Recent progresses on single-atom catalysts for the removal of air pollutants

The booming industrialization has aggravated emission of air pollutants, inflicting serious harm on environment and human health. Supported noble-metals are one of the most popular catalysts for the oxidation removal of air pollutants. Unfortunately, the high price and large consumption restrict their development and practical application. Single-atom catalysts (SACs) emerge and offer an optimizing approach to address this issue. Due to maximal atom utilization, tunable coordination and electron environment and strong metal-support interaction, SACs have shown remarkable catalytic performance on many reactions. Over the last decade, great potential of SACs has been witnessed in the elimination of air pollutants. In this review, we first briefly summarize the synthesis methods and modulation strategies together with the characterization techniques of SACs. Next, we highlight the application of SACs in the abatement of air pollutants including CO, volatile organic compounds (VOCs) and NOx, unveiling the related catalytic mechanism of SACs. Finally, we propose the remaining challenges and future perspectives of SACs in fundamental research and practical application in the field of air pollutant removal.


Introduction
Air pollutants, such as carbon monoxide (CO), volatile organic compounds (VOCs), nitrogen oxides (NO x ) and so on, are very hazardous to environment and human health (Liu Z. S. et al., 2019;He et al., 2019;Chen et al., 2021;Dong et al., 2021;He et al., 2021;Peng et al., 2021). With the surging industrialization, ever-growing emission of air pollutants has attracted great attention. Catalytic oxidation can transform diverse air pollutants into non-toxic or less harmful substances at room or low temperature, which is deemed promising to overcome gaseous pollutant problem. Catalytic oxidation techniques can be categorized into thermo-catalysis, photocatalysis and photothermal catalysis. Thermo-catalysis requires high temperature to activate oxygen species to oxidize air pollutants; and photocatalysis can produce reactive radicals by photo-generated carriers, which are responsible for degradation of air pollutants. Photothermal catalysis is a novel technique coupling with the advantages of thermo-catalysis and photocatalysis, exhibiting higher activity than individual process. Supported noble-metal catalysts, such as Au, Pt, Pd, etc., are the most widely used materials for air pollutant elimination owing to their high catalytic performance (Bai et al., 2019;Gan et al., 2019;Lu et al., 2019;Chen H. X. et al., 2020;Yang et al., 2021;Li J. T. et al., 2022;Chen et al., 2022). Traditionally, most noble-metals are supported as nanoparticles or clusters, but only a small portion of metal atoms can work as active sites, giving low atom utilization (Shang et al., 2021). Besides, high cost is also an obstacle to their large-scale applications. Therefore, tremendous efforts have made to further improve the atom utilization and reduce the consumption of noble-metals.
In 2011, Zhang et al. first defined the concept of single-atom catalysts (SACs) for isolated single Pt atoms anchored to the surface of FeO x (Pt 1 /FeO x ), whose TOF for CO oxidation was 2-3 times higher than that on Pt nanoparticles loaded on FeO x (Qiao et al., 2011). Henceforth, single-atom catalysis becomes an up-growing topic and has grown into a research hotspot in heterogeneous catalysis. The full exposure of active sites maximizes the atom utilization, accomplishing atomic economy and resource conservation of noble metals (Zhang L. et al., 2019;Zhou et al., 2020;. As distinct from nanoparticles, unique coordination environments and electronic structures of SACs endow them excellent activity and selectivity (Jiang and Wang, 2018;Gu Y. et al., 2021;Zhang et al., 2021).
The catalytic performance of supported metal catalysts greatly relies on the properties of supports. In 1978, Tauster et al. developed the concept of strong metal-support interaction (SMSI) to depict the phenomenon that Pt nanoparticles were encapsulated by sub-oxide TiO 2-x species deriving from TiO 2 support (Tauster et al., 1978). In addition to the migration and reconstruction of chemical components, metal-support interactions (MSI) are frequently accompanied by electron transfer and interfacial effects. Hence, MSI are expanded into strong metal-support interaction (SMSI), electronic metalsupport interaction (EMSI) and interfacial perimeter in supported metal-nanoparticle catalysts. Due to the ununiform distribution of metal nanoparticles and complex interfaces, it is very challenging to quantitatively clarify the nature of the MSI effects. While, in SACs, the well-defined single-atom site provides a good model to remove these barriers. When the supported metal exists as single atom, there are no complex interfaces between metals and supports. The catalyst systems have been largely simplified. The MSI originating from the chemical bonding between metal single-atoms and supports can be maximized to significantly influence their catalytic performances. The coordination bonding between metal atoms and supports can stabilize the isolated metal atoms with high surface energy to prevent their migration and aggregation. The EMSI, resulting from the difference in chemical potentials of metals and supports, will induce the charge redistribution of the whole SAC systems, thus modulating the adsorption and activation behaviors towards reactants and intermediates and enhancing their catalytic performances Wan X. et al., 2022).
Thanks to their unequalled advantages, SACs have shown conspicuously improved performance on a range of reactions in energy and environment fields Weon et al., 2020). For the abatement of air pollutants, the application of SACs has extended from initial CO to other pollutants, e.g., formaldehyde (HCHO), benzene (C 6 H 6 ), toluene (C 7 H 8 ), NO x and so on. For instance, the single Ag atoms anchored at the tunnel openings of the hollandite-type manganese oxide (HMO) were fabricated by Huang and co-workers (Huang et al., 2012), on which the TOF of HCHO oxidation was 7 times higher than that on Ag nanoparticles loaded on HMO. The high activity of Ag single-atoms was ascribed to the enhanced activation ability of both lattice oxygen and molecule oxygen. Subsequently, Ag 1 / HMO was also demonstrated to be reactive for benzene oxidation (Chen et al., 2017c). Hence, SACs have been demonstrated to be appealing for the catalytic removal of air pollutants. In the last decade, SACs have achieved great progresses on the catalytic oxidation of air pollutants and it is very necessary and highly desired to make a systematic summary towards current advances.
In this review, we first illustrate the synthesis and modulation methods together with the characterization techniques of SACs. Further, we focus on the application of SACs in elimination of air pollutants and summarize the corresponding intrinsic catalysis mechanism. More importantly, the structure-function relationship of SACs is discussed to give a hand for the rational design of SACs. Finally, the challenges and prospects of future research on SACs for elimination of air pollutants are provided ( Figure 1).

Synthesis of single-atom catalysts for air pollutant removal
The isolated atoms tend to agglomerate to form nanoparticles or nanoclusters due to its high surface energy. So, how to prevent the agglomeration of single atoms is the main challenge in the fabrication of SACs. Recently, variety of methods have been developed to synthesize SACs, such as wet-chemistry precipitation, atomic layer deposition (ALD), spatial confinement, defectengineering and atom trapping (AT), etc.
The wet-chemistry method, including coprecipitation and impregnation, is easy to be operated without special equipment (Pei et al., 2017;Cao et al., 2020;Sun et al., 2020;Muravev et al., 2021;Hai et al., 2022). The crucial point is to control the metal content and select appropriate support. Pt 1 /FeO x was successfully built by a coprecipitation method (Qiao et al., 2011); FeO x support with large surface area were used and low amount (0.17 wt%) of Pt was loaded, ensuring atomic dispersion of Pt single atoms. However, increased loading amount of metal atoms is a big issue for wet-chemistry method. Notably, Hai and co-workers (Hai et al., 2022) introduced a two-step annealing approach to realize ultra-high-density (UHD) of SACs with metal content up to 23 wt%. Regulating the bonding of metal precursors to supports by stepwise ligand removal can prevent their thermally induced aggregation into nanoparticles. The annealing temperature (T 1 ) of the first step should be lower than the decomposition temperature of metal precursors and the annealing temperature (T 2 ) of the second step should be high enough to remove the remaining ligands and transform the chemisorbed metal precursors into UHD-SACs.
The atomic layer deposition (ALD) is a promising tool to achieve large-scale synthesis of SACs. The ALD process to fabricate SACs includes two main steps: metal precursors react with adsorbed oxygen on support surface; then, metal precursor ligands are oxidized to M-O species (M: metal atoms) by oxygen pulse, resulting in the formation of single atoms (Zhang L. et al., 2018). Sun's group (Sun et al., 2013) first synthesized isolated Pt atoms anchored to graphene nanosheets using MeCpPtMe 3 as the precursor and O 2 as counter reactant by ALD techniques. Pt single-atoms loaded on Co 3 O 4 , CeO 2 and ZrO 2 were also prepared using ALD methods . The metal oxides were exposed in MeCpPtMe 3 precursor and O 3 was adopted to burn off the ligand of MeCpPtMe 3 to obtain Pt SACs. The content of Pt can be tuned via changing exposure time of MeCpPtMe 3 and also ALD cycle numbers. However, metal atoms are apt to be adsorbed on the deposited metal atoms to aggregate into cluster, thus, high metal loading is difficult to be achieved.
Defects on support, such as cation vacancies, oxygen vacancies and step edges etc., are good candidate sites to anchor metal precursors; then enhancing charge-transfer interaction between single atoms and defective sites can stabilize isolated metal atoms to restrict their migration. Chen and co-workers  created Mn vacancies by the redox reaction between MnO 2 and H 2 O 2 in acidic medium, and redox reaction as follows: MnO 2 (s) + H 2 O 2 (aq) + 2H + (aq) = Mn 2+ (aq) + 2H 2 O (l) + O 2 (g). Later, Au atoms were anchored on Mn vacancies. This strategy can be extended to prepare SACs using other oxides as supports (Xia et al., 2018;Xia et al., 2019;Yi et al., 2020). Ni 2+ vacancies in Ni(OH) x also had strong stabilizing effect on single-atomic Pt species (Zhang J. et al., 2018). Besides, Li's group (Wan et al., 2018) reported oxygen vacancies of TiO 2 stabilizing Au atoms through Ti-Au-Ti structure. Except for the above common defects, the step edges of CeO 2 can also anchor Pt singleatoms to form PtO 4 units (Dvorak et al., 2016).
The spatial confinement of pore structure has been found to be capable of achieving spatial segregation of metal precursors; then, the single atoms are generated after ligand removal. Zeolite is considered as good host materials to encapsulate isolated metal atoms due to its abundant microporous channels (Shan et al., 2017;Liu et al., 2020). Tang et al. (Tang et al., 2018) introduced Rh cations to the internal surface of micropores of ZSM-5 through a method integrating vacuum pumping and incipient wetness impregnation. EXAFS results disclosed that Rh single atoms were dispersed in micropores of ZSM-5 by Rh 1 O 5 coordination structure. Metal-organic frameworks (MOFs) with regular pore structures also have great potential to separate metal precursor (Chen Y. J. et al., 2017;Wang et al., 2018). For example, the cage of ZIF-8 can package Ru (acac) 3 precursor, because the molecular size of Ru (acac) 3 (9.7 Å) is smaller than the cavity diameter (11.6 Å) and larger than the pore diameter (3.4 Å) of ZIF-8 (Ji et al., 2019). Ru single-atoms were produced after removing ligand of Fe (acac) 3 by thermal reduction treatment below the thermal decomposition temperature of ZIF-8.
Atom trapping (AT) can directly transform metal nanoparticles to single atoms (Xiong et al., 2017;Jiang et al., 2020;Pham HN et al., 2022;Zhuang et al., 2022). Datye et al. (Jones et al., 2016) found that Al 2 O 3 supported Pt nanoparticles converted into Pt single-atoms on CeO 2 when the physical mixtures of Pt/Al 2 O 3 and CeO 2 were aged in air at 800°C. This is due to that volatile PtO 2 was emitted from Pt nanoparticles and trapped by CeO 2 support during the heat process. Meanwhile, Fe 2 O 3 substrate is also able to capture volatile PtO 2 units to generate Pt/Fe 2 O 3 SACs in air at 800°C ( Figure 2A) (Lang et al., 2019). But the smaller nanoparticles were formed at lower temperature ( Figure 2B) and the larger nanoparticles were emerged in Ar ( Figure 2C). Thus, AT methods need three basic conditions: a high temperature to break metal-metal bond to produce mobile Pt species; an oxidizing atmosphere to produce volatile PtO 2 units; a strong interaction between support and Pt species to anchor Pt single atoms.

Regulation of single-atom catalysts for air pollutant removal
The nature of SACs depends on their unique coordination environment and electron structure, as well as their interactions with the support (Jiang et al., 2021a), so the regulation of SACs mainly focuses on these aspects. Unfortunately, in the field of air pollutant purification, regulations on SACs have rarely been investigated, so we just briefly discuss the current status in this section.
The selection and modification of supports has great impact on the catalytic behavior of SACs. The reducibility of supports can alter the charge state of Au single-atoms and promote its stability . On irreducible supports, e.g., MgO, Al 2 O 3 and ZrO 2 etc., Au single atoms exhibit little electron transfer and weak bonding to CO. In contrast, on reducible supports, e.g., CeO 2 , TiO 2 and Fe 2 O 3 , charge transfer from Au single-atoms to supports generates positively charged Au + species, which are very stable upon CO adsorption. Heteroatom-doping can adjust the property of supports to improve the activity of SACs (Zhou X. et al., 2022;Meng et al., 2022). The CO oxidation activity of Pd/CeO 2 can be enhanced after Pr doping (Deng et al., 2022), which reduces the formation energy of oxygen vacancies around Pd sites, promotes the dissociation of O 2 and reaction rate of CO oxidation. Hydrogen reduction treatment is an effective path to tune the properties of supports. Gao et al. (Gao et al., 2022) found that reduced TiO 2 was conducive to the generation of Pt single atoms with high electron deficiency, which can suppress the strong CO adsorption and profit the adsorption/activation of O 2 .
Strong metal-support interaction (MSI), as an important feature of SACs, can prevent the migration of isolated atoms and mediate electron transfer between metal atoms and supports (Weon et al., 2020;Yan et al., 2020). The adjustment of the electronic metal-support interaction (EMSI) is valid for modulating the catalytic performance of SACs (Hu et al., 2014;He et al., 2018). Jiang et al. (2022) manipulated EMSI between Pt and CuO to acquire dramatic increment of the performance on acetone oxidation. The strong EMSIs facilitates the charge redistribution through electron donation from Pt atoms to CuO, thus producing sufficient positively charged Pt species, which strengthen the adsorption/activation of acetone. Meanwhile, the activation of adjacent lattice oxygen, the crucial active oxygen species, is facilitated by the strong EMSI ( Figure 3A).
However, the over-strong interaction between isolated atoms and supports could decrease the activity of SACs. The abovementioned Pt 1 /CeO 2 prepared by atom trapping showed excellent thermal stability, but it was inactive for CO oxidation at low temperatures due to the over-stabilized Pt 2+ in a perfect square-planar Pt 1 O 4 coordination (Jiang et al., 2021a). To break such strong interaction, Wang's group developed a thermal-shock (TS) method to create an asymmetric Pt 1 O 4 geometry, which was partially reduced into Pt 1 O 4-x unit during CO oxidation, forming active Pt 1 δ+ species to enhance low-temperature catalytic performance. Zhang's group also found the MSI on Pt/CoFe 2 O 4 was weakened by simple water soaking treatment and thus the C-H bond activation was enhanced in the catalytic combustion reaction of CH 4 ( Figure 3B) . The critical function of water was disclosed to turn the coordination structure of Pt single atoms by forming Pt 1 -O(H + )-Fe unit. The coordinated H + reduces the oxidation state of Pt and elongates Pt-O bond, so the MSI decreases ( Figures 3C,D). This strategy is general for other metal-supported catalyst systems with MSI.

Characterizations of single-atom catalysts for air pollutant removal
With the fast-growing research on SACs, advanced characterization techniques have been developed to identify the existence, spatial distribution, coordination environment, electron structure and dynamic evolution of SACs. They can provide indispensable information to clarify structure-activity relationships and guide rational design of SACs. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), high-resolution microscopes, X-ray absorption spectroscopy (XAS) and theoretical calculations are "four treasures" to explore SACs. DRIFTS using CO as a probe molecule is widely used to investigate the nature of supported platinum group metals (PGMs) (Pt, Pd and Au etc.) species (Jones et al., 2016;DeRita et al., 2017;DeRita et al., 2019;Jiang et al., 2020;Sarma et al., 2020), which can distinguish single atoms from nanoparticles by the vibration mode difference of adsorbed CO molecules. For example, the adsorption band of CO at 2,116 cm −1 is ascribed to the adsorption of CO over isolated Pt single atom of Pt 1 -Co 3 O 4 , whereas the band at 2089 cm −1 is caused by linear CO adsorption over metallic Pt nanoparticles of Pt/Co 3 O 4 . Meanwhile, the CO-DRIFTS can also monitor the   Figures 4A,B). High-resolution microscopes can offer visual image to clearly verify the location of isolated atoms, particularly the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM), which can directly observe various metal atoms by Z (atomic-number) contrast between metal atoms and supports . Typically, for Cu-TiO 2 , the bright spots can be clearly viewed on the Ti atom rows, corresponding to Cu single atoms located at Ti vacancies ( Figure 4E) . But it is out of operation when metal atoms are lighter than support atoms, such as Pd/CeO 2 (Kim et al., 2021). Scanning tunneling microscope (STM) is also an atomic-level imaging technique, more than that, it can visualize the bonding of isolated atoms with reactant molecules. On Ni/CuO , the adsorption behaviors of CO and O 2 on Ni cations can be viewed under low-temperature STM. The isolated Ni cations (Ni c ) are inert for CO adsorption and highly active for O 2 dissociation at room temperature, resulting in"Ni c -O" species like isosceles triangles ( Figure 4F).
X-ray absorption spectroscopy (XAS) based on synchrotron radiation can evaluate the coordination and electronic structures of SACs. Based on absorption energies, an XAS spectrum is divided into the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES can give geometric structures and oxidation states of specific atoms, while the coordination environment, such as interatomic distance and coordination number, can be determined by EXAFS. For instance, on Pd1/FeO x , the normalized XANES spectra show that the absorption edge positions of Pd1/FeO x were between those of Pd foil and PdO, suggesting positively charged Pd species ( Figure 4D). In EXAFS, two prominent peaks at~1.5 and~2.9 Å originate from the Pd-O and Pd-O-Fe coordination, respectively, and Pd-Pd scattering is absent, excluding the possibility of the existence of Pd nanoparticles or clusters ( Figure 4C) .
Theoretical calculations can assist to predict and confirm the coordination and electronic structures combining with XAS results (Qiao et al., 2011;Hoang et al., 2020). For example, Jin et al. studied the structure of Ni atoms on g-C 3 N 4 substrate by XAS (Jin et al., 2020). EXAFS exhibited only a peak at about 2.0 Å, attributed to Ni-C/N bonding; and the fitting results revealed that the coordination number of Ni atoms was~5. Unfortunately, the similar scattering factor between C and N results made it difficult to distinguish the contributions from coordinated C and N atoms. Then, density functional theory (DFT) calculation was used to determine that Ni atom bonded with four N atoms and one C atom was the most stable structure.
5 Applications of single-atom catalysts for air pollutant removal 5.1 Carbon monoxide oxidation CO is a toxic gas, mainly from automobile exhaust. CO oxidation at low-temperature has been the research focus. Additionally, CO oxidation, as an important model reaction, is often adopted to evaluate catalyst performance and reveal the underlying catalysis mechanism (Qiao et al., 2016;Dong et al., 2021).
CeO 2 is regarded as an outstanding substrate because of its high redox ability and oxygen storage/release properties (Tan et al., 2020;Deng et al., 2022). CeO 2 supported PGM single-atoms (such as Pt (Maurer et al., 2020), Au , Pd (Kim et al., 2021), and Rh (Jeong et al., 2020a)) have been widely explored and used for CO oxidation due to their excellent activity, particularly Pt singleatoms loaded on CeO 2 (Table 1). A lot of efforts have been made to improve the catalytic activity of Pt/CeO 2 .
Varied preparation methods can lead to obvious difference in catalytic activity of Pt/CeO 2 (Pereira-Hernandez et al., 2019). Pt/ CeO 2 prepared by atom trapping is more active than that obtained by wet chemical route because of the stronger interaction between Pt and CeO 2 , which can facilitate the reducibility of lattice oxygen. The presence of water can remarkably improve the CO conversion on Pt/CeO 2 SACs by directly participating in CO oxidation . The adsorbed H 2 O can be easily dissociated to a hydroxyl (OH) and a lattice hydroxyl (O lattice H). Then CO reacts with the OH to yield a carboxyl intermediate, which dehydrogenates with the help of O lattice H to generate CO 2 and water. This overall reaction route is more facile than the direct reaction of CO with lattice oxygen. Besides, the active O lattice [H] derived from steam treatment at high temperature (750°C) can also activate Pt/CeO 2 , resulting in T 100 dropping by 172°C compared with the untreated catalysts ( Figure 5A) (Nie et al., 2019). H 2 -TPR analysis shows that new type of active surface lattice oxygen is generated during steam treatment ( Figure 5B). It is revealed that H 2 O molecules can fill oxygen vacancies (V O ) to produce two neighboring active O lattice [H] in the vicinity of Pt single atoms under steam-treatment conditions ( Figure 5C), providing dramatically enhanced catalytic activity. Different from the unstable surface OH from water dissociation, this active O lattice [H] could be thermally stable up to 767°C.
The activity of Pt/CeO 2 can also be promoted by H 2 reduction treatment (Jeong et al., 2020b). The oxidation state of Pt single-atoms could be controlled by varying the reduction temperature. The metallic Pt SACs show higher activities than Pt NPs, whereas highly oxidized Pt SACs display poorer activities than Pt NPs. TPD measurements show the variation of oxidation state can modulate the adsorption strength of reactants and products, resulting in great disparity of catalytic performance.
The surface functionalization of CeO 2 can boost thermal stability and catalytic activity of Pt/CeO 2 in CO oxidation. The Frontiers in Chemistry frontiersin.org oxygen plasma pre-treatment on CeO 2 surface has been demonstrated to serve as an effective strategy (Wan W. et al., 2022), which led to two crucial modifications of CeO 2 surface: 1) surface restructuring, surface roughening that suppressing the diffusion and aggregation of Pt atoms; 2) a significant amount of surface peroxide (O 2 2− ) powerfully anchoring Pt single atoms. These features are favor of a dense and uniform distribution of active Pt single atoms to boost CO oxidation.

Volatile organic compounds degradation
VOCs are a kind of organic compounds with boiling points in the scope of~50-260°C under ambient pressure (101.325 kPa) . In addition to the danger for human health, VOCs also participate in the formation of photochemical smog and secondary aerogel (Shayegan et al., 2018;Zheng et al., 2022). In view of the serious harm of VOCs, their efficient elimination by advanced oxidation techniques is of great significance.
Recently, more and more SACs have been developed for VOCs degradation, e.g., formaldehyde, benzene and toluene (Table 2).

Formaldehyde degradation
Formaldehyde (HCHO) is the most prevalent indoor gaseous pollutant, which is released from construction and decoration materials (Yu and Crump, 1998;Salthammer et al., 2010). SACs for HCHO abatement consist of metal-oxide supported SACs (Hu et al., 2015;Chen et al., 2017b;Chen et al., 2019;Chen J. et al., 2020)and carbon supported SACs Tang et al., 2022) according to the variety of supports.
In catalytic oxidation of HCHO, metal-oxide supported SACs usually obey the Mars-van-Krevelen (MVK) mechanism. Tang's group (Hu et al., 2015) investigated the relationship between the electronic structure of single-atomic Ag active centers and the enhanced activity in HCHO oxidation. They found that the catalytic centers should include the single-atomic Ag sites and their vicinal lattice oxygen atoms, and their electronic states took a critical role. Single-atomic Ag sites with higher electron density facilitate the activation of O 2 , and the surface lattice oxygen with more negative charge has stronger nucleophilicity to oxidize HCHO. Subsequently, it is unveiled that the electronic environment of lattice oxygen is more significant than that of single-atomic metal sites in the catalytic centers for HCHO oxidation (Chen et al., 2017b). The surface lattice oxygen species of Na 1 /HMO is more Frontiers in Chemistry frontiersin.org negatively charged than that of Ag 1 /HMO, reaching higher efficiency in the oxidation abatement of HCHO. Chen et al.  synthesized single-atomic Au doped α-MnO 2 via the above-mentioned H 2 O 2 -assisted chemical etching method, removing HCHO of 500 ppm at 75°C and weight hourly space velocity (WHSV) of 60,000 ml/g·h. Au single-atoms facilitate the formation of surface oxygen vacancies and the mobility of lattice oxygen. The same method was also conducted to prepare Pt/Mn-TiO 2 (Mn doped TiO 2 ) . Because of the strong interaction between Pt single atoms and Mn-TiO 2 support, the redox properties of the catalyst, especially the reactivity of surface lattice oxygen, were improved, so the as-prepared catalyst exhibited enhanced performance on the low temperature oxidation of HCHO.
Zhang et al. (Zhang L. et al., 2022) adopted the strategy proposed by Nie et al. (Nie et al., 2019) to prepare Pt 1 /CeO 2 with steam treatment, achieving 100% conversion of HCHO at 25°C. The O lattice H generated during the steam treatment was revealed as highly active sites for catalytic oxidation of DMO (dioxymethylene) and facilitates the further oxidation of formate species.
For HCHO oxidation over carbon supported SACs, Langmuir-Hinshelwood (L-H) mechanism is appropriate and molecular oxygen activation is pivotal. N-doped carbon stabilized iridium single atoms (Ir 1 -N-C) could deliver high HCHO removal efficiency (>95%) under high/low concentrations at 20°C . Ir single atoms, coordinating with four N atoms, serve as the active sites to adsorb and dissociate O 2 by the strong electron coupling between Ir 5d orbital and O 2 2p orbital.

Toluene oxidation
Toluene is a common VOC, emitted from decoration, furniture, and paint industries (Qu et al., 2019). Toluene with aromatic ring is harder to be degraded at low temperatures. Fortunately, the appearance of SACs has led to new advancement. In this section, the latest achievement on SACs for toluene oxidation is summarized, including thermo-catalysis, photocatalysis and photothermal catalysis.
For thermo-catalysis, toluene oxidation over SACs mainly follows the (MVK) Wang et al., 2020;Feng et al., 2022c) or L-H Zhao et al., 2020) mechanism. The crucial active oxygen species are surface lattice oxygen in MVK model. Feng et al. (Feng et al., 2022c) found the balance between lattice oxygen mobility and toluene adsorption improved the stability of Pt 1 /MnO x . The hydrogen reduction treatment can weak Mn-O bond and lower coordination number of Pt-O, causing superior mobility of lattice oxygen and appropriate toluene adsorption ability. Whereas, in L-H model, surface adsorbed oxygen is responsible for VOCs oxidation. Zhao et al.  reported the introduction of H 2 O improved the catalytic activity of Pt/MgO catalyst in toluene oxidation and the T 90 dropped by 50°C in humid environment than under dry conditions. On oxygen vacancies, dissociated molecular oxygen reacts with H 2 O to generate ·OH, acting as dominant active oxygen species for toluene decomposition. Additionally, Zhang et al.  also disclosed that strong oxidative hydroxyl radicals (·OH) on Pt/ MnO 2 surface contributed to excellent low-temperature catalytic activity at high space velocity. In-situ DRIFTS show that the vibration peaks of hydroxyl group negatively increase with reaction time, proving that the important role of surface hydroxyl.
On photocatalysis, noble metals, such as Pt, Au, and Ag, are ideal co-catalysts to boost catalytic activity. Wang et al. (2022) developed a simple two-step electrochemical approach to synthesize an atomically dispersed Au-loaded WO 3 /TiO 2 , achieving a 95.4% conversion and 85.5% mineralization rate for toluene removal. It is disclosed that Au single-atoms, anchored by oxygen vacancies on the WO 3 surface, significantly enhanced the separation and transfer of photogenerated carriers and the adsorption of toluene. The increased MSI of the Au single atoms and WO 3 /TiO 2 nanotubes ensures thermodynamic stability of Au single atoms. Qu et al. (2019) found that Pt sing-atoms can increase absorption of visible light and suppress the recombination of electron-hole pairs to improve photocatalytic performance on heterojunction system of Pt-MoS 2 /TiO 2 .
Photothermal catalysis is disclosed in two ways including lightdriven thermocatalysis and photothermal synergistic catalysis. Light-driven thermocatalysis is actually thermocatalysis   synthesized Pt 1 /Fe 2 O 3 catalyst with high light-driven thermo-catalysis performance, achieving toluene conversion of 95% and CO 2 yield of 87% under the irradiation of 720 mW/cm 2 . It was ascribed to the high surface temperature stemming from excellent light-thermal conversion ability and the enhanced low-temperature reducibility by Pt single-atoms. Besides, the synergistic effect between photocatalysis and thermocatalysis over Pt 1 /CuO-CeO 2 can achieve higher activity than individual thermocatalysis, with toluene conversion elevated by about 48% at 180°C (Feng et al., 2022b). Reactive oxygen species (·OH and·O 2 − ) of photocatalysis further promote rapid transformation of the intermediates, accelerating release of the active sites. The Pt single-atoms could facilitate the utilization of photogenerated carriers and the adsorption/activation of oxygen molecules.

Benzene oxidation
Nowadays, SACs are also employed on thermocatalytic oxidation of benzene (Chen et al., 2017c;Yang et al., 2019;Hao et al., 2021;Hou et al., 2021;Fang et al., 2022). Chen et al. (Chen et al., 2017c) dispersed Ag single-atoms on the surface of nanostructured hollandite manganese oxide (HMO), achieving 100% conversion at 220°C and high space velocity of 23,000 h −1 . The reaction orders of benzene and O 2 are~0.9 and~0.7, respectively, indicating the MVK mechanism is valid on Ag 1 / HMO. The activation ability toward both surface lattice oxygen and gaseous oxygen was improved after the loading of Ag singleatoms, accounting for its high catalytic activity. Dai's group (Hou et al., 2021) prepared a Pd 1 Co 1 /Al 2 O 3 single-atom catalyst for benzene oxidation and L-H model was suitable for this system. Pd site and Co site act as the benzene and oxygen adsorption sites, respectively, inhibiting their competitive adsorption. Furthermore, Pd 1 Co 1 /Al 2 O 3 showed good sulfur resistance, which was attributed to the good regeneration ability of the active sites due to the rapid decomposition of sulfite or sulfate covered on Pd and Co sites.

Nitrogen oxides removal
NO x is a sort of typical air pollutants mainly from fuel combustion and automobile exhausts (Gu M. L. et al., 2021) and severely impairs environment and human health, including acid rain, photochemical smog, global warming and respiratory diseases of humans (Boyjoo et al., 2017;Jiang et al., 2017;Nguyen et al., 2020;Pham VV et al., 2022). The emission control and purification treatment of NO x are urgent problems to be solved.
Photocatalysis is an effective technique to remove lowconcentration (~ppb level) NO x at ambient temperature. NO x adsorbed on catalyst surface can be converted into harmless nitrate via strong-oxidizing radical, such as superoxide radical and hydroxyl radical. Then, the generated nitrate is easily removed by simple washing to recycle photocatalysts (Boyjoo    Nguyen et al., 2020). Many SACs (e.g. Pt (Feng H. et al., 2022), Pd (Fujiwara and Pratsinis, 2017;Fujiwara and Pratsinis, 2018;Liu et al., 2021) and Fe (Hu et al., 2020) etc.) have been reported for photocatalytic NO x removal (Table 3). Liu et al. (Liu et al., 2021) fabricated Pd single atoms confined by carbon vacancies of g-C 3 N 4 , displaying high activity and good stability in converting NO. The introduction of Pd single atoms restrains the recombination of photogenerated carriers and increases their average lifetime due to the low work function of Pd, allowing increased electrons to produce superoxide radical (·O 2 − ), which oxidizes NO to NO 3 − . Hu et al. (Hu et al., 2020) doped Fe singleatoms on Ti vacancies of TiO 2 ( Figure 6A), boosting the photoreactivity towards NO oxidation and achieving NO removal rate as high as 47.91%. The electron transfer from Fe to Ti modulates the electronic structure of the bonded Ti atoms, resulting in the formation of dual active sites of Fe-Ti, which enhanced the activation of NO and O 2 on Fe and Ti sites, respectively ( Figures  6B,C). The synergistic effect of the dual active sites leads to a drastic promotion of photocatalytic oxidation of NO. In addition, the introduction of Fe single atoms sharply suppresses NO 2 byproduct. But, during the process of NO oxidation, the generated nitrates would cover active sites to retard the catalytic performance. Therefore, high resistance to nitrate poisoning for SACs is demanded (Fujiwara and Pratsinis, 2018).
The abatement of NO x can also be accomplished by a twostep process, which involves an oxidation section to convert NO to NO 2 and a reduction section to transform NO 2 to N 2 . Jiang's group ) developed a single-atom Fe immobilized N-doped carbon catalyst (Fe 1 -N 4 -C), which showed ultrahigh catalytic activity for oxidizing NO to NO 2 at low and room temperature ( Figure 6D), owing to the low energy barriers of NO oxidation on Fe 1 -N 4 sites. Moreover, Fe 1 -N 4 -C afforded robust stability against sulfur poisoning, benefiting from preferable adsorption of reactants rather than SO 2 and SO 3 on Fe 1 -N 4 sites. Strikingly, the activity of NO oxidation was enhanced by 3% after introducing SO 2 ( Figure 6E), which was ascribed to that SO 2 oxidation accelerated consumption of adsorbed O atoms on Fe 1 -N 4 sites, thus leading to the regeneration of Fe 1 -N 4 sites ( Figure 6F).

Conclusions and perspectives
In this review, we introduce the synthesis and modulation strategies together with the characterization tools of SACs and their application in air pollutant removal, especially the abatement of CO, VOCs and NO x . It can be concluded that SACs have displayed immense superiority for effective treatment of various air pollutants because of its utmost atom utilization, tunable coordination and electron structure. Undoubtedly, satisfactory accomplishment of SACs has been made by persistent efforts; but it should be noted that SACs used in the field of air pollutant purification are still at the initial stage and face considerable challenges in fundamental research and practical engineering.
In terms of fundamental research of SACs, more attentions need to be paid to the following aspects. First, subtle difference of coordination and electron structure could cause distinct catalytic behavior despite identical single sites. So, the precise control to local environment of SACs has been a hard task. For non-noble metal single atoms, the increased metal loading conduces to improved catalytic activity. Although plentiful methods have been presented, the high content (>10wt%) of non-noble metal single atoms is still rarely reported. Besides, complex chemical reactions involving multiple reactants or reaction steps demand more than one active site to cooperate, resulting in that SACs are inactive for them. The synthesis of fully-exposed multi-atom catalysts has become a burning question. Furthermore, most synthesis approaches cannot receive high yields of SACs, which is averse to the demand of large-scale production in industry. So, facile and general synthesis approaches are highly desired. Apart from controllable synthesis, wide gaps are existing to in-depth understanding of structure-activity relationship of SACs during the air pollutant removal reactions. Ex-situ characterizations are still the main ways to explore the catalysis mechanism of SACs, resulting in great difficulties to detect the dynamic evolution of SACs during the catalytic reactions. With this in mind, in situ/operando techniques have risen to track dynamic changes of SACs in real reaction conditions. The enhancement of catalytic performance originates from the introduction of single atoms, so we focus on the change of single atoms during the in situ/operando characterization. However, for traditional nanoparticles or clusters, many factors need to be considered, such as the role of multiple active atoms and interface effect etc. In situ TEM/STM can directly observe surface morphology and local structure changes around isolated atoms. In situ XAS offers real-time electron and coordination structures of SACs and AP-XPS provides valence state information of SACs during catalytic reactions or under quasi-conditions of reactions. But in situ microscope only gets local information of fixed area; the beam time of in situ XAS is restricted and AP-XPS suffers from low atom-resolution. Hence, limitations of each techniques make them still far from enough to decode structure-performance relationship of SACs and tight integration of various in situ techniques is essential to unlock the black boxes of reaction mechanisms. On the other hand, due to the advantages of the well-defined single atomic site, calculation model of SACs is closer to the actual configuration; therefore, SACs are ideal model systems to operate theoretical study for gaining atomic-level insight into catalytic reaction mechanisms. Nowadays, theoretical calculations are easily conducted for small molecules, e.g., CO and HCHO. However, the adsorption and reaction behaviors of large molecules (e.g., C 6 H 6 and C 7 H 8 ) on SACs seem to be complicated. Because of the more possible configurations of these larger molecules on catalyst surface, the Frontiers in Chemistry frontiersin.org complication involves the establishment and search of adsorption active sites, transition states and intermediates, together with the calculation of reaction barriers. In-depth research closely combining in-situ explorations and DFT calculations will bring new understanding on catalysis mechanism. On the modulations of the coordination environment and electron structures, little efforts have been devoted. The nature of supports has significant effects on the chemical environment of SACs; and single atoms usually work together with the surrounding atoms of support to achieve high catalytic performance in some reactions. Besides, rational regulations on the MSI effects of SACs have been confirmed as very crucial issues to optimize their catalytic performance. Hence, the function of supports deserves more concerns in the future research; additionally, the generic descriptor of MSI effects demands to be established to screen optimal combination of metal atoms and supports. For practical applications, SACs also confront a serious of challenges. Apart from single pollutant in laboratory, real gas mixtures to be treated probably contain some halogenated or sulfur-containing pollutants, which react with catalysts to cause their deactivation. Thus, high demands are imposed for improving the resistance ability of SACs to poison components. Another challenge is to ensure stability of SACs during long-term operation. High-temperature and reaction media may induce migration of isolated atoms, then inhibit the catalytic activity of SACs. So, how to maintain atomic dispersion of metals on SACs under working conditions is worth careful considerations. Finally, in available literatures, in most cases SACs work in powder form; but industrial catalysis requires monolithic catalysts and set reaction devices. Therefore, it can be predicted that the integration of SACs into set reactors will be a top priority for their large-scale applications.

Author contributions
YW wrote original manuscripts. MW revised the manuscript and is responsible for this work. All authors discussed and approved the final manuscript version to be submitted.