Monolithic Mesh-Type Fe-Pd/γ-Al₂O₃/Al Bifunctional Catalysts for Electro-Fenton Degradation of Rhodamine B

Qianwen Shao, Guiru Zhang, Linhui Lu, Qi Zhang^*

Department of Chemical Engineering, East China University of Science and Technology, Shanghai, China.

DOI: 10.4236/mrc.2021.102004                

Abstract

A novel Fe-Pd bifunctional catalyst supported on mesh-type γ-Al₂O₃/Al was prepared and applied in the degradation of Rhodamine B (RhB). The monolithic mesh-type Fe-Pd/γ-Al₂O₃/Al bifunctional catalyst could be separated from the solution directly and could synthesize H₂O₂ in situ. The characterization results showed that Fe could improve the dispersion of Pd⁰, and the electronic interactions between Pd and Fe could increase the Pd⁰ contents on the catalyst, which increased the productivity of H₂O₂. Furthermore, DFT calculations proved that the addition of Fe could inhibit the dissociation of O₂ and promote the nondissociative hydrogenation of O₂ on the surface of Fe-Pd/γ-Al₂O₃/Al, which resulted in the increasement of H₂O₂ selectivity. Finally, the in-situ synthesized H₂O₂ by Pd was furtherly decomposed in situ by Fe to generateOH radicals to degrade organic pollutants. Therefore, Fe-Pd/ γ-Al₂O₃/Al catalysts exhibited excellent catalytic activity in the in-situ synthesis of H₂O₂ and the degradation of RhB due to the synergistic effects between Pd and Fe on the catalyst. It provided a new idea for the design of bifunctional electro-Fenton catalysts. Ten cycles of experiments showed that the catalytic activity of Fe-Pd/γ-Al₂O₃/Al catalyst could be maintained for a long time.

Keywords

Rhodamine B, Fe-Pd/γ-Al2O3/Al Catalyst, Electro-Fenton, Hydrogen Peroxide, Synergistic Effects

1. Introduction

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 [1] [2]. 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 [3], 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 [4] [5]. On the other side, a novel electro-Fenton process that can continuously synthesize H₂O₂ in situ has attracted great interests [6] [7]. 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 [8].

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 [14]. Many researchers were interested in adding a second metal to Pd catalysts to improve the selectivity of H₂O₂ [15] - [20]. 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 [15]. 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 [16]. 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 [17]. 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. Experimental

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 Fe_x/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 Fe_x-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) [21]. Perdew-Burke-Ernzerhof (PBE) was used for self-consistent description of exchange-correlation functions [22]. 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 [23]. 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 [24] and vibration frequency analysis.

The adsorption energy (E_ads) of the adsorbate on slab model surface was defined as

$E_{ads}=E_{ads/sub}-E_{ads}-E_{sub}$ (1)

where E_ads/sub, E_ads, and E_sub were the energies of adsorbed species stably adsorbed on the surface, adsorbed species, and a clean surface, respectively.

The reaction barrier (E_a) was defined as

$E_a=E_{TS}-E_{IS}$ (2)

where E_TS and E_IS 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 S_BET and V_p 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 [25]. 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 [17] [26]. 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 [27].

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) [28] [29]. 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 Fe_7.8Pd_1.9/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 [30]. 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 Fe_7.8Pd_1.9/γ-Al₂O₃/Al was higher than Fe_4.1Pd_1.9/γ-Al₂O₃/Al and Fe_12.1Pd_2.0/γ-Al₂O₃/Al, which was due to the highest Pd⁰ contents on Fe_7.8Pd_1.9/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 [31]. 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 Fe_7.8Pd_1.9/γ-Al₂O₃/Al was higher than Fe_4.1Pd_1.9/γ-Al₂O₃/Al and Fe_12.1Pd_2.0/γ-Al₂O₃/Al, which was due to the high H₂O₂ concentration generated by Fe_7.8Pd_1.9/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 [32] [33]. The O₂ nondissociative hydrogenation to OOH intermediate was generally considered the rate-determining step for H₂O₂ synthesis [34] [35]. 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 $\equiv$ 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 $\equiv$ Fe²⁺ and $\equiv$ 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 $\equiv$ Fe²⁺/ $\equiv$ 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 [36]. 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 [9]. 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.

4. Conclusion

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.

Acknowledgements

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.

Conflicts of Interest

The authors declare no conflicts of interest.

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