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All issues Volume 1 / No 2 (2022) Natl Sci Open, 1 2 (2022) 20220025 Full HTML
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Natl Sci Open
Volume 1, Number 2, 2022
Special Topic: Emerging Pollution and Emerging Pollutants
Article Number 20220025
Number of page(s) 24
Section Earth and Environmental Sciences
DOI https://doi.org/10.1360/nso/20220025
Published online 02 September 2022

© The Author(s) 2022. Published by China Science Publishing & Media Ltd. and EDP Sciences.

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Introduction

It is well known that urbanization and industrialization have resulted in rapidly increasing emissions of air pollutions (e.g., volatile organic compounds (VOCs), nitrogen-containing compounds (NO x ), and sulfur dioxide (SO2)) [1–5]. Most emitted air pollution can lead to the formation of secondary pollutants (e.g., tropospheric ozone, peroxyacetyl nitrate), secondary organic aerosols, and photochemical smog [6,7], which is the major cause of a sharp decrease in air quality in numerous regions around the world [8–10]. Long-term exposure to air pollutants induces respiratory diseases and increases the incidence of respiratory diseases and cancer [11–14]. Besides, air pollutants also pose a serious threat to the survival and well-being of local plants and animals. According to the World Health Organization health report, in 2012, air pollution caused about 7 million deaths worldwide, of which 3.7 million were caused by outdoor pollution, 4.3 million by indoor pollution, and the remaining 1 million by a combination of the above both. There are manifold research initiatives currently under development to meet the challenging air pollution problem. Adsorption technology is the leading approach for air purification currently [15]. However, adsorption capacity remarkably decreases when air pollutions are present at low concentrations (usually sub-ppm levels) [16]. In addition, the replacement, cleaning, and disposal of adsorbent materials are costly processes and may cause secondary pollution. Thermal incineration is also an efficient technology, but it typically requires high temperatures (≥800°C) and consumes high energy [17]. Furthermore, incomplete thermal oxidation of pollutants also produces numerous noxious byproducts. Besides, non-thermal plasma inevitably produces undesirable reaction by-products such as ozone and NO x [18–20].

Solar-driven photocatalysis is a promising alternative for air purification, which has broad-spectrum activity toward various inorganic to organic pollutants under mild conditions. The principle of photocatalysis is founded on the solid energy band theory [21]. When a semiconductor photocatalyst initiates by an appropriate light source with energy greater than its bandgap energy, the electrons (e−) can transfer from the valence band (VB) to its conduction band (CB), and then leave holes (h+) in the corresponding position of the VB. The photogenerated e− and h+ will undergo redox reactions with the water/oxygen molecules adsorbed on the surface of the photocatalyst to generate reactive oxygen species (ROS) [22,23]. Subsequently, the adsorbed air pollutants can react with the generated ROS and eventually be oxidized to form a stable end product [24]. In recent years, numerous research results have been reported on the photocatalytic air pollutant degradation technology, and the current research is focused on the design of photocatalytic materials and the mechanism of pollutant degradation [25–28]. The conversation and degradation mechanism of air pollutants requires sufficient research, which is essential for elevating the overall photocatalytic efficiency. Hu et al. [29] synthesized a single-atom Au-supported TiO2 photocatalyst to expand the light absorption performance and enhance the photocatalytic degradation efficiency of acetone. He et al. [30] reported a strategy to introduce an in-situ water layer on WO3 by coating hygroscopic periodic acid to dramatically enhance the photocatalytic removal of hydrophilic VOCs (formaldehyde (CH2O) and acetone) in the air. Dong and co-workers [31] comprehensively analyzed the degradation mechanism of the CH2O and toluene (C7H8) mixture to seek the optimal routes for their decomposition and mineralization. However, the refractory by-products produced by the degradation of photocatalytic gas pollutants under long-term operation would accumulate on the surface of the photocatalyst, blocking the active site and leading to the deactivation of the photocatalyst. The occurrence of photocatalyst deactivation will seriously limit the practical application of photocatalysis technology. Hence, there is still a huge gap between fundamental research and industrial application in this field. The specific aspects of the gap include the design of suitable photocatalysts, the revealing of pollutants degradation mechanisms, the strategies to regulate the reaction pathways, and the development of commercial products for different application scenarios.

Here, we intend to identify this gap and provide a comprehensive review of the recent development of photocatalysis for air purification, including photocatalyst types, reaction pathways of typical air pollutants, and application scenarios (Scheme 1). Firstly, we analyze the published articles on photocatalytic degradation of air pollutants and categorize them based on the types of photocatalytic materials and air pollutants. In addition, photocatalyst reaction mechanisms according to the types of air pollutants, various methods to identify intermediates or by-products, and strategies for regulating reaction pathways are discussed extensively. Subsequently, the application scenarios of photocatalytic air purification are summarized. Lastly, the major challenges for promoting applications of photocatalytic technology are critically discussed and some prospects are delivered.

thumbnail Scheme 1

The structure of the review for photocatalytic purification of typical air pollutants.

Photocatalytic purification of typical air pollutants

Nitrogen oxide (NO x )

Why is it important?

Among various air pollutants, NO x is mainly composed of nitric oxide (NO) and nitrogen dioxide (NO2). In the atmospheric environment, NO is usually in low concentrations but extremely hazardous and can lead to acid rain when it comes in contact with air [32]. Although NO itself is not active, it can be easily oxidized by O2 in the air to form NO2, which is a kind of highly corrosive and toxic gas. Meanwhile, photochemical smog, which results from the reaction of NO with other hydrocarbons in the atmosphere, can cause serious harm to human health, and the formation of photochemical smog would be further facilitated with sufficient UV light [33]. Besides, NO can also damage the human respiratory tract, cause methemoglobinemia, and even endanger human life safety [34]. In addition, when NO and VOCs co-exist in the environment and are exposed to sunlight, they can easily interact with each other to produce ozone, thus directly leading to the increase of near-ground ozone concentration [35]. So, how to effectively remove atmospheric NO x is still a pressing issue to be solved. Currently, the traditional means of purifying NO x mainly include adsorption, selective catalytic reduction, and thermal catalysis [36]. However, these processes usually require high temperature, high-pressure environments, reducing agents, and even precious metals, such as Ru, Pd, and Pt. Given the harsh conditions, the high energy consumption, and the cost of traditional technologies. It is, therefore, necessary to develop energy-efficient and environmentally friendly NO x purification technology [37–39]. Photocatalysis, which enables the efficient use of solar energy under mild conditions, has shown great potential in the field of photocatalytic NO x purification.

What are the chemical reaction pathways?

The reaction pathways involved in photocatalytic purification of NO x have been widely studied, and the general mechanism is illustrated in Figure 1A. When illuminated with light, photo-electrons (e−) and holes (h+) are generated on the surface of the photocatalyst, and then react with small molecules adsorbed on the surface, such as O2 and H2O, respectively, to form ROS through the following reaction process.

thumbnail Figure 1

(A) Possible mechanisms of NO x photocatalytic purification [36]; (B) calculated reaction pathway and structures of initial states, transitional states, and final states for NO photooxidation by on CN and Au@CN [40]; (C) design diagram for localized excess electrons that decrease the reaction activation energies and increase the photocatalysis efficiency [41]; (D) DFT calculated adsorption energy and bond length of several major intermediate adsorption products (upward for CN and downward for Sr intercalated CN), all lengths are given in Å [42]; (E) detailed mechanism of photocatalytic NO oxidation on -OH intercalated BiOI [43]; (F) diagram of the reduction of the surface by NO and e− [44]; (G) proposed schematic diagram for the separation and transfer of photogenerated carriers and the photocatalytic process over the insulator–semiconductor heterojunction [45].

(1)

(2)

(3)

Subsequently, NO pollutant would be oxidized by and , respectively, and eventually, yield the final product of nitrate via two main intermediates HNO2 and NO2 [2].

(4)

(5)

(6)

(7)

(8)

(9)

(10)

In recent years, Dong’s group has explored the reaction mechanism of NO oxidation extensively through a combination of in-situ DRIFTS and DFT calculations and gained deep insight into the rate-determining step. As shown in Figure 1B, they took g-C3N4 as the research model to explore the tune of NO reaction pathways with or without Au decorating [40]. For pristine g-C3N4, the processes of NO→NO2→ are both energetically unfavorable, resulting in the accumulation of NO2 on the surface. The dissociation of the O2 molecule is identified as the rate-determining step. In contrast, NO could be easily oxidized to nitrate by on the surface of Au@g-C3N4. These results show that Au as the active center of electron accumulation could effectively reduce the energy barrier, regulate the reaction pathway, inhibit the production of toxic by-products and promote the transformation of NO to final products. The schematic diagram in Figure 1C effectively illustrates the reduction of reaction activation energy and the improvement of photocatalytic efficiency by electron localization [41]. Figure 1D describes the calculated adsorption energy and bond length of several major intermediate adsorption products for g-C3N4 and Sr intercalated g-C3N4. Different from the above literature, two new intermediates cis-N2O2 and trans-N2O2 were detected on the g-C3N4 surface with or without Sr doping. For the pristine g-C3N4, the reactions of the NO→cis-N2O2→trans-N2O2 process are energy favorable. However, further oxidation of trans-N2O2 requires external energy, which is not conducive to occurring. Surprisingly, all the pathways are exothermic reactions for Sr intercalated g-C3N4 and are all energy favorable. Yang et al. [43] proposed that the intercalation of -OH groups in BiOI can effectively improve the adsorption and activation of NO, thus reducing the reaction energy barrier and promoting the conversion of NO to , as shown in Figure 1E.

In addition to the adsorption and activation of reactants, the production of ROS and the stability of the final product nitrate are also important indexes to measure photocatalytic efficiency. As shown in Figure 1F, Wang et al. [44] found that the N-O bond on the surface can be activated by NO molecule due to the overlap of 2p orbitals, and then the NO−, generated by NO getting an electron, can react with under light irradiation via the following pathway.

(11)

Meanwhile, NO− is the key species that triggers the decomposition on the surface. Figure 1G depicts the photocatalytic process over the insulator–semiconductor heterojunction, which is accompanied by some other intermediates such as NO+ and . The above results imply that the existing state of the intermediate is easily affected by the surface structure of the photocatalyst, which will further change the reaction path. But how to stabilize the final product on the catalyst surface is still a challenge.

Which are the most suitable photocatalysts?

According to the available literature, we screen photocatalysts suitable for photocatalytic NO purification and divide the most representative photocatalysts into four categories, which are summarized in Table 1. The vast majority of photocatalysts used for NO purification are applicable in visible light. In this way, it is more practical when dealing with low concentration NO in the air.

At present, although a series of efficient catalysts for photocatalytic NO purification have been developed, and the reaction mechanism has been deeply explored through in-situ DRIFTS and DFT calculations, there are still many challenges. The photocatalytic NO purification performance lacks a uniform evaluation criterion, which affects the objective evaluation of catalyst performance. In the later stage, it was essential to establish a scientific evaluation system in combination with the actual pollution scene. Besides, it is necessary to further investigate the influence of coexisting molecules such as H2O, SO2, O3, and VOCs in the atmosphere on the photocatalytic purification of NO, and then the photocatalysts with excellent performance should be selected to expand their applicability. The photocatalysts for NO purification reported at this stage only show excellent performance in laboratory research. However, there is still a big gap for practical application. It is necessary to further optimize the synthesis process and loading process of catalysts, systematically evaluate the economic and environmental effects of these photocatalysts and provide technical support for the large-scale application of high-efficiency photocatalytic purification of NO.

Table 1

Summary of various photocatalysts for NO purification

Formaldehyde

Why is it important?

Formaldehyde (HCHO), as one of the dominating VOCs emitted from industry activities, consumer products, and furnishing materials, endangers both urban environments and ecological health. Formaldehyde exists both in an indoor environment where the concentration is low and outdoor location where the concentration is high. Due to the mild reaction conditions and the green energy required, photocatalysis technology has been widely utilized for HCHO oxidation removal. However, there are still concerns about low photocatalytic efficiency and secondary pollution caused by toxic by-products. To achieve the overall improvement of photocatalytic HCHO decomposition performance, the binding form and conversion mechanism of HCHO on the photocatalyst surface demand to be revealed.

What are the chemical reaction pathways?

TiO2-based catalyst is one of the most common photocatalysts used for HCHO degradation. Generous hydroxyl groups are usually present on the surface of TiO2. Therefore, HCHO is adsorbed on the hydroxyl groups of the catalyst surface via hydrogen bonding, as shown in Figure 2A [58]. In the dry condition, most of the hydroxyl sites on the TiO2 surface are occupied by HCHO, and the superoxide radicals play an essential role in triggering the HCHO oxidation. In humid conditions, abundant water vapor is introduced to enhance the generation of hydroxyl radicals, resulting in hydroxyl radical dominated HCHO oxidation. The strong oxidizing properties of superoxide and hydroxyl radicals cause the HCHO to be oxidized to the intermediate (formate), followed by the final products (CO2 and H2O). The humidity has a positive effect on the photocatalytic oxidation of HCHO because the introduction of water leads to an accumulation of the hydroxyl radicals on the catalyst surface. Therefore, the high degradation rate of formaldehyde in a humid environment can be achieved. Notably, humidity becomes a negative character when the humidity level is above 2000 ppmv. In addition to the effect of moisture, Zhu et al. [59] identified the contribution of visible light and proposed new insights into the mechanism for photocatalytic HCHO oxidation over plasmonic Au/TiO2. As shown in Figure 2B, HCHO undergoes four sequential reaction steps (κ1, κ2, κ3, κ4) with different reaction conditions. At step κ1, HCHO is oxidized to trioxymethylene (DOM). At step κ2, DOM is oxidized to formate. At step κ3, the formate is further oxidized to carbonate. At step κ4, carbonate is decomposed to CO2 in the presence of moisture. The results of the conditional experiment prove that steps of κ1 and κ2 are much faster than κ3 and κ4. The rate-determining steps of the overall reaction are formate oxidation to carbonate (κ3) and carbonate decomposition (κ4). Moisture can accelerate DOM oxidation to formate and is indispensable to carbonate decomposition. Visible light can enhance the rate-determining steps of formate oxidation to carbonate and carbonate decomposition. As a result, the formation rate of intermediates and final products increased significantly.

thumbnail Figure 2

(A) Proposed reaction scheme for the photocatalytic oxidation of formaldehyde on the pure TiO2: superoxide radicals as the oxidant; hydroxyl radicals as the oxidant [58]; (B) schematic diagram of the proposed pathway for photocatalytic oxidation of formaldehyde over Au/TiO2 under visible light [59]; (C) schematic illustration for the binary reaction pathways for HCHO degradation [60]; (D) calculated activation energies and reaction energies for the primary and secondary reaction pathways of HCHO degradation, positive and negative depict energy absorption and release respectively [60]; (E) schematic for photocatalytic oxidation of HCHO through selective breakage of C-H in intermediate species (HCOOH) on the surface of Sn self-doped CaSn(OH)6 [61]; the charge difference distribution after HCHO adsorption and the corresponding plot of in situ DRIFTS tests for HCHO interaction in the dark on the HBPO (F) and MBOP (G) surface [62].

For the multiple elementary reactions of photocatalytic air purification, the generation of toxic intermediates is a troublesome issue that remains the major hindrance to the advancement of photocatalysts for practical applications. Li et al. [60] proposed the particular binary channel reactions in HCHO oxidation, including pathway I of HCHO→HCOOH→H2CO3→CO2 (path 1–3) and pathway II of HCHO→HCOOH→ CO→H2CO3→CO2 (path 1, 4, and 5) (Figure 2C). The HCOOH oxidation is the rate-determining step (Figure 2D). Hence, the HCHO degradation efficiency is directly related to the activation energy of HCOOH transformation. However, CO plays a disgusting role as the secondary pollutant during photocatalytic HCHO oxidation. To realize safe and efficient photocatalytic HCHO oxidation, Wang et al. [61] constructed the photocatalyst surface with abundant free electrons distribution to realize the selective breakage of C-H bonds in HCOOH formed as the intermediate of HCHO oxidation (Figure 2E). Therefore, the reaction pathway can be altered to avoid the generation of toxic by-products (CO). Additionally, the controlling of surface atomic arrangement can modulate the adsorption model of HCHO from single-point to bridging. The efficient conversion of intermediate (HCOOH*) can be facilitated while suppressing CO generation (Figure 2F and 2G) [62].

The above results show that the degradation pathways and rate-determining steps of photocatalytic HCHO oxidation have been relatively clearly revealed, which is attributed to the fact that HCHO is a simple small molecule. The key intermediate, HCOOH, is the dominant driver during the HCHO degradation. By tailoring the electronic structure of the photocatalyst surface and the adsorption configuration of the reactants, the reaction pathway can be effectively regulated. However, most of the state-of-the-art structure-activity relationships of photocatalytic HCHO degradation are still established under the condition of ultraviolet light (UV). It is urgent to achieve continuous and efficient visible-light degradation of HCHO. In addition to optimizing the reaction conditions, such as humidity, light intensity, and residence time, developing photocatalysts with a more positive valence band and visible light response may be a more effective method. As a C1 feedstock and reaction intermediate for CO2 reduction, the HCHO can be reused to produce value-added chemicals, such as photocatalytic reduction of HCHO to CH4 fuels.

Which are the most suitable photocatalysts?

There have been numerous studies with a variety of photocatalysts in photocatalytic HCHO degradation, which are summarized in Table 2. At present, getting the photocatalytic purification of HCHO to CO2 and H2O at ambient temperature exhibits the advantages of no energy consumption, complete purification, no secondary pollution, and high removal efficiency, which is the mainstream development direction of HCHO photocatalytic purification. However, most of the catalysts with the above capabilities are noble metal-based catalysts or precious metals as active additives. It is generally known that precious metals on the earth are rare and expensive. Therefore, the development of non-noble metal-based photocatalysts for photocatalytic purification of HCHO at ambient temperature is an important research goal in the future.

Table 2

Summary of various photocatalysts for HCHO oxidation

Benzene series

Why is it important?

The benzene series represent the main typical VOCs emitted in the air and result in high health risks due to its inherent chemical toxicity. They are relatively difficult to be decomposed because of the stable benzene ring. The co-existence of VOCs and NO x in the atmosphere would promote the generation of ozone via photochemical processes. For complete mineralization of VOCs with rings, the ring-opening reaction is regarded as the rate-determining step which should be fully considered when designing catalysts. Required by the demand for sustainable development, effective and eco-friendly technology is desired for VOCs removal. Photocatalytic oxidation is a promising purification technology, especially for the degradation of benzene series, due to its strong oxidation capacity at room temperature and atmospheric pressure.

What are the chemical reaction pathways?

During photocatalytic toluene degradation, there are two main different reaction pathways, including (Pathway I) hydrogen abstraction and (Pathway II) OH addition (Figure 3A) [67]. In pathway I, the oxidation takes place on the side chain of the methyl group. The reaction mechanism could probably follow the scheme: toluene→benzyl alcohol→benzaldehyde→benzoic acid. For pathway II, •OH could attack the aromatic ring to replace the aromatic hydrogen. The possible reaction scheme is presented as follows: toluene→cresol→hydroquinone or methyl-p-benzoquinone. Subsequently, these intermediates of two reaction pathways could further convert to benzoquinone. With further oxidation, the aromatic ring is broken, and all the intermediate products are oxidized to form CO2 and H2O with enough oxidizing radicals. The intermediate products are different for the two pathways. Besides, the pathway I is believed to be the primary reaction path for toluene degradation because the weakest bond in the toluene molecule is the C-H bond of the methyl group. By calculating the primary reaction pathways in photocatalytic toluene decomposition, as shown in Figure 3B, no energy barriers are found for the benzyl oxidation part (steps 1–3), indicating that toluene can be favorably converted to ring-containing intermediates (benzyl alcohol, benzaldehyde, and benzoic acid) [68]. However, for complete mineralization of toluene, the ring-opening pathway of the benzene ring in these intermediates becomes the rate-determining steps (4–7). Fortunately, low activation energies are required for the ring-opening reaction at benzaldehyde and benzoic acid. It is worth noting that the lowest energy barrier is the ring-opening starting from benzoic acid. These results indicate that a more efficient ring-opening reaction can be realized as the benzyl gets fully oxidized to benzoic acid. Accordingly, as shown in Figure 3C, the overall decreased reaction energy on the SrTiO3 surface during the toluene oxidation path realizes the highly selective formation of intermediate (benzoic acid) to accelerate the ring-opening and mineralization process of photocatalytic toluene degradation [69].

thumbnail Figure 3

(A) Possible reaction mechanisms of toluene degradation and catalyst regeneration [67]; (B) the calculated reaction coordinates for benzyl oxidation (path 1–3) and aromatic ring-opening (path 4–7) [68]; (C) free energy of conversion processes of toluene to benzoic acid via the route of toluene→benzyl alcohol→benzaldehyde→benzoic acid on STO and CTO, IS (initial state), FS (final state) [69]; (D) illustration of the role of Ba active sites in photocatalytic toluene decomposition on SBTO [70]; (E) illustration of the viral effect for lattice oxygen activation in photocatalytic toluene decomposition on Sr2Sb2O7 [71]; (F) schematic for photocatalytic oxidation of toluene over ZT-LDH [72].

In addition, the establishment of active sites on the photocatalyst surface is the critical factor to achieve highly selective benzyl oxidation. As shown in Figure 3D, the local electronic center constructed at Ba active sites intensifies the charge transfer between catalyst surface and adsorbed molecules [70]. The strong oxidative species generated by enhanced activation facilitate the selective conversion of toluene to generate benzoic acid. Consequently, the photocatalyst displays the efficient degradation and stable mineralization of photocatalytic toluene. For metal oxide, the activated lattice oxygen on the photocatalyst surface is favorable for adsorption and activation of toluene and intermediates and then realizes the selective conversion of benzoic acid (Figure 3E) [71]. Interestingly, CO2 desorption and lattice oxygen reactivation through H2O avoid the competitive adsorption between final products and specific intermediates. Additionally, as shown in Figure 3F, the hydrogen atoms on the hydroxyl groups in the LDH can selectively attract the oxygen atoms in the C=O bond of benzaldehyde and benzoic acid, thereby improving the selectivity of photocatalysis and reducing the generation of toxic by-products [72].

Although the highly selective oxidation of methyl group to benzoic acid facilitates efficient photocatalytic toluene degradation, some intermediates in pathway I (toluene→benzyl alcohol→benzaldehyde→benzoic acid) are also easy to cause the deactivation of the photocatalyst with poor oxidation ability. This dramatically diminishes the commercial value of photocatalysis as a practical flue gas purification technology. Hence, the intrinsic reason for the deactivation of the photocatalyst is revealed to provide a new theoretical reference to design efficient and stable photocatalysts for toluene degradation. Chen et al. [73] found that a considerable accumulation of benzaldehyde and weak ability of ring-opening on P25 are the direct causes of deactivation. The vast accumulation of benzaldehyde and benzoic acid on the catalyst surface blocks the active sites and causes a decrease in photocatalytic efficiency. Differently, on the surface of Ga2O3, adsorbed intermediates quickly and easily open the aromatic ring and further mineralize into small inorganic molecules (Figure 4A). The different ring-opening abilities of intermediates on the surface Ga2O3 and P25 decide their durability differences during prolonged toluene degradation. Alternatively, a direct and effective method is to modulate the reaction pathway to avoid the generation of benzoic acid on the photocatalyst surface. As shown in Figure 4B, the benzaldehyde was unlikely converted to benzoic acid on the In(OH)3 (IO) catalyst surface due to the presence of thermodynamic energy barriers [74]. Hence, the benzaldehyde was unlikely converted to benzoic acid on the IO catalyst. More efficiently, the aromatic ring can be directly opened, which is observed over the modified boron carbonitride photocatalyst (BCN-C) [75]. As shown in Figure 4C, this could bring out a more efficient and concise ring-opening pathway, producing fewer carbonaceous intermediates. The delocalized electrons on the photocatalyst surface are essential to enable the direct ring-opening of the aromatic ring. The strong oxidation ability of •OH generated over BCN-C and enhanced the interaction between the reactants and the catalyst made it possess the ability to attack the aromatic ring of toluene directly. Hence, BCN-C exhibits remarkable durability, a high mineralization rate, and immensely enhanced photocatalytic degradation performance.

thumbnail Figure 4

(A) The intermediate speciation and chemistry of the photocatalytic toluene oxidation on 120-β-Ga2O3 [73]; (B) proposed C7 conversion pathway and calculated reaction energies of hydroxyl induced toluene photooxidation on catalysts [74]; (C) schematic illustration for different paths of photocatalytic gas toluene degradation [75]; (D) the illustration of roles of methyl number on mechanisms and performance of photocatalytic decomposition of benzene, toluene, and o-xylene [76]. GC-MS chromatogram for the determination of reaction intermediates in the gas-phase photocatalytic decomposition of toluene: as a single individual component (E) and as part of a mixture of CH2O and C7H8 (F) [31]; (G) diagram of Gibbs free energy variation on the SnO2 surface [31].

In order to improve the ring-opening efficiency of the benzene ring, except for the abovementioned interaction of toluene with different photocatalyst surfaces, changes in the groups on the benzene ring play a vital role in the ring-opening. The intrinsic relationship between the methyl number and ring-opening reaction efficiency is worthy of investigation. As shown in Figure 4D, the methyl number on the benzene ring was dominantly responsible for the elevated ring-opening and decomposition efficiency [76]. The introduction of methyl groups greatly influences the distribution of electrons on the phenyl ring, which enhances charge transfer and reduces the stability of the conjugated π bond. Therefore, the introduction of carbon-containing groups on the benzene ring can effectively activate the benzene ring and reduce the ring-opening energy barrier of the reaction. Additionally, during the photocatalytic mixed degradation reaction of HCHO and toluene, the addition of HCHO into toluene leads to the generation of o-tolualdehyde as key intermediates (Figure 4F), which can elevate the adsorption and activation of the aromatic ring [31]. Compared with the vital intermediate (o-methylphenol) of OH addition at the aromatic ring of toluene (Figure 4E), the reaction energy for HCHO addition is lower than that of OH addition (Figure 4G). Besides, the addition of HCHO on toluene efficiently decreases the Gibbs free energy for enhanced ring-opening.

The above investigations indicate that breaking the stability of benzene ring is a key factor to realize efficient photocatalytic mineralization, including the direct addition of strong •OH radicals, selective generation of benzoic acid with lower ring-opening energy barriers, or synergistic addition between mixed pollutants. Since the toluene oxidation process involves a variety of complex intermediates, the results of the ring-opening pathways of benzene ring studied above are mostly derived from theoretical simulations. To reveal the reaction mechanism more accurately, direct experimental evidence remains to be provided in terms of in-situ formed reactive species and intermediates with high resolution. Isotopic labeling of O2 or H2O molecules combined with in situ infrared spectroscopy would be an advanced characterization tool to track the molecular conversion processes.

Which are the most suitable photocatalysts?

There have been numerous studies with a variety of photocatalysts applied in toluene degradation, which are summarized in Table 3. Photocatalytic technology can degrade toluene to CO2 and H2O at room temperature. However, some toxic by-products may inevitably be formed in this process. Therefore, it is necessary to develop more efficient photocatalysts. At present, doping, crystal surface regulation, or heterojunction construction are effective means to overcome the inherent limitations of photocatalysts and improve the photocatalytic activity under visible light. Meanwhile, the electron and hole recombination can be effectively suppressed. In addition, it is essential to explore the doping amount and the proportion of components in the heterojunction.

Table 3

Summary of various photocatalysts for toluene oxidation

Various photocatalytic application scenarios

The main challenges for practical application of photocatalytic air purification are (1) mass transfer, (2) durability and deactivation, (3) utilization of photons, (4) catalyst adhesion to substrates, (5) by-product generation and its adverse health consequences [79]. Achieving high-efficiency long-term stability is an important goal for the commercialization of photocatalytic applications.

Application in indoor air purification

NO x and VOCs released by furniture and building materials will seriously affect indoor air quality, including NO, NO2, benzene, toluene, polycyclic aromatic hydrocarbons (especially benzo[a]pyrene), trichloroethylene, perchloroethylene, and formaldehyde. Compared with the outdoor environment, the level of indoor air pollutants can reach higher values. The concentration of indoor NO2 is between 15 and 200 ppb (1 ppb=1 μg L−1). While in industrial workplaces and gas stove equipment, NO2 concentration may reach 1–2 ppm (1 ppm=1 mg L−1) [80]. For the treatment of indoor pollutants by photocatalytic technology, laying photocatalytic tile is an effective strategy for indoor air purification. As shown in Figure 5A, Jiang et al. [81] coated the N, F, and Fe ion triple-doped TiO2 photocatalytic function layer on tiles for removing NO air pollutants and organic compounds in the indoor and outdoor air under visible light. In addition to improving air quality, the modified ceramic tile also possesses low water absorption, good fastness, and antibacterial ability to reduce the risk of bacterial infection. Therefore, photocatalytic tile is a promising candidate for practical application in indoor and outdoor environment improvement.

thumbnail Figure 5

(A) Photocatalytic and ordinary tiles [81]; (B) manufacturing and assembling processes of PC-Trombe wall system [82]; (C) an air cleaner (AX7000, Samsung) equipped with two 001-TNT filter units and 30 UV-LEDs (Seoul Viosys, CUN66A1B) [25]; (D) schematic diagrams of the VUV-PCO air purifier [83]; (E) photocatalytic reactor of biological sampling system (left) and reactor frontal view (right) [84]; (F) a test chamber (8 m3) with the air purifier for VOC removal experiments [85]; (G) schematic representation of the photocatalytic air purifier prototype [86].

Long-term exposure to indoor VOCs may greatly increase the risks of allergy, respiratory illness, and even cancer. Using light energy can not only purify the VOCs in the air but also adjust the indoor temperature. Yu et al. [82] constructed a new type of solar gradient photocatalysis-Trombe wall system (Figure 5B), which can achieve the dual functions of space heating and removal of indoor formaldehyde. Under solar radiation, photocatalytic oxidation of formaldehyde is activated by ultraviolet light, and the remaining visible and infrared light is collected to heat the indoor environment. The daily clean air and formaldehyde degradations are 164.0 m3/(m2 day) and 100.0 mg/(m2 day), respectively. It is estimated that the initial investment of the photocatalysis-Trombe wall system will be recovered in about 12 years. The indoor air purifier can improve the air environment quickly and efficiently, and has been sought after and loved by many families in recent years, owing to its small and convenient characteristics. Through a new method that is easy to scale up, Weon et al. [25] synthesized (001)-surface exposed TiO2 nanotubes (001-TNT) and was successfully amplified and installed on a commercial air purifier, as seen in Figure 5C. The air purifier achieves an average VOC removal efficiency of 72.0% (running for 30 min) in an 8 m3 laboratory, meeting the air purifier standard agreement. Xu et al. [83] have constructed an innovative vacuum ultraviolet photocatalytic oxidation (VUV-PCO) air purifier (Figure 5D), which simultaneously eliminates VOC and O3 in a closed real room for the first time. The air purifier has a high removal efficiency of formaldehyde, and considerable removal efficiency of benzene, toluene, m-xylene, o-xylene, valeraldehyde, octanal, and nonanal. The air purifier showed good stability during the removal of formaldehyde decomposition and TVOC during the intermittent three-time on/off operation. The ozone content was reduced by 70% when the device was heated. Efficient portable air purifiers are expected to be a potential method for indoor air quality improvement, due to their small size and low price.

As active components of air pollution particulate matter, bacteria, fungi, and viruses cannot be ignored for indoor air quality and safety control. As shown in Figure 5F, a practical-grade photocatalytic air purifier equipped with a TiO2/H-ZSM-5 composite bead filter was established by Kim et al. [85], which is proved to effectively remove indoor VOC and viruses under UVA-LED lighting. The photocatalytic air purifier can effectively remove the atomized virus particles of the bacteriophage Phi-X 174. More than 99.999% of the viral RNA of SARS-CoV-2 is removed within 1 h, eliminating the infectivity of the virus. The air purifiers equipped with composite bead filters are ready for practical applications in homes and hospitals. Sánchez et al. [84] propose that the monolithic transparent polymer coated with a sol-gel TiO2 film was used to treat real indoor air in a laboratory-scale single-step annular photocatalytic reactor (Figure 5E). The photocatalytic reactor can realize disinfection and sterilization while eliminating indoor VOC. Martínez-Montelongo et al. [86] has prepared TiO2-Cu2+@perlite and Ag@TiO2-Cu2+/perlite supported materials. A prototype of a photocatalytic air purifier was constructed with novel materials (Figure 5G), showing a better air disinfection activity at lower doses. The growth inhibition of Gram-negative and Gram-positive bacteria reached up to 99.0%. In addition to eliminating indoor chemical pollutants, disinfection and sterilization of indoor air through portable air purifiers is also a crucial way to improve indoor air quality.

Application in outdoor air purification

In the case of outdoor air purification by photocatalysis, there are some differences compared to indoor air purification, such as the irradiation source, the type of contaminants, and the operation conditions [87]. For outdoor air purification, solar light can be used as the driving force to initiate photocatalytic reactions. The main target air pollutant from outdoor is nitric oxides which come from automobile exhaust, rather than VOCs. The formed nitrates can be simply washed off with rainwater, and the photocatalyst is thereby self-regenerated without additional treatment [88]. Moreover, outdoor applications are exposed to a harsh environment. The applications are required to withstand high shear rates.

The practical outdoor applications focus on utilizing construction materials with a large surface area that can serve as platforms for photocatalytic air decontamination, like canyon streets, tunnels, bridges, car garages, public outdoor furniture, and external surfaces of buildings [89]. Photocatalysts can be incorporated into building materials (plaster, mortar, paint, or cement) and applied in various forms. Anything made of concrete has the potential for photocatalytic air purification.

TiO2 is the most popular photocatalyst incorporated in concrete. Meng Chen and co-workers conducted a series of real-scale applications on the road. They utilized permeability technology to immobilize TiO2 on the surface of porous asphalt roads for the removal of NO x from vehicle emissions [90]. The purifying rate of vehicle pollutants ranged from 6.0% to 12.0% in the actual outdoor road traffic environment, showing a good environment purification function. As shown in Figure 6A, another study used a TiO2 and activated carbon permeable spray solution to coat concrete roads, aiming at the degradation of NO x [91]. High purification efficiency, self-regeneration ability, and good repetition performance were achieved. In addition, they found that the photocatalytic purification of NO x was related to light intensity and temperature. The reaction rate increased as the light intensity increased, while the reaction rate decreased as temperature increased. Guerrini [92] reported the NO x levels of the “Umberto I” tunnel in Italy before and after the renovation work by the photocatalytic treatment (Figure 6B). A NO x reduction of about 20.0% was calculated in the center of the tunnel as an absolute comparison between the two periods. The photocatalytic paint exhibited an efficient abatement of pollutants in the tunnel vault. Costa et al. [93] applied TiO2-based photocatalytic paint in a swine weaning unit and examined the concentrations and emissions of pollutants (NH3, CH4, CO2, N2O, and PM10) (Figure 6C). The photocatalytic treatment has beneficial effects on significant decreases in methane concentration (ca. 27.0%, P<0.05) and particulate matter emission (ca. 17.0%, P<0.01) in swine husbandry with respect to reference one. The limestone surfaces coated with water dispersed TiO2 nanoparticles were exposed to an urban site for one year to investigate the photocatalytic performance (Figure 6D) [94]. The obtained surface through TiO2 coating showed better preservation of the surface color early after exposure. But the self-cleaning efficiency was reduced to negligible after eight months due to TiO2 loss and deactivation.

thumbnail Figure 6

(A) Photocatalytic Nano-TiO2 on concrete road [91]; (B) simulation design of lighting system in the tunnel [92]; (C) treated swine weaning unit [93]; (D) samples exposed on the top of the Bari Cavour meteoclimatic station in the city centre [94]; (E) sense-city experimental campaign pictures [95]; (F) application scenario of the self-powered photocatalytic degradation system [96].

Other photocatalysts have also been studied for practical application. Le Pivert et al. [95] developed construction materials functionalized by low-cost hydrothermal direct growth of ZnO. Bitumen road was produced to evaluate the photocatalytic activity of simultaneous removal of various pollutants from a real car exhaust (O3, CO x , NO x , VOCs) (Figure 6E). All car exhaust components showed a downward trend in the presence of ZnO. Fu et al. [96] built a piston-based triboelectric nanogenerator (P-TENG) to enhance gaseous acetaldehyde photocatalytic degradation as shown in Figure 6F. The photocatalyst NH2-MIL-125(Ti) was evenly distributed on a conductive substrate which is connected to a P-TENG. The P-TENG can convert wind energy into electrical energy and induce an electric field that could generate more radicals and holes. The photocatalytic removal rate of the system reached 63.0% within 30 min.

Incorporating photocatalyst with construction material is a workable way to scale up the application of photocatalytic air purification. But big challenges remain and wait to be solved, such as the long-term stability, durability under harsh weather and environment, the corrosion of buildings, and the replacement of photocatalysts. Due to the complexity of the application environment, the outdoor application faces more difficulties than the indoor application.

The bottleneck of practical application

Currently, photocatalytic air purification technology has been developed for decades, and photocatalyst technology has been applied in indoor and outdoor air purification. However, the actual application effect is not ideal. There are still few research achievements to realize the industrialization and application, which has become a major scientific problem faced by all mankind. Fundamentally, there are three major bottlenecks.

(1) Low photocatalytic efficiency. Photocatalytic efficiency is one of the important indexes to evaluate the performance of a catalyst. High photocatalytic efficiency can completely degrade organic pollutants in a short time and effectively alleviate environmental pollution. Meanwhile, the high-efficiency photocatalysts can also achieve better purification of air pollution in a small amount. However, most photocatalysts exhibit high photocatalytic activity only under UV light but low activity with visible-light irradiation, such as commercial photocatalyst P25, while the UV light only accounts for about 5% of the sunlight. Therefore, the light absorption efficiency of commercial photocatalysts under sunlight is very low, which leads to low photocatalytic efficiency and limited practical application effect. However, if the catalytic material can display high catalytic activity under visible light, the efficient catalytic purification of air pollutants under LED lights can be realized, so as to effectively solve the problem of air pollution.

(2) Difficult realization of industrial production. In the actual scale-up production process, the preparation conditions are not as controllable and stable as the laboratory conditions, there are many uncontrollable factors, and the problems of cost, energy consumption, environmental protection, stability, and so on need to be considered. Therefore, the development of a feasible and stable preparation method is the key to realizing the industrialized application of photocatalysts.

(3) The loading of powder photocatalyst. Although a great deal of work has been done on the basic research of photocatalysis, there is still a gap between laboratory and industrial applications. For example, the commercialized P25, as a benchmark for evaluating the performance of catalysts, shows a stable photocatalytic purification capacity in photocatalysis, which has also realized mass industrial production. However, the powder photocatalyst is easy to be blown out in the reaction process, which greatly affects the air purification efficiency of P25 and its practical application effect. Therefore, the additional process needs to be carried out, such as the loading of powder photocatalysis, and then pay a lot of cost and energy. This is the gap between material properties and industrial applications.

Conclusions and perspectives

This review summarizes recent advances in photocatalytic purification of typical air pollutants, including NO x , HCHO, and benzene series. The insights into the mechanisms of pollutant conversion have been discussed extensively for the design of efficient and safe photocatalytic systems. Additionally, the applications of photocatalytic air purification technology indoors and outdoors have gradually set the tone for its contribution to the future of environmental remediation and human health. However, the photocatalytic air pollutant purification process is complicated and the applied research in this field is still limited. There are still major challenges that need to be further investigated.

(1) Even though some intermediates have been observed, more solid experimental evidence should be provided regarding their transient intermediates, especially for the photocatalytic degradation of multiple gaseous pollutants, which is more in line with the practical application. The combination of isotope labeling and in situ characterization techniques is essential for identifying reactant migration and transformation on the photocatalyst surface. Besides, the antagonistic or synergistic effects between multiple pollutants and the generation of harmful by-products (e.g., ozone) during photocatalytic degradation should also be further revealed.

(2) The stability of photocatalysts deserves to be taken seriously. The development of deactivation-resistant photocatalysts based on mechanistic understanding is a feasible strategy. Alternatively, the design of a coupled reactor to regenerate the photocatalyst or cover the photocatalyst surface with a porous protective layer may be an effective method.

(3) For outdoor air purification, it is necessary to develop high-performance visible-light-driven photocatalysts to make full use of sunlight, and a balance between visible light absorption efficiency and oxidation capacity is required to achieve continuous and efficient degradation of outdoor air pollution.

(4) For indoor air purification, there are many efficient photocatalysts available. Future work should be focused on the design of reactor structure, systematically considering the relationship among factors such as light, mass transfer, heat transfer, and reaction.

(5) The reuse of typical air pollutants is also worth considering, which can provide a more valuable alternative to conventional environmental governance. The tunable oxidation and reduction capability of photocatalytic technology are expected to enable the targeted conversion of air pollutants to valuable fuels or chemicals.

Funding

This work was supported by the National Natural Science Foundation of China (22176029, 21822601) and Excellent Youth Foundation of Sichuan Province in China (2021JDJQ0006).

Conflict of interest

The authors declare that there are no conflicts of interest to disclose.

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All Tables

Table 1

Summary of various photocatalysts for NO purification

In the text
Table 2

Summary of various photocatalysts for HCHO oxidation

In the text
Table 3

Summary of various photocatalysts for toluene oxidation

In the text

All Figures

thumbnail Scheme 1

The structure of the review for photocatalytic purification of typical air pollutants.
In the text
thumbnail Figure 1

(A) Possible mechanisms of NO x photocatalytic purification [36]; (B) calculated reaction pathway and structures of initial states, transitional states, and final states for NO photooxidation by on CN and Au@CN [40]; (C) design diagram for localized excess electrons that decrease the reaction activation energies and increase the photocatalysis efficiency [41]; (D) DFT calculated adsorption energy and bond length of several major intermediate adsorption products (upward for CN and downward for Sr intercalated CN), all lengths are given in Å [42]; (E) detailed mechanism of photocatalytic NO oxidation on -OH intercalated BiOI [43]; (F) diagram of the reduction of the surface by NO and e− [44]; (G) proposed schematic diagram for the separation and transfer of photogenerated carriers and the photocatalytic process over the insulator–semiconductor heterojunction [45].
In the text
thumbnail Figure 2

(A) Proposed reaction scheme for the photocatalytic oxidation of formaldehyde on the pure TiO2: superoxide radicals as the oxidant; hydroxyl radicals as the oxidant [58]; (B) schematic diagram of the proposed pathway for photocatalytic oxidation of formaldehyde over Au/TiO2 under visible light [59]; (C) schematic illustration for the binary reaction pathways for HCHO degradation [60]; (D) calculated activation energies and reaction energies for the primary and secondary reaction pathways of HCHO degradation, positive and negative depict energy absorption and release respectively [60]; (E) schematic for photocatalytic oxidation of HCHO through selective breakage of C-H in intermediate species (HCOOH) on the surface of Sn self-doped CaSn(OH)6 [61]; the charge difference distribution after HCHO adsorption and the corresponding plot of in situ DRIFTS tests for HCHO interaction in the dark on the HBPO (F) and MBOP (G) surface [62].
In the text
thumbnail Figure 3

(A) Possible reaction mechanisms of toluene degradation and catalyst regeneration [67]; (B) the calculated reaction coordinates for benzyl oxidation (path 1–3) and aromatic ring-opening (path 4–7) [68]; (C) free energy of conversion processes of toluene to benzoic acid via the route of toluene→benzyl alcohol→benzaldehyde→benzoic acid on STO and CTO, IS (initial state), FS (final state) [69]; (D) illustration of the role of Ba active sites in photocatalytic toluene decomposition on SBTO [70]; (E) illustration of the viral effect for lattice oxygen activation in photocatalytic toluene decomposition on Sr2Sb2O7 [71]; (F) schematic for photocatalytic oxidation of toluene over ZT-LDH [72].
In the text
thumbnail Figure 4

(A) The intermediate speciation and chemistry of the photocatalytic toluene oxidation on 120-β-Ga2O3 [73]; (B) proposed C7 conversion pathway and calculated reaction energies of hydroxyl induced toluene photooxidation on catalysts [74]; (C) schematic illustration for different paths of photocatalytic gas toluene degradation [75]; (D) the illustration of roles of methyl number on mechanisms and performance of photocatalytic decomposition of benzene, toluene, and o-xylene [76]. GC-MS chromatogram for the determination of reaction intermediates in the gas-phase photocatalytic decomposition of toluene: as a single individual component (E) and as part of a mixture of CH2O and C7H8 (F) [31]; (G) diagram of Gibbs free energy variation on the SnO2 surface [31].
In the text
thumbnail Figure 5

(A) Photocatalytic and ordinary tiles [81]; (B) manufacturing and assembling processes of PC-Trombe wall system [82]; (C) an air cleaner (AX7000, Samsung) equipped with two 001-TNT filter units and 30 UV-LEDs (Seoul Viosys, CUN66A1B) [25]; (D) schematic diagrams of the VUV-PCO air purifier [83]; (E) photocatalytic reactor of biological sampling system (left) and reactor frontal view (right) [84]; (F) a test chamber (8 m3) with the air purifier for VOC removal experiments [85]; (G) schematic representation of the photocatalytic air purifier prototype [86].
In the text
thumbnail Figure 6

(A) Photocatalytic Nano-TiO2 on concrete road [91]; (B) simulation design of lighting system in the tunnel [92]; (C) treated swine weaning unit [93]; (D) samples exposed on the top of the Bari Cavour meteoclimatic station in the city centre [94]; (E) sense-city experimental campaign pictures [95]; (F) application scenario of the self-powered photocatalytic degradation system [96].
In the text

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