Novel Mesoporous and Multilayered Yb/N-Co-Doped CeO₂ with Enhanced Oxygen Storage Capacity

Abstract

A cubic fluorite-type CeO₂ with mesoporous multilayered morphology was synthesized by the solvothermal method followed by calcination in air, and its oxygen storage capacity (OSC) was quantified by the amount of O₂ consumption per gram of CeO₂ based on hydrogen temperature programmed reduction (H₂–TPR) measurements. Doping CeO₂ with ytterbium (Yb) and nitrogen (N) ions proved to be an effective route to improving its OSC in this work. The OSC of undoped CeO₂ was 0.115 mmol O₂/g and reached as high as 0.222 mmol O₂/g upon the addition of 5 mol.% Yb(NO₃)₃∙5H₂O, further enhanced to 0.274 mmol O₂/g with the introduction of 20 mol.% triethanolamine. Both the introductions of Yb cations and N anions into the CeO₂ lattice were conducive to the formation of more non-stoichiometric oxygen vacancy (V_O) defects and reducible–reoxidizable Ceⁿ⁺ ions. To determine the structure performance relationships, the partial least squares method was employed to construct two linear functions for the doping level vs. lattice parameter and [V_O] vs. OSC/S_BET.

1. Introduction

Cerium oxide (ceria, CeO₂) and CeO₂-based binary or multiple composites are typical oxygen storage materials, which have a wide range of applications in VOC abatement, partial alkyne hydrogenation, oxidative dehydrogenation, and water gas shift [1,2,3,4,5,6,7], even in biomedical applications [8]. CeO₂ is appealing because of its unique structure and intrinsic oxygen vacancy (V_O) defect, which can be rapidly formed and eliminated on the surface of CeO₂, giving the redox cycle of Ce⁴⁺ ⇔ Ce³⁺. This is the source of the oxygen storage capacity (OSC), which could be described by Reaction (1) and could also be written using Kroger and Vink notations as Reaction (2) [9]:

$$\mathrm{Ce}{\mathrm{O}}_2\underset{\mathrm{Oxygen} \mathrm{storage}}{\overset{\mathrm{Oxygen} \mathrm{release}}{\rightleftarrows }}\mathrm{C}{\mathrm{e}}_{1-x}^{4+}\mathrm{C}{\mathrm{e}}_x^{3+}{\mathrm{O}}_{2-x/2}{V}_{\mathrm{O}}{}_{x/2}+x/4{\mathrm{O}}_2$$

(1)

$${\mathrm{Ce}}_2{\mathrm{O}}_3\stackrel{{2\mathrm{CeO}}_2}{\rightleftarrows }{2\mathrm{Ce}}_{\mathrm{Ce}}^{\prime }+3O_{\mathrm{O}}^{\times }+V_{\mathrm{O}}^{••}$$

(2)

Doping CeO₂ with other metallic cations proved to be efficient in enhancing its OSC [10,11]. Inspired by La/N [12] and Sm/N [13] co-doped TiO₂ for enhanced photocatalytic activities, Yb/N doping should be a feasible method for modifying the OSC of CeO₂ in this work. However, no reports had been found on the OSC of cationic and anionic co-doping of CeO₂. Based on the similarity–intermiscibility theory and the similar ionic radii of the ytterbium cation (Yb³⁺; 0.98 Å) and the cerium cation (Ce⁴⁺; 0.97 Å), the doping of Yb cations into the CeO₂ lattice was feasible, which could be supported by previous reports [14,15]. Nitrogen (N) was a favorite and preferred non-metal dopant due to its relatively small ionization energy and similar ionic radii to oxygen (O). However, besides N-doped TiO₂ [16,17], there have been very limited reports on doping with N anions into the CeO₂ lattice. Since the O²⁻ anion (1.38 Å) has a larger size than that of the Ce⁴⁺ cation (0.97 Å) in the ionic crystal of CeO₂, it was more difficult to be substituted by larger N³⁻ anions (1.46 Å) [18,19]. In our previous work, we successfully synthesized N-doped CeO₂ using ammonium persulfate as an inorganic nitrogen source [20].

Hence, the modification of CeO₂ by co-doping with Yb cation and N anion should be an effective method to enhance its OSC. In our strategy, Yb-/N-doped CeO₂ was prepared by a solvothermal method followed by calcination in air with Yb(NO₃)₃∙5H₂O as a Yb source and triethanolamine (TEA) as a N source. For Yb/N-co-doped CeO₂ samples, the nominal content of Yb was 5 mol.%, namely, Yb/(Yb + Ce) (mol.%) = 5, while the molar ratio of N was TEA/Ce(NO₃)₃∙6H₂O = 5, 10, 15, 20, 25, and 30 mol.%, and the as-obtained samples were denoted 5%N/5%Yb-, 10%N/5%Yb-, 15%N/5%Yb-, 20%N/5%Yb-, 25%N/5%Yb-, and 30%N/5%Yb-doped CeO₂, respectively. Moreover, 1~7 mol.% Yb-doped CeO₂ was synthesized in the absence of TEA, while pure or undoped CeO₂ was synthesized in the absence of Yb(NO₃)₃∙5H₂O and TEA.

2. Experimental Procedure

2.1. Starting Materials

Ce(NO₃)₃∙6H₂O (99.95%), Yb(NO₃)₃∙5H₂O (99.9%), and triethanolamine (TEA, ≥99.0%) were supplied by Aladdin Co. Ltd. (Shanghai, China). Ethylene glycol (AR) and ethanol (≥99.7%) were supplied by Chengdu Kelong Chemical Co. Ltd. (Chengdu, China). Distilled water was used in all experiments, and all chemicals were used as received without further purification.

2.2. Synthesis of Yb/N-Co-Doped CeO₂

Yb/N-co-doped CeO2 was prepared by a solvothermal method followed by calcination in air. Typically, 3.8 mmol Ce(NO3)3∙6H2O, 0.2 mmol Yb(NO3)3∙5H2O, and the desired amounts of TEA were dissolved in a 30 mL mixed solution of ethylene glycol and distilled water (10 vol.% H2O) in a 50 mL Teflon bottle. Then, the bottle was sealed in a stainless-steel autoclave, transferred to an electric oven, and maintained at 200 °C for 24 h. After the autoclave naturally cooled to room temperature, the resulting precipitate was washed with distilled water and ethanol and then dried at 60 °C for 24 h. Finally, the Yb and N co-doping CeO2 powders were obtained by following calcination at 500 °C for 2 h in air. For the Yb/N-co-doping CeO2, Yb/(Yb + Ce) (mol.%) = 5, TEA/Ce(NO3)3∙6H2O (mol.%) = 5, 10, 15, 20, 25, and 30 mol.%, the as-obtained samples designated as 5%N/5%Yb-, 10%N/5%Yb-, 15%N/5%Yb-, 20%N/5%Yb-, 25%N/5%Yb-, and 30%N/5%Yb-doped CeO2, respectively.

2.3. Synthesis of Yb-Doped CeO₂

Yb-doped CeO₂ with different molar concentrations of Yb cations was synthesized using the same procedure as controls, but in the absence of TEA. Typically, appropriate amounts of Ce(NO₃)₃∙6H₂O and Yb(NO₃)₃∙5H₂O with a total of 4.0 mmol were dissolved in a 30 mL mixed solution of ethylene glycol and distilled water (10 vol.% H₂O) in a 50 mL Teflon bottle. Then, the bottle was sealed in a stainless-steel autoclave, transferred to an electric oven, and maintained at 200 °C for 24 h. After the autoclave naturally cooled to room temperature, the resulting precipitate was washed with distilled water and ethanol and then dried at 60 °C for 24 h. Finally, the Yb-doped CeO₂ powders were obtained by following calcination at 500 °C for 2 h in air. The as-obtained Yb-doped CeO₂ powders with different molar concentrations of Yb were labeled as 1%Yb-doped CeO₂, 2%Yb-doped CeO₂, 3%Yb-doped CeO₂, 4%Yb-doped CeO₂, 5%Yb-doped CeO₂, 6%Yb-doped CeO₂, and 7%Yb-doped CeO₂.

2.4. Synthesis of Undoped CeO₂

Pure or undoped CeO₂ was synthesized using the same procedure as controls, but in the absence of Yb(NO₃)₃∙5H₂O and TEA. Typically, 4.0 mmol Ce(NO₃)₃∙6H₂O was dissolved in a 30 mL mixed solution of ethylene glycol and distilled water (10 vol.% H₂O) in a 50 mL Teflon bottle. Then, the bottle was sealed in a stainless-steel autoclave, transferred to an electric oven, and maintained at 200 °C for 24 h. After the autoclave naturally cooled to room temperature, the resulting precipitate was washed with distilled water and ethanol and then dried at 60 °C for 24 h. Finally, the pure CeO₂ powders were obtained by following calcination at 500 °C for 2 h in air.

2.5. Characterization

The crystallographic phases of samples were characterized by X-ray diffraction (XRD, D/MAX 2200 PC, Rigaku, Japan) with 40 kV tube voltages and 40 mA current. The morphologies of samples were evaluated by field-emission scanning electron microscopy (SEM; JEOL–7500F, Tokyo, Japan) with an acceleration voltage of 5 kV. N₂ adsorption–desorption isotherms were measured using a QuadraSorb SI surface area analyzer (Quantachrome, Boynton Beach, FL, USA), and the BET-specific surface areas were determined using the Brunauer–Emmett–Teller method. The surface composition and binding energy of samples were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, Waltham, MA, USA). The oxygen vacancy defects of samples were characterized using a Raman spectrometer (LabRAM Aramis, Horiba Jobin–Yvon, Paris, France) with a He–Cd laser of 325 nm.

2.6. Evaluation of OSC

Hydrogen temperature-programmed reduction (H₂–TPR) measurements were employed to evaluate the OSC of Yb/N-doped CeO₂ samples, which were performed using a TP–5080 instrument with a thermal conductivity detector (TCD). Briefly, 0.05 g of sample was pre-treated in a 5 vol.%–O₂/N₂ flow (30 mL/min) at 500 °C for 1 h and was cooled down under this flowing O₂/N₂. Then, the sample was purged with high-purity N₂ to remove the excess O₂ on the surface. Finally, a 5 vol.%–H₂/N₂ flow (30 mL/min) was introduced into the reactor, which was heating to ~960 °C (10 °C/min), and the change in H₂ concentration of the outlet gases was monitored online by TCD.

3. Results and Discussion

Figure 1a shows the XRD patterns of undoped, 5%Yb-doped, 25%N/5%Yb-doped, and 30%N/5%Yb-doped CeO₂. All identified peaks matched well with the standard CeO₂ (JCPDS no. 34–0349) pattern, indicating that the CeO₂ phase with a fluorite structure was obtained, and no other phases (such as CeN, YbN, and Yb₂O₃) were detected. Moreover, the corresponding lattice parameters were estimated using Bragg’s equation; the results and the appropriate linear fitting are shown in Figure 1b. The lattice parameters of Yb-doped CeO₂ and Yb/N-co-doped CeO₂ were greater than those of undoped CeO₂, which revealed that the possibly partial substitutions of Ce⁴⁺ (0.97 Å) and O²⁻ (1.38 Å) ions by the larger Yb³⁺ (0.98 Å) and N³⁻ (1.46 Å) ions happened in the CeO₂ lattice, and the local lattice distortion (such as lattice expansion) occurred as a result. For Yb-doped CeO₂, the lattice parameters increased linearly with increasing Yb content until 5%, suggesting that the concentration of Yb in CeO₂ reached the solid solubility limit with the addition of 5% Yb. Upon the introduction of TEA, the lattice parameters of 5%Yb-doped CeO₂ continued to increase almost linearly until 20%N, implying that N anions were saturated in the 5%Yb-doped CeO₂ lattice with the addition of 20%N. In addition, when Yb and N ions reached the solid solubility limit in CeO₂, and the amount of Yb(NO₃)₃∙5H₂O or TEA continued to increase, their lattice parameters decreased, which indicated that the solid solubility limits of Yb and N in CeO₂ were supersaturated concentrations. This was attributed to the fact that CeO₂ itself had a large number of defect structures, leaving CeO₂ in a metastable state.

SEM was used to study the effect of Yb/N doping on the morphologies of CeO₂. As seen in Figure 1c, the morphology of undoped CeO₂ was a multilayered structure consisting of flakes, and these flakes intertwined to form an open porous structure. After the addition of Yb and N ions, the morphologies of 5%Yb-doped CeO₂ and 25%N/5%Yb-doped CeO₂ still maintained the multilayered structure, as shown in Figure 1d,e, respectively. Surprisingly, 30%N/5%Yb-doped CeO₂ in Figure 1f has a completely different morphology from the original multilayered structure. It could be concluded that the addition of TEA had a certain effect on the morphology of CeO₂. Due to TEA’s alkalescence, excessive TEA addition would destroy the original equilibrium of the self-assembled multilayer morphology in the solvothermal system, which promoted the formation of spheroid particle aggregates.

To further clarify the porous structures of the as-obtained CeO₂, N₂ adsorption–desorption experiments were conducted. Figure 1g shows the N₂ adsorption–desorption isotherms of undoped, 5%Yb-doped, 25%N/5%Yb-doped, and 30%N/5%Yb-doped CeO₂. All isotherms were consistent with type IV hysteresis loops, confirming their mesoporous structure [21]. The specific surface area (m²/g) was an objective, reliable, and physically meaningful size metric for porous materials, and the well-known Brunauer–Emmett–Teller (BET) equation was usually used to determine the specific surface area from the physical adsorption of a gas on a solid surface (labeled as S_BET) [22]. The S_BET of undoped, 5%Yb-doped, 25%N/5%Yb-doped, and 30%N/5%Yb-doped CeO₂ powders was estimated and is shown in Figure 1h. The S_BET of 5%Yb-doped CeO₂ was almost constant with a value of 93.1 m²/g, while the S_BET value of 25%N/5%Yb co-doped CeO₂ was 107.3 m²/g, slightly higher than that of the undoped sample (96.0 m²/g), but the S_BET value of 30%N/5%Yb-doped CeO₂ was only 52.5 m²/g. It could be concluded that the introduction of TEA had a certain effect on the S_BET, especially when the TEA addition exceeded a certain value. The morphology not only changed significantly, but the S_BET also decreased sharply.

XPS analysis was performed to determine the chemical composition of Yb/N doped CeO₂ and identify the influence of Yb/N doping on V_O defects and Ceⁿ⁺ ions in the CeO₂ crystal. Figure 2a shows the XPS survey spectrum of 20%N/5%Yb-doped CeO₂. All wide-scan spectra showed clear CeO₂ features by the signals ascribed to Ce 3d and 4d and O KLL and 1s. Fortunately, the Yb signal could also be detected by the presence of the Yb 4d peak. In addition, the weak peak of the N element could also be clearly observed from the high-resolution XPS spectrum of N 1s in the Figure 2a inset. Figure 2b shows the Ce 3d XPS core-level spectrum of 20%N/5%Yb-doped CeO₂. The curve of the Ce 3d spectrum was fitted by eight peaks; the bands u₄, u₃, and u₁ (and those for v_i) were attributed to the Ce⁴⁺ state, while the u₂ and v₂ bands were due to the Ce³⁺ state [23]. Moreover, the O 1s XPS peak of 20%N/5%Yb-doped CeO₂ is shown in Figure 2c, which was divided into four separate peaks by Gaussian distributions, indicative of the presence of four kinds of oxygen species on CeO₂. The peaks labeled β, γ, and δ could be assigned to lattice oxygen (β for O–Ce⁴⁺ species, γ for O–Ce³⁺ species, and δ for O–Yb species), whereas the peak labeled α could be assigned to the chemisorption of oxygen or/and weekly bonded oxygen species related to V_O defects [24].

The relative concentration of Ce³⁺ ions in CeO₂, labeled as [Ce³⁺], could be calculated by comparing the integrated peak areas of the peak related to Ce³⁺ ions to those of all peaks in Figure 2b. Meanwhile, the relative oxygen vacancy content in CeO₂ (labeled as [V_O]) could also be estimated from O 1s XPS in Figure 2c by the ratio of the integrated area of the α peak to that of all peaks. Figure 2d shows the calculated [Ce³⁺] and [V_O] values of undoped, 5%Yb-doped, and 20%N/5%Yb-doped CeO₂. Both [Ce³⁺] and [V_O] values of 5%Yb-doped CeO₂ were higher than those of undoped CeO₂, which could be explained as follows. Yb³⁺ cations were introduced into the CeO₂ lattice by the substitution of Ce⁴⁺ ions, and a substoichiometric CeO_2–x unit was formed with increasing V_O defects based on the vacancy compensation mechanism, accompanied by an increase in [Ce³⁺] due to the activation effect of doping. The substitution reaction of Ce⁴⁺ by Yb³⁺ cations could be written using Kroger and Vink notations as Reaction (3). Moreover, for 20%N/5%Yb-doped CeO₂, the [Ce³⁺] and [V_O] values were increased further compared with those of 5%Yb-doped CeO₂, indicating that N anions had been incorporated into the CeO₂ lattice to form solid solutions, as shown in Reaction (4) using Kroger and Vink notations:

$${\mathrm{Yb}}_2{\mathrm{O}}_3\stackrel{{2\mathrm{CeO}}_2}{\rightleftarrows }{2\mathrm{Yb}}_{\mathrm{Ce}}^{\prime }+3{\mathrm{O}}_{\mathrm{O}}^{\times }+V_{\mathrm{O}}^{••}$$

(3)

$$\mathrm{CeN}\stackrel{{\mathrm{CeO}}_2}{\rightleftarrows }{\mathrm{Ce}}_{\mathrm{Ce}}^{\prime }+{\mathrm{N}}_{\mathrm{O}}^{\prime }+V_{\mathrm{O}}^{••}$$

(4)

Raman scattering is a very powerful tool for identifying the nature of surface V_O defects [25]. Figure 3a compares the Raman spectra of undoped, 5%Yb-doped, and 20%N/5%Yb-doped CeO₂. For undoped CeO₂, the visible Raman spectrum was dominated by the strong F_2g mode of the CeO₂ fluorite phase at ~462 cm⁻¹, as well as a weak band at ~592 cm⁻¹ assigned to the defect-induced mode, related to V_O defects. This finding indicated that a certain number of intrinsic V_O defect sites existed in undoped CeO₂. Both the band intensities at ~462 and ~596 cm⁻¹ had changed with the introduction of Yb and N ions in the CeO₂ lattice. Typically, compared with the defect-induced band of undoped CeO₂ at ~592 cm⁻¹, that of 5%Yb-doped CeO₂ had comparable intensity as the F_2g mode at ~462 cm⁻¹. In the case of 20%N/5%Yb-doped CeO₂, the band intensity of the defect-induced band was even stronger than that of the F_2g band. Moreover, an alternative approach was provided to estimate the relative concentration of V_O defects in CeO₂, namely, the relative number of V_O defects in CeO₂ could be determined by the intensity ratio of the bands at ~592 and ~462 cm⁻¹, labeled as I₅₉₂/I₄₆₂. As observed in Figure 3b, with the increasing amount of N in 5%Yb-doped CeO₂, the relative concentration of V_O defects, that is, the value of I₅₉₂/I₄₆₂, increased almost in a straight line and reached a maximum in 20%N/5%Yb-doped CeO₂. This proved that the additive amount of N anions in fluorite CeO₂ had an important influence on the formation of V_O defects in Yb/N-co-doped CeO₂.

Figure 4a displays the H₂–TPR profiles of 5%Yb-doped and 20%N/5%Yb-doped CeO₂, as well as undoped CeO₂ for comparison. For the H₂–TPR spectrum of undoped CeO₂, the reduction occurred at 200 °C and reached two maximum H₂ consumptions at ~505 and ~776 °C, implying the existence of at least two kinds of oxygen species at various coordination environments. In other words, the reduction behavior of pure CeO₂ was divided into two steps: The reduction band at 200~600 °C corresponded to the reduction of surface oxygen species, while the reduction band above 600 °C was assigned to the reduction of bulk oxygen, which could migrate to the CeO₂ surface through V_O defects and react with H₂. Compared to undoped CeO₂, the 5%Yb-doped and 20%N/5%Yb-doped CeO₂ could release a certain amount of oxygen below 200 °C, and there appeared to be a visible shoulder from 400 °C in the H₂–TPR profiles. Moreover, the reduction bands at ~600 °C were far higher than the baseline. These findings indicated that Yb/N doping could effectively improve the OSC of CeO₂ by increasing the number of V_O defects. In addition, 5%Yb-doped and 20%N/5%Yb-doped CeO₂ exhibited a slightly higher redox temperature than that of undoped CeO₂, which was attributed to the substitutions of Ce⁴⁺ and O²⁻ with the larger Yb³⁺ and N³⁻ and improved the stability of the CeO₂ lattice.

OSC is a fundamental characteristic and an important indicator of oxygen storage materials. In CeO₂-supported catalysts, the porous structure of CeO₂ is conducive to the loading of other active components, while the excellent OSC favors regulating the oxygen content of the catalytic system. Therefore, the quantification of OSC was the key to comparing their oxygen storage/release properties. For that, the OSC was quantified by the amount of H₂ consumption per gram of CeO₂ (mmol H₂/g CeO₂) based on H₂–TPR curves. Finally, the quantified OSC was obtained by calculating the amount of O₂ released per gram of sample (mmol O₂/g CeO₂) according to the H₂ consumptions, and the quantified OSC at low temperatures is shown in Figure 4b. Compared with the OSC of undoped CeO₂ (0.115 mmol O₂/g), that of 5%Yb-doped CeO₂ reached as high as 0.222 mmol O₂/g with a 93.04% increase. Upon the introduction of N anions, the OSC of Yb/N-co-doped CeO₂ continued to increase until 20%N, reached a maximum of 0.274 mmol O₂/g with a 138.26% increase, decreasing at higher N content. In our previous work, the OSC of CeO₂ increased by 119.02% and 40.22% upon 5% Hf-doping and 3% Sn-doping, respectively [26]. In our previous study, the OSC of N-doped CeO₂ microspheres was 0.73 mmol O₂/g [20]. Among the existing reported OSC data [27,28,29,30,31,32,33], our OSC in this work was above the average level. Even so, the idea of enhancing the OSC of CeO₂ by cation/anion co-doping has been proven to be feasible in this work.

S_BET is an important factor affecting the OSC of CeO₂, so we developed a function for the OSC/S_BET vs. lattice parameter as well as the OSC/S_BET vs. V_O concentration. These data were linearly fitted based on the partial least squares method, as shown in Figure 5a,b, and the calculated parameters are given as tables in Figure 5a,b. Both Figure 5a,b showed a linear dependence; however, the fitting of the OSC/S_BET vs. lattice parameter showed a higher correlation coefficient (R² = 0.98257) than that of OSC/S_BET vs. V_O concentration (R² = 0.92349), implying that a larger lattice distortion produced a higher OSC per S_BET for Yb/N-co-doped CeO₂. This lattice distortion includes not only the V_O defects, but also the lattice expansion, as well as the reducible–reoxidizable Ceⁿ⁺ ions, and so on. Thus, the OSC/S_BET could be predicted as a function of the lattice parameter for Yb/N co-doping CeO₂.

4. Conclusions

In summary, Yb/N-doped mesoporous CeO₂ with a multilayered structure was synthesized by a solvothermal method using Yb(NO₃)₃∙5H₂O and triethanolamine as cationic and anionic dopants, respectively. Both the original morphologies and fluorite crystal structure of undoped CeO₂ could be maintained even after the incorporation of 25%N/5%Yb, but the incorporation of Yb/N into CeO₂ could slightly affect their S_BET. Both the introduction of Yb cations and N anions could cause lattice expansion, accompanied by the formation of more V_O defects and reducible–reoxidizable Ceⁿ⁺ ions. Compared with the undoped CeO₂ (0.115 mmol O₂/g), the OSC of 5%Yb-doped CeO₂ increased to 0.222 mmol O₂/g with a 93.04% increase, while that of 20%N/5%Yb-doped CeO₂ enhanced to 0.274 mmol O₂/g with a 138.26% increase.