**Michalis Konsolakis \* and Maria Lykaki**

Industrial, Energy and Environmental Systems Lab (IEESL), School of Production Engineering and Management, Technical University of Crete, GR-73100 Chania, Greece; mlykaki@isc.tuc.gr

**\*** Correspondence: mkonsol@pem.tuc.gr; Tel.: +30-28210-37682

Received: 11 December 2019; Accepted: 22 January 2020; Published: 1 February 2020

**Abstract:** Catalysis is an indispensable part of our society, massively involved in numerous energy and environmental applications. Although, noble metals (NMs)-based catalysts are routinely employed in catalysis, their limited resources and high cost hinder the widespread practical application. In this regard, the development of NMs-free metal oxides (MOs) with improved catalytic activity, selectivity and durability is currently one of the main research pillars in the area of heterogeneous catalysis. The present review, involving our recent efforts in the field, aims to provide the latest advances—mainly in the last 10 years—on the rational design of MOs, i.e., the general optimization framework followed to fine-tune non-precious metal oxide sites and their surrounding environment by means of appropriate synthetic and promotional/modification routes, exemplified by CuOx/CeO<sup>2</sup> binary system. The fine-tuning of size, shape and electronic/chemical state (e.g., through advanced synthetic routes, special pretreatment protocols, alkali promotion, chemical/structural modification by reduced graphene oxide (rGO)) can exert a profound influence not only to the reactivity of metal sites in its own right, but also to metal-support interfacial activity, offering highly active and stable materials for real-life energy and environmental applications. The main implications of size-, shape- and electronic/chemical-adjustment on the catalytic performance of CuOx/CeO<sup>2</sup> binary system during some of the most relevant applications in heterogeneous catalysis, such as CO oxidation, N2O decomposition, preferential oxidation of CO (CO-PROX), water gas shift reaction (WGSR), and CO<sup>2</sup> hydrogenation to value-added products, are thoroughly discussed. It is clearly revealed that the rational design and tailoring of NMs-free metal oxides can lead to extremely active composites, with comparable or even superior reactivity than that of NMs-based catalysts. The obtained conclusions could provide rationales and design principles towards the development of cost-effective, highly active NMs-free MOs, paving also the way for the decrease of noble metals content in NMs-based catalysts.

**Keywords:** copper-ceria; rational design; size; shape; electronic/chemical functionalization; CO oxidation; N2O decomposition; preferential oxidation of CO (CO-PROX); water gas shift reaction (WGSR); CO<sup>2</sup> hydrogenation

#### **1. Introduction**

The fast growth rate of population in the last decades has led to an unprecedented increase in energy demands. However, the main energy sources fulfilling global demands originate from fossil fuels, rising significant concerns in relation to sources availability and environmental degradation. To this

end, the development of emerging energy technologies towards the production of environmentally friendly fuels besides the establishment of cost-effective environmental technologies for climate change mitigation has become a main priority in the scientific and industrial community. Clean and reliable energy supply in conjunction with environmental protection by means of highly- and cost-effective technologies is one of the most significant concerns of the 21st century [1–4]. To this end, the development of emerging energy technologies towards the production of environmentally friendly fuels besides the establishment of cost-effective environmental technologies for climate change mitigation has become a main priority in the scientific and industrial community. Clean and reliable energy supply in conjunction with environmental protection by means of highlyand cost-effective technologies is one of the most significant concerns of the 21st century [1–4]. In view of the above aspects, heterogeneous catalysis is expected to have a key role in the near

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fuels, rising significant concerns in relation to sources availability and environmental degradation.

In view of the above aspects, heterogeneous catalysis is expected to have a key role in the near future towards sustainable development. Heterogeneous catalysis has received considerable attention from both the scientific and industrial community, as it is a field of diverse applications, including the petrochemical industry with the production of high quality chemicals and fuels, the fields of energy conversion and storage, as well as the remediation of the environment through the abatement of hazardous substances, signifying its pivotal role in the world economy [1,3,5–7]. future towards sustainable development. Heterogeneous catalysis has received considerable attention from both the scientific and industrial community, as it is a field of diverse applications, including the petrochemical industry with the production of high quality chemicals and fuels, the fields of energy conversion and storage, as well as the remediation of the environment through the abatement of hazardous substances, signifying its pivotal role in the world economy [1,3,5–7]. Several types of catalysts have been employed for energy and environmental applications,

Several types of catalysts have been employed for energy and environmental applications, which can be generally classified into: Noble metal (NMs)-based catalysts and NMs-free metal oxides (MOs), such as bare oxides, mixed metal oxides (MMOs), perovskites, zeolites, hexaaluminates, hydrotalcites, spinels, among others. Among these, NMs-based catalysts have been traditionally used in numerous processes, such as CO oxidation [8–11], nitrous oxide (N2O) decomposition [12–17], water-gas shift reaction [18–20], carbon dioxide (CO2) hydrogenation [21–25], etc., exhibiting high activity and selectivity. However, their scarcity and extremely high cost render mandatory the development of highly active, stable and selective catalysts that will be of low cost, nonetheless [26,27]. which can be generally classified into: Noble metal (NMs)-based catalysts and NMs-free metal oxides (MOs), such as bare oxides, mixed metal oxides (MMOs), perovskites, zeolites, hexaaluminates, hydrotalcites, spinels, among others. Among these, NMs-based catalysts have been traditionally used in numerous processes, such as CO oxidation [8–11], nitrous oxide (N2O) decomposition [12–17], water-gas shift reaction [18–20], carbon dioxide (CO2) hydrogenation [21–25], etc., exhibiting high activity and selectivity. However, their scarcity and extremely high cost render mandatory the development of highly active, stable and selective catalysts that will be of low cost, nonetheless [26,27].

On the other hand, metal oxides (MOs) prepared from earth-abundant, and inexpensive transition metals have attracted considerable attention as alternatives to rare and expensive NMs, due to their particular features, such as enhanced redox properties, thermal stability and catalytic performance in conjunction to their lower cost [2,3,5,28–43]. The latter is clearly manifested in Figure 1, which schematically depicts the cost of noble metals in comparison with copper (a typical transition metal massively involved in MOs) for the past five-year period. It is evident that the price of noble metals is larger than that of copper by about four orders of magnitude. On the other hand, metal oxides (MOs) prepared from earth-abundant, and inexpensive transition metals have attracted considerable attention as alternatives to rare and expensive NMs, due to their particular features, such as enhanced redox properties, thermal stability and catalytic performance in conjunction to their lower cost [2,3,5,28–43]. The latter is clearly manifested in Figure 1, which schematically depicts the cost of noble metals in comparison with copper (a typical transition metal massively involved in MOs) for the past five-year period. It is evident that the price of noble metals is larger than that of copper by about four orders of magnitude.

**Figure 1.** Relative comparison of noble metal and copper metal prices over the past five-year period. Data taken from https://www.infomine.com. **Figure 1.** Relative comparison of noble metal and copper metal prices over the past five-year period. Data taken from https://www.infomine.com.

Mixed metal oxides (MMOs) appropriately prepared by admixing two or more single metal oxides in a specific proportion, have lately gained particular attention, since they exhibit unique structural and surface properties, which are completely different from that of parent oxides. Amongst the numerous MOs, transition metal-based oxides have attracted particular attention, due to their peculiar chemisorption capacity, linked to partially filled d-shells [44,45]. For instance, Cu-based oxides can catalyze a variety of reactions following a redox-type mechanism (e.g., photocatalysis), due to the wide range of Cu oxidation states (mainly Cu<sup>0</sup> , Cu<sup>I</sup> , CuII), which enables reactivity in multi-electron pathways. On the other hand, reducible oxides, such as ceria, not only provide the basis of active phase dispersion, but could have a profound influence on the intrinsic catalytic activity, through metal-support interactions, as will be further discussed in the sequence. In particular, ceria or cerium oxide (CeO2) has attracted considerable attention, due to its unique properties, including enhanced thermal stability, high oxygen storage capacity (OSC) and oxygen mobility, as well as superior reducibility driven by the formation of surface/structural defects (e.g., oxygen vacancies) through the rapid interplay between the two oxidation states of cerium (Ce3+/Ce4+) [2,6,38,46–48]. Besides bare ceria's exceptional properties, its combination with transition metals leads to improved catalytic performance, due to the synergy between the metal phase and the support, related to electronic, geometric and bifunctional interactions [40,49–54]. In this regard, the combination of CuO<sup>x</sup> and CeO<sup>2</sup> oxides towards the formation of CuOx/CeO<sup>2</sup> binary oxides, offers catalytic activities comparable or even better to NMs-based catalysts in various applications, such as CO oxidation, N2O decomposition, preferential oxidation of CO (CO-PROX), as lately reviewed [40].

The peculiar reactivity of CuOx/CeO<sup>2</sup> system arises not only from the distinct characteristics of individual CuO<sup>x</sup> and CeO<sup>2</sup> phases, but mainly from their synergistic interactions. More specifically, the synergistic effects between the different counterparts of MOs can offer unique characteristics (e.g., improved reducibility, abundant structural defects, etc.), reflected then on the catalytic activity [40,55–60]. Various interrelated factors are usually considered under the term "synergy", involving among others:


In view of the above, very recently, the modulation of metal-support interactions as a tool to enhance the catalytic performance was thoroughly reviewed, disclosing that up to fifteen-fold productivity enhancement can be achieved in reactions related to C1 chemistry by controlling metal-support interactions [61]. However, it is well established today—thanks to the rapid development of sophisticated characterization techniques—that various interrelated factors, such as the composition, the size, the shape, and the electronic state of MOs different counterparts can exert a profound influence on the local surface chemistry and metal-support interactions, and in turn, on the catalytic activity of these multifunctional materials [6,48,51,52,62–74]. In view of this fact, the fine-tuning of MOs towards the development of catalytic materials with the desired cost, activity, selectivity and stability could be considered the "Holy Grail" in the field of catalyst manufacturing. Size, shape, porous, redox and electronic adjustment by means of appropriate synthetic and promotional/modification routes can provide the vehicle to substantially modify not only the reactivity of metal sites in its own right, but also the interfacial activity, offering highly active and stable materials for real-life energy and environmental applications (Figure 2).

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**Figure 2.** Schematic illustration of metal oxides (MOs) fine-tuning by adjusting their size, shape, **Figure 2.** Schematic illustration of metal oxides (MOs) fine-tuning by adjusting their size, shape, composition and electronic/chemical state.

**.** 

In light of the aforementioned issues, the present review article aims to gain insight into the particular effect of each adjusted parameter on MOs physicochemical characteristics and in turn, on their catalytic performance, towards revealing rigorous structure-property relationships. The basic principles of MOs fine-tuning by modulating the size, shape and electronic/chemical state by means of appropriate synthetic/modification routes will be initially presented. The implications of size, shape and electronic/chemical effects in catalysis will be next exemplified on the basis of state-of-theart catalytic applications of CuOx/CeO2 binary oxide, involving CO oxidation, N2O decomposition, preferential oxidation of CO (CO-PROX), water gas shift reaction (WGSR), and CO2 hydrogenation to value-added chemicals/fuels. It should be noted here that the scope of the present article is not to provide a complete survey in relation to the fundamental understanding and practical applications of CuOx/CeO2 system, which can be found in various comprehensive reviews [40,49,75–77]. Herein, the CuOx/CeO2 system is used as an excellent benchmark to reveal how we can adjust the local surface chemistry and in turn, the catalytic activity of MOs. The obtained conclusions can provide rationales and design principles towards the development of cost-effective, highly active NMs-free MOs of various compositions, paving also the way for the decrease of noble metals content in NMs-based catalysts. The term "CuOx" instead of CuO is used throughout the text to denote the differentiation of Cu oxidation state depending on synthesis procedure and reaction environment, as discussed in In light of the aforementioned issues, the present review article aims to gain insight into the particular effect of each adjusted parameter on MOs physicochemical characteristics and in turn, on their catalytic performance, towards revealing rigorous structure-property relationships. The basic principles of MOs fine-tuning by modulating the size, shape and electronic/chemical state by means of appropriate synthetic/modification routes will be initially presented. The implications of size, shape and electronic/chemical effects in catalysis will be next exemplified on the basis of state-of-the-art catalytic applications of CuOx/CeO<sup>2</sup> binary oxide, involving CO oxidation, N2O decomposition, preferential oxidation of CO (CO-PROX), water gas shift reaction (WGSR), and CO<sup>2</sup> hydrogenation to value-added chemicals/fuels. It should be noted here that the scope of the present article is not to provide a complete survey in relation to the fundamental understanding and practical applications of CuOx/CeO<sup>2</sup> system, which can be found in various comprehensive reviews [40,49,75–77]. Herein, the CuOx/CeO<sup>2</sup> system is used as an excellent benchmark to reveal how we can adjust the local surface chemistry and in turn, the catalytic activity of MOs. The obtained conclusions can provide rationales and design principles towards the development of cost-effective, highly active NMs-free MOs of various compositions, paving also the way for the decrease of noble metals content in NMs-based catalysts. The term "CuOx" instead of CuO is used throughout the text to denote the differentiation of Cu oxidation state depending on synthesis procedure and reaction environment, as discussed in the sequence.

#### **2. Fine-Tuning of Metal Oxides (MOs)**

the sequence.

composition and electronic/chemical state.

**2. Fine-Tuning of Metal Oxides (MOs)**  Heterogeneous catalysis traditionally refers to a chemical reaction on the surface of a solid catalyst, involving adsorption and activation of reactant(s) on specific active sites, chemical transformation of adsorbed species and products desorption. Thanks to the rapid development of both in situ and ex situ characterization techniques, it is well acknowledged that the elementary reaction steps are strongly dependent on several parameters involving the size, the shape, the electronic state of individual particles, as well as on their interfacial interactions. Hence, the macroscopic catalytic behavior can be considered as the outcome of interactions between reactants, intermediates and products with the micro(nano)scopic coordination environment of surface atoms, involving geometric arrangements, electronic confinement, and interfacial effects, among others. In view of this fact, the modulation of the above-mentioned parameters can profoundly affect the local surface structure and chemistry with great implications in catalysis. It should be mentioned, Heterogeneous catalysis traditionally refers to a chemical reaction on the surface of a solid catalyst, involving adsorption and activation of reactant(s) on specific active sites, chemical transformation of adsorbed species and products desorption. Thanks to the rapid development of both in situ and ex situ characterization techniques, it is well acknowledged that the elementary reaction steps are strongly dependent on several parameters involving the size, the shape, the electronic state of individual particles, as well as on their interfacial interactions. Hence, the macroscopic catalytic behavior can be considered as the outcome of interactions between reactants, intermediates and products with the micro(nano)scopic coordination environment of surface atoms, involving geometric arrangements, electronic confinement, and interfacial effects, among others. In view of this fact, the modulation of the above-mentioned parameters can profoundly affect the local surface structure and chemistry with great implications in catalysis. It should be mentioned, however, that due to the interplay between structural and chemical factors, it is quite challenging to disclose the fundamental origin of catalytic performance.

however, that due to the interplay between structural and chemical factors, it is quite challenging to disclose the fundamental origin of catalytic performance. Thus, it is of vital importance to establish Thus, it is of vital importance to establish reliable structure-property relationships, unveiling the particular role of each factor. The latter could lead to rational design instead of trial-and-error methods by utilizing the fundamental knowledge at the nanoscale. reliable structure-property relationships, unveiling the particular role of each factor. The latter could lead to rational design instead of trial-and-error methods by utilizing the fundamental knowledge at the nanoscale.

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Moreover, taking into account that the majority of MOs consists of at least two different counterparts, this triggers unique opportunities towards designing various MOs of the same composition, but of different reactivity by adjusting the above-mentioned parameters either in one or both counterparts. For instance, by modulating the size, morphology and electronic/chemical state of ceria carrier in CuOx/CeO<sup>2</sup> composites, a different extent of metal-support interactions can be attained with implications in catalysis. In a similar manner, the co-adjustment of the shape of both CuO<sup>x</sup> and CeO<sup>2</sup> could lead to different synergistic interactions, providing CuOx/CeO<sup>2</sup> systems of peculiar reactivity. The proposed adjusting approach on the way to fine-tune MOs is schematically illustrated in Figure 3, clearly revealing the unique opportunities in the field of materials synthesis and engineering towards the development of low-cost and highly-effective composites. Moreover, taking into account that the majority of MOs consists of at least two different counterparts, this triggers unique opportunities towards designing various MOs of the same composition, but of different reactivity by adjusting the above-mentioned parameters either in one or both counterparts. For instance, by modulating the size, morphology and electronic/chemical state of ceria carrier in CuOx/CeO2 composites, a different extent of metal-support interactions can be attained with implications in catalysis. In a similar manner, the co-adjustment of the shape of both CuOx and CeO2 could lead to different synergistic interactions, providing CuOx/CeO2 systems of peculiar reactivity. The proposed adjusting approach on the way to fine-tune MOs is schematically illustrated in Figure 3, clearly revealing the unique opportunities in the field of materials synthesis and engineering towards the development of low-cost and highly-effective composites.

**Figure 3.** Indicative pathways towards the fine-tuning of a binary metal oxide of the general formula AB by adjusting the size (S), morphology (M), electronic (E) and chemical (C) state of one or both of the individual counterparts A and B. For instance, CuOx (A)/CeO2 (B) binary oxides of different reactivity can be obtained by combining the morphology engineering of CeO2 {denoted as M (B)} with the size engineering of CuOx {denoted as (S (A)} or by combining both the size and morphology engineering of both counterparts {S(A) + M(A) + S(B) + M (B)}. The scheme is just indicative of the different approaches that can be followed to adjust the local surface chemistry of MOs, without exhausting the margins of all possible combinations. **Figure 3.** Indicative pathways towards the fine-tuning of a binary metal oxide of the general formula AB by adjusting the size (S), morphology (M), electronic (E) and chemical (C) state of one or both of the individual counterparts A and B. For instance, CuO<sup>x</sup> (A)/CeO<sup>2</sup> (B) binary oxides of different reactivity can be obtained by combining the morphology engineering of CeO<sup>2</sup> {denoted as M (B)} with the size engineering of CuOx {denoted as (S (A)} or by combining both the size and morphology engineering of both counterparts {S(A) + M(A) + S(B) + M (B)}. The scheme is just indicative of the different approaches that can be followed to adjust the local surface chemistry of MOs, without exhausting the margins of all possible combinations.

In the following, the basic principles of size, shape, and electronic/chemical effects are provided in separate sections. It should be stressed, however, that this district presentation does not also mean a district effect of each factor in catalysis. Almost all parameters are interrelated; thus, the discrete role of each one in the catalytic activity of MOs cannot be easily disclosed, as further discussed below. In the following, the basic principles of size, shape, and electronic/chemical effects are provided in separate sections. It should be stressed, however, that this district presentation does not also mean a district effect of each factor in catalysis. Almost all parameters are interrelated; thus, the discrete role of each one in the catalytic activity of MOs cannot be easily disclosed, as further discussed below.

#### *2.1. Size E*ff*ects*

The rapid development of nanotechnology in the last years enables the fabrication of MOs with tunable size and shape at the nanometer scale. Nowadays, it has been both experimentally and theoretically revealed that the surface, structural and electronic properties of nanoparticles (NPs) differ essentially from the corresponding bulk properties. In general, by decreasing the particle size of metal oxide particles down to few nanometers (e.g., <10 nm), a dramatic increase in activity can be generally obtained, attributed to "size effects". This size-dependent reactivity can be ascribed to different contributions, namely: (i) Quantum size effects, (ii) presence of low coordinated atoms into NPs surface, (iii) electronic state of the surface, (iv) strong interparticle interactions. Hereinafter, the particular effect of every contribution is shortly presented for the sake of following discussions in relation to the fine-tuning of MOs. For additional reading, several comprehensive articles in this topical area are recommended [40,52,62,66,70,71,74,78].

In particular, by decreasing the size of a material down to nanometer scale, the surface-to-volume ratio is largely increased, resulting in an increased population of surface sites, being the active sites in catalysis. Besides the modulation of the fraction of atoms on the topmost surface layer, the number of atoms at corners and edges, being considered more active than those at planes, is considerably increased by decreasing the size. More specifically, size decrease leads to a high density of under-coordinated atoms with exceptional adsorption and catalytic properties [52,62,70,79–83]. Typically, surface sites with low coordination number (CN) demonstrate stronger adsorption ability as compared to those of high CN [66,70,84]; linear relationships between the adsorption energy of various adsorbates and the coordination number have been found for several transition metals, including among others non-precious metals, such as Cu, Ni, and Co [85,86]. Thus, from the geometrical point of view, size decrease has a direct effect on both the number and type of active surface sites reflected then on catalytic activity.

Aside from the "geometric size effects", the electronic state of surface atoms can undergo substantial modifications upon decreasing the particle size down to nanometer scale. In particular, when a bulk material with a continuous electron band is subjected to size decrease down to the nanoscale, the so-called quantum effect or confinement effect is taking place, arising from the presence of discrete electronic states as in the case of molecules [62,70,74,78,87]. For instance, it has been reported that a higher electron density, with a d band close to the Fermi level, can be obtained for Au NPs smaller than ca. 2 nm as compared to bigger ones, with great implications in CO oxidation [88–91].

Recently, thanks to the introduction of new generation sophisticated characterization techniques (e.g., high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), extended X-ray absorption fine structure (EXAFS)) and computational methods (e.g., DFT calculations), an indirect size effect linked to the metal-support interactions is clearly revealed. More specifically, even small perturbations between metal nanoparticles and oxide carriers, due to charge transfer between particles, local electric fields, morphological changes, "ligand" effect, etc., can induce a substantial modification in catalytic activity [40,50,55,92–95]. To more accurately describe these phenomena, the term Electronic Metal Support Interactions (EMSI) has recently been proposed by Campbell [96] in contrast to classical Strong Metal Support Interactions (SMSI). In view of this concept, tiny metal clusters composed of a few or even single atoms could play a dominant role in catalysis, despite the fact that they do not account for more than a few percent of the total metal content [40,50]. In view of this fact, it has been shown that by controlling metal (Ni, Pd, Pt) nanocrystal size, the length of metal-ceria interface is appropriately adjusted with significant implications in CO oxidation; normalized reaction rates were dramatically increased with decreasing the size, due to the increased boundary length (Figure 4).

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**Figure 4.** The calculated number of sites with a particular geometry (surface and perimeter or corner atoms in contact with the support) as a function of diameter and the turnover frequency (TOF) at 80 °C for ceria-based metals (Ni, Pd, Pt). Reproduced with permission from Reference [52]. Copyright© 2013, American Association for the Advancement of Science (AAAS). **Figure 4.** The calculated number of sites with a particular geometry (surface and perimeter or corner atoms in contact with the support) as a function of diameter and the turnover frequency (TOF) at 80 ◦C for ceria-based metals (Ni, Pd, Pt). Reproduced with permission from Reference [52]. Copyright© 2013, American Association for the Advancement of Science (AAAS).

As an additional implication of size-dependent behavior, the significant effect of particle size on structural defects of reducible carriers (such as ceria) should be mentioned. In fact, a close relationship between the crystal size of ceria and the concentration of oxygen vacancies has been revealed; the large surface-to-volume ratio in conjunction to the exposure of under-coordinated sites can facilitate the formation of oxygen vacancies and the Ce3+ fraction in non-stoichiometric CeO2-δ NPs [71,97–103]. Moreover, an inverse correlation between the lattice parameters of CeO2 NPs and particle sizes has been established (Figure 5a), attributed to the increase of Ce3+ and oxygen vacancies concentration [71]. A similar trend was recorded between the surface-to-bulk oxygen ratio and particle size (Figure As an additional implication of size-dependent behavior, the significant effect of particle size on structural defects of reducible carriers (such as ceria) should be mentioned. In fact, a close relationship between the crystal size of ceria and the concentration of oxygen vacancies has been revealed; the large surface-to-volume ratio in conjunction to the exposure of under-coordinated sites can facilitate the formation of oxygen vacancies and the Ce3<sup>+</sup> fraction in non-stoichiometric CeO2-<sup>δ</sup> NPs [71,97–103]. Moreover, an inverse correlation between the lattice parameters of CeO<sup>2</sup> NPs and particle sizes has been established (Figure 5a), attributed to the increase of Ce3<sup>+</sup> and oxygen vacancies concentration [71]. A similar trend was recorded between the surface-to-bulk oxygen ratio and particle size (Figure 5b).

5b). Closing this part concerning the size effects, it should be noted, that although particle size decrease has, in general, a positive catalytic effect, there is a variation in relation to size-activity relationships depending on catalyst type and reaction environment. For instance, a positive size effect could be obtained if the rate determining step (rds) involves the bond cleavage of a reactant molecule on surface atoms with low coordination number. However, if the same under-coordinated atoms strongly bind dissociated species (e.g., oxygen atoms), this could lead to the poisoning of catalyst surface, and thus, to the negative size effect. In particular, in reactions with no structure sensitivity, the activity remains unaffected by changes in the particle size, while it could decrease with decreasing particle size, referred as negative particle size effect or antipathetic structure sensitivity, or increase as the particle size decreases, referred as positive particle size effect or sympathetic structure sensitivity [79]. Moreover, the activity may reach a maximum when small particles exhibit a negative Closing this part concerning the size effects, it should be noted, that although particle size decrease has, in general, a positive catalytic effect, there is a variation in relation to size-activity relationships depending on catalyst type and reaction environment. For instance, a positive size effect could be obtained if the rate determining step (rds) involves the bond cleavage of a reactant molecule on surface atoms with low coordination number. However, if the same under-coordinated atoms strongly bind dissociated species (e.g., oxygen atoms), this could lead to the poisoning of catalyst surface, and thus, to the negative size effect. In particular, in reactions with no structure sensitivity, the activity remains unaffected by changes in the particle size, while it could decrease with decreasing particle size, referred as negative particle size effect or antipathetic structure sensitivity, or increase as the particle size decreases, referred as positive particle size effect or sympathetic structure sensitivity [79]. Moreover, the activity may reach a maximum when small particles exhibit a negative effect, and larger particles show a positive one [79].

effect, and larger particles show a positive one [79].

**Figure 5.** (**a**) Lattice expansion of ceria as a function of size and (**b**) the inverse relationship of surface oxygen to bulk oxygen (TPR) and the correlation of surface oxygen ratio with the theoretical surface to volume ratio. Adapted from Reference [71]. Copyright© 2010, Royal Society of Chemistry. **Figure 5.** (**a**) Lattice expansion of ceria as a function of size and (**b**) the inverse relationship of surface oxygen to bulk oxygen (TPR) and the correlation of surface oxygen ratio with the theoretical surface to volume ratio. Adapted from Reference [71]. Copyright© 2010, Royal Society of Chemistry.

#### *2.2. Shape E*ff*ects*

could be obtained.

*2.2. Shape Effects* Nanostructured catalysts possess unique properties originating from nanoscale phenomena linked mainly to size effects, commented above, and shape effects. The latter refers to the modification of catalytic activity through the preferential exposure of specific crystallographic facets on the reaction environment, also termed as morphology-dependent nanocatalysis [51,65,66,70,82,104–106]. In particular, the catalytic cycle and hence the reaction efficiency, is determined on reactants adsorption/activation and products desorption processes, being strongly influenced by the surface planes of catalysts particles. In this regard, the simultaneous modulation of size and shape at the nanometer scale can determine the number and the nature of exposed sites, and thus, the catalytic performance. This particular topic is an essential issue within the field of Nanostructured catalysts possess unique properties originating from nanoscale phenomena linked mainly to size effects, commented above, and shape effects. The latter refers to the modification of catalytic activity through the preferential exposure of specific crystallographic facets on the reaction environment, also termed as morphology-dependent nanocatalysis [51,65,66,70,82,104–106]. In particular, the catalytic cycle and hence the reaction efficiency, is determined on reactants adsorption/activation and products desorption processes, being strongly influenced by the surface planes of catalysts particles. In this regard, the simultaneous modulation of size and shape at the nanometer scale can determine the number and the nature of exposed sites, and thus, the catalytic performance. This particular topic is an essential issue within the field of nanocatalysis, aiming to the control of a specific chemical reaction through co-adjusting these parameters at the nanometer scale.

nanocatalysis, aiming to the control of a specific chemical reaction through co-adjusting these parameters at the nanometer scale. Thanks to the latest advances in materials science, nanostructured catalysts with well-defined crystal facets can be fabricated by precisely controlling nanocrystals nucleation and growth rate [48,63,66,67,78,105]. The obtained crystal morphology is the result of several synthesis parameters, involving temperature, pressure, concentration, and pH, among others. Several reviews have been devoted to the subject [6,38,67,82,97,105,107,108]. Various structures with similar or different Thanks to the latest advances in materials science, nanostructured catalysts with well-defined crystal facets can be fabricated by precisely controlling nanocrystals nucleation and growth rate [48,63,66,67,78,105]. The obtained crystal morphology is the result of several synthesis parameters, involving temperature, pressure, concentration, and pH, among others. Several reviews have been devoted to the subject [6,38,67,82,97,105,107,108]. Various structures with similar or different dimensions in all directions, such as nanospheres, nanocubes, nanowires, nanorods, nanosheets, etc., could be obtained.

dimensions in all directions, such as nanospheres, nanocubes, nanowires, nanorods, nanosheets, etc.,

The shape control of ceria and its implications in catalysis is most probably the most extensively investigated system among metal oxides in heterogeneous catalysis [2,6,38,48,51,64,65,73,82,97,104, 107,109–112]. The growth rate mechanism of ceria nanocrystals can be affected by various parameters, such as the basicity or polarity of the solvent [113,114], the aging temperature [115,116], the precursor compound [117,118], and the impregnation medium [119]. Regulation of nanocrystals nucleation and growth processes results in specific shapes, such as rods and cubes [48,67]. Moreover, by altering the physicochemical conditions during the synthesis procedure (e.g., by the usage of a capping agent), blocking of certain facets or continuous growth of others may occur. The shape control of ceria and its implications in catalysis is most probably the most extensively investigated system among metal oxides in heterogeneous catalysis [2,6,38,48,51,64,65,73,82,97,104,107,109–112]. The growth rate mechanism of ceria nanocrystals can be affected by various parameters, such as the basicity or polarity of the solvent [113,114], the aging temperature [115,116], the precursor compound [117,118], and the impregnation medium [119]. Regulation of nanocrystals nucleation and growth processes results in specific shapes, such as rods and cubes [48,67]. Moreover, by altering the physicochemical conditions during the synthesis procedure (e.g., by the usage of a capping agent), blocking of certain facets or continuous growth of others may occur.

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There are several synthetic approaches for the preparation of ceria nanoparticles, including precipitation [119–121], thermal decomposition [121,122], template or surfactant-assisted method [123–126], microwave-assisted synthesis [127–129], the alcohothermal [124,130] or hydrothermal [121,124,131–135] method, microemulsion[133,136,137], solutioncombustion[138,139], sol-gel[140–142], sonochemical[143,144], etc. However, not all methods lead to particles of well-ordered size and shape with uniform dispersion on the catalyst surface [145]. Among the different methods, the hydrothermal one has attracted considerable attention, due to the simplicity of the precursor compounds used, the short reaction time, the homogeneity in morphology, as well as the acquisition of various nanostructures, such as rods, polyhedra, cubes, wires [107,117,121,124,131,134,135,146–159]. Ceria nanocrystals have three low-index lattice facets of different activity and stability, namely, {100}, {110}, {111} [48,106], as shown in Figure 6. There are several synthetic approaches for the preparation of ceria nanoparticles, including precipitation [119–121], thermal decomposition [121,122], template or surfactant-assisted method [123–126], microwave-assisted synthesis [127–129], the alcohothermal [124,130] or hydrothermal [121,124,131–135] method, microemulsion [133,136,137], solution combustion [138,139], sol-gel [140– 142], sonochemical [143,144], etc. However, not all methods lead to particles of well-ordered size and shape with uniform dispersion on the catalyst surface [145]. Among the different methods, the hydrothermal one has attracted considerable attention, due to the simplicity of the precursor compounds used, the short reaction time, the homogeneity in morphology, as well as the acquisition of various nanostructures, such as rods, polyhedra, cubes, wires [107,117,121,124,131,134,135,146– 159]. Ceria nanocrystals have three low-index lattice facets of different activity and stability, namely, {100}, {110}, {111} [48,106], as shown in Figure 6.

**Figure 6.** Structural models of CeO2 (**a**) unit cell, (**b**) (111), (**c**) (100), and (**d**) (110) surfaces without structural optimizations. Reproduced with permission from Reference [106]. Copyright© 2014, Royal Society of Chemistry. **Figure 6.** Structural models of CeO<sup>2</sup> (**a**) unit cell, (**b**) (111), (**c**) (100), and (**d**) (110) surfaces without structural optimizations. Reproduced with permission from Reference [106]. Copyright© 2014, Royal Society of Chemistry.

The selective exposure of ceria reactive facets can strongly affect the redox properties of ceria and in turn, its intrinsic characteristics as an active phase or supporting carrier. Popular ceria shapes, mainly, involve nanorods (NR), nanocubes (NC) and nanopolyhedra (NP). Ceria nanorods, mostly, expose the {110} and {100} facets, whereas, nanocubes and nanopolyhedra preferentially expose the {100} and {111} facets, respectively [48,51,106]. By means of both experimental [82,104,109,146,160,161] and theoretical [112,162–166] studies, it was shown that the energy formation of anionic vacancies is dependent on the exposed facets, following the order: {111} > {100} > {110}. In this regard, the reactivity of ceria NR is, in general, increased upon increasing the fraction of {110} and {100} facets [65]. The selective exposure of ceria reactive facets can strongly affect the redox properties of ceria and in turn, its intrinsic characteristics as an active phase or supporting carrier. Popular ceria shapes, mainly, involve nanorods (NR), nanocubes (NC) and nanopolyhedra (NP). Ceria nanorods, mostly, expose the {110} and {100} facets, whereas, nanocubes and nanopolyhedra preferentially expose the {100} and {111} facets, respectively [48,51,106]. By means of both experimental [82,104,109,146,160,161] and theoretical [112,162–166] studies, it was shown that the energy formation of anionic vacancies is dependent on the exposed facets, following the order: {111} > {100} > {110}. In this regard, the reactivity of ceria NR is, in general, increased upon increasing the fraction of {110} and {100} facets [65].

In view of the above, it has been clearly revealed that the activity and selectivity are strongly affected by the exposed crystal planes. For instance, the formation rate of ammonia on Fe crystals follows the sequence: {111} >> {100} > {110} [167]. Similar morphology-dependent effects have been demonstrated for several noble metal [105,168] and metal oxide [51] catalyzed processes.

Very recently, we showed that among ceria nanoparticles of different morphology (nanocubes, nanorods and nanopolyhedra), as shown in Figure 7a, ceria nanorods with {100} and {110} crystal planes, exhibited the optimum redox properties in terms of loosely bound oxygen species population [115,121]. Figure 7b depicts the H<sup>2</sup> temperature-programmed reduction (H2-TPR) profiles of ceria nanocubes, nanopolyhedra and nanorods, where two main peaks at ca. 550 and 800 ◦C can be identified, ascribed to the reduction of surface (Os) and bulk (Ob) oxygen, respectively. Notably, the surface-to-bulk (Os/Ob) ratio is strongly dependent on ceria morphology following the order: NC (0.71) < NP (0.94) < NR (1.13). It is also worth mentioning that the onset reduction temperature follows the reverse order, i.e.,: NC (589 ◦C) > NP (555 ◦C) > NR (545 ◦C), implying the lower temperature reduction of {110} and {100} surfaces as compared to {111}. demonstrated for several noble metal [105,168] and metal oxide [51] catalyzed processes. Very recently, we showed that among ceria nanoparticles of different morphology (nanocubes, nanorods and nanopolyhedra), as shown in Figure 7a, ceria nanorods with {100} and {110} crystal planes, exhibited the optimum redox properties in terms of loosely bound oxygen species population [115,121]. Figure 7b depicts the H2 temperature-programmed reduction (H2-TPR) profiles of ceria nanocubes, nanopolyhedra and nanorods, where two main peaks at ca. 550 and 800 °C can be identified, ascribed to the reduction of surface (Os) and bulk (Ob) oxygen, respectively. Notably, the surface-to-bulk (Os/Ob) ratio is strongly dependent on ceria morphology following the order: NC (0.71) < NP (0.94) < NR (1.13). It is also worth mentioning that the onset reduction temperature follows the reverse order, i.e.,: NC (589 °C) > NP (555 °C) > NR (545 °C), implying the lower temperature reduction of {110} and {100} surfaces as compared to {111}.

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follows the sequence: {111} >> {100} > {110} [167]. Similar morphology-dependent effects have been

In view of the above, it has been clearly revealed that the activity and selectivity are strongly

**Figure 7.** (**a**) TEM images and (**b**) H2-TPR profiles for ceria nanoparticles of different morphology, i.e., nanorods, nanopolyhedra and nanocubes; (**c**) CO oxidation performance (T50) of ceria nanoparticles as a function of ceria redox properties expressed in terms of surface-to-bulk (Os/Ob) TPR ratio and ID/IF2g Raman ratio [115]. **Figure 7.** (**a**) TEM images and (**b**) H<sup>2</sup> -TPR profiles for ceria nanoparticles of different morphology, i.e., nanorods, nanopolyhedra and nanocubes; (**c**) CO oxidation performance (T50) of ceria nanoparticles as a function of ceria redox properties expressed in terms of surface-to-bulk (Os/Ob) TPR ratio and ID/IF2g Raman ratio [115].

More recently, we explored the impact of ceria exposed facets on structural defects by means of in situ Raman spectroscopy following a novel approach involving sequential spectra acquisition under alternating oxidizing and reducing atmospheres [115]. The in situ Raman measurements perfectly corroborated the aforementioned arguments in relation to the impact of crystal planes on the reducibility; the relative abundance of defects and oxygen vacancies exhibited by the ID/IF2g ratio, as well as the relative reducibility expressed by the detachment of O atoms and the partial Ce4<sup>+</sup> <sup>→</sup> Ce3<sup>+</sup> reduction, follow the same trend, i.e., NR > NP > NC.

These results unambiguously indicate that CeO2-nanorods exhibit the highest concentration of weakly bound oxygen species, linked to enhanced reducibility and oxygen mobility. Interestingly, an almost linear relationship is revealed between the redox properties, expressed either as Os/O<sup>b</sup> or ID/IF2g ratio, and the CO oxidation performance, in terms of half-conversion temperature (T50), of ceria nanoparticles (Figure 7c), clearly demonstrating the implications of shape modulation in catalysis.

#### *2.3. Electronic E*ff*ects*

Besides modulating the local surface structure of MOs by size and shape effects, described above, the fine-tuning of electronic structure by appropriate promoters can be considered as an additional modulating tool. Promoters hold a key role in heterogeneous catalysis towards optimizing the catalytic activity, selectivity and stability by modifying the physicochemical features of MOs, and can be classified into two general categories: Structural promoters and electronic promoters. The first category mainly involves the doping of supporting carrier to enhance its structural characteristics and in turn, the stabilization of active phase (e.g., incorporation of rare earth dopants into three-way catalysts [5]). On the other hand, electronic promoters can modify catalysts surface chemistry either directly or indirectly. The former mainly includes the electrostatic interactions between the reactant molecules and the local electric field of promoters. The latter denotes the promoter-induced modifications on metal Fermi level, which is then reflected on the chemisorptive bond strength of reactants and intermediates with great consequences in catalysis. In particular, "promoter effect" is related to the changes in the work function (Φ) of the catalysts surface upon promoter addition, accompanied by substantial modification of its chemisorption properties. The vast majority of electronic promotion over metal oxide catalysts refers to alkali modifiers. It has been well documented that alkali addition can drastically enhance the activity and selectivity of numerous catalytic systems, involving among others Pt-, Pd-, Rh-, Cu-, Fe-based catalysts, in various energy and environmental related reactions (e.g., [169–174]). Various comprehensive studies have been devoted to the role of promoters in heterogeneous catalysis, to which the reader can refer for further reading [175–177].

Figures 8 and 9 depict the "promoter effect" in the case of alkali-doped Co3O<sup>4</sup> oxides during the N2O decomposition [178,179]. A close relationship between the catalytic performance (in terms of half-conversion temperature, T50) and the work function (Φ) was disclosed revealing the electronic nature of alkali promotion; electropositive modifiers (such as alkalis) can decrease the work function of the catalyst surface, thus, activating the adsorption/decomposition of electron-acceptor molecules (such as N2O) [178]. However, at high alkali coverages, depolarization occurs, due to the strengthening of the alkali-alkali bond at the expense of the alkali-surface bond, resulting in a work function increase [180].

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**Figure 8.** Correlation of half-conversion temperature (T50) with the work function of alkali promoted Co3O4. Reaction conditions: 5.0% N2O; mcat = 300 mg; GHSV = 7000 h<sup>−</sup>1; alkali coverage = ~2 at/nm2. Adapted from Reference [178]. Copyright© 2009, Elsevier. **Figure 8.** Correlation of half-conversion temperature (T50) with the work function of alkali promoted Co3O<sup>4</sup> . Reaction conditions: 5.0% N2O; mcat = 300 mg; GHSV = 7000 h−<sup>1</sup> ; alkali coverage = ~2 at/nm<sup>2</sup> . Adapted from Reference [178]. Copyright© 2009, Elsevier. **Figure 8.** Correlation of half-conversion temperature (T50) with the work function of alkali promoted Co3O4. Reaction conditions: 5.0% N2O; mcat = 300 mg; GHSV = 7000 h<sup>−</sup>1; alkali coverage = ~2 at/nm2. Adapted from Reference [178]. Copyright© 2009, Elsevier.

**Figure 9.** (**a**) The half-conversion temperature of N2O (T50), (**b**) apparent activation energy (Ea) and (**c**) work function changes (ΔΦ) as a function of potassium loading (ΘΚ) introduced from K2CO3 and (c **Figure 9.** (**a**) The half-conversion temperature of N2O (T50), (**b**) apparent activation energy (Ea) and (**c**) work function changes (ΔΦ) as a function of potassium loading (ΘΚ) introduced from K2CO3 and (c **Figure 9.** (**a**) The half-conversion temperature of N2O (T50), (**b**) apparent activation energy (Ea) and (**c**) work function changes (∆Φ) as a function of potassium loading (ΘK) introduced from K2CO<sup>3</sup> and (c insert) KOH precursor. Reproduced with permission from Reference [179]. Copyright© 2008, Springer Nature.

In this point, it should be mentioned that, depending on the support nature and crystal planes, alkali adsorption may lead to surface reconstruction. This surface reconstruction can be explained by taking into account the structural/electronic perturbations induced by the formation of the alkali-surface bond [181]. As mentioned previously, the crystallographic orientation of the support plays an important role in the diffusion rate of the adsorbed species, as well as in their in-between interaction, resulting in different structural stabilization [181]. For instance, potassium promoter was shown to stabilize certain iron facets in K-promoted iron catalysts, by inducing changes in the crystal growth rate, thus, enabling the formation of small particles with abundance in active facets and affecting the activity and selectivity of the overall system [182]. As shown in Figure 10, by increasing the K/Fe surface atomic ratio, the crystal facets become more stable and the surface energy is decreased [182]. This clearly manifests the pivotal role of alkali addition towards co-adjusting the structural and electronic properties of the catalyst surface, and in turn, the catalytic performance. In this point, it should be mentioned that, depending on the support nature and crystal planes, alkali adsorption may lead to surface reconstruction. This surface reconstruction can be explained by taking into account the structural/electronic perturbations induced by the formation of the alkalisurface bond [181]. As mentioned previously, the crystallographic orientation of the support plays an important role in the diffusion rate of the adsorbed species, as well as in their in-between interaction, resulting in different structural stabilization [181]. For instance, potassium promoter was shown to stabilize certain iron facets in K-promoted iron catalysts, by inducing changes in the crystal growth rate, thus, enabling the formation of small particles with abundance in active facets and affecting the activity and selectivity of the overall system [182]. As shown in Figure 10, by increasing the K/Fe surface atomic ratio, the crystal facets become more stable and the surface energy is decreased [182]. This clearly manifests the pivotal role of alkali addition towards co-adjusting the structural and electronic properties of the catalyst surface, and in turn, the catalytic performance.

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**Figure 10.** Surface energy variation versus the surface atomic ratio of K to Fe. Reproduced with permission from Reference [182]. Copyright© 2011, John Wiley and Sons. **Figure 10.** Surface energy variation versus the surface atomic ratio of K to Fe. Reproduced with permission from Reference [182]. Copyright© 2011, John Wiley and Sons.

Besides the alkali-induced modifications on the chemisorption properties, significant alterations on the surface oxygen mobility have been demonstrated; alkali addition could facilitate suprafacial recombination of oxygen towards molecular oxygen desorption, thus, liberating active sites [40]. Both the electronic and redox modifications induced by alkali addition can exert remarkable effects on catalytic activity and selectivity, demonstrating the key role of "promoter effect" as an additional adjusting parameter in catalysis. Besides the alkali-induced modifications on the chemisorption properties, significant alterations on the surface oxygen mobility have been demonstrated; alkali addition could facilitate suprafacial recombination of oxygen towards molecular oxygen desorption, thus, liberating active sites [40]. Both the electronic and redox modifications induced by alkali addition can exert remarkable effects on catalytic activity and selectivity, demonstrating the key role of "promoter effect" as an additional adjusting parameter in catalysis.It should be noted here, as mentioned in the case of size and shape effects, that the promoter

It should be noted here, as mentioned in the case of size and shape effects, that the promoter effect is not always positive. The latter strongly depends on reactants type (electron donor or electron acceptor adsorbates) and the work function changes (increase or decrease) induced by the promoter (electropositive or electronegative). Thus, besides the structure-sensitivity, commented above, the electronic-sensitivity of a given reaction should always be taken into account, when attempting to coadjust the size, shape and electronic state. Although, this is not an easy task, it could be an effective approach towards designing cost-effective and highly active composites, as revealed in the sequence. effect is not always positive. The latter strongly depends on reactants type (electron donor or electron acceptor adsorbates) and the work function changes (increase or decrease) induced by the promoter (electropositive or electronegative). Thus, besides the structure-sensitivity, commented above, the electronic-sensitivity of a given reaction should always be taken into account, when attempting to co-adjust the size, shape and electronic state. Although, this is not an easy task, it could be an effective approach towards designing cost-effective and highly active composites, as revealed in the sequence.

#### *2.4. Chemical Modifiers*

Besides the extensive use of alkalis or alkaline earths as promoters, numerous other chemical substances can be employed to modulate the local surface chemistry/structure and in turn, the activity, selectivity and long-term stability of parent catalyst (e.g., [74]). In this regard, metal alloys (e.g., Au-Ni alloys as reforming catalysts [183], Pt-Sn alloys for ethanol oxidation [184]) are extensively employed in catalysis towards obtaining highly active and cost-effective catalytic formulations. Several mechanisms are considered responsible for the enhanced performance of bimetallic systems, involving mainly structural (strain effects) and electronic (charge-transfer effects) modifications that can be induced by the interaction between the different counterparts. The latter substantially modifies the binding energy of adsorbates and the path of chemical reactions with major consequences in catalysis [50,74].

In a similar manner, chemical substances with unique physico-chemical properties, such as carbon-based materials, have lately received considerable attention as chemical modifiers or supporting carriers [62,185]. Various carbon materials, such as carbon nanotubes (CNTs), reduced graphene oxide (rGO), ordered mesoporous carbon (OMC), carbon nanofibers (CNFs), and graphitic carbon nitride (g-C3N4), have received particular attention in catalysis after the significant progress in controlled synthesis and the fundamental understanding of their properties. In general, nanocarbons (NCs) possess unique physical (large surface area, specific morphology, appropriate pore structure) and chemical (electronic structure, surface acidity/basicity) properties arising from their nanoscale confined structures [185].

The combination of metal nanoparticles (NPs) with carbon materials by means of various synthetic approaches can exert significant modifications on the structural and electronic surrounding of NPs with subsequent implications in catalysis [62,185]. As for example, confined Fe NPs in CNTs exhibit an almost twice yield to C5<sup>+</sup> hydrocarbons as compared to Fe particles during the syngas conversion to liquid hydrocarbons [186]. The latter was mainly ascribed to the modified structural and redox properties of confined Fe NPs within CNTs [62]. Moreover, the application of graphene in catalysis allows the fabrication of multifunctional materials with distinct heterostructures, which offer quite different properties as compared to individual materials [185,187,188]. In general, carbon materials with exceptional structural and electronic characteristics can be effectively employed either as supporting materials or chemical modifiers, offering unique opportunities towards modulating the intrinsic reactivity of MOs. For instance, it has been found that the homogeneous distribution of copper atoms on the surface of rGO in combination with the outstanding electronic properties of rGO lead to high electrocatalytic activity, due to the synergy between the two components [189].

Metal-organic frameworks (MOFs) are another type of supporting carriers/chemical modifiers consisting of inorganic metal ions or clusters that are bridged with organic ligands in order for one or more dimensional configurations to be formed [190]. These materials exhibit unique properties, such as high surface area and porosity, while their complex network consisting of various channels allows passage in small molecules [191]. The fabrication of MOF-based MOs composites is of great interest, as it results in the development of materials with tunable properties and functionality. Metal nanoparticles regarded as the active centers can be stabilized by MOFs through confinement effects [192]. As for example, Cu, Ni, and Pd nanoparticles encapsulated by MOFs exhibited high catalytic efficiency, ascribed mainly to the synergistic effects of nanoconfinement and electron-donation offered by MOF framework [193–198]. Furthermore, by changing the MOFs functional groups, products distribution may differ, as a consequence of variations induced in the chemical environment of the catalytically active sites [199].

### *2.5. Pretreatment E*ff*ects*

Besides the advances that can be induced by adjusting the size, shape and electronic state of MOs, special pretreatment protocols or activation procedures could be applied to further adjust the local surface chemistry of MOs (e.g., [200,201]). In particular, the local surface chemistry of the MOs can be further tailored by appropriate pretreatment protocols, including thermal or chemical pretreatment. According to the pretreatment protocol followed, different properties get affected, resulting in diversified catalytic behavior. By way of illustration, it has been reported that defect engineering by a low-pressure thermal process instead of atmospheric pressure activation, could notably increase the concentration of oxygen vacancy defects and in turn, the CO oxidation activity of ceria nanoparticles, offering an additional tool towards the fine-tuning of MOs [200]. Moreover, it has been documented that the pretreatment protocol (oxidation or reduction) induces significant effects on the local surface structure of cobalt-ceria oxides affecting the dehydroxylation process in ammonia synthesis [202]. In a similar manner, oxidative pretreatment of cobalt-ceria catalysts resulted in an impoverishment of catalyst surface in cobalt species, due to the preferential existence of cerium species on the outer surface, whereas, cobalt and cerium species are uniformly distributed on the catalyst surface through the reduction pretreatment, which gives rise to the formation of oxygen vacancies [33]. In addition, a strong interaction between gold and ceria has been observed after O<sup>2</sup> pretreatment, due to the electron transfer from Au<sup>0</sup> to ceria, giving rise to oxygen vacancy formation, lattice oxygen migration, as well as to the formation of Auδ+-CO and surface bicarbonate species, favoring, thus, the adsorption of CO and the desorption of CO<sup>2</sup> [203]. In terms of T100, CO oxidation performance showed the following order: O<sup>2</sup> pretreatment (74 ◦C) < N<sup>2</sup> pretreatment (142 ◦C) < 10% CO/Ar pretreatment (169 ◦C) [203]. In view of the above short discussion, the pretreatment conditions can affect the facilitation with which certain active species are formed on the catalyst surface, the oxygen mobility or the formation of oxygen defects, with great implications in the catalytic performance.

#### **3. Implications of MOs Fine-Tuning in Catalysis Exemplified by CuOx**/**CeO<sup>2</sup> Binary System**

In this section, the implications of metal oxides fine-tuning by means of the above-described size, shape and electronic/chemical effects are presented, on the basis of the CuOx/CeO<sup>2</sup> binary oxide system. This particular catalytic system is selected as representative MOs, taking into account the tremendous fundamental and practical attention lately devoted to the copper-containing cerium oxide materials. More specifically, the abundant availability of copper and ceria and consequently, their lower cost (about four orders of magnitude, Figure 1) render CuOx/CeO<sup>2</sup> composites strongly competitive. Moreover, their excellent reactivity—linked to peculiar metal-support interactions—in conjunction to their remarkable resistance to various substances, such as carbon dioxide, water and sulfur is of particular fundamental and practical importance [57,76,204]. Remarkably, copper-containing ceria catalysts appropriately adjusted by the aforementioned routes demonstrated catalytic activity similar or even better than NMs-based catalysts in various applications, such as CO oxidation, the decomposition of N2O and the water-gas shift reaction, among others [115,124,159,205–216].

For instance, the inverse CeOx/Cu(111) system exhibits superior CO oxidation performance at a relatively low-temperature range (50–100 ◦C), in which the noble metals do not function well, exhibiting activity values of about one order of magnitude higher than those measured on Pt(100), Pt(111), and Rh(111) [217–219]. The latter has been mainly attributed, on the basis of the most conceptual experimental and theoretical studies, to the existence of Ce3<sup>+</sup> at the metal-oxide interface which binds O atoms weaker as compared to bulk Ce3<sup>+</sup> [217,220].

In light of the above aspects, in this section, the main implications of size, shape and electronic/chemical effects on the catalytic performance of CuOx/CeO<sup>2</sup> system during some of the most relevant applications in heterogeneous catalysis will be discussed. It should be stressed that it is not the aim of this section to provide an extended overview of CuOx/CeO<sup>2</sup> catalytic applications, which can be found in several comprehensive reviews [3,49,57,75,76,221]. It mainly aims to provide a general optimization framework towards the development of highly active and cost-effective MOs, paving also the way for the decrease of precious metal content in NMs-based catalysts.

#### *3.1. CO Oxidation*

CO oxidation is probably the most studied reaction in heterogeneous catalysis, due to its practical and fundamental importance. The catalytic elimination of CO is of great importance in various applications involving, among others, automotive exhaust emissions control and fuel cell systems. More importantly, CO oxidation can serve as a prototype reaction to gain insight into the structure-property relationships.

Regarding, at first, the CO oxidation activity of individual CuO<sup>x</sup> phase, it has been clearly revealed that it is strongly dependent on oxidation state, size and morphology. In particular, the following activity order: Cu2O > metastable cluster CuO > CuO has been revealed, closely related to the ability to release lattice oxygen [222,223]. On the other hand, the exposed crystal planes of CuO<sup>x</sup> phase drastically affect the CO oxidation; truncated octahedral Cu2O with {332} facets displayed better activity than low index {111} and {100} planes [224]. Similarly, CuO with exposed {011} planes is more active that close-packed {111} planes [225]. In view of this fact, it has been found that the CO oxidation activity of CuO mesoporous nanosheets with high-index facets is about 35 times higher than that of the commercial sample [226]. In general, surface vacancies, originated from coordinately unsaturated surface Cu atoms, can easily activate oxygen species towards their reaction with the reducing agent [3].

In a similar manner, theoretical and experimental studies have shown that the energy of anionic vacancies formation over bare ceria follows the order: {111} > {100} > {110}, as previously analyzed [82,104,109,112,146,160–166]. Moreover, a large increase in oxygen vacancies concentration has been found for ceria crystal size lower than ca. 10 nm [98], revealing the interrelation between size and shape effects. In this regard, we recently showed that ceria nanorods with {100} and {110} exposed facets demonstrated the optimum CO oxidation activity amongst ceria samples of different morphology; a close relationship between crystal planes-oxygen exchange kinetics-CO oxidation activity was disclosed [115].

In view of the above aspects, it could be argued that by adjusting the shape and size of individual counterparts of MOs (CuO<sup>x</sup> and CeO<sup>x</sup> in the case of CuOx/CeO<sup>2</sup> mixed oxides), significant modifications in their redox and catalytic properties can be obtained. However, in view of the fact that "the whole is more than the sum of its part", the majority of catalytic studies in heterogeneous catalysis is based on CuOx/CeO<sup>2</sup> mixed oxide than on individual oxides [52,227,228]. The underlying mechanism of this synergistic effect linked to metal-support interactions is the subject of numerous theoretical and experimental studies in catalysis. The latest advances in the field of CuOx-CeO<sup>2</sup> interactions and their implications in catalysis have been recently reviewed by one of us [40]. In general, the superiority of binary oxides can be ascribed to various interrelated phenomena, involving among others: (i) Electronic perturbations between nanoparticles, (ii) redox interplay between interfacial sites, (iii) facilitation of the formation of structural defects, (iv) improved reducibility and oxygen mobility, (v) unique reactivity of interfacial sites [40]. However, all of these factors are closely related with the intrinsic and extrinsic characteristics of individual oxides, triggering unique opportunities towards the development of highly active MOs by engineering the size and shape of individual oxides and in turn, the interfacial reactivity . Moreover, chemical or electronic effects induced by aliovalent doping can exert a profound influence on the catalytic performance, offering an additional tool towards the rational design of MOs (Figure 2). In the sequence, the optimization of CO oxidation activity of CuOx/CeO<sup>2</sup> catalysts by means of the above-mentioned approaches is presented, as an indicative example of MOs rational design.

Recently, we thoroughly explored the impact of ceria nanoparticles shape effects on the CO oxidation activity of CuOx/CeO<sup>2</sup> catalysts. The results clearly revealed the significant role of morphology in the CO oxidation activity, following the order: Nanorods (NR) > nanopolyhedra (NP) > nanocubes (NC), Figure 11. However, more importantly, CuO incorporation to different ceria carriers boosted the catalytic performance, without affecting the order observed for bare CeO<sup>2</sup> (Figure 11), demonstrating the crucial role of support.

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**Figure 11.** CO conversion as a function of temperature for bare CeO2 and CuOx/CeO2 samples of different morphology (NR, NC and NP, as indicated in each curve). Reaction conditions: 2000 ppm CO, 1 vol.% O2, GHSV = 39,000 h<sup>−</sup>1 [115]. **Figure 11.** CO conversion as a function of temperature for bare CeO<sup>2</sup> and CuOx/CeO<sup>2</sup> samples of different morphology (NR, NC and NP, as indicated in each curve). Reaction conditions: 2000 ppm CO, 1 vol.% O<sup>2</sup> , GHSV = 39,000 h−<sup>1</sup> [115].

The CuOx/CeO2-NR sample exhibited a half-conversion temperature (T50) of ca. 70 °C, which is much lower to that required for a typical noble metal oxidation catalyst, such as Pt/Al2O3 (T50 = 230 °C), as shown in Figure 12. Based on a thorough in situ and ex situ characterization study, a perfect relationship between the CO oxidation performance and the following parameters was disclosed: (i) Relative population of Cu+/Ce3+ redox pairs, (ii) abundance of loosely bound oxygen species, expressed in terms of surface-to-bulk oxygen reducibility, (iii) relative concentration of oxygen vacancies, evidenced by the ID/IF2g Raman ratio (Figure 7c) [115]. Similar conclusions in relation to the key role of ceria morphology in the CO oxidation activity have been reported by several groups [117,134,154,158,210,212,229–231], most of these revealing the superiority of nanorods. The CuOx/CeO2-NR sample exhibited a half-conversion temperature (T50) of ca. 70 ◦C, which is much lower to that required for a typical noble metal oxidation catalyst, such as Pt/Al2O<sup>3</sup> (T<sup>50</sup> = 230 ◦C), as shown in Figure 12. Based on a thorough in situ and ex situ characterization study, a perfect relationship between the CO oxidation performance and the following parameters was disclosed: (i) Relative population of Cu+/Ce3<sup>+</sup> redox pairs, (ii) abundance of loosely bound oxygen species, expressed in terms of surface-to-bulk oxygen reducibility, (iii) relative concentration of oxygen vacancies, evidenced by the ID/IF2g Raman ratio (Figure 7c) [115]. Similar conclusions in relation to the key role of ceria morphology in the CO oxidation activity have been reported by several groups [117,134,154,158,210,212,229–231], most of these revealing the superiority of nanorods.

However, it should be noted that similar or even better catalytic activities can be obtained by different morphologies (e.g., [124,211,212]). In this regard, it was recently shown that sub-nanometer copper oxide clusters (1 wt.% Cu loading) deposited on ceria nanospheres (NS) exhibited superior performance as compared to that deposited on nanorods (T100 = 122 °C vs. 194 °C) [124]. Extensive characterization investigations revealed that the copper species in nanorods samples existed in both Cu-[Ox]-Ce and CuOx clusters, while CuOx clusters dominated in nanospheres. Among these species, only CuOx clusters could be easily reduced to Cu(I) when they were subjected to interaction with CO, which is considered to be the reason of the enhanced reactivity of CuOx/CeO2-NS samples [124]. However, it should be noted that similar or even better catalytic activities can be obtained by different morphologies (e.g., [124,211,212]). In this regard, it was recently shown that sub-nanometer copper oxide clusters (1 wt.% Cu loading) deposited on ceria nanospheres (NS) exhibited superior performance as compared to that deposited on nanorods (T<sup>100</sup> = 122 ◦C vs. 194 ◦C) [124]. Extensive characterization investigations revealed that the copper species in nanorods samples existed in both Cu-[Ox]-Ce and CuO<sup>x</sup> clusters, while CuO<sup>x</sup> clusters dominated in nanospheres. Among these species, only CuO<sup>x</sup> clusters could be easily reduced to Cu(I) when they were subjected to interaction with CO, which is considered to be the reason of the enhanced reactivity of CuOx/CeO2-NS samples [124].

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**Figure 12.** A comparison between bare ceria, copper-ceria and a noble metal catalyst, in terms of halfconversion temperature (T50) for the oxidation of CO. Reaction conditions: 2000 ppm CO, 1 vol.% O2, GHSV = 39,000 h<sup>−</sup>1. **Figure 12.** A comparison between bare ceria, copper-ceria and a noble metal catalyst, in terms of half-conversion temperature (T50) for the oxidation of CO. Reaction conditions: 2000 ppm CO, 1 vol.% O<sup>2</sup> , GHSV = 39,000 h−<sup>1</sup> .

So far, numerous synthesis routes and different precursors have been employed to adjust the structural and morphological characteristics of CuOx/CeO2 composites, mostly summarized by Prasad and Rattan [76]. For instance, it has been found that the use of Ce(III) instead of Ce(IV) precursors can lead to CuOx/CeO2 catalysts with superior reducibility and CO oxidation activity [232]. In particular, it was experimentally shown that Ce(III)-derived samples contained a higher amount of Cu+ species, through the redox equilibrium Cu2+ + Ce3+ ↔ Cu+ + Ce4+, which are responsible for their enhanced oxidation performance [232]. Moreover, CuOx/CeO2 samples prepared from copper acetate precursor demonstrated better CO oxidation performance as compared to those prepared from nitrate, chloride and sulfate precursors [233]. Avgouropoulos and co-workers [234,235] recently employed a novel hydrothermal method for the synthesis of atomically dispersed CuOx/CeO2 catalysts, offering high CO oxidation performance. By means of various complementary techniques, it was shown that the catalytic activity is mainly related to the nature of highly dispersed copper species rather than the structural/textural characteristics. In a similar manner, Elias et al. [236] reported on the facile synthesis of phase-pure, monodisperse ~3 nm Cu0.1Ce0.9O2-x crystallites via solution-based pyrolysis of heterobimetallic Schiff complexes. An increase of CO oxidation activity by one and three orders of magnitude compared to ceria nanoparticles (3 nm) and microparticles (5 μm), respectively, was attained (Figure 13). So far, numerous synthesis routes and different precursors have been employed to adjust the structural and morphological characteristics of CuOx/CeO<sup>2</sup> composites, mostly summarized by Prasad and Rattan [76]. For instance, it has been found that the use of Ce(III) instead of Ce(IV) precursors can lead to CuOx/CeO<sup>2</sup> catalysts with superior reducibility and CO oxidation activity [232]. In particular, it was experimentally shown that Ce(III)-derived samples contained a higher amount of Cu<sup>+</sup> species, through the redox equilibrium Cu2<sup>+</sup> <sup>+</sup> Ce3<sup>+</sup> <sup>↔</sup> Cu<sup>+</sup> <sup>+</sup> Ce4+, which are responsible for their enhanced oxidation performance [232]. Moreover, CuOx/CeO<sup>2</sup> samples prepared from copper acetate precursor demonstrated better CO oxidation performance as compared to those prepared from nitrate, chloride and sulfate precursors [233]. Avgouropoulos and co-workers [234,235] recently employed a novel hydrothermal method for the synthesis of atomically dispersed CuOx/CeO<sup>2</sup> catalysts, offering high CO oxidation performance. By means of various complementary techniques, it was shown that the catalytic activity is mainly related to the nature of highly dispersed copper species rather than the structural/textural characteristics. In a similar manner, Elias et al. [236] reported on the facile synthesis of phase-pure, monodisperse ~3 nm Cu0.1Ce0.9O2-x crystallites via solution-based pyrolysis of heterobimetallic Schiff complexes. An increase of CO oxidation activity by one and three orders of magnitude compared to ceria nanoparticles (3 nm) and microparticles (5 µm), respectively, was attained (Figure 13).

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**Figure 13.** CO oxidation on annealed 3 nm Cu0.1Ce0.9O2−x, 3 nm CeO2 and commercial 5 μm CeO2 (Sigma-Aldrich). (**A**) "Light off" curves and (**B**) area-normalized Arrhenius plots, measured in 1.0% CO, 2.5% O2 balanced in He at a flow rate of 1300 mL min<sup>−</sup>1 gcat−1 for 20 mg catalyst loadings. Reproduced with permission from Reference [236]. Copyright© 2014, American Chemical Society. **Figure 13.** CO oxidation on annealed 3 nm Cu0.1Ce0.9O2−x, 3 nm CeO<sup>2</sup> and commercial 5 µm CeO<sup>2</sup> (Sigma-Aldrich). (**A**) "Light off" curves and (**B**) area-normalized Arrhenius plots, measured in 1.0% CO, 2.5% O<sup>2</sup> balanced in He at a flow rate of 1300 mL min−<sup>1</sup> gcat −1 for 20 mg catalyst loadings. Reproduced with permission from Reference [236]. Copyright© 2014, American Chemical Society.

Besides the engineering of shape and size, porous structure engineering could exert a significant influence on the CO oxidation activity of CuOx/CeO2 catalysts [123,213]. As for example, threedimensional CuOx-doped CeO2 prepared by a hard template method exhibited complete CO conversion at temperatures as low as 50 °C, due to their improved textural and redox properties [213]. Besides the engineering of shape and size, porous structure engineering could exert a significant influence on the CO oxidation activity of CuOx/CeO<sup>2</sup> catalysts [123,213]. As for example, three-dimensional CuOx-doped CeO<sup>2</sup> prepared by a hard template method exhibited complete CO conversion at temperatures as low as 50 ◦C, due to their improved textural and redox properties [213].

Regarding the influence of CuOx/CeO2 composition on the CO oxidation activity, most of the studies revealed an optimum Cu/(Cu+Ce) atomic ratio in the range of 15–30% [212,231,232,237,238]. Within this specific range, the optimum physicochemical characteristics and interfacial interactions can be achieved, reflected then on catalytic activity. Regarding the influence of CuOx/CeO<sup>2</sup> composition on the CO oxidation activity, most of the studies revealed an optimum Cu/(Cu+Ce) atomic ratio in the range of 15–30% [212,231,232,237,238]. Within this specific range, the optimum physicochemical characteristics and interfacial interactions can be achieved, reflected then on catalytic activity.

Apart from the above-described approaches that have been put forward to improve the CO oxidation performance of CuOx/CeO2 oxides, the addition of aliovalent elements as structural/surface promoters should be mentioned. In view of this fact, it has been found that the modification of ceria support by Mn [239] or Sn [240] can drastically modify the dispersion of CuOx and the redox interplay Apart from the above-described approaches that have been put forward to improve the CO oxidation performance of CuOx/CeO<sup>2</sup> oxides, the addition of aliovalent elements as structural/surface promoters should be mentioned. In view of this fact, it has been found that the modification of ceria support by Mn [239] or Sn [240] can drastically modify the dispersion of CuO<sup>x</sup> and the redox

between Cu species and support, thus, enhancing the CO oxidation performance. Very recently, the tuning of the interfacial properties of CuOx/CeO2 by In2O3 doping was also explored [241]. It was found that the CO oxidation performance of In2O3-CuOx/CeO2 sample greatly exceeds that of parent interplay between Cu species and support, thus, enhancing the CO oxidation performance. Very recently, the tuning of the interfacial properties of CuOx/CeO<sup>2</sup> by In2O<sup>3</sup> doping was also explored [241]. It was found that the CO oxidation performance of In2O3-CuOx/CeO<sup>2</sup> sample greatly exceeds that of parent oxide, offering complete CO conversion at temperatures as low as 100 ◦C [241]. By means of complementary characterization studies and density functional theory calculations, it was proved that In2O<sup>3</sup> could modify the geometric structure of CuO<sup>x</sup> particles by reducing their size. The latter results in more metal-support interfacial sites and abundant defects. Moreover, the interaction between In and Cu could modify the electronic state of Cu atoms towards the stabilization of partially reduced Cu sites at the interface [241].

Recently, copper-ceria nanosheets were synthesized by using graphene oxide as a sacrificial template, in an attempt to increase the concentration of active interfacial sites [242]. The copper-ceria interaction was further adjusted by appropriate pretreatment, with the catalyst calcined at 600 ◦C exhibiting complete CO conversion at 90 ◦C, due to the high concentration in active copper species and oxygen vacancies [242]. Moreover, a sword-like copper-ceria composite derived by a Ce-based MOF with 5 wt.% Cu loading, exhibited superior CO conversion performance (T<sup>100</sup> = 100 ◦C) in comparison to other irregular-shaped catalysts, due to the good interfacial contact, which resulted in the abundance of Cu<sup>+</sup> active species and oxygen vacancies [191]. Very recently, triple-shelled CuOx/CeO<sup>2</sup> hollow nanospheres were synthesized by MOFs, exhibiting high CO conversion performance (T<sup>100</sup> = 130 ◦C) [243]. This was mainly ascribed to the porous structure of the triple-shelled morphology, offering an enhanced synergistic interaction between copper and ceria [243].

Table 1 summarizes, at a glance, indicative attempts followed to adjust the interfacial properties and in turn, the CO oxidation performance of CuOx/CeO<sup>2</sup> binary oxides. It is evident that extremely active composites can be obtained by adjusting the shape, size and electronic/chemical state by means of appropriate synthetic and/or promotional routes. It is of worth pointing out the superiority of finely-tuned CuOx/CeO<sup>2</sup> samples as compared to noble metal-based catalysts, offering unique opportunities towards the rational design of highly active metal oxide catalysts. Moreover, as further guidance, it would be of particular importance to explore the combining effect of different adjusted parameters (e.g., CuOx/CeO<sup>2</sup> nanorods co-doped with main-group elements) towards further optimization.


**Table 1.** Indicative studies towards adjusting the CO oxidation performance of CuOx/CeO2 oxides.

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