Keywords

13.1 Introduction

Iron is the fourth most abundant element on earth constituting nearly 5 wt% of the earth’s crust. The most dominant forms of iron are oxyhydroxide, oxides and sulfides. It is not only an important component of the functional proteins in living systems but has been an integral part of the civilizations since the iron age [1]. Iron oxide-based materials have been used in catalysis, biomedicine, environmental remediation and energy storage devices due to their unique chemical, thermal, optical, electronic and magnetic properties. Among the 16 available oxide forms, haematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4) are the most studied ones. Haematite is the most stable form, which acts as a precursor for other oxides. It is an n-type semiconductor with a bandgap of 2.1–2.3 eV, whereas magnetite has very low resistivity and lower bandgap of 0.1 eV. Owing to its high abundance, low toxicity, high sensitivity/activity and high corrosion resistance, haematite is most widely used. In α-Fe2O3, Fe3+ occupies two thirds of octahedral sites with oxygen in hexagonal close packed arrangements, while in Fe3O4, oxide ions are in cubic close packed array resulting in an inverse spinel structure with Fe2+ in half of octahedral sites and Fe3+ in remaining octahedral and tetrahedral sites. The second most stable oxide of iron is γ-Fe2O3, which has a cubic spinel structure, where oxygen ions are in cubic close packed array and Fe3+ occupies the octahedral and tetrahedral sites [2] (Fig. 13.1).

Fig. 13.1
A flow chart exhibits an overview of the iron oxide. The occurrences include rocks and soils, freshwater and seawater, and aerosol and dust. The sources are mining, synthesis, and biomineralization. The applications are steel and catalysis, diagnostics and therapy, and pigments.

Occurrence, sources and general applications of iron oxide [1]

Full size image

An ever-increasing population coupled with aspirations for higher living standards has a direct or indirect impact on the air quality. Natural events and anthropogenic activities also release air pollutants in significant amounts making respiratory diseases as one of the major causes for death in recent years. The management of air quality is a perennial challenge that not only requires strict enforcement of policy level decisions but also development and deployment of new and efficient technologies to continuously monitor and curb it at the source. As clean air is an absolute necessity for a healthy living, the necessity to mitigate contaminants from the air has assumed enormous significance in recent decades. VOCs, CO, NOx and particulate matter arising from the combustion of fossil fuels by stationary and mobile sources are major pollutants that must be mitigated. CO is not only a toxic gas upon inhalation but also aids in the formation of ground level ozone, which leads to severe environmental pollution. Similarly, brownish nitrogen oxides are also poisonous and affect respiratory system. Along with VOCs, NOx creates photochemical smog during hot summer. The reaction between VOC and NOx occurs in the presence of sunlight producing brownish haze. In the recent years, air pollution has significantly increased even indoors. In developing countries, indoor air pollution has outgrown outdoor pollution such that it causes 2 million deaths every year [3]. Indoor air contains particulate matter and VOCs, which originates from construction materials, cooking, furniture and different packing materials, which are considered as serious air pollutants. Different government agencies have their own measures and policies to reduce and mitigate gaseous emissions. For mitigating the emissions, thermal incineration systems or catalytic systems can be used. Catalytic systems are more preferred over incineration because of low energy demand. Lots of research worldwide are trying to develop an efficient thermal/photo/plasma/electrocatalytic system, which can mitigate air pollutants at the ambient conditions. Iron-based catalytic materials have successfully found their way to play a reliable role in the technologies to purify the polluted environment.

Brief Introduction to Catalytic Process

According to IUPAC, a catalyst is a substance that increases the rate of a reaction without altering the standard Gibbs energy change of a reaction. Catalysis can be of two types depending on the phase in which reactants and catalyst exists. If both reactant and catalyst exist in different phases, catalysis is known as heterogeneous catalysis. If not, it is homogenous catalysis. Most of the industrial processes are heterogeneous catalysis, which in general contains catalysts in solid phase and reactant in gas or liquid phase. Heterogeneous catalysts are more robust over a range of harsh reaction conditions. They are also separable making them prepared over homogeneous catalyst. The steps involved are adsorption, surface reaction and termination. Adsorption can be physisorption or chemisorption depending on the energetics of interaction between catalysts and reactant molecule. Physisorption is uneventful as far as reactions are concerned. Chemisorption of intermediate strength has relevance to catalysis.

On the surface of the catalyst, there are certain regions known as active sites, which directly participate in the reaction. They are usually under-coordinated surface atoms or defects, which can satisfy their coordination by bonding with the reactant molecule. This step is normally understood as chemisorption and invariably causes the activation of certain bonds on the reactant molecule. Activation of a molecule is understood in terms of weakening (or elongation) of the bonds that eventually breaks and combines with other reactants or surface adsorbed species to form new intermediates or products. Desorption of the newly formed product and regeneration of the active sites are crucial parts to complete a catalytic cycle. According to Sabatier’s principle, the energy of interaction between the catalytic surface (adsorbent) and reactant (adsorbate) should be optimum for an eventful chemical reaction [4]. Number and nature of the active sites are critical parameters in evaluating catalytic performances, which are indicated by turn over numbers or frequency and selectivity of products. Catalysts are developed based on deep understanding of the chemical processes involved in the synthesis of the catalyst structure of active sites and energetics of the surface chemical reactions leading to products. Catalytic activity is also often found to be directly proportional to the surface area [5]. The process of heterogeneous catalysis relies on the adsorbate-adsorbent interactions, catalyst surface morphology, size of the particles, defects, etc.

Catalytic Oxidation Technology

In general, catalytic oxidation follows four steps. The first step is the adsorption step, where the reactant molecules (VOC/COx/NOx) and oxygen adsorb onto the active sites of the catalyst. The second step involves the surface diffusion of atoms to adjacent adsorption/active sites. Molecular rearrangements or the reaction between oxygen and reactant molecule occur in the third step resulting in the oxidation products of the original reactant molecule. The last step is desorption of the products of the reaction from the surface of the catalyst [6] (Fig. 13.2).

Fig. 13.2
A flow chart demonstrates the steps involved in thermal catalytic oxidation. The steps are adsorption, surface diffusion, molecular rearrangement at active sites, and desorption.

Steps of thermal catalytic oxidation [6]

Full size image

There are three kinetic models proposed to explain the mechanisms of catalytic VOC oxidation. They are:

  1. a.

    Langmuir–Hinshelwood (L–H) Mechanism: L–H mechanism proposes that both oxygen and the other reactant (e.g., VOCs) are adsorbed on to the surface and react to form the products (e.g., carbon dioxide and water). The rate determining step is the step in which reaction occurs between adsorbed oxygen and adsorbed reactant molecules. It can be of two types, single site and dual site based on same or different active site at which oxygen and reactant got absorbed.

  2. b.

    Eley–Rideal (E–R) Mechanism: Either oxygen or reactant gets adsorbed onto the surface, which reacts with the molecule present in the atmosphere. So, the reaction occurs between either the adsorbed oxygen with reactants in air or between adsorbed reactants and oxygen in air which is the slowest step and hence the rate determining step.

  3. c.

    Mars van Krevalan (MVK) Mechanism: According to the model, the reaction is triggered by the interaction of reactants and surface lattice oxygen in two consecutive steps. In the first step, the surface oxygen sites get reduced by reacting with reactant molecule and these sites will be regenerated either by the consumption of gaseous oxygen or by transfer of bulk oxygen to the surface in the second step [7] (Fig. 13.3).

    Fig. 13.3
    A schematic diagram presents the 3 kinetic models for the mechanism of thermal catalytic oxidation. It includes LangmuirHinshelwood, Eley-Rideal, and Mars van Krevalan. They exhibit the stepwise reaction for the interaction of oxygen and reactant molecules.

    Adapted from ref [7] with permission from Elsevier

    Different mechanisms of thermal catalytic oxidation.

    Full size image

Types of Catalysis Using Iron Oxides and General Materials Consideration

Depending on the nature of energy inputs, the reactive species might be different which can open a new reaction pathway on the surface of the catalyst. In the context of environmental catalysis, following types of catalytic reactions are known.

  1. a.

    Photocatalysis: Photocatalysis is a catalytic oxidation of VOCs/NOx/CO using a catalyst and light energy, particularly the visible region of the sunlight. Photocatalysts are mainly semiconductor nanoparticles with a specific band gap suitable enough to produce high energy excitons (photo-induced electron–hole pair) when light is shined on them. Absorption edge (λg) is related to the band gap (Eg) of the photocatalyst (λg = 1240/Eg). The photo-generated excitons can then produce reactive radicals, which can react with reactants such as VOCs to completely oxidize them. Mitigation of reactants by photocatalysis is dependent on the quantity of photon flux available. On the other hand, deactivation of the catalysts is caused by surface contamination or aggregation of active sites. Photocatalysts are poised to treat the indoor pollution effectively when used in combination with construction materials such as window glasses, whereby visible spectrum of the sunlight or indoor light source can be utilized. A photocatalyst material that can efficiently utilize visible light without undergoing photo-induced damage is a challenge [8, 9].

  2. b.

    Photothermal catalysis: Thermo-catalytic oxidation of reactants such as VOCs consumes relatively less energy compared to thermal oxidation. For the conventional photocatalysis, utilization efficiency of solar energy is small even after modification of the catalysts. In photo-thermo-catalysis, both thermal and photo processes are combined, whereby catalytic efficiency and durability of thermocatalysis combined with low energy utilization of photocatalysis are expected to coexist. The thermal effects of vis-IR are utilized in photocatalysis to increase reaction efficiency or a broad range of solar spectrum is explored which simultaneously induces thermochemical and photochemical processes which synergistically catalyze the reaction [10] (Fig. 13.4).

    Fig. 13.4
    A schematic diagram exhibits the concept of photothermal catalysis. The photothermal energy from the light source is converted into heat energy in the narrow bandgap photocatalysts. The metal and chalcogenide molecules are arranged in the lattice structure. H 2 O reduced to O H O 2 + reduced to O 2.

    Diagram representing photo-thermal catalysis Adapted from ref [11] with permission from Wiley VCH GmbH

    Full size image

It can be divided into two types based on the reaction pathway:

Photo-assisted thermal catalysis: Photothermal conversion can be defined as the conversion of sunlight into heat energy by collection and absorption system. Generally, there are three kinds of materials which can be used such as narrow band gap photocatalysts that can be excited by IR light, visible/near-IR plasmonic photocatalysts which carry out photochemical reactions by using plasmonic hot electrons, defective materials which can handle both light and heat like non-plasmonic oxide materials. Graphene is one of the materials which shows photothermal activity and some materials such as Mn-based catalysts doped with Fe, Cu and Mg [12]. Surface plasmon resonance effect is another process by which noble metals (Au, Ag and Pt) absorb light and scatter it. Incorporating these metals into the catalyst will improve light harvesting ability hence the photo-thermo catalytic activity.

Photo-thermal synergistic catalysis: The advantages of both thermal catalysis and photocatalysis are combined to have the synergistic effect, which enhances the catalytic performance. A semiconductor material with narrow band gap usually shows excellent performance [13].

  1. c.

    Plasma catalysis: When a gas is exposed to an electric field of desired magnitude, it will get ionized to ions and electrons. The partially ionized gas with electrons, ions and neutral species interacts with each other giving rise to reactive environment called plasma. In plasma catalysis, both plasma and catalyst enhance the reaction rate independently and inter dependably. Plasma initiates a few processes on the surface such as sputtering, [14] etching [15, 16] formation of hotspots and charging. The interdependence of plasma and catalysts known as synergistic behavior, which improves energy efficiency, conversion percentile and selectivity for the reaction. At the plasma catalyst interface, plasma establishes an electric field and changes the gas composition by producing reactive species, ions, electrons and photons to the surface, whereas catalyst lowers activation energy for some reactions. At the interdependent state, plasma alters the surface morphology or work function of the catalyst and dielectric constant, or morphology of the catalyst affect energy distribution in plasma. Vibrationally excited species are important because they influence plasma surface interactions more. Generally, for a nanoparticle to be active, the requirements are size, faceting, uncoordinated surface atoms and other defects, active sites, strain, oxidation state, charge transfer, etc. During plasma catalysis, all these factors will be modified by plasma thus effecting the catalytic activity. The requirements of a material to be tried for plasma catalysis can be summarized as having a large contact area for strong interaction and a higher dielectric constant of the catalyst. A shorter distance between plasma and catalyst facilitates the reaction of the short-lived radicals to the maximum. Most of the active catalysts are transition metals [17]. Plasma catalysis not only prevents coke formation and catalytic poisoning which are inevitable in thermal catalysis but synergism in plasma catalysis even modifies the reaction pathway (Fig. 13.5).

    Fig. 13.5
    2 diagrams for catalytic activities. In the thermal and plasma catalysis mechanisms, the free radicals are adsorbed by the surface and undergo surface reactions with the help of heat. Then the radicles undergo desorption in thermal catalysis, and the radicals undergo bombardment in plasma catalysis.

    Diagram differentiating thermal and plasma catalysis [17]

    Full size image

Synthesis of Iron Oxide-Based Catalysts

Proper identification of experimental conditions for the preparation of catalysts is extremely important for the activity of nanoparticles [2]. Iron oxide nanoparticles and nanocrystals are considered as 0 D architectures of iron oxide, used mostly for biomedical and catalytic applications. Desirable features of nanoparticles like narrow particle size distribution, good dispersion and stability can be achieved by optimizing reaction conditions like choice of iron precursor, solvent, concentration of reducing agent, pH, reaction temperature and time. Co-precipitation is often used to prepare iron oxide nanoparticles in water which involves adding a base to the precursor solution which contains Fe(II) or Fe(III) ions to promote the precipitation of ferrihydrites and subsequent dehydration generates iron oxide nanoparticles. Employing different bases, ionic medium, pH and iron precursors the formation of desired phase of iron oxide can be achieved [18]. Solvothermal methods have the added advantages of being low cost, high yield and greater control over the phases and requirement of moderate temperature. In a typical solvothermal synthesis by adjusting concentrations of ferrous chloride and ratio of ethanol/water mixture, particle size can be tuned from 15 to 31 nm. Higher particle concentrations of FeCl2 create more nuclei and finally to smaller particles and more the amount of ethanol in the solvent also created smaller particles by inhibiting the particle growth [19].

13.2 Catalytic Oxidation of VOCs by Iron Oxide-Based Catalysts

Air pollutants are mainly of two types: Particulate pollutants, which can be removed by adsorption techniques, and Gaseous pollutants (in sub ppm levels), which consists of VOCs as a major share. Among them, VOCs are not easily removable and their mitigation has been under the special focus for several years. The European Union defines any organic compound having an initial boiling point ≤ 250 °C measured at standard atmospheric pressure of 101.3 kPa as VOCs. They are acknowledged as hazardous to human health as well as to the environment mainly because of its high volatility, persistence in the environment, ability to spread over long distances from the point of release and ability to get transformed to other hazardous compounds by chemical reactions, often under normal atmospheric conditions. In India, the Clean Air Act 1990 (Amendment) and the Factory Act 1986 (Amendment) limit the emission of hazardous chemicals including VOCs [20]. Due to the current scenario of strict regulations on the concentration of VOCs in indoor as well as outdoor environment, either recovery techniques or oxidative techniques are applied. Recovery techniques involve both separation of the VOCs and particulate matter from the air followed by its recovery from the adsorbents. Adsorption, membrane separation and condensation methods come under the category of recovery techniques. Adsorption method is suitable only for dilute VOC emissions like the removal of solvent vapors in the atmosphere. Membrane separation and absorption methods are expensive because of the maintenance of the setup. Condensation is useful for VOCs with higher boiling points where the oversaturation is achieved by cooling or pressurizing the gas stream. The major disadvantage associated with these methods includes the lack of solutions to dispose of the VOCs recovered from the adsorbent. On the other hand, oxidative techniques involve the complete oxidation of the VOCs into carbon dioxide and water. Thermal incineration is a viable alternative despite being highly energy-demanding. The important disadvantage of the incineration method is the production of incompletely oxidized volatile byproducts, which invariably happens. This certainly aggravates air pollution instead of mitigating it. Hence, catalytic oxidation is a very powerful technology to completely oxidize the volatiles to relatively harmless carbon dioxide and H2O at reasonable temperatures. Belessi et al. carried out kinetic studies of the deep oxidation of CH4 on oxide solids La0.7Ce0.3FeO3, La0.7Sr0.3FeO3, La0.7Sr0.1Ce0.2FeO3 oxide solids having mixed oxide and perovskite phases. From the kinetic analyses of the reaction rate, it is found that adsorbed oxygen in dissociative form reacted with gaseous methane, following the Eley–Rideal model. The existence of a SrFeO3±x perovskite crystal phase, which can uptake large amounts of oxygen accounts for the large value of heat of adsorption of oxygen, \(\lambda_{{{\text{O}}_{2} }}\)  = 53–211 KJmol−1 [21].

Catalytic oxidation of VOCs by Fe-based materials using thermal, photo, photothermal, and plasma techniques is summarized in Tables 13.1, 13.2 and 13.3. Low-temperature catalytic activity is shown by Fe–Ce mixed oxides on SBA-15(Sl no 4) hence even better than supported Au catalyst (SI no 32). The higher activity can be due to the better dispersion of metal oxides over the porous structure of the material. Higher conversion with lower specific input energy (SEI = plasma power (kW)/flow rate(L/min)X 60 s/min) was shown by Fe-doped Mn octahedral sieves. Modifying defects and surface area of the catalysts by various methods improved the production of active species in plasma and oxygen mobility on the surface to enhance the mitigation efficiency of the catalysts for various VOC oxidation. Generally, iron oxides show good activity among the other oxides in the oxidation of chlorinated VOCs with the added advantages of high efficiency, low cost and environmental friendliness. Recently, studies were carried out with Fe–Mn mixed oxides for the oxidation of toluene, formaldehyde, chlorobenzene and dichlorobenzene [23, 58]. Among those, chlorinated VOCs are more important because of their contributions to the production of secondary organic aerosols, peroxyacetyl nitrate, tropospheric ozone and the greenhouse effect. Noble metal catalysts have the disadvantage of chlorinated compounds due to deactivation by HCl, Cl2 poisoning and chlorination of products other than oxidation [22]. Ru is known to be resistant to Cl poisoning by promoting deacon reaction. It is known that supports do affect the catalytic activity by altering the crystallite dimensions and thermal stability of the metal species. Wang et al. studied the possible application of Ru doped on mesostructured Fe–Mn oxides. The mesoporous Fe–Mn bimetallic oxide with a metal atomic ratio of 1:2 was prepared by oxalate pyrolysis without any template followed by metal impregnation for doping with Ru. The XRD data did not show any active phases of Ru and no significant diffraction peaks of Mn which means that the active phase of Ru is stabilized in the mesoporous substrate by strong metal-support interactions [23]. In a recent study by Fan et al., HCHO oxidation was carried out using Fe–Ce–O catalyst supported on mesoporous silica prepared by simple impregnation-calcination resulting in the formation of small solid solution particles with good dispersion in confined spaces. CeO2 materials are routinely investigated along with other transition metal oxides for VOC oxidation but suffer from having a low surface area and limited efficacy for low-temperature oxidation. Many methods were explored to lower the activation barrier such as doping other metal atoms and creating more oxygen vacancies. Activity achieved at a low temperature of 60 °C was associated with the presence of Fe content which along with the calcination temperature influenced the activity by increasing oxygen vacancy [25]. Xiaodong et al. tried mesoporous Ti-doped iron oxide for the degradation of ortho-dichlorobenzene. Surprisingly, the Ti-doped iron oxide was showing better activity compared to the individual counterparts, which can be due to the presence of Ti4+ which had a higher affinity toward chlorinated VOCs, and iron oxide which showed a better oxidation power. It has also been proven that TiO2 alone has a slower activity which proves the active component is iron oxide. The study mostly concentrates on regulating the composition and hence the structure of the TiO2–Fe2O3 system to gain more knowledge about the interface interactions between both. It was prepared by CTAB as the structure-directing agent [22]. For example, Bismuth ferrite with a rhombohedrally distorted perovskite structure is a multiferroic compound that exhibits ferroelectricity and weak ferromagnetism above room temperature. Due to the narrow band gap energy (2.2 eV) and excellent chemical stability, these materials have applications in visible light photocatalysis. However, the electrical and multiferroic potential applications of BFO magnetic nanoparticles are hindered due to the following properties such as weak ferroelectricity, remanent polarization, high leakage current density, poor ferroelectric reliability and inhomogeneous weak magnetization. The substitution of any of the A-site and B-site had significant improvement in the multiferroic and photocatalytic properties [60].

Table 13.1 Catalysts for VOC oxidation by thermal catalysis
Full size table
Table 13.2 Catalysts for VOC oxidation by photo and photothermal catalysis
Full size table
Table 13.3 Catalysts for VOC oxidation by plasma catalysis
Full size table

Materials Consideration for Heterogeneous Catalysts for VOC Oxidation

If the VOC is an N-containing one, then there is a chance for the generation of nitrogen oxides, and if it is S-containing, the formation of metal sulfates is inevitable and which leads to the deactivation of catalysts. The key step for catalytic oxidation is the adsorption of the compound onto the surface of the catalyst with subsequent activation. In activating oxygenated VOCs, the abundance of OH groups on the surface plays a significant role. Introducing alkali metal salts onto the supported metal increases the concentration of –OH groups. For example, using noble metal catalysts for chlorinated VOCs has the disadvantage of easy deactivation as well as generating polychlorinated pollutants as byproducts. Even though increasing the acidity of the support decreases the selectivity toward polychlorinated byproducts, the application of noble metal catalysts for chlorinated VOCs is not advisable. Hence, selecting a catalyst also depends on the functional groups present, and in the case of a mixture of compounds, the mutual chemical interaction plays a crucial role in the choice of metal, support, precursor, preparation method, reaction conditions and catalytic reactor to be chosen. Hence, designing new catalysts which are active at low temperatures with high selectivity and low cost is critically important in practical applications.

Supported Noble Metal Catalysts (SNMCs) and Transition Metal Oxides (TMOs) are in general used as catalysts where the former have superior activity, selectivity and ease of regeneration despite being costly. So, the noble metal atoms are dispersed on a support which can be simple oxides such as silica or alumina, transition metal oxides or molecular sieves. Based on the involvement in chemical transformation, supports are classified into active or inert. Active support gets involved in catalytic oxidation, whereas inert support does not show any catalytic activity but provides surface and pore structure for the uniform dispersion of active metal catalysts. The presence of these supports not only reduces the amount of noble metal used, which is an economical advantage but also increases the dispersion of active sites thereby increasing the number of molecules interacting with the active site. In addition, the acid–base properties of support help in more dispersion of noble metal over the catalyst. Despite being less active, TMOs are good alternatives to SNMCs due to low cost, reducibility and thermal stability, and most importantly resistance to poisoning which is most common with noble metal catalysts. The activity of a catalyst or the reaction rate of the oxidation mainly depends on physical and chemical properties as well as the morphologies of active metal and support.

13.3 CO Oxidation

CO is a gaseous air pollutant produced due to the incomplete combustion of fuels from automobiles, industrial processes and power generation. Indoor sources include leaking chimneys, unvented kerosene and gas space heaters and gas stoves. At very high concentrations in indoor or enclosed environments, CO causes dizziness, confusion, unconsciousness or even death. CO is a colorless and odorless gas and can irreversibly bind with haemoglobin causing depletion in the oxygen levels in cells, which ultimately leads to cell death. CO is aptly described as a “silent killer”. Developing an efficient catalytic oxidation system for CO is highly desired, not only because it is poisonous but its industrial significance in the synthesis of methanol and other fuel and pure hydrogen in proton membrane exchange fuel cells [73].

The state-of-the-art catalysts for CO oxidation are noble metals but their use is limited because of high cost, low abundance and poisoning. High dissociation probability, low adsorption energy due to the half-filled d-bands and low cost have made TMOs as an attractive alternative [73]. SNMCs are also actively pursued as it shows increased exposure of active sites and decreases the consumption of expensive metal component. Bimetallics of noble metals with transition metals such as Mn, Fe, Co, Ni and Cu retain the superior catalytic property of the noble metals at a lower cost. Au is the most investigated noble metal catalyst due to the presence of Lewis acid sites, ease of size and shape [2]. In SNMCs, the nature of the support plays an important role since the reaction either takes place at the interface of the metal and the oxide support or as a spillover of reactive species from or into the oxide layer. Also, semiconductor metal oxides like TiO2, Fe2O3 and NiO form more stable catalysts than insulating metal oxides such as Al2O3 and SiO2. The Turnover Frequency (TOF) of Au catalysts changes significantly compared to Pt with the preparation method. The table given below indicates clearly that contact structure determines activity in supported Au catalysts [74] (Table 13.4).

Table 13.4 Comparison of the activity of Au and Pt-supported catalysts for CO oxidation [74]
Full size table

In general, Strong Metal Support Interaction (SMSI) can be explained as the change in catalytic reactivity of reducible oxides supported by group VIII metals after the high-temperature reduction process. Au particles supported on reducible oxides such as TiO2, Fe2O3 and CeO2 possess improved oxidation activity compared to unsupported ones. SMSI induces both electronic and geometric effects on the catalyst and it can be strong or weak. Contact at the interface induces electronic redistributions leading to the formation of a new phase at the metal-support interface [75]. As a geometric effect, metals supported on reducible oxides exhibit SMSI, which is the encapsulation of metal by a thin layer from the oxide support. It is shown that Pt supported on Fe2O3 (111) also exhibits SMSI via encapsulation due to the strong adhesion energies between Pt and iron oxide. Pt particles heated above 800 K in a vacuum exhibit structure that is like ultrathin FeO (111) film on Pt (111) single crystal. The catalyst showed higher activity compared to Pt (111) surface which is explained due to the formation of inverted catalysts or highly dispersed FeOx nanoparticles on the Pt (111) surface [76] (Fig. 13.6).

Fig. 13.6
4 schematic diagrams of the effects of catalysts. It exhibits the W M S I and S M S I with electronic and geometric effects through electronic properties, electronic properties tuned by S M S I, no encapsulation, and encapsulation.

Diagram showing electronic effects (a, b) and geometric effects (c, d) of SMSI Adapted from ref [75] with permission from Elsevier

Full size image

A significant number of studies on low-temperature CO oxidation catalysts are based on Au nanoparticles supported on iron oxide. Since the oxidation reaction occurs at the interface of Au nanoparticles and support, several factors like the size of the Au particle, the nature of support and the contact structure between support and Au were reported [74] to be controlling the catalytic activity. Cui et al. reported that increasing the pH value during synthesis is proportional to the catalytic activity due to stronger interaction between Au–OH–Fe or Au–Fe–O structure. They also found that calcination temperatures above 673 K eliminated surface hydroxyl groups which resulted in lower catalytic activity [2]. Oxidation of CO on iron oxides can occur in presence of oxygen and it is considered to proceed through dissociative adsorption of oxygen and removal of oxygen by CO through an Eley–Rideal mechanism [76]. Given below is a schematic representation of CO oxidation catalyzed by Au supported on Fe-doped TiO2 which clearly indicates that Fe catalyzes the dissociation of oxygen molecules [77] (Fig. 13.7, Table 13.5).

Fig. 13.7
A schematic of the oxidation mechanism of the C O. The O and T i are arranged randomly in a zigzag pattern, with O at the top and T i at the bottom. The pattern reacts with F e, and central T i replaces F e, which further reacts with A u, resulting in the A u F e O T i and T i A u O structures.

Mechanism of CO oxidation over Fe-doped TiO2 Adapted from ref [77] with permission from Wiley VCH GmbH

Full size image
Table 13.5 Comparison of activity of catalysts for CO oxidation
Full size table

Iron oxide-based catalysts tested for CO oxidation along with the temperature and conversion are given below as a table. Complete conversion of CO at lower temperatures is exhibited by catalysts with noble metals. Besides, the activity of the supported catalysts is in the order of Au > Pt > Pd. Particularly, among transition metal oxide catalysts, good activity was observed when iron was combined with Cu followed by Co. When iron oxide is used as the support, geometric and electronic effects of SMSI improve the catalytic activity.

13.4 Selective Catalytic Reduction (SCR) of NOx

Nitrogen oxides (NO, NO2, and their derivatives), in general, denoted as NOx is a gaseous pollutant that can cause photochemical smog, acid rain and ozone depletion leading to global warming and eutrophication problems. Major sources are automobiles and other industries and there are regulations for power plants and engines regarding emissions. According to the United States Environmental Protection Agency, the NOx emission of an engine should be within the limit of 0.02–0.10 g/bhp-hr. Amount of NOx and VOCs is responsible for the presence of hydroxyl radicals hence the oxidizing ability of the stratosphere. In urban areas where a high concentration of NOx is present, it controls radical production hence the oxidizing ability, and in rural areas with relatively low concentrations of NOx VOCs control the radical production. The reaction of NO with VOCs which is responsible for the photochemical smog is shown below (Fig. 13.8).

Fig. 13.8
A chemical reaction of N O and V O C. It includes the reactions of 2 N O carrying lone pairs of electrons, N O 2 carrying pairs of electrons, O + O 2, N O + O 3, and V O Cs + N O + O 2. A general global reaction reads, V O Cs + N O reacts in the presence of sunlight to give O 3 + H N O 3 + organics.

Reaction of NO with VOCs generating photochemical smog [93]

Full size image

Photochemical smog is brown in color and affects the throat, causes chest pain, the difficulty of breathing for humans, cracking of rubber and damages plant life too. Hence, reducing and mitigating NOx is important.

SCR of NO by NH3 is widely used for removing NO from the atmosphere. In SCR, NOx is converted to dinitrogen and water in the presence of a catalyst as well as a reducing agent. Commonly used reducing agents are anhydrous or aqueous ammonia or urea solution which is added to the stream of gas which flows through the catalyst. Commercially used catalytic systems for SCR reactions are V2O5–MoO3/TiO2 and V2O5–WO3/TiO2 [94, 95]. The commercial system has some drawbacks such as a reduction in the selectivity of N2 in the temperature range of 573–673 K, the need to reheat the stack gas, sulfur poisoning and the toxicity of vanadium pentoxide [96,97,98]. TMOs are investigated the most due to the easy gain and loss of d electrons, low cost and stability [99]. A few Fe-based catalysts that showed good conversion in the low-temperature range were Fe–MnOx and FexTiOy [100, 101]. Mn is explored extensively due to the variable oxidation states and redox stability hence the activity at the lowest temperature. But the drawback of Mn is with low selectivity of N2 and sensitivity toward SO2 in the gas flow [102,103,104]. Due to the environment-friendly nature and low cost, easy reducibility and mobility of active surface oxygen, iron oxides have also been explored nowadays either in combination with other transition metals or as support with other elements [100, 105]. The drawback associated with iron oxide is low surface acidity, low N2 selectivity above 623 K and low resistance to SO2 and H2O.

Mechanism of SCR with NH3

The possible mechanism for SCR of NO in presence of NH3 catalyzed by \({\text{Mn}}_{{2}} {\text{O}}_{3}^{ - }\) Doped Fe2O3 is shown in Fig. 13.9. Four types of species are indicated by DRIFTS spectra after the adsorption of NO on the catalyst, they are gaseous NO2, bidentate nitrates, linear nitrites and monodentate nitrites. Adsorbed NH4+ ions combine with \({\text{NO}}_{2}^{ - }\) to form NH4NO2. Fe and Mn are involved in a redox cycle to continue the reaction, while NH3 acts as the hydrogen source for the reduction. After each cycle, Fe3+ is regenerated by Mn4+. Mn2+ is again oxidized by oxygen to continue the cycle of reaction.

Fig. 13.9
A S C R mechanism diagram of N O in the presence of N H 3 at F e 2 O 3. In the redox cycle, F e 2 + N H 2 is reduced to F e 2 +, followed by F e 3 +, F e 3 + N H 2, and M n 3 + is reduced to M n 4 +, followed by F e 2 +. Similarly, N O 2 ion + N H 4 + is reduced to N H 4 N O 2 and N 2 + 2 H 2 O.

Adapted from ref [106, 107] with permission from Elsevier and American Chemical Society

Mechanism of SCR with NH3 in presence of \({\text{Mn}}_{{2}} {\text{O}}_{3}^{ - }\) doped Fe2O3 hexagonal microsheets.

Full size image

Different catalytic systems involving iron as a dopant or support, the temperature of SCR and the percentage conversion are given in Table 13.6. Fe–Mn system is investigated the most and it shows the maximum activity at the lowest possible temperature. Activity at the lowest temperature is exhibited by porous MnOx–FeOx nanoneedles (Sl no 3) which can be due to the uniform distribution of the ions, availability of redox sites, porous structure and strong acidic sites. Various catalysts were tried for the NH3-catalyzed SCR reaction. It has been observed that the modified catalyst exhibits improved activity over the commercial catalysts known and have good selectivity toward nitrogen.

Table 13.6 Comparison of activity of catalysts for SCR reaction
Full size table

13.5 Fenton Reaction

The Fenton process was developed in 1894 by a French scientist Henry J Fenton who discovered that at pH 2–3, tartaric acid can be oxidized by the Fe2+/H2O2 system. Fenton’s process comes under the category of Advanced Oxidation Processes (AOP). The importance of this process is that toxic organic pollutants cannot be easily removed by conventional wastewater treatment processes and the abundant availability of resources such as iron and H2O2. Depending on the number of phases involved in the reaction, it can be classified into a homogenous Fenton process where only a single phase is involved and a heterogenous Fenton process where dual phases are present. The classical reaction is a homogenous Fenton process with Fe2+ interacting with H2O2 in the solution state. In the presence of H2O2, which acts as an oxidizing agent, ferrous ions (Fe2+) are converted to ferric ions (Fe3+) (Eq. 13.1). This oxidation is the first step in Fenton’s reaction. This results in the formation of a hydroxide ion (OH−) and a hydroxyl radical (·HO) as byproducts. To continue the activation of H2O2, Fe2+ can be obtained via the reduction of Fe3+ by H2O2 (Eq. 13.2). The reduction of Fe3+ is very slow and generates hydroperoxyl radicals (HO2·) which has a weaker oxidation ability than ·HO. Two different oxygen free radicals are formed when hydrogen peroxide molecules undergo disproportionation (Eq. 13.3). The produced hydroxide ions and protons combine to form water (Eq. 13.4).

$${\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}}_{2} \to {\text{Fe}}^{3 + } + {\text{OH}}^{-} + \cdot {\text{HO}}$$
(13.1)
$${\text{Fe}}^{3 + } + {\text{H}}_{2} {\text{O}}_{2} \to {\text{HO}}_{2}^{ \cdot } + {\text{Fe}}^{2 + } + {\text{H}}^{ + }$$
(13.2)
$$2{\text{H}}_{2} {\text{O}}_{2} \to {\text{HO}}_{2}^{ \cdot } +^{ \cdot } {\text{HO}} + {\text{H}}_{2} {\text{O}}$$
(13.3)
$${\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}}_{2} + {\text{H}}^{ + } \to {\text{Fe}}^{3 + } +^{ \cdot } {\text{HO}} + {\text{H}}_{2} {\text{O}}$$
(13.4)

To improve the efficiency of the process, Fenton’s process is often coupled with electro/UV/sono such as to increase the amount of ·HO and the regeneration of Fe2+ to Fe3+ [120]. Even then there are some disadvantages associated with the process. A continuous supply of H2O2 is required which makes the process economically non-viable and the formation of solid ferric sludge needs to be removed periodically along with the loss of iron ions. Overall, the process becomes expensive [121,122,123]. Increased production of reactive oxygen species was observed in hybrid systems where two of the modified Fenton process were combined like in Photo-Electro-Fenton (PEF). Even though an enhancement in activity is observed, compared with the real situation of huge amounts of wastewater reduced chemical consumption cannot be equated to electricity loss. Considering the example of the electro-Fenton process even though the process efficiency is higher, Fe is getting consumed at the anode. Recovery and reuse of iron sludge in this context may be addressed by using acid treatments and ion exchange. Ferrites thus prepared were again employed in the process.

Introducing iron in the heterogeneous solid phase has proved to be safe, efficient and cost-effective. Homogenous and heterogeneous Fenton processes differ in the fact that in heterogenous Fenton, ·HO production occurs on the surface of the catalyst, and also the reactant molecules get adsorbed on the active sites and after the reaction, they get desorbed and active sites are free for the next cycle [124, 125].

$$X - {\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}}_{2} \to X - {\text{Fe}}^{3 + } + \cdot {\text{OH}} + {\text{OH}}^{ - }$$
(13.5)
$${\text{Fe}}^{3 + } + {\text{H}}_{2} {\text{O}}_{2 } \to X - {\text{Fe}}^{2 + } + \cdot {\text{O}}_{2} {\text{H}} + {\text{H}}^{ + }$$
(13.6)

Reaction rate increases with higher surface area and porosity as usual with general catalysts are observed here too [126].

Advantages of heterogenous Fenton process [127,128,129] (Fig. 13.10, Table 13.7).

Fig. 13.10
An illustration exhibits the heterogenous Fenton process. It includes iron nanomaterials, iron on supports, iron in the zerovalent metal state, and iron minerals and waste-derived catalysts.

Forms of iron catalysts employed for heterogenous Fenton process

Full size image
Table 13.7 Iron-based heterogeneous catalysts used for the degradation of various pollutants
Full size table

  • Operating under a wide temperature range

  • Zero formation of iron sludge hence reducing additional removal costs

  • Easy handling and safe storage

  • Easy recovery by sedimentation, filtration and magnetic separation

13.6 Conclusion

Mitigation of environmental pollutants by catalytic technology is one of the promising areas of research that can directly address the goals set by the UN and other international agreements. Particularly, the literature suggests that iron and iron oxide-based catalysts or supports have been extensively tested due to their intrinsic activity or promotional activity as support, high natural abundance, low cost and no toxicity. The chapter discussed iron oxide applied in four environmentally important reactions such as VOC oxidation, CO oxidation, SCR reaction of NOx, and Fenton reaction. After comparing the catalytic activity, it can be understood that Fe plays a significant part in improving the catalytic activity. Although the major challenges with respect to activity and selectivity remain to be addressed. It is however possible that economically viable large-scale catalytic processes are likely to contain iron in their composition.