Advanced oxidation technology has been widely used in the degradation of organic pollutants in wastewater and the remediation of groundwater based on the generation of strong oxidizing •OH radicals  . Among the AOPs, Fenton oxidation is particularly effective for the treatment of organic wastewater that is difficult to degrade by biological or general chemical oxidation. However, in the traditional Fenton oxidation process, a large amount of iron sludge is produced, which causes secondary pollution , and the addition of hydrogen peroxide is needed, which is dangerous and expensive for the production, transportation and storage.
Heterogeneous Fenton oxidation can overcome the shortcomings of iron sludge, in which, the active components Fe or Cu are fixed in the structure of catalysts  . On the other side, a novel electro-Fenton process that can continuously synthesize H2O2 in situ has attracted great interests  . By combining heterogeneous Fenton reaction with electro-Fenton reaction, the H2O2 synthesized in situ can be simultaneously decomposed by Fe on the catalysts into •OH radicals, which finally degrading or even mineralizing organic contaminants.
Table 1 summarized the application of different heterogeneous electro-Fenton catalysts in organic pollutants degradation. It shows that there are two ways to synthesize H2O2 in situ: 1) H2O2 is synthesized from H2 and O2 under the catalysis of Pd; 2) H2O2 is synthesized from two-electron reduction of O2 at the cathode. Compared to the continuous pumping of O2 to the cathode, the use of electro-generated H2 and O2 was a safer and more controllable way to continuously synthesize H2O2 .
It was also noted that all catalysts were particles or powders. Although it was convenient for research, it was difficult to recycle in industrial applications. Therefore, how to structure the catalysts is a big problem for the heterogeneous electro-Fenton degradation of organic pollutants.
In addition, though H2O2 can be synthesized by Pd catalysts, the selectivity is
Table 1. List of heterogeneous electro-Fenton catalysts for organic pollutants degradation.
limited. In the process of in-situ synthesis of H2O2, the nondissociative hydrogenation of O2 on Pd surface is the main side reaction . Many researchers were interested in adding a second metal to Pd catalysts to improve the selectivity of H2O2  - . Suli Wang et al. demonstrated that the addition of Zn could modify the geometric structure of Pd catalysts, and the electronic interactions between Zn and Pd could improve the selectivity of H2O2 . Jun Li et al. compared the in-situ synthesis of H2O2 from H2 and O2 on the surface of Au-Pd(111) and Pd(111), and found that on the Au-Pd(111) surface, the main reactions for H2O2 synthesis exceeded all side reactions due to the co-adsorption of H atoms . Doudou Ding et al. reported that Sb could inhibit the oxidation of Pd, and increase the proportion of Pd monomer sites that were beneficial for H2O2 formation . Therefore, how to prepare an easy-to-recycle structured electro-Fenton catalyst with high H2O2 selectivity and removal efficiency should be an important research item.
In this research, a novel monolithic mesh-type Fe-Pd/γ-Al2O3/Al bifunctional catalyst was synthesized and applied in electro-Fenton degradation of Rhodamine B (RhB). Firstly, the geometric change of catalysts caused by Fe addition was studied by SEM, BET and XRD. Secondly, the synergistic effects between Pd and Fe on the in-situ synthesis of H2O2 and the decomposition of H2O2 to produce •OH radicals in situ were investigated by experiments and calculations. Based on the above research, a possible reaction mechanism for organic pollutants degradation by Fe-Pd/γ-Al2O3/Al was proposed. Finally, the durability of Fe-Pd/γ-Al2O3/Al catalyst was evaluated by ten cycles of experiments.
2.1. Catalyst Preparation
The γ-Al2O3/Al support was prepared by anodizing technology. First, the commercial Al mesh was washed with 10 wt% NaOH solution for 4 min, and washed with 10 wt% HNO3 solution for 2 min, and then washed with deionized water. Second, the washed Al mesh was put into a 0.4 mol/L oxalic acid solution at 25˚C and anodized for 10 hours, the current density was set to 25 A/m2. The anodized Al mesh was calcined at 350˚C for 1 h to form anodized aluminum oxide (AAO). Third, the AAO was hydrated in 80˚C deionized water for 1 h. Finally, the hydrated AAO was dried at room temperature followed by calcining at 500˚C for 4 h to form γ-Al2O3/Al support.
A series of Fe-Pd/γ-Al2O3/Al bimetallic catalysts with different Fe contents were synthesized via successive impregnation methods. First the γ-Al2O3/Al supports were impregnated in 0.05 mol/L ferric nitrate solution for different time, then dried and calcined at 500˚C for 4 h. Second, the prepared Fex/γ-Al2O3/Al catalysts were impregnated in 0.08 g/L palladium nitrate solution for 12 h, then dried and calcined at 550˚C for 4 h. Finally, the calcined catalysts were reduced in 0.05 mol/L sodium borohydride solution for 1 h. The resulting catalysts were denoted as Fex-Pd/γ-Al2O3/Al, where x is the contents of Fe.
2.2. Catalyst Characterization
The specific surface areas and pore size distribution of the support and different catalysts were measured by N2 adsorption and desorption method through ASAP 2020-M instrument (Micromeritics, USA). The actual contents of Pd and Fe on the catalysts were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Varian, USA). The phases of crystals on the catalysts were detected by X-ray diffraction (XRD) patterns using D/max 2250 VB/PC diffractometer (Rigaku, Japan). Scanning electron microscopy (SEM, JEM-6360LV, JEOL) was used to observe the cross-sectional morphology and the surface structure of catalysts. The chemical states of elements on different catalysts were elucidated by X-ray photoelectron spectroscopy (XPS) using ESCALAB 250Xi electron spectrometer (Thermo Scientific Corporation, USA). The calibration of binding energies was referred to C 1s peaks at 284.8 eV.
2.3. Catalyst Activity Test
Batch electrolytic experiments were carried out in a glass beaker with a capacity of 250 mL. As shown in Figure 1, two flakes of Pt (1 × 1 cm) were used as the cathode and anode. For each test, 250 mL solution containing 10 mg/L RhB and 0.1 M Na2SO4 were added into the glass beaker, 0.2 g (2.5 × 3 cm) mesh-type catalyst was suspended in the solution. The initial pH of the solution was adjusted to 2 by the addition of 0.1 M H2SO4 using a pH-meter. The beaker was put in a magnetic stirring water bath to maintain a certain temperature and stirring rate. A constant current was provided by a DC power supply (MN-3205D, 32V/5A). During the electrolysis, about 2 mL of electrolysed RhB solution was taken out from the beaker at regular time intervals, and the residual RhB concentration was measured by the ultraviolet-visible spectrophotometer (UV752, Shanghai YoKe Instrument Co., LTD) at 554 nm.
Figure 1. Schematic diagram of electro-Fenton degrading RhB.
2.4. DFT Method
DFT calculations were carried out by Vienna ab initio simulation package (VASP) . Perdew-Burke-Ernzerhof (PBE) was used for self-consistent description of exchange-correlation functions . The energy cutoff of plane wave expansion was set to 500 eV. The surface Brillouin zone was sampled with a 3 × 3 × 1 Monkhorst-Pack k-points grid mesh for all slabs . The interaction between adjacent slabs was eliminated by setting a 15 Å vacuum space in the vertical direction. The top layer of atoms and the adsorbates were relaxed, while the two layers of atoms in the bottom were fixed in corresponding positions. The transition states were searched by climbing image nudged elastic band (cNEB) method  and vibration frequency analysis.
The adsorption energy (Eads) of the adsorbate on slab model surface was defined as
where Eads/sub, Eads, and Esub were the energies of adsorbed species stably adsorbed on the surface, adsorbed species, and a clean surface, respectively.
The reaction barrier (Ea) was defined as
where ETS and EIS were the energies of transition states and initial states, respectively.
3. Results and Discussion
3.1. Characterization of Catalysts
Figure 2(a) shows the well-dense and ordered pore structure on the surface of AAO/Al. After hot water treatment and calcination, the surface of γ-Al2O3/Al became rough and formed a honeycomb porous structure (Figure 2(b)). For Pd/γ-Al2O3/Al (Figure 2(c)), there were obvious Pd particle aggregates covering the surface of γ-Al2O3/Al support. As for Fe-Pd/γ-Al2O3/Al catalyst (Figure 2(d)), there were no obvious Pd particles on the surface, indicating that adding Fe could reduce the particle size and improve the dispersion of Pd.
Table 2 shows textural properties of the support and catalysts. It was found that the specific surface area of AAO/Al was only 8 m2/g and the average pore diameter was 12.6 nm. After hot water treatment and calcination, the specific surface area of γ-Al2O3/Al was increased to 127 m2/g and the average pore diameter was decreased to 5.19 nm. It confirmed that the support had a high specific surface area. After loading Pd or Fe on the support, both the specific surface areas and pore volume reduced due to pore blockage. However, compared to Pd/γ-Al2O3/Al, the SBET and Vp of Fe-Pd/γ-Al2O3/Alslightly increased, this change could be explained as the addition of Fe promoted the dispersion of Pd.
The crystal phases of the support and different catalysts were detected by XRD measurements. From Figure 3, it can be seen that the characteristic diffraction peaks of γ-Al2O3/Al at 2θ = 45.8˚ and 67.0˚ were present in all catalysts. For Pd/γ-Al2O3/Al catalyst, there was an additional diffraction peak at 40.1˚ corresponding to (111) crystal plane of metallic Pd0, which was benefit for H2O2 generation . Meanwhile, it can be found that different from Pd/γ-Al2O3/Al, the intensity of the characteristic diffraction peak for Pd (111) in Fe-Pd/γ-Al2O3/Algets weak, which can be attributed to the interaction between Pd and Fe, that is, doping with Fe can improve the dispersion of Pd  . However, no diffraction peaks of iron oxide could be detected in the patterns of Fe-Pd/γ-Al2O3/Al, it
Figure 2. The SEM images of AAO/Al (a); γ-Al2O3/Al (b); Pd/γ-Al2O3/Al (c) and Fe-Pd/γ-Al2O3/Al (d).
Figure 3. XRD patterns of support γ-Al2O3/Al and different catalysts.
Table 2. Chemical compositions and textural properties of all catalysts.
could be explained that the particle size of iron oxide was very small or amorphous .
The chemical states of Pd and Fe on different catalysts were identified by XPS. Figure 4 shows the Fe 2p and Pd 3d XPS spectra of different catalysts. From Figure 4(a), it can be seen that the XPS spectra of Fe 2p can be deconvoluted into Fe2+ (red line) and Fe3+ (blue line) peaks, and compared with Fe/γ-Al2O3/Al, the binding energy of Fe 2p on Fe-Pd/γ-Al2O3/Al catalysts shifts toward high values. At the same time, the XPS spectra of Pd 3d in Figure 4(b) can also be divided into Pd0 (red line) and Pd2+ (blue line) peaks. Different from the chemical shift of Fe 2p, the binding energy of Pd 3d on Fe-Pd/γ-Al2O3/Al catalysts shifted toward low values compared with Pd/γ-Al2O3/Al. The differences in chemical shift of Pd and Fe were due to the transfer of electrons from Fe to Pd, which was consistent with the higher electronegativity of Pd (2.2) than that of Fe (1.8)  . To visually observe the Pd and Fe contents on different catalysts, Table 3 summarized the relative contents of Pd and Fe on different catalysts, which were calculated from the curve fitting in XPS spectra. The data show that Fe7.8Pd1.9/γ-Al2O3/Al catalyst has the highest Pd0 contents, it confirms that there is an optimal ratio between Pd and Fe.
3.2. Application of Fe-Pd/γ-Al2O3/Al Catalysts in Electro-Fenton Reaction
The activity of catalysts was evaluated by RhB degradation. For comparison, Fe/γ-Al2O3/Al catalyst was used to degrade RhB by Fenton-like reaction without electricity. As shown in Figure 5(a), H2O2 plays an important role in the Fenton-like system. Without H2O2, RhB would not be degraded by Fe/γ-Al2O3/Al catalyst. With the increase of H2O2 concentrations, the degradation rate of RhB was also increased, and when the concentration of H2O2 reached 1200 ppm, 80% of RhB was degraded within 2 hours. However, in the electro-Fenton system, H2O2 was not needed. Figure 5(b) discussed the effects of electric currents on the degradation of RhB. The result showed that the degradation rate of RhB increased with the electric currents increasing. When the electric current was 50 mA, 98% of RhB was degraded within 2 hours. Figure 5(c) exhibited that as the
Table 3. Fe and Pd concentrations of different catalysts.
Figure 4. XPS spectra of Fe 2p (a) and Pd 3d (b) for different catalysts.
pH value decreased, RhB degradation increased significantly, and when the pH value was 2, 96% of RhB could be degraded within 1 hours.
In the meantime, in order to verify the synergistic effects between Pd and Fe, different catalysts that were Pd/γ-Al2O3/Al, Fe/γ-Al2O3/Al, the mixture of Pd/γ-Al2O3/Al and Fe/γ-Al2O3/Al, and Fe-Pd/γ-Al2O3/Al were applied in electro-Fenton degradation of RhB. Figure 5(d) shows that the degradation rate of
Figure 5. Degradation of RhB under different conditions. (a) Fenton-like reaction without electricity under different H2O2 concentration; (b) electro-Fenton reaction under different electric currents; (c) electro-Fenton reaction under different pH; (d) electro-Fenton degradation RhB over different catalysts. Unless otherwise specified, the reactions were carried out under the conditions of 250 mL of 10 mg/L initial RhB, pH 2, 50 mA, 0.8 g/L catalyst, 25˚C, 300 rpm.
RhB on monometallic Pd/γ-Al2O3/Al and Fe/γ-Al2O3/Al catalysts is very low, which indicates that the catalytic activity of monometallic catalysts is very small. In addition, the mixture of Fe/γ-Al2O3/Al and Pd/γ-Al2O3/Al was used, and the total amounts were equal to Fe-Pd/γ-Al2O3/Al. The results show that the degradation rate of RhB by Pd/γ-Al2O3/Al and Fe/γ-Al2O3/Al mixture is much lower than that by Fe-Pd/γ-Al2O3/Al. It indicated that the synergistic effects between Pd and Fe really existed.
To verify H2O2 was synthesized in situ in this electro-Fenton system, the concentration of H2O2 synthesized in water was measured at 400 nm after coloring with potassium titanium oxalate . Figure 6(a) shows that without catalyst, almost no H2O2 is formed, indicating that the amount of H2O2 synthesized by O2 reduction in this system is very small. Interestingly, the concentration of H2O2 synthesized by the mixture of Fe/γ-Al2O3/Al and Pd/γ-Al2O3/Al increased with time, while the concentration of H2O2 synthesized by Fe-Pd/γ-Al2O3/Al had a decreasing process. It was because the H2O2 synthesized in situ by Fe-Pd/ γ-Al2O3/Al could be immediately decomposed by Fe which was on the catalyst. In addition, the concentration of H2O2 synthesized by Fe7.8Pd1.9/γ-Al2O3/Al was higher than Fe4.1Pd1.9/γ-Al2O3/Al and Fe12.1Pd2.0/γ-Al2O3/Al, which was due to the highest Pd0 contents on Fe7.8Pd1.9/γ-Al2O3/Al catalyst.
In order to test the generation of •OH radicals, electro-Fenton degradation of
Figure 6. In situ generation of H2O2 (a), changes in absorbance at 300 nm (b) over different catalysts. The reactions were carried out under the conditions of 250 mL solution, pH 2, 50 mA, 0.8 g/L catalyst, 25˚C, 300 rpm.
100 mg/L of benzoic acid was carried out. Since benzoic acid could be decomposed by •OH radicals to produce salicylic acid, the generation of salicylic acid could reflect the generation of •OH radicals. The formation of salicylic acid could be measured at 300 nm absorbance . Figure 6(b) shows that the increase rate of •OH radicals generated by Fe-Pd/γ-Al2O3/Al is far higher than that of by the mixture of Fe/γ-Al2O3/Al and Pd/γ-Al2O3/Al, which corresponds to the change in H2O2 concentration and the difference in catalytic activity. Similarly, the concentration of •OH radicals generated by Fe7.8Pd1.9/γ-Al2O3/Al was higher than Fe4.1Pd1.9/γ-Al2O3/Al and Fe12.1Pd2.0/γ-Al2O3/Al, which was due to the high H2O2 concentration generated by Fe7.8Pd1.9/γ-Al2O3/Al.
The above experimental results indicated that by combining Pd in-situ synthesis of H2O2 and Fe decomposition of H2O2 to generate •OH radicals, the organic pollutants in the solution could be effectively removed.
DFT calculations were further used to discuss the effects of Fe on the selectivity of H2O2 synthesized by Pd. As mentioned above, the selectivity of H2O2 depended on the competition between the nondissociative hydrogenation reaction and the dissociation reaction of O-O bond species on the catalyst surface  . The O2 nondissociative hydrogenation to OOH intermediate was generally considered the rate-determining step for H2O2 synthesis  . First, the dissociation of O2 on the surface of Fe-Pd/γ-Al2O3/Al and Pd/γ-Al2O3/Al was investigated. From Figure 7(a), it can be seen that the reaction barrier of O2 dissociation on the Fe-Pd/γ-Al2O3/Al surface is 1.47 eV, which is much higher than 0.66 eV on the surface of Pd/γ-Al2O3/Al. It indicated that the O2 dissociation on the Fe-Pd/γ-Al2O3/Al surface was relatively difficult. After that, we calculated the nondissociative hydrogenation energy barrier of O2 on the surface of Fe-Pd/ γ-Al2O3/Al and Pd/γ-Al2O3/Al. Figure 7(b) shows that the reaction barrier of O2nondissociative hydrogenation on Fe-Pd/γ-Al2O3/Al surface is 0.44 eV, lower than 0.56 eV on the surface of Pd/γ-Al2O3/Al. It meant that the nondissociative hydrogenation reaction of O2 was more likely to occur on the Fe-Pd/γ-Al2O3/Al surface than the dissociation reaction. Based on the DFT calculation results, we can infer that adding Fe can improve the selectivity of H2O2.
Figure 7. Energy diagram for (a) the dissociation of O2 and (b) the non-dissociative hydrogenation of O2 on the surface of Fe-Pd/γ-Al2O3/Al (red line) and Pd/γ-Al2O3/Al (blue line).
3.3. Possible Mechanisms of Fe-Pd/γ-Al2O3/Al Electrocatalytic System
Radicals scavenging experiments were carried out to determine the contribution of •OH radicals to RhB degradation. Isopropanol was widely used to scavenge •OH radicals. From Figure 8, it can be seen that after adding 100 mM isopropanol to the reaction system, the RhB degradation rate is remarkably inhibited, which validated that •OH radicals were the main active oxygen species that degrade RhB in the electro-Fenton system.
Therefore, according to the above experimental results, a possible reaction mechanism of Fe-Pd/γ-Al2O3/Al catalysts for organic pollutants degradation was proposed, as shown in Figure 9. First, H2 and O2 were produced by the electrolysis of water, the produced H2 and O2 adsorbed on Fe-Pd/γ-Al2O3/Al surface, then H2O2 was synthesized under the catalysis of Pd. In H2O2 synthesis step, Fe could improve the selectivity of H2O2, due to the addition of Fe could increase the contents and dispersion of Pd0, also inhibit the dissociation of O2 and promote the nondissociative hydrogenation of O2. Finally, the in-situ synthesized H2O2 was decomposed by Fe2+ on the catalyst to produce •OH radicals, which ultimately degraded organic pollutants into CO2 and other by-products. At the same time, comparing the content of Fe2+ and Fe3+ on Fe-Pd/γ-Al2O3/Al catalysts before and after reaction (56.8% and 43.1%, 55.7% and 44.3%, respectively), it was found that there was no obvious change. It confirmed that the electronic interactions between Pd and Fe could promote the circulation of Fe2+/ Fe3+ on the catalyst.
3.4. Durability of Fe-Pd/γ-Al2O3/Al Catalyst
The reusability of Fe-Pd/γ-Al2O3/Al catalyst was tested through ten RhB degradation experiments. After each cycle experiment, fresh RhB was added into the beaker to make the initial concentration reach 10 mg/L. Figure 10 shows that the degradation rate of RhB can be maintained above 90% in each cycle experiment, indicating that the catalyst can be reused for a long time.
The concentration of leached Fe ions during the electrolysis was measured,
Figure 8. Effects of radical scavenging agent on RhB degradation. Unless otherwise specified, the reactions were carried out under the conditions of 250 mL of 10 mg/L initial RhB, pH 2, 50 mA, 0.8 g/L catalyst, 25˚C, 300 rpm.
Figure 9. Proposed mechanism of Fe-Pd/γ-Al2O3/Al electrocatalytic system to degrade organic pollutants.
and the results were shown in Figure 11. The concentration of leached Fe2+ was measured at 510 nm using the modified 1,10-phenanthroline method, and the concentration of leached Fe3+ was measured at 525 nm by generating ferric-salicylic acid complexes . It can be seen that the concentration of leached Fe3+ increases with time and reaches a plateau value of about 1.6 mg/L after 60min, while the concentration of leached Fe2+ increases with time, reaches the maximum at 90min, and then decreases with time. The fluctuation was mainly due to the homogeneous Fenton oxidation. Generally, in the process of homogeneous electro-Fenton reaction, the added or leached Fe2+ concentration was greater than 10 mg/L . But the lower concentration of leached Fe2+ and Fe3+ in this electro-Fenton system indicated that the degradation of RhB by
Figure 10. Durability of Fe-Pd/γ-Al2O3/Al. Unless otherwise specified, the reactions were carried out under the conditions of 250 mL of 10 mg/L initial RhB, pH 2, 50 mA, 0.8 g/L catalyst, 25˚C, 300 rpm.
Figure 11. Accumulation of leached Fe2+ and Fe3+ ions during electrolysis. Unless otherwise specified, the reactions were carried out under the conditions of 250 mL of 10 mg/L initial RhB, pH 2, 50 mA, 0.8 g/L catalyst, 25˚C, 300 rpm.
Fe-Pd/γ-Al2O3/Al was mainly due to the Fe ions on the catalyst.
A novel monolithic mesh-type Fe-Pd/γ-Al2O3/Al bifunctional catalyst was synthesized and applied in electro-Fenton degradation of RhB. The Fe-Pd/γ-Al2O3/Al catalyst showed the high catalytic activity in the in-situ synthesis of H2O2 and the degradation of RhB. The addition of Fe could promote the dispersion of Pd, and due to the electronic interactions between Pd and Fe, electrons were transferred from Fe to Pd. Therefore, the dispersion and content of Pd0 on the Fe-Pd/ γ-Al2O3/Al catalyst increased, which could provide more active sites for the synthesis of H2O2. The addition of Fe could increase the reaction barrier of O2 dissociation, and reduce the reaction barrier of O2 nondissociative hydrogenation on the surface of Pd, which increased the selectivity of H2O2. Subsequently, the H2O2 synthesized in situ was immediately decomposed by Fe on the Fe-Pd/γ-Al2O3/Al catalyst to generate •OH radicals, which ultimately degraded RhB. For ten cycles of RhB degradation experiments, Fe-Pd/γ-Al2O3/Al catalyst showed good catalytic activity. However, the productivity of H2O2 and the durability of the catalyst needed further study.
This work was financially supported by the National Key Research and Development Project (Grant NO. 2018YFB0105603) and the Shandong Greenwater Environmental Protection Technology Co., Ltd.
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