Technological solutions for NO_x, SO_x, and VOC abatement: recent breakthroughs and future directions

Abstract

NO_x, SO_x, and carbonaceous volatile organic compounds (VOCs) are extremely harmful to the environment, and their concentrations must be within the limits prescribed by the region-specific pollution control boards. Thus, NO_x, SO_x, and VOC abatement is essential to safeguard the environment. Considering the importance of NO_x, SO_x, and VOC abatement, the discussion on selective catalytic reduction, oxidation, redox methods, and adsorption using noble metal and non-noble metal-based catalytic approaches were elaborated. This article covers different thermal treatment techniques, category of materials as catalysts, and its structure–property insights along with the advanced oxidation processes and adsorption. The defect engineered catalysts with lattice oxygen vacancies, bi- and tri-metallic noble metal catalysts and non-noble metal catalysts, modified metal organic frameworks, mixed-metal oxide supports, and their mechanisms have been thoroughly reviewed. The main hurdles and potential achievements in developing novel simultaneous NO_x, SO_x, and VOC removal technologies are critically discussed to envisage the future directions. This review highlights the removal of NO_x, SO_x, and VOC through material selection, properties, and mechanisms to further improve the existing abatement methods in an efficient way.

Introduction

Industrial revolution and growing global energy needs have been the prominent cause of air pollution. Uncontrolled release of pollutants and emissions contribute to global warming, climate change, acid rain, and other environmental changes (Munsif et al. 2021). These emissions include SO_x, NO_x, volatile organic compound (VOC), and CO emissions which are primary pollutants and are directly produced from industrial processes (Roberts 2021). VOCs react with NO_x (mainly nitrogen dioxide) to form fine particles or secondary pollutants and accumulation of these secondary pollutants (ozone, fine particulates) causes smog. Apart from affecting the environment, adverse health effects are possible with emissions. According to a report by the economic consequences of outdoor air pollution, the worrying situation of air pollution is increasing premature deaths. The Organization for Economic Co-operation and Development’s (OECD) ENV-Linkages model, a computable general equilibrium (CGE) model, suggests the possibility of increase in deaths from 3 to 9 million in a span of 2010 to 2060. The number of premature deaths is unequally distributed across the world. The highest number of deaths takes place in non-OECD economies and particularly in China and India. Other major impacts of air pollution are as follows: reduced labor productivity, increased health expenditures and crop yields. Globally these impacts continue to increase significantly relative to gross domestic product (GDP). In contrast, agricultural impacts are relatively stable over time in percentage of GDP, i.e., in absolute terms these impacts grow more or less at the same speed as GDP. Taken together, the total annual market costs of outdoor air pollution are projected to rise from 0.3% in 2015 to 1.0% by 2060 (Highlights 1978).

Major pollutant like carbon monoxide emissions can directly cause poisoning when intake reaches high ppm levels and other emissions are responsible for respiratory problems like asthma, central nervous systems diseases, chronic pulmonary disease, and cardiovascular diseases occur in individuals with long-term exposure to air pollution. Repeated exposure to them can also lead to skin-related problems like melisma and photodamage. Prolonged exposure to NO_x and SO_x can cause breathing issue, bronchospasm, and pulmonary edema. Exposure to VOCs can cause headache, nausea, and along with respiratory issues. The longer exposure to the compound can affect liver and kidney and can even cause cancer (Puri et al. 2017; Manisalidis et al. 2020; Shi et al. 2021a, d). Damage up to this extent mainly occurs in urban and industrial areas due to industrial emissions and vehicular emissions (Ma et al. 2021b). To decrease such adverse effects, several countries have set up emission standards for industries and automobiles so that these emissions are limited. As of 2020, 124 countries (about two thirds) were found to have national ambient air quality standards which is 17% more than reported in 2016. However, only 9% of these adhere to the limits established by the World Health Organization (WHO) guidelines.

US Environmental Protection Agency implemented National Ambient Air Quality Standards where there are limits for each pollutant. Similarly, South Korea follows Clean Air Conservation Act and Japan follows Environmental Quality Standards for air pollution limits. Also, several countries have set up emission standards for industries and automobiles so that these emissions are limited. There is a standard for automobile emission where the current standard follows Euro 6 by most of the countries including South Korea, European Union, Thailand, Indonesia, etc. The Central Pollution Control Board (CPCB) of India has set emission standards for 31 categories of industries for control of air pollution. Different standards and concentration limits of the pollutants are explained in Table 1. Therefore, the need to render these pollutants harmless before releasing them into the environment is extremely important, for which several treatment technologies have to be implemented.

Table 1 Pollutant concentration and standards in different countries

Types of emissions

NO_x

Nitric acid and nitrogen dioxide (NO and NO₂) are major contributors to air pollution and take a part in global warming and greenhouse effect. Industrial sources include power plants, industrial boilers, cement kilns, turbines, and other major sources are automobiles. Another oxide of nitrogen, N₂O has 300 times higher greenhouse impact on a per molecular basis than carbon dioxide (Boningari and Smirniotis 2016; Skowron et al. 2021). They have adverse effects on human health too and therefore strict regulations and standards have been set up to limit NO_x emissions which might have to be tightened further in future (Liu et al. 2018). Several methods have been found for NO_x abatement but many of them are complex, produce undesirable products, and are not cost-effective (Roy et al. 2009; Martinez-Oviedo et al. 2021). Selective catalytic reduction (SCR) in presence of ammonia is one of the most effective ways for control of industrial NO_x emissions (Tian et al. 2021a).

SO_x

SO_x emissions are majorly caused by combustion of fossil fuels at industries and power plants. SO_x have acidifying effect and can harm both land and water by means of acid rain thereby reducing the growth of plants. Also, it can affect the respiratory tract and can cause lung infection (Jiang et al. 2011; Liu and Guo 2021). For SO_x abatement, several pre-combustion and post-combustion techniques can be used like pressurized fluidized bed combustion which provides to about 98% SO₂ reduction and sorbents can be used for removal of SO_x (Rahmaninejad et al. 2012).

Carbon compound emissions

VOCs are all organic chemicals that include compounds of carbon. They also play a role in formation of secondary organic aerosols via photo-oxidation along with nitrogen oxides and sunlight which are found in airborne particulate matter and are the key precursors in formation of ground level ozone (Atkinson and Arey 2003). As far as human health is concerned, BTEX (group of VOCs of benzene, toluene, ethylbenzene, and xylenes) have carcinogenic effects and can lead to toxicity on inhalation even at low concentrations. Other health effects include headache, neurological diseases, eye irritation, etc. (Ueno et al. 2001; Kampa and Castanas 2008, 2018). Owing to stringent regulations, VOC abatement treatments have gained importance in the past few years and catalytic oxidation is regarded as one of the most efficient and promising approaches for VOC removal (Wu et al. 2016; Méausoone et al. 2019; Liu et al. 2021). Other major pollutant is carbon monoxide (CO) and carbon dioxide (CO₂) which is present as a pollutant in troposphere and plays a significant role in global carbon cycle and greenhouse effect (Streets et al. 2006; Jiang et al. 2015). Catalytic oxidation and water–gas shift reactions are promising methods for CO abatement (Gokhale et al. 2008; Zhu and Wachs 2015; Chen and Chen 2020), where normally in water–gas shift reaction, CO reacts with water to form CO₂ and H₂. CO₂ hydrogenation and reforming reactions are helpful to address CO₂ abatement, where in reforming, CO₂ reacts with CH₄ to give syngas (Kathiraser et al. 2015; Cai and Hu 2019; Yang et al. 2019; Neha and Vir Singh 2020). However, in this review, our discussion is limited to NO_x, SO_x, and VOC abatement.

Treatment technologies

The research and adaptation of promising, cost-effective methods for industrial emission control have taken a faster pace in the past few years. A lot of technologies like photocatalysis (Roy et al. 2007b, a), catalytic ozonation (which is more cost effective in comparison to single ozonation), plasma catalysis, selective catalytic reduction/oxidation, storage and reduction (Roy and Baiker 2009; Roy et al. 2010, 2012), and adsorption are found to be effective methodologies. For automobile and vehicular emissions, control catalytic converters prove to be a promising method (Casagrande et al. 2020; Guo et al. 2020; Naveenkumar et al. 2020; Li et al. 2021c; Shang et al. 2021; Wang et al. 2021b; Tan et al. 2022).

Catalytic oxidation/reduction

Techniques such as catalytic oxidation, selective catalytic reduction, and selective catalytic oxidation occurs in presence of catalysts that decrease the activation energy, thereby increases the rate of reactions. Packed and fluidized bed catalytic reactors are found to be economically feasible, produces low concentrations of secondary pollutants at large scale, and can occur at low temperatures than other treatment methods (Zhao et al. 2020b; Guo et al. 2021b; Li et al. 2021b). As most of the catalysts require high temperatures for the emission removal, the obvious energy needs become greater and which significantly increases the operating cost of the process. Hence, research on the development of efficient catalysts is essential to bring down the operating costs. The catalytic efficiency can be increased in several ways such as tuning the catalyst preparative method, doping the active metal over other oxides/metals or by changing the support of the catalyst and modifying the catalyst morphology (Roy et al. 2007d, c, 2008c; Roy and Hegde 2008; Jurado et al. 2021; Zhang et al. 2021c). Recently, a lot of focus has been laid upon research of low temperature catalysts because of high return and low operational cost. Figure 1 shows the number of published research papers on low-temperature reactions from 2000 to 2021 as obtained from Scopus data. It indicates the growing scientific interest on pollution control through catalytic routes.

Fig. 1

Number of published research papers on low-temperature reactions

Adsorption

Adsorption is widely recognized as a highly effective and non-destructive method for the removal of volatile organic compounds (VOCs) and nitrogen oxides (NO_x). It offers several advantages, including the ability to recover and reuse the adsorbed VOCs, making it an environmentally friendly approach. This has led to increased attention on commercially available adsorbents such as activated carbon, silica gel, biochar, and zeolites, which have proven to be highly efficient and cost-effective options for adsorption processes. Activated carbon is one of the most commonly used adsorbents due to its large surface area and high adsorption capacity. It exhibits excellent performance in removing a wide range of VOCs and NO_x from various sources. Silica gel, another popular adsorbent, possesses a high affinity for polar molecules and is particularly effective in adsorbing moisture and certain types of VOCs. Biochar, derived from biomass pyrolysis, has emerged as a promising adsorbent in recent years. Its porous structure and surface chemistry make it suitable for VOC and NO_x adsorption. Zeolites, with their well-defined pore structure and high thermal stability, are also extensively utilized for adsorption applications. They exhibit selective adsorption properties for specific VOCs and have shown remarkable performance in pollutant removal. Research studies conducted by Wu et al. (2019; Wu et al. 2021; Zhang et al. 2021a) have further highlighted the effectiveness of these adsorbents in VOC and NO_x removal. Their findings demonstrate the successful application of these materials in various industries, emphasizing the potential of adsorption as a reliable and efficient technique.

Redox methods

Several advanced oxidation processes (AOPs) with good prospects for developing air cleaning technologies are UV/ozonation, H₂O₂/ozonation, or photocatalysis. The advantage of photocatalysis implementation in indoor air treatment is given by the absence of additional oxidizers (such as H₂O₂). The photocatalysis can occur at low temperatures and catalyst is used for longer hours (Sekiguchi et al. 2010; Challagulla et al. 2019).

The present comprehensive review aims to provide a consolidated overview of recent advancements in the field of NO_x, SO_x, and VOC abatement through catalysis. Though there are several review articles on NO_x reduction and VOC abatements (He et al. 2019; Chen et al. 2021a; Zhao et al. 2022; Li et al. 2023), here we present a review which encompasses a wide range of topics, including NO_x, SO_x reduction, and VOC abatement by advanced catalysis techniques and adsorption. The development and optimization of catalytic materials play a vital role in achieving efficient and sustainable reduction of harmful emissions, contributing to improved air quality and a healthier environment. The performance characteristics of noble metal and non-noble metal catalysts are dealt in this study in the controlling mechanism point of view. The effect of mono-, bi-, and tri-metallic dopants on the reaction mechanism is also investigated. Further, redox and adsorption methods are also investigated for the removal of toxic gases. This will enable the researchers to choose a suitable material to abate the harmful gases in terms of energy efficiency, abundancy, and economics for catalytic/non-catalytic processes. By providing an amalgamated overview of the recent progress made in the field, this report aims to enhance understanding and knowledge regarding the significant role played by catalysis in the abatement of NO_x, SO_x, and VOCs, paving the way for further research and technological advancements in the field. A detailed schematic on the abatement technologies for NO_x/SO_x, VOC is shown in Fig. 2.

Fig. 2

Schematic diagram explaining the NO_x, SO_x, VOC emission effects and solution by abatement technologies

SCR for NO_x removal

SCR is seen as an efficient method for NO_x reduction to N₂. Commercially used catalysts operate at 300–400 °C. But at higher operating temperature, sulfur contamination in the feed can cause sintering of the catalyst and affect its stability. Thus, sulfur-tolerant and low-temperature SCR is a welcoming strategy with efficient catalysts (Pappas et al. 2016; Yan et al. 2021). The demand for efficient NO_x removal has led to the development of several classes of SCR catalysts. SCR mainly involves three reactions named as standard SCR, fast SCR, and NO₂-SCR (Zhou et al. 2020). SCR can be categorized into noble metal based (Pt, Pd, Au, Ag) and non-noble or transition metal-based catalysts (Mn, Ce, Cu, Fe, V, W) (Roy et al. 2008a, b, d; Li et al. 2017b; Liu et al. 2017; Jiang et al. 2018a; Chang et al. 2020; Zhang et al. 2021b).

Noble metal-based catalysts for SCR: Pt, Ru, Rh, Au, Pd

Noble metals like Pt, Pd, Ru, Rh, etc. are used as the catalyst for SCR. Very high conversions to NH₃ and good N₂ selectivity are possible with noble metals like Pt, Ru, Rh, and Pd. Some of the noble metal-based catalyst systems for NO_x conversion are shown in Table 2 (Sato et al. 2016), and Pt-based catalysts are also important for NO_x reduction (García Cortés et al. 2007). Another important aspect emerged is the metal-support interaction. Synergistic effect of noble metal with other metals as metal-support increases the activity (Xue et al. 2018). Pt/La_0.7Sr_0.2Ce_0.1FeO₃ catalyst can convert 87% of 2500 ppm NO at 150 °C (Costa et al. 2002). Yang et al. has shown that doping with noble metals such as Pt, Pd, Rh, Ru on MnO_x-CeO₂ matrix has increased the catalytic activity compared to undoped structure (Yang et al. 2018).

Table 2 Noble metal-based catalyst for NO_x conversion and N₂ selectivity

Non-noble metal-based catalysts

Non-noble metal catalysts like manganese, cerium, and vanadium are widely accepted as SCR catalysts due to its abundancy, low cost, high activity towards NO_x conversion, nitrogen selectivity, and sulfur resistance. Vanadium-based catalysts such as V₂O₅/TiO₂ and V₂O₅/WO₃/TiO₂ are currently commercially used SCR catalysts that operate at temperatures above 350 °C (Cha et al. 2016; Wang et al. 2018a; Yang et al. 2020; Inomata et al. 2021; Wu et al. 2021; Zhang et al. 2021a). Furthermore, details on non-noble metal catalysts are elaborated in the following sections.

Non-noble metal oxides

Non-noble metal oxides generally constitute transition metals and rare earth metals. They have gained significant attention because of their decent catalytic performance for the potential applications in SCR. Yarong et Al. prepared a series of Co₃O₄ nanoparticles inlaid in porous carbon (CoOx@PC-T) using pyrolysis of ZIF-67 in nitrogen at different temperatures. For the sample of CoOx@PC-800, greater than 80% conversion was seen in the temperature range of 150–175 °C with high nitrogen selectivity as shown in Fig. 2d–f. The greater ratio of Co³⁺/Co²⁺, larger surface area, and oxidizing capability of the catalyst were responsible for its performance as observed from the XPS shown in Fig. 3a–c (Bai et al. 2019). Haoxi et al. (Jiang et al. 2018b) synthesized manganese-based catalysts by a solvo-thermal method with several capping agents of PVP, P123, HAc, and CTAB to expose crystal facets of α-MnO₂. MnO₂-PVP sample with exposed (200) crystalline facets showed 100% NO conversion at 200 °C which was better than other samples. These samples also had adsorption capabilities among which MnO₂-PVP had the highest adsorption capacity. The (200) crystalline facet has maximum exposed acid sites for MnO₂-PVP, and it helped the reactant leading to maximum conversion at 200 °C.

Fig. 3

Reproduced from Elsevier 2019 (Bai et al. 2019)

XPS spectra of Co 2p in CoO_x@PC-600 (a), CoO_x@PC-800 (b), and CoO_x@PC-1000 (c). NH₃-SCR activity of Co@PC-800 and CoO_x@PC-800 (d). NH₃-SCR activity (e) and N₂ selectivity (f) of CoO_x@PC-600, CoO_x@PC-800, and CoO_x@PC-1000. Reaction conditions: [NO_x] = [NH₃] = 500 ppm, [O₂] = 5 vol%, Ar balance, and GHSV = 14,000 h.⁻¹.

Ole et al. (Bjørkedal et al. 2022) used zirconia as support and developed Cu/ZrO₂ by sol–gel method with different loading amounts of Cu (3%, 6%, 15%) which showed promising SCR performance, providing 75% NO conversion at 150 °C for 6 wt.% catalyst. Even though increase in copper loading (15%) increased the SCR activity at 150 °C, it was found to undergone sintering above 350 °C and showed poor selectivity. Therefore, 6% Cu on ZrO₂ is found to be better catalyst among the others.

Table 3 lists the variety of other non-noble metals used for the NO_x conversion and further emphasis on the role of mono-metallic and bi-metallic non-noble catalysts for the NOx reduction are briefly showcased.

Table 3 NO_x conversion by non-noble metal catalysts

Non-noble metal based bimetallic catalysts

Bimetallic catalysts are combination of two different metals that exhibits several new and improved catalytic properties. For instance, Liu et al. (2020b) prepared Sm-Mn/TiO₂ by modifying Mn/TiO₂ by Samarium using ultrasonic impregnation method which improved the surface dispersion of active metal cation and Mn/TiO₂ were also prepared for comparison. Mn/TiO₂ showed best catalytic activity with optimum Mn loading of 20% showing only 52% NO_x conversion at 120 °C whereas 10 wt.% Sm modified Mn/TiO₂ showed best low-temperature catalytic activity. The optimal interaction between Mn and Sm species was achieved with 10 wt.% Sm and 20 wt.% Mn loading forming 20Mn-10Sm/TiO₂ which exhibited greater than 80% conversion in 110 − 250 °C. Similarly, Liu et al. (2020a) used urea instead of ammonia as a reductant and binary catalyst Cu-Mn/NUAC and Cu-Co/NUAC were synthesized by ultrasonic-assisted impregnation method for LTU-SCR, and it was found that catalytic performance of binary catalysts was better than unary catalyst. Their study shows the synergic effect between the two metals (Cu-Co/Mn) which will enhance the catalytic properties compared to the individual metals. One hundred percent NO conversion for Cu_0.5Mn_0.5/NUAC was observed as the temperature increased from 50 to 80 °C due to the synergy between the oxides of active metal increases in this range. Synergistic mechanism of a catalyst involves distinct catalytic sites acting on different substrates sites to enhance the catalytic activity.

Vanadium-based catalysts generally offer high SO₂ resistance, high catalytic activity due to that these materials preferred commercially (Xu et al. 2019; Kwon et al. 2021). Zhu et al. (2018) looked for the effect of niobium oxide as a promoter on V/WT (vanadium-tungsten-titanium) catalyst with different contents of dopant. Out of different contents, 6 wt.% niobium oxide (3V₆Nb/WTi) was found to be appropriate and catalyst deactivation was found to be negligible at around 250 °C. Similarly, the commercially used V₂O₅-WO₃/TiO₂ was mixed with several Fe₂O₃ samples (Fe(N), Fe(P), and Fe(C)). NO_x conversion increased with 95% for VW/Ti + Fe at 300 °C. Addition of Fe₂O₃ inhibited the formation of ammonium sulphate thereby preventing catalyst deactivation (Zhu et al. 2018). In another study, Quanming et al. and Woojoon et al. (Cha et al. 2016; Liang et al. 2021) studied the effect of addition of cerium oxide on commercially used binary V₂O₅-WO₃/TiO₂ for NH₃-SCR. CeO₂-V₂O₅-WO₃/TiO₂ was prepared via one-step impregnation method using 0% and 3% amount of CeO₂. It was observed that 3Ce-3VWT could achieve 99.3% NO conversion at 160 °C. V⁴⁺ and V⁵⁺ was interchanged by the addition of 3% CeO₂, thereby increasing the NO₂/NO_x ratio in flue gas that improved the low temperature catalyst.

Yunfan et al. (Xu et al. 2019) studied about its effect of V₂O₅-MoO₃/TiO₂ and V₂O₅-WO₃/TiO₂ monolithic catalysts. NO_x conversion was observed to increase from 70 to 88% at 220 °C for V₂O₅-MoO₃/TiO₂ while it decreased for V₂O₅-WO₃/TiO₂. Manganese based catalysts are seen to have excellent redox ability at low temperature and have various valence states due to that these materials are highly recommended as SCR catalysts (Fang et al. 2019; Tang et al. 2020). Specifically, the mixed metal oxides such as MnO_x-CeO₂ can reduce NO_x even below 250 °C. Support materials also directly affect the catalytic performance and plays an important role in SCR. Its structure, size, and surface area have a critical role in catalytic performance (Patel and Sharma 2021). Bora Ye et al. prepared nitrogen-doped reduced graphene oxide as a support for Mn-Ce oxide catalyst (Mn-Ce/N-rGO). The catalytic activity was seen to provide 80% conversion at 200 °C. This could be achieved instead of using TiO₂ as support as the nanoparticles were evenly distributed without any aggregation on N-rGO which remains thermally stable at temperatures lower than 300 °C (Ye et al. 2021). Carbon monoxide can also be used as a reductant for SCR instead of NH₃. It has good reducing capability to convert NO into CO₂ and N₂ (Zhang et al. 2020b). Recently, metal organic frameworks (MOFs) have found to be capable for SCR due to high surface area, porosity, and synthetic tunability (Jiang et al. 2016, 2018a). Shi et al. (2021b) prepared bimetallic catalyst Ni_1-xMn_x-MOF-74 with different molar ratio of Mn/(Mn + Ni) (x = 0.1, 0.2, 0.35, and 0.5) via one-pot solvothermal method which all reported higher conversion percentages in comparison to monometallic Ni-MOF-74 and Mn-MOF-74. Among them, Ni_0.65Mn_0.35-MOF-74 achieved highest conversion of nearly 100% at 175 °C and 100% nitrogen selectivity at around 200 °C. Similarly, Zhong et al. (Wang et al. 2020) prepared a novel ball-flowerlike bimetallic CoMnO_x-BF catalyst using hydrothermal method for NH₃-SCR. It was observed that it was SO₂ resistant and had better catalytic performance in terms of conversion (in range of 150 to 350 °C) and N₂ selectivity than CoMnO_x catalyst and was also more durable catalyst to provide high NO_x conversion till 36 h. This is attributed to the large surface area of the CoMnO_x-BF catalyst, also the strong interaction between Mn and Co.

Non-noble metal-based tri/poly-metallic catalysts

Some dopants like Co and Ce can be used to enhance the performance of bimetallic catalysts and resistance against SO₂ deactivation (Wang et al. 2019a). Liu et al. (2019b) studied the effect Co doping onto Mn-Sm/Ti for NH₃-SCR using ultrasonic-assisted impregnation method with various amounts of doping. For 5 wt.% Co loading, 5CoMnSm/Ti achieved greater than 90% conversion at 100–200 °C having a GHSV of 60,000 h⁻¹. Qinghua et al. (Yan et al. 2018) prepared a Cu_0.5Mg_1.5Mn_0.5Al_0.5O_x synthesized from layered double hydroxides via co-precipitation method. It showed high conversion ranging from 87 to 96.6% in 100–250 °C. The highest NO_x conversion was observed for Cu_0.5Mg_1.5Mn_0.5Al_0.5O_x providing 96.7% conversion at 150 °C in comparison to others Cu_0.5Mg_1.5Mn_0.5Al_0.5O_x (96.7%) > Mg₂Mn_0.5Al_0.5O_x (93.3%) > Cu₁Mg₁- Mn_0.5Al_0.5O_x (92.2%) > Cu_1.5Mg_0.5Mn_0.5Al_0.5O_x (91.3%) > Cu₂Mn_0.5Al_0.5O_x (91.2%). Similarly, Wei et al. (Wei et al. 2018) used co-precipitation method to synthesize a series of iron-samarium mixed oxide catalysts modified by zirconium, cobalt, and titanium. Ti_0.1Sm_0.075Fe_0.825O_x-400 sample possessed the best activity up to 95% at 150 °C and N₂ selectivity of Ti_0.1Sm_0.075Fe_0.825O_x-400 and Zr_0.1Sm_0.075Fe_0.825O_x-400 was above 95% which was higher than that of Sm_0.075Fe_0.825O_x-400 between 100 and 250 °C, suggesting that modification by transition metals can enhance the catalytic activity of iron-samarium mixed oxides. Tourmaline, a manganese-iron-cerium-oxide composite, plays an important role in enhancing the catalytic activity. Zhao et al. (Zhao et al. 2020a) observed the effect of tourmaline on CeMnFeO_x catalyst. Manganese-iron composite catalyst and a manganese-iron-cerium composite catalyst were prepared and named as M7F3 and MFC-0.02. For 2% addition of tourmaline, NO_x conversion for MFCT-2% was 100% at 170 °C which was highest among all the other samples. Addition of tourmaline promotes the uniform distribution of layered composite materials on the surface and even small quantities of tourmaline can disperse the particles more effectively. The structure of catalysts also plays a critical role in its activity. Zhou et al. (Zhou et al. 2020) synthesized a two-dimensional MnFeCo layered double oxide (MnFeLDO, MnCo-LDO, and MnFeCo-LDO), based on the transition metal chemical compositions. Layered double oxides have large specific surface area and various active sites that enhance the activity. It was found that at 100 °C, 100% NO conversion was achieved and even at 50 °C, 86% conversion was observed. In presence of 5% H₂O vapor and 100 ppm SO₂, NO conversion was seen to be 89% and this further shows layered double oxide materials as the highly effective catalysts even under these operating conditions.

Alkali metal promoted NO_x reduction

The electro-positive nature of alkali metals as promoters are gaining interest in scientific community for NO_x reduction. Matsouka et al. have studied the un-promoted and sodium promoted Pt catalyst on supports such as Al₂O₃, CeO₂/La₂O₃ modified Al₂O₃. The study was conducted at under automobile exhaust conditions with CO, NO, C₃H₆, and O₂. Pt on Al₂O₃ support mainly has formats and acetates, leading to carbonyl species formation. But when Na is added as a promoter 100% NO reduction was at 395 °C. Konosolakiss et al. studied two different promotions separately to observe the synergy effect on de-NOx properties. Pd/Al₂O₃-TiO₂ system and K doped Pd(K)/Al₂O₃-TiO₂ system was thoroughly studied and found that K doping improves NO_x reduction (85% at 100–400 °C) and N₂ selectivity. This indicates a synergy effect in the latter catalyst. This is because Na usually strengthens the metal-NO bond and that, in turn, weakens the N–O bond due to its electro-positive nature to adsorb the electron accepter molecule NO. This was confirmed by performing TPD over model Na on Pt (111) surface. Yentekasis et al. explored a comparative study by a promoted Pt/γ-Al₂O₃ for three reactions C₃H₆ + NO + O₂, C₃H₆ + O₂, and NO + O₂ at similar conditions with excess oxygen. Their studies showed how Na affecting electron-donating hydrocarbon and electron-accepting NO at lower temperatures. At Na loading of 2.6%, the NO dissociation is found to be limited with significant NO₂ formation. In another study by Consuega et al., Pt/K-γAl₂O₃/Pt was used as electrochemical catalyst for NO_x storage/reduction properties. Here, the catalytic system allows for the electrochemical storage of NO_x as potassium nitrates and simultaneously decomposes by the positively polarized film without changing the reaction atmosphere. The electro-positive nature of alkali metals supports NO decomposition and, thereby, NO_x reduction. This happens via bond strengthening of NO on alkali metal-induced metal.

Hydrocarbons/H₂ for NO_x reduction

Currently, there is a significant focus on reducing NO_x emissions using hydrocarbons or carbon monoxide, mainly by taking out gas mixtures from exhaust gases like CO. In NO_x reduction with hydrocarbon, first NO_x will be oxidizing the surface nitrates along with conversion of hydrocarbons to surface oxygenates. Further reaction between these intermediates leads to the formation of NCO⁻, CN⁻ species and finally forms N₂ and CO₂. Boutros et al. has studied the NO_x reduction by ethanol using Ag and Al supported mesoporous SBA-15. The first catalyst, Ag-SBA-15 showed a better catalytic activity as Ag was doped in the catalytic system efficiently because of incipient wetness impregnation method (Boutros et al. 2009). In another study by Oton et al. NO_x reduction was done by CO by using Pt, Ni, Co, Fe, or Ni nanoparticles dispersed on porous alumina. The interaction and synergy between Pt/Ni nanoparticle and the support Al₂O₃ made Pt/Al₂O₃ and NiPt/Al₂O₃ as better catalysts among the other. The porous nature and Lewis-acidity sites also contributed to high performance of the catalyst. According to the kinetics based on based on Eley–Rideal and Langmuir–Hinshelwood models, here Pt⁰ receives an electron from NO_x (or CO) and forms PtO_x. PtO_x then adsorbs CO (than NO_x) leading to the formation of NCO intermediates on Pt sites. Further CO oxidation to CO₂, along with N₂ reduction from NO_x monodentate occurs simultaneously leaving PtO₂ for the next redox cycle (Oton et al. 2020). Seo et al. used H₂ for the reduction of NO_x and CO over 0.5Pt-2CeO₂/TiO₂/ZrO₂ SCR catalyst at lower temperature. By the addition of ZrO₂ at 100 °C, the catalyst showed highest NO_x reduction. The presence of CO promoted water gas shift reaction which further improved NO_x reduction at 100 °C. The structural property change was because of the following: the oxygen storage capacity due to doping Pt was high, and improvement in surface acidity in presence of CO (Choong-kil 2022).

N₂O reduction

N₂O is another class of NO_x, which is a significant anthropogenic greenhouse gases and one of the major reason for ozone depletion (Zhang et al. 2019b). Thus, reduction of N₂O to N₂ is important reaction. Recently, hydrocarbons are used for the conversion of N₂O to N₂. Hevia et al. have studied Fe-ZSM-5 zeolite for the same. Different C1-C3 alkanes, alkenes, and alkynes are used as the reductant and thoroughly checked for the N₂O reduction. Alkanes showed better catalytic activity compared to alkenes, and alkynes (very less reactive) because of their reactivity with oxygen and cost. Methane and ethane were best reductant for the N₂O reaction (Hevia 2008). Similarly, Zhang et al. have explored Mans–van Krevelen mechanism on phosphotungstic acid supported single-atom catalysts for the N₂O reduction by CO. Here the CO get oxidized by the surface oxygen present in the phosphotungstic acid, and N₂O occupies the oxygen vacancy and converts to N₂ (Zhang et al. 2019b).

SCO for NH₃ emissions

Selective catalytic reduction with ammonia (NH₃-SCR) is one of the most proficient techniques for NO_x removal because of excellent removal efficiency and low maintenance cost in which NH₃ is used as a reductant. Apart from high efficiency, this technique also has some shortcomings. For instance, high NH₃/NO_x ratio and catalyst deactivation by SO₂/H₂O poisoning leads to slipping of ammonia, initiating secondary pollution (Chen et al. 2019; Nakamura et al. 2021). NH₃ emissions are harmful to both environment and human health. Excess ammonia leads to eutrophication and also contributes in creation of secondary particulate aerosols. NH₃ emissions have adverse health effects, its high exposure can result in blindness and permanent lung damage (Gheorghe and Ion 2011). The techniques used for the removal of ammonia emissions are adsorption, catalytic decomposition and selective catalytic oxidation (Cardenas et al. 2021; Chen et al. 2021b; Pinzón et al. 2021). Selective catalytic oxidation (SCO) is one of the favorable approaches which can be used in complimenting to NH₃-SCR system to convert ammonia into nitrogen directly because of high nitrogen selectivity, low cost, and excellent efficiency (Guo et al. 2019; Gao et al. 2021). Catalysts play a key role in selective catalytic oxidation of ammonia. V₂O₅-TiO₂ oxide with addition of WO₃ or MoO₃ is used as commercial catalyst for SCO of ammonia. Several noble metals based, non-noble metal based, zeolites-based catalysts are used as active catalysts for selective catalytic oxidation of ammonia (Jabłońska and Palkovits 2016; Li et al. 2017a). Different categories of SCO catalysts have been elaborated below.

Noble metal-based catalysts

Many noble metals like Pt, Pd, Rh, Ru, and Ag show high catalytic activity for selective oxidation of ammonia in terms of high NH₃ conversion and N₂ selectivity (Zhang and He 2009; Hung 2010; Hung 2012; Ma and Schneider 2020). Similarly, some transition metal oxides have gained more attention than noble-metal based catalysts due to high nitrogen selectivity (Wang et al. 2009; Zhang et al. 2017a). Wang et al. (2019b) utilized sol–gel method to synthesize RuO₂-Fe₂O₃ composite oxide catalysts with different contents of RuO₂ (0.5%, 1%, 1.5%, and 2%) for selective catalytic oxidation of ammonia and results were compared with pure Fe₂O₃. At 100 °C, pure Fe₂O₃ achieved 100% NH₃ conversion with 79% N₂ selectivity which is less in comparison to RuO₂-Fe₂O₃ composite oxide catalyst. The sample prepared with 1.5% of RuO₂ content showed best catalytic activity with 100% NH₃ conversion and 89% N₂ selectivity at 225 °C. Addition of RuO₂ had a significant effect on surface acidity of catalyst and also increased surface area. Pt/V/TiO₂ can even over-oxidize NH₃ to NO_x in spite of showing excellent catalytic activity at low temperatures (Kim et al. 2018; Dann et al. 2019). Therefore, support plays a vital role in activity of SCO catalysts. For instance, Liu et al. (2019a) studied the promotional effects of ethylenediamine over Pt/SiAlO_x (Pt/SiAl-E) catalysts using co-impregnation method. Pt/SiAl-E showed better low-temperature activity than Pt/SiAl for NH₃-SCO. For 90% NH₃ conversion, the operating temperature was lower for Pt/SiAl-E (219 °C) in comparison to Pt/SiAl (235 °C) with almost similar nitrogen selectivity as shown in Fig. 4. Addition of ethylenediamine led to reduction of particle size which in turn led to larger surface/bulk atomic ratio increasing the active sites for SCO of ammonia.

Fig. 4

Reproduced from Elsevier

a The NH₃ conversion of catalysts. b The N₂ yield of catalysts. Experimental condition: 200 ppm NH₃, 10% O₂, 8% CO₂, 5% H₂O and balance N₂. The space velocity was 100,000 h⁻¹ (Liu et al. 2019a).

Similarly, Sun et al. (2017a) modified Pt/ZrO₂ catalyst by the addition of W using co-impregnation method and loadings of Pt and W were 1.5 wt.% and 5 wt.%, respectively. With decrease in activation energy from 113.4 to 96.2 kJ∙mol⁻¹, the light off temperature for ammonia also shifted from 284 to 249 °C with same nitrogen selectivity for both of them. Redox properties and acidic nature of the catalysts are improved due to the addition of W and it also resulted in electron transfer from W species to Pt species increasing the electron density of Pt. Pt-WO₃/ZrO₂ had larger number of surface acid sites which contributed to improved catalytic activity. Zhang et al. (2009) studied the role of silver species on Al₂O₃ using incipient wetness impregnation and sol–gel methods to prepare 10 wt.% Ag/Al₂O₃ catalyst and then were pre-treated by H₂. Pre-treatment of Ag/Al₂O₃ prepared by impregnation method resulted in excellent catalytic activity of 100% conversion at 160 °C. However, for fresh Ag/Al₂O₃ showed complete conversion at 220 °C. Ag⁰ was concluded as the most active species at low temperatures.

Metal oxide and zeolites-based SCO catalysts

Zeolite-based materials are another class of catalysts that show promising results and have been extensively used for selective catalytic oxidation of ammonia. Their activity can be increased by addition of transition metals (e.g., Cu, Fe, Co) or noble metals (e.g., Pt/Ag/Au). The shape selectivity and high surface area often attributes to excellent SCO activity (Qi et al. 2004; Jabłońska et al. 2014; Li et al. 2017a; Rutkowska et al. 2017, 2019; Wang et al. 2021c). Therefore, many scientific studies are being done for low temperature zeolites that operate below 200 °C. With respect to low operating temperature, Wang et al. (2019c) synthesized hollow ZSM-5 encapsulating Ag particles (Ag/ZSM-5-OH) in Al₂O₃, ZSM-5 and hollow ZSM-5. For 100% NH₃ conversion, operating temperature for Ag/ZSM-5 catalyst was 110 °C. By modification of Ag nanoparticles, activation energy for Ag/ZSM-5 was found to be the lowest measuring 16.4 kJ∙mol⁻¹. High content of Ag⁰ species contributed to higher catalytic activity. Leaching of Ag particles was prevented via hollow structure of ZSM-5 zeolite.

Similarly, Sun et al. (2017b) modified Pt/ZSM-5 by adding Cu via co-impregnation method with loading of 1.5 wt.% of both Pt and Cu. Catalytic performance of PtCu/ZSM-5 was better than original Pt/ZSM-5. It was observed that Cu acted as promoter lowering T_90% temperature from 250 to 245 °C as shown in Fig. 5. It also promoted N₂ selectivity (79% at 200 °C) which was 64% for Pt/ZSM-5. It was observed that Cu species adjusted the states of Pt species thereby increasing the electron density. Dealuminated zeolites are having higher Brønsted as well as Lewis acidity, and this contributed to higher nitrogen selectivity. Metal oxide-based SCO catalysts (MnO₂, CuO, Fe₂O₃, Co₃O₄) have higher nitrogen selectivity than noble metal-based catalysts but, they generally have lower catalytic activity. CuO/Fe₂O₃ catalysts for SCO have higher nitrogen selectivity than noble metal-based catalysts, however, majority of the catalysts offered lower catalytic activity. CuO/CeO₂ metal oxides can provide excellent catalytic activity as well as nitrogen selectivity (Wang et al. 2013; Zhang et al. 2017a). Further modification to the metal oxide catalysts enhances the catalytic activity. Preparation method, support and structure of catalyst plays an important role in catalytic activity and further to favorable results (Nassos et al. 2007; Jabłońska et al. 2017b, a; Wang et al. 2021a). For example, Song and Jiang (2012) prepared CuO/CNTs (carbon nanotubes) for SCO of ammonia. Nanocomposites were named as CuO/CNTs-80, CuO/CNTs-100, CuO/CNTs-120, CuO/CNTs-140, and CuO/CNTs-180. Higher the defect density of CNTs, higher catalytic activity was observed. Highest nitrogen selectivity of 98.7% was achieved on CuO/CNTs-140 with 100% NH₃ conversion at 189 °C. By these carbon nanotube defects, electron transfer was promoted and copper oxide was activated by these nanotubes. Similarly, Qu et al. (2015) used urea assisted hydrothermal method. MnO₂(UH) catalyst showed outstanding catalytic activity of 90% NH₃ conversion at 140 °C while 100% conversion at 170 °C which was better in contrast to MnO₂(H) (200 °C) and commercial MnO₂ (260 °C). However, some by-products like N₂O, NO, and NO₂ were obtained that led to reduced nitrogen selectivity. Urea-assisted synthesis successfully changed the surface properties of the catalyst by imparting more adsorption sites for NH₃ adsorption and activation. Similarly, Duan et al. (2010) improved Cu-Mn compounds catalysts with trivalent rare earth oxide Ce₂O₃ and La₂O₃ respectively by incipient wet impregnation method. Also, comparative analysis was done for different preparation methods of 2.5%Ce-5%Cu-5%Mn/TiO₂ namely incipient wet impregnation, co-precipitation and sol–gel as follows. By addition of La and Ce, better distribution of Cu and Mn species was observed along with increased interaction between supporter and Cu-Mn, better oxidation ability. Selective catalytic oxidation for NH₃ is discussed in Table 4 in detail.

Fig. 5

Reproduced from Elsevier

a NH₃ conversion, b N₂ selectivity, c Arrhenius plots of NH₃ oxidation, and d TOF profiles versus reaction temperature over the Ag/ZSM-5, Ag/Al₂O₃, and Ag/ZSM-5-OH catalyst. Reaction conditions: [NH₃] = 1000 ppm, [O₂] = 10 vol%, He balance, and GHSV = 35,000 h⁻¹ (Wang et al. 2019c).

Table 4 SCO catalysts to control NH₃ emission

Different mechanistic pathways of NO_x abatement are explained in Fig. 6. Mechanisms such as Langmuir–Hinshelwood and Eley–Rideal are explained in the schematics.

Fig. 6

Mechanistic pathway for NO_x abatement by NH₃ selective catalytic oxidation and hydrocarbon selective catalytic reduction

In the Langmuir–Hinshelwood mechanism, NH₃ adsorbs on the acid site of the catalyst to form NH₄⁺. Further, NO will be physically adsorbed on the metallic surface. The metal with higher oxidation state undergoes reduction in order to oxidize NO to NO_x species. Then, the NO_x species and the adsorbed NH₄⁺ species react to form NH₄NO_x. Further, NH₄NO_x decomposes to N₂ or N₂O. Similarly, in Eley–Rideal mechanism, activation of NH₃ species by metallic site takes place forming NH₂⁻. This NH₂⁻ reduces NO species to N₂ or N₂O.

Photo/electro catalysts for the NO_x removal

Numerous studies have explored the application of electro- and photocatalysis for the removal of NO_x, SO_x, and VOCs, highlighting the diverse catalyst materials and reaction mechanisms involved. Electrocatalytic reduction has been extensively studied for the removal of NO_x. Tan et al. (2016) explored the electrochemical reduction of NO_x using various electrode materials, including metal oxides, metal alloys, and conductive polymers. The study revealed that the electrochemical reduction of NO_x can effectively convert nitrogen oxides into harmless nitrogen under specific reaction conditions, presenting a viable pathway for NO_x abatement One such catalyst category is using graphene-based materials. Lee et al. (2019) studied the electrocatalytic reduction of NO_x using graphene-based catalysts. They reported enhanced catalytic activity and stability, suggesting graphene as a promising material for NO_x removal. The utilization of visible light-responsive catalysts, such as carbon-based materials and metal–organic frameworks, was discussed as a promising avenue for improved NO_x degradation. Si et al. (2021) investigated the photocatalytic reduction of NO_x using TiO₂-based catalysts, highlighting the influence of factors such as crystalline structure and surface area on the catalytic performance. They reported high conversion rates under UV irradiation than visible light.

Conjugate polymers and their composites are renowned for their metallic conductivity and impressive physical properties subjected to doping. Foreign elements inclusion can greatly enhance the photocatalytic activity under visible light by improving the separation of charge carriers and expanding the range of light absorption. Additionally, combining conductive polymers with oxide semiconductor photocatalysts creates a synergistic effect, resulting in highly efficient photocatalytic degradation. Ajmal et al. (2023) critically evaluates the previously unexplored utilization of conjugated polymers (CPs) in combination with common inorganic semiconductors as innovative photocatalysts. The distinctive characteristics of CPs, such as conductivity, exceptional light responsiveness, effective sorption capability, superior redox charge generation, and separation abilities facilitated by a delocalized π-electron system, differentiate them from inorganic semiconductors. Ajmal et al. (2022) discussed the advancements in photocatalytic removal of three major air pollutants, namely CO₂, NO_x, and VOCs, using Conjugate Polymer based photocatalysts. Additionally, the synergistic effects observed when conjugated polymers are combined with inorganic semiconductors are comprehensively summarized. Notably, the combined system enhances charge generation and separation, potentially activating the adsorb and shuttle process, wherein conjugate polymers may play a crucial role in the sorption process. Semiconductor-based catalysts are studied for the photochemical removal of NO_x, such as WO₃ and Bi₂WO₆ (Dong et al. 2016; Zhang et al. 2019a) These catalysts facilitated the efficient photodecomposition of NO_x species, offering potential solutions for NO_x abatement. Photo catalysis has shown promise for the removal of SO_x pollutants. The research demonstrates the potential of various catalyst materials, such as TiO₂-based catalysts, graphene, MOFs, and semiconductor-based catalysts, for efficient pollutant degradation. Additionally, the combination of photo and electrocatalysis has shown synergistic effects, leading to enhanced removal efficiency. However, further research is required to optimize catalyst design, improve stability, and scale up these catalytic systems for practical applications.

Adsorption techniques for the NO_x/SO_x removal

Air pollution caused by nitrogen oxides (NO_x) and sulfur oxides (SO_x) has serious environmental and health implications. Adsorption is another effective method for the removal of these pollutants from industrial exhaust gases. Several studies have investigated the adsorption of NO_x, SO_x, and VOCs using various adsorbents and process conditions. Mergbi et al. (2023) emphasizes the relationship between the synthesis method or surface modification of carbon materials derived from biomass waste and their effectiveness in removing organic, NO_x, SO_x, and heavy metal pollutants by photocatalysis. Surface modification of activated carbon is a technique that can enhance the presence of immobilized functional groups, leading to improved removal of NO_x from the air. Activated carbon is used for the removal of NO_x and SO_x. The primary role of activated carbon is to initially convert NO to NO₂, which can be easily adsorbed. Catalytically oxidizing NO to NO₂ could be a practical approach for removing NO_x from fuel gas, as NO₂ can be readily adsorbed.

The adsorption process for NO_x, SO_x, and VOC removal typically involves passing the gas stream through an adsorbent bed. The adsorbent materials, such as activated carbon, zeolites, and metal oxides, are selected based on their specific affinity for the target pollutants as shown in Fig. 7. The efficiency of the adsorption process depends on factors such as temperature, pressure, contact time, and adsorbent characteristics (Zhu et al. 2020; Sabzehmeidani et al. 2021; Zhu and Xu 2022). Despite the promising results, challenges persist in adsorption-based pollutant removal. Saturation of adsorbents, competition between different pollutants, and the regeneration of spent adsorbents are major hurdles that need to be addressed. The development of efficient regeneration methods and the treatment of complex gas streams are areas of ongoing research.

Fig. 7

Mechanistic pathway for the SO_x abatement by activated carbon

VOC abatement

Volatile organic compounds (VOCs) comprise organic compounds formed from burning fossil fuels, chlorination in water treatment, components of petroleum, dry cleaning agents, paint industries, etc. VOCs such as volatile benzene, toluene, ethylbenzene, and xylenes (BTEX) levels are usually higher in both industrial areas and cities (Montero-Montoya et al. 2018). Exposure to these pollutants can cause both minor and major health effects including nose, eye irritation, headache, nausea and extreme effects like kidney or liver failure, damage to nervous system and can be even carcinogenic in some cases. VOCs have adverse effect on environment and also generates secondary pollutants. Considering the toxicity and long-term adverse effects on human health and environment, strict regulations have been set up for release of VOCs into the environment; therefore, industries need to render them harmless before releasing them into the environment (Tsimpidi et al. 2012; Manisalidis et al. 2020). Several treatment technologies for VOC abatement are being currently used such as thermal oxidation, catalytic oxidation, adsorption, photocatalysis, bio-treatments including bio trickling filters, bio-scrubbers and membrane separation (Abou Saoud et al. 2020; Zhang et al. 2020a; Cheng et al. 2021; Tian et al. 2021b; Wantz et al. 2021). Among all the possible techniques, the focus has been given to low-temperature catalytic techniques for VOC removal as it possesses low operating costs compared to other techniques.

Low-temperature catalysts for VOC oxidation

Noble metal-based catalysts

Wu et. al. (2016) synthesized three-dimensionally mesoporous silica (KIT-6) was studied for toluene oxidation as shown in Fig. 8. The three-dimensionally ordered mesoporous Cr₂O₃ (meso-Cr₂O₃) was fabricated using the ultrasound-assisted strategy with KIT-6. The meso-Cr₂O₃-supported Au − Pd samples were prepared and Au and Pd loading (x) of 0.90 and 1.00 wt.% were loaded and indicated as xAu/meso-Cr₂O₃ and xPd/meso-Cr₂O₃ samples. The bimetallic 1.95Au₁Pd₂/meso-Cr₂O₃ sample performed exceptionally for toluene decomposition and its T_90% was 165 °C. Small particle size of the noble metals, high concentration of adsorbed oxygen species, low-temperature reducibility, and dispersion of noble metals contributed to excellent catalytic activity.

Fig. 8

Reproduced from Elsevier

A Toluene conversion as a function of reaction temperature of the (filled squares) meso-Cr₂O₃, (filled circles) 0.50Au₁Pd₂/meso-Cr₂O₃, (filled triangles) 0.97Au₁Pd₂/meso-Cr₂O₃, (filled diamonds) 1.95Au₁Pd₂/meso-Cr₂O₃, (empty triangles) 0.90Au/meso-Cr₂O₃, and (empty diamonds) 1.00Pd/meso-Cr₂O₃ samples at SV = 20,000 mL g⁻¹ h⁻¹. B Effect of SV on the catalytic activity of 1.95Au₁Pd₂/meso-Cr₂O₃ for toluene oxidation (Wu et al. 2016).

Similarly, Hu et al. (2018) synthesized ruthenium (Ru) nanoparticles with mass loading ranging from 1.5 to 3.2 wt.% supported on cerium dioxide (CeO₂) for application in the catalytic combustion of propane. Once Ru was loaded on the CeO₂ or Al₂O₃ support, the catalytic activities improved drastically, which clearly suggests that Ru has intrinsic catalytic properties for propane oxidation, which is due to the interface between Ru and CeO₂ or Al₂O₃ forms oxygen defects and attributes to strong metal support interaction, that can help in the propane oxidation. These findings confirmed that the Ru/CeO₂-X catalysts exhibited higher catalytic activities than the Ru/Al₂O₃-X catalysts in the total oxidation of propane. Also, CeO₂ support can act as a reservoir for oxygen and provide additional sites for propane adsorption. Shi et al. (2021c) prepared a series of Pt/CeO₂-TiO₂ with 0.5 wt% Pt via a modified ethylene glycol reduction method and their activity for catalytic oxidation for benzene and 1,2-dichloroethane (DCE) were tested. It was observed that Pt/CeTi-11 (where 11 represents the pH value of the synthesis solution) with the smallest average Pt particle size (1.53 nm) showed 90% conversion of benzene at 152 °C. The high performance was seen due to stronger interaction between PtO_x and CeO₂-TiO₂ support results in both the formation of more Pt²⁺ species and strong low-temperature redox properties of Pt/CeO₂-TiO₂ catalyst. Zeng et al. (2020) prepared Fe-doped Mn₃O₄ hexagonal plates with reactive (1 1 2) facet in one step and 4 nm Pd particles were loaded on Fe_xMn_3−xO₄-HP. The methyl acetate conversion as a function of temperature is shown in Fig. 9. The stability of the catalyst was tested for 50 h and methyl acetate conversion found to be slightly decreased during the 50-h time on stream.

Fig. 9

Reproduced from Elsevier

a Catalytic activity of Fe_xMn_3−xO₄-HP, Pd/Mn₃O₄-HP, and Pd/Fe_xMn_3−xO₄-HP for methyl acetate combustion. b Stability test of Pd/Fe_0.135Mn_2.865O₄-HP at 190 °C with an initial conversion of 75% (Zeng et al. 2020).

It was found that Fe doping increases the oxidation state of the Pd that supports the formation of highly active oxygen species in greater extent. The targeted VOC was methyl acetate and Pd/Fe_0.135Mn_2.865O₄ exhibited best catalytic performance of the lowest T_100% of 200 °C. Zhao et al. (2020b) prepared Ru/Ti_xSn_1-x catalysts via incipient-wetness with aqueous solution of RuCl₃ and co-precipitation method for catalytic oxidation of chlorobenzene and dichloromethane. As a result of the interaction between Ru and TiO₂/SnO₂, higher concentration of surface oxygen was observed. This is because of Ru–O-Ti and Ru–O-Sn interactions where they enhance strong basic surface lattice oxygen is possible and it further increase surface oxygen mobility. Moreover, the interface Ru–O-Sn has high Lewis acidity which enhances the transfer of surface oxygen to active site making suitable for dichloromethane oxidation.

Non-noble metal-based catalysts

Similarly, Zhenxuan Zhao et al. prepared three-dimensionally ordered macro-porous La_0.6Sr_0.4FeO_3-α (LSF) for oxidation of toluene. Among the prepared LSF catalysts, LSFPEG (PEG-polyethylene glycol) has high catalytic performance for toluene combustions T_10%, T_50%, and T_90% of the LSFPEG were 54, 225, and 280 °C, respectively. Low-temperature reducibility, high specific surface area, and high oxygen ad-species concentration of LSFPEG contributed to achieve improvement in catalytic activity.

Peng et al. (2020) performed low-temperature oxidative degradation of formaldehyde (HCHO) via novel manganese dioxide (MnO₂)/N-doped carbon nanotubes (NCNT) composites having varying MnO₂ content and the catalyst was defined as 70% MnO₂/CNTs (40% MnO₂/CNTs, 20% MnO₂/CNTs). When the temperature increased above 100 °C, the composite with the highest MnO₂ content (70% MnO₂/CNTs) showed the best activity with 100% HCHO removal efficiency at 150 °C. A complete 100% conversion of HCHO to CO₂ on 40% MnO₂/NCNT was obtained at as low as 100 °C, which was about 50 °C lower than the equivalent MnO₂/CNT catalyst. Synergy between well-formed MnO₂-CNT interfaces and strong electron transfer contributed to excellent catalytic activity. The detailed mechanism proposed for the formaldehyde oxidation is shown in Fig. 10.

Fig. 10

Reproduced from Elsevier

a Proposed formaldehyde oxidation mechanism on MnO₂-CNTs/NCNT. b Catalytic cycle of formaldehyde oxidation on MnO₂-based on DFT calculations and the oxygen molecule activation on CNT and NCNT. Where red = O, white = H, grey = C, blue = N and purple = Mn (Peng et al. 2020).

Li et al. (2021a) prepared amorphous Co-Mn binary oxides with defects for catalytic oxidation of propane. Highly defective Co₁Mn₃O_x showed highest catalytic activity of T_90% at 207 °C and high space velocity of 18,000 mL g⁻¹ h⁻¹. It possesses numerous oxygen vacancy defects that weaken the Mn–O bond and improve the mobility of surface lattice oxygen which in turn leads to activation of C-H bond and helps in oxidation. Zhao et al. (2020c) synthesized monolithic Co₃O₄ and Co₃O₄@MnO_x by a hydrothermal method and among the different catalysts synthesized bamboo leaf like Co₃O₄-NF-10 showed T_90% at 193 °C. The catalyst prepared with NH₄F as template agent and gave stronger adherence onto the Ni foam. Then, another catalyst Co₃O₄@MnO_x–NF was prepared by secondary hydrothermal method which showed T_90% at 177 °C which was 16 °C lower than Co₃O₄-NF-10. The improved performance is attributed to abundant Co³⁺ and increased the concentration of surface adsorbed oxygen species.

VOC removal using AOPs

VOC can be removed by using other AOPs such as photocatalysis and catalytic ozonation. Catalytic ozonation is predominantly focused by Ikhlaq et al. and Fujita et al. in presence of zeolites, TiO₂, and Al₂O₃ (Fujita et al. 2004; Ikhlaq et al. 2014). Manganese oxide-based catalysts are preferred for catalytic ozonation of VOCs and manganese addition can improve the decomposition rates of certain VOC like toluene. These metal-oxide catalysts are cost effective and have high redox potential (Xu et al. 2017; Shao et al. 2021; Gan et al. 2021).

Catalytic ozonation can degrade pollutants at lower temperature in comparison to catalytic oxidation which makes the process to be more feasible and safer for industrial use (Shao et al. 2021). In a study, Chen et al. (2020) performed catalytic ozonation of chlorobenzene over some MnO_x-based catalysts using different supports namely Al₂O₃, TiO₂, SiO₂, CeO₂, and ZrO₂. The manganese oxide-based catalysts were synthesized via an impregnation method and were tested at 120 °C. Mn/Al₂O₃ showed highest chlorobenzene conversion efficiency of 89.2% owing to its excellent textual properties, O₂ desorption, redox ability, its desirable surface adsorbed oxygen species and acidity. The other catalyst performance was in the following order: Mn/TiO₂ > Mn/SiO₂ > Mn/CeO₂ > Mn/ZrO₂. Another study by Amir et al. (Ikhlaq and Kasprzyk-Hordern 2017) revealed that as opposed to alumina, zeolites promoted decomposition of VOCs and catalytic ozonation of volatile organic compounds on zeolites takes a non-radical path which involves reactions of molecular ozone with pollutants adsorbed on the surface of zeolites since the presence of hydroxyl radical did not have significant effects on removal rates of VOCs.

Photocatalysis for VOC removal

Heterogenous photocatalysis in combination with catalytic ozonation also provides better results for VOC removal. The combination of these techniques overpowers the limitations of individual techniques and provides a high synergistic effect (Ebrahimi et al. 2017; Gérardin et al. 2021; Saqlain et al. 2021). But photocatalytic ozonation is difficult to scale-up therefore design optimization is required in order to make it industrially feasible. The advantage of photocatalytic route is the low operating cost as it occurs at room temperature with sun light or artificial light source that contain UV/Visible light spectrum.

The photocatalytic process involves adsorption of VOC on the surface sites of adsorbents followed by chemical reactions that convert harmful VOCs into carbon dioxide and water. As shown in Fig. 11, the semiconductors like titanium dioxide, ZnO are highly recommended which get activated by absorbing a photon via UV irradiation source which in turn leads to release of an electron from the valance band to the conduction band causing the oxidation of VOC absorbed on the surface (Zou et al. 2006; Hu et al. 2020). The photocatalysis mechanism proceeds via absorption of visible light by the catalyst, thereby creating electron hole pair in the conduction and valance band, respectively. These pairs can either recombine or can participate in the surface redox reaction. In order to maximise the charge separation and minimising the recombination many modification by doping, heterojunction and morphology enhancement taken care of (Zhao et al. 2022). Fiorenza et al. studied noble metal free co-catalyst for VOC removal. MnO_x-ZrO₂ mixed oxides used for removal of toluene and ethanol in the gas phase with the multi-catalytic solar photothermal approach. Among the photocatalytic, thermo-catalytic and the photo-thermo-catalytic methods for removal of VOCs, first method has the advantage of working at room temperature and that with the MnO_x-5% ZrO₂, it reached a similar activity of the most used TiO₂-based materials, showing the importance of the material (Fiorenza et al. 2022). Similarly, Enesca et.al. worked on a titania-free heterostructure based on CuS/SnO₂/WO₃ material and compared with single WO₃ for the removal of formaldehyde and acetaldehyde. While WO₃ showed 41% and 52%, the CuS/SnO₂/WO₃ material exhibits a superior photocatalytic activity of 62.9% and 78.5% for acetaldehyde and formaldehyde respectively (Alexandru Enesca and Viorel Sisman 2022). Detailed comparison of the catalyst for VOC abatement is explained in Table 5.

Fig. 11

Reproduced from Elsevier

Schematic of the basic mechanism of photocatalysis (Almaie et al. 2022).

Table 5 Literature comparison on VOC abatement

Adsorbents for VOC abatement

Adsorption is a simple and cost effective regenerative method for removal of volatile organic compounds at low temperature (Zhang et al. 2017b; Ma et al. 2021a). For the industrial application of VOC removal, wide variety of materials are used as adsorbents, namely, activated carbon, CNT, zeolites, metal organic frameworks, carbon nanofibers, hyper cross-linked polymer, microporous polymer, and silica gel (Wang et al. 2018b; Ojstršek et al. 2020; Zhang et al. 2020c; Anand et al. 2021; Kutluay and Temel 2021). MOFs are known to have high adsorption capacity towards VOCs due to its peculiar properties of high specific surface area, large porous volume, and numerous metal sites which can interact with gaseous molecules of VOCs. MOFs consist of metal clusters that are connected by organic ligands and can form many structures (Britt et al. 2008; Bahri et al. 2017; Ongari et al. 2017). Therefore, metal organic frameworks make excellent adsorbents. For instance, Rui Ou et al. found that structural defects in UiO-666 can actually lead to improved performance. The modified UiO-66 by adding acetic acid (HAc) via a hydrothermal method has increased the surface area and pore volume of the initial adsorbent. The modified adsorbent UiO-66–2.0 HAc has been studied for adsorption of benzene and toluene at various HAc concentrations. The adsorbent UiO-66–1.0HAc has tuning concentration of HAc/Tac = 24, and it showed highest adsorption capacity of 367.13 mg g⁻¹ for benzene which was 41.9% higher than non-defected UiO-66. When the concentration of Hac/Tac was increased to 48 molar ratios, the adsorption capacity of UiO-66–2.0Hac was achieved with the highest capture values of 410.21 mg g⁻¹ of toluene at 25 °C which was 93% greater than original UiO-66. The high adsorption capacities can be attributed to addition of acetic acid which controllably modulated the number of defects by missing linkers (Ou et al. 2021). Similarly, Yang et al. carried out pyrolysis of Zn based MOF(ZIF-8) to synthesize Zn-containing graphite carbon (Zn-GC) which targeted the adsorption of formaldehyde. The adsorption capacity of it was found to be 736 times greater than commercial activated carbon and 5.6 times that of ZSM-5 adsorbents. The adsorption capacities of Zn-GC-550, Zn-GC-650, Zn-GC-750, and Zn-GC-850 for formaldehyde adsorption were 16.67, 17.57, 13.49, and 11.32 mg g⁻¹ of formaldehyde, respectively, among which Zn-GC-650 exhibited the best performance. This excellent adsorption capacity is attributed to the hierarchical porous structure of the framework which enhanced the capture of formaldehyde (having a strong affinity for formaldehyde molecules) through hydrogen bonding as the porous structure provided abundant space to pass through and hold the water vapor in the air (Yang et al. 2021a).

Kutluay (2021) modified magnetic Fe₃O₄/AC@SiO₂ nanoparticles with 8-hydroxyquinoline-5-sulfonic acid via the co-precipitation and sol–gel methods to form Fe₃O₄/AC@SiO₂@8HQ5SA as effective adsorbent for BTX (benzene, toluene, xylene) vapors. The maximum adsorption capacities of the BTX vapours by Fe₃O₄/AC@SiO₂@8HQ5SA were seen to be 555.85, 620.80, and 745.54 mg g⁻¹, respectively. After five consecutive adsorption–desorption cycle tests, Fe₃O₄/AC@SiO₂@8HQ5SA maintained the reuse efficiencies of 91.92%, 91.17%, and 90.65% for the BTX vapors which makes them cost-effective (Kutluay 2021). The dynamic adsorption capacities of Fe₃O₄, Fe₃O₄/AC, Fe₃O₄/AC@SiO₂, and Fe₃O₄/AC@SiO₂@8HQ5SA for the removal of the BTX vapors is shown in Fig. 12. Activated hydrochars derived hickory wood and peanut hull with H₃PO₄ and KOH using hydrothermal carbonization method have also improved the VOCs of acetone and cyclohexane removal efficacy. Hydrochar derived from hickory wood and peanut hull were abbreviated as HH and PH. Those modified by KOH and H₃PO₄ were abbreviated as HHK, PHK, HHP, and PHP. The VOC adsorption (acetone) of the activated hydrochars (50.57–159.66 mg g⁻¹) were greater than that of the nonactivated hydrochars (15.98–25.36 mg g⁻¹), which was mainly due to increase in surface area. The adsorption capacities for acetone on HHP and PHP were 147.77 mg g⁻¹ and 113.94 mg g⁻¹, which were higher than that on HHK and PHK. After several adsorption and desorption cycles, the reusability decreased slightly which means that they can be used as effective adsorbents (Yu et al. 2020) as shown in Fig. 13.

Fig. 12

Reproduced from Elsevier

The comparison of dynamic adsorption capacities of Fe₃O₄, Fe₃O₄/AC, Fe₃O₄/AC@SiO₂, and Fe₃O₄/AC@SiO₂@8HQ5SA for the removal of the BTX vapors (Kutluay 2021).

Fig. 13

Reproduced from Elsevier

Breakthrough curves of a acetone, b toluene, c acetic ether on PCs, and d adsorption capacities of PCs; 300 mg of PC at 300 K. Total adsorption flow rate-500 mL min⁻¹, with 1000 ppmv VOCs and N₂ as balance gas (Yu et al. 2020).

Plata-Gryl et al. made chemical modifications to nitrated asphaltenes and used these as an active layer coated on a surface of diatomaceous earth. The AsfNitro adsorbent was prepared using the methods of coating of stationary phases onto solid support. The adsorption capacities for benzene, pyridine, and 1-nitropropane increased to 26, 12, and 8 times respectively by addition of AsfNitro. Comparing to commercial carbotrap adsorbent, twice adsorption capacity was observed for benzene. However, in high humidity, there was a decrease in performance due to competitive adsorption of water molecules (Plata-Gryl et al. 2022). A schematic explaining VOC abatement mechanism on carbon surface to form CO₂ and H₂O as the final products is shown in Fig. 14. Here, L–H model is Langmuir–Hinshelwood model, where the interaction of two molecules m adsorbed at the metal surface reacts to form CO₂ and H₂O. In the second model, Eley–Rideal mechanism, reaction between gaseous O₂ and reactant adsorbed at the surface is explained. In the third mechanism, Mars-Van-Krevelen model, the catalyst surface is assumed to be an oxygen surface and thus explains the interaction of oxygen surface of the catalyst and the reactant molecules.

Fig. 14

Mechanistic pathway explaining the VOC abatement on carbon surface

Catalytic converters, adsorption systems, and filtration minimize VOC emissions. These abatement procedures help meet regulatory criteria and make living situations cleaner and healthier. To design the large-scale systems, the mechanism and reaction pathway investigations are critical.

Challenges and opportunities

Abatement technologies offer unique challenges and opportunities for the abatement of nitrogen oxides (NO_x), sulfur oxides (SO_x), and volatile organic compounds (VOCs).

Challenges

Catalyst-reactant interaction

Achieving efficient catalytic conversion of NO_x, SO_x, and VOCs requires a strong interaction between the catalyst surface and the pollutants. However, the adsorption and activation of these molecules on the catalyst surface can be challenging due to their diverse chemical nature and the presence of interfering species in the reaction mixture.

Reaction kinetics

The kinetics of the catalytic reactions for NO_x, SO_x, and VOC abatement can be complex. Multiple reactions, including oxidation, reduction, and acid–base chemistry, may co-occur leading to challenges in controlling reaction selectivity and optimizing conversion efficiency.

Catalyst stability

Heterogeneous catalysts used for NO_x, SO_x, and VOC abatement must withstand harsh reaction conditions, such as high temperatures, corrosive environments, and exposure to water and other reactants. Maintaining catalyst stability and preventing deactivation or degradation over time is a significant challenge.

Cost and energy efficiency

Implementing abatement technologies can be costly, particularly for large-scale industrial applications. Ensuring cost-effectiveness and energy efficiency in NO_x, SO_x, and VOC abatement is crucial for widespread adoption.

Opportunities

Catalyst design and development

Heterogeneous catalysis offers opportunities for tailored catalyst design. Catalyst composition, structure, and active sites can be optimized to enhance catalytic activity, selectivity, and stability. Incorporating promoters, modifiers, or nano-structuring the catalyst surface can improve NO_x, SO_x, and VOC abatement performance.

NO_x abatement in hydrogen combustion engines

Optimizing lean-burn combustion, employing advanced combustion techniques, utilizing exhaust gas after treatment systems like SCR and LNT (Lean NO_x Trap-NO_x adsorber to control NO_x emission from gasoline or lean burn engines), developing advanced catalyst materials, implementing engine optimization and control strategies, and establishing a robust hydrogen infrastructure are to be considered in designing efficient H₂ combustion engines. These efforts aim to minimize NO_x emissions and make hydrogen combustion engines more environmentally friendly and sustainable for transportation.

Novel catalyst materials

Exploration and development of novel catalytic materials can lead to NO_x, SO_x, and VOC abatement breakthroughs. For example, advanced metal oxides, zeolites, MOFs, or hybrid catalysts combining different materials can provide higher activity, selectivity, and resistance to deactivation.

Reaction engineering

Understanding and optimizing the reaction parameters and reactor design can improve catalytic performance. Strategies such as reactor configuration, flow pattern, catalyst bed geometry, and inert additives can enhance reactant/product distribution. These strategies will also facilitate better contact time with the catalyst, and minimize mass transfer limitations, and on overall improve the abatement efficiency.

Synergistic catalytic systems

Combining multiple catalysts or employing multifunctional catalysts can lead to synergistic effects and improved NO_x, SO_x, and VOC abatement performance. The overall abatement process can be enhanced by integrating complementary catalytic functions, such as oxidation and reduction.

Catalyst regeneration and recycling

Developing efficient regeneration techniques or catalyst recycling methods is crucial for the economic viability of heterogeneous catalytic processes. Designing catalysts with self-cleaning properties or developing regeneration strategies, such as controlled oxidation or reduction, can extend the catalyst’s lifespan and reduce the operational costs.

Process integration and scale-up

Integrating heterogeneous catalytic systems into industrial processes, such as power plants, refineries, or chemical manufacturing facilities, offers opportunities for large-scale NO_x, SO_x, and VOC abatement. Catalytic technologies can be effectively implemented by optimizing process parameters, considering the overall energy efficiency, and addressing safety and economic aspects.

Innovative adsorbents and absorbents

Developing novel adsorbents and absorbents with high selectivity and capacity for NO_x, SO_x, and VOC removal presents opportunities for efficient abatement. Materials such as activated carbon, zeolites, and MOFs show promise in this regard.

Conclusion and future aspects

The gradual increase in the emission of hazardous gases like NO_x, SO_x, and VOCs must be addressed on priority to safeguard the environment. Even though a large number of treatment technologies for reducing these gases are available, making these highly efficient is essential to reduce the adverse effect of the gaseous pollutant mixtures before releasing them into the atmosphere. The industries and R&D laboratories need to take radical steps in implementing technology transfer at a quicker pace. Low-temperature technologies like catalytic oxidation/reduction, non-destructive adsorption technology, redox methods like photo- and electrocatalytic ozonization are of great use with low operating cost and easily scalable for large-scale application, which are indeed required globally.

• Noble metal catalysts for NO_x reduction are preferred due to its low-temperature selective catalytic reduction, and non-noble metals like Mn, Ce, and V were started to use because of their high conversion, sulfur resistance, and nitrogen selectivity. However, cost and availability of the materials is the paramount concern for the scalability. A few transition metal oxides, zeolites, MOFs, and their modifications also have led to achieve excellent NO_x conversion and N₂ selectivity.

• Recently flue gases containing hydrocarbon, CO are mostly highlighted for the NO_x reduction. Redox reactions such as NO_x to N₂ and hydrocarbon to CO₂ are extremely important for environment safety. The catalyst can be selected so as to have high surface area, surface acidity and oxygen storage capacity.

Oxidation of NH₃ can be considered as another important step in NO_x reduction, mainly in exhaust treatment. Four-way catalytic convertors with combination of oxidation and reduction units are getting attention nowadays for the oxidation of unreacted and excess NH₃. Thus, selective oxidation of NH₃ has its own importance.

• High conversions of NH₃ selective reduction were possible because of the increased synergic effect of non-noble metal-based bimetallic catalysts. Promoters such as NbO inhibits the catalyst deactivation even at elevated temperature.

• Zeolite-based catalysts because of their shape selectivity and high surface area attributes to excellent SCO activity. Preparation method, support, and structure of catalyst plays an important role in catalytic activity and tuning these aspects can lead to favourable results.

• Promoters such as V and W that can increase the Brønsted acidity and catalyst that has high oxygen vacancy are important for the SCO of NH₃. There are promoters like W that decreases the activation energy there by increasing the activity.

It is well known that small particle size of noble metals, high adsorbed oxygen species concentration, and low-temperature reducibility contributed to excellent catalytic activity for the VOC removal. Non-noble metal-based catalysts with large surface area, high oxygen ad-species concentration is used for VOC abatement.

• Catalytic ozonation and photocatalysis are used to overpower the limitation of VOC abatements to convert CO₂ and H₂O. Further, less-expensive adsorption technique is also efficient method to eliminate VOCs. With the help of activation, the surface nature and porosity of MOF and its derivatives can be tuned for the VOC removal.

• Inexpensive activated hydrochars can attributed to increased surface area there by increased adsorption of VOCs. By introducing linkers, desired pore tunability can be achieved to enhance the adsorption capacity and thereby limiting the emission of pollutant gases.

Through this review, we have tried to combine the technologies for removing the gases like NO_x, SO_x, and VOC. The main application for the abatement of these gases requires in the automobile industries. Along with conventional catalytic technologies like selective catalytic reduction, advanced technologies such as advanced oxidation for the removal, adsorption, and photo-electro catalysis for the NO_x/SO_x removal have also been explored in the review. Similarly, VOC removal technologies have included low-temperature oxidation, advanced catalytic oxidation, adsorption, and other catalytic technologies. We have included how the different structures of the catalyst by modifying their surface properties have assisted in improving the catalytic properties. Dispersing active metal species like platinum, palladium, and rhodium as nanoparticles or clusters on the catalyst surface is an effective technique. This increases catalytic reaction surface area and catalyst-reactant contact, enhancing NO_x reduction efficiency. Similarly, maximizing catalyst surface area with porous or high-surface-area supports (e.g., zeolites, mesoporous materials) can boost catalytic activity. These structures increase NO_x adsorption and reaction, enhancing NO_x reduction. Likewise, functional groups or additions on the catalyst surface improve NO_x reduction selectivity and stability. Alkaline-earth metals like barium and strontium can help catalysts store and release NO_x as well as VOC species, enhancing pollutants conversion efficiency. Similarly, cerium and other transition metal oxide catalysts can alter oxidation states during catalysis. Redox behaviour helps convert NO_x as well as VOC species by transferring oxygen atoms between catalyst and reactants. Also, optimising catalyst support material affects NO_x reduction efficiency. Support acidity/basicity, thermal stability, and oxygen storage capacity can optimise NO_x reduction reaction catalytic activity and stability.

The future aspects of the abatement of gaseous pollutants involve low-cost technology. As several transition metals/carbon-based materials are abundant, relying on transition metal modified/CNT and other carbon-based supports as catalysts is a viable option from an economic viewpoint. A comprehensive understanding of the structure–activity relation, kinetic studies, and other related reaction engineering studies is yet to be explored. It is challenging to scale up the process and establish the removal in real-life applications. As discussed in the review, exploring and understanding the challenges and gaps and investigating future opportunities would be tremendous.