A highly active bimetallic oxide catalyst supported on γ-Al₂O₃/TiO₂ for catalytic wet peroxide oxidation of quinoline solutions under microwave irradiation

Abstract

A new heterogeneous wet oxidation catalyst, Cu–Ni bimetallic oxides supported on γ-Al₂O₃/TiO₂, was synthesized using a wet impregnation method. The physicochemical characteristics of the as-synthesized catalyst were assessed using various modern methods such as scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), N₂ adsorption–desorption, and X-ray photoelectron spectroscopy (XPS). Compared with monometallic Cu or Ni oxide catalysts, this new supported Cu–Ni bimetallic oxide catalyst presented a higher activity and stability in quinoline mineralization. The impact of the microwave (MW) power, temperature, pH and H₂O₂ dosage was studied on the basis of the TOC mineralization. At a MW power of 500 W, pH 7, 333 K and a 22.75 mmol L⁻¹ H₂O₂ dosage, an 81% abatement of the TOC was reached. Based on the results of metal leaching and XPS analysis, the stability of the Cu–Ni bimetallic supported catalyst, along with the role of the copper and nickel, is discussed.

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1. Introduction

Advanced oxidation processes (AOPs) involve sets of redox processes planned for water remediation. AOPs have been acknowledged as an attractive technique to eliminate biorefractory pollutants in effluent.¹ Among them, catalytic wet peroxide oxidation (CWPO) is a promising approach due to the almost complete removal of organic pollutants, without causing secondary pollution, at a mild temperature (<373 K) and atmospheric pressure range.² Iron species, as traditional catalysts, show excellent catalytic activities, but iron-containing catalysts are pH dependent (pH 3–4)³ and the oxidation–reduction reaction between H₂O₂ and iron is relatively unstable.⁴ Such weaknesses as those mentioned prompted a great effort to advance CWPO.

Recently, copper species, which are less pH dependent,⁵ have received a lot of attention in the last ten years because the redox reaction between H₂O₂ and Cu species is more stable than with iron.⁴ Such characteristics promote the application of copper catalysts in water remediation. K. M. Valkaj et al.⁶ reported the removal of phenol using Cu/13X zeolite as a catalyst, and the degradation of 0.9 mg L⁻¹ phenol reached 100% within 180 min at temperatures below 353 K. V. Priyanka et al.⁷ examined the degradation of nitrobenzene using Cu/AC as a heterogeneous CWPO catalyst, and the mineralization of 100 mg L⁻¹ nitrobenzene reached 89% after 4 h using the conditions: C_catalyst = 0.25 g L⁻¹, C_0H2O2 = 1.05 mol L⁻¹, pH = 3.0, and T = 328 K. Additionally, the introduction of another metal into the copper species to form a bimetallic oxide catalyst has become a focus in the area of CWPO.^8–10 Considering Ni presents a great catalytic performance in the treatment of phenol and its derivatives,^11–13 two-metal materials of copper and nickel had enhanced catalytic activity owing to the synergistic effects of the two metals.

On the other hand, microwave-enhanced catalytic oxidation has gained the attention of numerous scientific communities in the field of wastewater treatment,^13–15 because it has higher energy efficiency and shorter reaction times, and it promotes ion flexibility, the transmission of charges in the support to the surface.¹⁶ Additionally, earlier research has confirmed that the non-thermal effects of MWs can promote H₂O₂ decomposition into hydroxyl radicals (˙OH).^17,18 However, to our knowledge, there is no report on quinoline mineralization by MW-enhanced CWPO. Therefore, it is necessary to investigate the oxidation degradation of quinoline by MW-enhanced CWPO using a Cu/Ni bimetallic oxide catalyst.

This paper exhibits our study on the catalytic properties of a Cu–Ni bimetallic catalyst supported on γ-Al₂O₃/TiO₂, which was utilized to improve the CWPO oxidation of quinoline under microwave irradiation. Physicochemical characterization of the bimetallic Cu–Ni/γ-Al₂O₃/TiO₂ supported catalyst was executed, and the applicability of the Cu–Ni bimetallic catalyst in CWPO reactions under microwave irradiation was assessed on the basis of the influence of the main variables (e.g., microwave power, temperature, pH, and H₂O₂ concentration) and the Cu–Ni bimetallic supported catalyst stability, along with investigation of the role of the copper and nickel.

2. Experimental

2.1. Materials

Quinoline (Shanghai Wulian Chemical Factory, P. R. China), Cu(NO₃)₂ and Ni(NO₃)₂ (Chengdu Xiya Chemical Factory, P. R. China) were used in the experiments. The main characteristics of quinoline are presented in Table 1. All reagents were utilized as received without additional purification. All solutions were prepared with Milli-Q water (Millipore, France).

Table 1 Chemical structure and characteristics of quinoline

Chemical structure
λmax (nm)	313
MW (g mol^−1)	129.16
Chemical class	Analytical reagents

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2.2. Catalyst preparation and characterization

2.2.1. Catalyst preparation. In a typical wet impregnation synthesis procedure, Cu(NO₃)₂·3H₂O and Ni(NO₃)₂·6H₂O were utilized as the metallic sources. The pretreatment and element content of the carrier are detailed in Text S1 and Table S1 of the ESI,† respectively. In a typical synthesis process, known amounts of Cu(NO₃)₂·3H₂O and Ni(NO₃)₂·6H₂O were dissolved in 50 mL of deionized water to form solution A. After stirring for 1 h, 20 g of the carrier γ-Al₂O₃/TiO₂ was added into solution A. The mixed system was kept under shaking conditions for 20 h in an oscillator, followed by aging at 378 K for 16 h. The as-synthesized products were dried at 383 K, after being repeatedly washed several times with deionized water. Finally, calcination was conducted in a muffle furnace using a heating rate of 1 K min⁻¹ to reach 773 K, and holding for 5 h. The calcined products were designated as C4. Finally, a washing process was carried out by immersing 10 g of the calcined samples in 500 mL of a dilute HNO₃ solution (pH 3) for 8 h to eliminate the loosely attached metal particles. The washed products were filtered, dried, and calcined at 773 K for 2 h. The molar ratios of Cu(NO₃)₂·3H₂O to Ni(NO₃)₂·6H₂O (total 1.5 mol L⁻¹) were 1 : 1, 3 : 1, 5 : 1, and 7 : 1. The results of a separate experiment (ESI, Fig. S1†) showed that the best removal efficiency for the TOC was attained with the molar ratio of 5 : 1 (total 1.5 mol L⁻¹). Therefore, the sample of Cu–Ni bimetallic oxides supported on γ-Al₂O₃/TiO₂ with a molar ratio of 5 : 1 was selected for the mineralization experiments and characterization in the present work. A Ni-containing γ-Al₂O₃/TiO₂ catalyst and a Cu-containing γ-Al₂O₃/TiO₂ catalyst, which were denoted as C2 and C3, respectively, were prepared following a similar synthesis method without adding Cu(NO₃)₂·6H₂O or Ni(NO₃)₂·6H₂O into solution A.

For the blank reactions, the pure carrier γ-Al₂O₃/TiO₂ material was designated as C1.

2.2.2. Catalyst characterization. Various characteristics of the catalyst were investigated using different detection techniques. Scanning electron microscopy (FEI QUANTA 200), SEM, was used to study the surface morphology of the prepared catalysts and the carrier. The content of O, Al, Ti, Cu and Ni in the support and the Cu/Ni catalyst was investigated utilizing energy dispersive X-ray (EDX) (Oxford INCA, Germany) combined with SEM. X-ray diffraction (XRD, D/max-rB, Japan) using a Siemens model with Cu Kα radiation (λ = 0.15405 nm) was used to identify the crystal phases of the catalysts and support. The surface areas, pore volumes and average pore sizes of the catalysts and the carrier were calculated using conventional BET and BJH methods. X-ray photoelectron spectroscopy (XPS) measurements were obtained using a RBD upgraded PHI-5000C ESCA system (PerkinElmer Co., USA). The peaks of binding energy (BE) for Cu and Ni were standardized using the C 1s BE of 284.6 eV.

2.3. CWPO experiments

Quinoline degradation was performed in a glass flask of 500 mL equipped with a mechanical agitator, a reflux condenser and a thermocouple under MW irradiation. An experimental microwave oven, whose power could be attuned continuously using a dial, was employed to regulate the MW energy. To avoid the extreme conditions (high temperature and pressure) produced by MW irradiation, the buffer pool was linked to a condenser tube which was connected to the atmosphere. Furthermore, the buffer pool was fixed in a constant-temperature bath, so that the reaction was carried out at a mild temperature and atmospheric pressure.

The prepared catalyst (4 g L⁻¹) was put into a 500 mL quinoline aqueous solution (100 mg L⁻¹) under continuous stirring. A H₂O₂ solution was added to achieve a specific H₂O₂/quinoline ratio with stabilization of the temperature/energy power of the reactor. The catalytic activity was assessed by measuring the TOC at different reaction times, after filtering the samples through a 0.45 μm filter. Each experiment was conducted in triplicate, and the average values and standard deviations are shown.

2.4. Analytical methods

The concentration of quinoline was determined using HPLC analysis (LC-10AVP HPLC, Kyoto, Japan) with a mixed mobile phase (1% acetic acid and HPLC grade methanol) at a flow rate of 1 mL min⁻¹ and 313 nm. The TOC of the sample was quantified using a Multi 2100 TOC/TN analyzer (Shimadzu, 5000A).

For assessing the catalytic activity, both the quinoline removal efficiency and TOC removal efficiency were calculated as shown below:¹⁹

where C₀ and C_t are the initial and final concentration of quinoline, or the initial and final TOC of the solution, respectively. The efficiency of the utilization of H₂O₂ (η) is defined as the ratio of the amount of H₂O₂ used for degradation of the quinoline (Δ[H₂O₂]_degradation) to the total amount of consumed H₂O₂ (Δ[H₂O₂]_{decomposition}) in the reaction. The calculated equation is shown below:²⁰

The concentration of the total Cu and Ni ions in the reaction solution was measured by utilizing an Atomic Absorption Spectrophotometer model (Shimadzu AA6650) and the minimum detectable limit is 0.05 ppm. An iodimetric titration method was utilized for the concentration of H₂O₂.²¹

3. Results and discussion

3.1. Physicochemical characterization

The physical and chemical characterization results of C1 and C3 are provided in Fig. S2–S9 of the ESI.† Physicochemical properties of the catalysts explored in this research are presented in Table 2.

Table 2 Summary of the physicochemical properties of C1, C2, C3 and C4

Materials	SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)	Removal of quinoline after 9 min (%)	TOC removal
C1	156.35	0.40	10.44	16.97	9.78
C2	141.37	0.39	10.29	41.82	24.97
C3	138.87	0.37	10.21	62.69	34.98
C4	136.67	0.34	10.11	72.21	40.05

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A SEM image of C4 is presented in Fig. 1a. C4 was characterized as a mesoporous sample with a two-dimensional mesoporous structure. Compared with the SEM image of C1 (Fig. S2†), the addition of Cu/Ni into C1 did not impact on the framework of the macrostructure. Nevertheless, Ni addition favors the presence of a CuO-segregated phase on the surface in comparison with the SEM image of C3 (Fig. S7†). The experimental result is in good agreement with the previous results.^22,23

Fig. 1 Characterization of C4. (a) SEM image. (b) XRD pattern. (c) Nitrogen adsorption–desorption isotherms for C4. (d) Pore size distribution curve for C4.

EDX analysis, as shown in Fig. S10 (ESI†), coupled with SEM verified the coexistence of O, Al, Ti, Cu and Ni in C4 and the average values are presented in Table 3.

Table 3 The content of O, Al, Ti, Cu and Ni in C4

Sample	O (wt%)	Al (wt%)	Ti (wt%)	Cu (wt%)	Ni (wt%)
C4	22.88	59.82	4.27	10.67	2.36

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Fig. 1b shows the X-ray diffractogram of the C4 catalyst. Peaks characteristic of a CuO crystal can be observed in the pattern of the C4 catalyst, and the existence of a sharp peak at a 2θ angle of 35.1° indicated the existence of CuO.^24,25 Compared with the XRD pattern of the CuO crystal in the pattern of C3 (Fig. S6†), the peaks characteristic of a CuO crystal become stronger in the pattern of the C4 catalyst, which confirms that the introduction of Ni enhances the dispersion of the CuO particles and provides a smaller crystal size of CuO. This fact is consistent with the preference of Ni²⁺ to occupy octahedral sites as stated by other authors.²⁶ For this type of catalyst, the introduction of Ni promotes the formation of a CuO-segregated phase at the surface of C1, in agreement with the SEM. Peaks characteristic of NiO crystals can also be observed in the XRD pattern of C4, and the existence of a sharp peak at a 2θ angle of 43.2° indicated the existence of NiO.²⁷

As exposed in Fig. 1c, the C4 samples presented type IV N₂ adsorption–desorption isotherms and H3 hysteresis loops according to the Brunauer–Deming–Deming–Teller classification²⁸, which shows that C4 is typical of a mesoporous structure. The corresponding pore size distribution in Fig. 1d also verified that the C4 sample is chiefly mesoporous.^29,30 The BET surface area, pore size, and pore volume of C4 were 136.67 m² g⁻¹, 0.34 cm³ g⁻¹ and 10.11 nm, respectively (Table 2).

3.2. Catalytic activity of C4

A series of experiments were conducted under the employed conditions (MW = 500 W, pH 7, temperature = 333 K, and initial quinoline concentration = 100 mg L⁻¹) to estimate the catalytic activity of C4. Fig. 2 shows the change of the removal efficiencies for quinoline and the TOC mineralization with the reaction time for these experiments (Table S2†).

Fig. 2 Removal of quinoline and TOC under different conditions: (a) 4 g L⁻¹ C4 without H₂O₂; (b) 22.75 mmol L⁻¹ H₂O₂; (c) 4 g L⁻¹ C1 and 22.75 mmol L⁻¹ H₂O₂; (d) 4 g L⁻¹ C2 and 22.75 mmol L⁻¹ H₂O₂; (e) 4 g L⁻¹ C3 and 22.75 mmol L⁻¹ H₂O₂; (f) 0.67 g L⁻¹ C2, 3.33 g L⁻¹ C3 and 22.75 mmol L⁻¹ H₂O₂; (g) 4 g L⁻¹ C4 and 22.75 mmol L⁻¹ H₂O₂, (h) 4 g L⁻¹ C4 and 22.75 mmol L⁻¹ H₂O₂ without MW irradiation. Other reaction conditions: MW = 500 W, pH 7, temperature = 333 K, initial quinoline concentration = 100 mg L⁻¹.

The results in Fig. 2 reveal that H₂O₂ only led to a minor removal of the quinoline (curve b Fig. 2a) and TOC (curve b Fig. 2b) after 18 min of reaction. Additionally, utilizing C1 as the catalyst for H₂O₂, a quinoline removal of 26.73% (curve c Fig. 2a) and a TOC abatement value of 15% (curve c Fig. 2b) were reached in 18 min. Compared with C2 (Ni addition, curve d Fig. 2), using C3 (Cu addition) as the catalyst with H₂O₂ could attain a quinoline removal of 100% (curve e Fig. 2a) and a TOC removal of 61.67% after 18 min (curve e Fig. 2b), respectively, which obviously evidenced the main role of Cu in the catalytic wet peroxide oxidation under microwave irradiation. With 4.0 g L⁻¹ of the C4 catalyst only, the quinoline and TOC removal values were about 10% (curve a Fig. 2a) and 8% (curve a Fig. 2b), respectively, chiefly owing to surface adsorption, which was insignificant compared to the fast quinoline and TOC removal by catalytic wet peroxide oxidation under microwave irradiation. In the company of H₂O₂, the removal of the quinoline and the TOC using C4 as the catalyst was remarkably higher than that for C3 (Table 2), indicating that the catalytic activity was improved by the incorporation of Ni. Moreover, the removal of quinoline and the TOC abatement using a physical mixture of C2 and C3 with H₂O₂ was examined. Complete removal of the quinoline (curve f Fig. 2a) and 68.77% removal of the TOC (curve f Fig. 2b) were accomplished after an 18 min reaction, consisting of a rapid removal period. By comparison, the C4 catalytic activity was higher than that of the physical mixture composite (curve g Fig. 2), implying that there might be a synergy effect in C4, thus improving the mass transfer rates to active sites and the chemical reactions at reactive sites. Additionally, in the presence of MW irradiation and the Cu–Ni bimetallic oxides catalyst, the mineralization efficiency in the heterogeneous MW-CWPO was higher than for the supported Cu–Ni bimetallic oxides/H₂O₂ without MW on the basis of the TOC abatement (curve h Fig. 2b) and quinoline degradation (curve h Fig. 2a), indicating that there is a synergistic effect between the MWs and catalyst that improves the quinoline mineralization.

3.3. Effect of the parameters on the microwave-enhanced catalytic wet peroxide oxidation of quinoline solutions

Regenerated C4 catalysts (designated as C4-3rd), which had been utilized twice, were used for the next set of tests to reduce the effect of homogeneous reactions. Based on the results of Section 3.2., the TOC was chosen as the evaluation criterion.

3.3.1. Effect of microwave power. In order to investigate the effect of the microwave power on the mineralization of the quinoline solution, a batch of experiments were conducted at five different microwave powers (pH 7, catalyst dosage = 4 g L⁻¹, temperature = 333 K, initial quinoline concentration = 100 mg L⁻¹ and H₂O₂ dosage = 22.75 mmol L⁻¹). The results are shown in Fig. 3.

Fig. 3 Effect of microwave power on quinoline degradation.

As revealed in Fig. 3, the different microwave powers resulted in different quinoline mineralization efficiencies. The TOC abatement can be heightened by increasing the microwave power (300, 400 and 500 W, Fig. 3). This is attributed to the reasons that follow: (1) H₂O₂ in solution generates extra HO˙ when it is present in the field of MW irradiation (eqn (1));³¹ (2) the effect of the microwaves could improve the quinoline movement and the transmission of charge carriers to the surface of the catalyst;^16,17 (3) violent motion of the H₂O₂ and quinoline molecules (which are polar compounds) caused by MW radiation can promote the collision numbers and excitation of the molecules to a higher excited state, resulting in the intensity of the molecular bonds being reduced;³² (4) hot spots with a high temperature on the surface of C4 can be induced by the Maxwell–Wagner effect of the MW.^33,34 These hot spots could accelerate the reactions of H₂O₂ and quinoline molecules that are adsorbed on the surface of C4.

H₂O₂ + microwave energy → HO˙ (1)

Nevertheless, greater microwave powers (600 and 700 W, Fig. 3) can also promote the thermal conversion of H₂O₂ into O₂ and H₂O at the surface of the samples because of the higher temperature of hot spots on the surface of the samples.^35,36 Hence, it is essential to select an optimal microwave power for the microwave-enhanced catalytic wet peroxide oxidation reaction.

3.3.2. Effect of temperature. In order to confirm the influence of the reaction temperature on the catalytic wet oxidation of the quinoline solution under microwave irradiation, a batch of experiments were executed at five different temperatures (MW = 500 W, pH 7, catalyst dosage = 4 g L⁻¹, initial quinoline concentration = 100 mg L⁻¹ and H₂O₂ dosage = 22.75 mmol L⁻¹). The results are presented in Fig. 4.

Fig. 4 Effect of temperature on quinoline degradation.

Apparently, increasing the temperature has a positive influence on the abatement of the quinoline solution.³⁷ The TOC elimination is elevated from 68% to 81% as the temperature increases from 313 to 333 K (Fig. 4). A higher temperature seems to promote the conversion of H₂O₂ into HO˙,^37,38 and consequently increases degradation. Nevertheless, a greater temperature can also promote the thermal conversion of H₂O₂ into O₂ and H₂O, and can subsequently increase H₂O₂ consumption and reduce the H₂O₂ utilization efficiency.³⁹ Hence, it is essential to select an optimal reaction temperature for the catalytic wet peroxide oxidation reaction under microwave irradiation.

3.3.3. Effect of pH. The pH is one of the important factors in the catalytic wet peroxide oxidation process.^40,41 It impacts on the activity of the metal species, the oxidant and the substrate, and the conversion of H₂O₂.⁴² The quinoline solution was adjusted to five different pH values to investigate the impact of pH on the catalytic wet oxidation reaction under microwave irradiation and to assess the catalytic activity of C4 (MW = 500 W, catalyst dosage = 4 g L⁻¹, temperature = 333 K, initial quinoline concentration = 100 mg L⁻¹ and H₂O₂ dosage = 22.75 mmol L⁻¹). Fig. 5 demonstrates the degradation efficiency for quinoline with different pH values.

Fig. 5 Effect of pH on quinoline degradation.

It was observed that the TOC removal decreases at correspondingly higher pH values. This is because at a higher pH, the H₂O₂ would rapidly convert into O₂ and H₂O but not decompose into HO˙, based on an oxidation ability loss.⁴³ Furthermore, it should be noted that at pH 9 (Fig. 5), the reused C4 still provided a relatively high performance owing to the effect of the microwave (as mentioned eqn (1)). The selection of Cu as the main active species would be one cause for such an observation.

3.3.4. Effect of H₂O₂ concentration. The effect of the H₂O₂ concentration, ranging from 13.65 to 36.4 mmol L⁻¹, on the quinoline mineralization was studied under the employed conditions (MW = 500 W, pH 7, catalyst dosage = 4 g L⁻¹, temperature = 333 K and initial quinoline concentration = 100 mg L⁻¹) and the results are presented in Fig. 6.

Fig. 6 Effect of H₂O₂ dosage on quinoline degradation.

When the H₂O₂ dosage was raised from 13.65 to 22.75 mmol L⁻¹, the TOC removal increased correspondingly from 42% to 81% (Fig. 6). Increasing the H₂O₂ dosage in the reaction system can actually promote the production of HO˙.^44–46 However, when the H₂O₂ dosage was increased from 22.75 to 36.40 mmol L⁻¹, the TOC abatement was not further increased but reduced. This can be attributed to the fact that at a high H₂O₂ dosage, HO˙ consumption is a competition between the H₂O₂ and the substrate.⁴⁷

H₂O₂ + HO˙ → HOO˙ + H₂O (2)

HOO˙ + HO˙ → H₂O + O₂ (3)

As presented in eqn (2) and (3), H₂O₂ at a high dosage acts as a highly efficient scavenger of HO˙, and the HO˙ would recombine to produce H₂O and O₂.^47,48 Hence, it is crucial to choose an appropriate H₂O₂ dosage.

3.4. Stability studies for C4

In order to investigate the stability and reusability characteristics of C4, successive batch experiments were conducted for six runs under the same reaction conditions. The experimental results in Fig. 7 present that the catalytic behavior of C4 declined step by step during the first three successive batch runs, possibly owing to leaching of the active components from the surface of C4, which implies that the mineralization rate for quinoline steadily decreased with the repeated use of the catalyst. Conversely, after the first three successive runs, the mineralization of the quinoline was practically stable with the reused catalyst, which implied a probability of using C4 for a longer operation time.

Fig. 7 Reuse of C4 after subsequent reactions. Reaction conditions: H₂O₂ dosage = 22.75 mmol L⁻¹, catalyst dosage = 4 g L⁻¹, reaction temperature = 333 K, MW = 500 W, initial quinoline concentration = 100 mg L⁻¹ and pH = 7.

Additionally, atomic absorption spectroscopy was utilized to measure whether there were any free Cu and Ni ions in the reaction solution after each reaction. The amount of metal leaching was analyzed after each experimental run (Table 4). Due to the metal Cu and Ni leaching, the quinoline mineralization rate was reduced in the third experimental run (Fig. 7). This agrees well with previous reports that show that the catalysts were deactivated, caused by the active ingredient leaching.^49,50 However, dissolved Cu and Ni ions could not be detected from the fourth to the sixth run and the TOC abatement was almost stable (Fig. 7), implying that C4 was relatively stable in the catalytic wet oxidation processes under microwave irradiation. Further work is required to reduce the leaching of the active ingredient from the surface of C4 and to find some effective regeneration processes.

Table 4 Extent of metal leaching using reused catalysts

Sample	Metal leached (mg L^−1)
	1st run		2nd run		3rd run		4th run		5th run		6th run
	Cu	Ni	Cu	Ni	Cu	Ni	Cu	Ni	Cu	Ni	Cu	Ni
C4	0.071	0.023	0.037	0.013	0.007	0.004

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3.5. Decomposition of H₂O₂

Besides the TOC abatement, the decrease of H₂O₂ was investigated utilizing C3-3rd and C4-3rd as the catalyst, respectively.

Like the reduction of the TOC, the dosage of H₂O₂ in the two solutions diminished as the reaction advanced (Fig. 8). Nevertheless, the reduction in H₂O₂ was much more than the theoretical value predicted using eqn (4).

C₉H₇N + 23.5H₂O₂ → 9CO₂ + 27H₂O + NO₂ (4)

Fig. 8 H₂O₂ and TOC reduction tendency for C3-3rd and C4-3rd: (a) C3 theoretical values of H₂O₂ decomposition for quinoline mineralization; (b) C3 practical consumption of H₂O₂; (c) C4 theoretical values of H₂O₂ decomposition for quinoline mineralization; (d) C4 practical consumption of H₂O₂; (e) C3 TOC abatement and (f) C4 TOC abatement. Reaction conditions: MW = 500 W, pH 7, catalyst dosage = 4 g L⁻¹, temperature = 333 K, initial quinoline concentration = 100 mg L⁻¹ and H₂O₂ dosage = 22.75 mmol L⁻¹.

It could be appreciated that a part of the H₂O₂ was decayed to H₂O and O₂, which had no role in the TOC elimination. Furthermore, the reduction of H₂O₂ was quicker in the C4 system than in the C3 system. The witnessed improvement is chiefly attributable to the introduction of Ni. After 18 min of reaction, about a 72% decrease in the H₂O₂ concentration occurred, equivalent to 62% elimination of the TOC, whereas the theoretical H₂O₂ usage was about 50% and the utilized amount of H₂O₂ was only 69% for the C3 system. On the other hand, 72% of the H₂O₂ was reduced, equivalent to removal of 81% of the TOC, and equivalent to 87% H₂O₂ utilization in the C4 system. It is apparent that the utilization of H₂O₂ was improved in the C4 system, which advocated that in the C4 system a higher proportion of H₂O₂ was decayed into HO˙.

3.6. Role of copper and nickel

In order to investigate the function of the copper and nickel, XPS was employed to analyse the surface of the C4 catalyst before and after the quinoline mineralization reaction. The XPS spectra of Cu and Ni are exhibited in Fig. 9. As shown in Fig. 9a, the signals of the Ni elements in the C4 composite catalyst before and after utilization could be identified as Ni³⁺ and Ni²⁺ with a binding energy near to 856.1 and 854.6 eV (Fig. 9a), respectively.^51,52 In addition, the ratio of Ni³⁺/Ni²⁺ changed before and after the wet oxidation reaction under microwave irradiation as presented in Table 5. The change between Ni²⁺ and Ni³⁺ could create a charge imbalance and vacancies on the catalyst surface.^53,54 The above results indicate that the existence of the redox cycle between Ni²⁺ and Ni³⁺ on the surface of the C4 catalyst might have a certain contribution in the heterogeneous CWPO reaction. On the other hand, the spectra of Cu 2p in the C4 catalyst before the reaction show a spin–orbit doublet with binding energies of 953.6 eV for Cu 2p_1/2 and 933.6 eV for Cu 2p_3/2 (Fig. 9b). According to ref. 55, the Cu elements could be identified as Cu²⁺. After the reaction with H₂O₂ under microwave irradiation, the binding energy of the Cu elements was stable at 933.5 eV, which is near to 933.6 eV. The stabilization of the Cu state might indicate that the oxidation–reduction cycle of the Cu elements on the surface of the C4 catalyst was associated with the decomposition of H₂O₂ to ˙OH.

Fig. 9 XPS spectra of C4 before and after a 10 min reaction: (a) Ni 2p; (b) Cu 2p.

Table 5 XPS binding energies of the Ni in the C4 catalyst before and after reaction

Element	Conditions	Ni 2p3/2 (eV)
Ni	Before reaction	854.88 (Ni^2+) (68.02%)
		856.27 (Ni^3+) (31.98%)
	After reaction	854.82 (Ni^2+) (66.09%)
		856.22 (Ni^3+) (33.91%)

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3.7. Possible catalytic mechanism

The actual reactive species in the MW-CWPO process was investigated by detecting the impact of n-butanol and KI, as radical scavengers, on the quinoline degradation, as presented in Fig. 10.^56–58

Fig. 10 Effect of radical scavengers on the degradation of quinoline (quinoline concentration: 100 mg L⁻¹, temperature: 333 K, MW: 500 W, H₂O₂: 22.75 mmol L⁻¹, catalyst: 2 g L⁻¹).

Excessive n-butanol in the bulk liquid scavenges all the ˙OH generated in the MW-CWPO system and iodide ions react with ˙OH produced at the supported Cu/Ni catalyst surface, both of which decrease the number of oxidizing species in the MW-CWPO system. The results obviously display that the quinoline removal was totally constrained in the presence of 1000 mg L⁻¹ of n-butanol, implying that the quinoline was oxidized by the attack of ˙OH both at the catalyst surface and in solution.

With the addition of excess KI (20 mg L⁻¹), the quinoline degradation was reduced in the presence of KI, demonstrating that ˙OHads plays a central role. Subsequently, about 100% quinoline removal was attained at 15 min in the absence of scavenger, primarily attributed to the action of ˙OH in solution.

Based on all the information obtained above, a possible catalytic mechanism for the H₂O₂ activation by C4 under microwave irradiation is displayed in Fig. 11.

Fig. 11 Schematic diagram of the reaction mechanism for H₂O₂ activated by C4 under microwave irradiation.

As described by other researchers,^59,60 the initially generated Cu²⁺ can react with H₂O₂ to form surface-bound ˙OH₂ (eqn (5)), where Cu²⁺ stands for Cu²⁺ on the catalyst surface. Some more Cu ions are generated through reaction of the previously generated Cu ions with H₂O₂ (eqn (6)) and HO₂˙ (eqn (7)).⁶¹ The standard redox potential of Ni²⁺/Ni³⁺ is 1.74 V, while that of Cu⁺/Cu²⁺ is 0.15 V; hence, the transfer of electrons from Cu⁺ to Ni³⁺ (eqn (8)) is thermodynamically approved.^62,63 Ni is capable of redox cycling in the presence of H₂O₂ and generates ˙OH (eqn (9)–(12)).⁵⁴ Finally, quinoline is decomposed mainly by ˙OH, including ˙OH on the surface of C4.

≡Cu^II + H₂O₂ → ≡Cu^I + HO₂˙ + H⁺ (5)

≡Cu^I + H₂O₂ → ≡Cu^II + ˙OH + OH⁻ (6)

≡Cu^II + HO₂˙ → ≡Cu^I+ + O₂ + H⁺ (7)

≡Ni^III + ≡Cu^I ↔ ≡Ni^II + ≡Cu^II (8)

≡Ni^II + H₂O₂ → ≡Ni^III + ˙OH + OH⁻ (9)

˙OH + H₂O₂ → HO₂˙ + H₂O (10)

≡Ni^III + HO₂˙ → ≡Ni^II + O₂ + H⁺ (11)

˙OH + quinoline → degraded products (12)

4. Conclusions

A Cu–Ni bimetal-based γ-Al₂O₃/TiO₂ catalyst for wet oxidation catalysis under microwave irradiation was effectively synthesized via a wet impregnation method. The as-synthesized catalyst had a typical mesoporous structure with a two-dimensional macrostructure, as illustrated using SEM-EDX, XRD and N₂ adsorption–desorption. An increased catalytic activity and stability for quinoline mineralization were witnessed. Such important reaction conditions as the MW power, temperature, pH and H₂O₂ concentration were explored. It was established that the MW and higher temperature had a positive effect on the mineralization; excess H₂O₂ would act as a scavenger of HO˙; copper species could improve the as-synthesized sample to preserve the activity at neutral pH. The as-synthesized sample was verified to be an attractive substitute for use in the heterogeneous wet peroxide oxidation system under microwave irradiation, and application of the bimetallic oxides catalyst exposed new tactics in the advancement of novel effective microwave-enhanced catalytic wet peroxide oxidation catalysts.

Conflict of interest

The authors declare that there are no conflicts of interest.

Acknowledgements

The present paper is supported by SKLUWRE of HIT (No. 2016DX12 and No. 2013DX09).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08576h

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