3.1. Soft Template Methods

The soft-template routes result in the direct formation of porous structure by assembly of precursor molecules on “soft materials” with non-rigid shapes. There are plenty of materials which can be used as templates, such as organic macromolecules, ionic liquids, and surfactants. These materials can assemble into supramolecular aggregates forming flexible matrix for micro-/meso-/macrostructures. Varying the composition of the reaction medium and such parameters as temperature and pressure during formation of the soft template structure provides an easy way for controllable tuning of the chemical composition, structure, morphology, size, orientation, pore distribution, and internal and external surface properties of the synthesized porous materials. Some authors [68] use the name “endo-templating” for methods involving soft templates because in this case precursor molecules surround aggregates of template molecules and form solid ceria phase outside the template structure (template is situated inside the ceria precursor, inside = “endo”). The voids remaining in the material after removal of the template form a porous system. It is important to note that there is a confusion in literature about the name of these methods. Several authors [68,69] call them as “exo-templating” (the precursor is outside the matrix, outside = “exo”). Further in this review, we will stick to the former name. It seems that the classification of soft materials only as “endo”-templates is not quite correct, because there are direct or inverse micelle assembly mechanisms resulting in the formation of endo- or exo-structures, respectively.

Micelles and vesicles formed by amphiphilic molecules when their concentrations exceed critical values are widely used as soft templates. Their packing pattern can be easily controlled by varying the concentration of surfactant, ionic strength, temperature, pH values, or by introduction of additives, which allows producing structures with different shapes including spherical, cylindrical, and bilayer spherical aggregates [70]. Amphiphilic compounds (surfactants and polymer materials) can be classified into three groups: cationic, anionic, and non-ionic.

A striking example of widely used cationic surfactants is alkyltrimethylammonium salts, especially cetyltrimethylammonium bromide (CTAB). A lot of papers report the use of CTAB to design novel high surface area ceria-based materials for applications in catalysis including methane dry reforming [22], chlorobenzene destruction [26], toluene oxidation [71], solvent-free oxidation of benzyl alcohol [28], transesterification of ethylene carbonate (EC) with methanol to dimethyl carbonate (DMC) and other cyclic carbonates and alcohols [29] etc. Other cationic templates have also been applied in the field of catalysis, but they are not so popular as CTAB and its counterparts. Summarizing the results of plenty of works, CTAB template creates a mesoporous structure of ceria with the narrow pore size distribution. It also improves S_BET, reducibility of cerium, oxygen mobility, and can influence the acid-basic properties, which is important for adsorption of reactants and in acid-base catalysis.

A series of cobalt and/or iron-modified ceria catalysts for diesel soot combustion was synthesized using CTAB-assisted co-precipitation method [23]. It was shown that all prepared systems had mesoporous texture and monomodal pore size distribution. The specific surface area of the ternary metal oxides was relatively large (85–115 m²/g). Interestingly, the specific surface area of co-doped oxides increases with increasing the iron loading. The Co-Fe/CeO₂ systems were especially efficient in the soot oxidation under both tight and loose contact conditions because of their unique mesoporous structure and additional oxygen vacancies on the surface. The large surface area and high porosity of the catalyst are known to increase the number of contacts between the catalyst and soot particles, thereby improving the catalyst activity in the soot oxidation.

The double oxide CeZrO_x (CZ) materials prepared using the CTAB template or by precipitation with urea were compared in [30]. The template method provided higher S_BET value for the samples with high Zr content, in contrast to the precipitation with urea, in which the highest specific surface areas were observed for pure oxides and at Ce:Zr = 1. In the case of CZ, the influence of Ce:Zr ratio on the texture, structure, morphology, and surface properties is more complex than in the case of CeO₂-TiO₂ composite. When CTAB is added, the concentration of mesopores increases, with their size distribution and shape being dependent on the Zr modifier loading. The high catalytic activity and selectivity to ethyl acetate total oxidation is explained in this work by the improved Lewis acidity and reduction ability of materials with the relatively high extent of lattice and surface defects formed for three reasons: as a result of smaller crystal size, on the crystal planes with higher Miller indices, and by incorporating zirconium ions into the CeO₂ lattice and vice versa.

The evaporation-induced self-assembly (EISA) is a widely used method for the preparation of ceria-based oxides [27,31,72]. In this process the self-association of individual components into an organized structure is stimulated by the slow solvent evaporation. Thus, in our scientific group CTAB and citric acid were used as a template and complexing agent, respectively, to synthesize Ce_0.8Zr_0.2O₂ (CZ) and MnO_x-Ce_0.8Zr_0.2O₂ (Mn-CZ) oxide catalysts by EISA method (Figure 3). These catalytic systems were tested in CO oxidation [31]. It was noticed that the catalytic action depended on the way manganese was added to ceria-zirconia oxide: during the self-assembly of oxide precursors on a micelle template, as in “one-pot” method used to prepare Mn-CZ, or by post-impregnation of the CZ prepared by EISA method with a manganese precursor followed by calcination to remove the CTAB template (Mn-CZ IM). Both ways led to a substantial decrease in S_BET (by about half) compared to the non-modified Ce-Zr sample for which S_BET was relatively high (83 m²/g). Such deterioration of porosity could be explained by the poor ability of manganese ions to form stable complexes with citric acid. The lowest S_BET value of MnO₂ prepared by EISA (20 m²/g) confirmed this assumption. In both modified samples manganese oxides were partially distributed on the surface and partially sealed inside CZ particles. However, electron paramagnetic resonance study of Mn-CZ revealed that only the minor fraction of Mnⁿ⁺ ions are incorporated or intercalated in the bulk or in the subsurface layer of CZ oxide crystal lattice, which confirmed the triple oxide formation. No traces of such ions were found in Mn-CZ IM. Interestingly, despite the low specific surface area, the ternary oxide system prepared with impregnation step exhibited higher CO conversions values in the whole studied temperature range (100–450 °C) than CZ and Mn-CZ. This fact can be explained by the mosaic surface structure formed by alternating surface areas enriched with CZ and MnO_x that can provide the additional adsorption sites on the surface of Mn-CZ IM sample. Thus, it was clearly demonstrated in our work that the method of adding the third component to cerium-zirconium systems has a significant effect on the catalytic properties.

CO oxidation over Mn-CZ prepared by one-pot EISA method using CTAB template, and Mn-CZ IM, where MnO_x was supported on CZ by impregnation (adapted from [31]).

The modification of cerium oxide with nickel makes it possible to produce catalysts for various processes. Thus, binary nickel-cerium oxide materials were synthesized by the soft-template method using CTAB as a template and modified with nickel using two techniques: the “one-pot” method where ceria and nickel oxide precursors were co-precipitated in the presence of CTAB, providing high S_BET values of 170–210 m²/g, and the post-impregnation by depositing Ni on the soft-templated ceria support through incipient wetness impregnation, in which case S_BET was slightly lower (155–170 m²/g). The catalytic tests were performed in CO₂ methanation [33] after mild H₂ treatment (400 °C). Both series were found to be highly active and selective towards methanation. The catalytic results were explained in this work in terms of the CO₂ and H₂ activation on different phases. NiO nanocrystals of about 4 nm in size were identified in the “one-pot” samples, regardless of the Ni loading. According to the author’s explanation, such small nanocrystals did not sinter due to the strong metal-support interaction. Nickel deposition by the impregnation led to larger NiO particles (about 20 nm), and they agglomerated during reduction. Interestingly, despite the significantly different NiO crystal size, comparable CO₂ conversion values were observed for all catalysts, but “one-pot” samples exhibited superior performance at increased space velocities (CO₂ conversion was 50 and 3 mol.% over the “one-pot” and impregnated catalysts with the same 1.5 Ni:Ce molar ratio). The authors underlined the important role of the highly uncoordinated Ni atoms at the metal-support interface, the number of which is higher in small Ni crystallites. These atoms are responsible for hydrogenation of CO₂ species activated on nearby ceria sites.

Similar study was performed in [32], where the series of NiO/CeO₂-ZrO₂ mixed oxides with the same Ni content and different Ce:Zr molar ratios were also prepared by the “one-pot” CTAB-templated method. In this work, a mixture of carbon oxides was subjected to methanation. In all ceria-containing samples the Ni⁰ particle size was the same, about 6 nm. Ternary oxides demonstrated remarkably high S_BET (200 m²/g and more) and provided complete CO methanation, whereas CO₂ conversion was much lower and increased with Ce content, at least up to Ce:Zr = 1. Simultaneously, the specific surface areas decreased. The NiO/CeO₂-ZrO₂ sample with Ce:Zr = 1 was stable during at least 50 h time-on-stream. The authors explained the beneficial effect of the Ce content by the increased NiO reducibility and the higher ability of CeO₂ to adsorb and activate CO₂. However, at high Ce:Zr ratio the larger amount of activated hydrogen would favor the reverse methane dry reforming reaction producing CO₂. As a result of these balanced processes the overall CO₂ concentration remained almost constant.

Ceria-titania composites with 2:8, 5:5 or 8:2 Ce:Ti molar ratios were synthesized using the CTAB template with the subsequent hydrothermal treatment and calcination at 600 °C [73]. They were tested in methanol decomposition to produce hydrogen as a potential alternative fuel, and ethyl acetate oxidation to check their ability in VOCs disposal. These oxide materials were combined to overcome their individual disadvantages: wide bandgap of TiO₂ and poor thermostability of CeO₂. No mixed oxide formation was found by Raman spectroscopy and XRD, but according to XPS the intimate contact between individual oxides resulted in the generation of Ce³⁺ and Ti³⁺ ions as well as oxygen vacancies. The increase of the temperature of hydrothermal treatment weakened the contact between individual oxides. The highest S_BET values were observed for the composites with Ce:Ti = 2:8 hydrothermally threated at 100 °C (166 m²/g) and 140 °C (127 m²/g) with predominantly cylindrical pores. The increase in Ce content led to the changes in pore shape from the “cage-like” at Ce:Ti = 5:5 to “slit-like” at 8:2 with the simultaneous deterioration of homogeneity of mesopore size distribution. The best texture was achieved at low Ce:Ti ratio and low temperature of the hydrothermal treatment. This material comprised highly dispersed CeO₂ particles anchored on the oxygen vacancies of TiO₂, high concentration of Lewis acid centers and mobile oxygen. All these features ensured the high catalytic activity and selectivity in total oxidation of ethyl acetate to CO₂ and methanol decomposition to syngas. In contrast, bulk ceria crystallites partially substituted with Ti with worse texture parameters are formed at high Ce:Ti ratio, providing higher density of Lewis acid centers, decrease in the concentration of Ti³⁺ and Ce³⁺, deterioration of catalytic activity, and change in selectivity. Ethyl acetate hydrolysis to ethanol, and methanol decomposition to methane were registered.

Skillful use of the template allows synthesizing complex anisotropic structures. In [74] nanosized Au@CeO₂ core-shell catalyst was produced by controlled hydrolysis of cerium acetate precursor in the presence of CTAB as a soft template. The template hinders hydrolysis of the ceria precursor to produce mushroom-like structure in which golden nanorods are half-covered with CeO₂, whereas without the template metal nanorods are completely covered with the ceria shell. Anisotropic structure of the catalyst comprising golden nanorods with the specified aspect ratio partially covered with ceria provides appropriate activity in photocatalytic reduction of 4-nitrophenol under near-infrared laser irradiation due to improved plasmon absorption.

Among different surfactants (CTAB, sodium dodecyl sulfate, dodecyltrimethylammonium bromide, cetyltrimethylammonium chloride, polyvinylpyrrolidone, and KBr) only CTAB one led to the desired mushroom-like morphology [74]. It seems that bromide ions play an important role during the anisotropic growth, possibly due to stronger (compared to Cl⁻) interaction with noble metals, affecting the growth of surface nanostructures.

Summarizing the data on the ceria systems prepared using the CTAB template, and modified with manganese, Co-Fe or nickel dopants, the following conclusions can be drawn:

Template can significantly improve the textural properties of both unmodified and modified ceria, but the careful choice of dopant is needed, because some modifiers can hinder the pore structure formation in synthesized material. For instance, the one-step CTAB-templated method results in the formation of the Ni/CeO₂ and Ni/CeZrO_x (Ni/CZ) oxide systems active in CO₂ methanation while the similar technique used for modification of CeZrO_x with Mn in [31] did not lead to highly effective catalysts for CO oxidation;

A well-developed porous structure is a beneficial quality for a heterogeneous catalyst, but many other factors may outweigh its influence on catalytic properties. Nickel particle size [33] or the degree of supported nickel reduction [32] were proposed as the key factors, which determine the catalytic action of Ni/CZ in the methanation of carbon oxides, but tuning of reaction conditions and composition of reaction mixture provide the way to achieve the desirable values of these parameters;

The nature of cation and anion in polar templates can play significant role during ceramic synthesis. Thus, the presence of residual anions in the oxides prepared by CTAB-assisted method can affect the surface morphology and therefore catalytic activity. However, the degree of exposure to such ions is difficult to predict, since the effect depends on their concentration, nature of template, preparation conditions, catalyst composition, and type of catalytic reaction.

Anionic surfactants form another group of soft-templates with the long hydrocarbon tail combined with the negatively charged “head-group”. A prominent representative of this class of amphiphilic compounds is sodium dodecyl sulfate (SDS), which is widely used for the synthesis of different oxides (TiO₂, SiO₂, zinc oxide, etc.), but there are only scarce references about the use of this or other anionic surfactants for the synthesis of ceria-based oxides.

Thus, the ceria-zirconia mixed oxides were prepared using SDS via the sol-gel route in [24]. After calcination at 500 °C they exhibited regular pore structure and the appropriate values of specific surface area of about 100 m²/g. The authors believed that these materials could be successfully tested for various catalytic applications; however, the article lacks the data on the catalytic tests. SDS can also play a role of the foaming agent and hydrophobic modifier for ceria NPs on the surface of hard template (silica hollow spheres) [75].

Another group of soft templates comprises non-ionic surfactants, such as fatty alcohols, esters and ethers with long carbon chain, and block copolymers containing uncharged hydrophilic and hydrophobic moieties. These templates are distinguished by the absence of counterions, which, as mentioned above, can significantly affect the properties of the resulted product.

In [27] the mesoporous ceria catalysts prepared both by EISA method using Pluronic F127 triblock copolymer as a soft template and by nanocasting using SBA-15 mesoporous silica as a hard template were compared in benzene oxidation. The hard-templated ceria showed relatively large specific surface area (92 m²/g) and highly defective internal structure, while the soft-templated sample exhibited the morphology of 3D-linked filaments with the low content of internal defects and lower specific surface area of 32 m²/g. The hard-templated sample demonstrated the improved benzene oxidation activity with 50% benzene conversion achieved at 257 °C, which was much lower than that for the soft-templated CeO₂ (384 °C). However, the catalytic properties of soft-templated ceria can be significantly improved by etching with NaOH, which increases the number of surface defects. They can adsorb active oxygen species providing low-temperature benzene oxidation. Thus, a simple modification of the soft template methods can improve the properties of the produced oxides.

Polymer templates from the pluronic series have been widely used for the synthesis of nickel catalysts supported on pure and modified cerium oxide.

Two strategies, the template one using triblock copolymer Pluronic F127 and the polymerizable complex method, were compared for the synthesis of modified ceria [76]. A wide range of modifiers (Gd, La, Mg) were tested to select a suitable support for nickel catalysts. Different ceria precursors were used (chloride and nitrate, respectively), whereas all modifiers were introduced as nitrates in both the preparation strategies. The conditions of thermal treatment of Ce_1−xM_xO_y systems were varied in a wide range (final temperature was 300, 500 or 800 °C, duration 4 or 24 h, atmosphere of air or reductive mixture of 30% H₂/Ar). Ni was added by impregnation. Compared to the competitive technique the template method provided the higher specific surface area nearly for all dopants at air calcination temperatures of 300 and 500 °C with exception of pure CeO₂ calcined at 500 °C. Doping with lanthanum and calcination at 500 °C lead to the highest S_BET both for the support and Ni catalyst; to diminish a decrease in the specific surface area due to the Ni addition, the thermal treatment in H₂ + Ar atmosphere was recommended. The enhancement of the nickel-support interaction, which stabilizes highly dispersed Ni species and improves thermal and cocking stability in autothermal reforming of ethanol at 200–700 °C, was achieved with a high molar fraction of the dopant and a decrease in the calcination temperature, which was demonstrated by the example of La-modified samples.

In [25] the NiO/ZrO₂-CeO₂ composites were prepared by the soft-template method using a different triblock copolymer Pluronic P123 (Ce:Zr molar ratio of 9:1). The produced catalyst showed suitable textural and structural properties for the use in catalysis or as an anode in the solid-oxide fuel cell (SOFC). The choice of calcination conditions allowed tuning the crystal size and promoting phase stabilization. The calcination at lower temperature of 400 °C resulted in the larger total pore volume, higher specific surface area and smaller crystallite size (mostly cubic shape) of biphasic NiO/ZrO₂-CeO₂, enhancing NiO and CeO₂ reducibility, while calcination at higher temperature of 540 °C only improved the Ce⁴⁺ reducibility. Both systems were equally active in methane conversion, but for the sample thermally treated in milder conditions no signs of carbonaceous deposits formation were noticed. This enhancement in the ceria redox properties may be responsible for the improved surface oxygen exchange, allowing the gasification of carbon species and completely preventing carbon deposits formation, which is typical for the catalysts with similar composition but different morphology synthesized by co-precipitation [77].

Similar Pluronic P123 template was used to produce a catalyst comprising niobium oxide confined by ceria nanotubes for the selective catalytic reduction (SCR) of NO_x [78]. Interestingly, in this study CeCl₃·7H₂O was used as a ceria precursor instead of the most common nitrate one, and ethanol washing for template removal instead of the more common calcination. The produced composite with the nanotube morphology showed better synergistic effect than its counterpart comprising niobium oxide on ceria nanoparticles, demonstrating higher SCR activity and remarkable resistance to potassium, phosphorus, and lead poisons. Indeed, nanotubular catalyst ensured more than 90% NO_x conversion in a broad temperature region of 275–450 °C, while nanoparticulate material demonstrated a similar efficiency only in the narrow temperature range around 350 °C. Similarity in the crystal structure and specific surface area of the both materials led the authors to the conclusion about the crucial role of morphology in ensuring the high efficiency of niobium-cerium nanotubular oxide in the SCR of NO_x.

Tin oxide is another promising modifier due to the ability to form solid solutions with cerium oxide and the relatively low cost as compared to zirconia. A CO oxidation activity of CeSnO_x (CS) and CuO_x/CeSnO_x (Cu-CS) catalysts prepared by CTAB or Pluronic P123 was compared in our scientific group [79]. Catalytic properties of prepared systems strongly depend on the template nature and copper modification technique because these parameters determine degree of interaction between different components in Cu–Ce–Sn oxide systems. The combination of CTAB-templated method and “one-pot” copper addition technique led to the more uniform distribution and partial incorporation of copper ions into the CS lattice, which provided high oxygen mobility and defectiveness. These facts explain why Cu-CS CTAB sample (S_BET = 84 ± 8 m²/g) exhibited excellent catalytic properties over the entire temperature range studied. In contrast, Pluronic 123-templated counterpart (S_BET = 96 ± 9 m²/g) showed 40–60% conversion of CO only at relatively low temperatures, and it was less effective in the high-temperature range.

Not only the template, but also the nature and concentration of a swelling agent can influence the texture and other properties of ceramics. Thus, the influence of the weight ratio of template (Pluronic P123) and swelling agent (tri-isopropyl-benzene, TIPB) on the properties of porous double oxide Zr_0.1Ce_0.9O₂ as well as 3 and 10 wt.% Ni/CZ catalysts produced by the post-impregnation was studied in [80]. The template was removed by calcination at an unusually low temperature of 400 °C. The produced materials were mainly mesoporous with S_BET of about 110 m²/g; but the growth of the swelling agent concentration increased microporosity. In this way the gas permeability and S_BET that are important characteristics for Intermediate Temperature-Solid Oxide Fuel Cell (IT-SOFC) and catalytic applications can be increased.

Several petrochemical processes, e.g., methane oxidation, can be efficiently performed only at high temperature, at which the sintering of the particles of active component is highly probable. Thus, new strategy was developed for the synthesis of ceria-supported nanorods of the noble metals, confined in the shell of silica [36]. The mixture of templating agent surfactant NP-5 (polyethylene glycol mono-4-nonylphenyl ether), solvent and hexane was used to form the micro-emulsion system. A solution of precursor salts of the noble metal and cerium oxide was added to the organic mixture and the fine Pd-CeO_x wires were precipitated with alkali. The silica shell was created by adding a silica precursor, e.g., tetraethoxysilane (TEOS), in the last step. The final Pd-CeNW@SiO₂ catalyst obtained by calcination at 600 °C showed outstanding stability toward moisture and SO₂ during methane combustion [36]. The temperature of 100% methane conversion to CO₂ and H₂O was the lowest for Pd-CeNW@SiO₂ (350 °C) compared to Pd@SiO₂ (375 °C), and the commercial Pd/Al₂O₃ catalyst (425 °C), and it decreased even more in the cooling cycle during catalytic test. The Pd-Ce-O_x core phase remained isolated even after 50 h time-on-stream at 800 °C. Its Pt-containing counterpart demonstrated very good catalytic properties in CO and toluene oxidation [38]. 2%Pt-CeO₂NW@SiO₂ catalyst was stable during 100 h time-on-stream in the oxygen-enriched CO + O₂ + N₂ reaction mixture, and the temperature of 50% CO conversion was 75 °C lower than over the 2%Pt-CeO₂NW/SiO₂ counterpart simply supported on silica rather than confined into silica shell. TPR-H₂ data showed that the reduction of both surface and bulk Ce⁴⁺ proceeded at much higher temperatures than in the case of the common Pt/CeO₂/SiO₂ catalyst (the difference was about 75 and 150 °C, respectively), which was attributed to the more intimate contact of ceria and silica. However, the similar effects can be caused by diffusion limitations due to water formation during catalyst interaction with H₂. The activity of Pd-Ce-O_x or Pt-Ce-O_x phases resulted from the higher degree of interaction between the metal and cerium oxide, while the stability is ensured by a protective microporous silica shell.

Thus, the use of the soft template methods is a convenient way to synthesize porous ceria-based oxide systems, and the template is easily removable by calcination in air under relatively mild conditions. However, the nature of ceria precursor (e.g., cerium chloride or nitrate) and the presence of additional ions of doping metals (e.g., rare-earth metals, Pd, Pt, Ni, Cu, etc.) in the reaction solution can significantly influence the textural properties of the resulted materials. The interaction of such ions with the template molecules/aggregates can affect hydrolysis and polymerization processes and change the degree of interaction between the modifier and CeO₂.