Novel Nanomaterials for Indoor Air Chemical Purification: A Review

Abstract

Indoor air pollution, listed by the World Health Organization (WHO) as one of the top 10 environmental risk factors for human health, significantly elevates the risk of respiratory diseases, cardiovascular diseases, and cancers upon long-term exposure. Traditional indoor air purification technologies dominated by physical adsorption and filtration have inherent limitations, including mere pollutant phase transfer, easy saturation, and secondary pollution, while chemical purification centered on pollutant mineralization and degradation is the core development direction for radical elimination of indoor air pollution. Novel nanomaterials, featuring ultra-high specific surface area, precisely tunable active sites and electronic structures, and excellent room-temperature catalytic activity, have become the research focus in this field. This review systematically summarizes the characteristics of typical indoor air pollutants and purification scenario requirements, clarifies the core advantages of chemical purification technologies, details the research progress of novel nanomaterial systems in indoor air chemical purification, and dissects the reaction mechanisms and material optimization strategies of core pathways (photocatalysis, room-temperature thermal catalysis, electrocatalysis, plasma catalysis). We also outline the engineering application status and bottlenecks of these nanomaterials, propose systematic future development directions targeting existing challenges, and aim to provide a reference for fundamental research and industrial application of novel nanomaterials in indoor air purification.

1. Introduction

With the acceleration of urbanization and shifts in residential lifestyles, modern humans spend approximately 80%~90% of their time in indoor environments, and indoor air quality (IAQ) directly determines human health outcomes and quality of life [1]. According to the Indoor Air Pollution and Health report released by the World Health Organization (WHO), approximately 4.3 million people worldwide die annually from various diseases induced by indoor air pollution. Long-term exposure to indoor air pollutants significantly elevates the risk of respiratory diseases, cardiovascular diseases and cancers. These pollutants enter the human body primarily through respiratory inhalation, inducing sustained oxidative stress, chronic inflammatory responses, and even genetic material damage, which constitute the core mechanisms underlying the associated adverse health outcomes [2]. The concentration of indoor pollutants is typically 2 to 5 times higher than that outdoors and can even exceed 10 times in some enclosed and confined spaces [3]. In scenarios including civil buildings, public spaces, and enclosed ship cabins/vehicle compartments, the non-compliance rate of volatile organic compounds (VOCs) such as formaldehyde and benzene series has remained persistently high, making such pollutants the core control targets for indoor air pollution [4,5,6].

Among conventional indoor air purification technologies, physical adsorption methods (e.g., activated carbon, molecular sieves) can only achieve pollutant enrichment and phase transfer rather than harmless degradation, with inherent limitations that are highly dependent on pollutant concentration conditions. Under high-concentration pollutant scenarios, the limited adsorption sites of porous adsorbents are occupied rapidly, leading to fast adsorption saturation. In this case, the adsorption system can only reduce contaminants to a certain residual level, failing to achieve deep and complete purification, and the adsorption capacity drops sharply once saturation is reached, resulting in frequent filter media replacement [7,8,9]. In contrast, under low-concentration conditions, the concentration gradient driving force between the gas phase and the adsorbent surface is significantly weakened, leading to poor mass transfer efficiency, slow purification kinetics, and extremely low removal efficiency. Traditional chemical purification technologies, such as ozonation and chemical spraying, are limited by harsh reaction conditions, high energy consumption, and secondary pollution issues, which make it difficult to meet the core requirements of indoor environments: ambient temperature and pressure, low pollutant concentrations, and human occupancy compatibility [10,11,12].

The rise of nanomaterials has opened up a new avenue for breakthroughs in indoor air chemical purification technologies. Herein, the novel nanomaterials specifically refer to nanomaterial systems that are distinct from traditional micron-scale materials and conventional nanoparticles. These systems feature precisely controllable morphology, pore structure, and active sites and can achieve efficient catalytic degradation performance under ambient temperature and pressure through functional modification [13,14,15,16]. Their ultra-high specific surface area provides abundant reactive active sites, while their tunable electronic structure enables targeted optimization of catalytic activity. Meanwhile, multi-component compositing can realize adsorption–catalysis integration and multi-path synergistic purification, which can better adapt to the scenario requirements of indoor air purification, thus attracting extensive attention from the academic community [17,18,19].

To date, numerous works have summarized research progress in single directions such as nano-photocatalytic materials and formaldehyde catalytic oxidation materials [20,21,22,23]. However, reviews that focus on full indoor scenario adaptability and systematically cover various novel nanomaterials as well as the full spectrum of chemical catalytic pathways remain relatively scarce. On this basis, this review focuses on the core field of indoor air chemical purification, systematically sorts out the research advances of novel nanomaterials in this field, clarifies the applicable scenarios and core bottlenecks of different material systems and catalytic pathways, and proposes future research and development directions. This work aims to provide theoretical and practical references for the research and development of high-efficiency indoor air chemical purification technologies.

2. Characteristics of Indoor Air Pollutants and Typical Purification Application Scenarios

2.1. Major Indoor Air Pollutants

2.1.1. Volatile Organic Compounds

Volatile organic compounds (VOCs) are the most prevalent and highly hazardous pollutants in indoor environments. Among them, formaldehyde, mainly emitted from wood-based panels, adhesives, coatings and other decoration and finishing materials, is characterized by long-term emission and potent carcinogenicity even at low concentrations and is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC) [24]. Benzene series (including benzene, toluene, xylene, etc.) are mainly derived from paints, organic solvents and furniture coating processes, and long-term exposure to these compounds can induce hematopoietic system damage and neurotoxicity [25]. In addition, polycyclic aromatic hydrocarbons (PAHs), total volatile organic compounds (TVOCs), chlorinated and sulfur-containing organic compounds, among others, all exert acute and chronic toxicity on the human body, making them the primary control targets for indoor air purification [26].

2.1.2. Inorganic Hazardous Gases

The main inorganic hazardous gases include carbon monoxide (CO), nitrogen oxides (NO_x), sulfur dioxide (SO₂), ozone (O₃), etc. Among them, CO is mainly generated from indoor coal combustion, gas combustion and tobacco smoking, which readily binds to hemoglobin to cause hypoxic poisoning [27]. NO_x and SO₂ are mainly derived from outdoor atmospheric infiltration and indoor combustion processes; they exhibit strong irritancy and can damage the respiratory tract mucosa [28]. Ozone is mainly emitted from office equipment, negative ion air purification devices and outdoor photochemical products, and excessive ozone exposure can induce respiratory tract inflammation [29]. Such inorganic gases usually coexist with VOCs to form a complex pollution system, which imposes higher requirements on the broad-spectrum performance of purification technologies [30,31].

2.1.3. Bioaerosols and Microbial Pollutants

Bioaerosols and microbial pollutants, including bacteria, fungi, viruses and other pathogens, are mainly sourced from human activities, microbial proliferation in humid environments, and intrusion from outdoor air [32]. These pollutants can not only trigger respiratory allergies and infections, but some pathogenic bacteria can also cause infectious diseases via aerosol transmission. Conventional filtration technologies can only achieve physical interception of these contaminants without inactivation, whereas chemical catalytic technologies can realize the integration of sterilization, disinfection and pollutant degradation by disrupting the cell membranes and nucleic acid structures of microorganisms [33].

2.1.4. Particulate Pollutants

Particulate pollutants mainly include fine particulate matter (PM_2.5), inhalable particulate matter (PM₁₀) and ultrafine particles (UFPs), which are primarily derived from outdoor atmospheric infiltration, indoor combustion, cooking activities and human activities [34]. These particulates can not only directly enter the human pulmonary alveoli and blood circulation but also act as carriers for VOCs, heavy metals and microorganisms, thereby exacerbating their adverse health effects. Conventional filtration technologies can achieve high-efficiency interception of particulates, while novel nanomaterials can realize the interception of particulates and simultaneous degradation of their surface-attached pollutants via adsorption–catalysis synergy [35,36].

2.2. Typical Application Scenarios and Core Requirements

2.2.1. Civil Residential Scenarios

Civil residential scenarios are the most fundamental and widely applicable purification scenarios, covering residential buildings, apartments, bedrooms, kitchens and other daily living spaces [37]. The core characteristics of this scenario include ambient temperature and pressure, sustained emission of pollutants at low concentrations, stringent requirements for safety and quietness in the living environment, and high sensitivity to equipment energy consumption. Meanwhile, the applied catalysts are required to achieve efficient degradation of low-concentration formaldehyde and benzene series under ambient temperature and pressure, with no secondary pollution and activity decay during long-term operation [38].

2.2.2. Public Building Scenarios

Public building scenarios include office buildings, shopping malls, schools, hospitals, gymnasiums and other public indoor spaces, with core characteristics of high personnel density, large space volume, highly variable ventilation conditions, and complex pollutant types. Among them, hospital scenarios additionally require simultaneous high-efficiency sterilization and disinfection as well as nosocomial infection prevention and control [39]. For this type of scenario, the purification system is generally required to achieve broad-spectrum and high-efficiency purification under high air volume, synergistic degradation of multiple pollutants, and long service life with excellent anti-poisoning performance.

2.2.3. Confined and Restricted Space Scenarios

Confined and restricted space scenarios are special scenarios with the highest purification difficulty, including ship cabins, engine rooms, spacecraft and other enclosed spaces. The core characteristics of this scenario are a fully/semi-enclosed structure and poor natural ventilation capacity. Taking ship cabins as an example, the interior contains not only formaldehyde and benzene series emitted from decoration materials but also complex pollutants such as CO, NO_x and SO₂ generated from fuel combustion in the engine room, as well as cooking fumes from the galley [40]. Meanwhile, the marine environment with high humidity and high salt spray imposes extremely high requirements on the stability and corrosion resistance of materials [41]. Therefore, catalytic materials applicable to this scenario are required to have high stability, moisture resistance and anti-poisoning performance, compact size with high purification efficiency, the capability to achieve synergistic purification of multiple pollutants under complex working conditions, and low energy consumption to adapt to the energy supply constraints of confined spaces.

3. Core Value of Indoor Air Purification Technologies and Novel Nanomaterials

3.1. Core Differences Between Physical Purification and Chemical Purification

The essential difference between physical purification and chemical purification lies in the final fate of pollutants. Physical purification only transfers pollutants from the gas phase to the solid-phase carrier through physical effects such as adsorption and sieving, without altering the intrinsic chemical structure of the pollutants; thus, their toxicity remains intact [42]. Its purification performance is severely constrained by the initial concentration of pollutants, and at high concentrations, the rapid occupation of adsorption sites leads to premature saturation and loss of purification capacity, making it unable to achieve deep purification of pollutants. At low concentrations, the insufficient adsorption driving force results in sluggish reaction kinetics and low removal efficiency, which cannot meet the demand for long-term control of trace-level pollutants in indoor environments.

In contrast, chemical purification breaks the chemical bonds of pollutants through catalytic oxidation/reduction reactions, completely mineralizes them into harmless small-molecule substances such as CO₂ and H₂O, and eliminates the toxicity of pollutants at the source [43]. Unlike the concentration-dependent performance of physical adsorption, chemical catalytic systems can maintain stable degradation activity for both high-concentration pollutant shocks and long-term low-concentration sustained emission scenarios. For indoor environments dominated by low-concentration pollutants, chemical purification can continuously decompose pollutants at the molecular level without a saturation effect, achieving long-term stable purification without frequent material replacement.

Given the inherent characteristics of indoor pollutants, namely sustained emission at low concentrations, a single physical purification technology can hardly achieve long-term stable purification performance. By virtue of its capability to completely degrade pollutants, chemical purification technology has become the core development direction of indoor air purification technologies. Notably, physical adsorption can serve as a synergistic approach for chemical purification. Through the adsorption effect of porous nanomaterials, low-concentration pollutants are enriched around the catalytic active sites, which increases the local pollutant concentration, thereby enhancing the catalytic degradation efficiency and realizing the integrated cycle of “adsorption–enrichment in situ catalytic degradation” [44].

3.2. Core Advantages of Novel Nanomaterials

3.2.1. Ultrahigh Specific Surface Area and Abundant Active Sites

Materials at the nanoscale exhibit an extremely high specific surface area. For instance, the specific surface area of two-dimensional g-C₃N₄ nanosheets can reach over 200 m²/g [45]; it is worth noting that the specific surface area of conventional g-C₃N₄ is generally much lower than that of high-surface-area activated carbon (typically 1000–2500 m²/g). From the perspective of physical adsorption, its low specific surface area results in a far lower saturated adsorption capacity for gaseous pollutants compared with activated carbon, which limits its ability to rapidly enrich low-concentration indoor pollutants via physical adsorption. However, unlike traditional porous adsorbents represented by activated carbon, which merely achieve phase transfer of pollutants through physical interactions, g-C₃N₄ functions as a catalytic material that can completely mineralize pollutants via surface redox reactions. Its purification performance is determined mainly by the intrinsic activity of the material, the accessibility of surface active sites, and the separation efficiency of photogenerated carriers, rather than relying solely on the specific surface area.

Metal–organic frameworks (MOFs) can achieve even higher values. Yang et al. [46] synthesized MOF-177 via a solvothermal method, which delivered a specific surface area up to 2790 m²/g and excellent adsorption performance toward VOCs, including acetone, benzene, and BTEX. In addition, Yang et al. [47] synthesized a novel metal–organic framework material, MIL-101, via the hydrothermal method. This material possesses an ultra-high specific surface area of 5870 m²/g. Experimental tests have verified that MIL-101 is applicable for the removal of various VOCs, including polar acetone and nonpolar benzene, toluene, ethylbenzene, and xylene, demonstrating excellent potential for practical applications. A sufficient specific surface area of the catalyst can expose a large number of catalytic active sites while providing ample space for the adsorption and enrichment of pollutants, making it perfectly suited to the demand for efficient catalytic degradation of low-concentration indoor pollutants.

3.2.2. Controllability of Morphology and Electronic Structure

Precise design of the crystal facets, defects, pore structures, and active sites of materials can be realized through the regulation of nanomaterial synthesis strategies. For example, introducing oxygen vacancies into metal oxides via defect engineering can significantly enhance the oxygen adsorption and activation capacity of the materials, enabling efficient catalytic oxidation of formaldehyde at room temperature [48]. Sun et al. [49] prepared a series of MnO_x/γ-Al₂O₃ catalysts via the incipient wetness impregnation method, where the γ-Al₂O₃ support was pretreated with acid, alkali, and hydrogen peroxide, respectively. Among them, the MnAl-II catalyst (with the support pretreated by acetic acid) was classified as an oxygen-vacancy-rich catalyst owing to its optimized metal dispersion, abundant oxygen vacancies, and acidic sites. This catalyst achieved a formaldehyde conversion of nearly 100% at 30 °C with an O₃/HCHO molar ratio of 2.0. While Liou et al. [50] proposed a post-thermal treatment strategy to regulate oxygen vacancies and lattice oxygen in MnO_x catalysts. This enables precise modulation of surface redox properties and achieves efficient photothermocatalytic oxidation of formaldehyde (HCHO). After thermal treatment optimization at 80 °C, the modified catalyst exhibited excellent formaldehyde removal performance, with a removal efficiency exceeding 0.25 ppm·g⁻¹·min⁻¹.

3.2.3. Excellent Room-Temperature Catalytic Activity and Multifunctional Synergy

Through component regulation and structural design, novel nanomaterials enable efficient catalytic reactions under ambient temperature and pressure without additional harsh conditions such as high temperature and high pressure, thus meeting the low-energy-consumption requirements of indoor living environments. Meanwhile, multi-component composite modification can endow a single material with multiple functions, including photocatalysis, room-temperature thermal catalysis, and sterilization, satisfying the demand for synergistic purification of multiple indoor pollutants [51].

3.2.4. Favorable Environmental Stability and Scenario Adaptability

Strategies including surface modification, core–shell structural design, and support immobilization can significantly improve the water resistance, anti-poisoning performance, and anti-corrosion properties of nanomaterials, adapting them to indoor environments with high humidity and coexisting complex pollutants. Meanwhile, they can be fabricated into various forms such as powders, coatings, filter elements, and thin films via molding processes, which are compatible with a wide range of application scenarios, including household air purifiers, fresh air systems, passive purification building materials, and special equipment for confined spaces [52].

4. Novel Nanomaterial Systems for Indoor Air Chemical Purification

4.1. Noble Metal-Based Novel Nanomaterials

Noble metal-based nanomaterials are among the earliest catalytic materials applied to indoor air chemical purification. Benefiting from their outstanding oxygen activation capacity, room-temperature catalytic activity, and anti-poisoning performance, they have become a core material system for the room-temperature catalytic oxidation of indoor pollutants, mainly comprising noble metal components such as Pt, Pd, Au, Ag, and Rh [53]. These noble metals exhibit distinct catalytic activities and applicable conditions during pollutant removal. Pt effectively promotes the oxidation of carbon monoxide at high temperatures, rapidly converting toxic indoor CO into harmless carbon dioxide, and is suitable for CO pollution control under high-temperature operating conditions. Au delivers outstanding activity toward CO oxidation at low temperatures. It can efficiently catalyze this reaction without heating, making it highly suitable for the removal of low-concentration CO in indoor ambient environments. Pd enables the efficient oxidative degradation of VOCs such as formaldehyde and benzene at moderate and low temperatures, converting these major indoor gaseous pollutants into carbon dioxide and water, which matches conventional indoor temperature conditions. Ag possesses excellent catalytic oxidation performance for polar VOCs (e.g., formaldehyde) at room temperature. Meanwhile, it shows a certain adsorptive–catalytic removal effect on low-concentration odorous pollutants, including hydrogen sulfide and ammonia, enabling the simultaneous treatment of multiple mild contaminants. In addition, Rh exhibits superior catalytic reduction activity for NO_x at moderate and high temperatures. It can convert indoor NO_x generated from air infiltration or combustion into harmless nitrogen and water and performs remarkably well, especially in the simultaneous removal of multiple pollutants.

However, traditional noble metal nanoparticles suffer from inherent drawbacks, including low atomic utilization, high cost, and easy deactivation via agglomeration. In recent years, research efforts have been highly focused on the precise regulation of nanostructures and the optimization of noble metal loading. Monodisperse noble metal nanocrystals can expose highly active crystal facets through facet engineering, thereby boosting the purification efficiency of harmful gases [54]. Meanwhile, the core–shell structure design can inhibit the agglomeration of noble metal nanoparticles via the protection of an inert shell layer and simultaneously enhance catalytic activity through the electron transfer effect between the core and shell. Zhao et al. [55] fabricated hierarchical core–shell structured Al₂O₃@Pd-CoAlO (Pd-CoAlO-Al) microspheres and applied them to the catalytic oxidation of toluene. Experimental results demonstrated that the core–shell structured Pd-CoAlO-Al catalyst exhibited excellent catalytic efficiency. This performance was attributed to the uniform distribution of Pd-CoAlO nanosheets on the Al₂O₃ support, as well as the strong metal–support interaction (SMSI) between the catalytically active Pd-CoAlO nanosheets and the Al₂O₃ support, which effectively prevented the high-temperature agglomeration of Pd nanoparticles.

Noble metal single-atom materials have emerged as a research hotspot in this field in recent years. Their atomically dispersed active sites achieve 100% atomic utilization, enabling an order-of-magnitude improvement in catalytic activity while drastically reducing noble metal consumption [56]. Zhang et al. [57] synthesized Ag single-atom catalysts supported on Mn₂O₃ nanowires via an in situ molten salt method. The 0.06 wt% Ag/Mn₂O₃ catalyst exhibited outstanding catalytic activity for toluene oxidation, with the temperatures required for 50% and 90% toluene conversion (T₅₀ and T₉₀) being as low as 170 °C and 205 °C, respectively, at a gas hourly space velocity (GHSV) of 40,000 mL/(g·h). Its catalytic activity outperformed that of 5.8 wt% Au/3DOM Mn₂O₃ and was comparable to that of 1.0 wt% AuPd_1.85/3DOM Mn₂O₃. To improve the stability of the Ag single-atom catalyst, CeO₂ was introduced for modification. The obtained 0.63 wt% CeO₂-0.06 wt% Ag/Mn₂O₃ catalyst displayed excellent catalytic stability, with only a 10% decrease in toluene conversion after 50 h of continuous reaction at 195 °C. STEM and EDX elemental mapping images (Figure 1) reveal that both Ag single atoms and CeO₂ are highly dispersed on the surface of the Mn₂O₃ nanowire support. The oxygen species formed at the oxygen vacancies on the CeO₂ surface can efficiently migrate to the active sites at the Ag-Mn₂O₃ interface and replenish the surface-active lattice oxygen in a timely manner, which is the core mechanism underlying its superior catalytic activity.

Noble metal-based nanomaterials hold core advantages of high room-temperature catalytic activity, excellent stability, and strong anti-poisoning capability. Nevertheless, their high production cost poses a critical barrier to large-scale application in civilian fields. Future research priorities will be focused on the low-cost, scalable fabrication of single atom/cluster materials and the modification and optimization of non-noble metal supports to further reduce the noble metal loading while simultaneously enhancing the long-term operational stability of the materials.

4.2. Non-Noble Metal-Based Novel Nanomaterials

Non-noble metal-based nanomaterials, with the advantages of wide availability of raw materials, low cost, and high structural tunability, have become the most promising material system for industrialization in the field of indoor air chemical purification, as well as the most extensively studied material category at present.

Transition metal oxides and composite oxides are the core representatives of this system, mainly including single metal oxides such as MnO₂, Co₃O₄, CeO₂, TiO₂ and ZnO, as well as spinel-type and perovskite-type composite oxides. Manganese-based oxides, featuring multiple variable valence states and abundant oxygen vacancies, are representative materials for the room-temperature catalytic oxidation of gaseous pollutants [58,59]. Xu et al. [60] synthesized honeycomb-like δ-MnO₂ nanomaterials, which introduced massive surface oxygen vacancies via defect engineering and achieved efficient degradation of toluene at room temperature. In contrast, cerium-based oxides, with their excellent oxygen storage capacity and oxygen migration performance, are commonly used as supports and active components to construct composite oxides with other transition metals. For example, Huang et al. [61] synthesized Mn-Ce composite oxides via the citric acid complex method. The synergistic effect between Mn and Ce significantly enhanced the oxygen vacancy concentration and redox performance of the material, enabling efficient formaldehyde degradation at room temperature. In addition, spinel-type composite oxides such as Co₃O₄ and NiCo₂O_x exhibit outstanding performance in both room-temperature thermal catalysis and photocatalysis owing to their tunable metal sites and excellent electron conduction properties [62]. Wang et al. [63] prepared a series of spinel-type NiCo₂O_x nickel–cobalt-based composite oxide catalysts doped with Mg, Fe, Cu and Ce via the coprecipitation method and systematically investigated the effects of doping with different metal elements on their CO catalytic oxidation performance, as well as their water resistance and sulfur tolerance. The study revealed that Fe doping remarkably improved the low-temperature CO catalytic activity and water resistance of NiCo₂O_x, with a CO catalytic efficiency of 91.72% at 100 °C and a CO conversion still maintained at 98.37% at 140 °C in the presence of 10% water vapor (Figure 2).

Non-noble metal single-atom materials have emerged as a cutting-edge research focus in this field over recent years. By atomically dispersing non-noble metal atoms (e.g., Fe, Co, Ni, Cu, Mn) onto supports, these materials achieve atomic-level exposure of active sites and nearly 100% atomic utilization efficiency, thus overcoming the critical limitation of insufficient active sites in conventional non-noble metal catalysts. As shown in Figure 3, Qu et al. [64] investigated the selective catalytic reduction of NO with NH₃ (NH₃-SCR) reaction. Aiming to elucidate the general structural characteristics of highly active catalytic sites for SCR, the authors designed and synthesized a series of single-atom catalysts, namely Mo₁/Fe₂O₃, W₁/Fe₂O₃, and Fe₁/WO₃, and explored the effects of catalytic site structure and acid-redox properties on SCR reaction performance. It was revealed that isolated acidic metal ions in the single-atom catalysts and adjacent redox-active metal ions on the support surface can form uniform dual-site acid-redox sites, which were identified as the core highly active sites for the SCR reaction. The abundance of these sites exhibited a linear positive correlation with the SCR reaction rate, and modulation of the acidic or redox properties could directly tune the catalytic activity. This work, for the first time, verified in a single-atom catalyst system that dual-site acid-redox sites serve as the general structural feature of high-activity SCR catalytic sites. The authors further proposed a design strategy for developing high-performance SCR catalysts by optimizing the acid-redox properties of the dual sites, which provides novel design guidelines and theoretical support for the development of advanced catalysts for the chemical purification of indoor nitrogen oxides (NO_x). Furthermore, dual-atom materials can further boost catalytic activity via the synergistic effect between two metal atoms. For instance, in Fe-Co dual-atom catalysts, the synergy between the two metal sites enables simultaneous optimization of pollutant adsorption and activation as well as reactive oxygen species generation, thereby realizing the synergistic degradation of multiple pollutants [65].

In addition, metal sulfide, phosphide and nitride nanomaterials exhibit excellent performance in the fields of visible-light photocatalysis and electrocatalysis, benefiting from their intrinsic features of narrow band gap and high electrical conductivity. Layered double hydroxides (LDHs) and their derivatives have been widely applied in room-temperature catalysis and photocatalysis owing to their unique two-dimensional layered structure, tunable metal sites and abundant surface hydroxyl groups [66].

The core advantages of non-noble metal-based nanomaterials lie in their low cost and wide availability of raw materials. However, their room-temperature catalytic activity is still inferior to that of noble metal-based counterparts. Future research priorities will focus on strategies including defect engineering, heterostructure construction and single-atom modification to further enhance the room-temperature catalytic activity, water resistance and anti-poisoning performance of these materials.

4.3. Novel Porous Nanoframe Materials

Novel porous nanoframe materials are a class of crystalline porous materials featuring periodic pore channel structures, ultrahigh specific surface area, and precisely designable pore structures and functional groups. This category mainly includes metal–organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs) and has emerged as an emerging material system in the field of indoor air chemical purification in recent years.

4.3.1. MOFs

MOFs are self-assembled from metal nodes and organic ligands via coordination bonds [67]. Their specific surface area can reach as high as over 7000 m²·g⁻¹ [68], and their pore structures and functional groups can be precisely regulated through ligand engineering, which perfectly matches the requirements for adsorption and enrichment as well as in situ catalytic degradation of indoor low-concentration pollutants [69]. Representative MOF systems include the MIL series [70], UiO series [71], and ZIF series [72], among which UiO-66 has become the most widely used MOF material in indoor air purification due to its excellent chemical stability.

Pristine MOFs have limited catalytic activity, and their catalytic performance is generally improved through strategies including metal node modification, ligand functionalization, and active component immobilization. Chen et al. [73] prepared an acid–base tunable Deep Eutectic Solvent (DES) using 2-methylimidazole and p-toluenesulfonic acid as raw materials and developed a green and rapid synthesis method for the zirconium-based metal–organic framework UiO-66 (Figure 4). The as-prepared UiO-66-DES nanoparticles possessed high crystallinity, a large specific surface area, and abundant open Zr Lewis acid active sites and exhibited outstanding performance in catalyzing the acetalization reaction of benzaldehyde and methanol at room temperature, with a conversion of 94% achieved within 1 h. This work overcame the long reaction time and non-environmentally friendly solvents associated with conventional MOF synthesis. The UiO-66 prepared in this study shows promising application prospects in the acetalization of carbonyl compounds due to its high-efficiency Lewis acid catalytic performance and provides a potential nanomaterial candidate for the catalytic degradation of aldehyde-containing pollutants in indoor air. In addition, MOF-derived materials can retain the porous structure of pristine MOFs through high-temperature calcination, while generating highly active metal/metal oxide nanoparticles and nitrogen-doped carbon supports. These materials combine excellent stability and high catalytic activity and deliver superior performance in both photocatalysis and room-temperature thermal catalysis fields [74].

4.3.2. COFs

COFs are constructed from organic monomers linked via covalent bonds [75]. Compared with MOFs, COFs possess higher chemical and water stability and enable precise regulation of their band structures and functional groups through rational monomer design, thus exhibiting tremendous application potential in the field of visible-light photocatalysis [76].

Chen et al. [77] systematically investigated the structural characteristics (Figure 5a–c) and CO oxidation catalytic performance of a palladium single-atom catalyst supported on a triazine-based covalent organic framework (Pd₁/trzn-COF) using Density Functional Theory (DFT). It was found that Pd single atoms could be stably anchored on the pore channel walls and surface sites of trzn-COF via d–π interactions and exhibited dynamic diffusion behavior between adjacent optimal sites. The CO oxidation catalytic activity of this catalyst could be modulated by the anion–π interactions of the support and the electron-withdrawing effect of amino groups. The optimal reaction pathway followed the termolecular Eley–Rideal mechanism, with the lowest energy barrier of the rate-determining step at the W3 site, enabling efficient CO oxidation at room temperature (Figure 5d–f). This work revealed the synergistic mechanism between the COF support and noble metal single atoms and provides important theoretical reference and design guidelines for the development of novel covalent organic framework-based nano-sized single-atom catalysts for the chemical purification of harmful indoor gases such as CO. POPs feature high stability and excellent designability. Through the introduction of specific functional groups, POPs can achieve targeted adsorption and catalytic degradation of characteristic pollutants and thus hold promising application prospects in the field of purification of characteristic pollutants in industrial indoor environments [78].

The core advantages of porous nanoframe materials lie in their ultrahigh specific surface area and precise controllability over pore structures and functional groups, enabling the integrated process of “adsorption–enrichment coupled with in situ catalytic degradation”. However, they are still constrained by the high cost of large-scale preparation and insufficient water stability of some materials. Future research priorities will focus on the development of high-stability MOFs and COFs and the optimization of low-cost, large-scale synthesis processes, as well as functional modification to boost their catalytic activity.

4.4. Novel Two-Dimensional Nanomaterials

Two-dimensional (2D) nanomaterials feature atomic-level thickness, ultrahigh specific surface area, tunable band structures, and excellent electron transport properties. They can expose abundant catalytic active sites on their surface and have become a research hotspot in the field of indoor air chemical purification. This category mainly includes graphene and its derivatives, graphitic carbon nitride (g-C₃N₄), and two-dimensional transition metal dichalcogenides (TMDs).

Among them, graphene and its derivatives are the most representative, including graphene oxide (GO) and reduced graphene oxide (rGO). They possess ultrahigh electron mobility, a large specific surface area, and favorable chemical stability and are generally used as catalytic supports and synergistic components to compound with active components such as metal oxides and MOFs so as to improve the electron transport performance and adsorption capacity of the composite materials [79,80]. For example, Winayu et al. [81] prepared a composite photocatalyst of rGO and S, N co-doped TiO₂ via a solvothermal method and investigated its visible-light photocatalytic degradation performance for toluene, a typical indoor VOC, as well as the relevant environmental influencing factors. It was found that the optimal doping ratio of rGO was 0.1 wt%. At this ratio, the rGO/S_0.05N_0.1TiO₂ exhibited significantly better toluene degradation efficiency than pristine TiO₂ and single S, N co-doped TiO₂ (Figure 6), which was attributed to the enhanced specific surface area, suppressed recombination of photogenerated carriers, and broadened visible-light absorption. It is worth noting that a higher content of rGO leads to a decrease in the visible-light photocatalytic degradation efficiency of toluene. The primary reason is that excessive rGO produces a shielding effect, which directly blocks light from penetrating the catalyst surface and drastically reduces the absorption and utilization of visible light by TiO₂ active sites. Meanwhile, excessive rGO increases the recombination probability of photogenerated electron–hole pairs, weakening charge separation and transport efficiency. Furthermore, high rGO content tends to cause agglomeration of catalyst particles, reducing the specific surface area and the number of active sites of the material. The combined effect of these multiple factors ultimately results in a significant decline in the photocatalytic performance for toluene degradation.

4.5. Novel Multifunctional Composite Nanomaterials

Single-component nanomaterials generally suffer from inherent limitations such as single function, insufficient catalytic activity, and poor stability. In contrast, multifunctional composite nanomaterials can achieve complementary advantages and synergistic effects among different components through rational multi-component design and structural optimization and have become the core development direction of materials for indoor air chemical purification.

The design principles of composite nanomaterials mainly include the following three aspects. Firstly, the integration of adsorption and catalysis. By compounding porous adsorption supports (activated carbon, molecular sieves, MOFs) with catalytic active components, rapid enrichment and in situ catalytic degradation of pollutants can be realized, addressing the key challenge of low catalytic efficiency for indoor low-concentration pollutants [82]. Secondly, the synergy of multiple catalytic pathways. Through the combination of photocatalytic components and room-temperature thermal catalytic components, full-time continuous purification under both light and dark conditions can be achieved. Alternatively, the coupling of plasma catalysis and photocatalysis can enhance the degradation efficiency of pollutant gases via multi-field synergy. For instance, in our previous work [83], we prepared a porous Fe-doped CeO₂ catalyst (Fe@CeO₂-T) via a carbon template method and coupled it with dielectric barrier discharge (DBD) plasma to construct an efficient toluene removal system. The 5 wt% Fe-doped porous CeO₂, with abundant oxygen vacancies, excellent redox properties and photoelectric response characteristics, could regulate the plasma discharge into a more uniform mode and simultaneously utilize the photo-effect generated by plasma discharge (Figure 7a), thus significantly strengthening the plasma–catalyst interaction. As shown in Figure 7b,c, this system exhibited a 1.2-fold enhancement in toluene conversion compared with the conventional CeO₂ system at an input power of 8 W, with the CO₂ selectivity maintained at approximately 80% and extremely low CO selectivity. The superior performance of this system was attributed to the triple synergy among the (photo)catalytic activity of the catalyst, the plasma activation capability, and the interaction between the two. Thirdly, the integration of pollutant degradation and sterilization. By compounding catalytic components with antibacterial components, efficient inactivation of bacteria and viruses can be simultaneously achieved with the degradation of organic pollutants, meeting the application requirements of scenarios such as hospitals and public buildings [84].

5. Core Catalytic Systems of Novel Nanomaterials for Indoor Air Chemical Purification

5.1. Photocatalytic Purification System

Photocatalytic purification technology is the most extensively studied and commercially mature technology system in the field of indoor air chemical purification. Its core mechanism is based on the following process: when semiconductor nanomaterials are irradiated by light with energy higher than their band gap width, valence band electrons are excited to the conduction band, forming photogenerated electron–hole pairs. The photogenerated electrons and holes migrate to the material surface and react with adsorbed oxygen and water molecules to generate strongly oxidizing reactive oxygen species (ROS), including hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻). These ROS can break the chemical bonds of pollutants and completely mineralize them into harmless small-molecule substances such as CO₂ and H₂O [85]. The core advantages of this technology lie in mild reaction conditions, feasibility at ambient temperature and pressure, drivability by sunlight or indoor visible light, no secondary pollution, and excellent adaptability to the environmental requirements of civil residential scenarios.

Traditional photocatalytic materials are represented by TiO₂, which has a band gap of 3.2 eV and only responds to ultraviolet (UV) light. However, UV light accounts for less than 5% of the total light in indoor environments, resulting in extremely low efficiency of TiO₂ in practical applications [86]. Accordingly, recent research has focused on the development and performance optimization of novel visible-light-responsive nano-photocatalytic materials. Binas et al. [87] systematically investigated the preparation, performance and application of transition metal-doped modified TiO₂ photocatalysts. Through experiments including environmental chamber simulation and irradiation under indoor/simulated sunlight, the authors tested the degradation performance of modified TiO₂-based building materials (coatings, calcareous/cement-based matrices) against pollutants such as NO_x, toluene and acetaldehyde. Meanwhile, inactivation experiments on pathogenic bacteria (including Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus), fungi and viruses in water and indoor air were carried out, and the types and concentrations of carbonyl by-products generated during the photocatalytic process were detected. As confirmed in Figure 8, 0.1% Mn-doped TiO₂ was a high-efficiency photocatalytic material under indoor visible light. Its NO degradation rate in calcareous/cement-based matrices reached up to 95%, and it also exhibited excellent degradation performance against VOCs such as toluene and acetaldehyde. It was clarified that the modified TiO₂-based photocatalysts could achieve efficient inactivation of multiple pathogenic bacteria and viruses via ROS generation. For example, 1 wt% Mn-doped TiO₂ completely inactivated Klebsiella pneumoniae in water under simulated sunlight. And Fe/Al/Cr-doped TiO₂ achieved a 7-log reduction of Staphylococcus aureus within 30 min. The improvement in its performance is not caused by a single factor but by the synergistic effect of the modified band structure, enhanced charge separation ability, increased oxygen vacancy concentration, and enlarged specific surface area. Doping with metal ions such as Fe, Al and Cr introduces impurity levels that narrow the band gap of TiO₂, extending its photoresponse to the visible-light region; the dopant ions can act as trapping sites for photogenerated electron–hole pairs, significantly suppressing carrier recombination and enhancing charge separation efficiency. Doping also induces lattice distortion in TiO₂, increasing the oxygen vacancy concentration, surface active sites and pollutant adsorption capacity, while appropriate doping can regulate the crystallite size and increase the specific surface area of the material, further improving the contact area and reaction rate of the catalytic reaction. The combined action of multiple factors ultimately leads to a significant improvement in photocatalytic performance. Meanwhile, it was found that photocatalytic coatings generated carbonyl by-products such as formaldehyde and acetaldehyde due to the degradation of organic components, and pre-irradiation pretreatment could significantly reduce the emission of such by-products.

5.2. Room-Temperature Thermal Catalytic Purification System

Room-temperature thermal catalytic purification technology, also known as room-temperature catalytic oxidation technology, refers to a technology system that achieves complete oxidative decomposition of pollutants by oxygen in air through the activation of catalysts at room temperature without additional light irradiation or heat input. The active sites on the catalyst surface can activate oxygen and pollutant molecules at room temperature to break the chemical bonds of pollutants. Through reaction pathways including the Mars–van Krevelen mechanism and Langmuir–Hinshelwood mechanism, pollutants are gradually oxidized and finally mineralized into CO₂ and H₂O [88].

Compared with the photocatalytic system, the room-temperature thermal catalytic system requires no light irradiation, enables full-time continuous purification with no secondary pollution and fast reaction rates, and is well compatible with light-free and low-light indoor environments. The core research focus of this system is the development of room-temperature catalytic nanomaterials with high activity and excellent stability, which are currently mainly divided into two categories: noble metal-based and non-noble metal-based systems.

5.2.1. Room-Temperature Thermal Catalysis over Noble Metal-Based Materials

Noble metal-based room-temperature catalytic materials are the best-performing system reported to date. As shown in Figure 9, Ahmad et al. [89] prepared oxygen vacancy-rich anatase TiO₂ (V_O-TiO₂) via chemical vapor condensation combined with post-thermal treatment in an N₂ atmosphere and constructed a low-loading Pt-supported catalyst. The valence state and particle size of Pt were regulated through two thermal treatment routes (oxidation and reduction). Via DFT calculations, it was confirmed that the F-type oxygen vacancies in V_O-TiO₂ can transfer excess electrons to Pt through the electronic metal-support interaction (EMSI). This not only achieved high dispersion of Pt clusters but also stabilized the metallic state of Pt and promoted the activation and dissociation of O₂. The as-prepared 0.086 wt% Pt/V_O-TiO₂-r achieved 100% conversion of 10 ppm formaldehyde at room temperature with excellent stability for over 250 min. Its mass-specific reaction rate was remarkably superior to that of conventional Pt/TiO₂ and previously reported similar catalysts. This work provides novel and high-efficiency catalytic material for the room-temperature chemical purification of indoor low-concentration VOCs.

5.2.2. Non-Noble Metal-Based Materials for Room-Temperature Thermal Catalysis

The core representatives of non-noble metal-based room-temperature catalytic materials are manganese-based and cerium-based oxides and their composite oxides. Their core advantage is low cost, while their room-temperature catalytic activity is still inferior to that of noble metal-based counterparts. Recent studies have significantly boosted the room-temperature catalytic activity of non-noble metal materials through strategies including defect engineering, multi-metal composites, and morphology regulation. For example, Zong et al. [90] combined Ce-Eu co-doped modified TiO₂ with a palmitic acid–decanoic acid phase change material to fabricate Ce–Eu/TiO₂ phase change composites and further compounded them with gypsum to obtain a gypsum-based wall functional material integrating photocatalytic formaldehyde removal, phase change heat storage and humidity regulation functions, realizing comprehensive regulation of the indoor environment via building materials. In this work, the optimal sample with mechanical properties meeting the national standard requirements was obtained by optimizing the compounding ratio of Ce–Eu/TiO₂ phase change composites and gypsum. Full-scale tests showed that the material achieved a 68% degradation rate of 1 mg/m³ formaldehyde within 11 h under visible light. Its phase change temperature matched the human comfort range, leading to a maximum indoor temperature difference of 3.9 °C between the test room and the control room, while stabilizing the indoor relative humidity in the range of 38.40–62.30%. This work, for the first time, introduced rare earth-modified TiO₂ photocatalytic materials and phase change heat storage materials into gypsum-based building materials and realized the functional synergy between air purification and building thermal and humidity environment regulation. It provides new design ideas and technical references for the development of multifunctional green indoor building materials, reduction of building energy consumption and improvement of indoor air quality.

However, the room-temperature thermal catalytic system still faces several critical limitations: insufficient room-temperature activity of non-noble metal materials, which makes it difficult to achieve complete mineralization of low-concentration pollutants; significant decrease in catalytic activity under high-humidity environments due to the competitive adsorption of water molecules and pollutant molecules on active sites; and catalyst deactivation caused by the accumulation of reaction intermediates and carbon deposition during long-term operation. Future research priorities will focus on the breakthrough improvement of the room-temperature activity of non-noble metal materials, the optimization of water resistance and anti-poisoning performance, and the enhancement of long-term stability. Meanwhile, research on the catalyst molding process should be carried out to meet the application requirements of commercial purifier filter elements.

5.3. Electrocatalytic Purification System

Electrocatalytic purification technology enables electron transfer on the catalyst surface under an applied voltage, where adsorbed oxygen and water molecules are reduced/oxidized into reactive oxygen species (ROS), including ·OH, ·O₂⁻ and H₂O₂. These ROS react with pollutant molecules in the gas phase to completely mineralize them into harmless small-molecule substances [91]. This technology has received extensive attention owing to its strong reaction controllability, high degradation efficiency, no limitation by light conditions, adaptability to high-concentration pollutant scenarios, as well as its capability to achieve precise control of the catalytic reaction rate by regulating voltage and current.

Wang et al. [92] focused on the core challenges of sluggish proton transfer and severe competition from the hydrogen evolution reaction (HER) in the electrocatalytic NO reduction to ammonia (NORR). The authors proposed a sulfur-mediated interface engineering strategy and designed and prepared an S-Cu@Co/C dual-site electrocatalyst, achieving a substantial improvement in NORR performance and resource utilization of NO pollutants. In this work, a metal–organic framework was used as the precursor, Cu-Co dual sites were constructed via pyrolysis, and sulfur was introduced for interface regulation. Combined with X-ray Photoelectron Spectroscopy (XPS), X-ray Absorption Near Edge Structure (XANES) and Density Functional Theory (DFT) calculations, the sulfur-induced electron rearrangement mechanism at the Cu-Co interface was elucidated. Performance tests showed (Figure 10) that the catalyst achieved an ammonia yield rate of 439.73 μmol h⁻¹ cm⁻² with a Faradaic efficiency of 92.4% in an H-type electrolytic cell. The ammonia yield rate was further increased to 655.3 μmol h⁻¹ cm⁻² in a flow cell, with stable continuous electrolysis for 100 h. The assembled Zn-NO battery also exhibited excellent energy output and ammonia production capability. This work revealed the dual-site synergistic catalytic mechanism for NORR, provides a new paradigm for the rational design of high-efficiency electrocatalytic reduction catalysts, and opens up a new avenue for the chemical purification and resource utilization of indoor/industrial NO pollutants.

At present, the application of electrocatalytic purification technology in the field of indoor air purification is still at the laboratory research stage. Its core bottlenecks lie in three aspects: the low contact efficiency between gaseous pollutants and the electrode surface, which leads to mass transfer limitation and insufficient degradation efficiency; conventional electrocatalytic systems require electrolytes, making them difficult to adapt to gaseous purification scenarios; and high energy consumption, which fails to meet the low-energy demand of civil residential scenarios.

In recent years, researchers have developed all-solid-state electrocatalytic reactors. Replacing conventional liquid electrolytes with solid electrolytes addresses the problems of electrolyte leakage and volatilization, while the design of gas diffusion electrodes enhances the mass transfer efficiency of gaseous pollutants, laying a solid foundation for the indoor application of electrocatalytic technology. Future research priorities will focus on the development of low-energy-consumption, high-activity electrocatalytic materials, the optimized design of all-solid-state gas-phase electrocatalytic reactors, and the in-depth elucidation of the electrocatalytic degradation mechanism for low-concentration gaseous pollutants.

5.4. Non-Thermal Plasma Synergistic Catalytic Purification System

Non-thermal plasma (NTP) synergistic catalytic purification technology is a novel purification system that combines non-thermal plasma technology with nanocatalysis technology. Non-thermal plasma is generated via high-voltage discharge, in which high-energy electrons collide with oxygen and water molecules in the air to produce abundant active species, including active free radicals and excited-state molecules. Meanwhile, the high-energy electrons in the plasma can break the chemical bonds of pollutants to achieve preliminary decomposition of pollutants. The nanocatalyst can further enhance the generation of active species and simultaneously catalyze the decomposition of by-products such as ozone and toxic intermediates generated from the plasma reaction, thus realizing complete mineralization of pollutants. This improves purification efficiency while controlling secondary pollution [93].

The core advantages of this system include a fast reaction rate, large air-handling capacity, synergistic degradation of multiple pollutants, and excellent compatibility with confined space scenarios featuring high pollutant concentration and large air volume. Single non-thermal plasma technology suffers from inherent drawbacks such as high energy consumption, abundant by-products, and excessive ozone emissions. Whereas its combination with nanocatalysis technology can significantly reduce the reaction energy consumption and simultaneously achieve precise control of by-products to meet the safety requirements of indoor environments [94].

Lu et al. [95] focused on the challenges of low efficiency and poor anti-airflow interference performance of single catalytic technology for treating indoor low-concentration formaldehyde and proposed a purification scheme coupling TiO₂/activated carbon (TiO₂/AC) nano photocatalytic film with NTP technology. In this work, the TiO₂/AC composite film was prepared via an impregnation method, and two NTP electrode configurations (needle-plate and wire-plate) were compared (Figure 11a,b). It was found that the wire-plate configuration delivered better synergistic purification performance due to its higher discharge intensity and larger corona current. By regulating parameters including applied voltage, airflow velocity and humidity, it was confirmed that the coupled system exhibited a significant synergistic effect, with formaldehyde removal efficiency and reaction rate far exceeding the sum of individual photocatalytic oxidation (PCO) and individual NTP systems. The formaldehyde removal efficiency reached nearly 87% under the optimal humidity condition (Figure 11c,d). This work systematically verified the application potential of the PCO-NTP-coupled technology in indoor formaldehyde purification for the first time, clarified the key optimization parameters and synergistic mechanism, and provided an important experimental basis and technical reference for the development of high-efficiency and anti-interference indoor air purification technologies.

In addition, Roland et al. [96] conducted research on air pollutant purification using non-thermal plasma (NTP)-coupled catalysis. By introducing ferroelectric materials (BaTiO₃, PbZrO₃–PbTiO₃) and catalytically active materials (LaCoO₃, porous Al₂O₃/SiO₂) into the NTP discharge region, they compared the performance of homogeneous plasma, ferroelectric packed beds, in-plasma catalytic reactors (IPCRs), post-plasma catalytic reactors (PPCRs), and a two-layer combined system. It was found that ferroelectric materials can strengthen the local electric field, increase electron energy and pollutant conversion efficiency, and reduce energy consumption but cannot improve CO₂ selectivity. In-plasma catalysis allows short-lived active species (O (³P), OH, etc.) to enter the catalyst pores, greatly enhancing CO₂ selectivity and reducing polymeric by-products, while also forming an active oxygen reservoir on the catalyst surface. In contrast, post-plasma catalysis relies only on ozone and shows no degradation effect on refractory fixed-state pollutants. The two-layer combination of the two achieves synergy between high conversion efficiency and high selectivity.

In terms of activity, the in-plasma catalysis/combined system is the best, followed by the ferroelectric packed system, then the post-catalysis system, with homogeneous plasma being the worst. For stability, ferroelectric materials are more stable, whereas catalytically active materials tend to deactivate due to polymer deposition. Regarding by-product control, in-plasma catalysis significantly reduces CO and polymer formation and improves CO₂ selectivity; ferroelectric systems only partially reduce polymers, while post-catalysis and homogeneous systems exhibit poor by-product control ability.

Notably, the core bottlenecks of this system lie in the energy consumption and noise issues in civil residential scenarios, the potential generation of toxic by-products during the discharge process, and catalyst deactivation caused by carbon deposition during long-term operation.

5.5. Novel Synergistic Catalytic Purification System

Single catalytic systems generally suffer from inherent limitations such as limited applicable scenarios and insufficient purification efficiency. In contrast, the novel synergistic catalytic system achieves complementary advantages of different catalytic systems by coupling two or more catalytic pathways, which significantly improves the purification efficiency and scenario adaptability, and has emerged as a cutting-edge research focus in this field over recent years.

The photothermal synergistic catalytic system is the most widely studied synergistic system at present. It converts light energy into thermal energy through photothermal conversion materials to raise the local temperature on the catalyst surface and simultaneously combines the dual effects of photocatalysis and thermal catalysis to significantly boost the catalytic degradation efficiency. This system can be driven by sunlight or indoor visible light. While realizing the photocatalytic reaction, it enhances the thermal catalytic activity via the photothermal effect, achieving continuous, full-time purification under both light and dark environments. For example, Liu et al. [97] prepared porous carbon spheres using glucose as the raw material and fabricated MnO_x-modified porous carbon sphere photothermal catalysts by loading MnO_x via a KMnO₄ impregnation method. The authors conducted research on the photothermal catalytic elimination of formaldehyde, a typical indoor VOC, under visible light at room temperature. In this work, the optimal catalyst 0.05MnC-30-500, was obtained by optimizing the activation temperature of the carbon spheres, KMnO₄ concentration and impregnation time. It reached a surface temperature of 93.8 °C under visible light, achieved an 87.5% removal efficiency of 160 ppm formaldehyde (Figure 12), and exhibited no obvious attenuation of catalytic performance after five cycling tests. The catalyst prepared in this work was synthesized from low-cost raw materials with a facile preparation process and realized efficient elimination of formaldehyde under visible light at room temperature via the photothermal synergistic effect. This work provides a new approach for the development of nanomaterials for indoor low-concentration VOC purification, and the as-prepared catalyst also has promising application potential in practical indoor air purification.

Furthermore, the adsorption–catalysis cycle system can realize long-term cyclic utilization of materials through the adsorption–enrichment and in situ catalytic regeneration of porous materials. The integrated degradation–sterilization catalytic system can achieve efficient inactivation of microorganisms while degrading organic pollutants, meeting the application requirements of scenarios such as hospitals and public buildings [98].

5.6. Priority Recommendation of Catalytic Systems

The core difference between laboratory research and industrial application of indoor air purification technologies lies in that the former focuses on the optimal degradation performance under ideal conditions, while the latter prioritizes the comprehensive economy, long-term stability, working condition adaptability, and engineering feasibility of the technology under complex actual indoor environments. Based on the above discussion, this paper systematically compares the industrial application characteristics of the core catalytic systems and catalyst materials covered in this review, as shown in

Table 1. Industrial application performance comparison of core catalytic systems.

Catalytic System	Core Applicable Catalyst Materials	Core Advantages for Industrial Application	Priority Adaptation Scenarios
Room-Temperature Thermal Catalytic Purification	1. Noble metal-based: Low-loading Pt/Pd/Ag nano/single-atom catalysts 2. Non-noble metal-based: Mn-Ce composite oxides, Mn-based oxides, spinel-type composite oxides	Full-time purification without light/heat input; no secondary pollution, human-friendly; mature process, compatible with existing equipment; non-noble metal systems, cost-effective	Civil residential buildings, office buildings, schools, and other daily indoor scenarios with long-term human occupancy
Photocatalytic Purification	1. Transition metal-doped TiO2 composite catalysts 2. g-C3N4-based composite photocatalysts 3. MOFs/COFs-based visible-light photocatalysts	Mild conditions, visible-light-driven, low energy consumption; synchronous degradation and sterilization; can be made into passive purification coatings/building materials; modified TiO2 has mature production and ultra-low cost	Civil residential buildings, public buildings with good lighting conditions, passive purification building materials, and household small air purifiers
Non-Thermal Plasma Synergistic Catalytic Purification	Mn/Ce-based composite oxides, Fe-doped CeO2, BaTiO3-supported catalysts, honeycomb monolith catalysts	Fast reaction, strong large air volume processing capacity, high pollution shock resistance; synchronous purification, sterilization and dust removal	Confined and restricted spaces, industrial workshops, and public buildings with intermittent high pollution load
Electrocatalytic Purification	Transition metal dual-site catalysts, metal sulfide/phosphide electrocatalysts, all-solid-state gas diffusion electrode catalysts	Strong controllability, no light limitation, adjustable degradation rate; excellent high-concentration pollutant degradation efficiency; can realize pollutant resource utilization	Industrial indoor environments with high-concentration NOx pollution and special confined spaces with controllable energy supply
Adsorption–Catalysis Integrated Composite System	MOFs-derived materials, activated carbon/zeolite-supported catalysts, porous carbon-based composite catalysts	Adsorption–catalysis cycle solves low-concentration pollutant treatment pain point; synchronous removal of particulate and gaseous pollutants; pore structure designable for targeted purification	Industrial environments with characteristic pollutants, high-end civil purification equipment, and hospital high-efficiency sterilization and purification scenarios

.

Based on the above systematic evaluation of industrial application suitability, room-temperature thermal catalytic systems represented by low-loading noble metals and Mn-Ce composite oxides, as well as modified TiO₂-based visible-light photocatalytic systems, are currently the most industrialized material systems. In contrast, non-thermal plasma-assisted catalysis and MOF-derived integrated adsorption–catalysis systems exhibit great industrialization potential under specific scenarios.

6. Challenges and Future Perspectives

6.1. Challenges

Despite the initial commercial application of novel nanomaterials in the field of indoor air purification, there are still core bottlenecks restricting the whole industrial chain from laboratory research and development to large-scale industrialization. First is the challenge of large-scale preparation. High-performance nanomaterials developed in the laboratory mostly rely on elaborate synthesis processes, which generally suffer from complex procedures, high cost, and poor batch-to-batch consistency, making it difficult to achieve low-cost, large-scale controllable mass production. For example, noble metal single-atom catalysts and MOF/COF materials exhibit excellent performance in gram-scale laboratory preparation. However, during ton-scale industrial production, they are prone to agglomeration of active sites, structural collapse, and performance degradation. Meanwhile, the persistently high preparation cost fails to meet the cost–performance requirements of the civil market [99]. Second is the insufficient adaptability to actual working conditions. Laboratory performance tests are mostly conducted under ideal conditions with a single pollutant and constant temperature and humidity, while the actual indoor environment is a complex working condition featuring coexistence of multiple pollutants, large temperature and humidity fluctuations, interference from impurity gases, and continuous release of low-concentration pollutants. This leads to the actual purification performance of the materials being far lower than the laboratory data. The competitive adsorption between water molecules and pollutants under high humidity, as well as the competitive reaction of components in multi-pollutant systems, will cause a significant decrease in catalytic activity and degradation efficiency [100].

6.2. Future Development Directions

In view of the core challenges faced by nanocatalytic materials in the current field of indoor air chemical purification, future research in this field will be systematically promoted following the whole-chain innovation logic, with a core focus on four major dimensions. In the basic research dimension, machine learning and high-throughput technologies will be leveraged to empower the precise design and controllable synthesis of catalytic materials and break through the large-scale preparation technology of low-cost, high-performance catalytic materials. Meanwhile, via the combination of in situ characterization and theoretical calculation, the catalytic reaction mechanism in complex indoor environments will be deeply elucidated, providing theoretical support for the rational design of materials. In the application technology dimension, focusing on diversified scenarios including civil residential buildings, indoor low-light environments, confined spaces, and large public buildings, room-temperature thermal catalysis, visible-light photocatalysis, and plasma/photoelectric synergistic catalytic systems will be optimized and innovated, respectively, to achieve precise matching between purification technologies and scenario requirements.

In the engineering and industrialization dimensions, core engineering bottlenecks including large-scale molding of nanomaterials, efficient immobilization, reactor structure optimization, and regeneration of catalytic modules will be systematically broken through. Low-energy-consumption, high-adaptability civil purification equipment will be developed to open up the transformation path from laboratory research to commercial application. In the industry guarantee and cross-innovation dimension, a sound biosafety evaluation system for nanocatalytic materials will be established to standardize the development of the industry. Meanwhile, the in-depth integration of nanocatalysis technology with Internet of Things (IoT) and artificial intelligence (AI), building materials, and heating, ventilation and air conditioning (HVAC) technology will be promoted to comprehensively expand the application boundary of the technology and realize the intelligent, passive and large-scale upgrading of indoor air purification.

7. Conclusions

Indoor air pollution, which poses severe hazards to human health, has become a globally concerned environmental and public health issue. Chemical purification technology with the complete mineralization of pollutants as the core is the primary development direction for eliminating indoor air pollution from the root. Novel nanomaterials have become the research core in the field of indoor air chemical purification by virtue of their ultrahigh specific surface area, precisely tunable active sites, excellent room-temperature catalytic activity, and multifunctional synergy. In this review, we systematically sort out the characteristics of typical indoor air pollutants and the requirements of purification scenarios and clarify the core advantages and positioning of chemical purification technology. The research progress of different novel nanomaterial systems is classified and elaborated. The reaction mechanism and optimization strategies of core chemical purification systems, including photocatalysis, room-temperature thermal catalysis, electrocatalysis, and plasma synergistic catalysis, are deeply analyzed. The engineering application status and core bottlenecks of novel nanomaterials are summarized. Finally, in view of the challenges faced by current research, future development directions are proposed, including precise material design, scenario-based customization, in-depth mechanism elucidation, engineering transformation, improvement of the standard system, and multi-technology integration. Although remarkable research progress has been achieved for novel nanomaterials in the field of indoor air chemical purification, there are still many bottlenecks to be broken through from laboratory basic research to large-scale industrial application. In the future, through the interdisciplinary integration of materials science, chemistry, environmental science, material molding, and equipment development, the low-cost and large-scale preparation of high-performance nanocatalytic materials, in-depth elucidation of the catalytic mechanism, and systematic breakthrough of engineering application bottlenecks will be realized. This will promote the wide application of novel nanocatalytic purification technology and provide core technical support for improving indoor air quality and protecting human health.