A new type of photocatalytic material, double-perovskite oxides, LnBaCo<SUB>2</SUB>O<SUB>5+δ</SUB> (Ln = Eu, Gd, and Sm) was synthesized via a conventional solid-state reaction process using Ln <SUB>2</SUB>O<SUB>3</SUB>, BaCO<SUB>3</SUB> and Co <SUB>2</SUB>O<SUB>3</SUB> as raw materials. X-ray diffraction results show that the crystalline structures are a pure orthorhombic lattice and are consistent with LnBaCo<SUB>2</SUB>O<SUB>5+δ</SUB> microparticles. The photocatalytic activity of the LnBaCo<SUB>2</SUB>O<SUB>5+δ</SUB> (Ln = Eu, Gd, and Sm) powders was further demonstrated in the degradation of Congo red (CR) under ultraviolet light irradiation with the dye solution concentration of 25 or 50 mg·L-1. The double-perovskite oxides LnBaCo<SUB>2</SUB>O<SUB>5+δ</SUB> show a certain photocatalytic activity during the degradation of CR under ultraviolet light, which means that they are one kind of the promising photocatalytic materials for the degradation of the azo dyes.
With the society and economic development, the problem of environmental pollution is outstanding day by day. Especially, the contamination of water due to color effluents coming from different industries, such as textiles, dyestuff, and paper, which are toxic and low-biodegradable, discharges into the aquatic systems and leads to the serious destroy of the survival environment as well as serious damage to human health [
Perovskite oxides with the general formula of ABO3 in which A-site (with ionic radius of larger than 1.0 Å) is a rare-earth or an alkali-earth element and B-site (having ionic radius in the order of 0.6 - 0.8 Å) is typically a 3D transition metal become the hotspot of material science because of their stable structure and unique physical and chemical properties [
Double-perovskite oxides which named after perovskites ABO3 are a large family and have wide application fields. Among them, the oxygen deficient ordered double perovskite cobaltates with the general formula LnBaCo2O5+δ have drawn significant interest as potential IT-SOFC (Intermediate Temperature-Solid Oxide Full Cell) cathodes, chemical gas censors, magnetic and oxygen permeation materials because of their high electronic con- ductivity, oxygen transport properties, oxygen surface exchange coefficient and superior oxide ionic diffusivity [
In this paper, the mainly research is focused on the preparation process and removal properties of organic pollutants CR from the wastewater using the double-perovskite oxides LnBaCo2O5+δ (Ln = Eu, Gd, and Sm).
All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification and treatment.
LnBaCo2O5+δ (Ln = Eu, Gd, and Sm) powders were prepared by a conventional solid-state reaction method. Ln2O3, BaCO3 and Co2O3 served as the starting materials for the necessary metal ions. The process flow chart is shown in
The crystal structures of the synthesized materials were characterized by X-ray powder diffraction (XRD) using
a Rigaku D/Max-3B instrument with copper target and Kα radiation (λ = 1.54056 Å). The accelerating voltage and applied current were 40 kV and 200 mA. The samples were scanned from 10˚ to 90˚ (2θ) in steps of 0.02˚.
CR (C32H22N6Na2O6S2, molecular weight: 696.67 g∙mol−1, from Guangzhou Reagent Corporation, China) was used as a model dye to evaluate the photocatalytic activity of the LnBaCo2O5+δ (Ln = Eu, Gd, and Sm) samples. The degradation tests of CR were conducted in an aqueous solution at room temperature under ultraviolet light irradiation with the dye solution concentration of 25 or 50 mg∙L− 1 . In a typical degradation process, under ultraviolet light irradiation and stirring conditions, 20 mg of the synthesized samples were added into 50 mL CR aqueous solutions with an initial concentration of 25 or 50 mg∙L−1 in a quartz beaker, respectively. At a given time, 4 mL of the dispersion was continually extracted and subsequently centrifuged to separate LnBaCo2O5+δ powders and dye solutions at 4000 rev∙min-1 for 30 min. The reaction mixtures were irradiated by a Xe-lamp (30 W) with the wavelength range of 320 - 400 nm in which a peak wavelength of 365 nm for UV light and 400 - 800 nm for visible light, and magnetically stirred throughout the photocatalytic experiment under air. The initial concentration (C0 ) and the instant concentration (C ) of the aqueous solution of CR were determined with a UV-2401PC spectrophotometer at λmax = 498 nm . The degradation rate of CR dye was calculated as C/C0.
The as-synthesized LnBaCo2O5+δ (Ln = Eu, Gd, and Sm) powders with well crystal were used for photocatalytic tests under ultraviolet light irradiation. CR was used as a test contaminant since it has been extensively used as
an indicator for the photocatalytic activity. After photocatalysis experiments, we found that double-perovskite oxides LnBaCo2O5+δ could remove the CR effectively, which mean they have photocatalytic properties.
To further clarify the photocatalytic performance and mechanism of CR, the continuous UV-Vis spectra of the centrifuged solution after catalytic reactions at the different intervals were used to record and in contrast to initial CR solution.
achieved about 47%. Complete decolorization of CR solution (C0 = 25 mg∙L−1) was achieved within 26 h. These results are in agreement with the analysis shown in inset. It is clearly seen that the cardinal red color of starting solution gradually disappears along with increasing the degradation time, which also depicts the degradation process of CR.
The possible catalytic mechanism of LnBaCo2O5+δ (Ln = Eu, Gd, and Sm) powders is proposed like that of the perovskite oxides. We suppose the photocatalytic property of double-perovskite is derived from the BO6 octahedron. In LnBaCo2O5+δ, 2p-orbit of O2? constitutes the valence band, while the 3d-orbit of Co3+ constitutes the conduction band. Under the ultraviolet light irradiation, the valence bands electrons of LnBaCo2O5+δ were excited and transferred into conduction bands, generating highly reactivity electron-hole pairs which can induce oxidation of organic molecule, by leaving holes (h+) in the valence bands. The hole (h+) is a strong oxidant on the surface of LnBaCo2O5+δ and could oxidize the adsorbed water molecules or hydroxyl ions into ・OH radicals (H2O or OH? + h+ → ・OH ) which is highly radicals to oxidize or degrade the adjacent CR dye molecules. At the same time, the adsorbed oxygen reacted with electrons to form
Double-perovskite oxides, LnBaCo2O5+δ (Ln = Eu, Gd, and Sm) microparticles were successfully prepared by a conventional solid-state reaction method. The samples were characterized by X-ray diffraction (XRD), showing that the resulting particles were highly crystalline LnBCO particles. The photocatalytic activity of the LnBaCo2O5+δ (Ln = Eu, Gd, and Sm) powders was investigated by the degradation of CR, and the results revealed that LnBCO had a certain photocatalytic activity, which indicated that they could be a promising photocatalyst for the degradation of organic molecules.
This work was supported by National Natural Science Foundation of China (Grant No.51262029), the Key Project of the Department of Education of Yunnan Province (ZD2013006), Program for Excellent Young Talents, Yunnan University (XT412003), Yunnan University Graduate Program for Research and Innovation, and the Department of Science and Technology of Yunnan Province via the Key Project for the Science and Technology (Grant No.2011FA001).