Monolithic Mesh-Type Fe-Pd/ γ -Al 2 O 3 /Al Bifunctional Catalysts for Electro-Fenton Degradation of Rhodamine B

A novel Fe-Pd bifunctional catalyst supported on mesh-type γ-Al 2 O 3 /Al was prepared and applied in the degradation of Rhodamine B (RhB). The monolithic mesh-type Fe-Pd/γ-Al 2 O 3 /Al bifunctional catalyst could be separated from the solution directly and could synthesize H 2 O 2 in situ. The characterization results showed that Fe could improve the dispersion of Pd 0 , and the electronic interactions between Pd and Fe could increase the Pd 0 contents on the catalyst, which increased the productivity of H 2 O 2 . Furthermore, DFT calculations proved that the addition of Fe could inhibit the dissociation of O 2 and promote the nondissociative hydrogenation of O 2 on the surface of Fe-Pd/γ-Al 2 O 3 /Al, which resulted in the increasement of H 2 O 2 selectivity. Finally, the in-situ synthesized H 2 O 2 by Pd was furtherly decomposed in situ by Fe to generate •OH radicals to degrade organic pollutants. Therefore, Fe -Pd/ γ-Al 2 O 3 /Al catalysts exhibited excellent catalytic activity in the in-situ synthesis of H 2 O 2 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-Al 2 O 3 /Al catalyst could be maintained for a long time.


Introduction
Advanced oxidation technology has been widely used in the degradation of organic pollutants in wastewater and the remediation of groundwater based on the 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 H 2 O 2 in situ has attracted great interests [6] [7]. By combining heterogeneous Fenton reaction with electro-Fenton reaction, the H 2 O 2 synthesized in situ can be simultaneously decomposed by Fe on the catalysts into •OH radicals, which finally degrading or even mineralizing organic contaminants.  [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 H 2 O 2 can be synthesized by Pd catalysts, the selectivity is

Catalyst Characterization
The specific surface areas and pore size distribution of the support and different catalysts were measured by N 2 adsorption and desorption method through The calibration of binding energies was referred to C 1s peaks at 284.8 eV.

Catalyst Activity Test
Batch electrolytic experiments were carried out in a glass beaker with a capacity of 250 mL. As shown in Figure

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.
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 where E TS and E IS were the energies of transition states and initial states, respectively.     could be explained that the particle size of iron oxide was very small or amorphous [27].

Characterization of Catalysts
The chemical states of Pd and Fe on different catalysts were identified by XPS. [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 7.8 Pd 1.9 /γ-Al 2 O 3 /Al catalyst has the highest Pd 0 contents, it confirms that there is an optimal ratio between Pd and Fe.

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-Al 2 O 3 /Al catalyst was used to degrade RhB by Fenton-like reaction without electricity. As shown in Figure 5(     To verify H 2 O 2 was synthesized in situ in this electro-Fenton system, the concentration of H 2 O 2 synthesized in water was measured at 400 nm after coloring with potassium titanium oxalate [30]. Figure 6(

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-Al 2 O 3 /Al catalysts for organic pollutants degradation was proposed, as shown in Figure 9.

Durability of Fe-Pd/γ-Al2O3/Al Catalyst
The reusability of Fe-Pd/γ-Al 2 O 3 /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,   and the results were shown in Figure 11. The concentration of leached Fe 2+ was measured at 510 nm using the modified 1,10-phenanthroline method, and the concentration of leached Fe 3+ was measured at 525 nm by generating ferric-salicylic acid complexes [36]. It can be seen that the concentration of leached