Ceria as an Efficient Nanocatalyst for Organic Transformations

Abstract

Valuable chemicals, fuels and pharmaceuticals obtained by the transformation of raw materials have fascinated a lot of researchers in past few decades. However, to reduce problems related to these transformations different green, sustainable and economic techniques have been developed to carry out such organic transformations. Development of nanostructured catalysts has been preferred to accomplish heterogeneous catalytic organic transformations because of greater number of surface-active sites for catalytic processes, high catalyst recovery rate, environment friendly nature and their ease of synthesis. Besides the advances in nanocatalysis, certain challenges including not well-defined morphologies due to loss of control over it and loss of catalytic activity during operation need to be addressed. Ceria is actively investigated in field of catalysis. As a ubiquitous component in catalytic system, its inception is like an irreplaceable component in organic transformations. In this chapter, we appropriately reported various fabricating approaches to synthesize Cerium and CeO2-rooted nanoparticles and cerium nanoparticles supported on various support materials, accompanied with multimetallic schemes that show notable contribution to the field of catalysis. This comprehensive chapter will provide an improved understanding of nanostructured CeO2 and will provide deeper insight in the catalysis of Ce-based nanostructured materials and further widen their ambit of applications.

1. Introduction

Deterioration of environment by excessive dependency on fossil fuel reservoirs encounters an array of challenges for our ecological system. Presently, some other practices such as producing harmful pharmaceuticals, hazardous by-products in manufacturing, etc., results in the severe environmental problems [1]. To encounter these challenges catalysts are needed, but their multiplicity and complexity demand a breakthrough in the approaches in which these catalysts are designed and used [2]. In chemical reactions, catalysts act as unsung heroes which have marked impact on human society [3]. By approaching recent catalytic constituents, moving beyond simple modifications and making efforts to understand the elementary principles, it has been made possible to synthesize and choose suitable catalysts for a provided set of reactants to obtain desired products [4]. These challenges can be resolved by utilizing various techniques like computational modeling, atomic resolution microscopy and atomic scale measurements [5]. With the enhancements in chemical sciences, the property which is named as catalysis adorns the chemical reactions in appropriate aspects. Discussions regarding differentiation between homogeneous and heterogeneous catalysts have dominated research in the recent years [6, 7, 8]. Therefore, in this chapter we explore novel catalytic systems, which fulfill the specifications of both homogeneous and heterogeneous catalysts like higher activity and better reproducibility.

At this point, nano catalysis comes into the picture and dramatically shaped queries on combining the properties of both the catalytic systems in recent years. As considering nanodimensions, nanocatalysts possess high surface area which provide a better surface to reactants and this property resembles to homogeneous catalysis. The catalyst can act like heterogeneous due to the insolubility in the reaction solvent and hence carried out an adequate separation from the reaction mixture. In view of these significances, nanocatalysts has propelled to the forefront in investigations in recent years. Much attention has been drawn to explore a lot in the field of nanocatalysis and synthesis of nanomaterials for organic transformations. This encourages researchers to develop a simple, efficient, mild, environmentally benign, ligand free, heterogeneous, and reusable nanocatalyst for organic transformations [6]. Prior research has thoroughly investigated nanoscale catalysts in several reactions [9]. It has been recently studied that due to the high surface area and high activity, nanoparticles (NPs) find much importance in catalysis.

Rare earth metals and their complexes exhibit rich variety of solid-state properties and characteristic behavior which make them interesting subjects for catalyzing many organic transformations. There are 14 lanthanides included in the rare earths which include yttrium and scandium also. Among all the rare earths, several researchers corroborate cerium in the field of catalysis. The abundance of cerium in the upper crust is 64 ppm which is relatively higher than copper, tin and any other rare earth elements. Nanoceria has been regarded as the potential catalyst by employing under ligand free conditions in the form of metal, metal oxides for various organic reactions. Cerium due to its outstanding catalytic efficiency and enough abundance makes it useable for a variety of reactions which makes it the selective element for validating the catalytic conversion of the exhaust system of automobiles in automotive industry [10]. Several studies carried out have revealed that due to the high surface area and reactive morphologies of ceria-based nanomaterials can be effectively used as catalysts for organic transformation reactions such as oxidation, reduction, hydrogenation, coupling reactions and many more [11, 12, 13] as represented in Figure 1. The rare earth elements occur in many minerals inside the earth’s crust with quite higher abundance. Being actively investigated, cerium oxide is most imperative and well-known among light rare earth oxides, especially its use in catalysis [14]. Jons Jakob Berzelius and Wilhelm Hisinger were first who discovered elemental cerium in 1803. The exact values of crustal abundance of cerium (average concentration in the earth’s crust) are still contentious. For instance, Kleber and Love stated 46 ppm for the cerium crustal abundance in 1963, while Jackson and Christiansen reported the value of 70 ppm in 1993; McGill reported a wide range from 20 to 46 ppm in 1997. Lide reported the value of 66.5 ppm in 1997, representing the intermediate of the various reported values, was commonly accepted for present discussions [15]. Light rare-earth metals occur mainly in the minerals of fluorocarbonate form called bastnasite and phosphate form called monazite. The elemental distribution varies in both minerals and locations. In bastnasite, cerium content is 49.1% with respect to all the rare earth content from Mountain Pass, California, U.S.; while the content is 50.0% located in Bayan Obo, Inner Mongolia, China. The cerium content is 45.8% in monazite minerals at North Staradbroke, Australia and 47% in East Coast Brazil [16]. In the lanthanide group, Cerium is an element with an atomic number of 58. Cerium often shows +3 oxidation state, where it acts as typical rare earth and it also exceptionally has a stable +4 oxidation state.

Figure 1.

Nanostructured ceria catalyzed organic transformations.

Metallic cerium, Ce(OH)₃ and other oxosalts of cerium like oxalate, nitrate allowed to heat in air or oxygen for the production of its oxide form, i.e., cerium (IV) oxide (CeO₂) [17]. CeO₂ is a well-known cerium compound which is pale yellow/white powder synthesized by cerium oxosalts calcination and generally used as a catalyst or as three-way catalysts (TWCs). The ceria with fluorite structure has tetrahedral holes with oxide ions residing within it, which are formed by the face-centered cubic array of cerium ions, and vacancies are at the octahedral holes. Each cerium ion equivalently surrounded by eight oxygen anions and four cerium cations are tetrahedrally coordinated to these anions. The lattice constant of each unit cell is 5.411 Å. These eight coordination sites are occupied by a cerium cation. It is determined that its structure possess large vacant octahedral holes which intensify its further applications. Ce(III) trioxide (Ce₂O₃) also occurs under ambient conditions [18]. Ce₂O₃ is very unstable against oxidation and as a pressure of 10–40 atm of oxygen is applied, it gets oxidized and then CeO₂ begins to form. Characterization techniques such as X-ray diffraction studied at different temperatures reveals that Cerium oxide have also been observed in other phases. For example, a disordered non-stoichiometric fluorite-related phase of α-phase cerium oxide, is stable above 685°C (CeOx, 1.714 < x < 2) [19, 20]. A β-phase formed at room temperature with a rhombohedral structure (CeOx, 1.805 < x < 1.812) remains stable until 400°C [21, 22].

Over the last years, cerium’s price has continually dropped as compared to the other rare earth elements. Although fascinating, the cost of cerium oxide cost is going below the price of lanthanum oxide and has observed a sheer incline in its application. CeO₂ has been focused for a plethora of studies both in industry and in academia [23].

2014 and 2015 are the most profused years which have recorded about 2300 publications related to ceria materials. It must be because 1301 publications on catalytic applications in 2015 mark a booming interest in ceria catalysis for the first time become 50% of the total [24, 25, 26, 27, 28, 29, 30]. Though, it is worthy to reveal the number of studies on the utilization of ceria in new scientific areas such as biology and pharmaceuticals. Ceria has been used as a support for stem cells cultured in vitro [31] or as a vehicle for intracellular drug delivery [32]. One more noteworthy study reveals that ceria nanoparticles could treat ischemia as well as reduce ischemic brain damage by interruption of the blood-brain barrier after ischemia [33]. Even more fascinatingly, the thermal water and CO₂ splitting by the employment of CeO₂ in solar reactors for fuel generations has been evolving as a novel and exciting investigation topic while accumulation of ceria-based compounds in photocatalysis which merits special mention is another rising field [34, 35].

2. Structure and morphology affecting the catalytic properties of ceria

Structure and morphology play a key role in determining the application of the material by influencing its surface properties. Extensive studies have been conducted to unveil different applications which depend on morphology of nanocrystals. The crystal plane is one of the most common morphological parameters being considered in cerium oxide crystals. The surface of materials is important in various physical and chemical processes that involve the reaction on inorganic oxides such as catalysis and crystallization [29]. CeO₂ in cubic fluorite structure possesses three low-index planes: (100), (110) and (111) as shown in Figure 2. The (100) planes contain scattered charged planes which establish a dipole moment perpendicular to the surfaces which are not stable. However, they could be sustained by charge-counteracting species for example, ligands or surfactants or by defects present. The (110) surfaces are charge neutral which consists of anions and cations in stoichiometric proportions in each plane, which exhibit negligible dipole moment perpendicular to the surface. The (111) surfaces also results no dipole moment perpendicular to the surface. Unlike the (110) planes, (111) surfaces consist of a neutral three-plane replicating subdivision ended with a single anion plane. The (100) facet exhibits 2.0 eV of surface energy, the highest one among these three low-index facets and the (111) plane is calculated as the most stable facet irrespective of different potentials used in simulation, both before and after relaxation according to the work done by Vyas. While (110) plane is the other highly stable facet, comprising a surface energy of 1.5 eV from Butler potential calculation [36]. Hence, different shapes of nanostructured ceria particles have different crystal surfaces and plane properties, which further enhances their performances in different systems including catalysis by affecting the interactions between the ceria surface and adsorbed molecules. Fronzi et al. stated similar results on the three low-index surfaces of CeO₂ as they performed density functional theory (DFT) investigations. The stoichiometric (111) surface is the most stable surface structure with a surface free energy of 0.060 eV under oxygen rich conditions calculated by “ab initio atomistic thermodynamics.” The subsurface oxygen vacancies of (111) surface has been found to be the most stable one with a surface free energy of −0.001 eV/Ų in a reducing environment. While in a highly reducing environment, a Ce-terminated (111) surface is the most stable one. CeO₂ (110) surface with surface oxygen vacancies has 0.012 eV/Ų surface free energy, which is 0.006 eV higher than CeO₂ (111) surface with same oxygen vacancies. The surface free energies of CeO₂ (100) surface having the same type and amount of surface oxygen vacancies terminated with oxygen and cerium are 0.575 and 0.016 eV/Ų respectively, which are both larger than those of CeO₂ (111) and CeO₂ (110) surface [37]. Sayle and coworkers reported the surface energies of 11.577 and 2.475 J/m² for (331) planes before and after relaxation through applying energy minimization code MIDAS [38]. Other crystal planes of cerium oxides, such as (200), (220), (331) planes, etc., have also been investigated and characterized in both experimental and simulation studies [10, 13]. For example, {220} facets were found in a slightly truncated cerium oxide nanocubes with predominate (100) facets synthesized by Kaneko et al. [39, 40]. Moreover, the feasibility of tailoring the metal oxide morphology have upgraded due to recent advancements in materials chemistry, and the required crystal planes of the cerium oxide materials can be favorably exposed through precise control of the growth kinetics. However, these three low-index planes are the most commonly observed and the most studied facets on synthesized cerium oxide structures [30, 41, 42]. It is also reported that perception about the nanocatalysis must be explained by intrinsic properties of nanoparticles which include (Figure 3) (i) quantities such as bond length and binding energy; (ii) quantities related to cohesive energy per discrete atom and the activation energy for atomic dislocation and diffusion, etc.; (iii) properties such as the Hamiltonian which demonstrate band structure, band gap and (iv) properties from the combined effect of binding energy density and atomic cohesive energy like surface area, surface strength, etc. [43].

Figure 2.

Diagrammatic representation of CeO₂ facets (100), (110), and (111). Cerium and oxygen ions are represented by red and gray spheres. Reprinted with permission from Ref. [42]. Copyright 2017 American Chemical Society.

Figure 3.

Intrinsic properties affecting catalytic activity of nanomaterials.

2.1. Mechanism of reactivity and catalytic activity of ceria

In the growing field of catalysis, ceria attributed oxygen storage capacity (OSC) as its fortune. CeO₂ shows multi valence nature which give large number of oxygen vacancies to produce under stoichiometric CeO₂-x at reducing temperatures, which can be oxidized back to CeO₂ in an atmosphere containing oxygen. As cerium (III) (Ce³⁺) switches to cerium (IV) (Ce⁴⁺) states, it results in high oxygen mobility in the ceria lattice that in turn leads to a strong catalytic potential and it happens without any structural modification of the fluorite ceria lattice. The change in energy can cause largely a surface effect to heterogeneous catalyst [44, 45, 46].

Thus, scientists have been looking for maximizing the formation of oxygen vacancies of ceria-based catalysts to improve their activity, which needs high temperatures and a reducing atmosphere. Yan’s group had made the first observation during the study of preparation of ceria nanostructures, which depict that as compared to octahedral ceria nanoparticles, nanocubes and nanorods had a higher capacity to store and release oxygen at high temperature. Recently, the precise fabrication of functional nanostructured ceria is turn out a routine. Though, some other materials also demonstrate very discrete catalytic activity due to defect sites effect and exposed crystal facet, even from materials which have similar structures [16, 47, 48, 49, 50, 51]. The values which are reported in Table 1 display the exposure of {100} and {110} planes in nano-size ceria accompanying the improved oxygen storage capacity (OSC) and show the following order of OSC nanocubes > nanorods ≫ nanopolyhedra [41]. An appropriate comparison is done with determined hypothetical surface area-normalized OSC which is calculated on more reducible surfaces and specify that OSC is not only defined to the surface, but it also takes place in the bulk [52]. On introduction of defects into the lattice, it is investigated that through controlling synthetic and postproduction parameters, preferred crystal orientation is précised. It is also noticed that the reactivity of the surface of crystal is greatly affected by variable lattice defects critically.

	OSC^a (μmol O/g)	OSC/B.E. T (μmol O/m^2)	Calcd OSCb (μmol O/m^2)
Nanopolyhedra	318	5.1	6.2
Nanorods	554	9.1	4.9
Nanocubes	353	10.6	5.7

Table 1.

Oxygen storage capacity of variable nanostructures.

^a

CO-OSC measured at 400°C.

^b

Calculated according to the theoretical OSC of exposed surfaces. See Ref. [41] for details.

Reprinted with permission from Ref. [41]. Copyright 2005 American Chemical Society.

Vacancies included in the lattice defects are (a) oxygen vacancy defects, (b) self-interstitials, (c) interstitial impurity atoms and (d) edge dislocations [53]. In the fluorite lattice of ceria, the degree of oxygen mobility accredited to its size, dispersion, and value of oxygen vacancy defects (OVD) [54, 55, 56, 57]. The empty 4f states of cerium located electrons by surrounding a vacancy in the ceria support lattice establishes defect sites [17, 58, 59, 60, 61]. These defects are shown to be mobile with high oxygen mobility and arise around cerium (III) ions only. On introducing subsurface vacancies into the lattice, the mobility of the vacancies, and therefore the defects, is decreased significantly. The formation of vacancy clusters is in the three or six surrounding cerium ions of the material’s surface. An oxygen vacancy defect tends to form under low partial pressure of oxygen [17]. After approaching a favorable oxygen by another oxygen, a bond is formed, and from the surface of the crystal the oxygen molecule can diffuse away. Respectively, the oxygen molecule that is obtained, further diffuses away from the surface and two electrons are left back to be distributed between three cerium atoms. Due to this, cerium atoms undergo partial reduction to a valency between the 3+ and 4+ states. It occurs in a manner to leave behind triads of vacancies which are surrounded by nine cerium atoms sharing eight electrons [62, 63, 64, 65]. It is widely proposed that change in the adsorption energy regarding carbon monoxide and oxygen can cause the change in activity of a surface with lattice strain. It is noteworthy that the similar researches reported a lesser increment in the adsorption energy related to carbon dioxide [66, 67]. The adsorption energy for oxygen is about five times superior than for carbon monoxide. Furthermore, the activation barriers respecting to dissociation of oxygen and formation of carbon dioxide are greatly dropped. However, the ease of formation of the oxygen vacancies facilitate the reaction. A molecule of carbon monoxide adsorbs on the surface of the ceria and readily reacts with oxygen existing on its surface and further diffuses away in the form of carbon dioxide leaving an oxygen vacancy. This oxygen vacancy results in a weakened bond between the oxygen atoms, as it allows an adsorbed oxygen molecule to react with the surface. Across the surface of the catalyst, a carbon monoxide diffuses until it encounters the excess oxygen and diffuses away from the surface [68, 69, 70, 71].

2.2. Nanoarchitectured ceria and its influence in the behavior to supported metals

The vast expansion of the usage of the nanomaterials offer is just incredible. Nanotechnology revolution has revolutionized the research arena as matter of the fact that it creates the vast possibilities to fabricate the materials with nanodimensions. Catalysis, fuels and microelectronics are different fields of applications where nanostructured cerium oxides grow rapidly and reflecting their importance in enhancing the performances of those systems. Variable morphologies of ceria nanoconstructs have been explored in these applications, for instances nanocubes, nanorods, octahedron polyhedron, tube and many more. Ceria nanostructures with various shapes possess the different crystal planes and surface morphologies, which influences the interactions between the ceria surface and adsorbed molecules, and hence changes the performances in different systems. Zhou et al. described many strategies for synthesis of well-controlled morphologies of nanostructured ceria. Now, Ce-based materials with controlled morphologies which exhibit zero-, one-, two-, and three-dimensional structures are possibly synthesized. (Figure 4). The categories defined on basis of number of dimensions which cannot be restricted to the nano-range (<100 nm).

Figure 4.

1D, 2D and 3D nanostructures. Reprinted with permission from Ref. [2]. Copyright 2010 Springer Nature.

Nanostructured ceria with Zero-dimensional (0D) possessing isotropic cubic phase of the fluorite structure can be observed distinctively. According to results, it presented a lack of fortunate growth direction of seeding crystals. Hence, (0D) nanostructures have most straightforward synthesis. Mono-dimensional (1D) CeO₂ nanoparticles possess the different properties due to which they have been explored more than the 2D and 3D architectures and a variety of synthesis procedures were proposed. 1D hexagonal Ce₂O nano-rods (NRs) synthesized by template-free electrochemical growth method on a Ti substrate which mainly exposes the {110} planes and displayed outstanding photocatalytic activity in hydrogen evolution, with H₂ yield reaching 741 mmol g⁻¹ [26].

The preparation of 2D and 3D architectures are drawing significant attention and they also evolving as good alternatives in various catalytic and energy applications. For the construction of a spongy mesoporous CeO₂ microspheres an analogous concept was assumed in which in-situ formation of the removable template by graft polymerization reaction between acrylamide and glucose takes place [72]. Ceria nanocubes synthesis is important because their possession of high surface energies usually exhibit specific activities due to the unsaturated coordination atoms, atomic steps and ledges [73, 74, 75]. Numerous fabrication methods have been reported for spherical nanostructures [76, 77, 78, 79]. Planes in ceria octahedron have gained much attention as these planes are exposed on their surfaces [80]. Nanostructured ceria with different morphologies, such as nanotubes, spindles, nanosheets, etc. have been synthesized [45, 80, 81]. Due to their enormously developed activities ceria nanorods have increased wide-ranging interest than those of ceria with other shapes in many different reactions, such as CO oxidation, NO reductions and 1,2-dichloroethane and ethyl acetate oxidation [57, 82, 83]. By means of the most stable (111) planes on the surface, ceria octahedra demonstrated the least catalytic activity being studied when compared to the activities of nanocubes, nanorods and other shapes in many reactions, such as CO oxidation and ethyl acetate oxidation [80, 83], Notable progress has been made to achieve these ceria nanomaterials. However, synthesizing these morphologically different nanostructures with well-controlled size and homogeneity is still difficult due to their uncommon shapes. For instance, it is difficult to prepare nanosheets due to their exceptionally small thickness and possible quantum size effects [84].

In Prospect, the synthesis of ceria NPs requires the interaction with metal nanoparticles that act as the “active sites” for catalysis. Schelter and co-workers reported a ligand to vary the stability of the Ce (III)/Ce (IV) redox couple, in their synthesized 1,3-bis [(20 tertbutyl) hydroxyamino phenyl]-benzene Ce complex, [85]. Recently, a hydrothermal process has been used for the synthesis of Au@CeO₂, presenting core-shell systems grounded on other precious metal core-shell [86], while Ag@CeO₂ was also synthesized by reverse micelle/redox reaction [86, 87, 88, 89, 90]. Among different characterization, an exclusive strength of the STM technique is the ability to enquire the atomic structure of surfaces, down to the level of distinct defects and adsorbates. Figure 5 shows one such image, obtained on the surface of a CeO₂ (111)/Pt (111) system [91].

Figure 5.

STM image of the CeO₂ (111)/Pt (111). Reprinted with permission from Ref. [91]. Copyright 2010 American Chemical Society.

Catalytic characteristics of supported metal nanoparticles depend on the role of the support as well as on the composition, shape, particle size, and chemical state too. The catalytic reactivity is directly related to the atomic interaction within support and metal nanoparticles which is termed as metal-support interaction which has attained significant attention nowadays (Figure 6). Due to the possession of unique properties by the ceria by virtue of which it makes oxygen species readily available to the metal site which make its outstanding applications in large number of catalytic reactions. This way, noble metals on ceria are activated for various oxidation reactions at low temperatures [92].

Figure 6.

Metal-oxide configurations used in studies with ceria model catalysts. Reprinted with permission from Ref. [92]. Copyright 2017 Royal Society of Chemistry.

A pivotal role is played in the activity of catalysts by the nature of their support. Vayssilov et al. has studied the origin of interactions proposed between the various support effects like the active (metal) phase and support which include interaction of electrons among both components [93], destabilization or stabilization of particle sizes or shapes [94], surface transport of adsorbates through the boundary (spillover, reverse spillover capture zone effects); [95] and the stabilization/destabilization of oxidized active phases by the support or strong “metal-support interactions” relating movement of partially reduced oxides onto the active phase [96, 97, 98]. The metal oxide intervenes in the catalytic process as well as an inert support [99].

Pure ceria, CeO₂, undergo degradation with time at elevated temperatures which minimize its performance due to reduction in its surface area as well as oxygen storage capacity (OSC), also it has been presented that pure ceria accommodate “active” weakly bound oxygen species, which relates bulk rather than to the surface by using steady-state CO oxidation kinetics and/or temperature-programmed desorption (TPD) [100, 101, 102]. Thus, development of CeO₂ based nanocatalysts for chosen activities is surely done with a keen understanding about metal-ceria support interaction in supported metal catalysts (Figure 7). E. Mamontov used pulsed neutron diffraction to investigate the nature of these “active” oxygen species in pure ceria. The study of oxygen position in oxides by neutron diffraction demonstrates a comparable scattering contrast of oxygen and metal ions. In the real space the oxygen defects in CeO₂ examined by both pulsed neutron diffraction data and atomic pair-distribution function (PDF) analysis whereas in the reciprocal space, it is analyzed by the Rietveld refinement [103].

Zirconia, ZrO₂, has been actively investigated in many studies and have been characterized. The enhanced OSC of ceria-zirconia related to ceria as well as known to improve partial degradation of ceria at high temperatures. It is also probable that the necessary oxygen mobility which is essential for the functioning of CeO₂ as a catalytic support is provided by the interstitial oxygen ions which act as “active” sites [104, 105, 106, 107, 108, 109, 110, 111].

Figure 7.

Real (STM, LEEM) and reciprocal space (LEED) of ceria structures on a range of metals. Reprinted with permission from Ref. [112]. Copyright 2016 American Chemical Society.

CeO₂ impart high oxygen storage and release capacity and it is predicted as the finest supporting material for catalysis at Au NPs following other properties such as facile oxygen vacancy formation, and narrow Ce f-band. Au gets oxidized once in contact with CeO₂. The atomic and electronic interaction between reduced CeO₂ and supported Au NPs is highly contributed by the electrons located on the occupied 4f-orbital of Ce³⁺ ions. Ceria particles were also deposited on the surfaces of Au, Pt, Re, Rh, Ru, Cu to produce inverse oxide/metal catalysts and these supports can cause dramatic effect on the structure of ceria islands in an inverse catalyst [112, 113]. The electron transfer is induced from metal to the support because of the presence of platinum over ceria nanoparticles leading to the formation of a small fraction of Ce³⁺ cations. Among transition metal core-Pt shell nanoparticles, altering Au NPs with alloying elements would be an interesting strategy for lowering CO adsorption energy, as well as deducing and enhancing saturated CO and O₂ concentration, correspondingly. Vayssilov et al. studied that a crystalline atomic arrangement exhibited by the interaction of an illustrative metal cluster Pt8 with two group of model ceria nanostructures [114]. A Pt8 cluster was selected as a model which supported on a stoichiometric Ce₄₀O₈₀ nanoparticle. The metal group relates five Pt atoms with ceria particle found as most stable structure for Pt8/Ce₄₀O₈₀. Liu and colleagues [115] prophesied adsorption and dissociation of oxygen and transport processes on the two most stable Ag (111) and Ag (110) surfaces and reveals a binding energy of CeO₂ catalysts and on a monolayer silver supported by CeO₂ (111) surfaces with or without oxygen vacancies by DFT with PAW method. The computed energies of these reactions display that the process of oxygen reduction and the combination of the dissociated oxygen ions in the oxide electrolyte prefer taking place in the triple phase boundaries (TPB) region with oxygen vacancies [116].

2.3. Synthesis and characterization of ceria nanoparticles

This segment of the chapter encompasses the detailed outline of various synthesis techniques and the conditions applied for the reaction which influences the final product. According to the applications in various fields, the synthesis of ceria nanoparticles with desired morphology is very important; therefore, thorough investigations were done by researchers to investigate several approaches. The studies on Ceria based nanoparticles demonstrated that conditional to the synthesis methodology, wide variations may occur in shape, size, crystal structure, and properties of nanostructures, as well as the physical and chemical conditions employed during the reaction process. Current literature revealed many chemical methods, such as, hydrothermal method, co-precipitation method, a micro emulsion mediated approach, and other methods like sol-gel synthesis have been employed to synthesize cerium based nanostructured materials.

Hydrothermal method refers to the oxide synthesis and crystal growth in aqueous solutions under high temperature and pressure using a sealed heated vessel which is known as autoclave. It is well-established method for the laboratory and industrial scale synthesis of nanoceria materials. Two big advantages of this method are that: the reaction temperature is below the melting point of reactants, and the operational parameters such as reaction temperature, duration, autoclave types can be easily tuned to modify the reactivity of synthesized inorganic solids [117]. A facile hydrothermal method for the synthesis of ceria nanocubes with six {100} facets using oleic acid as the surfactant was developed by Wang and co-workers. The as-prepared ceria was single crystalline, confirmed by uniform crystal lattice fringes. The sizes were well controlled with side lengths from 9 to 17 nm [92]. A single-step hydrothermal method is utilized in the fabrication of uniform CeO₂ nanoparticles with diameter approving nano-size, i.e., 13–17 nm and further, hexamethylenetetramine is added for the formation of (220)-dominated surface structure.

Co-precipitation synthetic method is another extensively used approach for preparing nanomaterial. The strategy is best choice for commercial synthesis of CeO₂ and due to very low solubility of ceria, it attains great advantage. This method is simple and rapid preparation process which makes it easy to synthesize controllable particle size and flexible in altering overall homogeneity of the particle with its surface state [118]. The cerium precursors are generally inorganic cerium salt, such as Ce(NO₃)₃, CeCl₃, (NH₄)₂Ce (NO₃)₆, and the precipitating agents are usually NaOH, NH₄OH, hydrazine and oxalic acid [119, 120, 121, 122]. Abimanyu and coworkers performed co-precipitation method and applied ionic liquid as a template to prepare magnesium and cerium mixed oxides. To overcome the difficulty of controlling particle size, template-assisted co-precipitation, carbonate co-precipitation, redox co-precipitation, etc. have been presented in conventional co-precipitation method [118].

Sol-gel method is highly suitable for the fabrication of metal oxides (Figure 8). This method is widely used in ceramics industry and materials science for producing solid materials such as ceramic fibers and dense films. It is easy to accomplish and does not need any special conditions and equipment [117]. The process involves conversion of metal alkoxide/chloride solution into a colloidal suspension (sol) and gelation of the sol to form discrete particles or network polymers in a continuous liquid phase (gel) [123]. Gnanam et al. successfully prepared nanocrystalline cubic fluorite/bixbyite CeO₂ or α-Mn₂O₃ via simple sol-gel method using cerium (III) chloride/manganese (II) chloride as the precursor by using methanol as a solvent calcined at 400°C [124].

Figure 8.

Diagrammatic illustration of different stages of sol-gel method. Reprinted with permission from Ref. [118].

The reverse micelle technique offers the greatest control over size and morphology. It is a wet chemical method in which pools of water are enclosed by surfactant molecules in an excess volume of oil. During synthesis procedure, surfactant molecules retain particles separated and confine particle growth this keeps control on size and shape of particles. The root for the technique is the use of a surfactant to stabilize variable aqueous droplet sizes in hydrocarbon medium. Metal salt precursors are transformed by a reactant from the hydrocarbon phase and are contained in the aqueous portion. Mutually, the structure of the surfactant and the steric size can produce metals having a wide range of grain sizes. Masui et al. [125] synthesized ceria nanoparticles by using reverse micelles and reported fluctuation of bandgap values from 3.38 to 3.44. Ganguli et al. fabricated monophasic nanoshaped oxides by performing similar versatile methodology [126, 127].

The worth of morphology-activity association is clearly recognized with the implementation of different examples as breakthrough. With the advanced characterization techniques included SEM, HR-TEM, STM, uncountable studies investigated that morphology/exposed facet combination still contain some uncertainties and, so, on the mechanism of crystal growth. Some focus is also bounded by surface reactivity analysis or through TEM. The elucidation of this area is must to simplify instrumentation acquaintance [128, 129]. The powder XRD patterns of the CeO₂ nanorods, nanocubes and nanopolyhedra are of pure cubic phase shows its fluorite structure with lattice constants of 5.414(3), 5.436(3), and 5.405(3) Å, respectively (Figure 9a). The enlargement of the reflections indicated their nanocrystalline nature, recognized to the polyhedron and rods distinctly. The sharper reflections for cubes implied their larger sizes as compared with the former two samples. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) techniques investigate the oxidation state of cerium ions in ceria nanoparticles. The remark of the existence of the Ce³⁺/Ce⁴⁺ shifts in150 mm active region leads to the inference that the lateral electron transport and surface reaction kinetics on the thin ceria electrodes are co-limiting processes. The XPS spectrum of the CeO₂ nanorods is shown in Figure 9b. It illustrates six consistent Ce 3d binding energy (BE) peaks for the rods with the former report on Ce⁴⁺, signifying +4 was the main valence of rods in cerium [41]. The surface termination of oxide-based nanoparticles can be easily determined by the transmission electron microscopy (TEM) [130, 131]. The specific surfaces of catalytic CeO₂ nanostructures during a reversible beam induced redox reaction examined by combination of direct aberration corrected TEM and computational exit wavefunction restoration at ambient temperature.

Figure 9.

(a) XRD patterns of CeO₂ nanorods, nanocubes and nanopolyhedra and (b) XPS wide spectrum of the CeO₂ nanorods. Reprinted with permission from Ref. [41]. Copyright 2005 American Chemical Society.

Mesoporous ceria being versatile attracted researchers as catalysts and catalyst-support which possess increased dispersion of active secondary components and offer high surface area (Figure 10). Additionally, an issue which is mandatory to discuss is that as surfactant is removed during synthesis procedure, it shows its poor thermal stability at elevated temperatures precepted to be caused by collapsing of structure. So, mesoporous CeO₂ has been developed by a novel hydrothermal method for high performance catalysts with excellent thermal stability [72, 132, 133, 134, 135]. Hojo et al. analyze cationic reconstruction by using in-situ phase contrast HR-TEM with spherical aberration correction [136]. STEM, EELS, and theoretical calculations were performed to inspect the atomic structure of grain boundary which is selected as a model grain boundary in thin films of CeO₂ [38, 137, 138].

Figure 10.

Illustrative SEM images (a and b) and TEM images (c and d) of the flowerlike CeO₂ microspheres. Reprinted with permission from Ref. [72]. Copyright 2006 American Chemical Society.

2.4. Cerium oxide catalyzed organic transformations

Organic synthesis occupied one of the most protruding places in the field of chemistry research. Additionally, the space of organocatalytic reactions is well-reviewed and widely examined. Further, initiating with certain reports to describe the organocatalytic applications of ceria nanostructures. Investigations on CeO₂ as catalytic support or recently as catalyst for conversions in organic reactions are liberated in accumulated manner. Although, the versatile behavior of this material is selectively observed in various catalytic applications. By Mars-van Krevelen mechanism, computer stimulation techniques were performed to predict the higher reactivity of ceria {110} and {100} surfaces towards carbon monoxide oxidation, which stated that CO first interacts with surface ceria oxygen and produce CO₂ by leaving an oxygen vacancy which is then filled with gas phase oxygen [139, 140]. The CO oxidation frequency turnover is higher on {110} as compare to {100} and {111} surfaces, presented as the opposite order of oxygen vacancy formation energy. CeO₂–ZnO composite catalyst utilized in hydrogen transfer reaction by Mishra et al. for cyclohexanone with isopropanol and it showed 51.3 mol% conversion of cyclohexanone [82]. Acetalization of cyclohexanone with methanol also reported by Rose et al. using different transition metals [141]. Tamizhdurai synthesized CeO₂ and inspect it with various spectroscopic and analytical techniques. Afterwards, its oxidation effect was investigated on benzyl alcohol which reveals better conversion and selectivity. The catalytic oxidation properties of ceria closely tied with its redox and oxygen storage behavior, and CO oxidation can work as a model reaction to probe the redox properties of CeO₂ [142] (Scheme 1).

Figure 11.

(A) CO oxidation over ceria rods, cubes, and octahedra. (B) Comparison of CO turns over frequency oversurface planes (110), (100) and (111). Reprinted with permission from Ref. [143]. Copyright 2011 Elsevier.

Zhou et al. [143] explored oxidation of carbon monoxide over ceria nanostructures in their study where they compared nanorods and irregular nanoparticles of same surface area; the former attributed exposed planes of {100} and {110} surfaces with higher proportion has higher activity. This study initiated the investigation of CO oxidation with nanoparticles, and several investigations were followed which clearly establish the correlation between ceria shapes and CO oxidation as shown in Figure 11 [143, 144, 145, 146, 147, 148, 149, 150]. CeO₂ nanospheres fabricated sonochemically in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim] [Tf2N] show the best presentation for low-temperature CO oxidation [151].

Scheme 1.

Selective oxidation of benzyl alcohol to benzaldehyde. Reprinted with permission from Ref. [142]. Copyright 2017 Scientific Reports.

CeO₂ nanoparticles explored by Deori K. for the para-xylene oxidation to terephthalic acid as a heterogenous catalyst (Scheme 2). The synthesis procedure is environmentally friendly, and water was used as a solvent during catalysis reaction. The ceria nanostructures which were synthesized acquire 15 nm sized particles and high surface area of 268 m² g⁻¹ [152].

Scheme 2.

Oxidation process for para-xylene to terephthalic acid. Reprinted with permission from Ref. [152]. Copyright 2017 Royal Society of Chemistry.

Scheme 3.

Representation of oxidation process of para-chlorobenzyl alcohol/toluene to benzaldehyde by ceria nanocubes. Reprinted with permission from Ref. [153]. Copyright 2017 Royal Society of Chemistry.

The advancement in catalytic performance of the cube shaped CeO₂ nanoparticles displayed by the conversion of benzyl alcohol (BA) and para-chlorobenzyl alcohol (PCBA) to their respective aldehydes (>99%) (Schemes 3). Accompanying, in toluene (PhCH₃) oxidation, this CeO₂ nanocube catalyst was found to be very effective, as well as being more effective than the nanorods. Besides, reusable property of CeO₂ nanocatalyst also proposed for several cycles which display obtainment of the desired products without any deterioration in selectivity and activity in all cases [153].

CeO₂ is actively used in hydrogenation reactions as a promoter or carrier of noble metal nanomaterials for many years [154, 155]. As compared to oxidation reaction on CeO₂, hydrogenation owing to the specific role of adjacent oxygen on stabilizing hydroxyl intermediates is favored over low-vacancy surfaces whereas reverse effect applied for hydrogenation reactions, where nanoparticles are more active than nanocubes [156, 157]. Hydrogen activation on CeO₂ is often regarded as the limiting step of the reaction, even for other functional groups which includes substituted nitroarenes [28, 158]. As already discussed, partial hydrogenation over CeO₂, they have also been employed for the hydrogenation of olefins and carbonyl bonds. For example, a good yield of 1-butene can be obtained by reduction of 1,3-butadiene by Pd/CeO₂ catalyst supported on alumina [159]. Ceria nanostructures evidencing their value in other more complex organic reactions, apart from oxidations and hydrogenations, in advanced and controlled fabrication, promoting it as attractive and versatile nanocatalyst. Coupling reactions including aldol condensation (Scheme 4) [160, 161], Mannich reaction (Scheme 5) [162], Suzuki-Miyaura [163], Knoevenagel condensation (Scheme 6) [164] or Sonogashira cross couplings [165] have also been reported.

Scheme 4.

Cu supported catalysts on ceria-zirconia catalyzing cross-aldol condensation of acetone and n-butanol into aliphatic ketones. Reprinted with permission from Ref. [160]. Copyright 2017 Catalysts.

Yadav et al. [166, 167] have cast-off CeO₂ in synthesis. Recent Literature exposed that very few reagents have been reported for the bis-Michael addition reactions and most of the reagents, yielded the mono-Michael addition product [168]. Javad Safaei-Ghomi progressively synthesize CeO₂ nanoparticles and further utilize them by pseudo five-component reaction of acetylenedicarboxylates, phenylhydrazine and aromatic aldehydes in preparation of C-tethered bispyrazol-5-ols at 70°C in water [169]. In organic conversions, as carbon-carbon (C▬C) bond formation reactions, the catalytic activity of free-CeO₂ NPs has not been studied extensively to the unsurpassed of our knowledge.

Scheme 5.

Mannich reaction catalyzed by Sulfated Ce_xZr_1−xO₂ catalyst. Reprinted with permission from Ref. [162]. Copyright 2006 Elsevier.

Scheme 6.

Possible mechanism of Knoevenagel condensation. Reprinted with permission from Ref. [164]. Copyright 2009 Elsevier.

3. Conclusion

Various synthetic strategies of Cerium oxide nanoparticles and their progress in the field of catalysis of organic transformations are selectively highlighted in this comprehensive chapter. We presented that at nano level, ceria structures are manipulated with different techniques which allows direct control over catalytic behavior in various reactions. Cerium oxide occupies widespread attention in research on new catalysts with improved properties for organic synthesis due to its very rich chemistry.

Ceria, firstly used by Ford Motor Company as an oxygen storage component, further stepping towards growth in its applications, as consider an “inert” support can stabilize metal nanoparticles which are actively practiced for its catalytic activities, that directly takes part in the reaction with lattice oxygen, afterwards a cocatalyst, and more recently a catalyst. Applications of Ce and Ce-based nanoparticles in different forms of catalysis with recent advances in their preparation methods are properly introduced in the chapter. The synthesis section included different preparation procedures such as hydrothermal, reverse micelle, Co-precipitation and sol-gel method for synthesis of Ce and Ce-based NPs and their characterization. These procedures show their importance in designing and development of Ce-based nanostructures by controlling the morphology of these nanosystems with featured catalytic applications ranging from organic transformations to photocatalysis, and so on. The catalytic improvements of ceria-based nanostructures followed two major directions. First, the surface area is increased with the enhancement of its thermal stability. Second, the nanostructures with well controlled shape and size are obtained by the advent of nanotechnology. While ceria-based materials effectively promoted several oxidation reactions as well as other emerging applications are also proposed.

In addition to this enormous applications of Ce nanocatalysts are reported for organic conversions such as hydrogenation, reduction, alkyne-azide cycloaddition, coupling reactions including A3, coupling. CeO₂ has a good feature as follows: their redox ability and the acid base properties whether they are doped with transition metals or alone. The activation of complex organic molecules with further possible transformation can possibly proceed due to these parameters. Certain acid-base and redox properties can adjust with various cerium-based mixed oxides and to control the number of active sites and their strength for the specific reaction. Latest advances in ceria nanocrystals synthesis with controlled morphologies such as nanocubes, nanorods, polyhedras, etc. should be leading towards encounter of novel catalysts with better selectivities and higher activities in catalysis and organic chemistry.

Acknowledgments

TA thanks to CSIR (Grant No. 01(2897)/17/EMR-II) and SERB-DST (Grant No. EMR/2016/001668), New Delhi, Govt. of India for financial support to research projects. FN and UF are thankful to UGC, New Delhi for Non-NET Research Fellowship.

Conflict of interest

The authors have no conflict of interest.