Available data on catalytic applications of the iron-containing nanomaterials are reviewed. Main synthesis methods of nZVI, nano-sized iron oxides and hydroxides, core-shell and alloy structures, ferrites, iron-containing supported forms, and composites are described. Supported structures include those coated and on the basis of polymers or inert inorganic materials (i.e., carbon, titania or silica). Description of catalytic processes includes the decomposition reactions (in particular photocatalytic processes), reactions of dehydrogenation, oxidation, alkylation, C–C coupling, among a series of other processes. Certain attention is paid to magnetic recovery of catalysts from reaction systems and their reuse up to several runs almost without loss of catalytic activity.
- Iron nanomaterials
- bimetallic nanoparticles
- core-shell nanoparticles
1.1. General information on the iron-containing nanostructures
Nanomaterials on iron basis mainly include zero-valent iron (ZVI and nZVI (nano zero-valent iron) are nowadays classic terms), iron-based nanoalloys or core-shell nanoparticles, iron(II and III) oxides, and ferrites, among others. Metallic iron is normally covered with iron(II) and iron(III) oxides . The iron oxides (iron oxide nanoparticles are also referred in several reports to as superparamagnetic iron-oxide nanoparticles (SPIONs) although SPIONS have inducible magnetic properties) [2, 3], belong to the most technologically important oxides of transition metals. The collective term “iron oxides” is also used for oxides, hydroxides, and oxy-hydroxides containing Fe(II) and/or Fe(III) cations and OH- and/or O2- anions. In total, sixteen pure iron oxide phases, i.e., oxides, hydroxides or oxy-hydroxides, are currently known. These compounds are Fe(OH)3, Fe(OH)2, Fe5HO8⋅4H2O, Fe3O4, FeO, five polymorphs of FeOOH and four of Fe2O3. In these oxide compounds, which are generally low soluble and possess brilliant colors, the iron is present in the form of Fe(III). The extremely important advantages of nanostructured iron, in comparison with other nanomaterials, are its relatively low toxicity and capacity to be biodegradable. This metal, in addition, is non-expensive and commonly widespread material .
Particle diameters of nZVI are normally in the range from 10 to 100 nm , exhibiting a classic core-shell structure. Their core contains metallic iron phase, meanwhile the oxidation products of zero-valent iron form mixed valent [i.e., Fe(II) and Fe(III)] oxide shell. If stabilizers in excess are present, these core-shell nanoparticles could be protected against further oxidation . Among such stabilizers, a series of organic compounds can be used for nZVI functionalization to stabilize nZVI aqueous dispersions, inhibiting strongly their further agglomeration. Such compounds can be used to satisfy this purpose, for example, PEG, polyacrylic acid, 4-butanediphosphonic acid, and methoxyethoxyethoxyacetic acid (MEEA) .
The magnetite (Fe3O4) and maghemite (γ-Fe2O3) are of a particular interest talking about iron oxides (SPIONs). The magnetite structure corresponds to an inverse spinel ferrite. The oxygen ions are the part of a close-packed cubic lattice, containing the iron ions between two different interstices, tetrahedral sites (A), and octahedral sites (B). In a chemical point of view, the magnetite/maghemite can be represented by the following formula: Fe3+ [Fe2+1-y Fe3+1-y Fe3+1.67y▯0.33y]O4, where y=0 for pure magnetite and y=1 for pure maghemite (completely oxidized magnetite). From room temperature up to Curie temperature (Tc=860 K), the A sites are filled by Fe3+ ions and the B sites are filled by Fe3+ and Fe2+ ions in equal quantity. Although the lepidocrocite (γ-FeOOH) dehydration transforms into γ-Fe2O3, industrial fabrication of maghemite is based on a multistep process (1):
In addition to the nZVI and SPIONs, a variety of composite inorganic iron-based nanomaterials have been discovered, in particular core-shell Fe(or FexOy)/Au or more complex trimetallic nanoparticles such as (Fe60Co49)core/Aushell . These nanoparticles were classified  on the basis of on their complexity levels: 1) the nanostructures based of an iron-containing material with magnetic properties different from iron oxide; 2) the nanostructures with a non-spherical morphology (e.g. hollow structure); 3) the nanostructures with multi-material composition, i.e. each of them is constructed ≥2 more domains of joined together different inorganic materials.
2. Main synthesis methods
A number of currently used methods, shown below, are nowadays used for preparation of Fe-containing nanomaterials. At the same time, some well-known conventional wet chemistry routes have not been forgotten and are applied for nZVI synthesis  (for instance, by a borohydride reduction in laboratory scale) , Fe2O3 (sol-gel technique  or electrochemical deposition ), or Fe3O4 (urea- and NaOH-assisted hydrolysis of Fe3+ and Fe2+ salts and further ultrasonic treatment of FeO(OH)/Fe(OH)2). Particle sizes and morphologies of the formed nanomaterials, synthesized by distinct methods, can vary depending on the synthesis conditions. For example, the Fe3O4 nanoparticles , obtained by radio frequency nitrogen plasma technique, represent a regular spherical form, meanwhile the nanoparticles synthesized by wet chemistry synthesis were seen as well shaped cubic form. In case of the plasma prepared iron oxides, their size distribution was wider resulting small (25–80 nm) and larger (>100 nm) particles.
2.1. nZVI and Fe-M nanoalloys and core-shell nanostructures
To produce nanopowders, the method of electrical explosion of wire was applied [15, 16], among other physical methods. Intermetallic phases can also be obtained by the arc discharge technique, for example Fe-Sn bimetallic nanoparticles . Thus prepared nanoparticles have a core/shell structure consisting of a SnO2 shell (5–10 nm in thickness) and a core containing polycrystalline intermetallic compounds. The intermetallic compounds FeSn2 and Fe3Sn2 were shown to be generated; they coexist with the Sn phase as a single nanoparticle. Microwave irradiation (which is used to fabricate normally inorganic materials and composites, lesser organic- or organometallic-based materials), is an alternative method in comparison with classic heating of product precursors. Metal transformations in microwave field are extensively reviewed in a comprehensive recent book . Elemental metal nanoparticles in various forms can also be fabricated using this method. For example, iron-based nanoparticles were obtained applying microwave–polyol route in ethyleneglycol at 100 and 150 °C with additives of polyvinyl pyrrolidone and dodecyl amine . Also, pulsed excimer laser radiation (248 nm) was used to ablate a feedstock of permalloy (~2 μm, Ni 81%:Fe 19%) under both normal atmospheric conditions (in air) and in other gases, as well as under pressures . α-Fe nanoparticles were also obtained with use of a modified metal-membrane incorporation method applying diffusing metal ions through a dialysis membrane  (the diffusion-time ≤15 min).
To get nZVI in laboratory conditions , the classic and usual synthesis technique is the reduction of Fe(II or III) salts using NaBH4, NaAlH4 or LiAlH4 as reductants. Thus, nZVI was synthesized (reaction 2)  in ethanol medium by the method of reduction of FeXn (X= Cl, OH, OR, CN, OCN, SCN) using sodium borohydride under atmospheric conditions .
A patent  describes a route to metal nanoparticles by thermal decomposition of iron acetate Fe(OOCCH3)2, placed in a reaction vessel with a passivating solvent such as a glycol ether. Discussing the pyrolysis method, it should be noted the preparation of iron nanoparticles (embedded in a carbon matrix) from metal phthalocyanine as precursor  and carbon-encapsulated iron nanoparticles (size 5–20 nm) via a picric acid-detonation-induced pyrolysis of ferrocene as precursor; this route has such peculiarities as self-heating and extremely fast process . Also, the “greener” techniques [28, 29, 30, 31, 32] have been applied for nanoparticle fabrication. Use of plant extracts and other natural products on polyphenole basis in these syntheses as reductants and capping agents at the same time for obtaining nZVI and several other Fe-containing nanoparticles is intriguing  as well. For instance, the herbal tea extracts were applied to reduce iron(III) chloride to elemental iron nanoparticles (50 nm) .
In addition, a variety of general physico-chemical methods have been applied for the production of as Fe-containing bi- and polymetallic alloys as core-shell nanostructures. For instance, high entropy Nd-Fe-Co-Ni-Mn alloy nanofilms were prepared  by electrodeposition at r.t. After preliminary preparation of alumina nanotemplates, Fe, Fe-Ni, and Fe-Pd nanowires were successfully electrodeposited within their porous structure. Also, the Fe-Pt nanocrystalline magnetic films (200 nm of thickness) with planar texture were obtained with use of magnetron sputtering and crystalline annealing in magnetic field .
2.2. Supported and coated iron nanoparticlesA number of publications are devoted to carbon-supported ZVI nanomaterials . This type of protective carbon-cage encapsulation of iron nanoparticles can result hybrid core-shell nanomaterials with unique properties . This way, carbon encapsulated iron core-shell nanoparticles (15–40 nm in size) were obtained via confined arc plasma method . Resulting nanoparticles possessed a clear core-shell structure. The core (16 nm in diameter) of the particles corresponded to a BCC iron structure, and the shell (thickness 6–8 nm) was shown to be disorder carbon phase. A closely related arc discharge technique  is also frequently used for obtaining a variety of nanomaterials, in particular the iron containing ones. For instance, a simple, inexpensive and one-step arc discharge synthesis technique to prepare metal-containing carbon nanocapsules in aqueous solution is known . It was established that iron nanoparticles can be in situ encapsulated in carbon shells when the arc discharge process was carried out in aqueous solutions of FeSO4.
Combustion synthesis of iron oxide/iron-coated carbons such as cellulose fiber, anthracite, and activated carbon was reported a classic microwave oven with inverter technology . The size of the iron oxide/iron nanoparticle-coated samples were determined to be in the range of 50–400 nm. It was found that iron oxide/iron nanoparticles exist in 4 main phases: γ-Fe2O3, α-Fe2O3, Fe3O4, and Fe, some of them had significant arsenic adsorption. In addition, carbon-coated iron nanoparticles with well-developed quasi-spherical shape were prepared with Fe(NO3)3⋅9H2O and starch as carbon source [43, 44]. This is an efficient approach for the mass production of nanocage structures under mild conditions, which needs to be further explored for preparing various carbon coated metal nanomaterials. Radiation methods are being applied more widely last 10–15 years, in that case for preparation of Fe-carbon-supported nanostructures. This way, 2 types of amorphous carbon films (15 at.% iron containing film) were deposited onto Si substrates by a sputtering method and further exposed to an electron flow, where the energy and dose rate were much smaller compared to the electron beam in a TEM. In this case, graphitic structures were observed in amorphous matrix at temperatures up to 450 K. It was established that the graphitization progressed more intensively during the electron irradiation than in annealing at 773 K. This was attributed to thermal and catalytic effects, strongly related to grain growth of metal clusters.
2.3. Free and supported iron oxides and ferrites
Zeolites and closely related supporting materials represent an ideal basis for iron oxide nanocomposites. This way, the zeolite loading with nanoiron oxide by a simple chemical process was described . Final crystallite sizes of the doped nanomaterials were in the range of 4–6 nm. It was shown that the zeolites become to have magnetic properties after being doped with nanoiron oxide. Mesoporous nanocomposites “iron oxide/silicate” Fe2O3-SBA-15 (SBA-15 is an abbreviation for hexagonally ordered mesoporous silica) with iron loadings of 1.2–35.8 wt.% were prepared hydrothermally . It was revealed that these composites contain well-dispersed iron oxide nanoclusters in the walls of ordered mesoporous silica and high surface area. Certain number of composite nanomaterials based on Fe3O4 is known, for instance core/shell Fe3O4 coated gold nanoparticles (diameter 50–100 nm) . Their possible formation mechanism was proposed as follows: pH-sensitive polymer owing to a shrunken or stretched structure of polyethyleneimine (PEI), led to the aggregation of the Fe3O4-gold seed nanoparticles, then gold reduces onto the surface of Fe3O4-gold seed nanoparticles. It was concluded that these core/shell multifunction nanomaterials will not only have external magnetic separation by the core of Fe3O4 but also detect the large biological molecules using the shell of gold. In addition, iron phthalocyanine prepolymer/Fe3O4 nano hybrid magnetic material  can be applied as high temperature-resistant polymer magnetic composite material. At last, ferrites having different sizes, from ultrasmall (2 nm) to 50 nm, can be fabricated by distinct techniques  mainly co-precipitation method (CPM), sometimes without using any capping agents/surfactants.
3. Catalytic applications
3.1. nZVI and supported Fe0 nanocomposites
3.1.1. Catalyzed removal or decomposition of pollutants
nZVI has been extensively reported to resolve a series of environmental problems, related with destruction, adsorption, precipitation, reduction or oxidation of heavy metals, salt anions, hydrocarbons and halogenated organic pollutants (Fig. 1) , leading to their conversion to final non-hazardous products. In these processes, iron nanoparticles have shown high efficiency and practically no damage for the environment because of absence of toxicity. Thus, the efficiencies of nitrate removal from aqueous solution by single (TiO2 and nZVI), and composite (nano-TiO2–Fe0 composite, NTFC) system under UV illumination were studied . Among the three systems, both nZVI and NTFC can effectively remove nitrate. However, only NTFC can achieve satisfactory transformation of nitrate to N2. Reactive materials for catalytic degradation of chlorinated organic compounds in water at ambient conditions have been prepared on the basis of silica-supported Pd-Fe nanoparticles . Nanoscale Fe-Pd particles were synthesized inside porous silica supports using (NH4)3[Fe(C2O4)3] and [Pd(NH3)4]Cl2 or Pd acetate as reaction precursors. The reduction of these supported precursors using hydrogen led to materials, revealed high activity in the processes of perchloroethene (PCE) degradation and 2-chlorobiphenyl (2-ClBP) dechlorination. It was established that highly dispersed amorphous Fe-Pd bimetallic nanoparticles on silica support possess superior catalytic activity against PCE dechlorination, comparing with the free-standing Fe-Pd nanoparticles. It was also established that the addition of vitamin B12 (it is known to be an effective electron mediator, having strong synergistic effects with nZVI for reductive dehalogenation reactions) can significantly enhance the reductive dechlorination of PCE by nZVI . A remarkable reductive dechlorination of PCE (0.25±0.01 h−1) was observed in nZVI suspension (0.05 g/24 mL) with 0.5 mM vitamin B12 in 6 h, while no significant reductive dechlorination of PCE was observed in the nZVI suspension without vitamin B12. Similar composite material based on deposition of nZVI particles and cyanocobalamine (vitamin B12) on a diatomite matrix was also used for catalytic transformation of PCE and other organic contaminants in water . The composite material rapidly degrades or transforms completely a large spectrum of water contaminants, including halogenated solvents like TCE, PCE, and cis-DCE, pesticides like alachlor, atrazine and bromacyl, and common ions like nitrate, within minutes to hours. In a related publication , iron nanoparticles were applied for remediation of PCB-contaminated soil, taking into account a maximization of PCB destruction in each treatment stage. The efficiency of PCB destruction during the first step treatment (mixing of soil and iron nanoparticles in water) can be improved by increasing the water temperature. The PCB destruction efficiency of minimum 95% can be achieved. In air at 300 °C, Fe2O3 is also a good catalyst for remediating PCB-contaminated soils. In addition, photo-Fenton like method using nZVI/UV/H2O2 was applied  for removing total petroleum hydrocarbons (TPH) and determining the optimal conditions using Taguchi method. The removal rate in optimal conditions was between 95% and 100%. The nZVI particles can be reused in a magnetic field. This process may enhance the rate of diesel degradation in polluted water and could be used as a pretreatment step for the biological removal of TPH from diesel fuel in the aqueous phase. Among other degradation applications, we note the use of nZVI/AC for catalytic wet peroxide oxidation of phenol . The catalytic activity of phenol degradation was improved by application of nZVI/AC catalysts compared to that of Fe/AC. For the range 150–1,000 mg/L, the phenol conversion ≥90% can be achieved using these nanocatalysts during 15 min of the reaction in presence of the stoichiometric hydrogen peroxide for complete mineralization. At last, as an example of classic organic synthesis, we emphasize the conversion of synthesis gas to C2 through C4 olefins with up to 60% selectivity by carbon , using catalysts which comprise Fe promoted nanoparticles (5–30 nm in diameter) homogeneously dispersed on weakly interactive α-alumina or carbon nanofiber supports.
Degradation of dyes with use of nZVI is carried out mainly on carbon-based nanocomposites. Thus, nZVI/activated carbon (nZVI/AC) was investigated as heterogeneous Fenton catalyst in 3D electrode system for methyl orange (MO) degradation . The mineralization of MO was significantly improved by 20–35% compared to 2D AC system at the optimum conditions. A possible mechanism for decolorization and mineralization was proposed, which was attributed to the combination of adsorption, anodic oxidation, and Fenton oxidation in 3D nZVI/AC system. As an example of application of CNTs-based system, an ozone catalyst capable of working on acidic solution environments (labeled as CNTs-Fe0), which was prepared by immobilizing nZVI onto the surface of MWCNTs and used for decomposition of methylene blue (MB) by formed hydroxyl radicals (HO⋅) . At pH 3, the production of HO⋅ was found to be considerably accelerated in the presence of CNTs-Fe0 about 80 times in comparison with results using plain ozone. In the process of CNTs-Fe0 catalytic ozonation, the CNTs support was analyzed to perform as effective “promoter” allowing the fast surface-mediated reactions, owing to the combination of its surface-active nature, conductivity, and chemical stability. All these and above technologies perfectly fit into advanced water treatment technologies, whose additional representative example is as follows. Thus, the in situ synthesis of air-stable nZVI embedded in cellulose fibers led to the assembly of highly reactive magnetic filter papers (membrane nanocomposites) . This nZVI@FP nanocomposite (Fig. 2) showed high activity towards the removal of hexavalent chromium as well as an excellent catalytic ability to convert phenols into catechols, by simple filtration processes of the contaminated water solutions.
3.2. Fe–M nanoalloys, bimetallic NPs and core–shell nanostructures
Degradation of pollutants. Iron-based alloys, core-shell, and bimetallic nanoparticles, especially with noble metals, have been extensively applied in the catalysis and reviewed [62, 63, 64], so we will present in this section only their most representative recently reported examples. As well as nZVI described above, bimetallic Fe-containing nanoparticles (Fe with Pt, Ru, Rh, Ni, Co, Au, Cu, Ag) are used for the catalytic elimination of environmental pollutants. Reactions between them (pollutants and nanoparticles) can be mainly divided in 4 types : a) catalytic replacement reactions for removal of heavy metals, b) hydrodehalogenation reactions (in case of halogenated hydrocarbons), c) azo and nitro hydrogenation reactions (for azo and nitro) compounds, and d) hydrodeoxygenation reactions (for oxyanions). In comparison with monometallic iron nanoparticles, the bimetallic iron NPs have considerable capacity to be separated and catalytic ability for degradation of non-biodegradable pollutants. Among them, Fe-Pt NPs are of an especial interest. Thus, Pt-Fe application as heterogeneous Fenton-like catalysts was reported for hydrogen peroxide decomposition and the decolorization of methylene blue . FePt (and also Fe3O4, see more information below) NPs were prepared and tested as heterogeneous Fenton-like catalysts to evaluate and compare their efficiency toward the decolorization of MB dye in solution. Both FePt and Fe3O4 exhibited high activity toward the MB decolorization reaction though FePt exhibited a reaction rate that was 100 times faster for a 5 ppm catalyst concentration. Both FePt and Fe3O4 NPs are superparamagnetic and thus can be easily separated with a magnet and reused for subsequent catalytic cycles. The same objective was reached using core-shell NPs on the nZVI basis. Thus, a series of nanocomposites consisting of nZVI encapsulated in SiO2 microspheres were applied for the degradation of organic dyes was investigated using MB as the model dye in the presence of H2O2 . The degradation efficiency and apparent rate constant of the degradation reaction were significantly enhanced with increased nZVI encapsulated in SiO2 microspheres, whereas the dosage of H2O2 remarkably promoted degradation rate without affecting degradation efficiency.
It should be also noted that Fe-containing thin films can be applied for dye degradation. Thus, 2D nano-TiO2 and Fe-doped nano-TiO2 thin films with large sizes were obtained  at low temperature in an aqueous system via molecular self-assembly approach. Degradation of methyl orange solution under action of UV and visible light radiation was applied for evaluation of the photocatalytic activity. The doped iron presence was shown to improve the TiO2 photocatalytic activity. The degradation yields of methyl orange were 98.62% and 89.24%, respectively, under illumination by UV lamp and using visible light. As an example of another degradation process, we note that the longevity and reactivity of nZVI and palladized bimetallic particles (BNP) were evaluated in batch and column experiments for remediation of a trichloroethene (TCE)-contaminated plume within a clayey soil . The particle behavior was found to be severely affected by clay sediments. Results of butch studies testified that TCE degradation in ORR clayey soil corresponds to a pseudo-first-order kinetic model with reaction rate constants (k) of 0.05–0.24 day−1 at varied iron-to-soil ratios. Despite of elevated reactivity in water phase, the BNP were less effective in the site-derived clay sediment resulting calculated TCE removal efficiencies of 98.7% and 19.59%, respectively.
Transformations of ketones using stabilized nZVI and bimetallic Fe NPs are known. Among these catalytic processes, we note that porous Pt-Fe bimetallic nanocrystals were found to effectively facilitate the manufacturing of 2-propanol from acetone . It was suggested that the high reactivity is strictly related to the interface consist of bimetallic Pt-Fe alloy and the Fe2O3-x. As an example of reduction of substituted aromatic ketones to alcohols, iron nanoparticles (size 9 nm, 14 nm, and 17 nm), stabilized by polyethylene glycol (PEG), carboxymethyl cellulose (CMC), and poly N-vinyl pyrrolidone (PVP), were used as catalysts in the hydrogenation reaction of various substituted aromatic ketones to alcohols with NaBH4 . Fe-PEG NPs were found to act as better catalyst than Fe-CMC NPs and Fe-PVP NPs. The trend in the catalytic activity among metals falls in the line of decreasing size effect of the nanoparticles i.e., the order of the nanoparticle sizes increase as Fe-PEG < Fe-CMC < Fe-PVP. Also, effects of substituents in the aromatic ring of ketones revealed that +I substituents are better catalyzed than –I substituents. In addition, bimetallic ruthenium-iron nanoparticles constitute a magnetically recoverable heterogeneous catalyst for transfer hydrogenation with a pronounced selectivity for ketones over aldehydes and nitro groups . The nanoparticles are recyclable up to five times without significant decrease in activity or leaching.
Other processes. An Fe group ternary nanoalloy FeCoNi (NA) catalyst (its synthesis see Fig. 3) enabled selective electrocatalysis towards CO2-free power generation from highly deliverable ethylene glycol (EG) . This FeCoNi nanoalloy catalyst exhibited the highest selectivities toward the formation of C2 products and to oxalic acid, i.e., 99% and 60%, respectively, at 0.4 V vs. the reversible hydrogen electrode (RHE), without CO2 generation. The key feature was the formation of an atomically mixed FeCoNi alloy to enhance the synergetic effect of the Fe group elements on anti-self-oxidation and selective oxidation of EG to oxalic acid. Reduction processes also required polymetallic particles. Thus, trimetallic core/shell Pd/FePt NPs were applied in oxygen reduction reaction (ORR) catalysis . The uniform FePt shell was formed by controlled nucleation of Fe(CO)5 in the presence of a Pt salt and Pd NPs at designated reaction temperatures.
Iron-iron oxide core–shell nanoparticles were used as a catalyst for the hydrogenation of olefins and alkynes (reaction 3) under mild conditions in ethanol and in an aqueous medium . The system is active in respect of a row of substrates and considerably selective for alkenes and alkynes over aromatic and carbonyl groups. The authors supposed that the presence of an oxide shell does not decrease its activity and provides a certain protection against oxidation by oxygen and water. In addition, highly active and well-defined AuPt nanoalloys, supported on the surface of ellipsoidal Fe@SiO2 nanoparticles, were prepared by a method involving the loading of Pt NPs on the Fe2O3@SiO2 nanocapsules via Sn2+ linkage and reduction, then in situ fabrication of Au nanoparticles by the galvanic replacement reaction between Au and Pt, and finally calcination and reduction to convert the nonmagnetic Fe2O3 to Fe core with high saturation magnetization . The obtained Fe@SiO2/AuPt samples exhibited a remarkably higher catalytic activity in comparison with the supported monometallic counterparts toward the reduction of 4-nitrophenol to 4-aminophenol by NaBH4. The catalyst can be reused for several cycles with convenient magnetic separation.
3.3. Nano-Fe2O3 phases and their composites
Destruction of dangerous substances and pollutants. As well as Fe(0)-containing nanocomposites, oxidated iron forms are successfully used for degradation of several substances, both inorganic and organic. Thus, the decrease in temperature of decomposition of ammonium perchlorate in the presence of nano-ferric oxide was investigated  (see also section below on ferrites). It was shown that addition of nanometer-sized ferric oxide leads to a significant decrease in higher decomposition temperature of ammonium perchlorate. The catalytic activity of a colloidal catalyst (based on iron(III) oxides and obtained by hydrolysis followed by peptization of FeCl3⋅6H2O salt in water in the presence of 1% ethanol) in decomposition of H2O2 was studied . The obtained catalyst is mainly composed of α-Fe2O3 crystals with an admixture of other crystalline structures of iron oxides, as well as carbon-containing compounds. Its activity with respect to Н2О2 decomposition varies nonlinearly and nonmonotonically and its particle size grows starting from 1–3 nm with an increase in the initial concentration of FeCl3⋅6H2O used to synthesize the catalyst. In addition, thermal decomposition of silver acetate at 200 °C in the presence of iron oxide microspheres in diphenyl ether led to the formation of iron oxide@Ag core–shell nanoparticles, exhibiting superparamagnetic behavior with a blocking temperature of about 42 K . Their good catalytic activity and magnetic recovery was demonstrated by using two reactions, namely, reduction of 4-nitrophenol and reduction of methylene blue in aqueous solution. An especial case is the use of iron oxide / TiO2 nanoparticles . Thus, photocatalytic oxidation with TiO2 nanoparticles (6–20 nm) was investigated as a promising water-treatment process [81, 82]. TiO2 nanoparticles, after UV irradiation, are able to adsorb and degrade a huge variety of organic contaminants present in the environment. For example, in case of organic arsenic species (monomethylarsonic [MMA] and dimethylarsinic [DMA] acids), the strong affinity between the TiO2 nanoparticles surface results a covalent bonding between MMA or DMA and the nanoparticle surface via formation of bidentate (AsMMA-Ti 3.32 Å) and monodentate (AsDMA-Ti 3.37 Å) inner-sphere complexes, respectively. Dopation of TiO2 nanoparticles with Fe3+ ions at 0.1–0.5% may significantly increase the photocatalytic activity. The doped ions act as charge separators of the photoinduced electron–hole pair and enhanced interfacial charge transfers. Finally, an interesting application is known for adsorption of elemental sulfur. Thus, photoinduced sulfur desorption from the surfaces of Au Nps loaded on a series on metal oxides, in particular Fe2O3, was studied . Elemental sulfur S8 was selectively adsorbed on the Au Nps surfaces of Au/metal oxides in an atomic state. This phenomenon is applicable to the low temperature cleaning of sulfur-poisoned metal catalysts.
Organic synthesis. Free iron oxide(III) NPs and its nanocomposites have found numerous application in organic synthesis. Thus, hematite α-Fe2O3 NPs (diameters in the 7–18 nm range, synthesized by thermolysing a PVA-Fe(OH)3 gel matrix at moderate temperatures) can effectively catalyze the epoxidation of styrene with tert-butyl hydroperoxide (TBHP) as the terminal oxidant . Iron oxide nanoparticles supported on zirconia were tested in the gas-phase conversion of cyclohexanol to cyclohexanone in a fixed-bed flow type, Pyrex glass reactor, at 433–463 K . Major detected products were cyclohexanone, cyclohexene and benzene, depending on the used catalyst. Experimental results showed that there was no leaching of metal, and that the catalyst was thus truly heterogeneous. In addition, core-shell Fe2O3/Pt nanoparticles with amorphous iron oxide cores exhibited superior catalytic activity with lower peak potential and enhanced CO2 selectivity toward methanol electrooxidation in acidic medium . This catalytic performance may be attributed to the uniform distribution of Pt particles on the amorphous Fe2O3 surface as well as interactions between the Pt particles and amorphous Fe2O3 cores. The catalytic activity of core-shell Fe2O3/Pt nanoparticles first increases and then decreases with decreasing Pt content. These nanomaterials also were found to have much higher structural stability and tolerance to the intermediates of methanol oxidation.
A magnetically separable core–shell iron oxide@nickel (IO@Ni) nanocatalyst, synthesized by reduction of Ni2+ ions in the presence of iron oxide (Fe2+, Fe3+) by a one-pot synthetic route using NaBH4 as a reducing agent and starch as a capping agent, was found to have excellent activity for the hydrogenation reactions of aromatic nitro compounds under mild conditions using water as a green solvent (reaction 4). Excellent chemoselectivity and recyclability up to 30 cycles for the nitro group reduction was demonstrated. Nano propylsulphonated γ-Fe2O3 (NPS-γ-Fe2O3, reaction 5) was applied as a magnetically recyclable heterogeneous catalyst for the efficient one-pot synthesis of bis(pyrazolyl)methanes in water (reaction 6) . The catalyst was easily isolated from the reaction mixture by a magnetic bar and reused at least five times without significant degradation in activity. Nanoporous α-Fe2O3 nanoparticles (about 100 nm in size and containing pores <10 nm) were synthesized via a hydrothermal method and applied in the catalytic benzylation of benzene and benzyl chloride (BC) in the fabrication of diphenylmethane (DPM) . The BC conversion reached 100% in a reaction time of 3 min with 97.76% selectivity to DPM. The nanoporous α-Fe2O3 nanoparticles also have potential applications in other Friedel-Crafts alkylations, especially in large molecular reactions. Another example is related with chemical recycling of PET. Thus, easily recoverable superparamagnetic γ-Fe2O3 nanoparticles (10.5 nm in size and 147 m2.g−1 surface area, produced by calcining Fe3O4 nanoparticles prepared by the co-precipitation method) were used as a reusable catalyst for PET glycolysis . At 300°C and a 0.05 catalyst/PET weight ratio, the maximum bis(2-hydroxyethlyl) terephthalate (BHET) monomer yielded reached more than 90% in 60 min. The catalyst was reused 10 times, giving almost the same BHET yield each time. In addition, heterogeneous photo-Fenton reaction which utilizes nanosized iron oxides as catalyst for maximizing the activity due to the enhanced physical or chemical properties brought about by the unique structures was described .
Antibacterial activities. The doping effects of silver(I) and iron(III) on photocatalytic results using TiO2 thin films were studied . Ag and Fe doping and co-doping contents on nano-titania photocatalytic bactericidal films were obtained by sol-gel technique, thus uniting three classic active antibacterial species (Ag, Fe and TiO2). The photocatalytic activity of TiO2 films was confirmed by the sterilizing rate of the E-coli in each case. Applying fluorescent light irradiation, the optimal doping amounts of silver(I)/titania and iron(III)/titania were found to be 0.05% and 0.1%, respectively. In addition, a photocatalytic technique using visible light and carbon nanotubes and nano-sized Fe2O3 powder was used to inhibit pathogenic bacterial growth in water . It was suggested that after careful design, this system can be used to disinfect drinking water, making it free of pathogenic bacteria.
3.4. Nano-Fe3O4 phases and their composites
Iron(II,III) oxide based nanostructures are slightly less explored in the organic synthesis. Thus, haemin-functionalized magnetic iron(II,III) oxide nanoparticles (Fe3O4/haemin) exhibited pronounced electrocatalytic activity towards trichloroacetic acid (TCA) like haemin itself (a linear detection range of 5–80 M and a detection limit of 0.3 M at 60 °C) . This activity towards TCA was affected by detection temperature, which was controlled via electrically heated carbon paste electrodes. The maximal catalytic current was 5.8 times higher at 60 °C than at room temperature (25 °C). A process capable of synthesizing minor fractions of aromatic hydrocarbons (benzene, toluene, xylenes, and mesitylene) from CO2 and H2 at modest temperatures (T = 380 to 540°C) employing Fe/Fe3O4 nanoparticles as catalyst was designed . The authors consider this technology as principally compatible with solar heat and hydrogen technology and having the potential to mitigate the impacts of global warming by making use of the existing distribution technology for gasoline. Also, trisubstituted imidazoles can be synthesized condensation reaction from 1,2-diketones, aromatic aldehydes, and ammonium acetate (reaction 7) in high yield in the presence of sulphamic acid functionalized magnetic Fe3O4 nanoparticles (reaction 8) as a solid acid catalyst under solvent-free classical heating conditions or using microwave irradiation .
Among other reactions, we note an efficient one-pot, three-component condensation reaction between 4-hydroxycoumarin, aryl glyoxals, and malononitrile catalyzed by Fe3O4 nanoparticles, which was carried out for the synthesis of several dihydropyrano[c]chromenes . Also, an inexpensive and non-hazardous sulfuric acid functionalized magnetic Fe3O4 nanoparticles (efficiently catalyze one-pot multicomponent condensation of β-naphthol with aromatic and aliphatic aldehydes and amide derivatives (reaction 9) at 100 °C under solvent-free conditions to afford the corresponding amidoalkyl naphthols in excellent yields and in very short reaction times . Silver(0) nanoparticles supported on silica-coated Fe3O4 (synthesis see reactions 10) serve as an efficient and recyclable heterogeneous catalyst for oxidant-free dehydrogenation of alcohols to the corresponding carbonyl compounds . The catalyst can be easily recovered and reused for 8 reaction cycles without considerable loss of activity. At last, a nanocomposite of functionalized Ni(II) complex containing surface of pyridine, methoxysilanyl and amino groups with iron(II,III) oxide, Fe3O4@[-Ni(bpy)2(py-tmos)] was found to be highly efficient green catalyst for the synthesis of a diverse range of 3,4-dihydropyrimidin-2(1H)-ones under solvent free conditions, and in addition it could be easily recovered by a simple magnetic separation and recycled at least 5 times without deterioration in catalytic activity .
Cobalt ferrites having different sizes, from ultrasmall (2 nm) to 50 nm, can be fabricated by distinct techniques , mainly co-precipitation method (CPM), sometimes without using any capping agents/surfactants. Thus, the CPM was used to synthesize ultrasmall CoFe2O4 superparamagnetic nanoparticles (SPMNPs, 2–8 nm of an average size and high surface area of 140.9 m2/g) without any surfactant . Their catalytic activity was verified in the preparation of arylidene barbituric acid derivatives (reaction 11) applying CoFe2O4 SPMNPs in aqueous ethanol as a reusable catalyst, which can be magnetically separated from the reaction system. Main advantages of this approach were found to be high yields, short reaction time and high turnover frequency, a clean reaction methodology, and chemoselectivity, among several others. More large-size CoFe2O4 magnetic nanoparticles (25 nm) were used as a catalyst for the oxidation of various alkenes in the presence of tert-butylhydroperoxide (t-BuOOH) with almost quantitative yields (reaction 12) . It seemed that this heterogeneously catalysis system proceeds by coordination of t-BuOOH to the metal (Fe3+ cations) on the surface of the catalyst. The separation of the catalyst from the reaction media was easily achieved with the aid of an external magnet, and the catalyst can be reused several times with no loss of activity. In addition, combination of synthesis techniques can also be used for cobalt ferrite preparation. Thus, synthesis of spinel CoFe2O4 MNPs (average sizes 40–50 nm) was achieved by a combined sonochemical and co-precipitation technique in aqueous medium, also without any surfactant or organic capping agent . These uncapped NPs were utilized directly for aldol reaction in ethanol (reaction 13). After the reaction was over, the nanoparticules were compartmented by using an external magnet.
Nickel ferrites. Pure or doped nickel ferrites out of the nano-range size are common and frequently used in several catalytic processes. For instance, high reactivity of NiFe2O4 (111) surfaces (higher that in Fe3O4) is well-known; NiFe2O4 is an effective metal-doped ferrite catalyst in a typical industrial process such as the water-gas shift (WGS) reaction . Similarly, NiFe2O4 was examined as catalyst in photocatalytic water oxidation using [Ru(bpy)3]2+ as a photosensitizer and S2O82− as a sacrificial oxidant . The catalytic activity of NiFe2O4 is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O2 yield and O2 evolution rate under ambient reaction conditions. As an example of non-nano-sized doped ferrites, their catalysts (granules of ~1 mm diameter) of nickel, cobalt and copper, prepared by co-precipitation hydrothermal route and impregnated with palladium, cerium and lanthanum as promoters , were tested for carbon monoxide oxidation activities. The catalysis of NiFe2O4 nanoparticles on the hydrogen storage performances of magnesium hydride synthesized by high-energy ball milling was studied , showing that the initial dehydrogenation temperature of 7 mol.% NiFe2O4-doped MgH2 is 191°C, which is 250°C lower than that of pristine MgH2. The enhancement in the H2 storage performances of MgH2 by adding NiFe2O4 nanoparticles is primarily ascribed to intermetallic Fe7Ni3 and (Fe, Ni) phases during the desorption procedure, which act as the real catalyst in the 7 mol.% NiFe2O4-doped sample. As an example of another application, a magnetic acidic catalyst comprising Preyssler (H14[NaP5W30O110]) heteropoly acid supported on silica coated nickel ferrite nanoparticles (NiFe2O4@SiO2) was investigated for the synthesis of bis(dihydropyrimidinone)benzene and 3,4‐dihydropyrimidin‐2(1H)‐ones derivatives by the Biginelli reaction . With the catalyst, the reactions occurred in less than 1 h with good to excellent yields.
Copper ferrites. Non-nanosized range copper ferrites have certain catalytic applications, such as, for example, for CO conversion to CO2 . In a difference of pure nickel ferrites, copper ferrite NPs are applied in organic catalysis in more uniform particle size (mainly about 20 nm). Thus, nano material on the basis of copper ferrite (20 nm) was applied as reusable heterogeneous initiator in the preparation of 1,4-dihydropyridines. The interaction of substituted aromatic aldehydes, ethyl acetoacetate and ammonium acetate (reaction 14) was observed in presence of CuFe2O4 nano powders in ethanol at ambient conditions. The nano catalyst can be magnetically recovered and reused . The same 20-nm size copper ferrite nano material also was reported as reusable heterogeneous initiator in the synthesis of β,γ-unsaturated ketones and allylation to acid chlorides in THF at r.t. without any additive/co-catalyst (reactions 15–16) . The notable advantages are less expensive, heterogeneous reusable catalyst; mild reaction conditions, high yields of products, shorter reaction times, no isomerization during the reaction, and easy workup. In addition, 20-nm CuFe2O4 was applied as reusable heterogeneous initiator in the synthesis of α-aminonitriles by one-pot reaction of different aldehydes with amines and trimethylsilyl cyanides at r.t. in water as a solvent (reactions 17–18) . α-Aminonitriles are important in preparing a wide variety of amino acids, amides, diamines, and nitrogen containing heterocycles. In addition, a strategy for the synthesis of benzoxazoles from substituted N-(2-halophenyl)benzamides (reaction 19) was developed , where inexpensive, readily available, air-stable, recyclable copper(II) ferrite serves as a nanocatalyst. Also, larger-size cubic copper ferrite CuFe2O4 nanopowders (24–51 nm in size) were synthesized via a hydrothermal route using industrial wastes (ferrous sulfate containing free sulfuric acid ≈10%, 0.01% Zn2+ and 2% silica; copper waste 12.5% Cu, 8.7% Cl- with minor Ni 0.001%) . Study of photocatalytic degradation of the methylene blue (MB, C16H18ClN3S) dye using copper ferrite powders showed a good catalytic efficiency (95.9%) at hydrothermal temperature 200 °C for hydrothermal time 24 h at pH = 12 due to high surface area (118.4 m2/g).
Zinc ferrite. Non-nano-sized zinc ferrites (ZnFe2O4) have been used in oxidative organic reactions. Thus, the catalytic behavior for oxidative conversion of methane and oxidative coupling of methane was investigated over pure and neodymium substituted zinc ferrites prepared by combustion method . The catalytic activity proved to be strongly related to the oxide structure as well as to the specific defects created by substitution. The pure zinc ferrite (ZnFe2O4) and ZnNd2O4 exhibited high activity for coupling reaction whereas the neodymium substituted ferrites (ZnFe1.75Nd0.25O4, ZnFe1.5Nd0.5O4 and ZnFeNdO4) was low active in this reaction. The order of the catalytic activities expressed as yields to C2+ were ZnNd2O4 > ZnFe2O4 > ZnFe1.75Nd0.25O4 > ZnFeNdO4 > ZnFe1.5Nd0.5O4.
Analyzing pure zinc ferrite nanocatalysts, we note that mainly ultrasmall particles are currently applied in catalytic purposes. Thus, a nanosized highly ordered mesoporous zinc ferrite (ZF, 7–10 nm in size) was synthesized via co-precipitation method, further sulfated with ammonium sulfate solution to obtain sulfated ZF (SZF) and was used for the synthesis of nopol by Prins condensation of β-pinene and paraformaldehyde (reaction 20) . 70% conversion of β-pinene with 88% selectivity to nopol was observed; the spent catalyst was regenerated and reused successfully up to four cycles with slight loss in catalytic activity. The influence of various reaction parameters such as solvent, reaction temperature, effect of substrate stoichiometry and catalyst loading was investigated. In particular, very low conversion (9%) of β-pinene was observed in protic solvents such as methanol (solvent effect); in case of apolar–aprotic solvents such as hexane, ethyl acetate and toluene, increase in the β-pinene conversion as well as nopol selectivity was observed. In case of reaction temperature, as the temperature increased to 110 °C, β-pinene conversion increased to 72% with slight drop in selectivity for nopol (57%). 95 °C was identified as optimal reaction temperature for further studies. At molar ratio of β-pinene to paraformaldehyde 1:3 molar ratio, maximum conversion was observed. The β-pinene conversion increased with an increase the catalyst loading (0.12 to 0.16 g) without affecting the nopol selectivity.
Other simple ferrites. The catalytic behavior of supported Au NPs for the process of oxidation of benzyl alcohol was elucidated in presence of gold nanoparticles . Mg2+ ions, being present in the ferrite structure, led to an improvement of the catalytic activity of supported Au NPs to ca. 35% conversion, when an additional base was absent. Modifying the support with addition of MgO, the catalytic activity of supported Au nanoparticles was further improved to ca. 50% conversion; however, the catalyst was found to be deactivated in successive recycling tests. As well as nano-Fe2O3 mentioned above in the corresponding section, nano-MnFe2O4 particles (20–30 nm in size), synthesized by co-precipitation phase inversion method and low-temperature combustion method, using MnCl2, FeCl3, Mn(NO3)2, Fe(NO3)3, NaOH and C6H8O7, were applied for thermal decomposition of ammonium perchlorate . The catalytic mechanism was explained by the favorable electron transfer space provided by outer d orbit of transition metal ions and the high specific surface absorption effect of MnFe2O4 particles. Manganese ferrite nanoparticles were also applied in the synthesis of spirooxindoles (compounds I-III) via a one-pot and three-component reaction of isatins, malononitrile, and anilinolactones in the presence of a catalytic amount of MnFe2O4 NPs in PEG-400, as a nontoxic, green, and reusable solvent .
Mixed-metal or core-shell ferrites. Ferrites containing 2 metal ions, additionally to iron, are much more widespread in the nano-catalysis; their nanoparticle size can vary in a broad range, from ultrasmall particles (5–8 nm) up to 100 nm or more (in case of supported NPs). Both nano-sized and out-of-nano-sized mixed-metal ferrite NPs can be synthesized by a variety of methods, in particular classic sol-gel and co-precipitation methods or microwave heating (MnZnFe2O4 ).
Cobalt-based ferrite nanoparticles. For cobalt-containing ferrite NPs, as well as for zinc ferrite above, one of important applications is the methanol decomposition to CO and hydrogen. Thus, Cu1-xCoxFe2O4 (0<x<1, 8–40 nm in size) was applied as a nanodimensional powder for this purpose . The stabilization of the cubic structure with the substitution of copper ions by cobalt in mixed Cu-Co ferrites was observed. Cobalt containing ferrites exhibited higher and more stable catalytic activity and selectivity in methanol decomposition to CO and hydrogen in comparison with the CuFe2O4 one. Photocatalytic properties of the cobalt zinc ferrite Co1-xZnxFe2O4 (0<x<1) nanoparticles (10.5−14.8 nm in size), prepared by a hydrothermal method, were studied on the example of degradation of methyl blue in aqueous solution . It was elucidated that the oxidation-reduction potential of methyl blue aqueous solution in presence of the ferrite nano-particles at pH=7 under natural sunlight irradiation was negative and increased with increase in Zn content. The degradation rate of methyl blue also decreases as increase in Zn content in sunlight.
Nickel-based ferrite nanoparticles. Similar to cobalt ferrites, several nickel-containing mixed or core-shell ferrites have been reported as nanocatalysts but in more narrow size range (18–50 nm). Thus, a magnetically separable catalyst consisting of ferric hydrogen sulfate (FHS) supported on silica‐coated nickel ferrite nanoparticles (50 nm) was prepared . This catalyst was shown to be an efficient heterogeneous catalyst for the synthesis of 1,8‐dioxodecahydroacridines (reaction 21) under solvent‐free conditions. The catalyst can be recycled several times with no significant loss of catalytic activity.
Other mixed-metal ferrite nanoparticles. Ferrite nanoparticles, containing other metals and applied in the catalysis, are represented more chaotically in the available literature. Thus, the spinel ferrites Cu1-xCdx[Fe1-xAlxCr1-xMnx]O4, where 0<x<1, having unknown particle size, were prepared by the coprecipitation technique . Catalytic studies using decomposition of H2O2 as a model reaction between 303 and 343 K for 1–5 h using first order rate law suggested higher catalytic power for the composition x = 0 and then it decreases gradually. For the mixed spinel ferrite system Mn1-xCuxFe2O4 (x = 0, 0.25, 0.5, 0.75, 1.0), the formation of phase pure spinels with FCC cubic structure with particle size ranging from 5.21 nm to 20 nm was observed at 333 K applying co-precipitation method with MnCl2, Fe(NO3)3.9H2O and Cu(NO)3.3H2O as precursors . These ferrites were used as catalysts in the alkylation of aniline, showing a maximum conversion of 80.5% of aniline with selectivity of 98.6% towards N-methylaniline at 673 K, methanol/aniline molar ratio of 5:1 and weight hour space velocity of 0.2 h-1. It was found that the yield is maximum for CuFe2O4. In addition, the catalytic performance of the ferrites was found to be proportional to surface area as well as acidity.
Iron-based nanoparticles, utilized in catalytic reactions described in this chapter, possess different sizes, from ultrasmall (2 nm) to 100 nm. They are obtained mainly by wet-chemical sol-gel or co-precipitation methods, sometimes combined with simple calcination at high temperatures, sonochemical technique, mechanical high-energy ball milling, or spark plasma sintering, among other methods. Microwave heating or hydrothermal route are also frequently used. Due to magnetic properties, these nano catalysts can be easily recovered from reaction systems and reused up to several runs almost without loss of catalytic activity.
Catalytic processes with application of iron-based nanocomposites are in a wide range. Notable attention is paid to methanol decomposition to CO and methane or to CO and hydrogen. Other catalyzed organic reactions consist of oxidation of various alkenes, aldol, alkylation and dehydrogenation reactions, synthesis of various organic compounds such as, for example, quinoxaline derivatives , β,γ-unsaturated ketones, arylidene barbituric acid derivatives, α-aminonitriles, nopol, 1,4-dihydropyridines, and 1,8‐dioxodecahydro-acridines. Degradation/decomposition processes are also reported, for instance decomposition of H2O2 or photocatalytic degradation of methylene blue. Some of catalyzed reactions might have great practical applications, for instance transesterification of soybean oil to biodiesel. In addition, small iron-based particles could also be considered  as substituents of noble metals in a variety of catalytic transformations. Several iron nanomaterials could have biological applications, such as peroxidase-like catalytic activity of Fe3O4 ultrasmall NPs .
We note that the total number of nano-iron composites applications for catalytic purposes is still not high, so it could be a perfect research niche for further applications of these nanomaterials in a variety of organic processes.