*3.2. N2O Decomposition*

Nitrous oxide (N2O) has been lately recognized as one of the most potent greenhouse gases and ozone depleting substances [246]. In view of this fact, the catalytic abatement of N2O has received particular attention as one of the most promising remediation methods. Although noble metals exhibit satisfactory activity, their high cost and sensitivity to various substances (e.g., O2, H2O) hinder widespread applications. Hence, as previously stated, noble metal-free composites have gained particular attention as potential candidates. The recent advances in the field of N2O decomposition over metal oxides have been recently reviewed by Konsolakis [246]. It was clearly revealed that MOs could be effectively applied for N2O decomposition, demonstrating comparable or even better catalytic performance compared to NMs-based catalysts. More interestingly, and in line with the aim and scope of the present article, it was shown that very active and stable MOs could be obtained by adjusting their size, shape and electronic state through appropriate synthesis and promotional routes [246].

Herein, we shortly present the main approaches lately followed to improve the deN2O performance of MOs, exemplified by the CuOx/CeO<sup>2</sup> system. Table 2 presents indicative studies towards this direction, involving our recent advances in the field [215,247]. It is of worth noticing the comparable or even superior deN2O performance of finely-tuned CuOx/CeO<sup>2</sup> samples as compared to noble metal-based catalysts (Table 2).

Recently, we explored the impact of synthesis procedure (impregnation, co-precipitation and exotemplating methods) on the deN2O performance of CuOx/CeO<sup>2</sup> mixed oxides [247]. Co-precipitation method resulted in the optimum performance, offering complete N2O conversion at ca. 550 ◦C. On the basis of characterization results (XPS, TPR, micro-Raman), the superiority of precipitated samples was ascribed to their enhanced reducibility and the facilitation of Ce4+/Ce3<sup>+</sup> and Cu2+/Cu<sup>+</sup> redox cycles. In an attempt to further improve the deN2O performance of CuOx/CeO<sup>2</sup> samples, very recently, we explored the potential to further adjust the local surface chemistry and metal-support interactions by means of electronic (alkali) promotion. Notably, the results showed that by co-adjusting the synthesis procedure and the electronic state, highly active deN2O catalysts could be obtained; the sample with a Cs content of 1.0 at Cs/nm<sup>2</sup> offers a half-conversion temperature (T50) about 200 ◦C lower as compared to the commercial sample (Figure 14) [215]. The superiority of Cs-doped samples was ascribed to the electronic effect of alkali doping towards stabilizing partially reduced Cu+/Ce3<sup>+</sup> pairs, which play a pivotal role in the deN2O process [215,246].


**Table 2.** Indicative studies followed to adjust the deN2O performance of CuOx/CeO2 oxides.

*Catalysts* **2020**, *10*, 160

WHSV: Weight hourly space velocity [=] mL g−1 h −<sup>1</sup> ; GHSV: Gas hourly space velocity [=] h−1

.

**Figure 14.** Optimization of deN2O performance of CuOx/CeO2 mixed oxides by co-adjusting synthesis parameters (co-precipitation method) and electronic state (alkali addition). For comparison, the corresponding performance of CuOx supported on commercial ceria is included. Reaction conditions: 0.1% N2O balanced with He; WHSV = 90,000 mL g<sup>−</sup>1 h−1 [215]. **Figure 14.** Optimization of deN2O performance of CuOx/CeO<sup>2</sup> mixed oxides by co-adjusting synthesis parameters (co-precipitation method) and electronic state (alkali addition). For comparison, the corresponding performance of CuOx supported on commercial ceria is included. Reaction conditions: 0.1% N2O balanced with He; WHSV = 90,000 mL g−<sup>1</sup> h −1 [215].

The effect of ceria morphology (nanorods, nanocubes, nanopolyhedra) on the deN2O performance of CuOx/CeO2 composites was extensively investigated by Pintar and co-workers [146]. Copper clusters located on {100} and {110} planes—preferentially exposed on ceria nanorods—exhibit a normalized activity ca. 20% higher compared to {111} planes of polyhedra (Figure 15). In terms of conversion performance, the 4.0 wt.% CuOx/Ceria-nanorods exhibited a half-conversion temperature (T50) of about 430 °C compared to 440 °C and 470 °C of nanopolyhedra and nanocubes, respectively. On the basis of a thorough characterization study, it was disclosed that the oxygen mobility and the regeneration of active Cu phase are easier on ceria nanorods, which in turn, facilitates the deN2O activity through oxygen desorption and replenishment of active sites [146]. In a similar manner, CuOx supported on CeO2 nanospheres exhibited high deN2O performance (T50 = 380 °C, Table 2), ascribed mainly to the high population of CuOx clusters on the high surface area CeO2 nanospheres [205]. These findings clearly demonstrate the significant advances that can be achieved in the deN2O process by engineering the size and shape of metal oxide composites. The effect of ceria morphology (nanorods, nanocubes, nanopolyhedra) on the deN2O performance of CuOx/CeO<sup>2</sup> composites was extensively investigated by Pintar and co-workers [146]. Copper clusters located on {100} and {110} planes—preferentially exposed on ceria nanorods—exhibit a normalized activity ca. 20% higher compared to {111} planes of polyhedra (Figure 15). In terms of conversion performance, the 4.0 wt.% CuOx/Ceria-nanorods exhibited a half-conversion temperature (T50) of about 430 ◦C compared to 440 ◦C and 470 ◦C of nanopolyhedra and nanocubes, respectively. On the basis of a thorough characterization study, it was disclosed that the oxygen mobility and the regeneration of active Cu phase are easier on ceria nanorods, which in turn, facilitates the deN2O activity through oxygen desorption and replenishment of active sites [146]. In a similar manner, CuO<sup>x</sup> supported on CeO<sup>2</sup> nanospheres exhibited high deN2O performance (T<sup>50</sup> = 380 ◦C, Table 2), ascribed mainly to the high population of CuO<sup>x</sup> clusters on the high surface area CeO<sup>2</sup> nanospheres [205]. These findings clearly demonstrate the significant advances that can be achieved in the deN2O process by engineering the size and shape of metal oxide composites.

*Catalysts* **2019**, *9*, x FOR PEER REVIEW 26 of 57

**Figure 15.** The activity of nanoshaped CuOx/CeO2 catalysts measured at T = 375 °C. Adapted from Reference [146]. Copyright© 2015, American Chemical Society. **Figure 15.** The activity of nanoshaped CuOx/CeO<sup>2</sup> catalysts measured at T = 375 ◦C. Adapted from Reference [146]. Copyright© 2015, American Chemical Society.

#### *3.3. Preferential Oxidation of CO (CO-PROX) 3.3. Preferential Oxidation of CO (CO-PROX)*

below.

The copper-ceria binary oxides are amongst the most widely investigated catalytic systems in the preferential oxidation of carbon monoxide (CO-PROX), a reaction used for the production of highly purified hydrogen and the removal of CO. CuOx/CeO2 catalysts have gained particular attention in CO-PROX process, due to their superior performance, which is mainly ascribed to the peculiar properties of copper-ceria interface [40]. The copper-ceria binary oxides are amongst the most widely investigated catalytic systems in the preferential oxidation of carbon monoxide (CO-PROX), a reaction used for the production of highly purified hydrogen and the removal of CO. CuOx/CeO<sup>2</sup> catalysts have gained particular attention in CO-PROX process, due to their superior performance, which is mainly ascribed to the peculiar properties of copper-ceria interface [40].

In the light of the above-mentioned size, shape and electronic/chemical effects, numerous efforts have been put forward towards optimizing the CO-PROX performance. Indicative approaches followed to fine-tune the CO-PROX performance are summarized in Table 3, and further discussed In the light of the above-mentioned size, shape and electronic/chemical effects, numerous efforts have been put forward towards optimizing the CO-PROX performance. Indicative approaches followed to fine-tune the CO-PROX performance are summarized in Table 3, and further discussed below.



**Table 3.** Indicative studies towards adjusting the CO preferential oxidation performance of CuOx/CeO2 oxides.

Several copper-ceria catalytic systems of various copper loadings have been synthesized by different methods, with the optimum Cu loading varying between 5 and 10 wt.% [250,252–254,265]. A further increase in Cu content from 10 to 15 wt.% has been reported to reduce the catalytic activity, due to the large CuO<sup>x</sup> agglomerates on the catalyst surface [252]. It was revealed, by means of both ex situ and in situ characterization studies, that the desired CO oxidation process is related to partially reduced Cu<sup>+</sup> species, whereas, highly reduced copper species not strongly associated with CeO<sup>2</sup> favor the undesired H<sup>2</sup> oxidation [40,250,265–267]. In view of this fact, extensive research efforts have been put forward to control the two competitive oxidation processes by appropriately adjusting the geometric and electronic interactions between copper and ceria through the above-described fine-tuning approaches. different methods, with the optimum Cu loading varying between 5 and 10 wt.% [250,252–254,265]. A further increase in Cu content from 10 to 15 wt.% has been reported to reduce the catalytic activity, due to the large CuOx agglomerates on the catalyst surface [252]. It was revealed, by means of both ex situ and in situ characterization studies, that the desired CO oxidation process is related to partially reduced Cu+ species, whereas, highly reduced copper species not strongly associated with CeO2 favor the undesired H2 oxidation [40,250,265–267]. In view of this fact, extensive research efforts have been put forward to control the two competitive oxidation processes by appropriately adjusting the geometric and electronic interactions between copper and ceria through the above-described finetuning approaches. Regarding the shape effect, different copper-ceria nanostructures (rods, cubes, spheres,

Several copper-ceria catalytic systems of various copper loadings have been synthesized by

Regarding the shape effect, different copper-ceria nanostructures (rods, cubes, spheres, octahedra, spindle or multi-shelled morphologies) have been synthesized and studied for the CO-PROX reaction. It was revealed that the shape-controlled synthesis of ceria nanoparticles has a profound influence on the CO-PROX activity and selectivity. In particular, it was found that rod-shaped and polyhedral copper-ceria systems exhibited higher CO conversion performance (T<sup>50</sup> = 68 ◦C) at low-temperatures, as compared to plates (T<sup>50</sup> = 71 ◦C) and cubes (T<sup>50</sup> = 89 ◦C) [157]. The latter was mainly attributed to the smaller CuO<sup>x</sup> clusters subjected to a strong interaction with the ceria carrier, which, in turn, facilitates the formation of Cu<sup>+</sup> sites and oxygen vacancies [157]. More importantly, a close relationship between measurable physicochemical parameters, such as the amount of Cu<sup>+</sup> species and the A584/A<sup>454</sup> Raman ratio (related to oxygen vacancies) with the catalytic performance was obtained (Figure 16); rodand polyhedral-shaped samples exhibited the highest values on Cu<sup>+</sup> species and oxygen vacancies, demonstrating, also, the optimum CO-PROX performance [157]. octahedra, spindle or multi-shelled morphologies) have been synthesized and studied for the CO-PROX reaction. It was revealed that the shape-controlled synthesis of ceria nanoparticles has a profound influence on the CO-PROX activity and selectivity. In particular, it was found that rodshaped and polyhedral copper-ceria systems exhibited higher CO conversion performance (T50 = 68 °C) at low-temperatures, as compared to plates (T50 = 71 °C) and cubes (T50 = 89 °C) [157]. The latter was mainly attributed to the smaller CuOx clusters subjected to a strong interaction with the ceria carrier, which, in turn, facilitates the formation of Cu+ sites and oxygen vacancies [157]. More importantly, a close relationship between measurable physicochemical parameters, such as the amount of Cu+ species and the A584/A454 Raman ratio (related to oxygen vacancies) with the catalytic performance was obtained (Figure 16); rod- and polyhedral-shaped samples exhibited the highest values on Cu+ species and oxygen vacancies, demonstrating, also, the optimum CO-PROX performance [157].

**Figure 16.** (**a**) TOF (60 ◦C)/T50% values versus Cu<sup>+</sup> content and (**b**) TOF (60 ◦C)/T50% values versus A584/A<sup>454</sup> ratios for CuO/CeO<sup>2</sup> catalysts with different morphologies. Adapted from Reference [157]. Copyright© 2016, Royal Society of Chemistry.

Copyright© 2016, Royal Society of Chemistry.

In this point, it should be mentioned that in relation to which ceria shape is the most active or selective for the CO-PROX process, inconclusive results are acquired, due to the different reaction conditions applied (see Table 3) in conjunction to the complexity of CO-PROX process, which is affected to a different extent by the various interrelated parameters (e.g., reducibility, metal dispersion, oxygen vacancies, oxidation state, metal-support interactions). Under this perspective, it was reported that copper-ceria nanocubes exhibited higher CO<sup>2</sup> selectivity than copper-ceria nanorods or nanospheres, due to the difficulty of nanocubes to fully reduce the copper oxide species under CO-PROX conditions [133,268], while, at the same time, exhibiting lower CO conversion than nanorods [268] and nanospheres [133]. In a similar manner, CuOx/CeO<sup>2</sup> spheres and spindles, exposing {111} and {002} facets, showed the highest CO conversion (T<sup>50</sup> = 69 and 74 ◦C, respectively), as well as a wide temperature window for total CO conversion (95–195 ◦C for spheres and 115–215 ◦C for spindles), in comparison with octahedrons, cubes and rods [116]. Interestingly, in different shaped ceria nanostructures, a close relationship is found between the concentration of oxygen vacancies and the amount of reduced copper species (Figure 17), clearly revealing the key role of exposed facets towards adjusting the catalytic performance. These findings were further substantiated by DFT calculations (Figure 18), showing the high population of oxygen vacancies at the intersection of {111} and {002} facets in opposition to CeO<sup>2</sup> {111} surface [116]. conditions applied (see Table 3) in conjunction to the complexity of CO-PROX process, which is affected to a different extent by the various interrelated parameters (e.g., reducibility, metal dispersion, oxygen vacancies, oxidation state, metal-support interactions). Under this perspective, it was reported that copper-ceria nanocubes exhibited higher CO2 selectivity than copper-ceria nanorods or nanospheres, due to the difficulty of nanocubes to fully reduce the copper oxide species under CO-PROX conditions [133,268], while, at the same time, exhibiting lower CO conversion than nanorods [268] and nanospheres [133]. In a similar manner, CuOx/CeO2 spheres and spindles, exposing {111} and {002} facets, showed the highest CO conversion (T50 = 69 and 74 °C, respectively), as well as a wide temperature window for total CO conversion (95–195 °C for spheres and 115–215 °C for spindles), in comparison with octahedrons, cubes and rods [116]. Interestingly, in different shaped ceria nanostructures, a close relationship is found between the concentration of oxygen vacancies and the amount of reduced copper species (Figure 17), clearly revealing the key role of exposed facets towards adjusting the catalytic performance. These findings were further substantiated by DFT calculations (Figure 18), showing the high population of oxygen vacancies at the intersection of {111} and {002} facets in opposition to CeO2 {111} surface [116].

*Catalysts* **2019**, *9*, x FOR PEER REVIEW 30 of 57

**Figure 16.** (**a**) TOF (60 °C)/T50% values versus Cu+ content and (**b**) TOF (60 °C)/T50% values versus A584/A454 ratios for CuO/CeO2 catalysts with different morphologies. Adapted from Reference [157].

selective for the CO-PROX process, inconclusive results are acquired, due to the different reaction

**Figure 17.** Plots of the number of oxygen vacancies and reduced copper species for the CuO/CeO2 catalysts with different support shapes. Adapted from Reference [116]. Copyright© 2018, Elsevier. **Figure 17.** Plots of the number of oxygen vacancies and reduced copper species for the CuO/CeO<sup>2</sup> catalysts with different support shapes. Adapted from Reference [116]. Copyright© 2018, Elsevier.

[116]. Copyright© 2018, Elsevier.

which facilitates reactants accessibility [260].

*Catalysts* **2019**, *9*, x FOR PEER REVIEW 31 of 57

**Figure 18.** (**a**) DFT-calculated adsorption energy for molecularly adsorbed CO on {111}, {002} facets and their intersection sites of a Ce60O120 cluster obtained, (**b**) Models of the optimized geometries of Ce60O119 with an oxygen vacancy in the highlighted positions: {111}, {002} facets and their intersection, and values of oxygen vacancy formation energy (Evac) below models. (Color code: O in red, Ce in grey, oxygen vacancy in the highlighted circle labeled "V"). Reproduced with permission from Reference **Figure 18.** (**a**) DFT-calculated adsorption energy for molecularly adsorbed CO on {111}, {002} facets and their intersection sites of a Ce60O<sup>120</sup> cluster obtained, (**b**) Models of the optimized geometries of Ce60O<sup>119</sup> with an oxygen vacancy in the highlighted positions: {111}, {002} facets and their intersection, and values of oxygen vacancy formation energy (Evac) below models. (Color code: O in red, Ce in grey, oxygen vacancy in the highlighted circle labeled "V"). Reproduced with permission from Reference [116]. Copyright© 2018, Elsevier.

In an attempt to optimize the CO-PROX performance through size and shape engineering singleand multi-shelled copper-ceria hollow microspheres were synthesized [260,269]. By tuning the number of shells, the catalytic activity was notably improved, with the triple-shelled structure (Figure 19) exhibiting the highest activity and selectivity (100% CO conversion and 91% CO2 selectivity at 95 °C), as well as a wide temperature window for complete CO conversion (95–195 °C) [260]. The increase in the number of shells enhances the electronic and geometric interaction between copper and ceria, offering a high population of exposed active sites and an increased space inside the catalyst In an attempt to optimize the CO-PROX performance through size and shape engineering singleand multi-shelled copper-ceria hollow microspheres were synthesized [260,269]. By tuning the number of shells, the catalytic activity was notably improved, with the triple-shelled structure (Figure 19) exhibiting the highest activity and selectivity (100% CO conversion and 91% CO<sup>2</sup> selectivity at 95 ◦C), as well as a wide temperature window for complete CO conversion (95–195 ◦C) [260]. The increase in the number of shells enhances the electronic and geometric interaction between copper and ceria, offering a high population of exposed active sites and an increased space inside the catalyst which facilitates reactants accessibility [260].

*Catalysts* **2019**, *9*, x FOR PEER REVIEW 32 of 57

**Figure 19.** Schematic diagrams for CO-PROX over the CuOx/CeO2 hollow microsphere catalysts with triple shells. Reproduced with permission from Reference [260]. Copyright© 2019, Royal Society of Chemistry. **Figure 19.** Schematic diagrams for CO-PROX over the CuOx/CeO<sup>2</sup> hollow microsphere catalysts with triple shells. Reproduced with permission from Reference [260]. Copyright© 2019, Royal Society of Chemistry.

Taking into account the pivotal role of nanoparticles crystallite size/shape and their consequent effect on metal-support interactions, different preparation routes have been investigated for the synthesis of copper-ceria composites, such as the hydrothermal method, the template-assisted method, the solid-state preparation method, sol-gel, co-precipitation, freeze-drying, depositionprecipitation, etc. [250,251,253–255,257,262,270]. For instance, template-assisted synthesis resulted in small ceria crystallite sizes (ca. 5.6 nm), and thus, in a high population of copper-ceria interfacial sites with implications in Cu oxidation state and CO-PROX activity [257]. Moreover, the precursor compounds or the template agent used during the synthesis procedure can affect the pore size and volume or the reducibility of the materials [255,271]. Interestingly, ethanol washing during the preparation of CuOx/CeO2 oxides leads to decreased particle sizes, as it affects the dehydration process between precursors particles, resulting in decreased adsorbed water and improved dispersion [272]. Very recently, a novel ultrasound-assisted precipitation method was employed to adjust the defective structure of CeO2 and in turn, the CO-RPOX activity [273]. By means of characterization techniques and theoretical calculations, it was shown that only two-electron defects on ceria surface (i.e., defects adsorbing oxygen to form peroxides instead of superoxide species which are formed on one-electron defects) were responsible for the formation of Cu+ and Ce3+ species, which were intimately involved in the CO adsorption and oxygen activation processes [273]. In particular, the adsorption of O2 on two-electron defects resulted in peroxides formation, followed by Cu ions incorporation towards the development of Cu-O-Ce structure. Meanwhile, the two additional electrons in the two-electron defects facilitate the electronic re-dispersion in Cu-O-Ce structure, leading to the creation of Cu+ (CO adsorption sites) and Ce3+ (oxygen activation sites). Another approach in the direction of catalysts functionalization that has attracted considerable attention in recent years is the preparation of inverse catalytic systems. In particular, the co-existence of Cu+ and Cu2+ ions was observed in star-shaped inverse CeO2/CuOx catalysts which exhibited high Taking into account the pivotal role of nanoparticles crystallite size/shape and their consequent effect on metal-support interactions, different preparation routes have been investigated for the synthesis of copper-ceria composites, such as the hydrothermal method, the template-assisted method, the solid-state preparation method, sol-gel, co-precipitation, freeze-drying, deposition-precipitation, etc. [250,251,253–255,257,262,270]. For instance, template-assisted synthesis resulted in small ceria crystallite sizes (ca. 5.6 nm), and thus, in a high population of copper-ceria interfacial sites with implications in Cu oxidation state and CO-PROX activity [257]. Moreover, the precursor compounds or the template agent used during the synthesis procedure can affect the pore size and volume or the reducibility of the materials [255,271]. Interestingly, ethanol washing during the preparation of CuOx/CeO<sup>2</sup> oxides leads to decreased particle sizes, as it affects the dehydration process between precursors particles, resulting in decreased adsorbed water and improved dispersion [272]. Very recently, a novel ultrasound-assisted precipitation method was employed to adjust the defective structure of CeO<sup>2</sup> and in turn, the CO-RPOX activity [273]. By means of characterization techniques and theoretical calculations, it was shown that only two-electron defects on ceria surface (i.e., defects adsorbing oxygen to form peroxides instead of superoxide species which are formed on one-electron defects) were responsible for the formation of Cu<sup>+</sup> and Ce3<sup>+</sup> species, which were intimately involved in the CO adsorption and oxygen activation processes [273]. In particular, the adsorption of O<sup>2</sup> on two-electron defects resulted in peroxides formation, followed by Cu ions incorporation towards the development of Cu-O-Ce structure. Meanwhile, the two additional electrons in the two-electron defects facilitate the electronic re-dispersion in Cu-O-Ce structure, leading to the creation of Cu<sup>+</sup> (CO adsorption sites) and Ce3<sup>+</sup> (oxygen activation sites).

catalytic activity [274]. Moreover, the alteration of Ce/Cu molar ratio and/or the pH value in the inverse CeO2/CuOx catalysts notably affects the morphology and the particle size, which in turn, favor the contact interface between ceria and copper, and thus, the CO oxidation at the expense of H2 oxidation in PROX process [44]. In addition, a multi-step synthetic approach has been applied for a high concentration of oxygen vacancies to be successfully anchored at the interfaces of the inverse CeO2/CuOx system, leading to outstanding CO-PROX activity (~100% CO conversion at a wide temperature window 120–210 °C) and adequate stability [275]. Another approach in the direction of catalysts functionalization that has attracted considerable attention in recent years is the preparation of inverse catalytic systems. In particular, the co-existence of Cu<sup>+</sup> and Cu2<sup>+</sup> ions was observed in star-shaped inverse CeO2/CuO<sup>x</sup> catalysts which exhibited high catalytic activity [274]. Moreover, the alteration of Ce/Cu molar ratio and/or the pH value in the inverse CeO2/CuO<sup>x</sup> catalysts notably affects the morphology and the particle size, which in turn, favor the contact interface between ceria and copper, and thus, the CO oxidation at the expense of H<sup>2</sup> oxidation in PROX process [44]. In addition, a multi-step synthetic approach has been applied for a high concentration of oxygen vacancies to be successfully anchored at the interfaces of the inverse CeO2/CuO<sup>x</sup> system, leading to outstanding CO-PROX activity (~100% CO conversion at a wide temperature window 120–210 ◦C) and adequate stability [275].

The doping effect on the CO-PROX performance has also been studied in the inverse copper-ceria catalysts [276,277]. It was reported that doping ceria with transition metals (e.g., Fe, Co, Ni) induces changes in the ceria lattice and in the formation of oxygen vacancies [276]. The doping element affects the reducibility of the CeO2/CuO<sup>x</sup> catalysts, while promoting the formation of Ce3<sup>+</sup> ions and oxygen vacancies, with the NiO-doped CeO2/CuO<sup>x</sup> catalyst exhibiting the highest activity (T<sup>50</sup> = 68 ◦C) and the widest temperature window for total CO conversion (115–155 ◦C) [276]. In the inverse copper-ceria catalysts, it has also been found that the presence of Zn improves the CO-PROX performance, as it has the ability to hinder the CuO reduction to highly reduced copper sites which provide the active sites for the H<sup>2</sup> oxidation [277]. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 33 of 57 The doping effect on the CO-PROX performance has also been studied in the inverse copperceria catalysts [276,277]. It was reported that doping ceria with transition metals (e.g., Fe, Co, Ni) induces changes in the ceria lattice and in the formation of oxygen vacancies [276]. The doping element affects the reducibility of the CeO2/CuOx catalysts, while promoting the formation of Ce3+ ions and oxygen vacancies, with the NiO-doped CeO2/CuOx catalyst exhibiting the highest activity (T50 = 68 °C) and the widest temperature window for total CO conversion (115–155 °C) [276]. In the inverse copper-ceria catalysts, it has also been found that the presence of Zn improves the CO-PROX

By applying appropriate pretreatment protocols, the CO-PROX performance may also be greatly affected. In particular, the pretreatment of copper-ceria catalysts in an oxidative or reductive atmosphere affects the amount and dispersion of the active species, and consequently, the catalytic performance [262]. The pretreatment with hydrogen led to a breakage of the Cu-[Ox]-Ce structure, which resulted in enhanced catalytic performance, indicating the significance of the highly dispersed CuO<sup>x</sup> clusters in the CO-PROX process [262]. Furthermore, the pretreatment in an acidic or a basic environment affects the interaction between the two oxide phases. For instance, the pretreatment of ceria spheres in a basic solvent (2M NaOH), followed by etching in an ionic liquid for the acquisition of ceria nanocubes, resulted in the best catalytic activity at temperatures lower than 150 ◦C, due to the strong interaction between the highly dispersed CuO<sup>x</sup> clusters and ceria support [263]. An acidic treatment with nitric acid in nanorod-shaped CuOx/CeO<sup>2</sup> catalysts has also been performed by Avgouropoulos and co-workers [264]. It was found that a highly acidic environment (pH < 4) led to an enrichment of catalysts surface in Cu<sup>+</sup> species and to high concentrations of oxygen vacancies and Ce3<sup>+</sup> species, while facilitating the formation of surface hydroxyls that are considered responsible for controlling the interfacial interactions in the copper-ceria binary system [264]. All the above-mentioned characteristics in conjunction with the better copper dispersion and the improved reducibility of the highly acidic catalysts resulted in enhanced catalytic performance (T<sup>50</sup> ' 84 ◦C) [264]. The same group has also investigated the pretreatment effect of employing ammonia solutions in copper-ceria nanorods [278]. It was revealed that the textural and structural properties of the modified catalysts remained almost unaffected after treatment, whereas, increasing the Cu:NH<sup>3</sup> ratio to 1:4 resulted in higher reducibility and gave rise to Cu<sup>+</sup> and surface lattice oxygen species, leading, thus, to improved catalytic performance [278]. As shown in Figure 20, close relationships between the half-conversion temperature (T50) and the main Raman peak shift or the concentration of Ce3<sup>+</sup> and oxygen vacancies were observed [278]. performance, as it has the ability to hinder the CuO reduction to highly reduced copper sites which provide the active sites for the H2 oxidation [277]. By applying appropriate pretreatment protocols, the CO-PROX performance may also be greatly affected. In particular, the pretreatment of copper-ceria catalysts in an oxidative or reductive atmosphere affects the amount and dispersion of the active species, and consequently, the catalytic performance [262]. The pretreatment with hydrogen led to a breakage of the Cu-[Ox]-Ce structure, which resulted in enhanced catalytic performance, indicating the significance of the highly dispersed CuOx clusters in the CO-PROX process [262]. Furthermore, the pretreatment in an acidic or a basic environment affects the interaction between the two oxide phases. For instance, the pretreatment of ceria spheres in a basic solvent (2M NaOH), followed by etching in an ionic liquid for the acquisition of ceria nanocubes, resulted in the best catalytic activity at temperatures lower than 150 °C, due to the strong interaction between the highly dispersed CuOx clusters and ceria support [263]. An acidic treatment with nitric acid in nanorod-shaped CuOx/CeO2 catalysts has also been performed by Avgouropoulos and co-workers [264]. It was found that a highly acidic environment (pH < 4) led to an enrichment of catalysts surface in Cu+ species and to high concentrations of oxygen vacancies and Ce3+ species, while facilitating the formation of surface hydroxyls that are considered responsible for controlling the interfacial interactions in the copper-ceria binary system [264]. All the abovementioned characteristics in conjunction with the better copper dispersion and the improved reducibility of the highly acidic catalysts resulted in enhanced catalytic performance (T50 ≃ 84 °C) [264]. The same group has also investigated the pretreatment effect of employing ammonia solutions in copper-ceria nanorods [278]. It was revealed that the textural and structural properties of the modified catalysts remained almost unaffected after treatment, whereas, increasing the Cu:NH3 ratio to 1:4 resulted in higher reducibility and gave rise to Cu+ and surface lattice oxygen species, leading, thus, to improved catalytic performance [278]. As shown in Figure 20, close relationships between the half-conversion temperature (T50) and the main Raman peak shift or the concentration of Ce3+ and oxygen vacancies were observed [278].

**Figure 20.** T50 vs. (i) shift of the main peak (F2g Raman vibration mode) of fluorite CeO2 and (ii) surface concentration of Ce3+ and oxygen vacancies determined via XPS analysis. Adapted from Reference [278]. Copyright© 2018, John Wiley and Sons. **Figure 20.** T<sup>50</sup> vs. (i) shift of the main peak (F2g Raman vibration mode) of fluorite CeO<sup>2</sup> and (ii) surface concentration of Ce3<sup>+</sup> and oxygen vacancies determined via XPS analysis. Adapted from Reference [278]. Copyright© 2018, John Wiley and Sons.

Another adjusted parameter that can exert a profound influence on the catalytic performance is the electronic promotion mainly induced by alkali modifiers, as it may affect the chemisorption ability of active sites, as well as the copper-ceria interactions. In that context, it was found that the presence of K <sup>+</sup> ions in CuOx/CeO<sup>2</sup> catalysts has a beneficial effect on CO-PROX process in the presence of both CO<sup>2</sup> and H2O, since a proper K<sup>+</sup> content was proved to alleviate the CO<sup>2</sup> and H2O adsorption on the reaction sites and thus, enhancing the catalytic performance [279]. Potassium has also been found to stabilize Cu<sup>+</sup> active species by affecting Cu-Ce interactions [280].

An additional engineering approach towards enhancing the CO-PROX reactivity of CuOx/CeO<sup>2</sup> oxides involves the employment of chemical substances of specific architecture and textural properties, such as the carbon-based materials (rGO, CNTs, etc.). These materials favor the dispersion of copper and ceria, while affecting the reducibility and the population of oxygen vacancies, thus, resulting in enhanced catalytic performance at low-temperatures [256,281–284]. As for example, the introduction of rGO resulted in abundant Ce3<sup>+</sup> species and oxygen vacancies, offering high catalytic activity at temperatures below 135 ◦C and good resistance to CO<sup>2</sup> and H2O [283].

Interestingly, by combining electronic (alkali promotion) and chemical modification (carbon nanotubes), highly active multifunctional composites can be obtained. In copper-ceria catalysts supported on carbon nanotubes (CNTs) with a specific alkali/Cu atomic ratio, i.e., 0.68, the nature of the alkali metal (Li, Na, K, Cs) has been shown to affect the dispersion of ceria over CNTs and the copper-ceria interaction [261]. K-promoted CuOx/CeO<sup>2</sup> oxides combined with CNTs exhibited high catalytic activity (T<sup>50</sup> ' 109 ◦C as compared to 175 ◦C of un-promoted catalyst), attributed to the K-induced modification on redox/electronic properties [261].

#### *3.4. Water-Gas Shift Reaction (WGSR)*

The water-gas shift reaction (WGSR) plays a key role in the production of pure hydrogen, through the chemical equilibrium: CO + H2O ↔ CO<sup>2</sup> + H2. Among the different catalytic systems, copper-ceria oxides have gained particular attention, due to their low cost and adequate catalytic performance. Moreover, significant efforts have been put forward towards optimizing the low-temperature WGS activity by means of the above discussed methodologies. Regarding CuOx/CeO<sup>2</sup> catalyzed WGSR, two main reaction mechanisms have been proposed, namely, the redox and the associative mechanism. The first one involves the oxidation of adsorbed CO by oxygen originated by H2O dissociation. The second one involves the reaction of CO with surface hydroxyl groups towards the formation and subsequent decomposition of various intermediate species, such as formates [161,285].

A thorough study concerning the nature of active species and the role of copper-ceria interface for the low-temperature WGSR has been recently performed by Chen et al. [285]. It was revealed that the activity of copper-ceria catalysts is intrinsically related with the Cu<sup>+</sup> species present at the interfacial perimeter, with the CO molecule being adsorbed on the Cu<sup>+</sup> sites, while water being dissociatively activated on the oxygen vacancies of ceria [285,286]. In a similar manner, Flytzani-Stephanopoulos and co-workers [287] have earlier shown that strongly bound Cu-[Ox]-Ce species, probably associated with oxygen vacancies of ceria, are the active species for the low-temperature WGSR, whereas, the weakly bound copper oxide clusters and CuO<sup>x</sup> nanoparticles act as spectators.

Although the distinct role of copper and ceria and their interaction is not well determined, it is generally accepted that the activation of H2O, linked to copper-ceria interface and oxygen vacancies, is the rate-determining step [285]. Therefore, particular attention has been paid to modulate the interfacial reactivity via the above discussed adjusting approaches. Indicative studies towards modulating the WGSR performance are summarized in Table 4, and further discussed below.


*Catalysts* **2020**, *10*, 160

WHSV: Weight hourly space velocity [=] mL g−1 h −<sup>1</sup> . GHSV: Gas hourly space velocity [=] h−1

10.0% CO + 12.0% CO2 + 60.0% H2

vapor:gas = 1:1; WHSV = 2337 mL g−1 h

15.0% CO + 6.0% CO2 + 55.0% H2

vapor:gas = 1:1; GHSV = 4500 h−1

;

;

electronic/chemical state (doping with yttrium by co-precipitation)

pretreatment

(with 20 CO2

/2H2

calcination in O2

)

followed by

Y-doped Cu/CeO2 25 wt.% CuO, 2 wt.% Y2O3

93.4% at 250 ◦C [209]

10 wt.% Cu/CeO2 86% at 350 ◦C [290]

.

−<sup>1</sup>

The preparation method can affect various characteristics, such as the specific surface area, the total pore volume, the dispersion of the active phase or the crystallite size [206,291]. For instance, copper-ceria catalyst prepared by a hard template method showed higher WGSR activity as compared to the one prepared by co-precipitation (62 vs. 54% CO conversion at 450 ◦C), due to its larger surface area and higher CuO<sup>x</sup> dispersion, while they both exhibited a similar amount of acidic surface sites [291]. Among CuOx/CeO<sup>2</sup> catalysts synthesized by different precipitation methods, the catalysts prepared by stepwise precipitation showed the highest CO conversion, due to their higher reducibility and oxygen defects [208]. Precipitation was also found to give catalysts with higher WGSR activity, namely, 91.7% CO conversion at 200 ◦C, in comparison with the hydrothermal (82%) or sol-gel methods (64.5%), due to their abundance in oxygen vacancies, associated with the small CuO<sup>x</sup> crystals and large pore volume [206]. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 36 of 57 The preparation method can affect various characteristics, such as the specific surface area, the total pore volume, the dispersion of the active phase or the crystallite size [206,291]. For instance, copper-ceria catalyst prepared by a hard template method showed higher WGSR activity as compared to the one prepared by co-precipitation (62 vs. 54% CO conversion at 450 °C), due to its larger surface area and higher CuOx dispersion, while they both exhibited a similar amount of acidic surface sites [291]. Among CuOx/CeO2 catalysts synthesized by different precipitation methods, the catalysts prepared by stepwise precipitation showed the highest CO conversion, due to their higher reducibility and oxygen defects [208]. Precipitation was also found to give catalysts with higher WGSR activity, namely, 91.7% CO conversion at 200 °C, in comparison with the hydrothermal (82%)

The precipitating agent used could also exert a significant impact on the physicochemical properties of CuOx/CeO<sup>2</sup> catalysts, with the great implication in the catalytic behavior [207,292]. By employing ammonia water instead of ammonium and potassium carbonate, the WGSR activity is notably enhanced (91.7% CO conversion at 200 ◦C in contrast to 78.3% and 46.2%, respectively), due to the better dispersion of copper species and the stronger copper-ceria interactions [207]. Moreover, the copper precursor compound (nitrate or ammonium ions) and the preparation temperature can notably affect the WGSR activity [292]. or sol-gel methods (64.5%), due to their abundance in oxygen vacancies, associated with the small CuOx crystals and large pore volume [206]. The precipitating agent used could also exert a significant impact on the physicochemical properties of CuOx/CeO2 catalysts, with the great implication in the catalytic behavior [207,292]. By employing ammonia water instead of ammonium and potassium carbonate, the WGSR activity is notably enhanced (91.7% CO conversion at 200 °C in contrast to 78.3% and 46.2%, respectively), due to the better dispersion of copper species and the stronger copper-ceria interactions [207]. Moreover,

Recently, it was found that the dispersion of differently formed copper structures (particles, clusters, layers) on ceria of rod-like morphology is dependent on copper loading, with low copper loadings (1–15 mol.%) exhibiting monolayers and/or bilayers of copper, while a further increase in copper loading up to 28 mol.% results in faceted copper particles and multi-layers of copper [286]. At copper loadings up to 15 mol.%, a linear relationship between the CO conversion and the copper content was observed (Figure 21), indicating that the number of the active interfacial sites (Cu+-Vo-Ce3+) is significantly increased along with Cu content up to 15 mol.% [286]. the copper precursor compound (nitrate or ammonium ions) and the preparation temperature can notably affect the WGSR activity [292]. Recently, it was found that the dispersion of differently formed copper structures (particles, clusters, layers) on ceria of rod-like morphology is dependent on copper loading, with low copper loadings (1–15 mol.%) exhibiting monolayers and/or bilayers of copper, while a further increase in copper loading up to 28 mol.% results in faceted copper particles and multi-layers of copper [286]. At copper loadings up to 15 mol.%, a linear relationship between the CO conversion and the copper content was observed (Figure 21), indicating that the number of the active interfacial sites (Cu+-Vo-Ce3+) is significantly increased along with Cu content up to 15 mol.% [286].

**Figure 21.** Low-temperature WGS reaction over the Cu/CeO2 catalysts. CO conversion as a function of the copper content in the respect catalysts. Reaction conditions: 1.0 vol.% CO/3.0 vol.% H2O/He, 40,000 h<sup>−</sup>1, 200 °C. Adapted from Reference [286]. Copyright© 2019, Elsevier. **Figure 21.** Low-temperature WGS reaction over the Cu/CeO<sup>2</sup> catalysts. CO conversion as a function of the copper content in the respect catalysts. Reaction conditions: 1.0 vol.% CO/3.0 vol.% H2O/He, 40,000 h−<sup>1</sup> , 200 ◦C. Adapted from Reference [286]. Copyright© 2019, Elsevier.

The morphological features of both copper and ceria counterparts notably affect the WGSR

activity. In a comprehensive study by Zhang et al. [293], it was reported that Cu cubes exhibit high WGSR activity in contrast to Cu octahedra with the Cu–Cu suboxide (CuxO, x ≥ 10) interface of Cu(100) surface being the active sites. In a similar manner, it was shown that ceria nanoshapes (rods, cubes, octahedra) exhibit different behavior during interaction with CO and H2O, due to their diverse The morphological features of both copper and ceria counterparts notably affect the WGSR activity. In a comprehensive study by Zhang et al. [293], it was reported that Cu cubes exhibit high WGSR activity in contrast to Cu octahedra with the Cu–Cu suboxide (CuxO, x ≥ 10) interface of Cu(100) surface being the active sites. In a similar manner, it was shown that ceria nanoshapes (rods, cubes, octahedra) exhibit different behavior during interaction with CO and H2O, due to their diverse defect chemistry [294]. Upon CO exposure, ceria nanocubes, exposing {100} planes, favor the formation of oxygen defects at the expense of the existing anti-Frenkel defects, while in nanorods and nanooctahedra (exposing mainly {111} planes) both types of defects are formed [294]. By combining Raman and FTIR results, it was revealed that H2-reduced ceria rods and octahedra could be further reduced in

CO, resulting in the formation of both defects. In contrast, cubes cannot be further reduced by CO; thus, oxygen is available to form carbonates and bicarbonates by converting Frenkel defects to oxygen vacancies [294]. These findings clearly revealed the key role of both copper and ceria nanoshape on the defect chemistry of individual counterparts. It should be noted, however, that in the binary copper-ceria system, where multifaceted interactions are taking place, the relationships between shape effects and catalytic activity can be rather complex, leading to inconsistent conclusions [119,136,159].

Very recently, Yan et al. [288] reported on a novel structural design approach towards optimizing the WGS activity of CuOx/CeO<sup>2</sup> system. In particular, inverse copper-ceria catalysts of high efficiency were developed through the fabrication of highly stable bulk-nano interfaces under reaction conditions. Nano-sized ceria particles (2–3 nm) were stabilized on bulk copper resulting in abundant ceria-copper interfaces [288]. This inverse catalyst showed outstanding WGS conversion (T<sup>100</sup> = 350 ◦C), due to the high amount of interfacial sites and the strong copper-ceria interaction, which facilitated the dissociation of water and the oxidation of CO [288].

The doping approach has also been employed to enhance the WGSR activity of CuOx/CeO<sup>2</sup> system [209,295]. For instance, copper-ceria catalysts doped with 2 wt.% yttrium have shown excellent WGSR activity and high thermal stability, as yttrium favored the oxygen vacancy formation on ceria [209]. Recently, Wang et al. [295] performed DFT calculations in order to theoretically investigate the alkali effect on the WGSR activity of Cu(111) and Cu(110) surfaces. It was found that potassium enhances the WGSR activity as it favors the dissociation of H2O and induces stronger promotion on the (111) surface. With regard to other alkali metals (Na, Rb, Cs), the promoting effect on the dissociation of water differentiates with their electronegativities which induce changes in the work function, i.e., the lower the work function, the stronger the promoting effect of the alkali [295].

Finally, the WGSR activity and the sintering resistance of the CuOx/CeO<sup>2</sup> catalysts can be further enhanced by improving the metal-support interactions through appropriate pretreatment protocols [290,296]. As for example, the treatment of CuOx/CeO<sup>2</sup> catalyst in a gas mixture of 20CO2/2H<sup>2</sup> led to highly active catalysts, due to the electron enrichment of copper atoms via electronic metal-support interactions [290]. Moreover, ceria pretreatment in different atmosphere (air, vacuum or H2) affected the WGSR performance of CuOx/CeO<sup>2</sup> catalysts, with the H2-pretreated samples exhibiting the highest conversion performance, due to the strong synergism between the two oxide phases, the small CuO<sup>x</sup> particle size, and the high concentration in oxygen vacancies [296].
