Visible light decomposition of ammonia to N₂ with Ru(bpy)₃²⁺ sensitizer

Abstract

Visible light decomposition of aqueous NH₃ to N₂ was investigated using a photocatalyst aqueous solution based on molecular photoelectron relay systems of either sensitizer (tris(2,2′-bipyridine)ruthenium(II), (Ru(bpy)₃²⁺)/potassium peroxodisulfate(K₂S₂O₈) or Ru(bpy)₃²⁺/methylviologen dichloride(MV²⁺)/O₂, capable of using visible light instead of UV-driven semiconductors such as TiO₂. It was confirmed by using an in situ visible absorption spectral change under irradiation that the Ru(II) complex is oxidized to the Ru(III) complex by K₂S₂O₈, and that the Ru(III) complex formed is stable without NH₃, while the added NH₃ was oxidized by the Ru(III) complex to produce the Ru(II) complex. In the presence of 1 mM NH₃ aqueous solution, the Ru(III) complex was the predominant species under the photostationary state, but in the presence of 100 mM NH₃, Ru(II) predominated. Gas-chromatographic analysis of the gaseous phase in the presence of 8.1 M NH₃ showed that the photochemical oxidation of ammonia yielded N₂. It was also demonstrated by using the in situ visible absorption spectrum under irradiation of the NH₃ (1 M)/Ru(bpy)₃²⁺ (0.1 mM)/MV²⁺ (10 mM) system under Ar that MV⁺˙ is accumulated, showing that NH₃ works as an electron donor for MV⁺˙ accumulation with simultaneous formation of the oxidized product of ammonia ((NH₃)ox) without producing N₂. It was suggested that the reduced product (MV⁺˙) and the oxidized product ((NH₃)ox) are in a kind of dynamic equilibrium prohibiting further oxidation of (NH₃)ox by Ru(bpy)₃³⁺ to N₂. In the O₂ atmosphere, the oxidation of MV⁺˙ to MV²⁺ takes place to accumulate Ru(III) complex, so that (NH₃)ox was further oxidized to N₂. The high activity of IrO₂ as a cocatalyst in this system was demonstrated.

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Introduction

Our ecosystem discharges a large amount of nitrogen compounds, which belong to the four major categories of global pollutants.¹ As for such compounds of nitrogen, there are nitric oxide (NO), nitrogen dioxide (NO₂), ammonia (NH₃), nitrous oxide (N₂O), peroxy acetyl nitrate, nitric acid (HNO₃), ammonium (NH₄⁺), etc. The largest emission of nitrogen compounds in our society is NH₃, the amount of which is larger than NO_x (including NO and NO₂).² In the ground NH₃ is rapidly converted to NH₄⁺ because of the abundance of hydrogen ions. The bacteria, including Nitrobacter and Nitrosomonas, contribute to the conversion of NH₄⁺ to NO₃⁻, which is referred to as nitrification.³ Under anaerobic conditions, NO₃⁻ is converted to N₂ gas by some bacteria,^4,5 which is defined as denitrification. In total, these processes are known as the nitrogen cycle,^6,7 which has kept our environment safe with regard to nitrogen problems.^8,9 However, our society is now releasing too much NH₃ especially from livestock wastes, which far exceeds the amount that is decomposed into safe N₂ by the natural nitrogen cycle. It is an important and urgent issue to decompose NH₃ into N₂ to protect our environment against pollution.

Generally, treatments of waste water are biological, chemical and electrochemical. Treatments other than the electrochemical method require a higher construction cost, a longer time, an additive process, and heat energy, etc. The electrochemical processes can completely convert organic contaminants into gases such as N₂, CO₂, and H₂, etc.⁸ However, a photochemical process using visible light from sunshine is more favorable for solving these problems because of its economical and environmental compatibility with nature. Since nitrogen compounds in livestock wastes are converted finally to ammonia by enzymes such as urease, if ammonia can be converted to N₂, it can solve the livestock waste problem with regard to nitrogen.

Many organic and inorganic compounds have been decomposed by a photocatalyst such as TiO₂.^10–15 The photodecomposition of ammonia in neutral water has also been reported by using TiO₂-supported Pt (Pt–TiO₂) or Pd (Pd–TiO₂) catalyst, but only biohazardous nitrogen oxides were obtained.¹⁶ Photodecomposition of NH₃ to produce N₂ has also been demonstrated by using platinized TiO₂.^17,18 We found recently that Pt–TiO₂ could photodecompose aqueous ammonia into N₂ and H₂ with a stoichiometric 1 : 3 ratio (volume) if reacted under alkaline conditions. However, visible light conversion of NH₃ to N₂ has not been reported except in our recent communication.¹⁹ This system is composed of either tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)₃²⁺)/potassium peroxodisulfate (K₂S₂O₈) or Ru(bpy)₃²⁺/methylviologen dichloride (MV²⁺)/O₂.²⁰ Since ∼50% of the solar spectrum is visible light composed of the wavelength of 400–760 nm, it is more beneficial to use visible light for decomposing NH₃ to N₂. In the present paper, we will report the details of visible light decomposition of ammonia into N₂ as a candidate for a future artificial solar nitrogen cycle system.

Experimental

Materials

Tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)₃²⁺) and iridium(IV) oxide (IrO₂) were purchased from Aldrich Co. Ltd., and methylviologen dichloride (MV²⁺) from Tokyo-Kasei Co. Ltd. Pt-black and an ammonia aqueous solution (30%) were obtained from Kanto Chemical Co., Inc. Potassium peroxodisulfate (K₂S₂O₈) was from Wako Pure Chemical Ind. Ltd. and Ar gas with >99% purity was obtained from Nippon Sanso Corp. All the chemicals were of the purest grade available and used as received. Ru(bpy)₃(ClO₄)₂ was synthesized as follows: Ru(bpy)₃Cl₂ (4.7 × 10⁻⁴ mol per 50 ml aqueous solution) and NaClO₄ (8.5 × 10⁻³ mol per 50 ml aqueous solution) were mixed, filtered, washed with pure water and dried in a vacuum.

In situ measurement of the visible absorption spectrum under photoirradiation

A diode array spectrophotometer, Shimadzu Multispec-1500, was used for the in situ spectroscopy under photoirradiation conditions.²¹ In this instrument the mechanical shutter speed to take in the monitoring light onto the diode array is about 0.1 s thus allowing the minimum time interval between measurements to be 0.1 s. A cell box was designed and fabricated to minimize the effects of scattering excitation light. The spot shape of the monitoring light (light intensity 0.54 mW cm⁻²) on the solution surface was an ellipse (the line of upsides 8 mm and the minor axis 5 mm). It was confirmed that the excitation light does not affect the photochemical process in sample solutions. A quartz cell with a 1 cm light pass length and a 5 cm height was used. A 3 ml aqueous solution containing 0.1 mM Ru(bpy)₃²⁺, 10 mM K₂S₂O₈, and NH₃ was used, deaerated by bubbling argon gas for 10 min, and all the measurements were carried out at 25 °C. The solution surface (area, 1 cm × 3 cm) was irradiated from the horizontal direction by the excitation light from a 500 W xenon lamp through a UV cut-off (L-42) and IR cut-off filters (IRA-25S) (light intensity 101 mW cm⁻²). Visible light from a 100 W halogen lamp without filters (light intensity 167 mW cm⁻²) was irradiated for gaseous product analysis. In some cases the visible absorption of a solution before irradiation was taken as a base line, and the net change of the absorbance due to photoreaction was recorded.

Photodecomposition of an NH₃ aqueous solution using Ru(bpy)₃²⁺, MV²⁺ and O₂

A 5 ml aqueous solution containing 0.1 mM Ru(bpy)₃²⁺, 10 mM MV²⁺, and 8.1 M NH₃ in a cylindrical reactor (12 ml) was reacted under irradiation with or without 1 mM IrO₂. The reactor was sealed with a rubber septum, through which the gas phase could be sampled with a syringe. The photoirradiation was carried out using a 100 W halogen lamp without filters (light intensity 167 mW cm⁻²) at 25 °C. A sample aliquot of 100 µl gas was taken with a 100 µl syringe for analysis by gas chromatography.

Gas analysis. Analysis of the nitrogen gases evolved was carried out using a gas chromatograph (Shimadzu GC-4CPT) at 80 °C with a 5 Å molecular sieve column using Ar as the carrier gas.

Results and discussion

The typical in situ visible absorption spectrum of the solution composed of 0.1 mM Ru(bpy)₃²⁺ and 10 mM K₂S₂O₈ in the photoirradiated state is shown in Fig. 1(a). The absorption peak at 452 nm due to Ru(bpy)₃²⁺ disappeared quickly on irradiation, reaching the absorption by Ru(bpy)₃³⁺ having a peak at 420 nm. The visible absorption of the above solution before irradiation was taken as a base line, and the net change of the solution by photoreaction was monitored and the result is shown in Fig. 1(b). The absorbance change of the peak at 452 nm is shown in Fig. 1(c). When switching on the irradiation, the absorbance at 452 nm decreased steeply and reached saturation within 100 s. After switching off the irradiation, the absorbance increased slightly. This spectral change means that the Ru(bpy)₃³⁺ formed was almost stable in the present system. Further irradiation caused a slight decrease of the absorbance showing total recovery of the Ru(III) complex by K₂S₂O₈.

Fig. 1 In situ visible absorption spectral change of the aqueous solution of 0.1 mM Ru(bpy)₃²⁺ and 10 mM K₂S₂O₈(pH not adjusted), under visible light irradiation from a 500 W xenon lamp through a UV cut-off filter (L-42) and IR cut-off filter (IRA-25S) with a light intensity of 101 mW cm⁻² under an Ar atmosphere. (a) Typical absorbance of Ru(bpy)₃²⁺ and Ru(bpy)₃³⁺, (b) spectral change after irradiation with the absorbance before irradiation taken as the base line, (c) time-course of the absorbance change at 452 nm. The spectrum was measured with a 5 s interval in (b).

The spectral changes in the presence of different concentrations of ammonia (1 mM and 100 mM) are shown in Fig. 2(a) and (b), respectively. The different NH₃ concentrations greatly affected the absorbance change. In the 1 mM NH₃ solution the absorbance change was quick and after switching off the irradiation reached a stable state. It should be noted that the irradiation of the aqueous solution of Ru(bpy)₃²⁺ by visible light in the presence of K₂S₂O₈ (10 mM) induced the formation of Ru(bpy)₃³⁺ by oxidation of the photoexcited Ru complexes by K₂S₂O₈ in a similar way as in Fig. 1(c). It is shown that in the 1 mM NH₃ solution, almost the same reaction occurs except that there is a partial existence of the Ru(II) complex in the photoirradiated state. On the other hand, in the presence of 100 mM NH₃, the absorbance change of Ru(bpy)₃³⁺ was much slower and more incomplete than that with 1 mM NH₃, as shown in Fig. 2(b). The absorbance decreased slowly and after switching off the irradiation the absorbance increased clearly, showing a quick reaction of the Ru(III) complex with NH₃. Further irradiation induced the absorbance decrease again. These absorbance changes indicate that the Ru(II) and Ru(III) complexes are in a dynamic equilibrium with NH₃.

Fig. 2 In situ visible absorption spectral change of the aqueous solution of 1 mM or 100 mM NH₃, 0.1 mM Ru(bpy)₃²⁺ and 10 mM K₂S₂O₈, under visible light irradiation from a 500 W xenon lamp through a UV cut-off filter (L-42) and IR cut-off filter (IRA-25S) with a light intensity of 101 mW cm⁻² under an Ar atmosphere. (a) 1 mM NH₃ pH 5.6, (b) 100 mM NH₃ (pH 11.0). The spectrum was measured with a 5 s interval for 5 min. The right figures represent the time dependent absorbance changes at 452 nm over 5 min.

The photochemical reaction product was analysed for the NH₃/Ru(bpy)₃²⁺/K₂S₂O₈ system by gas chromatography showing N₂ formation as reported earlier.²⁰ During the irradiation fine bubbles of N₂ in the NH₃ (8.1 M)/Ru(bpy)₃²⁺ (0.1 mM)/K₂S₂O₈ (10 mM) system were observed and the time course of the N₂ evolution is shown in Fig. 3. The N₂ evolution increased linearly up to 5 min and then reached a plateau at 298 µl, which is 73% of the theoretical value (407 µl) when the 5 ml aqueous solution of 10 mM K₂S₂O₈ (sacrificial acceptor) is completely consumed in a two-electron reduction process. At this plateau, the decomposition ratio of NH₃ is 0.4% of the initial concentration. i.e., this decomposition ratio is limited by the amount of the acceptor K₂S₂O₈ (10 mM). In a separate experiment gas evolution was not observed without Ru(bpy)₃²⁺ showing evidently that photodecomposition does not occur in an NH₃/K₂S₂O₈ system.

Fig. 3 Time dependent photochemical N₂ evolution in the aqueous NH₃(8.1M)/Ru(bpy)₃²⁺(0.1 mM)/K₂S₂O₈ (10 mM) system at pH 12.9 under an Ar atmosphere. The dashed line indicates the theoretical amount of N₂ evolution when K₂S₂O₈ (sacrificial acceptor, 10 mM in 5 ml aqueous solution) is completely consumed. Visible light irradiation from a 100 W halogen lamp with a light intensity of 167 mW cm⁻².

To investigate the intermediate of the NH₃ photodecomposition into N₂, the photochemical decomposition of either NH₃ or N₂H₄ in an aqueous solution of Ru(bpy)₃(ClO₄)₂ (1 mM)/K₂S₂O₈ (0.1 mM) was studied and the result is shown in Table 1. The concentration of the NH₃ solution (100 mM) is 10 times as much as that of the N₂H₄ solution (10 mM), but the N₂ evolution by N₂H₄ in 1 h was five times as much as that by NH₃. It means that N₂H₄ is more easily oxidizable than NH₃ in this system. The spectral change in the N₂H₄ (100 mM)/Ru(bpy)₃²⁺(0.1 mM)/K₂S₂O₈ (10 mM) system by taking the absorbance before irradiation as the base line is shown in Fig. 4 where the absorbance change at 452 nm was not clearly observed supporting a very quick decomposition of N₂H₄ by Ru(bpy)₃³⁺ to N₂. N₂H₄ could be a candidate for an intermediate in the NH₃ decomposition, but the present results could not confirm conclusively the nature of the intermediate due to the rapid N₂H₄ reaction. Anyway we can at least say that the reaction occurs by photochemical oxidation of the Ru(II) complex to Ru(III) by K₂S₂O₈, leading to the oxidation of NH₃ by the Ru(III) complex to yield N₂ (Scheme 1) possibly via short-lived N₂H₄ under the present conditions.

Table 1 Photochemical N₂ evolution in the aqueous system, NH₃(100 mM) or N₂H₄ (10 mM)/Ru(bpy)₃ (ClO₄) (1 mM)/K₂S₂O₈ (0.1 mM) for 1 h visible light irradiation from a 500 W xenon lamp through a UV cut-off filter (L-42) and IR cut-off filter (IRA-25S) with a light intensity of 101 mW cm⁻² under an Ar atmosphere

System^a	N2 evolution
a pH not adjusted.
NH3 (100 mM)/Ru(bpy)3(ClO4)2 (1 mM)/K2S2O8 (0.1 mM)	130.2
N2H4 (10 mM)/Ru(bpy)3(ClO4)2 (1 mM)/K2S2O8 (0.1 mM)	681.3

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Fig. 4 In situ visible absorption spectral change of the aqueous solution, 100 mM N₂H₄, 0.1 mM Ru(bpy)₃²⁺ and 10 mM K₂S₂O₈ at pH 11.0, under visible light irradiation from a 500 W xenon lamp through a UV cut-off filter (L-42) and IR cut-off filter (IRA-25S) with a light intensity of 101 mW cm⁻² under an Ar atmosphere. The spectrum was measured with a 5 s interval for 5 min by taking the absorbance before irradiation as the base line.

Scheme 1

Methylviologen (MV²⁺) is a well-known mediator for proton reduction towards H₂ evolution in the presence of a sacrificial donor.^11–13 The in situ visible absorption spectrum under irradiation of the NH₃ (1 M)/Ru(bpy)₃²⁺ (0.1 mM)/MV²⁺ (10 mM) system under Ar is shown in Fig. 5(a) where the accumulation of the viologen cation radical (MV⁺˙) having absorption maxima at 395 and 603 nm was clearly observed. It should be noted here that MV⁺˙ is not accumulated in the two components system, Ru(bpy)₃²⁺/MV²⁺, since the recombination reaction between Ru(bpy)₃³⁺ and MV⁺˙ is very rapid. In the above NH₃/Ru(bpy)₃²⁺/MV²⁺ system, we confirmed by another experiment that NH₃ did not quench the photoexcited Ru(bpy)₃²⁺ complex. It has been well established that electron transfer takes place from the photoexcited Ru(bpy)₃²⁺ to MV²⁺. For these reasons, it could be concluded that in this system methylviologen works as an acceptor from the photoexcited Ru complex to produce MV⁺˙; ammonia works as an electron donor to the formed Ru(III) complex resulting in the oxidized compound of ammonia ((NH₃)ox). The time course of the absorbance change at 606 nm due to MV⁺˙ for the NH₃/Ru(bpy)₃²⁺/MV²⁺ system is shown in Fig. 5(b) where the change of the absorbance increased steeply initially and then became a gentle slope within 1 h. In the 1 h irradiation, N₂ evolution was not observed under Ar. This reaction mechanism is represented in Scheme 2 where the NH₃ molecule works as the electron donor for the MV²⁺ reduction, resulting in accumulation of the oxidized product of ammonia ((NH₃)ox). As a candidate for the (NH₃)ox, hydrazine or NH₃⁺˙ might be possible, but the structure cannot yet be determined. The accumulated MV⁺˙ disappeared quickly upon switching off the irradiation. It is strongly suggested that the reduced product (MV⁺˙) and the oxidized product ((NH₃)ox) are in a kind of dynamic equilibrium prohibiting further oxidation of (NH₃)ox by Ru(bpy)₃³⁺ to N₂.

Fig. 5 In situ visible absorption spectral change of the aqueous solution, 1 M NH₃, 0.1 mM Ru(bpy)₃²⁺ and 10 mM MV²⁺(pH 11.5), under visible light irradiation from a 500 W xenon lamp through a UV cut-off filter (L-42) and IR cut-off filter (IRA-25S) with a light intensity of 101 mW cm⁻² under an Ar atmosphere. The spectrum was measured with a 5 s interval by taking the absorbance before irradiation as the base line; total photoreaction time being 1 h in (a). The right figure represents the time dependent absorbance change at 606 nm.

Scheme 2

As published in our earlier communication,²⁰ under an O₂ atmosphere, 860 µl N₂ was produced by irradiation of an aqueous solution of NH₃ (10 M)/Ru(bpy)₃²⁺(0.1 mM)/MV²⁺(10 mM) after 9 h without MV⁺˙ accumulation. This mechanism can be represented in Scheme 3. In this system, when O₂ was absent (Scheme 2), the predominant species in the reaction mixture are MV⁺˙, (NH₃)ox, and Ru(bpy)₃²⁺, because of the absence of the absorbance change around 450 nm due to the Ru(II) complex (see Fig. 5(a)). In the O₂ atmosphere, the oxidation of MV⁺˙ to MV²⁺ takes place to accumulate Ru(III) complex and (NH₃)ox leading to further oxidation to N₂.

Scheme 3

We investigated the effect of catalysts on the photodecomposition of NH₃ by Ru(bpy)₃²⁺/MV²⁺ in an aqueous solution under air and the result is shown in Fig. 6. In this figure, the effect of IrO₂ as a catalyst on the photochemical decomposition of NH₃ under air is very large. Under these reaction conditions, N₂ evolution was not efficient without IrO₂. On the contrary, when using IrO₂ as a catalyst, NH₃ decomposed quickly to N₂ and the volume of the evolved N₂ gas reached a plateau at 5 h after a few hours induction time and the following steep increase. In Scheme 3 the reduction of MV²⁺ to MV⁺˙ by Ru(bpy)₃^2+* and also the oxidation of MV⁺˙ by O₂ are very rapid, so that the oxidation of NH₃ by Ru(bpy)₃³⁺ must be rate-determining. It is therefore inferred that the IrO₂ catalyst would catalyse the oxidation of NH₃ to N₂ in Scheme 3. The catalyst would collect positive charges to enhance the multi-electron process of the reaction. Other catalysts such as RuO₂, Pt black, and RuO₂/Pt black co-catalyst, have also been tested, but did not give good results.

Fig. 6 Time dependent photochemical N₂ evolution in aqueous NH₃ (8.1 M)/IrO₂(1 mM)/Ru(bpy)₃²⁺ (0.1 mM)/MV²⁺(10 mM) system (pH not adjusted) (5 ml solution) under air by visible light irradiation from a 100 W halogen lamp without filters with a light intensity of 167 mW cm⁻².

The decomposition ratio of NH₃ in the experiment shown by Fig. 6 was 0.2% of the theoretical value. This low decomposition ratio of NH₃ might be explained by the following. At first, we considered the number of electrons involved in this photodecomposition reaction. It should be noted that N₂ evolution is caused by the oxidation of NH₃, in which 6 electrons are involved. If the O₂ were consumed by two-electron reduction, the amount of O₂ consumed should be 120 µmol when the evolved N₂ is 40 µmol (after 22 h). This estimation does not coincide with the experimental result in Fig. 6. In contrast, when assuming that O₂ was consumed by four-electron reduction, the amount of the O₂ consumed should be 60 µmol when the evolved N₂ is 40 µmol after 22 h. However, in our experimental result we observed that the O₂ consumed was about 40 µmol after 22 h. Therefore, the excess of reduction power (40 µmol × 6–40 µmol × 4 = 80 µmol electrons) must have been utilized in some other reduction process. As for possible candidates, the two-electron irreversible reduction of MV²⁺ to MV⁰ is the most probable. If this is the case, the amount of the MV²⁺reduced irreversibly is calculated to be 40 µmol, which is equivalent to 80% of MV²⁺ in the cell (50 µmol). It is likely that this irreversible two-electron reduction of MV²⁺ leads to termination of the oxidation reaction of NH₃. When the volume of N₂ evolution reached a plateau, the amount of electrons consumed by the NH₃ oxidation is 240 µmol (= 40 µmol × 6), so that the turnover number (TN) for the Ru complex (5 × 10⁻⁷ mol present in the reaction cell) is calculated to be 480 in 22 h, while for MV²⁺ (5 × 10⁻⁵ mol present in the same cell) it is 4.8 in 22 h. It is concluded that both the Ru complex and the MV²⁺ work as catalysts.

Thus, in both the NH₃/Ru(bpy)₃²⁺/K₂S₂O₈ under an Ar atmosphere and NH₃/IrO₂/Ru(bpy)₃²⁺/MV²⁺ under air, the visible light decomposition of NH₃ to N₂ in an aqueous solution was investigated by using in situ visible absorption spectra and gas chromatography analysis. These molecule-based photocatalysts can work under visible light, and could be utilized for solving the NH₃/N₂ circulation problem by an artificial nitrogen cycle converting NH₃ to N₂ with the help of solar irradiation.

Acknowledgements

The present work has been supported by the Grant-in-Aid for Scientific Research (No. 18550164) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.

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