide (CO) and hydrogen (H2), is the main route to produce the value-added chemicals from natural gas. Thus, development of a direct process for CH4 conversion is desirable. Oxidative coupling of methane into ethane (C2H6) and ethene (C2H4) over heterogeneous catalysts has attracted much attention as a potential direct process, but there is a trade-off between the conversion of CH4 and the selectivity for C2 hydrocarbons.4 A C2 of yield less than 30% is obtained from successive oxidation into thermodynamically favorable byproducts such as carbon dioxide (CO2). The nonselective overoxidation is caused by high-temperature reaction with O2 to activate extremely inert CH4, which has a high C−H bond energy (438 kJ mol−1) and large energy gap between its highest occupied and lowest unoccupied molecular orbitals.6 The activation of CH4 at low temperature might be an attractive approach to retard the formation of undesired byproducts. Heterogeneous photocatalytic process can activate © XXXX American Chemical Society

Received: December 13, 2018 Accepted: January 10, 2019

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DOI: 10.1021/acsenergylett.8b02436 ACS Energy Lett. 2019, 4, 502−507

Letter

Cite This: ACS Energy Lett. 2019, 4, 502−507

Letter

ACS Energy Letters

Figure 1. Photoelectrochemical (PEC) system for gas-phase CH4 activation. (a) Membrane electrode assembly (MEA) composed of a proton exchange membrane (PEM) sandwiched between a WO3 electrode and Pt catalyst electrode for photoelectrolysis of CH4 into C2H6 and H2. (b) PEC membrane flow reactor with an optical window to allow visible light irradiation for gaseous CH4 conversion.

Figure 2. Prepared WO3/Ti fiber electrode. (a) SEM image of Ti microfiber felt. The inset is a photograph of the electrode. (b) SEM images of WO3 particles deposited on Ti microfibers (WO3/Ti fiber). Cross-sectional SEM images are shown on the right. (c) Schematic illustration of the photoanodic reaction over the WO3/Ti fiber electrode. (d) PEC triple-phase boundary (left) and SEM image (right) of the Nafion ionomer-coated WO3/Ti fiber. (e) Photograph of the membrane electrode assembly composed of the ionomer-coated WO3/Ti fiber electrode and Nafion membrane. An ionomer-coated Pt catalyst film was attached to the other side.

We selected tungsten trioxide (WO3) with a band gap of 2.7 eV as an example of n-type semiconductors that absorb blue light.16,17 The valence band (VB) maximum of WO3 is more positive than the standard electrode potential (E°) of the oneelectron oxidation of CH4. This indicates that photogenerated h+ in WO3 can energetically oxidize CH4 into a methyl radical (•CH3).

in water are very low. PEC activation of CH4 is less developed and the experiment was performed in aqueous electrolyte; CO and carbonate are produced from CH4 over TiO2 photoanode under UV irradiation.19 Figure 1a shows a concept of “gas− solid” PEC conversion of CH4 to C2H6 using a membrane electrode assembly (MEA). The protons generated over the WO3 photoanode are passed through a solid polymer electrolyte membrane and reduced into H2 over a Pt catalyst cathode using e− via an external circuit. The reaction sites are separated by the membrane in the same manner as in a proton exchange membrane (PEM) electrolyzer to evolve H2.20 Although the conduction band (CB) minimum of WO3 is more positive than E° of H2 evolution reaction, it is possible to drive the reaction by applying an external bias voltage. Overall, in the PEC system, CH4 is converted to C2H6 and H2 by an input of energy larger than the Gibbs free energy (ΔG) and the external potential difference (ΔEext).

CH4 → •CH3 + H+ + e− E°(•CH3/CH4) = +2.06 V vs SHE18

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The thermodynamically downhill coupling of •CH3 in the gas phase is considered to be promoted in catalytic oxidative coupling of CH4 to produce C2 hydrocarbons.4 The challenge here is the development of a high-performance PEC reaction system at a gas−solid interface to activate CH4 in the gas phase since the solubility of hydrophobic CH4 503

DOI: 10.1021/acsenergylett.8b02436 ACS Energy Lett. 2019, 4, 502−507

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ACS Energy Letters

Figure 3. Photoelectrolysis of CH4 in the photoelectrochemical flow reactor at 1.2 V under blue light irradiation. (a) Current−time curve for the WO3 photoanode with increasing CH4 concentration from 10 to 97 vol % under pulsed photoirradiation. The numbers show incident photon-to-current conversion efficiency (%IPCE). (b) Time course of H2 evolution in the cathode compartment. The numbers show Faraday efficiency (%FE) of H2. (c) Time course of product formation in the photoanode compartment. The numbers show C2H6 selectivity (%, C-based). (d) Effect of CH4 concentration on the product formation rate. Reaction conditions: 453 nm blue LED, irradiance 6.8 mW cm−2, photoirradiation area 16 cm2, applied voltage 1.2 V, Ar/CH4/H2O = balance/0−97/3 for the photoanode, Ar/H2O = 97/3 for the cathode, flow rate 20 mL min−1, temperature 298 K, pressure 0.1 MPa.

measured specific surface area (SBET) of the WO3 particles in the WO3/Ti fiber electrode was 7.5 m2 g−1. The average diameter calculated from SBET assuming that each particle was a sphere was 110 nm, which corresponds to the diameter observed by scanning electron microscopy (SEM). The presence of the monoclinic WO3 was confirmed by Xray diffraction (XRD) and Raman spectroscopy measurements (Figure S1). The triclinic phase was not observed in the XRD pattern. Both monoclinic and triclinic WO3 show two intense Raman modes located at 710 and 810 cm−1, which are assigned to W−O stretching in WO6 octahedral units, and 12 Raman modes observed in the 150−500 cm−1 spectral region, which are assigned to W−O deformations.28 The formation of highly crystalline monoclinic WO3 was indicated by the Raman mode at 572 cm−1 (mode F).28 The sharp Raman peaks of WO3/Ti fiber suggest the high crystallinity of the monoclinic phase, because the higher the crystallinity, the sharper the Raman peaks. XRD pattern of the thermally oxidized Ti microfibers shows the formation of rutile TiO2 phase on the surface. However, the TiO2 crystalline phase was not confirmed in the WO3 electrode, suggesting that the interfacial titanium oxide layer is not crystallized. The electrode composed of WO3 nanoparticles deposited on Ti microfibers (denoted as WO3/Ti fiber) was used for gasphase CH4 activation (Figure 2c). Conventional PEC study in aqueous electrolyte revealed that the prepared WO3/Ti fiber electrode exhibited high performance in water oxidation to evolve O2.17 The enhanced PEC performance might be brought by the existence of the interfacial oxide layer. In contrast, the PEC performance was significantly decreased in the case of the gas-phase condition.26 Because WO3 is not an

2CH4 → C2H6 + H 2 ΔG298K = 68.6 kJ mol−1, ΔEext = 0.36 V18

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The PEM-based PEC cell facilitates the separation of reaction products while reducing the electrolyte resistance to minimal.21 This setup can be operated for gaseous reactants such as water vapor and volatile organic compounds in air unlike the conventional “liquid−solid” batch system in aqueous electrolytes.22−26 The design of high surface area semiconductor electrodes will need to maximize the triple-phase boundary between the electrolyte, the electrode, and the gaseous reactant.20,25,26 Here, we used perfluoropolymer with sulfonic acid for the electrolyte since the strong C−F bonds is more stable than CH4.27 The PEM-based flow reactor was applied to the activation of gaseous CH4 at the PEC triplephase boundary for the first time (Figure 1b). We prepared a bimodal (meso and macro) porous WO3 semiconductor electrode to facilitate gas diffusion and promote proton transport in the vertical direction of the electrode. A sintered Ti microfiber felt (Figure 2a) was used as a macroporous three-dimensional conductive substrate.17 The specific surface area of the felt (445 cm2 g−1) was calculated to be 83 times higher than that of a conventional two-dimensional Ti sheet substrate when the thickness was 1.0 mm.26 WO3 nanoparticles with a size of approximately 100 nm were connected with each other, providing interparticle mesopores and e− transport pathways (Figure 2b). The WO3 nanoparticles were deposited on the Ti microfibers through contact with the thermally oxidized surface layer, suggesting a good junction formed at the nanoparticle−microfiber interface. The 504

DOI: 10.1021/acsenergylett.8b02436 ACS Energy Lett. 2019, 4, 502−507

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ACS Energy Letters Table 1. Effect of CH4 Concentration on the PEC Reaction at 1.2 V under Blue Light Irradiationa Faraday efficiency (%) %CH4 0 10 50 70 97

%IPCE 8.6 10.5 11.2 11.1 10.9

H2 98.7 97.3 98.4 98.6 98.5

O2 90.4 36.7 4.6 1.6 0.8

CO2 c

9.1 56.7 81.7 78.8 75.3

selectivity (%, C-based)

C2H6

CO

sumb

CO2

C2H6

CO

0 0.5 4.8 7.7 12.0

0

99.5 93.9d 91.0d 88.1d 94.4e

100 93.4d 68.1d 56.0d 41.8

0 6.6d 32.0d 44.0d 53.5

0

6.3

4.7

a

The reaction conditions are the same as those in Figure 3. The data used for the calculation were obtained just before switching the light off. bThe sum in the photoanode compartment. cThe CO2 formation suggests the decomposition of Nafion ionomer in the absence of CH4. dThe selectivity was calculated without considering CO formation. The analysis of CO was possible only in the absence of Ar or CH4. eA trace amount of C3H8 was observed, but its quantification was difficult.

selectivities for CO2 and CO were 41.8% and 4.7% under a 97/3 CH4/H2O stream. The sum of FE values was 94%, suggesting that there are some unidentified products; a trace amount of propane (C3H8) was produced, but methanol was not observed in the exhaust gas. The PEC water oxidation to evolve O2 was inhibited as CH4 concentration increased, and the IPCE was increased from 8.6% to 11% by the presence of CH4. These results suggest that access of gaseous CH4 to the WO3 photoanode surface is important to induce preferential C−H bond activation by photogenerated h+ and C−C bond formation in the PEC system. Therefore, it is considered that the C2H6 selectivity is improved when the WO3 surface is modified to be hydrophobic. On the basis of the relationship between E° and the VB position of WO3, the dehydrogenative coupling of CH4 (2CH4 → C2H6 + 2H+ + 2e−) may proceed by the following mechanism.

ion conductor at room temperature, its surface was coated with perfluoropolymer with sulfonic acid, DuPont Nafion ionomer, to improve its proton conductivity (Figure 2d). The resulting triple-phase boundary facilitates proton-coupled electron transfer from gaseous CH4 and the transport/diffusion of them and products. The ionomer-coated WO3/Ti fiber electrode was hot pressed onto a membrane with an ionomer-coated Pt catalyst film on the opposite side (Figure 2e). Photoelectrolysis of CH4 was performed in the PEC membrane flow reactor under atmospheric pressure at room temperature (Figure S2). The applied voltage between the WO3 photoanode and gas-diffusion Pt catalyst cathode was set to 1.2 V. A continuous flow of humidified gas mixtures of CH4 and Ar was supplied to the photoanode compartment separated from the cathode compartment by the Nafion ionomer membrane. Water vapor was cofed since the membrane must be hydrated to maintain the high proton conductivity. The action spectrum of the photocurrent in the PEM-based PEC cell was consistent with the diffuse reflectance UV−vis spectrum of the WO3/Ti fiber electrode, suggesting that the photocurrent response is due to the bandgap photoexcitation of WO3.26 Figure 3a shows the photocurrent response of the reactor under 453 nm (blue) irradiation. The incident photon-to-current conversion efficiency (IPCE), which is also called the external quantum efficiency, of the system was 11%. At the same time, H2 was evolved in the cathode compartment with a Faraday efficiency (FE) close to 100% (Figure 3b). The H2 is separated from CH4 and products in photoanode compartment owing to the membrane partition by PEM. At the WO3 photoanode, only a small amount of C2H6 was produced under the stream of Ar with 10 vol % CH4 and 3 vol % H2O (Figure 3c). The dominant reactions were water oxidation to evolve O2 and complete oxidation of CH4 with H2O to CO2. When water vapor was not present, O2 was not evolved (Figure S3). Under the anhydrous condition, CO2 formation gradually decreased along with a decrease of photocurrent. This suppression is explained by the increase of the proton transfer resistance of the Nafion ionomer and membrane during anhydrous operation. In contrast, C2H6 formation slightly increased in the absence of water vapor. This suggests that the multiple-electron oxidation of CH4 is associated with water. We found that the rate of C2H6 formation increased monotonically with the concentration of CH4 fed into the photoanode compartment (Figure 3d). When the CH4 concentration was raised to 97 vol %, the C2H6 selectivity reached 53.5% on a per carbon basis (Table 1). The

CH4 + h+ → •CH3 + H+ •CH3 + •CH3 → C2H6

k1

(3)

k2

(4)

We applied the steady-state approximation to the concentration of the •CH3 intermediate, which is low and constant during the reaction. The rate equation of C−C coupling can be expressed as second-order with respect to the •CH 3 concentration, which is denoted as [•CH3]. Therefore, we can write the rate law of the C2H6 formation as follows. r(C2H6) = k 2[•CH3]2 = 0.5k1[CH4][h+]

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The first step involving •CH3 formation is rate determining, because the rate equation for the overall reaction is simply the rate equation for the first step, which is pseudo-first-order with respect to the concentration of CH4 and that of photogenerated holes. The rate equation corresponds to the kinetic data, which indicated that C2H6 formation was nearly proportional to the CH4 concentration (Figure 3d). The rate of C2H6 formation also increased almost linearly with the intensity of blue light. This suggests that the oxidation of C−H bond proceeds via a single electron transfer process.29 We propose that proton-coupled electron transfer is an important step of the CH4 activation. The C2H6 formation is expected to be accelerated by the high concentration of •CH3, since the C−C bond formation proceed between neighboring •CH3 species. It should be noted that the overall reaction is not limited by the diffusion of CH4 in the ionomer coated on WO3, since there was no significant effect of the flow rate of CH4 on the PEC reaction rate. Therefore, it is possible to discuss the kinetic data. 505

DOI: 10.1021/acsenergylett.8b02436 ACS Energy Lett. 2019, 4, 502−507

ACS Energy Letters The developed PEC system enables H2 production from CH4 under visible light irradiation. To the best of the author’s knowledge, this is the first report of the visible-light-driven H2 evolution from CH4 (Table S1). The H2 formation rate under UV was greater than 3 μmol min−1, which corresponds to 1000 μmol h−1 g−1. In the PEC system, so-called “photocatalytic steam reforming of methane” were partly promoted.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b02436. Experimental details, XRD patterns and Raman spectrum of WO 3/Ti fiber (Figure S1), system for photoelectrolysis of CH 4 (Figure S2), effect of humidification on photoelectrolysis of 10 vol % CH4 (Figure S3), and comparison with the reported photocatalytic and PEC activation of CH4 (Tables S1) (PDF)

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ACKNOWLEDGMENTS

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REFERENCES

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The half of the evolved H2 comes from CH4. Note that the Faraday efficiency of H2 was close to 100% in the cathode compartment. The purity of the H2 evolved over Pt catalyst can be sustained by the membrane separation from CH4 and oxidized products. The developed PEC system based on WO3 photoanode can utilize blue light (453 nm, 2.7 eV) but requires an external bias (1.2 V) to show high IPCE value (11%). The conventional photocatalytic system works without external bias but requires deep UV light (254 nm, 4.9 eV) to obtain high quantum efficiency (5%), as shown in Table S1. This indicates that the present PEC system is more efficient than the reported photocatalytic system in view of the total energy input. The thermodynamic minimum voltages for dehydrogenative coupling and steam reforming are 0.36 and 0.15 V as shown in eqs 2 and 6. Therefore, the PEC reaction is not photoenergy conversion system unless the external bias voltage is reduced. In conclusion, we have succeeded in developing the PEC homocoupling of CH4 to produce C2H6 and H2 at room temperature for the first time. Visible-light-driven methane activation was achieved in a high quantum efficiency by using WO3 photoanode in a gas-phase PEC system. The design concept of the photoelectrode and PEC reactor is possible to apply to other semiconductors with narrow band gaps and high CB minimum energies (negative in potential), realizing solar CH4 conversion into H2 and value-added chemicals without any applied bias. The CH4 conversion (approximately 0.1%) and C2H6 selectivity (54%, C-based) also should be increased by the improvement in the accessibility of CH4 on the photoanode surface.

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This work was supported by the JST, Precursory Research for Embryonic Science and Technology (PRESTO), grant number JPMJPR15S1 and JPMJPR18T1. F.A. thanks Prof. Hiroshi Kitagawa from Kyoto University and Prof. Yasushi Sekine from Waseda University for valuable discussions and encouragement.

CH4 + 2H 2O(g) → CO2 + 4H 2 ΔG298K = 114 kJ mol−1, ΔEext = 0.15 V18

Letter

AUTHOR INFORMATION

Corresponding Author

*Fumiaki Amano. E-mail: [email protected]. Tel: +8193-695-3372. ORCID

Fumiaki Amano: 0000-0003-2812-5799 Notes

The authors declare no competing financial interest. 506

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