Ordered mesoporous La–Co–Ce–O₃ through nanocasting: an effective catalyst for the photodegradation of phenol

Abstract

The design of highly active photocatalysts for degradation of phenol can be particularly challenging for solar energy utilisation. Herein, a series of mesoporousLaCo_1−xCe_xO₃catalysts have been synthesised and studied for degradation of phenol under UV irradiation. Compared with LaCoO₃-SG catalyst which is formed through sol-gel method, optimised LaCo_1−xCe_xO₃-NC prepared by nanocasting method showed remarkably higher activity for the degradation of phenol due to the higher specific surface area, and the order of degradation rates is LaCo_0.85Ce_0.15O₃-NC > LaCoO₃-NC > LaCoO₃-SG. Moreover, Au/LaCo_1−xCexO₃ catalysts were synthesized via Au deposition–precipitation method, and the catalytic activity for photodegradation of phenol increased with the loading of Au on the support.

Keywords:

• mesoporous La–Co–Ce–O3
• nanocasting
• photodegradation of phenol

1. Introduction

As industry wastewater entering into rivers, lakes and seas, serious water pollution is caused by organic pollutants. In particular, phenol is one of most polluted materials in the water environment owing to its stable chemical structure as well as extensive usage in the chemical industry [1]. Currently, photocatalytic technology with semiconductor as catalyst is known as an effective method to treat refractory organic compounds [2]. Compared with simple oxides such as TiO₂, Fe₂O₃, ZnO, WO₃, perovskite-type composite oxides (ABO₃) are the most promising photocatalysts in the photodegradation process due to their flexible design, high thermal and chemical stability.

The ABO₃ composite oxides have a narrow band gap and can absorb visible light and promote electron transitions, so that ABO₃ composite oxides show good photocatalytic activity for photodegradation of organic compounds. Especially, the doping of different ions at A or B sites further improve the catalytic performance of the composite oxides under visible light without changing perovskite-type structures, in which the B-site ion is always the active centre plays a decisive role in the catalytic reactions. Therefore, the selection of B-site elements is very important in the design or modification of ABO₃ catalyst.

It was reported that the partial replacement of Co³⁺ ions in LaCoO₃ by Fe³⁺, Cu²⁺, Mn³⁺ and Ce³⁺ ions could improve the catalytic performance for the degradation of phenol, resulting from the formation of oxygen vacancies [3–6]. However, the rate of phenol degradation using perovskite oxides obtained by traditional methods as photocatalyst is very limited owing to low surface area of the catalyst and poor electron–hole separation rate in photochemistry process. To obtain perovskite-type oxides with high specific surface area, several preparation methods were put forward [7–9]. Template route method including hard and soft template is used widely. For example, Lu [10] synthesized three-dimensional ordered mesoporous LaCoO₃ oxide with a specific surface of 96.7 m².g⁻¹ using cubic ordered mesoporous vinyl silica as template. Wang [11] used KIT-6 as hard template to get mesoporous LaCoO₃ oxide with specific surface area of 270 m².g⁻¹.

The perovskites with high specific surface area and mesoporous structure could provide a large number of active sites, accelerate the diffusion of the reactants and products, shorten the length of charge carrier diffusion and effectively inhibit the electron–hole recombination in semiconductors [12]. Furthermore, the deposition of Au on the perovskite could enlarge the absorption range of sunlight through surface plasmon resonance, which inhibit the recombination of electron–hole pairs and improve the catalytic activity [13–15].

Motivated by the above observations, we developed a novel and promising photocatalysts with mesoporous perovskite materials and doped rare earth ion (La–Co–Ce–O₃) for the degradation of phenols. The ordered mesoporous La–Co–Ce–O₃-NC catalyst through nanocasting method with high surface area showed an interesting performance in the degradation reaction rather than the catalyst through sol-gel method (La–Co–O₃-SG). In order to implement the degradation of phenol under visible light irradiation, Au/LaCo_1−xCe_xO₃ photocatalyst was prepared, and the photocatalytic performance can be further improved.

2. Experimental

2.1. Catalyst preparation

The mesoporous LaCo_1−xCe_xO₃ (x = 0, 0.10, 0.15, 0.20) catalysts were prepared via nanocasting method by using SBA-15 as the template. In a typical synthesis procedure, stoichiometric amounts of lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), cerium nitrate (Ce(NO₃)_3·6H₂O) and citric acid were mixed in 15 mL ethanol containing 5 mL of deionised water. The molar ratio of the citric acid to total metal ions (La³⁺, Co²⁺ and Ce⁴⁺) was 2:1. A clear solution was obtained after stirring at room temperature for 30 min. Then, 1.0 g SBA-15 was added into the complex solution with stirring at 40 °C until it became viscous, then dried at 80 °C overnight. Subsequently, the obtained solid was calcined at 300 and 700 °C for 2 h (ramp 2 °C/min) in air, respectively. Finally, 2 M NaOH aqueous solution was used to remove silica template at room temperature for three times. After the solid was washed with deionised water and ethanol, and dried at 80 °C for 12 h, black powder was obtained, which was recorded as LaCo_1−xCe_xO₃-NC, respectively.

For comparison, LaCoO₃ catalyst was prepared by sol-gel method, and was recorded as LaCoO₃-SG. Except without template, this method is consistent with the above procedure.

About 3% Au/LaCo_1−xCe_xO₃-NC catalyst was prepared by deposition–precipitation method. The support was added to the HAuCl₄·6H₂O solution, followed by adding 0.1 M NaOH solution to adjust pH to 8, after continuous stirring for 2 h at 70 °C and subsequent aging for 6 h, the precipitate was filtered and washed with deionised water until being free from C1⁻ ion, then the catalyst was obtained after calcination at 250 °C for 3 h.

2.2. Catalyst characterisation

The composition and crystal phase of samples were characterised by powder X-ray diffraction meter with Cu Ka radiation. Nitrogen adsorption–desorption measurements were determined using an ASAP 2020 M analyser utilising the Brunauer–Emmett–Teller (BET) model and Barrett–Joyner–Halenda (BJH) method for the calculation of specific surface areas and pore size distribution. Before measurement, the samples of 0.1–0.15 g were degassed in vacuum at 100 °C for 5 h. Hydrogen temperature programmed reduction (H₂-TPR) measurements were carried out on a self-assembled apparatus with thermal conductivity detector (TCD) as detector under a flow rate of 50 mL·min⁻¹ (5% H₂/Ar). Approximately 50 mg of sample was pre-treated with Ar at 100 °C for 2 h, prior to running the H₂-TPR experiments, and the temperature was raised from room temperature to 800 °C at a rate of 10 °C·min⁻¹. X-ray photoelectron spectroscopy (XPS) also has been registered using ESCALAB 250xi Electron Spectrometer.

2.3. Photocatalytic evaluation

The photocatalytic degradation of phenol reactions were carried out using a 300 W Mercury lamp (without filter) as light source. Briefly, 10 mg of catalyst was suspended in 10 mL of aqueous solution containing 20 ppm phenol. The reaction temperature was kept at 20 °C. Before irradiation, stirred for 30 min to reach an adsorption/desorption equilibrium. Samples were withdrawn from the reactor after the reaction and filtered through a 0.22 µm water filter film. Analysis of the evolved phenol was performed by high-performance liquid chromatography produced by Shimadzu Company of Japan. The dark reaction results show that the prepared catalyst has better photocatalytic effect.

3. Results and discussion

3.1. X-ray diffraction

Wide-angle X-ray diffraction (XRD) pattern of the samples is shown in Figure 1. LaCoO₃-SG, LaCoO₃-NC and LaCo_0.85Ce_0.15O₃-NC exhibit characteristic diffraction peaks at 2θ = 22.9°, 32.7°, 40.3°, 47.6°, 58.3°, 68.6°, 69.5° and 78.2°, respectively, and similar to characteristic peaks of the XRD standard card PDF (JCPDS 48-0123), which indicate the pure rhombohedral perovskite phase is formed by nanocasting and sol-gel method. The intensity of characteristic diffraction peaks of LaCoO₃-NC is lower than that of LaCoO₃-SG, indicating that the crystallinity of LaCoO₃-NC is weaker. La, Co and Ce oxide phases are not observed in LaCo_1−xCe_xO₃-NC, which indicates that Ce ions occupy the lattice position of Co ions [16]. Au/LaCoO₃-NC and Au/LaCo_0.85Ce_0.15O₃-NC catalysts have obvious diffraction signal at 2θ = 38.3° attributable to Au (JCPDS PDF# 04-0784), suggesting that gold is loaded on the surface of the support.

Figure 1. XRD patterns of the catalysts.

3.2. N2 adsorption–desorption

As shown in Figure 2a, the N₂ adsorption–desorption isotherms of LaCoO₃-NC and LaCo_0.85Ce_0.15O₃-NC correspond to type-IV isotherm with type-H₂ hysteresis loop appeared in the relative pressure range of 0.45–0.80, which is the feature of mesoporous materials [10]. Figure 2b shows that the pore size distributions of the two supports are narrow. The average pore diameters are 4 and 4.6 nm, respectively, and the specific surface area is 149 and 213 m².g⁻¹ (Table 1). The specific surface area of LaCoO₃-SG is 5 m².g⁻¹. Furthermore, the pore size distribution and specific surface area of the 3% Au/LaCo_0.85Ce_0.15O₃-NC were not change significantly, indicating the mesoporous structure remained.

Figure 2. N₂ adsorption–desorption isotherms (a) and pore size distributions (b) plot of samples.

Table 1. N₂ adsorption–desorption measurement results of the samples.

Sample	BET surface area (m^2·g^−1)	BJH pore size (nm)	Pore volume (cm^3·g^−1)
LaCoO3-SG	5	–	–
LaCoO3-NC	149	4.0	0.264
3%Au/LaCoO3-NC	146	4.4	0.283
LaCo0.85Ce0.15O3-NC	213	4.6	0.327
3%Au/LaCo1−xCexO3-NC	202	5.2	0.344

3.3. Transmission electron microscopy

The structure and size of the catalysts were characterised by transmission electron microscopy (TEM). The average particle size of the nanoparticles of LaCoO₃-SG is 61 nm according to the software of particle size calculation (Figure 3(a, b)). The result is corresponding to XRD analysis. It can be seen from Figure 3c–f that some mesoporous structures have been obtained. At the same time, there are some large particles on the surface of the samples, which indicates that the nanopore filling is not completely. Compared with the LaCoO₃-SG, the mesoporous structure can provide good mass transfer performance and easy accessibility to the active sites in the catalytic process, and finally improve the catalytic efficiency. Figure 3g,h shows that the average particle sizes of Au are 7.81 and 6.05 nm, respectively.

Figure 3. TEM patterns of catalysts. (a, b) LaCoO₃-SG; (c, d) LaCoO₃-NC; (e, f) LaCo_0.85Ce_0.15O₃; (g) 3%Au/LaCoO₃-NC; (h) 3%Au/LaCo_0.85Ce_0.15O₃.

3.4. Hydrogen temperature programmed reduction

Two main reduction steps were observed for the LaCoO₃-SG (Figure 4(a)): the reduction peaks at 466 and 658 °C were the reduction of surface Co³⁺ to Co²⁺, and Co²⁺ to Co0 [17], respectively. The reduction peaks of LaCoO₃-NC and LaCo_0.85Ce_0.15O₃ shifted to 450 and 448 °C (Figure 4(b, d)). After Au loading, the reduction peaks of Au appeared at 200–300 °C for 3% Au/LaCoO₃ and 3% Au/LaCo_0.85Ce_0.15O₃ corresponding to the literature [14]. The reduction peak of catalysts shifted to low temperature, indicating that the catalytic efficiency were improved, which may result from nanocasting method facilitates loading of Au.

Figure 4. H₂-TPR patterns of the samples. (a) LaCoO₃-SG; (b) LaCoO₃-NC; (c) 3%Au/LaCoO₃-NC; (d) LaCo_0.85Ce_0.15O₃; (e) 3%Au/LaCo_0.85Ce_0.15O₃.

3.5. X-ray photoelectron spectroscopy

The surface element composition and metal oxidation state of the samples were characterised by XPS. The 3D spectra of lanthanum in the catalyst were shown in Figure 5a.The peaks of La³⁺at 856.0–852.5 eV and 837.4–833.7 eV appeared at La 3d_3/2 and La 3d_5/2 [18], respectively. The BE = 779.8 and 797.9 eV ascribe to the surface Co³⁺ species, BE =780.0 and 781.9 eV ascribe to Co⁴⁺ [19–20] (Figure 5(b)). In order to maintain the charge balance of the sample, some Ce⁴⁺ ions were formed in the process of substituting Ce3⁺ for Co3⁺ ions at B site. It is reported that two main peaks at BE = 822.0 and 866.0 eV (Ce 3d_5/2 signal), BE = 899.5 and 903.6 eV (Ce 3d_3/2 signal), and BE = 917.7 eV(Ce 3d_3/2 signal), attributable to the surface Ce³⁺ and Ce⁴⁺ species [21]. This conclusion is confirmed by Ce 3D spectra (Figure (5d)), which is helpful to improve catalytic activity.

Figure 5. XPS patterns of samples. (a) LaCoO₃-SG; (b) LaCoO₃-NC; (c) 3%Au/LaCoO₃-NC; (d) LaCo_0.85Ce_0.15O₃; (e) 3%Au//LaCo_0.85Ce_0.15O₃.

As shown in Figure 5c, the asymmetrical O 1 s spectrum of each sample could be decomposed into two components at BE = 532.2 and 530.6 eV, which can be assignable to the surface oxygen and lattice oxygen [22], respectively. The surface oxygen content of LaCoO₃-NC (Figure 5(b)) and LaCo_0.85Ce_0.15O₃-NC (Figure 5(d)) is higher than LaCoO₃-SG. Meanwhile, the catalytic activity of LaCo_0.85Ce_0.15O₃-NC is higher than that of LaCoO₃-NC, as the presence of Ce⁴⁺ increasing the oxygen vacancy of materials, facilitating the adsorption, activating and migration of oxygen sources, accelerating the formation of more surface oxygen [9].

The Au 4f spectra of 3%Au/LaCoO₃-NC and 3% Au/LaCo_0.85Ce_0.15O₃-NC are shown in Figure 5e. and Table 2. Both Au0 and Au^δ+(0 < δ < 3) species coexisted on the surface of the samples. The BE values correspond to 4f_7/2 and 4f_5/2 peaks at 84.0, 84.9, 87.6 and 88.9 eV, respectively [23]. After loading Au onLaCoO₃-NC and LaCo_0.85Ce_0.15O₃-NCsupport, the surface oxygen O_S content increased (Table 3), which might be attributed to the strong interaction between Au and oxygen species [14].

Table 2. Au4f_7/2 and O 1 s binding energies for catalysts.

Catalysts	O1s BE (eV)	Oxygen species	Peak area (%)	Au4f7/2 BE (eV)	Oxidation state	Peak area (%)
3%Au/LaCo0.85Ce0.15O3-NC	530.9 532.5	OL OS	38.03% 61.97%	84.4 84.9 88.1 88.9	Au^0 Au^δ+	32.66% 67.34%
3%Au/LaCoO3-NC	530.6 532.3	OL OS	38.41% 61.59%	84.0 86.0 87.6 90.2	Au^0 Au^δ+	24.31% 75.29%
LaCo0.85Ce0.15O3-NC	530.6 532.2	OL OS	38.35% 61.65%
LaCoO3-NC	530.6 532.3	OL OS	40.81% 59.19%
LaCoO3-SG	528.8 531.1	OL OS	44.52% 55.48%

Table 3. Composition of surface metal elements of catalysts determined by XPS.

Catalysts	Atomic ratio(%)
	La	Co	Ce	Au
3%Au/LaCo0.85Ce0.15O3-NC	7.96	7.4	1.29	0.42
3%Au/LaCoO3-NC	8.49	6.53		0.22
LaCo0.85Ce0.15O3-NC	7.79	7.83	1.35
LaCoO3-NC	8.96	6.52
LaCoO3-SG	9.28	8.83

3.6. Catalytic performance

The degradation rate of phenol is shown in Table 4. LaCoO₃-NC demonstrated higher activity than LaCoO₃-SG for the photodegradation of phenol under UV irradiation. The replacement of Co by Ce could improve the activity and reach the highest activity with 15% Ce doping. After loading Au on the LaCoO₃-NC and LaCo_0.85Ce_0.15O₃-NC support, the degradation rates of phenol were increased to 53.59 and 57.52%, respectively, indicating that gold had certain catalytic activity for phenol. The larger specific surface area of the LaCoO₃-NC and LaCo_0.85Ce_0.15O₃-NC means more opportunities to react with phenol, which may enhance the photocatalytic performance (Figure 2(a)).

Table 4. The activity of catalysts.

Catalyst	Degradation rate of phenol (%)
	Dark	UV
LaCoO3-SG	4.2	30.56
LaCoO3-NC	14.90	41.26
LaCo0.95Ce0.05O3-NC		47.74
LaCo0.90Ce0.10O3-NC		48.97
LaCo0.85Ce0.15O3-NC		49.95
LaCo0.80Ce0.20O3-NC	18.82	48.74
3%Au/LaCoO3-NC	13.75	53.59
3%Au/LaCo0.85Ce0.15O3-NC		57.52

* Experimental conditions: The dosage of catalyst: 1 g·L⁻¹, neutral pH, the concentration of phenol: 20 mg· L⁻¹, working volume of phenol: 10 mL, reaction time: 6 h, lamp source: 300 W Mercury lamp.

Figure 6 shows that gold modified LaCoO₃-NC and LaCo_0.85Ce_0.15O₃-NC,to some extent, have degradation effect on phenol under visible light. Degradation rate of phenol followed the order LaCo_0.85Ce_0.15O₃-NC > LaCoO₃-NC > LaCoO₃-SG. This is consistent with the results of photodegradation of phenol under ultraviolet irradiation, indicating that Au modification is beneficial to the absorption of visible light.

Figure 6. Activity patterns of LaCo_1−xCe_xO₃.

Experimental conditions: The dosage of catalyst: 1 g·L⁻¹, neutral pH, the concentration of phenol: 20 mg L⁻¹, working volume of phenol: 10 mL, reaction time: 6 h, lamp source: 500 W Xenon lamp.

4. Conclusions

In summary, the ordered mesoporous catalysts with Ce-doped perovskite oxides at LaCoO₃ B site were prepared successfully by nanocasting method for the first time. LaCo_0.85Ce_0.15O₃-NC catalysts with abundant surface area achieved 49.74% photocatalytic activity for phenol degradation at 6 h of UV irradiation, which was better than other photocatalysts for phenol degradation. After loading Au on the LaCo_0.85Ce_0.15O₃-NC support, the degradation rate of phenol (57.52%) increased. The BET and TEM characterisation results show that Au/LaCo_0.85Ce_0.15O₃catalyst has a high specific surface area (213 m².g⁻¹) and small gold particles, which is conducive to improving the dispersion of active component Au. XPS analysis indicate that the presence of Ce⁴⁺ ions increased the oxygen vacancy of the material, which can be advantages to improve the catalytic performance of the catalyst. In addition, the presence of Au^δ+ (0＜δ＜3) and more lattice oxygen on the surface of 3% Au/LaCo_0.85Ce_0.15O₃ catalyst also contributed to the high activity of the catalyst.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported financially by the University Scientific research project Foundation of Inner Mongolia (NJZY18281)