An efficient round-the-clock La₂NiO₄ catalyst for breaking down phenolic pollutants

Abstract

A novel La₂NiO₄ photocatalyst in layered perovskite crystal was synthesized by co-alcoholysis of La(NO₃)₃ and Ni(NO₃)₂ under solvothermal conditions. It exhibited high activity for mineralizing 4-chlorophenol under visible light irradiation and even in the dark, leading to round-the-clock photocatalysis. The photocatalytic mechanism of the La₂NiO₄ in the absence of light irradiation was discussed based on both the structural characterizations and the kinetic studies. Both the La₂NiO₄ crystals and the organic molecules played key roles for starting the degradation reaction in dark. The 4-chlorophenol firstly ionized into anions, followed by donating electrons to La₂NiO₄ owing to the positively charged surface. Then, the electrons could react with dissolved O₂ to produce ˙O₂⁻, which subsequently reacted with H⁺ to form ˙OH radicals. These highly active ˙OH radicals could oxidize 4-chlorophenol into CO₂, leading to complete degradation.

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Introduction

Organic compounds have been considered as the principal source of industrial pollution. The phenolic compounds represent a typical family of organic pollutants widely present in wastewater and air coming from petrol, coal, and other chemical industries,^1–5 which might cause various diseases including cancer, angiocardiopathy and gastroenterology etc., even at very low concentrations.^6,7 For these reasons, the US Environmental Protection Agency (US EPA) has enforced an effluent limit for phenolic compounds, where in the aquatic environment there is to be no more than 1.0 mg L⁻¹.⁸ Due to the stable benzene ring and its recalcitrant nature, most phenolic compounds, e.g., 4-chlorophenol (4-CP), are usually difficult to be mineralized by a biodegradation method, especially when its concentration is higher than 1.0 mg L⁻¹.⁹ Adsorption is also employed to remove phenolic compounds, but it usually results in secondary environmental pollution.^10,11 Fenton-like reactions have also been proved as another useful techniques to eliminate phenolic compounds, but their practical application is still limited by the high costs due to the consumption of much H₂O₂ and the multifarious procedures in subsequent treatment of the ferrous slurry for regenerating the catalysts.^12–14 Other techniques including solvent extraction,¹⁵ chemical oxidation,¹⁶ and electrochemical methods^17,18 have also been reported for the removal of 4-CP, but they usually display low efficiencies and also generate secondary pollution due to the formation of by-products.

Photocatalysis has been widely used in degrading organic pollutants owing to the energy saving, easy operation and the absence of secondary pollution. Most studies are focused on the TiO₂ photocatalyst owing to its strong oxidizing ability, excellent stability and low toxicity.^19,20 However, TiO₂ can only be excited by UV light due to its wide band gap (E_g = 3.2 eV), which accounts for merely 4–5% sun light spectrum on the earth surface. To date, great achievements have been obtained for fabricating visible-light photocatalysts by doping TiO₂ or designing non-TiO₂ photocatalysts.^21–23 However, these catalysts can work well only in ther daytime and cannot work in the dark due to the absence of light irradiation. Recently, Wu et al. reported a novel approach to mineralize azo dyes in the absence of light irradiation.²⁴ Rosseinsky and co-workers also found that HCa₂Nb₃O₁₀ could oxidize methyl orange at room temperature in the dark.²⁵ However, these photocatalysts exhibited low efficiencies in the dark and some co-catalysts or the additional oxidant reagents were required for enhancing the activity. Meanwhile, the degradation mechanism in the dark was not clearly elucidated. Herein, we report for the first time a layered perovskite La₂NiO₄ crystal, acting as a round-the-clock photocatalyst for breaking down 4-CP with excellent performance and strong durability at ambient conditions. The degradation mechanism in the absence of light irradiation was examined based on both the structural characterizations and kinetic studies, which revealed that the La₂NiO₄ could generate photoelectrons via activation with visible light irradiation and also trap electrons from some reactant molecules (e.g., 4-CP) in the dark, leading to round-the clock degradation reactions.

Experimental

La₂NiO₄ preparation

In a typical synthesis, 0.79 g La(NO₃)₃·6H₂O, 0.39 g Ni(NO₃)₂·6H₂O and 0.47 g citric acid were added into a mixture containing 35 mL ethanol and 5.0 mL glycerol under ultra-sonication. The clear solution was then transferred into a 50 mL autoclave, followed by aging at 433 K for 24 h. After being cooled down to room temperature, the solid product was centrifuged, washed thoroughly with distilled water and ethanol, vacuum dried at 353 K overnight, and calcined at 1273 K for 3 h at a heating rate of 4.0 K min⁻¹.

For comparison, the traditional visible-light photocatalyst (TiO_2−xN_x) was also synthesized according to the method reported elsewhere.³⁰ Briefly, 17 mL tetrabutyl orthotitanate was added dropwise into 100 mL aqueous solution containing 1.0 wt% NH₃. After being stirred for 1 h and aged at 25 °C for another 24 h, the solid product was filtrated, washed thoroughly with water, and dried at 60 °C, followed by calcining at 60 °C for 2 h.

Characterization

X-Ray diffraction measurements were carried out using a Rigacu Dmax-3C Advance X-ray diffractometer (Cu-Kα radiation, λ = 1.5406 Å). The morphologies and particle size were observed by a field emission scanning electron microscope (FESEM, S-4800). The selected area electronic diffraction (SAED) pattern and transmission electronic micrograph (TEM) were recorded on a JEM-2010. The N₂ adsorption–desorption isotherms were measured on a NOVA 4000 at 77 K, before which all samples should be degassed at 423 K and 10⁻⁶ Torr for 24 h. The Brunauer–Emmett–Teller approach was used to calculate the surface area. The surface electronic state was analyzed by X-ray photoelectron spectroscopy (XPS, Versa Probe PHI 5000). All the binding energy (BE) values were calibrated using the standard BE value of contaminant carbon (C1s = 284.6 eV) as a reference. Meanwhile, the spectral response and light absorption ability were analyzed by UV-vis diffuse reflectance spectra (DRS, MC-2530). The ˙OH-trapping photoluminescence spectra were collected on a Varian Cary-Eclipse 500 spectrophotometer by using terephthalic acid as a ˙OH-captor. The zeta potential was measured on a Zetasizer Nano ZS90 (Malvern company). The Tafel plots were recorded on an electrochemical working station (CHI-660D) in 0.20 M Na₂SO₄ aqueous solution using the platinum plate as a counter electrode, the saturated calomel electrode (SCE) as a reference electrode, and the unmodified or La₂NiO₄-modified graphite electrode as a working electrode.

Activity test

The liquid-phase degradation of 4-chlorophenol (4-CP) was carried out at 303 K in a 80 mL self-designed quartz reactor containing 0.075 g catalyst and 50 mL 4-CP aqueous solution (5.0 mg L⁻¹). The catalytic reaction was performed under magnetic stirring at a speed of 500 rpm either under visible light irradiation (λ > 420 nm, 300 W Xenon lamp) or in the dark. At given time intervals, the reaction mixture was sampled and centrifuged to remove the solid catalyst, followed by analysis on a UV spectrophotometer (UV 7504/PC) to determine the degradation yield of 4-CP at its characteristic wavelength (λ_4-CP = 224 nm). Degradation reactions of other organic compounds including phenol, methyl orange (MO), and rhodamine B (RhB) were performed following the similar procedures and under the same reaction conditions. HPLC-MS (Agilent 1200 series) was used to detect the reaction intermediates. The reproducibility of each activity test was checked by repeating the results at least three times and was found to be within acceptable limits (±5%).

Results and discussion

The XRD pattern in Fig. 1 demonstrates that the as-prepared La₂NiO₄ sample was present in pure perovskite layered crystals with tetragonal structures (see Fig. 1), corresponding to the diffraction peaks at 2θ of 24.1, 28.1, 31.4, 32.8, 42.7, 43.7, 47.0, 53.5, 54.9, 55.7, 56.0, 57.6, 58.1, 65.3, 68.7, 76.1, 77.5, 78.2° (JCPDF 34-0314).²⁶ As shown in Fig. 2a and b, the FESEM and TEM images show that the La₂NiO₄ was composed of blocks with average size ranging from 100 to 200 nm. The SAED image (Fig. 2c) also demonstrated a well-defined single-crystal structure. The HRTEM image (Fig. 2d) further confirmed the layered perovskites La₂NiO₄, corresponding to the fringes with a neighboring lattice space of 0.207 nm.

Fig. 1 XRD pattern (upper) and crystal cell of the La₂NiO₄. a: (100), b: (010), c: (001).

Fig. 2 FESEM (a), TEM (b), SAED (c) and HRTEM (d) images of the La₂NiO₄ sample.

Fig. 3a revealed that La₂NiO₄ displayed a type-II N₂ adsorption–desorption isotherm indicative of a non-porous structure, corresponding to a low surface area (7.3 m² g⁻¹). Fig. 3b demonstrated that the La₂NiO₄ showed spectral responses in the whole UV-Vis-IR region, which could possibly be attributed to the presence of multiple consecutive energy levels.

Fig. 3 (a) Nitrogen adsorption–desorption isotherm and (b) UV-Vis-IR adsorption spectrum of the La₂NiO₄ sample.

4-CP was chosen as the model phenolic compound to test the photocatalytic performance of the as-prepared La₂NiO₄ crystals. Preliminary experiments demonstrate that no significant degradation of 4-CP was observed in the absence of La₂NiO₄, regardless of the light irradiation. Fig. 4 shows the influence of reaction temperature or reaction time on the 4-CP degradation yield under visible light irradiation and in the dark, where C₀, C, k, and t refer to the initial concentration and the real-time concentrations of 4-CP, the reaction constant and the reaction time. Obviously, 4-CP could be effectively degraded by the La₂NiO₄ crystals both under visible light irradiation and in the dark. The linear relationship between Ln(C₀/C) and t demonstrates that the reaction is pseudo-first-order with 4-CP concentration.²⁷ The reaction constants at 10, 30 and 50 °C were calculated as following: k_L50 (0.27 h⁻¹) > k_L30 (0.16 h⁻¹) > k_L10 (0.10 h⁻¹) under visible light irradiation and k_D50 (0.11 h⁻¹) > k_D30 (0.074 h⁻¹) > k_D10 (0.060 h⁻¹) in the dark. These k values reveal that the reaction rate increases rapidly with increasing temperature. At the same reaction temperature, the k values obtained under visible light irradiation are much higher than those obtained in the dark, suggesting that 4-CP could be more easily degraded by photocatalysis. According to the influence of the reaction temperature on the reaction rate, the apparent activation energies were calculated as 73 kJ mol⁻¹ and 61 kJ mol⁻¹ for the 4-CP degradation reactions under visible light irradiation and in the dark, respectively. The difference in either the k value or apparent activation energy implied that the 4-CP degradation on La₂NiO₄ crystals under visible light irradiation and in the dark might follow different reaction mechanisms.

Fig. 4 Kinetic curves of 4-CP degradation obtained under visible light irradiation (λ > 420 nm) (a) and in the dark (b). Reaction conditions: 75 mg La₂NiO₄, 50 mL 5.0 × 10⁻⁶ M 4-CP aqueous solution, reaction T = 10 °C (-★-), 30 °C (-▼-), and 50 °C (-■-).

As shown in Fig. 5, the HPLC-MS spectra display a peak at retention time of around 7.0 min is indicative of a 4-CP molecule (m/z = 127). The other two peaks observed at retention times of around 3.0 min were ascribed to the injection fluctuation. With the increase of reaction time either under visible light irradiation or in the dark, the signal corresponding to the 4-CP molecule decreases gradually owing to the degradation reaction. No other peaks are detected by HPLC-MS during the reaction until the complete removal of 4-CP. It suggests that, different from those reported so far,²⁸ the 4-CP molecule is directly mineralized without formation of relatively stable by-products including hydroquinone, benzoquinone, and hydroxyhydroquione etc., possibly owing to the super strong oxidizing ability of the active species generated from the La₂NiO₄ crystals. By measuring the total organic carbon (TOC), we found that a more than 95% mineralization degree of 4-CP was obtained after reaction for 12 h in the dark. This is in good accordance with the above product analysis by HPLC-MS.

Fig. 5 HPLC-MS chromatograms of the resulting solution after photocatalytic degradation of 4-CP on La₂NiO₄ crystals for different times under visible light irradiation and in the dark, respectively.

Table 1 summarized the degradation efficiencies of La₂NiO₄ and TiO_2−xN_x (standard visible-light photocatalyst) for different organic pollutants under different conditions. Both La₂NiO₄ and TiO_2−xN_x showed high activity under visible light irradiation since they could be easily activated owing to the narrow energy band gaps. La₂NiO₄ exhibited a slightly lower activity than TiO_2−xN_x in 4-CP degradation due to its extremely low surface area (7.3 m² g⁻¹). Under visible light irradiation, La₂NiO₄ exhibited a higher activity for MO degradation than that for 4-CP degradation since the MO molecule was more active to be oxidized than the 4-CP owing to the presence of N=N double bond (see the structural formula in Scheme 1). Similarly, the phenol degradation yield was higher than the 4-CP degradation yield on La₂NiO₄ under visible light irradiation because the C–Cl bond was quite stable against oxidative cleavage. In comparison with the 4-CP, phenol, and MO degradation, La₂NiO₄ showed the lowest activity in RhB degradation since the RhB molecule was even more stable against oxidation due to the conjugated π system (see the structural formula in Scheme 1). Interestingly, it was found that the activity of La₂NiO₄ in the dark was quite different from that observed under light irradiation. The degradation yield decreased in the order of 4-CP > MO > phenol > RhB. Meanwhile, one could see that 4-CP degradation on La₂NiO₄ in the dark was almost completely inhibited by either bubbling of the reaction solution with N₂ flow to remove dissolved O₂, or by introducing benzoquinone for trapping ˙O₂⁻ radicals, or by decreasing the pH value from 7.1 to 5.6 to inhibit the ionization of 4-CP into 4-CP⁻ anions. Moreover, although TiO_2−xN_x showed high activity in 4-CP degradation under visible light irradiation, it was almost inactive in the dark. These results further confirmed that the degradation mechanism in the dark was different from that under visible light irradiation, taking into account that no photoelectrons and holes could be produced directly in the dark due to the absence of light irradiation.

Scheme 1 The structural formula of MO and RhB molecules.

Table 1 Degradation yields under different conditions^a

Photocatalyst	Irradtiation	Reactant	Gas	pH	Time	Yield (%)
a Reaction conditions: 75 mg photocatalyst, 50 mL 5.0 × 10^−6 M 4-CP aqueous solution, 30 °C. b BQ = benzoquinone (10 ppm).
La2NiO4	> 420 nm	4-CP	Air	7.1	4	45
La2NiO4	> 420 nm	Phenol	Air	7.1	4	61
La2NiO4	> 420 nm	MO	Air	7.1	4	88
La2NiO4	> 420 nm	RhB	Air	7.1	4	10
TiO2-xNx	> 420 nm	4-CP	Air	7.1	4	60
La2NiO4	In dark	4-CP	Air	7.1	12	93
La2NiO4	In dark	4-CP	Air	5.6	12	4
La2NiO4	In dark	4-CP	N2	7.1	12	9
La2NiO4	In dark	4-CP + BQ^b	Air	7.1	12	7
La2NiO4	In dark	Phenol	Air	7.1	12	20
La2NiO4	In dark	MO	Air	7.1	12	70
La2NiO4	In dark	RhB	Air	7.1	12	12
TiO2−xNx	In dark	4-CP	Air	7.1	12	∼0

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As shown in Fig. 6, the La₂NiO₄ could be used repetitively for more than 6 times without any significant decrease in activity during 4-CP degradation in the dark, showing excellent durability. More importantly, it also demonstrated that the 4-CP degradation reaction in the dark was not induced by the light originally stored in the La₂NiO₄ crystals.

Fig. 6 Recycling test of the La₂NiO₄ photocatalyst for 4-CP degradation in the dark. Reaction conditions are shown in Fig. 4.

Scheme 2 illustrated a plausible mechanism for the 4-CP degradation on La₂NiO₄ in the dark. Firstly, the 4-CP molecules ionized into H⁺ cations and 4-CP⁻ anions in aqueous solution. Then, the 4-CP⁻ anions interacted with the La₂NiO₄ to donate electrons to La₂NiO₄, together with the formation of 4-CP radical (4-CP˙). Similar to the photo-induced electrons, the as-trapped electrons could react with dissolved O₂ to produce ˙O₂⁻ radicals, which subsequently reacted with H⁺ to form ˙OH radicals. These highly active ˙OH radicals could oxidize 4-CP˙ radicals into CO₂, leading to 4-CP degradation. Obviously, the degradation efficiency of the La₂NiO₄ in dark was mainly dependent on the ability to trap electrons from organic pollutants.

Scheme 2 A plausible mechanism of 4-CP degradation reaction in dark.

There are several pieces of evidence which support the as-proposed mechanism. On one hand, the zeta potential spectra in Fig. 7 revealed that the La₂NiO₄ crystal in pure water was positively charged and turns to neutral in the presence of 4-CP molecules, which confirmed the above conclusion that the La₂NiO₄ crystal could trap electrons from the 4-CP⁻ anions, together with the formation of 4-CP˙ radicals. On the other hand, the XPS spectra in Fig. 8 revealed that both the Ni(II) and La(III) species displayed negative shifts of binding energy after the La₂NiO₄ was pre-treated with 4-CP aqueous solution, which also confirmed that the 4-CP donated partial electrons to La₂NiO₄.

Fig. 7 Zeta potential spectra of the La₂NiO₄ in H₂O (green line) and 4-CP aqueous solution (red line).

Fig. 8 XPS spectra of La₂NiO₄ crystals before (a) and after (b) being pre-treated with 4-CP aqueous solution.

The electrochemical measurements could further support the above mechanism. Fig. 9a shows the Tafel plots of pure and La₂NiO₄ modified carbon/glassy carbon (C/GC) electrodes in Na₂SO₄ aqueous solution with and without adding 4-CP. The equilibrium electric potential on the La₂NiO₄ modified C/GC electrode decreased from −0.094 to −0.120 V by adding 4-CP, which further confirmed the electron donation from 4-CP⁻ to La₂NiO₄. On one hand, it enhanced the electric current indicative of 4-CP oxidation since more electrons released. On the other hand, it decreased electric current indicative of O₂ reduction since partial electrons directly transferred from La₂NiO₄ to O₂. Similar results could also be observed on the pure C/GC electrode with and without adding 4-CP. However, the C/GC electrode without La₂NiO₄ modification displayed much lower electric current indicative either the 4-CP oxidation or the O₂ reduction than the La₂NiO₄-modified C/GC electron, suggesting that the La₂NiO₄ favored to trap electrons from 4-CP⁻. Meanwhile, the CV curves in Fig. 9b revealed that the O₂ reduction peak potential on the La₂NiO₄ modified C/GC electrode shifted positively and electric current greatly enhanced compared with that of the pure C/GC electrode, suggesting that the presence of La₂NiO₄ crystals promoted the O₂ reduction.²⁹ This could be attributed to the layered perovskite structure of the La₂NiO₄ crystal which facilitated the O₂ transfer. Furthermore, the CV curves in Fig. 9c revealed that the La₂NiO₄-modification promoted the 4-CP reduction on the C/GC electrode, suggesting that the La₂NiO₄ could more easily trap electrons from 4-CP⁻ owing to the positive surface (Fig. 7).

Fig. 9 (a) Tafel plots of unmodified and La₂NiO₄ modified C/C/GC electrode with and without 4-CP (1.0 mg mL⁻¹), (b) CV curves of the C/GC electrode and La₂NiO₄–C/GC electrode in O₂ saturated solution, and (c) CV curves of the C/GC electrode and La₂NiO₄–C/GC electrode with 4-CP (1.0 mg mL⁻¹). All those experiments were conducted in the dark.

To further confirm the above mechanism and account for the changes in degradation activity of La₂NiO₄ in the dark, the hydroxyl radicals (˙OH) were detected in the dark by measuring photoluminescence excited at 312 nm, with terephthalic acid (TA) as the ˙OH trapping agent.²⁹Fig. 10 shows the intensities of photoluminescence emission peaks around 426 nm, which are characteristic of 2-hydroxyterephthalic acid (TAOH), obtained at 120 min under different conditions. La₂NiO₄ displayed a strong TAOH peak intensity in the 4-CP solution in the dark. However, almost no TAOH peak was observed when the solution was bubbled with N₂ flow, suggesting that the ˙OH radicals were mainly generated from dissolved O₂ rather than from H₂O. Meanwhile, it was also found that the TAOH peak intensity abruptly decreased upon introduction of benzoquinone for trapping the ˙O₂⁻ radicals, which confirmed that the ˙OH radicals were mainly resultant from the reaction between ˙O₂⁻ and H⁺. In addition, an abrupt decrease in the TAOH peak intensity was found when the pH value decreased from 7.1 to 5.6, indicating that the 4-CP⁻ anions played an important role for donating electrons and the high acidity might inhibit the 4-CP ionization to 4-CP⁻. The La₂NiO₄ exhibited a stronger TAOH peak intensity in the presence of 4-CP than that in the presence of MO, suggesting that the 4-CP molecule could donate electrons to La₂NiO₄ for forming ˙OH radicals more easily than the MO. This could accountfor the higher degradation yield of 4-CP than that of MO on La₂NiO₄ in the dark, although the MO molecule was more active to be degraded under light irradiation. An abrupt decrease in the TAOH peak intensity was found on La₂NiO₄ when using phenol instead of 4-CP, obviously due to the lower ionization degree of phenol (pK_a = 9.99) than that of 4-CP (pK_a = 7.65), leading to the lower ability to donate electrons to La₂NiO₄, which could account for the lower degradation yield of phenol than that of 4-CP. No significant TAOH peak was observed in the presence of RhB since RhB could ionize into RhB⁺ cations rather than anions, which exhibited a poor ability to donate electrons to La₂NiO₄, leading to the extremely low degradation yield of RhB on La₂NiO₄ in the dark. Although the TiO_2−xN_x exhibited strong TAOH peak intensity under visible light irradiation (λ > 420 nm) even in the absence of 4-CP, it showed no significant TAOH peak in the dark. Thus, it displayed almost no activity towards the degradation in the dark. From these results, we could conclude that the interaction between La₂NiO₄ and the anions of the organic pollutants with strong electron-donating abilities was essential for producing ˙OH radicals in dark. The traditional visible photocatalyst (TiO_2−xN_x) displayed a poor ability to trap electrons from organic pollutants and thus, could not produce ˙OH radicals in the dark, meaning that it could only work well under light irradiation.

Fig. 10 ˙OH-trapping photoluminescence spectra with terephthalic acid. (a) La₂NiO₄–4-CP, (b) La₂NiO₄–4-CP–N₂, (c) La₂NiO₄–4-CP–benzoquinone (10 ppm), (d) La₂NiO₄–4-CP at pH 5.6, (e) La₂NiO₄–MO, (f) La₂NiO₄–phenol, (g) La₂NiO₄–RhB, (h) TiO_2−xN_x–4-CP, (i) TiO_2−xN_x–4-CP under visible light (λ > 420 nm) irradiation. All the photoluminescence spectra were obtained at 120 min. Besides (i), all the other photoluminescence spectra were obtained in the dark.

Conclusions

In summary, this work developed a novel La₂NiO₄ photocatalyst which could efficiently degrade phenolic pollutants not only under visible light irradiation but also in the dark. The 4-CP degradation which occurs in the dark could be attributed to the electron-donation of the 4-CP⁻ anions to La₂NiO₄, followed by reaction with dissolved O₂ to form ˙O₂⁻ radicals and then reaction with H⁺ to generate ˙OH radicals, which could deeply oxidize 4-CP˙ radicals into CO₂, leading to the complete degradation of 4-CP. The present work may provide deep insights into the photocatalytic mechanism, and also offer more opportunities for the practical applications of photocatalysis in environmental cleaning.

Acknowledgements

This work was supported by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the National Natural Science Foundation of China (21007040, 20825724, 21047009, 20907032), the Research Fund for the Doctoral Program of Higher Education (20103127120005), and the Project supported by the Shanghai Government (10160503200, 11PJ1407500, 11ZR1426300, 12YZ079, 10QA1405300, SK201104).

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