Cobalt Oxides and Co-Al Mixed Oxides as Thermo-, Photo- and Electrocatalytic Materials: Properties and Perspectives of Industrial Applications

Abstract

The literature data on the solid-state and surface chemistry of cobalt and cobalt–aluminum oxide and hydroxide systems are reviewed. The actual and potential applications of these materials in the fields of catalysis, electrocatalysis, photocatalysis, adsorption and sensor technologies are reviewed. A comprehensive analysis of the peculiar redox and acid–base properties of these cobalt-based systems, both at the solid–gas and at the solid–water solution interface, is conducted. Evidence is provided for the exceptional versatility of these systems and on their relevant potential for optimal applications, in particular, in several catalytic total oxidation reactions, in N₂O catalytic decomposition, in ammonia catalytic oxidation to NO, as electrocatalysts in water splitting reactions, as active elements in supercapacitors, and as precursors of cobalt metal-based systems. It is underlined that these systems could successfully substitute more critical and expensive noble metal-based systems in several technological fields.

1. Introduction

Cobalt is considered as a quite critical element in occidental countries, mainly because most of its production occurs in the Democratic Republic of Congo and most of its processing is realized in China [1]. Additionally, some cobalt compounds have genotoxic and carcinogenic activity [2]. In spite of these limitations, cobalt and its oxides have many actual and potential applications. Their main application (>72%) in 2022 has been in the production of rechargeable lithium-ion batteries [1] such as the LCO batteries (based on LiCoO₂ electrodes) and NMC batteries (based on Li(Mn,Ni,Co)O₂ electrodes) [3]. Catalytic applications of cobalt are also relevant. It has a wide use as a homogeneous catalyst in liquid phase [4], such as cobalt acetate which is the homogenous catalyst of p-xylene oxidation to terephthalic acid (the monomer to produce polyesters such as PET, PolyEthyleneTerephthalate [5]). It is also used as a metallic Co/Al₂O₃ catalyst for Fischer Tropsch process [6,7], and as the dopant of molybdenum sulphide -based catalysts (CoMo), largely applied in hydrocarbon hydrodesulphurization processes in refineries [8]. The latter have also been recently applied for the catalytic hydrodeoxygenation of vegetable oils to produce Hydrogenated Vegetable Oil, the renewable Diesel fuel (HVO [9]). Cobalt also has relevant application in the production of alloys characterized by good corrosion and wear resistance, high-temperature strength, fatigue strength and biocompatibility, used in such applications as jet engine turbines and gas turbine generators, in nuclear power and chemical-processing industries as well as for orthopaedic implants [10]. Additionally, it has application in the biomedical field [11].

Metal oxide technologies also represent a very relevant industrial sector, such as in the fields of construction materials, electronic and electrocatalytic materials, insulators, electroceramics, pigments, glasses, refractories, abrasives, industrial catalysts, adsorbents, etc. [12]. In fact, pure and mixed cobalt oxides are extremely interesting materials for many of their bulk and surface properties. In addition to applications related to optical properties (i.e., for pigments used in glazes, glass, and enamels [13]), and to their electronic and chemical properties (such as in batteries [3]) they have practical or potential interest in many surface related applications, such as in the fields of adsorption, catalysis, electrocatalysis and supercapacitor technologies. Cobalt oxides also have interesting activities as catalysts used in environmental remediation [14]. Most of these applications are related to the electronic state of Co³⁺ and Co²⁺ ions and their redox properties. In several cases, to take advantage of these properties and improve stability, they are mixed with or supported on aluminas. In fact, aluminas are among the most applied supports in the field of heterogeneous catalysis and adsorption technologies [15]. Additionally, cobalt aluminates have many actual and potential applications. In this review, the preparation, the solid-state chemistry and the surface-related applications of cobalt oxides and cobalt-aluminium mixed oxides will be reviewed, together with their actual and potential industrial applications.

2. Solid-State Chemistry of Pure Cobalt Cobalt-Containing Phases

Several cobalt oxide and hydroxide phases exist in different conditions. For reference on the thermodynamic stability of different phases, in Figure 1 the cobalt-oxygen state diagram [16,17,18] is reported while in Figure 2 the Pourbaix diagram of the cobalt-water system [19,20] is reported. These diagrams allow us to interpret the metallurgy of cobalt [21] as well as the behavior of cobalt—based oxides and hydroxides towards several of their actual applications.

2.1. Cobalt Hydroxides

Co (II) hydroxide, usually denoted as β-Co(OH)₂, has a layered structure, isostructural to brucite Mg(OH)₂, with high spin (HS: e⁴t₂³) Co²⁺ ions occupying distorted octahedral sites in a cationic layer sandwiched in between two hydroxy-group layers [22,23] with a d-spacing of 4.6 Å. It shows typically a pink color [23]. The phase usually denoted as α-Co(OH)₂ is actually not a Co(II) hydroxide polymorph. In fact, it is a Co(II)-Co(III) layered hydroxycarbonate, hydroxynitrate or hydroxychloride [24,25,26,27,28] with a structure similar to that of hydrotalcite (Mg₆Al₂(OH)₁₆·CO₃·4H₂O) characterized by positively charged [Co(OH)₂]^x+ layers containing both divalent and trivalent cobalt ions intercalating balancing anions, that can be exchanged with each other such as carbonate, nitrate and chloride. These solids with different anions have a variable color, i.e., red [29] or green [23]. Both α- and β-Co(OH)₂ phases are intrinsic p-type semiconductors [30].

Heating β-Co(OH)₂ in N₂ atmosphere at 473–493 K [31] and in vacuum at 410 K [32] is reported to produce crystalline rock salt type CoO or an amorphous material that crystallizes to rock-salt-type CoO at 473–573 K [33], also in the presence of saturated water vapor [34]. The decomposition of the hydrotalcite-type α-phase is a more complex phenomenon and, in any case, calcination in air of both phases at 423–523 K produces Co₃O₄ [24].

According to Pankratov [35], two different phases of trivalent cobalt hydroxide Co(OH)₃ exist, a green phase and a brownish-black phase. Yang et al. reported the synthesis and the characterization, including XRD pattern, of Co(OH)₃ nanobelts [36]. However, the crystal structure of Co(OH)₃ phases does not seem to have been identified.

Trivalent cobalt oxy-hydroxide phases certainly exist, forming the mineral Heterogenite, an economically important source of cobalt, extracted mainly in the Democratic Republic of Congo (DRC). This mineral is composed of at least two polytypes of the CoOOH, which can also be precipitated individually. These polymorphs have the same layered structure, with different crystal stackings [37]. The polytype generally denoted as β-CoOOH has 3R crystal stacking (space group R $\overline{3}$m), and a brown color [29,38], while the γ-CoOOH polytype has 2H crystal stacking (space group P6₃/mmc) [39,40,41], a black color and high electrical conductivity [29,38]. In both polytypes Co³⁺ is bonded to six equivalent O²⁻ atoms to form edge-sharing CoO₆ octahedra, and takes the Low Spin (LS) electronic configuration (τ⁶_2ge⁰_g). All Co-O bond lengths are 2.02 Å. H⁺ is bonded in a linear geometry to two equivalent O²⁻ atoms of two different layers with a strong H-bond 1.21 Å long. O²⁻ is bonded in a 4-coordinate geometry to three equivalent Co³⁺ ions and one H⁺ ion. Both β-CoOOH [42] and γ-CoOOH [29] can be partially oxidized containing tetravalent cobalt. On the other hand, solids with H_xCoO₂ stoichiometry and x ~ 0.3, i.e., hydroxides of predominantly tetravalent cobalt, have also been prepared, obtained through cation exchange from M_xCoO₂ oxides (M = Li, Na) [43,44].

It can also be mentioned that a crystalline Co(III) hydroxycarbonate phase with stoichiometry Co(OH)₂(CO₃)_0.5 [45] or Co(CO₃)_x(OH)_y, is also reported to exist, and produces Co₃O₄ by decomposition [46].

As for the precipitation of cobalt hydroxides from water solutions, cobalt (II) hydroxide, whose pKs (logarithm of the solubility constant Ks) is reported to be 14.8 [47], can be prepared by precipitation [48,49,50] or hydrothermal synthesis [51] from Co²⁺-containing water solutions at pH > 8.0 (Figure 2). Co(OH)₂ is actually an amphoteric hydroxide which is essentially insoluble at pH 10 ÷ 13, where it can act as the passivation layer for metallic cobalt (Figure 2), but dissolves in neutral and acidic solution, as well as in strongly basic conditions (pH > 13). Both α- and β-Co(OH)₂ can also be prepared through electrodeposition techniques [52,53,54]. Solid Co(OH)₂ can be electrochemically oxidized to CoOOH [55].

“Cobaltic hydroxide” Co(OH)₃, whose pKs of is reported to be 44.3 [47], can be precipitated from Co³⁺-containing solutions, obtained oxidizing Co²⁺-containing solutions, at a pH as low as -1 to 0 [48,56,57], and is consequently stable also in acidic solutions, as indeed reported for CoOOH [58]. CoOOH is considered to be insoluble at all pHs [59].

We have also to mention that tetravalent cobalt can be produced electrochemically at high voltages, but is essentially unstable in contact with water causing water oxidation [60,61].

2.2. Cobalt Oxides

As reported in the cobalt oxygen state diagram (Figure 1), two thermodynamically stable cobalt oxide phases exist: rock salt-type CoO and spinel Co₃O₄ [16,17,18,62]. CoO, which is stable at reduced oxygen pressures, and in air above near 1100 K [63,64], until its melting point which is reported at 2086 K [18] or at 2208 K [65], crystallizes in the cubic rock-salt or periclase (MgO) structure, with octahedrally coordinated High Spin (e⁴t₂³) Co²⁺ ions [66]. It has an antiferromagnetic behavior below the Néel temperature of 293 K [67]. When stoichiometric, CoO is a charge-transfer insulator, with energy gap of about 6 eV [68], which is strongly reduced in case of nanoparticles [69] and defective structures [70]. When treated in air or oxygen, it easily becomes non stoichiometric with formula CoO_1+x with x ≤ 0.0159 and a p-type semiconducting behavior [71]. In oxygen-rich atmosphere the XPS features of CoO_1+x are similar to those of Co₃O₄ [72,73]. Kinetically limited conversion of rock salt type CoO to Co₃O₄ is commonly realized by calcination in air at temperatures > 523 K depending on CoO morphology [64,74,75]. Co₃O₄ grows on the surface of CoO in case of nanoparticles [74] and both at the surface and in the bulk for bulk particles [64].

On the other hand, in reducing conditions, or upon cobalt metal partial oxidation, CoO_1-x oxygen poor phases may also exist [76]. Depending on stoichiometry, particle size and morphology, the UV-vis-NIR spectrum of CoO is quite variable [77,78], and, accordingly, CoO can appear as olive-green or pink crystals, or as black powder [79]. Combination of cobalt monoxide with other oxides gives rise to a variety of colored pigments such as cobalt blue CoAl₂O₄ (see below), cobalt green (Co,Zn)O, cobalt violet Co₃(PO₄)₂, etc. The color of these materials is due to d-d transitions of the Co²⁺ ion.

With special preparation procedures, cobalt monoxide alternative polymorphs can be produced, with zinc blende and wurtzite structures [80,81], which are expected to have a complex frustrated antiferromagnetic ground state with no net magnetic moment in the bulk [82]. Atomistic simulation calculations predict the lattice energies of the three polymorphs to be relatively close, with the rock salt polymorph being most stable, followed by the wurtzite form and lastly zinc blende [62,80]. As evident from the Pourbaix diagram (Figure 2), CoO is thermodynamically unstable at room temperature and atmospheric pressure with respect to hydration producing Co(OH)₂ [83], which, as said, decomposes to CoO in inert atmosphere at 400–500 K.

As mentioned, Co₃O₄ is the thermodynamically stable form of cobalt oxide under ambient room temperature and oxygen partial pressure. Basically, it has the normal spinel structure with octahedrally coordinated low spin (τ⁶_2ge⁰_g) Co³⁺ ions and tetrahedrally coordinated high spin (e⁴t₂³) Co²⁺ ions at relatively low temperatures. However, a certain degree of inversion is found, depending on particle size and on the preparation temperature [84] as well as on measurement temperature [17,85]. Co₃O₄ is antiferromagnetic at very low temperature, with a Néel temperature varying from around 15 to 40 K depending on particle size [86], while being paramagnetic at room and higher temperatures. However, weak ferromagnetism can be found in nanoparticles [87]. It is a semiconductor with a bandgap of 2.11 eV [88]. In oxidizing conditions an enrichment in Co³⁺ is found, in particular at the surface, according to XPS data [89,90]. In these conditions Co₃O₄ is a p-type semiconducting material [91]. According to its strong absorption in the entire visible region (Figure 3), Co₃O₄ usually has a dark blackish color, making it a black pigment [92] usually commercialized as “cobalt black pigment” or pigment black 13.

As said, Co₃O₄ can be prepared by calcination of precipitated Co(OH)₂ and CoOOH at > 573 K [55,94,95], as well as by calcination of the cobalt nitrate and carbonate salts. It is also formed by calcining CoO in air at 523–873 K (depending on CoO morphology) [64,74,75]. Practically, Co₃O₄ preparation procedures are multiple, as summarized e.g., by Zhao and Ma [94], as reported in

Table 1. Various synthesis methods of Co₃O₄ and advantages and disadvantages, according to [96].

Entry Name of the Synthetic Process	Advantages	Disadvantages
Hydrothermal reaction	The crystalline powder can be obtained directly without high-temperature sintering, the crystallinity is high, and the control the particle size of the produced crystal is easy	Strong dependence on production equipment
Thermal decomposition	Thermal instability; pyrolysis products are different	The combustible gas is large and the residual carbon slag is small
Solution combustion	The process is simple, the operation is convenient, the purification efficiency is high, and the heat energy can be recovered	When the combustible component content is low, preheating energy consumption is required
Vapor deposition method	The film-forming device is simple and raw materials are easy to obtain	High reaction temperature
Coprecipitation	The process is simple, the cost is low, the preparation conditions are easy to control, and the synthesis cycle is short	Agglomeration or uneven composition
Sol–gel	Easy doping, uniform composition, low reaction temperature required	Poor film density; volume shrinkage
Template method	Easy synthesis and size control, especially for nanomaterials	High pH and ionic strength of the solution required
Chemical reduction method	Simple reagents and equipment; low cost	Reaction process not easy to control; impurities easily appear
Wet synthesis	Simple operation; can be a large number of syntheses	Hidden dangers in emissions and cooling methods
Ionic liquid-assisted method	Low melting point, good thermal stability	Complex process, high cost, conductivity low

.

As will become clear later on, one of the most relevant features of Co₃O₄ is the possibility to prepare it in a variety of different nanostructures, that can offer enhanced activity in different technologies. In fact, a number of approaches have been developed to produce different morphologies of Co₃O₄ nanoparticles [97,98,99] and in order to expose more reactive faces, e.g., for catalytic combustion [100] and CO oxidation [101]. In Figure 4, reproduced from ref. [102], methods for preparing cobalt-based nanoparticles are summarized. In Figure 5, reproduced from ref. [103], Field Emission Scanning Electron Microscopy (FE-SEM) images of cobalt oxide nanoparticles are reported, showing that morphology can significantly change even with small changes of synthesis temperature. On the other hand, looking at their industrial applications, economic aspects of such preparation procedures must also be taken into consideration [104], along with the stability of the resulting nanopowders (see below).

Co₃O₄ decomposes to CoO at about 1110–1140 K in inert atmosphere [24,105] and at 1150–1230 K in air [24,106]. In Figure 6 the evolution of the XRD pattern of Co₃O₄ with the calcination temperature is shown, providing evidence of the appearance of the peaks of the CoO phase at 1173 K and the full disappearance of the peaks of the Co₃O₄ phase at 1373 K, when calcination is realized for 1 h [106].

Taking into account the remarkable reaction enthalpy (844 kJ/kg) in the reversible Co₃O₄ ⇆ CoO + ½ O₂ reaction this system is considered for thermochemical energy storage [107]. However, it has been found that sintering causes deterioration of the behavior upon thermochemical cycles.

Under vacuum, the decomposition of Co₃O₄ to CoO can be complete at a temperature as low as 673 K [108]. CoO-Co₃O₄ composite particles can also be produced [109]. Interestingly they appear to be ferromagnetic at r.t. [110]. Co₃O₄ is reported to convert into CoOOH when in contact with both acidic and basic solutions [58] without significant dissolution.

The possible existence of Co₂O₃ phases, either with corundum-type or bixbyite-type structure, is doubtful and a little bit controversial, based on theoretical calculations [111,112]. Apparently, corundum type Co₂O₃ has been synthesized under high pressure by Chenavas and Joubert [113] but is likely unstable at atmospheric pressure [62]. It can be remarked, however, that several trivalent-only cobalt mixed oxides exist, such as e.g., lanthanide cobaltites LnCoO₃ with bixbyite [112] and perovskite [114] structures, where Co³⁺ is in octahedral coordination.

CoO₂, with tetravalent cobalt, is the end term of the Li_xCoO₂ and Na_xCoO₂ series with x~0. It and can be obtained through an electrochemical method by deintercalation of Li⁺ ions from Li_xCoO₂ [115,116,117]. Li_xCoO₂ [3] and Na_xCoO₂ [118] can be used as cathodes of Li- ion and Na- ion batteries, respectively, but, during the battery recharge, deintercalation is only partial. However, CoO₂ can be produced as an essentially stoichiometric material and is converted by hydrogen Temperature Programmed Reduction (TPR) to CoO at a little above 473 K [117]. More recently, CoO₂ has also been prepared by converting bulk Ca₃Co₂O₆ via a hydrothermal treatment [119]. CoO₂ is a layered structure with dark color and metallic paramagnetic behavior [120].

In

Table 2. Reference data of cobalt oxides and hydroxides.

Formula or “Symbol”	Structure Type	Space Group	Color	References
CoO1±x	rock salt	Fm $\overline{3}$m	variable with x	[16,17,18,62,63,64,65,66,67,68,69,70,71]
	wurtzite	P63mc	green	[62,79,80,81,82]
	zinc blende	F $\overline{4}$3m		[62,79,80,82]
Co3O4	spinel	Fd $\overline{3}$m	dark blackish	[16,17,18,62,84,85,86,87,88,89,90,91]
Co2O3	corundum	R3c	black	[62,111,112,113]
CoO2	O1	P $\overline{3}$m1	dark	[115,116,117,119,120]
β-Co(OH)2	brucite	P $\overline{3}$m1	pink	[22,23]
“α-Co(OH)2”	hydrotalcite	P $\overline{3}$m1	variable	[24,25,26,27,28]
Co(OH)3			brownish-black or green	[35,36,47,48]
β-CoOOH		R $\overline{3}$m	brownish	[28,38,39,40,41,42]
γ-CoOOH		P63/mmc	black	[28,38,39,40,41,42]
“HxCoO2”	O1	P $\overline{3}$m1		[43,44]

a summary on the structures of cobalt oxides and hydroxides is given.

2.3. Metallic Cobalt

The thermodynamically stable phase of metallic cobalt at ambient temperatures is hexagonal close-packed (hcp), with ferromagnetic behavior and a Curie temperature of about 800 K. However, it usually converts, by heating, into the face-centered close packed phase (fcc), which is more stable above about 700 K [121,122,123]. This phase is ferromagnetic too and becomes paramagnetic at the Curie temperature of about 1400 K, while it melts near 1700 K. The same phases can also appear in nanoparticles [124,125], that can sometimes appear as amorphous and superparamagnetic [126,127].

2.4. Redox Chemistry of Pure CoOx Phases

It has already been mentioned that Co₃O₄ decomposes to CoO by simple outgassing at ~673 K, but it has been reported that most of the CoO phase is reduced to metallic Co by simple vacuum annealing at temperatures above 633 K [128]. Both Co₃O₄ and, at slightly higher temperature, CoO are reduced to metallic cobalt at 473–673 K in hydrogen, although the position of the TPR peak strongly depends on experimental conditions [105,129,130,131,132,133].

Conversely, when a clean cobalt surface is exposed to air at room temperature, an 8–10~film of Co(OH)₂ forms in seconds or less. Exposure to air at 373–500 K results in the growth of a film of CoO. At 700 K in air the film appears to be a mixture of CoO and Co₃O₄ [134]. At high temperature (>1300 K) fcc cobalt can dissolve up to 0.25 at% of oxygen in its own structure (Figure 1). Oxidation of cobalt metal nanoparticles at 573 K in oxygen results in the formation of polycrystalline CoO followed by the formation of Co₃O₄ [135].

3. Solid-State Chemistry of Pure Aluminum-Containing Phases

The chemistry of aluminum oxides and hydroxides is a very complex one and has been the object of several reviews [136,137,138]. Although two oxyhydroxide phases and four different trihydroxide phases exist, the precipitation from an Al³⁺-containing water solutions commonly produces, at low temperatures, either bayerite α-Al(OH)₃ or gibbsite γ-Al(OH)₃, depending mainly on pH, while boehmite (γ-AlOOH) and its poorly crystallized form pseudoboehmite are produced at higher temperatures (i.e., 350 K).

The thermal decomposition of hydroxides and oxyhydroxides gives rise to the many different alumina phases. The most common phases are more or less distorted defective spinel phases, i.e., cubic γ-Al₂O₃, which is usually prepared through the calcination of gelatinous pseudoboehmite or crystalline boehmite at around 723 K, or η-Al₂O₃, usually prepared through the decomposition of bayerite at 523–573 K. By further heating these phases, a distorted spinel phase forms, denoted as δ-Al₂O₃. At 970–1170 K, θ-Al₂O₃, which is isostructural with β-Ga₂O₃, also a spinel derived phase, forms.

Indeed, γ-Al₂O₃ is the most used material as a catalyst support due to its sufficiently high surface area (150–300 m²/g), tunable porosity, stability up to 773 K and more, high dispersion ability for supported phases, and moderate cost and easy preparation.

All aluminum oxides and hydroxides finally produce, through calcination at 773–1273 K, the thermodynamically stable phase α-Al₂O₃ (corundum), a ceramic abrasive and refractory material (T_melt 2313 K), also used as an inert and thermally stable support of catalysts [139].

It can be taken into consideration that aluminas are unreducible by hydrogen, based on thermodynamics, until maybe very high temperatures [140,141], and that, accordingly, alumina reduction does not occur in H₂-Temperature Programmed Reduction experiments until 1273 K. Being also not further oxidable, alumina is a fully stable compound, in terms of its redox behavior, in normal conditions.

4. Solid-State Chemistry of Mixed Co-Al Phases

4.1. Co-Al Mixed Hydroxide Phases

The precipitation of Co²⁺-Al³⁺ ion-containing solutions with a Co/Al ratio = 3 produces solids with a Co₆Al₂(OH)₁₆X·4H₂O composition, where X^- is most commonly the carbonate anion, with the structure of the mineral hydrotalcite [142,143,144], i.e., the isostructural Mg compound, best known among Layered Double Hydroxide (LDH) anionic clays [145,146]. This phase is constituted by brucite (Mg(OH)₂)-like layers with water and anions in the interlayer region to compensate the positive charge of these layers when trivalent elements are present. Actually, hydrotalcite-like structures are obtained by precipitating a solution with a Co/Al ion molar ratio from 1 to 6 [147,148,149] with an expansion of the unit cell parameters by increasing the Co/Al ratio in the 1–4 range only. The constancy of the cell parameters in the samples with Co/Al 4–6 is consistent with the copresence of Co²⁺ and Co³⁺ in the samples with Co/Al ratio > 3, according to the size of the ions (Co²⁺ > Al³⁺~Co³⁺). On the other hand, samples with a Co/Al ratio < 3 represent solid solutions of the hydrotalcite-like double hydroxide phase and the so-called α-Co(OH)₂ phase (see above). The calcination in air of all these phases at 773 K for 5 h produces Co-Al mixed oxide spinel phases. However, if the calcination of the stoichiometric hydrotalcite-like compound Co₆Al₂(OH)₁₆CO₃·4H₂O is realized in nitrogen, the XRD analysis shows the production of a spectrum with relative broad peaks, due to the monophasic solid with a defective rock salt or periclase structure, i.e., Al³⁺-containing CoO [150]. This phase is unstable and converts into Co_3−xAl_xO₄ spinel in an air atmosphere at 473 K.

The formation of Co-Al ammonium hydroxycarbonates with formula NH₄CoAl₂(OH)₅(CO₃)₂·2H₂O has also been reported, which can produce CoAl₂O₄ spinel by decomposition [151].

4.2. Bulk Co-Al Mixed Oxides

4.2.1. Rock Salt-Type Phases

The calcination in an inert atmosphere of Co-Al layered hydroxides with an Co/Al ratio > 1 can produce poorly crystallized rock salt-type solid solutions, i.e., CoO containing moderate amounts (Co/Al ≥ 3) of Al³⁺, i.e., Al³⁺-doped CoO [150]. However, these phases are thermodynamically unstable and crystallize in separate phases (CoO + CoAl₂O₄ spinel) by further heating. On the other hand, a small solubility of Al³⁺ in the CoO phase is also reported to occur at high temperatures in the CoO-Al₂O₃ phase diagram [152,153].

4.2.2. Co-Al Mixed Oxide Stoichiometric Spinel Phases

The CoO-Al₂O₃ phase diagram at high temperatures (>1273 K) has been reported by Mori [152]. It shows the spinel CoAl₂O₄ (melting point at 2253 K) as the only thermodynamically stable intermediate phase in between rock salt-type CoO and α-Al₂O₃, with a large solubility of Al₂O₃ into CoAl₂O₄ only above 1373 K, and no solubility with CoO.

Cobalt aluminate CoAl₂O₄ is a blue-colored normal spinel, with high-spin Co²⁺ in tetrahedral sites and Al³⁺ in octahedral sites, largely used for centuries as a pigment. The blue ceramic pigment CoAl₂O₄, denoted as Thénard’s blue, cobalt blue, or Pigment Blue 28, is commonly prepared through a solid-state reaction between Co₃O₄ and Al₂O₃ at about 1573 K [154]. It is used for the coloration of different materials such as plastics, porcelain, enamel, paint, paper, rubber, glass, concrete, and glaze, and as a pigment in optical devices. When the calcination temperature is lower, the presence of Co³⁺ results in a darker color. As the temperature increases, a partial inversion of the spinel structure will occur [155].

Looking at what happens in an oxidizing environment, as said, Co₃O₄ spinel is the stable cobalt oxide phase. CoAl₂O₄ and Co₃O₄ are isostructural normal spinel solids, and a complete solubility in the compositional range Co/Al ∞ ÷ 0.5, with an intermediate stoichiometry of Co₂AlO₄, is usually assumed. However, XRD data are interpreted by some authors to reveal a CoAl₂O₄-Co₃O₄ spinel phase separation occurring at high temperatures [156], although this might be an artifact due to the non-homogeneity of the samples.

In fact, the isostructural compounds Co₃O₄, Co₂AlO₄ and CoAl₂O₄ [157,158,159,160,161] have essentially the same XRD patterns with small peak shifts [162], while the increase in cobalt content in the spinel is reported to result in a slight decrease of the unit cell size or cubic lattice parameter [163]. Differentiation can be attempted using the intensity of the 111 peak at 2θ = 19.0, which is supposed to be most intense for Co₃O₄ [164], and the I(220)/I(311) intensity ratio, which is expected to be smallest for the CoAl₂O₄ phase [165,166]. Also, the skeletal IR spectra of the three stoichiometric Co-spinel phases Co₃O₄, Co₂AlO₄, and CoAl₂O₄ [127,164,167,168,169,170,171] were reported to differ very slightly for the peak position.

On the other hand, the end stoichiometric spinel phases Co₃O₄ and CoAl₂O₄, when crystalline and calcined at relatively high temperatures, have a different color and are consequently well distinguished through visible spectroscopy (Figure 3). Bulk Co₃O₄ is dark near to black, and accordingly has a strong absorption in the visible region with two broad and very strong absorption doublets centered near 400 and 700 nm [167,172] similar to that reported for CoAl₂O₄ if calcined at low temperature, which is black too [157,163,173]. Instead, as said, when calcined at a high temperature, CoAl₂O₄ is blue, and its visible spectrum shows a strong absorption triplet near 545, 582 and 627 nm, due to Co²⁺ d-d transitions [93,163,174,175] (Figure 3). Detailed spectroscopic studies indicate that CoAl₂O₄ may have a significant degree of spinel inversion when calcined at low temperatures (black), while it becomes a fully normal spinel (with high-spin Co²⁺ in tetrahedral sites, Al³⁺ and low-spin Co³⁺ in octahedral sites [162]) with a blue color when heated above 1073 K [174,176]. The XPS spectra show that stoichiometric CoAl₂O₄ may contain, at least at the surface, significant amounts of Co³⁺ [147], but samples treated at high temperatures do not show Co³⁺ [175,176].

Thus, the electronic spectrum indicates that CoAl₂O₄ calcined at low temperatures may contain Co₃O₄ species, or, in any case, a Co³⁺-containing cobalt-rich spinel phase at least at the surface. To develop a blue color, cobalt ions must be entirely reduced to Co²⁺, and this is realized through heat treatment even in air at T > 1073 K.

Co₂AlO₄ also presents similar visible spectra, depending on the calcination temperature, to CoAl₂O₄ [158]. The XPS spectra of Co₂AlO₄ calcined at 1023 K presents both features of Co²⁺ and Co³⁺ [159].

As already mentioned, Co-Al mixed oxide phases can be prepared at low temperatures through the decomposition of Co-Al mixed hydroxide phases or from sol–gel prepared solids [177]. Nanocrystalline cobalt aluminate was also successfully prepared using both the conventional and microwave combustion methods starting from nitrates and aloe vera plant extract [178], as well as through a number of soft chemistry techniques [179,180,181]. Solid state reaction of mixtures of separated cobalt oxide and alumina phases, which is, as said, the most common way to produce cobalt aluminate pigments, is a slow process, being predominant only above 1323 K [182] and normally realized above 1473 K [154].

4.2.3. Co-Doped Alumina Non-Stoichiometric Spinel Phases

Through the (co-)precipitation of Co-Al mixed solutions followed by calcination, Co₃O₄-Co₂AlO₄-CoAl₂O₄-γ-Al₂O₃ spinel-type solid solutions in the entire compositional range Co/Al = ∞ ÷ 0 can be obtained. However, in the range Co/Al = 0.5 ÷ 0, these solid solutions are cation-deficient spinels, also denoted as Co-doped γ-aluminas. Such phases are (as with γ-Al₂O₃) metastable. Their thermal treatment reveals that Co²⁺ slightly inhibits the γ-Al₂O₃ to α-Al₂O₃ phase transition, and at 1273 K produces CoAl₂O₄/γ-Al₂O₃/α-Al₂O₃ composites [183,184]. Calcination at 1373 K results in blue colored biphasic CoAl₂O₄/α-Al₂O₃ [185] in agreement with the data from the phase diagram [152].

The mixing of alumina and Co₃O₄ (0.40 mol concentration of Co₃O₄ or lower) followed by sintering at 1823 K or a higher temperature results in green cobalt sapphire powders, i.e., Co-doped corundum [186], characterized by visible main absorptions at 436 and 652 nm assigned to Co³⁺ ions [187,188]. After the reduction in hydrogen at 1623 K, the spectrum is modified, now showing a triplet at 540, 580, and 627 nm, assigned to Co²⁺ coming from Co³⁺ reduction [188,189], similar to that found on CoAl₂O₄, corresponding to a change in color from dark green to brownish blue. Similarly, the calcination of cobalt-doped boehmite gives rise to cobalt-doped corundum species, with nearly the same visible spectrum [190].

4.3. CoO_x/Al₂O₃ Materials Produced Through Impregnation Techniques

Supported cobalt oxide on alumina, used as an oxide catalyst or as a precursor of alumina-supported cobalt metal catalysts, is mostly prepared using γ-Al₂O₃ as the support and cobalt nitrate hexahydrate, Co(NO₃)₂·6H₂O, as an impregnating salt, followed by calcination and/or reduction [191,192]. This is mostly due to the relatively easy and complete decomposition of this salt, as well as to the lack of any residue left on the surface when using this salt.

The impregnation of small amounts of Co²⁺ salts (i.e., nitrate) onto γ-Al₂O₃ followed by moderate temperature calcination results in the dispersion of Co ions, forming highly dispersed Co²⁺ ions on alumina, while the loading of larger amounts of cobalt species produces a surface layer with a spinel-like structure, whose XPS and UV-vis-NIR spectra are closely similar to those of CoAl₂O₄ [193], with the presence of Co²⁺ and the absence of Co³⁺ [194,195]. of these defective spinel materials results in a shift in the 440 XRD peak in the range 600–1100 K, providing evidence for the partial penetration of Co ions in the defective spinel structure of γ-Al₂O₃, and in the crystallization of CoAl₂O₄ and α-Al₂O₃ only at 1290 K [131], in agreement with the behavior of Co-doped γ-Al₂O₃ (see above). H₂-TPR experiments show that highly dispersed Co²⁺ species are almost non-reducible up to 1000 K, while the surface spinel layer is reduced in H₂-TPR conditions in the 700–900 K range [131,132,194] (Figure 7).

If the loaded amount exceeds a level that can be calculated to be sufficient for covering the entire support surface (monolayer amount), the features of the sample become those of a biphasic material. Taking into account that typical γ-Al₂O₃ supports have 150–250 m²/g surface areas, the amount of cobalt on near-monolayer catalysts is 15–20 wt%. Over the theoretical monolayer loading, the visible and XPS spectra become analogous to those of Co₃O₄ (or Co_3−xAl_xO₄) stoichiometric spinel particles, which are also highly evident in the XRD pattern, present together with the features of the γ-Al₂O₃ support [193,194,195,196].

4.4. CoO_x/Al₂O₃ Materials Produced Through Other Techniques

According to a recent paper, low-Co-loading CoO_x/γAl₂O₃ samples can be prepared using a dry solid-state technique, i.e., mixing and calcinating separated Co nitrate and dehydroxylated γAl₂O₃ [197]. These materials, in spite of their low cobalt loading, would show a high catalytic activity because of the presence of undispersed Co₃O₄ agglomerates. However, the real deposition of such particles on alumina, as well as the stability of these materials as catalysts, has not been ascertained. Zavyalova et al. [198] reported on a combustion synthesis method starting from impregnated cobalt nitrate on alumina for preparing CoO_x/Al₂O₃ total oxidation catalysts.

4.5. Reduction of CoOx-Al₂O₃ Materials as Precursors of Supported Cobalt Metal Catalysts and Their Industrial Application

As already mentioned, cobalt oxides are reduced by hydrogen to metallic cobalt at temperatures ranging from 473 to 673 K. In contrast, alumina is not reducible by hydrogen in normal conditions up to 1273 K. In parallel, the H₂ reduction at T ≤ 773 K of biphasic CoO_x/Al₂O₃ samples, i.e., those presenting CoO- or Co₃O₄-phase particles within the material, results in the production of cobalt metal particles. This occurs, in particular, in the case of CoO_x/Al₂O₃ prepared by impregnation of γ-Al₂O₃ with an over-monolayer loading of cobalt oxide, and in the case of coprecipitated spinels with Co/Al ratio > 0.5. The H₂-reduction of these materials results in the formation of metallic cobalt particles interacting with or supported on alumina or Co²⁺-containing alumina [131,132,194,195,196,199].

On the other hand, the presence of small amounts of Al³⁺ ions in cobalt oxides hinders cobalt reduction, thus increasing the stability of cobalt oxides in hydrogen reducing conditions [200]. Instead, samples based on bulk CoAl₂O₄, co-doped aluminas, and catalysts constituted by well-dispersed Co oxides in small amounts on alumina are much more resistant to reduction, starting to be reduced only above 1000 K in TPR conditions [131,132,194]. Nevertheless, the formation of small amounts of small-size dispersed cobalt metal particles or cobalt clusters supported by or interacting with alumina is possible already after a prolonged reduction also at 673–773 K [171,194,196]. The “surface spinel layer” predominant on Co₃O₄/Al₂O₃ samples, with a near-monolayer coverage, is mostly reduced in TPR experiments in the 700–900 K range (Figure 7).

CoO_x/Al₂O₃ materials have commercial applications as the precursors of metallic Co/Al₂O₃ catalysts, mainly used for hydrogenation reactions, such as the Fischer Tropsch synthesis of hydrocarbons from CO/CO₂/H₂ syngases [6,7]. In many cases, such precursors contain a cobalt oxide loading slightly exceeding the monolayer loading, thus producing small metal particles after pre-reduction, as occurs, e.g., with Co/γ-Al₂O₃ Fischer Tropsch catalysts [7,201] which usually contain nearly 20 wt% cobalt with alumina supports of 150–200 m²/g [6,202]. Similar materials and procedures are also applied commercially in the manufacturing of Co/Al₂O₃ catalysts for other hydrogenation, dehydrogenation, amination and steam reforming catalysts [203,204].

It can be mentioned that the preparation of metallic alumina-supported cobalt catalysts can also be obtained using an impregnation of the support with cobalt carbonyls Co₂(CO)₈ or Co₄(CO)₁₂ in organic solutions [6]. In this case, the calcination step would not be needed; thus, oxidic precursors are not necessarily used.

5. Surface Chemistry of Catalytic Materials Based on Co and Al Oxides

5.1. Surface Chemistry of Cobalt Hydroxides

Very few data are reported on the surface chemistry of cobalt hydroxides. The surface of Co(OH)₂ polymorphs is reported to be covered by hydroxy groups that can be exchanged by carboxylate and anhydride groups, thus improving the electrocatalytic properties [205]. A point which recently raised much attention refers to the spin configuration of Coⁿ⁺ surface sites on the surfaces of CoOOH. Authors have, in fact, concluded that surface Co³⁺ in the HS state (different from its LS state in the bulk) has a higher activity for the electrochemical Oxygen Evolution Reaction. According to Zhang et al., HS-surface coordinatively unsaturated Co³⁺ sites can be generated on CoOOH samples through the electrochemical oxidation of cobalt sulfides [206].

5.2. Surface Chemistry of CoO and Co₃O₄

Given the periclase structure of CoO and the only slightly larger lattice constant of CoO with respect to MgO (4.263 vs. 4.213 Å [207]), due to the slightly larger ionic radius of octahedrally coordinated Co²⁺ with respect to Mg²⁺ (0.745 Å vs. 0.72 Å [208], CoO is expected to have a highly ionic character and a predominant basic character at its surface [209,210], only slightly lower than that of MgO [211]. In fact, surface Co²⁺ ions are expected to act as weak Lewis acid sites, and, in parallel, surface oxide anions should be markedly basic. In contrast, the definitely higher polarizing power of Co³⁺ with respect to Co²⁺ due to a higher charge and smaller ionic radius (~0.6 Å) would result in a higher Lewis acidity of such sites when exposed at the surface.

On the other hand, it has been already said that CoO, CoO_1+x and Co₃O₄ easily transform each into the other, at least at the surface, through air, inert or vacuum pretreatments at moderate temperatures [212]. Most surface studies, in particular those realized through IR spectroscopy, are realized after outgassing at moderately high temperatures, such as at T ≥ 673 K, in order to desorb water species, and this leads to the at least partial decomposition of Co₃O₄ to CoO [108,129,193]. Thus, the surface studies of cobalt oxides may give very different results depending on the pretreatments.

In fact, surface studies have been realized for Co₃O₄ with different pretreatments [129]. The slight reduction of a Co₃O₄ pure powder pressed disk even through simple outgassing, or, even more, in hydrogen (200 Torr, 523 K, 10 min) followed by outgassing at 373 K, strongly increases the transmittance to IR radiation and results in a hydroxylated surface. In fact, a main band at 3680 cm⁻¹ is observed, similar to that previously observed and attributed to the surface OHs on CoO [213]. Over this pre-reduced sample, essentially transformed into CoO, methoxy species are formed from methanol at room temperature (C-O stretching at 1080 cm⁻¹), and Lewis-bonded ammonia (1611 cm⁻¹ NH₃ asymmetric deformation, 1175 cm⁻¹ NH₃ symmetric deformation) is formed from NH₃, confirming the ionic nature of the CoO surface, the moderate Lewis acidity of Co²⁺ surface sites, and their limited oxidation ability [129]. The IR spectra of NO adsorbed on Co₃O₄ outgassed at 773 K, mostly transformed into CoO according to XRD, have been reported by Topsøe and Topsøe [193], producing dinitrosyl species absorbing at 1865 and 1780 cm⁻¹, confirming the activity of surface cobalt sites, which are likely bivalent.

Studies of CO adsorption on outgassed Co₃O₄, essentially constituted by CoO, show the presence of a band at 2140 cm⁻¹, assigned to CO interacting with Co²⁺ centers, but also a number of other bands in the 2100–1700 cm⁻¹ range that are attributed to different polycarbonyl species on reduced cobalt centers. This suggests that CO is able to reduce Co²⁺ to Co⁺ or even to Co metal centers at room temperature [129], or that a partial decomposition of CoO to Co metal already occurred under outgassing [135].

Over simply outgassed Co₃O₄ samples, likely still incompletely converted to CoO, methanol is oxidized on the surface at room temperature, producing carbonate ions and dioxymethylene species. Also, ammonia is oxidized on the simply outgassed Co₃O₄ surface, producing at room temperature amide, imide and NO⁺ species [129]. This confirms the strong oxidizing power of the Co₃O₄ surfaces. On the other hand, the adsorption of CO at 150 K over this simply outgassed Co₃O₄ surface gives rise to a single CO stretching band whose frequency decreases slightly with increasing coverage (from near 2195 to near 2170 cm⁻¹) attributed to CO on Co³⁺ surface sites, while a C-O stretching at 2140 cm⁻¹ was again assigned to CO interacting with Co²⁺ surface sites [129].

On the other hand, CO adsorption on unreduced Co₃O₄ produces CO₂ and surface carbonates at very low temperatures, such as T ≥ 323K [90,214] and CO adsorbed on reduced cobalt centers produces carbonyl species absorbing at 2120, 2070 and 2000 cm⁻¹ (Figure 8). Structural and computational studies suggest that CO can adsorb and be oxidized even at 77 K over Co₃O₄ nanorods, and that the active sites for CO adsorption in an oxygen-containing atmosphere are Co³⁺ [215].

A more recent paper [216] reports an IRAS study of CO adsorption at 110 K on a Co₃O₄ (111) monocrystal face deposited on Ir(111). Similar bands were observed with a slightly different assignment, as reported in Figure 9. In any case, these data confirm the very strong oxidizing ability of the Co₃O₄ surface due to reduction of Co³⁺ to Co²⁺ with the release of surface oxygen species, while the surface of CoO retains an oxidation ability, together with a relevant acid–base behavior shifted towards basicity.

Also, in the case of cobalt oxide, the spin configuration of Coⁿ⁺ surface sites has relevance with respect to the activity in OER. The amount of surface octahedral HS Co³⁺ on Co₃O₄ can be increased by expanding the surface lattice constant through a complex preparation procedure [217]. Enhancing the concentration of surface HS Co³⁺ ions can be obtained in mixed oxide phases containing cobalt such as Zn-Co spinels [218] and PrBaCo₂O₆ (PBCO) double perovskite [219].

5.3. Surface Chemistry of CoAl₂O₄

Studies on the CoAl₂O₄ surface show that it is hydroxylated after outgassing at 773 K [160,220]. IR spectra of NO adsorbed on CoAl₂O₄ have been reported by Topsøe and Topsøe [193]. Also, in this case, like for Co₃O₄, the doublet at 1860, 1780 cm⁻¹ is attributed to dinitrosyl species on Co²⁺. However, a different intensity ratio between the two bands observed on CoAl₂O₄ and on Co₃O₄ suggests that the angle between the Co-nitrosyl species is different in the two cases.

CO adsorbed at 170 K and at room temperature on CoAl₂O₄ outgassed at 773 K is responsible for the main C-O stretching band at 2170 cm⁻¹ and the shoulder near 2140 cm⁻¹ [93,160,220]. They were assigned to CO coordinated on octahedral-like Al³⁺ and tetrahedral-like Co²⁺, respectively, in agreement with the normal spinel nature of this solid. However, by increasing the contact time at r.t., the spectrum modifies, with the growth of bands in the region 2120–1950 cm⁻¹ and also of absorptions in the carbonate region. This suggests that CO is oxidized by Co²⁺, which is reduced to monovalent or zerovalent cobalt centers, as observed also for pure cobalt oxide (see above).

Pyridine adsorption on outgassed CoAl₂O₄ [93,160,220] confirms the location at the surface of both Al³⁺ and Co²⁺, according to the splitting of the 8a vibrational mode of coordinated pyridine (1623 and 1608 cm⁻¹, respectively). Thus, CoAl₂O₄ surfaces retain a moderate oxidation ability after treatments in a vacuum due to the surface Co²⁺ ions and maybe residual Co³⁺ ions, as well as a marked acido-basic character due to the strong Al³⁺ Lewis acid centers and weaker Co²⁺ Lewis acid centers, the latter also showing a moderate oxidizing ability.

5.4. Surface Chemistry of Unreduced CoO_x/Al₂O₃ Materials

IR studies of CO adsorbed at a low temperature over outgassed CoO_x/Al₂O₃ [93,171] show two main components in the C-O stretching band, with a high frequency shoulder at 2190 cm⁻¹ assigned to CO adsorbing on Al³⁺ and Co³⁺ ions and a main band near 2170 cm⁻¹ assigned to CO adsorbing on Co²⁺ sites. However, the formation of CO₂ adsorbed species is also observed, confirming the strong oxidizing ability of part of surface cobalt oxide species over these samples.

The IR spectra of NO adsorbed on CoO_x/Al₂O₃ have been reported by Topsøe and Topsøe [193]. NO is reported to adsorb as a dinitrosyl species (doublet at 1860, 1780 cm⁻¹), similar to those found over CoAl₂O₄ and over cobalt cationic centers dispersed on the surface of low-loading samples. Instead, for the higher-loading sample, the spectrum becomes more similar to that found over Co oxide, confirming the coalescence of cobalt oxide particles [193].

The data indicate that the surface properties of largely dispersed sub-monolayer-loading CoO_x/Al₂O₃ have comparable properties to those of CoAl₂O₄ surfaces, in agreement with the formation of a cobalt aluminate spinel-like surface layer [193,194,196]. Higher-loaded samples, bypassing the monolayer coverage, contain Co₃O₄ or CoO particles, behaving in quite a similar way to unsupported oxides.

6. Catalytic Activity of Cobalt Oxides and CoO_x/Al₂O₃ Materials for Gas Phase Total Oxidations

6.1. CO Oxidation Catalysis

6.1.1. Co₃O₄ as a Low-Temperature CO Oxidation Catalyst

CO oxidation is a relevant process for removing the strongly poisonous CO molecule from breathing air [221]. Co₃O₄ is an active catalyst for CO oxidation even below room temperature [14] or at 300–400 K [97], but may suffer deactivation [222]. According to Cunningham et al. [223] Co₃O₄ is active in CO oxidation even at 217 K and retains a good stability even if the feed is wet. This was confirmed by Whang et al. [224]. As reviewed by Dei and Dhal [101], Co₃O₄-based nanomaterials are among the most active catalytic materials for very-low-temperature CO oxidation to CO₂. In fact, according to Xie et al. [215], Co₃O₄ nanorods not only catalyze CO oxidation at temperatures as low as −350 K but also remain stable in a moist stream of normal feed gas. The high activity in different cobalt oxide-based solids was attributed to electron-mobile Co²⁺-O²⁻-Co³⁺ linkages at the surface of the Co₃O₄ spinel [225] and depends on the concentration of surface Co³⁺ ions [215]. According to Tang et al. [72], bulk Co₃O₄ is covered by a CoO surface layer during low-temperature CO oxidation (353–413 K), while it is covered by a more active Co₃O₄ surface layer at higher temperatures. On the other hand, surface spectroscopic studies show a CO oxidation to surface carbonates at room temperature over Co₃O₄, due to a reaction with surface oxide species and Co³⁺ reducible cations [129]. In conclusion, Co₃O₄- and Co₃O₄-based materials are promising practical catalysts for low-temperature CO oxidation, competitive with noble metal-based catalysts and with copper and manganese oxide-based materials [226].

6.1.2. CoO_x/Al₂O₃ Catalysts for CO Oxidation

Mixed spinels with a Co²⁺/Al³⁺ ratio of 1.5 and 3.0 show a higher activity and stability in CO oxidation at 300–373 K than unsupported Co₃O₄ [222]. Also, supported CoO_x/γ-Al₂O₃ catalysts find relevant activity in CO oxidation, which can be further improved through reduction + reoxidation pretreatments, leading to catalysts active at 423–523 K, in the case of 10–30 wt% Co/γAl₂O₃ catalysts [227]. high activity is reported for catalysts prepared from cobalt acetate and precalcined in nitrogen [228], while the preparation method and properties of mesoporous alumina support have a considerable effect [229]. On the other hand, Thormählen et al. [230] produced a monolithic catalyst based on a Co₃O₄-CoAl₂O₄-Al₂O₃ system, with a 20% nominal Co₃O₄ content, and found that, after pre-oxidation, it was very active at low temperatures and with a light-off temperatures of 210 K, even lower than that of similarly prepared platinum-containing catalysts [231]. In summary, the addition of alumina to cobalt oxide may give rise to more stable and very active catalysts for low temperature CO oxidation, which can even compete with noble metal-based catalysts.

6.1.3. Cobalt Oxides in Preferential CO Oxidation (PROX) Catalysis

The preferential CO oxidation reaction allows the oxidation of CO in mixtures with hydrogen to CO₂, favoring the later purification of H₂ with washing or adsorption techniques. While noble metals such as platinum are mostly studied for this reaction, cobalt oxides are potentially very interesting too [232,233] The Co₃O₄ catalyst exhibits a very high activity, maintaining a 100% CO conversion and CO₂ yield without any hydrogen consumption between 423 and 448 K [234]. According to Lukashuk et al. [235], Co₃O₄ is reduced by H₂ to CoO and ultimately to metallic Co during CO-PROX, and this results in a loss of oxidation activity and in the coproduction of methane by CO hydrogenation on metallic cobalt. Catalysts based on Co₃O₄/γ-Al₂O₃ are also active in the PROX reaction at 323–473; the more active, the lower the Co oxide particle size [236]. On the other hand, a full selectivity in CO oxidation is found below 498 K, while at higher temperatures hydrogen consumption is found and Co₃O₄ is reduced to CoO [236]. Studies on Co₃O₄/α-Al₂O₃ catalysts show that the catalytic activity implies a key role of Co³⁺ ions and that the 100 spinel face of Co₃O₄ is the most active one [237].

6.2. Hydrocarbon Total Oxidation Catalysis

6.2.1. Co Oxides as Methane Combustion Catalysts

Co₃O₄ was found as the most active bulk transition metal oxide catalyst for methane combustion in the early study of Anderson et al. [238], and this was confirmed more recently [239,240], although its stability is limited [241], lower than that of Mn₂O₃. On the other hand, mixed oxides, such as, e.g., cobalt–chromium spinels [239] and Co₃O₄-CeO₂ systems [97], may even have an increased methane combustion activity.

The catalytic combustion of methane and natural gas is used in combined cycle technologies to limit NOx emissions in electrical energy production [242,243]. In this case, a relatively high methane concentration is fed in air; thus, high temperatures are produced on the catalyst, such as 773–1273 K—in any case, sufficiently low to prevent the production of relevant amounts of NOx. To have a stable methane combustion activity at high temperatures, solid-state stabilization is needed. Low-surface-area (3 m²/g) Co₃O₄ was found to be remarkably active in high-temperature methane combustion [244], although it was definitely less active than platinum-based catalysts. To increase stability, cobalt oxide must be mixed with refractory oxides, thus producing mixed phases as perovskites (e.g., LaCoO₃) and β-aluminas (e.g., SrCo_xAl_12−xO_19−δ) [243,244,245].

Low-temperature methane combustion represents a technology for burning methane in low concentrations in air to reduce fugitive methane emissions [246] or to treat the flue gases of NG-fueled engines [247,248]. Thanks to the lower concentration of methane, heat evolution is limited, and the temperature in the catalyst bed is moderate, i.e., 500–873 K. In fact, Co₃O₄ is active in fully burning 1% methane in air at a gas hourly space velocity (GHSV) = 30,000 mLg⁻¹h⁻¹ at 723 K [249] and in 1% methane and 10% O₂ in nitrogen (e.g., oxygen-depleted air) at GHSV 36,000 mLg⁻¹h⁻¹ at 640 K [250]. Working at GHSV 60,000 mLg⁻¹h⁻¹ with a gas composed of 1000 ppm CH₄, 200 ppm NO, 3% CO₂, 5% H₂O, 20% O₂, and N₂ balance, the half-conversion temperatures of different mesoporous samples of Co₃O₄ were in the range 681–746 K [251].

6.2.2. Co Oxides as Higher Hydrocarbon Total Oxidation Catalysts

The potential use of cobalt oxides as catalysts for the total combustion of higher hydrocarbons and other Volatile Organic Compounds (VOCs) has been the subject of some reviews [14,97,252,253,254]. Co₃O₄ is reported to catalyze the total oxidation of aliphatic and aromatic hydrocarbons, as well as oxygenated compounds, in air or oxygen at very low temperatures. Co₃O₄ has the weakest metal–oxygen bond, and the easy reduction of Co³⁺ in Co₃O₄ to Co²⁺ could accelerate the formation of oxygen vacancies at low temperatures [255].

Cobalt oxides have also been tested in propane total oxidation, observed to occur, e.g., at 423–473 K (1000 ppm of C₃H₆ + 9% of O₂ in He at WHSV 36,000 mL g⁻¹ h⁻¹) on Co₃O₄ (11 m²/g) [97]. Propane combustion catalytic activity was found to be mainly dependent on the crystallite size, decreasing with an increase in the crystallite size, correspondingly depending on the surface area [256].

The light-off oxidation temperature of both propane and propene is similar for commercial Co₃O₄ in diluted O₂/He in flow conditions [257], but spectroscopic surface studies show that they are oxidized at the surface of Co₃O_4+x already at or just above room temperature, respectively. Propene is oxidized “selectively” at the allylic methyl group, giving rise to acrylate species, while propane is probably attacked both at C(2) and at C(1), giving rise at 373 K to propanoates and acetates + formates. Acrylates are also formed from propane, probably arising from 2-propoxides via propylene. At higher temperatures, the adsorbed species are fully oxidized to CO₂ [258]. Surface spectroscopy data show that the full oxidation catalysis of transition metal oxides such as Co₃O₄ is the result of successive partial oxidation steps at the surfaces essentially involving oxide anions and reducible high-valency cations such as Co³⁺ [259].

Co₃O₄ also shows a high activity in the oxidation of aromatic hydrocarbons such as benzene and toluene at temperatures as low as 513–543 K [260,261]. It has been pointed out that pre-calcination conditions have a relevant effect on the activity of Co₃O₄ for the combustion of toluene [261,262]. Slow and weak deactivation is observed in both toluene and benzene’s full oxidation on Co₃O₄ after 20–40 h.

6.2.3. CoOx/Al₂O₃ as Hydrocarbons Total Oxidation Catalysts

Supporting Co₃O₄ on alumina was found to decrease its activity in methane combustion [238,248,263]. Co₃O₄-γAl₂O₃ catalysts are reported to be less active than magnesia-, titania-, and, in particular, zirconia-supported Co₃O₄ [264] as well as ZnAl₂O₄-supported Co₃O₄ [263] and MgAl₂O₄-supported Co₃O₄ [265] in methane combustion. However, due to the stability and well-known properties of gamma alumina, the potential use of Co₃O₄/γ-Al₂O₃ as a methane combustion catalyst has been the subject of investigation [198,248,266]. According to Solsona et al. [256], Co₃O₄ supported on low-surface-area α-Al₂O₃ is more active as a propane combustion catalysts than Co₃O₄/γ-Al₂O₃, and this is due to the less dispersing properties of corundum surfaces with respect to surfaces, as well as to the low surface area, resulting in an agglomeration of bulk Co₃O₄ particles in the case of Co₃O₄/α-Al₂O₃. On the other hand, the activities in catalytic combustion of the supported cobalt catalysts depend obviously on the cobalt loading [241,256]. Indeed, the activities in propane combustion of 50% Co₃O₄/α-Al₂O₃ and of 50% Co₃O₄/γ-Al₂O₃ are only slightly lower than those of bulk Co₃O₄ at 423–523 K [256]. On the other hand, it was found that the activity of 10 wt% Co₃O₄/γ-Al₂O₃ in propane combustion can be greatly enhanced through structural modulation, i.e., through the reduction–passivation treatment of core shell materials, up to be capable of outperforming unsupported Co₃O₄ [267].

Supported Co₃O₄/γAl₂O₃ [268] with an up to 21 wt% cobalt loading has also been tested in benzene oxidation. Benzene is completely burnt in the range of 473–573 K to CO₂, with a maximum activity for the sample with 21 wt% cobalt prepared using the equilibrium deposition filtration technique. Also, ex-hydrotalcite coprecipitated Co₃O₄-CoAl₂O₄ spinel samples are very active in benzene catalytic combustion, with a maximum activity for samples with a Co/Al atomic ratio = 5 [149] while details on the preparation also have some effect on catalytic activity [269]. Ex-hydrotalcite-type coprecipitated Co₃O₄-CoAl₂O₄ catalysts (Co/Al a.r. 2) also catalyzed full benzene oxidation at 373–573 K, but the activity of samples precalcined in nitrogen is better compared to that of samples pre-calcined in air, likely due to a higher surface area [150]. Similar catalysts also catalyze toluene combustion in similar conditions [147]. Also, cobalt-containing hierarchical laminated Al₂O₃ (3 and 5 wt% Co) catalyzes the total oxidation of toluene near 573 K [270].

6.3. Total Oxidation of Oxygenated Compounds and of Other VOC’s

6.3.1. Co₃O₄ as a Catalyst for the Total Oxidation of Oxygenated Compounds

Co₃O₄ has high activity in catalyzing burning of formaldehyde, the most important gaseous pollutant in indoor environment primarily released from building materials, furniture, indoor combustion processes, such as cooking, incense burning, and smoking, already at room or slightly higher temperature [271]. It is also able to catalyze full combustion of ethanol at 430 K [272], but the more toxic product acetaldehyde is produced at lower temperature when ethanol conversion is incomplete. Finocchio et al. reported on the isopropanol and acetone full oxidation on Co₃O₄ [258,260]. It was found that at moderate conversion, isopropanol is selectively oxy-dehydrogenated to acetone (80% selectivity at 500 K), but at higher temperature (i.e., 650 K) both isopropanol and acetone are fully converted to carbon oxides (Figure 10). The activity in isopropanol combustion was found to significantly depend on morphology of mesoporous Co₃O₄ catalyst powder [273].

6.3.2. Co₃O₄ as a Catalyst of Total Combustion of Chlorinated Hydrocarbons

The catalytic combustion of chlorinated VOCs represents a particular field [274] due to the effect of chlorine on catalyst stability, as well as on the fate of chlorine in the products. In fact, noble metal catalysts may suffer a deactivation by chlorine upon catalytic oxidation, while some transition metal can give rise to volatile chlorides. Also, zeolite structures can suffer instability in the presence of chlorine. Finally, while the evolution of HCl, which can be more easily washed down from combustion gas, is desirable, the oxidation of chlorine to Cl₂ should be avoided. DeRivas et al. [275] reported on the high activity of Co₃O₄ in the catalytic combustion of Ethylene Dichloride (EDC, 1.2-dichloroethane) with a 100% C-selectivity towards CO₂ and with a coproduction of HCl and Cl₂ at relatively low temperatures (623–673 K). Samples obtained via precipitation proved to be considerably more active than the supported noble (Pt, Pd) metals, protonic zeolites, and Ce/Zr and Mn/Zr mixed oxides, and were found to be highly stable despite the retention of chlorine on the catalyst surface. Yuan et al. [276] reported on the catalytic activity of Co₃O₄ in Vinyl Chloride Monomer (VCM, chloroethylene) catalytic combustion. Over catalysts produced through the template method and calcined at 673 K, working at 563 K with a space velocity of 30,000 mLg_cat⁻¹.h⁻¹ and VCM concentration of 1000 ppm in air, the initial conversion is complete, while it exceeded 96% after 60 h of continuous reaction, with HCl as the main coproduct. At incomplete conversion, traces of chlorinated by-products (CHCl₃, CCl₄, C₂HCl₃, C₂H₃Cl₃) were formed. Depositing cobalt oxide over mesoporous silica was reported to improve catalytic performances in the combustion of chloride organics [277,278].

6.4. Cobalt Oxides as Catalysts for the Combustion of Hydrogen and of Fuel Cell Anode Tail Gas

The catalytic combustion of hydrogen may have relevant applications in the near future [279,280]. Cobalt oxide Co₃O₄ is reported to be the most active transition metal oxide as a catalyst for hydrogen combustion [281] and can be used as a component of non-noble metal catalysts [278,282]. A particular case can be represented by the anode tail gas of hydrogen fuel cells such as Solid Oxide Fuel Cells and Polymeric Electrolite Membrane Fuel Cells. These gases contain hydrogen together with different amounts of CO and methane, and must be partially purged to avoid concentrations of such gases in the recycling of unconverted hydrogen. Such anode tail gas purge can be catalytically burnt to minimize the CH₄ and CO emissions and provide energy to the system. CoO_x/γ-Al₂O₃ catalysts were used for the combustion of anode tail gas produced in a proton-exchange membrane (PEM) fuel cell [283].

6.5. Diesel Soot Oxidation Catalysis

Co₃O₄ is the most active single-phase catalyst for the total oxidation of Diesel soot [284] at 623–723 K, which is further increased by doping with other elements such as silver, cerium, nickel, lanthanum, potassium, lead, and Pt as well, down to 523–623 K [285,286]. Also, CoAl₂O₄ spinels, in particular, after milling, show a high combustion activity similar to that of Pt/Al₂O₃ catalysts [287]. In fact, cobalt oxides are reported to be possible components of commercial Diesel catalyzed anti-particulate filters [288,289].

6.6. Stability and Deactivation of Cobalt Oxide-Based Catalysts for Gas-Phase Oxidation Reactions

As already remarked, the gas-phase oxidation catalytic activity of Co₃O₄ based catalysts strongly depends, in most cases, on morphology, being essentially the stronger the larger the surface area. In fact, an impressive number of papers have been recently published on the preparation of high-surface-area cobalt oxide- based nanoparticles with sophisticated soft chemistry methods [98,99,100,101,102,103,104]. On the other hand, even with classical methodologies Co₃O₄ with very high surface area can be obtained: as for example, 202 m²/g from decomposition of Co(OH)₂ [290]. Indeed, quite few data are available on the thermal stability of such high surface area materials. Data indicate that Co₃O₄ samples heated above 773 K in oxidizing conditions tend to sinter rapidly, with increase of particle size and decrease of surface area [107,291]. As for example, in the case of cobalt oxide microflowers the surface area decreases from 77 to 12 m²/g by increasing calcination temperature from 573 to 773 K [96]. In Figure 11, the TEM images of cobalt oxide nanoparticles are reported after heating in the TEM chamber, i.e., at a low pressure of <10⁻⁵ Pa [108]. XRD analyses show that the starting sample is still Co₃O₄, while, already at 673 K it is fully converted into CoO, as at the final temperature of 1073 K. The TEM image shows that after heating at a temperature of 673 K, the surfaces of the nanocrystals became uneven and more faceted, due preferential exposition of particular crystallographic facets, but no significant particle size growth is observed. Instead, after heating at 1073 K particle coalescence clearly occurred [108]. Early data show that heat treatment of ex-oxalate Co₃O₄ for 24 h already causes a decreases of S_BET from 57 m²/g to 33 m²/g at 573 K, to 19 m²/g at 673 K and to 5 m²/g at 873 K [289]. Thus, sintering can be the cause of progressive loss in catalytic activity at moderate to high temperature depending also on time on stream [261,262]. On the other hand, high surface area can be not necessary for high temperature catalytic oxidation activity, such as e.g., for methane high temperature catalytic combustion at 773–1273 K [244,292], but also for CO oxidation at 333 K [289].

It is also well evident that catalytic activity is reduced when so high a temperature is raised when Co₃O₄ starts to decompose into CoO. On the other hand, in the presence of reductants, Co₃O₄ may be reduced at the surface at much lower temperature forming an epitaxial layer of CoO [237,293], loosing oxidation activity. Reversely, at low temperature in the presence of oxygen, CoO can become covered by a Co₃O₄ layer, thus acting as a very active oxidation catalyst [72]. Thus, catalytic activity in oxidation depends on the relative concentration of oxygen and reductants in the feed, even at low temperatures. The presence of water in the reaction medium, taking into account that water is also a coproduct of organic compound total oxidation, also negatively influences catalytic activity [263] due to the basicity of water competing with oxygen and organic molecules for the adsorption of cobalt ions.

Supporting cobalt oxides on alumina tend to reduce low temperature catalytic activity in oxidation, but may certainly increase solid state stability by limiting the sintering and surface area loss of the active cobalt oxide phase. This is the likely reason for the increased stability of catalytic activity finally producing even more performant catalysts at moderate temperatures [218].

7. Catalytic Activity of Cobalt Oxides in the Chemistry of Ammonia and Nitrogen Oxides

7.1. Ammonia Oxidation Catalysis

Ammonia oxidation to NO is the first step in the Ostwald process for nitric acid synthesis [294]. This reaction, where the co-production of the most stable products N₂ and N₂O must be limited, is realized at 1073–1173 K using Pt metal-based gauges. However, this commercial process suffers the serious drawback of losing platinum as volatile PtO. As an alternative, the development of monolithic honeycomb oxide catalysts has been considered. Among catalytic materials for such an application, cobalt oxide-based materials appear to have excellent properties [295] associated with minimal N₂O coproduction [296]. In fact, the pure oxide Co₃O₄ shows the maximum NO selectivity in ammonia oxidation at 923 K, but can be used only under atmospheric pressure and at a low ammonia concentration, otherwise it transforms to less selective CoO [294,295]. This effect is hindered by doping Co₃O₄ with ceria, thus increasing the high selectivity to NO at 993–1093 K [297]. Also supporting Co₃O₄ on ZnAl₂O₄ spinel improves Co₃O₄ stability in ammonia oxidation to NO [298]. In fact, several cobalt oxide based catalysts, including Co-Al mixed oxides, have been patented for such an application [299,300].

In contrast, supporting cobalt oxide on γ-Al₂O₃, at least at low and moderate coverages, results in catalysts with a low activity in oxidizing ammonia [297]. Ammonia conversion over CoO_x/γ-Al₂O₃ increases with cobalt loading [197,301] producing both NOx and N₂. However, 2% Co/γ-Al₂O₃ reduced in hydrogen at 873 K, which still predominantly contains Co²⁺, gives rise to a stable, quite high selectivity (>80%) for N₂ [300], which can be useful for performing the so-called Selective Catalytic Oxidation (SCO) of ammonia to reduce ammonia slip, e.g., after DeNOxing devices for Diesel aftertreatment using Selective Catalytic Reduction with ammonia (SCR).

7.2. Catalysis for Oxidation of NO to NO₂

The NO-to-NO₂ oxidation reaction is a relevant process in Diesel car afterburner systems, previous to the so-called fast SCR reaction, i.e., the reduction of NO₂ to N₂ by ammonia [302]. It can also speed up the Ostwald process for nitric acid synthesis [303]. Co₃O₄ is a very effective catalyst for the high conversion of NO to NO₂ at 523–573 K and a high space velocity of 238,000 h⁻¹ [304]. The reaction raises the thermodynamic equilibrium near 550 K over Co₃O₄ [305]. However, the activity is significantly negatively affected by the presence of SO₂ in the stream [303]. This can limit its application in Diesel aftertreatment systems.

7.3. Catalysts for the Abatement of Nitrogen Oxides

7.3.1. N₂O Decomposition Catalysis

N₂O is a powerful Greenhouse Gas mainly produced through adipic acid and nitric acid synthesis processes, and, to a lesser extent, combustion processes. It could also be produced in ammonia-fueled internal combustion engines [306,307]. To abate it from waste gases, either its reduction by methane or its thermocatalytic decomposition may be accomplished [308]. Cobalt oxides [309], including Co₃O₄ and other cobalt-containing spinels [310] and perovskites [307,308], are among the most active catalysts for N₂O decomposition to N₂ + O₂ and are mentioned among commercially available catalysts. Alkali doping is reported to further enhance the catalytic activity of Co₃O₄ in N₂O decomposition [311,312,313]. Also Co-Al oxides produced through the calcination of hydrotalcite like hydroxycarbonates display a high N₂O decomposition activity [314,315,316]. The catalytic activity of Co₃O₄ in N₂O decomposition is inhibited in the presence of methane and of oxygen as well [317]. Doubly doped (K,Zn) α-Al₂O₃-supported Co₃O₄ [318] also has a high activity and a slight inhibition by oxygen, but is strongly inhibited by NO in the gas phase.

Co-Al spinels are able to catalyze the full decomposition of 5000 and 10,000 ppm of N₂O with and without the presence of oxygen in the feed gas near 673 K, the catalytic activity being slightly inhibited by oxygen [315]. Nevertheless, Co_3-xAl_xO₄ catalysts have been patented for N₂O decomposition [319], and seem to be commercially available too [320].

Actually, it seems that reduced cobalt oxide species are the most active in N₂O decomposition centers, as seen for a catalytic material Co/CoO_x@carbon catalyst obtained by pyrolyzing cobalt-containing zeolitic–imidazolate framework precursors (ZIF-67) at 923 K in a N₂ atmosphere [321]. This material allows the complete conversion of N₂O at 673 K. In fact, the samples obtained through pyrolysis in air contain Co₃O₄ and show an activity at much higher temperatures, as with pure Co₃O₄.

7.3.2. Cobalt Oxide Catalysts for the Selective Catalytic Reduction of NO_x by Ammonia (NH₃-SCR)

The Selective Catalytic Reduction of NO_x by ammonia (as a water solution or coming from the decomposition of urea water solution [322] is commercially used to abate NOx from waste gases of nitric acid plants, thermal power stations and Diesel engines. Commercial catalysts are based on V₂O₅-MoO₃(WO₃)-TiO₂ [323] or Fe- or Cu-containing zeolites [324,325]. Many transition metal oxides also have a high activity for this reaction [326,327], spinel-type oxides and mixed oxides [328]. Co₃O₄ is an active catalyst for the SCR of NOx by ammonia at low temperatures (e.g., 423 K), but its activity strongly depends on morphology factors and is reduced above 473 K [327,329]. However, SO₂ promotes the activity of Co₃O₄ at T > 473 K [328]. Co₃O₄ nanorods have a higher activity than other Co₃O₄ nanoparticles [330]. Pre-oxidized Co₃O₄ nanorods are more active than slightly reduced ones, suggesting that surface Co³⁺ ions act as the active sites for the reaction.

7.3.3. Cobalt Catalysts for the Selective Catalytic Reduction of NOx by Hydrocarbons (HC-SCR)

CoO_x/Al₂O₃ catalysts, both coprecipitated [331,332] and prepared through wet impregnation [333,334], find an interesting activity in NO_x SCR with propane at 623–723 K, which can be slightly further increased by doping with Sn [330], Zn, Ag, Ni [331], and Ba [333]. Interestingly, a small cobalt loading (e.g., 1% Co) is more effective in the SCR reaction, while at a higher Co content (3% Co) the combustion by oxygen of propane limits SCR activity [333]. Dispersed surface Co²⁺ ions are supposed to be responsible for the SCR activity, while cobalt oxide particles are active in propane combustion [332,333].

7.3.4. Cobalt Oxide Catalysts for the Simultaneous NO_x and Soot Abatement in Diesel Engine Aftertreatment Systems

Bulk Co₃O₄-CoAl₂O₄ spinel phases were found to be very active for the simultaneous removal of NO_x and diesel soot particulates. In particular, the spinel with a Co/Al atomic ratio of 5 and calcination temperature of 1073 K is the best one, with a medium activity (T_i = 563 K) and high selectivity to N₂ formation, as well as a low selectivity to N₂O [335]. The presence of the Co₃O₄ spinel is likely a determinant. In fact, while low-defectivity CoAl₂O₄ has a low activity, the ball-milled CoAl₂O₄ spinel shows a high activity in soot oxidation in NO_x/O₂ [287]. In fact, the addition of cobalt oxide allows us to decrease the emissions of NO₂ in commercial catalyzed Diesel particulate filters [336].

8. Catalysts for Ozone Activation and Decomposition

Co₃O₄ shows a very high activity in decomposing 40 ppm of ozone in dry oxygen-enriched air at 298 K and WHSV 1,200,000 mL/gh, with a slight decline of activity with time [130]. However, the activity is almost lost in wet feed. A much higher stability was found for mixed MgO-CoO oxides. By comparing the activity of different Co-containing oxides, it was deduced that octahedral Co cations give rise to the most active sites. In contrast, according to the data of Liu et al. [337] tetrahedral Co²⁺ sites would give rise to the most active catalysts for this reaction. According to these authors, CoAl₂O₄ has a comparable activity with respect to Co₃O₄, but much lower if the reaction rate is normalized for surface area.

Also cobalt hydroxides have a high activity for ozone decomposition. β-Co(OH)₂ shows a ~90% ozone decomposition efficiency under a high space velocity (WHSV = 1,200,000 mLg⁻¹ h⁻¹, 298 K, 40 ppm), which is far higher than that of α-Co(OH)₂ (~5%) [23].

CoO_x/Al₂O₃ shows a lower activity than Co₃O₄ [130], but is still of interest for both the reactions of ozone decomposition and oxidation with ozone, the catalyst remaining active with the time [338]. The catalytic activity with respect to the complete oxidation of iso-propanol and the oxidation of carbon monoxide in the presence of ozone is higher than in presence of oxygen. Co₃O₄ thin layers on oxidized aluminum honeycomb catalysts are also reported to be very active in ozonation in wastewater purification [339].

9. Cobalt Oxide/Alumina Catalysts for Liquid-Phase Oxidation Reactions

9.1. Catalysts for Aerobic Selective Oxidations in Liquid Phase

Pure Co₃O₄ was recognized as an excellent and reusable catalyst for the aerobic oxidation of vanillyl alcohol to vanillin, with an 80% conversion and 98% selectivity to vanillin [340] in a NaOH/isopropanol solution with 6.8 bar of O₂. A high surface area, small particle size and high exposition of Co³⁺ ions were considered as key factors for high efficiency in this reaction.

Co₃O₄ supported on alumina (9.6% Co) has been tested as a catalyst for the aerobic oxidation of complex alcohols to the corresponding carbonyl compounds in water solution in the presence of bases at total reflux conditions [341]: benzaldehyde yields from benzyl alcohol up to 93% were obtained.

The oxidation of styrene with oxygen in a dioxane solvent was tested using mesoporous cobalt/alumina catalysts, producing high selectivities to styrene oxide or benzaldehyde depending on the reaction conditions [342].

9.2. Co₃O₄ as an Active Catalyst in Advanced Oxidation Processes (AOP) for Water Contaminant Degradation with Peroxide Compounds

Co₃O₄ is reported to act as an outstanding catalyst for the activation of peracetic acid (PAA) in the degradation of water pollutants [343] such as orange G dye [344] and sulphonamides [345]. Similarly, Co₃O₄-based materials were found to effectively activate peroxymonosulfate (PMS: KHSO₅) for the degradation of organic pollutants such as paracetamol [346], 4-chlorophenol [347], several aromatic compounds [348], imidacloprid [349], tetracycline (TC) [350], enrofloxacin [351]. Nanoparticles and defect-engineered samples show a higher activity than commercial Co₃O₄ [346,347]. Electron paramagnetic resonance characterization, radical quenching, and probe oxidation experiments indicate that tetravalent cobalt ions are the dominant reactive species on the surface of Co₃O₄ in contact with PMS [348].

10. Cobalt Oxide Catalysts for the Hydrolysis of Hydride Compounds

Metal hydrides have a potential interest for hydrogen storage and transport. In particular, NaBH₄ is considered as a potential candidate for this application. In this context, the hydrogen generation obtained through hydrolysis represents a crucial step. This reaction is slow; thus, catalysis is needed to accelerate it [352]. Co₃O₄ [353], CoO-Co₃O₄ composites [354], carbon supported Co₃O₄ [355] and graphene supported Co₃O₄ [356] as well as a number of cobalt-containing metallic and oxide materials, are reported to act as among the most performant catalysts for the hydrolysis of NaBH₄ [356,357]. In fact, it seems that the best catalysts are based on Co-B alloys [358]. However, it has been concluded that the cobalt oxide phase serves as a key catalyst component that improves water activation in catalysts based on metallic Co-based alloys [359].

11. Cobalt Oxides as Photocatalysts

The semiconductor nature of Co₃O₄ with a bandgap of 1.8–2.2 eV also makes it suitable for photocatalysis applications under visible light [360]. However, these materials do not seem to be really competitive with other photocatalytic materials [361,362,363] mainly due to the fast electron hole recombination, which can only be partially improved by metal doping.

Co₃O₄-based materials have an interesting activity in the solar light photodegradation of water pollutants [364]. In particular, they are active in dye abatement technologies [365,366,367,368,369]. Also, Co hydroxides have a potentiality for this application [370].

CoO is reported to be an active solar photocatalyst for neutral water splitting [69]. Co₃O₄ shows hydrogen evolution activity with visible light in sulphuric acid [371] and in triethanolamine solution [372]. Lai et al. [373] developed superhydrophilic and superaerophobic Co₃O₄/Nickel-Molybdenum-Foam (NMF), efficient and stable as a bifunctional electrocatalyst for the overall water splitting reaction. Also, cobalt hydroxides have a water splitting photocatalytic activity [374]. In fact, even if cobalt-containing mixed oxides are considered as components of efficient water splitting photocatalysts [375,376] cobalt oxides do not seem to be competitive materials for this application with respect to other more active materials [375,377].

Cobalt oxides containing materials also have been reported as active for the Visible Light-Driven photocatalysis of CO₂ reduction, including ultrathin CoO atomic layers with varying concentrations of oxygen vacancies [378], Co₃O₄ with special morphology control [379], Co₃O₄ modified by nickel [380,381] and Co₃O₄ activated by N-bromosuccinimide [382].

12. Electrocatalytic Activity of Cobalt Oxide-Based Materials

Given the redox properties of cobalt in its oxides and hydroxides, their low solubility in neutral water and in moderately acid and basic solution [383,384] and their semiconducting behavior, they are active materials in several electrocatalytic systems [385], as for example, on both sides of the electrochemical water splitting reaction [386,387]. electrical conductivity is a main drawback for some of these applications, which can be overcome by depositing or combining cobalt oxides on/with carbon-based or metal-based conductive components [386]. The electrocatalytic semireactions involved are summarized in

Table 3. Electrochemical semireactions catalyzed by cobalt-based materials.

Reaction Name	pH	Semireaction	Technologies
Oxygen Evolution Reaction (OER)	Alkaline	4 OH−(aq) → O2(g) + 2 H2O(aq) + 4e−	AEL, MAB
	Acidic	2 H2O(aq) → O2(g) + 4 H+ (aq) + 4 e−	PEMEC
Oxygen Reduction Reaction (ORR)	Alkaline	O2(g) + 2 H2O(aq) + 4e− → 4 OH−(aq)	AFC, AEMFC, MAB
	Acidic	O2(g) + 4 H+ (aq) + 4 e− → 2 H2O(aq)	PEMFC
	Gas	½ O2 + 2e− → O2−	SOFC
Hydrogen Evolution Reaction (HER)	Alkaline	2H2O(aq) + 2e− → H2(g) + 2OH−(aq)	AEL
	Acidic	2H+(aq) + 2e− → H2(g)	PEMFC

AEL: Alkaline Electrolysis; MAB: metal–air batteries; PEMEC: Polymeric Membrane Electrolysis Cells; AFC: Alkaline Fuel Cells; AEMFC: Alkaline Electrolite Membrane Fuel Cells; PEMFC: Polymeric Membrane Fuel Cells; SOFC: Solid Oxide Fuel Cells.

.

12.1. Activity of Cobalt Oxides and Hydroxides in the Oxygen Evolution Reaction (OER)

The Oxygen Evolution Reaction, OER, is the slow step in the electrochemical water splitting process [388]. Co₃O₄ has long been mentioned as a very active and stable electrocatalyst for OER. The stabilization of high valence cobalt is a key factor: in fact, high-valency oxides are reported to show a superior OER activity to their low-valence counterparts [389] According to a recent study [390], the presence of low-spin Co³⁺ on the surface of Co oxide-based particles is essential for high OER activity, because it promotes surface reconstruction, while the presence of high-spin Co²⁺ is detrimental. The spin state of surface Co³⁺ ions can be tuned in cobalt-containing mixed oxides such as perovkites [219]. The activity of Co₃O₄-based materials strongly depends on the surface area, particle size and morphology, with a high activity for a number of different nanoparticle shapes and sizes [391]. According to Liu et al. [386], the exposition of the 111 crystallographic plane enhances activity in the OER.

Cobalt oxides have been considered to replace more expensive RuO₂-IrO₂ based electrodes for PEM electrolyzers for water splitting, and have been patented for such an application [392,393]. In fact, Co₃O₄ demonstrates high stability in strongly acidic conditions for OER [394], which can be further increased by metal doping. On the other hand, Co₃O₄ is also of interest in basic solutions due to its high stability in these conditions. A number of cobalt-containing complex oxides or oxyhydroxides are of interest for OER [388].

It must be considered that cobalt oxide-based electrocatalyst may undergo reconstruction during OER e.g., producing high oxidation state hydroxides or oxyhydroxides [395]. In fact, it has been supposed that the surface active phase for OER implies the formation of CoOOH [58,396]. Indeed, CoOOH is a very active electrocatalyst for OER in basic solutions, with enhanced activity through defect engineering [397] and strain engineering to increase surface concentration of high spin Co³⁺ ions [398], as well as in acidic solutions [58]. Operando XAS measurements combined with DFT calculations suggest that a potential-dependent deprotonation reaction occurs during the OER, producing Co³⁺/⁴⁺OOH_1−x, which acts as an active phase for the reaction [399]. However, CoOOH is not stable at high current densities due to Co dissolution [58].

The OER can also occur in nearly neutral conditions at the anodic side of CO₂ electrolysis cells. In this case the use of Co₃O₄ is considered promising in substitution of the most common iridium-based electrocatalysts [400].

The OER also occurs at solid/gas interfaces at the anodes of Solid Oxide Electrolysis Cells, where Co-based perovskite may act [401].

12.2. Cobalt Oxides for Hydrogen Evolution Reaction

Co₃O₄ is also studied as an electrocatalyst for the HER [386,391], the cathodic side of electrochemical water splitting, mostly tested in strongly basic conditions, such as 1 M KOH water solutions, but sometimes also in acidic conditions (e.g., 0.5 M H₂SO₄) [402]. Although its intrinsic activity is low due to limited electrical conduction, a combination with electrically conductive phases strongly increases its activity [402], such as in the case of three-dimensional crystalline/amorphous Co/Co₃O₄ core/shell nanosheets [403].

12.3. Cobalt Oxides for Oxygen Reduction Reaction ORR

The oxygen reduction reaction (ORR) occurs at the cathodic side of hydrogen fuel cells. Pure Co₃O₄ has some catalytic activity in ORR [402], whose mechanism involves two electron reductions of O₂ to HO₂⁻, followed by disproportionation. The activity of Co₃O₄ increases when it is combined with active and conducting components producing very interesting devices for ORR [385,386,391,404]. example, for CoO combined with N-doped graphene, it is reported to exhibit a similar catalytic activity but superior stability to Pt in alkaline solutions [405].

The ORR occurs also at the cathodes of Solid Oxide Fuels Cells (SOFC), in this case in gas/solid phase, producing oxide anions O²⁻ from oxygen from air. It has been reported that the deposition of a layer of CoO_x grains onto the internal surface of a Lanthanum Strontium Manganite/Yttria-Stabilized Zirconia (LSM/YSZ) composite cathode enhances the reaction rate of ORR [406]. On the other hand, cobalt-containing perovskites appear to be even more efficient than the traditional LSM-based cathodes for the low-temperature operation of SOFC [407,408].

12.4. Cobalt Oxides as OER/ORR Bifunctional Electrocatalysts for Metal–Air Batteries

Bifunctional OER/ORR electrocatalysts are needed as electrode materials for metal–air batteries [409]. In fact, cobalt oxide- based materials represent an interesting option for metal-air batteries [410], and, in particular, for Zn-air batteries [411]. In particular, cobalt oxide nanoparticles combined with different types of supports have been tested for Mg-, Li- and Zn- air batteries [410,412,413]. The use of cobalt oxides for metal air batteries has been the object of recent patents [414].

12.5. Cobalt Oxides for Photoelectrocatalytic Reactions

Co₃O₄ has emerged as a candidate to serve as a photoelectrocatalyst specifically for the oxidation of water with oxygen in these materials to promote the production of intermediate reactive species (hydroxyl radicals and superoxide radicals), in particular for the abatement of water pollutants [94]. According to these authors, the preparation technology suitable for producing a high surface area and useful morphology for this practical application still needs to be investigated.

Cobalt oxide-containing materials, such as cobalt–phosphate amorphous oxyhydroxide [415], are also considered among promising photoelectrocatalysts for hydrogen evolution from water splitting reactions [416].

12.6. Cobalt Oxides for Lithium-Ion Battery (LIB) and Sodium-Ion Battery (NIB) Anodes

CoO and Co₃O₄ have recently generated great interest as the most attractive alternatives to commercial graphite as anode materials for LiBs [391,417,418]. The high theoretical capacity of 890 mAhg⁻¹ for Co₃O₄, which is almost three times higher than that of the commercially used graphite anode (less than 372 mAhg⁻¹), is particularly interesting. This value is due to the eight lithium ions reacting with one cobalt oxide unit in each formula unit. Depending on the cobalt oxide morphology, the following reactions

Co₃O₄ + 2 Li⁺ + 2e⁻ ⇆ 3 CoO + Li₂O

3CoO + 6 Li⁺ + 6 e⁻ ⇆ 3 Co + 3 Li₂O

occur stepwise, or the overall reaction

Co₃O₄ + 8 Li⁺ + 8e⁻ ⇆ 3 Co + 4 Li₂O

occurs directly [419]. The corresponding reactions are observed using sodium ions [420,421]. Disadvantages of cobalt-based anodes for LIBs and SIBs include the fast loss of capacity due to low electronic conductivity and large volume expansion during charge/discharge [421].

12.7. Cobalt Oxides as Components of Cathodes of Lithium−Sulphur Batteries

Cobalt oxides are reported to act as a useful component of the cathodes of lithium–sulphur batteries. In fact, CoO. as such or formed over Co₃O₄, acts as a catalyst to facilitate the adsorption of polysulphides [422] and the conversion of lithium polysulphides (LiPSs) to Li₂S₂/Li₂S during the charge−discharge process, allowing for a high sulfur utilization in these batteries [423].

13. Cobalt Oxides as Adsorbents for H₂S Abatement

The high activity of cobalt oxide- and hydroxide-based systems in absorbing H₂S at a low temperature down to room temperature [424] has also been the object of interest. Mesoporous Co₃O₄ finds a much higher activity than commercial Co₃O₄ [424] for this application. Cobalt oxide-containing composite systems such as cobalt oxide–silica [425] or cobalt oxide–graphite oxide composites [426] also have an interesting activity. Co₃O₄-based solids absorb H₂S, producing Co₉S₈ + Co₃O₄ mixtures [427], or CoSOH and CoS, depending on the conditions [425]. Cobalt oxides can, however, be regenerated through oxidation only at quite high temperatures with the release of SO₂ [425,428].

14. Cobalt Oxide Materials as Sensor Materials

Taking into account the high catalytic combustion properties of Co₃O₄, as well as the redox and semiconducting behavior of both Co₃O₄ and CoO, cobalt oxide materials have potential applications as gas-phase sensors for molecules such as H₂, CO, NO, NO₂, NH₃, CH₄, toluene, xylene, formaldehyde, acetone, and ethanol [429,430]. These properties are strongly related to the surface area, porous nature and morphologies of the oxide, and can be increased by combining cobalt oxide with other components. In fact, a remarkable resistance response was found for Co₃O₄ films exposed to CH₄, H₂, CO, NO₂ and NH₃ at 513 K, such as in a CO gas sensor [431]. CoOOH has an even better response than Co₃O₄ at low temperatures and can detect CO concentrations of 1 ppm at 353 K [432]. It can also be applied to CH₄, H₂, NO₂, ethanol and water [432]. Also cobalt-doped alumina was found to act as an excellent gas sensor for benzene [433].

Cobalt oxides are also useful as optical gas sensors. In fact, the transmittance in visible light of stoichiometric CoO is far higher than that of oxidized Co₃O₄, which is a dark solid (Figure 3). Therefore, cobalt oxides are active in sensing carbon monoxide in air, as seen using λ = 625 nm and 200 ppm of CO gas in dry air at 623 K, because the Co₃O₄ ⟶ CoO phase transition occurs with a strong increase in light transmittance [434].

Co₃O₄ nanomaterials have been explored as effective electrode materials for enzyme-free glucose sensing in solutions, including Co₃O₄ spherical nanoparticles, porous nanorod and nanoflower shapes [435], Co₃O₄ nanoparticles on graphene [436], Co₃O₄ nanosheets [437] and straw-sheaf-like Co₃O₄ [438]. The mechanism of glucose detection is based on the oxidation of glucose to gluconolactone by electrochemically generated surface Co(IV) in basic solutions [435]. Despite the very interesting behavior, glucose detection is limited due to the poor electrical conductivity of cobalt oxide semiconductors and the associated sluggish reaction kinetics. To enhance performance, the photothermal effect-assisted electrochemical detection of glucose can be utilized. Co₃O₄ nanowire arrays supported on nickel foam were proposed for showing the good feasibility and reliability of this technique for glucose determination in human serum samples [439].

In similar ways, cobalt oxide-based sensors are investigated for the detection of other substances, e.g., hydrazine [440] and uric acid [441], and for the measure of chemical oxygen demand (COD) in polluted water [442].

15. Cobalt Oxides and Hydroxides in Supercapacitor Technologies

Supercapacitors are objects of much interest to be used as energy storage devices [443]. In fact, they exhibit a high power density and long-lasting lifespan with a high cyclic stability and reversibility, and they can be used in a wide temperature range (230–360 K).

Co₃O₄ is particularly attractive for supercapacitor applications [444,445], due to its low cost, remarkable redox activity, and the extremely high theoretical specific capacitance for Co₃O₄ of around 3560 Fg⁻¹. Co₃O₄-based materials have been patented for such an applications [446]. Its practical use is actually hindered by a poor electrical conductivity, low power density, and structural instability. Co₃O₄ supercapacitors use KOH solutions and work with a pseudocapacitor mechanism [444,445,447,448,449,450,451]. In fact, the electrochemical potential of Co₃O₄ in contact with KOH solutions is associated with the following reversible reactions:

Co₃O₄ + OH⁻ + H₂O ⇆ 3 CoOOH + e⁻

CoOOH + OH⁻ ⇆ CoO₂ + H₂O + e⁻

The rate and extent of these reactions depend strongly on the morphology (size and shape [103]) of the nanoparticles used and in particular on the surface area. In practice, the real capacitance obtained is in the range 400–1820 Fg⁻¹ [451]. The maximum value was obtained for Co₃O₄ nanoflakes with a 66 m²/g surface area grown on Ni foam, produced through a hydrothermal method, with 2M KOH as the electrolyte [450]. This material retains 92% capacitance after 1000 cycles.

Also, cobalt hydroxides can be used for this application [370]. In particular, β-Co(OH)₂ has potential as a pseudocapacitor phase [451,452,453], due to its high capacitance, stability, and cyclability. According to a recent study [454] the oxidation reaction

Co(OH)₂ + OH⁻ → CoOOH + H₂O + e⁻

is almost irreversible, and is followed by the reversible redox couple

3 CoOOH + e⁻ ⇆ Co₃O₄ + H₂O + OH⁻

while the reaction

CoOOH + OH⁻ ⇆ CoO₂ + H₂O + e⁻

only occurs at very high voltages. The reaction mechanism is summarized in Scheme 1.

Also, Co-Al layered double hydroxides of the hydrotalcite type are reported to effectively act as supercapacitor materials [455]. An enhancement of the specific capacitance was observed for samples with Co:Al a.r. 2 (i.e., the richest in electrochemically inert Al and poorer in electrochemically active Co); thus, capacitance would be associated with an increased basal space, and with an increase in the electrochemical surface area and higher electrolyte diffusion.

Indeed, the combination of cobalt oxides together with other oxide materials, forming doped Co₃O₄, Co-containing spinels and other mixed oxides, results in a number of very active materials for supercapacitors [444,445].

16. Mechanistic Aspects of the Surface Activity of Cobalt Oxide-Based Systems

As said, Co₃O₄-containing materials show a very high activity in the catalytic combustion of hydrocarbons and VOCs. The catalytic oxidation activity of transition metal oxides is most commonly explained using the redox mechanism first described by Mars and Van Kreveken [456,457], implying the reduction of the oxide surface by the reactant as the rate determining step and the reoxidation of the oxide surface by O₂ from the gas phase. However, alternative mechanisms are possible, such as the so-called Langmuir−Hinshelwood (LH) route, where adsorbed oxygen species (such as O₂⁻, O₂²⁻, or O⁻) react with an adsorbed form of the reducing reactant, and the Eley−Rideal (ER) route, where gas-phase oxygen reacts with the adsorbed reductant or vice versa.

The validity of the Mars–Van Krevelen or redox mechanism of CO oxidation on Co₃O₄ is supported by theoretical and experimental studies [90,221,458] as well as by spectroscopic studies [129], involving the Co³⁺/Co²⁺ redox couple and oxygen vacancy formation. Bridging surface carbonate species should act as intermediates [129,459,460] The redox mechanism also occurs for CO oxidation on CoO_x/Al₂O₃ catalysts [231]. In the case of methane combustion over Co₃O₄, the redox mechanism was considered [461] but an interplay of redox and Langmuir−Hinshelwood mechanisms is likely occurring, depending on the reaction conditions (temperature and reactants to molar ratio [462]). In the VOC oxidation catalysis over Co₃O₄, the redox mechanism is considered to be predominant [254]. In fact, spectroscopic data show that VOC molecules, including hydrocarbons such as propane, propene, and toluene, but also alcohols, are progressively selectively converted to more oxidized surface species, up to full oxidation, with the reduction of the surface through a redox mechanism [257,258,463]. Also, the oxidation of ammonia to NO over Co₃O₄ is supposed to occur through a Mars–Van Krevelen mechanism involving Co³⁺ and Co²⁺ [464] as with other metal oxides [465].

A slightly different situation likely occurs in the case of the oxidation of NO to NO₂. The kinetics of this reaction over Co₃O₄ were interpreted as being limited by the rate-determining step constituted by the dissociation of adsorbed molecular oxygen, assisted by gaseous NO to form NO₂ and surface-activated oxygen atoms in [305,466]. These species also react with NO to produce NO₂ and the surface where Co³⁺ is reduced to Co²⁺. Also in this case, an interplay of redox and Langmuir−Hinshelwood mechanisms is likely occurring.

In order to have high catalytic combustion activity, morphological and structural stability at the reaction temperature are also needed. For this reason, supporting or combining cobalt oxides with quite refractory materials such as aluminas may increase performances in this field. This makes cobalt aluminate and Co₃O₄-Al₂O₃ composite materials stable and active catalytic materials for most gas-phase oxidation processes.

The ability of cobalt ions to change their redox state is the key feature of cobalt oxides and hydroxides for their electrocatalytic behavior. In this case, however, thanks to the anodic voltage that can be applied to cobalt oxide-containing electrodes, the oxidation of Co³⁺ to Co⁴⁺ also becomes possible, thus allowing the Co⁴⁺/Co³⁺ couple to be active together with the Co³⁺/Co²⁺ couple too. This makes Co₃O₄-based systems very active in the OER, thanks to the oxidation of water to oxygen by surface Co⁴⁺ ions. The mechanisms of catalytic oxidation and the electrocatalytic water splitting behavior of Co₃O₄ are schematized in Figure 12.

However, for electrochemical applications in contact with water solutions, other properties are needed. First of all, it is necessary to have an extremely low solubility in water in practical conditions. Indeed, the extremely low solubility of Co³⁺-containing compounds (i.e., Co(OH)₃, CoOOH and Co₃O₄) at a low pH makes them very attractive for anodic applications at acidic pH. On the other hand, cobalt oxides and hydroxides also have a low solubility at neutral and medium basic pHs, allowing their use also in contact with moderately basic solutions.

Additionally, a sufficient electrical conductivity is needed for electrochemical applications. This is the main drawback of cobalt oxides for electrochemical applications. In fact, the most stable cobalt oxides and hydroxides are semiconducting solids with moderate conductivity. For this reason, to exploit the excellent redox and solubility properties of these materials in electrocatalysis, they may be combined with conducting solids, e.g., carbons or metallic foams. The same point is relevant for sensor applications of cobalt oxides. In most cases, combining cobalt oxide is detrimental due to further reduced conductivity.

The strong visible light absorbance by Co₃O₄, combined with its poor solubility and high redox activity, makes it interesting for visible photocatalytic applications, although, in this case, fast electron–hole recombination represents a strong drawback.

As seen, the ability of cobalt oxide and hydroxide phases to interconvert each other reversibly and quickly with redox behavior in contact with basic solutions, in particular, the CoOOH-Co₃O₄ and CoO₂-CoOOH systems, gives these systems an excellent behavior for supercapacitor applications with pseudocapacitor mechanisms.

A common additional property of these materials for all these applications is the possibility of producing them in a large variety of nanostructures. In fact, cobalt oxide and hydroxide nanomaterials provide an enhanced performance for most of these applications and also significant stability and reversibility.

17. Modification of Cobalt Oxides to Improve Performances

In several cases, as specified above, the production of modified cobalt oxides can improve performances in some of the mentioned applications. This can be obtained in different ways. One way is to combine it with alumina. In fact, as described above, the different types of alumina–cobalt oxide mixed phases can allow us to improve morphology and stability factors. The combination of cobalt oxide with alumina may allow the production of cobalt oxide-supported nanoparticles which retain a high redox activity with higher stability and better morphology. On the other hand, cobalt–alumina mixed oxide phases also allow the production of different cobalt oxide species, like more or less dispersed and even monoatomic cobalt ions on alumina surfaces, with peculiar redox properties for catalysis and adsorption. Cobalt alumina complex oxide phases frequently have a higher morphological and chemical stability than cobalt oxides. In

Table 4. A summary of surface-related applications of CoO_x-Al₂O₃ materials.

Technology	Reference
Precursors of Co/Al2O3 Fischer Tropsch synthesis catalysts	[6,7,202,203]
Low-temperature CO oxidation	[222,227,228,229,230,231]
Preferential CO oxidation	[236,237]
Hydrocarbon combustion	[241,256]
Combustion of anode tail gas of PEMFC	[283]
Diesel soot combustion	[287,336]
N2O decomposition	[308,310,314,315]
NOx SCR by propane	[331,332,333,334,335]
Simultaneous removal of NOx and Diesel soot particulates	[335,336]
O3 activation and decomposition	[338]
Liquid-phase aerobic oxidations	[342]
Supercapacitor	[455]
Benzene sensor	[433]

, surface-related applications of cobalt–aluminum mixed oxide phases are reported.

Another way to increase or modify the catalytic activity of Co₃O₄ is doping. In Figure 13, the effect of doping Co₃O₄ with metallic elements on N2O’s decomposition activity is summarized, also showing the structural effect of doping [313]. As shown, doping with alkali occurs at the surface and increases catalytic activity. The addition of Al³⁺ modifies the bulk and causes a partial deactivation, although, as said, it may increase stability.

However, to improve photo- and electrochemical behavior, an improvement of the electrical conduction properties may be needed. This can be done by supporting or mixing cobalt oxides on/with metal phases and foams, carbon materials and condicting ceramics. Alternatively or additionally, the combination of cobalt oxide phases with other metal elements can improve redox properties and electrical conduction and can also generate ionic conduction. In fact, mixed oxides generally have a higher conductivity and capacitive activity than single-metal oxides [467]. In fact, mixed cobalt oxides have a number of potential applications [468]. Examples of applications of cobalt-containing mixed oxide phases are summarized in

Table 5. Examples of Co-containing mixed oxide phases and their application.

Structure Type	Element	Formula	Technology	Ref.
Composites	Lithium	Li2O-Co3O4	LIBs anodes	[417,418,419]
	Sodium	Na2O-Co3O4	SIBs anodes	[418,420]
	Cerium	CeO2-Co3O4	catalytic combustion	[255]
			CO oxidation	[97]
Doping	Alkali	K-, Cs-doped Co3O4	N2O decomposition	[311,312,313]
	Lithium	LixCoO2	LIBs cathodes	[3]
	Sodium	NaxCoO2	SIBs cathodes	[118]
Amorphous	Phosphorus	Co3O4-CoPO4	photoelectrocatalysis	[415]
Rock salt	Magnesium	Mg1−xCoxO	ozone decomposition	[130]
Spinel	Magnesium	MgCo2O4	N2O decomposition	[310]
Spinel	Chromium	Co1+xCr2−xO4	catalytic combustion	[255]
Spinel	Iron	(Co1−xFex)3O4	supercapacitor	[444,445]
Spinel	Nickel	Ni1xCo3−xO4	supercapacitor	[444,445]
Spinel	Manganese	(Co1−xMnx)3O4	CO oxidation	[226]
			Catalytic combustion	[254]
			supercapacitor	[444,445]
Spinel	Copper	CuxCo3−xO4	CO oxidation	[226]
			catalytic combustion	[254]
Spinel	Zinc	ZnxCo3−xO4	electrocatalysis OER	[218]
			N2O decomposition	[313]
Perovskites	Lanthanum	LaCoO3	catalytic combustion	[243,244,245]
			N2O decomposition	[307,308]
			electrocatalysis OER	[400]
	Strontium	SrCoO3	SOFC anodes	[406,407]
	Praseodimium, Barium	PrBaCo2O6	electrocatalysis OER	[400]
β-aluminas	Aluminum, Strontium	SrCoxAl12−xO19−δ	catalytic combustion	[243,244,245]

.

18. Industrial Applications and Perspectives

The data summarized above demonstrate the exceptional versatility of cobalt oxides and their combination with aluminas in a number of different applications. While most of the papers cited above report on academic studies, several of them are related to industrial research and patents, showing their enormous industrial interest at least in some fields.

In

Table 6. Actual and potential industrial applications of cobalt oxides and cobalt-aluminium mixed oxides.

Application		Materials	State	Ref.
pigments		CoAl2O4 “cobalt blue” Co3O4 “cobalt black”	commercial	[13,92,154]
catalysts	low/medium temperature CO oxidation	Co3O4 CoOx/γ-Al2O3 Co3O4/α-Al2O3	might substitute noble metal-based systems	[14,96,221,222,223,224,225,226,227,228,229,230,231]
	chlorinated VOC combustion	Co3O4 Co3O4/SiO2	more stable than noble metals, zeolites	[274,275,276,277,278]
	soot combustion	CoAl2O4 Co3O4	patented as components of Diesel active catalytic antiparticolate filters	[288]
	N2O decomposition	Co3−xAlxO4	patented, commercial	[319,320]
	ammonia oxidation to NO	Co3O4, Co3−xAlxO4	patented for nitric acid synthesis process	[299,300]
	ozone activation and decomposition	Co3O4, CoOx/γ-Al2O3	under study	[23,337,338,339]
precursors of cobalt metal catalysts	precursor for Fischer Tropsch synthesis catalysts	CoOx/γ-Al2O3	commercial	[6,7,201,202,203,204,205]
	precursor for other metal catalysts	CoOx/γ-Al2O3	commercial	[203,204]
electrocatalysts and battery electrodes	anodes for water splitting	Co3O4/CoOOH/CoO2	patented	[385,386,387,388,389,390,391,392,393,394,395,396,397,398,399]
	electrode for metal-air batteries	Co3O4	patented	[409,410,411,412,413,414]
	cathodes of lithium-ion batteries	LixCoO2	commercial	[3]
	anodes of lithium-ion batteries	Co3O4	very high theoretical capacity	[33,159,417,418,419,420,421]
supercapacitors	active layer of supercapacitors	CoO2/CoOOH/Co3O4/ Co(OH)2	patented	[103,370,443,444,445,446,447,448,449,450,451,452,453,454,455]

, the industrial applications of these materials and technologies with preindustrial studies and potential future applications are documented. It is evident that, while in some fields cobalt-based oxide materials already represent industrial-level products, in several fields they are promising candidates to substitute more expensive and more critical materials such as those based on noble metals and/or to introduce new more performant technologies.

19. Conclusions

The data summarized here emphasize the enormous versatility of cobalt oxide-based materials for a number of at least potential technological applications. This is associated with the redox properties of Coⁿ⁺ ions (n = 0,2,3,4), the related electrical conduction properties of their solid compounds, and also the magnetic and optical properties of these materials. It seems quite evident that cobalt oxide-based materials are in some way underutilized practically today but represent interesting alternatives to more expensive and critical materials, e.g., those based on platinum group metals and noble metals. It seems likely that the industrial application of cobalt oxide-based materials will strongly increase in the near future to sustain new technologies needed for the energy transition expected to occur shortly. To do this, the search and discovery of new cobalt resources would be beneficial, together with establishing processing technologies in more countries. On the other hand, recovering and reuse technologies should also be developed.