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

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 emissions, etc. (Figure 1). However, since 2001, the EU has listed NH3 as one of the four main types of atmospheric pollutants (among NO, 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 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).

Figure 1

Schematic representation of NH3 application and abatement, as well as the impact of NH3 on human health and the environment.

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 by NH3 (SCR-DeNO, 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)):4NH 2NH 4NH 4NH

Thus, NH3-SCO catalysts with enhanced activity, N2 selectivity, stability (also in the presence of H2O, SO, and CO, 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) [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 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 and NH3 oxidation to yield NO 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-OH interfacial sites [9]. Still, compared to NH3-SCR-DeNO, 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−M(III)(OH)2]+(A−)) mH2O, where M(II) and M(III) represent divalent and trivalent metal cations, respectively, A− 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.

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

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.

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 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].

Figure 3

Results of temperature-programmed desorption of NH3 in (a) pure He or (b) 5 vol.% O2/He, adsorption: 70 °C, 1 vol.% NH3/He, (c) comparison of conversion of NH3 and NO, and (d) comparison of the space velocities (SV) over the CuFeAl hydrotalcite-derived mixed metal oxides. Reprinted from [31] with permission of Springer.

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., Cu1−MgAl2O4 of lower reducibility, which caused higher N2 selectivity [35].

Figure 4

(a) Results of NH3-SCO over the CuMgAl mixed metal oxides. Reprinted from [34] with permission from Elsevier, and (b) results of NH3-SCO over the hydrotalcite-derived mixed metal oxides calcined at 600 and 900 °C. Reprinted from [28] with permission from Springer.

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	CuMgAln(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	CuMgAln(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	CuMgFen(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	CuMgAln(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	CuMgAln(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	CuMgAln(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* CeCuMgAln(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	CuMgAln(Cu)/n(Mg)/n(Al) = 10–15/52–57/33, mol.%* (4.1 wt.%)CeCuMgAln(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	CuMgAln(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	CuZnAln(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	CoMnAln(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-Zrn(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-Fe2O3n(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-Fe2O3n(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-CeO2n(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/TiO2n(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]

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 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):Ce Ce

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 and/or SO), 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, 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) 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).

Figure 5

(a) Two competitive routes for NH3 oxidation over V2O5(010). Reprinted from [55] with permission from ACS Publications; (b) schematic representation of the mechanism of NH3-SCO over Cu-Ce/V2O5-WO3-TiO2. Reprinted from [53] with permission from Elsevier.

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, CuO+ (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. 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.

Figure 6

Reaction mechanism of NH3-SCO over Cu/TiSnO2. Reprinted from [85] with permission of Elsevier.

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 NH3− 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 to form N2 or N2O (Equations (13) and (14)). NH NH NH + O → HNO O HNO + O → NO + OH NH + O NH NH + NO → N H + OH → H

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 Ce1−ZrO2 (0.2 ≤ x ≤ 0.8) mixed oxide catalysts, among which particularly Ce0.4Zr0.6O2 reached the total NH3 oxidation of about 360 °C (N2 selectivity > 90%). Ce0.4Zr0.6O2 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 and -HNO intermediates, Figure 7a) [98].

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.

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 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].

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 Cu1+Mn2−O4 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(z)-TiO2 [106] (z = 0.1–0.3) prepared by the sol-gel method. The optimum MnO(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 SmMn2O5. 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 over mullite catalyst), which stays contrary to the above-mentioned studies. The i-SCR mechanism was also proposed over the LaSr1-MnO3 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.

Figure 8

Schematic representation of (a) Nb2O5 modification to enhance the N2 selectivity of SmMn2O5. Reprinted from [108] with permission from Elsevier; (b) NH3-SCO mechanism over modified LaSr1-MnO3 perovskite. Reprinted from [110] with permission from ACS Publications.

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.

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, SO 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.