Ion-Exchange of Cu²⁺ Promoted Layered Perovskite K₂La₂Ti₃O₁₀ for Photocatalytic Degradation Chlorobenzene under Simulated Solar Light Irradiation

Abstract

The layered perovskite, K₂La₂Ti₃O₁₀ was prepared by sol-gel method. Ion-exchange of Cu²⁺ was prepared to improve the photocatalytic activity of K₂La₂Ti₃O₁₀ for chlorobenzene degradation under simulated solar light irradiation. The original K₂La₂Ti₃O₁₀ and Cu²⁺/K₂La₂Ti₃O₁₀ were characterized by power X-ray diffraction, UV-visible diffuse reflectance spectroscopy, and specific surface area measurement. The XRD analysis shows that Cu²⁺ ions is incorporated in place of K⁺ ions and the grain growth is inhibited by ion-exchange. With the rise of calcination temperature, more interlayer Cu²⁺ was converted into new crystal phase CuO. The degradation ratio reaches 51.1% on Cu²⁺/K₂La₂Ti₃O₁₀ calcined at 500 °C in air, which is higher 16.9% than the original K₂La₂Ti₃O₁₀. It should be ascribed to the narrow interlayer distance, the formation of CuO, smaller grain size, and the high visible light absorption on the surface of Cu²⁺/K₂La₂Ti₃O₁₀ calcined at 500 °C. It is found that the exposure of CO₂ could promote the photocatalytic activity of Cu²⁺/K₂La₂Ti₃O₁₀. It also suggests that CO₂ is involved in the reduction to form benzaldehyde during decomposition of chlorobenzene.

1. Introduction

Volatile organic compounds (VOCs) were found to be toxic, malodorous, carcinogenic, mutagenic, and teratogenic to human health [1]. Among them, chlorobenzene is listed by the US Environmental Protention Agency as a priority pollutant [2]. Chlorobenzene as one of volatile organic compounds (VOCs) is widely used in the pharmaceutical industry as a solvent and in dye processes [3]. Figure 1 presents the framework of chlorobenzene. As one of bio-refractory and hydrophobic organics, it cannot be removed by biological treatment [2]. Presently, TiO₂ with a band gap of 3.2 eV has been the most accepted photocatalyst for decomposition of various pollutants, including refractory VOCs, due to its thermal stability, facile synthesis, low cost, and low toxicity [4]. However, the general drawbacks of TiO₂ as a photocatalyst, such as low reaction rates and limited response range toward light, require improvement [5,6]. Therefore, developing new types of photocatalysts with higher photocatalytic activity and visible-light response has received increasing interest.

In recent years, several layered perovskites, such as K₂La₂Ti₃O₁₀, Sm₂Ti₂S₂O₅, H₄Nb₆O₁₇ and Sr₃Ti₂O₇, have been considered to have photocatalytic activity [7,8,9]. The higher photocatalytic activity of the perovskites were ascribed to its specific layered structure. They use the interlayer space as reactions sites to separate the electron–hole pairs [6]. Moreover, the interlayer ions are ion-exchangeable with various foreign species to obtain photocatalysts with higher activity [4,10]. However, most of the studies were mainly on photochemical splitting of water to produce hydrogen, there were a few reports on photodcatalytic degradation of chlorobenzene, especially under simulant solar light irradiation. Benzaldehyde has been known to be formed from the photocatalytic degradation of chlorobenzene, which reacted with formaldehyde formed earlier [1]. On the other hand, gaseous CO₂ could be converted to formaldehyde by the photo reduction on photocatalyst. Formic acid, formaldehyde, methanol, and methane were detected as the CO₂ reduction products at a Cu electrode and nitrogen-doped TiO₂ thin film as a photoanode under illumination [11]. A successive double electron reduction of CO₂ to CH₃OH or CH₄ via CH₂O by photocatalysis was also considered [12]. Therefore, we think that CO₂ may improve the separation of electron-hole pairs and promote the degradation of chlorobenzene.

In this paper, we synthesized the layered metal oxide, K₂La₂Ti₃O₁₀, with sol-gel method. The Cu²⁺/K₂La₂Ti₃O₁₀ powder was obtained by ion-exchange reaction with the original K₂La₂Ti₃O₁₀. The photocatalytic activity for chlorobenzene decomposition with K₂La₂Ti₃O₁₀ and Cu²⁺/K₂La₂Ti₃O₁₀ powders under simulated solar irradiation were investigated. We also discussed the influences of ion-exchange on physical and chemical properties of the prepared photocatalysts and the effects of CO₂ concentration on the photocatalytic activity of Cu²⁺/K₂La₂Ti₃O₁₀.

2. Results and Discussion

2.1. Characterization of the Powders

The UV-vis absorption spectra of original K₂La₂Ti₃O₁₀ and Cu²⁺/K₂La₂Ti₃O₁₀ calcined at different temperature are shown in Figure 2. In the present study, the sharp UV absorption edge of the original K₂La₂Ti₃O₁₀ is identified near 351 nm. However, ion-exchange of Cu²⁺ causes a red shift of the absorption edge and the Cu²⁺/K₂La₂Ti₃O₁₀ photocatalysts calcined at different temperature show different light response behaviors in the visible light range from 400 to 700 nm. Moreover, the highest visible light absorption appears on the surface of Cu²⁺/K₂La₂Ti₃O₁₀ calcined at 500 °C.

Figure 3 shows that the XRD patterns of Cu²⁺/K₂La₂Ti₃O₁₀ and original K₂La₂Ti₃O₁₀ calcined at different temperatures. K₂La₂Ti₃O₁₀ is composed of negatively charge lanthanum titanium perovskite layers interleaved by K⁺ ions and the adjacent triple perovskite sheets, La₂Ti₃O₁₀, are stacked with a displacement by 1/2 along the (110) direction [13]. The interlayers are bonded by static force [6]. It belongs to the space group of I4/mmm and symmetry of C4v, and the parameters of K₂La₂Ti₃O₁₀ cell are a = b = 0.387 nm, c = 2.978 nm [4,13,14]. In the present study, the powder X-ray diffraction pattern of the original K₂La₂Ti₃O₁₀ (Figure 3g–i) agreed well with the K₂La₂Ti₃O₁₀ crystal phase (PDF#48-0982). The distinctive peaks at 2θ = 5.90°, 11.84°, 17.80°, 23.12°, 23.86°, 24.62°, 27.44°, 31.20°, 32.66°, 37.46°, 40.88°, 42.46°, 46.86°, 48.88°, 53.26°, 54.58°, 57.40°, corresponding to the K₂La₂Ti₃O₁₀ (0 0 2), (0 0 4), (0 0 6), (1 0 1), (0 0 8), (1 0 3), (1 0 5), (1 0 7), (1 1 0), (1 1 6), (1 1 8), (0 0 14), (2 0 0), (2 0 4), (2 1 1), (1 1 14), (2 1 7) crystal planes are observed in original K₂La₂Ti₃O₁₀ sample. The XRD pattern of original K₂La₂Ti₃O₁₀ calcined at 650 °C (Figure 3f) shows weak characteristic lines, which indicates that it cannot form K₂La₂Ti₃O₁₀ stucture when the precursor is calcined below 650 °C. The peak positions of Cu²⁺/K₂La₂Ti₃O₁₀ calcined at different temperatures are different from the original K₂La₂Ti₃O₁₀. Compared to original K₂La₂Ti₃O₁₀ sample, the Cu²⁺/K₂La₂Ti₃O₁₀ samples show a significant shift of the (002) peak. There is no shift of the (002) peaks of original K₂La₂Ti₃O₁₀ with increase of calcination temperature (Figure 3g–i). By comparing the characteristic lines of Figure 3g–i with Figure 3a–e), it is found that the shift of (002) peak should be attributed to ion-exchange of Cu²⁺, rather than the increase of calcination temperature. Moreover, the d values for the (002) reflection changes from 1.497 nm for the original K₂La₂Ti₃O₁₀ to 1.429 nm (300 °C), 1.424 nm (400 °C), 1.420 nm (500 °C), 1.406 nm (600 °C), and 1.415 nm (700 °C) for the Cu²⁺/K₂La₂Ti₃O₁₀ powders. It is clear from the above observation that Cu²⁺ ions is incorporated in place of K⁺ ions. And the decrease in d (002) value and the peak shift possibly result from the smaller ion radius of Cu²⁺ (0.70 Å) compared with that of K⁺ (1.65 Å) [4,10,15,16]. Furthermore, some weak peaks appeared at 38.80° and 53.78° for Cu²⁺/K₂La₂Ti₃O₁₀ samples, which can be attributed to CuO (PDF#01-1117), become more obvious as the calcination temperature increases. However, the d value decreases, except for Cu²⁺/K₂La₂Ti₃O₁₀ sample at 700 °C. It indicates that, with the rise of the temperature, more interlayer Cu²⁺ is converted into new crystal phase CuO. The full widths at half maximum of Cu²⁺/K₂La₂Ti₃O₁₀ samples are broader than that of original K₂La₂Ti₃O₁₀, indicating that the grain growth is inhibited by ion-exchange, which results in small grain size (

Table 1. Grain size, specific surface area, and chlorobenzene degradation ratio of Cu²⁺/K₂La₂Ti₃O₁₀ at different calcination temperatures and original K₂La₂Ti₃O₁₀ in air.

Samples	Grain Size (nm) a	Specific Surface Area (m2/g)	Degradation Ratio (%)
Original K2La2Ti3O10 at 950 °C	61.8	2.0	34.2
Cu2+/K2La2Ti3O10 at 300 °C	13.0	5.8	46.0
Cu2+/K2La2Ti3O10 at 400 °C	15.5	5.0	48.0
Cu2+/K2La2Ti3O10 at 500 °C	13.0	4.6	51.1
Cu2+/K2La2Ti3O10 at 600 °C	13.8	3.4	45.0
Cu2+/K2La2Ti3O10 at 700 °C	20.3	2.7	44.0

^a According to Scherrer equation, the grain size was calculated from the full width at half maximum of the peak at 23.12°.

). Surface characteristics of Cu²⁺/K₂La₂Ti₃O₁₀ calcined at different temperatures and original K₂La₂Ti₃O₁₀ samples measured by N₂ adsorption are summarized in Table 1. After ion-exchange reaction, the specific surface areas of Cu²⁺/K₂La₂Ti₃O₁₀ calcined at different temperature are higher than the original K₂La₂Ti₃O₁₀ sample, ranging from 2.7 to 5.8 m²/g. Table 1 also shows the chlorobenzene degradation ratio of Cu²⁺/K₂La₂Ti₃O₁₀ calcined at different temperatures and original K₂La₂Ti₃O₁₀. It was found that degradation ratio of Cu²⁺/K₂La₂Ti₃O₁₀ at 500 °C reached to 51.1%, about 16.9% higher than that of the original K₂La₂Ti₃O₁₀ (34.2%). It should be ascribed to the narrow interlayer distance, the formation of CuO, and smaller grain size [17,18,19]. The unique structure of layered peroskite and the heterojunction between the CuO and the perovskite are in favor of the separation of electron-hole pairs for higher photocatalytic activity. With the increase of calcination temperature, the degradation ratio increase and reach the maximum at 500 °C. It results from the high visible light absorption on the surface of Cu²⁺/K₂La₂Ti₃O₁₀ calcined at 500 °C (Figure 2), except for narrow interlayer distance and the formation of CuO. However, excessive calcination temperature decreases the photocatalytic activity, which is ascribed to the lower specific surface area (Table 1) and low visible light absorption.

Figure 4 presents the comparison of photocatalytic activity of Cu²⁺/K₂La₂Ti₃O₁₀ with other ions exchanged K₂La₂Ti₃O₁₀ (Ni²⁺, Sn²⁺, Ce²⁺ and Fe²⁺). The degradation ratio order for chlorobenzene is Cu²⁺/K₂La₂Ti₃O₁₀ > Fe²⁺/K₂La₂Ti₃O₁₀ > Ce²⁺/K₂La₂Ti₃O₁₀ > Sn²⁺/K₂La₂Ti₃O₁₀ > Ni²⁺/K₂La₂Ti₃O₁₀. The degradation ratios of the last three catalysts for chlorobenzene are even less than that of the original K₂La₂Ti₃O₁₀.

2.2. Effect of Initial Concentration on the Photocatalytic Activity

Figure 5 presents the relationship between the initial concentration of chlorobenzene and the degradation ratio. It is observed that the degradation ratio increases with the initial concentration increase of chlorobenzene up to 13.3 μg/mL. When the initial concentration exceeds this value, the degradation ratio decreases. With the rise of initial concentration, the active sites will be covered by chlorobenzene [20], which may decrease the degradation ratio due to the loss of active sites. Moreover, based on the results of Zhang [21], the intermediates (o-, m-, p-chlorophenol) are likely to accumulate on the surface of the photocatalyst during the photocatalytic degradation of chlorobenzene, which will lead to a decrease in the degradation ratio with irradiation time.

2.3. Effect of CO₂ Concentration on the Photocatalytic Activity

In order to investigate the effect of CO₂ concentration on the photocatalytic activity, the reactor was purged by 100% N₂, 70%N₂ + 30% CO₂, 50% N₂ + 50% CO₂, 30% N₂ + 70% CO₂, and 100% CO₂ for 30 min to fill the reactor and the other experiment conditions were the same as those in photocatalytic experiments. Figure 6 shows the effect of CO₂ concentration on the chlorobenzene degradation ratio of Cu²⁺/K₂La₂Ti₃O₁₀ at 500 °C. The degradation ratio is lower (only 48%) under simulant solar light irradiation for 180 min in the absence of CO₂. After CO₂ is introduced, the degradation ratio increase with the CO₂ concentration. It can reach 68% at 50%CO₂ + 50%N₂. The apparent rate constant (min⁻¹) (K_app) and corresponding correlation coefficient (R²) is also calculated according to the pseudo-first-order equation [22], as shown in

Table 2. Degradation ratio, apparent rate constant, and correlation coefficient (R²) of chlorobenzene with Cu²⁺/K₂La₂Ti₃O₁₀ at 500 °C under different CO₂ concentrations.

Irradiation Time (min)	Degradation Ratio (%)							Apparent Rate Constant (min−1)	R2
	0	30	60	90	120	150	180
100% N2	0	5	16.5	23.0	33.0	42.0	48.0	0.0038	0.9882
70% N2 + 30% CO2	0	3.8	15.6	24.9	34.4	45.2	49.9	0.0041	0.9836
50% N2 + 50% CO2	0	10.1	22.4	31.7	53.8	59.6	67.8	0.0066	0.9714
30% N2 + 70% CO2	0	8.5	22.5	33.1	47.0	58.6	61.4	0.0057	0.9823
100% CO2	0	6.1	17.1	25.8	43.3	55.7	57.4	0.0053	0.9601

. Figure 7 shows the chlorobenzene degradation ratio and apparent rate constant of Cu²⁺/K₂La₂Ti₃O₁₀ at 500 °C as a function of CO₂ concentration. With the increase of the CO₂ concentration, the degradation ratio rises and then decreases. The apparent rate constant follows a similar trend. It demonstrates that the exposure of CO₂ can improve the photocatalytic activity of Cu²⁺/K₂La₂Ti₃O₁₀. However, excessive CO₂ concentration can decrease the photocatalytic activity of Cu²⁺/K₂La₂Ti₃O₁₀, and the reason is still not clear. Further in-depth experiments would be required to clearly understand the reason of the lower activity of Cu²⁺/K₂La₂Ti₃O₁₀ with excessive CO₂ concentration.

As one of bio-refractory organics, heterogeneous photocatalytic oxidation and other advanced oxidation processes have been developed to remove chlorobenzene. Dilmeghani and Zahir reported that the observed pseudo first-order rate constants in air-saturated solution, oxygen-saturated solution, and under anaerobic condition are 5.1 × 10⁻⁴, 6.4 × 10⁻⁴, and 1.8 × 10⁻⁴ s⁻¹ for UV-induced degradation of chlorobenzene with pH value at 3.5 [23]. Tseng et al. reported that 42.8% of monochlorobenzene was degraded under UV light irradiation for 240 min, and the extent of monochlorobenzen degradation within 240 min of UV irradiation was approximately 60.2%, under the condition of H₂O₂ dosage equal to 22.5 mg/L [24]. The rate constant of monochlorobenzene degradation in UV/TiO₂/O₂ photocatalysis was increased from 0.016 to 0.046 min⁻¹, as the initial concentration of dissolved oxygen (DO) was increased from 1.6 to 28.3 mg/L [24]. The apparent rate constant of Cu²⁺/K₂La₂Ti₃O₁₀ during the photocatalytic degradation of chlorobenzene is 1.1 × 10⁻⁴ s⁻¹ and 6.8 × 10⁻⁵ s⁻¹ (corresponding to degradation ratio of 67.8% and 51.1%) under the reactor filled with 50% CO₂ + 50% N₂ and air, respectively. Cu²⁺/K₂La₂Ti₃O₁₀ exhibits high photocatalytic activity for chlorobenzene degradation under simulated solar light irradiation, rather than UV light, and without H₂O₂.

Formaldehyde is an important product of the photoreduction of gaseous CO₂ on semiconductor-based photocatalysis [25]. Based on literatures [1,12,25,26], a pathway might occur during the photocatalytic degradation of chlorobenzene. Dimitrijevic et al. have found that CO₂ could be reduced to CH₃O radical and HCOOH on titania under UV illumination [26]. CH₂O (or HCHO) was generated from the interaction between HCOOH and hydrogen radicals (H^•) [11] or the interaction between HCOOH, H⁺ and e _ CB during CO₂ reduction on titania under illumination [25,26]. Moreover, we found formaldehyde generation by photo degradation of chlorobenzene on Zn/KLaTi₂O₆ photocatalyst, and then chlorobenzene converted to benzaldehyde by dechlorination and reaction with formaldehyde formed earlier [1]. In this paper, the chlorobenzene degradation ratio increases with the CO₂ concentration on Cu²⁺/K₂La₂Ti₃O₁₀. It implies that benzaldehyde was similarly generated through reaction with formaldehyde (HCOH) intermediate during CO₂ reduction in the photocatalytic reaction on Cu²⁺/K₂La₂Ti₃O₁₀, which is in favor of the immobilization of greenhouse gases CO₂.

3. Experimental

3.1. Preparation of Photocatalysts

The K₂La₂Ti₃O₁₀ powder was prepared by sol-gel process. 0.02 mol La(NO₃)₃·nH₂H and 0.04 mol KNO₃ were dissolved in 22 mL water to form solution A. 0.03 mol titanium tetrabutyl (Ti(OC₄H₉)₄) was dissolved in 80 mL isopropanol ((CH₃)₂CHOH) with stirring for 30 min to form solution B. Solution A was added dropwise into solution B with vigorous stirring for 20 min to get the gelatinous solution. The resulted gelatinous solution was aged for 12 h at room when water and isopropanol were separated from the gel. The gel was then dried at 80 °C, crushed and calcined under different calcination temperatures for 4 h to obtain original K₂La₂Ti₃O₁₀ powder.

The Cu²⁺/K₂La₂Ti₃O₁₀ powder was synthesized by ion-exchange reaction. The original K₂La₂Ti₃O₁₀ powder at 950 °C (1 g) was added into 0.1 mol/L Cu(NO₃)₂ aqueous solution (100 mL). The solution was maintained at 333 K for two days. The sample after washing with distilled water to remove Cu²⁺ adsorbed on the surface of the sample was dried at room temperature and calcined at 300, 400, 500, 600, and 700 °C for 2 h to obtain Cu²⁺/K₂La₂Ti₃O₁₀ powder.

For comparison, Ni²⁺/K₂La₂Ti₃O₁₀, Sn²⁺/K₂La₂Ti₃O₁₀, Ce²⁺/K₂La₂Ti₃O₁₀, and Fe²⁺/K₂La₂Ti₃O₁₀ were also synthesized by the same ion-exchange reaction of the original K₂La₂Ti₃O₁₀ with 0.1 mol/L Ce(NO₃)₃, Ni(NO₃)₂, SnCl₂, and Fe(NO₃)₃, respectively.

3.2. Characterization of Photocatalysts

The X-ray diffraction (XRD, Rigaku Corporation, Tokyo, Japan) patterns of the powders were obtained on a Rigaku D/max 2500 X-ray diffraction analyzer with Cu $\mathrm{K}\alpha$ X-ray source at a scanning rate of 8°/min in the 2θ range between 5° and 80°. The accelerating voltage and the applied current were 40 kV and 200 mA, respectively. The specific surface area of samples was measured by nitrogen gas adsorption at 77 K using an automated adsorption apparatus (BELSOROP-MINI II). UV-visible diffuse reflectance spectroscopy of the powders was collected with Shimadzu Corporation UV-2540 (Kyoto, Japan) over the spectral range 240–800 nm. BaSO₄ was used as a reference.

3.3. Photocatalytic Experiments

Photocatalytic reaction was carried out in a self-made cylindroid glass reactor using a 350 W xenon arc lamp (ShenZhen AnHongDa Opto Technology Co., Ltd., Shenzhen, China) with similar characteristic spectrum with sun light as a light source of simulant solar light. A typical experiment of degradation was carried out as follows: a suspension containing 13.23 g/L the catalyst in distilled water was prepared. The surface of a frosted glass plate (25.0 × 53.0 mm, 13.25 cm²) was cleaned with acetone. The suspension was poured on the glass plate and then it was dried at 110 °C. Deposition process was performed four times to obtain a catalyst loading density of 3.77 × 10⁻² g/cm², as estimated by weighing the plate before and after the deposition process. The plate was dried at 110 °C and placed at the center of the reactor. 1 μL chlorobenzene liquid was injected into the sealed reactor filled with air and allowed to evaporate with a small magnetic stirring fan. Illumination was initialized after it got to adsorption equilibrium. The small fan guaranteed the gases circulated inside the reactor. 500 μL gases was withdrawn from the reactor with a syringe periodically and further analyzed by gas chromatography (SP-6890, Shandong Lunan Ruihong Chemical Instruments Co., Ltd., Tengzhou, China) to obtain the chlorobenzene concentration. All of the tests were conducted at the room temperature (298 K). Each degradation experiment was repeated three times. The degradation ratios at different irradiation times of three parallel experiments were determined using degradation ratio equation [22] and the corresponding chlorobenzene concentration. The average value and standard deviation of the degradation ratio of parallel experiments were taken as the final degradation ratio and error bars.

The photocatalytic activity for chlorobenzene decomposition with different initial concentration of chlorobenzene were investigated by varying the amount of chlorobenzene liquid injected into the reactor, and the initial concentrations of chlorobenzene were quantified by GC.

4. Conclusions

In summary, original K₂La₂Ti₃O₁₀ and Cu²⁺/K₂La₂Ti₃O₁₀ were successfully prepared. The XRD analysis demonstrates that Cu²⁺ ions are incorporated in place of K⁺ ions. With the rise of calcination temperature, more interlayer Cu²⁺ was converted into new crystal phase CuO. The degradation ratio of Cu²⁺/K₂La₂Ti₃O₁₀ is improved after ion-exchange, which should be ascribed to the narrow interlayer distance, the formation of CuO, smaller grain size, and the high visible light absorption on the surface fof Cu²⁺/K₂La₂Ti₃O₁₀ calcined at 500 °C. Moreover, the exposure of CO₂ can improve the photocatalytic activity of Cu²⁺/K₂La₂Ti₃O₁₀.