A Comparative Mini-Review on Transition Metal Oxides Applied for the Selective Catalytic Ammonia Oxidation (NH₃-SCO)

Abstract

The selective catalytic oxidation of NH₃ (NH₃-SCO) into N₂ and H₂O is an efficient technology for NH₃ abatement in diesel vehicles. However, the catalysts dedicated to NH₃-SCO are still under development. One of the groups of such catalysts constituted transition metal-based catalysts, including hydrotalcite-derived mixed metal oxides. This class of materials is characterized by tailored composition, homogenously dispersed mixed metal oxides, exhibiting high specific surface area and thermal stability. Thus, firstly, we give a short introduction to the structure and composition of hydrotalcite-like materials and their applications in NH₃-SCO. Secondly, an overview of other transition metal-based catalysts reported in the literature is given, following a comparison of both groups. The challenges in NH₃-SCO applications are provided, while the reaction mechanisms are discussed for particular systems.

1. Introduction

Ammonia (NH₃) is one of the most important chemicals in the world, e.g., used to produce fertilizers, synthetic fibers, dyes and synthetic foam, as well as to reduce NO_x emissions, etc. (Figure 1). However, since 2001, the EU has listed NH₃ as one of the four main types of atmospheric pollutants (among NO_x, SO₂, and non-methane volatile organic compounds (NMVOC)), and has released the EU National Pollutant Discharge Inventory (NECD) every year. Twelve Member States (including, e.g., Germany, France, Austria, etc.), and the United Kingdom need to reduce NH₃ emissions by up to 10% against 2018 levels to attain their 2020 and 2030 emission reduction commitments. Denmark and Lithuania need to reduce emissions by more than 10% [1]. Ammonia emitted from livestock, industrial processes or NH₃ emitted to the atmosphere through either the large-scale usage of fertilizers or gas slippage from the NH₃-SCR-DeNO_x applications can cause serious damage to human health (i.e., to the eyes, throat, nose, etc., if its concentration exceeds 50–100 ppm) [2], and environment (e.g., acidification, formation of haze, Figure 1).

To control the ammonia slip, several different techniques used for the elimination of NH₃ (e.g., adsorption, absorption, catalytic decomposition, etc.) have been applied [3]. However, the selective catalytic oxidation of ammonia into nitrogen and water vapor is an ideal technology for removing NH₃ from O₂-containing waste gases, after the selective catalytic reduction of NO_x by NH₃ (SCR-DeNO_x, by the use of stoichiometric or even excess amount of NH₃) from stationary and mobile sources. Thus, ideally, the residual NH₃ (slip) could be selectively oxidized to N₂ and H₂O (i.e., inert, and non-toxic products, Equation (1). However, the N₂ selectivity is affected by the undesired oxidation of ammonia to N₂O, NO, and NO₂ (Equations (2)–(4)):

4NH₃ + 3O₂ → 2N₂ + 6H₂O

(1)

2NH₃ + 2O₂ → N₂O + 3H₂O

(2)

4NH₃ + 5O₂ → 4NO + 6H₂O

(3)

4NH₃ + 7O₂ → 4NO₂ + 6H₂O

(4)

Thus, NH₃-SCO catalysts with enhanced activity, N₂ selectivity, stability (also in the presence of H₂O, SO_x, and CO_x, up to 600–700 °C in the cycle of diesel particulate filter regeneration) and low cost, at the same time are of both scientific and industrial importance. However, catalysts of sufficient activity, selectivity and stability under application-relevant reaction conditions are not yet available. To date, many catalysts have been proposed for NH₃-SCO and they can be classified into several groups, including noble metal-based catalysts, transition metal-based catalysts and noble/transition metal (bimetallic)-containing catalysts, etc. Particularly, the catalysts containing copper species are recognized as the most active and N₂ selective among other transition metal-containing materials. Copper oxide species and their redox properties were found to determine their catalytic properties [4]. Recently, we published on noble metal-based catalysts (including Pt-, Pd-, Ag-, and Au-, Ru-based catalysts) [5] and Cu-containing zeolite-based catalysts (e.g., Cu-SSZ-13 commercialized in NH₃-SCR-DeNO_x) [6]. Thus, in the current mini-review, we focused on the application of transition metal oxides, excluding catalysts modified with noble metals or zeolite-based catalysts. However, the publications and results on the subject of transition metal-based catalysts increased recently, therefore, we found the mini-review timely. Particularly, we aim to compare hydrotalcite-based mixed metal oxides with other transition metal catalysts presented in the literature, and based on that propose an active, N₂ selective, and stable catalytic system (either single one or as a component for hybrid catalyst) for NH₃-SCO. The hybrid catalyst consists of an SCR catalyst for NO_x control and catalyst with oxidation functionality for ammonia conversion. The different arrangements of the hybrid catalysts, i.e., dual-layer, inverse dual-layer, hybrid dual layer, dual mixed layers, etc., were so far tested in the literature (e.g., [7]). The combination of the two active components for NH₃-SCR-DeNO_x and NH₃ oxidation to yield NO_x arise from the internal selective catalytic reduction mechanism (i-SCR) [3,4]. However, despite research in this direction, a debate remains on the elementary reaction steps and the active sites in NH₃-SCO. Moreover, for the i-SCR mechanism, the imide mechanism (with the formation of imide (-NH) and nitrosyl (-HNO) as intermediates), and the hydrazine mechanism (involving hydrazine (N₂H₄) as an intermediate) are reported as the main mechanisms for NH₃-SCO. Recently, the N₂⁻ mechanism (with adsorbed N₂ anion regarded as the intermediate), was also proposed to explain the high reactivity of nano-size Al₂O₃-supported Ag species [8]. Contrary to the micro-size Al₂O₃ supported Ag species, the i-SCR mechanism was proposed, evidencing that the reaction mechanism depends on the applied catalytic systems. Another example can constitute Cu species deposited on Al₂O₃. Depending on the applied treatment (calcination versus dynamic construction), NH₃-SCO over the catalysts can follow different routes. For example, the fast i-SCR mechanism characterized by the presence of consumable NO₂ adsorbed species was proposed on CuO_x-OH interfacial sites [9]. Still, compared to NH₃-SCR-DeNO_x, the reaction mechanisms of NH₃-SCO are not frequently discussed in the literature. Hence, it is vital to study the reaction mechanisms over more materials thoroughly and then rationally design high-performing NH₃-SCO catalyst with appropriate promotion strategies.

Although some review articles [3,4,10] have outlined the advances of transition-metal-based catalysts in NH₃-SCO, only examples of such materials have been given. Thus, in the current mini-review, we thoroughly discuss the hydrotalcite-derived mixed metal oxides and other transition metal oxides applied in NH₃-SCO, evidencing that for these materials their application as catalysts is quite relevant. Our objective was not to make a systematic review of the hydrotalcite-like compounds because several reviews have already been published, including preparation and physico-chemical characterization (e.g., [11,12,13]), catalytic applications (e.g., [14,15])—in particular nitrogen oxides removal [16,17]. Despite not being mentioned in the text, selective ammonia oxidation over (mixed) metal oxides into NO [18,19,20,21,22] or N₂O [23,24] was reported, in the present work we concentrate only on the ammonia oxidation into N₂ and H₂O. Throughout the mini-review, we highlight the structure-activity/selectivity correlations and try to narrow the gap between research and industrial applications. We hope that based on these correlations, a knowledge-driven industrial catalyst design and its optimization becomes possible, which will allow keeping the NH₃ emissions of diesel-powered vehicles at a very low level under various boundary conditions.

2. Hydrotalcite-Derived Mixed Metal Oxides

Hydrotalcite-like compounds (HT), otherwise referred to as anionic clays or layered double hydroxides (LDHs) are described by the general formula [M(II)_1−xM(III)_x(OH)₂]^q+(Aⁿ⁻)_q/n) mH₂O, where M(II) and M(III) represent divalent and trivalent metal cations, respectively, Aⁿ⁻ represents interlayer anions of charge n⁻. Usually, the structure of the HT-like compounds is better visualized by analyzing the structure of brucite. Mg(OH)₂ octahedra of Mg²⁺ coordinated with six OH⁻ share edges to form successive sheets, with the hydroxide ions located perpendicularly to the plane of the layers. The resulting sheets are stacked on top of each other and held together by hydrogen bonds. When Mg²⁺ ions are substituted by Al³⁺, a positive charge is created in the hydroxyl part of the layer. The positive charge is neutralized by the negative CO₃²⁻ anions, which are located between the layers of brucite, along with H₂O that is also present in the interlayer space (Figure 2) [25,26]. To obtain a pure hydrotalcite-like phase, the x in the general formula of the material should be in the range of 0.15 and 0.34 [25]. Coprecipitation is the most common method for the synthesis of hydrotalcite-like compounds.

The structure and physico-chemical properties of the hydrotalcite-like compounds are dependent on the kind and amount of metal ions present in the brucite-like layers, the type and position of anions and water in the interlayer region, and the type of stacking between the layers (i.e., rhombohedral (3R) versus hexagonal (2H)) [27]. It is possible to synthesize hydrotalcite-like compounds with more than two different metals (regarding different oxidation states, e.g., Li, Mg, Mn, Fe, Co, Al, Mn, Fe, Co, Ni, Cr, Ga) or anions (halides: Cl⁻, F⁻, I⁻, oxo-anions: CO₃²⁻_, NO₃⁻, SO₄²⁻, BrO₃⁻, organic acids: adipic, oxalic, sebacic or malonic acid, oxo and polyoxo-metallates: (PW₁₂O₄₀)³⁻, (PMo₁₂O₄₀)³⁻, chromate, dichromate, anionic complexes: ferro and ferricyanide, PdCl₄²⁻, etc.). Consequently, the size of the interlayer region varies depending on the introduced anions. e.g.,

Table 1. Products obtained from the preparation of the CuM(II)M(III)CO₃ hydrotalcite-like compounds. Reprinted from [25] with permission from Elsevier.

Cations	Cations’ Ratio	Compounds Identified
CuAl	1.0/1.0	Amorphous species
CuZnAl	2.0/1.0/1.0	HT + R
CuZnAl	3.3/1.6/1.0	HT + R
CuZnAl	1.6/0.8/1.0	HT + R
CuZnAl	1.5/1.5/1.0	HT (HT + R)
CuZnAl	1.2/1.2/1.0	HT
CuZnAl	0.8/0.8/1.0	HT
CuCr	1.0/1.0	Amorphous species
CuZnCr	1.5/1.5/1.0	HT
CuCoCr	2.0/2.0/1.0	HT + M
CuCoCr	1.5/1.5/1.0	HT
CuZnCr	1.5/1.5/1.0	HT
CuMgCr	1.5/1.5/1.0	HT
CuMnCr	1.5/1.5/1.0	MnCO3 + HT
CuCoZnCr	1.4/0.1/1.5/1.0	HT
CuZnAlCr	3.0/3.0/1.0/1.0	HT
CuZnFe	1.5/1.5/1.0	Au

HT—hydrotalcite-like compounds; M—Cu₂CO₃(OH)₂ (malachite); R—(Cu,Zn)₂CO₃(OH)₂ (Rosasite); Au—aurichalcite.

lists the products obtained from the preparation of CuM(II)M(III)CO₃ hydrotalcite-like compounds.

Figure 2. In situ XRD diffraction patterns of the Cu-Mg-Al hydrotalcite-like material recorded in oxidizing conditions. HT—hydrotalcite-like compounds, P—MgO (periclase), C—Cu₂O (cuprite), S—MgAl₂O₄ (magnesium aluminate) and/or CuAl₂O₄ (copper aluminate), B—CuAlO₂: Reprinted from [28] with permission from Springer.

Figure 2. In situ XRD diffraction patterns of the Cu-Mg-Al hydrotalcite-like material recorded in oxidizing conditions. HT—hydrotalcite-like compounds, P—MgO (periclase), C—Cu₂O (cuprite), S—MgAl₂O₄ (magnesium aluminate) and/or CuAl₂O₄ (copper aluminate), B—CuAlO₂: Reprinted from [28] with permission from Springer.

The hydrotalcite-like compounds are used as the precursors for the catalysts more often than as layered materials themselves. During the thermal treatment, the HT-like compounds transform first to an amorphous oxide and then, at higher temperatures, to crystalline mixed metal oxides (Figure 2). The hydrotalcite-derived mixed metal oxides are characterized by key features such as relatively high specific surface area, homogenous dispersion of active metal ions, non-stoichiometry, and high thermal stability, etc. [15].

To the best of our knowledge, Trombetta et al. [29] reported for the first time the catalytic activity and N₂ selectivity over the CuMgAl hydrotalcite-derived mixed metal oxides in NH₃-SCO in 1997. They tested CuMgAl with n(Cu)/n(Mg)/n(Al) of 4.6–7.2/63.8–66.4/29 and found nearly full NH₃ conversion at ca. 400–500 °C and N₂ selectivity below 80%. Following such studies, Chmielarz et al. [30] studied the activity of hydrotalcite-derived mixed metal oxides (M(II, III)Mg(II)Al(III)) containing Ni, Fe, Cu or Co, and pointed out that both the kind and the number of metal ions introduced into hydrotalcite-like structure influenced the activity and selectivity in NH₃-SCO. Among the investigated compositions, Cu-containing catalysts (CuMgAl; n(Cu)/n(Mg)/n(Al) = 5/66/29, 10/61/29, 20/51/29) were the most active in NH₃-SCO, while the Fe-containing one (FeMgAl; n(Fe)/n(Mg)/n(Al) = 10/61/29) revealed enhanced N₂ selectivity. Based on these results, the catalytic properties were further optimized by the combination of both metals, i.e., the introduction of copper and iron ions into the brucite-like structure. The CuMgFe mixed metal oxides with different compositions, such as n(Cu)/n(Mg)/n(Fe) = 0–1/2/1, mol.% (with the optimum composition guaranteeing enhanced NH₃ conversion and N₂ selectivity being n(Cu)/n(Mg)/n(Fe) = 0.5/2/1 [31]) or n(Cu)/n(Mg)/n(Fe) = 5–15/52–62/33 (with the optimum composition of n(Cu)/n(Mg)/n(Fe) = 12/55/33 [32]) were reported. The temperature-programmed studies, i.e., sorption of NH₃ and desorption in He or O₂/He, as well as NH₃-SCR-DeNO_x and NH₃-SCO with different spaces velocities, revealed that the reaction over the CuMgFe hydrotalcite-derived mixed metal oxides proceeds according to the i-SCR mechanism and NH₃ oxidation to NO is a rate-determining step (Figure 3) [31]. Thus, the modification of hydrotalcite-derived materials with noble metals (Pt, Pd, Rh) arose based on those studies [33].

Copper loading at about 5–8 mol.% in the CuMgAl mixed metal oxides allowed reaching full NH₃ conversion at 375–600 °C with N₂ selectivity above 60% [34]. The increase in copper loading led to the formation of bulk-like copper oxide species. The N₂ selectivity varied depending on the catalysts’ composition and the method used for the preparation of the hydrotalcite-like precursor. CuMgAl mixed metal oxides prepared via coprecipitation (cop.), and subsequent calcination of the hydrotalcite-like precursor revealed a significantly higher activity and N₂ selectivity compared to the material with similar composition obtained via rehydration (reh.) of calcined Mg-Al hydrotalcite-like compounds or thermal decomposition (decom.) of nitrate precursors (Figure 4a). This effect was ascribed to the presence of the highly dispersed copper oxide species in hydrotalcite-derived mixed metal oxides. Additionally, the catalysts containing the same copper content, but with variations in the n(Mg)/n(Al) ratio presented similar catalytic activity [34,35]. Beyond the optimization of the kind and loading of metal species, the optimization of the calcination temperature is also relevant. The thermal treatment of hydrotalcite-like materials influences the composition of mixed metal oxides and their physico-chemical properties and thus, the activity and N₂ selectivity in NH₃-SCO [28,35]. Hydrotalcite-derived CuMgAl, CuZnAl, CuMgFe mixed metal oxides calcined at 900 °C revealed significantly lower activity compared to the materials calcined at 600 °C (Table 2, pos. 5, Figure 4b), which was ascribed to the different copper oxide phases and their redox properties. Enhanced activity at low temperatures together with a drop of N₂ selectivity at higher temperatures were driven by the easily reduced copper oxides species. Otherwise, calcination temperature at ca. 800 °C led to the formation of the spinel phases, e.g., Cu_1−xMg_xAl₂O₄ of lower reducibility, which caused higher N₂ selectivity [35].

Regarding the use of other metals as dopants in Cu-containing mixed metal oxides, Jabłońska et al. [36] introduced Ag, Ce and Ga (y = 0–1 mol.%) to the CuMgAl mixed metal oxides (n(M)/n(Cu)/n(Mg)/n(Al) = y/5/66-y/29). The redox properties determined the catalytic properties for materials with the loading of y ≤ 0.25, while for the higher metal loading (y ≥ 0.25) the catalytic properties were driven mainly by the metal oxide phases. Górecka et al. [37] investigated hydrotalcite-derived (5 mol.%) CuMgAl mixed metal oxides, also impregnated with cerium (4 wt.%) over different feed compositions, i.e., NH₃ and O₂ (2 vol.% versus 20 vol.%). Higher O₂ concentration enhanced NH₃ conversion, while N₂ selectivity dropped (the opposite effect was found for higher NH₃ concentration in the feed). The effect of the enhanced NH₃ activity over Cu-doped samples was ascribed to the synergetic effect of Ce-Cu redox pairs, which activated the lattice oxygen to react with NH_x species towards the formation of N₂ (Equation (5)). The oxygen vacancies were filled again with the surface oxygen as the cerium and copper species were oxidized (Equation (6), where ⎕ represents oxygen vacancies):

Ce⁴⁺ − O²⁻ − Cu²⁺ + NH_x → Ce³⁺ − ⎕ − Cu⁺ + N₂ + H₂O

(5)

Ce³⁺ − ⎕ − Cu⁺ + O₂ → Ce⁴⁺ − O₂ – Cu²⁺

(6)

However, from such studies, it was not clear why a rather high Ce loading was applied, since previous research showed that lower cerium loading (0.5 wt.% versus 3 wt.%) led to improved catalyst activity in NH₃-SCO [38]. Nevertheless, even higher Ce loading (8.14 wt.%) was applied in further studies over the hydrotalcite-derived CuZnAl mixed metal oxides [39]. Overall, such systems revealed significantly lower N₂ selectivity compared to the Ce/CuMgAl mixed metal oxides (Table 2, pos. 10). N₂O was a minor by-product. Nevertheless, it is also worth mentioning that the Co-Mn-containing materials were reported to selectively oxidize NH₃ to N₂O (ca. 100% below 250 °C) [40].

Concluding, the above-mentioned examples show that the hydrotalcite-derived mixed metal oxides offer a large variety of possible modifications and tuning of their properties, which makes them suitable for NH₃-SCO applications. Mainly Cu-containing hydrotalcite-derived mixed metal oxides were applied for NH₃ oxidation to N₂. Overall, the full NH₃ conversion between 375–650 °C and N₂ selectivity above 70% (based on the data gathered in Table 2, depending on the catalyst composition and preparation, reaction conditions, etc.), were achieved over the Cu-containing hydrotalcite-derived mixed metal oxides. Thus, further optimization of both chemical and phase compositions could lead to enhanced NH₃ conversion and N₂ selectivity below 350 °C. Still, intensive studies focused on the development of such catalytic systems in NH₃-SCO are required under application-relevant reaction conditions (i.e., minor NH₃ slip (O₂ excess), up to 600–700 °C (in the cycle of diesel particulate filter regeneration) in the presence of H₂O, CO_x and/or SO_x), including an investigation of the reaction mechanisms.

3. Other Metal Oxides

In the literature, various types of other metal oxides (still excluding noble metal-doped catalysts and zeolite-based materials) have been reported for NH₃-SCO. The investigation of Co₃O₄, MnO₂, CuO, Fe₂O₃ and V₂O₅ in NH₃-SCO was reported in the early studies of Il’chenko and Golodets in 1975 [41,42]. The specific catalytic activities at 230 °C (p(NH₃) = 0.1 atm, p(O₂) = 0.9 atm) of the selected metal oxides decreased in the following sequence: Co₃O₄, MnO₂ > CuO > NiO > Bi₂O₃ > Fe₂O₃ > V₂O₅ > TiO₂ > ZnO > WO₃. Among the transition metals, V₂O₅, MoO₃ and WO₃ exhibited nearly 100% N₂ selectivity at 230 °C [43]. A similar activity order (MnO₂ > Co₃O₄ > CuO > Fe₂O₃ ≈ V₂O₅ > NiO) was found by Hinokuma et al. [44] under 1 vol.% NH₃, 0.75 vol.% O₂, He balance. CuO reached higher N₂ selectivity than other oxides [43,45].

NH₃-SCO (as a side process) has been frequently studied with NH₃-SCR-DeNO_x, thus, the following studies focused on the supported V-containing catalysts, e.g., V₂O₅/TiO₂ [46,47] V₂O₅/TiO₂-SiO₂ [48], V₂O₅-WO₃/TiO₂ [46,49], V₂O₅-WO₃/ZrO₂ [50], etc. E.g., Ueshima et al. [51] examined different supports for V₂O₅-WO₃ and found the decreasing order of catalytic activity in terms of support as follows: TiO₂-SiO₂ (binary oxide) > TiO₂ (anatase) > TiO₂ (rutile) > SiO₂. Contrary to such studies, V₂O₅ supported on the rutile form of TiO₂ led to a more active and N₂ selective (below 400 °C) catalyst [47]. In another study, V₂O₅/MgO was the least active among vanadium oxide supported on TiO₂, SiO₂ or MgO [52]. Furthermore, the commercial V₂O₅-WO₃-TiO₂ catalyst was modified with Cu (1 wt.%) and Ce (1–10 wt.%) species. Overall, the catalyst with the composition of (1.02 wt.%)Cu-(4.79 wt.%)Ce/V₂O₅-WO₃-TiO₂ showed enhanced activity at 300 °C, while its modification with Ce enhanced H₂O and sulfur resistance [53]. The V₂O₅-WO₃-TiO₂ catalyst modified with Cu or Fe (1.0 wt.%) ions revealed the highest activity and N₂ selectivity above 350 °C (among other materials modified with Mn or Co species) [54]. Furthermore, K₂O was also suggested (but not experimentally tested) as an effective promoter for the V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ catalysts for NH₃-SCO at 500 °C. Regarding, the reaction mechanisms in NH₃-SCO, Yuan et al. [55] performed density functional theory (DFT) calculations in conjunction with cluster models on the V₂O₅ surfaces. According to such mechanisms (Figure 5a), NH₃⁺ appears as the initial intermediate from the activated NH₃ which transfers an electron to the metal oxide surfaces. Further routes, depending on the availability of the O₂ species, can arise, i.e., the direct route appears in the case of limited O₂ or its absence. Consequently, formed N₂H₄ was oxidized to N₂ on V=O sites. In the presence of O₂, NH₃O₂⁺ complex was formed, which further decomposes to NO (followed by NH₃-SCR-DeNO_x) with the N₂ formation. Such an indirect route (i.e., i-SCR mechanism) was reported also over the V-based catalysts [53,54] (e.g., Figure 5b). Recently, Liu et al. [56] identified based on the first-principle calculation method of DFT the adsorption sites of NH₃ on V₂O₅(001).

Carley et al. [57] revealed the structural characteristics of imide strings formed when a Cu(110) surface was exposed to the NH₃-O₂ mixture (30 vol.%–1 vol.%, 52 °C, 10⁻⁸ mbar —UHV investigations). Contrary to that, no imide species were found on the surface of polycrystalline copper in NH₃-SCO at 1.2 mbar [58]. CuO was found to selectively oxidize NH₃ to N₂O, while Cu₂O to N₂, respectively [58,59]. Hirabayashi and Ichihashi [60] investigated reactions of copper oxide cluster cations, Cu_nO_m⁺ (n = 3–7, m ≤ 5) with NH₃ at near thermal energies using a guided ion beam tandem mass spectrometer. Depending on the applied clusters, H₂O, O₂ or -HNO were released, while the release of N₂ was observed in the multiple-collision reactions of Cu₅O₃⁺ and Cu₇O₄⁺ clusters. Nevertheless, it is recognized that supported copper species (e.g., Cu/Al₂O₃ [61,62], CuO/carbon nanotubes [63]) or the combination of CuO and other transition or rare earth metal oxides, e.g., Fe₂O₃, CeO₂, La₂O₃, CuCr₂O₄ and CuCrO₂ or La₂Ce₂O₇ (nonporous pyrochlore structure—A₂B₂O₇) (e.g., [61,62,64,65,66,67]) result in catalysts with enhanced activity and N₂ selectivity in NH₃-SCO. E.g., Gang et al. [61,62] and afterward other authors [68,69,70,71] have proved the high catalytic activity of copper species deposited on γ-Al₂O₃. They claimed that copper species dispersion becomes poorer on Al₂O₃ at metal loading higher than ca. 10 wt.% (among 5–15 wt.%) [62]. Furthermore, many studies about similar catalytic systems concur on ca. 10 wt.% as an optimum loading of copper species [4]. The full NH₃ conversion of this group appears at 350–500 °C with N₂ selectivity above 75% (Table 2) depending on the preparation methods (e.g., copper precursors [70,72], treatment strategies [9]), and reaction conditions (e.g., fuel-lean/rich conditions [71,73,74]). Based on the studies of Lenihan and Curtin [69], the stability of Cu/Al₂O₃ through the dry, wet and subsequently dry conditions was proved. Moreover, the activity of CuO/Al₂O₃ catalyst was further enhanced by its dopping with PbO, NiO, CoO or SnO (with ≤ 1 wt.% loading of dopant) [68]. Not only has γ-Al₂O₃ been applied as the catalyst support, η-Al₂O₃ has also been employed [75]. Still, the full NH₃ conversion appeared around 550 °C over (1–2 wt.%)Cu/η-Al₂O₃. The catalytic properties of Cu/Al₂O₃ were further increased via its modification with Li₂O and CeO_x. Above 250 °C the materials were only N₂ selective [76]. In addition, increasing the c(O₂)/c(NH₃) ratio enhanced the conversion. Recently, Machida et al. [77] investigated several nanometer-thick transition metal (mainly noble metals but also Cu or Co) overlayers formed on a Fe-Cr-Al metal (SUS) foil by pulsed cathodic arc-plasma deposition. The activity of the Co- and Cu-based materials decreased significantly in the presence of 10 vol.% H₂O (while those of the catalysts containing Pt and Ir remained nearly unchanged, i.e., preserved the active metallic surface during NH₃-SCO in the presence of O₂/H₂O).

Other than Cu, other metal species such as Ni, Mn, Fe, Co, Mg and Zn were supported on Al₂O₃ [78]. For the (10 wt.%)Ni-containing catalysts, the activity was found to decrease in the order of γ-Al₂O₃, ZrO₂, MgO > SiO₂ > TiO₂ >ZSM-5, with full NH₃ conversion being reached at 550 °C for Ni/Al₂O₃ (possibly due to presence of NiAl₂O₄). The catalytic properties of the samples loaded above 10 wt.% tend to become similar to that expected for pure NiO. Furthermore, Mn/Al₂O₃ and Fe/Al₂O₃ were proved to be more active (full NH₃ conversion at 300–500 °C with N₂ selectivity > 70%) than Ni/Al₂O₃ (550 °C) for NH₃ oxidation, possibly because of their enhanced redox properties. In addition, several groups have investigated Ni-based catalysts above 500 °C (e.g., prepared via microemulsion) [79,80].

He et al. [65] demonstrated that TiO₂ is a more suitable support (due to the higher oxygen mobility and lower oxygen bonding strength) than Al₂O₃ for copper-based catalysts, which was represented by the enhanced catalytic properties of Cu/TiO₂ compared to Cu/Al₂O₃. Contrary to that, for the Cu-Mn species, deposition on Al₂O₃ (compared to TiO₂) guaranteed enhanced activity [81]. In the case of Cu/TiO₂, NH₃ conversion was reported to depend on the Cu species loading, e.g., for (10 wt.%)Cu/TiO₂ full conversion occurs at about 250 °C with 95% N₂ selectivity [65], while for (1 wt.%)Cu/TiO₂—425–500 °C with < 60% N₂ selectivity was reported [82]. Duan et al. [83] investigated (10 wt.%) V, Cr, Zn and Mo supported on TiO₂. Comparatively tested Cu/TiO₂ and Cr/TiO₂ revealed lower NO and NO₂ selectivity over chromium-containing catalysts. The applied O₂ content in the feed gas ranging from 0.5 vol.% to 5 vol.% revealed similar NH₃ conversion (with an exception below 150 °C where NH₃ conversion was higher in the presence of 0.5 vol.% O₂). Moreover, NO selectivity was not affected by the different O₂ content during NH₃-SCO. TiO₂ anatase is the most common catalyst support, while the catalytic properties are affected by the properties of the support (i.e., anatase versus rutile) [84]. NH₃-SCO over Cu/TiSnO₂ and Cu/TiO₂ (10.5–11.8 wt.% of Cu) was reported to follow the i-SCR and imide (-NH) mechanisms, respectively [85]. For the i-SCR mechanism (Figure 6), NH₃ was first adsorbed on both Lewis and Brønsted acid sites. After that, it reacted with surface-active oxygen species to form nitrate species (intermediates in NH₃-SCO), which finally reacted with the remaining NH₃ with the formation of N₂ and H₂O. Gaseous NH₃ recombined with the released acid sites to participate in the next cycles.

The bands assigned to nitrate species were also found using in situ DRIFTS in the spectra of a series of CuO-Fe₂O₃ catalysts (with an optimum at n(Cu):n(Fe) molar ratio of 5:5 with full NH₃ conversion > 250 °C) recorded at 250 and 350 °C [86]. The authors proposed the molecular steps of the i-SCR mechanism, in which the nitrosyl (-HNO) species were formed via a reaction between adsorbed NH_3−x species with atomic oxygen (Equations (7)–(10)). Then, the -HNO was oxidized by oxygen atoms from O₂ to form NO species (Equations (11) and (12)). Additionally, the -NH species interacted with O₂ to form NO. Meanwhile, the in situ-formed NO could react with -NH_x to form N₂ or N₂O (Equations (13) and (14)).

NH₃ → NH₂ + H

(7)

NH₂ → NH + H

(8)

NH + O → HNO

(9)

O₂ → 2O

(10)

HNO + O → NO + OH

(11)

NH + O₂ → NO + OH

(12)

NH₂ + NO → N₂ + H₂O

(13)

NH + NO → N₂O + H

(14)

H + OH → H₂O

(15)

To reveal the impact of calcination temperature on the catalytic properties of the CuO-Fe₂O₃ catalysts (at n(Cu)/n(Fe) molar ratio of 1/1), the materials were calcined between 400 and 700 °C [87]. Among them, CuO-Fe₂O₃ calcined at 500 °C revealed full NH₃ conversion at ca. 225 °C, i.e., at about 25 °C lower than for the material calcined at 400 °C. The increase in the calcination temperature (up to 600–700 °C) resulted in a decrease in the activity in NH₃-SCO, while selectivity was not affected. The simultaneous addition of H₂O and SO₂ to the feed gas led to a drop in activity and N₂ selectivity. Regarding the application of the Cu-Fe-containing spinel, Yue et al. [88] found that for the mesoporous CuFe₂O₄—prepared with KIT-6 as the hard template, NH₃ was nearly completely consumed at 300 °C while the N₂ selectivity dropped below 90% up to 600 °C. CuMoO₄, CoMoO₄ or FeMoO₄ were significantly less active in NH₃-SCO [45]. For CuMoO₄, activity and N₂ selectivity were completely inhibited by water vapor (10 vol.%). Beyond fully synthesized materials, natural vermiculite and phlogopite [89,90,91] or attapulgite [92] modified with Cu or Fe species are also active and N₂ selective catalysts for NH₃-SCO (Table 2, pos. 42–46).

Similar to pure CuO and NiO, for CeO₂ the catalytic activity was also poor [93,94]. Despite this, the (10 wt.%)Ce/TiO₂ catalyst (calcined at 400–500 °C) revealed enhanced activity in NH₃-SCO between 300–350 °C but did not reach full NH₃ conversion [94]. Furthermore, the catalytic activity increased from 50 to 90% at 300 °C for (10 wt.%)Ce/TiO₂ after its modification with vanadium (2 wt.%) [95]. This effect was assigned to the dispersion of Ce⁴⁺ species on TiO₂. The V/Ce/V/TiO₂ catalyst showed resistance to SO₂ poisoning due to the reduced formation of the NH₄HSO₄ species. The Ce-containing mixed metal oxides constitute a representative group of catalysts for NH₃-SCO. E.g., Wang et al. [93] investigated a series of Ce_1−xZr_xO₂ (0.2 ≤ x ≤ 0.8) mixed oxide catalysts, among which particularly Ce_0.4Zr_0.6O₂ reached the total NH₃ oxidation of about 360 °C (N₂ selectivity > 90%). Ce_0.4Zr_0.6O₂ was also subjected to further modifications with Ru species [96]. Cu-Ce-Zr catalyst prepared by a citric acid sol-gel method exhibited the highest activity among other materials (prepared via the homogenous precipitation and incipient wetness impregnation methods) achieving full NH₃ conversion at 230 °C with > 90% N₂ selectivity [97]. These results were attributed to the finely dispersed CuO, the Cu-Ce-Zr solid solution and the monomeric Cu²⁺ ions in octahedral sites (in contrast to monomeric Cu²⁺ in the square-planar pyramidal sites). Moreover, the adsorbed oxygen species were more active than the bulk lattice oxygen species in NH₃-SCO. The co-presence of SO₂ and H₂O or CO₂ in the feed resulted in the NH₃ conversion decreasing to 92 and 81%, respectively. NH₃-SCO over the catalysts prepared via different techniques followed the i-SCR mechanism (with the -NH_x and -HNO intermediates, Figure 7a) [98].

Lou et al. [99] have reported nearly complete NH₃ conversion at temperatures as high as 400 °C with an overall N₂ selectivity varying from 19 to 82% over Cu-Ce mixed oxides prepared by coprecipitation with an optimum at n(Cu)/n(Ce) = 6/4 (among 6–9/1–4). Afterward, the CuO-CeO₂ catalysts prepared by a surfactant-templated method exhibited full NH₃ conversion below 300 °C with more than 90% N₂ selectivity [100]. However, the thermal resistance of CuO-CeO₂ mixed oxides needs to be further enhanced. The finely dispersed CuO species as well as a strong synergetic interaction between the copper oxide species and cerium oxides significantly decreased the operation temperature. Thus, activated ammonia reacted with lattice oxygen in the Cu-O-Ce solid solution generating N₂ and H₂O, while gaseous O₂ regenerated the oxygen vacancies in the Cu-O-Ce solid solution to maintain Ce⁴⁺/Ce³⁺ redox couple (Figure 7b). NH₃ was oxidized over CeO₂ to NO, which in the next step reacted with NH_x forming N₂ over CuO (according to the i-SCR mechanism). CuO/La₂O₃ (n(Cu)/n(La) = 6–9/1–4 with an optimum at 8/2) showed a significantly lower activity and N₂ selectivity of 93 and 53% at 400 °C, respectively [64], compared to CuO-CeO₂ (e.g., 98–99% NH₃ conversion with 85–86% N₂ selectivity at 400 °C for n(Cu)/n(Ce) = 6/4) [101,102].

Figure 7. (a) Relationship of activity-adsorption-structure and reaction pathway over Cu-Ce-Zr prepared via different synthesis routes; SOL—citric acid sol-gel method; HP—homogeneous precipitation, IW— incipient wetness impregnation. Reprinted from [98] with permission from Elsevier; (b) NH₃-SCO oxidation over CuO-CeO₂. Reprinted from [100] with permission from Elsevier.

Figure 7. (a) Relationship of activity-adsorption-structure and reaction pathway over Cu-Ce-Zr prepared via different synthesis routes; SOL—citric acid sol-gel method; HP—homogeneous precipitation, IW— incipient wetness impregnation. Reprinted from [98] with permission from Elsevier; (b) NH₃-SCO oxidation over CuO-CeO₂. Reprinted from [100] with permission from Elsevier.

Mn-based catalysts have been demonstrated to be active in NH₃-SCO. E.g., natural manganese ore (NMO, consisting of manganese oxides and small amounts of Fe₂O₃, CaO, MgO, SiO₂, Al₂O₃) was recognized as a low-cost catalyst possessing similar activity (ca. 50 % NH₃ conversion) to that of MnO₂ below 150 °C. Above 150 °C, Mn₂O₃ was the most active one. Across the studied temperatures between 50–250 °C, N₂ selectivity decreased as follows: NMO > MnO₂ > Mn₂O₃ [103]. Mn₂O₃ supported on TiO₂ (anatase containing 1.15 wt.% sulfate) revealed significantly lower activity at 277–307 °C than the supported Cu-Mn mixed oxides (n(Cu)/n(Mn) = 20–80/20–80) [104]. It was found that the most active catalyst (with n(Cu)/n(Mn) = 20/80) contained a Cu_1+xMn_2−xO₄ crystalline spinel phase and X-ray amorphous Mn²⁺-containing species. Unfortunately, the selectivity was not reported. The activity of Cu-Mn/TiO₂ was further enhanced via its modification with Ce or La species [105]. Ce-Cu-Mn/TiO₂ prepared through the sol-gel method was the most active among the catalysts prepared via impregnation or coprecipitation and reached complete NH₃ conversion at ca. 200 °C, however with 96% NO selectivity. Song et al. [106] investigated a series of MnO_x(z)-TiO₂ [106] (z = 0.1–0.3) prepared by the sol-gel method. The optimum MnO_x(0.25)-TiO₂ showed nearly full NH₃ conversion around 200 °C with N₂ selectivity of more than 60% up to 350 °C. NO was formed only above 250 °C. Based on the in situ DRIFTS studies, they claimed that N₂ appeared as a product of the reaction between -HNO and -NH species. N₂O was formed from the combination of two -HNO species at low temperatures, as well as from the reaction between adsorbed NH₃ and nitrite/nitrate species at high temperatures. Additionally, the i-SCR mechanism was proposed over Fe₂O₃-Al₂O₃, Fe₂O₃-TiO₂, Fe₂O₃-ZrO₂ and Fe₂O₃-SiO₂ prepared by the sol-gel method [107]. The materials prepared from iron sulfate led to a higher N₂ selectivity than those prepared from nitrate. The higher N₂ selectivity was reported earlier for CuO/TiO₂ prepared from CuSO₄ compared to the corresponding catalyst prepared from Cu(NO₃)₂ [52], which is also valid for the pre-sulfated samples [68].

Chen et al. [108] developed a series of mullite-based AMn₂O₅ (A = Sm, Y, Gd) catalysts, among which SmMn₂O₅ achieved complete NH₃ conversion at 175–250 °C (albeit with a rather low N₂ selectivity of barely more than 45%). The imide mechanism was reported for NH₃-SCO over SmMn₂O_5. Furthermore, its modification with niobium oxide (5 wt.%)Nb₂O₅/SmMn₂O₅ stood out with N₂ selectivity > 60%. The niobium oxide was supposed to enhance the catalyst surface attraction to the N atom lone-pair electron. Consequently, the reaction between the increased amount of -NH and -HNO species towards the formation of N₂ was favored (Figure 8a). In the other approach, SmMn₂O₅ was mixed with Cu-SAPO-34 [109]. Still, N₂ selectivity varied between 20–60% in the range of 150–400 °C. For the mixed catalysts, the i-SCR mechanism was proposed (NH₃ oxidation to NO_x over mullite catalyst), which stays contrary to the above-mentioned studies. The i-SCR mechanism was also proposed over the La_xSr_1-xMnO₃ perovskite-based catalysts post-modified with a 3 M solution of HNO₃ (0.1–72 h, Figure 8b) [110]. As the treatment time increased, the perovskite phase changed from a mixture of perovskite and MnO₂ (10 h treatment) to pure MnO₂ (72 h treatment). Additionally, the materials subjected to a 72 h treatment were the most active, albeit selective to NO and N₂O, as well as poorly resistant to sulfur species.

Concluding, several transition metal-based catalysts were proposed for NH₃ oxidation to N₂. Among them, mainly Cu/Al₂O₃, Cu-Ce-Zr, CuO-Fe₂O₃ or CuO-CeO₂ were the most frequently studied materials (according to data gathered in Table 2). In the case of Cu/Al₂O₃, the optimal loading of 10 wt.% Cu guarantees an enhanced NH₃ conversion and N₂ selectivity (full NH₃ conversion at 350–500 °C with N₂ selectivity above 75%). For the other mentioned materials, i.e., Fe- or Ce-containing systems, the catalytic studies were carried out only in a narrow temperature range. The complete NH₃ conversion was achieved between 225/250–300 °C with N₂ selectivity above 80% (Table 2). Further attention should be given to the stability under conditions simulating real exhaust from diesel engines up to 600–700 °C of the mesoporous CuFe₂O₄ spinel.

Table 2. Comparison of complete NH₃ conversion and N₂ selectivity in the same temperature range over hydrotalcite-derived mixed metal oxides and other transition metal-based catalysts reported in the literature (related data are marked with asterisks).

Pos.	Sample	Preparation	Reaction Conditions	Operation Temperature for Achieving 100% NH3 Conversion/°C	N2 Selectivity/%	Refs.
Hydrotalcite-derived mixed metal oxides
1	CuMgAl n(Cu)/n(Mg)/n(Al) = 4.6/66.4/29, mol.%	Coprecipitation, calcination, 650 °C, air, 14 h	0.5 vol.% NH3, 1.75 vol.% O2, He balance, GHSV 10,000–12,000 h−1	500	>80	[29]
2	CuMgAl n(Cu)/n(Mg)/n(Al) = 5/66/29, mol.%	Coprecipitation, calcination, 600 °C, air, 16 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, GHSV 30,000 h−1	400–650	>80	[30]
3	CuMgFe n(Cu)/n(Mg)/n(Fe) = 0.5/2/1, mol.%	Coprecipitation, calcination, 600 °C, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, GHSV 15,400 h−1	400–450	>70	[31]
4	CuMgAl n(Cu)/n(Mg)/n(Al) = 8/63/29, mol.%	Coprecipitation, calcination, 600 °C, air, 6 h	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, WHSV 24,000 mL h−1 g−1 * 0.5 vol.% NH3, 2.5 vol.% O2, N2 balance, WHSV 137,000–140,000 mL h−1 g−1 ** 0.5 vol.% NH3, 2.5 vol.% O2, 10 vol.% CO2, 5 vol.% H2O, N2 balance, WHSV 137,000–140,000 mL h−1 g−1	400–600 * 450–600 ** 600	>60 * >60 ** >55	[34] *,** [111]
5	CuMgAl n(Cu)/n(Mg)/n(Al) = 0.6/1.4/1.0, mol.%	Coprecipitation, calcination, 600 °C, *900 °C, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	375–500 * 500	>70 * >40	[28]
6	CuMgAl n(Cu)/n(Mg)/n(Al) = 5/62/33, mol.%	Coprecipitation, calcination, 600 °C, 800 °C *, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	475–500 * 475–500	>60 * >85	[35]
7	GaCuMgAl * CeCuMgAl n(Ga/Ce)/n(Cu)/n(Mg)/n(Al) = 0.25/5/65.75/29	Coprecipitation, calcination, 600 °C, air, 6 h	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, WHSV 24,000 mL h−1 g−1	375–500 * 375–500	>80 * >50	[36]
8	CuMgAl n(Cu)/n(Mg)/n(Al) = 10–15/52–57/33, mol.% * (4.1 wt.%)CeCuMgAl n(Cu)/n(Mg)/n(Al) = 5/62/33, mol.%	Coprecipitation, calcination, 800 °C, air, 9 h * Impregnation, calcination, 800 C, air, 9 h	0.035 vol.% NH3, 20 vol.% O2, N2 balance, WHSV 30,000 mL h−1 g−1	350 * 350	<20 * <70	[37]
9	CuMgAl n(Cu)/n(Mg)/n(Al) = 5/62/33, mol.% * (3 wt.%)CeCuMgAl ** (0.5 wt.% Ce)CuMgAl	Coprecipitation, calcination, 600 °C, air, 12 h, *,** Impregnation, calcination, air, 600 °C, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	500–600 * 500–600 ** 450–600	>60 * >75 ** >55	[38]
10	CuZnAl n(Cu)/n(Zn)/n(Al) = 10–15/52/33, mol.% * (8.14 wt.%)CeCuMgAl		0.035 vol.% NH3, 20 vol.% O2, N2 balance, WHSV 30,000 mL h−1 g−1	350 * 350	<30 * <40	[39]
11	CoMnAl n(Co)/n(Mn)/n(Al) = 4/1/1	Coprecipitation, calcination, 500 °C, air, 4 h; * Mechanochemical method, calcination, 500 °C, air, 4 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	250–500 * 250–500	>40 * >45	[40]
Other metal oxides
12	CuO/monolith	Precursors calcination on the monolith, 600 °C, air, 6 h	0.05 vol.% NH3, 3 vol.% O2, N2 balance, GHSV 40,000 h−1	450–550	67–85	[45]
13	(10 wt.%)V/TiO2	Impregnation, calcination, 550 °C, air, 6 h	0.05 vol.% NH3, 2.5 vol.% O2, N2 balance, GHSV 35,385 h−1	225–300	-	[83]
14	(10 wt.%)Cu/TiO2			200–300	-
15	(10 wt.%)Cu/TiO2	Impregnation, rotary evaporator, calcination, 450 °C, air, 3 h	0.04 vol.% NH3, 10 vol.% O2, He balance, GHSV 50,000 h−1 * 0.04 vol.% NH3, 10 vol.% O2, 3 vol.% H2O, He balance, GHSV 50,000 h−1	250–300 * 350–375	>95 * >95	[65]
16	(10 wt.%)Cu/Al2O3			400	>95
17	(10–15 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 24 h	1.14 vol.% NH3, 8.21 vol.% O2, He balance, WHSV 2 240 mL h−1 g−1	350	>90	[61]
18	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 24 h	1.14 vol.% NH3, 8.21 vol.% O2, He balance, WHSV 2240 mL h−1 g−1 * 1.14 vol.% NH3, 8.21 vol.% O2, He balance, WHSV 2240 mL h−1 g−1	350 * 350	94 * 95	[62]
19	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 3 h, * Cu(CH3COO)2 as precursor ** Cu(NO3)2 as precursor	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 50,000 h−1	* 350–400 ** 375–400	* >85 ** >95	[70]
20	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 6 h	0.5 vol.% NH3, 2.5 vol.% O2, N2 balance, WHSV 137,000–140,000 mL h−1 g−1 * 0.5 vol.% NH3, 2.5 vol.% O2, 10 vol.% CO2, 5 vol.% H2O, N2 balance, WHSV 137,000–140,000 mL h−1 g−1	450–600 * 600	>60 * >50	[111]
21	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2 Ar balance, WHSV 24,000 mL h−1 g−1	425–500	>75	[112]
22	(10 wt.%)Cu/Al2O3 * (10 wt.%)Cu/Al2O3	Imprgnation, rotary evaporation, calcination, 500 °C, air, 2 h * Impregnation, rotary evaporation, 500 °C, H2/N2, 2 h; 0.05 vol.% NH3, 5 vol.% O2, N2 balance	0.05 vol.% NH3, 5 vol.% O2 N2 balance, GHSV 60,000 h−1	330 * 300–330	not shown * not shown	[9]
23	(1.3 wt.%)Cu/Al2O3 * (1 wt.%)Cu/CeOx/Li2O/Al2O3	Impregnation, calcination, 350 °C, air, time not given; homogenous deposition precipitation, H2 reduction, 400 °C, 2 h	2 vol.% NH3, 2 vol.% O2, Ar balance, GHSV 2500 h−1	400 * 325–400	100 * 100	[76]
24	(3.4 wt.%)Cu/Al2O3	Impregnation, calcination, 450 °C, air, 5 h	0.54 vol.% NH3, 8 vol.% O2, He balance, WHSV 240 mL h−1 g−1	400–450	not shown	[69]
25	(20 wt.%)Cu/Al2O3/monolith	Impregnation, calcination, 800 °C, air, 4 h	0.04 vol.% NH3, 8.2 vol.% O2, 1.3 vol.% CH4, 3.9 vol.% CO2, 4.1 vol.% CO, 2.9 vol.% H2, GHSV 100,000 h−1	400–500	0	[71]
26	(1 wt.%)PbO-(4.3 wt.%)Cu/Al2O3	Impregnation, calcination, 450 °C, air, time not shown	0.54 vol.% NH3, 8 vol.% O2, He balance, WHSV 800 mL h−1 g−1	325	95	[68]
27	(1–2 wt.%)Cu/η-Al2O3	Impregnation, Rotary evaporator, calcination, 500 °C, air, 10 h; pre-treatment conditions: 20 vol.% O2/He, 550 °C, 1 h	0.1 vol.% NH3, 8 vol.% O2, 3.5 vol.% H2O, He balance, WHSV 250,000 mL h−1 g−1	550	not shown	[75]
28	CuO/CNTs (carbon nanotubes, 9.85 wt.% Cu)	Impregnation, ultrasonic treatmnet, evaporation, 350 °C, He, 3 h	0.1 vol.% NH3, 2 vol.% O2, He balance, WHSV 60,000 mL h−1 g−1	189–250	>98	[63]
29	Cu/graphene (2.57–3.42 wt.%)	Impregnation, ultrasonic treatment, 400 °C, N2, 3 h, * Cu(CH3COO)2 H2O as precursor ** Cu(NO3)2·H2O as precursor	0.05 vol.% NH3, 1 vol.% O2, N2 balance, GHSV 35,000 h−1	* 300 ** 250–300	* >80 ** >80	[72]
30	(5 wt.%)Ni/Al2O3	Impregnation, calcination, 800 °C, air, 8 h	0.1 vol.% NH3, 18 vol.% O2, N2 balance, GHSV 61,000 h−1	550–800	>55	[78]
31	(5 wt.%)Mn/Al2O3			300–800	>55
32	(10.5 wt.%)CuO/TiSnO2	Impregnation, calcination, 450 °C, air, 4 h	0.05 vol.% NH3, 3 vol.% O2, N2 balance, WHSV 60,000 mL h−1 g−1	300–400	>70	[85]
33	(5 wt.%)CuOx/La2Ce2O7	Impregnation, calcination, 600 °C, air, 1 h	0.05 vol.% NH3, 5 vol.% O2, N2 balance, GHSV 20,000 h−1	275–425	>80	[67]
34	(10 wt.%)Ce/(2 wt.%)V/TiO2	Impregnation, calcination, 400 °C, air, 4 h; pre-treatment conditions: 8 vol.% O2/N2, 400 °C, 0.5 h	0.02 vol.% NH3, 8 vol.% O2, 6 vol.% H2O, N2 balance, GHSV 120,000 h−1	300–350	>90	[95]
35	Ce0.4Zr0.6O2	Surfactant-templated method, calcination, 550 °C, air, 3 h	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 40,000 h−1	360–380	>90	[93]
36	(6 wt.%)Cu-Ce-Zr n(Si)/n(Al) = 4	Sol-gel method, calcination, 450 °C, air, 3 h	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 40,000 h−1	230	>90	[97]
37	CuO-Fe2O3 n(Cu)/n(Fe) = 1:1	Sol-gel method, calcination, 500 °C, air, 4 h	0.08 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 60,000 h−1	225–300	>80	[87]
38	CuO-Fe2O3 n(Cu)/n(Fe) = 5:5	Sol-gel method, calcination, 400 °C, air, 4 h	0.08 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 60,000 h−1	250–300	>80	[86]
39	CuFe2O4 (8.59 wt.% Cu, 7.45 wt.% Fe)	Hard-template method, 600 °C, air, 6 h	0.1 vol.% NH3, 0.2 vol.% O2, He balance, GHSV 35,000 h−1	350–600	>75	[88]
40	CuO-CeO2 n(Cu)/n(Ce) = 6/4	Coprecipitation, calcination, 500 °C, air, 4 h	0.1 vol.% NH3, 4 vol.% O2, 12 vol.% H2O, He balance, WHSV 92,000 mL h−1 g−1	400	82	[99]
41	CuO-CeO2 (10 wt.% Cu)	Surfactant templated method, 500 °C, air, 3 h	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 40,000 h−1	250–300	>90	[100]
42	(1 wt.%)Cu-PILC-Verm (Alumina pillared vermiculites)	Ion-exchange, calcination, 450 °C, air, 3 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	500–550	>95	[89]
43	(5.7 wt.%)Fe-PILC-Phlog (Alumina pillared phlogopite)			500–550	>70
44	(0.59 wt.%)Cu-PCH (Porous clay heterostructures)	Ion-exchange, 450 °C, air, 3 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	500–550	>90	[53]
45	(1.43 wt.%)Cu-PCH (Porous clay heterostructures)			400–550	>90	[90]
46	Cu/attapulgite (5–10 wt.% Cu)	Impregnation, 400 °C, air, 4 h	0.005 vol.% NH3, 4 vol.% O2, N2 balance, GHSV 150,000 h−1	450–500	>75	[92]
47	natural manganese ore	Fluidization, 12 h	0.05 vol.% NH3, 3 vol.% O2, He balance, GHSV 15,000–80,000 h−1	240	>70	[103]
48	MnO2	Calcination, 400 °C, air, 2 h		210	>60
49	Cu-Mn/TiO2 n(Cu)/n(Mn) = 20/80	Impregnation, rotary evaporator, calcination, 550 °C, air, 2 h	0.06 vol.% NH3, 6 vol.% O2, N2 balance, WHSV 200,000 mL h−1 g−1	307	not shown	[104]
50	MnOx-TiO2 (27.8 wt.% Mn)	Sol-gel method, calcination, 500 °C, air, 4 h	0.05 vol.% NH3, 5 vol.% O2, He balance, WHSV 240,000 mL h−1 g−1	200–350	>60	[106]
51	SmMn2O5	Organic solution combustion methods, 700 °C, air, 8 h	0.05 vol.% NH3, 10 vol.% O2, N2 balance, WHSV 120,000 mL h−1 g−1	175–250	>45	[108]
52	(5.0 wt.%)Nb2O5/SmMn2O5	Impregnation, 450 °C, air, 2 h		200–250	>60
53	(30 wt.%)SmMn2O5/Cu-SAPO	Grinding the mixture; * after hydrothermal aging treatment conditions: 21 vol.% O2, 10 vol.% H2O, N2 balance, 800 °C, 5 h	0.05 vol.% NH3, 21 vol.% O2, N2 balance, GHSV 100,000 h−1	225–400 * 300–400	>20 * not shown	[109]
54	LaxSr1−xMnO3	Hydrothermal method, 400 °C, air, 2 h, post-treatment in 3 M HNO3	0.05 vol.% NH3, 3 vol.% O2, N2 balance, WHSV 120,000 mL h−1 g−1	300–450	not shown	[110]

Table 2. Comparison of complete NH₃ conversion and N₂ selectivity in the same temperature range over hydrotalcite-derived mixed metal oxides and other transition metal-based catalysts reported in the literature (related data are marked with asterisks).

Pos.	Sample	Preparation	Reaction Conditions	Operation Temperature for Achieving 100% NH3 Conversion/°C	N2 Selectivity/%	Refs.
Hydrotalcite-derived mixed metal oxides
1	CuMgAl n(Cu)/n(Mg)/n(Al) = 4.6/66.4/29, mol.%	Coprecipitation, calcination, 650 °C, air, 14 h	0.5 vol.% NH3, 1.75 vol.% O2, He balance, GHSV 10,000–12,000 h−1	500	>80	[29]
2	CuMgAl n(Cu)/n(Mg)/n(Al) = 5/66/29, mol.%	Coprecipitation, calcination, 600 °C, air, 16 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, GHSV 30,000 h−1	400–650	>80	[30]
3	CuMgFe n(Cu)/n(Mg)/n(Fe) = 0.5/2/1, mol.%	Coprecipitation, calcination, 600 °C, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, GHSV 15,400 h−1	400–450	>70	[31]
4	CuMgAl n(Cu)/n(Mg)/n(Al) = 8/63/29, mol.%	Coprecipitation, calcination, 600 °C, air, 6 h	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, WHSV 24,000 mL h−1 g−1 * 0.5 vol.% NH3, 2.5 vol.% O2, N2 balance, WHSV 137,000–140,000 mL h−1 g−1 ** 0.5 vol.% NH3, 2.5 vol.% O2, 10 vol.% CO2, 5 vol.% H2O, N2 balance, WHSV 137,000–140,000 mL h−1 g−1	400–600 * 450–600 ** 600	>60 * >60 ** >55	[34] *,** [111]
5	CuMgAl n(Cu)/n(Mg)/n(Al) = 0.6/1.4/1.0, mol.%	Coprecipitation, calcination, 600 °C, *900 °C, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	375–500 * 500	>70 * >40	[28]
6	CuMgAl n(Cu)/n(Mg)/n(Al) = 5/62/33, mol.%	Coprecipitation, calcination, 600 °C, 800 °C *, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	475–500 * 475–500	>60 * >85	[35]
7	GaCuMgAl * CeCuMgAl n(Ga/Ce)/n(Cu)/n(Mg)/n(Al) = 0.25/5/65.75/29	Coprecipitation, calcination, 600 °C, air, 6 h	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, WHSV 24,000 mL h−1 g−1	375–500 * 375–500	>80 * >50	[36]
8	CuMgAl n(Cu)/n(Mg)/n(Al) = 10–15/52–57/33, mol.% * (4.1 wt.%)CeCuMgAl n(Cu)/n(Mg)/n(Al) = 5/62/33, mol.%	Coprecipitation, calcination, 800 °C, air, 9 h * Impregnation, calcination, 800 C, air, 9 h	0.035 vol.% NH3, 20 vol.% O2, N2 balance, WHSV 30,000 mL h−1 g−1	350 * 350	<20 * <70	[37]
9	CuMgAl n(Cu)/n(Mg)/n(Al) = 5/62/33, mol.% * (3 wt.%)CeCuMgAl ** (0.5 wt.% Ce)CuMgAl	Coprecipitation, calcination, 600 °C, air, 12 h, *,** Impregnation, calcination, air, 600 °C, 12 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	500–600 * 500–600 ** 450–600	>60 * >75 ** >55	[38]
10	CuZnAl n(Cu)/n(Zn)/n(Al) = 10–15/52/33, mol.% * (8.14 wt.%)CeCuMgAl		0.035 vol.% NH3, 20 vol.% O2, N2 balance, WHSV 30,000 mL h−1 g−1	350 * 350	<30 * <40	[39]
11	CoMnAl n(Co)/n(Mn)/n(Al) = 4/1/1	Coprecipitation, calcination, 500 °C, air, 4 h; * Mechanochemical method, calcination, 500 °C, air, 4 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	250–500 * 250–500	>40 * >45	[40]
Other metal oxides
12	CuO/monolith	Precursors calcination on the monolith, 600 °C, air, 6 h	0.05 vol.% NH3, 3 vol.% O2, N2 balance, GHSV 40,000 h−1	450–550	67–85	[45]
13	(10 wt.%)V/TiO2	Impregnation, calcination, 550 °C, air, 6 h	0.05 vol.% NH3, 2.5 vol.% O2, N2 balance, GHSV 35,385 h−1	225–300	-	[83]
14	(10 wt.%)Cu/TiO2			200–300	-
15	(10 wt.%)Cu/TiO2	Impregnation, rotary evaporator, calcination, 450 °C, air, 3 h	0.04 vol.% NH3, 10 vol.% O2, He balance, GHSV 50,000 h−1 * 0.04 vol.% NH3, 10 vol.% O2, 3 vol.% H2O, He balance, GHSV 50,000 h−1	250–300 * 350–375	>95 * >95	[65]
16	(10 wt.%)Cu/Al2O3			400	>95
17	(10–15 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 24 h	1.14 vol.% NH3, 8.21 vol.% O2, He balance, WHSV 2 240 mL h−1 g−1	350	>90	[61]
18	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 24 h	1.14 vol.% NH3, 8.21 vol.% O2, He balance, WHSV 2240 mL h−1 g−1 * 1.14 vol.% NH3, 8.21 vol.% O2, He balance, WHSV 2240 mL h−1 g−1	350 * 350	94 * 95	[62]
19	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 3 h, * Cu(CH3COO)2 as precursor ** Cu(NO3)2 as precursor	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 50,000 h−1	* 350–400 ** 375–400	* >85 ** >95	[70]
20	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 6 h	0.5 vol.% NH3, 2.5 vol.% O2, N2 balance, WHSV 137,000–140,000 mL h−1 g−1 * 0.5 vol.% NH3, 2.5 vol.% O2, 10 vol.% CO2, 5 vol.% H2O, N2 balance, WHSV 137,000–140,000 mL h−1 g−1	450–600 * 600	>60 * >50	[111]
21	(10 wt.%)Cu/Al2O3	Impregnation, calcination, 600 °C, air, 12 h	0.5 vol.% NH3, 2.5 vol.% O2 Ar balance, WHSV 24,000 mL h−1 g−1	425–500	>75	[112]
22	(10 wt.%)Cu/Al2O3 * (10 wt.%)Cu/Al2O3	Imprgnation, rotary evaporation, calcination, 500 °C, air, 2 h * Impregnation, rotary evaporation, 500 °C, H2/N2, 2 h; 0.05 vol.% NH3, 5 vol.% O2, N2 balance	0.05 vol.% NH3, 5 vol.% O2 N2 balance, GHSV 60,000 h−1	330 * 300–330	not shown * not shown	[9]
23	(1.3 wt.%)Cu/Al2O3 * (1 wt.%)Cu/CeOx/Li2O/Al2O3	Impregnation, calcination, 350 °C, air, time not given; homogenous deposition precipitation, H2 reduction, 400 °C, 2 h	2 vol.% NH3, 2 vol.% O2, Ar balance, GHSV 2500 h−1	400 * 325–400	100 * 100	[76]
24	(3.4 wt.%)Cu/Al2O3	Impregnation, calcination, 450 °C, air, 5 h	0.54 vol.% NH3, 8 vol.% O2, He balance, WHSV 240 mL h−1 g−1	400–450	not shown	[69]
25	(20 wt.%)Cu/Al2O3/monolith	Impregnation, calcination, 800 °C, air, 4 h	0.04 vol.% NH3, 8.2 vol.% O2, 1.3 vol.% CH4, 3.9 vol.% CO2, 4.1 vol.% CO, 2.9 vol.% H2, GHSV 100,000 h−1	400–500	0	[71]
26	(1 wt.%)PbO-(4.3 wt.%)Cu/Al2O3	Impregnation, calcination, 450 °C, air, time not shown	0.54 vol.% NH3, 8 vol.% O2, He balance, WHSV 800 mL h−1 g−1	325	95	[68]
27	(1–2 wt.%)Cu/η-Al2O3	Impregnation, Rotary evaporator, calcination, 500 °C, air, 10 h; pre-treatment conditions: 20 vol.% O2/He, 550 °C, 1 h	0.1 vol.% NH3, 8 vol.% O2, 3.5 vol.% H2O, He balance, WHSV 250,000 mL h−1 g−1	550	not shown	[75]
28	CuO/CNTs (carbon nanotubes, 9.85 wt.% Cu)	Impregnation, ultrasonic treatmnet, evaporation, 350 °C, He, 3 h	0.1 vol.% NH3, 2 vol.% O2, He balance, WHSV 60,000 mL h−1 g−1	189–250	>98	[63]
29	Cu/graphene (2.57–3.42 wt.%)	Impregnation, ultrasonic treatment, 400 °C, N2, 3 h, * Cu(CH3COO)2 H2O as precursor ** Cu(NO3)2·H2O as precursor	0.05 vol.% NH3, 1 vol.% O2, N2 balance, GHSV 35,000 h−1	* 300 ** 250–300	* >80 ** >80	[72]
30	(5 wt.%)Ni/Al2O3	Impregnation, calcination, 800 °C, air, 8 h	0.1 vol.% NH3, 18 vol.% O2, N2 balance, GHSV 61,000 h−1	550–800	>55	[78]
31	(5 wt.%)Mn/Al2O3			300–800	>55
32	(10.5 wt.%)CuO/TiSnO2	Impregnation, calcination, 450 °C, air, 4 h	0.05 vol.% NH3, 3 vol.% O2, N2 balance, WHSV 60,000 mL h−1 g−1	300–400	>70	[85]
33	(5 wt.%)CuOx/La2Ce2O7	Impregnation, calcination, 600 °C, air, 1 h	0.05 vol.% NH3, 5 vol.% O2, N2 balance, GHSV 20,000 h−1	275–425	>80	[67]
34	(10 wt.%)Ce/(2 wt.%)V/TiO2	Impregnation, calcination, 400 °C, air, 4 h; pre-treatment conditions: 8 vol.% O2/N2, 400 °C, 0.5 h	0.02 vol.% NH3, 8 vol.% O2, 6 vol.% H2O, N2 balance, GHSV 120,000 h−1	300–350	>90	[95]
35	Ce0.4Zr0.6O2	Surfactant-templated method, calcination, 550 °C, air, 3 h	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 40,000 h−1	360–380	>90	[93]
36	(6 wt.%)Cu-Ce-Zr n(Si)/n(Al) = 4	Sol-gel method, calcination, 450 °C, air, 3 h	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 40,000 h−1	230	>90	[97]
37	CuO-Fe2O3 n(Cu)/n(Fe) = 1:1	Sol-gel method, calcination, 500 °C, air, 4 h	0.08 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 60,000 h−1	225–300	>80	[87]
38	CuO-Fe2O3 n(Cu)/n(Fe) = 5:5	Sol-gel method, calcination, 400 °C, air, 4 h	0.08 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 60,000 h−1	250–300	>80	[86]
39	CuFe2O4 (8.59 wt.% Cu, 7.45 wt.% Fe)	Hard-template method, 600 °C, air, 6 h	0.1 vol.% NH3, 0.2 vol.% O2, He balance, GHSV 35,000 h−1	350–600	>75	[88]
40	CuO-CeO2 n(Cu)/n(Ce) = 6/4	Coprecipitation, calcination, 500 °C, air, 4 h	0.1 vol.% NH3, 4 vol.% O2, 12 vol.% H2O, He balance, WHSV 92,000 mL h−1 g−1	400	82	[99]
41	CuO-CeO2 (10 wt.% Cu)	Surfactant templated method, 500 °C, air, 3 h	0.1 vol.% NH3, 10 vol.% O2, He balance, GHSV 40,000 h−1	250–300	>90	[100]
42	(1 wt.%)Cu-PILC-Verm (Alumina pillared vermiculites)	Ion-exchange, calcination, 450 °C, air, 3 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	500–550	>95	[89]
43	(5.7 wt.%)Fe-PILC-Phlog (Alumina pillared phlogopite)			500–550	>70
44	(0.59 wt.%)Cu-PCH (Porous clay heterostructures)	Ion-exchange, 450 °C, air, 3 h	0.5 vol.% NH3, 2.5 vol.% O2, He balance, WHSV 24,000 mL h−1 g−1	500–550	>90	[53]
45	(1.43 wt.%)Cu-PCH (Porous clay heterostructures)			400–550	>90	[90]
46	Cu/attapulgite (5–10 wt.% Cu)	Impregnation, 400 °C, air, 4 h	0.005 vol.% NH3, 4 vol.% O2, N2 balance, GHSV 150,000 h−1	450–500	>75	[92]
47	natural manganese ore	Fluidization, 12 h	0.05 vol.% NH3, 3 vol.% O2, He balance, GHSV 15,000–80,000 h−1	240	>70	[103]
48	MnO2	Calcination, 400 °C, air, 2 h		210	>60
49	Cu-Mn/TiO2 n(Cu)/n(Mn) = 20/80	Impregnation, rotary evaporator, calcination, 550 °C, air, 2 h	0.06 vol.% NH3, 6 vol.% O2, N2 balance, WHSV 200,000 mL h−1 g−1	307	not shown	[104]
50	MnOx-TiO2 (27.8 wt.% Mn)	Sol-gel method, calcination, 500 °C, air, 4 h	0.05 vol.% NH3, 5 vol.% O2, He balance, WHSV 240,000 mL h−1 g−1	200–350	>60	[106]
51	SmMn2O5	Organic solution combustion methods, 700 °C, air, 8 h	0.05 vol.% NH3, 10 vol.% O2, N2 balance, WHSV 120,000 mL h−1 g−1	175–250	>45	[108]
52	(5.0 wt.%)Nb2O5/SmMn2O5	Impregnation, 450 °C, air, 2 h		200–250	>60
53	(30 wt.%)SmMn2O5/Cu-SAPO	Grinding the mixture; * after hydrothermal aging treatment conditions: 21 vol.% O2, 10 vol.% H2O, N2 balance, 800 °C, 5 h	0.05 vol.% NH3, 21 vol.% O2, N2 balance, GHSV 100,000 h−1	225–400 * 300–400	>20 * not shown	[109]
54	LaxSr1−xMnO3	Hydrothermal method, 400 °C, air, 2 h, post-treatment in 3 M HNO3	0.05 vol.% NH3, 3 vol.% O2, N2 balance, WHSV 120,000 mL h−1 g−1	300–450	not shown	[110]

4. Conclusions and Future Perspectives

In this short mini-review, we have discussed recent trends, limits and opportunities offered by hydrotalcite-derived mixed metal oxides as opposed to the other transition metal-based catalysts applied in NH₃-SCO. Although there are relatively several catalytic systems proposed in the literature, at the same time, their systematic investigations and further improvement are scarce. Furthermore, there is a lack of systematic investigations of the reaction mechanisms. The mechanisms of NH₃-SCO have been explored mainly by the application of in situ DRIFTS and the indication of the characteristic intermediates of the imide, hydrazine or i-SCR mechanism.

Overall, based on our comparison, two transition metal-based catalytic systems can be selected for the preparation of the next-generation catalysts, i.e., the CuMgAl hydrotalcite-derived mixed metal oxides and Cu/Al₂O₃. The complete NH₃ conversion activity between 375–650 °C and N₂ selectivity above 70% were reached over hydrotalcite-derived mixed metal oxides. Similarly, Cu/Al₂O₃ being the most frequently studied catalyst reached full NH₃ conversion at 350–500 °C with N₂ selectivity above 75%. Our revision is further supported by our previous study [111], where the activity and N₂ selectivity in NH₃-SCO over hydrotalcite-derived CuMgAl (n(Cu)/n(Mg)/n(Al) = 8/63/29, mol.%) mixed metal oxides and (10 wt.%)Cu/Al₂O₃ were tested under NH₃/O₂/CO₂/H₂O/N₂ conditions by applying ca. 6.5–6.7 g of catalysts. The mixture of the highly dispersed easily reducible copper oxide and bulk copper oxide species allowed for enhanced activity, N₂ selectivity and stability. Still, further work is needed on the systematic catalysts’ chemical and phase optimization and catalyst tests, including investigations under more applied reaction conditions concerning either reaction mixture composition (NH₃ concentration of about 100 ppm with O₂ concentrations of about 10 vol.%; in the presence of CO_x, SO_x and H₂O) or temperature (up to 600–700 °C), should follow. Furthermore, thermal stability should be tested, as well as catalyst poisoning via the typical components of the lubricating oil for diesel engines (e.g., Ca, Zn, P and S species). A comprehensive understanding of the involved active species, e.g., through operando technologies under realistic working conditions, could facilitate a knowledge-based catalyst optimization to obtain desired NH₃ slip catalysts.