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Article

Kinetic and Mechanistic Study of Rhodamine B Degradation by H2O2 and Cu/Al2O3/g-C3N4 Composite

by
Chunsun Zhou
†,
Zhongda Liu
†,
Lijuan Fang
,
Yulian Guo
,
Yanpeng Feng
and
Miao Yang
*
School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
†
Authors contributed equally to this work and are recognized as co-first authors.
Catalysts 2020, 10(3), 317; https://doi.org/10.3390/catal10030317
Submission received: 25 January 2020 / Revised: 6 March 2020 / Accepted: 9 March 2020 / Published: 10 March 2020
(This article belongs to the Special Issue Sustainable and Environmental Catalysis)
Browse Figures
Versions Notes

Abstract

:
The classic Fenton reaction, which is driven by iron species, has been widely explored for pollutant degradation, but is strictly limited to acidic conditions. In this work, a copper-based Fenton-like catalyst Cu/Al2O3/g-C3N4 was proposed that achieves high degradation efficiencies for Rhodamine B (Rh B) in a wide range of pH 4.9–11.0. The Cu/Al2O3 composite was first prepared via a hydrothermal method followed by a calcination process. The obtained Cu/Al2O3 composite was subsequently stabilized on graphitic carbon nitride (g-C3N4) by the formation of C−O−Cu bonds. The obtained composites were characterized through FT-IR, XRD, TEM, XPS, and N2 adsorption/desorption isotherms, and the immobilized Cu+ was proven to be active sites. The effects of Cu content, g-C3N4 content, H2O2 concentration, and pH on Rh B degradation were systematically investigated. The effect of the catalyst dose was confirmed with a specific reaction rate constant of (5.9 ± 0.07) × 10−9 m·s−1 and the activation energy was calculated to be 71.0 kJ/mol. In 100 min 96.4% of Rh B (initial concentration 20 mg/L, unadjusted pH (4.9)) was removed in the presence of 1 g/L of catalyst and 10 mM of H2O2 at 25 °C, with an observed reaction rate constant of 6.47 × 10−4 s−1. High degradation rates are achieved at neutral and alkaline conditions and a low copper leaching (0.55 mg/L) was observed even after four reaction cycles. Hydroxyl radical (HO·) was identified as the reactive oxygen species by using isopropanol as a radical scavenger and by ESR analysis. HPLC-MS revealed that the degradation of Rh B on Cu/Al2O3/CN composite involves N-de-ethylation, hydroxylation, de-carboxylation, chromophore cleavage, ring opening, and the mineralization process. Based on the results above, a tentative mechanism for the catalytic performance of the Cu/Al2O3/g-C3N4 composite was proposed. In summary, the characteristics of high degradation rate constants, low ion leaching, and the excellent applicability in neutral and alkaline conditions prove the Cu/Al2O3/g-C3N4 composite to be a superior Fenton-like catalyst compared to many conventional ones.

1. Introduction

With the rapid development of industry, persistent organic pollutants in water have attracted widespread attention due to their persistence, bioaccumulation, and high toxicity [1,2]. So far, advanced oxidation processes (AOPs) have been found to be one of the most promising methods to treat persistent organic pollutants in water [3,4,5,6,7]. Due to the in-situ formed highly reactive and non-selective hydroxyl radicals (HO·) during the process, AOPs are capable of mineralizing almost all organic compounds to CO2, H2O, and small organic compounds [8,9,10,11].
As a typical AOP, Fenton reaction is efficient for HO⋅ production, but still faces some limitations, such as the strict acidic pH range (pH ˂ 4) [12,13], formation of iron sludge [14,15], and high cost for catalyst recycling [16,17]. A number of non-ferrous metals, such as copper [18], manganese [19], and titanium [20], have been developed as alternatives. In particular, Cu+ reacts with H2O2 in a similar manner to Fe2+, but with a much higher reaction rate constant (as shown in Equations (1) and (2)). In contrast to Fe3+ that forms the insoluble [Fe(H2O)6]3+ complex at pH > 5, Cu2+ forms the aquo complex [Cu(H2O)6]2+ that predominates at neutral conditions, making Cu+ own a wider pH range for application [21]. Additionally, it is known that Cu2+ could form certain complexes with organic degradation intermediates, which could react with H2O2 to generate more HO⋅ [22].
Fe2+ + H2O2 → Fe3+ + HO· + OH−  (k = 63 − 76 M−1·s−1)
Cu+ + H2O2 → Cu2+ + HO· + OH−  (k = 1.0 × 104 M−1·s−1)
However, Cu+ ions are prone to disproportionation in acidic aqueous solution and can be easily oxidized by dissolved O2, thereby limiting the application in the aqueous environment [23]. One common strategy for the preparation of copper-based catalysts is the immobilization of copper species on support materials, for example, the immobilization of Cu+/Cu2+, copper oxide, or the copper-organic complex [24] on various matrixes like metal oxides [25,26,27], molecular sieve [28,29], and graphitic carbon nitride (g-C3N4) [30,31].
With a suitable band gap (2.7 eV) and high response to visible light, g-C3N4 has been intensively explored as a photocatalyst for energy and environmental applications [3,32,33,34]. Apart from this, g-C3N4 could also be applied as a Fenton-like catalyst in the absence of light irradiation through the combination with other materials [35,36]. g-C3N4 has a 2D planar structure in which tri-s-triazine units are connected by tertiary amines. Typical π-conjugated graphitic planes are formed via the sp2 hybridization of carbon and nitrogen atoms [37], which brings a large specific surface area, thus providing more reaction sites for heterogeneous reaction. Besides, g-C3N4 shows strong affinity for H2O2 and could easily adsorb them to the surface, thereby providing larger chances for their contact with other catalytic materials loaded on g-C3N4 [34]. Additionally, numerous external groups (−NH2, −NH, −N, and −OH) on the surface may serve as strong Lewis base sites for nanoparticle deposition or metal inclusion [38]. The incorporation of metallic elements into the g-C3N4 matrix could induce the production of delocalized electrons, which promotes the catalytic reactions [36,39]. As a typical example, Xu et al. constructed the Cu-Al2O3-g-C3N4 system, in which a small amount of g-C3N4 was used to coordinate with Cu ions to induce the formation of an electron-rich Cu centre and decrease the electron density of the π-electron conjugated system through cation-π interactions [40].
Inspired by the above-mentioned research, a similar but distinguishing efficient Cu/Al2O3/g-C3N4 composite was proposed in this work, with which H2O2 activating was promoted in a different approach. Copper species were bonded to the Al2O3 framework to act as the catalytic component and g-C3N4 was used in large amounts to act as support for the Cu/Al2O3 composite. The decomposition of H2O2 was promoted by the strong adsorption of H2O2 on the g-C3N4 matrix. The obtained composites were characterized through Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption isotherms. The optimal synthetic parameters and experimental conditions, including the Cu content, g-C3N4 content, H2O2 concentration, and pH value were determined. Besides, the effect of the catalyst dose and temperature were confirmed by calculating specific reaction rate constant and activation energy, and the durability and low leaching were evaluated in recycling experiment. In addition, the reactive oxygen species generated in the present system were identified by scavenging experiments and electron spin resonance (ESR) analysis, while the degradation products of Rh B were identified by high performance liquid chromatography-mass spectrometry (HPLC-MS) analysis. Based on the results, the catalytic mechanism of Rh B degradation on Cu/Al2O3/g-C3N4 composite was proposed.

2. Results and Discussion

2.1. Structural Characterization of Composites

Cu12/Al2O3 and Cu12/Al2O3/CN1.3 (the naming rules of catalysts are indicated in experimental section) were selected as the typical Cu/Al2O3 and Cu/Al2O3/CN composite to compare their structures with Al2O3 and CN. The FT-IR spectra of Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN samples were recorded to distinguish the functional groups. As shown in Figure 1, the two peaks at 1513 cm−1 and 1636 cm−1 in the FT-IR spectra of Al2O3 and Cu/Al2O3 are attributed to the C=C vibrations and C=O vibrations, which are derived from glucose added in the preparation process [40]. As observed for CN, the sharp peak at 810 cm−1 is ascribed to the breathing mode of tri-s-triazine units, and the peaks in the region of 1200−1600 cm−1 are assigned to the stretching vibration of the CN heterocycle [31]. In addition, the peak at 3173 cm−1 corresponds to the N−H stretching vibration [36]. All these characteristic peaks could be observed in the Cu/Al2O3/CN composite, indicating that the chemical structure of CN was not affected by the introduction of Cu/Al2O3.
The X-ray diffraction patterns of Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN samples were investigated to determine the crystalline structures, as presented in Figure 2. It is clearly observed that the major crystalline structure of Al2O3 is the γ-alumina phase (JCPDS No. 10-0425) [41]. In comparison with pure Al2O3, new peaks are founded at 35.2°, 38.5°, 48.8°, 53.4°, 58.2°, 66.2°, and 68.1° in the XRD pattern of Cu/Al2O3, which represent the (002), (111), (20-2), (020), (202), (31-1), and (220) planes of the copper oxide phase (JCPDS No. 48-1548), respectively [40]. The peak at 61.5° is referred to the diffraction of the (220) plane of cuprous oxide phase (JCPDS No. 65-3288) [42]. These results imply that the copper species were successfully bonded to the Al2O3 framework. From the pattern of CN, two main diffraction peaks could be found at 13.0° and 26.9°, which correspond to the interlayer stacking (100) plane of tri-s-triazine units and the (002) plane arising from the interlayer stacking of aromatic systems (JCPDS No. 87-1526) [43]. All the diffraction peaks of Cu/Al2O3 and CN could be found in the pattern of Cu/Al2O3/CN composite, indicating the successful synthesis of the Cu/Al2O3/CN composite.
The microstructures of Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN samples were analyzed by TEM. The results are shown in Figure 3. As can be seen in Figure 3A,B, the lattice spacing was confirmed to be 0.29 nm, which corresponds to the (220) lattice planes of γ-alumina [44], and was consistent with the results obtained by XRD. In Figure 3C, it can be seen that the copper bonded alumina framework was rod like. The lattice spacing in Figure 3D was confirmed to be 0.24 nm, which corresponds to the (111) lattice planes of copper oxide confirmed by XRD [45]. In Figure 3E, it is clear that CN shows a two-dimensional structure with a wide pore size distribution [35]. According to Figure 3F, it can be observed that the rod like Cu/Al2O3 was embedded in the CN matrix, indicating the successful combination of Cu/Al2O3 and CN.
The XPS spectra of the Cu/Al2O3/CN composite before and after the reaction were obtained to investigate its surface chemical states and elemental composition. The results are shown in Figure 4 and Table 1. The peaks of the C 1s spectrum at 281.5 eV and 284.6 eV were assigned to C−H and C−C, respectively. Notably, the peak at 285.1 eV was attributed to C−O−H or C−O−metal, which could confirm that Cu/Al2O3 was stabilized on the CN matrix by the C−O−Cu bonds [36,40]. For N 1s, the peaks at 395.1 eV, 395.7 eV, and 397.1 eV were ascribed to triazine rings C−N−C, tertiary nitrogen N−(C)3, and sp2-type C=N bond, indicating that the structure of CN was not changed [36]. The XPS spectrum of Cu 2p in Figure 4C displayed three main peaks at 932.0 eV, 934.0 eV, and 941.5 eV, which correspond to Cu+, Cu2+, and satellite peaks of copper species, respectively [40,41]. As the binding energy of Cu+ and Cu0 are very close, it is difficult to distinguish between Cu+ and Cu0 by the XPS feature of Cu 2p3/2. The Cu LMM peak is normally applied to distinguish between the Cu0 and Cu+ according to previous studies [46,47]. However, due to the relatively low copper content in the sample, the presence of Cu+ cannot be judged by the Cu LMM peak. The results of the XRD patterns could be used instead to confirm the presence of Cu+. Two peaks of the Al 2p spectrum were observed at 74.1 eV and 75.0 eV in Figure 4D, which were attributed to Al−O−Al and Al−O−Cu, indicating that copper species were bonded to Al2O3 framework [46]. The Cu 2p XPS spectrum of the used Cu/Al2O3/CN composite is shown in Figure 4E. The peaks at 931.7 eV, 933.6 eV, and 941.5 eV still represent Cu+, Cu2+, and satellite peaks of copper species, respectively. The Cu+ to Cu2+ ratio on the Cu/Al2O3/CN composite was 2.34 before reaction and decreased to 1.42 after reaction, indicating that copper species were the active sites and the conversion of Cu+ to Cu2+ was involved in the reaction [48].
The N2 adsorption/desorption isotherm and the pore size distribution of the Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN samples are shown in Figure 5. The specific surface area of Al2O3 reaches 133.1 ± 0.5 m2/g with a pore width of 6.7 nm, and the introduction of copper species leads to a decline to 119.5 ± 1.0 m2/g and 5.3 nm, respectively. All the CN containing samples show typical IV isotherms with the H3 hysteresis loop, indicating the existence of a typical mesoporous structure with slit-like pores in CN [22]. Additionally, when Cu/Al2O3 was stabilized on the CN matrix, the specific surface area increased from 72.2 ± 0.9 m2/g for CN to 146.6 ± 1.0 m2/g for Cu/Al2O3/CN, which is more beneficial for the adsorption of organic pollutants and H2O2 on the catalyst surface and proves that the fabrication of the Cu/Al2O3/CN composite is not merely a physical mixing process. In summary, a higher specific surface area and smaller pore width are achieved in the Cu/Al2O3/CN composite.

2.2. Catalytic Performance of Composites

The effect of Cu content, CN content, H2O2 concentration, and pH value were investigated to obtain the optimal synthetic parameters and reaction conditions. Besides, the effect of the catalyst dose and temperature was confirmed by calculating the specific reaction rate constant and activation energy (Ea). The durability and ion leaching of the composite were evaluated by the recycling experiments. In addition, the reactive species generated were identified by ESR analysis and radical scavenging experiments, and then the catalytic mechanism of Rh B on Cu/Al2O3/CN composite was proposed by confirming intermediates using HPLC-MS.

2.2.1. Effect of Cu Content

In order to obtain the optimal Cu content for catalyst fabrication, a series of Cux/Al2O3 composites (x = 0, 4, 7, 9, 12, and 15 wt %) were prepared to compare their catalytic performances on the removal of Rh B in the presence of H2O2. As shown in Figure 6, in the absence of copper decoration, Al2O3 exhibited negligible catalytic capacity for Rh B degradation, with 3.0% Rh B removed in 100 min. The degradation was enhanced by copper doping and the rate progressively increased from x = 0 to x = 12, after which the degradation rate started to decrease. During the synthesis process of Cu/Al2O3 composite, copper species were bonded to the Al2O3 framework. However, there is an upper limit (x = 12) for the amount of copper species that could enter the Al2O3 framework, so part of the copper species would be present as the extra framework if excess copper was introduced [47]. The extra framework copper species may impede the contact between H2O2 and framework copper species, thereby limiting the decomposition of H2O2 [22,41]. Therefore, the optimal Cu content was confirmed to be x = 12 and Cu12/Al2O3 was selected as optimal components for further studies.

2.2.2. Effect of CN Content

A fixed amount (0.1 g) of the Cu12/Al2O3 composite was loaded on different amounts of CN to determine the optimal CN content. The degradation of Rh B in the presence of Cu12/Al2O3/CNy (y = 0, 0.7, 1.3, 2.0, and 2.7 wt %) is presented in Figure 7. As shown, the Cu12/Al2O3 composite shows acceptable catalytic property for Rh B degradation, removing 34.6% Rh B in 100 min. Additionally, it is apparent that the introduction of CN leads to a corresponding enhancement of the degradation rate. With CN incorporated, part of Al2O3/Cu0.9 was stabilized on CN sheets by the formation of C−O−Cu bonds. Aqueous H2O2 are easily bonded to the surface of CN [34], providing larger chances for its contact with immobilized Cu+, which brought a significant improvement in catalytic property. Consequently, the degradation ratio within 100 min increased from 34.6% to 96.4% in the CN range of y = 0 to y = 1.3. However, excess CN incorporation from y = 1.3 to y = 2.7 did not further improve the degradation ratio. In the presence of adequate CN (y = 1.3), Cu12/Al2O3 is fully loaded on the CN interlayers, resulting in the largest degradation ratio exhibited in Figure 7. When the amount of CN was largely in excess to Cu12/Al2O3, the mass ratio of Cu12/Al2O3 in Cu/Al2O3/CN decreased accordingly, thereby limiting the further improvement of catalytic property. Therefore, the optimal CN content was confirmed as y = 1.3 and Cu12/Al2O3/CN1.3 was selected as the typical composite for further studies. The Cu/Al2O3/CN composite in the following text refers to the Cu12/Al2O3/CN1.3 composite unless addition descriptions were used.

2.2.3. Synergistic Effect

To verify the synergistic effect of Cu12/Al2O3 and CN on the degradation of Rh B, four samples were prepared and their catalytic properties were compared in the same condition. The results are presented in Figure 8. It can be seen that both Cu12/Al2O3 and CN show a weak catalytic property. Notably, CN exhibited a fine adsorption capacity for Rh B, which is attributed to its large specific surface area and multiple groups on the surface. By physically mixing Cu12/Al2O3 and CN, the adsorption capacity for Rh B decreased but a higher degradation rate was observed. This may be attributed to the fact that immobilized copper species form an adsorption competition with Rh B on CN sheets, leading to a portion of adsorptive sites occupied by immobilized copper, which lowers the adsorption of Rh B but accelerates its degradation. In the Cu/Al2O3/CN composite, much stronger bonds were formed between Cu and C−O−H as C−O−Cu, leading to a much lower adsorption capacity and a synergistically enhanced degradation property for Rh B.

2.2.4. Effect of pH

The homogeneous Fenton reaction is strictly limited to acidic conditions (pH < 4), which limits its application in neutral and alkaline conditions [4,12]. Therefore, the effect of pH is an important criterion for the evaluation of catalytic performance of catalysts, especially the ones that involve potential dissolved metal ion species. To evaluate the practical applicability of the Cu/Al2O3/CN composite in various pH conditions, a series of experiments were carried out ranging from pH 3.0 to pH 11.0. The initial pH value of 20 mg/L Rh B is 4.9, so this value (unadjusted pH) was used as a replacement of pH 5.0. The Rh B solutions were adjusted to predetermined pH values with the addition of 1 M HCl or NaOH. In all the experiments, the initial pH values underwent a slight increase with the extent of 1.0 ± 0.3 units, which could be attributed to the formation of OH− in the H2O2 decomposition process. The degradation of Rh B is shown in Figure 9. It is evident that rapid degradations of Rh B were achieved at pH values higher than pKa of Rh B (3.7), and a lower pH value of 3.0 led to a remarkable repression. This is mainly attributed to the different existence forms of Rh B at different pH conditions. At pH values higher than 3.7, the carboxyl group of cationic RhB+ is deprotonated and the dye is transformed into zwitterionic RhB± with a higher hydrophilic character that is prone to reacting with HO⋅ radical [49]. Additionally, the scavenging effect of HO⋅ radical by H+ becomes stronger at low pH values [50,51], and H2O2 would become more stable under strong acid conditions to form oxonium [H3O2]+, which inhibits its reaction with active species to generate HO⋅ radical in the presence of a large amount of H+ [48]. According to the Eh-pH diagram of Cu-H2O [52,53], CuO and Cu2O remain stable under alkaline conditions, which is beneficial for the catalytic degradation of Rh B. Despite the fact that more O2·− radicals are produced in alkaline conditions that consumes HO⋅ radical and H2O2, the degradation rates did not decline in higher pH conditions and even remarkably increased at pH 11.0, which may be attributed to the change of hydrophobic property of the dye and its interaction with negatively charged catalyst surface in strong alkaline conditions [49]. In summary, a remarkable degradation rate could be achieved in neutral and alkaline conditions, which is superior to classic Fenton reaction. Since there is no drastic difference in the removal of Rh B in the range of pH 4.9−9.0, Rh B solution with the unadjusted pH value of 4.9 was used for subsequent experiments.

2.2.5. Effect of H2O2 Concentration

Since H2O2 is the main source of HO⋅ in the Fenton-like reaction, the effect of H2O2 on the degradation of Rh B in the presence of Cu/Al2O3/CN composite was investigated. As calculated by Equation (3), the stoichiometric amount of H2O2 to achieve total mineralization of 20 mg/L Rh B was calculated to be 3.0 mM, and the effect of H2O2 concentration was investigated in the range of 1–10 mM.
C28H31ClN2O3 + 73 H2O2 → 28 CO2 + 87 H2O + HCl + 2 HNO3
The evolution of H2O2 with an initial concentration of 10 mM was taken as an example for kinetics analysis, the result of which is presented in Figure S1A. The residual concentration of H2O2 evolves linearly with reaction time, demonstrating the zero-order reaction kinetics during the reaction, which is attributed to the fact that H2O2 was present in large excess compared to Rh B. Only 11.9% of H2O2 was consumed in this experiment, which is far less significant than the Rh B decline that reached up to 96.4%. Therefore, more emphasis was stressed on the evolution of Rh B in this work. The normalized concentration of Rh B ([Rh B]/[Rh B]0) and its logarithm are plotted as a function of the reaction time in Figure S1B,C for kinetic analysis. The plot of ln([Rh B]/[Rh B]0) versus reaction time was linearly fitted with an R2 of 0.9913, which demonstrated that the Rh B decline followed a pseudo first-order kinetics, as confirmed by some photocatalytic and Fenton-like systems [48,54]. The negative slope of the fitted curve represents the observed reaction rate constant kobs, which was 5.14 × 10−4 s−1 for this experiment. The decline of Rh B with different initial H2O2 concentrations was recorded in Figure 10 together with the corresponding reaction rate constants. As expected, the degradation rate increased with increasing H2O2 concentration. The initial H2O2 concentration of 10 mM led to an efficient degradation with a rate constant of 5.14 × 10−4 s−1, which is a satisfactory value for subsequent activity evaluations. Certainly, larger reaction rates could be achieved with higher H2O2 concentrations, but considering the fact that H2O2 also acts as an HO⋅ scavenger (as shown in Equation (4)) that lowers the efficiency of using H2O2, exorbitant concentrations were not used [4]. The emphasis is not to seek for an ‘optimal’ H2O2 concentration, so [H2O2]0 was fixed at 10 mM for other experiments in this work.
H2O2 + HO·→ HO2·+ H2O

2.2.6. The Effect of the Catalyst Dose

The value of kobs describes the rate that a reaction happens, and this value is dependent on the dose of the catalyst, which means a larger amount of the catalyst leads to a larger reaction rate constant. In this work, different amounts of catalyst (0.4–1.0 g/L) were used to catalyze Rh B degradation to check the effect of the catalyst dose on the reaction rate. Considering the facts that surface reaction dominates in heterogeneous systems and surface area plays a significant role, it would be unfair to use mass concentration for intersystem comparisons, which does not give consideration to the particle size and shape. Thus, in this work, the catalyst dose is represented by surface area concentration (SA/V), which is the product of mass concentration (g/L) and specific surface area (m2/g) and has a unit of m−1. The concept of surface area concentration was first proposed by Matheson and Tratnyek as a general independent variable to correlate with the contaminants removal rate constants, as it incorporates most of the effects of grain size and shape [55], and now it is widely used in many heterogeneous systems for kinetic study [35,56,57,58,59]. One example for the unit conversion is exhibited in Equation (5), with a mass concentration of 1 g/L and the specific surface area of 146.6 m2/g for the Cu/Al2O3/CN composite.
S A V = 1   g / L × 146.6   m 2 / g = 146.6   m 2 / L = 146.6   m 2 / ( 10 − 3   m 3 ) = 1.466 × 10 5   m 2 / m 3 = 1.466 × 10 5   m − 1
Figure 11A depicts ln([Rh B]/[Rh B]0) as a function of time with different surface area concentrations (SA/V) of catalyst at 25 °C. All the ln([Rh B]/[Rh B]0) plots were linearly fitted with reaction time, reconfirming the pseudo first-order kinetics in all cases. The key parameters of the fitted curves are listed in Table 2, and kobs is plotted as a function of SA/V in Figure 11B. As shown, a linear fitted curve was achieved (R2 = 0.9948), demonstrating that kobs was linearly related to SA/V within certain limits, which is in accordance with previous reports [35,60,61]. The slope of this fitted line represents the specific reaction rate constant kSA, which is the normalization of kobs to SA/V (i.e., kSA = kobs normalized to SA/V) [62]. The units for kobs and SA/V are s−1 and m−1, respectively, so the unit for kSA should be m·s−1. The specific reaction rate constant kSA could be regarded as a general descriptor of reactivity of heterogeneous catalysts [55,62], and is calculated to be (5.9 ± 0.07) × 10−9 m·s−1 in this work. Judging from the kobs data obtained in this work, much more rapid degradation of Rh B is achieved in the presence of the proposed catalyst compared with many other conventional ones like CuO nanowires, CuO nanoparticles, CuO nanoflowers, and commercial CuO [63,64].

2.2.7. Activation Energy

Generally, the activation energy (Ea) of the reaction could be calculated by the Arrhenius equation: k o b s = k 1 · e − E a / R T , which can be integrated as
l n ( k o b s ) = − E a R T + l n ( k 1 )
where Ea, R, and T represent the activation energy for the reaction, the gas constant, and the absolute temperature, respectively [35]. To estimate the activation energy of the reaction, a series of experiments were conducted at different temperatures from 20 to 40 °C. The ln([Rh B]/[Rh B]0) was plotted as a function of time in Figure 12A to obtain kobs at different temperatures. The key parameters of the fitted curves were listed in Table 3, and ln(kobs) was plotted as a function of 1/T (K−1) in Figure 12B to obtain Ea.
As can be seen from Figure 12A, with the increase of temperature, the degradation rate of Rh B increased significantly. In addition, all the ln([Rh B]/[Rh B]0) plots were linearly fitted with reaction time, reconfirming the pseudo first-order kinetics at different temperatures. The ln(kobs) is linearly fitted with 1/T with an R2 of 0.9926 as shown in Figure 12B. According to the Arrhenius equation, the slope of this fitting curve equals to −Ea/R, and the activation energy Ea was calculated to be 71.0 kJ/mol. This value was lower than a series of other similar reactions, such as the degradation of Congo red by Fenton reagent (85.9 kJ/mol), the degradation of phenol by diamond supported gold nanoparticles (90.0 kJ/mol), and the iron oxide coating Fenton-like catalyst (96.9 kJ/mol) [65,66,67].

2.2.8. Recycling Experiments

In order to evaluate the stability of the Cu/Al2O3/CN composite, recycling experiments were performed. After each cycle, the suspension was filtered, washed, and dried to obtain the remaining composite. The recovered composite was subsequently subjected to a second cycle under the same reaction conditions. The leaching of copper species was measured at the end of each reaction cycle. The results are presented in Figure 13 and Table 4. Despite that the reaction rate constant decreased to some extent, efficient removal of Rh B was still achieved with a degradation ratio of 89.7% in 50 min after four cycles with a kobs value of 0.77 × 10−3 s−1, indicating that the catalytic property of the composite was efficient and long-lasting. The slight decline in catalytic performance may be attributed to the loss of Cu species in catalyst. The leaching of copper species increased from 0.16 mg/L in the first cycle to 0.55 mg/L in the fourth cycle, which shows an increasing trend. This could be regarded as a shortcoming of the proposed catalyst, and it also provides a direction for further improvement. It is remarkable that, despite the fact that copper leaching increases with cycle numbers, only 0.55 mg/L copper species were released to the solution even after four reaction cycles, i.e., copper leaching was remained at a very low level, which is far below the European standard of effluent (2 mg/L) [48].
In some systems the dissolved metal species could form certain complexes that are able to catalyze the decomposition of H2O2, thereby persistently generating HO· [36,68]. To determine the effect of dissolved copper species on Rh B degradation, the suspension containing Cu/Al2O3/CN composite was filtered at 40 min to remove catalyst particles, and the residual Rh B in the filtrate was monitored over time. As shown in Figure S2, negligible Rh B degradation was observed in the filtrate, indicating that the dissolved copper species had little effect on Rh B degradation, so only surface reaction dominates in the system containing the Cu/Al2O3/CN composite. With a tiny loss of copper species, the Cu/Al2O3/CN composite remains highly reactive and could efficiently catalyze the degradation of Rh B. The main features of the proposed Cu/Al2O3/CN composite and some other copper-based Fenton-like catalysts on Rh B degradation are listed in Table S1 [27,40,48,63,64,69,70,71,72,73]. The superiority of the Cu/Al2O3/CN composite in this work is supported by its high degradation rate constant, low copper leaching, and excellent applicability in neutral and alkaline conditions.

2.3. Catalytic Mechanism

In order to ascertain the Rh B degradation mechanism by the Cu/Al2O3/CN composite, both the reactive species and degradation products were identified in the Rh B degradation process. The reactive species were identified by a radical scavenger and ESR analysis, and HPLC-MS analysis was used to separate the degradation products and to identify each component. A tentative mechanism was proposed based on the abovementioned results.

2.3.1. Scavenging Experiments

Isopropanol (IPA) was employed as the HO· scavenger due to its rapid reaction rate with HO· (1.9 × 10−9 M−1·s−1) [38]. As shown in Figure 14A, by increasing the IPA concentration from 10 to 100 mM, the scavenging effect became more significant, which indicates that HO⋅ radical is the primary oxidation species in the catalytic reaction. This conclusion is also supported by the ESR experiment in which different radicals were trapped by DMPO to form different adducts that have different spectra [74,75]. As shown in Figure 14B, no ESR signal was observed in the absence of H2O2. With the addition of H2O2, a four-line ESR signal was observed with the relative intensity of 1:2:2:1, which is the characteristic spectrum of the DMPO-HO⋅ adduct. This result reconfirmed the existence of HO· as main reactive species. Additionally, the characteristic spectrum of DMPO-O2·− with a relative intensity of 1:1:1:1 was also observed in Figure 14C. O2·− may arise from the reaction between HO· and H2O2 as shown in Equation (4). Notably, the intensity of the DMPO-HO· signal reached maximum at 10 min, while the intensity of DMPO-O2·− signal continuously increased over time, which supports that the observed O2·− radicals were generated from HO· radicals.
Moreover, the effect of dissolved oxygen on Rh B degradation was checked by N2 bubbling. The Rh B solution was purged by N2 for 30 min before the reaction to dissipate dissolved oxygen. As shown in Figure 14A, the presence and absence of dissolved oxygen exerted no significant influences on Rh B degradation, indicating that oxygen was not the source of HO·, and the HO· radicals mainly arose from H2O2. In summary, HO· was identified as the major reactive species generated in the reaction, and the O2·− was generated via the reaction of HO· and H2O2.

2.3.2. HPLC-MS Analysis

Since HO· was confirmed as the main contributing reactive species, the Cu/Al2O3/CN system exhibited a universal degradation peroperty for other organic pollutants as expected. As shown in Figure S3, the degradation ratios of methyl orange (MO), Rh B, and methylene blue (MB) in 100 min were 55.3%, 96.4%, and 93.1%, respectively. Particularly, the degradation products of Rh B were identified by HPLC-MS analysis.
The HPLC chromatograms of samples extracted at different reaction times were recorded in Figure S4. The two main peaks at 14.7 and 16.9 min corresponded to N-de-ethylated intermediate and Rh B, respectively, which were confirmed by MS identification [76]. It is clear that the peak of Rh B decreased with time, which was accompanied with the formation and subsequent elimination of other peaks. This implies that some transformation products were formed in the degradation process, and these produced compounds were further degraded into smaller products.
Based on the results above and previous reports [76,77,78,79], the possible degradation pathways of Rh B are proposed in Figure 15, which could be mainly summarized as N-de-ethylation, chromophore cleavage, ring opening and mineralization process. The MS spectra of each product were shown in Figure S5. A compound with a m/z value of 443 was identified to be Rh B, and the intermediates with m/z values of 415 and 387 were identified as N, N-diethyl-N′-ethyl rhodamine (DER), and N, N-diethyl rhodamine (DR), respectively. The intermediates with the m/z value of 387 could also be N-ethyl-N′-ethyl rhodamine (EER), which is the isomer of DR and has an ethyl on each side. These intermediates prove that in the first stage Rh B was degraded through a stepwise N-de-ethylation pathway, which finally forms an intermediate with an m/z value of 318 [79]. At the same time, the de-carboxylation process occurs and forms the product with a m/z value of 274 [80], and the central carbon of Rh B was also attacked by HO⋅ radicals to form the hydroxylated product (m/z = 459) via the additive reaction [78]. There were two unidentified peaks with m/z values of 453 and 475, which could be regarded as the [M + H]+ and [M + Na]+ adducts of an unknown intermediate. To the best of our knowledge, no such pseudomolecule ion with m/z = 453 was previously reported in Rh B degradation, and we deemed that this compound might not exert a significant influence on the whole degradation pathway. Thus, it is marked unidentified in the present work. In the next stage, non-selective HO⋅ directly attacks the center carbon of these N-de-ethylated intermediates, leading to the cleavage of the conjugated xanthene structure [81]. The intermediates with m/z values of 340, 245, 230, and 223 were identified from this process [82,83,84,85]. After the chromophore cleavage process, the ring opening process occurs and forms smaller molecules. The ring opening products such as tricarballylic acid, 7-oxooctanoic acid, and propionic acid with m/z values of 177, 158, and 74 were identified [81]. In the final stage, these small molecules were possibly mineralized to CO2, H2O, NO3−, NH4+, etc.
Based on the results and analysis above, a tentative mechanism for the catalytic performance of the Cu/Al2O3/CN composite was proposed. Copper species were bonded to the Al2O3 framework, which facilitates the conversion of H2O2 to HO· by Fenton-like reaction and broadens the pH range for application. CN provides a large surface area and multiple functional groups to load Cu/Al2O3 composite by the formation of C−O−Cu bonds. In the presence of the Cu/Al2O3/CN composite, aqueous H2O2 could be easily adsorbed to the surface of CN, increasing its chance to contact with immobilized copper species to generate HO·. The catalytic process is surface predominant and the tiny leaching of copper species leads to an efficient and long-lasting catalytic property. O2·− is generated via the reaction of HO· and H2O2, and the highly reactive HO· attacks Rh B molecules, leading to the degradation of Rh B into smaller molecules.

3. Experimental

3.1. Materials

The chemicals used in the present work included aluminum nitrate nonahydrate (Al(NO3)3·9H2O, CAS[7784-27-2], 99%), copper(II) nitrate trihydrate (Cu(NO3)3·3H2O, CAS[10031-43-3], 99%), D(+)− glucose monohydrate (C6H12O6·H2O, CAS[14431-43-7], 99%), urea (CH4N2O, CAS[57-13-6], 99%), Rhodamine B (Rh B, C28H31ClN2O3, CAS[81-88-9], 99.7%), methylene blue trihydrate (MB, C16H16CIN3S·3H2O, CAS[7720-79-3], 82%), methyl orange (MO, C14H14N3NaO3S, CAS[547-58-0], 98.5%), hydrogen peroxide (H2O2, CAS[7722-84-1], 30 wt %), potassium iodide (KI, CAS[7681-11-0], 99%), acetic acid (C2H4O2, CAS[64-19-7], 99.5%), sodium acetate anhydrous (CH3COONa, CAS[127-09-3], 99%), isopropanol (IPA, C3H8O, CAS[67-63-0], 99.7%) (Sinopharm, Beijing, China), and ammonium molybdate (di) ((NH4)2Mo2O7, CAS[27546-07-2], Mo 56.5%, Macklin, Shanghai, China). All chemicals were used without further purification. All the solutions were prepared in deionized water.

3.2. Preparation of Composites

3.2.1. Preparation of Cu/Al2O3

Cu/Al2O3 composite was prepared via a hydrothermal method followed by a calcination process [40]. Typically, 7.5 g of Al(NO3)3·9H2O, 5.0 g of C6H12O6, 0.9 g of Cu(NO3)2·3H2O, and 60 mL of ultrapure water were added to a 100 mL breaker, and stirred at 25 °C for 1 h. The solution was transferred into a 100 mL Teflon-lined autoclave, then heated to 180 °C and kept for 20 h in an oven. After suction filtering, and water washing to neutral, the obtained product was added to an alumina crucible with a cover and then heated to 550 °C with a ramping rate of 5 °C/min in a muffle furnace and hold at 550 °C for 4 h. After naturally cooling down to room temperature, the product Cu/Al2O3 was obtained, and labeled as Cu12/Al2O3, where 12 represents the initial mass ratio of Cu(NO3)2·3H2O to Al(NO3)3·9H2O in percentage. Following the same procedure, a series of Cux/Al2O3 composites (x = 4, 7, 9, 12, and 15 wt %) were prepared in order to obtain the optimal Cu content.

3.2.2. Preparation of g-C3N4

Pure g-C3N4 was prepared following the thermal polymerization process reported in a previous study [35]. Typically, 50 g of urea was added to an alumina crucible with a cover and then heated to 550 °C with a ramping rate of 5 °C/min in a muffle furnace and hold at 550 °C for 3 h. After naturally cooling down to room temperature, the product g-C3N4 was obtained, and labeled as CN.

3.2.3. Preparation of Cu/Al2O3/CN

The Cu/Al2O3/CN composite was prepared using the samples obtained in Section 3.2.1 and Section 3.2.2. Typically, 0.1 g of the Cu12/Al2O3 composite and 0.1 g of CN were immersed in 100 mL ultrapure water. The suspension was dispersed by ultrasound for 30 min. After drying at 80 °C and grinding to powder, the product Cu/Al2O3/CN composite was obtained, and labeled as Cu12/Al2O3/CN1.3, where 1.3 represents the initial mass ratio of CN to Al(NO3)3·9H2O in percentage. Additionally, a series of Cu12/Al2O3/CNy composites (y = 0.7, 1.3, 2.0, and 2.7 wt %) were prepared by the same synthetic process to obtain the optimal CN content.

3.3. Characterization of Prepared Composites

The Fourier transform infrared (FT-IR) spectra of prepared samples were recorded by a Nicolet iS5 (Thermo Fisher Scientific, Waltham, MA., USA) FT-IR spectrometer with KBr pellets to confirm the functional groups of materials.
X-ray diffraction (XRD) patterns were recorded with D8 advance (Bruker, Karlsruhe, Germany) diffractometer using Bragg–Brentano geometry over the 2θ range of 10–70° with a Cu Kα irradiation (λ = 1.54 Å) to obtain the crystal structure of materials.
The morphology of the synthesized samples was visualized by a JET-2100F (JEOL, Akishima, Japan) transmission electron microscope (TEM).
X-ray photoelectron spectroscopy (XPS) data was recorded at 100 W power with a 20 eV pass energy on an ESCALAB 250Xi instrument (Thermo Fisher Scientific, Waltham, MA., USA).
The Brunauer–Emmett–Teller (B.E.T) method via isothermal adsorption and desorption of high purity nitrogen using a TriStar II 3020 (Micromeritics, Norcross, GA., USA) instrument was used to determine the specific surface area of synthesized samples.
The degradation intermediates were identified by a high performance liquid chromatography-mass spectrometry (HPLC-MS, thermo TSQ Quantum Ultra, Waltham, MA., USA). The samples were chromatographically separated by a 2.1 mm × 150 mm XB C18 column with a particle size of 3 μm (Welch, Shanghai, China) with acetonitrile/0.1% formic acid (8:92, v/v) as the mobile phase The flow rate was 0.3 mL/min and the injection volume was 20 μL. The eluent was detected by a diode array detector at the wavelength of 555 nm. The total ion current was recorded by quadrupole in a positive mode with electrospray ionization as ion source.
The concentration of the dissolved copper species was measured by a Prodigy 7 (Teledyne Leeman Labs, Hudson, NH., USA) inductively coupled plasma-optical emission spectrometer (ICP-OES).
The electron spin resonance (ESR) signals of radicals spin-trapped by 5,5-dimethyl-pyrroline-N-oxide (DMPO) were detected by a Bruker/A300 spectrometer (Bruker, Karlsruhe, Germany) with or without the addition of H2O2 in different air saturated methanol/aqueous dispersions.
The concentration of H2O2 and Rh B was measured by the UV-5500PC UV-visible spectrophotometer (METASH, Shanghai, China) and V-5600 spectrophotometer (METASH, Shanghai, China). The samples were weighted using a ME204E microbalance (Mettler Toledo, Shanghai, China). The pH of the solution was measured by Starter 2100 pH meter (Ohaus, Shanghai, China) with an accuracy of ± 0.01 pH units.

3.4. Catalytic Experiments

Rh B was selected to evaluate the catalytic performance of the synthesized samples. All experiments were performed in dark conditions. Typically, 0.1 g of sample was placed into 100 mL of 20 mg/L Rh B solution, and then the suspension was stirred continuously for 1 h prior to the addition of H2O2, in order to achieve Rh B adsorption/desorption equilibrium. Afterwards, predetermined amount of H2O2 was added in the suspension under constant stirring to trigger the reaction. At given time intervals, 4 mL samples were collected with a syringe and filtered using a Millipore filter (220 nm, Tianjin, China). The concentrations of Rh B and H2O2 in the filtrate were measured and plotted as a function of time.
The concentrations of Rh B and H2O2 in solutions were measured using a UV-visible spectrophotometer due to its convenience and easy operation. The produced intermediates may interfere the measurements, but the experimental error in the determination of Rh B and H2O2 concentration was less than 2%, which is acceptable for subsequent discussions. Generally, the absorbance of Rh B was measured at 555 nm in which Rh B had the maximum absorption. The concentration of H2O2 was determined by the Ghormley triiodide method reported in previous work [68]. The I− could be oxidized by H2O2 to form I3− in the presence of catalyst (NH4)2Mo2O7 (ADM). The I3− has the maximum absorption peak at the wavelength of 350 nm, thus the standard curve of the absorbance of I3− as a function of H2O2 concentration can be obtained.
Radical scavenger experiments were carried out to investigate the reactive oxygen species in the system. The scavenger IPA was added into the suspension after the adsorption/desorption equilibrium prior to the addition of H2O2. Additionally, Rh B solution was purged with N2 prior to the addition of composite to exclude O2.

4. Conclusions

An efficient Fenton-like Cu/Al2O3/CN composite was proposed in this work. The characterizations show that Cu species were immobilized on the Al2O3 framework in the form of Cu2+ and Cu+, and the introduction of CN increases its specific surface area and adsorption capacity for Rh B. The Cu/Al2O3/CN composite showed an excellent catalytic performance in a wide range of pH (4.9–11.0). The specific reaction rate constant of Rh B degradation was calculated as (5.9 ± 0.07) × 10−9 m·s−1, and the activation energy was calculated to be 71.0 kJ/mol. The recycling experiment demonstrated its durability for Rh B removal and proved that the degradation reaction was surface doninated, with a negligible leaching of copper species in solution. HO· and O2·− were both major reactive oxygen species generated in the reaction, and HO· was mainly responsible for Rh B degradation, which involves N-de-ethylation, hydroxylation, de-carboxylation, chromophore cleavage, ring opening, and the mineralization process. This work shows that Cu/Al2O3/CN composite is a promising Fenton-like catalyst with high activity and stability for the treatment of water pollution.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/3/317/s1, Figure S1: (A) [H2O2], (B) [Rh B]/[Rh B]0, and (C) ln ([Rh B]/ [Rh B]0) as a function of time in the presence of 1 g/L of the Cu/Al2O3/CN composite at room temperature. Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, pH 4.9 (unadjusted); Figure S2: [Rh B]/[Rh B]0 as a function of time in nonfiltered Cu/Al2O3/CN suspension and filtrates obtained at selected time intervals. Reaction conditions: [catalyst] = 1 g/L, [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, 25 °C, pH 4.9 (unadjusted); Figure S3: C/C0 as a function of time with different organic pollutants in the presence of 1 g/L of the Cu/Al2O3/CN composite. Reaction conditions: [dye] = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, 25 °C, pH 4.9 (unadjusted); Figure S4: HPLC chromatograms of samples extracted at different reaction times. Reaction conditions: [catalyst] = 1 g/L, [dye] = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, 25 °C, pH 4.9 (unadjusted); Figure S5: Mass spectrograms of the degradation products in sample extracted at 60 min in Rh B degradation.; Table S1: Comparison of the catalytic activities of the Cu/Al2O3/CN composite with other copper-based Fenton-like catalysts.

Author Contributions

M.Y. conceived the experiments; C.Z. and Z.L. designed and performed the experiments; L.F. and Y.G. analyzed the data; Y.F. contributed analysis tools; C.Z., Z.L. and M.Y. wrote the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by National Natural Science Foundation of China (21707108) and Independent Innovation Foundation of Wuhan University of Technology (20411057 and 20410962).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. FT-IR spectra of Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN samples.
Figure 1. FT-IR spectra of Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN samples.
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Figure 2. XRD patterns of the synthesized samples (a) Cu/Al2O3/CN, (b) Cu/Al2O3, (c) CN, and (d) Al2O3.
Figure 2. XRD patterns of the synthesized samples (a) Cu/Al2O3/CN, (b) Cu/Al2O3, (c) CN, and (d) Al2O3.
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Figure 3. TEM images of (A,B) Al2O3, (C,D) Cu/Al2O3, (E) CN, and (F) Cu/Al2O3/CN.
Figure 3. TEM images of (A,B) Al2O3, (C,D) Cu/Al2O3, (E) CN, and (F) Cu/Al2O3/CN.
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Figure 4. (A) C 1s, (B) N 1s, (C) Cu 2p, and (D) Al 2p XPS spectra of Cu/Al2O3/CN composite before reaction, and (E) Cu 2p spectrum of the Cu/Al2O3/CN composite after reaction.
Figure 4. (A) C 1s, (B) N 1s, (C) Cu 2p, and (D) Al 2p XPS spectra of Cu/Al2O3/CN composite before reaction, and (E) Cu 2p spectrum of the Cu/Al2O3/CN composite after reaction.
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Figure 5. N2 adsorption/desorption isotherm and the pore size distribution (insert) of the Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN.
Figure 5. N2 adsorption/desorption isotherm and the pore size distribution (insert) of the Al2O3, Cu/Al2O3, CN, and Cu/Al2O3/CN.
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Figure 6. [Rh B]/[Rh B]0 as a function of time in the presence of 1 g/L Cux/Al2O3 composite (x = 0, 4, 7, 9, 12, and 15 wt %). Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted).
Figure 6. [Rh B]/[Rh B]0 as a function of time in the presence of 1 g/L Cux/Al2O3 composite (x = 0, 4, 7, 9, 12, and 15 wt %). Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted).
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Figure 7. [Rh B]/[Rh B]0 as a function of time in the presence of 1 g/L Cu12/Al2O3/CNy composite (y = 0, 0.7, 1.3, 2.0, and 2.7 wt %). Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted).
Figure 7. [Rh B]/[Rh B]0 as a function of time in the presence of 1 g/L Cu12/Al2O3/CNy composite (y = 0, 0.7, 1.3, 2.0, and 2.7 wt %). Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted).
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Figure 8. [Rh B]/[Rh B]0 as a function of time in the presence of 1 g/L different samples. Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted).
Figure 8. [Rh B]/[Rh B]0 as a function of time in the presence of 1 g/L different samples. Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted).
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Figure 9. [Rh B]/[Rh B]0 as a function of time with the different pH conditions in the presence of 1 g/L of the Cu/Al2O3/CN composite. Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C.
Figure 9. [Rh B]/[Rh B]0 as a function of time with the different pH conditions in the presence of 1 g/L of the Cu/Al2O3/CN composite. Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C.
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Figure 10. [Rh B]/[Rh B]0 as a function of time and the corresponding reaction rate constants (insert) with different H2O2 concentrations in the presence of 1 g/L of the Cu/Al2O3/CN composite at room temperature. Reaction conditions: [Rh B]0 = 20 mg/L, V = 100 mL, pH 4.9 (unadjusted).
Figure 10. [Rh B]/[Rh B]0 as a function of time and the corresponding reaction rate constants (insert) with different H2O2 concentrations in the presence of 1 g/L of the Cu/Al2O3/CN composite at room temperature. Reaction conditions: [Rh B]0 = 20 mg/L, V = 100 mL, pH 4.9 (unadjusted).
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Figure 11. (A) ln([Rh B]/[Rh B]0) as a function of reaction time with different surface area concentrations (5.86−14.66 × 104 m−1). Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted). (B) kobs as a function of SA/V. The kobs values were obtained from Figure 11A.
Figure 11. (A) ln([Rh B]/[Rh B]0) as a function of reaction time with different surface area concentrations (5.86−14.66 × 104 m−1). Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, T = 25 °C, pH 4.9 (unadjusted). (B) kobs as a function of SA/V. The kobs values were obtained from Figure 11A.
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Figure 12. (A) ln([Rh B]/[Rh B]0) as a function of reaction time at different temperatures (20–40 °C) in the presence of 1 g/L of the Cu/Al2O3/CN composite. Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, pH 4.9 (unadjusted). (B) ln(kobs) as a function of 1/T. The kobs values are obtained from Figure 12A.
Figure 12. (A) ln([Rh B]/[Rh B]0) as a function of reaction time at different temperatures (20–40 °C) in the presence of 1 g/L of the Cu/Al2O3/CN composite. Reaction conditions: [Rh B]0 = 20 mg/L, [H2O2]0 = 10 mM, V = 100 mL, pH 4.9 (unadjusted). (B) ln(kobs) as a function of 1/T. The kobs values are obtained from Figure 12A.
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Figure 13. Recycling performance of the Cu/Al2O3/CN composite for Rh B degradation in the presence of 1 g/L of the Cu/Al2O3/CN composite. Reaction conditions: [Rh B]0 = 10 mg/L, [H2O2]0 = 10 mM, V = 100 mL, 25 °C, pH 4.9 (unadjusted).
Figure 13. Recycling performance of the Cu/Al2O3/CN composite for Rh B degradation in the presence of 1 g/L of the Cu/Al2O3/CN composite. Reaction conditions: [Rh B]0 = 10 mg/L, [H2O2]0 = 10 mM, V = 100 mL, 25 °C, pH 4.9 (unadjusted).
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Figure 14. (A) [Rh B]/[Rh B]0 as a function of time in the presence of different concentrations of IPA and N2 bubbling. ESR spectra of (B) DMPO-HO· adducts in aqueous solution and (C) DMPO-O2·− adducts in methanol solution recorded with the Cu/Al2O3/CN composite.
Figure 14. (A) [Rh B]/[Rh B]0 as a function of time in the presence of different concentrations of IPA and N2 bubbling. ESR spectra of (B) DMPO-HO· adducts in aqueous solution and (C) DMPO-O2·− adducts in methanol solution recorded with the Cu/Al2O3/CN composite.
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Figure 15. Illustration of the proposed Rh B degradation pathways.
Figure 15. Illustration of the proposed Rh B degradation pathways.
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Table
Table 1. XPS results of the Cu/Al2O3/CN composite.
Table 1. XPS results of the Cu/Al2O3/CN composite.
BE/eVChemical Bonds BE/eVChemical Bonds
C 1s281.5C−HN 1s395.1C−N−C
284.6C−C 395.7N−(C)3
285.1C−O 397.1C=N
Cu 2p932.0/931.7Cu+Al 2p74.1Al−O−Al
934.0/933.6Cu2+ 75.0Al−O−Cu
BE denotes the binding energy.
Table
Table 2. The key parameters of the fitted curves in Figure 11A.
Table 2. The key parameters of the fitted curves in Figure 11A.
Catalyst Dose
(g/L)		SA/V
(105 m−1)		kobs
(10−4 s−1)		Standard Deviation
(10−5 s−1)		R2
(%)
0.40.5861.330.31799.72
0.60.8792.730.59299.73
0.81.1724.630.85999.83
0.91.3195.440.40999.98
1.01.4666.471.04299.87
Table
Table 3. The key parameters of the fitted curves in Figure 12A.
Table 3. The key parameters of the fitted curves in Figure 12A.
Temperature (°C)kobs (10−3 s−1)Standard Deviation (10−5 s−1)R2 (%)
200.3580.68999.70
250.6471.04299.87
301.072.18499.71
351.572.08199.91
402.348.08199.53
Table
Table 4. kobs and Cu leaching in the recycling experiments.
Table 4. kobs and Cu leaching in the recycling experiments.
Recycling Cycles1st2nd3rd4th
Removal ratio96.8%95.5%91.8%89.7%
kobs (10−3 s−1)1.371.120.830.77
Cu leaching (mg/L)0.160.370.430.55

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Zhou, C.; Liu, Z.; Fang, L.; Guo, Y.; Feng, Y.; Yang, M. Kinetic and Mechanistic Study of Rhodamine B Degradation by H2O2 and Cu/Al2O3/g-C3N4 Composite. Catalysts 2020, 10, 317. https://doi.org/10.3390/catal10030317

AMA Style

Zhou C, Liu Z, Fang L, Guo Y, Feng Y, Yang M. Kinetic and Mechanistic Study of Rhodamine B Degradation by H2O2 and Cu/Al2O3/g-C3N4 Composite. Catalysts. 2020; 10(3):317. https://doi.org/10.3390/catal10030317

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Zhou, Chunsun, Zhongda Liu, Lijuan Fang, Yulian Guo, Yanpeng Feng, and Miao Yang. 2020. "Kinetic and Mechanistic Study of Rhodamine B Degradation by H2O2 and Cu/Al2O3/g-C3N4 Composite" Catalysts 10, no. 3: 317. https://doi.org/10.3390/catal10030317

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