Recent Progress and Current Status of Photocatalytic NO Removal

Abstract

Air pollution has become a globally prominent environmental problem in which nitrogen oxide (NOx, 95% NO and NO2) has been considered as one of the most serious harmful gaseous pollutants that can cause haze, photochemical smog, and acid rain. Exposure to NO (∼ppb) harms human health with a risk of respiratory and cardiopulmonary diseases. As such, much attention is focused on the throughout removal, effective control, and precise monitoring of NO, especially for those of NO with low concentration (∼ppb). Semiconductor-based photocatalysis is a practical approach for pollutant treatments, especially for low concentrations but highly toxic ones, for example, NO (∼ppb) removal in indoor and outdoor atmospheres. This work aims to introduce the main process, methods and summarize the critical scientific issues during the photocatalytic NO treatment and review the latest progress in semiconducting materials. This work also surveys the newly emerged photocatalysts such as metal oxides, Bi-based semiconductors, including Bi2O2CO3, BiOX (X = Cl, Br, and I), Bi-metal-based defective photocatalysts, and other Bi-based catalysts with well-defined surface/interface characters for the complete NO removal, specific conversion mechanisms and controlling the generation of the toxic intermediate (NO2) is highlighted. The challenges/bottlenecks of the practical applications in the field are also highlighted at the end.

1. Introduction

According to the World Health Organization (WHO) health report in 2021, air pollution caused about 7 million deaths worldwide, of which 4.2 million die prematurely every year by outdoor pollution, and 3.8 million by indoor pollution (Figure 1a) [1]. Air pollutants also affect local plants and animals’ survival and cause environmental issues. Nitrogen oxides (NO_x, mainly NO + NO₂), primarily emitted from power stations, factories, and automobiles are regarded as the major source of atmospheric contaminations, which remarkably influence the tropospheric chemistry and become the main cause of the greenhouse effects, acid rain, photochemical smog, and PM_2.5 [2]. Generally, with an unpaired electron {(σ2s)²(σ^*2s)²(σ2px)²(π2py)²(π2pz)²(π^*2py)¹}, NO is chemically active and reacts with oxygen readily to generate NO₂. However, the reaction activity significantly descends when the concentration of NO is lower than the ppm level and it can exist stably in the air for a long time. Long-term exposure of NO even at such a low level still causes serious respiratory diseases, such as lung cancer, emphysema, and asthma. Moreover, the concentration of NO in the indoor environment is up to 200 ppb, because of cooking or smoking activities. Concerning such severe harms, strict legislation and policies have been enacted to control the emissions of NO in many countries. For instance, in 2004, the USA claimed the maximum emission rate of NO is required not to exceed 553.5 mg m⁻³ for tangentially fired boilers [3]. The limit of NO from light-duty vehicles is set at 35 mg km⁻¹ in China (GB 18352.6-2016) [4]. According to the WHO guideline, the lowest NO_x concentration emission threshold under the current was set at 40 μg m⁻³ [5]. Thus, it is urgent to develop environmentally friendly, effective methods and evolution of technical measurements that could control emissions, decrease the concentrations, and reduce the harm of nitrogen oxides, for example, NO at the level of hundreds of ppb in indoor circumstances. Conventionally, methods such as physical/chemical adsorption, advanced wet oxidation, and post-combustion reduction technologies such as selective catalytic reduction with NH₃ (SCR-NH₃) and hydrocarbons (SCR-HC), are used for minimizing the NO emission [6]. The chemical adsorption method refers to water or aqueous solutions of acids, bases, and salts to absorb the nitrogen oxides in the exhaust gas so that the exhaust gas can be purified. This method has low investment in equipment and low operating costs. However, the absorption efficiency is not high, and the purification effect is poor for the exhaust gas containing more NO, and it is not suitable for treating the exhaust gas with large volumes. SCR is also regarded as an efficient method to reduce the values of NO. For instance, Dong et al. reported the “anchoring (NH₃/He plasma)-reduction (O₂/N₂ plasma)” plasma-catalytic circular system for the NO reduction at room temperature in the presence of O₂ and revealed that NO was firstly oxidized to NO₂ by plasma, then NO₂ experienced part-disproportionation reactions during adsorption on oxygen vacancies, producing NO, N₂O, N_2, and nitrate [7]. However, these approaches require high temperatures, special handling systems, and sophisticated equipment to avoid NH₃ slip, reducing agents, or cocatalysts, meanwhile suffering from high costs and yields of more toxic byproducts. Moreover, the above techniques are no longer suitable for the complete removal of dilute air pollutants, for example, gaseous NO at the ppb level. Utilizing endless solar light as the driving force, semiconductor-based photocatalysis has received considerable attention to remove the atmospheric gaseous hazards from the atmosphere, especially for the ones in low concentration but highly toxic, for example, NO (∼ppb), in an economically attractive and environmentally friendly manner [8, 9, 10]. In view of the current state of NO removal, photocatalytic oxidation is the most studied way to reduce NO concentration from the air. As shown in Figure 1b, the number of papers on photocatalytic NO removal and their citations has rapidly increased in the past 13 years.

Figure 1.

(a) WHO 2021 report on the correlations between mortality and environmental pollution; (b) numbers and citations of publications about photocatalytic NO removal since 2010.

2. Methods, basic principles, and main scientific issues for photocatalytic NO removal

There are three commonly accepted methods for the removal of NO_x by photocatalysis: photodecomposition, photo-selective catalytic reduction (Photo-SCR), and photooxidation. Both direct and photo-SCR decomposition can be classified as the typical reduction pathways, while NO photooxidation belongs to the photoinduced oxidation methods. Through these approaches, the concentrations of NO in the atmosphere can successfully be decreased or will be completely removed by their conversion into the other N, O contained species such as nontoxic harmless N₂, O₂, or intermediates of NO₂, N₂O₄, or even the deeper oxidation products of HNO₃. Evidently, direct NO decomposition into nontoxic and harmless products of stoichiometric N₂ and O₂ offers one of the most ideal routes for the NO treatment, in which no additional reductants are required, and the side reactions are hence minimized. Although this reduction process is exothermic and thermodynamically favored, several kinetically sluggish steps such as the breakage of N〓O bonds (153.3 kJ mol⁻¹) and subsequent reconstruction of structurally stable N☰N triple bonds (940.95 kJ mol⁻¹) are involved, both of which require to overcome the huge energetic barriers [8, 11]. The Photo-SCR process occurs on a photocatalyst surface and involves the reduction of NO_x in the presence of reducing agents such as NH₃ and hydrocarbons under light irradiation. However, the trigger of this multi-step reaction requires a temperature higher than 500°C, and undesired compounds intermediate byproducts (e.g., N₂O) are released instead of the stoichiometric formation of harmless N₂ and O₂, affecting the reaction efficiency as well as selectivity [12]. Compared with traditional NO removal methods, more selective and nonselective products, such as nitric acid or nitrates, are generated via NO photooxidation and thus reduce its harm. However, these oxidative species will block the surface-active sites of photocatalysts, restrict further redox reactions and consequently reduce the reaction efficiency. Hence, these species are required to be removed rapidly from the surface of the photocatalyst to avoid catalysts deactivation. The main methods/ aims, advantages, and disadvantages of photocatalytic NO removal are summarized in Figure 2. Obviously, the photocatalytic oxidation of dilute NO to NO₃⁻ with well-defined photocatalysts is preferred as the ideal pathway that is low-cost, environmentally friendly, and suitable for the removal of low concentrations of NO.

Figure 2.

A summary of advantages and disadvantages of photocatalytic NO removal.

The main principles of photocatalytic NO removal as followings: (i) formations of electron-hole pairs: when the photocatalyst is irradiated with photons with the same or higher energy than its band gap, the electrons are excited from the valence band to the conduction band of semiconductor, and leave same amounts of holes in the VB, hence, resulting in the formations of electron-hole pairs; (ii) charge carrier separation and migration: the photoinduced electrons and holes will migrate to the surface, where photocatalytic redox reaction will further be initiated; partial electron and holes migrate at the bulk or surface of the photocatalysts, while some of them will lead to the emission of light or heat; (iii) surface redox reaction: the photogenerated electrons are trapped by surface adsorbed molecular oxygen to generate various reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂), single line state oxygen (¹O₂), superoxide radicals (·O₂⁻), and hydroxyl radicals (·OH), which react with NO through different pathways to generate oxidation species such as NO⁺, N₂O₄, NO₂⁻, or be deeply oxidized into NO₃⁻; Under oxygen free system, the photoinduced electrons would generate assorted intermediates, such as N₂O^* or ^*NO₂., and subsequently converted into completely harmless N₂ or O₂, and reduce the concentrations of NO (Figure 3a). Among them, ·O₂⁻ can completely convert NO to the final nitrate (NO + ·O₂⁻ → NO₃⁻), while ¹O₂ will oxidize NO to unwanted NO₂ (NO +¹O₂ → NO₂). On the other hand, HNO₂ is the main oxidation species at the initial state and is closely adhered to the surfaces of the catalysts. Note that the main intermediate NO₂ possesses nearly 9-fold higher toxicity than that of dilute gaseous NO and should be minimized or restricted during the photocatalytic NO conversion [13]. It was reported that the adsorbed HNO₂ will further dissociate to NO₂⁻ with further light illuminations, and subsequently oxidized into NO₃⁻, which can be washed away from the catalyst’s surfaces (Figure 3b). Unfortunately, the product nitrate does not desorb and adheres to the catalyst’s surface and occupies the active surface sites of the catalysts to reduce its activity, resulting in massive deactivation of the catalyst’s powder, and subsequently blocking further conversion. Figure 3c displays the main reaction setup for photocatalytic NO removal, in which the catalysts powders were exposed to light, and the effluent NO and NO_x (NO + NO₂) concentrations were continuously recorded using the online chemiluminescence NO_x analyzer. The concentration changes of the intermediate NO₂ were simultaneously calculated from concentration gaps between NO and NO_x during the tests. The final conversion products are analyzed by online separation methods, for example, NO₃⁻ can be detected by ionic chromatography, while the decomposition products of N₂ and O₂ are detected by gas chromatography analyses. Generally, the NO removal process includes the initial state, transition state, and final step, during which the conversion products are closely related to the reaction conditions such as continuous reactor, NO concentration, flow rate, temperature, water (relative humidity), catalysts loading, reactor sizes, air flow and light parameters (wavelength or intensity), detected conversion products and so on [14]. With tuning the above reaction conditions, the main conversion products of NO can be controlled, and reverse reactions can be prevented to enhance reaction efficiencies. Given upon assorted conversion products during the photocatalytic NO removal, the reaction displays certain selectivity toward specific products such as selectivity for NO₃⁻, N₂, or NO₂. For the typical photocatalytic NO conversions, the corresponding removal efficiency, NO_x conversions (%), N₂ and NO₂ selectivity are calculated by the following equation [15, 16, 17]:

Figure 3.

(a, c) Schematic diagram of the photocatalytic NO conversion process, (b) reaction setup, and (d) in situ DRIFTS tests for photocatalytic removal of NO under room temperature.

$$\mathrm{NO}\text{conversion}\left({\mathrm{\eta }}_{\mathrm{NO}},\%\right)=\left(1-\frac{{\left[\mathrm{NO}\right]}_{\text{in}}}{{\left[\mathrm{NO}\right]}_{\text{out}}}\right)\times 100\%$$E1

$${\mathrm{NO}}_{\mathrm{x}}\text{conversion}\left({\mathrm{\eta }}_{\mathrm{NOx}},\%\right)=\left(1-\frac{{\left[\mathrm{NOX}\right]}_{\text{out}}}{{\left[\mathrm{NOX}\right]}_{\text{in}}}\right)\times 100\%$$E2

$${\mathrm{N}}_2\text{selectivity}\left({\mathrm{S}}_{{\mathrm{N}}_2},\%\right)=\frac{{\left[{\mathrm{N}}_2\right]}_{\text{out}}}{{\left[{\mathrm{N}}_2\right]}_{\text{out}}-{\left[{\mathrm{NO}}_2\right]}_{\text{out}}}\times 100\%$$E3

$${\mathrm{NO}}_2\text{selectivity}\left({\mathrm{S}}_{{\mathrm{NO}}_2},\%\right)=\frac{{\left[{\mathrm{NO}}_2\right]}_{\text{out}}}{{\left[\mathrm{NO}\right]}_{\text{in}}-{\left[\mathrm{NO}\right]}_{\text{out}}}\times 100\%$$E4

where [NO]_in, [NO]_out is the initial and final concentrations of NO, while [NO_x]_in, [NO]_out and [NO₂]_out represent the initial and final concentrations of NO_x and NO₂ in ppb level, respectively.

The main NO oxidation products NO₂, NO₃⁻, N₂O, and N₂O₄ etc., are strongly dependent on the surface characters of semiconducting materials and the assorted NO oxidation products can be obtained. The time-dependent generation of intermediates and products of NO on the surface of catalysts powders are monitored by the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements (Figure 3d).

3. Latest progress in photocatalytic NO removal

Many successful state-of-the-art photocatalysts have been explored for NO removal since the photocatalytic process can be conducted under ambient conditions without the addition of extra redox reagents, which is specifically applicable to indoor circumstances. It can be found that great progress has actively been pursued in many research groups and successful state-of-the-art catalysts, for example, TiO₂ and TiO₂-based oxides, Bi-based compounds BiOX (X=Cl, Br, and I), metal-free catalysts, for example, g-C₃N₄ and some plasmonic metals (Au, Ag, and Bi), etc., have been developed for photocatalytic NO removal. These informative and fundamentally important studies provided encouraging results for commercial applications of photocatalysts for air cleaning and NO removal. However, the photoconversion efficiency remains low (apparent quantum yield 2.5%) due to hard control energy band gaps causing inevitable charge recombination and most of these reactions involved releasing more toxic gas NO₂ or the photocatalysts have been suffering from deactivation. In view of the current state of photocatalytic NO removal, the development of effective and green NO control technologies is of great significance to controlling air quality. It can be seen from the state-of-the-art that the most widely studied semiconductor photocatalysts for photocatalytic oxidation and reduction to remove low-concentration NO are mainly TiO₂ and metal-based materials, bismuth-based materials, graphite phase carbon nitride (g-C₃N₄)-based materials and other heterojunction systems. We have made a specific summary of these photocatalysts used for the removal of low-concentration NO at room temperature.

3.1 TiO₂ and other metal oxide-based materials

To remove dilute NO from the air by transforming NO into HNO₃ to form HNO₂ and NO₂ species upon light illumination, the previous investigations focused on the famous star catalyst TiO₂ and TiO₂-based photocatalysts. Wang et al. proposed the additional reactions, in which NO conversions over TiO₂ by the process of charge carrier generation, the trapping of a hole and an electron to produce “active” hydroxyl and oxygen radicals, and further oxidized to final product by NO → HNO₂ → NO₂ → HNO₃ pathway [18]. Following this, Hunger et al. investigated kinetic models for photo-removal of NO_x over TiO₂ under UV light and proposed that the kinetic parameters of NO oxidation were determined by the concentration of NO and NO₂, flow rate, relative humidity, and light parameters [19]. As evidenced, the NO conversion products over TiO₂ hugely influence the reaction selectivity [20]. In this regard, the blue TiO₂ with highly abundant oxygen deficiency was investigated and displayed the highest selectivity of 99% under visible-light irradiation for the photocatalytic NO oxidation to nitrate without NO₂ yields. It was claimed that oxygen defects could not only activate molecule O₂ to generate ·O₂⁻ that facilitated the selective NO transformation toward nitrate under ambient conditions but also could annihilate the photogenerated holes to further inhibit the byproduct NO₂ formation [16]. The elusive NO₃⁻ conversion mechanism over P25 is revealed by Dong et al., and it was revealed that the N▬O bond in surface NO₃⁻ could be activated by NO molecules, arising from the significant overlap of the 2p orbitals between the N in NO and the O in NO₃⁻. Then, photogenerated electrons (e⁻) captured by NO drive the transformation of NO₃⁻ under light irradiation via the NO₃⁻ + NO⁻ → 2NO₂⁻ route. Additionally, although photogenerated holes (h⁺) and ·OH radicals could oxidize NO into NO₃⁻, the rate of production of NO₃⁻ is much slower than that of photochemical transformation by NO⁻. Hence, the photochemical transformation of surface NO₃⁻ can be solved only by preventing the formation of NO⁻ during photocatalytic NO oxidation (Figure 4) [21]. The nitrate photolysis on photoactive TiO₂ particles in the presence of SO₂ was investigated through density functional theory (DFT) simulation and in situ DRIFTS analysis. It was found that the nitrate was oxidized to ^*NO₃ radicals by the holes generated on the surface of TiO₂, followed by reactive nitrogen species generation via NO₃ radicals’ reduction by the photoinduced electrons. It was indicated that photogenerated h⁺ plays a key role in nitrate photolysis on photoactive mineral dusts with or without the coexistence of SO₂, providing new insights into the source of NO_x and HONO in complex air-polluted areas during the daytime [22].

Figure 4.

Experimental and theoretical investigations of surface NO₃⁻ reduction by NO and e⁻ with light illumination on the surfaces of P25. Reproduced with permission from Ref. [21], Copyright © 2021, American Chemical Society.

In addition, other metal oxide-based semiconductors have been investigated to improve NO conversions as well as reaction selectivity. Lei et al. have investigated the defective α-Bi₂O₃ and β-Bi₂O₃ to explore the synergistic effects of crystal structure and Vo on photocatalysis and reported the highly efficient photocatalytic NO removal. With surface defects, the photocatalytic NO removal over β-Bi₂O₃ was increased from 25.2 to 52.0%, while α-Bi₂O₃ indicated NO enhancement just from 7.3 to 20.1%. The improved NO performances were attributed to Vo, which could synergistically regulate the electron transfer pathway [23]. To tackle the bottlenecks of the sluggish carrier separation, catalysts deactivation, and incomplete oxidation during the photocatalytic NO treatments, Hailili et al. fabricated a series of ZnO nanostructures with gradient Vo and investigated their NO oxidations. Results showed that with higher Vo on the unusual nonpolar facets, Vo-rich ZnO exhibited 5.43 and 1.63 times enhanced NO removal with fewer toxic product NO₂ formations than its counterparts pristine and Vo-poor ZnO due to the promoted carrier separation, massive productions of ·O₂⁻ radicals from the molecular oxygen activation, and effective adsorptions of small molecules (O₂, H₂O, and NO) on the defective surface [15]. Continuing their interesting studies, they investigate the influences of defect-induced surface interface for NO removal. It was found that with well-positioned band edges, defect-associated carrier separations, and strengthened surface-interface reaction, ZnO displayed 4.16 folds enhanced efficiency and 2.76 times decreased NO₂ yields, indicating the significance of surface-interface regulations and surface defect controlling [24].

3.2 Bismuth-based materials

The synthesis of highly efficient photocatalysts and the revealing of the interfacial reaction mechanism are two major prerequisites for the commercial application of photocatalytic technology. Among all the studied systems, Bismuth-based photocatalysts have received extensive attention due to their unique layered structure, electronic configurations and photoelectric properties. The layered structure polarizes the internal atomic orbitals, leading to the formation of the internal electric field, which in turn promotes the separation of photogenerated electrons and holes. Moreover, this unique two-dimensional layered structure has abundant active sites and an easily adjustable band gap. Many bismuth-based photocatalysts with excellent photocatalytic activity have been explored for the removal of dilute NO (∼ppb) from the environment. However, limited visible-light absorptions, rapid carrier recombination, and ambiguous reaction mechanism with uncleared NO conversion products hindered their practical implementations. Methods such as surface defect engineering and metal doping, especially with Bi-metal incorporations turn out to be the applicable way to tackle such bottlenecks, in which the presence of surface Vo can affect all three basic steps of photocatalytic NO oxidation: i) photon energy absorption by semiconductor photocatalysts to generate photogenerated carriers, ii) separation and transfer of photogenerated electrons and photogenerated holes, and iii) surface reactions between electron holes and substrate molecules (NO, O₂, H₂O, etc.). Herein, we present several types of mostly investigated Bi-based photocatalysts that are utilized for the removal of NO by oxidation reaction.

3.2.1 Bi₂O₂CO₃-based materials

With low toxicity, controllable structure, and facile preparation, Bi₂O₂CO₃ with non-centrosymmetric crystal structure containing unique [Bi₂O₂]²⁺ and CO₃²⁻ layers have been investigated as promising photocatalysts in the field of environmental photocatalysis, especially in NO purification. For photocatalytic NO removal, a significant challenge is to achieve catalytic stability while maintaining high conversion efficiency. N-doped Bi₂O₂CO₃ with (001) and (110) exposed facets were synthesized by tuning the pH in the hydrothermal processes and displayed crystal facet dependent NO conversion. Results showed that the N-doped Bi₂O₂CO₃ with (110) exposed facets are more beneficial for the activation and adsorption of NO molecules, and further reduce the activation energy, thus promoting the selective conversion of NO to the target products to inhibit the formations of toxic intermediate NO₂ [25]. The B-doped Bi₂O₂CO₃ hierarchical microspheres exhibited remarkably enhanced visible light NO conversion to the nitrates via important NO₂⁺ intermediates due to the massive production of active specie, NO molecule activation and subsequently promoted charge carrier separations [26]. In further investigation, La-doped Bi₂O₂CO₃ was studied to simultaneously improve the photocatalytic NO conversion efficiency and selectivity to target products (NO₂⁻/NO₃⁻). The experimental and theoretical simulations indicated that the O₂ and NO could exchange electrons with localized excess electronic and get activated to produce more active species [27]. To highly maintain NO removal efficiency and NO₂ production, a Bi₂O₂CO₃/β-Bi₂O₃ heterostructure is developed and further decorated with graphene quantum dots. By construction of such an efficient interfacial charge transport channel, the charge carrier separation is hugely promoted in this heterojunction displaying high efficiency and stable visible light NO-photooxidation [28]. Zhu et al. investigated the influences of the two different crystallographic positions of oxygen atoms in the [Bi₂O₂]²⁺ layer of Bi₂O₂CO₃ for reactive oxygen species generations as well as NO oxidations. Results showed that samples showed 50.0 and 41.6% NO removal efficiencies with generations of 15.6 and 16.54 ppb NO₂, respectively. The formation mechanism of the position-manipulated Vos and the mechanism of photooxidative NO removal over the BOC were well-disclosed (Figure 5) [29]. Lee et al. reported the p-n type of Bi₂O₂CO₃/ZnFe₂O₄ heterojunction for the removal of NO and obtained improved NO conversion due to the massive production of ·O₂⁻ radicals and carrier separation induced from an internal electric field at the interface of the catalysts [30]. Furthermore, Vo-induced heterojunctions Bi₂O₃/Bi₂O₂CO₃ exhibited superior gas adsorption and improved NO oxidation due to the effective production of ·OH and ·O₂⁻ radicals, and the presence of surface defects changed the NO removal pathways [31]. A heterojunction Bi₂Mo₃O₁₂@Bi₂O₂CO₃ was designed and favorably synthesized in a hydrothermal way and further employed for NO removal. Results showed that promote NO oxidation was achieved over this heterojunction, and reaction process was monitored by in situ DRFTS, which revealed the detailed NO adsorption and conversion process as final product (NO₃⁻) via several important intermediate products (NO⁻, NO₂⁻, and NO₂), all raised from the effective carrier separation, migration, and conversion of photoinduced electron-hole pairs [32]. The hybrid of two-dimensional/two-dimensional (2D/2D) Mo-g-C₃N₄ (Mo-CN) and Bi₂O₂CO₃ (BOC) materials displayed 45% NO removal since the interfaces led to stronger interfacial interaction and Vo due to the introduction of Mo atom in contrast with bare graphitic carbon nitride (g-C₃N₄; CN) and BOC [33].

Figure 5.

Proposed schematic diagram for the migration and separation of electron-hole pairs and photocatalytic process over surface Vos-induced Bi₂O₃/Bi₂O₂CO₃ heterostructure photocatalyst Reproduced with permission from Ref. [29], Copyright © 2021, American Chemical Society.

3.2.2 BiOX (X = Cl, Br, and I)-based materials

Bismuth halide oxide BiOX (X = Cl, Br, and I) is a sillén-structured bismuth-based semiconductor material consisting of two interlaced layers of halogen atoms and [Bi₂O₂]²⁺ layers. Controlling and blocking the generation of highly toxic intermediates through regulating the reactive species during the NO oxidation was investigated over these kinds of Bi-based photocatalysts. For instance, Vo containing BiOCl was investigated and demonstrated enhanced NO removal from 5.6 to 36.4% as well as obviously inhabited NO₂ generation was found under visible-light irradiation. Results revealed that Vo on the surface of BiOCl speeds the trapping and transfer of localized electrons to activate the O₂ to produce ·O₂⁻ radicals, which avoid NO₂ formation, resulting in complete oxidation of NO (NO + O₂⁻ → NO₃⁻) [34]. Dong et al. revealed the dynamic evolution of surface defects in BiOCl during the gas-solid photocatalytic reaction at the electronic level by in situ electron paramagnetic resonance (EPR) technology. It was disclosed that the in situ-generated surface defects are the real active sites and can effectively activate the reactant molecules via directional single-electron transfer [35]. Zhang et al. reported the photocatalytic NO conversion with 99% selectivity using a defective BiOCl with (001) surface. Mechanism investigation disclosed that Vo on its prototypical (001) surface of BiOCl allows the selective and efficient activation of O₂ to ·O₂⁻ in different geometric structures that thermodynamically suppressed the terminal end-on O₂⁻ associated NO₂ emission and selectively oxidized NO to nitrate (Figure 6a) [17]. In their further investigations, they developed the two-electron-trapped V_O of BiOCl, in which a prototypical F center (V_O^′′), is a superb site to confine O₂ toward efficient and selective NO oxidation to nitrate. Upon solar light illumination, V_O^′′ completes NO oxidation via a two-electron charging (V_O′ + O₂ → V_O′′-O₂²⁻) and subsequent one-electron de-charging process (V_O′′-O₂²⁻ + NO → V_O-NO₃⁻ + e⁻). The back-donated electron is re-trapped by V_O to produce a new single-electron-trapped Vo (V_O^′), simultaneously triggering a second round of NO oxidation (V_O′-O₂ + NO → V_O-NO₃⁻) (Figure 6) [9]. Yuan et al. reported that Mn₃O₄/BiOCl achieves about 75% of NO removal within 10 min, and not only exhibited superior inhibition for NO₂ under light irradiation, but the activities gradually decreased due to the accumulation of products. Moreover, the NO removal efficiency increased with the addition of 5–10% H₂O gas, meanwhile displayed remarkably reduced NO₂ inhibition [36].

Figure 6.

(a) Free energy change against the reaction coordinate for the oxidation of NO by ·O₂⁻ on BiOCl (001) surface in different geometries; (b) free energy change during O₂²⁻- and ·O₂⁻-mediated NO oxidation on the Vo of BOC-010 and BOC-001, respectively. Reproduced with permission from Refs. [9, 17], Copyright © 2018 and 2019, American Chemical Society.

The simultaneous incorporations of Ba and Vo into BiOBr nanosheets were investigated and displayed ∼10 times enhanced NO removal than the pristine BiOBr under visible-light irradiation [37]. Utilizing the layered structure stacked by the Bi-O layer and halogen anions layer, Zhang et al. reported the novel BiOCl_xBr_1-x, (0 ≤ x ≤ 1) catalysts and demonstrated that the mixture anions products have a preferable reaction thermodynamics for NO removal with the highest efficiency of 60%, especially the BiOCl_xBr_1–x–3:1. It was presented that the improved activity was not linear dependence with light harvesting and charge conversions but was mainly decided to the optimized reaction thermodynamics, as BiOCl_xBr_1–x–3:1 possess the lowest thermodynamic barrier for NO oxidation [38]. The BiOBr/SnO₂ heterojunction exhibited more efficient NO oxidation but also inhibited the production of toxic NO₂ due to the effective carrier transportation and separation under the influence of internal electric field [39]. To reveal true reaction mechanisms, Zhang et al. investigated the NO removal over surface boronized BiOBr and proposed that the robust excitonic effect of BiOBr nanosheets, which is prototypical for ¹O₂ production to partially oxidize NO into a more toxic NO₂, can be weakened by surface boronizing via inducing a staggered band alignment from the surface to the bulk and simultaneously generating more surface Vo. They proposed that O₂⁻ radicals enable the complete oxidation of NO into nitrate with high selectivity under visible-light irradiation (Figure 7) [40].

Figure 7.

(a) In situ FTIR spectra of B-BiOBr for photocatalytic NO oxidation; (b) long-term NO removal; (c) free energy changes against the reaction coordinate for NO oxidation on modeled BiOBr and B-BiOBr surfaces, and (d) schematic illustration of the exciton-dominated and charge-carrier-involved photocatalytic NO oxidation processes. Reproduced with permission from Ref. [40], Copyright © 2022, American Chemical Society.

Zhang et al. reported highly efficient NO conversion over BiOI, and they found that NO removal pathways could be changed from nonselective oxidation to selective oxidation to produce nitrogen dioxide [41]. The ▬OH functionalization could enhance the reactants’ activation capacity to exhibit the excellent photocatalytic NO conversion performances of BiOI to generate stable final products by activating O₂ molecules to generate active species [42]. For the heterojunction system, Lee et al. reported a visible-light heterojunction formed between insulator SrCO₃ and photosensitizer BiOI for NO conversion and proposed corresponding reaction pathways as NO → NO⁺ and NO₂⁺ → nitrate or nitrite routes by in situ FTIR study [43]. To disclose the specific atomic interfacial electronic structure of heterostructure and its effect on the reaction, Sun et al. designed an insulator-based heterojunction CaSO₄-BiOI and further investigated the visible light NO conversions. Results suggest that the electronic environment of the interface/surface exerted great impacts on the active species formation, and the `intermediate transformation was supported by in situ DRIFT [44].

3.2.3 Bi-metal-based materials

The deposition of metals with local surface plasmon resonance effects such as Ag, Au, and Bi and Bi-vacancy on semiconductor surfaces is a common application, often in concert with Vo to promote photocatalytic NO oxidation (Figure 8) [45, 46]. Bismuth (Bi) is a commonly accepted semimetal that exhibits a highly anisotropic Fermi surface, low carrier density, small carrier effective mass, and long carrier mean free path, and investigated in the field of environmental pollution control widely due to its instinct characteristics such as broaden the optical ranges and promote the carrier separation. Although most of the research works are mainly focused on the NO removal efficiency instead of paying attention to the toxic byproduct formations during the NO conversion process. Many efforts have been devoted to tackling such a challenge. The defective Bi@Bi₂Ti₂O₇ photocatalyst was investigated for NO removal due to the co-effect of Bi-/Vo and displayed superior oxidation to NO₃⁻ compared to the defect-free Bi₂Ti₂O₇ counterpart [47]. Defective Bi/BiOBr nanoflowers were synthesized and further displayed 63% NO removal due to the effective carrier separation induced by bismuth and Vo. Importantly, the NO removal efficiency was negligibly affected by humidity, in which the generation of toxic NO₂ intermediate was reduced progressively from 87 to 29 ppb as the humidity increased from 5 to 100%, further indicating the significance of high humidity in promoting the transformation of toxic intermediate NO₂ to NO₃⁻ [48]. Due to the energetic hot electrons from the surface plasmon resonances of metallic Bi and superoxide generation from the molecular oxygen activation, Bi-metal-BiPO₄ (Bi-BPO) nanocomposites displayed 32.8% NO oxidation under illumination with visible light [49]. Dong et al. developed ultrathin Bi₂MoO₆ nanosheets modified by MoO₃ clusters, in which the ultrathin structure shortens the carrier transmission distance to reduce carrier recombination, while surface clusters highly favor the interfacial charge transfer, leading to fire-new electron migration pathway, and displayed highly efficient NO removal under relative humidity from 25 to 100% [50].

Figure 8.

(a–d) Schematic illustrations of the subnanometer Ag/AgCl clusters incorporated on atomically thin defective Bi₁₂O₁₇C_l2 nanosheets via rebinding with unsaturated Cl atoms; (e–g) visible light NO removal activity and proposed mechanism over Ag-SrTiO₃. Reproduced with permission from Refs. [45, 46], Copyright © 2016, 2021, American Chemical Society.

All these state-of-the-art indicate the significant roles of surface defects in improving NO conversion via facilitating the activation and adsorption of NO molecules on the surface of the photocatalysts. However, the instability and deactivation of surface Vo of photocatalysts in the continuous photocatalytic NO removal reaction results in a decrease in reaction selectivity, which needs to be improved with surface modifications. Taking advantage of surface defects and plasma effects of Bi metal, Dong et al. reported, the Bi nanoparticle decorated Bi₂O₂CO₃ nanosheets with Vo and obtained significantly enhanced NO conversion to generate NO₃⁻ with remarkably inhibited toxic intermediates NO₂ under visible-light illumination. The in situ DRFTS and DFT simulations indicate that the Bi nanoparticles and surface Vo act as active sites to activate the surface adsorbed O₂ and H₂O to produce molecular oxygen activation, and subsequently favored the NO oxidation to NO₃⁻. and surface defects in promoting carrier separations are systematically investigated [51]. Bi-metal @ Bi₂O₂[BO₂(OH)] with Vo was investigated for photocatalytic oxidation of NO under visible light and displayed the unique electron transfer covalent loop ([Bi₂O₂]²⁺ → Bi-metal → O₂⁻), which was confirmed by experimental and theoretical simulations. The Vo improve the charge separation efficiency and the yield of active oxygen species, while Bi-metal has functioned as electron donors to activate NO molecules and form NO⁻ and induces a new reaction path of NO → NO⁻ → NO₃⁻ to achieve the harmless conversion of NO, effectively restraining the generation of noxious intermediates (NO₂, N₂O₄) [52]. A series of Bi and surface defects co-modified Zn₂SnO₄ were developed for NO removal and obtained enhanced NO conversions, which can be basically attributed to the synergistic effect of Vo and surface plasmon resonance (SPR) effects of Bi elements [53].

3.2.4 Other Bi-based materials

The roles of surface-interface investigated and displayed superior NO conversion in other Bi-based photocatalysts. For instance, Zhu et al. reported the Au nanoparticles loaded Vo-rich Bi₄Ti₃O₁₂ (Au/Bi₄Ti₃O₁₂) displayed 48% selectivity for NO removal and significantly reduced NO₂ production under visible light due to the obviously promoted carrier separation induced from the built-in electric field of surface plasmon resonance effect from Au nanoparticles [54]. The effects of Br⁻ on Vo construction over Bi₂MoO₆ with different facets exposed were systematically investigated, and the OVs concentration optimized Bi₂MoO₆ (BMO-001-Br) exhibited superior activity with 62.89% NO removal and 93.61% selectivity for complete NO oxidation [55]. The carbonate-intercalated defective Bi₂WO₆ facilitated the electron-hole pairs converting to reaction radical and reactants activation, including the H₂O oxidized to ·OH, O₂ reduced to ·O₂⁻ and resulted in 55% oxidation of NO without generating secondary pollution of toxic intermediates [56]. To improve the transportation of the charge carriers, a Z-scheme heterojunction of 2D/2D BP/monolayer Bi₂WO₆ (MBWO) was designed and exhibited 67% NO oxidation owing to the intimate face-to-face contact between BP nanosheets and ultrathin MBWO [57]. Lee et al. reported the role of Vo in optimizing the performance of Bi₂Sn₂O_7−x hollow nanocubes, which displayed 32% NO removal and suppression of NO₂ [58]. A Z-scheme n-Bi₁₂SiO₂₀/p-Bi₂S₃ displayed 56% photocatalytic NO removal and the diminished generation of NO₂ (3 ppb) within 4 min visible-light irradiation [59]. The n-p heterojunctions Bi₁₂GeO₂₀-Bi₂S₃ with surface Vo showed reinforced NO removal under visible light with 96% selectivity for NO₂⁻/NO₃⁻ species, avoiding the generation of toxic NO₂ [60]. To tackle the bottlenecks of a few intrinsic active sites and inefficient carrier separation of photocatalysts during the NO removal, Lv et al. introduced Vo into Bi₃TaO₇ and achieved 5.4 folds higher NO removal efficiencies than bare Bi₃TaO₇. It was revealed that the intermediate products of defective-Bi₃TaO₇ are helpful to promote the deep oxidation of NO to NO₃⁻, while pristine Bi₃TaO₇ is more likely to produce toxic intermediate NO₂, which greatly hinders the deep photocatalytic oxidation of NO [61]. Other Bi-Ti-based layered structured photocatalysts, such as Pb₂Bi₄Ti₅O₁₈ (Figure 9), SrBi₂M₂O₉ (M = Nb, Ta), and Bi-/Bi₁₂TiO_20, also displayed appreciable NO conversion and toxic intermediate generation, providing the encouraging results for the effective NO control in the field of environmental science [11, 62, 63].

Figure 9.

Photocatalytic NO removal, the relative change in NO₂ concentration, photochemical stability, and NO removal mechanisms over layered Bi-based Pb₂Bi₄Ti₅O₁₈. Reproduced with permission from Ref. [61], Copyright © 2017, American Chemical Society.

All in all, Bi-based photocatalysts with pure and defective structures have been widely used for the removal of NO, during which the surface-interface controlling such as surface defects, doing with plasmatic metal, or their coeffects for the removal of NO have been witnessed. The other semiconducting materials such as metal-free g-C₃N₄ and Ti-based perovskite have been extensively investigated, though reaction efficiency remains low due to massive and rapid recombination of photoinduced electron and holes, and cannot fulfill the requirements of both high efficiency and long-term robustness. Meanwhile, exact reaction mechanisms and conversion pathways of NO over such catalysts are still under debate, a lack of systematic investigations. More attention should be focused on the effective control of the toxic byproducts NO₂ and preventing catalyst deactivations.

4. Conclusions and perspectives

NO_X is harmful to both the environment and the human body, and thus the development of efficient NO_X control technology is of great significance for preventing and controlling air pollution. Compared with traditional physical or chemical adsorption technology, photocatalytic reactions can achieve the conversion or degradation of pollutants removal at room temperature and pressure with less secondary pollution, which is a new and efficient environment-friendly purification strategy. In view of the current state of photocatalytic NO removal, mainly for photooxidation, the lack of investigations on semiconductor-based photocatalysts and their industrial implementations on NO conversion is mainly due to the following factors:

1. To improve NO conversion efficiency, efforts should also be continued in other kinds of modification strategies such as vacancy and intercalation embedding engineering instead of merely being limited by the introduction of oxygen vacancies, Bi deposition, and the construction of Bi-based heterojunctions; moreover, although NO removal efficiency has been enhanced by the currently investigated semiconductors, their applications are still far from the industrial level; further investigations should be focused to achieving the greater degree of performance enhancement.

2. More attention still needs to be focused on developing nontoxic products that can oxidize NO to nitric acid or reduce it to N₂ to avoid the production of NO₂ to the maximum extent. Many issues of controlling the single reaction pathways on oxidation or reduction reaction, and its dependence on the surface structural and electronic features of the interface are still open. Thus, effective control of selectivity or NO conversion products and unearthing reaction mechanisms are highly desired.

3. The powdered photocatalytic semiconductors are not easily recoverable due to the occupation of surface-active sites by the final product NO₃⁻ as the reaction time increases, so NO₃⁻ should be removed by water washing and other means in a timely manner; Moreover, development of powder material solid-loading techniques, photocatalytic film macro-preparation, and preparation of photocatalytic devices are highly desired in anticipation of further adaptation of photocatalytic technology to practical applications

4. In the process of photocatalytic NO oxidation, ·O₂⁻ is the most critical as the reactive oxygen species that can completely oxidize NO to NO₃⁻, however, the role of ·OH should not be wasted and neglected, and further oxidation of NO₂ from ·OH activation is not a point worth exploring. In addition, the role of other ROS in this process and the mechanism are also worth discussing.

5. To study the reaction process and true mechanism of photocatalytic NO oxidation, more parameters continuous reactor, NO concentration, flow rate, temperature, water (relative humidity), catalysts loading, reactor sizes, air and light parameters (wavelength or intensity), detected conversion products and so on should be investigated deeply; meanwhile, more in situ characterizations and theoretical calculations at the atomic level should be served to disclose the real reaction pathways.

6. For the reduction pathway, the direct decomposition of NO into stoichiometric N₂ and O₂ under ambient conditions merely with light as the driving force remains offers one of the most ideal routes for NO treatment. However, its realization has been tremendously restricted by the huge activation barrier for both the cleavage of N=O bonds and the subsequent reconstruction of N≡N triple bonds, which has been hardly achieved under ambient conditions. More detailed investigations should be devoted to opening this gate.

7. During the photocatalytic NO removal, the chemical nature and concentration of particular matter, such as benzene, toluene, volatile organic compounds (VOCs), and other gases such as CO₂, SO₂, NH_3, and Hg^o, may compete with NO or reaction intermediates lead to reducing in the selectivity as well as removal efficiency of NO. To tackle this, future research needs to consider the interference of the above factors and other gases to establish more applicable methods. The possible solution is still closely related to the structure and interface characters of photocatalysts. Moreover, the source and concentration of interfering substances should also be considered to reduce the response of catalysts to interfering substances by adjusting the actual reaction conditions in advance.

Acknowledgments

Financial support by the National Natural Science Foundation of China (no. 22376008) is gratefully appreciated.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this contribution.

Appendices and nomenclature

WHO	World Health Organization
NOx	NO nitrogen oxides
SCR	selective catalytic reduction
Photo-SCR	photo-selective catalytic reduction
H2O2	hydrogen peroxide
h+	photoinduced holes
1O2	single line state oxygen
·O2−	superoxide radicals
·OH	hydroxyl radicals
DRIFTS	in situ diffuse reflectance infrared Fourier transform spectroscopy
DFT	Density Functional Theory
Vo	oxygen vacancies or surface oxygen defects
2D/2D	two-dimensional/two-dimensional
EPR	electron paramagnetic resonance
SPR	surface plasmon resonance