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

There are three commonly accepted methods for the removal of NOx 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<sup>1</sup> ) and subsequent reconstruction of structurally stable N☰N triple bonds (940.95 kJ mol<sup>1</sup> ) 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 NOx 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.

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

### **Figure 2.**

*A summary of advantages and disadvantages of photocatalytic NO removal.*

### **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.*

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 (<sup>1</sup> O2), 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 <sup>1</sup> O2 will oxidize NO to unwanted NO2 (NO +<sup>1</sup> O2 ! 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

### *Recent Progress and Current Status of Photocatalytic NO Removal DOI: http://dx.doi.org/10.5772/intechopen.112485*

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 NOx (NO + NO2) concentrations were continuously recorded using the online chemiluminescence NOx analyzer. The concentration changes of the intermediate NO2 were simultaneously calculated from concentration gaps between NO and NOx 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, NOx conversions (%), N2 and NO2 selectivity are calculated by the following equation [15–17]:

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

$$\text{NO}\_{\text{x}}\text{ conversion}\ (\eta\_{\text{NOx}}, \text{\textquotedblleft}\text{\textquotedblright}) = \left(1 - \frac{[\text{NOX}]\_{\text{out}}}{[\text{NOX}]\_{\text{in}}}\right) \times 100\text{\textquotedblright}\tag{2}$$

$$\text{N}\_2\text{ selectionity} \left(\text{S}\_{\text{N}\_2}, \text{@}\right) = \frac{\text{[N}\_2\text{]}\_{\text{out}}}{\text{[N}\_2\text{]}\_{\text{out}} - \left[\text{NO}\_2\right]\_{\text{out}}} \times 100\text{\textdegree }\tag{3}$$

$$\text{[NO}\_2\text{ selectionity} \ (\text{S}\_{\text{NO}\_2}, \text{\%}) = \frac{[\text{NO}\_2]\_{\text{out}}}{[\text{NO}]\_{\text{in}} - [\text{NO}]\_{\text{out}}} \times 100\text{\%} \tag{4}$$

where [NO]in, [NO]out is the initial and final concentrations of NO, while [NOx]in, [NO]out and [NO2]out represent the initial and final concentrations of NOx 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 lowconcentration 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 NOx 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 NdO 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<sup>+</sup>* ) 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 *Recent Progress and Current Status of Photocatalytic NO Removal DOI: http://dx.doi.org/10.5772/intechopen.112485*

**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.*

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<sup>+</sup>* 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 NOx and HONO in complex air-polluted areas during the daytime [22].

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 CO3 <sup>2</sup> 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 <sup>+</sup> 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 NOphotooxidation [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 welldisclosed (**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

*Recent Progress and Current Status of Photocatalytic NO Removal DOI: http://dx.doi.org/10.5772/intechopen.112485*

### **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.*

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/twodimensional (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].

### *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 singleelectron 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 VO of BiOCl, in which a prototypical F center (VO <sup>00</sup>), is a superb site to confine O2 toward efficient and selective NO oxidation to nitrate. Upon solar light illumination, VO <sup>00</sup> completes NO oxidation *via* a two-electron charging (VO <sup>0</sup> + O2 ! VO <sup>00</sup>-O2 <sup>2</sup>�) and subsequent one-electron de-charging process (VO <sup>00</sup>-O2 <sup>2</sup>� + NO ! VO-NO3 � + e�). The back-donated electron is re-trapped by VO to produce a new single-electron-trapped Vo (VO 0 ), simultaneously triggering a second round of NO oxidation (VO 0 -O2 + NO ! VO-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].

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 BiOClxBr1-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 BiOClxBr1–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 BiOClxBr1–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 <sup>1</sup> O2 production to partially oxidize NO into a more toxic NO2, can be weakened by surface boronizing *via* inducing a

### **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 O2 <sup>2</sup>*�*- 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.*

*Recent Progress and Current Status of Photocatalytic NO Removal DOI: http://dx.doi.org/10.5772/intechopen.112485*

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].

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 dOH 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<sup>+</sup> and NO2 <sup>+</sup> ! 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

### **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.*

### **Figure 8.**

*(a–d) Schematic illustrations of the subnanometer Ag/AgCl clusters incorporated on atomically thin defective Bi12O17Cl2 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.*

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].

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

### *Recent Progress and Current Status of Photocatalytic NO Removal DOI: http://dx.doi.org/10.5772/intechopen.112485*

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+ ! Bimetal ! 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 electronhole 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 Bi2Sn2O7�<sup>x</sup> hollow nanocubes, which displayed 32% NO removal and suppression of NO2 [58]. A Zscheme 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-/ Bi12TiO20, 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].

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

### **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.*

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**

NOX is harmful to both the environment and the human body, and thus the development of efficient NOX 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.

*Recent Progress and Current Status of Photocatalytic NO Removal DOI: http://dx.doi.org/10.5772/intechopen.112485*

