Comparison of Textural Characteristics of Ceria Solids Prepared via Triton X Reverse Micelles and in Situ Synthesized Ce(OⁱPr)₄ and Ce(OⁱPr) ₃ Precursors

Polyxeni Paschalidou, Charis R. Theocharis^*

Porous Solids Group, Department of Chemistry, University of Cyprus, Nicosia, Cyprus.

DOI: 10.4236/msa.2019.109042                

Abstract

The textural and surface properties of ceria solids prepared in the presence of Triton X reverse micelles and using in situ synthesized Ce(OⁱPr)₄ and Ce(OⁱPr)₃ precursors are compared with one another to investigate the effect of the two different precursors on the ceria particles formation. A wide spectrum of analytical methods such as UV-Vis absorption, isothermal N₂ adsorption/desorption, powder XRD, FTIR, ATR-FTIR, DRUV-Vis solid state, TGA and DSC has been employed for the characterization of the solids. Generally, the ceria solids obtained when the Ce(OⁱPr)₄ precursor is used to possess better textural and surface properties, and more defects (e.g. oxygen vacancies) in their crystal structure.

Keywords

Ceria Solids, Triton X Reverse Micelles, Ce(OⁱPr)₄, Ce(OⁱPr) ₃, Surface Characteristics

1. Introduction

The lanthanide metal compound cerium oxide (CeO₂), is a very reactive rare earth metal oxide and has attracted the interest of the research community. The impetus for this was the wide range of its applications in catalysis, oxygen sensors, solid-state fuels, decontamination technologies, biotechnology, and optoelectronic applications. Of particular interest is also the storage capacity of CeO₂ in oxygen [1] [2] [3] , which is related to the fluorite crystalline structure of CeO₂. The structure is very stable and does not collapse even when lattice oxygen atoms are removed. The difference in charges in the lattice is compensated by the formation of Ce(III), resulting in non-stoichiometric CeO_(2-x) solids [4] [5] .

According to literature data [6] [7] the use of ammonium cerium nitrate results in ceria solids with high surface area and small mean crystalline particle size, then following in descending order the cerium nitrate and cerium chloride salts [6] . According to the studies of Liu and coworkers [7] , the CeO₂ nano-particles prepared from the (ΝΗ₄)₂Ce(NO₃)₆ salt exhibit greater deformation in their crystalline lattice compared to the CeO₂ nano-bars obtained from Ce(NO₃)₃, which results in higher activity for NO reduction. This effect can be attributed to quantum particle nano-sized phenomena that improve the intrinsic reductivity of surface oxygen atoms and favor the formation of oxygen anion vacancies [7] .

In recent years, the study of possible pathways for preparation of mesoporous ceria has been widely investigated by scientists with the ultimate goal to adjust the porosity and prepare solids with increased surface area [8] [9] . In this context, the use of reverse microemulsions allows particles size and follows surface area control of CeO₂ solids [10] - [15] .

According to the literature [9] [16] [17] [18] [19] , the use of electrically neutral surfactant Triton X is associated with the following advantages. First, weak repulsion interactions result in bigger degree of condensation and polymerization, second, improvement of thermal stability and mesoporosity of the products and thirdly, easy removal of the matrix by simple solvent extraction or calcination without the subsequent collapse of the mesoporous structure. It has to be noted that there is a limited number of studies related to the effect of the Triton X surfactants with varying polar tail length on the surface properties of ceria solids. Furthermore, we chose cerium isopropoxide as a precursor compound because it provides the optimal control over the hydrolysis, polycondensation and polymerization rate. Second, the compound acts partially as a structure-directing agent, and thirdly favors optimal stereochemical stabilization. In addition, this molecule leads to the minimizing of organic residues, and finally, replaces inorganic precursors’ salts leading to abrupt and non-homogeneous precipitation [20] [21] . In addition, due to the fact that cerium tetra-isopropoxide is relatively unstable and is immediately oxidized in the presence of air, and therefore has to be prepared in-situ [22] the number studies involving cerium isopropoxide which is relatively limited [20] [21] [23] [24] .

The present study deals with the comparison of textural characteristics of CeO₂ solids prepared using non-ionic Triton X reverse micelles of varying polar tail length and Ce(OⁱPr)₄ and Ce(OⁱPr)₃ precursor compounds. Furthermore, the effect of the surfactant polar tail length, the calcination temperature and the precursor compounds (e.g. Ce(OⁱPr)₄ vs Ce(OⁱPr)₃) on the ceria surface properties (e.g. porosity, structure and morphology) have been investigated for the first time. Finally, the nucleation mechanism of the CeO₂ particles in the water cores of the reverse micelles has been investigated and described. This paper represents the first systematic study of the differences between samples prepared in the presence of inverse micelles from Ce(III) and Ce(IV) precursors.

2. Experimental

2.1. Ce(OⁱPr)₄ and Ce(OⁱPr)₃ Preparation

The tetravalent (Ce(OⁱPr)₄) and trivalent (Ce(OⁱPr)₃) ceria precursor compounds have been synthesized as described elsewhere [15] [25] [26] [27] and the precursor concentrates in DME have been used for the synthesis of the ceria nanoparticles.

2.2. Reversed Micelle-Mediatedceria Synthesis

Three different mixtures of reversed micelles have been prepared by adding the neutral surfactants Triton X-100, Triton X-114 and Triton X-45 in cyclohexane (1.22 mol∙kg⁻¹) containing 0.83 mol equivalents of water at 30˚C and under N₂-atmosphere and continuous stirring for homogeneous dispersion of the water molecules within the micelles. The ceria precursors have been added in equimolar ratio to water and the resulting sol has been left to gelate in a glass beaker under ambient conditions for 48 h. Following, aliquots of the gels were calcined at 400˚C, 500˚C and 600˚C for 2 hours [28] . In order to distinguish between the ceria solids prepared from the tetravalent precursor (CeO₂), the solids obtained from the trivalent precursor are indicated with an asterisk ( ${\text{CeO}}_2^{*}$ ).

2.3. Physicochemical Characterization of Gels and Powders

The precursor compounds, Ce(OⁱPr)₄ & Ce(OⁱPr)₃, the gels and the powders resulted after calcination were characterized using a variety of methods. The measurements of UV-Vis absorption spectrophotometry, isothermal N₂ adsorption/desorption (employing a slit-shaped pore model), powder X-ray diffraction spectroscopy, Fourier-Transform Infrared spectrophotometry, ATR-FTIR (attenuated total reflectance) Fourier-Transform Infrared spectroscopy, UV-Vis Diffuse Reflectance solid state spectroscopy, Thermo Gravimetric analysis and Differential Scanning Calorimetry were carried out as described elsewhere [15] .

3. Results and Discussion

Comparison of the Surface Characteristics of Ceria Solids Obtained from Ce(III) and Ce(IV) Precursors Using Triton X Reversed Micelles

The isotherms of the CeO₂ and ${\text{CeO}}_2^{*}$ samples after thermal treatment at 400˚C for 2 h of the corresponding gels, are shown in Figure 1(a) and Figure 1(b), respectively. Examination of the isotherms indicates that in the case of the CeO₂ solids the specific area increases whereas the specific pore volume and average pore diameter decrease with increasing the tail length of the Triton X surfactants. On the other hand, regarding the ${\text{CeO}}_2^{*}$ solids the specific area and pore volume increase and the average pore diameter declines with increasing the tail

(a)(b)(c)

Figure 1. Ν₂ ad/desorption isotherms of the (a) CeO₂ and (b) ${\text{CeO}}_2^{*}$ solids after thermal treatment of the corresponding gels at 400˚C for 2 h, and (c) the specific area of the solids as function of different calcination temperatures of the corresponding gels.

length of the surfactant (Table S1, Supplementary Information). Generally, all isotherms are of type II/IV with an H3 hysteresis loop and the CeO₂ samples possess significantly higher specific surface area (Figure 1(c)) [29] [30] [31] . According to the XRD measurements (Figure 2(a) and Figure 2(b)) the CeO₂ and ${\text{CeO}}_2^{*}$ solids have face centered cubic structure and belong to Fm3m space group [32] .

Moreover, evaluation of the main peak corresponding to the (111) plane indicates that with increasing the tail length of the Triton X surfactants the ceria crystallites become smaller, more amorphous and with higher thermal stability. As the calcination temperature increases the condensation processes occur and the crystal size is increased as follows: TrX-114 < TrX-100 < TrX-45 for the CeO₂ and TrX-100 < TrX-114 < TrX-45 for the ${\text{CeO}}_2^{*}$ solids (Table S1, Supplementary Information and Figure 3(a)).

According to the XRD and UV-Vis data of the different ceria gels summarized in Table 1 with increasing the tail length of the surfactant the core diameter of the reverse micelles increases accordingly resulting in a larger space available for the ceria particles formation [33] [34] [35] . However, only in the case of the CeO₂ solids increasing the surfactants tail length results in the formation of larger particles, whereas when the Ce(OⁱPr)₃ precursor is used smaller ceria particles are formed. The average particle size is affected by the surface tension of the precursor molecule (e.g. (ΝΗ₄)₂Ce(NO₃)₆ = 63.23 mN/m and Ce(NO₃)₃∙6H₂O = 64.92 mN/m). SEM (scanning electron microscopy) studies have indicated that the samples are made-up of roughly cubic aggregates of 50 μm average size. When the precursor molecule has low surface tension migrates easier within the smaller surfactant micelles resulting in the formation of smaller ceria particles [6] [36] . The higher surface tension of the Ce(III) precursor compounds results in a stronger interaction with the hydrophilic tail of the Triton X surfactants within the aqueous core of the micelles and the subsequent formation of smaller

(a) (b)

Figure 2. XRD profiles of (a) CeO₂ and (b) ${\text{CeO}}_2^{*}$ samples corresponding to the different Triton X reverse microemulsion systems gels calcinated at 400˚C for 2 h.

(a) (b)

Figure 3. (a) Crystal size of the ceria solids as a function of the calcination temperature and (b) UV-Vis diffuse reflectance spectra of the CeO₂ and ${\text{CeO}}_2^{*}$ solids obtained after thermal treatment of the corresponding gels at 400˚C for 2 h.

Table 1. UV-Vis and XRD data of the ceria gels obtained from three different Triton X surfactants.

ceria particles [37] . The stronger interaction of Ce(III) with the hydrophilic tail of the surfactant is indicated by the TGA and DSC data schematically summarized in Figures 4(a)-(d). A more detailed presentation of the data is presented as Supplementary Information (Table S2). The Ce(III) precursor compound has a lower surface energy and anisotropy compared to the Ce(IV) compound because the Ce(III) cation has larger ionic radius (128.3 pm) than the Ce(IV) cation (111 pm) and therefore higher polarizability. The latter favors stronger interaction of Ce(III) with the hydrophilic surfactant tails of the Triton X matrix resulting in lower surface energy of the developing particles [7] [38] .

Moreover, according to the UV-Vis diffuse reflectance data (Figure 3(b)) the ${\text{CeO}}_2^{*}$ samples present higher energy gaps and adsorb in lower wavelengths [39] because of their smaller particle size (Table S1, Supplementary Information).

As the calcination temperature increases the CeO₂ and ${\text{CeO}}_2^{*}$ solids obtained

(a) (b) (c) (d) (e)

Figure 4. TGA and DSC graphs of the (a) & (b) Ce(IV) gels and (c) & (d) Ce(III) gels corresponding to three different Triton X surfactants and (e) schematic illustration of possible interaction between TrX-114 and the ceria precursor molecules.

from the TrX-100 surfactant present ad/desorption isotherms, which are getting closer to type IV(a) isotherms with a hysteresis loop of Η2(b) type (Figure 5(a)

(a)(b)(c)

Figure 5. Ν₂ ad/desorption isotherms of the (a) CeO₂ and (b) ${\text{CeO}}_2^{*}$ solids obtained from the TrX-100 gel and (c) schematic illustration of possible interactions of the TrX-100 surfactant molecules with the inorganic precursor molecules.

Figure 6. FTIR spectra of the CeO₂ and ${\text{CeO}}_2^{*}$ solids after thermal treatment of the Triton X-100, 114 and 45 reverse micelle gels at 400˚C for 2 h.

and Figure 4(b)) as if they were real mesoporous solids. It is obvious that the interactions of the TrX-100 surfactant with the inorganic precursor molecules favor the formation of mesoporous solids [15] (Figure 5(c)).

On the other hand, the TrX-114 surfactant presents the strongest interaction with the inorganic precursor molecules favoring the formation of a long-range pseudo-mesoporous network for both CeO₂ and ${\text{CeO}}_2^{*}$ solids [13] . This strong interaction is indicated (Figures 4(a)-(d) and Table S2, Supplementary Information) by the corresponding TGA and DSC graphs, which are shifted accordingly and present the highest endothermic peak (~30 J/g).

In addition, the FTIR spectra of the CeO₂ and ${\text{CeO}}_2^{*}$ solids (Figure 6) show similar absorption peaks indicating the same crystal cubic phase for both ceria solids [15] .

4. Conclusions

The main conclusions drawn from this study can be summarized as follows:

· Increasing the length of Triton X poly(ethylenoxy) chain results in increasing surface area and decreasing specific volume and average pore diameter of the CeO₂ and ${\text{CeO}}_2^{*}$ solids.

· The studied ceria samples present type II isotherms with a type H3 hysteresis loop.

· The tail length of the Triton X surfactant determines the diameter of the aqueous core of the micelles and interacts differently with the inorganic precursors (Ce(OⁱPr)₄and Ce(OⁱPr)₃), affecting the surface properties and the particle size of the ceria solids formed.

· Generally, the ceria solids obtained from the Ce(OⁱPr)₄ precursor possess have significantly higher specific surface and better surface properties.

· Increasing the length of the surfactant polar tail results in the formation of smaller and amorphous particles with higher thermal stability.

· According to XRD and UV-Vis measurements, the ${\text{CeO}}_2^{*}$ solids obtained from TrX-100 and TrX-114 possess the smaller crystal size than corresponding CeO₂ solids.

· With increasing temperature, the samples obtained from TrX-100 present isotherms which are similar to type IV with an H2 hysteresis loop.

· The interaction between the TrX-114 surfactant and the precursor molecules is the strongest as indicated by the TGA and DSC data and results in the formation of long-range pseudo-mesoporous network and crystal lattice.

Supplementary Information

Table S1. Surface analysis of CeO₂ & ${\text{CeO}}_2^{*}$ solids obtained from three different Triton X reverse micelle gels and calcinated at 400˚C for 2 h.

(a) (b) (c) (d)

(a)**The weight loss values are normalized. and were obtained from the TGA studies.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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