Photocatalytic NO_x removal and recovery: progress, challenges and future perspectives

Abstract

The excessive production of nitrogen oxides (NO_x) from energy production, agricultural activities, transportation, and other human activities remains a pressing issue in atmospheric environment management. NO_x serves both as a significant pollutant and a potential feedstock for energy carriers. Photocatalytic technology for NO_x removal and recovery has received widespread attention and has experienced rapid development in recent years owing to its environmental friendliness, mild reaction conditions, and high efficiency. This review systematically summarizes the recent advances in photocatalytic removal, encompassing NO_x oxidation removal (including single and synergistic removal and NO₃⁻ decomposition), NO_x reduction to N₂, and the emergent NO_x upcycling into green ammonia. Special focus is given to the molecular understanding of the interfacial nitrogen-associated reaction mechanisms and their regulation pathways. Finally, the status and the challenges of photocatalytic NO_x removal and recovery are critically discussed and future outlooks are proposed for their potential practical application.

---

1 Introduction

Nitrogen oxides (NO_x, consisting of 95% NO and NO₂) are pervasive air pollutants which are implicated in various environmental events (acid rain, photochemical smog, and global warming) and cause detrimental impacts on public health.^1–3 They serve as key precursors for the formation of emerging tropospheric ozone, PM_2.5 as well as secondary organic aerosol pollution.^2,4 Although there are NO_x from natural sources, anthropogenic activities, particularly fossil fuel combustion, account for a major contribution to the global NO_x emissions in the atmosphere.^5,6

While NO_x is a pollutant, it can also be recognized as an energy feedstock. With a high reactivity, NO_x possesses the potential to be utilized for the synthesis of valuable N-containing chemicals. Ammonia (NH₃) is indeed an indispensable chemical for fertilizers,⁷ dyes,⁸ polymers,⁹ explosives,¹⁰ resins,¹⁰etc., and can serve as a carbon-neutral energy carrier.¹¹ The industrial production of NH₃ synthesis currently heavily relies on the Haber–Bosch route, which requires harsh conditions (400–500 °C, 150–300 atm), resulting in more than 2% of global energy consumption.^12,13 This high energy consumption is also largely derived from fossil fuels.¹⁴ Faced with global energy shortages, recovery of NO_x to NH₃ gives an alternative route for both environmental control and nitrogen resource utilization.

Photocatalytic technology powered by renewable solar energy has been reckoned as one of the most viable strategies that could convert NO_x into less harmful or useful products under mild conditions.^15–17 At present, conventional photocatalytic technologies, including NO_x oxidation, NO_x decomposition, and selective NO_x reduction, have been vigorously developed, spanning from material design to elucidation of the underlying mechanisms.^6,18–20 Meanwhile, the tendency of photocatalytic removal of NO_x has been gradually transformed from oxidation, purification, and then to resource utilization. Particularly, the photocatalytic NO_x reduction for ammonia synthesis has achieved milestone progress in recent years.

The review aims to summarize recent advancements in photocatalytic NO_x removal and recovery. The cutting-edge developments in photocatalytic removal in terms of NO_x oxidation, NO_x reduction to nitrogen, and NO_x upcycling to ammonia are comprehensively reviewed. We also highlighted the one-step (NO → NH₃) and two-step (NO → NO₃⁻ → NH₃) methods as new ideas for simultaneous NO_x removal and resource utilization. Finally, we discuss the current challenges and provide some perspectives on future directions for NO_x removal and recovery. The illustration of this review on NO_x removal and recovery is shown in Scheme 1. We hope that this review will inspire significant research and essential advancements in the field of photocatalytic NO_x removal.

Scheme 1 The NO_x removal and recovery by different photocatalytic approaches.

2 NO_x oxidation

2.1 Single NO_x oxidation

2.1.1 NO_x oxidation mechanisms. Photocatalytic oxidation removal is mainly applied to low-concentration pollutant purification. During the photocatalytic reaction process, electron–hole pairs are generated in the catalyst under light irradiation, initiating the oxidation reactions. These pairs migrate to the surface of the photocatalysts and react with adsorbed H₂O and O₂ molecules, leading to the generation of reactive oxygen species (ROS) such as ˙OH, ˙O₂⁻, and ¹O₂. These ROS then participate in a series of oxidation reactions with NO, ultimately forming the final product NO₃⁻. Detailed reaction pathways and intermediates involved in this photocatalytic NO oxidation process can be described using specific equations, which are supported by relevant ref. 21 and 22:

Photocatalysts + hν → h⁺ + e⁻ (1)

H₂O + h⁺ → ˙OH + H⁺ (2)

NO + ˙OH → HNO₂ (3)

HNO₂ + ˙OH → NO₂ + H₂O (4)

NO₂ + ˙OH → NO₃⁻ + H⁺ (5)

O₂ + e⁻ → ˙O₂⁻ (6)

NO + ˙O₂⁻ → NO₃⁻ (7)

O₂⁻ + h⁺ → ¹O₂ (8)

NO + ¹O₂ → NO₃⁻ (9)

2.1.2 Materials for NO_x oxidation. There are three main factors affecting the catalytic performance of photocatalytic oxidation: light absorption, charge carrier separation and transfer, and active site construction.²³ Based on these factors, various modification methods were conducted on TiO₂-based materials,^24–27 Bi-based materials,^28–30 g-C₃N₄-based materials,^31,32 and perovskite-type materials.³³ Modification methods include but are not limited to metal/non-metal doping, defects, heterojunctions, and sensitization.³⁴

Single-atom catalysts have attracted much attention in the field of materials due to their unique activity and effective utilization of active atoms. Several studies have confirmed that single atoms can improve the performance of photocatalysts for NO_x removal and provide a convenient way to regulate the generation of ROS for effective pollutant elimination. Liu et al.³⁵ achieved the stabilization of Pt on carbon-defective g-C₃N₄ by leveraging its affinity with nitrogen atoms resulting from carbon vacancies. The resulting Pd-C_v-CN catalyst enhances charge transfer efficiency, generating sufficient photoelectrons for the formation of ˙OH and ˙O₂⁻ species. This, in turn, facilitates the removal of NO and enhances the selectivity towards NO₃⁻. Hu et al.³⁶ discovered that single Pt atom bridged within a metal–organic framework (MOF)-derived ZnO/C via carbon atoms (Pt–ZnO/C) exhibits enhanced adsorption energy and charge transfer characteristics for O₂ and H₂O (Fig. 1a and b). In contrast to ˙OH, ˙O₂⁻ plays a pivotal role in the conversion of NO_x to NO₃⁻ on a single atom site at the catalytic surface (Fig. 1c). Zhang et al.³⁷ incorporated Zn atoms into the interlayer of g-C₃N₄, forming the ZnN₃ structure by linking zinc with three nitrogen atoms. The presence of a single zinc atom facilitated the formation of a Zn–O₂–NO structure upon adsorption of O₂ and NO, thereby promoting the dissociation of O₂ and the direct conversion of NO to nitrate (Fig. 1d). Simultaneously, the formation of the toxic byproduct NO₂ was inhibited. A similar result enhancement was observed upon doping TiO₂ hollow microspheres (TiO₂-HMSs) with single atomic Fe.³⁸ Particularly noteworthy is the effect of silver atoms loaded onto BiOCl (001) facet-exposed nanosheets, which constructed triangular Cl–Ag₁–Cl sites on Cl-terminated BiOCl. These sites selectively activated molecular oxygen to ˙O₂⁻ and enhanced nitrate adsorption by altering the coordination mode of nitrate on the catalyst, thereby preventing nitrate decomposition (Fig. 1e and f).³⁹ The NO_x removal efficiency of Ag₁/BiOCl reached up to 90%, with 97% selectivity for NO₃⁻ and minimal emission of NO₂ (Fig. 1g–i).

Fig. 1 DFT calculation of (a) O₂ adsorption and (b) H₂O adsorption on ZnO/C and Pt–ZnO/C surfaces. (c) The NO conversion rate and selectivity of NO₂ formation over the Pt–ZnO/C catalyst in the presence and absence of moisture and O₂.³⁶ (d) NO oxidation process over g-C₃N₄ and Zn–CN.³⁷ (e) Photocatalytic NO removal over Ag₁/BiOCl using different scavengers. (f) DRIFTS spectra of Ag₁/BiOCl for NO oxidation. (g) NO oxidation of P25, BiOCl, Ag₁/P25 and Ag₁/BiOCl. (h) NO₃⁻ selectivity of Ag₁/P25 and Ag₁/BiOCl under visible light irradiation. (i) The amount of NO₂ generation.³⁹

In summary, the introduction of single atoms can significantly enhance the adsorption of O₂ and H₂O molecules and the generation of ˙O₂⁻ and ˙OH, thereby improving both the performance and nitrate selectivity of the photocatalyst. This provides a facile pathway to manipulate the ROS generation for efficient and selective pollutant removal.

2.1.3 Machine learning as an analysis tool for NO_x oxidation. It is well-known that the performance of NO_x removal not only depends on catalyst properties but also closely relies on preparation methods and reaction conditions. Although catalyst design and optimization of experimental conditions have been fully explored, the dominant factors affecting the NO_x removal rate remain unclear. Machine learning (ML) has achieved remarkable progress over the past few decades and has become the most potent tool in the field of data mining and analytics.^40,41 ML methods leverage algorithms to extract insights from vast, intricate, and multidimensional datasets, enabling rapid and precise predictions. Recently, progress has been made in employing ML methods in the photocatalytic NO_x oxidation process. Li et al.⁴² achieved success in predicting the NO removal rate in the photocatalytic purification of g-C₃N₄ catalysts using ML methods like gradient boosting decision trees, eXtreme gradient boosting, and random forests. Their findings indicate that catalyst characteristics, reaction process, and preparation conditions are the primary empirical categories influencing the NO removal rate. They have also unveiled the intricate relationships between the photocatalytic NO removal rate and various influencing factors. This approach, to some extent, mitigates the downsides of traditional experimental work, including high costs, lengthy timelines, and demanding labor.

2.1.4 NO₃⁻ decomposition on the photocatalyst. The formation of nitrate is a key process of NO_x oxidation, as nitrate has been considered a permanent sink of NO_x.⁴³ However, it undergoes photolysis processes under light irradiation (also called detoxification), especially on the surface of the photocatalyst, which in turn serves as a source of atmospheric nitrogenated compounds.^44–49 TiO₂, the predominant photocatalyst,^50,51 has been shown to promote nitrate photolysis processes through photochemical interaction.⁴⁴ Therefore, exploring the decomposition mechanism of nitrate on the surface of photocatalysts assumes pivotal significance for stabilizing nitrate on the catalyst surface. The enhanced photolysis of nitrate by TiO₂ under UV irradiation can be explained using the following reactions^52–54 (eqn (10)–(19)):

TiO₂ + hν → h⁺ + e⁻ (10)

NO₂ + hν → NO + O(³P) (14)

NO₂ + e⁻ → NO₂⁻ (15)

NO₂ + 2e⁻ + 2H⁺ → NO + H₂O (16)

2NO + 2e⁻ + 2H⁺ → N₂O + H₂O (17)

NO₂⁻ + H₂O → HONO + HO˙ (18)

HONO + hν → NO + HO˙ (19)

There are two main factors affecting the nitrate decomposition mechanism on the TiO₂, which are co-adsorbed cations and the coexisting atmosphere. It is suggested that the cations on TiO₂ could inhibit the decomposition of nitrates. The presence of cations enhanced the adsorption of NO₃⁻ by forming ion pairs with NO₃⁻ and prevented NO₃⁻ from binding with light-induced h⁺ to generate ˙NO₃.^55,56 The decomposition of nitrates is facilitated by co-existing pollutants in the surrounding environment such as SO₂ and volatile organic compounds (VOCs). SO₂ preferentially reacts with ˙NO₃ radicals rather than photoelectrons, resulting in a significant improvement in nitrate decomposition.^57,58 Similar to SO₂, formaldehyde (HCHO) participated in the transformation of NO₃ to HNO₃ through hydrogen abstraction to promote nitrate decomposition. HCHO converts NO₃⁻ on particle surfaces into HNO₃ by reacting with ˙NO₃, and then HNO₃ photolyzes at a faster rate.⁵⁵ Interestingly, NO produced from nitrate decomposition may be re-adsorbed on the surface of TiO₂ to promote nitrate decomposition.⁵⁹ The N–O bond of NO₃⁻ could be activated by NO molecules. Subsequently, photogenerated electrons, captured by NO, facilitate the transformation of NO₃⁻ under light irradiation through the NO₃⁻ + NO⁻ → 2NO₂⁻ pathway.⁵⁹ It is proposed that addressing the photochemical transformation of surface NO₃⁻ may entail suppressing the formation of NO⁻ during the photocatalytic NO oxidation process.

Although the well-known photochemical activity of nitrate decomposition is excited around 300 nm,⁶⁰ it has been verified that nitrate decomposition could be induced by visible light (λ > 380 nm) on TiO₂. Wang et al.⁴⁷ investigated the decomposition of surface NO₃⁻ on nmTiO₂ under visible light exposure. By regulating the reaction atmosphere (dry argon, wet argon, and wet air), distinct production of NO and NO₂ is observed (Fig. 2a–c). Through meticulous control of in situ DRIFTS experiments, they uncovered different decomposition pathways stemming from different surface coordination modes of the NO₃⁻ (Fig. 2d and e). Nitrates generated on photocatalysts exhibit two coordination modes: monodentate nitrate (m-NO₃⁻) and bidentate nitrate (b-NO₃⁻). The m-NO₃⁻ decomposition initiated by photo-induced electrons primarily leads to the production of NO and NO₂. It is indicated that H₂O and O₂ have a remarkable influence on the decomposition products of NO₃⁻. The introduction of H₂O molecules results in dissociation on nmTiO₂, forming surface hydroxyl groups, which then facilitate the conversion of b-NO₃⁻ to m-NO₃⁻ through hydrogen bonding interactions.⁴⁷ Subsequently, visible-light-driven photocatalytic decomposition led to an enhancement in the concentration of NO_x. Although O₂ can promote nitrate regeneration through the oxidation process, the effect of H₂O on NO₃⁻ decomposition could not be restrained in the presence of O₂.

Fig. 2 Generation of gas-phase products in (a) dry argon atmosphere, (b) humidity argon atmosphere, and (c) humidity air atmosphere under visible light irradiation. (d) Proposed decomposition mechanism of b-NO₃⁻ triggered by visible light photocatalysis: (a) conversion pathway of b-NO₃⁻ to m-NO₃⁻ under visible light irradiation. (b) Photocatalytic decomposition pathway of produced m-NO₃⁻ to NO. (c) Photocatalytic decomposition pathway of produced m-NO₃⁻ to NO₂. (e) Proposed decomposition mechanism of m-NO₃⁻.⁴⁷

To sum up, the decomposition of nitrate on the surface of active particulate matter TiO₂ under various conditions has been confirmed. Therefore, when exploring the mechanism of oxidative NO_x removal, it is essential to consider the nitrate decomposition process. This perspective offers valuable insights for a more comprehensive understanding of the photocatalytic reaction.

2.2 Synergistic NO_x oxidation

In the real scenario, NO_x is typically present alongside other air pollutants. The interaction or combination of NO_x with other pollutants, such as volatile organic compounds (VOCs) and sulfur dioxide (SO₂), can lead to the formation of more severe secondary pollutants, especially under sunlight irradiation.^61–65 In addition to designing effective photocatalysts, it is crucial to address the complexities arising from these multiple pollutants when implementing photocatalytic NO oxidation technology in real reactors at pilot or larger scales.⁶⁶

2.2.1 Synergistic effect between NO_x and VOCs. It has been revealed that there is a certain synergistic effect between NO_x and VOCs on the specific photocatalysts to inhibit the generation of by-products. Xue et al.⁶⁷ proposed an enhanced photocatalytic NO removal through the addition of acetaldehyde to Sr₂Sb₂O₇ to prevent secondary peroxyacetyl nitrate (PAN) formation. The intermediates NO₂⁻ from NO and ˙CH₃ from acetaldehyde tend to bond and further oxidize to CH₃ONO₂, thus promoting NO removal. Density functional theory (DFT) calculations show that the Gibbs free energy required for the conversion of NO to NO₃⁻ in the mixed degradation process is lower compared to that in the individual removal pathways, thereby favoring the deep oxidation to NO₃⁻. Li et al.⁶⁸ investigated the simultaneous degradation of NO and C₇H₈ (toluene) mixed pollutants. On the In(OH)₃ photocatalyst, NO and toluene exhibit coupling reaction effects and give rise to a new NO conversion pathway, leading to NO deep oxidation to NO₃⁻ (Fig. 3a and b). The key intermediate C₇H₇NOH is favorable for the NO oxidation to NO₂ to inhibit O₃ formation (Fig. 3c and d). Then NO₂ was inclined to bond with toluene to form C₇H₆NO₂. The incorporation of the nitro group alters the charge distribution within the benzene ring, disrupting the typical distribution of electrons within the π–π stacked structure (Fig. 3e). The rearrangement of electrons opens the benzene ring more readily than toluene, allowing further NO₃⁻ formation. This synergistic effect of NO and toluene also occurs in InOOH. The increase in humidity could suppress catalyst deactivation and improve the conversion of mixed pollutants, as shown in Fig. 3f.⁶⁹ The yield of nitrate is also consistent with the conversion efficiency (Fig. 3g). This finding indicates that the extensive occupation of the catalyst surface by nitrate serves as a deactivation factor in mixed pollutant reactions.⁶⁹ Under high humidity conditions, H₂O and NO₃⁻ indicate similar adsorption configurations on InOOH, but H₂O generates more free radicals to directly enhance the conversion of C–N intermediates, thus promoting the conversion of NO to NO₃⁻ (Fig. 3h and i).⁶⁹ It was also proposed that NO could prevent the deactivation of the photocatalyst due to the incomplete oxidation product NO₂ in the process of o-xylene degradation.⁷⁰ NO₂ ensures the generation of a sufficient amount of TiO₂ (˙OL) radicals, allowing for the complete mineralization of o-xylene and inhibiting the deactivation of TiO₂.⁷⁰

Fig. 3 (a) The removal of NO and C₇H₈ at different mixing ratios. (b) The nitrogen-containing products identified by IC. (c) The generation of O₃ without and with the photocatalyst under light irradiation. (d) In situ DRIFTS spectra for the photocatalytic reaction of NO and C₇H₈.⁶⁸ (e) Charge distribution of C₇H₈ and C₇H₇NO₂ on the In(OH)₃ photocatalyst; blue and yellow clouds represent charge depletion and accumulation, and the isosurface level is set to 0.0007 eV Å⁻³. (f) Conversion of mixed pollutants at different relative humidities. [NO] = [C₇H₈] = 30 ppm. (g) NO₃⁻ production in mixed and single systems at different relative humidities. (h) Simulation of reactant and product binding on the catalyst. (i) Illustration of the synergistic interaction between NO and C₇H₈ and the promoting effect of H₂O.⁶⁹

2.2.2 Inhibition effect between NO_x and SO₂. In contrast to VOCs, research indicates that SO₂ tends to hinder the removal of NO_x. According to the studies of Ao et al.,⁷¹ the presence of SO₂ resulted in an 8% decrease in NO conversion and a 10% increase in NO₂ generation. The formation of sulfate ions on the catalyst surface obstructed the adsorption sites of TiO₂ for converting NO₂ to HNO₃, consequently increasing the exit concentration of NO₂. Chen et al.⁶⁶ further validated this deactivation mechanism through DFT calculations. The result showed that surface hydroxyls on the (101) facet of TiO₂ create unsaturated coordination of adjacent Ti or O atoms, promoting the adsorption of NO and SO₂. However, there is a competitive adsorption between NO and SO₂, as evidenced by the decreasing adsorption energy and charge density. In the oxidation process, SO₂ significantly competes with NO for reaction with ˙O₂⁻. More valence electrons are involved in the oxidation of SO₂, resulting in higher binding energy between *O–SOOO* than *O–NOO*.

3 NO_x reduction to nitrogen

Contemporary research in photocatalytic NO abatement predominantly centers on oxidizing NO to NO₃⁻. Nevertheless, concerns arise from the production of the more toxic by-product, NO₂, catalyst deactivation resulting from the obstruction of reaction sites by nitrate products, and the denitrification of NO₃⁻. These challenges impede the practical application of photocatalysts. An alternative approach for NO_x removal involves the reduction of NO to N₂ without causing deactivation or secondary pollution. This can be achieved through two approaches: direct reduction of NO to N₂ and O₂ under oxygen-free conditions (NO_x decomposition), and selective reduction of NO to N₂ in a reducing atmosphere (NO_x photo-selective reduction).

3.1 NO_x decomposition

The decomposition of NO into harmless N₂ and O₂ represents one of the most desirable pathways of NO removal, as it eliminates the need for additional reductants. Transition metal oxides have emerged as typical photocatalysts for NO_x decomposition.^72–82 Utilizing zeolite supports allows for the production of highly dispersed powders, enhancing the performance of NO decomposition.⁸³ Previous studies have demonstrated the significant contribution of transition metal ions to the photocatalytic NO decomposition. As shown in eqn (20), highly dispersed and isolated transition metal oxides form the charge-transfer excited states under irradiation, which involve electron transfer from O_(l)²⁻ to M_(l)ⁿ⁺:

The selectivity of N₂ increases as the coordination number of the M–O bond decreases, which triggers the photocatalytic decomposition of NO.⁸³ It was shown that the Ti-oxide/Y-zeolite catalysts with tetrahedral coordination exhibit higher reactivity and selectivity for N₂ generation and lower selectivity for N₂O formation than those with octahedral coordination.⁸³ The performance of the photoreduction reaction could also be improved by oxygen vacancies and doping the catalyst with transition metals.^81,82 van de Krol et al.⁸² doped TiO₂ nanoparticles with Fe³⁺, which changed the photocatalytic NO removal route from oxidation to reduction (Fig. 4a). The Fe³⁺ substituted Ti⁴⁺ in the lattice, and the negative charge of this acceptor-type dopant contributed to the stabilization of positivity-charged oxygen vacancies. The oxygen vacancies could act as active sites to capture the molecular O of the NO molecule and provide light-induced electrons (Fig. 4b). Besides, Ag⁰ and Ag⁺ coexist on the TiO₂ surface, which both play different roles in the enhancement of N₂ selectivity: the Ag⁰ nanoparticles enhance the light absorption of the material, while Ag⁺ can decompose the attachment product N₂O into N₂.⁸¹ A recent study shows that layered structured perovskite SrBi₂Nb₂O₉ with ultrathin nanostructure and oxygen vacancies (SBNO-UT) is capable of decomposing NO into N₂ and O₂ at the close-to-stoichiometric ratio (Fig. 4c).⁸⁴ As can be seen from Fig. 4d, N₂O intermediates were formed by NO reduction during the photocatalytic process. The in situ formed N₂O bounds to the defective surface, ensuring further reduction (Fig. 4e). Finally, the breakage of the N=O bond completes the reduction of NO to N₂.

Fig. 4 (a) Photocatalytic conversion of NO to N₂ and O₂ over 1% Fe-doped TiO₂. The sample was irradiated with UV light, and the target pollutant was 100 ppm NO in He. (b) Possible elementary reaction routes of NO on Fe-doped TiO₂.⁸² (c) Formation of photocatalytic NO decomposition products on SBNO and SBNO-UT, respectively, in a sealed system with 13.5 ppm NO. (d) In situ FTIR spectra of SBNO-UT during the photocatalytic NO abatement (0–20 min). The background was collected after the equilibrium of NO adsorption was reached but before illumination. (e) Possible mechanism of the photocatalytic NO decomposition on SBNO-UT.⁸⁴

3.2 NO_x photo-selective reduction

The method of selectively converting NO to N₂ by introducing reducing agents such as NH₃, CO, H_2, and hydrocarbons into the system is known as selective catalytic reduction (SCR). Despite extensive study,^85,86 the technology typically operates at high temperatures (300–400 °C),^87,88 leading to relatively high energy consumption and potentially excessive CO₂ emissions. In addressing these challenges, photo-assisted SCR (photo-SCR) of NO_x with NH₃ using photocatalysts under light irradiation and low-temperature conditions has been developed as an alternative for NO_x reduction.

NH₃ serves as a common reductant for NO_x removal by photo-SCR, with TiO₂ being the most extensively utilized catalyst in this system. The reaction mechanism of NO photo-SCR with NH₃ on TiO₂ is depicted in Fig. 5a.⁸⁹ There are numerous TiO₂-based materials for photo-SCR, e.g., TiO₂/SiO₂,⁹⁰ TiO₂ nanotube arrays,⁹¹ Bi₂WO₆/TiO₂ Z-scheme heterojunctions,⁹² g-C₃N₄/TiO₂ (ref. 93) and Pd-loaded TiO₂,⁹⁴ and Si/TiO₂.⁹⁵ Tanaka et al.⁹⁶ reported achieving the photocatalytic selective reduction of NO with NH₃ using TiO₂ under xenon lamp illumination at conversion temperatures as low as 50 °C. They also investigated the activity of TiO₂ surfaces loaded with 1 wt% of various transition metal oxides.⁹⁷ Their findings revealed that Nb, Mo, and W oxides promoted the photo-SCR activity of TiO₂, with WO₃/TiO₂ exhibiting the highest activity (Fig. 5b). The number of acidic sites on TiO₂ as active sites for the photo-SCR reaction determines the reactivity of the photocatalyst.⁹⁸ Modification of TiO₂ with WO₃ resulted in increased polarizability, enhancing the surface acidity and thus facilitating the reduction of NO.^99,100 Subsequent studies concluded that agglutination occurs with increasing WO₃ addition, with isolated W species enhancing the chemical reduction activity of NO, while agglutinated W species remain inactive.¹⁰¹

Fig. 5 (a) Proposed reaction mechanism of the photo-SCR over a TiO₂ photocatalyst under UV-light irradiation.⁸⁹ (b) Conversion of NO in the photo-SCR over various metal oxide-promoted TiO₂ photocatalysts.¹⁰² (c–e) NH₃-SCR activity of single- and multi-element doped CaTiO₃.¹⁰³

Perovskite (ABO₃) structured materials with flexible structures have recently garnered scientific interest for their application in photo-SCR. Researchers have synthesized Ce–Fe–Mn doped CaTiO₃ perovskite from titanium-containing solid waste using the molten salt method.¹⁰³Fig. 5c–e illustrate that CaTiO₃ showed no photocatalytic activity for NO removal at temperatures ranging from 100 to 300 °C. However, (Ca, Ce) (Ti, Mn, Fe) O₃ demonstrated almost 100% NO conversion under light irradiation at 135 °C (GHSV = 72 000 h⁻¹). Both Mn and Fe are incorporated into the B-site of CaTiO₃, while Ce occupies the A-site. Mn doping enhances material light absorption and the capacity to capture NO. After Fe doping, the oxidation center for NO-to-NO₃⁻ shifts from the five-coordinated Mn_5c site to the Fe_5c site, significantly altering the reaction pathway of conventional SCR. Furthermore, Pr doping of LaCoO₃ supported on a natural palygorskite (Pal) surface eliminates over 95% of NO_x within the low-temperature range of 150–250 °C.¹⁰⁴ Appropriate Mn doping into LaFeO₃ to form LaFe_1−xMn_xO₃/attapulgite (ATP) nanocomposites extends its visible light absorption range, resulting in increased NO conversion, reaching a maximum of 85%, with N₂ selectivity close to 100%.⁸⁷ Ni-doped LaFeO₃ nanocomposites exhibit a higher NO_x conversion rate of 92%.¹⁰⁵ Moreover, nitrogen-doped carbon quantum dot modified PrFeO₃/Pal, with abundant acid sites, demonstrate 93% NO removal and 100% N₂ selectivity under visible light illumination, showing considerable tolerance to SO₂ and H₂O.¹⁰⁶

4 NO_x upcycling to ammonia

The upcycling of NO_x to ammonia presents an environmentally friendly opportunity for both NO_x pollutant elimination and sustainable ammonia production. While electrocatalytic synthesis of ammonia has garnered considerable attention, photocatalytic reduction of NO to ammonia remains in its nascent stage. Nevertheless, recent years have seen significant advancements in photocatalytic ammonia synthesis. The authors outline two potential approaches for NO_x resource utilization via photocatalysis. One method involves the use of a chelating agent with the dissolution of NO in water, followed by a one-step reduction to ammonia via photocatalysis (one-step method). Another approach entails oxidizing NO to the highly water-soluble NO₃⁻ initially, followed by its reduction to ammonia through an eight-electron reduction process (two-step method).

4.1 One-step method (NO → NH₃)

The direct one-step synthesis of NH₃ from NO is an ideal strategy for transforming waste into valuable resources. While research efforts in NO_x reduction reactions (NO_xRR) have predominantly focused on enhancing NH₃ synthesis rates, achieving high NO_x conversion efficiency remains a crucial yet less explored objective.^107,108 Typically, NO_xRR experiments necessitate efficiency tests with high concentrations (>10 000 ppm) of NO to ensure an adequate feedstock for enhancing ammonia production.¹⁰⁹ However, limited NO conversion persists as a key challenge for the sustainability of the NO-to-NH₃ reduction pathway.

The ultra-low solubility of NO in water (1.94 mmol L⁻¹) is one of the primary reasons for limited NO conversion.¹¹⁰ To address this challenge, a novel one-step NO_x photoreduction pathway, referred to as the on-site coupling system, has been recently developed. This system enables NO direct upcycling under ambient conditions (Fig. 6a).¹¹¹ Specifically, the solution was supplemented with Fe(II)EDTA as a NO chemical absorbent, generating Fe(II)EDTA-NO chelates, while formaldehyde (HCHO) served as an antioxidant to prevent the Fe(III) formation from Fe(II) oxidation. This simultaneous chemical absorption and photocatalytic reduction system enabled continuous NO adsorption, NO reduction, and Fe(II)EDTA regeneration on-site.¹¹¹ TiO₂ decorated with 11.6 mg L⁻¹ Au nanoparticles could provide ample active sites to facilitate charge separation, thereby enhancing NH₄⁺ generation. Using this on-site coupling system, the performance of Au_NPs–TiO₂ is shown in Fig. 6b and c, demonstrating exceptional NO conversion efficiency (89.0%), ammonia production selectivity (95.6%), and ammonia recovery efficiency (>90%). Virtually no other side products are detected. Subsequently, the generated ammonia was recovered via a simple ion exchange method, with recovery rates still exceeding 90% within 50–200 mg L⁻¹ ammonia production (Fig. 6d). Moreover, the NO conversion efficiency and the NH₄⁺ selectivity remained unaffected even in the presence of resistance factors such as H₂O, SO₂, and metal ions (K⁺, Ca²⁺, Cd²⁺, and Pb²⁺) (Fig. 6e). These high recovery efficiencies and anti-poisoning capacities underscore the environmental practicality of NO_x removal in flue gas. The on-site coupling strategy achieves significant efficiency milestones for sustainable NO removal and also offers insights into NO upcycling for the future of carbon neutrality.

Fig. 6 (a) Illustration of the on-site coupling system of continuous chemical absorption and catalytic reduction. (b) NH₄⁺ synthesis rate between the pristine and Au-decorated TiO₂. (c) Selectivity test. (d) Ammonia recovery evaluation after the continuous absorption and photoreduction of NO. (e) Poisoning resistance against vapor, SO₂, and metals.¹¹¹

The simultaneous presence of NO and SO₂ in flue gas poses significant challenges for their concurrent removal and recovery.^112,113 Since SO₂ is easily dissolved and oxidized, it can serve as a potential reductant in the NO photoreduction reaction. Furthermore, the on-site coupled system demonstrated the synergistic removal and recycling of NO and SO₂ without the need for scavengers.¹¹⁴ The formation of the SO₂–NO redox pair facilitates high conversion rates of both NO and SO₂ in continuous flow. Achieving a remarkable selectivity for both NO-to-NH₃ upcycling (97%) and SO₂-to-SO₄²⁻ purification (92%) simultaneously underscores the practicability of value-added conversion of air pollutants (Fig. 7a and b).¹¹⁴ Maintaining the efficiency of NO conversion and reduction crucially depends on preventing the oxidation of Fe(II) to Fe(III) in Fe(II) EDTA. Through verified in situ ATR-FTIR experiments, it was revealed that SO₂ exhibited a promoting effect on Fe(II) maintenance, while no interaction was observed between SO₂ and EDTA (Fig. 7c and d).

Fig. 7 The continuous selectivity evaluation for (a) NO-to-NH₃ reduction and (b) SO₂-to-SO₄²⁻ oxidation. In situ ATR-FTIR spectra for the recording of Fe(II)EDTA characterization signals (c) without and (d) in the presence of SO₃²⁻ provision. (e) In situ EPR measurements for the consumption of TEMPO in the photocatalytic reduction process of NO without (up) and with (down) SO₃²⁻ provision, respectively, (f) In situ EPR measurements for the production of DMPO-˙OH by H₂O oxidation (up) and DMPO-˙SO₃⁻ by SO₃²⁻ oxidation (down), respectively.¹¹⁴

The chemical redox pair of SO₂–Fe(III) is illustrated in eqn (21)–(23).¹¹⁴

Fe(II) → Fe(III) (spontaneous oxidation) (21)

SO₂ + H₂O → SO₃²⁻ + 2H⁺ (dissolution) (22)

2Fe(III) + 2SO₃²⁻ + O₂ → 2Fe(II) + 2SO₄²⁻ (chemical redox pair) (23)

As depicted in Fig. 7e and f, compared to h⁺-induced ˙OH production, the h⁺-induced SO₂ oxidation reaction to ˙SO₃⁻ promotes charge separation, thereby generating more e⁻, and enhancing the efficiency of the nitrogen oxide reduction reaction to produce NH₃. The overall mechanism for synchronous NO and SO₂ conversion is illustrated in eqn (24)–(26):

SO₂ + H₂O → SO₃²⁻ + 2H⁺ (dissolution) (24)

5SO₃²⁻ + NO + 5h⁺ + 5e⁻ + 5H⁺ →5˙SO₃⁻ + NH₄⁺ + OH⁻ (photocatalytic redox pair) (25)

5˙SO₃⁻ + NO + 5h⁺ + 5e⁻ + 5H⁺ + 9OH⁻ → 5SO₄²⁻ + NH₄⁺ + 5H₂O (photocatalytic redox pair) (26)

Thus, a one-step NO_x reduction method has been devised by concurrently absorbing and converting NO_x. This system also exhibits great resistance to toxicity, including resistance to K⁺, Ca²⁺, and Cd²⁺ metal ions, respectively. The effective separation and retrieval of reaction products provide this system with sustainable stability and potential economic viability. Continuous NO_x removal and upcycling under ambient conditions with high conversion efficiencies were achieved, marking significant advancements in photocatalytic NO_x removal and recycling compared to SCR and chemical absorption technologies.

4.2 Two-step method (NO → NO₃⁻ → NH₃)

Among the redox products of NO_x, NO₃⁻ is the dominant and accessible substance on the surface of the photocatalyst.^39,67 Significant quantities of NO₃⁻ are released into the environment through processes such as the combustion of fossil fuels and agricultural activities, driven by the progress of urbanization and industrialization.¹¹⁵ The surface of the photocatalyst is the overlooked storage site for NO₃⁻, where the generated nitrate could be easily removed from the catalyst and subsequently concentrated in liquid form.^69,116 However, excessive amounts of NO₃⁻ in water can pose a pollution concern. To address this issue, photocatalytic technologies have been employed to convert excess nitrate into valuable NH₃.

Photocatalytic reduction of NO₃⁻ to ammonia via multiple electron transfer (eqn (27)), could emerge as a potent method for ammonia production under ambient conditions of room temperature and atmospheric pressure.¹¹⁷ N₂ and H₂ formation reactions are two side reactions that consume photoexcited electrons (eqn (28) and (29)).^117,118 Therefore, it is necessary to enhance not only the ammonia yield but also the ammonia selectivity. A summary of studies on the reduction process of nitrate to ammonia by various photocatalysts is presented in Table 1.

NO₃⁻ + 9H⁺ + 8e⁻ → NH₃ + 3H₂O (1.20 V vs. NHE) (27)

NO₃⁻ + 6H⁺ + 5e⁻ → 1/2N₂ + 3H₂O (1.25 V vs. NHE) (28)

2H⁺ + 2e⁻ → H₂ (0.00 V vs. NHE) (29)

Table 1 Comparison of the results of NH₃ production with different photocatalysts

Photocatalysts	Hole scavengers	Light source	pH	NO3^− conversion (%)	NH4^+ production (mmol gcat^−1 h^−1)	NH4^+ selectivity (%)	Ref.
Cu2ONCs-TNS	Formaldehyde	300 W Xe lamp	N/A	94.2	42.6	98.6	119
CuOx@TNS	Ethylene glycol	300 W Xe lamp	8.3	100	16	96.1	120
Cu-TNS	Formic acid	300 W Xe lamp	3.5–3.6	>99	0.1	97.6	118
BaONCs@TNS	Ethylene glycol	300 W Xe lamp	7	∼3.5	89.8	97.7	121
Photo-reductive TiO2	N/A	450 W medium pressure UV	7	∼71	N/A	60	122
LaFeO3/HTCC nanocomposites	Acetic acid	300 W Xe lamp	2	94.6	N/A	88.7	123
LaFeO3/biochar nanocomposites	Formic acid	300 W Xe lamp	2	98	N/A	97	124
Ni/HxWO3−y hybrids	Ethylene glycol	300 W Xe lamp	N/A	∼92	10.5	98.3	125
Carbon/bismuth/Bi2O3	Ethylene glycol	Simulated sunlight	N/A	N/A	0.4	95	126
CuAg/TiO2	Methanol	Black light	6	96	N/A	85	127
Ni2P/Ta3N5	N/A	Fluorescent lamps	2	79	0.2	68	128
PdSn/NiO/NaTaO3:La	Formic acid	125 W high pressure UV	N/A	100	N/A	72	129
JRC-TIO-6 (rutile TiO2)	Formic acid	2 kW Xe lamp	2.4–3	79	0.02	97	117
CuPd/TiO2	Methanol	UV	N/A	>95	N/A	78	130

---

It can be inferred from Table 1 that TiO₂-based materials are prominently utilized as catalysts for NO₃⁻-to-NH₃ reduction, particularly highlighting metal-modified TiO₂ nanosheets (TNSs). Recently, as shown in Fig. 8a, alkaline-earth oxide clusters constructed on TiO₂ nanosheets (MO_NCs-TNSs, M = Mg, Ca, Sr or Ba) were reported to enhance NO₃⁻-to-NH₄⁺ production, among which BaO_NCs-TNSs achieve both exceedingly high NH₄⁺ selectivity (97.7%) and high NH₄⁺ yield (89.8 mmol g_cat⁻¹ h⁻¹).¹²¹ To the best of our knowledge, this is the highest production of ammonia by the photocatalytic reduction reaction. Remarkably, there's an incremental rise in the rate of ammonia synthesis throughout the reaction, attributed to the concurrent Operando formation of BaO_NCs and the ammonia synthesis reaction (Fig. 8b and c). Upon achieving stabilization of BaO_NCs on TNSs, a notable and consistent production of ammonia on BaO_NCs-TNS composites was realized.

Fig. 8 (a) Catalytic efficiency tests showing the enhancement of Operando construction of alkaline-earth oxide clusters on TNS surfaces. (b) Quasi in situ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images showing the evolution course from isolated Ba single atoms (Ba_SAs) to subnanometric (BaO_NCs) at the irradiation time of 5 min (up) and 10 min (down). (c) Variation of Ba²⁺ concentration during the Operando construction of BaO_NCs detected by ion chromatography.¹²¹ (d) NH₃ photosynthesis rate of the dynamic process. (e) Efficiency comparison between the different composites of Cu species and the TNS substrate. (f) The evolution process of the different chemical states of Cu²⁺ and Cu⁺ under the catalysis conditions. (g) Snapshots of the photosynthesis reactor along with the photosynthesis reaction time for the observation of the dynamic evolution of Cu species. (h) Schematic illustrations for the synchronous Cu₂O NCs construction and NH₃ photosynthesis in the realistic catalysis condition.¹¹⁹

Chen et al.¹¹⁹ further elucidated the criticality of the on-site formation of authentic active sites under realistic catalytic conditions. They engineered Cu₂O_NCs-TNS materials, wherein dynamic Cu₂O sub-nanoclusters were on-site constructed on TNSs using CuCl₂ solution through a photoinduced pseudo-Fehling's route. Different from BaO_NCs-TNS which remains stable once it has been constructed, the morphology and chemical state of Cu₂O exhibit dynamic evolution, as the actual reaction cannot progress over preformed Cu₂O sites (Fig. 8d and e). Numerous in situ experiments revealed a photoswitchable reversible conversion pattern between Cu²⁺ and Cu⁺ in the photosynthesis reaction, aligning with the performance of ammonia synthesis by nitrate reduction (Fig. 8f and g).¹¹⁹ NH₃ photosynthesis was directly attributed to the enhanced charge separation and transformation capacity from the Cu₂O active sites. Consequently, the establishment of a genuine structure–activity correlation was achieved by uncovering the synchronous formation of the Cu₂O active site and catalytic reaction (Fig. 8h).

In addition, a groundbreaking perspective has emerged regarding the interplay between single atoms and oxygen vacancies (OVs) (Fig. 9).¹¹⁸ The incorporation of single-atom Cu into TiO₂ nanosheets (Cu-TNSs) resulted in a 62-fold increase in ammonia production compared to pristine TiO₂, with a selectivity of 97.6%. The introduction of Cu atoms, replacing Ti sites, induces the generation of OVs and lattice strain. NO₃⁻ preferentially adsorbs onto Ti atoms neighboring Cu atoms. The near-isolated Cu atoms and OVs facilitate multiple NO₃⁻ adsorptions at a single site, effectively inhibiting the release of NO₂⁻ intermediates and minimizing N-to-N interactions, thus ensuring high NH₃ selectivity.

Fig. 9 NO₃⁻-to-NH₃ reaction mechanism on Cu-TNSs. TiO₂ with isolated Cu²⁺ atomic sites (blue) and defects generates photoexcited electrons and holes under light irradiation.¹¹⁸

Furthermore, CuO_x@TNS was designed for NO₃⁻ reduction with a 100% NO₃⁻ conversion rate and 96.1% ammonia selectivity by coupling the nitrate reduction reaction with a glycol oxidation reaction system.¹²⁰ The active site for improving ammonia formation is identified as the CuO_x species with an amorphous Cu–O–Ti bimetallic oxide cluster structure (Fig. 10a).¹²⁰ Given that redox reactions occur concurrently, it's evident that the oxidation half-reaction also significantly influences NH₃ production. To explore the impact of various oxidation half-reactions on NO₃⁻ reduction, oxidative reactants including deionized water (DI), methanol (CH₃OH), ethylene glycol ((CH₂OH)₂), and acetic acid (HCOOH) are introduced into the reaction system (Fig. 10b and c). It is revealed that the NO₃⁻ reduction process is hindered by the formation of potent oxidizing ˙OH radicals (Fig. 10d). The presence of ethylene glycol expedites the consumption of h⁺ during the formation of alkoxy radicals (˙R), thus mitigating ˙OH production. Moreover, the Cu–O–Ti sites could promote the preferential oxidation of ethylene glycol, thereby enhancing both the efficiency and selectivity of ammonia production (Fig. 10e). These findings underscore the pivotal role of the oxidation half-reaction. The deliberate design of the oxidation half-reaction not only enhances the effective utilization of electrons and holes but also modulates the reaction pathway, thus promoting the progress of the NO₃⁻ reduction half-reaction.

Fig. 10 (a) Magnified HAADF-STEM image of 0.52 wt% CuO_x@TNS. NH₃ photosynthesis yields under different cooperative oxidative half-reactions for (b) TNSs and (c) 0.52 wt% CuO_x@TNS after 1 h irradiation. (d) EPR spectra of DI, CH₃OH, (CH₂OH)₂, and HCOOH oxidative half-reactions on TNSs and 0.52 wt% CuO_x@TNS catalyst after light irradiation for 5 min. (e) Illustration of the directed NO₃⁻RR by coordination of different oxidative reactions.¹²⁰

In summary, the photocatalytic performance of NO₃⁻-to-NH₃ photocatalytic reduction highly relies on the strategic construction of active sites and the coordination of redox reactions. This offers an innovative perspective on experimental design aimed at achieving high NH₃ selectivity and production. Recent studies have shown that NO₃⁻ originating from NO oxidation has achieved nearly 100% NH₃ selectivity. It is desired to employ a two-step process (NO → NO₃⁻ → NH₃) to achieve a highly selective conversion of NO to NH₃.

5 Conclusions and perspectives

5.1 Conclusions

Table 2 summarizes and compares the application scenarios, reaction conditions, photocatalysts, and reaction activity for NO_x removal under the different reaction pathways. It is found that the application of photocatalytic technology has transcended indoor NO_x treatment and is now addressing challenges posed by complex pollutant compositions and industrial flue gases. Interestingly, TiO₂ remains the most extensively studied photocatalyst in both NO_x oxidation and NO_x reduction, probably due to its outstanding carrier separation capability. Besides, the overall redox properties also depend on catalyst modification or adjustment of environmental atmospheres in different systems. The photocatalytic performance showed that the NO_x conversion and product selectivity have exceeded 90% for most of the redox pathways except NO_x decomposition. This marks a significant advancement towards the future application of photocatalytic NO_x removal and recovery.

Table 2 Conclusion and comparison of NO_x removal by various reaction pathways

Reaction pathway		Product	Scenarios of application	Typical photocatalysts	NOx concentration	Temperature (°C)	Flow rate (L min^−1)/GHSV (h^−1)	Carrier gas	Supplements	Conversion (%)	Selectivity (%)
NOx oxidation	Single oxidation	NO3^−	Air purification	TiO2, g-C3N4, WO3, Bi-based materials	∼600 ppb	Room temperature	1 L min^−1	Air	N/A	∼90	∼97
	Mixed oxidation	NO3^−	Air purification	TiO2, In(OH)3, Sr2Sb2O7	30 ppm	Room temperature	1 L min^−1	Air	VOCs, SO2	∼100	N/A
NOx reduction	NOx decomposition	N2	Air purification	TiO2, g-C3N4	<1 ppm	N/A	1 L min^−1	Air/Ar	N/A	∼50	N/A
	NOx photo-SCR	N2	Flue gas treatment	TiO2, perovskite	1000 ppm	<200	50 000 h^−1	3% O2, 97% N2	NH3, H2, hydrocarbon, etc.	>90	∼100
NOx upcycling	One-step method	NH3	Flue gas treatment	TiO2	500 ppm	25	25 000 h^−1	N/A	Fe(II)EDTA/hole scavengers (e.g. HCHO, SO2)	97.6	∼97.1%

---

In conclusion, photocatalytic technology is a promising method for NO_x removal and recovery, owing to its mild reaction conditions and eco-friendliness. In recent years, significant strides have been made in the field of photocatalytic NO_x removal, spanning from the optimization of photocatalysts to the exploration of mechanisms and the modulation of reaction pathways, resulting in unprecedented performance in NO_x removal. The emerging photocatalytic reduction of NO_x to ammonia offers a viable alternative for NO_x recovery. This review summarizes the latest advancements in photocatalytic removal, encompassing NO_x oxidation (both single and synergistic removal, as well as NO₃⁻ decomposition), NO_x reduction to nitrogen, and the upcycling of NO_x into ammonia. Furthermore, we highlight photocatalysis-based NO_x recovery, including one-step and two-step methods, as a promising approach to tackle existing environmental and energy challenges.

5.2 Future outlook

The utilization of photocatalytic technology for the removal and recovery of NO_x is currently in the research phase, with the practical application still a considerable distance away. We recommend that future studies systematically address the following issues:

(1) Comparing the performance of photocatalysts across different studies in the realm of NO_x oxidation proves challenging due to the diverse array of reaction conditions such as light source, gas flow rate, initial NO_x concentration, mass of the photocatalyst, reactor design, and others. To comprehensively evaluate photocatalyst performance and identify truly efficient photocatalysts, there is a pressing need to establish uniform performance standards.

(2) Despite notable advancements in the performance of photocatalytic NO_x removal in recent years, a gap persists when compared to thermal catalytic NO_x removal. One of the primary reasons for this gap is the insufficient exploration of the interfacial reaction mechanism governing photocatalytic NO_x removal. To address this, in situ detection methods with higher resolution, such as in situ EPR, DRIFTS, and Raman spectroscopy, should be more extensively employed. These techniques can unveil the intricacies of the photocatalytic reaction mechanism, subsequently informing catalyst design and modification efforts.

(3) In the one-step method, the yield of ammonia is significantly influenced by the solubility of NO, thus necessitating further enhancement of NO solubility in water. Introducing chemical absorbents to adsorb NO suggests the need for additional methods to separate the resulting ammonia, thereby increasing costs and rendering it less favorable for practical applications. An absorbent-free approach is preferable to augment the solubility of high concentrations of NO in water and to develop photocatalysts capable of achieving an ammonia yield that meets recovery criteria. Moreover, photocatalysts can be selectively designed to further reduce NO_x to N₂ instead of ammonia, offering an alternative solution to address current environmental challenges.

(4) In the two-step process, the primary challenge lies in effectively collecting the nitrates produced from NO_x. During the photocatalytic NO_x oxidation process, nitrates gradually accumulate on the catalyst surface. However, due to the limited availability of adsorption sites on the surface and the simultaneous decomposition of nitrates, only a small portion of nitrates remains on the photocatalyst surface. It is imperative to enhance the thorough oxidation of NO_x and suppress the decomposition of nitrates. Additionally, nitrates are commonly found in relatively high concentrations in wastewater, presenting an opportunity to enhance ammonia production.

(5) Machine learning offers a potent tool for predicting catalytic reaction performance accurately, aiding chemists in tasks like material screening, experiment optimization, and mechanistic studies. Nonetheless, it's crucial to emphasize that machine learning should be applied judiciously. While it's a valuable tool, it's not a universal solution and cannot substitute well-considered experimental designs. In practice, a combination of expertise is still vital to ensure that machine learning and traditional methods work together harmoniously to achieve broader environmental science objectives.

Author contributions

All of the authors contributed to the literature search, writing and editing of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22261142663, 22225606, and 22176029).

References

1. J. Lelieveld, J. S. Evans, M. Fnais, D. Giannadaki and A. Pozzer, The contribution of outdoor air pollution sources to premature mortality on a global scale, Nature, 2015, 525, 367–371.
2. S. C. Anenberg, J. Miller, R. Minjares, L. Du, D. K. Henze, F. Lacey, C. S. Malley, L. Emberson, V. Franco, Z. Klimont and C. Heyes, Impacts and mitigation of excess diesel-related NO_x emissions in 11 major vehicle markets, Nature, 2017, 545, 467–471.
3. D. Shindell, G. Faluvegi, M. Walsh, S. C. Anenberg, R. Van Dingenen, N. Z. Muller, J. Austin, D. Koch and G. Milly, Climate, health, agricultural and economic impacts of tighter vehicle-emission standards, Nat. Clim. Change, 2011, 1, 59–66.
4. N. Li, C. Wang, K. Zhang, H. Lv, M. Yuan and D. W. Bahnemann, Progress and prospects of photocatalytic conversion of low-concentration NO_x, Chin. J. Catal., 2022, 43, 2363–2387.
5. M. G. Lawrence and P. J. Crutzen, Influence of NO_x emissions from ships on tropospheric photochemistry and climate, Nature, 1999, 402, 167–170.
6. J. Lasek, Y. H. Yu and J. C. S. Wu, Removal of NO_x by photocatalytic processes, J. Photochem. Photobiol., C, 2013, 14, 29–52.
7. V. Smil, Detonator of the population explosion, Nature, 1999, 400, 415.
8. S. Su, Y. Liang, G. Yin, Q. Wang, Y. Cai, X. Peng, M. N. Pervez and L. Lin, Anhydrous dyeing processes of ramie fiber in liquid ammonia, Cellulose, 2019, 26, 8109–8120.
9. T. Wu, X. Zhu, Z. Xing, S. Mou, C. Li, Y. Qiao, Q. Liu, Y. Luo, X. Shi, Y. Zhang and X. Sun, Greatly Improving Electrochemical N₂ Reduction over TiO₂ Nanoparticles by Iron Doping, Angew. Chem., Int. Ed., 2019, 58, 18449–18453.
10. R. Schlögl, Catalytic Synthesis of Ammonia—A “Never-Ending Story”, Angew. Chem., Int. Ed., 2003, 42, 2004–2008.
11. J. Egerer, V. Grimm, K. Niazmand and P. Runge, The economics of global green ammonia trade - “Shipping Australian wind and sunshine to Germany”, Appl. Energy, 2023, 334, 120662.
12. Q. An, M. McDonald, A. Fortunelli and W. A. Goddard III, Si-Doped Fe Catalyst for Ammonia Synthesis at Dramatically Decreased Pressures and Temperatures, J. Am. Chem. Soc., 2020, 142, 8223–8232.
13. X. Guo, P. Wang, T. Wu, Z. Wang, J. Li, K. Liu, J. Fu, M. Liu, J. Wu, Z. Lin, L. Chai, Z. Bian, H. Li and M. Liu, Aqueous Electroreduction of Nitric Oxide to Ammonia at Low Concentration via Vacancy Engineered FeOCl, Angew. Chem., Int. Ed., 2023, e202318792.
14. K. A. Brown, D. F. Harris, M. B. Wilker, A. Rasmussen, N. Khadka, H. Hamby, S. Keable, G. Dukovic, J. W. Peters, L. C. Seefeldt and P. W. King, Light-driven dinitrogen reduction catalyzed by a CdS: nitrogenase MoFe protein biohybrid, Science, 2016, 352, 448–450.
15. Q. Guo, Z. B. Ma, C. Y. Zhou, Z. F. Ren and X. M. Yang, Single Molecule Photocatalysis on TiO₂ Surfaces, Chem. Rev., 2019, 119, 11020–11041.
16. T. Wu, X. J. Liu, Y. Liu, M. Cheng, Z. F. Liu, G. M. Zeng, B. B. Shao, Q. H. Liang, W. Zhang, Q. Y. He and W. Zhang, Application of QD-MOF composites for photocatalysis: Energy production and environmental remediation, Coord. Chem. Rev., 2020, 403, 213097.
17. F. He, S. Weon, W. Jeon, M. W. Chung and W. Choi, Self-wetting triphase photocatalysis for effective and selective removal of hydrophilic volatile organic compounds in air, Nat. Commun., 2021, 12, 6259.
18. S. Wang, W. Cui, B. Lei, X. A. Dong, Y. Tang and F. Dong, Targeted NO Oxidation and Synchronous NO₂ Inhibition via Oriented ¹O₂ Formation Based on Lewis Acid Site Adjustment, Environ. Sci. Technol., 2023, 57, 12890–12900.
19. R. Hailili, Z.-Q. Wang, H. Ji, C. Chen, X.-Q. Gong, H. Sheng and J. Zhao, Mechanistic insights into the photocatalytic reduction of nitric oxide to nitrogen on oxygen-deficient quasi-two-dimensional bismuth-based perovskites, Environ. Sci.: Nano, 2022, 9, 1453–1465.
20. J. C.-C. Yu, V.-H. Nguyen, J. Lasek and J. C. S. Wu, Titania nanosheet photocatalysts with dominantly exposed (001) reactive facets for photocatalytic NO_x abatement, Appl. Catal., B, 2017, 219, 391–400.
21. V.-H. Nguyen, B.-S. Nguyen, C.-W. Huang, T.-T. Le, C. C. Nguyen, T. T. Nhi Le, D. Heo, Q. V. Ly, Q. T. Trinh, M. Shokouhimehr, C. Xia, S. S. Lam, D.-V. N. Vo, S. Y. Kim and Q. V. Le, Photocatalytic NO_x abatement: Recent advances and emerging trends in the development of photocatalysts, J. Cleaner Prod., 2020, 270, 121912.
22. X. Shi, P. Wang, W. Li, Y. Bai, H. Xie, Y. Zhou and L. Ye, Change in photocatalytic NO removal mechanisms of ultrathin BiOBr/BiOI via NO₃^- adsorption, Appl. Catal., B, 2019, 243, 322–329.
23. K. Li, H. Wang, J. Li and F. Dong, Design and mechanism of photocatalytic oxidation for the removal of air pollutants: a review, Environ. Chem. Lett., 2022, 20, 2687–2708.
24. Q. Li, J. Zhao, H. Shang, Z. Ma, H. Cao, Y. Zhou, G. Li, D. Zhang and H. Li, Singlet Oxygen and Mobile Hydroxyl Radicals Co-operating on Gas-Solid Catalytic Reaction Interfaces for Deeply Oxidizing NO_x, Environ. Sci. Technol., 2022, 56, 5830–5839.
25. A. Martinez-Oviedo, Y. K. Kshetri, B. Joshi and S. W. Lee, Surface modification of blue TiO₂ with silane coupling agent for NO_x abatement, Prog. Nat. Sci.: Mater. Int., 2021, 31, 230–238.
26. Y. Bu, B. Wang, M. Yue, Q. Ren, L. Guo and Y. Fu, Photocatalytic NO removal based on TiO₂ photonic crystals through slow photon effect, Mater. Lett., 2023, 346, 134537.
27. M. Kamran, T. A. Kandiel, S. Abdel-Azeim, M. A. Morsy and D. W. Bahnemann, Mechanistic Insights into the High Selectivity and Photocatalytic Activity of Brookite TiO₂ toward NO_x Abatement, J. Phys. Chem. C, 2023, 127, 7707–7717.
28. Y. Xin, Q. Zhu, T. Gao, X. Li, W. Zhang, H. Wang, D. Ji, Y. Huang, M. Padervand, F. Yu and C. Wang, Photocatalytic NO removal over defective Bi/BiOBr nanoflowers: The inhibition of toxic NO₂ intermediate via high humidity, Appl. Catal., B, 2023, 324, 122238.
29. Q. Zhu, Y. Wang, J. Wang, J. Luo, J. Xu and C. Wang, Synergistic polarization and oxygen vacancies engineering for enhancing photocatalytic NO removal over Bi₄Ti₃O₁₂ nanowires, Appl. Catal., B, 2024, 346, 123734.
30. Y. Shi, Z. Yang, L. Shi, H. Li, X. Liu, X. Zhang, J. Cheng, C. Liang, S. Cao, F. Guo, X. Liu, Z. Ai and L. Zhang, Surface Boronizing Can Weaken the Excitonic Effects of BiOBr Nanosheets for Efficient O₂ Activation and Selective NO Oxidation under Visible Light Irradiation, Environ. Sci. Technol., 2022, 56, 14478–14486.
31. D. Liu, D. Chen, N. Li, Q. Xu, H. Li, J. He and J. Lu, Surface Engineering of g-C₃N₄ by Stacked BiOBr Sheets Rich in Oxygen Vacancies for Boosting Photocatalytic Performance, Angew. Chem., Int. Ed., 2020, 59, 4519–4524.
32. Y. Geng, D. Chen, N. Li, Q. Xu, H. Li, J. He and J. Lu, Z-Scheme 2D/2D α-Fe₂O₃/g-C₃N₄ heterojunction for photocatalytic oxidation of nitric oxide, Appl. Catal., B, 2021, 280, 119409.
33. Z. Chen, H. Yin, R. Wang, Y. Peng, C. You and J. Li, Efficient Electron Transfer by Plasmonic Silver in SrTiO₃ for Low-Concentration Photocatalytic NO Oxidation, Environ. Sci. Technol., 2022, 56, 3604–3612.
34. H. Shang, H. Jia, P. Li, H. Li, W. Zhang, S. Li, Q. Wang, S. Xiao, D. Wang, G. Li and D. Zhang, Highly selective and efficient photocatalytic NO removal: Charge carrier kinetics and interface molecular process, J. Nano Res., 2024, 17, 1003–1026.
35. G. Liu, Y. Huang, H. Lv, H. Wang, Y. Zeng, M. Yuan, Q. Meng and C. Wang, Confining single-atom Pd on g-C₃N₄ with carbon vacancies towards enhanced photocatalytic NO conversion, Appl. Catal., B, 2021, 284, 119683.
36. L. Hu, J. Liu, X. Huang, Q. Nie, P. Liu, Z. Tan and H. Yu, Carbon-bridged atomically dispersed platinum on MOF-derived ZnO/C for selective photocatalytic oxidation of NO into Nitrates and Nitrites, Carbon, 2023, 214, 118299.
37. R. Zhang, Y. Cao, D. E. Doronkin, M. Ma, F. Dong and Y. Zhou, Single-atom dispersed Zn-N₃ active sites bridging the interlayer of g-C₃N₄ to tune NO oxidation pathway for the inhibition of toxic by-product generation, Chem. Eng. J., 2023, 454, 140084.
38. Z. Hu, X. Li, S. Zhang, Q. Li, J. Fan, X. Qu and K. Lv, Fe₁/TiO₂ Hollow Microspheres: Fe and Ti Dual Active Sites Boosting the Photocatalytic Oxidation of NO, Small, 2020, 16, 2004583.
39. F. Guo, C. Mao, C. Liang, P. Xing, L. Yu, Y. Shi, S. Cao, F. Wang, X. Liu, Z. Ai and L. Zhang, Triangle Cl-Ag₁-Cl Sites for Superior Photocatalytic Molecular Oxygen Activation and NO Oxidation of BiOCl, Angew. Chem., Int. Ed., 2023, e202314243.
40. A. M. Żurański, J. I. Martinez Alvarado, B. J. Shields and A. G. Doyle, Predicting Reaction Yields via Supervised Learning, Acc. Chem. Res., 2021, 54, 1856–1865.
41. S. Zhong, K. Zhang, M. Bagheri, J. G. Burken, A. Gu, B. Li, X. Ma, B. L. Marrone, Z. J. Ren, J. Schrier, W. Shi, H. Tan, T. Wang, X. Wang, B. M. Wong, X. Xiao, X. Yu, J.-J. Zhu and H. Zhang, Machine Learning: New Ideas and Tools in Environmental Science and Engineering, Environ. Sci. Technol., 2021, 55, 12741–12754.
42. J. Li, X. Liu, H. Wang, Y. Sun and F. Dong, Prediction and interpretation of photocatalytic NO removal on g-C₃N₄-based catalysts using machine learning, Chin. Chem. Lett., 2024, 35, 108596.
43. B. J. Finlayson-Pitts and J. N. Pitts, Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications, 1999.
44. Y. Cao, Q. Ma, B. Chu and H. He, Homogeneous and heterogeneous photolysis of nitrate in the atmosphere: state of the science, current research needs, and future prospects, Front. Environ. Sci. Eng., 2023, 17(4), 48.
45. J. Schuttlefield, G. Rubasinghege, M. El-Maazawi, J. Bone and V. H. Grassian, Photochemistry of adsorbed nitrate, J. Am. Chem. Soc., 2008, 130, 12210.
46. X. L. Zhou, H. L. Gao, Y. He, G. Huang, S. B. Bertman, K. Civerolo and J. Schwab, Nitric acid photolysis on surfaces in low-NO_x environments: Significant atmospheric implications, Geophys. Res. Lett., 2003, 30, 2217.
47. H. Wang, Y. Sun and F. Dong, Insight into the Overlooked Photochemical Decomposition of Atmospheric Surface Nitrates Triggered by Visible Light, Angew. Chem., Int. Ed., 2022, 61, e202209201.
48. Q. W. Shi, Y. Tao, J. E. Krechmer, C. L. Heald, J. G. Murphy, J. H. Kroll and Q. Ye, Laboratory Investigation of Renoxification from the Photolysis of Inorganic Particulate Nitrate, Environ. Sci. Technol., 2021, 55, 854–861.
49. C. Ye, X. Zhou, D. Pu, J. Stutz, J. Festa, M. Spolaor, C. Tsai, C. Cantrell, R. L. Mauldin III, T. Campos, A. Weinheimer, R. S. Hornbrook, E. C. Apel, A. Guenther, L. Kaser, B. Yuan, T. Karl, J. Haggerty, S. Hall, K. Ullmann, J. N. Smith, J. Ortega and C. Knote, Rapid cycling of reactive nitrogen in the marine boundary layer, Nature, 2016, 532, 489–491.
50. H. Schwartz-Narbonne, S. H. Jones and D. J. Donaldson, Indoor Lighting Releases Gas Phase Nitrogen Oxides from Indoor Painted Surfaces, Environ. Sci. Technol. Lett., 2019, 6, 92–97.
51. F. Karagulian, C. Santschi and M. J. Rossi, The heterogeneous chemical kinetics of N₂O₅ on CaCO₃ and other atmospheric mineral dust surrogates, Atmos. Chem. Phys., 2006, 6, 1373–1388.
52. M. Ndour, P. Conchon, B. D’Anna, O. Ka and C. George, Photochemistry of mineral dust surface as a potential atmospheric renoxification process, Geophys. Res. Lett., 2009, 36, L05816.
53. Y. Bedjanian and A. El Zein, Interaction of NO₂ with TiO₂ Surface Under UV Irradiation: Products Study, J. Phys. Chem. A, 2012, 116, 1758–1764.
54. G. Rubasinghege and V. H. Grassian, Photochemistry of Adsorbed Nitrate on Aluminum Oxide Particle Surfaces, J. Phys. Chem. A, 2009, 113, 7818–7825.
55. Y. Liu, X. Wang, J. Shang, W. Xu, M. Sheng and C. Ye, The positive effect of formaldehyde on the photocatalytic renoxitication of nitrate on TiO₂ particles, Atmos. Chem. Phys., 2022, 22, 11347–11358.
56. M. S. Rosseler, V. Nahuel Montesinos, A. Shavorskiy, V. Keller, N. Keller, M. I. Litter, H. Bluhm, M. Salmeron and H. Destaillats, Chemistry of NO_x on TiO₂ Surfaces Studied by Ambient Pressure XPS: Products, Effect of UV Irradiation, Water, and Coadsorbed K⁺, J. Phys. Chem. Lett., 2013, 4, 536–541.
57. Q. Ma, C. Zhong, J. Ma, C. Ye, Y. Zhao, Y. Liu, P. Zhang, T. Chen, C. Liu, B. Chu and H. He, Comprehensive Study about the Photolysis of Nitrates on Mineral Oxides, Environ. Sci. Technol., 2021, 55, 8604–8612.
58. H. Shang, Z. Chen, X. Wang, M. Li, H. Li, C. Mao, L. Yu, J. Sun, Z. Ai and L. Zhang, SO₂-enhanced nitrate photolysis on TiO₂ minerals: A vital role of photochemically reactive holes, Appl. Catal., B, 2022, 308, 121217.
59. H. Wang, K. Li, J. Li, Y. Sun and F. Dong, Photochemical Transformation Pathways of Nitrates from Photocatalytic NO_x Oxidation: Implications for Controlling Secondary Pollutants, Environ. Sci. Technol. Lett., 2021, 8, 873–877.
60. M. S. Gen, R. F. Zhang, D. D. Huang, Y. J. Li and C. K. Chan, Heterogeneous Oxidation of SO₂ in Sulfate Production during Nitrate Photolysis at 300 nm: Effect of pH, Relative Humidity, Irradiation Intensity, and the Presence of Organic Compounds, Environ. Sci. Technol., 2019, 53, 8757–8766.
61. J. N. Pitts, A. M. Winer, R. Atkinson and W. P. Carter, Comment on “Effect of nitrogen oxide emissions on ozone levels in metropolitan regions”, “Effect of nitrogen oxide (NO_x) emission rates on smog formation in the California South Coast Air Basin”, and “Effect of hydrocarbon and nitrogen oxide (NO_x) on photochemical smog formation under simulated transport conditions, Environ. Sci. Technol., 1983, 17, 54–57.
62. Y. Qiu, Z. Ma, K. Li, W. Lin, Y. Tang, F. Dong and H. Liao, Markedly Enhanced Levels of Peroxyacetyl Nitrate (PAN) During COVID-19 in Beijing, Geophys. Res. Lett., 2020, 47, e2020GL089623.
63. C. Li, Q. Zhu, X. Jin and R. C. Cohen, Elucidating Contributions of Anthropogenic Volatile Organic Compounds and Particulate Matter to Ozone Trends over China, Environ. Sci. Technol., 2022, 12906–12916.
64. A. Mellouki, T. J. Wallington and J. Chen, Atmospheric Chemistry of Oxygenated Volatile Organic Compounds: Impacts on Air Quality and Climate, Chem. Rev., 2015, 115, 3984–4014.
65. Z. Meng, D. Dabdub and J. H. Seinfeld, Chemical coupling between atmospheric ozone and particulate matter, Science, 1997, 277, 116–119.
66. Z. Chen, H. Yin, C. Wang, R. Wang, Y. Peng, C. You and J. Li, New Insights on Competitive Adsorption of NO/SO₂ on TiO₂ Anatase for Photocatalytic NO Oxidation, Environ. Sci. Technol., 2021, 55, 9285–9292.
67. T. Xue, L. Chen, K. Li, B. Lei, H. Wang, F. Dong and Y. Yang, Highly Enhanced Photocatalytic NO Removal and Inhibited Peroxyacetyl Nitrate Formation in Synergistic Acetaldehyde Degradation, Environ. Sci. Technol., 2023, 57, 8174–8182.
68. K. Li, H. Wang, L. Chen, J. Li and F. Dong, Synergistic degradation of NO and C₇H₈ for inhibition of O₃ generation, Appl. Catal., B, 2022, 312, 121423.
69. K. Li, T. Xue, L. Chen, J. Li, F. Dong and Y. Sun, Dual function of H₂O on interfacial intermediate conversion and surface poisoning regulation in simultaneous photodegradation of NO and toluene, Environ. Res., 2024, 240, 117526.
70. X. Wang, A. Mahmood, G. Lu, X. Xie and J. Sun, Synergistic photocatalytic treatment of aromatic hydrocarbons/NO_x mixtures over TiO₂: The activation and replenishment of lattice oxygen, Chem. Eng. J., 2022, 450, 138168.
71. C. H. Ao, S. C. Lee, S. C. Zou and C. L. Mak, Inhibition effect of SO₂ on NO_x and VOC_s during the photodegradation of synchronous indoor air pollutants at parts per billion (ppb) level by TiO₂, Appl. Catal., B, 2004, 49, 187–193.
72. M. Anpo, S. G. Zhang, H. Mishima, M. Matsuoka and H. Yamashita, Design of photocatalysts encapsulated within the zeolite framework and cavities for the decomposition of NO into N₂ and O₂ at normal temperature, Catal. Today, 1997, 39, 159–168.
73. J. L. Zhang, Y. Hu, M. Matsuoka, H. Yamashita, M. Minagawa, H. Hidaka and M. Anpo, Relationship between the local structures of titanium oxide photocatalysts and their reactivities in the decomposition of NO, J. Phys. Chem. B, 2001, 105, 8395–8398.
74. M. Anpo, M. Matsuoka, H. Yamashita, W. S. Ju, S. E. Park and Y. G. Shul, Photocatalytic decomposition of NO on transition metal ion-exchanged zeolite catalysts, J. Ind. Eng. Chem., 2000, 6, 133–143.
75. Y. Hu, S. Higashimoto, G. Martra, J. L. Zhang, M. Matsuoka, S. Coluccia and M. Anpo, Local structures of active sites on Ti-MCM-41 and their photocatalytic reactivity for the decomposition of NO, Catal. Lett., 2003, 90, 161–163.
76. Y. Hu, G. Martra, J. L. Zhang, S. Higashimoto, S. Coluccia and M. Anpo, Characterization of the local structures of Ti-MCM-41 and their photocatalytic reactivity for the decomposition of NO into N₂ and O₂, J. Phys. Chem. B, 2006, 110, 1680–1685.
77. P. Y. Dong, N. Xu, Y. Xu and X. F. Wang, A study of Pt/WO₃-carrier catalysts for photocatalytic purification of NO gas, Catal. Commun., 2016, 84, 142–146.
78. N. Bowering, G. S. Walker and P. G. Harrison, Photocatalytic decomposition and reduction reactions of nitric oxide over Degussa P25, Appl. Catal., B, 2006, 62, 208–216.
79. M. Anpo, M. Takeuchi, K. Ikeue and S. Dohshi, Design and development of titanium oxide photocatalysts operating under visible and UV light irradiation.: The applications of metal ion-implantation techniques to semiconducting TiO₂ and Ti/zeolite catalysts, Curr. Opin. Solid State Mater. Sci., 2002, 6, 381–388.
80. M. Anpo and M. Takeuchi, Design and development of second-generation titanium oxide photocatalysts to better our environment-approaches in realizing the use of visible light, Int. J. Photoenergy, 2001, 3, 90–94.
81. M. Xu, Y. Wang, J. Geng and D. Jing, Photodecomposition of NO_x on Ag/TiO₂ composite catalysts in a gas phase reactor, Chem. Eng. J., 2017, 307, 181–188.
82. Q. Wu and R. van de Krol, Selective Photoreduction of Nitric Oxide to Nitrogen by Nanostructured TiO₂ Photocatalysts: Role of Oxygen Vacancies and Iron Dopant, J. Am. Chem. Soc., 2012, 134, 9369–9375.
83. M. Anpo, T.-H. Kim and M. Matsuoka, The design of Ti-, V-, Cr-oxide single-site catalysts within zeolite frameworks and their photocatalytic reactivity for the decomposition of undesirable molecules—The role of their excited states and reaction mechanisms, Catal. Today, 2009, 142, 114–124.
84. R. Hailili, Z. Q. Wang, H. W. Ji, C. C. Chen, X. Q. Gong, H. Sheng and J. C. Zhao, Mechanistic insights into the photocatalytic reduction of nitric oxide to nitrogen on oxygen-deficient quasi-two-dimensional bismuth-based perovskites, Environ. Sci.: Nano, 2022, 9, 1453–1465.
85. L. Han, S. Cai, M. Gao, J.-Y. Hasegawa, P. Wang, J. Zhang, L. Shi and D. Zhang, Selective Catalytic Reduction of NO_x with NH₃ by Using Novel Catalysts: State of the Art and Future Prospects, Chem. Rev., 2019, 119, 10916–10976.
86. D. Wang, Q. Chen, X. Zhang, C. Gao, B. Wang, X. Huang, Y. Peng, J. Li, C. Lu and J. Crittenden, Multipollutant Control (MPC) of Flue Gas from Stationary Sources Using SCR Technology: A Critical Review, Environ. Sci. Technol., 2021, 55, 2743–2766.
87. X. Li, X. Yan, S. Zuo, X. Lu, S. Luo, Z. Li, C. Yao and C. Ni, Construction of LaFe_1−xMn_xO₃/attapulgite nanocomposite for photo-SCR of NO_x at low temperature, Chem. Eng. J., 2017, 320, 211–221.
88. L. Ma, J. Li, Y. Cheng, C. K. Lambert and L. Fu, Propene Poisoning on Three Typical Fe-zeolites for SCR of NO_x with NH₃: From Mechanism Study to Coating Modified Architecture, Environ. Sci. Technol., 2012, 46, 1747–1754.
89. K. Teramura, T. Tanaka, S. Yamazoe, K. Arakaki and T. Funabiki, Kinetic study of photo-SCR with NH₃ over TiO₂, Appl. Catal., B, 2004, 53, 29–36.
90. T. Tanaka, K. Teramura, T. Yamamoto, S. Takenaka, S. Yoshida and T. Funabiki, TiO₂/SiO₂ photocatalysts at low levels of loading: preparation, structure and photocatalysis, J. Photochem. Photobiol., A, 2002, 148, 277–281.
91. Y.-C. Chou and Y. Ku, Preparation of high-aspect-ratio TiO₂ nanotube arrays for the photocatalytic reduction of NO in air streams, Chem. Eng. J., 2013, 225, 734–743.
92. R. Yuan, M. Wang, L. Liao, W. Hu, Z. Liu, Z. Liu, L. Guo, K. Li, Y. Cui, F. Lin, F. Tao and W. Zhou, 100% N₂O inhibition in photocatalytic NO_x reduction by carbon particles over Bi₂WO₆/TiO₂ Z-scheme heterojunctions, Chem. Eng. J., 2023, 453, 139892.
93. Y. Ren, Q. Han, J. Yang, Y. Zhao, Y. Xie, H. Wen and Z. Jiang, A promising catalytic solution of NO reduction by CO using g-C₃N₄/TiO₂: A DFT study, J. Colloid Interface Sci., 2022, 610, 152–163.
94. J. Lasek, Y.-H. Yu and J. C. S. Wu, Water and temperature effects on photo-selective catalytic reduction of nitric oxide on Pd-loaded TiO₂ photocatalyst, Environ. Technol., 2012, 33, 2133–2141.
95. R. Jin, Z. Wu, Y. Liu, B. Jiang and H. Wang, Photocatalytic reduction of NO with NH₃ using Si-doped TiO₂ prepared by hydrothermal method, J. Hazard. Mater., 2009, 161, 42–48.
96. S. Yamazoe, T. Okumura, K. Teramura and T. Tanaka, Development of the efficient TiO₂ photocatalyst in photoassisted selective catalytic reduction of NO with NH₃, Catal. Today, 2006, 111, 266–270.
97. S. Yamazoe, Y. Masutani, T. Shishido and T. Tanaka, Metal oxide promoted TiO₂ catalysts for photo-assisted selective catalytic reduction of NO with NH₃, Res. Chem. Intermed., 2008, 34, 487–494.
98. T. Tanaka, K. Teramura, K. Arakaki and T. Funabiki, Photoassisted NO reduction with NH₃ over TiO₂ photocatalyst, Chem. Commun., 2002, 2742–2743.
99. J. Papp, S. Soled, K. Dwight and A. Wold, Surface Acidity and Photocatalytic Activity of TiO₂, WO₃/TiO₂, and MoO₃/TiO₂ Photocatalysts, Chem. Mater., 1994, 6, 496–500.
100. S. Yamazoe, Y. Masutani, K. Teramura, Y. Hitomi, T. Shishido and T. Tanaka, Promotion effect of tungsten oxide on photo-assisted selective catalytic reduction of NO with NH₃ over TiO₂, Appl. Catal., B, 2008, 83, 123–130.
101. S. Yamazoe, Y. Masutani, T. Shishido and T. Tanaka, XAFS study of active tungsten species on WO₃/TiO₂ as a catalyst for photo-SCR, EXAFS Spectrosc.: Tech. Appl., Proc. Symp. Appl. EXAFS Mater. Sci., 2007, 696.
102. A. Yamamoto, K. Teramura and T. Tanaka, Selective Catalytic Reduction of NO by NH₃ over Photocatalysts (Photo-SCR): Mechanistic Investigations and Developments, Chem. Rec., 2016, 16, 2268–2277.
103. Y. Ji, B. Ding, W. Ni, X. Li, X. He, Z. Chen, S. Ran and H. Lü, Tailoring the crystal structure of CaTiO₃ by multielement doping for photo-assisted activation of NO, Chem. Eng. J., 2022, 450, 138255.
104. K. Wei, X. Yan, S. Zuo, W. Zhu, F. Wu, X. Li, C. Yao and X. Liu, Photo-assisted Catalytic Removal of NO_x Over La_1–xPr_xCoO₃/Palygorskite Nanocomposites: Role of Pr Doping, Clays Clay Miner., 2019, 67, 348–356.
105. X. Li, H. Shi, W. Zhu, S. Zuo, X. Lu, S. Luo, Z. Li, C. Yao and Y. Chen, Nanocomposite LaFe_1-xNi_xO₃/Palygorskite catalyst for photo-assisted reduction of NOx: Effect of Ni doping, Appl. Catal., B, 2018, 231, 92–100.
106. X. Li, H. Shi, X. Yan, S. Zuo, Y. Zhang, T. Wang, S. Luo, C. Yao and C. Ni, Palygorskite Immobilized Direct Z-Scheme Nitrogen-Doped Carbon Quantum dots/PrFeO₃ for Photo-SCR Removal of NO_x, ACS Sustain. Chem. Eng., 2018, 6, 10616–10627.
107. L. Chen, D. Shen, B. Li, Z. Xiao, W. Sun, X. Liu, J. Ma, C. Li and W. Wang, The efficient electrocatalytic and photocatalytic reduction of nitric oxide into ammonia over 0D/3D g-C₃N₄ quantum dots/3DOMM-TiO_2-x heterojunction, Ceram. Int., 2023, 49, 23129–23139.
108. L. Ju, X. Tang, Y. Zhang, X. Li, X. Cui and G. Yang, Single Selenium Atomic Vacancy Enabled Efficient Visible-Light-Response Photocatalytic NO Reduction to NH₃ on Janus WSSe Monolayer, Molecules, 2023, 28(7), 2959.
109. D. Yao, C. Tang, P. Wang, H. Cheng, H. Jin, L.-X. Ding and S.-Z. Qiao, Electrocatalytic green ammonia production beyond ambient aqueous nitrogen reduction, Chem. Eng. Sci., 2022, 257, 117735.
110. I. G. Zacharia and W. M. Deen, Diffusivity and Solubility of Nitric Oxide in Water and Saline, Ann. Biomed. Eng., 2005, 33, 214–222.
111. J. Li, J. Wang, S. Shen, R. Chen, M. Liu and F. Dong, Beyond Purification: Highly Efficient and Selective Conversion of NO into Ammonia by Coupling Continuous Absorption and Photoreduction under Ambient Conditions, Environ. Sci. Technol., 2023, 57, 5445–5452.
112. W. Chen, R. Zou and X. Wang, Toward an Atomic-Level Understanding of the Catalytic Mechanism of Selective Catalytic Reduction of NO_x with NH₃, ACS Catal., 2022, 12, 14347–14375.
113. Z. Si, Y. Shen, J. He, T. Yan, J. Zhang, J. Deng and D. Zhang, SO₂-Induced Alkali Resistance of FeVO₄/TiO₂ Catalysts for NO_x Reduction, Environ. Sci. Technol., 2022, 56, 605–613.
114. R. Chen, J. Li, J. Wang, W. Yang, S. Shen and F. Dong, Continuous NO Upcycling into Ammonia Promoted by SO₂ in Flue Gas: Poison Can Be a Gift, Environ. Sci. Technol., 2023, 57, 12127–12134.
115. W. Yang, J. Wang, R. Chen, L. Xiao, S. Shen, J. Li and F. Dong, Reaction mechanism and selectivity regulation of photocatalytic nitrate reduction for wastewater purification: progress and challenges, J. Mater. Chem. A, 2022, 10, 17357–17376.
116. H. Li, H. Zhu, Y. Shi, H. Shang, L. Zhang and J. Wang, Vacancy-Rich and Porous NiFe-Layered Double Hydroxide Ultrathin Nanosheets for Efficient Photocatalytic NO Oxidation and Storage, Environ. Sci. Technol., 2022, 56, 1771–1779.
117. H. Hirakawa, M. Hashimoto, Y. Shiraishi and T. Hirai, Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts, ACS Catal., 2017, 7, 3713–3720.
118. H. S. Moon, B. Song, J. Jeon, T.-H. Lai, Y.-P. Chang, Y.-D. Lin, J. K. Park, Y.-G. Lin, Y.-J. Hsu, H. Shin, Y. Yun and K. Yong, Atomically isolated copper on titanium dioxide for ammonia photosynthesis via nitrate reduction with unprecedently high apparent quantum yield, Appl. Catal., B, 2023, 339, 123185.
119. J. Li, R. Chen, J. Wang, K. Wang, Y. Zhou, M. Xing and F. Dong, Dynamic in situ Formation of Cu₂O Sub-Nanoclusters through Photoinduced pseudo-Fehling’s Reaction for Selective and Efficient Nitrate-to-Ammonia Photosynthesis, Angew. Chem., Int. Ed., 2024, 63, e202317575.
120. R. Chen, S. Shen, K. Wang, J. Wang, W. Yang, X. Li, J. Li and F. Dong, Promoting the efficiency and selectivity of NO₃^--to-NH₃ reduction on Cu-O-Ti active sites via preferential glycol oxidation with holes, Proc. Natl. Acad. Sci. U.S.A., 2023, 120, e2312550120.
121. J. Li, R. Chen, J. Wang, Y. Zhou, G. Yang and F. Dong, Subnanometric alkaline-earth oxide clusters for sustainable nitrate to ammonia photosynthesis, Nat. Commun., 2022, 13, 1098.
122. A. Sanchez, Z. Ye, Y. Yin and H. Liu, Photochemical conversion of nitrate to ammonium ions by a newly developed photo-reductive titanium dioxide catalyst: implications on nitrogen recovery, Environ. Sci.: Water Res. Technol., 2023, 9, 3318–3324.
123. L. Wei, Y. Zhang, C. Zhang, C. Yao, C. Ni and X. Li, In Situ Growth of Perovskite on 2D Hydrothermal Carbonation Carbon for Photocatalytic Reduction of Nitrate to Ammonia, ACS Appl. Nano Mater., 2023, 6, 13127–13136.
124. W. Liu, X. Li, X. Chu, S. Zuo, B. Gao, C. Yao, Z. Li and Y. Chen, Boosting photocatalytic reduction of nitrate to ammonia enabled by perovskite/biochar nanocomposites with oxygen defects and O-containing functional groups, Chemosphere, 2022, 294, 133763.
125. Y. Wang, H. Yin, X. Zhao, Y. Qu, A. Zheng, H. Zhou, W. Fang and J. Li, Photocatalytic ammonia synthesis from nitrate reduction on nickel single-atom decorated on defective tungsten oxide, Appl. Catal., B, 2024, 341, 123266.
126. D. Hao, Y. Wei, L. Mao, X. Bai, Y. Liu, B. Xu, W. Wei and B.-J. Ni, Boosted selective catalytic nitrate reduction to ammonia on carbon/bismuth/bismuth oxide photocatalysts, J. Cleaner Prod., 2022, 331, 129975.
127. R. Kato, M. Furukawa, I. Tateishi, H. Katsumata and S. Kaneco, Novel Photocatalytic NH₃ Synthesis by NO₃^- Reduction over CuAg/TiO₂, Chem. Eng., 2019, 3(2), 49.
128. L. Wei, M. A. S. Adamson and J. Vela, Ni₂P-Modified Ta₃N₅ and TaON for Photocatalytic Nitrate Reduction, ChemNanoMat, 2020, 6, 1179–1185.
129. N. Tong, Y. Wang, Y. Liu, M. Li, Z. Zhang, H. Huang, T. Sun, J. Yang, F. Li and X. Wang, PdSn/NiO/NaTaO₃: La for photocatalytic ammonia synthesis by reduction of NO₃^- with formic acid in aqueous solution, J. Catal., 2018, 361, 303–312.
130. M. Yamauchi, R. Abe, T. Tsukuda, K. Kato and M. Takata, Highly Selective Ammonia Synthesis from Nitrate with Photocatalytically Generated Hydrogen on CuPd/TiO₂, J. Am. Chem. Soc., 2011, 133, 1150–1152.

---