Nitrogen Fixation with Water on Carbon-Nitride-Based Metal-Free

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Nitrogen Fixation with Water on Carbon-Nitride-Based Metal-Free Photocatalysts with 0.1% Solar-to-Ammonia Energy Conversion Efficiency Yasuhiro Shiraishi, Shingo Shiota, Yusuke Kofuji, Masaki Hashimoto, Kiyomichi Chishiro, Hiroaki Hirakawa, Shunsuke Tanaka, Satoshi Ichikawa, and Takayuki Hirai ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00829 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Applied Energy Materials

Nitrogen Fixation with Water on Carbon-Nitride-Based Metal-Free Photocatalysts with 0.1% Solar-to-Ammonia Energy Conversion Efficiency Yasuhiro Shiraishi,*,†,‡ Shingo Shiota,† Yusuke Kofuji,† Masaki Hashimoto,† Kiyomichi Chishiro,† Hiroaki Hirakawa,† Shunsuke Tanaka,‖ Satoshi Ichikawa,§ and Takayuki Hirai† †

Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan ‡ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan ‖ Department of Chemical, Energy and Environmental Engineering, Kansai University, Suita 564-8680, Japan § Institute for NanoScience Design, Osaka University, Toyonaka 560-8531, Japan ABSTRACT: Ammonia (NH3), which is an indispensable chemical, is produced by the Haber-Bosch process using H2 and N2 under severe reaction conditions. Although photocatalytic N2 fixation with water under ambient conditions is ideal, all previously reported catalysts show low efficiency. Here, we report that a metal-free organic semiconductor could provide a new basis for photocatalytic N2 fixation. We show that phosphorus-doped carbon nitride containing surface nitrogen vacancies (PCN-V), prepared by simple thermal condensation of the precursors under H2, produces NH3 from N2 with water under visible light irradiation. The doped P atoms promote water oxidation by the photoformed valence-band holes, and the N vacancies promote N2 reduction by the conduction-band electrons. These phenomena facilitate efficient N2 fixation with a solar-to-chemical conversion (SCC) efficiency of 0.1%, which is comparable to the average solar-to-biomass conversion efficiency of natural photosynthesis by typical plants. Thus, this metal-free catalyst shows considerable potential as a new method of artificial photosynthesis. KEYWORDS: photocatalysis· carbon nitride · nitrogen fixation · ammonia · artificial photosynthesis

INTRODUCTION Owing to its role in various biological processes, ammonia (NH3) is an essential compound for all forms of life.1 In recent years, NH3 has attracted considerable attention not only as a potential hydrogen carrier because of its high hydrogen density (17.8 wt%) and low liquefying pressure (~8 atm),2,3 but also as a fuel for electricity generation because the theoretical potential of the NH3/O2 fuel cell is 1.17 V,4 which is comparable to that of the conventional H2/O2 cell (1.23 V).5 NH3 is manufactured via the Haber-Bosch process using H2 and N2. However, this process requires significantly high pressures and high temperatures,6 with huge amounts of H2 produced from fossil fuels.7 Catalytic N2 fixation using an earth-abundant reductant at atmospheric pressure and room temperature is desired for sustainable NH3 synthesis. Photocatalysis is a desirable method for N2 fixation since it is a simple method that can use earth-abundant water as a reductant.8 The valence-band holes (VB h+) oxidize water (eq.1), and the conduction-band electrons (CB e) reduce N2 (eq. 2). NH3 can be formed by sunlight under ambient conditions (eq. 3). Owing to its large free-energy gain,9 it is a potential candidate as a new artificial photosynthesis, with the reactions such as water splitting (eq. 4)10–12 and hydrogen peroxide generation (eq. 5).13–15

2H2O + 4h+  O2 + 4H+ (1.23 V vs. NHE)

(1)

N2 + 6H+ + 6e–  2NH3 (–0.09 V vs. NHE)

(2)

1/2N2 + 3/2H2O  NH3 + 3/4O2 (ΔG = 339 kJ mol ) (3) –1

H2O  H2 + 1/2O2 (ΔG = 237 kJ mol1) –1

H2O + 1/2O2 → H2O2 (ΔG = 117 kJ mol )

(4) (5)

Several semiconductors have been used for N2 fixation. Most of them16–22 exhibit low activity owing to their poor capability of water oxidation (eq. 1), and they require sacrificial electron donors. Bismuth oxybromide promotes the reactions under visible light,23 but its quantum yield is low (~0.2% at 420 nm) and it suffers from low photostability.24 Robust TiO2 photocatalysts produce NH3 with relatively high efficiency, but they require UV light and a noble metal (Pt,25 Ru, Rh, or Pd26) as a co-catalyst. Recent report has indicated that a localized surface plasmon resonance (LSPR)-based photoelectrode (Au/NbSrTiO3/Zr/ZrOx) produces NH3 with a high quantum yield (~1%) under visible light, but it requires a noble metal and a chemical bias to the cells (anode: pH 1, cathode: pH 13).27 Therefore, robust and inexpensive catalysts that efficiently promote water oxidation and N2 reduction under visible light are desired for ultimate green NH3 synthesis.

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Table 1. Properties of Catalysts.

catalyst

surface area / m2 g–1 a

average pore diameter / nm b

pore volume / cm3 g–1 b

elemental composition / mol% c,d C

N

P

N/C / mol/mol d

bandgap / eV e

CN

7

43.4

56.6

1.30

2.68

CN-V

12

47.2

52.8

1.12

2.53

PCN

10

41.6

56.2

2.2

1.35

2.54

PCN-V

11

45.2 (45.4)

52.3 (52.1)

2.5 (2.3)

1.16 (1.15)

2.43

m-PCN-V(64)

64

10.6

0.12

45.6

52.0

2.4

1.14

2.45

m-PCN-V(98)

98

10.6

0.19

46.1 (45.8)

51.7 (51.8)

2.2 (2.5)

1.12 (1.13)

2.47

m-PCN-V(164)

164

12.1

0.30

45.7

52.3

2.0

1.14

2.48

a Brunauer–Emmett–Teller

(BET) surface area determined by N2 adsorption/desorption analysis. b Determined by Barrett–Joyner–Halenda (BJH) method. c Determined by the respective peak areas for XPS charts with atomic sensitivity factors (C1s, 1.00; N1s, 1.22; P2p, 0.42). d The numbers in the parenthesis are the data for the catalysts obtained after the photoreactions for 24 h. e Determined by a plot of the Kubelka– Munk function versus the energy of light absorbed.

The step that determines the rate of N2 reduction is the cleavage of the NN bond.28 Therefore, the active sites for efficient NN cleavage is necessary. Recently, we showed that TiO2 with surface oxygen vacancies successfully reduces N2 with water under UV light.29 These vacancies behave as N2 adsorption sites via electron donation from the adjacent Ti3+ species and promote N2-to-NH3 reduction with a solar-to-chemical conversion (SCC) efficiency of 0.02%, which is higher than that of previously reported systems. Thus, defective reduction sites created on the surface of inexpensive and visible-light-responsive semiconductor powders may promote efficient N2 fixation. Our strategy for N2 fixation involves the use of carbon nitride (CN), a visible-light-responsive polymeric semiconductor with a graphitic stacking structure of melem sheets. 30,31 Theoretical calculations of graphitic materials suggest that N2 is strongly adsorbed onto the defective sites of graphite or carbon nanotubes and its NN bond is weakened significantly, whereas it is scarcely adsorbed onto perfect surfaces.32,33 Furthermore, it has been noted that pristine CN exhibits low activity for water oxidation.31 Several methods, such as loading of metal oxide cocatalysts34 and incorporation of aromatic imides,13,15 have been proposed for the enhancement of oxidation activity. In particular, doping of the boron,35 sulfur,36 or phosphorus atom37,38 within the melem framework is the simplest and most effective method because it involves simple calcination of the precursors. These preliminary findings imply that defective sites created on the surface of CN materials with enhanced water oxidation ability may promote efficient N2 reduction with water as a reductant. Here, we report that simply prepared metal-free CN photocatalysts successfully promote N2 fixation with water. We fabricated phosphorus-doped CN containing surface nitrogen vacancies (PCN-V) by thermal polymerization of the precursors under H2 atmosphere. In this case, the doped P atoms promote water oxidation, and the N vacancies promote N2 reduction. These redox properties successfully promote N2 fixation with water under visible light. Furthermore, we emphasize that mesoporous PCN-V (m-PCN-V) with large surface area, prepared by silica-templated polymerization, exhibit enhanced activity and promote N2 fixation with an SCC efficiency of 0.1%, which

is comparable to the average solar-to-biomass conversion efficiency of natural photosynthesis by typical plants. RESULTS AND DISCUSSION Catalyst preparation. Pristine CN catalyst was prepared by calcination of dicyandiamide under N2.30 CN containing N vacancy (CN-V catalyst) was prepared by calcination under H2 flow.20 P-doped CN (PCN catalyst) was prepared using hydroxyethylidene diphosphonic acid (HEDP) as a phosphorus source:37,38 A mixture of HEDP and dicyandiamide was calcined under N2 at 773 K. P-doped CN with N vacancy (PCN-V catalyst) was prepared by calcination of the above-mentioned mixture under H2 flow at 773 K (See Experimental Section). As summarized in Table 1, the surface area of PCN-V (11 m2 g–1) is similar to that of CN, CN-V, and PCN (7–12 m2 g–1). The corresponding transmission electron microscopy (TEM) images show sheet-like structures (Figure S1, Supporting Information), representing a layered architecture. The X-ray diffraction (XRD) patterns of the catalysts show distinctive peaks assigned to (100) and (002) packing of the melem sheets at 2θ = 13.1 (d = 0.675 nm) and 27.4 (d = 0.325 nm),30 respectively (Figure S2). These data suggest that PCN-V has a layered stacking structure of melem sheets, as does CN.30 Structural properties of catalysts. Elemental mapping of PCN-V by scanning electron microscopy (SEM) shows homogeneously distributed P elements (Figure S3). X-ray photoelectron spectroscopy (XPS) of PCN and PCN-V at the P2p level (Figure S4) shows a signal at 133.3 eV assigned to the P–N group,39,40 whereas CN and CN-V are silent. The elemental compositions of the catalysts (C, N, and P) can be determined by their respective peak areas in the XPS charts with individual atomic sensitivity factors (Table 1).41 The C and N compositions of pristine CN are 43.4 mol% and 56.6 mol%, respectively, where the N/C ratio is consistent with the theoretical stoichiometric value (1.30). PCN has a similar N composition (56.2 mol%), but the C composition (41.6 mol%) decreases by the P doping (2.2 mol%). This implies that the P atoms are doped in place of the melem C atoms. The P composition of PCN-V (2.5 mol%) is similar to that of PCN, whereas the N composition of

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PCN-V (52.3 mol%) is smaller than that of PCN (56.2 mol%). Elemental analysis by the combustion method also shows decreased N compositions of the catalysts prepared under H2 (Table S1). These data indicate that thermal polymerization of the precursors under H2 produces a CN framework containing N vacancies.42,43 Figure 1a shows a schematic representation of the melem units on the respective catalysts. XPS charts of the catalysts at the N1s level (Figure S5) show three components assigned to sp2-hybridized N atoms of melem (Npyridine) at 398.4 eV, trigonal N atoms of the melem center and tertiary amine of the melem peripheral (Ncenter + Ntertiary) at 399.7 eV, and primary and secondary amines of the melem peripheral (Nprimary + Nsecondary) at 400.9 eV.44 Deconvolution of these signals determines the composition of the respective N atoms (Table S2). CN and PCN contain 77% Npyridine, whereas CN-V and PCN-V contain 70% Npyridine. This implies that thermal polymerization of the precursors under H2 creates N vacancies at the Npyridine sites (Figure 1a). As shown in Figure 2a, electron spin resonance (ESR) analysis of the catalysts measured at 77 K shows a Lorentzian line originating from the unpaired electron in the C atom (C3+) adjacent to the Npyridine vacancy,20 where the intensity of PCN-V is higher than that of CN. Therefore, the structure of PCN-V can be proposed as shown in Figure 1a. The P atoms are doped in place of C atoms, and parts of the Npyridine atoms are absent owing to calcination under H2, creating Npyridine vacancies. As shown in Figure 1b, the melem sheets containing P atoms and N vacancies have multiple layers owing to stacking interactions, forming a graphitic layered architecture. Optical and electrochemical properties of catalysts. The diffuse-reflectance UV-vis spectrum of PCN-V shows larger absorption in the visible region than that of CN (Figure 2b). This difference can be attributed to the incorporated P atoms and N vacancies, as evidenced by the spectra of PCN and CNV; the bandgap of PCN-V (Table 1) decreases to 2.43 eV (= 540 nm). The electrochemical Mott-Schottky plots of the catalysts (Figure S6) show a typical n-type response. Thus, the obtained flat-band potentials and bandgap energies provide the band structures of the catalysts (Figure 2c). The CB levels become positive by the incorporation of P atoms and N vacancies, but they are still more negative than the reduction potential of N2 (– 0.09 V vs. NHE, pH 0),18 whereas the VB levels scarcely g = 2.004

a

change. The CB and VB levels of CN are mainly contributed by the C2p and N2p orbitals, respectively.30 Substitution of C(2p) with electronegative P(3p) causes the CB level to become more positive.45 In addition, at the N vacancy (Figure 1a), the donor level of the unpaired electron located in the adjacent C 3+ atom may lie below the CB level,42 as is the case for TiO2 with surface oxygen vacancies.46 The defect-derived state partially overlaps the CB and causes the level to become more positive. These effects due to the incorporation of the P atoms and N vacancy may thus contribute to the decrease in the bandgap of PCN-V. a

b

melem sheet

P atom Npyridine vacancy

Figure 1. Proposed structure of catalysts. (a) Melem units for the respective catalysts and (b) three-dimensional architecture of PCNV catalyst.

b

c –0.76 V

vs. NHE (pH 0) –0.62 V

m-PCN-V(98)

–0.57 V

2

PCN-V

–0.46 V CB

PCN

CN-V

F(R∞)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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m-PCN-V(164) CN

2.68 eV

PCN-V

N2 / NH3 –0.09 V 2.53 eV

2.54 eV

1

2.43 eV CN-V

H2O / O2 1.23 V

CN

0 3360

3380 Magnetic field / G

3400

400

500 600  / nm

700

1.82 V CN

1.91 V CN-V

1.97 V PCN

VB 1.97 V PCN-V

Figure 2. Properties of catalysts. (a) ESR spectra (77 K), (b) DR UV-vis spectra, and (c) electronic band structures of the catalysts.

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b 40

10

c 1

1.5

1

6

0.5

−

20

A

+

NH3 O2

0.5

H2

0 0

0 300

0 24

12 t/h

PCN-V

d

400

500  / nm

600

0 700

e 100

20

4

AQY / 

m-PCN-V(64)

1 e /h

8

Products / mol

m-PCN-V(98)

RCT

m-PCN-V(164)

2 CN-V

m-PCN-V(98)

15 10

0.05 PCN-V

5

PCN CN

0 0

6

12 t/h

18

24

0

0 0

2

4

6

t /h

SCC efficiency /  −Z'' / k

0.1

NH3 formed / mol

NH3 formed / mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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RS PCN

CDL

50 CN-V

CN

PCN-V

0 0

50 Z' / k

100

Figure 3. Photocatalytic performance and properties. (a) Formation of NH3 by photoreaction. Conditions: water (100 mL), catalyst (200 mg), N2 (0.3 L min−1), λ >420 nm (Xe lamp, light intensity at 420−600 nm: 40.7 W m−2). (b) Change in the product amounts and the e–/h+ balance {= [NH3]  3/([O2]  4)} during photoreaction (λ >300 nm) on PCN-V in water with N2 using a closed gas circulation system (0.04 MPa). (c) Absorption spectrum of PCN-V and action spectrum for NH3 formation obtained under N2 bubbling, where AQY was determined by the eq. 6. (d) The NH3 amount and SCC efficiency under simulated AM1.5G sunlight (1-sun) irradiation with N2 bubbling, where the efficiency was determined by the eq. 7. The light intensity at 300−600 nm is 291 W m−2. (e) Nyquist plots of the catalysts under visible light at a bias of 0.8 V (vs Ag/AgCl) in 0.1 M KCl. The equivalent circuit model contains ohmic resistance (RS), double layer capacitance (CDL), and charge transfer resistance (RCT).

Photocatalysis. Photoreactions were carried out by visible light irradiation (λ > 420 nm) of pure water (100 mL) with catalyst (200 mg) using a Xe lamp under N2 bubbling (0.3 L min– 1 ) at 303 K. Figure 3a shows the time-dependent change in the amount of NH3 formed. In all reactions, hydrazine, nitrate, or nitrite was not detected by UV-vis analysis47 or ion chromatography analysis, indicating that NH3 is the sole nitrogen-containing product. The pH of the solutions after the reactions is ca. 5; therefore, the produced NH3 exists as a protonated NH4+ form (pKa 9.3).48 Pristine CN scarcely produces NH3, and CN-V and PCN produce only m-PCN-V(64) > PCN-V (11), which is in good agreement with the photocatalytic activity, indicating that greater numbers of exposed P atoms and N vacancies as active sites enhance the photocatalysis. By contrast, mPCN-V(164) generates a much smaller photocurrent despite its large surface area. As shown in Figure 2a, the intensity of the ESR signal for the unpaired C3+ electrons adjacent to the N vacancies on m-PCN-V(98) is higher than that on PCN-V, whereas that on m-PCN-V(164) is much lower. This is probably because a large number of N vacancies act as defective sites that decrease the electronic conductivity or promote recombination of the photoformed charge carriers,56,57 thereby decreasing the photocatalytic activity. These findings suggest that creation of appropriate numbers of N vacancies by controlling the surface area is necessary for the best catalytic performance. Notably, as shown in Figure 3d, the SCC efficiency for N2 photofixation on m-PCN-V(98) reaches 0.1%, which is twice as high as that on nonporous PCN-V and is comparable to the average solar-tobiomass conversion efficiency (photosynthetic efficiency) of natural photosynthesis by typical plants.58 It must be noted that a solar-to-glucose efficiency is significantly larger (~8%) although much of the energies are consumed by several processes such as photorespiration and growth of the roots. 59 In addition, the solar-to-biomass conversion efficiency is significantly larger on biofuel crops (~5%)60 and C3 and C4 plants (~5%).61 These data indicate that the SCC efficiency of the present N2 photofixation (0.1%) is still much lower than that of natural photosynthesis. Furthermore, the efficiency is lower than that of artificial photosynthesis (~0.2%) achieved by powdered catalysts in the cases of overall water splitting10–12 and H2O2 production.13–15 However, N2 fixation on a cheap and simple metalfree catalyst that stably produces NH3 under sunlight shows considerable potential as a new method of artificial photosynthesis. CONCLUSION We demonstrated that metal-free semiconductor photocatalysts, e.g., P-doped carbon nitride containing N vacancies (PCN-V), efficiently produce NH3 from water and N2 by visible light and ambient conditions. The P atoms and N vacancies behave as sites for water oxidation and N2 reduction, respectively, promoting efficient N2 fixation with water as an electron donor. The SCC efficiency of N2 fixation on the mesoporous PCN-V catalyst is 0.1%, which is higher than that of previously reported photocatalytic N2 fixation systems and is comparable to the average solar-to-biomass conversion efficiency by typical plants.

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ACS Applied Energy Materials This new method of artificial photosynthesis has the following advantages: (i) water and sunlight can be used for the reaction, (ii) a metal-free catalyst prepared by simple calcination is available, and (iii) an easily transportable hydrogen carrier (NH3) can directly be obtained. Although enhancing the catalytic activity is necessary for practical applications, the new concept for N2 photofixation on metal-free carbon-nitride-based catalysts could contribute to the new methods of artificial photosynthesis for storing renewable energy. EXPERIMENTAL SECTION Catalyst. CN was prepared by calcination of dicyandiamide (4.0 g) under N2 at 773 K (heating rate, 2 K min−1; holding time, 4 h). CN-V was prepared by calcination under H2 flow (0.2 mL min−1) at 773 K. PCN was prepared as follows: dicyandiamide (4.0 g) and HEDP (0.12 g) were added to water (50 mL) and concentrated by evaporation of water under stirring at 393 K. The mixture was calcined under N2 at 773 K. PCN-V was prepared by calcination of the above-mentioned mixture under H2 flow at 773 K. Further, m-PCN-V(x) (x = 64, 98, 164) were prepared as follows: dicyandiamide (4.0 g) and HEDP (0.12 g) were added to water (50 mL) containing a Ludox HS40 solution (40% solution of ~12 nm silica particles; 6.3, 7.5, or 2.6 g) and stirred at 333 K for 12 h. The resultant was calcined under H 2 flow at 773 K. The obtained powders were stirred in 4 M HF (200 mL) at room temperature for 24 h to remove the silica particles. The resultant was washed thoroughly with water until the pH of the solution became ca. 7.0. Photoreaction. Catalyst (200 mg) and water (100 mL) were added to a Pyrex glass tube (φ 45 mm; capacity, 200 mL) and dispersed by ultrasonication for 10 min. The tube was photoirradiated at λ > 420 nm by a 2 kW Xe lamp (USHIO Inc.) 62 with magnetic stirring under N2 bubbling (0.3 L min−1) unless otherwise noted. For action spectrum analysis, reactions were performed for 24 h under irradiation by monochromated light. 63 Further, AQY (%) was determined as follows: 𝛷AQY (%) =

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS website. TEM images (Figure S1), XRD (Figure S2), elemental-mapping SEM micrographs (Figure S3), P2p XPS charts (Figure S4), N1s XPS charts (Figure S5), electrochemical Mott-Schottky plots (Figure S6), indophenol assay for isotopic labeling experiments (Figure S7), light emission spectra (Figure S8), results for O2 evolution (Figures S9) and N2 reduction (Figure S10), photocurrent response (Figure S11), DRIFT spectra of water (Figure S12), ESR spectra (Figure S13), DRIFT spectra of N2 (Figure S14), interfacial plots of models (Figure S15), N2 adsorption/desorption isotherm (Figure S16), DR UV-vis spectra (Figure S17), photocurrent response of mesoporous catalysts (Figure S18), elemental composition (Table S1), N component composition (Table S2), photocatalytic NH3 formation under different conditions (Table S3), and Cartesian coordinates for model structures (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions

[NH3 generated (mol)] × 3 × 100 [photon number entered into the reactor (mol)]

The manuscript was written through contributions of all authors.

(6) To determine the SCC efficiency, reactions were performed using a solar simulator, where the efficiency was calculated as SCC efficiency (%) [G for NH3 generation (J mol−1 )] × [NH3 formed (mol)] × 100 = [total input energy (W)] × [reaction time (s)]

(7) –1 9

a previously described procedure.14 DRIFT analysis was performed using an FT/IR system equipped with an in-situ DR cell.29 The cell containing the catalyst (20 mg) was evacuated (0.9 Pa) at 423 K for 3 h. Water (21 μmol), N 2 (42 μmol), or NH3 (84 μmol) was injected in the gas phase at 100 K and subjected to analysis. The ESR spectra were recorded as follows:64 the catalyst (20 mg) was placed in a quartz tube, evacuated at 423 K for 3 h, and cooled to room temperature. The tube was used for analysis at 77 K. Then, N2 (20 Torr) was injected into the tube and maintained at 298 K for 6 h, and the tube was used for analysis (77 K). DR UV-vis (BaSO4 as a reference), XRD, XPS (Mg Kα radiation), N2 adsorption/desorption analysis, SEM, and TEM (200 kV) observations were carried out according to the procedures described previously.65

The free energy for NH3 formation is 339 kJ mol . The overall irradiance (300–2500 nm) is 1000 W m–2, and the irradiation area is 3.14×10–4 m2. Thus, the total input power is 0.314 W. After the reactions, the catalyst was removed by centrifugation, and the NH3 amount in the solution was analyzed using an ion chromatograph equipped with a conductivity detector. Analysis. Electrochemical analysis was performed using 0.1 M Na2SO4 (pH 6.6) in a three-electrode cell with a Pt wire and an Ag/AgCl electrode as the counter and reference electrodes, respectively. The working electrode was prepared according to

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Precursory Research for Embryonic Science and Technology (PRESTO, JPMJPR1442) program of the Japan Science and Technology Agency (JST).

REFERENCES (1) Smil, V. Detonator of the Population Explosion. Nature 1999, 400, 415. (2) Klerke, A.; Christensen, C. H.; Nørskov, J. K.; Vegge, T. Ammonia for Hydrogen Storage: Challenges and Opportunities. J. Mater. Chem. 2008, 18, 2304–2310. (3) Lan, R.; Irvine, J. T. S.; Tao, S. Ammonia and Related Chemicals as Potential Indirect Hydrogen Storage Materials. Int. J. Hydrogen Energy 2012, 37, 1482–1494. (4) Lan, R.; Tao, S. Direct Ammonia Alkaline Anion-Exchange Membrane Fuel Cells. Electrochem. Solid-State Lett. 2010, 13, B83– B86.

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