A Short Review of Layered Double Oxide-Based Catalysts for NH₃-SCR: Synthesis and NO_x Removal

Abstract

Nitrogen oxides are one of the main atmospheric pollutants and pose a threat to the ecological environment and human health. Selective catalytic reduction (NH₃-SCR) is an effective way of removing nitrogen oxides, with the catalyst being the key to this technology. Two-dimensional nanostructured layered double oxide (LDO) has attracted increasing attention due to the controllability of cations in the layers and the exchangeability of anions between layers. As a derivative of layered double hydroxide (LDH), LDO not only inherits the controllability and diversity inherent in the LDH structure but also exhibits excellent performance in the catalytic field. This article contains three main sections. It begins with a brief discussion of the development of LDO catalysts and analyzes the advantages of the LDO structure. The later section introduces the synthesis methods of LDH, clarifies the conversion relationship between LDH and LDO, and summarizes the modification impacts of the properties of LDO catalysts. The application of LDO catalysts used in NH₃-SCR under wild temperature conditions is discussed, and the different types, reaction processes, and mechanisms of LDO catalysts are described in the third section. Finally, future research directions and outlooks are also offered to assist the development of LDO catalysts and overcome the difficult points related to NH₃-SCR.

1. Introduction

Nitrogen oxide (NO_x) pollution is a pressing issue globally, posing a significant threat to both the environment and human health. In the last few decades, the main emissions of NO_x have been from the combustion of coal-based products [1,2,3]. However, with the continuous improvement of environmental standards and strict governance, traditional emission sources of NO_x have been gradually reduced and transferred to non-electricity industries. The NO_x emission standards of non-electricity industries are far lower than those of coal-fired power station industries, so the treatment of NO_x emitted by non-electricity industries has become an important issue. Moreover, compared with NO_x discharged by traditional industries, the NO_x emitted by non-electricity industries has the following characteristics: (1) large fluctuations in gas volume; (2) wide temperature range; (3) complex composition; and (4) unstable flue gas [4,5]. At the same time, the lack of economically feasible technical solutions and unstable industry profitability make the treatment of NO_x flue gas very difficult. Therefore, replicating the experience of ultra-low electricity emissions is difficult, necessitating the updating of existing technologies and the development of advanced ones.

Selective catalytic reduction (SCR) of nitrogen oxides utilizing ammonia as a reducing agent was discovered in the United States by the Engelhard Corporation in 1957 [6]. After commercialization by IHI Corporation (Japan) in 1978 [7], the technology of NH₃-SCR has achieved rapid development (Figure 1). Until now, the advantages of SCR such as considerable efficiency (70–95%), appropriate cost (the typical cost of an SCR device for power plant boilers is between 7.8 and 28.2 USD/kW), huge flue gas treatment ability, and simple operation motivate it to become the most widely used denitrification technology in the industry all over the world. Although reaction temperature, flow rate, and gas type are important factors affecting NH₃-SCR efficiency, catalysts are the core part of NH₃-SCR equipment. The NO_x conversion, N₂ selectivity, working temperature window, anti-poisoning ability, and relative treatment parameters are determined by the composition of catalysts. Thus, the design of catalysts and the study of the reaction process play important roles in NH₃-SCR for converting nitrogen oxides. In the past few decades, the most successful commercial catalyst has been V₂O₅-WO₃/TiO₂, whose denitrification conversion is over 90% within the 300~400 °C window temperature [8,9]. However, vanadium-based materials are easy to sublimate and release into the environment, causing adverse effects on human health and the environment. Moreover, the temperature of flue gas in non-electric industries is lower than that of the working windows of V₂O₅-WO₃/TiO₂, which requires secondary heating and high energy consumption. Thus, the utilization of this V₂O₅-WO₃/TiO₂ catalyst has been restricted in non-electronic fields, and developing new NH₃-SCR catalysts is imperative.

As we mentioned above, one of the difficulties in flue gas denitrification in non-power industries is that the catalyst must adapt to wide temperature conditions (normally between 150 and 350 degrees). In this case, the design of catalysts should maintain two qualities: one is good structural thermal stability in a wide temperature range and the other is that the catalysts can exhibit considerable denitrification efficiency across various temperature stages. Among all the types of catalysts developed, LDO is highly promising due to its highly stable structure and diverse composition. In general, LDO materials are the products of LDH pyrolysis and can retain many unique properties of LDH after the heating process [10,11]. Because LDH is a typical two-dimensional layered material and the interaction between inlayers is weak, excellent expanding properties can be obtained by changing the internal layer composition of LDH, which has been applied in many fields [12,13,14,15]. Based on previous studies, the use of LDH as a precursor for LDO has several benefits: (i) The growth of LDH crystals. The active metal cations can be distributed on the atomic surface uniformly due to the special layered structure, promoting a positive reaction. (ii) Because strong metal–metal bonds exist in the structure of LDH, during the calcination process, these special bonds can prevent the sintering of metal particles. (iii) LDH and LDO have memory effects. At low calcination temperatures (300–500 °C), LDO can reconstruct and regain the original layered structure in certain solutions and turn back into LDH. Therefore, LDO materials have gradually captured the attention of researchers and have been reported in the field of catalytic reactions.

Based on our knowledge, there have been few published summaries of LDO catalysts in NH₃-SCR in the past. Thus, in this review, we discussed the development of LDO catalysts for NH₃-SCR in recent years. Different kinds of preparation methods used to synthesize LDH and LDO catalysts are introduced in detail and the relationship between structural properties and modification methods is analyzed. The application of LDO catalysts is discussed regarding NH₃-SCR, including LDO catalyst compositions, reaction processes, and mechanisms. In addition, useful strategies are presented to take advantage of the excellent properties of LDO catalysts to improve NH3-SCR performance. Finally, future research directions and challenges are offered to give useful ideas about LDO catalysts to readers from diverse backgrounds.

2. Synthesis of LDO

2.1. LDH Properties and Preparation

Based on the previous papers [16,17], most LDO materials are obtained by the pyrolysis of LDH (calcination temperature from 400 to 600 °C), so the design and composition of LDH structures have an important relationship with the properties of LDO materials. In general, the molecular formula of LDH is [M²⁺_1-xM³⁺_x(OH)₂]^x+[(Aⁿ⁻)_x/n·mH₂O]^x−, where M²⁺ presents metal divalent ions such as Ni²⁺, Zn²⁺, Mg²⁺, Ca²⁺, Mn²⁺, and Fe²⁺, while M³⁺ represents trivalent metal ions such as Al³⁺, Cr³⁺, Fe³⁺, Co³⁺, Mn³⁺, Mo³⁺, Co³⁺, Ga³⁺, In³⁺, etc. Moreover, Aⁿ⁻ is an anion inserted into the interlayer for charge compensation (CO₃²⁻, NO₃⁻, SO₄²⁻, OH⁻, Cl⁻, PO₄³⁻, etc.) and n is the charge number. x represents the molar ratio of [M³⁺/(M²⁺+M³⁺)], which is usually between 0.17 and 0.33, and m shows the number of water molecules located in the interlayer [18,19,20]. As shown in Figure 2, the M²⁺ and M³⁺ cations in LDH form a hydroxide octahedral coordination structure, and the main layer is formed by sharing hydroxyl groups. The layers are connected to each other through hydrogen bonds (OH-Aⁿ⁻-OH or OH-H₂O-Aⁿ⁻-OH) between interlayer water and layer hydroxyl groups. The M²⁺ and M³⁺ cations on the layer maintain charge balance with the interlayer An- anions, making the LDH layers as a whole electrically neutral. In addition, some crystal water molecules between the layers play a role in stabilizing the layered structure. Under an appropriate heating temperature, these crystal water molecules can be removed without destroying the layered structure.

Different synthesis methods of LDH have been reported by some groups [26,27,28]. Unlike traditional catalyst systems, the methods of LDH synthesis are simple and efficient, which is suitable for industrial-scale production. After calcination, a highly stable and uniformly distributed multi-metal mixture LDO can be formed. Thus, research and discussion on the preparation method of LDH can be used to further study the basic structure and properties of LDO. Herein, three main synthesis methods of LDH are introduced as follows.

2.1.1. Coprecipitation

Coprecipitation is the most common method for preparing LDH materials. The divalent and trivalent metal salt solutions are mixed and stirred under alkaline conditions in certain ways to produce the sediment, producing the LDH products after necessary treatments. The most critical influencing factor among the synthesis conditions (concentration, addition rate, temperature, water volume during aging, and washing method) is the pH value, which must arrive at a certain value for two solutions to form precipitates. When the mixed solution contains a high pH value, hydroxides are formed. On the contrary, a low pH solution results in the conformation of amorphous compounds, and the growth of LDH grains is chaotic. Moreover, the reaction temperature and aging time also affect the physical–chemical properties of LDH, such as the intensity of crystallinity, the dimension of particles, and the size of the specific surface area. Due to the unique structure of LDH, the composition of the layers of LDH can be conveniently adjusted by changing the type and ratio of divalent and trivalent metal salt solutions during the coprecipitation process. Figure 3 displays a typical coprecipitation process of the synthesis of Zr-LDH; the Zr-LDO was easily obtained by calcination under limited oxygen conditions [29]. However, this coprecipitation method requires a long period of aging and asynchronous reaction steps, the particle size of the prepared LDH is not uniform, and the crystallinity is relatively low, which needs to be modified in the future. Almojil and Othman concluded a series of binary and ternary LDHs for phosphate absorption using a coprecipitation method [30]. By limiting the ratio of M²⁺/M³⁺ to between 2 and 4, different structures of LDH could be obtained. Furthermore, the exchange capacity of LDH could be increased by increasing the number of trivalent cations in the LDH, thereby increasing the net positive charge in the interlayer region. Many previous studies have shown the effects of controlling the ratio of M²⁺ and M³⁺ in the coprecipitation method of LDH, and the structural parameters of LDH and its phosphate adsorption capacity are shown in

Table 1. LDH structural parameters and phosphate adsorption capacity.

Materials	Preparation Method	Size (nm)	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)	Phosphate Sorption Quantity (mg/g)	Refs.
Fe3O4@Zn–Al–LDH	Co-precipitation	200	133	0.59	17.6	36.9	[31]
Fe3O4@Mg–Al–LDH	Co-precipitation	200	71.9	0.15	7.81	31.7	[31]
Fe3O4@Ni–Al–LDH	Co-precipitation	200	50.9	0.34	28.4	26.5	[31]
AC/MgAl LDH	Co-precipitation	——	584.124	584.124	——	337.2	[32]
LDHns-U80	Co-precipitation	30	97	0.28	13	98 ± 11	[33]
Fe/Mg-LDH-BC	Co-precipitation	——	267.3	0.0879	——	117.2	[34]
ZnAlLa-LDH	Co-precipitation	200	28.631	0.138	20.316	105.42	[35]
SBAC100MgFe	Co-precipitation	30	169	0.21	1.82	104	[36]
GLDA@MgAl–LDH	Co-precipitation	50	60.15	0.13	1.76	44.17	[37]

[31,32,33,34,35,36,37].

2.1.2. Urea Hydrothermal Method

The urea hydrothermal method is performed by mixing the metal salt solution and a certain amount of urea thoroughly to form a uniform solution and then transferring the above solution to a hydrothermal reactor. The reactor is put at a high temperature and high pressure for a while, and the LDH products are finally obtained through centrifugation, washing, drying, grinding, and other treatments. Urea (CO(NH₂)₂) is highly soluble in water and slowly hydrolyzes in water to produce NH⁴⁺ and NCO⁻, which can keep the internal pH of the solution consistent. During the growth process of LDH, the rate of urea decomposition can be regulated by adjusting the reaction temperature and duration, ultimately influencing the structure and morphology of LDH. LDH materials prepared by the urea hydrothermal method generally contain good crystallization and uniform particle size. Moreover, compared to synthesizing LDH using an alkaline solution, synthesizing LDH with urea facilitates easier cleaning and yields purer samples, eliminating the need for repeated cleaning to remove alkali metal ions and unnucleated cations. For example, Tao and his co-workers used four Co sources to obtain four different hydrotalcite precursors by the urea hydrothermal method (Figure 4). Based on their paper, CoFe-LDH had good crystallinity and exhibited a multilayered structure formed by the accumulation of nanosheets. After calcination, CoFe-LDO still maintained the multilayered structure, but the smooth nanosheets changed to porous nanosheets [38]. However, this method results in the product of LDH containing carbonate anions, which is an unfavorable factor for some catalytic reactions and needs to be improved.

2.1.3. Ion Exchange Method

The ion exchange method is an extension of the above-mentioned methods when the designed LDH materials cannot be directly obtained. According to previous studies [39], many monovalent anions are easily exchanged in the layers of LDH, and the ability of migration is arranged as CO₃²⁻ > NYS²⁻ > SO₄²⁻ > OH⁻ > F⁻ > Cl⁻ > Br⁻ > NO³⁻ > I⁻ [40], but, for multi-ion exchange, the reaction conditions can be more complex. Moreover, the unstable divalent and trivalent ions in the brucite-like layers can be also replaced by other metal ions, and it should be noted that the exchanged metal ions need to have similar radii and properties of the original metal ions, ensuring that the structure of LDH will not be destroyed. Different groups have utilized this method to obtain their designed LDH. For example, Rouahna’s lab used the coprecipitation method to prepare Mg-Al-CO₃ LDH in a mixed alkaline solution of NaOH and Na₂CO₃. Then, they found that the D2EHPA groups easily replaced CO₃²⁻ ions to obtain Mg-Al-D2EHPA LDH, and the anion exchange capacity was equivalent to 100% [41]. Wu et al. (Figure 5) used CuMgAl-NO₃-LDH and DSO- as the main and exchangeable interlayer anions, respectively, to prepare the CuMgAl-SDSO-LDH precursor by a one-step in situ method, and calcined it to obtain the CuMgAl-SDSO-LDO catalyst. Compared with CuMgAl-NO₃-LDO and CuMgAl-NO₃-LDO/SDSO prepared by the traditional ion exchange method, CuMgAl-SDSO-LDO has better SCR activity and stronger resistance to H₂O/SO₂, which may be due to the moderate carbon doping content that gives Cu greater dispersion and a more suitable valence distribution [42]. Generally, the ions used for exchange should follow several rules, the exchanged ions should have similar properties, the host LDH layers should not repel exchanged ions, and suitable pH values and reaction temperatures should be employed. However, the ion exchange method is a two-step reaction, and the cost and complexity are greater than those of the above-mentioned two methods. Also, due to the structural properties of LDH, neutral species are not allowed to intercalate into the inner LDH layers, resulting in a limited range of applications.

In addition, with the continuous development of synthetic technology and the demands for LDH with various properties, the calcination recovery method [43], nucleation/crystallization isolation method [44], microwave method [45], and reverse microemulsion [46] method, etc., are also reported for the construction of LDH. Based on the rich chemical composition and controllable structural characteristics of LDH, the appropriate preparation method can be selected and designed to satisfy the different catalytic reactions [47], which is a major advantage of LDH materials. From all the above preparation methods, it can be concluded that some novel preparation methods have unique advantages. Yaseneva and his co-workers used a two-step process: co-precipitation in a millimeter-scale (mesoscale) continuous flow reactor, followed by aging and the continuous synthesis of doped layered double hydroxides (Figure 6). The specific process is as follows: the metal salt solution and the mixed alkali solution (NaOH and Na₂CO₃) are respectively dripped into a constant temperature beaker containing a certain amount of deionized water (T = 65~95 °C) through a controllable speed injection pump and then the precipitate is washed to neutral and dried to obtain the product. Compared with conventional batch synthesis, the pH in this continuous process is not controllable and is determined by the composition of the feeding solution. It is impossible to precisely specify the pH at the mixing position, but the starting pH of the solution can be easily measured in batch experiments. In conventional batch synthesis, the pH value is usually adjusted by adding NaOH to the precipitation stirring vessel. Results show that compared with the conventional coprecipitation method, the synthesis is not repeatable and the properties of the batch coprecipitation products are significantly different. The surface area of the samples obtained by the new method (80~150 m²g⁻¹) is greater than that of coprecipitation batch synthesis samples. The samples have high crystallinity and small grain size (in the range of 9.5~11.9 nm, depending on the composition), and the standard deviation between samples is very low [48].

LDH can also be directly synthesized using solid raw materials by avoiding the utilization of salt solutions and alkali solutions. Zadaviciute S. et al. [49] used magnesium carbonate and γ-Al₂O₃ formed by calcining magnesium hydroxide at 475 °C for 5 h as raw materials to prepare MgAl-LDH, where the molar ratio of Mg/Al was 1 and 2 and the water/solid ratio was kept at 10. Studies have shown that under hydrothermal and microwave conditions, the formation of hydrotalcite is significantly affected by the molar ratio of the initial mixture and the synthesis time, and microwave treatment is suitable for the direct synthesis of hydrotalcite from solid raw materials [49].

Overall, in this part, we have discussed the common methods for the synthesis of LDH, and we find that these methods are controllable. Through the detailed design of LDH, various methods can be employed to modify its properties, aiding our further investigation into its pyrolysis products—LDO.

2.2. The Transition from LDH to LDO

Generally, LDO is the product resulting from the pyrolysis of LDH, effectively preserving the structural framework and certain characteristics of LDH during the pyrolysis process. However, since this step involves the transformation and release of functional groups in LDH, a discussion of the pyrolysis process is also imperative. The study of the pyrolysis process can also contribute to elucidating the growth mechanism and associated characteristics of LDO. Many researchers have studied LDH pyrolysis. Frost et al. studied the thermal decomposition behavior of natural hectorite (Ni₆Al₂(SO₄/CO₃)(OH)₁₆·4H₂O) and hydropyroxene (Ni₆Fe₂(SO₄/CO₃)(OH)₁₆·7H₂O) by high-resolution thermogravimetric analysis and mass spectrometry and found that the thermal decomposition of these hydrotalcite minerals could be attributed to five steps: (a) water desorption, (b) dehydration, (c) dehydroxylation, (d) decarbonation, and (e) desulfurization [50]. Similarly, Zohre Karami’s team detected the thermal behavior of Mg-Al-CO₃-LDH samples via thermogravimetric (TGA) analysis and derivative thermogravimetric analysis (DTG). Figure 7 shows the analysis results, and was found that four main endothermic processes occurred during the thermal decomposition of LDH. Corresponding endothermic peaks were observed on the DTG curve. The total weight loss of Mg-Al-LDH was measured to be 54.78%. Specifically, 18.47% of the total weight loss was attributed to the removal of weakly adsorbed water and interlayer water on the LDH layer below 150 °C, the 35.29% loss was due to the dihydroxylation of the LDH structure from 150 to 320 °C, and the fourth step loss was about 1.02%, stemming from the final decarbonization of the LDH powder and the removal of interlayer anions [51].

In this case, the thermal decomposition process of LDH can be concluded with four main steps: interlayer water removal, interlayer anion extraction, layered hydroxyl dehydration, and new phase formation. At different calcination temperatures, the generated oxide phases are different, which has a great influence on the structure, formation, and NH₃-SCR performance of LDO. Choi et al. calcined Co-Al and Co-Mg-Al mixed oxides at different temperatures and discussed the phase transition of products. When the calcination temperature was over 500 °C, the Co-containing hydrotalcite was converted into a spinel phase. As the heat treatment continued, the spinel phase gradually decomposed into CoO, and the content of Co³⁺ in the mixed oxide decreased. The isothermal NO_x adsorption test showed that the presence of O₂ and the low calcination temperature were conducive to the NO_x adsorption of LDO [52]. As shown in Figure 8, Xu’s lab studied the topological transformation process of ZnCo-LDH by various microscopic and spectroscopic techniques. The results showed that below 200 °C, ZnCo-LDH maintained a layered structure with an exposed high-energy catalytic interface, which was beneficial to electrochemical water oxidation. When the calcination temperature was 200–400 °C, the zincite ZnO and spinel Co₃O₄ phases formed with the LDH structure collapsed slowly. Moreover, Zn ions were inserted into the spinel Co₃O₄, creating ZnCo₂O₄ with slight Co doping of ZnO, resulting in a large number of interfaces. Increasing the calcination temperature from 400 °C to 600 °C led to grain growth of the zincite ZnO and spinel Co₃O₄ with the reduction of the crystal face. When the temperature was increased from 600 °C to 800 °C, the zincite ZnO and spinel Co₃O₄ sintering and the intertwined interfaces disappeared [53].

In addition, the specific surface area, pore parameters, and crystal phase of LDO are easily affected by the thermal decomposition temperature. Thus, choosing a suitable calcination temperature is key to improving the NH₃-SCR activity of LDO catalysts. Wu et al. calcined Ni₃Mn₁Ti₁-LDO catalysts at different temperatures (400 °C, 500 °C, and 600 °C) and tested their denitration performance. The results showed that Ni₃Mn₁Ti₁-400 had the best NH₃-SCR activity compared to Ni₃Mn₁Ti₁-500 and Ni₃Mn₁Ti₁-600, with a NO_x conversion rate of more than 80% at 150 °C. As the temperature increased from 210°C to 330 °C, the NO_x conversion rate approached 100% and then decreased as the temperature continued to rise. Although the NH₃-SCR activity of Ni₃Mn₁Ti₁-LDO has different changes at a wild temperature window, the N₂ selectivity was always close to 90% in all reaction processes. When the calcination temperature was 400 °C, the diffraction peaks corresponded to the NiO crystal phase. After increasing the calcination temperature, the particle crystallinity improved, the average grain size gradually enlarged, and new phases were generated. The anatase TiO₂ phase was detected after calcining at 500 °C, and the rutile TiO₂ phase and NiTiO₃ phase were formed when the calcination temperature was 600 °C. Moreover, the specific surface area and pore volume decreased sharply, while the pore size showed the opposite trend in the range of 400–600 °C. According to the NH₃-TPD analysis, the acid content of Ni₃Mn₁Ti₁-LDO decreased significantly with the temperature increase. Therefore, the Ni₃Mn₁Ti₁-400 catalyst had the richest acid content, which was conducive to the adsorption and activation of the reactants, thus showing the best catalytic performance [54]. These examples clearly illustrate that pyrolysis is a crucial step in the transformation of LDH to LDO, and varying temperatures significantly affect the structural composition and NH₃-SCR activity of LDO.

3. LDO NH₃-SCR Catalysts

We have summarized the research status of LDO catalysts used in NH₃-SCR reactions in recent years. In this section, the preparation method, morphological structure, active ingredients, working temperature window, NO_x conversion rate, and N₂ of LDO catalysts are analyzed. Selectivity and resistance to H₂O/SO₂ are introduced.

3.1. The Influence of Preparation Methods and Structure on Catalytic Activity

The selection of the preparation method is closely related to the structure of the formed LDO, which, in turn, affects the efficiency of the denitrification reaction. Even when synthesized using different methods with the same raw materials, the structure of LDO can exhibit significant variations. Wu et al. used TiO₂ to modify NiO and obtained three NiTi mixed oxides (NiTi-IMP, NiTi-SOL, and NiTi-LDO) through the solution impregnation method, one-pot sol–gel method, and urea homogeneous precipitation method and performed denitration performance testing. It was found that compared to NiTi-IMP, NiTi-SOL, and NiO, NiTi-LDO had the best SCR performance, with a more than 90% NO_x conversion rate at 240~360 °C and close to 95% at 150~390 °C. It had N₂ selectivity and good stability (Figure 9). After continuous operation for 50 h at 240 °C, the DeNO_x activity of NiTi-LDO only fluctuated slightly by around ±2%. In addition, the catalyst was also tested against poisoning. The results showed that NiTi-LDO maintained a NO_x conversion rate of 90% even in the presence of 100 ppm SO₂ and 10vol% H₂O at 240 °C. Furthermore, after eliminating SO₂ and H₂O, the NO_x conversion rate of LDO steadily reverted to its initial value of 92%, indicating its strong resistance to SO₂ and H₂O (Figure 10) [55]. In addition to conventional preparation methods, some improved and innovative methods have also been proposed and implemented and have shown considerable NH₃-SCR performance. Tian and his labmates synthesized MnO_x-modified MgAl layered double oxide (Mn/MgAl-LDO) using a simple and rapid pour-assisted co-precipitation (FP-CP) process. Compared with Mn/MgAl-LDO obtained by slow drop-assisted co-precipitation (SD-CP), Mn/MgAl-LDO (FP-CP) had greater low-temperature activity than Mn/MgAl-LDO (SD-CP), with a NO conversion rate of 76–100% at 25–150 °C, 97% NO conversion rate at 100 °C, and N₂ selectivity of 97.3% [56]. Zhao’s team successfully prepared the MnAl-LDO catalyst using the flash nanoprecipitation method (FNP) and the results showed that the MnAl-LDO (FNP) catalyst had significant low-temperature catalytic activity, with a NO conversion rate over 90% at 150–300 °C [57]. From these examples, it is evident that when designing and utilizing various methods to prepare LDO, a preparation method that results in a thinner and smaller layered structure, as well as the creation of more oxygen vacancies and pores, is conducive to facilitating the positive progression of NH₃-SCR.

In addition to the above effects, the advantages of the LDO catalyst structure also include easier exposure of active sites and more controllable interface interactions compared with traditional multimetal oxides. Du et al. synthesized a series of NiMnTi mixed metal oxides (Ni/Mn-TiO₂, Mn/NiTi-LDO, TiO₂/NiMn-LDO, NiMnTi-LDO) using different assembly methods and evaluated their NH₃ selective catalytic reduction of NO_x. NiMnTi-LDO had the best denitrification performance among all samples, with a NO_x conversion rate of over 90% and N₂ selectivity of 95% in the temperature window of 150~360 °C. Through analysis, it was found that the strong interaction between Ni, Mn, and Ti not only accelerated the redox cycle in the reaction (Mn⁴⁺+Ni²⁺=Mn³⁺+Ni³⁺, Ni³⁺+ Ti³⁺=Ni²⁺+Ti⁴⁺, and Mn⁴⁺+Ti³⁺=Mn³⁺+Ti⁴⁺) but also caused an appropriate amount of nickel ions, manganese ions, and surface oxygen to be evenly distributed on the surface of the LDO catalyst, effectively avoiding the coverage of active centers, thereby promoting the efficiency of the denitrification reactions [58].

In summary, the synthesis methods of LDO catalysts are intimately linked to the formation of its structure and significantly impact the activity of NH₃-SCR. Typically, the chosen synthesis method in design should ensure a constant pH value during the synthesizing process. Secondly, the synthesis method should prioritize rapid reactions, promoting the formation of small and thin LDH layers, which can then be transformed into small and thin LDO structures. Lastly, it is crucial to expose more active sites and maintain the interaction among multiple metals, thereby enhancing the overall NH₃-SCR activity of LDO catalysts.

3.2. The Influence of Morphology and Support on Catalytic Activity

As an LDO catalyst possessing a two-dimensional layered structure, its morphology and support also exert a significant influence on NH₃-SCR activity. Previous studies reveal that the traditional co-precipitation method exhibits limited capacity for adjusting the morphology and particle size of LDO catalysts, resulting in most synthesized LDOs predominantly exhibiting a flaky morphology. To utilize the morphology with special functions such as hierarchical and shell–core structures to promote the performance of NH₃-SCR, many researchers have conducted extensive research on the morphological modification of LDO catalysts. Feng’s team prepared three morphologies of MnCo mixed oxides with the same composition, which is illustrated in Figure 11. It was discovered that compared to the MnCoO_x nanoflower (MnCo-NF) and MnCoO_x particle (MnCo-P), MnCoO_x layered double oxide (MnCo-LDO) had the largest surface area (SBET = 142 m²g⁻¹) and exhibited superior low-temperature catalytic activity and resistance to poisoning. Specifically, it achieved a NO_x conversion rate exceeding 90% and almost 100% N₂ selectivity within the temperature range of 60–270 °C [59]. Well-organized spherical mesoporous hollow @CuMgAl-LDH (Figure 12) was prepared by controlling the removal of SiO₂ microspheres in the SiO₂@CuMgAl-LDH core–shell. @CuMgAl-MOs (mixed oxides) were obtained and the NH₃-SCR performance was evaluated. Compared with the traditional hydrotalcite-like-structured CuMgAl-MO catalyst, @CuMgAl-0.5-MO had a higher NO conversion rate (89%) at the range temperature of 275–300°C, which may have been due to the larger specific surface area (S_BET = 296 m²g⁻¹> S_BET = 86 m² g⁻¹), mesoporous structure, and stronger surface acidity [60].

In addition, the in situ hydrothermal method was also used to grow diverse morphologies of LDO catalysts. A novel three-dimensional flower-like NiMnFe LDO on cylindrical iron wire meshes (IWM-C) was reported (Figure 13). Compared to NiMnFe-LDO and NiMnFe mixed metal oxides prepared using similar methods, NiMnFe-IWM-C exhibited excellent NO activity and a broader operating temperature range. The NiMnFe-IWM-C catalyst featured a uniform three-dimensional flower-like nanoarray, with evenly distributed active components, and the active substances were well integrated with the iron substrate, resulting in superior catalytic performance, even at high temperatures [61].

In recent years, the integration of supports with various morphologies and LDO catalysts to enhance catalytic activity has emerged as a key research focus. Anatase TiO₂ is commonly used in NH₃-SCR processes as a catalyst support due to its abundant Lewis acid sites. CuAl-LDO/TiO₂ and CuAl-LDO/TiO₂ NTs were prepared through in situ assembly and the performance of NH₃-SCR was evaluated. Compared to CuAl-LDO and CuAl-LDO/TiO₂, CuAl-LDO/TiO₂ NTs had the greatest surface area (80.5 m²/g) and pore volume (0.67 cm³/g) and the best ability to remove nitrogen oxides, with a NO_x conversion rate of over 80% and N₂ selectivity of over 90% at 210–330 °C (Figure 14). After introducing 100 ppm SO₂, the NO_x conversion rate of CuAl-LDO/TiO₂NTs decreased from 92% to 80%, while the NO_x conversion rate of the CuAl-LDO catalyst decreased from 93% to 70%. All the results proved that the TiO₂ supporter improved the performance and SO₂ resistance of CuAl-LDO. Further characterization revealed that the improvement in denitrification performance may have been due to the larger specific surface area, richer surface chemical adsorption of oxygen, and superior oxidation reduction performance of the catalyst after doping with TiO₂ NTs [62].

Similarly, CNTs were used to decorate CuAl layered double hydroxide due to the high thermal stability, large specific surface area, unique porous structure, and excellent reducibility. Three assembly methods were used to prepare CuAl-LDO/CNTs(I) (in situ assembly), CuAl-LDH/CNTs(S) (stir assembly), and CuAl-LDH/CNTs(M) (mechanical assembly) catalysts (Figure 15). It was found that among the three catalysts, CuAl-LDO/CNTs(I) had the best NH₃-SCR performance, with a NO_x conversion rate of over 80% and N₂ selectivity of over 90% in the range of 180~300 °C. Moreover, the CuAl-LDO/CNTs catalyst showed excellent catalytic stability and toxicity resistance, and the NO_x conversion could still maintain 65.7% after reacting for 56 h with 100 ppm SO₂ and 10% vol H₂O. SEM and TEM images revealed that the CuAl-LDO had a typical layered structure, but a large number of layers were severely stacked. After the introduction of CNTs, the stacking layers gradually dispersed, and CNTs could act as “spacer layers” to effectively inhibit the self-agglomeration of CuAl-LDH nanosheets. Compared to the other two assembly methods, further characterization revealed that CNTs effectively alleviated the accumulation of CuAl-LDH precursor during in situ assembly, achieving the high dispersion of copper-based metal oxides after calcination and effectively improving their surface acidity and redox ability, which was the reason for their excellent NH₃-SCR performance [63].

The modification of LDO catalysts through morphology and carriers primarily focuses on enhancing their specific surface area, pore size and distribution, diffusion capabilities, and surface acidity, thereby boosting their catalytic activity. Simultaneously, combined with the advantages of supports, the derived composite LDO materials can effectively utilize the advantages of each material and, through synergistic effects, further improve the catalyst’s resistance to medium toxicity and stability, enabling LDO catalysts to fully exhibit superior denitrification activity in a wide temperature range.

Table 2. Synthesis method and structural parameters of LDO catalysts.

Catalysts	Preparation Method of LDH	Calcination		Morphology of LDO	Size (nm)	Structure Parameters			Refs.
		T (°C)	Time (h)			Specific Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)
MnCo-LDO	Urea Hydrothermal	400	4	Layered structure	4000	142.89	0.17	5.01	[59]
Ni5Mn-LDO	Co-precipitation	400	5	Spherical	13.23	70	0.17	9.35	[64]
MnAl-LDO (FNP)	Flash Nanoprecipitation	550	6	Particles	114.9	121	0.22	7.13	[57]
Cu4AlOx	Co-precipitation	600	5	Flower-like	10.16	108.4	0.92	20.6	[65]
Cu1Ti1O	Homogeneous Precipitation	400	5	Particles	10	150.5	0.377	5.02	[66]
NiMnTi-LDO	Urea Homogeneous Precipitation	400	5	Flower-like	60	210	0.32	5	[58]
Ni2Mn2Ti1-LDO	Urea Homogeneous Precipitation	400	5	Flower-like	7.157	210	0.32	5	[54]
Ni0.5Mn0.5Fe0.5Ox	Co-precipitation	400	5	Flower-like	50	——	——	——	[67]
NiMnTi-LDO	Urea Homogeneous Precipitation	400	5	Flower-like	4.54	244	0.23	3.5	[68]
Co0.5Mn1Fe0.25Al0.75Ox-LDO	Co-precipitation	400	5	Layered structure	——	316	1.5	——	[69]
Ce0.5/Co1Mn0.5Al0.5Ox-LDO	Co-precipitation	500	5	Flower-like	200	210.1	0.77	7.29	[70]
Cu0.5Mg2.5Fe1-LDO	Co-precipitation	500	5	Sheet	10	102	0.34	10.4	[71]
Cu1Mn0.5Ti0.5Ox	Co-precipitation	400	5	Sheet	200	102.1	0.339	6.63	[72]
Cu-Mg-Al	Co-precipitation	600	16	——	——	——	——	——	[73]
Cu0.5Mg1.5Mn0.5Al0.5Ox	Co-precipitation	400	5	Flower-like	230	228.7	1.68	14.69	[74]
CuAl-LDO/CNTs(I)	In Situ Assembly	500	5	Stacked gauze-like	——	——	——	——	[63]
CoMnAl-LDO	Co-precipitation	500	5	Layered structure	250	140.9	——	——	[75]
CuMgAl-SDSO-LDO	Co-precipitation	500/400	5/4	Sheet	300	85	0.2	7	[42]
Ce2/Cu4Al1Ox-LDO	Co-precipitation	400	5	Nanosheet	90	130.8	0.86	13.14	[76]
NiTi-LDO	Urea Homogeneous Precipitation	400	5	Flake morphology	5	224.6	——	2.9	[55]
Mn/MgAl-LDO (FP-CP)	FP-CP	400	2	Stacked nanosheet	8	103.6	0.23	8.97	[56]
MnFeCo-LDO	Co-precipitation	500	4	Layered structure	200–300	92	——	——	[77]
CuAl LDH/CNTs-2	In Situ Assembly	500	5	——	——	138	——	6.2	[78]
MnCoGd0.2	Urea Hydrothermal	400	4	Nanoplates	——	181.1	0.18	5.88	[79]
Ce0.2-Mn2Cr1Ox-LDO	Co-precipitation	400	5	Flower-like	100	173.8	0.476	10.7	[80]
Mn1Fe0.25Al0.75Ox	AMOST	400	5	Flower-like	40	304.3	1.719	11.3	[81]
CuVOLDHc	Co-precipitation	470	5	Flower-like	80	72	0.344	——	[82]
(Cu4Zny)2Al-MMO	Co-precipitation	450	4	Sheet	2000	58	0.15	10.4	[83]
Co2Cr1Ox-LDO	Co-precipitation	400	5	Flower-like	500	124.8	0.401	12.9	[84]
Ni4-xMnxAlOy	Hydrothermal	400	4	Nanosheet	——	178.2	0.4	8.9	[85]
@CuMgAl_0.5-MO	Template	450	4	Core–shell particles	800	258	0.66	——	[60]
Ni4Ti1-400	Urea Homogeneous Precipitation	400	5	Nanosheet	5.6	224.6	——	2.9	[86]
Mg3Fe1-LDO/10CNTs	Co-precipitation	500	5	Thin sheets	——	112	0.62	15.4	[87]
Cu5Mg62Al33-Ce3.0%	Co-precipitation	600	12	——	——	242	——	——	[88]
Cu2Mn0.5Al0.5Ox	Co-precipitation	400	5	Flower-like	——	136.4	1.029	15.09	[89]
NiMnFe-IWM-C	In Situ Hydrothermal	500	5	3D flower-like	3000	——	——	——	[61]
Mn(0.25)/CoAl-LDO	Urea Homogeneous Precipitation	500	4	Nanosheet	2000	155.1	0.31	3.48	[90]
MnO2/CoAl-LDO	Ion-exchange/Redox Reaction	500	5	Particles	9	156.8	——	——	[91]
CuAl-LDO/TiO2NTs	In Situ Assembly	500	5	Nanosheet	——	80.5	0.67	28.7	[62]

summarizes the basic characteristics of reported LDO catalysts, and the toxic resistance and stability of these catalysts are displayed in

Table 3. The toxic resistance and stability of LDO catalysts.

Catalysts	Reaction Conditions							Stability				Refs.
	NO (ppm)	NH3 (ppm)	O2 (%)	GHSV (h−1)	T (°C)	NOx Conversion	N2 Selectivity	H2O (%)	SO2 (ppm)	T (°C)	Activity (%)
MnCo-LDO	500	500	5	36,000	60~270	>90%	~100%	5	100	120	91	[59]
Ni5Mn-LDO	600	600	5	45,000	180~360	>90%	>90%	10	100	240	80	[64]
MnAl-LDO (FNP)	500	500	5	60,000	150~250	95%	>90%	——	——	——	——	[57]
Cu4AlOx	500	500	5	80,000	200	91.1%	——	5	50	200	67.6	[65]
Cu1Ti1O	500	500	5	80,000	200	88.9%	——	——	50	200	76.4	[66]
NiMnTi-LDO	600	600	5	45,000	150~360	>90%	>95%	——	——	——	——	[58]
Ni2Mn2Ti1-LDO	600	600	5	45,000	150~360	>90%	>95%	10	100	210	94	[54]
Ni0.5Mn0.5Fe0.5Ox	500	500	5	60,000	250	>94%	91.6%	5	100	200	70.4	[67]
NiMnTi-LDO	600	600	5	45,000	150~360	>90%	>90%	10	100	210	94	[68]
Co0.5Mn1Fe0.25Al0.75Ox-LDO	500	500	5	60,000	80~250	>92%	>75%	5	100	150	>60	[69]
Ce0.5/Co1Mn0.5Al0.5Ox-LDO	500	500	5	80,000	100~250	>95%	>80%	5	100	150	92.4	[70]
Cu0.5Mg2.5Fe1-LDO	500	500	5	36,000	180~240	>80%	>85%	8	100	240	80	[71]
Cu1Mn0.5Ti0.5Ox	500	500	5	80,000	200	90%	99.4%	5	100	200	64.6	[72]
Cu-Mg-Al	2500	2500	2.5	7000	200~250	80~95%	93~97%	——	——	——	——	[73]
Cu0.5Mg1.5Mn0.5Al0.5Ox	500	500	5	60,000	100~250	87.2~96.6%	>90%	5	100	150	68.2	[74]
CuAl-LDO/CNTs(I)	600	600	5	45,000	180~300	>80%	>90%	10	100	240	83.3	[63]
CoMnAl-LDO	600	600	5	45,000	150~300	>80%	>88%	10	100	240	75	[75]
CuMgAl-SDSO-LDO	600	600	5	45,000	210	90%	75%	10	100	210	75/56	[42]
Ce2/Cu4Al1Ox-LDO	500	500	5	80,000	200	95.3%	99%	5	100	200	78.8	[76]
NiTi-LDO	600	600	5	45,000	240~360	>90%	>95%	10	100	240	90	[55]
Mn/MgAl-LDO (FP-CP)	500	500	5	60,000	25~150	76~100%	>90%	5	100	100	65	[56]
MnFeCo-LDO	500	500	5	30,600	50~400	>86%	>50%	5	100	120	92	[77]
CuAl LDH/CNTs-2	600	600	5	45,000	180~305	>90%	——	——	——	——	——	[78]
MnCoGd0.2	500	500	5	50,000	90~210	100%	~100%	5	100	240	92	[79]
Ce0.2-Mn2Cr1Ox-LDO	500	500	5	90,000	200	100%	>90%	5	100	240	43.3	[80]
Mn1Fe0.25Al0.75Ox	500	500	5	60,000	150	97.6%	——	——	100	150	76.6	[81]
CuVOLDHc	2000	2000	3	414,000	200~500	70~80%	>98%	——	——	——	——	[82]
(CuxZny)2Al-MMO	600	480	5	30,000	240	>80%	——	——	——	——	——	[83]
Co2Cr1Ox-LDO	500	500	5	90,000	150	100%	>90%	5	100	200	43.3	[84]
Ni4-xMnxAlOy	500	500	6.5	45,000	120~210	>90%	>50%	5	1000	210	<20	[85]
@CuMgAl_0.5-MO	2500	2500	2.5	24,000	275~300	89%	>99.7%	——	——	——	——	[60]
Ni4Ti1-400	600	600	5	45,000	240~360	>90%	~95%	10	100	240	85	[86]
Mg3Fe1-LDO/10CNTs	500	500	5	36,000	240	90%	>80%	——	500	240	~80	[87]
Cu5Mg62Al33-Ce3.0%	2500	2500	2.5	12,000	275	96%	>90%	——	——	——	——	[88]
Cu2Mn0.5Al0.5Ox	500	500	5	80,000	150	91.2%	——	5	100	150	62.18	[89]
NiMnFe-IWM-C	500	500	3	20,000	250~370	>80%	~100%	——	200	350	92	[61]
Mn(0.25)/CoAl-LDO	500	500	5	40,000	150~300	>95%	>70%	10	100	200	76.5	[90]
MnO2/CoAl-LDO	600	600	5	45,000	90~270	>90%	>95%	10	100	240	80	[91]
CuAl-LDO/TiO2NTs	600	600	5	45,000	210~330	>80%	>90%	——	100	240	80	[62]

.

3.3. Common LDO Catalysts Used in NH₃-SCR Reactions

In recent decades, numerous kinds of LDO catalysts have been developed. Common LDO catalysts used for NH₃-SCR include the following types.

3.3.1. Mn-Based LDO Catalysts

Manganese (Mn)-based catalysts have considerable catalytic activity for low-temperature NH₃-SCR due to their variable valence state and excellent redox ability. However, the Mn element, whether as a single metal, metal oxide, or composite, is sensitive to H₂O/SO₂ and high temperatures, resulting in decreased NO_x removal efficiency. Although Mn-based LDO also faces the same problems in NH₃-SCR, the unique layered structure gives them better adjustability and controllability. For example, Du and his co-workers studied the basic properties of MnAl LDO catalysts by doping Cu, Ni, and Co elements and discussed the NH₃-SCR performance at low temperatures. Among the three samples tested, CoMnAl-LDO demonstrated the highest activity, achieving an NO_x removal efficiency exceeding 80% and an N₂ selectivity exceeding 88% within the temperature range of 150~300 °C. Furthermore, the introduction of Co modification enhanced the SO₂ and H₂O tolerance of MnAl-LDO, as illustrated in Figure 16. In the presence of 100 ppm SO₂ and 10 vol% H₂O in the NH₃-SCR, the denitrification efficiency of CoMnAl-LDO remained at approximately 75% after 15 hours of reaction, whereas MnAl-LDO only managed to maintain 42.2%. [75]. Wang et al. utilized the hexagonal CoSn₃Al-LDO mixed with MnO_x, increasing the NH₃-SCR activity from 38.6% to 73.4% under the condition of 150 ppm SO₂ [92]. Chen’s team found that Co ions loaded on Mn₁Fe_0.25Al_0.75O_x-LDO could accelerate the electron transfer to suppress N₂O formation and obtain high N₂ selectivity for NH₃-SCR, and the NO_x conversion was 80% at 150 °C with the existence of 100 ppm SO₂ and 5% vol H₂O [69]. Xie’s lab adjusted the ratio of Ni and Mn in Ni_xMn-LDOs to achieve increased NH₃-SCR activity; Ni₅Mn-LDO exhibited above 90% NO_x conversion and N₂ selectivity at a temperature zone of 180–360 °C [64].

Another study showed that the performance of Mn-based LDO could be effectively improved by introducing Mn using different methods. CoMnAl CO₃-LDH was synthesized using the hexamethyl tetramine (HMT) hydrolysis method, and CoAl MnO₂ LDH was prepared as a precursor via an ion exchange/redox reaction. CoMnAl mixed metal oxides were obtained by calcination, namely, CoMnAl LDO and MnO₂/CoAl LDO, and tested as low-temperature NH₃-SCR catalysts. Among them, MnO₂/CoAl LDO had better denitrification activity and a wider operating temperature window, with over 90% NO_x conversion and over 95% N₂ selectivity in the range of 90~270 °C [91]. To further improve the activity and resistance to H₂O/SO₂ of Mn-based NH₃ SCR catalysts, introducing the Ce element into CoMnAl LDO is a good choice due to the excellent oxygen storage/release ability and high redox ability formed by the conversion between Ce⁴⁺and Ce³⁺. Ce can also be used as an active component in SCR reactions. After introducing Ce, Ce_0.5/Co1Mn_0.5Al_0.5O_x LDO had a NO_x conversion rate of over 95% in the range of 100–250 °C, with a conversion rate of 100% at 150 °C. Under the combined action of 100 ppm SO₂ and 5 vol% water vapors, the NO_x conversion rate of Ce_0.5/Co₁Mn_0.5Al_0.5O_x LDO after 1 h of reaction was 92.4%, higher than that of Co₁Mn_0.5Al_0.5O_x (79.9%) and Ce Co Mn/γ-Al₂O₃ (41.8%) (Figure 17). In addition, the Ce_0.5/Co₁Mn_0.5Al_0.5O_x LDO catalyst had good thermal stability and, even after calcination at 600 °C, the NO_x conversion rate remained close to 100% [70].

3.3.2. Cu-Based LDO Catalysts

Cu-containing oxides have become potential low-temperature NH₃-SCR catalysts due to their environmental friendliness, low price, and good redox ability. The activity and selectivity of CuO-based catalysts are highly dependent on the structure and dispersion state of copper oxide species. A new, highly dispersed Cu_yAlO_x (y = 2~4) mixture from AMO-Cu-Al-CO₃ LDHs was prepared. Compared with the supported CuO/γ-Al₂O₃ catalyst, the catalytic activity and thermal stability of the best Cu₄AlO_x catalyst were much higher than that of CuO/γ-Al₂O₃. At 200 °C, the NO_x conversion rates of Cu₄AlO_x and CuO/γ-Al₂O₃ were 91.1% and 81.8%, respectively, which dropped to 84.7% and 57.5%, respectively, after calcination at 700 °C. XRD and HR-TEM analyses showed that the Cu₄AlO_x catalyst mainly contained well-dispersed copper oxide nanoparticles with an average particle size of ∼10 nm, while the CuO/γ-Al₂O₃ catalyst contained CuAl₂O₄ nanoparticles with an average particle size of ∼20 nm (Figure 18). Larger specific surface areas and more highly dispersed CuO active species were the keys to the good catalytic activity of the Cu₄AlO_x catalyst. In addition, the Cu₄AlO_x catalyst showed better resistance to alkali metals (K and Na), SO₂, and H₂O [65].

In further studies, CeO_2-x nanoparticles were deposited on Cu₄Al₁O_x-LDO supports and the low-temperature NH₃-SCR was evaluated, specifically studying the effects of HCl, SO₂, and H₂O on the activity and selectivity of the catalyst (Figure 19). It was found that for Cu/Al₂O₃, Cu-Ce/Al₂O₃, and Cu₄Al₁O_x, the NO_x conversion rate at 200 °C dropped significantly from 81.9, 85.4, and 91.1% to 23.7, 30.7, and 45.8%, respectively. However, under the same reaction conditions, Ce₂/Cu₄Al₁O_x-LDO still had a high NO_x conversion rate (68.3%). The introduction of Ce improved the denitrification activity of the catalyst and also significantly improved its resistance to HCl and SO₂. Through transient kinetic studies, it was found that the L-H and E-R mechanisms coexisted on the Ce₂/Cu₄Al₁O_x LDO catalyst (Figure 20). The latter better promoted the oxidation of NO to NO₂. The presence of NO₂ promoted the formation of ammonium nitrite (NH₄NO₂) and accelerated the formation of ammonium nitrite (NH₄NO₂). This process was named the “fast SCR” reaction process [76].

Researchers have also considered whether the valence distribution of CuO_x has a significant impact on the NH₃-SCR reaction. To further study the interface synergistic catalytic mechanism between Cu₂O and CuO, a CuAl-LDO/CNT_s-x composite with stable Cu₂O and CuO was prepared. The prepared CuAl-LDO/CNT_s-2 catalyst showed the best catalytic performance, with a NO_x conversion rate of more than 90% in the temperature range of 180~305°C. The higher catalytic efficiency could be attributed to the appropriate valence distribution of CuO_x species, which could significantly improve the redox capacity and surface acidity of the catalyst, thereby promoting the adsorption and activation of reactants. After further theoretical calculations, the researchers preliminarily proposed the cooperative catalytic mechanism of Cu₂O/CuO in the NH₃-SCR reaction, that is, the CuO active center could serve as the main adsorption site for NO and NH₃, promoting the formation of NO⁺ active species and NH₃ dehydrogenation activation (NH²⁺). The Cu₂O active center could serve as an adsorption site for O and promote the formation of reactive oxygen species O. Therefore, the synergistic effect between Cu₂O and CuO could lead to the rapid formation of reaction intermediates to complete the catalytic cycle through the L-H and E-R reaction pathways [78].

3.3.3. Fe-Based LDO Catalysts

Fe-based catalysts have good SO₂ resistance, high-temperature stability, and excellent redox ability, which gives them good SCR catalytic activity at medium and low temperatures, but their low surface acidity limits further applications. Inducing LDO structures into Fe-based catalysts is a considerable way to construct incredible properties. Du et al. used a one-step in situ method to synthesize a highly dispersed CNTs-doped MgFe-LDO mixed oxide catalyst. The Mg₃Fe₁/10CNTs catalyst had good NH₃-SCR activity. At low temperatures (180–270 °C), the NO_x conversion rate exceeded 80%, which may have been because the doping of CNTs gave the catalyst a larger specific surface area and greater surface weak and medium-strong acid contents. Furthermore, because of Fe’s excellent sulfur resistance, it is often used as a doping element to improve the catalytic activity of LDO catalysts in the presence of SO₂ [87].

Zhou et al. synthesized a two-dimensional MnFeCo layered dioxide that exhibited excellent low-temperature activity. At 100 °C, the NO conversion rate of the MnFeCo-LDO catalyst was 100%. Even at 25 °C and 50 °C, the NO conversion rates reached 49% and 86%, respectively. In addition, the MnFeCo-LDO catalyst had a wide temperature range (50~400 °C) and achieved a more than 86% NO conversion rate and more than 50% N₂ selectivity. A resistance test was performed at a temperature of 120 °C and a GHSV of 30,600 h⁻¹. When 5vol% H₂O was added, the MnFeCo-LDO catalyst still had a conversion rate of 94%. Then, after adding 100 ppm SO₂ to the system, the conversion rate under the MnFeCo-LDO catalyst only dropped by about 2%. After removing H₂O and SO₂ from the flue gas, the MnFeCo-LDO catalyst returned to the initial level at 120 °C [77].

Overall, the active components of LDO catalysts significantly influence NH₃-SCR catalysis. Their variable composition and highly dispersed nature can enhance the oxidation–reduction cycle through synergistic effects, thereby improving the catalyst’s low-temperature activity. LDO catalysts with varying active components possess distinct advantages and disadvantages. Mn-based catalysts exhibit good low-temperature activity but are sensitive to SO₂, and they produce significant amounts of N₂O at high temperatures, reducing selectivity. Cu/Fe-based catalysts are cost-effective and demonstrate good denitrification performance. However, the agglomeration of CuO species and the weaker acidity of Fe₂O₃ hinder the further application of Cu/Fe oxide denitrification. Therefore, building upon these types of LDO catalysts, the development of multi-component LDO catalysts, such as those containing Ni, Ce, Co, and Cr, can further diversify the applications of LDO catalysts in NH₃-SCR catalysis, promoting their efficient performance across a wide temperature range.

3.4. The NH₃-SCR Process and Mechanism of LDO Catalysts

In the NH₃-SCR reaction, the goal is to maximize the production of N₂ while minimizing the generation of additional nitrogen oxides. Therefore, it is imperative to explore the reaction mechanism of NH₃-SCR on catalysts (Figure 21). Based on the reactive sites and reaction pathways, there are primarily two mechanisms for NH₃-SCR on metal oxides: the L-H (Langmuir–Hinshelwood) mechanism and the E-R (Eley–Rideal) mechanism. In the L-H mechanism, NH₃ is adsorbed onto the Lewis acid sites on the catalyst surface, forming a coordinated NH₃ species. This species is then oxidized by high-valent metal sites, resulting in the formation of nitrate species, which, in turn, produce the intermediate products NH₄NO₂/NH₄NO₃. Due to the instability of NH₄NO₂/NH₄NO₃, they further decompose into N₂ and H₂O. Simultaneously, the reduced low-valent metal sites are oxidized by O₂ back to high-valent metal sites, completing a cycle of oxidation–reduction reactions. For the E-R mechanism, NH₃ adsorbs onto acidic sites, either forming a coordinated NH₃ species on Lewis acid sites or an NH⁴⁺ species on Brønsted acid sites. It is then oxidized by high-valent metal sites to form active NH₂ species, which subsequently react with gaseous NO to form the intermediate products NH₄NO₂/NH₄NO, which are then decomposed into N₂ and H₂O. Simultaneously, the reduced low-valent metal sites are oxidized by O₂ back to high-valent metal sites, completing another cycle of oxidation–reduction reactions [93].

Studies have shown that the adsorption of NH₃ is considered to be the first step of the NH₃-SCR reaction. Whether it is the L-H mechanism or the E-R mechanism, the adsorption of NH₃ on the acidic site is essential, and the valence change of the metal site promotes the redox cycle, which is an indispensable part of the NH₃-SCR reaction. For the E-R mechanism, excellent redox cycles promote the denitrification performance of the catalyst, but excessive oxidation ability will lead to the production of N₂O and reduce N₂ selectivity. Generally speaking, the redox properties of the catalyst determine the low-temperature activity of the catalyst, while the surface acid properties determine the high-temperature activity of the catalyst [93]. At present, more and more studies are proving that the L-H mechanism and E-R mechanism on the catalyst can coexist. Improving the strength and number of the surface acid sites of catalysts and strengthening the redox ability of catalysts are the keys to improving the denitrification performance of catalysts in a wide temperature range. Since the d orbital of the valence layer of transition metal elements is not full, the ability to gain and lose electrons is strong and there is excellent redox ability. Among them, Mn, Ce, Cu, and Fe have variable valence states and show high denitrification activity at low temperatures. However, the catalytic efficiency of a single transition metal oxide is not satisfactory. Due to its strong redox ability, it is easy to cause ammonia overoxidation, reduce N₂ selectivity, and cause SO₂ poisoning, resulting in catalyst deactivation. In general, one strategy is to highly disperse the transition metal oxide on the carrier to expose more active sites and at the same time improve the catalytic and anti-poisoning activity by interacting with the carrier.

The NH₃-SCR reaction mechanism on LDO catalysts is reported to be similar to that of metal oxides. Due to the coexistence of multiple active components on LDO, the specific reaction mechanism is related to the active species and surface characteristics of the catalyst. The CuMnTiO_x catalyst prepared by Yan et al. [72] had high NH₃-SCR activity at low temperatures. The optimal composition Cu₁Mn_0.5Ti_0.5O_x had the highest NO_x conversion rate (90%) at 200 °C. The high conversion rate may have been due to the large specific surface area, high dispersion of active species MnO_x and CuO, and abundant acid sites. In situ drift studies show that both the E-R mechanism and the L-H mechanism exist on the surface of CuMnTiO_x catalysts (Figure 22). The coordinated NH₃ on the Lewis acid site and NH⁴⁺ on the Brønsted acid site can react with gaseous NO (E-R mechanism) and adsorbed NO (L-H mechanism). In the E-R mechanism, the absorbed NH₃ is converted into NH₂ species, which then react with gaseous NO to form highly active NH₄NO₂/NH₂NO intermediates. Because NH₄NO₂/NH₂NO is unstable, it quickly decomposes into N₂ and H₂O. In the L-H mechanism, the NH₃-SCR reaction occurs between adsorbed NH₃ and adsorbed NO. At the same time, the NO oxidation reaction can be enhanced through the electron transfer between Mn³⁺-Mn⁴⁺ and Cu²⁺-Cu⁺, as shown in the following formula, thereby improving the catalytic activity through the “fast SCR reaction”.

4. Conclusions and Prospects

This review summarized the preparation methods, thermal decomposition processes, and partial reaction mechanisms of LDO catalysts. The effects of preparation methods, morphology, and active components on the NO_x conversion rate, N₂ selectivity, operating temperature window, and poisoning resistance of catalysts were discussed. The advantages and disadvantages of various types of LDO catalysts were also compared. The preparation of an LDH precursor with subsequent calcination is the simplest and most effective way to obtain LDO. During the thermal decomposition process of LDH, calcination temperature is a key factor in the formation of new phases. LDO catalysts have the advantages of large specific surface area, highly uniform dispersion of metal elements at the atomic scale, good thermal stability, and excellent SCR performance and N₂ selectivity. The preparation method, morphology, structure, and active components of LDO catalysts have a significant impact on their catalytic performance. In addition, due to their rich and adjustable chemical composition and morphological structure, composite catalysts composed of doping (or adding) one or more other elements and/or materials often exhibit better denitrification performance and resistance to intermediate toxicity. Although significant progress has been made in LDO denitrification, there are still many issues that need to be addressed.

The wide-temperature (150–350 °C) activity of LDO catalysts still needs to be improved to satisfy the complex flue gases, especially in non-electrical industry applications. The relationships between the compositions between layers, the different inserted anions, and the reaction efficiency should be investigated clearly to clarify their synergistic effects. Moreover, different types of poisoning (SO₂, H₂O, alkali metals, HCl, etc.) on catalysts should be studied in depth. Currently, the research on the synergistic poisoning mechanisms of SO₂ and H₂O on catalysts is not yet complete, and the positive impact mechanism of H₂O on inhibiting the sulfation of active components and the positive effects of H₂O are not clear and still require further research. How to use the positive effects of H₂O to neutralize some of the negative effects of SO₂ becomes an important point. Finally, the reaction process for NH₃-SCR on LDO catalysts also needs to be studied. By utilizing the distinct interlaced structure of anion and cation layers in LDO, we can gain a deeper understanding of the electron transfer pathway, redox reaction cycle, and surface acidity changes. This, in turn, can lead to a significant enhancement in the NH₃-SCR catalytic activity of LDO materials.

Overall, layered double oxides have shown great potential to achieve an incredible performance for NH₃-SCR. However, in the future, reducing the production costs of LDO catalysts while maintaining effective NO_x conversion and facilitating its successful commercial application represents a significant research direction.