La2O3/Fe2O3-CeO2 Composite Oxide Catalyst and Its Performance

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 + qe. The maximum k-value of the sample degradation process obtained by coprecipitation method is 0.074 min-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]. CeO 2 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 CeO 2 catalyst generally are improved by adjusting morphology and particle size of CeO 2 , 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 CeO 2 . The lattice distortion of CeO 2 results in defects when CeO 2 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 Fe 3+ was the best. Machida et al. [9] found that the oxygen storage/release capacity of CeO 2 -Fe 2 O 3 composites was better than that of pure components CeO 2 and Fe 2 O 3 . They also pointed out that CeO 2 was the entrance of oxygen storage. The high temperature reaction activity and thermal stability of the La modified Ni/CeO 2 -ZrO 2 catalysts were improved because La 3+ doped into Ni/CeO 2 -ZrO 2 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 Fe 3+ and La 3+ doped into CeO 2 , 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

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 H 2 O 2 (6 ml) were introduced into 100 ml of aqueous dye solution containing methylene blue at a con-Advances in Materials Physics and Chemistry centration 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: where c 0 is the initial after the dark equilibrium, and c t is different times absorbance values of methylene blue, respectively.

XRD Analysis
The   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

BET Analysis
The texture structure of the samples was conducted by N 2 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 m 2 ·g −1 and the highest pore volume of 0.091 cm 3 ·g −1 . The smallest surface area of the sample-SG sample are 32.66 m 2 ·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 m 2 ·g −1 , 0.038 cm 3 ·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 La 2 O 3 /Fe 2 O 3 -CeO 2 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/P 0 ) 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 La 2 O 3 /Fe 2 O 3 -CeO 2 sample. These textural properties will directly affect the photocatalytic degradation performance of La 2 O 3 /Fe 2 O 3 -CeO 2 .   Figure 5 shows the time dependence of the methylene blue removal efficiency by different La 2 O 3 /Fe 2 O 3 -CeO 2 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 La 2 O 3 /Fe 2 O 3 -CeO 2 /γ-Al 2 O 3 , 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.

La2O3-Fe2O3-CeO2 Photocatalytic Degradation Analysis
The degradation reaction of methylene blue can be described by the first-order reaction kinetics: ln (c 0 /c t ) = kt + q e , where c 0 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.