Progress on Noble Metal-Based Catalysts Dedicated to the Selective Catalytic Ammonia Oxidation into Nitrogen and Water Vapor (NH₃-SCO)

Abstract

A recent development for selective ammonia oxidation into nitrogen and water vapor (NH₃-SCO) over noble metal-based catalysts is covered in the mini-review. As ammonia (NH₃) can harm human health and the environment, it led to stringent regulations by environmental agencies around the world. With the enforcement of the Euro VI emission standards, in which a limitation for NH₃ emissions is proposed, NH₃ emissions are becoming more and more of a concern. Noble metal-based catalysts (i.e., in the metallic form, noble metals supported on metal oxides or ion-exchanged zeolites, etc.) were rapidly found to possess high catalytic activity for NH₃ oxidation at low temperatures. Thus, a comprehensive discussion of property-activity correlations of the noble-based catalysts, including Pt-, Pd-, Ag- and Au-, Ru-based catalysts is given. Furthermore, due to the relatively narrow operating temperature window of full NH₃ conversion, high selectivity to N₂O and NO_x as well as high costs of noble metal-based catalysts, recent developments are aimed at combining the advantages of noble metals and transition metals. Thus, also a brief overview is provided about the design of the bifunctional catalysts (i.e., as dual-layer catalysts, mixed form (mechanical mixture), hybrid catalysts having dual-layer and mixed catalysts, core-shell structure, etc.). Finally, the general conclusions together with a discussion of promising research directions are provided.

1. Introduction

Ammonia (NH₃) is a corrosive, highly toxic, and reactive inorganic gas with a pungent odor under ambient conditions. It is an atmospheric pollutant that is dangerous to health and life because it could corrode skin, eyes, and lungs, and cause permanent injury or even death when the concentration is higher than 300 ppm [1,2]. Ammonia is also reported to be the most common pollutant found in water streams, further affecting human health as the consequence of eating toxic fish and drinking water [3]. The toxic action of ammonia on aquatic animals can lead to the extinction of the entire population, threatening many important ecosystems and fisheries worldwide [4,5]. NH₃ is referred to as one of four major atmospheric pollutants together with NO_x, SO_x, and nonmethane volatile organic compounds (NMVOC). Approximately 5600 kt y⁻¹ of ammonia are emitted into the atmosphere each year, i.e., up to 4-times higher emission levels than in the previous century, and it continues to increase [6]. NH₃ is emitted by several various processes, including nitric acid production, urea manufacturing, nitrogen fertilizer production, biomass, and coal gasification, petroleum refining and refrigeration, livestock waste, and animal agriculture, transport (as a gas slip from the process of selective catalytic reduction of NO_x using NH₃ or urea (SCR) in DeNO_x applications), etc. More attention was given to the removal of NH₃ from gaseous and waste streams, e.g., through its oxidation.

The selective catalytic oxidation of ammonia (NH₃-SCO) into nitrogen and water vapor is considered as the most promising method for the elimination of NH₃ from oxygen-containing exhaust gases (Equation (1)) [7]:

4 NH₃ + 3 O₂ → 2 N₂ + 6 H₂O

(1)

NH₃ is generated by an onboard aqueous urea dosing system. The obtained ammonia acts as a NO_x reductant in the DeNO_x process. There is a serious risk that unreacted ammonia is released into the atmosphere. Thus, the active SCO catalysts (so-called guard catalyst, ammonia slip catalyst—ASC, ammonia oxidation catalyst—AMOX) should operate in a broad temperature range (up to 600–700 °C—in the cycle of diesel particulate filter regeneration) in the presence of typical components of the exhausts (H₂O, CO_x, and SO_x), and additionally should selectively direct the reaction to the formation of N₂ and H₂O. Euro VI emission standards for heavy-duty vehicles (HDVs) introduced for the first time limits for NH₃ emissions up to 10 ppm [8]. Currently, there are no limits for NH₃ emitted from light-duty vehicles (LDVs, i.e., passenger cars), despite their high levels of emissions (e.g., [9,10]). Thus, potentially NH₃ will be considered next to ultra-fine particles smaller than 23 nanometers (PN₁₀) and nitrous oxide (N₂O) in the upcoming regulations, e.g., upcoming Euro emission standards.

Various kinds of catalysts were studied for NH₃-SCO, including noble metals (e.g., Ru [11]), supported noble metals (e.g., Au/Nb₂O₅, Au/ZrO₂ [12]), (mixed) metal oxides, supported metal oxides, modified zeolites (e.g., Pt-CuMgAlO_x [13], Ag-USY [13]), etc. These groups of catalysts investigated in NH₃-SCO were summarized by Jabłońska et al. [7,14], Gao et al. [15] and Lan et al. [16]. In general, noble metal-based catalysts tend to possess high activity at low temperatures (< 300 °C), while their high cost and relatively low N₂ selectivity have restrained their widespread application. Transition metal-containing oxides and zeolites show improved selectivity toward N₂ than noble metal-based catalysts; however, they need higher operating temperatures (300–600 °C). Consequently, the proper combination of these two metals (in the form of bifunctional catalysts) could produce the catalysts with enhanced activity and N₂ selectivity. In general, the concept of bifunctional catalysts is based on the internal selective catalytic reduction (i-SCR) mechanism. This mechanism consists of two main steps. In the first step, part of ammonia is oxidized to NO and NO₂—minor by-product, while in the second step, NO and NO₂ are reduced by ammonia (unreacted in the first step) with the formation of N₂ and H₂O (DeNO_x, NH₃-SCR). In this step, also the formation of N₂O is possible. Besides the i-SCR mechanism, other major reaction pathways, i.e., the imide (NH, in which NH₃ transforms to N₂ and N₂O as final products, with nitrosyl (HNO) as an intermediate) mechanism and the hydrazine (with the formation of a hydrazine-type (N₂H₄) intermediate) mechanism were proposed for NH₃-SCO over different types of catalysts. The details of the above-mentioned reaction mechanisms can be found in previous review articles (e.g., [7,14]). Additionally, due to lack of identification of the characteristic intermediates of the aforementioned three mechanisms, recently Wang et al. [17] proposed a N₂⁻ mechanism of NH₃-SCO on a Ag/nano-Al₂O₃. Li et al. [18] postulated an Eley–Ridel mechanism over perovskite-based catalysts, where gaseous NH₃ reacts with adsorbed -ONH₂ species to form the surface diazo species (-N=N-) with the rate-determining step depending on the catalysts composition.

Recent interest focuses on bifunctional catalysts consisting of noble metal-based catalyst and transition metal-based catalyst. Previous review articles, including ammonia oxidation, give a clear statement about high activity, N₂ selectivity, and stability over Cu-containing materials (Jabłońska et al., 2016, Jabłońska, 2020) [7,19]. From the noble metal-based catalysts the most frequently applied—Pt/Al₂O₃, provides high activity but also significant selectivity to byproducts (N₂O and NO_x). Besides Pt/Al₂O₃, also the Ag/Al₂O₃ catalyst has received extensive concerns on the low-temperature NH₃-SCO. Other systems were less investigated for NH₃-SCO than Pt/Al₂O₃ or Ag/Al₂O₃. Thus, the present mini-review aims to provide a broad picture of the property-activity correlations of noble metal-based catalysts investigated for NH₃-SCO up to now (including Pt-, Pd-, Ag- and Au-, Ru-based catalysts). The NH₃-SCO catalysts are classified considering their full NH₃ conversion and N₂ selectivity in the same temperature range. If not provided in the references, the catalytic activity and N₂ selectivity were roughly estimated based on NH₃ conversion-temperature profiles. This overview will shed light on future research directions regarding catalyst composition and architecture that maximizes the oxidation of NH₃ into N₂ over a broad temperature range and in the presence of the typical components of exhaust, such as H₂O, SO_x, and CO_x.

2. Pt- and Pd-Based Catalysts

Early work on NH₃ oxidation was given by Il’chenko et al. [20] Among metal-based catalysts, Pt and Pd are the most active for the ammonia oxidation (p(NH₃) = 0.1 atm, p(O₂) = 0.9 atm) but also the most selective to N₂O—according to the following order: Pt, Pd > Ni > Fe > W > Ti (note that Rh was not mentioned in Il’chenko review). The catalytic ammonia oxidation over platinum is a key step in the industrial manufacturing of nitric acid. The ammonia oxidation exhibits a moderate structure sensitivity, while the activity decreased in the order of: Pt foil > Pt(865) > Pt(533) > Pt(443) > Pt(100) due to different oxygen sticking coefficient [21]. Novell–Leruth et al. [22] used periodic slab density functional theory (DFT) calculations and found that NH₃ adsorbs preferentially on the top sites, NH₂ (dehydrogenation intermediates) on the bridge sites, while NH and N species on the hollow sites on both the (111) and (100) surfaces. The ammonia oxidation with atomic or molecular oxygen over Pt(100), Pt(111), stepped Pt(111)/Pt(211) or terrace Pt(111) orientations, etc., yields N₂, NO, N₂O, and H₂O, in varying amounts depending on reactant conditions. Steady-state reaction studies [23] under ultra-high vacuum (UHV) conditions with a stepped Pt(111) surface revealed that excess NH₃ lead to N₂ formation, while under excess O₂, NO formation was preferred. No other nitrogen-containing products, i.e., N₂O were detected in the gas phase (note that N₂O was never observed under UHV conditions [24]). Similar conclusions were given by Pérez–Ramirez et al. [25], who studied the sequence of steps in NH₃ oxidation (applying the isotope ¹⁵NH₃) over Pt, Pd, and Rh wires in the temporal analysis of products (TAP) reactor at relevant temperatures in industrial ammonia burners. High NO selectivity is favored at a high ratio of adsorbed n(O)/n(NH_x) species, e.g., at c(O₂)/c(¹⁵NH₃) = 0.1, the ¹⁵NO selectivity over Pt reached 45%, while at c(O₂)/c(¹⁵NH₃) = 10—ca. 100% selectivity. NO was found to be a primary reaction product in NH₃ oxidation, while N₂ and N₂O originate from consecutive NO transformations. Pd and Rh were more active for the reduction of nitric oxide by ammonia than Pt (Figure 1). Additionally, DFT calculations showed that the N₂O formation over Rh(100) plane needs higher activation energy than over Pt(100) or Pd(100). Furthermore, Rh(100) was more active in NH₃ decomposition (possessed a lower activation barrier for the NH₃ → NH₂ step) than Pt and Pd surfaces, and strongly stabilized the dehydrogenated NH and N species [22].

γ-Al₂O₃ and ZSM-5 are often used as the supports for noble metal-based catalysts for the selective catalytic oxidation of NH₃. A summary of Pt-, Pd-based catalysts is presented in Table 1. Pt/Al₂O₃ is usually applied to provide high low-temperature activity. The main drawback of such catalyst is low N₂ selectivity due to the formation of N₂O (below 250 °C) and NO_x (above 250 °C). The metallic Pt is significantly more active for NH₃-SCO than oxidized platinum [26,27], which provide limited sites for O₂ dissociation [28]. The operando XANES/EXAFS studies revealed the highest N₂ selectivity (ca. 80%) over H₂-reduced (2 wt.%)Pt/Al₂O₃. Nevertheless, under reaction conditions, at least 40% of Pt surface remains oxidized resulting in the formation of N₂O [29]. Otherwise, the metal-support interactions of Pt/TiO₂ were reported to stabilize Pt in the metallic state (also under reaction conditions) [30]. The Pt⁰ content can also be manipulated by preparation procedure of Pt/SiO₂-Al₂O₃, i.e., by adding ascorbic acid (vitamin C, vC; n(vC)/n(Pt) = 0.25–1.5) [31]. Ostermaier et al. [26] reported that small Pt particles (2.0 and 2.7 nm) of (1–2.93 wt.%)Pt/Al₂O₃ demonstrate lower activity in comparison to larger crystallites (15.5 nm). The catalysts with a small size of Pt⁰ crystallites were characterized by the strongest deactivation during NH₃ oxidation due to their oxidation to PtO_x, where x depends on the particle size [32]. Later, Sobczyk et al. [33,34] demonstrated with positron emission profiling (PEP) that the catalysts deactivate due to poisoning of the surface mainly by nitrogen species (NH and NH₂).

The dispersion of Pt species was increased after the introduction of ethylenediamine (from 2.8 to 2.0 nm) during the preparation of (1 wt.%)Pt/SiO₂-Al₂O₃ [35], and thus lead to higher activity in NH₃-SCO in the presence of CO₂ and H₂O, compared to that of unmodified samples. The N₂ selectivity remained nearly unaffected by the Pt particle size. Contrary to these studies, Slavinskaya et al. [36] found that a larger Pt particle size (ca. ~23 nm compared to ~1 nm) of (2 wt.%)Pt/Al₂O₃ enhanced activity. Additional measurements in the presence of CO₂ and H₂O did not change the trends of activity and selectivity of Pt/Al₂O₃ on the Pt dispersion and Pt state. Similar to the above discussed studies over Pt/SiO₂-Al₂O₃, N₂ selectivity did not depend on the Pt particle size, while in all cases, N₂ selectivity was below 70% (>300 °C). Furthermore, authors [27] showed no deactivation of the catalysts, i.e., the oxidation state of platinum in Pt/Al₂O₃ did not increase after the catalytic experiments. Also, the hydrothermal aging (in a feed containing O₂, H₂O, CO₂, at 550 °C over 250 h) of a Pt/Al₂O₃ washcoated monolith did not influence its activity below 250 °C [37]. Above 300 °C the activity significantly decreased with aging time (0, 122, 253 h) but the product selectivity remains the same. Recently, Machida et al. [38] found that a thin-film catalyst, which was prepared by deposing a nanoscale-thickness Pt(111) overlayer on a 50 μm-thick Fe-Cr-Al metal foil (Pt/SUS) achieved more than 180-fold higher TOF compared with the conventional (0.13 wt.%)Pt/Al₂O₃. The thermal stability of Pt/SUS was enhanced by the insertion of the Zr layer between the Pt and SUS foil. For the Pt surfaces, the NH mechanism was mostly proposed by experimental and DFT simulation studies; e.g., over Pt(100) or Pt(111) [39,40,41]. Also, the so-called NH mechanism (i.e., NH as the intermediates in the imide mechanism) occurred on Pt/Al₂O₃, while the HNO and N₂H₄ mechanism (i.e., HNO and N₂H₄ as the intermediates in the imide and hydrazine mechanism, respectively) coexisted on Pt/CeZrO₂ (Figure 2) [42].

Li and Armor [43] studied a series of zeolite ZSM-5 ion-exchanged with (4.07 wt.%)Pd, (2.66 wt.%)Rh, and (2.55 wt.%)Pt as catalysts for NH₃-SCO. Among them, relatively high activity and N₂ selectivity in the low-temperature range (≤300 °C) were found for the Pd-containing catalysts (i.e., full ammonia conversion with 91% N₂ selectivity at 300 °C in the presence of 5 vol.% H₂O). 58–61% of N₂O selectivity on (2.55 wt.%)Pt-ZSM-5 and 16–25% on (4.01 wt.%)Pd-ZSM-5 was obtained at 250–300 °C. The noble metal-exchanged ZSM-5 materials were less affected by water vapor than the corresponding Al₂O₃ supported catalysts. Similar results, i.e., 92% NH₃ conversion and 73% N₂ selectivity at 350 °C over (5.51 wt.%)Pd-ZSM-5 in NH₃-SCO (feed without H₂O), were also reported by Long and Yang [44]. Furthermore, Jabłońska et al. [45] investigated zeolites HY modified with palladium (0.05–2.5 wt.%). An increase in Pd loading leads to higher catalysts activity together with the drop in N₂ selectivity. The palladium oxide species (PdO_x) were found to be active sites for ammonia oxidation (based on FT-IR studies). A part of ammonia was stabilized against oxidation over the zeolite framework acid sites (in the form of NH₄⁺), and lead to enhanced N₂ selectivity in the higher temperature range. The analysis of the results of temperature-programmed (NH₃-SCO with various spaces velocity) and spectroscopic studies lead to the conclusion that the ammonia oxidation over Pd-Y followed the i-SCR mechanism. On the other hand, an appearance of hydrazine species (intermediates in the hydrazine mechanism) on the ammonia pre-adsorbed Pd-Y catalyst at 250 °C during FT-IR studies, suggested that the ammonia oxidation is more complicated and followed different parallel routes. Otherwise, Wells et al. [46,47] identified PdN_x under reaction conditions over H₂-reduced (1.5 wt.%)Pd/Al₂O₃ and Pd/Y (n(Si)/n(Al) = 2.6) as the dominant species during N₂ formation (based on combined operando spectroscopy and DFT calculations). As stated above, palladium-containing materials appear as promising NH₃-SCO catalysts, although, their stability (also in the presence of H₂O, SO_x, and CO_x) needs confirmation.

The state-of-the-art NH₃-SCO systems include a combination of a noble metal-based catalyst—usually Pt/Al₂O₃, and an SCR catalyst, e.g., Cu- or Fe-containing zeolite. Thus, a part of ammonia is oxidized over the noble metal-based catalyst to N₂ and NO_x which is further transformed to N₂ over the SCR catalyst. Different arrangements of both metals were reported in the literature, i.e., noble/transition metal deposited on one support in systems such as (0.05 wt.%)Pt/(1 wt.%)Cu/Al₂O₃ [48], (0.5–4 wt.%)Pt/(20 wt.%)Cu/Al₂O₃ [49], (1 wt.%)Pt/(20 wt.%)Cu/Al₂O₃ [50,51,52], (1.5 wt.%)Pt-(5.5 wt.%)Cu/ZSM-5 [53], (0.5 wt.%)Pt-(1.54 wt.%)Fe-ZSM-5 [54], (1.5 wt.%)Pt-(0.5 wt.%)Fe/ZSM-5 [55], (0.21 wt.%)Pt/CuMgAlO_x hydrotalcite-derived mixed metal oxides [13], (2 wt.%)PdO/(5 wt.%)CuO/Al₂O₃ [56], etc. Besides such form, the active components may be present in different configurations—dual-layer configuration, mixed and hybrid layer sample types, that are presented in Figure 3. The dual-layer catalytic systems consist of noble metal-based catalyst as a bottom layer, and transition metal-based catalysts as an upper layer, i.e., Pt/Al₂O₃ and Cu-ZSM-5, Pt/Al₂O₃ and Cu-SSZ-13, Pt/Al₂O₃, and Fe-ZSM-5 investigated by Shrestha et al. [57,58,59], Pt/Al₂O₃ and Fe-zeolite investigated by Scheuer et al. [60] and Colombo et al. [61]. E.g., Shrestha et al. [57] pointed out the increase of N₂ selectivity (with a corresponding decrease in NO selectivity) with increasing copper loading of Cu/ZSM-5 (NH₃-SCR layer), e.g., from 58% to 82% at 250 °C for 0.8 and 2.5 wt.% of Cu, respectively. Still, N₂O selectivity reached a maximum of about 40% at 260 °C in all cases. Similar N₂O selectivity was reported over Pt/Al₂O₃ and Fe-ZSM-5 arranged as both dual-layer and mixed catalysts [58]. Also, the NH₃ oxidation depended on the applied conditions, i.e., space velocity (66,000 versus 265,000 h⁻¹). At higher space velocity mixed catalyst revealed (ca. 7%) higher NH₃ conversion, while dual-layer catalyst provided higher N₂ selectivity (especially above 350 °C). The hybrid catalyst (bottom layer of mixed Cu-SSZ-13 + Pt/Al₂O₃, top layer of Cu-SSZ-13) allowed to achieve (ca. 5%) higher NH₃ conversion than that of the dual-layer catalyst [59]. Furthermore, Dhillon et al. [62] applied sacrificial agents (yeast or polymer) to generate macropores on a (2.90 wt.%)Cu-SSZ-13 top-layer washcoat supported on (1.47 wt.%)Pt/Al₂O₃, and thus to enhance NH₃ conversion (still below 90% up to 500 °C) without impact on N₂ selectivity. Recently, Gosh et al. [63] reported a Pt/Al₂O₃@Cu-ZSM-5 (0.05 wt.% Pt, 2.93 wt.% Cu) core-shell catalyst that allowed full NH₃ conversion at ca. 300 °C and 100% N₂ selectivity (up to 275 °C). The NH₃ conversion was negligibly affected by the variation in the shell thickness (0.5 µm versus 1.2 µm), while the ticker shell was beneficial in improving N₂ selectivity at higher temperatures. The addition of H₂O (5 vol.%) to the feed had a minor impact on the catalyst activity, while more tests in the presence of CO_x and SO_x are still required.

Concluding this part, as can be seen from the above presented data, a great variety of Pt- and Pd-based catalytic systems were developed and tested for NH₃-SCO. In particular, (1–2 wt.%)Pt/Al₂O₃ was reported as one of the most active catalyst that allowed to obtain full NH₃ conversion around 200–450 °C with N₂ selectivity of 15–87% (according to data gathered in Table 1), depending on the applied catalyst preparation procedure as well as pretreatment and reaction conditions. Pt⁰ serves as the active species responsible for high catalytic activity, and thus most of the catalytic systems were prepared by an impregnation method and subsequently reduced in H₂. Great research efforts aimed at determining the role of Pt dispersion in NH₃-SCO, but there is no clear consensus about it yet. Platinum (0.05–1.5 wt.%) was also applied as the most active noble metal component in the bifunctional catalysts. Such systems fully oxidized NH₃ in a broad temperature range of 195–500 °C with 44–100% N₂ selectivity (according to data gathered in Table 1) depending on their architecture, i.e., Pt deposited on one support with transition metal component, as dual-layer catalysts, mixed form, hybrid catalysts or incorporated in the core-shell structure, etc. Among them the (0.46 wt.%)Pt/Al₂O₃-(2.5 wt.%)Cu-ZSM-5 dual-layer catalyst (full NH₃ conversion at 250–500 °C, 82–100% N₂ selectivity) and (0.05 wt.%)Pt/Al₂O₃@(2.93 wt.%)Cu/ZSM-5 core-shell catalyst (full NH₃ conversion at 310–500 °C, 91–94% N₂ selectivity) appear as the most interesting systems for further catalysts optimization. However, the stability of these catalysts and the influence of the potential catalyst pollutants usually present in the exhausts, i.e., H₂O, SO_x and CO_x, were not fully provided within the scope of the studies. A similar conclusion can be given for other Pt- or Pd-containing catalysts (only a few materials were tested in the presence of H₂O and CO₂). Hence, further studies are required to understand structure-activity relationships and reaction mechanisms under application-relevant reaction conditions.

Figure 3. Schematic diagram representing bifunctional ammonia slip catalyst in three different washcoated structured methodologies. Reprinted from [64] with permission from Science Direct.

Figure 3. Schematic diagram representing bifunctional ammonia slip catalyst in three different washcoated structured methodologies. Reprinted from [64] with permission from Science Direct.

Table 1. Comparison of full NH₃ conversion and N₂ selectivity in same temperature range over Pt-based catalysts reported in literature.

Catalyst	Catalyst Preparation	Reaction Conditions	NH3 Conversion_N2 Selectivity/% (Temperature/°C)	Ref.
(3 wt.%)Pt-Rh	wash-coated on Al2O3, calcination, 400 °C, air	0.08 vol.% NH3, 4 vol.% O2, He balance, GHSV 92,000 h−1	100/62% (400 °C)	[65]
(4.4 wt.%)Pt-Rh	wash-coated on Al2O3, calcination, 500 °C, air	0.1 vol.% NH3, 4 vol.% O2, He balance, GHSV 92,000 h−1	100/80% (400 °C)	[66]
(1.2 wt.%)Pt/Al2O3	impregnation, calcination, 600 °C, air; reduction conditions not shown)	0.1 vol.% NH3, 10 vol.% O2, He balance, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/75% (200 °C)	[67]
		1.14 vol.% NH3, 8.21 vol.% O2, He balance, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/87% (200 °C)
(1.73 wt.%)Pt/Al2O3	impregnation, calcination, 400 °C, air; reduction, 250 °C, H2	0.1 vol.% NH3, 4 vol.% O2, He balance, 500 mL min−1, mass of the catalyst: 0.145 g, GHSV 120,000 h−1	100/40–60% (200–400 °C)	[27]
(2 wt.%)Pt/Al2O3	impregnation, calcination, 400 °C, air; oxidation, 400 °C, O2/He	0.1 vol.% NH3, 4 vol.% O2, He balance, 500 mL min−1, mass of the catalyst: 0.145 g, GHSV 120,000 h−1	100/40–50% (325–400 °C)	[36]
	impregnation, calcination, 400 °C, air; reduction, 350 °C, H2; calcination, 400 °C, Ar		100/30–60% (250–400 °C)
(1 wt.%)Pt/Al2O3	impregnation, calcination, 550 °C, air	0.02 vol.% NH3, 10 vol.% O2, N2 balance, 2.31 l min−1, mass of the catalyst: 0.34 g, WHSV 407,000 mL g−1 h−1	100/25–49% (250–450 °C)	[68]
(1 wt.%)Pt/CeO2-SiO2			100/15–42% (225–450 °C)
(1 wt.%)Pt/SiO2-Al2O3	impregnation, calcination, 550 °C, air; monolithic catalyst	0.02 vol.% NH3, 10 vol.% O2, 8 vol.% CO2, 5 vol.% H2O, N2 balance, GHSV 100,000 h−1	100/10–40% (300–450 °C)	[35]
(1 wt.%)Pt/SiO2-Al2O3	impregnation; treatment strategies—conditions not shown; monolithic catalyst; Vc—ascorbic acid	0.02 vol.% NH3, 10 vol.% O2, 8 vol.% CO2, 5 vol.% H2O, N2 balance, GHSV 100,000 h−1	100/28–60% (240–300 °C)	[31]
(1 wt.%)Pt/SiO2-Al2O3-vC			100/22–50% (240–300 °C)
(1 wt.%)Pt/Al2O3	impregnation, calcination, 550 °C, air; monolithic catalyst	0.02 vol.% NH3, 8 vol.% O2, N2 balance, GHSV 100,000 h−1	100/15–30% (300–400 °C)	[42]
(1 wt.%)Pt/CeZrO2			100/20–45% (325–400 °C)
(1.5 wt.%)Pt/ZrO2	impregnation, calcination, 550 °C, air	0.018 vol.% NH3, 8 vol.% O2, N2 balance, GHSV 100,000 h−1	100/25–60% (350–500 °C)	[53]
(1.5 wt.%)Pt-(5 wt.%)W/ZrO2			100/28–50% (300–500 °C)
(2.0 wt.%)Pt/TiO2	impregnation, calcination, 400 °C, air; oxidation, 400 °C, O2/He	0.1 vol.% NH3, 4.0 vol.% O2, He balance, 500 mL min−1, mass of the catalyst: 0.145 g, WHSV 206,897 mL h−1 g−1	100/38–55% (200–400 °C)	[30]
	pulsed laser ablation in liquids, calcination, 400 °C, air oxidation, 400 °C, O2/He		100/22–50% (175–400 °C)
(0.1 wt.%)Pt/(2 wt.%)V/TiO2	impregnation, reduction, 600 °C, H2/N2	0.02 vol.% NH3, 8.0 vol.% O2, 6.0 vol.% H2O, He balance, 500 mL min−1, mass of the catalyst: 0.25 g, GHSV 60,000 h−1	100/63–81% (250–350 °C)	[69]
(1.2 wt.%)Pd/Al2O3	impregnation, calcination, 600 °C, air; reduction—conditions not shown	1.14 vol.% NH3, 8.21 vol.% O2, He balance, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/98% (300 °C)	[67]
(4.2 wt.%)PdO/Al2O3	impregnation, calcination, 500 °C, air**reduction, 400 °C, H2/He	0.1 vol.% NH3, 4.0 vol.% O2, He balance, * 0.1 vol.% NH3, 4.0 vol.% O2, 5 vol.% H2O, He balance, 100 mL min−1, mass of the catalyst: 0.1 g, WHSV 60,000 mL h−1 g−1	100/67% (350 °C) ** 100/86% (300 °C)	[43]
(4.07 wt.%)Pd-ZSM-5	ion-exchange, calcination—conditions not shown		* 100/91% (300 °C)
(1.5 wt.%)Pd/Y	impregnation, calcination—conditions not shown)	0.5 vol.% NH3, 2.5 vol.% O2, He balance, 40 mL min−1, mass of the catalyst: 0.05 g, GHSV 15,400 h−1	100/80–90% (250–500 °C)	[45]
(0.05 wt.%)Pt/(1 wt.%)CuO/Al2O3	impregnation, calcination air, 600 °C; impregnation, calcination air, 500 °C	0.5 vol.% NH3, 2.5 vol.% O2, He balance, 40 mL min−1, mass of the catalyst: 0.05 g, GHSV 15,400 h−1	100/44–73% (325–500 °C)	[48]
(1 wt.%)Pt/(20 wt.%)CuO/Al2O3	impregnation, calcination air, 500 °C; impregnation, calcination air, 450 °C	0.07 vol.% NH3, 0.5 vol.% O2, He balance *0.07 vol.% NH3, 8 vol.% O2, He balance 1000 mL min−1, WHSV 180,000 mL h−1 g−1	100/88 (210–230 °C) * 100/95 (230 °C)	[49]
(4 wt.%)Pt/(20 wt.%)CuO/Al2O3			100/83 (195–230 °C) * 100/90 (220–230 °C)
(1.5 wt.%)Pt-(0.5 wt.%)Fe/ZSM-5	ion-exchange, calcination, air, 500 °C, impregnation calcination, air, 500 °C	0.1 vol.% NH3, 2 vol.% O2, He balance, 500 mL min−1, WHSV 500,000 mL h−1 g−1	100/61–88% (200–300 °C)	[55]
(0.5 wt.%)Pt-(1.54 wt.%)Fe/ZSM-5	ion-exchange, calcination, air, 500 °C, impregnation calcination, air, 500 °C	0.1 vol.% NH3, 2 vol.% O2, He balance, 500 mL min−1, GHSV 230,000 h−1	100/77–89% (250–400 °C)	[54]
(1.5 wt.%)Pt-(5.5 wt.%)Cu/ZSM-5	impregnation calcination, air, 550 °C	0.018 vol.% NH3, 8 vol.% O2, N2 balance, GHSV 100,000 h−1	100/56–73% (275–450 °C)	[53]
(0.21 wt.%)Pt/CuMgAlOx	impregnation, calcination, air, 500 °C	0.5 vol.% NH3, 2.5 vol.% O2, He balance, 40 mL min−1, mass of the catalyst: 0.05 g, GHSV 15,400 h−1	100/67–89% (350–500 °C)	[13]
(0.21 wt.%)Pd/CuMgAlOx			100/71–76% (425–500 °C)
(0.46 wt.%)Pt/Al2O3-(0.8 wt.%)Cu-ZSM-5	Pt/Al2O3: impregnation, calcination, 500 °C, air; reduction, 500 °C, H2/Ar; Cu-ZSM-5: preparation not provided; oxidation, 650 °C, O2/Ar; monolithic catalyst *dual layer catalyst	0.05 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 66,000 h−1	* 100/58–74% (250–500 °C)	[57]
(0.46 wt.%)Pt/Al2O3-(2.5 wt.%)Cu-ZSM-5			* 100/82–100% (250–500 °C)
(0.46 wt.%)Pt/Al2O3-Fe-ZSM-5	Pt/Al2O3: impregnation, calcination, 500 °C, air, reduction, 500 °C, H2/Ar, oxidation, 650 °C, O2/Ar; Fe-ZSM-5: commercial; oxidation, 650 °C, O2/Ar; monolithic catalyst *dual layer catalyst **mixed catalyst	0.05 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 66,000 h−1	* 100/48–93% (250–500 °C) ** 100/57–82% (250–500 °C)	[58]
(0.05 wt.%)Pt/Al2O3@(2.93 wt.%)Cu/ZSM-5	core-shell catalyst, Pt/Al2O3: impregnation, calcination, 550 °C, air; Cu-ZSM-5: ion-exchange, calcination, 500 °C, air	0.05 vol.% NH3, 5 vol.% O2, Ar balance, 100 mL min−1, mass of the catalyst: 0.18 g, GHSV 280,000 h−1	100/91–94% (310–500 °C)	[63]

Table 1. Comparison of full NH₃ conversion and N₂ selectivity in same temperature range over Pt-based catalysts reported in literature.

Catalyst	Catalyst Preparation	Reaction Conditions	NH3 Conversion_N2 Selectivity/% (Temperature/°C)	Ref.
(3 wt.%)Pt-Rh	wash-coated on Al2O3, calcination, 400 °C, air	0.08 vol.% NH3, 4 vol.% O2, He balance, GHSV 92,000 h−1	100/62% (400 °C)	[65]
(4.4 wt.%)Pt-Rh	wash-coated on Al2O3, calcination, 500 °C, air	0.1 vol.% NH3, 4 vol.% O2, He balance, GHSV 92,000 h−1	100/80% (400 °C)	[66]
(1.2 wt.%)Pt/Al2O3	impregnation, calcination, 600 °C, air; reduction conditions not shown)	0.1 vol.% NH3, 10 vol.% O2, He balance, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/75% (200 °C)	[67]
		1.14 vol.% NH3, 8.21 vol.% O2, He balance, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/87% (200 °C)
(1.73 wt.%)Pt/Al2O3	impregnation, calcination, 400 °C, air; reduction, 250 °C, H2	0.1 vol.% NH3, 4 vol.% O2, He balance, 500 mL min−1, mass of the catalyst: 0.145 g, GHSV 120,000 h−1	100/40–60% (200–400 °C)	[27]
(2 wt.%)Pt/Al2O3	impregnation, calcination, 400 °C, air; oxidation, 400 °C, O2/He	0.1 vol.% NH3, 4 vol.% O2, He balance, 500 mL min−1, mass of the catalyst: 0.145 g, GHSV 120,000 h−1	100/40–50% (325–400 °C)	[36]
	impregnation, calcination, 400 °C, air; reduction, 350 °C, H2; calcination, 400 °C, Ar		100/30–60% (250–400 °C)
(1 wt.%)Pt/Al2O3	impregnation, calcination, 550 °C, air	0.02 vol.% NH3, 10 vol.% O2, N2 balance, 2.31 l min−1, mass of the catalyst: 0.34 g, WHSV 407,000 mL g−1 h−1	100/25–49% (250–450 °C)	[68]
(1 wt.%)Pt/CeO2-SiO2			100/15–42% (225–450 °C)
(1 wt.%)Pt/SiO2-Al2O3	impregnation, calcination, 550 °C, air; monolithic catalyst	0.02 vol.% NH3, 10 vol.% O2, 8 vol.% CO2, 5 vol.% H2O, N2 balance, GHSV 100,000 h−1	100/10–40% (300–450 °C)	[35]
(1 wt.%)Pt/SiO2-Al2O3	impregnation; treatment strategies—conditions not shown; monolithic catalyst; Vc—ascorbic acid	0.02 vol.% NH3, 10 vol.% O2, 8 vol.% CO2, 5 vol.% H2O, N2 balance, GHSV 100,000 h−1	100/28–60% (240–300 °C)	[31]
(1 wt.%)Pt/SiO2-Al2O3-vC			100/22–50% (240–300 °C)
(1 wt.%)Pt/Al2O3	impregnation, calcination, 550 °C, air; monolithic catalyst	0.02 vol.% NH3, 8 vol.% O2, N2 balance, GHSV 100,000 h−1	100/15–30% (300–400 °C)	[42]
(1 wt.%)Pt/CeZrO2			100/20–45% (325–400 °C)
(1.5 wt.%)Pt/ZrO2	impregnation, calcination, 550 °C, air	0.018 vol.% NH3, 8 vol.% O2, N2 balance, GHSV 100,000 h−1	100/25–60% (350–500 °C)	[53]
(1.5 wt.%)Pt-(5 wt.%)W/ZrO2			100/28–50% (300–500 °C)
(2.0 wt.%)Pt/TiO2	impregnation, calcination, 400 °C, air; oxidation, 400 °C, O2/He	0.1 vol.% NH3, 4.0 vol.% O2, He balance, 500 mL min−1, mass of the catalyst: 0.145 g, WHSV 206,897 mL h−1 g−1	100/38–55% (200–400 °C)	[30]
	pulsed laser ablation in liquids, calcination, 400 °C, air oxidation, 400 °C, O2/He		100/22–50% (175–400 °C)
(0.1 wt.%)Pt/(2 wt.%)V/TiO2	impregnation, reduction, 600 °C, H2/N2	0.02 vol.% NH3, 8.0 vol.% O2, 6.0 vol.% H2O, He balance, 500 mL min−1, mass of the catalyst: 0.25 g, GHSV 60,000 h−1	100/63–81% (250–350 °C)	[69]
(1.2 wt.%)Pd/Al2O3	impregnation, calcination, 600 °C, air; reduction—conditions not shown	1.14 vol.% NH3, 8.21 vol.% O2, He balance, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/98% (300 °C)	[67]
(4.2 wt.%)PdO/Al2O3	impregnation, calcination, 500 °C, air**reduction, 400 °C, H2/He	0.1 vol.% NH3, 4.0 vol.% O2, He balance, * 0.1 vol.% NH3, 4.0 vol.% O2, 5 vol.% H2O, He balance, 100 mL min−1, mass of the catalyst: 0.1 g, WHSV 60,000 mL h−1 g−1	100/67% (350 °C) ** 100/86% (300 °C)	[43]
(4.07 wt.%)Pd-ZSM-5	ion-exchange, calcination—conditions not shown		* 100/91% (300 °C)
(1.5 wt.%)Pd/Y	impregnation, calcination—conditions not shown)	0.5 vol.% NH3, 2.5 vol.% O2, He balance, 40 mL min−1, mass of the catalyst: 0.05 g, GHSV 15,400 h−1	100/80–90% (250–500 °C)	[45]
(0.05 wt.%)Pt/(1 wt.%)CuO/Al2O3	impregnation, calcination air, 600 °C; impregnation, calcination air, 500 °C	0.5 vol.% NH3, 2.5 vol.% O2, He balance, 40 mL min−1, mass of the catalyst: 0.05 g, GHSV 15,400 h−1	100/44–73% (325–500 °C)	[48]
(1 wt.%)Pt/(20 wt.%)CuO/Al2O3	impregnation, calcination air, 500 °C; impregnation, calcination air, 450 °C	0.07 vol.% NH3, 0.5 vol.% O2, He balance *0.07 vol.% NH3, 8 vol.% O2, He balance 1000 mL min−1, WHSV 180,000 mL h−1 g−1	100/88 (210–230 °C) * 100/95 (230 °C)	[49]
(4 wt.%)Pt/(20 wt.%)CuO/Al2O3			100/83 (195–230 °C) * 100/90 (220–230 °C)
(1.5 wt.%)Pt-(0.5 wt.%)Fe/ZSM-5	ion-exchange, calcination, air, 500 °C, impregnation calcination, air, 500 °C	0.1 vol.% NH3, 2 vol.% O2, He balance, 500 mL min−1, WHSV 500,000 mL h−1 g−1	100/61–88% (200–300 °C)	[55]
(0.5 wt.%)Pt-(1.54 wt.%)Fe/ZSM-5	ion-exchange, calcination, air, 500 °C, impregnation calcination, air, 500 °C	0.1 vol.% NH3, 2 vol.% O2, He balance, 500 mL min−1, GHSV 230,000 h−1	100/77–89% (250–400 °C)	[54]
(1.5 wt.%)Pt-(5.5 wt.%)Cu/ZSM-5	impregnation calcination, air, 550 °C	0.018 vol.% NH3, 8 vol.% O2, N2 balance, GHSV 100,000 h−1	100/56–73% (275–450 °C)	[53]
(0.21 wt.%)Pt/CuMgAlOx	impregnation, calcination, air, 500 °C	0.5 vol.% NH3, 2.5 vol.% O2, He balance, 40 mL min−1, mass of the catalyst: 0.05 g, GHSV 15,400 h−1	100/67–89% (350–500 °C)	[13]
(0.21 wt.%)Pd/CuMgAlOx			100/71–76% (425–500 °C)
(0.46 wt.%)Pt/Al2O3-(0.8 wt.%)Cu-ZSM-5	Pt/Al2O3: impregnation, calcination, 500 °C, air; reduction, 500 °C, H2/Ar; Cu-ZSM-5: preparation not provided; oxidation, 650 °C, O2/Ar; monolithic catalyst *dual layer catalyst	0.05 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 66,000 h−1	* 100/58–74% (250–500 °C)	[57]
(0.46 wt.%)Pt/Al2O3-(2.5 wt.%)Cu-ZSM-5			* 100/82–100% (250–500 °C)
(0.46 wt.%)Pt/Al2O3-Fe-ZSM-5	Pt/Al2O3: impregnation, calcination, 500 °C, air, reduction, 500 °C, H2/Ar, oxidation, 650 °C, O2/Ar; Fe-ZSM-5: commercial; oxidation, 650 °C, O2/Ar; monolithic catalyst *dual layer catalyst **mixed catalyst	0.05 vol.% NH3, 5 vol.% O2, Ar balance, GHSV 66,000 h−1	* 100/48–93% (250–500 °C) ** 100/57–82% (250–500 °C)	[58]
(0.05 wt.%)Pt/Al2O3@(2.93 wt.%)Cu/ZSM-5	core-shell catalyst, Pt/Al2O3: impregnation, calcination, 550 °C, air; Cu-ZSM-5: ion-exchange, calcination, 500 °C, air	0.05 vol.% NH3, 5 vol.% O2, Ar balance, 100 mL min−1, mass of the catalyst: 0.18 g, GHSV 280,000 h−1	100/91–94% (310–500 °C)	[63]

3. Ag-Based Catalysts

Il’chenko et al. [20,70] reported that the specific activity of metal Ag at 300 °C was lower than that of Pt and Pd. Among silver-based catalysts, γ-Al₂O₃ impregnated with Ag species (mainly 10 wt.%) was widely investigated. Depending on the applied conditions, i.e., catalyst (its preparation, pre-treatment strategies, etc.) and reaction conditions, the full NH₃ conversion can be reached in the range of 150–400 °C with 45–95% N₂ selectivity over (10 wt.%)Ag/Al₂O₃ (according to data gathered in Table 2). However, at temperatures above 300 °C N₂ selectivity dropped due to the large NO production. For NH₃-SCO, the Ag/Al₂O₃ catalysts are mainly applied after H₂ pretreatment. E.g., Gang et al. [71,72] reported extremely high activity of Ag/Al₂O₃ at 160 °C (full NH₃ conversion with N₂ selectivity of about 82%), which was even superior to H₂-reduced Ir/Al₂O₃ or Pt/Al₂O₃. The activity of Ag/Al₂O₃ was also higher than over silver powder and Ag/SiO₂ [72], indicating that the applied support influenced the Ag particle dispersion. However, the difference in the Ag particle size of the Ag/Al₂O₃ (8.2 nm) and Ag/SiO₂ (24 nm) catalysts was not discussed in these studies. The authors correlated the NH₃ oxidation activity at low temperatures to the catalysts’ ability to promote dissociative or nondissociative adsorption of O₂. However, again the role of different oxygen species (i.e., adsorbed molecular oxygen, adsorbed atomic oxygen, subsurface oxygen, and bulk dissolved oxygen) in the activity and the reaction mechanisms was not fully explored. Zhang and He [73] reported that the dissociation of O₂ is a rate-determining step for NH₃-SCO. They concluded that molecular O₂ can be dissociatively chemisorbed on the surface of H₂-reduced Ag/Al₂O₃, i.e., metallic species (in contrast to fresh material) to form O species, and thus enhance NH₃-SCO activity [74]. Furthermore, the modification of Ag/Al₂O₃ with CeO₂ improved catalysts’ ability in the adsorption and activation of O₂ to form O species [75]. However, Wang et al. [76] claimed that the recovery of Brønsted acid sites via H₂ reduction (i.e., break of Ag-O bonds on the Ag/Al₂O₃ surface and formation of Ag clusters in the metallic state—Ag_n⁰, based on EXAFS analysis) is also responsible for the improved activity of H₂-reduced Ag/Al₂O₃. Highly dispersed particles of Ag⁰ (3.5–25 nm) were found to enhance the catalytic activity below 140 °C, whereas large particles (12–50 nm) of Ag⁰ were responsible for improved N₂ selectivity [74].

Furthermore, N₂ selectivity of about 85% above 300 °C over (2.2 wt.%)Ag/Al₂O₃ (c(NH₃):c(O₂) = 1:1–1:25), was assigned to the small Ag particle size (<5 nm, based on XRD analysis) [77]. Ag⁺ cations are the main active species in NH₃-SCO at temperatures above 140 °C. The adsorbed NH₃ mainly reacts with the gaseous O₂ over fresh Ag/Al₂O₃. Although besides Ag⁰ and Ag⁺, also Ag_n^δ+ species were evidenced by DR UV-Vis analysis, the authors did not specify their role in ammonia oxidation. Also, Qu et al. [78] obtained highly dispersed Ag⁰ particles with a size of 5–6 nm (based on XRD and DR UV-Vis analyses) on the H₂-reduced (10 wt.%) Ag/Al₂O₃ catalyst, which reached full conversion and 89% of N₂ selectivity at 180 °C. Yang et al. [79] and Jabłońska et al. [80] studied (1–10 wt.%)Ag/Al₂O₃ and claimed that the low N₂ selectivity above 200–300 °C over Ag/Al₂O₃ was ascribed to the formation of Ag₂O crystals.

Besides the influence of the Ag particle size, Wang et al. [17] investigated the effect of the different support particle size (micro-Al₂O₃ versus nano-Al₂O₃) on the activity of the final Ag/Al₂O₃ catalysts in NH₃-SCO. The catalyst characterization indicated that nano-Al₂O₃ was beneficial for Ag dispersion (the average Ag particle size of 3.7 nm, based on HRTEM analysis). The catalyst abundant acid sites (based on NH₃-TPD analysis) facilitated the adsorption and dissociation of NH₃, therefore resulting in an enhanced activity. Furthermore, the same research group [81] studied Ag supported on TiO₂, SiO₂ as well as their mixture—Ag/SiO₂-TiO₂. Although the (10 wt.%)Ag/SiO₂-TiO₂ catalyst reached full NH₃ conversion at 140 °C, N₂ selectivity in the whole studied range of 100–240 °C was below 70%. Significantly higher N₂ selectivity was obtained over Ag/TiO₂ (91–99% at 180–240 °C). Jabłońska et al. [82] compared commercial TiO₂ with mesoporous TiO₂ (prepared by evaporation induced self-assembly (EISA)) as support in NH₃-SCO. The activity and N₂ selectivity were favored over (1.5 wt.%)Ag-doped mesoporous TiO₂ (calcined at 600 °C, with predominant anatase phase). The easily reducible highly dispersed oxidized silver species were converted into Ag⁰ and possibly Ag_n^δ+ clusters through in situ H₂-pretreatment of catalyst. The metallic silver decomposed N₂O into N₂ and surface oxygen species, leading to higher N₂ selectivity. Further studies, concerning the stability tests, revealed that these materials are unstable, especially in the higher temperature range (>600 °C). However, temperature of the full conversion of NH₃ over (9.8 wt.%)Ag/Al₂O₃ also gradually increased after ca. 4 reaction cycles (from 150 to 250 °C). A higher stability in subsequent catalytic runs revealed (9.9 wt.%)Ag/ZSM-5 with the postsynthetic modified support. The micro-/mesoporous structure could prevent sintering and/or leaching of Ag particles during NH₃-SCO [83].

As stated above, while a broad number of studies examined NH₃-SCO over Ag-based catalysts, the mechanism of NH₃ oxidation and N₂ formation is still uncertain, and the studies are mainly based on temperature-programmed (TPD) or in situ DRIFTS studies. E.g., Gang et al. [84] investigated NH₃-SCO over powder silver by TPD and in situ FT-Raman spectroscopy. They found NO as the main reaction intermediate yielding N₂O and/or N₂ (Figure 4a). The dissociation of oxygen was believed to be the rate-controlling step for ammonia oxidation, while low surface coverage favors N₂ formation. Similar conclusions were given by Karatok et al. [85] The exposure of ozone on Ag(111) surfaces at −133 °C led to a disordered surface atomic oxygen overlayer (confirmed by LEED). Such oxygen species selectively catalyzed N-H bond cleavage, yielding mostly N₂ and minor amounts of by-products (NO and N₂O). Higher coverage O/Ag(111) surfaces at −133 °C led to bulk-like amorphous silver oxide species, forming NO and N₂O (Figure 4b). The ordered oxide surfaces—obtained through annealing of atomic oxygen-covered Ag(111) surface at 200 °C in UHV, showed only limited reactivity toward ammonia. Suppression of the N₂ formation at high oxygen coverages was also reported over Ir(510) and Ir(110) surfaces [86].

Zhang and He [73] investigated the reaction mechanisms over Ag/Al₂O₃ based on in situ DRIFTS studies and found that at low temperatures (<140 °C), NH₃ oxidation follows the -NH (imide) mechanism (Ag⁰ as the main active species), while above 140 °C, NH₃ oxidation follows an in situ selective catalytic reduction (i-SCR) mechanism (Ag⁺ as the main active species). Furthermore, they claimed that NH₃-SCO over Ag/nano-Al₂O₃ follows a reaction pathway called the N₂⁻ mechanism (based on in situ DRIFTS, kinetic measurements, and DFT calculation results) [17]. The intermediate N₂⁻ species appear from the combination of two -NH₂/NH species (considered to be the rate-determining step). In the next step, the N₂⁻ species are converted into N₂ and/or N₂O in the presence of O₂.

The activity and N₂ selectivity strongly depend on the loading of noble metal and can be steered into the desired direction by the introduction of transition metal. E.g., Yang et al. [79] studied Ag-Cu/Al₂O₃ with 5–5 or 10–10 wt.% of metal and indicated the material with the first composition as a highly efficient catalyst (full NH₃ conversion below 320 °C with N₂ selectivity of more than 95%). Unfortunately, the authors did not present results of catalytic tests above 350 °C. Gang et al. [71] investigated (7.5 wt.%)Ag-(2.5 wt.%)Cu/Al₂O₃ and found the full conversion of ammonia at 200–300 °C with 95% N₂ selectivity. Above 300 °C appeared significant amounts of by-products—NO and N₂O. A mechanical mixture of (10 wt.%)Ag/Al₂O₃ and (10 wt.%)Cu/Al₂O₃—applied for comparative purposes, showed comparable activity and N₂ selectivity to a silver-based catalyst. The same oxidation state for bimetallic (Ag-Cu/Al₂O₃ (5–5 wt.%, 7.5–2.5 wt.%) and monometallic (10 wt.% Ag/Al₂O₃ or 10 wt.% Cu/Al₂O₃) catalysts were approved (based on XPS analysis). Additionally, LEIS analysis over (10–2.5 wt.%, 2.5–7.5 wt.%, 5–5 wt.%, 9–1 wt.%) Ag-Cu/Al₂O₃ excluded formation of any Ag-Cu phases. Jabłońska et al. [80] found among all tested combinations—1–1, 1–10, 1.5–10, 5–5 wt.% of silver and copper, respectively, the (1.5 wt.%)Ag-(10 wt.%)Cu/Al₂O₃ catalyst with an optimum activity, N₂ selectivity (full ammonia conversion and 94% N₂ selectivity at 375 °C) and stability in NH₃-SCO under wet conditions and time-on-stream tests. (0.59–2.34 wt.%)Ag-promoted CuMgAlO_x hydrotalcite derived mixed metal oxides [87] with noble metal deposited inside the structure revealed relatively low NH₃ conversion below 350 °C. Silver loading of 2.34 wt.% (n(Ag)/n(Cu)/n(Mg)/n(Al) = 1/5/65/29) led to the formation of CuO_x and Ag₂O—that caused higher catalytic activity and the observed drop in N₂ selectivity. Significantly higher catalytic activity was reported for the Ag-Cu alloy nanoparticles (Figure 5a–d) synthesized by a solventless mix-bake-wash method. Ag-Cu (n(Ag)/n(Cu = 2/1, 77.25 wt.% of Ag) revealed full NH₃ conversion at 200 °C. The AgCu alloy structure maintains the metallic state of Ag and Cu as well as structure stability, which enhanced activity and thermal stability in NH₃ oxidation. The calcination of precursors of noble metal and transition metal did not form the alloy structure (Figure 5e–h). Besides, above-mentioned catalyst architectures, the (7.2 wt.%)Ag-(12.2 wt.%)Cu species were deposited onto wire-mesh honeycomb (WMH; characterized by open frontal area: 74.1%; geometric surface area: 16.2 cm² cm⁻³; pressure drop: 2.58 × 10⁻² Pa) [88]. Such catalyst revealed enhanced N₂ selectivity (above 89% at 180–320 °C) compared to Ag/WHM as well as stability in the presence of H₂ and CH₄.

Concluding this part, Ag-based catalysts (especially Ag/Al₂O₃) received extensive concerns in NH₃-SCO. As can be seen from the above examples, the research focuses mainly on the influence of the valance state of Ag species and particle size on the catalytic properties. As mentioned above, (10 wt.%)Ag/Al₂O₃ was suited for full NH₃ conversion at about 150–400 °C with 45–95% N₂ selectivity. Based on the presented above studies, the dispersed Ag⁰ with an average particle size in the range between 3.5–6.0 nm was found as the active species for NH₃-SCO below 200 °C. On the other hand, there are limited studies that discuss the stability (especially concerning oxidation state) of the Ag-based catalysts, i.e., in the consecutive reaction cycles or the presence of H₂O, SO_x, and CO_x. Furthermore, despite their high potential, only a few studies addressed Ag-containing bifunctional catalysts for NH₃-SCO. A rather high content of Ag species (compared to Pt-based bifunctional catalysts, e.g., (7.5 wt.%)Ag-(2.5 wt.%)Cu/Al₂O₃ or (7.2 wt.%)Ag-(12.2 wt.%)Cu/WHM) was necessary to reach high activity and N₂ selectivity (i.e., full NH₃ conversion with 81–95% N₂ selectivity at 200–320 °C, according to data gathered in Table 2). The catalysts containing lower content of Ag species, i.e., 0.59–1.5 wt.% required a higher temperature of 375–500 °C to fully oxidize NH₃. Another important aspect of NH₃-SCO over Ag-based catalysts is the investigation of the reaction mechanisms, which were explored mainly by the application of in situ DRIFTS and the indication of the characteristic intermediates of the imide, hydrazine, or i-SCR (internal) mechanism. Overall, FT-IR investigations suggest that NH₃-SCO may follow different parallel pathways. Thus, the combination of the advantages of in situ DRIFTS with the advantages of other (e.g., temperature-programmed and/or transient) techniques will be strategic to clarify the pathways of NH₃-SCO. Furthermore, the exploration of the reaction mechanisms should be ongoing in the realistic catalytic mixture containing besides NH₃ and O₂ in the inert gas also H₂O, SO_x, and CO_x.

Table 2. Comparison of full NH₃ conversion and N₂ selectivity in same temperature range over Ag-based catalysts reported.

Catalyst	Catalyst Preparation	Reaction Conditions	NH3 Conversion_N2 Selectivity/% (Temperature/°C)	Ref.
Ag powder	Ag2O, triple reduction, 400 °C, H2/He; oxidation, 400 °C, O2/He	0.1 vol.% NH3, 10 vol.% O2, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL g−1 h−1	100/33–77% (185–400 °C)	[72]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 500 °C, air; reduction, 400 °C, H2/He	0.1 vol.% NH3, 10 vol.% O2, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/70–88% (160–400 °C)	[72]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 500 °C, air; *reduction, 400 °C, H2/N2	0.05 vol.% NH3, 10 vol.% O2, N2 balance, 100 mL min−1, GHSV 28,000 h−1	100/93–95% (180 °C) *100/80–82% (160–180 °C)	[76]
(10 wt.%)Ag/Al2O3	calcination, 600 °C, air; reduction, 400 °C, H2/N2 impregnation	0.05 vol.% NH3, 10 vol.% O2, N2 balance, 200 mL min−1, mass of the catalyst: 0.2 g, WHSV 1000 mL h−1 g−1	100/45–55% (150–200 °C)	[74]
	incipient wetness impregnation		100/55–60% (150–200 °C)
	sol-gel		100/96% (300 °C)
(10 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air; reduction, 300 °C, H2/N2	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 400 mL min−1, mass of the catalyst: 0.4 g, GHSV 50,000 h−1	100/89 (180–260 °C)	[78]
(5 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/58–83% (275–500 °C)	[80]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air	1 vol.% NH3, 10 vol.% O2, He balance, 400 mL min−1, mass of the catalyst: 0.8 g, WHSV 30,000 mL h−1 g−1	100/70–83% (200–250 °C)	[79]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 500 °C, air	1.14 vol.% NH3, 8.21 vol.% O2, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/83% (300 °C)	[71]
(1.5 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/78% (325 °C)	[82]
(10 wt.%)Ag/Al2O3			100/94% (225 °C)
(2.2 wt.%)Ag/Al2O3	homogenous deposition precipitation, reduction, 400 °C, H2	2 vol.% NH3, 2 vol.% O2, Ar balance, 40 mL min−1, GHSV 2,500 h−1*0.15 vol.% NH3, 3.85 vol.% O2, Ar balance, 40 mL min−1, GHSV 2,500 h−1	100/100% (368–400 °C) *100/85–100% (342–400 °C)	[77]
(1.6 wt.%)Ag/CeOx/Li2O/Al2O3			100/95–100% (275–400 °C)
(10 wt.%)Ag/micro-Al2O3	impregnation, calcination, 500 °C, air	0.05 vol.% NH3, 10 vol.% O2, He balance, 100 mL min−1, GHSV 28,000–115,000 h−1 *0.05 vol.% NH3, 10 vol.% O2, 5 vol.% H2O, He balance, 100 mL min−1, GHSV 136,000 h−1	100/94–96% (160–180 °C)	[17]
(10 wt.%)Ag/nano-Al2O3			100/66–76% (120–180 °C) *100/74–90% (250–400 °C)
(9.8 wt.%)Ag/Al2O3	rotary evaporator, calcination, 500 °C, air; reduction, 400 °C, H2/Ar	0.1 vol.% NH3, 10 vol.% O2, He balance, 100 mL min−1, GHSV 35,000 h−1	100/70–74% (140–190 °C)	[83]
(9.9 wt.%)Ag/ZSM-5			100/82–89% (150–190 °C)
(10 wt.%)Ag/SiO2	impregnation, calcination, 500 °C, air; reduction, 400 °C, H2/He	0.1 vol.% NH3, 10 vol.% O2, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/40–73% (200–400 °C)	[72]
(10 wt.%)Ag/SiO2	impregnation, calcination, 600 °C, air; reduction, 300 °C, H2/N2	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 400 mL min−1, mass of the catalyst: 0.4 g, GHSV 50,000 h−1	100/50–63% (220–260 °C)	[78]
(10 wt.%)Ag/TiO2			100/64% (260 °C)
(10 wt.%)Ag/SiO2	impregnation, calcination, 500 °C, air	0.05 vol.% NH3, 10 vol.% O2, N2 balance, 100 mL min−1, GHSV 28,000 h−1	100/60–62% (180–240 °C)	[81]
(10 wt.%)Ag/TiO2			100/91–99% (180–240 °C)
(10 wt.%)Ag/SiO2-TiO2			100/60–70% (140–240 °C)
(1.5 wt.%)Ag/mesoTiO2	impregnation, calcination, 600 °C, air; *reduction, 600 °C, H2/Ar	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/74–76% (375–400 °C) *100/81–87% (350 °C)	[82]
(10 wt.%)Ag/mesoTiO2			100/81–87% (350–400 °C) *100/72–78(275–400 °C)
(7.3 wt.%)Ag/MnO2	multi-step process, calcination, 400 °C, air	0.005 vol.% NH3, 20 vol.% O2, Ar balance *0.005 vol.% NH3, 20 vol.% O2, 0.057 vol.% H2O, Ar balance 100 mL min−1, mass of the catalyst: 0.15 g, WHSV 40,000 mL h−1 g−1	100/98–99% (90–120 °C) *100/95–96% (115–130 °C)	[90]
(10 wt.%)Ag-Y	impregnation, calcination, 600 °C, air; reduction, 300 °C, H2/N2	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 400 mL min−1, mass of the catalyst: 0.4 g, GHSV 50,000 h−1	100/32–50% (220–260 °C)	[78]
(21 wt.%)Ag-Y	ion-exchange, calcination, 400–500 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, He balance *0.5 vol.% NH3, 2.5 vol.% O2, 3.2 vol.% H2O, He balance **0.5 vol.% NH3, 2.5 vol.% O2, 4.8 vol.% CO2, He balance 40 mL min−1, mass of the catalyst: 0.05 g, WHSV 48,000 mL h−1 g−1	100/90% (300 °C)	[91]
(21 wt.%)Ag-USY			100/92–95% (200–300 °C) *100/98% (200–300 °C) **100/90–95% (200–300 °C)
(33 wt.%)Ag-Y	ion-exchange, reduction, 400 °C, H2	0.05 vol.% NH3, 7 vol.% O2, N2 balance, 800 mL min−1, mass of the catalyst: 0.25 g, WHSV 192,000 mL h−1 g−1	100/70–80% (300–400 °C)	[92]
(5 wt.%)Ag-(5 wt.%)Cu/Al2O3	impregnation, calcination, 600 °C, air	1 vol.% NH3, 10 vol.% O2, He balance, 400 mL min−1, mass of the catalyst: 0.8 g, WHSV 30,000 mL h−1 g−1	100/95% (320 °C)	[79]
(7.5 wt.%)Ag-(2.5 wt.%)Cu/Al2O3	impregnation, calcination, 500 °C, air	0.1 vol.% NH3, 10 vol.% O2, He balance, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/95% (200–300 °C)	[71]
(10 wt.%)Ag/Al2O3+(10 wt.%)Cu/Al2O3 (mixture 3:1)	impregnation, calcination, 500 °C, air	1.14 vol.% NH3, 8.21 vol.% O2, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/82% (300 °C)	[71]
(1.5 wt.%)Ag-(10 wt.%)Cu/Al2O3	impregnation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance *0.5 vol.% NH3, 2.5 vol.% O2, 3.2 vol.% H2O, Ar balance 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/83–94% (375–500 °C) *100/83–94% (375–500 °C)	[80]
(0.59 wt.%)AgCuMgAlOx	coprecipitation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/88% (425–500 °C)	[87]
(2.34 wt.%)AgCuMgAlOx			100/78–92% (400–500 °C)
(77.25 wt.%)Ag-Cu nanoalloy	solventless mix-bake-wash method, 300 °C, air	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 100 mL min−1, GHSV 12,000 h−1	100/72–85% (210–240 °C)	[89]
(67.57 wt.%)AgCuOx	calcination, 500 °C, air		100/64–72% (300–340 °C)
(7.4 wt.%)Ag/WMH	WMH—wire-mesh honeycomb; impregnation, calcination, 500 °C, air	0.1 vol.% NH3, 10 vol.% O2, He balance, 300 mL min−1, GHSV 2,250 h−1	100/63–73% (210–320 °C)	[88]
(7.2 wt.%)Ag-(12.2 wt.%)Cu/WMH			100/81–89% (220–320 °C)

Table 2. Comparison of full NH₃ conversion and N₂ selectivity in same temperature range over Ag-based catalysts reported.

Catalyst	Catalyst Preparation	Reaction Conditions	NH3 Conversion_N2 Selectivity/% (Temperature/°C)	Ref.
Ag powder	Ag2O, triple reduction, 400 °C, H2/He; oxidation, 400 °C, O2/He	0.1 vol.% NH3, 10 vol.% O2, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL g−1 h−1	100/33–77% (185–400 °C)	[72]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 500 °C, air; reduction, 400 °C, H2/He	0.1 vol.% NH3, 10 vol.% O2, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/70–88% (160–400 °C)	[72]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 500 °C, air; *reduction, 400 °C, H2/N2	0.05 vol.% NH3, 10 vol.% O2, N2 balance, 100 mL min−1, GHSV 28,000 h−1	100/93–95% (180 °C) *100/80–82% (160–180 °C)	[76]
(10 wt.%)Ag/Al2O3	calcination, 600 °C, air; reduction, 400 °C, H2/N2 impregnation	0.05 vol.% NH3, 10 vol.% O2, N2 balance, 200 mL min−1, mass of the catalyst: 0.2 g, WHSV 1000 mL h−1 g−1	100/45–55% (150–200 °C)	[74]
	incipient wetness impregnation		100/55–60% (150–200 °C)
	sol-gel		100/96% (300 °C)
(10 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air; reduction, 300 °C, H2/N2	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 400 mL min−1, mass of the catalyst: 0.4 g, GHSV 50,000 h−1	100/89 (180–260 °C)	[78]
(5 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/58–83% (275–500 °C)	[80]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air	1 vol.% NH3, 10 vol.% O2, He balance, 400 mL min−1, mass of the catalyst: 0.8 g, WHSV 30,000 mL h−1 g−1	100/70–83% (200–250 °C)	[79]
(10 wt.%)Ag/Al2O3	impregnation, calcination, 500 °C, air	1.14 vol.% NH3, 8.21 vol.% O2, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/83% (300 °C)	[71]
(1.5 wt.%)Ag/Al2O3	impregnation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/78% (325 °C)	[82]
(10 wt.%)Ag/Al2O3			100/94% (225 °C)
(2.2 wt.%)Ag/Al2O3	homogenous deposition precipitation, reduction, 400 °C, H2	2 vol.% NH3, 2 vol.% O2, Ar balance, 40 mL min−1, GHSV 2,500 h−1*0.15 vol.% NH3, 3.85 vol.% O2, Ar balance, 40 mL min−1, GHSV 2,500 h−1	100/100% (368–400 °C) *100/85–100% (342–400 °C)	[77]
(1.6 wt.%)Ag/CeOx/Li2O/Al2O3			100/95–100% (275–400 °C)
(10 wt.%)Ag/micro-Al2O3	impregnation, calcination, 500 °C, air	0.05 vol.% NH3, 10 vol.% O2, He balance, 100 mL min−1, GHSV 28,000–115,000 h−1 *0.05 vol.% NH3, 10 vol.% O2, 5 vol.% H2O, He balance, 100 mL min−1, GHSV 136,000 h−1	100/94–96% (160–180 °C)	[17]
(10 wt.%)Ag/nano-Al2O3			100/66–76% (120–180 °C) *100/74–90% (250–400 °C)
(9.8 wt.%)Ag/Al2O3	rotary evaporator, calcination, 500 °C, air; reduction, 400 °C, H2/Ar	0.1 vol.% NH3, 10 vol.% O2, He balance, 100 mL min−1, GHSV 35,000 h−1	100/70–74% (140–190 °C)	[83]
(9.9 wt.%)Ag/ZSM-5			100/82–89% (150–190 °C)
(10 wt.%)Ag/SiO2	impregnation, calcination, 500 °C, air; reduction, 400 °C, H2/He	0.1 vol.% NH3, 10 vol.% O2, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/40–73% (200–400 °C)	[72]
(10 wt.%)Ag/SiO2	impregnation, calcination, 600 °C, air; reduction, 300 °C, H2/N2	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 400 mL min−1, mass of the catalyst: 0.4 g, GHSV 50,000 h−1	100/50–63% (220–260 °C)	[78]
(10 wt.%)Ag/TiO2			100/64% (260 °C)
(10 wt.%)Ag/SiO2	impregnation, calcination, 500 °C, air	0.05 vol.% NH3, 10 vol.% O2, N2 balance, 100 mL min−1, GHSV 28,000 h−1	100/60–62% (180–240 °C)	[81]
(10 wt.%)Ag/TiO2			100/91–99% (180–240 °C)
(10 wt.%)Ag/SiO2-TiO2			100/60–70% (140–240 °C)
(1.5 wt.%)Ag/mesoTiO2	impregnation, calcination, 600 °C, air; *reduction, 600 °C, H2/Ar	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/74–76% (375–400 °C) *100/81–87% (350 °C)	[82]
(10 wt.%)Ag/mesoTiO2			100/81–87% (350–400 °C) *100/72–78(275–400 °C)
(7.3 wt.%)Ag/MnO2	multi-step process, calcination, 400 °C, air	0.005 vol.% NH3, 20 vol.% O2, Ar balance *0.005 vol.% NH3, 20 vol.% O2, 0.057 vol.% H2O, Ar balance 100 mL min−1, mass of the catalyst: 0.15 g, WHSV 40,000 mL h−1 g−1	100/98–99% (90–120 °C) *100/95–96% (115–130 °C)	[90]
(10 wt.%)Ag-Y	impregnation, calcination, 600 °C, air; reduction, 300 °C, H2/N2	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 400 mL min−1, mass of the catalyst: 0.4 g, GHSV 50,000 h−1	100/32–50% (220–260 °C)	[78]
(21 wt.%)Ag-Y	ion-exchange, calcination, 400–500 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, He balance *0.5 vol.% NH3, 2.5 vol.% O2, 3.2 vol.% H2O, He balance **0.5 vol.% NH3, 2.5 vol.% O2, 4.8 vol.% CO2, He balance 40 mL min−1, mass of the catalyst: 0.05 g, WHSV 48,000 mL h−1 g−1	100/90% (300 °C)	[91]
(21 wt.%)Ag-USY			100/92–95% (200–300 °C) *100/98% (200–300 °C) **100/90–95% (200–300 °C)
(33 wt.%)Ag-Y	ion-exchange, reduction, 400 °C, H2	0.05 vol.% NH3, 7 vol.% O2, N2 balance, 800 mL min−1, mass of the catalyst: 0.25 g, WHSV 192,000 mL h−1 g−1	100/70–80% (300–400 °C)	[92]
(5 wt.%)Ag-(5 wt.%)Cu/Al2O3	impregnation, calcination, 600 °C, air	1 vol.% NH3, 10 vol.% O2, He balance, 400 mL min−1, mass of the catalyst: 0.8 g, WHSV 30,000 mL h−1 g−1	100/95% (320 °C)	[79]
(7.5 wt.%)Ag-(2.5 wt.%)Cu/Al2O3	impregnation, calcination, 500 °C, air	0.1 vol.% NH3, 10 vol.% O2, He balance, 50 mL min−1, mass of the catalyst: 0.1 g, WHSV 30,000 mL h−1 g−1	100/95% (200–300 °C)	[71]
(10 wt.%)Ag/Al2O3+(10 wt.%)Cu/Al2O3 (mixture 3:1)	impregnation, calcination, 500 °C, air	1.14 vol.% NH3, 8.21 vol.% O2, 74.7 mL min−1, mass of the catalyst: 0.2 g, WHSV 22,410 mL h−1 g−1	100/82% (300 °C)	[71]
(1.5 wt.%)Ag-(10 wt.%)Cu/Al2O3	impregnation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance *0.5 vol.% NH3, 2.5 vol.% O2, 3.2 vol.% H2O, Ar balance 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/83–94% (375–500 °C) *100/83–94% (375–500 °C)	[80]
(0.59 wt.%)AgCuMgAlOx	coprecipitation, calcination, 600 °C, air	0.5 vol.% NH3, 2.5 vol.% O2, Ar balance, 40 mL min−1, mass of the catalyst: 0.1 g, WHSV 24,000 mL h−1 g−1	100/88% (425–500 °C)	[87]
(2.34 wt.%)AgCuMgAlOx			100/78–92% (400–500 °C)
(77.25 wt.%)Ag-Cu nanoalloy	solventless mix-bake-wash method, 300 °C, air	0.1 vol.% NH3, 10 vol.% O2, N2 balance, 100 mL min−1, GHSV 12,000 h−1	100/72–85% (210–240 °C)	[89]
(67.57 wt.%)AgCuOx	calcination, 500 °C, air		100/64–72% (300–340 °C)
(7.4 wt.%)Ag/WMH	WMH—wire-mesh honeycomb; impregnation, calcination, 500 °C, air	0.1 vol.% NH3, 10 vol.% O2, He balance, 300 mL min−1, GHSV 2,250 h−1	100/63–73% (210–320 °C)	[88]
(7.2 wt.%)Ag-(12.2 wt.%)Cu/WMH			100/81–89% (220–320 °C)

4. Au- and Ru-Based Catalysts

Gold-based catalysts are well-known for their high catalytic activity at low temperatures, e.g., in the oxidation of CO [93,94]. However, such catalysts were rarely investigated in NH₃-SCO (Table 3). Interestingly, Lin et al. [95] investigated NH₃ oxidation over the in situ H₂-reduced (5 wt.%)Au/MO_x/Al₂O₃ (M = Cu, Fe, Ce, Li, and Ti) catalysts in the temperature range from 200 to 400 °C. Among all investigated catalysts, Au/Cu-Al₂O₃ was the most active and N₂ selective (full NH₃ conversion at 300 °C with 95% N₂ selectivity). The H₂-reduced Au/Al₂O₃ catalyst revealed significantly lower NH₃ conversion (ca. 30% above 400 °C). However, the NH₃ conversion increases with an increasing O₂ in the feed (c(NH₃):c(O₂) = 1:10; NH₃ conversion of 45% at 400 °C) [77]. Gong et al. [96] and Liu et al. [97] reported from experimental and theoretical studies that NH₃ did not dissociate on the Au(111) surface until it was precovered with oxygen atoms or hydroxyl groups. Thus, Lippits et al. [77] observed the enhanced NH₃ conversion (full NH₃ conversion at 338 °C with N₂ selectivity below 50%) over Au/Al₂O₃ after its doping with CeO_x (able to provide and store oxygen) and Li₂O (responsible for decreasing the catalyst acidity and thus improved oxygen adsorption). Recently, Lin et al. [12] reported the acidic metal-oxide-supported gold catalyst (Au/Nb₂O₅) with improved N₂ selectivity compared to other metal-oxide-supported gold catalysts (e.g., Au/SiO₂, Au/Al₂O₃, Au/Fe₂O₃, etc., Figure 6a). Specifically, Au/Nb₂O₅ contains both Brønsted and Lewis acid sites that allowed NH₃ oxidation according to hydrazine mechanism (N₂H₄ as the intermediate) and imide mechanism (HNO as the intermediate), respectively (Figure 6b). Overall, further detailed property-activity studies over Au-containing materials could path the way for further catalysts design and their optimization.

Some studies were carried out on RuO₂(110) surface characterized by two types of atoms with unsaturated bonds along [1] direction: a) the twofold coordinated oxygen atoms (O-bridge; O-br) and b) the fivefold coordinated Ru atoms (Ru-cus; the adsorption site for ammonia) [98]. NH₃ decomposes (to NH₂) at −183 °C, while successive annealing to −23–27 °C produces N [98,99]. N₂ is predominantly formed over polycrystalline RuO₂ in a direct combination of Ru-coordinated N atoms (at ambient pressure, 6% of NO at c(O₂):c(NH₃) = 2:1; 65% of NO at c(O₂):c(NH₃) = 140:1) [100]. The selectivity to NO increases with increasing temperature (100% around 257 °C, in UHV, p(NH₃) = 10⁻⁷ mbar, and c(O₂):c(NH₃) = 20:1) because of the high desorption temperature for NO (227 °C). At lower temperatures, NO-formation is hindered by surface water molecules [98,99]. Seitsonen et al. [99] estimated energy barriers to the elementary H-abstraction steps and the recombination of N and O atoms on RuO₂(110) surface by using DFT calculations and high-resolution core-level shift spectroscopy (Figure 7). The high activity of RuO₂(110) arose from low activation energies from the successive H-abstraction.

Also, RuO₂-supported catalysts are active for ammonia oxidation (Table 3). E.g., Cui et al. investigated the RuO₂-CuO/Al-ZrO₂ [101] and CuO/RuO₂ [102] catalysts with 5–30 wt.% and 70–95 wt.% of Ru loading, respectively. The catalysts possessed excellent activity and N₂ selectivity at low temperatures, i.e., for (20 wt.%)RuO₂-CuO/Al-ZrO₂ full NH₃ conversion at 195 °C with 100% N₂ selectivity [101], or for (10 wt.%)CuO/RuO₂ full NH₃ conversion at 180 °C and N₂ selectivity above 95% [102] were achieved. However, these catalysts are relatively expensive and therefore their commercialization is hindered. Thus, Chakrobaty et al. [103] investigated the Cu/Ru catalysts with varying overlaying thickness of Cu film (with the optimum of 0.8 monolayers) deposited by physical vapor deposition on (5 nm)Ru/TiO₂ (111). The synergistic interaction between Cu and Ru species led to a threefold higher ammonia conversion rate than was achieved over Ru-based catalyst. Concerning the powder materials, Wang et al. [104] studied a series of WO₃-modified RuO₂-Fe₂O₃ catalysts with a lower cost, i.e., with 1 wt.% of ruthenium. The introduction of 5 wt.% of WO₃ (among 1–9 wt.% of WO₃) tuned the surface acidity, and thus, enhanced activity and N₂ selectivity of RuO₂-Fe₂O₃ (full NH₃ conversion at 250–400 °C and 93–97% N₂ selectivity). In situ DRIFTS results indicated that NH₃-SCO over (1 wt.%)RuO₂-(5 wt.%)WO₃-Fe₂O₃ proceeds according to the i-SCR mechanism. The -NH₂ intermediate reacted with the in situ-generated NO_x ad-species with the formation of N₂. Furthermore, Chen et al. [105,106] studied (0.2 wt.%)Ru/Ce_0.6Zr_0.4O₂(PVP) or (0.2 wt.%)IrO₂/Ce_0.6Zr_0.4O₂(PVP) (PVP, polyvinylpyrrolidone) and claimed that -HNO appeared as an intermediate in the i-SCR mechanism of NH₃-SCO (Figure 8). The formed -HNO interacted with atomic oxygen with the formation of NO, which furthermore reacts with -NH_x (-NH₂ and -NH) species with the formation of N₂ and N₂O (minor by-product). The presence of SO₂ in the feed gas effectively inhibits the production of N₂O, i.e., the reactions between gaseous NO and -NH₂ will be enhanced (more adsorbed ammonia on the sulfated (acidic) surface). SO₂ can also inhibit NH₃ oxidation resulting in higher N₂ selectivity (up to 100%) in the absence of NO_x. Similar conclusions were given for RuO_x/TiO₂-SO₄²⁻, however, the time of the sulfated treatment (0.5–6 h) of the support varied activity and N₂ selectivity of the final catalysts (with an optimum at 2 h) [107].

Concluding this part, both Au- and Ru-based catalysts were significantly less investigated in NH₃-SCO. A highly loaded Ru-containing materials (10–20 wt.% Ru; (10 wt.%)CuO-RuO₂ or (20 wt.%)RuO₂-CuO/Al-ZrO₂) present a class of highly active and N₂ selective catalysts at relatively low temperatures (i.e., full NH₃ conversion at 180–350 °C with 95–100% N₂ selectivity; according to data gathered in Table 3). Otherwise, the materials with significantly lower content of Ru species (0.5–3 wt.%) were less active (full NH₃ conversion at 175–400 °C) and N₂ selective (43–99%). However, again, concerning the influence of the preparation variables (e.g., the different total amount of ruthenium, variety of applied metal promoters and supports) as well as pretreatment and reaction conditions, the comparison of activity and N₂ selectivity over Ru-based catalysts each other or even with other noble metal-based catalysts is limited. Furthermore, for both Au- and Ru-based catalysts, oxidized metal species (i.e., Au⁺/Au³⁺, Ru⁴⁺) ensure enhanced activity and N₂ selectivity (in contrast to the Pt- or Ag-based catalysts). Nevertheless, an in-depth understanding of the role of active species in NH₃-SCO is still lacking and needs to be demonstrated clearly in further studies (especially over Au-based catalysts). Furthermore, the presented catalytic systems were mainly investigated under ideal conditions (only NH₃ and O₂ diluted in inert gas) also concerning the investigation of the reaction mechanisms, i.e., through in situ DRIFTS experiments (NH₃ adsorption/desorption in inert gas or oxygen). Otherwise, surface reactions are fast (residence time in a range of seconds) and the reaction mechanism involves a series of parallel and consecutive reactions. Thus, the reaction intermediates and conversion of the substrate molecules on the catalyst surface should be followed with more detailed ex situ, in situ, and operando spectroscopic studies as well as transient kinetic investigations under-applied reaction conditions.

Table 3. Comparison of full NH₃ conversion and N₂ selectivity in same temperature range over Au- and Ru-based catalysts reported in literature.

Catalyst	Catalyst Preparation	Reaction Conditions	NH3 Conversion_N2 Selectivity/% (Temperature/°C)	Ref.
(5 wt.%)Au/CuO/Al2O3	impregnation, calcination, 300 °C, air; reduction, 300 °C, H2	2 vol.% NH3, 2 vol.% O2, He balance, 30 mL min−1, mass of the catalyst: 0.15 g, WHSV 12,000 mL h−1 g−1	100/95% (300 °C)	[95]
(4 wt.%)Au/CeOx/Li2O/Al2O3	homogenous deposition precipitation, reduction, 400 °C, H2	2 vol.% NH3, 2 vol.% O2, Ar balance, 40 mL min−1, GHSV 2500 h−1	100/34–49% (338–400 °C)	[77]
(10 wt.%)CuO-RuO2	conanocasting-replication method, calcination, 500 °C, air	0.1 vol.% NH3, 2 vol.% O2, Ar balance, 100 mL min−1, mass of the catalyst: 0.08 g, WHSV 75,000 mL h−1 g−1	100/95–97% (180–350 °C)	[102]
(20 wt.%)RuO2-CuO/ZrO2	impregnation, calcination, 550 °C, air	0.04 vol.% NH3, 5 vol.% O2, 6 vol.% H2O, Ar balance, 200 mL min−1, mass of the catalyst: 0.1 g, WHSV 120,000 mL h−1 g−1	100/68–98% (250–275 °C)	[101]
(20 wt.%)RuO2/Al-ZrO2			100/70–82% (248–325 °C)
(20 wt.%)RuO2-CuO/Al-ZrO2			100/100% (195–280 °C)
(1 wt.%)RuO2-Fe2O3	sol-gel route, calcination, 500 °C, air	0.08 vol.% NH3, 5 vol.% O2, Ar balance, 400 mL min−1, GHSV 60,000 h−1	100/67–90% (250–400 °C)	[104]
(1 wt.%)RuO2-WO3-Fe2O3			100/93–97% (250–400 °C)
(1.13 wt.%)Ru/Cu-SSZ-13	impregnation, 500 °C, air; pretreatment, 300 °C, O2/N2	0.05 vol.% NH3, 0.5 vol.% CO, 5 vol.% O2, N2 balance, GHSV 300,000 h−1	100/94–96% (220–300 °C)	[108]
(3 wt.%)RuO2/TiO2	impregnation, calcination, 400 °C, air	0.1 vol.% NH3, 5 vol.% O2, 3 vol.% H2O, N2 balance, 150 mL min−1, mass of the catalyst: 0.1 g, WHSV 90,000 mL h−1 g−1	100/47–72% (350–400 °C)	[109]
(3 wt.% Ru)RuO2/Na-Y	ion-exchange, pretreatment, 450 °C, O2/H2O/N2		100/51–99% (175–400 °C)
(1 wt.%)Ru/TiO2	impregnation, 400 °C, air; pretreatment, 400 °C, O2/N2	0.02 vol.% NH3, 10 vol.% O2, 6 vol.% H2O, N2 balance, 500 mL min−1, mass of the catalyst: 0.3 g, GHSV 60,000 h−1	100/54–63% (275–300 °C)	[110]
(0.5 wt.%)Ru/TiO2	impregnation, 450 °C, air; pretreatment, 400 °C, O2/N2	0.08 vol.% NH3, 5 vol.% O2, N2 balance, 400 mL min−1, GHSV 60,000 h−1	100/43–85% (200–400 °C)	[107]
(0.5 wt.%)Ru/TiO2-SO42−			100/65–93% (225–400 °C)

Table 3. Comparison of full NH₃ conversion and N₂ selectivity in same temperature range over Au- and Ru-based catalysts reported in literature.

Catalyst	Catalyst Preparation	Reaction Conditions	NH3 Conversion_N2 Selectivity/% (Temperature/°C)	Ref.
(5 wt.%)Au/CuO/Al2O3	impregnation, calcination, 300 °C, air; reduction, 300 °C, H2	2 vol.% NH3, 2 vol.% O2, He balance, 30 mL min−1, mass of the catalyst: 0.15 g, WHSV 12,000 mL h−1 g−1	100/95% (300 °C)	[95]
(4 wt.%)Au/CeOx/Li2O/Al2O3	homogenous deposition precipitation, reduction, 400 °C, H2	2 vol.% NH3, 2 vol.% O2, Ar balance, 40 mL min−1, GHSV 2500 h−1	100/34–49% (338–400 °C)	[77]
(10 wt.%)CuO-RuO2	conanocasting-replication method, calcination, 500 °C, air	0.1 vol.% NH3, 2 vol.% O2, Ar balance, 100 mL min−1, mass of the catalyst: 0.08 g, WHSV 75,000 mL h−1 g−1	100/95–97% (180–350 °C)	[102]
(20 wt.%)RuO2-CuO/ZrO2	impregnation, calcination, 550 °C, air	0.04 vol.% NH3, 5 vol.% O2, 6 vol.% H2O, Ar balance, 200 mL min−1, mass of the catalyst: 0.1 g, WHSV 120,000 mL h−1 g−1	100/68–98% (250–275 °C)	[101]
(20 wt.%)RuO2/Al-ZrO2			100/70–82% (248–325 °C)
(20 wt.%)RuO2-CuO/Al-ZrO2			100/100% (195–280 °C)
(1 wt.%)RuO2-Fe2O3	sol-gel route, calcination, 500 °C, air	0.08 vol.% NH3, 5 vol.% O2, Ar balance, 400 mL min−1, GHSV 60,000 h−1	100/67–90% (250–400 °C)	[104]
(1 wt.%)RuO2-WO3-Fe2O3			100/93–97% (250–400 °C)
(1.13 wt.%)Ru/Cu-SSZ-13	impregnation, 500 °C, air; pretreatment, 300 °C, O2/N2	0.05 vol.% NH3, 0.5 vol.% CO, 5 vol.% O2, N2 balance, GHSV 300,000 h−1	100/94–96% (220–300 °C)	[108]
(3 wt.%)RuO2/TiO2	impregnation, calcination, 400 °C, air	0.1 vol.% NH3, 5 vol.% O2, 3 vol.% H2O, N2 balance, 150 mL min−1, mass of the catalyst: 0.1 g, WHSV 90,000 mL h−1 g−1	100/47–72% (350–400 °C)	[109]
(3 wt.% Ru)RuO2/Na-Y	ion-exchange, pretreatment, 450 °C, O2/H2O/N2		100/51–99% (175–400 °C)
(1 wt.%)Ru/TiO2	impregnation, 400 °C, air; pretreatment, 400 °C, O2/N2	0.02 vol.% NH3, 10 vol.% O2, 6 vol.% H2O, N2 balance, 500 mL min−1, mass of the catalyst: 0.3 g, GHSV 60,000 h−1	100/54–63% (275–300 °C)	[110]
(0.5 wt.%)Ru/TiO2	impregnation, 450 °C, air; pretreatment, 400 °C, O2/N2	0.08 vol.% NH3, 5 vol.% O2, N2 balance, 400 mL min−1, GHSV 60,000 h−1	100/43–85% (200–400 °C)	[107]
(0.5 wt.%)Ru/TiO2-SO42−			100/65–93% (225–400 °C)

5. Conclusions and Outlook

NH₃-SCO is the most efficient method for ammonia removal from oxygen-containing exhausts. The number of publications related to this process successively increases with the main researchers’ interest in the development of the catalyst with high activity, N₂ selectivity, and stability in the broad temperature range. The present mini-review provides a broad picture of the property-activity correlations of noble metal-based catalysts investigated for NH₃-SCO. Among presented Pt-, Pd-, Ag- and Au-, Ru-based catalytic systems, mainly H₂-reduced (1–2 wt.%)Pt/Al₂O₃ and (10 wt.%)Ag/Al₂O₃ were recognized as the most active NH₃-SCO systems in the low temperatures (<300 °C). Unfortunately, they caused significant formation of N₂O and NO_x. Moreover, as can be seen from the above examples, although Pt- and Ag-based catalysts were more intensively investigated regarding their property-activity correlations—compared to that of the Pd-, Au-, and Ru-containing catalysts—there is still a lack of systematic studies concerning the nature and role of active species as well as the influence of (a) the particle size of active components and their aggregation state; (b) the catalyst supports (i.e., inorganic oxides versus zeolites); (c) the preparation methods (i.e., catalysts in the structured forms, e.g., monolith), and (d) feed composition (i.e., various c(NH₃):c(O₂) ratios (1:1–25—an excess of oxygen together with minor NH₃ slip, presence of H₂O, SO_x, CO_x, etc.) on activity, N₂ selectivity, and stability in catalysis. Contrary to the transition metal-based catalysts only a few examples concern catalytic studies over noble metal-based zeolites. Otherwise, concerning the discussed material requirements (i.e., enhanced ammonia conversion, N₂ selectivity and stability in the presence of typical components of exhaust gases and the broad temperature range up to 600–700 °C (in the cycle of diesel particulate filter regeneration)), the zeolite-based catalysts present a class of highly promising materials. For instance, ion-exchanged zeolites showed higher activity also in the presence of H₂O, compared to that of alumina-supported oxides with the same metal loading due to high dispersion of metal species and acid sites of high strength. Furthermore, the studies on the reaction mechanisms are rather scarce. The reaction mechanisms must be clarified to rationally develop a process for NH₃ oxidation to N₂ over applied catalysts. These problems highlight the importance of more detailed ex situ and in situ methods (i.e., temperature-programmed, spectroscopic, and/or transient methods) in studying the catalysts under real working environments.

Furthermore, a relatively narrow operating temperature window of full NH₃ conversion (Table 1, Table 2 and Table 3), high selectivity to N₂O and NO_x, and high costs of noble metals motivated researchers to develop suitable bifunctional systems. Cu-ZSM-5 or even Cu-chabazite (SSZ-13, SAPO-34) are already recognized (also discussed in the previous review articles concerning NH₃-SCO, Jabłońska et al., 2016, Jabłońska et al., 2020) as the active and N₂ selective catalysts for NH₃-SCR. Despite presented here developments in bifunctional catalysts (mainly Pt-Cu or Ag-Cu catalysts systems), challenges remain in achieving enhanced NH₃ conversion, N₂ selectivity, and stability (in the presence of real flue gases, such as H₂O, SO_x, and CO_x). Further investigations concerning bifunctional catalysts with a low number of noble metals, e.g., Au, Pt-Au, Pt-Rh, etc., constitute a promising research direction. Nevertheless, the present findings, indications, and thoughts given in the mini-review form a solid basis for further developments of structured catalysts (i.e., in the form of dual-layer or core-shell structure) and their optimization.