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Jul 17, 2017 - ABSTRACT: Ammonia (NH3) is an essential chemical in modern society. It is currently manufactured by the Haber−. Bosch process using H...
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Photocatalytic Conversion of Nitrogen to Ammonia with Water on Surface Oxygen Vacancies of Titanium Dioxide Hiroaki Hirakawa,† Masaki Hashimoto,† Yasuhiro Shiraishi,*,†,‡ 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 S Supporting Information *

ABSTRACT: Ammonia (NH3) is an essential chemical in modern society. It is currently manufactured by the Haber− Bosch process using H2 and N2 under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH3 production from water and N2 at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO2 with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N2, successfully produces NH3. The active sites for N2 reduction are the Ti3+ species on the oxygen vacancies. These species act as adsorption sites for N2 and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N2 to NH3. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO2 system therefore shows a potential as a new artificial photosynthesis for green NH3 production.

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INTRODUCTION Ammonia (NH3) is an indispensable chemical for synthesis of fertilizers and fibers.1,2 It has also received much attention as a potential hydrogen carrier due to its high hydrogen density (17.6 wt %) and low liquefying pressure (∼8 atm).3,4 Traditionally, NH3 has been manufactured by the Haber− Bosch process using H2 and N2 for over 100 years. This process, however, needs extremely high pressures (>200 bar) and high temperatures (>673 K),5 with large amounts of H2 produced by steam reforming of fossil fuels with a large concomitant emission of CO2.6 Catalytic processes that produce NH3 using N2 and earth-abundant reducing reagents at atmospheric pressure and room temperature are desired for a clean, safe, and sustainable NH3 synthesis. Some electrocatalytic systems for N2 reduction have been studied.7−9 Photocatalytic N2 reduction has also attracted much attention because it can use light energy.10,11 Photocatalysis on powder semiconductors is a promising method for NH3 production because it is simple and can use water as a reducing reagent. The basic concept is as follows:12 The photoformed valence band holes (VB h+) oxidize water (eq 1). N2 reduction by the conduction band electrons (CB e−) produces NH3 (eq 2). As a result of this, NH3 is produced from water and N2 under ambient conditions by using sunlight as energy source (eq 3). The large free energy gain of this reaction (ΔG° = 339 kJ mol−1)13 makes this a potential new artificial photosynthesis, © 2017 American Chemical Society

along with early reported uphill reactions such as overall water splitting (eq 4)14−16 and H2O2 production (eq 5).17,18 2H 2O + 4h+ → O2 + 4H+

(1)

N2 + 6H+ + 6e− → 2NH3

(2)

1/2N2 + 3/2H 2O → NH3 + 3/4O2 (ΔG° = 339kJ mol−1) (3)

H 2O → H 2 + 1/2O2 (ΔG° = 237kJ mol−1)

(4)

H 2O + 1/2O2 → H 2O2 (ΔG° = 117kJ mol−1)

(5)

Several inorganic or organic semiconductors have been used for NH3 production. Many of them such as Fe2Ti2O7, ZnO, SrTiO3, MoxNiyCdS, and graphitic carbon nitride (g-C3N4) are, however, less active due to their poor ability for water oxidation (eq 1) and need sacrificial electron donors such as alcohols.19−23 Bismuth oxybromide (BiOBr) is active for the reactions even under visible light (λ > 400 nm);24 however, it produces relatively small amount of NH3 (∼50 μM) and suffers from low photostability due to self-oxidation by the VB h+.25 Some robust TiO2-based catalysts have been investigated under Received: June 26, 2017 Published: July 17, 2017 10929

DOI: 10.1021/jacs.7b06634 J. Am. Chem. Soc. 2017, 139, 10929−10936

Article

Journal of the American Chemical Society UV irradiation (λ < 400 nm). The literatures report that bare TiO2 shows low activity, but loading of metal particles such as Pt,26 Ru, Rh, or Pd27 as co-catalysts produces NH3 relatively efficiently (∼80 μM). The use of noble metals, however, must be omitted for practical applications. Robust semiconductors that efficiently promote water oxidation and N2 reduction without noble metal are therefore necessary. The rate-determining step of the N2 reduction cycle is the cleavage of the NN bond. This bond has an extremely high dissociation energy (941 kJ mol−1).28−30 Creation of active sites that efficiently promote NN cleavage is therefore necessary. It is well-known that transition-metal complexes with Mo, W, Fe, or Ru cations efficiently promote the NN cleavage by strong coordination,31−36 where trivalent titanium (Ti3+) is one possible cation.37−39 Scheme 1 shows a typical role of Ti3+.40−42

Scheme 2. Proposed Photocatalytic Cycle for N2 Fixation on the Rutile TiO2 (110) Surfacea

a

Scheme 1. Catalytic Cycle for N2 Fixation by Ti3+Containing Complexes

The light blue spheres are the Ob atoms lying in the [001] azimuth. The red and blue spheres are the Ti and bulk O atoms, respectively. The green and yellow spheres are the O and H atoms in the surface −OH.

Two Ti3+L3 complexes (L = ligand) react with N2 via electron donation and create a Ti4+−azo complex with an end-on bridging mode (a → b).40 The NN bond formed in the step (a → b) is subsequently cleaved by strong reducing reagents such as Li and Mg, producing a Ti4+−hydrazo complex (b → c).41 Further reduction gives Ti4+−amine complexes (c → d) and finally produces NH3 with regeneration of Ti3+L3 (d → a).42 The reactions proceed efficiently at atmospheric pressure and room temperature. The above reactions imply that the Ti3+ species created on the surface of robust semiconductors capable of oxidizing water (eq 1) may reduce N2 by the photoformed CB e− (eq 2). This process may lead to NH3 production from water and N2 (eq 3) on a noble-metal-free photocatalyst under ambient conditions. Here we report that the Ti3+ species are inherently created on the surface defects of a commercially available TiO2 and behave as very active sites for N2 reduction. The TiO2 with a large number of surface defects, therefore, successfully produces NH3 from water and N2 under sunlight irradiation. Scheme 2 shows the rutile TiO2 (110) surface, which is the most stable facet of TiO2. This surface is characterized by alternate rows of 5-fold coordinated Ti4+ and bridging O (Ob) that run in the (001) direction.43 Surface defects are the Ob vacancies, where two excess electrons associated with Ob are transferred to the empty 3d orbitals of neighboring Ti4+, producing two exposed Ti3+.44 The donor levels of these Ti3+ lie at 0.1−0.3 eV below the CB bottom. They therefore act as trapping sites for the CB e−.45,46 We present here that these Ti3+ species behave as active sites for photocatalytic N2 reduction. In addition, the cheap, robust, and noble-metal-free TiO2 powders successfully produce NH3 very efficiently (180 μM) under sunlight.

RESULTS AND DISCUSSION Photocatalytic Activity. Photocatalytic NH3 production from water and N2 was carried out using several kinds of commercially available TiO2 with different crystalline phases, surface areas, and particle sizes. Milli-Q treated pure water (200 mL) containing each respective TiO2 (200 mg, 2.5 mmol) was photoirradiated by a high-pressure Hg lamp (λ > 280 nm) with magnetic stirring under N2 bubbling (0.3 L min−1) at 313 K. Table 1 summarizes the amount of NH3 formed in the course of the reaction on the respective catalysts for 48 h photoirradiation. It is noted that pKa of NH3 is 9.3,47 and almost all of the NH3 formed exists as a protonated NH4+ form. Most of the anatase and rutile TiO2 (1−6) scarcely produce NH3 ( 280 nm (intensity at 280−420 nm: 35 W m−2), time (48 h). bDetermined by N2 adsorption/desorption analysis. cDetermined by dynamic light scattering (DLS) analysis. dDetermined by ion chromatography (IC). eThe number of surface Ti3+ species on TiO2 determined by DRIFT analysis with nitrobenzene as a probe molecule (ref 57). fThe rate of NH3 formation during photoreaction (∼48 h, Figure S3). gJapan Reference Catalyst supplied from the Catalyst Society of Japan. hSupplied from Ishihara Sangyo, Ltd. (Japan). iSupplied from Toho Titanium Co., Ltd. (Japan). jDetermined by XRD analysis (ref 57; Figure S1). kSmaller particles are assigned to the dispersed anatase particles, and larger particles are assigned to the interwoven aggregates of anatase and rutile particles (ref 57). lCannot be determined due to very noisy DRIFT spectrum. a

indophenol by the reaction with phenol in basic condition.11 Water containing 0.5 wt % formic acid (10 mL) and catalyst 8 (20 mg) were added to a glass tube (capacity, 20 mL), and the tube was sealed with a rubber septum cap. 14N2 or 15N2 gas (20 mL) was then bubbled through the solution, and the tube was photoirradiated for 36 h. It is noted that the amounts of NH3 formed with 14N2 and 15N2 are similar (74.1 μM and 72.2 μM, respectively). The obtained solutions were treated with phenol in the presence of NaClO as an oxidizing reagent and Na2[Fe(CN)5NO] as a catalyst. As shown in Figure 2a, both solutions show distinctive absorption at 630 nm assigned to indophenol. The respective solutions were subjected to liquid

contaminants on TiO2 surface do not act as sacrificial electron donor. This is further confirmed by the half-reaction; as shown in Figure S4, photocatalytic N2 reduction on the catalyst 8 with 2-PrOH as a sacrificial electron donor produces NH3 more efficiently than in the water/N2 system because 2-PrOH is oxidized more easily than water by h+. This means that, in the present system, water behaves as an electron donor for photocatalytic N2 reduction. To confirm N2 gas as the source of NH3 in the present system, photocatalytic reaction with isotope-labeled 15N2 gas was performed using formic acid as a sacrificial electron donor. The formed NH3 was transformed to the corresponding 10931

DOI: 10.1021/jacs.7b06634 J. Am. Chem. Soc. 2017, 139, 10929−10936

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Journal of the American Chemical Society

Figure S5, the rate of O2 evolution in acidic pH (4.0) is much slower than that in neutral pH (7.0). In contrast, basic pH (10.0) enhances water oxidation, but is ineffective for NH3 formation (Figure 1b). This is because, at basic pH, the formed NH4+ exists as a deprotonated NH3 form (pKa 9.3)47 and is easily oxidized by the VB h+ (eq 6).52 This is confirmed by the photoreactions with NH3 as a starting substrate; as shown in Figure S6, it is stable at acidic or neutral pH (4.0 and 7.0), but is oxidized easily at pH 10.0. These findings indicate that photoirradiation at neutral pH (pure water) under a sufficiently high rate of N2 bubbling efficiently produces NH3. 2NH3 + 6h+ → N2 + 6H+

(6)

Photocatalytic Performance. Figure 1c shows the action spectrum for NH3 formation on JRC-TIO-6 (8) determined by monochromated light irradiation in water with an N2 bubbling rate 1.0 L min−1. The apparent quantum yields (ΦAQY) agree well with the absorption spectrum of the catalyst, indicating that its bandgap photoexcitation promotes the reactions. The ΦAQY at λ < 350 nm is ca. 0.7%, which is higher than that for previously reported systems (∼0.5%).24 Figure 1d shows the solar-to-chemical conversion (SCC) efficiency for the NH3 formation, determined by AM1.5G simulated sunlight (1-sun) irradiation (Figure S7).53 The SCC efficiency is determined to be 0.02% and almost constant even after prolonged irradiation. To the best of our knowledge, this is the first report of SCC efficiency for NH3 production. The SCC efficiency for NH3 production is lower than the highest efficiency (0.2%) of artificial photosynthesis on powdered catalysts such as overall water splitting (eq 4)15,16 and H2O2 production (eq 5)18 and the average efficiency of natural photosynthesis (∼0.1%).54 The cheap, robust, and noble-metal-free TiO2 that stably produces NH3 by sunlight, however, could potentially provide a new basis of artificial photosynthesis. Role of Surface Ti3+ Species. The remarkably high activity of the JRC-TIO-6 catalyst (8) is explained by a large number of surface Ti3+ species behaving as active sites for N2 reduction (Scheme 2). The N2 adsorption onto the Ti3+ species is confirmed by electron spin resonance (ESR) analysis. As shown in Figure 1e, the catalyst was measured at 77 K in vacuo, showing a distinctive signal (g = 2.004), which can be assigned to the bridging oxygen (Ob) vacancies.55 Addition of N2 to this sample leads to complete disappearance of the ESR signal. This indicates that the surface Ti3+ leads to adsorption of N2 via the electron donation from Ti3+ (Scheme 2). The Ti3+ species as the active sites for N2 reduction are confirmed by the loading of metal particles onto the catalyst. It is well-known that a conventional metal loading method consisting of impregnation of metal precursors followed by H2 reduction creates metal particles on the surface defects of TiO 2 . 56 Previous literatures26,27 reported that Ru, Pt, or Pd particles loaded on TiO2 act as reduction sites for N2 and produce NH3 more efficiently than bare TiO2. However, as shown in Table S1, Ru, Pt, or Pd particles loaded on JRC-TIO-6 (8) produce much smaller amounts of NH3 ( 350 nm) of the N2-adsorbed JRC-TIO6 (8) does not show any spectral change, indicating that absence of an electron donor scarcely promotes the reduction of the Ti4+−azo′ species.

Figure 3c shows the time-dependent change in DRIFT spectra of the N2-adsorbed catalyst under UV irradiation with water. As the time advances, the νN−H bands shift to higher wavenumbers along with the intensity increase, and their positions become similar to those of NH3 adsorbed on TiO2 (Figure 3d). This indicates that photoexcitation of TiO2 indeed produces NH3 with water as electron and proton donor. The N−H bond becomes stronger with an increase in the basicity of the N atom.63 The gradual νN−H shift by UV irradiation suggests that N2 reduction occurs in a stepwise manner. Photoexcitation of TiO2 with Ti4+−azo′ species (Scheme 3c) creates CB e− and VB h+ pairs. The CB e− are trapped on the surface defects45,46 and regenerate the surface Ti3+, while the VB h+ are located at the surface Ti−OH64 (Scheme 3d). The NN dissociation on the Ti3+ produces Ti4+−hydrazo species with the water oxidation by the h+ (e). Further photocatalysis of the species gives Ti4+−amine species (f) and finally produces NH3 with the regeneration of surface Ti3+ (a). These photocatalytic cycles around the surface defects of TiO2 facilitate efficient NH3 production with water at atmospheric pressure and room temperature.

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CONCLUSION We found that UV irradiation of a commercially available TiO2 with a large number of surface Ti3+ species efficiently produces NH3 from water and N2 under ambient conditions. The surface Ti3+ species behave as the active sites for N2 reduction and produce NH3 with solar-to-chemical conversion (SCC) efficiency 0.02%. It is the highest efficiency among the early reported powder photocatalysts, but lower than that of natural photosynthesis (0.1%) and artificial photosynthesis such as overall water splitting and H2O2 production (0.2%). Although an improvement of the catalytic activity is necessary, this cheap, 10933

DOI: 10.1021/jacs.7b06634 J. Am. Chem. Soc. 2017, 139, 10929−10936

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4.0, 7.0, 10.0) containing NH3 (0.5 mM) under air bubbling (0.3 L min−1). After the reactions, the gas phase was analyzed by GC-TCD (GC-8A, Shimadzu). The catalyst was recovered by centrifugation, and the amount of NH 3 in the solution was analyzed by ion chromatography (LC-20AD, Shimadzu) equipped with a conductivity detector (CDD-10A, Shimadzu) and IC-C4 cation-exchange column (Shimadzu). The amount of NH3 formed was confirmed with Nessler’s reagent;24 the obtained data are similar to those obtained by the ion chromatography analysis, where the error was ±5%. The formation of nitrate and nitrite was investigated by the ion chromatography system with IC-A1 anion-exchange column (Shimadzu). Isotopic Labeling Experiments. A 0.5 wt % formic acid solution (10 mL) and catalyst 8 (20 mg) were added to a Pyrex glass tube (capacity, 20 mL), and the tube was sealed with a rubber septum cap. 14 N2 or 15N2 gas (20 mL) was bubbled through the solution, and the tubes were photoirradiated for 36 h under magnetic stirring. The solutions were recovered by centrifugation and treated as follows: 2 mL of the aliquot was mixed with 2 mL of a phenolic solution (3.0 g of Na3PO4·12H2O, 3.0 g of trisodium citrate dihydrate, 0.3 g of ethylenediaminetetraacetic acid disodium salt dehydrate, 0.02 g of Na2[Fe(CN)5NO]·2H2O in 100 mL water), and 2 mL of a NaClO solution (2 mL of 5 wt % NaClO solution and 1.6 g of NaOH in 100 mL water) and stirred for 1 h at room temperature. The respective solutions were then analyzed on Shimadzu LC-MS system equipped with HRC-ODS column (Shimadzu). Action Spectrum Analysis. Photoreactions were carried out in pure water (200 mL) with JRC-TIO-6 (8, 200 mg) under N2 bubbling (1.0 L min−1) by a 2 kW Xe lamp (USHIO Inc.) for 4 h.66 The incident light was monochromated by the band-pass glass filters (Asahi Techno Glass Co.), where the full-width at half-maximum of the lights was 11−16 nm. The photon number entered into the reaction vessel was determined with a spectroradiometer (USR-40, USHIO Inc.). Determination of SCC Efficiency. The SCC efficiency was determined by photoreactions with a solar simulator SX-UID502XQ (USHIO Inc.).18 Reactions were performed in pure water (50 mL) with JRC-TIO-6 (8, 200 mg) under N2 bubbling (1.0 L min−1). The light irradiance was adjusted to the AM1.5 global spectrum.53 The SCC efficiency was calculated with the following equation:18

Figure 3. (a−c) DRIFT spectra of N2 adsorbed on JRC-TIO-6 (8) at 100 K. (a) Measurement was started in the dark after injection of N2 (42 μmol) to the cell containing the catalyst. The sample (a) left for 50 min after N 2 injection was measured at 100 K under photoirradiation (λ > 350 nm) (b) without and (c) with water (84 μmol). (d) DRIFT spectrum of NH3 (42 μmol) adsorbed on JRCTIO-6 in the gas phase at 100 K.

robust, and noble-metal-free TiO2 photocatalyst which stably produces NH3 under sunlight irradiation shows potential as a new artificial photosynthesis to be used for inexpensive, green, and sustainable solar-to-chemical energy conversion.

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SCC efficiency(%) =

[ΔG for NH3 generation(J mol−1 )] × [NH3 formed (mol)] × 100 [total input energy(W)] × [reaction time (s)]

(8)

EXPERIMENTAL SECTION

General. Pure water was purified by the Milli Q system. To remove the impurities on the catalysts, they were stirred in water for 12 h, rinsed thoroughly with water, and dried in vacuo. Ru, Pt, or Pd-loaded JRC-TIO-6 (8) [metal loading: M/TiO2 × 100 = 0.5 wt %] were prepared as follows:27 the TiO2 (400 mg) was added to water (20 mL) with RuCl3 (4.1 mg), H2PtCl6·6H2O (5.3 mg), or PdCl2 (3.3 mg), and the mixture was stirred at 393 K for 3 h. Water was removed by evaporation. The resultant was dried in air and reduced under H2 flow at 673 K with the heating rate and the holding time being 2 K min−1 and 2 h, respectively. Diffuse reflectance UV−vis spectra of the metalloaded catalysts were summarized in Figure S11. Photoreaction. Catalyst (200 mg, 2.5 mmol) was added to a solution (200 mL) within a glass vessel and dispersed well by ultrasonication (20 min). Milli Q-treated pure water, buffered 0.1 M KPi solutions with different pH (4.0, 7.0, or 10.0), or 2-PrOH/water (1/9 v/v) mixture were used as the solutions. A high-pressure Hg lamp (300 W; Eikohsha Co., Ltd., Japan)65 was immersed into the suspensions, and the suspension was photoirradiated (λ > 280 nm) with magnetic stirring under gas bubbling. The light intensity at 280− 420 nm was 35 W m−2, and the solution temperature during photoirradiation was 313 K, respectively. Photocatalytic water oxidation was performed with catalyst (50 mg) in 0.1 M KPi solution (30 mL; pH 4.0, 7.0, 10.0) containing NaIO3 (0.05 M) as a sacrificial electron acceptor under Ar (1 atm). Photodecomposition of NH3 was performed with catalyst (200 mg) in 0.1 M KPi solution (200 mL; pH

The free energy for NH3 generation from water and N2 is 339 kJ mol−1.13 The overall irradiance of AM1.5 global spectrum (300−2500 nm) is 1000 W m−2, and the irradiation area is 3.14 × 10−4 m2. The total input power over the irradiation area is therefore determined to be 0.314 W. ESR Measurement. ESR spectra were recorded at the X-band using a Bruker EMX-10/12 spectrometer with a 100 kHz magnetic field modulation.67 The magnetic field was calibrated with 1,1′diphenyl-2-picrylhydrazyl (DPPH). Catalyst (20 mg) was placed in a quartz ESR tube. The ESR tube was calcined with O2 (20 Torr) and evacuated at 423 K for 3 h. The tube was subjected to analysis at 77 K. N2 (20 Torr) was then introduced to the tube and left for 6 h at 298 K and then subjected to analysis at 77 K. Analysis. DRIFT analysis was carried out on a FT/IR 610 system (Jasco Corp.) equipped with an in situ DR cell (Heat Chamber cold +HC-500, ST Japan, Inc.).68 Catalyst (30 mg) was placed in the cell and evacuated (0.9 Pa) at 423 K for 3 h. N2 or NH3 (42 μmol) was introduced to the cell in the gas phase at 100 K, and measurement was started in the dark. The N2-adsorbed sample was then measured under photoirradiation at λ > 350 nm without or with water (84 μmol) at 100 K by a Xe lamp (300 W; Asahi Spectra Co. Ltd.; Max-302). UV− vis spectra were measured on an UV−vis spectrophotometer (Jasco Corp.; V-550 with Integrated Sphere Apparatus ISV-469) with BaSO4 as a reference. XRD analysis was carried out by Philips X′Pert-MPD spectrometer. 10934

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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/jacs.7b06634. XRD patterns (Figure S1), relationship between activity and some catalyst parameters (Figure S2), time profiles for NH3 formation (Figure S3), time profiles for NH3 formation with 2-PrOH (Figure S4), time profiles for photocatalytic water oxidation (Figure S5), photocatalytic NH3 decomposition (Figure S6), light emission spectra (Figure S7), DRIFT spectra of adsorbed nitrobenzene (Figure S8), DRIFT spectra of adsorbed N2 (Figures S9 and S10), DR UV−vis spectra (Figure S11), and photocatalytic NH3 formation under various reaction conditions (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Yasuhiro Shiraishi: 0000-0003-1812-0644 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by PRESTO (JPMJPR1442) from JST. H.H. thanks the Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists.

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