Catalytic Properties of the Spinel-Like Cu_xMn_3−xO₄ Copper Manganese Oxides—An Overview

Abstract

Copper manganese oxide spinels and related (multiphase) materials with the formula Cu_xMn_3−xO₄ are the active catalysts in a wide variety of industrially important processes due to their great diversity in their phase relations, metal ion valence/site distribution, and chemical properties. In this review, we summarize the preparation methods and their effects on the composition, properties, and catalytic properties of various Cu_xMn_3−xO₄ catalysts with various Cu/Mn ratios. The main summarized catalytic reactions are the oxidation of carbon monoxide, nitrogen oxide, and hydrogen sulfide and the oxidative removal of organic solvents such as benzene, toluene, and xylene from the air. Some industrially important reactions (steam reforming of methanol or synthesis gas) and the manufacture of organic chemicals (methyl formate, propylene oxide, and benzyl alcohol) catalyzed by Cu_xMn_3−xO₄ spinels are also reviewed.

1. Introduction

From both fundamental and applicative points of view, copper manganese oxides, especially the copper manganese spinel compounds and related (multiphase) materials with Cu_xMn_3−xO₄ formula, are important catalysts because of their great diversity in their phase relations, metal ion valence/site distribution, and chemical properties. Copper and manganese oxide-based materials are widely used catalysts in many industrially important processes such as room-temperature CO conversions, exhaust gas purification, oxidation of harmful organic pollutants, and methanol conversions [1,2,3,4,5]. The most important representatives of the system are the Cu_0.5Mn_2.5O₄, CuMn₂O₄, and Cu_1.5Mn_1.5O₄ phases, but the solid solutions of Cu_xMn_3−xO₄ spinels (0 < x < 3), and composite materials with inhomogeneous phase distribution are also known, which have special properties due to stresses in the mixed crystals/solid solutions/phase boundaries [1,2,3,4,5].

Hopcalite was the first copper manganese oxide catalyst used in the long term for CO removal from air [6], and depending on the production conditions, may contain various amounts and kinds of Cu_xMn_3−xO₄ spinels together with copper, manganese, and copper manganese oxides. The presence of Cu_xMn_3−xO₄ components in various copper manganese oxide-based catalysts strongly depends on the synthesis conditions, and sometimes on the catalytic reaction condition due to the reductive/oxidative nature of the atmosphere, and reactants also have a strong influence on the composition/phase relations of the catalysts used. The crystallinity, composition, and chemical state of phases and surface layers in Cu_xMn_3−xO₄ catalysts, including the presence of amorphous materials, distorted crystal lattices, and the number of oxygen and metal vacancies are key parameters in the development of catalysts with high activity. The newest trends belong to the so-called “defect and synergic engineering” methods with the aim of enhancing the oxygen replenishment capacity of Cu_xMn_3−xO₄ phases. The amount and distribution of copper and oxygen vacancies are regulated with the preparation and annealing conditions. The valence of copper and manganese ions and the distribution of the Cu^I,II and Mn^II,III,IV ions between the crystalline and amorphous phases are key parameters, both in the bulk and surface layer, including the distribution of these ions between the tetrahedral and octahedral spinel sites. These features of Cu_xMn_3−xO₄ spinels can be controlled with a wide variety of reaction and annealing conditions, including the reaction routes, Cu/Mn ratios, metal valences, counter-ions, reagents, metal compound concentrations, temperature, time, and other factors.

In this review, we focused on the effect of synthesis routes (and consequently, the composition and structure) on the catalytic efficiency of Cu_xMn_3−xO₄ spinels. We summarized the influence of the preparation conditions on the composition, structure, and catalytic properties of Cu_xMn_3−xO₄ spinels and mixed-phase composites with various Cu/Mn ratios containing Cu_xMn_3−xO₄ spinel components (Tables 1–3). It is a widely accepted fact that the Cu_xMn_3−xO₄ spinel phase is one of the main active phases in the mixed Mn−Cu oxides in various catalytic oxidation reactions [7,8]. The adsorbed surface oxygen concentration and the Cu²⁺ + Mn³⁺ ↔ Cu⁺ + Mn⁴⁺ redox cycle play a key role in these processes, and oxygen vacancies that form ensure the regeneration ability of the copper manganese oxide catalysts. The presence and activity of the active spinel phases depend on the distribution of ions between the crystalline sites, indirectly on the synthesis conditions. Therefore, we summarized and compared the influence of the synthetic routes (ceramic, precipitation, redox, and combined methods) on the composition and catalytic properties of the copper manganese oxide spinel catalysts.

The reviewed catalytic reactions were the oxidation of carbon monoxide (Table 4), the removal of organic and inorganic volatiles such as benzene, toluene, trichloroethylene, NO, or H₂S from air (Tables 5–9), the manufacturing of some organic chemicals (methyl formate, propylene oxide, and benzyl alcohol) (Tables 10–12) and the steam reforming of methanol (hydrogen production) (Table 13). These examples show the enormous industrial and environmental protection significance of these cheap and easily available noble metal-free spinel catalysts that contain copper and manganese oxide. Defect and synergism engineering can develop new kinds of copper manganese oxide catalysts with increased efficiency and selectivity in various reactions. Doping the known and newly prepared copper manganese oxide spinels is a prospective field of modifying and developing new noble metal-free catalyst systems.

2. Preparation Routes of Cu_xMn_3−xO₄ Spinels

The main spinel preparation methods consist of solid-phase reactions of copper and manganese oxides. These oxides can be prepared in situ from their precursors (hydroxide, carbonate, oxalate, or other compounds) precipitated from solutions with precipitation, gel-forming, and redox precipitation [9,10,11,12,13]. A special case of the stoichiometric CuMn₂O₄ synthesis is based on a solid-phase thermal decomposition of copper permanganate or its complexes with reducing ligands like ammonia [14].

The calculated Cu–Mn–O₂ phase diagram shows the temperature–composition relationships [10] (Figure 1).

In order to ensure homogeneous distribution of the reacting precursors, various methods have been developed over the simple grinding of oxide precursors, e.g.,

-

precipitation of copper and manganese hydroxides or carbonates or oxalates directly from aqueous solutions with bases or with special techniques such as spray drying, immersion, or alginate xerogel formation

-

hydrothermal and solvothermal methods with the transformation of the precursors containing metal into reactive materials; these methods include hydrolysis or co-precipitation with alkaline materials, such as ammonia that formed from the hydrolysis of urea or hexamethylenetetramine

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solution and solid–phase redox reactions, such as oxidation of low-valence Cu or Mn compounds with permanganate or other oxidants, with subsequent heat treatment or the thermal decomposition of copper permanganate or its complexes that have reducing ligands such as ammonia.

2.1. Ceramic and Related Processes from Solid Precursors

The stability area of the Cu_xMn_3−xO₄ spinels (Figure 2) shows that compounds with x > 1 at atmospheric oxygen pressure can be prepared from the corresponding copper and manganese oxides only above 900 °C. The copper-rich spinels such as Cu_1.5Mn_1.5O₄ (x = 1.5) can be prepared below 900 °C only at elevated oxygen pressures, but due to the rather slow diffusion of copper and manganese ions in oxides below 900 °C, long enough sintering time and/or highly reactive precursors have to be used. In the preparation of manganese-rich spinels, such as CuMn₂O₄ (x = 1), the clustering of Mn^III ions causes complications [9].

The high-temperature sintering of the finely divided copper and manganese oxides in the appropriate ratio is the most suitable method for the preparation of compounds Cu_xMn_3−xO₄ with x = ~1.0, because the reaction temperature can be selected as high as possible (Figure 2) [9]. For example, Cu_xMn_3−xO₄ compounds with 0.98 < x < 1.10 were prepared by sintering the mixtures of CuO and Mn₃O₄ in air at temperatures between 900 and 940 °C for 72 h. Quenching in water resulted in samples of slightly tetragonally distorted spinels with x < 1.06, whereas at x > 1.06 values (higher copper concentrations), cubic spinels are produced [9]. Spray granulation of CuO and Mn₃O₄ in a spouted bed system in a molar ratio of 1:2 with subsequent calcination at 1125 °C for 2 h resulted in Cu_1.5Mn_1.5O₄ [15]. A solid-state reaction of CuO and MnO₂ powders mixed in the desired proportions resulted in Cu_xMn_3−xO₄ spinels with x = 1.1–1.6 at 940 °C for 24 h in air with intermittent grindings [10]. Two cubic spinels, Cu_1.15Mn_1.85O₄ and Cu_1.47Mn_1.53O₄, were prepared similarly in the presence of 5 wt.% graphite as a pore-modifier additive at 950 °C for 6 h. These spinels contained macropores formed by the evolved CO₂ gas [12]. Precursors of metal oxides, e.g., copper and manganese carbonates mixed at Mn:Cu 3:1, 1:1, and 1:3 ratios, were used to prepare Cu–Mn spinels by heating at 750 and 1000 °C [11,16]. The stoichiometric CuMn₂O₄ was prepared analogously at 700 °C for 240 h [17]. Spray-drying of water-soluble precursors, e.g., metal nitrates, ensured avoiding macroscopic separation. The homogeneous aq. solution of copper(II) and manganese(II) nitrate mixtures were nebulized and dried with a hydrogen burner at 300 °C. The obtained solid mixed metal nitrate particles were partially decomposed into oxides when kept at 350 °C for 24 h. A series of cubic Cu_xMn_3−xO₄ spinels with 1.1 < x < 1.5 [9] was prepared in this way.

A supported mixed-phase Cu_1.4Mn_1.6O₄ catalyst with CuO and/or Mn₂O₃ content was synthesized by incipient wetness impregnation with the use of copper nitrate trihydrate and manganese(I) nitrate tetrahydrate and anatase. The aqueous solutions were mixed with anatase, the mixture was dried at 100 °C for 5 h, and calcined at 500 °C for 7 h in air [18].

A mechanochemical route to prepare solid spinel precursors was also developed [19]. Solid Mn(CH₃COO)₂·4H₂O, copper acetate Cu(CH₃COO)₂·4H₂O, and oxalic acid (H₂C₂O₄·2H₂O) were ball-milled in a molar ratio of Mn^II:Cu^II:oxalic acid of 1 − x:x:0.5 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) at room temperature for 3 h. The powdered mixture was calcined at 550 °C for 2 h in air [19]. A similar process based on oxalate precursor formation is when solid Cu₂CO₃(OH)₂ and MnCO₃ were premixed in a 1:2 molar ratio with 20% stoichiometric excess of solid oxalic or citric acid, and the mixtures were ground under ambient conditions in a planetary mill at a speed of 600 rpm for 2 h. Calcination at 300 and 500 °C in air for 4 h resulted in CuMn₂O₄ [20].

A short summary of the ceramic (solid-state) preparation methods of catalysts containing Cu_xMn_3−xO₄ phases is given in

Table 1. Synthesis conditions of Cu_xMn_3−xO₄ catalysts with solid-phase methods.

x	Precursors	Synthesis/Annealing Conditions	[Ref.]
0.98–1.10	CuO, Mn3O4	In air, 800–940 °C, 72 h, quenching in H2O	[9]
1.5	CuO, Mn3O4	Spray granulation, calcining at 1125 °C for 2 h	[15]
1.1–1.6	CuO, MnO2	Grinding, pressing, 940 °C for 24 h, annealing between 500 and 850 °C	[10]
1.15–1.47	CuO, MnO2	5 wt.% graphite as pore modifier, 950 °C for 6 h	[12]
0.98–1.10	CuO, MnCO3·H2O	In air, 800–940 °C, 72 h, quenching in H2O	[21]
1	CuCO3·Cu(OH)2, MnCO3	Heating at 750 and 1000 °C	[16]
1.5	CuCO3·Cu(OH)2, MnCO3	20% excess of solid oxalic or citric acid, grinding at 600 rpm for 2 h, calcining at 300–500 °C in air for 4 h	[20]
1	Cu(II) and Mn(II) nitrates	Heating in air to decompose into oxides then calcining at 700 °C for 240 h	[17]
1	Cu(II) acetate tetrahydrate, Mn(II) acetate tetrahydrate	Oxalic acid dihydrate, ball milling at room temperature for 3 h, calcining at 550 °C for 2 h in air	[19]

. The solid-phase thermal decomposition of copper permanganates is discussed separately.

The solid-state methods started from oxides or carbonate, or nitrate oxide precursors require elevated temperatures (>700 °C); thus these methods are favorable for preparing crystalline spinels (Table 1). The spinels with x = 0.98–1.6 can be prepared with these solid-phase methods, and the solid precursor oxides can be homogenized with classical intermittent grindings [10] or spray drying [15]. Either cubic or tetragonally distorted spinels can be prepared by selecting the appropriate precursor composition and annealing temperature (Figure 1 and Figure 2) [20]. Pore-forming agents such as graphite resulted in porous materials [12]. The intensive milling of the solid oxalic or citric acid with solid carbonate or acetate salts resulted in mechanochemical reactions and the formation of oxalate/citrate precursors. These precursors were transformed into CuMn₂O₄ at lower temperatures (300–550 °C) [17,19] than the appropriate metal oxides or a mixture of oxides and carbonates (800–940 °C) [21], or carbonates (750–1000 °C) [16].

2.2. In Situ Precipitation of Solid Precursors from Solutions

Mixing the dissolved water-soluble precursors in solutions and their transformation in situ into homogeneously mixed solid precursors—mainly hydroxides, carbonates, or oxalates—is one of the most frequently used methods (Scheme 1). Sodium hydroxide and ammonium hydroxide or sodium and ammonium carbonates are the most frequently used precipitation agents. Water-soluble copper and manganese salts (most frequently nitrates, sulfates, chlorides, and acetates) are used as precursors containing metal [21,22,23,24].

The single-phase cubic Cu_xMn_3−xO₄ spinels with 1.1 < x < 1.5 values were prepared by sintering the co-precipitated copper and manganese hydroxide mixtures in an oxygen atmosphere between 500 and 800 °C for 48 h [9]. Similarly, the aqueous solutions containing copper and manganese acetates or nitrates (1:2 molar ratio of copper to manganese) were mixed with 4 M aq. NaOH at 298 K until the final pH of 11 was reached. CuMn₂O₄ formed after 30 min aging at 298 K, drying in air at 50 °C for 24 h, and calcination at 300 °C for 2 h [25]. Instead of NaOH, organic bases can also be used, such as tetramethylammonium hydroxide. CuMn₂O₄ was prepared from the aqueous solutions of Cu(NO₃)₂·3H₂O and 2 equiv. of Mn(NO₃)₂·4H₂O [26,27], by adding aq. tetramethylammonium hydroxide at 25 °C for 20 min, followed by drying and calcining the precipitate at 100 °C for 24 h and 400 °C for 5 h, respectively [26].

If ammonium hydroxide acts as a hydroxide ion source in the synthesis of the spinel containing Cu, the ammonia has a double role: a precipitating and a complex-forming agent. The starting salts that contain copper immediately turn into [tetraamminecopper(II)] complexes, and the precipitation of copper(II) and manganese hydroxides arises from the self-protonation of the [tetraamminecopper(II)] ion [27,28]. A series of Cu–Mn mixed oxide catalysts with Cu/Mn molar ratios of 2:1, 1.5:1, 1:1, and 0.5:1 containing Cu_1.5Mn_1.5O₄ as the active phase was prepared with this method. A 1 M Mn(NO₃)₂ aqueous solution with pH = 3–4 was dropped into an ammoniacal copper salt solution with pH = 9–10 at 30–40 °C. The precipitate was aged for 5 h and then calcined at 550 °C for 4 h [29,30]. The ammonia can also be generated in situ in hydrothermal/solvothermal reactions of urea or hexamethylenetetramine [13,31]. The advantages of the precipitation methods are combined with the positive effect of harsh reaction conditions (high temperature and pressure), and the hydrolysis with H₂O. A hydrothermal reaction was used to prepare mixed oxide phases containing Cu_1.5Mn_1.5O₄ with an overall Cu/Mn ratio < 0.2 from copper nitrate, manganese nitrate, citric acid, urea, and a KMnO₄ solution (Mn(NO₃)₂/KMnO₄ and urea/citric acid/Mn^II + KMnO₄/Cu ratios were 1:2 and 1.5:1.5:1:0−0.2, respectively). Aqueous ammonia solution was added dropwise until a pH of 6–7 was reached. Then, the mixture was heated at 180 °C for 12 h in an autoclave. The precipitate was dried and calcined at 350 °C in air for 4 h [32].

The most frequently used method to prepare Cu and Mn-containing intermediates for spinel synthesis is the precipitation of a mixture of Cu and Mn carbonates/basic carbonates with the use of water-soluble carbonate compounds, mainly with sodium and ammonium carbonate. For example, CuMn₂O₄ was prepared from an aqueous solution containing copper nitrate trihydrate and manganese nitrate hexahydrate in a 1:2 molar ratio by treatment with 2 M Na₂CO₃ until pH = 8.3 was reached, with subsequent calcination of the precipitate formed at 400–500 °C for 2–18 h [33]. The Cu_1.5Mn_1.5O₄ spinel was prepared from copper and manganese acetates dissolved in a ratio of 1:1 in water by co-precipitation with an aqueous solution of sodium carbonate at room temperature. The solid obtained was dried and calcined at 500 °C for 6 h. In a similar route but varying the molar ratio of Cu to Mn from 5:1 to 1:5 and the calcination temperature between 400 and 800 °C, mixed-phase catalysts with Cu_1.5Mn_1.5O₄ content were prepared [34]. The effect of aging of the precipitates formed from aqueous Cu(NO₃)₂·3H₂O and Mn(NO₃)·6H₂O solutions (0.25 M each, mixed in a 1:2 ratio) at 80 °C with aqueous Na₂CO₃ (0.25 M) at pH = 8.9 was studied in detail. Cu_1.4Mn_1.6O₄ and CuMn₂O₄ spinels were formed (80 °C, pH = 8.9) by aging the precursor precipitate between 0 to 1440 min, after drying (120 °C, 16 h) and calcination at 500 °C for 17 h. [35]. Different variations in the Na₂CO₃ and metal salts concentrations, heat treatment times, and temperatures were used to prepare Cu–Mn spinels with various properties from nitrate salts of manganese and copper (80 °C, pH = 8.3, calcination at 700 °C for 7 h [36] and pH = 8.5, drying in air at 110 °C for 12 h, calcination at 300 or 500 °C in air for 4 h [20]) with an aqueous solution of Na₂CO₃ as precipitation agent. Instead of sodium carbonate, ammonium carbonate can also be used, especially in preparing metal-doped spinels to avoid odium contamination. For example, La-doped Cu_1.5Mn_1.5O₄ was also prepared by co-precipitation from Mn(CH₃COO)₂ 4H₂O and Cu(NO₃)₂·3H₂O solutions spiked with La(NO₃)₃·6H₂O in a water bath at 60 °C with 1.5 M (NH₄)₂CO₃ solution at a pH of 8. Calcination was done at 550 °C in air for 2 h [37].

Polycarboxylic acids such as citric acid or oxalic acid (or its precursor, e.g., ethylene glycol, under oxidative conditions) are used as precipitating and complex or gel-forming reagents in Cu–Mn spinel synthesis. The mixed copper–manganese oxalates, as thermally easily decomposable and reactive compounds, are important precursors of direct spinel synthesis. According to this, copper and manganese nitrates were dissolved in water and mixed with an aq. solution of oxalic acid in 20% excess at room temperature. The precipitate was dried at 110 °C overnight and calcined at 300–500 °C for 4 h [20]. Gel/xerogel/polymer forming is also a convenient way to prepare homogeneously mixed quasi–solid precursors with a controlled Cu and Mn salt content. Cu_1.5Mn_1.5O₄ and its composites with CuO (50 and 77% CuO) were prepared with the use of 2% ionotropic sodium alginate and 0.1 M (total metal salt) CuCl₂ and MnCl₂ solutions in various (including 1:1) Cu:Mn stoichiometric ratios by stirring at room temperature for 12 h and by subsequent alcoholic dehydration (with increasing alcohol concentration from 10 to 100%). The xerogel formed was calcined at 450 °C for 8 h [38]. A gel-like intermediate was prepared from citric acid monohydrate, copper nitrate trihydrate, and a 50% aq. manganese(II) nitrate solution (the nitrate:citric acid molar ratio was 1:1.2) and ethylene glycol (the nitrate:ethylene glycol molar ratio was 1:3). The mixture was stirred at 70 °C, and the gel that formed was calcined at 500 °C for 3 h [19]. Copper and manganese nitrates were dissolved in a mixture of ethylene glycol, water, and concentrated nitric acid. The solution was stirred under reflux (105 °C) for 2 h and dried at the same temperature for 16 h [39]. The method resulted in the formation of polymeric precursors consisting of ethylene glycol and its oxidation product (oxalic acid). The drying procedure was adjusted to the different hydrolysis rates of the precursor complexes resulting in variations in phase compositions and crystallinity. Calcination was done at a temperature of 350 and 500 °C for 5 h [39].

A simple way to prepare Cu_xMn_3−xO₄ spinels is based on the autoignition of a mixed solution of urea with manganese nitrate and copper nitrate (75% excess of urea) in an open muffle furnace at 550 °C for 1 h [36,40]. Manganese(II) acetate and a calculated amount of copper nitrate (and dopant metal precursors) were dissolved in water, then a KMnO₄ solution was added at room temperature, then the precipitate that formed was calcined to Cu_xMn_3−xO₄ spinels in air at 300 °C for 2 h [41]. The precipitation agent (ammonia) can be prepared in situ from the hydrolysis of hexamethylenetetramine [13].

The results of the preparation of Cu_xMn_3−xO₄ spinels from precursors made in solution-phase reactions are summarized in

Table 2. Synthesis conditions of Cu_xMn_3−xO₄ catalyst with solution-phase methods.

x	Precursors	Synthesis/Annealing Conditions	[Ref.]
0.30–1.05	Cu(II) and Mn(II) nitrates	Aq. soln., 25–100% excess of urea, jellifying at 100 °C, combustion at 4–500 °C, calcining at 550 °C for 1 h	[40]
1	Cu(II) and Mn(II) nitrates	80 °C, adding of Na2CO3, (2 M), pH = 8.3, calcining in static air at 200–400 °C for 0.5–6 h	[33]
1	Cu(II) and Mn(II) nitrates	Ethylene-glycol–water–nitric acid mixture, 2 h reflux at 105 °C, calcining at 4–500 °C for 5 h	[39]
1	Cu(II) and Mn(II) nitrates	Me4NOH, 25 °C, 20 min, drying at 100 °C for 24 h and calcining at 400 °C for 5 h	[26,42]
1–2	Cu(II) and Mn(II) nitrates	1−1 M aq. metal salts, 2 M fuel (glycerol, maleic acid, citric acid, ethyl acetoacetate, or nitric acid), 1 M NaCl, jellifying at 150 °C for 12 h, calcining at 400–700 °C for 6 h	[43]
1.1–1.5	Cu(II)– and Mn(II) nitrates	Spray-drying at 300 °C, firing at 350 °C for 24 h, sintering at 530–940 °C for 48 h, quenching in water	[9]
1.2–1.4	Cu(II) and Mn(II) nitrates	80 °C, aq. Na2CO3, pH = 8.9, 10 min, conditioning between 0 and 1440 min at 80 °C in the mother liq., calcining at 500 °C for 17 h	[35]
1.4	Cu(II) and Mn(II) nitrates	Incipient wetness impregnation, TiO2, 100 °C for 5 h then calcining at 500 °C for 7 h in air	[18]
1.5	Cu(II) and Mn(II) nitrates	NaOH, 298 K, pH = 11, 30 min, calcining at 300 °C for 2 h	[25]
1.1–1.5	Cu(II)– and Mn(II) nitrates	NaOH, firing in air at 530–940 °C for 48 h, quenching in H2O	[9,21,22,23]
1.5	Cu(II) and Mn(II) nitrates	Abs. ethanol, dip-coating, Al foil, room temp., drying at 300 °C for 3 h, calcining at 4–500 °C for 15–90 min	[44]
1.5	Cu(II) and Mn(II) nitrates	Urea, citric acid, 0.3 M KMnO4, pH = 6–7, 4 h, hydrothermal treatment at 180 °C for 12 h, calcining at 350 °C in air for 4 h	[32]
1.5	Cu(II) and Mn(II) nitrates	Autoignition, 75% excess of urea, open furnace at 4–500 °C, calcining at 550 °C for 1 h	[36,40]
1.5	Mn(II) and Cu(II) nitrates	Aq. Na2CO3, 80 °C, pH = 8.3, aging at 80 °C for 6 h, calcining at 700 °C for 7 h	[36,45]
1.5	Cu(II) and Mn(II) nitrates	1.2 M Na2CO3, 80 5C, pH = 8.5, calcining at 300–500 °C in air for 4 h	[20]
1.5	Cu(II) and Mn(II) nitrates	20% excess of aq. oxalic acid, room temperature, calcining at 300–500 °C for 4 h	[20]
0.5–2	Cu(II) and Mn(II) nitrates	Aq. citric acid, ethylene glycol, 30 °C for 2 h, jellifying at 70 °C, calcining at 500 °C for 3 h	[46]
1.5	Mn(II) nitrate, [Cu(NH3)4](II)–hydroxide	Mn nitrate (pH = 3–4) and [Cu(NH3)4](II)–hydroxide (pH = 9–10) solutions were mixed, 30–40 °C, calcining at 550 °C for 4 h	[29,30]
1.8	Mn(II) nitrate, Cu(II) acetate	Reverse microemulsion, water/triton–X100/n–octanol/cyclohexane, 25% aq. NH4OH. Stirring for 1 h, calcining at 450–1000 °C in oxygen	[47]
1.5	Mn(II) acetate, Cu(II) nitrate	Aq. Me4NOH, aging for 2 h, calcining in dry air at 400 °C for 5 h	[48]
1.5	Mn(II) acetate, Cu(II) nitrate	KMnO4, room temp., 24 h, calcining at 300 °C for 2 h	[41]
1.5	Mn(II) acetate, Cu(II) nitrate	Aq. soln., La nitrate, 1.5 M ammonium carbonate, pH = 8, 2 h aging, calcining at 550 °C for 2 h in air	[37]
1	Cu(II) and Mn(II) acetates	Aq. Na2CO3, 80 °C, pH = 8–10, 30 min, aging at 80 °C for 12 h, calcining at 500 °C for 20 min, 10 M NaOH, hydrothermal treatment at 110 °C for 20 h, calcining at 250 °C for 1 h	[49]
1.5	Cu(II) and Mn(II) acetates	0.2 M aq. Na2CO3, room temp., 4 h, calcining at 500 °C for 6 h	[33,34]
1.5	Cu(II) and Mn(II) acetates	NaOH, 298 K, pH = 11, 30 min, calcining at 300 °C for 2 h	[25]
1.5	Cu(II) and Mn(II) chloride	Na2CO3, 3 h aging, drying at 80 °C overnight, calcining at 450 °C for 5 h	[13]
1.5	Cu(II) and Mn(II) chlorides	Sodium alginate (2 wt.%), room temp. for 12 h, dehydration with successive ethanol−water mixture washing (10–100%), 15 min each, calcining at 450 °C for 8 h	[37]
1.2–1.5	Cu(II) and Mn(II) chlorides	pH = 4, sodium oxalate, 550–650 °C	[23,50]
1	Mn(II) sulfate, copper(II) chloride	Solvothermal method, water: methanol = 60:40, ethylene glycol, 1 M NaOH, pH = 10, 120–160 °C foe 12 h	[31]
1.1–1.5	Cu(II) and Mn(II) sulfates	NaOH, firing in air at 530–940 °C for 48 h, quenching in H2O	[9,21,22,23]

.

The solution-phase spinel precursor syntheses ensure the complete distribution of copper and manganese ions and the monophase spinel synthesis. The reactive species can be transformed into spinels at lower temperatures (>250 °C) than the solid oxides or oxide precursors. These methods are typical in the preparation of copper-rich spinels [9,13,21,22,23,38,50]. The copper manganese oxide spinels with x ≥ 1 were prepared from the appropriate sulfate, chloride, and acetate salts with such precipitating agents as NaOH, NaOH, Na₂CO₃, sodium oxalate, or sodium alginate. The most frequently used precursor salts are nitrate salts. For example, Cu_1.5Mn_1.5O₄ can be prepared from the Mn(II) acetate and Cu(II) nitrate via precipitating the solid precursors with ammonium carbonate or tetramethylammonium hydroxide, or with redox precipitation by KMnO₄ [20,37,41]. The solid precursors for the synthesis of spinels with x = 1.5 and 1.8 were deposited from manganese(II) nitrate solution with the use of [tetraamminecopper(II)] hydroxide, and ammonium hydroxide (in reverse microemulsions of water–surfactant–n-octanol–cyclohexane) as precipitating agents, respectively [29,30,47].

The Cu_xMn_3−xO₄ spinels with x = 0.3–1.5 can be prepared from copper(II) and manganese(II) nitrate solutions with various methods. The precursors for heat treatment were precipitated with Na₂CO₃ [20,33,35] Me₄NOH [26,42], NaOH [9,21,22,23,25,31], or oxalic acid [20]. Direct impregnation methods including dip-coating, incipient wetness impregnation and spray drying were also used with subsequent thermal decomposition of the solid metal nitrates into active oxide precursors [9,18,44]. Gel-forming materials such as urea, an ethylene-glycol–nitric acid mixture, citric acid [32,39,46], or autoignition methods with organic fuels were also used to prepare the Cu_xMn_3−xO₄ spinels with x ≥ 1 [36,40,43].

2.3. Thermal Decomposition of the Hydrated and Ammonia-Complexed Copper Permanganate

Anhydrous copper(II) permanganate has not been isolated yet, because the hydrated and the ammonia-complexed copper(II) permanganates decompose at lower temperatures than their ligand loss temperatures [14]. The hydrates of copper permanganate (Cu(MnO₄)₂·nH₂O (n = 2, 3, 4, 6, 8), however, were prepared easily via various synthesis methods [14,51]. Its composition ensures an optimal Cu:Mn = 1:2 ratio to prepare the regular CuMn₂O₄ spinel. Its thermal decomposition starts at 80 °C with oxygen evolution in an autocatalytic process [52]. The decomposition of the permanganate ion and water release are parallel processes during heating. Thus, the water content of copper permanganate (n = 1.7–4.1 with evaporation at 290–330 K in vacuum) does not influence the decomposition temperature or the composition of the thermal decomposition products. The product of the first decomposition step is a mixed-phase copper manganese oxide with a composition of CuMn₂O_5.45 [53]. Kinetic studies performed in the range of 335 and 370 K showed that in the median range of thermal decomposition, the dominant decomposition feature is the constant reaction rate without any acceleratory behavior, and the induction period was also short. The mixed-phase decomposition intermediate decomposes further when the temperature is increased to 695 K, and the amorphous product formed corresponds to the CuMn₂O₅ formula. Magnetic susceptibility measurements showed that the valence state of the copper above 725 K is exclusively Cu^I. Thus “CuMn₂O₅” cannot be a mixture of CuO + 2MnO₂. When the temperature was increased to 1175 K, the expected CuMn₂O₄ and its partial decomposition product, Mn₂O₃, were formed [54].

The basic copper(II) permanganate, Cu₂(OH)₃MnO₄, was prepared by an anion exchange of the layered Cu₂(OH)₃OAc with 1 M KMnO₄ in a day [55]. The thermal decomposition of Cu₂(OH)₃MnO₄ can be described as:

Cu₂(OH)₃MnO₄ = 0.5CuMn₂O₄ + 1.5CuO + 1.5H₂O + O₂

The amorphous mixed copper manganese oxide that formed catalyzes the oxidation of CO at 30 °C. The amorphous mixed copper manganese oxide crystallizes at 440 °C, and loses its catalytic activity above 500 °C but can be reactivated in oxygen at 400 °C. It decomposes at 940 °C into CuMnO₂ [55].

We developed a solid-phase reaction to obtain CuMn₂O₄ spinel materials with x = 1 at a low temperature (between 100 and 500 °C), via a quasi-intramolecular redox reaction of the ammonia ligand and permanganate anion in [Cu(NH₃)₄](MnO₄)₂ [56]. The decomposition reaction of [tetraamminecopper(II)] permanganate strongly depends on the reaction conditions. It explodes on fast heating at 65 °C and ignites at >8 atm O₂ in a violent reaction (dark red flame, 1500 K combustion temperature) [57], according to the equation.

[Cu(NH₃)₄](MnO₄)₂ = Cu(l) + 2/3 Mn₃O₄(l) + 5.34H₂O(g) + 0.44NH₃ + 1.78N₂

Thermogravimetric studies on the decomposition of [tetraamminecopper(II)] permanganate showed a stepwise ligand and oxygen-releasing process [58], and the solid decomposition end-product was identified on the basis of weight loss as CuMn₂O₄ [58]. Although the decomposition mechanism declared was proven to be incorrect, the end-product of the decomposition was an amorphous material with CuMn₂O₄ composition [56]. The decomposition residue did not dissolve in HNO₃ at all, and the decomposition process was formally written as [56].

[Cu(NH₃)₄](MnO₄)₂ = CuMn₂O₄ + 2NH₃ + NH₄NO₃ + H₂O

NH₄NO₃ = N₂O + 2H₂O

Solid-phase decomposition is frequently explosion-like; therefore, to control the decomposition process, the decomposition reaction was performed under inert solvents like CHCl₃ and CCl₄. In this way, the decomposition temperature cannot exceed the boiling point of the solvent, because the evaporation heat of the solvents absorbs the exothermic reaction heat. The solid products are the expected spinel and ammonium nitrate; every other reaction product is gas. Since ammonium nitrate can be dissolved out with water and decomposed thermally as well, we studied the effect of the synthesis temperature (CHCl₃–61 °C, CCl₄–77 °C) and the isolation methods (aqueous extraction or thermal decomposition of ammonium nitrate) on the composition and properties of the resulting spinel compounds [59,60].

This temperature-limited process prevents nucleation and crystallization of the formed copper manganese oxides driven by the exothermicity-derived reaction heat (289.8 kJ/mol) [57]. The amorphous copper manganese oxides were characterized with the formula of CuMn₂O_4+x from x = 0 (CuMn₂O_{4,spinel structure}) to x = 0.5 (Cu_0.89Mn_1.78□_0.32O₄ or Cu_0.67Mn_1.33MnO₃) with defect structure (□ is a defect in the spinel structure). Depending on the removal method of ammonium nitrate (washing with water or thermal decomposition) and the temperature and time of the subsequent heat treatments, the size and phase composition of the products can be controlled (Scheme 2). This method ensures a safe and easy route to prepare copper manganese oxide nanoparticles in various compositions [60]. The samples containing copper manganese oxide and ammonium nitrate prepared in CCl₄ at 77 °C were extracted with water to remove ammonium nitrate. Subsequent heat treatment of the extraction residue up to 400 °C for 1–8 h resulted in copper manganese oxides, which d(001) peak shows the initial stage of the formation of the spinel structure with 2 nm crystallite sizes. The crystallization of the spinels starts at 500 °C when crystallites with the size of 80 nm were obtained [60].

3. Composition and Crystal Structure of Cu_xMn_3−xO₄ Spinels

Hausmannite (Mn^IIMn^III₂O₄) occurs in both tetragonal (room temperature) and cubic (high temperature, >>1000 K) forms. The two phases co-exist in a wide temperature range [61]. The cubic spinel phase can be stabilized at lower temperatures by doping with copper [21] despite the fact that both Mn^III([Ar]d⁴) and Cu^II([Ar]d⁹) ions show Jahn–Teller activity in both octahedral and tetrahedral spinel sites. These Cu_xMn_3−xO₄ mixed oxides are solid solutions, where copper substitutes manganese in the tetragonal lattice of hausmannite (Mn₃O₄, Mn^IIMn^III₂O₄). Depending on the amount, site, and valence of copper (Cu^I or Cu^II), and manganese species (Mn^II, Mn^III, and Mn^IV), distorted cubic or tetragonal spinel lattices are built up. The Cu_xMn_3−xO₄ spinels with x ≈ 1 have a tetragonally deformed structure with a c/a ratio slightly varying between 1.05 and 1.03. Above x = 1.06, a cubic spinel structure was found with a diminishing lattice parameter upon increasing copper content [9]. Formally, the stoichiometric spinel structures can be formulated as [62,63,64]:

Cu^II[Mn^III₂O₄] and Cu^I[Mn^IIIMn^IVO₄]

In octahedral field symmetry, Mn^III and Cu^II d levels are split into t_2g³e_g¹ and (t_2g⁶)(e_g²)e_g¹ levels. In tetrahedral symmetry, they split into e²t₂² and (e⁴)(t₂⁴)(t₂¹), respectively [10]. The mixing of copper and manganese valences in the spinel lattice leads to subcritical levels of Jahn–Teller active ions, thereby causing no distortion [21,65]. As a result of that, the ionic configurations in Cu_xMn_3−xO₄ spinels can be written as:

Cu^I_aMn^II_bMn^III_1−a−b[Cu^II_x−aMn^III_3−2x−bMn^IV_x+a+b−1]O₄

The tetrahedral sites of the spinel lattice can mainly be occupied by Cu^I, Mn^II, and Mn^III, whereas the octahedral sites are mainly occupied by Cu^II, Mn^III, and Mn^IV ions. Thus, only the Mn^III ions can disperse between the A (tetrahedral) and B (octahedral) spinel sites, in any proportion; Cu^I and Mn^II strongly favor the tetrahedral, whereas Cu^II and Mn^IV the octahedral sites. The electrical conductivity of cubic Mn₃O₄ was explained by small polaron hopping between Mn^III and Mn^III on octahedral sites [66] and the disproportionation reaction between two Mn^III ions, resulting in Mn^II and Mn^IV [67] at the octahedral sites. The Mn^II in the tetrahedral sites is the consequence of the equation,

Mn^III_A + Mn^III_B = Mn^II_A + Mn^IV_B

which is shifted to the Mn^II + Mn^IV side [24] in the manganese-rich compounds.

Cu^II appeared in the octahedral site above 300 °C due to the reaction of tetrahedral Cu^I components and octahedral Mn^IV components [24], according to the equation.

Cu^I_A +Mn^IV_B = Cu^II_B + Mn^III_A

Furthermore, a thermally activated site exchange reaction was also detected by neutron diffraction measurement on the CuMn₂O₄ samples quenched from 1213 K due to the strong octahedral site preference of the Jahn–Teller Cu^II–ion compared to that of Mn^II [68]. As a result of these reactions, various cation distributions were found experimentally in the samples formally from the normal (M^IIM^III₂O₄) to inverse spinel (M^III[M^IIM^III]O₄ structures. Therefore, the cation valence and site distributions observed at room temperature strongly depends not only on the synthesis method but also on the annealing conditions and cooling rates as well [66,67,68,69,70,71,72]. For example, the CuMn₂O₄ crystals obtained from a 1:1 mixture of CuO and Mn₂O₃ in vacuum at 1323 K and with cooling at 6 K/h to room temperature resulted in Cu^I_0.2Mn^II_0.8[Cu^II_0.8Mn^III_0.2Mn^IV_1.0]O₄ [65], whereas the room temperature magnetic susceptibility measurements on quenched CuMn₂O₄ showed that all Cu is monovalent, and all manganese is tetravalent [69].

Neutron diffraction experiments showed that CuMn₂O₄ prepared in a solid-state reaction of oxides at 700 °C for 240 h has a partially inverted cation distribution (12–13% of copper ions located on octahedral sites [17]). Since the copper ions are mainly expected to be monovalent Cu^I at the tetrahedral sites, it means that all Mn⁴⁺ and Cu²⁺ ions should be located at the octahedral sites [13]. The magnetic susceptibility and Curie constant parameters of Cu_xMn_3−xO₄ spinels confirm the Cu^I(T–4)–Mn^IV(OC–6) configuration, and according to this, at x = 1.5 (Cu_1.5Mn_1.5O₄), the favored configuration is Cu^I[Cu^II_0.5Mn^IV_1.0Mn^III_0.5]O₄. The cubic structure (x > 1.06) is an indicative parameter of the low concentration of distorting Mn^III and Cu^II cations.

The surface composition of the copper manganese oxides is not identical to their bulk composition, e.g., typically, Cu^II is enriched at the surface layer of CuMn₂O₄ and Cu_1.5Mn_1.5O₄ [44]. Thin films of copper–manganese oxides were prepared on aluminum foil by dip coating from nitrate salts dissolved in ethanol at room temperature. The coatings were decomposed and annealed at 300 °C and between 400 and 500 °C, respectively, for periods between 15 and 90 min. The results summarized in

Table 3. Distribution of copper ions between the T–4 and OC–6 sites of the spinel lattice in Cu_xMn_3−xO₄ samples.

Cu/Mn	CuI/CuII	CuII (T–4) (A)	CuII(OC–6)(B)	CuI(T–4)	Preparation Method	Ref.
0.83	0.47	25	43	32	Dip-coating	[44]
0.88	0.47	25	43	32	Dip-coating	[44]
1.14	0	33	67	0	Dip-coating	[44]
1.08	0	23	77	0	Dip-coating	[44]
1.23	0.13	41	47	12	Dip-coating	[44]
2	3.16		24	76	Ceramic	[21]
2	1.00	50	25	25	Precipitation	[73]
2	2.00	0	33	66	Ceramic	[50]
2	0.39	48	24	28	Ceramic	[71]
2	0.67	0	60	40	Ceramic	[74]
1	2.00	0	33	66	Ceramic	[9,21]

show that the preparation method of copper manganese oxide spinels has a strong influence on the amount and distribution of the copper and manganese species with various valence at the different spinel crystal lattice sites. Furthermore, the precipitation and ceramic methods resulted in copper manganese oxide spinels with completely different metal ion valences and distributions, depending on the synthesis methods used. Not only the preparation method, but within one preparation route, the post-treatment conditions such as annealing time and temperature also have an enormous influence on the composition and properties of the Cu_xMn_3−xO₄ films (Figure 3).

For example, for the preparation of Cu_1.5Mn_1.5O₄, with the dip-coating of nitrates, the reaction has to be carried out above 450 °C and the full conversion of Cu and Mn oxides into Cu_1.5Mn_1.5O₄ requires at least 60 min. The high conductivity of the films prepared in this way may be attributed to the presence of Mn³⁺ and Mn⁴⁺ in octahedral and Cu⁺ and Cu²⁺ in tetrahedral sites, respectively. This is detected by XPS and may be attributed to the electron hopping between Cu^I and Mn^IV ions [44]. This unexpected distribution of copper valences in the film is similar to the distribution of copper valences in the surface layer of the bulk compound with the same composition.

The relative amount of copper, manganese, and the lattice defects, and also the distribution of valence states of copper and manganese ions and the locations of each type of ions at various spinel sites are one of the key factors in the catalytic activity of Cu_xMn_3−xO₄ spinels.

4. Overview of the Catalytic Activity of Cu_xMn_3−xO₄ Spinels

One of the oldest and most important application areas of copper manganese oxide catalysts, including Cu_xMn_3−xO₄ spinels and mixtures containing a Cu_xMn_3−xO₄ spinel is the removal of carbon monoxide from air. The Cu_xMn_3−xO₄ spinels, however, proved to be very active catalysts in such important processes, such as the recovery of aromatic and chlorinated hydrocarbons from air, methanol reformation to produce hydrogen, and in a series of industrially important reactions, which are summarized in

Table 4. Removal of carbon monoxide from the air in the presence of Cu_xMn_3−xO₄ catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Refs.	Remarks
x = 0.5	Aq. solutions of Cu and Mn nitrates in 1/2 molar ratio were mixed, aq. NaOH solution, 298 K, pH = 11, 300 °C for 2 h	1% CO, 20% O2 and 79% N2, space velocity was 20,000 mL h−1 g cat−1. The temperature was ramped at a rate of 1 °C min−1 from 0 °C to the final temperature, T50/T100 = 80/120 °C, resp.	[25]	SBET = 46 m2 g−1, Vp = 0.23 cm3 g−1
x = 0.5	Thermal decomposition of the [Cu2(OH)3MnO4] layered basic salt	It catalyzes CO oxidation at 30 °C	[55]	Mixed phase with CuO
x = 1.0	Mn–nitrate, Cu–nitrate, 0.25 M each, 2 M Na2CO3, pH = 8.3, 400 for 2	5000 ppm CO in air, 25 °C, 33,000 h−1, cooling due to 10 °C adiabatic temperature increase	[33]	SBET = 93 m2 g−1, mixed phase
x = 1.0	Metal nitrates (Cu:Mn = 1:4) in ethylene glycol–water–cc. nitric acid mixture, 2 h reflux, drying at 105 °C for 16 h, 450–500 °C	1 vol.% CO in synthetic air, 100–200 mm particles, 25 °C, flow rate: 30 000 mL h−1 g−1; Experiments with moist air were carried out near low-temperature fuel cell conditions using a different feed gas composition (1 vol.% CO, 4.88 vol.% synthetic air, 20 vol.% CO2 in hydrogen), 60–120 °C, 17–20% CO conversion after 1 h	[39]	SBET = 55–67 m2 g−1
x = 1.0	Cu–nitrate and Mn–nitrate in 1:2 ratio, 80 °C, 0.25 M Na2CO3, pH = 8.9, aging from 0 to 720 min	5% CO in He and O2 (1:10 v/v) were fed to the reactor, 20 °C; total GHSV of 33,000 h−1	[35]	SBET = 26 m2 g−1
x = 1.0	[Cu(NH3)4](MnO4)2 decomposition in CHCl3 at 61 °C, aq. extraction, 300 °C for 1 h	0.5% CO and 0.5% O2 in He, (1.37 gCO gcat−1 h−1), T10/T25/T50 = 94/132/163 °C, resp.	[60]	Mixed phase
x = 1.0	[Cu(NH3)4](MnO4)2 decomposition in CHCl3 at 61 °C, 300 °C for 1 h °C	0.5% CO and 0.5% O2 in He, (1.37 gCO gcat−1 h−1), T10/T25/T50 = 134/159/188 °C, resp.	[60]	–
x = 1.0	[Cu(NH3)4](MnO4)2 decomposition in CCl4 at 77 °C, with aq. extraction (NH4NO3 removal), 200–500 °C for 1–8 h	0.5% CO and 0.5% O2 in He, (1.37 gCO gcat−1 h−1), without heat treatment T10/T25/T50 = 121/130/161 °C, resp.; 200 °C for 1 and 8 h, T10/T25/T50 = 135/155/173 °C and 127/155/177 °C, resp.; 500 °C for 1 and 8 h, T10/T25/T50 = 111/142/170 and 137/164/190 °C, resp.	[60]	–
x = 1.0	[Cu(NH3)4](MnO4)2 decomposition in CCl4 at 77 °C, 500 °C for 1 h	0.5% CO and 0.5% O2 in He, (1.37 gCO gcat−1 h−1), T10/T25/T50 = 166/190/>200 °C, resp.	[60]	–
x = 1.4	Cu–nitrate and Mn–nitrate, 1:2 ratio, 80 °C, 0.25 M Na2CO3, pH = 8.9, aging from 0 to 300 and 1440 min	5% CO in He and O2 (1:10 v/v) were fed to the reactor, 20 °C; total gas hourly space velocity of 33,000 h−1	[35]	SBET = 23–30 m2 g−1, mixed phase
x = 1.4	Cu:Mn = 1:1, MnCl2, CuCl2, Na2CO3, 3 h aging,, 450 °C for 5 h	1000 ppm CO, GHSV = 50,000 mL g−1 h−1, T50/T90 = 62/76 °C, resp., reaction rate = 0.36 mmol g−1 h−1 and 1.45 mmol m−2 h−1 at 50 °C; reaction rates were measured at isotherm CO oxidation at 80–180 °C at less than 20% conversion	[13]	SBET = 18 m2 g−1, Vp =0.02 cm3 g−1, APD = 13
x = 1.5	Mn–acetate, Cu–nitrate, KMnO4, 300 °C for 2 h	1% CO (20% air in Ar) was fed into the reactor for the CO oxidation reaction, T50/T70/T90 = 59/79/93 °C, resp., the complete transformation was found at 100 °C, Co and Fe doping increases the efficiency	[41]	Mixed phases
x = 1.5	Copper and manganese acetates in 1/2 molar ratio in water, NaOH solution, 298 K, pH 11 300 °C for 2 h	1% CO, 20% O2 and 79% N2 as feed gas with a space velocity of 20,000 mL h−1 g cat−1. The temperature was ramped at a rate of 1 °C min−1 from 0 °C to the final temperature, T50/T100 = 72/110 °C, resp.	[25]	SBET = 70 m2 g−1, Vp = 0.36 cm3 g−1
x = 1.5	Hydrothermal method, aq. Cu and Mn nitrates, citric acid, urea, KMnO4 solution, Mn(NO3)2/KMnO4 = 1:2 and urea/citric acid/Mnall/Cu = 1.5:1.5:1:0–0.2; Aq. ammonia (25 wt.%), pH = 6–7, 180 °C for 12 h, 350 °C in air for 4 h	1.6 vol% CO, 20.8 vol% O2, and 77.6 vol% N2, a space velocity of 30,000 mL·g−1·h−1. It is active at room temperature. 100% CO oxidation was achieved at 65 °C	[32]	Mixed phase, SBET = 112–126 m2 g−1, 5.8–10.1 nm size

S_BET—specific surface area; V_p—the total adsorption pore volume at P/P₀ = 0.995; APD—average pore diameter; TOF—turnover frequency; GHSV—gas hourly space velocity.

,

Table 5. Copper manganese oxide spinel catalysts and their activity in NO oxidation reactions.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Ref.	Remarks
x = 1.5	Hydrothermal method, aq. Cu and Mn nitrates, citric acid, urea, KMnO4 solution, Mn(NO3)2/KMnO4 = 1:2 and urea/citric acid/Mnall/Cu = 1.5:1.5:1:0–0.2; aq. ammonia (25 wt.%), pH = 6–7, 180 °C for 12 h, 350 °C in air for 4 h.	5 vol% NO, 10 vol% CO, and 85 vol% He as diluents, GHSV 24,000 mL·g−1·h−1 The 100% NO conversion and 100% N2 selectivity were achieved at 275 °C	[32]	Mixed phase, SBET = 112–126 m2 g−1, 5.8–10.1 nm size

S_BET—specific surface area; GHSV—gas hourly space velocity.

,

Table 6. Desulfurization with Cu_xMn_3−xO₄ catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Ref.	Remarks
x = 1.15	CuO, MnO2, 5% graphite, 950 °C for 6 h	Atmospheric pressure, space velocity of 6000 h−1, 0.5 vol% H2S, 10 vol% H2, 15 vol% H2O(v), 5 vol% CO2, and 15 vol% CO, balance N2, 600 °C	[12]	Mixed oxide phase, porous, Vp = 0.16 cm3 g−1
x = 1.47	CuO, MnO2, 5% graphite, 950 °C for 6 h	Atmospheric pressure, space velocity of 6000 h−1, 0.5 vol% H2S, 10 vol% H2, 15 vol% H2O(v), 5 vol% CO2, and 15 vol% CO, balance N2, 600 °C	[12]	Mixed oxide phase, porous, Vp = 0.14 cm3 g−1

V_p—the total adsorption pore volume at P/P₀ = 0.995.

,

Table 7. Removal of chlorinated hydrocarbons from air in the presence of Cu_xMn_3−xO₄ catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Ref.	Remarks
x = 1.5	Tetramethylammonium hydroxide precipitating agent, aq. copper(II) and manganese(II) salt solution, dry air at 400 °C for 5 h	Decomposition of trichloroethylene in moist air by post–plasma catalysis, 300 °C, GHSV 60,000 mL g−1 h−1, relative humidity (20 °C) 15%, initial TCE concentration 300 ppm, 50% CO2 yield. T50 = 300 °C (244 °C for CO2 formation).	[48]	SiC support, SBET = 30 m2 g−1, Vp = 0.29 cm3 g−1

S_BET—specific surface area; V_p—the total adsorption pore volume at P/P₀ = 0.995; GHSV—gas hourly space velocity.

,

Table 8. Removal of benzene from the air in the presence of Cu_xMn_3−xO₄ catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Refs.	Remarks
x = 0.5	Citric acid monohydrate, copper nitrate and 50% manganese nitrate solutions, ethylene glycol, 70 °C, the gel was dried and calcined for 3 h at 500 °C	100 ppm benzene in synthetic air, GHSV = 60,000 mL g−1 h−1, T10/T50/T90 = 114/164/193 °C, resp.	[46]	Crystallite size 13.2 nm, SBET = 45.00 m2 g−1, Vp = 0.310 cm3 g−1
x = 1.0	Mn and Cu nitrate, NaHCO3 as precipitating agent, 400–600 °C	150 ppm benzene concentration, 10% O2 in the gas stream, 2250 ppm ozone, 70 °C	[42]	Mixture with oxide phases
x = 1.0	Citric acid monohydrate, copper nitrate and 50% manganese nitrate solutions, ethylene glycol, 70 °C, the gel was dried and calcined for 3 h at 500 °C	100 ppm benzene in synthetic air, GHSV = 60,000 mL g−1 h−1, T10/T50/T90 = 96/149/186 °C, resp.	[46]	Crystallite size 11.6 nm, SBET = 68.26 m2 g−1, Vp = 0.314 cm3 g−1
x = 1.5	Citric acid monohydrate, copper nitrate and 50% manganese nitrate solutions, ethylene glycol, 70 °C, the gel was dried and calcined for 3 h at 500 °C	100 ppm benzene in synthetic air, GHSV = 60,000 mL g−1 h−1, T10/T50/T90 = 136/188/216 °C, resp.	[46]	Crystallite size 14.7 nm, SBET = 26.53 m2 g−1, Vp = 0.161 cm3 g−1
x = 2	Citric acid monohydrate, copper nitrate and 50% manganese nitrate solutions, ethylene glycol, 70 °C, the gel was dried and calcined for 3 h at 500 °C	100 ppm benzene in synthetic air, WHSV = 60,000 mL g−1 h−1, T10/T50/T90 = 149/198/220 °C, resp.	[46]	Crystallite size 18.3 nm, SBET = 25.44 m2 g−1, Vp = 0.177 cm3 g−1

S_BET—specific surface area; V_p—the total adsorption pore volume at P/P₀ = 0.995; GHSV—gas hourly space velocity.

, Table 9, Table 10, Table 11, Table 12 and Table 13.

4.1. Removal of Carbon Monoxide from the Air

The Hopcalite catalyst used for the removal of CO from air is a mixed copper manganese oxide, the composition of which strongly depends on the synthesis conditions. Hopcalite was developed by the USA Chemical Warfare Service for gas masks against carbon monoxide during World War I. The heat treatment history of the Hopcalite catalyst is a key factor to ensure its catalytic activity in CO removal at room temperature [33,45]. The samples prepared between 400 and 500 °C for a calcination time of 2–18 h resulted in catalysts containing CuMn₂O₄, but their activity dropped to zero if the sample was prepared at 500 °C in 18 h. The sample containing crystalline CuMn₂O₄ was active for the room temperature oxidation of CO, but the amorphous precursors prepared by calcination at 300 °C showed higher activity than CuMn₂O₄ [33].

The lowest Cu content (x = 0.5) among the studied Cu_xMn_3−xO₄ catalysts proved to be active in CO oxidation was found in the samples prepared by the heat treatment of basic copper(II) permanganate, Cu₂(OH)₃MnO₄. This catalyst was active for CO removal at 30 °C [55]. The catalyst with the same composition prepared by precipitation from copper and manganese nitrates solution with NaOH and heated at 300 °C reached quantitative CO conversion in air at 120 °C [25].

The CuMn₂O₄ catalysts with x = 1, which are active in CO oxidation reactions, were prepared in various reaction routes (Table 4), including precipitation from metal nitrates with Na₂CO₃ [33,35], via the formation of complexes with citric acid and ethylene glycol (oxalate precursor) [39], and by the thermal decomposition of [Cu(NH₃)₄](MnO₄)₂ in CHCl₃ and CCl₄ at 61 and 77 °C [60]. The thermal decomposition of [Cu(NH₃)₄](MnO₄)₂ under inert solvents resulted in nanosized CuMn₂O₄ with a defect structure. The best catalytic performance in CO oxidation was found for the catalysts prepared in CHCl₃ and CCl₄ with the aqueous extraction of the NH₄NO₃ intermediate with subsequent annealing at 300 °C for 1 h (80 and ~95%, respectively). If the ammonium nitrate was removed by thermal decomposition, the catalytic activity of the Cu–Mn spinels in the course of CO oxidation [60] decreased. A catalyst (CCl₄, aq. leaching, 300 °C) was tested in 5 cycles (each cycle was 22–25 h). Its catalytic activity decreased with increasing reaction time from 5 × 10⁻³ to 2.5 × 10⁻³ mol CO₂ m⁻² h⁻¹ at 125 °C, mainly in the first three hours. This kind of activity profile was found for commercial Hopcalite catalysts as well [75].

Mixed-phase Cu_xMn_3−xO₄ spinel catalysts containing Mn₂O₃ were developed for fuel cells PROX (preferential CO oxidation in CO + H₂ mixtures). Copper and manganese nitrates were refluxed in a mixture of ethylene glycol, water, and concentrated nitric acid for 2 h and dried at 105 °C for an additional 16 h followed by calcination at a temperature of 350 and 500 °C for 5 h. The obtained samples converted 20 and 17% CO in 1 h at room temperature, respectively [39].

The precipitation method using Na₂CO₃ and metal nitrates resulted in Cu–Mn spinels with x = 1.0–1.4, depending on the aging time (from 0 to 1440 min). The conversion of CO was found to be ca. 50% for the unaged catalyst at room temperature and decreased with increasing aging time, but catalytic performance is restored after 720 min and reaches its maximum (the highest CO conversion). For this aging time, the composition of the catalyst is characterized by x = 1 (CuMn₂O₄) [35]. The copper-rich catalysts with x = 1.4–1.5 were prepared by precipitation methods, starting from the appropriate metal nitrates [35,41], acetates [25,41] and chlorides [13], by redox methods using KMnO₄ [41] and by combined methods using KMnO₄ and hydrothermal treatments with complex forming reagents, such as citric acid or oxalate precursors (ethylene glycol) [32]. Their detailed catalytic activities in CO oxidation are given in Table 4. The Cu_1.5Mn_1.5O₄ catalysts synthesized by the co-precipitation method from acetate or nitrate salt solutions of copper and manganese with NaOH and Na₂CO₃) were studied in detail [25] to maximize the efficiency of CO oxidation at room temperature. The activity of the oxidation of CO strongly depended on the combination of precipitant and precursor anions, ranking in the order (acetate + carbonate) > (nitrate + carbonate) > (acetate + hydroxide) > (nitrate + hydroxide) [25]. The redox reaction of Mn-acetate, Cu-nitrate, and KMnO₄ solution with calcination of the reaction product in air at 300 °C for 2 h resulted in a catalyst with the same composition (Cu_1.5Mn_1.5O₄), which was used in the low-temperature oxidation of CO. The presence of other transition metals added to the catalysts improved the catalytic properties. For example, the T₅₀ temperatures of CO conversion were 34 and 50 °C with Co- and Fe-doping, respectively [41]. The catalyst was prepared with a hybrid method, with the use of KMnO₄ as oxidant and citric acid as a complex-forming agent, together with ethylene glycol as oxalate precursor under hydrothermal conditions (180 °C for 12 h) and by calcination at 350 °C in air for 4 h contained nanosized (5–10 nm) particles [32]. The mechanism of the oxidation of CO with this catalyst is shown in Figure 4.

This catalyst shows activity even at room temperature, but 100% CO oxidation was achieved only at a temperature of 65 °C [32]. A plausible tentative mechanism for the carbon monoxide oxidation reaction over the copper manganese oxide spinel catalysts includes two main processes:

(1)

Molecular oxygen is preferentially adsorbed on the spinel surface and subsequently forms surface-active species containing adsorbed oxygen (O*) such as O₂⁻(ads) and O⁻(ads).

(2)

CO molecules are oxidized by O* species with the formation of gaseous carbon dioxide. The two-step reaction can be formulated as e.g., CO + O⁻(ads) → CO₂ + e⁻ and CO + 2O⁻(ads) → CO₃²⁻(ads).

The cooperative effect of the redox cycle between the metal centers in the spinel catalyst (Cu⁺ + Mn⁴⁺ ↔ Cu²⁺ + Mn³⁺) involving the Cu²⁺ and Mn³⁺ ions of the distorted spinel structure promotes the generation of Lewis acid sites on the surface, which play a beneficial role in the activation of reactants to form active species. These lead to the enhancement of catalytic activity (performance) in the CO oxidation reactions. The spinel catalyst transports the surface O* species. The Cu²⁺–O–Mn³⁺ units lose some oxygen to form CO₂ from CO and form oxygen defects. The spinel phase captures some oxygen from the gas phase to form surface-active O* species again. The detailed mechanism is given in [32].

4.2. Removal of NO from Air by Carbon Monoxide

The reaction of NO with carbon monoxide in the presence of copper manganese oxide catalysts, especially Cu_1.5Mn_1.5 spinels, results in CO₂ and N₂. The aging time of samples prepared from metal nitrates with sodium carbonate has crucial importance in the preparation of a catalyst for the NO SCR with carbon monoxide at ambient temperature [35]. The Cu_1.5Mn_1.5O₄ nanocatalyst given in Table 5 was prepared with a hybrid method. It can adsorb NO_x gases due to the presence of Cu⁺–CO active species. The surface-dispersed Cu^x+–O₂⁻–Mn^y+ active species can be reduced to Cu⁺–□–Mn^(4−x)+ active species at increasing adsorption temperature, and the synergetic effects between Cu and Mn (Cu^x+–O²⁻–Mn^y+ species) also play a role in the NO + CO SCR reaction [16]. The mechanism of the reaction is shown in Figure 5.

A possible tentative reaction mechanism for the NO + CO reaction over copper manganese oxide spinel catalysts (illustrated in Figure 5) is given in [32]. The formation of surface oxygen vacancies in the spinel catalysts contributes to the activation of NO and CO gases via the formation of highly reactive species. NO molecules are preferentially adsorbed on the spinel surface with the formation of surface NO_x species, thus only a small part of carbon monoxide molecules is oxidized by surface active oxygen (O*) derived from Cu–O–Mn linkages. The surface NO_x species are gradually dissociated into N and O atoms, with the formation of N₂O as an intermediate product [32], and surface O vacancies are also produced due to the synergistic effect between Cu and Mn.

The high or intermediate oxidation-state metal ions such as Mn⁴⁺, Mn³⁺, and Cu²⁺ are partially reduced by carbon monoxide with the formation of low or intermediate valence metal species, such as Mn³⁺, Mn²⁺, Cu⁺ or Cu⁰. The interaction between divalent and monovalent metal ions and surface CO_x species forms Mn²⁺–CO_x and Cu⁺–CO species. The higher the temperature, the more oxygen vacancies are generated, resulting in an enhancement of catalytic activity/performance. The catalytic cycle is when the CO molecules and the intermediate N₂O turn into gaseous CO₂ and N₂.

4.3. Removal of Other Pollutants from the Air and Other Gases

Copper manganese spinel catalysts are widely used materials in the oxidative recovery of various other inorganic and organic pollutants such as H₂S (Table 6), chlorinated hydrocarbons (Table 7), and aromatic hydrocarbons (Table 8 and Table 9). The recovery of hydrogen sulfide with porous copper-rich Cu_xMn_3−xO₄ spinel catalysts prepared from finely divided CuO, MnO₂, and graphite (x = 1.47 and x = 1.15) were tested from hot coal gas in the presence and absence of water. The pores in the catalysts are produced due to the release of the CO₂ gas that formed from the graphite and oxide phases. The regeneration of catalysts was performed with a regeneration gas containing 3 vol% O₂ (balance N₂) at 720 °C [12].

The oxidative removal of trichloroethylene from air was studied in the presence of Cu_1.5Mn_1.5O₄ catalyst carried out on SiC support. The decomposition of trichloroethylene in moist air (relative humidity was 15%) was done by post-plasma catalysis above 150 °C. Trichloroethylene removal is 19% and 50% at 250 °C and 300 °C, respectively. Up to 250 °C, the selectivity of the catalyst into CO₂ is 100%, whereas, at 300 °C, C₂Cl₄, CO, and CO₂ formed (Table 7) [58].

4.4. Removal of Aromatic Hydrocarbons from Air

The strong carcinogen effect of benzene generated a demand to remove it from the air. This may be done in the presence of Cu_xMn_3−xO₄ spinels with either synthetic air [46] or ozone [42]. The high reactivity of ozone towards benzene ensures a much lower reaction temperature (70 °C) than that of synthetic air (Table 8) [42,46]. The ozonation reaction was catalyzed with a mixed oxide phase containing CuMn₂O₄ prepared from Mn and Cu–nitrate solutions with NaHCO₃, by calcination between 400 and 600 °C. Oxidation with synthetic air was tested with a series of Cu_xMn_3−xO₄ spinels with x = 0.5, 1.0, 1.5, and 2. The Cu^II/Cu^I or Mn^III/Mn^IV ratios for the spinels with x = 0.5, 1.0, 1.5, and 2.0 were found to be 12.0, 20.6, 4.5, and 7.8 or 2.0, 2.5, 1.4, and 0.6, respectively. The catalytic oxidation of benzene was complete at a temperature lower than 200 °C. The Mn^III-occupied octahedral positions of the spinel lattice greatly improve catalytic activity, and the surface lattice O-species play a key role in the process [46].

The removal of toluene from air in the presence of various Cu_xMn_3−xO₄ spinels (x = 0.5–1.5) was studied in detail (Table 9). These catalysts were synthesized from the appropriate nitrate, acetate, and chloride salts by precipitation with hydroxides (NaOH [49], NH₄OH [47], Me₄NOH [26]) or carbonates (Na₂CO₃ [13,26] or (NH₄)₂CO₃) [37], including reverse emulsion formation [47] or hydrothermal reactions [49]. Incipient wetness impregnation and direct pyrolysis on γ–Al₂O₃ [76] and anatase [18] were used to prepare supported catalysts. A series of TiO₂-supported mixed-phase catalysts containing Cu_1.4Mn_1.6O₄ showed high catalytic activity with the total conversion (100%) of toluene into carbon dioxide above 240 °C, without any deactivation of the catalyst. The mobility of the network oxygen in the structure of the catalyst is a key factor in the catalytic process. The gradual loss of catalytic activity below 240 °C can be attributed to the adsorption of carbonaceous compounds on the surface, and according to this, catalytic activity can easily be regenerated by heating the sued catalysts in airflow at 300 °C [18].

Alcoholic dehydration of the xerogel made with sodium alginate from Cu(II) and Mn(II) salt solutions resulted in cubic nanosized Cu_1.5Mn_1.5O₄ [38]. This nanosized cubic Cu_1.5Mn_1.5O₄ proved to be a high-performance catalyst in the oxidation of toluene at atmospheric pressure. The CuMn₂O₄ prepared from copper acetate and manganese nitrate in reverse microemulsion (Triton–X–100 surfactant, n–octanol, and cyclohexane) by adding NH₄OH and calcining at 450–1000 °C in oxygen also catalyzed toluene combustion (0.35% toluene and 9% O₂ in inert gas). The ignition temperature of 210 °C and a combustion temperature of 220 °C resulted in the complete oxidation of toluene [47]. The oxidation of toluene in the presence of this catalyst was complete at ∼240 °C [38].

The catalyst containing stoichiometric CuMn₂O₄, which was prepared from nitrate salts with tetramethylammonium hydroxide, was active in toluene oxidation with the formation of CO₂ as the main product with 22 (10.45) and 1.9 (0.04) μmol g⁻¹ h⁻¹ (μmol m⁻² h⁻¹) decomposition rate at 170 and 150 °C, respectively. Only a minor amount of benzene and CO formed [26]. Oxygen is adsorbed by the surface (O_ads), but the subsurface oxygen content also plays a key role in the activity of the catalyst [26]. The rod-like nanosized (10–20 nm) copper manganese mixed oxides containing CuMn₂O₄ were prepared from copper and manganese acetate with Na₂CO₃ under hydrothermal conditions. These catalysts have significantly higher catalytic activity for toluene combustion at a temperature of 210 °C than that found for CuMn₂O₄ prepared without hydrothermal post-treatment even at a combustion temperature of 250 °C [49]. It was attributed to the higher surface concentration of high valence ionic species such as Cu²⁺, Mn³⁺, and Mn⁴⁺, and surface oxygen concentration in the nanorods, than those in the CuMn₂O₄ made without hydrothermal treatment [23]. The hydrothermal process increases the surface Mn⁴⁺ concentration up to 40% in the hydrothermally prepared nanorods due to the Cu²⁺ + Mn³⁺ ↔ 2 Cu⁺ + Mn⁴⁺ [77]. The excess surface oxygen combines more easily with toluene molecules than the adsorbed oxygen on the catalyst surface [78]. The π bond of the aromatic ring interacts with Cu²⁺ with the formation of a transition complex, and the d electrons of Cu²⁺ can be fed back to the π bond system of the toluene molecule [34].

The catalytic activity of the copper-rich Cu_1.5Mn_1.5O₄ spinels in toluene oxidation can be increased by La-loading [37]. Lanthanum acts as a structure promoter and increases the surface area and diminishes the crystallinity of the catalyst. As an electron promoter, lanthanum changes the valence state and redox properties of copper and manganese located at various spinel sites. The reducibility of the La-doped catalysts is significantly improved, and there was also a significant increase in the surface oxygen concentration [37].

Table 9. Removal of toluene from air in the presence of Cu_xMn_3−xO₄ catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Refs.	Remarks
x = 1.0	Cu–nitrate, Mn–acetate, Cu:Mn = 1:2, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 369/474 °C, resp.	[76]	Supported on Al2O3, SBET = ~101.3 m2 g−1
x = 1.0	Cu and Mn(II) acetate, co-precipitation at 80 °C with aq. Na2CO3, 500 °C	0.35% toluene and 9% O2 in Ar, 250 °C	[49]	Mixed phase, SBET = 73 m2 g−1
x = 1.0	Cu and Mn(II) acetate, hydrothermal treatment in 10 M NaOH at 110 °C for 20 h, 250 °C	0.35% toluene and 9% O2 in Ar, GHSV 36,000, 210 °C; T10/T50/T95 are 150, 170, 200 °C, resp.	[49]	Nanorods, mixed phase, SBET = 221 m2 g−1
x = 1.0	Mn and Cu nitrates, co-precipitation with Me4NOH, 400 °C for 5 h	800 ppm toluene in air, 150 °C for 24 h, GHSV = 27,800 mL g−1 h−1, T10/T50/T90 = 185/195/200 °C, resp.	[26]	SBET = 48 m2 g−1, Vp = 0.30 cm3 g−1
x = 1.2	Cu–nitrate, Mn–acetate, Cu:Mn = 1:0.5, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 330/383 °C, resp.	[76]	Supported on Al2O3, SBET = ~109.2 m2 g−1
x = 1.3	Cu–nitrate, Mn–acetate, Cu:Mn = 1:1.5, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 271–303/293–329 °C, resp., depending on loaded amount on Al2O3	[76]	Supported on Al2O3, SBET = ~99.0 m2 g−1, TOF = 0.05–0.28
x = 1.4	Mn and Cu–nitrates, anatase, incipient wetness impregnation, 500 °C for 7 h in air	Atmospheric pressure, 500 ppm toluene in air, 150–300 °C, 5 h, GHSV 5000 h−1	[18]	Mixed phase, SBET = 34–48 m2 g−1, supported on TiO2
x = 1.4	Cu:Mn = 1:1, MnCl2, CuCl2, Na2CO3, 450 °C for 5 h	500 ppm toluene in humid air, GHSV = 50,000 mL g−1 h−1, T50/T90 =265/280 °C, resp., reaction rate = 0.001 mmol g−1 h−1 and 0.005 mmol m−2 h−1 at 200 °C	[13]	SBET = 18 m2 g−1, Pv = 0.02 cm3 g−1, APD = 13
x = 1.5	Cu–nitrate, Mn–acetate, Cu:Mn = 1:2, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 300/343 °C, resp.	[76]	Supported on Al2O3 SBET = ~107.4 m2 g−1
x = 1.5	ionotropic sodium alginate, metal chlorides, alcoholic dehydration, and calcining the xerogel at 450 °C	Toluene in dry air, 1000 ppm, GHSV = 46,000 mL g−1 h−1, reaction rate = 1.03 mmol g−1 h−1 and 24.52 mmol m−2 h−1 at 200 °C	[38]	cubic, ∼10 nm size, SBET = 42 m2 g−1
x = 1.5	Mn–acetate, Cu–nitrate, 1.5 M ammonium carbonate, pH = 8, 2 h, calcining at 550 C for 2 h in air	1000 ppm toluene in air, GHSV 30,000 h−1, T50/T90 = 245/274 °C, resp.	[37]	Mixed phase, undoped, SBET = 784.1 m2 g−1, Vp (meso) = 0.246 and Vp (all) 0.25 cm3 g−1
x = 1.5	Mn–acetate, Cu–nitrate, La–nitrate, 1.5 M ammonium carbonate, pH = 8, 2 h, calcining at 550 C for 2 h in air	1000 ppm toluene in air, GHSV 30,000 h−1. T50/T90 for 2, 4, and 6% La content were 218/268, 217/255, and –/280 °C, respectively.	[37]	Mixed phase, 4% La-doping, SBET = 164.2 m2 g−1, Vp (meso) 0.426 and Vp (all) 0.45 cm3 g−1

S_BET—specific surface area; P_v—the total adsorption pore volume at P/P₀ = 0.995; APD—average pore diameter; TOF—turnover frequency; GHSV—gas hourly space velocity.

Table 9. Removal of toluene from air in the presence of Cu_xMn_3−xO₄ catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Refs.	Remarks
x = 1.0	Cu–nitrate, Mn–acetate, Cu:Mn = 1:2, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 369/474 °C, resp.	[76]	Supported on Al2O3, SBET = ~101.3 m2 g−1
x = 1.0	Cu and Mn(II) acetate, co-precipitation at 80 °C with aq. Na2CO3, 500 °C	0.35% toluene and 9% O2 in Ar, 250 °C	[49]	Mixed phase, SBET = 73 m2 g−1
x = 1.0	Cu and Mn(II) acetate, hydrothermal treatment in 10 M NaOH at 110 °C for 20 h, 250 °C	0.35% toluene and 9% O2 in Ar, GHSV 36,000, 210 °C; T10/T50/T95 are 150, 170, 200 °C, resp.	[49]	Nanorods, mixed phase, SBET = 221 m2 g−1
x = 1.0	Mn and Cu nitrates, co-precipitation with Me4NOH, 400 °C for 5 h	800 ppm toluene in air, 150 °C for 24 h, GHSV = 27,800 mL g−1 h−1, T10/T50/T90 = 185/195/200 °C, resp.	[26]	SBET = 48 m2 g−1, Vp = 0.30 cm3 g−1
x = 1.2	Cu–nitrate, Mn–acetate, Cu:Mn = 1:0.5, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 330/383 °C, resp.	[76]	Supported on Al2O3, SBET = ~109.2 m2 g−1
x = 1.3	Cu–nitrate, Mn–acetate, Cu:Mn = 1:1.5, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 271–303/293–329 °C, resp., depending on loaded amount on Al2O3	[76]	Supported on Al2O3, SBET = ~99.0 m2 g−1, TOF = 0.05–0.28
x = 1.4	Mn and Cu–nitrates, anatase, incipient wetness impregnation, 500 °C for 7 h in air	Atmospheric pressure, 500 ppm toluene in air, 150–300 °C, 5 h, GHSV 5000 h−1	[18]	Mixed phase, SBET = 34–48 m2 g−1, supported on TiO2
x = 1.4	Cu:Mn = 1:1, MnCl2, CuCl2, Na2CO3, 450 °C for 5 h	500 ppm toluene in humid air, GHSV = 50,000 mL g−1 h−1, T50/T90 =265/280 °C, resp., reaction rate = 0.001 mmol g−1 h−1 and 0.005 mmol m−2 h−1 at 200 °C	[13]	SBET = 18 m2 g−1, Pv = 0.02 cm3 g−1, APD = 13
x = 1.5	Cu–nitrate, Mn–acetate, Cu:Mn = 1:2, γ–Al2O3, 500 °C for 5 h	1000 ppm toluene in 7:3 N2:O2, 120,000 h−1 space velocity, T50/T90 = 300/343 °C, resp.	[76]	Supported on Al2O3 SBET = ~107.4 m2 g−1
x = 1.5	ionotropic sodium alginate, metal chlorides, alcoholic dehydration, and calcining the xerogel at 450 °C	Toluene in dry air, 1000 ppm, GHSV = 46,000 mL g−1 h−1, reaction rate = 1.03 mmol g−1 h−1 and 24.52 mmol m−2 h−1 at 200 °C	[38]	cubic, ∼10 nm size, SBET = 42 m2 g−1
x = 1.5	Mn–acetate, Cu–nitrate, 1.5 M ammonium carbonate, pH = 8, 2 h, calcining at 550 C for 2 h in air	1000 ppm toluene in air, GHSV 30,000 h−1, T50/T90 = 245/274 °C, resp.	[37]	Mixed phase, undoped, SBET = 784.1 m2 g−1, Vp (meso) = 0.246 and Vp (all) 0.25 cm3 g−1
x = 1.5	Mn–acetate, Cu–nitrate, La–nitrate, 1.5 M ammonium carbonate, pH = 8, 2 h, calcining at 550 C for 2 h in air	1000 ppm toluene in air, GHSV 30,000 h−1. T50/T90 for 2, 4, and 6% La content were 218/268, 217/255, and –/280 °C, respectively.	[37]	Mixed phase, 4% La-doping, SBET = 164.2 m2 g−1, Vp (meso) 0.426 and Vp (all) 0.45 cm3 g−1

S_BET—specific surface area; P_v—the total adsorption pore volume at P/P₀ = 0.995; APD—average pore diameter; TOF—turnover frequency; GHSV—gas hourly space velocity.

The Cu_1.4Mn_1.6O₄ spinel catalyst showed activity in the combustion of benzene, toluene, and xylene. The acidity of the catalyst used in the oxidation of toluene decreased from ca. 89% to 79% after 2500 min, and the surface concentrations of Cu^II and Mn^IV were 56.8 and 56.9%. Catalytic activity decreased when H₂O, CO₂, and SO₂ were added, due to the formation of surface species. However, the deactivation is reversible. The aromatic hydrocarbons were oxidized into carbon dioxide, and only a minor amount of CO was detected as a consequence of the incomplete conversion of CO into CO₂. This means the catalyst is less efficient in the oxidation of CO than in the combustion of aromatic hydrocarbons [13].

5. Copper Manganese Oxides as Catalysts in Industrial Production Processes

Hydrogen gas is an essential raw material for numerous industrial processes, including ammonia and fertilizer synthesis and fine chemical industry, therefore converting the carbon monoxide content of water gas into carbon dioxide and hydrogen, and the methanol steam reforming processes have a huge importance. These processes can be catalyzed with copper manganese oxide spinels [20,36,40]. The hydrogen-rich CO:H₂ mixtures (e.g., synthesis gas), however, resulted in the formation of important organic intermediates such as methyl formate in the presence of these copper–manganese oxide catalysts and the alkanol–DMF organic solvents [29,30].

5.1. Copper Manganese Oxide Spinel-Catalyzed Reactions of CO:H₂ Mixtures

The water–gas shift reaction is an enormously important industrial process for hydrogen generation through the conversion of CO gas produced by the steam reforming of hydrocarbons:

CO + H₂O ↔ CO₂ + H₂, ΔH₂₉₈ = −41 kJ mol⁻¹.

The catalytic performance of the catalysts containing Cu_1.5Mn_1.5O₄, which were prepared by the autoignition of copper and manganese nitrates with an excess of urea and by precipitation with sodium carbonate [36], was studied in the oxidative conversion of carbon monoxide (

Table 10. The water–gas shift reaction in the presence of Cu_xMn_3−xO₄ catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Refs.	Remarks
1.5	Autoignition, 75% excess of urea, manganese nitrate, and copper, heating at 550 °C for 1 h	Space velocity = 4000 h−1, partial pressure of water vapor = 31.1 kPa, 4.492 vol.% CO/Ar + H2O and 15 vol.% CO, 10 vol.% CO2, 63 vol.% H2, 12 vol.% N2 + H2O, CO conversion was 90% at 220 °C	[36]	Multiphase, SBET = 8.0 m2 g−1
1.5	aqueous solutions of manganese and copper nitrate, Na2CO3 at 80 °C, pH = 8.3, aging the resulting precipitate at 80 °C for 6 h, 700 °C for 7 h	Space velocity = 4000 h−1, partial pressure of water vapor = 31.1 kPa, 4.492 vol.% CO/Ar + H2O and 15 vol.% CO, 10 vol.% CO2, 63 vol.% H2, 12 vol.% N2 + H2O, 160–240 °C, CO conversion was 80% at 240 °C	[36]	Multiphase, SBET = 5.9 m2 g−1

S_BET—specific surface area.

). The measured CO conversion strongly depended on the preparation method. The sample prepared by autoignition showed 90% conversion at 220 °C. The catalyst prepared by co-precipitation was less active in the 160–240 °C interval, and ca. 80% CO conversion was reached at 240 °C. The effect of space velocity on CO conversion over the catalyst prepared by the autoignition method was studied at 180 °C when a significant decrease of CO conversion by ∼17 and 32% was observed at GHSV = 6000 and 8000 h⁻¹, respectively. The conversion CO at 180 °C increases when the partial pressure of water is increased from 5 to 10 kPa [26].

5.2. Synthesis of Methyl Formate

The copper manganese spinels with x = 0.5 prepared from manganese nitrate and [tetraamminecopper(II)] hydroxide solutions with subsequent calcination at 450, 550, and 650 °C were tested in the synthesis of methyl formate from synthesis gas at 160 °C in 30 min with the MeOH:DMF solvent [29,30] (

Table 11. Methyl formate synthesis from synthesis gas catalyzed by copper manganese oxide spinel containing catalysts.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Refs.	Remarks
x = 1.5	Mn(II)–nitrate:Cu(NH3)42+ (Cu:Mn = 1:1), 35 °C, 550 °C for 4 h	MeOH–DMF as liquid phase, H2/CO = 2:1, 160 °C, 30 min	[30]	Mixed phase, SBET = 15 m2 g−1, Vp = 0.03 cm3 g−1 size is 38.4 nm
1.5	Mn(II)–nitrate:Cu(NH3)42+ (Cu:Mn = 1:1), 35 °C, 550 °C for 4 h	CaO–ZrO2 as co-catalyst, 1.5:3.5 (v/v) methanol–DMF as a solvent, 2:1 H2:CO molar ratio, room temperature up to 4.0 MPa, heating up to 160 °C in 1 h, stirring for 8 h; 12.7–16.6% CO conversion, STY = 2.17–2.63 g L−1 h−1, methyl formate selectivity was 81.3–83.6%	[29]	Mixed phase, SBET = 10.5–18.4 m2 g−1 and Vp = 0.02–0.04 cm3 g−1, nanosized sample

S_BET—specific surface area; P_v—the total adsorption pore volume at P/P₀ = 0.995; STY—space–time yield.

).

2CO + 2H₂ = HC(=O)OCH₃

When the molar ratio of CO:H₂ was adjusted to 1:2, the selectivity of methyl formate formation (byproducts were CO₂, EtOH, and formaldehyde) was >70% [30] whereas using a CaO–ZrO₂ co-catalyst resulted in an increase in selectivity up to >80% [30]. The best methyl formate space–time yield and selectivity, 4.19 g L⁻¹ h⁻¹ and 88.1%, respectively, were obtained on the spinel formed at 450 °C due to its high specific surface area. CO conversion in ethanol/DMF was higher than in methanol/DMF as solvent. The hydrogenation of methyl formate into methanol according to the equation

HCOOCH₃ + 2H₂ = 2CH₃OH

(1)

was catalyzed with the spinel prepared at 550 °C. Methyl formate showed high conversion (86.5%) with high methanol space-time yield and selectivity (Table 11) [30].

5.3. Methanol Steam Reforming Process

The steam reforming of methanol is a reaction that produces high H₂ yield/per mole methanol:

CH₃OH + H₂O ↔ CO₂ + 3H₂

A series of Cu_xMn_3−xO₄ catalysts with x = 0.3–1.05 prepared by the urea–nitrate combustion method at 550 °C were tested in the methanol steam reforming reaction under atmospheric pressure. Dilute (5% MeOH and 7.5% H₂O) gas stream was used, and quantitative methanol conversion was achieved at 280 °C with the copper manganese oxide spinel catalysts with x = 0.6–1.05. The superior catalysts resulted in H₂ and CO₂ between 240 and 280 °C, and only a small amount of CO was formed (CO and H₂ selectivity were between 1.7–6.4 and 91.9–97.5%, respectively, depending on the value of x and the reaction temperature [40] (

Table 12. Steam reforming of methanol in the presence of catalysts containing copper manganese oxide spinel.

CuxMn3−xO4	Precursors, Preparation Method, and Conditions	Catalyst Efficiency and Reaction Conditions	Refs.	Remarks
x = 0.3	Combustion method, 50% excess of urea, Cu, and Mn–nitrate (4–500 °C), heat treatment at 550 °C for 1 h	Atmospheric pressure, 240–280 °C, W/F = 0.257 g s−1 cm−3, 5% MeOH, 7.5% H2O and He; 61/97% MeOH conversion, 1.7/5.4% CO and 96.3/94.6% H2 selectivity, at 240 and 280 °C, resp.	[40]	SBET = 4.1 m2 g−1
x = 0.6	Combustion method, 50% excess of urea, Cu, and Mn–nitrate (4–500 °C), heat treatment at 550 °C for 1 h	Atmospheric pressure, 240–280 °C, W/F = 0.257 g s−1 cm−3, 5% MeOH, 7.5% H2O and He; 83.6 1/100% MeOH conversion, 2.1/5.7% CO and 96.5/94.3% H2 selectivity, at 240 and 280 °C, resp.	[40]	SBET = 7.5 m2 g−1
x = 0.75	combustion method, 50% excess of urea, Cu, and Mn–nitrate (4–500 °C), heat treatment at 550 °C for 1 h	Atmospheric pressure, 240–280 °C, W/F = 0.257 g s−1 cm−3, 5% MeOH, 7.5% H2O and He; 91.6/100% MeOH conversion, 1.7/6.4% CO and 97.5/93.6% H2 selectivity, at 240 and 280 °C, resp.	[40]	SBET = 6.0 m2 g−1
x = 0.9	Combustion method, 50 % excess of urea, Cu, and Mn–nitrate (4–500 °C), heat treatment at 550 °C for 1 h	Atmospheric pressure, 240–280 °C, W/F = 0.257 g s−1 cm−3, 5% MeOH, 7.5% H2O and He; 93.1/100% MeOH conversion, 2.1/7.5% CO and 92.5/97.9% H2 selectivity, at 240 and 280 °C, resp.	[40]	SBET = 4.8–8.6 m2 g−1
x = 1.05	Combustion method, 50% excess of urea, Cu, and Mn–nitrate (4–500 °C), heat treatment at 550 °C for 1 h	Atmospheric pressure, 240–280 °C, W/F = 0.257 g s−1 cm−3, 5% MeOH, 7.5% H2O and He; 93.8/100% MeOH conversion, 2.1/9.1% CO and 91.9/97.9% H2 selectivity, at 240 and 280 °C, resp.	[40]	SBET = 6.2 m2 g−1
x = 1.5	1.2 M aqueous solution of Na2CO3, Cu, and Mn nitrates (each 0.5 M), pH at 8.5, 300 °C in air for 4 h	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 51.5/65.7% MeOH conversion, 1.2/1.1 CO, and 99.6/99.1. H2 selectivity at 240 and 260 °C, 246 and 157 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 55.2 m2 g−1, Vp = 0.14 cm3 g−1, dp = 8.5
x = 1.5	1.2 M aqueous solution of Na2CO3, Cu and Mn nitrates (each 0.5 M), pH = 8.5, 500 °C in air for 4 h	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 45.5/62% MeOH conversion, 1.7/1.5 CO, and 99.4/99.1 H2 selectivity at 240 and 260 °C, with 139 and 189 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 14.9 m2 g−1, Vp = 0.07 cm3 g−1, dp = 18.5 nm
x = 1.5	Aq. copper and manganese nitrate solution, 20% excess of oxalic acid at room temperature, 300 °C for 4 h	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 43/60% MeOH conversion, 1.5/1.4% CO and 99.1–99.1% H2 selectivity at 240 and 260 °C, with 131 and 182 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 9.6 m2 g−1, Vp = 0.06 cm3 g−1, dp = 18.2 nm
x = 1.5	Aq. copper and manganese nitrate solution, 20% excess of oxalic acid at room temperature, 500 C for 4 h	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 35/52.5% MeOH conversion, 1.5/1.5% CO and 99.1/99.4% H2 selectivity at 240 and 260 °C, with 106 and 160 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 4.7 m2 g−1, Vp = 0.05 cm3 g−1, dp = 35.6 nm
x = 1.5	A 1:2 molar ratio amount of Cu2CO3(OH)2 and MnCO3, 20% stoichiometric excess of solid oxalic acid dihydrate, grinding under ambient conditions in a planetary mill at a speed of 600 rpm for 2 h, 300 °C in air for 4 h	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 74/88.5% MeOH conversion, 0.8/0.8% CO and 99–4/99.4 H2 selectivity at 240 and 260 °C, with 226 and 170 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 96.1 m2 g−1, Vp = 0.35 cm3 g−1, dp = 12.5 nm
x = 1.5	A 1:2 molar ratio amount of Cu2CO3(OH)2 and MnCO3, 20% stoichiometric excess of solid oxalic acid dihydrate, grinding under ambient conditions in a planetary mill at a speed of 600 rpm for 2 h, 500 °C in air for 4 h	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 65/80.5% MeOH conversion, 1.3/1.5 CO, and 99.1/99.4% H2 selectivity at 240 and 260 °C, with 199 and 246 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 24.6 m2 g−1, Vp = 0.18 cm3 g−1, dp = 26.5 nm
x = 1.5	A 1:2 molar ratio amount of Cu2CO3(OH)2 and MnCO3 premixed with 20% stoichiometric excess of solid oxalic acid, grinding under ambient conditions in a planetary mill at a speed of 600 rpm for 2 h with higher intensity (more balls), calcination at 300 °C in air for 4 h.	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 79.9/92.9% MeOH conversion, 0.7/0.7% CO and 99.5/99.5% H2 selectivity at 240 and 260 °C, with 244 and 284 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 118.1 m2 g−1, Vp = 0.39 cm3 g−1, dp = 11.1 nm
x = 1.5	A 1:2 molar ratio of Cu2CO3(OH)2 and MnCO3 premixed with 20% stoichiometric excess of solid citric acid, grinding under ambient conditions in a planetary mill (600 rpm) for 2 h, 500 °C in air for 4 h	Atmospheric pressure, premixed water, and methanol with an H2O/MeOH molar ratio of 1.3; 57/69% MeOH conversion, 1.5/1.6% CO and 99.4/99.6% H2 selectivity at 240 and 260 °C, with 173 and 209 mmol gcat−1 h−1 production rate, resp.	[20]	SBET = 60.6 m2 g−1, Vp = 0.30 cm3 g−1, dp = 17.5 nm

S_BET—specific surface area; P_v—the total adsorption pore volume at P/P₀ = 0.995; d_p—pore diameter; W/F—space velocity.

).

The catalytic activities of nanostructured Cu–Mn spinel oxides prepared in various reactions including the mechanochemical reaction of solid oxalic acid and citric acid with basic copper carbonate and manganese carbonate with subsequent calcination at 300 and 500 °C) were compared in the steam reforming reaction of methanol (Table 12). The catalytic activity of the copper–manganese oxide spinels prepared by mechanochemical reaction routes strongly depended on the milling intensity and the chemical nature of the precursors. The Cu_1.5Mn_1.5O₄ spinel prepared with solid oxalic acid was extremely active and converted more than 92% of methanol at temperatures of 240 and 260 °C. The spinel prepared from citric acid precursors with the same composition converted only 68% of methanol. The samples prepared at 300 °C had much higher catalytic activity than those calcined at 500 °C. The superior activity of the spinel prepared by grinding oxalic acid and calcining it at 300 °C was attributed to the generation of the active Cu_1.5Mn_1.5O₄ spinel phase and the high surface area, which provides high component dispersion. The catalytic performance of the spinels obtained with the same stoichiometry by the conventional co-precipitation with aq. Na₂CO₃ is much inferior to the samples prepared by the mechanochemical methods, due to the low surface areas and the presence of less of the active phase of Cu_1.5Mn_1.5O₄ spinel [20].

5.4. Other Industrial Processes Catalyzed by Cu_xMn_3−xO₄ Spinels

The copper-rich catalysts containing Cu_1.5Mn_1.5O₄ spinel prepared with the conventional precipitation technique with sodium carbonate from the appropriate metal acetates were tested in the oxidation of toluene into benzyl alcohol. The catalysts (heat-treated between 400 and 800 °C) proved to be effective in the oxidation of toluene with molecular oxygen without using any solvents and additives, combining the advantages of minimized separation difficulties and equipment corrosion [34]. The main products were benzyl alcohol, benzaldehyde, and benzoic acid. The conversion of toluene (~17%) was higher than that found in the presence of other copper or manganese-based bimetallic Cu–(Zn, Si, Zr, Cr) or Mn–(Co, Ce, Fe) catalysts under 1.0 MPa oxygen pressure at 190 °C for 2 h. The distributions of the oxidation products strongly depend on the phase composition (heat treatment temperature and molar ratio of Cu and Mn acetate). The highest conversion of toluene (21.6%) was found with the spinel prepared at a 1:1 Cu:Mn ratio and the sintering temperature of 700 °C [34]. The benzyl alcohol was formed in 30 min in a high amount but transformed gradually into benzoic acid in 180 min. The highest selectivity to benzoic acid was in the sample made at 600 °C [34,43] (

Table 13. Other industrial-scale reactions catalyzed by Cu_xMn_3−xO₄ spinels.

x	Preparation Method, Precursors, and Conditions	Reaction, Catalyst Efficiency, Catalytic Reaction Conditions	Ref.	Remark
x = 1	Precipitation, Cu– and Mn–acetate, Na2CO3, calcination at 700 °C	Toluene conversion into benzyl alcohol, benzaldehyde, and benzoic acid, ~21.6% conversion at 190 °C and 1.0 MPa O2 pressure, 2 h	[34]	Size is 37 nm
x = 1–2	1 M copper nitrate and 1 M manganese nitrate 2 M fuel (glycerol), 1 M NaCl, 150 °C for 12 h, combustion at 400–700 °C for 6 h, self–propagating reaction	Propylene conversion into propylene oxide, 2:2:5 propylene:oxygen:helium volume ratio, 250–300 °C, 1 atm, GHSV = 20 000 h−1, 30–37 % selectivity at 1–3–1.5 % conversion	[43]	Multiphase, SBET = 6.2 m2 g−1, Pd 39.4 nm
x = 1.5	Spray granulation of CuO and Mn3O4 in a spouted bed system, calcining for 2 h at 1125 °C	Chemical Looping with Oxygen Uncoupling in coal combustion with CO2 capture, 50 h	[15]	Mixed phase, SBET = 1.0, porosity is 12.1–18.7%

S_BET—specific surface area; P_v—the total adsorption pore volume at P/P₀ = 0.995; P_d—pore diameter; GHSV—gas hourly space velocity.

).

A series of mixed-phase catalysts containing Cu_xMn_3−xO₄ (1 < x < 2) doped with NaCl were tested in the propylene epoxidation reaction. The catalysts were prepared by combustion synthesis (self–propagating reaction) with the use of a variety of fuels such as glycerol, maleic acid, citric acid, or ethyl acetoacetate. The optimal propylene oxide selectivity of 30−37% was achieved at 1.3−1.5% propylene conversion using a catalyst prepared with glycerol at 550 °C [43].

The total combustion of solid coals with inherent CO₂ separation was performed with the Chemical Looping with Oxygen Uncoupling (CLOU) process performed in the presence of mixed-phase catalysts containing Cu_1.5Mn_1.5O₄ as oxygen carriers. Full combustion was reached, but CO₂ capture efficiencies depended on coal rank [15].

6. Characterization and Operando Techniques Used in the Studying of Cu_xMn_3−xO₄ Spinel Catalysts

The phase characterization of Cu_xMn_3−xO₄ spinels was performed with powder and single-crystal XRD studies. Vibrational spectroscopy including IR, far-IR, and Raman methods was also used.

The surface of the catalysts was characterized by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and N₂ BET. Porosity was also characterized with BET isotherms measurements. Temperature-programmed desorption of O₂, H_2, and other gases used in the catalytic reactions (CO, NO, etc.) was also used. The decomposition reaction routes were monitored with DSC, TG, and TG–MS methods with simultaneous characterization of the solid phase by XRD, XPS, IR, SEM, EDX, and transmission electron microscopy (TEM).

Operando methods used to monitor the changes in the catalysts and conversion of reactants were mainly IR and GC methods. In situ diffuse-reflectance FT-IR spectra (DRIFTS) of fine powdery catalysts were recorded in in situ chambers. The reaction/desorption studies were performed by heating the adsorbed species and obtaining DRIFTS spectra at various targets.

The catalytic tests with gases were performed in various kinds of flow reactors, at atmospheric and high-pressure conditions and various temperatures. The liquid oxidation reactions were performed in an autoclave by adjusting the temperature/pressure conditions. The conversions were monitored by LC–MS or GC–MS.

7. Conclusions

The preparation and catalytic activity of Cu_xMn_3−xO₄ spinels and multiphase materials containing Cu_xMn_3−xO₄ spinels in a wide variety of industrial and environmental protection processes are reviewed. The great diversity of metal ion valences and distribution in the spinel lattice, and the relation of the preparation and annealing methods on the crystallinity, composition, and chemical state of phases and surface layers play an important role in the catalytic activity of the Cu_xMn_3−xO₄ spinels. The presence of amorphous materials, lattice distortions, and the number of oxygen and metal vacancies are key factors in their catalytic activity. These features of Cu_xMn_3−xO₄ spinels can be controlled by the synthesis conditions, including the reaction routes, Cu/Mn ratios, starting metal compound valences, counter-ions, reagents, metal compound concentrations, temperature, time, and a wide variety of other reaction and annealing conditions.

The properties and catalytic activities and performances of the catalyst prepared by traditional solid-phase and solution-phase precipitating reactions were compared with the properties of the catalyst prepared new and promising synthesis routes were developed. The low-temperature methods (<100 °C) based on a solvent-mediated thermal decomposition of [tetraamminecopper(II)] permanganate results in amorphous phases, which can be transformed into regular spinel or bixbyite-like copper manganese oxides with a defect structure. The size and crystallinity of the copper manganese oxides prepared in this reaction route can be controlled with annealing time and temperature, and nanosized catalysts can be prepared even at 500 °C. These kinds of catalysts have been tested only in CO oxidation reactions, yet their studying in other industrially important reactions promises exciting possibilities to increase the performance of copper manganese oxide spinel catalysts in these processes. This reaction to prepare spinel oxides can be made with co-crystals or solid solutions made with [tetraamminecopper(II)] permanganate and other analog salts with various divalent cations or anions containing metal (e.g., perrhenate). In this way, a wide variety of metal-doped copper manganese oxide spinels can be prepared, which gives an almost infinite possibility to change the properties of amorphous or partially crystalline nanosized catalysts.

These perspectives pose new challenges to defect and synergic engineering to enhance the oxygen replenishment capacity of the doped CuMn₂O₄ phases and control the amount and distribution of copper and oxygen vacancies in the spinel lattice sites. Developing (multi)-doped copper–manganese oxide catalysts to increase their efficiency and selectivity in various reactions helps substitute expensive and hardly available noble metal catalysts.