Simultaneous removal of nitrite and ammonia as dinitrogen in aqueous suspensions of a titanium(IV) oxide photocatalyst under reagent-free and metal-free conditions at room temperature

Abstract

Photocatalytic reduction of nitrite (NO₂⁻) in an aqueous suspension of metal-free titanium(IV) oxide in the presence of ammonia (NH₃) as a hole scavenger gave dinitrogen (N₂), indicating that NO₂⁻ and NH₃ are simultaneously removed from water as N₂ at room temperature without the use of an extra reagent.

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In nature, the nitrite ion (NO₂⁻) is a product formed in the process of ammonia (NH₃) oxidation by nitrite bacteria. Since NO₂⁻ reacts with secondary amines to yield nitrosoamines, which are carcinogens, ingestion of a large amount of NO₂⁻ should be avoided. Nitrite ion is often formed as a relatively stable intermediate in the processes of nitrate ion (NO₃⁻) reduction by catalysis,^1–9 electrocatalysis^10–17 and photocatalysis^9,18–33 using dihydrogen (H₂), electrically supplied electrons and photogenerated electrons, respectively. However, reduction of NO₃⁻ and NO₂⁻ by the above methods has mainly or partially yielded NH₃ or ammonium ion (NH₄⁺), in which nitrogen is over-reduced.

In our previous study, photocatalytic reaction of NO₂⁻ in aqueous suspensions of bare and metal-loaded titanium(IV) oxide (TiO₂) particles was examined in the absence of electron and hole scavengers under irradiation of UV light.³⁴ In the bare TiO₂ system, disproportionation of NO₂⁻ to dinitrogen (N₂) and nitrate (NO₃⁻) with both nitrogen balance (NB) and redox balance (ROB) close to 100% within experimental errors was observed (eqn (1)), although the reaction was slow. Eqn (1) consists of two half reactions caused by photogenerated electrons and holes as shown in eqn (2) and (3), respectively. Palladium-loaded TiO₂ (Pd-TiO₂) particles exhibited an extraordinarily large rate of disproportionation of NO₂⁻ in their aqueous suspension, i.e., NO₂⁻ was almost completely converted to N₂ and NO₃⁻, both the values of NB and ROB being close to unity.³⁴

5NO₂⁻ + 2H⁺ → N₂ + 3NO₃⁻ + H₂O (1)

2NO₂⁻ + 8H⁺ + 6e⁻ → N₂ + 4H₂O (2)

3NO₂⁻ + 3H₂O + 6h⁺ → 3NO₃⁻ + 6H⁺ (3)

From the point of view of denitrification of waste water, conversion of NO₂⁻ to N₂ without formation of NO₃⁻ is most favorable.

Two feasible methods for photocatalysis of TiO₂ for conversion of NO₂⁻ to N₂ in aqueous suspensions are shown in Fig. 1. As shown in Fig. 1(a), NO₃⁻ was selectively reduced back to NO₂⁻ over Ag–TiO₂ in the presence of a hole scavenger (HS) (eqn (4) and (5)), and NO₂⁻ was totally reduced to N₂ by combination with disproportionation of NO₂⁻ catalyzed by Pd-TiO₂.³⁵

e⁻ − h⁺ + HS → e⁻ + HSox⁺ (4)

NO₃⁻ + 2e⁻ + 2H⁺ → NO₂⁻ + H₂O (5)

Fig. 1 Photocatalytic reduction of NO₂⁻ to N₂ in an aqueous suspension of metal-loaded TiO₂ in the presence of a hole scavenger. (a) Combination of two photocatalytic systems: disproportionation of NO₂⁻ to N₂ and NO₃⁻ catalyzed by Pd-TiO₂ and selective reduction of NO₃⁻ to NO₂⁻ catalyzed by another metal-loaded TiO₂ in the presence of a hole scavenger and (b) simple photocatalytic system: hole scavenging with a sacrificial reagent and reduction of NO₂⁻ to N₂ by photogenerated electrons on Pd particles.

If HS is added together with NO₂⁻ to an aqueous suspension of TiO₂ or Pd-TiO₂ and holes are consumed by the hole scavenger (eqn (4)), the oxidation aspect of disproportionation of NO₂⁻ (eqn (3)) may be suppressed and only N₂ can be obtained without formation of NO₃⁻ as shown in Fig. 1(b). However, oxidation of NO₂⁻ by holes partly occurred (eqn (3)) even in the presence of HS (sodium oxalate), and ideal selective conversion of NO₂⁻ to N₂ in an aqueous suspension of Pd-TiO₂ as shown in Fig. 1(b) was not achieved.³⁵ From the point of view of practical application, metal-free reaction and the use of a cheap HS are highly desired in addition to high product selectivity to N₂ in NO₂⁻ conversion. In the present study, we investigated various kinds of HSs enabling us to achieve selective conversion of NO₂⁻ to N₂ and found that NH₃ was an excellent HS for conversion of NO₂⁻ to N₂.

Results of photoirradiation of UV light to NO₂⁻ in an aqueous suspension of bare TiO₂ (AEROXIDE® TiO₂ P25, Evonik) in the presence of NH₃ (as NH₄⁺) for 6 h are shown in Fig. 2(a). Only sodium nitrite (NaNO₂), ammonium sulfate ((NH₄)₂SO₄), water, TiO₂ and argon were present before the reaction. The amounts of NO₂⁻ and NH₄⁺ (both 500 μmol) were gradually decreased with photoirradiation, and NO₂⁻ (285 μmol) and NH₄⁺ (260 μmol) were removed from the liquid phase after photoirradiation for 6 h. Corresponding to the decreases in NO₂⁻ and NH₄⁺, N₂ (278 μmol, corresponding to 556 μmol nitrogen) was formed. During the reaction, NB was close to 100% within experimental errors as also shown in Fig. 2(a). The amounts of other detectable species with GC (NO, H₂ and O₂) were under their detection limits. After photoirradiation for 28 h, almost 100% of NO₂⁻ and 92% of NH₄⁺ were removed and 455 μmol of N₂ was evolved, the values of NB being close to unity (Fig. 2(b)). These results indicate that NO₂⁻ and NH₄⁺ reacted with almost 1 : 1, forming N₂. The oxidation states of nitrogen in NO₂⁻, NH₃ and N₂ are +3, −3 and 0, respectively. Therefore, this reaction includes change in the oxidation states of nitrogen in NO₂⁻ and NH₄⁺ (+3 and −3, respectively) to the same oxidation state (zero), i.e., redox reaction between NO₂⁻ and NH₄⁺, and formation of an N₂ product. We used various commercially available metal (double) oxides for photocatalytic reaction of NO₂⁻ and NH₄⁺ under the same conditions, and results obtained after 5 h photoirradiation are shown in Fig. S1 (ESI†), indicating that TiO₂ (P 25) exhibited the highest N₂ yield among the metal oxides used in this study.

Fig. 2 Time courses of photocatalytic conversions of NO₂⁻ (squares) and NH₄⁺ (circles) in aqueous suspensions of bare TiO₂ particles at pH 7 under irradiation of UV light for (a) 6 h and (b) 28 h at 298 K. Triangles: N₂, diamonds: NO₃⁻, open squares: N₂O, reversed triangles: NB.

It is known that NaNO₂ in a concentrated and boiling aqueous solution reacts with ammonium chloride in the solution to form N₂. Based on the thermal reaction of NO₂⁻ and NH₄⁺ in an aqueous solution, the simultaneous removal of NO₂⁻ and NH₄⁺ and formation of N₂ observed under the present conditions can be shown by eqn (6).

NO₂⁻ + NH₄⁺ → N₂ + 2H₂O (6)

A large molar heat capacity at constant pressure of liquid water (75 J K⁻¹ mol⁻¹ at 298 K) means that removal of NO₂⁻ and NH₄⁺ in waste water by boiling needs a large amount of energy. Therefore, the thermal reaction cannot be applied for the removal of NO₂⁻ and NH₄⁺ in waste water from the point of view of practical application. Since a moderate supply of heat to waste water would be possible by using exhaust heat in factories, thermal reaction of NO₂⁻ and NH₄⁺ in the dark at 348 K was examined to compare with the results of photocatalytic reaction at 298 K, and the results are shown in Fig. 3(a). Concentrations of NO₂⁻ and NH₄⁺ slightly decreased and a small amount of N₂ was evolved, indicating that thermal reaction of NO₂⁻ and NH₄⁺ occurs at 348 K. However, the reaction rate was much smaller than that of the photocatalytic reaction (Fig. 2(a)). Since NO₂⁻ absorbs UV light, photochemical reaction (not photocatalytic reaction) might occur under the present conditions. To elucidate the effect of photochemical reaction, a solution containing NO₂⁻ and NH₄⁺ was irradiated by UV light in the absence of TiO₂ at 298 K, and the results are shown in Fig. 3(b). Only a small decreases in NO₂⁻ and NH₄⁺ and formation of N₂ were observed, indicating that the contribution of photochemical reaction was negligible under the present conditions. From results of control experiments shown in Fig. 3, it can be concluded that a photocatalyst (TiO₂) and UV light were essential for the reaction of NO₂⁻ and NH₄⁺ to N₂ with a sufficient reaction rate. In factories producing and re-producing precious metals, a large amount of waste water containing NH₃ is formed in another process and should be denitrificated as well as waste water containing NO₂⁻. Therefore, simultaneous removal of NO₂⁻ and NH₄⁺ to N₂ over a bare TiO₂ photocatalyst by appropriate mixing of two kinds of waste water is an ideal method from the point of view of practical application for the following reasons: (1) since NH₃, which should be detoxificated, can be used as a reducing reagent (hole scavenger), an extra reagent for NO₂⁻ reduction such as methanol in a biochemical process is not required, (2) TiO₂ is a safe, stable and cheap catalyst material, (3) extra precious metals such as platinum and Pd are not required, (4) the reaction occurs under atmospheric pressure at room temperature, and (5) sunlight can be used as the energy source.

Fig. 3 Time courses of (a) thermal conversions of NO₂⁻ (squares) and NH₄⁺ (circles) at pH 7 and at 348 K in the absence of TiO₂ in the dark and (b) photochemical conversions of NO₂⁻ (squares) and NH₄⁺ (circles) in the absence of TiO₂ at pH 7 and at 298 K under irradiation of UV light. Triangles: N₂, diamonds: NO₃⁻, open squares: N₂O, reversed triangles: NB.

Tanaka and co-workers reported that photo-assisted gas-phase selective catalytic reduction of nitrogen oxide (NO) with NH₃ (photo-SCR) in the presence of oxygen (O₂) proceeded over a TiO₂ photocatalyst at room temperature^36–43 and that the process of decomposition of NH₂NO intermediates was a rate-determining step at room temperature in the presence of excess O₂ gas.⁴⁰ As well as their proposed reaction mechanism for the gas-phase photo-SCR of NO with NH₃ over TiO₂,⁴² the present NO₂⁻–NH₄⁺ reaction over TiO₂ would occur via reduction of NO₂⁻ and oxidation of NH₄⁺ (or NH₃) followed by the formation of NH₂NO intermediates as shown in eqn (7), although we did not observe NH₂NO intermediates and related compounds because of the difficulty in detection of these species in water.

NO₂⁻ + NH₄⁺ + (e⁻ − h⁺) → [NH₂NO + H₂O] → N₂ + 2H₂O (7)

Eqn (7) means that only one electron–hole pair contributes to N₂ formation. Therefore, apparent quantum efficiency (AQE) was calculated from eqn (8).

The value of AQE in this reaction under irradiation of relatively intense light at 366 nm (3.23 mW cm⁻²) at room temperature was determined to be 1.9%. Since there are various parameters controlling the reaction rate and AQE in this reaction system, we are now investigating a decisive parameter among them.

In summary, we succeeded in conversion of NO₂⁻ to N₂ in an aqueous suspension of a metal-free TiO₂ photocatalyst under irradiation of UV light in the presence of NH₃. Since NH₃, which should be detoxificated, works as a reducing reagent (hole scavenger) and an extra reagent for NO₂⁻ reduction is not required, this reaction system is ideal from the point of view of practical application.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure and Fig. S1. See DOI: 10.1039/c4ra09900a

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