La₂O₃/Fe₂O₃-CeO₂ Composite Oxide Catalyst and Its Performance

Youfeng Li^*, Jinliang Lin, Guangwei Wang

School of Chemistry and Chemical Engineering, Zunyi Normal College, Zunyi, China.

DOI: 10.4236/ampc.2019.912017                

Abstract

The La₂O₃/Fe₂O₃-CeO₂ composite oxide catalysts were prepared by coprecipitation method, sol-gel method and hydrothermal method. The effect of preparation methods on structure morphology and photocatalytic properties of La₂O₃/Fe₂O₃-CeO₂ samples was investigated. The results show that the cubic CeO₂ structure can be obtained at 600℃. The rod-shaped sample prepared by coprecipitation method, displays the highest crystalline and the strongest diffraction peak intensity. The spherical sample is acquired from sol-gel method, while the small granular sample prepared by hydrothermal method tends to aggregate. The maximum specific surface area of the sample prepared by coprecipitation method is 76.21 m²/g, the minimum specific area of the sample from sol-gel method is 32.66 m²/g and the maximum pore size is 13.84 nm, while the minimum pore volume and pore size of the sample by hydrothermal method are 0.038 cm³/g and 3.95 nm respectively. The band gap energy of catalyst samples is in the following order: sample-CP < sample-SG < sample-HT. The sample obtained by coprecipitation method has the best catalytic degradation performance for methylene blue. Under the excitation of visible light, the degradation rate was 99.58% at 50 minutes, which was higher than those of sol-gel method and hydrothermal method by 5.58% and 9.54% respectively. The catalytic degradation reaction is a first-order kinetic model: ln (c₀/ct) = kt + q_e. The maximum k-value of the sample degradation process obtained by coprecipitation method is 0.074 min^-1.

Keywords

La₂O₃/Fe₂O₃-CeO₂, Composite Oxide, Synthetic Method, Catalytic Degradation, Wastewater Treatment

1. Introduction

There is about 2.37 billion ton textile wastewater discharged from Chinese textile industry every year, among which dying wastewater accounts for about 80%. The color of dyeing wastewater is particularly serious pollution, and it is difficult to remove by general biochemical methods. The recycling rate of wastewater after treatment in the whole textile industry is only about 10%. Therefore, the treatment and emission reduction of dyeing wastewater are urgent for improving the quality of water environment. Photocatalytic technology is a good application prospect in the fields of environmental purification, solar energy utilization and self-cleaning [1] [2] [3].

CeO₂ has attracted much attention in photocatalytic degradation of wastewater contaminants for its unique crystal structure, high oxygen storage, and oxygen release capacity. The activity and selectivity of pure CeO₂ catalyst generally are improved by adjusting morphology and particle size of CeO₂, so its catalytic oxidation technology is insufficiently pushed to a wide range of practical applications [4] [5] [6]. Researchers have tried to synthesize multi-component Ce-based composite oxide catalysts by doping transition metal M into CeO₂. The lattice distortion of CeO₂ results in defects when CeO₂ interacts with transition metal M for its variable valence and excellent redox performance. Then it accelerates the flow of oxygen in the crystal phase and provides more active oxygen species for catalytic oxidation. So the catalytic activity and stability of the Ce-based composite oxide catalysts are greatly improved [7]. Choi et al. [8] studied the effect of various metal ions on the photocatalytic activity of titanium dioxide, among which the effect of Fe³⁺ was the best. Machida et al. [9] found that the oxygen storage/release capacity of CeO₂-Fe₂O₃ composites was better than that of pure components CeO₂ and Fe₂O₃. They also pointed out that CeO₂ was the entrance of oxygen storage. The high temperature reaction activity and thermal stability of the La modified Ni/CeO₂-ZrO₂ catalysts were improved because La³⁺ doped into Ni/CeO₂-ZrO₂ oxides could produce more oxygen vacancies during high temperature sintering, and then the oxygen mobility was enhanced [10]. In this paper, Ce-based composite oxide catalysts were prepared with transition metals of Fe³⁺ and La³⁺ doped into CeO₂, and their performances were also investigated.

The preparation methods of Ce-based composite oxide catalysts have shown great influence on their structure and morphology. The preparation methods are hydrothermal method [11], sol-gel method [12], combustion method [13], precipitation method [14], template method [15], and so on. In the paper, the La₂O₃/Fe₂O₃-CeO₂ catalysts with different morphologies were prepared by coprecipitation method, sol-gel method and hydrothermal method. The effects of preparation methods on the structure and performance of the La₂O₃/Fe₂O₃-CeO₂ catalysts were studied.

2. Experimental Sections

2.1. Preparation of La₂O₃/Fe₂O₃-CeO₂ Catalyst

The reagents Ce(NO₃)₃·6H₂O, Fe(NO₃)₃·9H₂O, La(NO₃)₃·6H₂O, polyethyleneglycol (PEG), ammonium bicarbonate, urea, and methylene blue from China National Medicines Corporation Ltd. were analytically pure. All the chemicals were AR grade, and used as received without further purification. Deionized water was used in all experiments.

Coprecipitation method: First, 0.868 g Ce(NO₃)₃·6H₂O, corresponding Stoichiometric ratio Fe(NO₃)₃·9H₂O and La(NO₃)₃·6H₂O were dissolved in deionized water, then the solution was added into 250 ml three-port bottle. The polyethylene glycol (PEG) solution was slowly added into the three-port bottle with 0.21 g PEG as complexing agent dissolved in 40 ml water. Then the precipitator NH₄HCO₃ was added into to mixed solution to aid precipitate at 80˚C under agitation. After aging at 100˚C for 10 h for 24 h, the precipitate was filtrated and washed with deionized water. The La₂O₃/Fe₂O₃-CeO₂ precursor samples were obtained after the precipitate being dried, followed by calcinating in a muffle furnace at 600˚C for 4 h. The corresponding La₂O₃/Fe₂O₃-CeO₂ composite sample was denoted as sample-CP.

Sol-gel method: First, 0.868 g Ce(NO₃)₃·6H₂O, corresponding Stoichiometric ratio Fe(NO₃)₃·9H₂O and La(NO₃)₃·6H₂O were dissolved in deionized water to obtain mixed solution, then the mixed solution was added into 250 ml three-port bottle. The polyethylene glycol (PEG) solution was slowly added into the three-port bottle with 0.21 g PEG as complexing agent dissolved in 40 ml water. Then the mixed solution was agitated at 80˚C to obtain sol. The scarlet La₂O₃/Fe₂O₃-CeO₂ gel precursor was obtained after the sol was dried at 100˚C for 10 h. The CeO₂ compound oxide samples were acquired by calcinating CeO₂ gel in a muffle furnace at 600˚C for 4 h. The corresponding La₂O₃/Fe₂O₃-CeO₂ composite sample was denoted as sample-SG.

Hydrothermal method: First 0.21 g PEG and 1.0 g urea were dissolved in distilled water, and continued stirring to make the mixture solution. Then 0.868 g Ce(NO₃)₃·6H₂O, corresponding Stoichiometric ratio Fe(NO₃)₃·9H₂O and La(NO₃)₃·6H₂O were added into the mixture solution and stirred for 0.5 h at 80˚C. Finally, the mixed solution was transferred to a polytetrafluoroethylene high-pressure hydrothermal kettle of 100 ml for 24 hours at 140 C. The precipitate was filtrated and washed with deionized water. The La₂O₃/Fe₂O₃-CeO₂ precursor samples were obtained after the precipitate being dried, followed by drying in oven at 80 C for 10 h then calcinating in a muffle furnace at 600˚C for 4 h. The corresponding La₂O₃/Fe₂O₃-CeO₂ composite sample was denoted as sample-HT.

The molar ratio of La³⁺:Fe³⁺ was 1:2, and the molar doping amount of La₂O₃ and Fe₂O₃ in La₂O₃/Fe₂O₃-CeO₂ was 3% in the experiment.

2.2. Catalytic Degradation Experiments

Catalytic degradation experiments were performed in a double beaker of 250 ml with water cooling. A portion of catalyst (10 mg) and H₂O₂ (6 ml) were introduced into 100 ml of aqueous dye solution containing methylene blue at a concentration of 100 mg·L⁻¹. Sodium hydroxide and hydrochloric acid solution were used to adjust the pH value of the solution. Adsorption equilibrium was attained by the reaction solution after magnetic stirring for 30 min in the dark. The degradation reaction was then performed using 300 W xenon lamps as visible light. Liquid samples were removed at regular 10 min intervals for analysis of the absorbance. To evaluate the catalytic activity of the catalysts, the dye removal efficiency (η) was calculated, as shown below:

$\eta =\frac{c_0-c_t}{c_0}\times 100\%$,

where c₀ is the initial after the dark equilibrium, and c_t is different times absorbance values of methylene blue, respectively.

2.3. Catalyst Characterization

A thermogravimetric (TG/DSC) analysis was conducted on an STD Q600 instrument at a rate of 5˚C·min⁻¹ under nitrogen protection. The crystalline phases of the as-prepared CeO₂ composite oxides were studied by x-ray diffraction (XRD) with a Bruker/AXS D8 Advance X-ray diffractometer operated at 40 kV and 200 mA with Cu Ka radiation (λ = 0.15418 nm). The morphology and dispersion of the powders were observed with the use of a transmission electron microscope (TEM, JEOL-6360LV). Textural properties of the samples were measured by the Brunauer-Emmett-Teller (BET) model on a Quantachrome NOVA instrument, with Aras the carrier gas and N₂ as the adsorbent at −196˚C. The visible light absorbance at the characteristic wavelength of 664 nm for methylene blue dye was measured with a UV-2500 spectrophotometer.

3. Results and Discussions

3.1. TG/DSC Analysis

Figure 1 shows the TG/DSC curves of the dried La₂O₃/Fe₂O₃-CeO₂ precursors with a heating rate of 5˚C·min⁻¹. DSC curves display endothermic peaks and exothermic peaks of the dried precursors. The about 150˚C endothermic peak can be explained by the removal of adsorbed water and crystal water. The about 200˚C exothermic peaks can be explained by the burning and decompose of PEG in the precursor powders. Major mass loss of the precursor samples occurs below 500˚C. The overall weight loss is 20% - 50%. The thermal decomposition of precursor from the coprecipitation method was showed in Figure 1(A). The main mass loss occurred below 300˚C. A small endothermic peak at 150˚C could be attributed to the loss of adsorbed water and crystal water, which accounting for about 10% of the total mass loss. There are two exothermic peaks between 200˚C and 300˚C, which were caused by the decomposition of Ce(OH)₃ and Ce₂(CO₃)₃ precursors, accounting for about 40% of the total mass loss. The different interaction between PEG and metal ions leads to varied decomposition temperature. When the temperature is about 520˚C, there is another exothermic

Figure 1. TG/DSC curves of precursors synthesized by different methods: (A) CP, (B) SG, (C) HT.

peak which shows crystal formation. The mass loss of precursor samples by coprecipitation method mainly occurs in two regions of 200˚C - 250˚C and 300˚C - 460˚C, with a total loss of about 25%. The corresponding exothermic peak at 200˚C - 250˚C can be attributed to thermal decomposition of residual PEG in precursor sample-SG. The strong endothermic peak at 300˚C - 460˚C can be decomposed of PEG additives (shown in Figure 1(B). While at about 350˚C - 550˚C, there occurs an exothermic peak of sample-HT precursors in the heating process which implies the decomposition of precipitates of precursor Ce(HCOO)₃ and the formation of CeO₂ crystal phases, with a total weight loss of about 20% (Figure 1(C). This is proved by the XRD results in Figure 2. From the above results, the intermolecular and ionic interactions in the precursor precipitates prepared by different methods are different. So the thermal stability of precursors and the decomposition temperature of samples are different.

3.2. XRD Analysis

The XRD patterns of La₂O₃/Fe₂O₃-CeO₂ samples are exhibited in Figure 2. The peaks around 2θ = ~28.5˚, ~33.0˚, ~47.5˚, ~57.5˚ and ~69.5˚ in agreement with the (111), (200), (220), (311) and (400) planes of the cubic fluorite structure of CeO₂, respectively. The crystal form is consistent with the standard JCPDS (34-0394) spectra [16]. The results show that the main active component of the catalyst is crystallized CeO₂ with cubic fluorite structure and this structure remains after calcinating process. However, the crystallinity of the samples varies greatly with different synthesis methods. Sample-CP shows the best crystallinity and strongest diffraction peak intensity. To resume, the crystallinity of catalyst samples is in the following order: sample-CP > sample-SG > sample-HT. According to Scherer formula, the particle sizes of La₂O₃/Fe₂O₃-CeO₂ samples prepared by the three methods were calculated to be about 100 nm (sample-CP), 32.56 nm (sample-SG) and 5.23 nm (sample-HT), respectively.

From the diffraction peak in Figure 2, we can see that there is no impurity phase peak, and the samples display high purity, while the diffraction peak of the sample obtained by hydrothermal method slightly shifts red. The doping elements La and Fe in the samples are not detected by XRD, indicating that La³⁺ and Fe³⁺ cations have been well inserted into the CeO₂ lattice, or La₂O₃-based and Fe₂O₃-based clusters are well dispersed on the surface of CeO₂ particles [17].

Figure 2. XRD of La₂O₃/Fe₂O₃-CeO₂ catalysts from different methods.

3.3. TEM Analysis

Figure 3 shows TEM photographs of La₂O₃/Fe₂O₃-CeO₂ samples synthesized by different methods. It can be seen that the rod-like sample, with a diameter of about 100 nm, obtained from coprecipitation is well dispersed from Figure 3(A). The sample-SG is spherical and well dispersed, with a diameter of 20 - 40 nm. The samples obtained by hydrothermal method are fine particles, some of which accumulate into electronic clouds. The results consist of the results of XRD. Compared with (A), (B) and (C) in Figure 3, it is obvious that the morphologies of samples synthesized by different methods vary greatly.

During the formation of sol-gel, the spherical structure is formed by nanoparticles self-assembly through hydrogen bonds between ethylene glycol molecules. When the gel is dried and calcined at high temperature, the nuclei grow along the isotropic state, and nanospheres are acquired [18]. There are Ce(OH)₃ and Ce₂(CO₃)₃ precipitators formed during coprecipitation. While Ce(OH)₃ and Ce₂(CO₃)₃ are polyhedrons constituted of a series of polyhedrons consisting of Ce³⁺ and OH^－, ${\text{CO}}_3^{2-}$. During coprecipitation, Ce³⁺ binds with OH^－and ${\text{CO}}_3^{2-}$ then form nanocrystals with very small size. Nanorods are formed when these nanocrystals grow along the crystal plane with high chemical activity [17]. Ce-OCH₂-CH₂-OH are formed between Ce(HCOO)₃ and polyethylene glycol in the hydrothermal reaction process, because polyethylene glycol was adsorbed on the positive charge Ce(HCOO)₃ crystal surface with high chemical activity. Which hindered the growth of Ce(HCOO)₃ along the crystal surface, resulting in fine nanoparticles. When calcined at high temperature, some of these fine nanoparticles aggregated. The morphology and particle size of La₂O₃/Fe₂O₃-CeO₂ samples directly affect their texture properties.

Figure 3. TEM photography of La₂O₃/Fe₂O₃-CeO₂ catalyst from different methods: (A) CP, (B) SG, (C) HT.

3.4. BET Analysis

The texture structure of the samples was conducted by N₂ adsorption and desorption method. The specific surface area, pore volume and average pore diameter results of the samples from three methods are shown in Table 1. Sample-CP has the highest surface area of 76.21 m²·g⁻¹ and the highest pore volume of 0.091 cm³·g⁻¹. The smallest surface area of the sample-SG sample are 32.66 m²·g⁻¹, and the largest pore diameter is 13.84 nm. While the surface area of sample-HT intervene sample-CP and sample-SG, the pore volume and pore diameter were the smallest, the values of them are 53.48 m²·g⁻¹, 0.038 cm³·g⁻¹ and 3.95 nm, respectively. This may be due to the different decomposition temperature, gas release rate and heat release rate of the precursors during thermal decomposition, which results in obvious difference in pore structure and surface area.

The pore size of the sample-SG shows obvious growth tendency, indicating that the sample-SG possesses better aging resistance and stronger adsorption capacity. During the sol-gel process, the organic dispersant PEG has been kept in the precursor. When calcined at high temperature, PEG decomposes and releases a large amount of gas, resulting in an increase in the pore size of the sample. However, its surface area is the smallest because the particle shape of the sample-SG is spherical. Most PEG is lost following the filtrate during precipitate filtrating for coprecipitation process and hydrothermal process. At the same time, the particle size is small and the particles agglomerated seriously of the sample-HT, which reduces the pore volume and pore size. The result is consistent with the thermal analysis results.

Figure 4 shows the adsorption and desorption isotherms of the La₂O₃/Fe₂O₃-CeO₂ samples. We can find that three kinds of samples present adsorption and desorption isotherms of mesoporous (2 - 50 nm) materials characteristics. There are also hysteresis loops in the isotherms due to incomplete uniformation of pore shapes and pore size, as well as phenomenon of capillary condensation. Of which, the hysteresis loops of sample-CP are the most obvious, and the hysteresis loops of sample-HT are the smallest. Whatmore, the pore shape of samples is bottle-shaped. Hysteresis loops are formed due to the phenomenon of capillary condensation and incomplete pore size [19]. The adsorption curves of the samples prepared by coprecipitation and sol-gel method increase sharply at the relative pressure (P/P₀) of 0.6 - 1.0. It may be due to the larger pore size and relatively concentrated pore size distribution of the sample, and the pore size is mainly concentrated in the mesoporous size. The results are consistent with the pore size distribution curves. It can also be seen from Figure 4 that among the three samples, desorption of sample-HT is the strongest, and the hysteresis loop formed by adsorption-desorption curve is the most obvious. The adsorption desorption value of the sample-CP is the largest, followed by the sol-gel method. The value of the hydrothermal method is the smallest, which is consistent with the analysis results of the surface area and pore structure of the La₂O₃/Fe₂O₃-CeO₂ sample. These textural properties will directly affect the photocatalytic degradation performance of La₂O₃/Fe₂O₃-CeO₂.

Table 1. Texture properties of different La₂O₃/Fe₂O₃-CeO₂ photocatalyst samples.

Figure 4. N₂ adsorption-desorption isotherms and pore volume distribution of La₂O₃-Fe₂O₃-CeO₂ catalyst samples: (A) CP, (B) SG, (C) HT.

3.5. La₂O₃-Fe₂O₃-CeO₂ Photocatalytic Degradation Analysis

Figure 5 shows the time dependence of the methylene blue removal efficiency by different La₂O₃/Fe₂O₃-CeO₂ catalysts under 300 W Xenon lamps. The methylene blue removal efficiencies are shown in Figure 5 for a degradation process with a catalyst concentration of 100 mg·L⁻¹ at solution pH = 10.0. The sample-CP catalyst shows the best catalytic activity during the catalytic degradation process. The degradation of methylene blue is almost complete at 50 min for La₂O₃/Fe₂O₃-CeO₂/γ-Al₂O₃, which represents an increase of 5.58% and 9.54%, compared with the efficiencies of sample-SG and sample-HT under the same reaction conditions, respectively. The removal efficiency of methylene blue by sample-CP achieves a maximum value of 99.58% at 50 min. The high catalytic activity is attributed to the texture and morphology of the catalyst sample.

The sample-CP has good adsorption and more active sites on surface for its large surface area. So the catalytic reaction effect is better than the other two. This indicates that catalytic oxidation process is major dependent on adsorption-desorption, and catalytic oxidation also plays an important role. The pore size and pore size of sample-CP are large, which increases the defects inside the lattice and decreases the band gap. It provides more reactive oxygen species for catalytic oxidation and improves the catalytic activity [20]. The sample-HT has small particle size and there is agglomeration among the particles, which leads to the reduction of the catalytic effect.

The degradation reaction of methylene blue can be described by the first-order reaction kinetics: ln (c₀/c_t) = kt + q_e, where c₀ and c_t are the concentration of methylene blue at the initial time and each interval time during irradiation, respectively, k is the first-order rate constant, t is the irradiation time and q_e is a constant. A high k-value usually implies the fast reaction rate of efficient catalysts [21]. The calculated k value (in Figure 6) is 0.074, 0.063 and 0.060 min⁻¹ for sample-CP, sample-SG and sample-HT, respectively, which indicates that the sample-CP catalyst exhibits the highest photocatalytic activity. The result is consistent with the results of Figure 5.

Figure 5. Degradation methylene blue results for different La₂O₃/Fe₂O₃-CeO₂ catalyst samples under Xenon light.

Figure 6. Linear fits of ln (c₀/c_t) versus time.

3.6. Mechanism Analysis

The electron-hole pairs on the surface of the catalyst react with H₂O, H₂O₂ and O₂, then form •OH and •O₂. In the degradation process, methylene blue is first adsorbed on the surface of the catalyst, then reacts with •OH and •O₂ and decomposed into CO₂ and H₂ [22]. Researchers found that doped with transition metals, electron-hole pairs on the surface of CeO₂ will be promoted [23] [24]. When transition metal La³⁺ is doped into CeO₂, some of Ce⁴⁺ lattices are substituted by La³⁺ and produce a large amount of reactive oxygen species, which greatly improves the catalytic activity of CeO₂ [25]. The doping of Fe³⁺ into TiO₂ results in the decrease of its energy level, catalytic reaction shifted to 400 - 700 nm in the visible region, and the catalytic activity of TiO₂ catalyst was improved [8]. Therefore, the doping of La³⁺ and Fe³⁺ into CeO₂ will result in lattice distortion and defect, decrease energy level, accelerate the flow of oxygen in the crystal phase, and provide more active oxygen species for catalytic oxidation. Therefore, the catalytic activity of La₂O₃/Fe₂O₃-CeO₂ samples is enhanced under visible light excitation.

Usually, the visible wavelength is 380 - 760 nm and its corresponding energy is 3.10 - 1.63 ev. CeO₂ has a large band gap of 3.2 ev, which makes it sensitive to ultraviolet radiation only [23]. The band gap between the valence and conduction bands of the CeO₂ and La₂O₃/Fe₂O₃-CeO₂ samples is determined using UV-vis diffuse reflectance spectra. As shown in Figure 7(A), the large absorption at wavelengths shorter than 400 nm can be attributed to intrinsic band-to-band absorption of La₂O₃/Fe₂O₃-CeO₂₃. Compared to the CeO₂, the La₂O₃/Fe₂O₃-CeO₂ compound oxide exhibit significantly increased absorption at above 430 nm, indicating that the loaded La³⁺ and Fe³⁺ may be helpful for utilizing visible light more effectively. The diffuse reflectance was transferred via Kubelka-Munk function into Tauc plots and the band gap energy of the samples was defined via Tauc plots that are shown in Figure 7(B). The band gap La₂O₃/Fe₂O₃-CeO₂ exhibits the energy gaps of sample-CP (2.50 ev), sample-SG (2.68 eV) and sample-HT (2.82 eV) respectively. It is deduced that the decrease

Figure 7. UV–Vis diffuse reflectance spectra of different La₂O₃/Fe₂O₃-CeO₂ catalyst samples (A); (ah_v)² versus energy (hv) plots of different La₂O₃/Fe₂O₃-CeO₂ catalyst samples (B).

of band gap increases the light absorption, and electrons through absorbing lower photon energy in the valence band more easily jump to the conduction band [24]. Thereby, optical results of La₂O₃/Fe₂O₃-CeO₂ imply more efficient processes for visible-light catalytic degradation of methylene blue. Sample-CP shows the best catalytic degradation effect for methylene blue.

4. Conclusion

La₂O₃/Fe₂O₃-CeO₂ composite oxide catalysts were prepared by coprecipitation method, sol-gel method and hydrothermal method. The catalyst was CeO₂ phase with cubic fluorite structure. The samples with different morphology were obtained due to varied interaction forces between ions and molecules in the precursors, and different decomposition processes. The rod-shaped sample from coprecipitation method was about 100 nm particles with good dispersivity, 20 - 40 nm spheres sample obtained by sol-gel method, and it was small agglomerated particles by hydrothermal method. The morphology and particle size of the sample directly affected its texture properties of surface area, pore volume and pore diameter. Catalytic degradation performance depends not only on structure and texture properties, but also on band gap of the catalyst. The catalytic degradation rate of methyl blue under visible light is in the following order: sample-CP > sample-SG > sample-HT.

Acknowledgements

This project was financially supported by the projects of “Qian Jiao He KY” [2019] 114 and “Talents of Qian Ke He Platform” ([2017]5727-11).

Conflicts of Interest

The authors declare no conflicts of interest.

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