Photo- and thermal catalysis of nitrate to nitrite by manganese ions under light up to 600 nm

Abstract

We report a catalytic system using manganese ion (Mn²⁺) for the photo- and thermal reduction of nitrate (NO₃⁻) to nitrite ions, in the presence of organic compounds, under ultraviolet and visible light irradiation up to 600 nm. This method enables NO₃⁻ reduction utilizing long-wavelength light and a novel Mn²⁺ catalytic system.

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The pollution of rivers, oceans, soil, and groundwater caused by nitrate ions (NO₃⁻) and persistent organic compounds such as aromatic compounds that exist in industrial water, agricultural water and wastewater, has evolved into a serious environmental problem.¹ In particular, NO₃⁻ (combined with phosphorus-containing compounds) are not only harmful to the human body but also cause eutrophication (overfertilization). The photocatalytic reaction has attracted significant attention because it is an environmentally friendly method for treating NO₃⁻; this type of reaction mainly employs semiconductor photocatalysts.^2–4 In the catalysis process, the holes (h⁺) generated in the photocatalyst by light irradiation oxidize organic compounds or water (H₂O), and the simultaneously generated excited electrons (e⁻) reduce NO₃⁻.^2–4 The main objective of the photocatalytic NO₃⁻ reduction is the detoxification to nitrogen (N₂) (reaction (2)) or conversion to ammonia (NH₃) (reaction (3)) in a multi-step, multi-electron process via nitrite ion (NO₂⁻) formation (reaction (1)).

NO₃⁻ + 2H⁺ + 2e⁻ → NO₂⁻ + H₂O (+0.84 V vs. RHE) (1)

NO₃⁻ + 6H⁺ + 5e⁻ → 1/2N₂ + 3H₂O (2)

NO₃⁻ + 9H⁺ + 8e⁻ → NH₃ + 3H₂O (3)

In parallel with NO₃⁻ reduction, advances in NO_x reduction have been achieved using catalytic approaches.⁵ Challenges such as selectivity, multi-electron transfer, and by-product suppression are common to both systems. Recent findings in NO_x reduction catalysis offer valuable insights for designing efficient NO₃⁻ reduction systems.

The introduction of metal co-catalysts in photocatalysts to enable e⁻ trapping and selective e⁻ transfer to achieve multi-electron chemical reduction has been reported in many reaction types (for example, reactions (2) and (3).³ Therefore, improving the efficiency of the two-electron reduction of NO₃⁻ to NO₂⁻ is highly important to efficient NO₃⁻ treatment.

Additionally, research and development have focused on the synthesis and reaction of photocatalysts that respond to visible light, which constitutes a significant portion of sunlight, to harness the inexhaustible sunlight energy. There have been also interesting and promising reports of visible-light-responsive photocatalysts that also achieve conversion of NO₃⁻ to N₂ or NH₃.⁴ However, there are still few candidate materials for photocatalysts that can utilize visible light in the long-wavelength region, which makes up the majority of sunlight. To achieve efficient conversion of NO₃⁻ to N₂ or NH₃ using solar light energy, the first step is to search for photocatalytic materials that enable efficient reduction of NO₃⁻ to NO₂⁻ even in visible light in the long-wavelength range.

In this study, we focused on divalent manganese ion (Mn²⁺), which is transition metal ions that absorb ultraviolet (UV) and visible light (up to ca. 700 nm) as novel photocatalyst to achieve efficient NO₃⁻ reduction; Mn²⁺ exhibits several highly oxidized ionic forms. Many manganese composite oxides and doped materials can be used as photocatalytic materials.⁶ However, the direct application of Mn²⁺ to photocatalytic NO₃⁻ reduction has not been reported, to the best of our knowledge. In this study, we investigate the potential application of Mn²⁺ as photo- and thermal catalysts in the reduction of NO₃⁻ to NO₂⁻ when they are irradiated with UV and visible light (up to 600 nm) while oxidatively degrading phenol, which is a model persistent organic compound. Details of the experimental procedures on the reaction and tracking mechanism are provided in the SI.

A comparison of the performance in the reduction of NO₃⁻ to NO₂⁻ at 30 °C in the presence or absence of Mn²⁺, visible light (λ > 420 nm) irradiation, and phenol is presented in Table 1. An aqueous solution of metal nitrate was used as a source of metal ions and NO₃⁻. In an aqueous solutions of metal nitrates other than manganese nitrate (Mn(NO₃)₂), no formation of NO₂⁻ was observed even under visible light irradiation in the presence of phenol. Regarding Mn²⁺, the NO₂⁻ formation was limited in the absence of visible light irradiation or phenol. The reduction of NO₃⁻ to NO₂⁻ was significantly achieved in the presence of Mn²⁺, visible light irradiation, and phenol. To track the specific effects of Mn²⁺ on the NO₃⁻ reduction under visible light irradiation, we investigated the effect of light irradiation wavelength (λ > 300, 420, 520, and 600 nm) on the reduction of NO₃⁻ to NO₂⁻ in the presence of phenol at 30 °C (Fig. 1(A)). It should be noted that the NO₂⁻ formation was achieved even under visible light irradiation in the long-wavelength region (up to 600 nm). In addition, the performance in the reduction of NO₃⁻ to NO₂⁻ significantly increased as the irradiation wavelength decreased. In this study, the visible light irradiation wavelength was varied by changing the cutoff filters attached to the 300 W Xenon (Xe) illuminator, indicating that the light intensity absorbed by Mn²⁺ increases as the irradiation wavelength decreases (the UV irradiation was performed without using cutoff filters). The NO₂⁻ formation is highly consistent with the absorption property of the Mn(NO₃)₂ aqueous solution. These wavelength-dependent results indicate that the reduction of NO₃⁻ to NO₂⁻ is caused by the absorption of light by Mn(NO₃)₂ in the aqueous solution. Fig. 1(B) shows the variation of the generated NO₂⁻ with the irradiation time in the presence of Mn²⁺, visible light (λ > 420 nm), and phenol at 30 °C under ambient air conditions, oxygen (O₂), and oxygen-free argon (Ar). In all reaction conditions, the generated NO₂⁻ increased with the irradiation time. Conversely, the variation of the generated NO₂⁻ when varying the reaction conditions (i.e., the O₂ concentration) was small. Under ambient air conditions, phenol conversion reached ca. 100% within 120 min; additionally, CO₂ was generated as a complete degradation product even under mild reaction conditions (Fig. S1). O₂ is reduced via photocatalytic two- or four-electron processes to generate hydrogen peroxide (H₂O₂) or H₂O, respectively (reactions (4) and (5)).⁷ From a thermodynamic perspective of the redox potential, the reduction of NO₃⁻ to NO₂⁻ (reaction (1)) occurs more readily than that of O₂ to H₂O₂ but less readily than the multi-electron reduction of O₂ to H₂O.

O₂ + 2H⁺ + 2e⁻ → H₂O₂ (+0.68 V vs. RHE) (4)

O₂ + 4H⁺ + 4e⁻ → 2H₂O (+1.23 V vs. RHE) (5)

Table 1 Comparison of the performance in the reduction of NO₃⁻ to NO₂⁻ at 30 °C for 3 h in the presence or absence of Mn²⁺, visible light (λ > 420 nm) irradiation, and an organic compound (phenol)

Catalyst and NO3^− source	Light irradiation (λ > 420 nm)	Phenol	Generated NO2^−/µmol
NaNO3	○	○	<0.1
Al(NO3)3	○	○	<0.1
Ca(NO3)2	○	○	<0.1
Cu(NO3)2	○	○	<0.1
Ni(NO3)2	○	○	<0.1
Co(NO3)2	○	○	0.4
Mn(NO3)2	—	○	0.2
	○	—	0.7
	○	○	28.1

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Fig. 1 (A) Reduction of NO₃⁻ to NO₂⁻ (reaction time: 60 min) under UV or visible light irradiation at ambient air conditions, and absorption spectrum of a 0.1 M aqueous solution of Mn(NO₃)₂; (B) effects of the reaction atmosphere on the reduction performance of NO₃⁻ to NO₂⁻ under visible light (λ > 420 nm) irradiation at 30 °C in an aqueous solution of Mn(NO₃)₂ and phenol.

However, the results in Fig. 1(B) indicate that the reduction of O₂ to H₂O₂ or H₂O using Mn²⁺ does not occur under the reaction conditions used in this study. Therefore, Mn²⁺ can be considered a promising visible-light-responsive photocatalyst for the reduction of NO₃⁻ to NO₂⁻ and the oxidation of persistent organic compounds. These results indicated that Mn²⁺ exhibits an excited e⁻ level (i.e., potential for the reduction reaction) between +0.68 and +0.84 V vs. RHE and is not subjected to multi-electron reduction reactions of more than four electrons.

To examine the catalytic properties of Mn²⁺, in detail, we investigated the effect of the reaction temperature on the reduction of NO₃⁻ to NO₂⁻ under visible light (λ > 420 nm) irradiation at ambient air conditions (Fig. 2(A) and Fig. S2). The amount of the generated NO₂⁻ increased with the reaction time at all reaction temperatures; additionally, it increased linearly as the reaction temperature increased up to 30 °C. Without light irradiation, little formation of NO₂⁻ was confirmed regardless of the reaction temperature. These results indicate that Mn²⁺ acts both as a photo- and thermal catalyst under light irradiation, i.e., it exhibits photo-induced thermal catalytic properties. To investigate these properties, we performed a NO₂⁻ degradation (oxidation) test using an ice bath (at ∼5 °C) and an water bath (at 30 and 60 °C) in an aqueous solution containing phenol and NO₂⁻ (Fig. 2(B)). Mn(NO₃)₂ or manganese chloride (MnCl₂) was used as an Mn²⁺ source with or without NO₃⁻ under visible light (λ > 420 nm) irradiation at ambient air conditions. In the case of MnCl₂ (i.e., in the absence of NO₃⁻), the variation in the amount of the generated NO₂⁻ was small at all reaction temperatures, indicating that the photo- and thermal catalytic properties of Mn²⁺ do not cause NO₂⁻ degradation (oxidation) in the presence of phenol. In the case of Mn(NO₃)₂, the NO₂⁻ amount increased significantly (i.e., reduction of NO₃⁻ to NO₂⁻ was achieved) even in the presence of NO₂⁻. These results indicate that the photo-induced thermal catalysis of Mn²⁺ facilitates only the reduction of NO₃⁻ to NO₂⁻ and the oxidation of phenol. Although the precise mechanism remains to be clarified, it is possible that an increase in reaction temperature promotes photoexcitation of Mn²⁺ species, facilitates charge carrier separation, and enhances interfacial electron transfer kinetics.

Fig. 2 Effects of reaction temperature on (A) the reduction performance of NO₃⁻ to NO₂⁻ in an aqueous solution of Mn(NO₃)₂ and on (B) the change amount of NO₂⁻ in an aqueous solution of [a] MnCl₂ or [b] Mn(NO₃)₂ containing initially added NO₂⁻ under visible light (λ > 420 nm) irradiation in the presence of phenol at ambient air conditions (reaction time: 180 min).

The photocatalysis mechanism of Mn²⁺ in the reduction of NO₃⁻ to NO₂⁻ was investigated by examining the effects of the light irradiation wavelength on the photoelectrochemical properties of Mn²⁺ using a Pt electrode immersed in a 0.1 M Mn(NO₃)₂ aqueous solution without organic compounds by switching the irradiation light ON and OFF in the range of λ > 300, 420, 520, and 600 nm (Fig. S3(A)). The photoreduction current (i.e., the increase in the cathode current under light irradiation) was observed under all light irradiation conditions. We found that the photoreduction current varies significantly with the irradiation wavelength. As shown in Fig. S3(B), the photoreduction current results are highly consistent with the absorption properties of the Mn(NO₃)₂ aqueous solution, indicating that this current is generated by the absorption of light by Mn²⁺. This behavior is similar to that observed in the results showing the effect of the irradiation wavelength on the NO₂⁻ generation (Fig. 1(A)). The results in Fig. S3 indicate that irradiating light to the Mn(NO₃)₂ aqueous solution leads to the reduction of NO₃⁻ to NO₂⁻ caused by excited e⁻ in Mn²⁺; additionally, the generated oxidized manganese ion (Mn^(2+x)+) species are electrochemically reduced and regenerated to Mn²⁺ (Fig. S4(A)). In addition, we investigated the effect of adding organic compounds (phenol, methanol, and formic acid) on the photoreduction current by irradiating visible light (λ > 420 nm) to a 0.1 M Mn(NO₃)₂ aqueous solution (Fig. 3). The photoreduction current varied depending on the type of organic compound; it decreased in the following order: no organic compound ≓ methanol ≓ formic acid ≫ phenol. In particular, the addition of phenol significantly caused decreasing the photoreductive current; this indicates that adding phenol in an Mn(NO₃)₂ aqueous solution inhibits the electrochemical e⁻ transfer (Fig. S4(A)) to the Mn^(2+x)+ species generated by the reduction of NO₃⁻ to NO₂⁻ under light irradiation, i.e., the Mn^(2+x)+ species are preferentially reduced and regenerated by the e⁻ transfer caused by phenol under the electrochemical conditions considered in this study (Fig. S4(B)). In fact, highest reduction performance of NO₃⁻ to NO₂⁻ was achieved in the presence of phenol among the organic compounds under visible light (λ > 420 nm) irradiation and ambient air conditions at 30 °C (Fig. S5). Furthermore, UV-vis measurements showed negligible absorption overlap between phenol and Mn²⁺ in the visible region (λ > 420 nm) (Fig. S6), indicating that phenol does not interfere with the photoexcitation process of Mn²⁺. These results indicate that phenol, which is a model persistent organic (aromatic) compound, acts as an effective reducing agent (e⁻ donor), thereby reducing and regenerating Mn^(2+x)+ species in the oxidized state generated by the reduction of NO₃⁻ to NO₂⁻.

Fig. 3 Effects of adding organic compounds (phenol, methanol, and formic acid) under visible light (λ > 420 nm) irradiation on the photoelectrochemical properties of a 0.1 M aqueous solution of Mn(NO₃)₂ on a Pt electrode at an applied voltage of +0.5 V vs. Ag/AgCl (light irradiation was performed every 5 s).

Based on the above results, the proposed mechanism of the reduction of NO₃⁻ to NO₂⁻ in an aqueous solution of Mn(NO₃)₂ and phenol under light irradiation is summarized in Fig. 4. Mn²⁺ absorbs UV and visible light (up to 600 nm), generating excited e⁻ in Mn²⁺ that achieve the two-electron reduction reaction from NO₃⁻ to NO₂⁻. The Mn^(2+x)+ species are reduced and regenerated to Mn²⁺ by oxidizing the aromatic compound present in the solution. These photocatalytic redox processes are significantly assisted by increasing the reaction temperature.

Fig. 4 Proposed mechanism for the reduction of NO₃⁻ to NO₂⁻ in an aqueous solution of Mn(NO₃)₂ and phenol under light irradiation.

In summary, compared with other metal ions, Mn²⁺ exhibited specific properties in the reduction of NO₃⁻ to NO₂⁻ and the oxidative degradation of phenol under visible light irradiation (λ > 420 nm). The wavelength dependence of NO₃⁻ reduction under light irradiation was closely linked to the light absorption properties of the Mn(NO₃)₂ aqueous solution, indicating that Mn²⁺ can utilize visible light (up to 600 nm). It should be noted that, even though Mn²⁺ was inactive in the oxidative decomposition of the generated NO₂⁻ and the reduction of the coexisting O₂, its ability to reduce NO₃⁻ to NO₂⁻ increased significantly with increasing reaction temperature under light irradiation. The photoelectrochemical properties of the Mn(NO₃)₂ aqueous solution, which was used as the electrolyte, showed a significant increase in the cathode current under light irradiation. The photoreduction current varied significantly with the light irradiation wavelength, showing good agreement with the aforementioned light absorption properties of the Mn(NO₃)₂ aqueous solution; additionally, it decreased significantly when adding phenol to the solution. These results suggest Mn²⁺ could act as a catalyst combining photo- and thermal effects under the conditions used in this study, as it can reduce NO₃⁻ to NO₂⁻ and oxidatively decompose persistent aromatic organic compounds upon irradiation with UV and visible light (up to 600 nm). This study contributes to the development of promising design guidelines to overcome global environmental problems via the environmentally friendly treatment of NO₃⁻ and persistent aromatic compounds contained in wastewater using only inexhaustible visible light energy. The ultimate objective of this study is to detoxify NO₃⁻ in wastewater to N₂ or convert it into NH₃, a valuable chemical product, using the photo- and thermal catalytic properties of Mn²⁺ as well as its broad light absorption across the UV-visible range. Although the formation of N₂ or NH₃ was not confirmed under the present conditions, this study focuses on the two-electron reduction of NO₃⁻ to NO₂⁻, and we recognize that achieving the multi-electron reduction to N₂ or NH₃ remains a significant challenge and an essential next step for practical applications. We are currently investigating the integration of Mn²⁺-based photocatalysis and thermalcatalysis with metal nanoparticle co-catalysts, which are known to facilitate multi-electron transfer reactions through efficient electron trapping and selective transfer. Additionally, we are developing heterogeneous Mn²⁺ catalysts to enable better control over the catalytic environment, facilitate catalyst separation and recovery after the reaction, and track the valence change of Mn during the reaction. These efforts using the properties of Mn²⁺ will provide mechanistic insights and design principles for future photocatalytic systems capable of achieving complete NO₃⁻ treatment using solar energy.

This work was partially supported by the Environment Research and Technology Development Fund (JPMEERF20233R03) of the Environmental Restoration and Conservation Agency provided by Ministry of the Environment of Japan, and JSPS KAKENHI (Grant Number 22K05293).

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this study have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5cc05059f.

Notes and references

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