Effective hydrodechlorination of 4-chlorophenol catalysed by magnetic palladium/reduced graphene oxide under mild conditions

Abstract

Magnetic palladium/reduced graphene oxide nanocomposites (Pd/rGO-Fe₃O₄) were prepared by depositing Pd nanoparticles on an rGO-Fe₃O₄ magnetic support. The as-prepared nanocomposites were investigated as catalysts for the hydrodechlorination of 4-chlorophenol in the aqueous phase. The complete conversion of 4-chlorophenol (with a concentration as high as 2.5 g L⁻¹) to phenol was obtained in a reaction time of 40 min at room temperature and balloon hydrogen pressure without any additives. The excellent catalytic activity of the Pd/rGO-Fe₃O₄ can be attributed to the small particle size of Pd, and an electron-deficient state of Pd in the catalyst as a result of the strong interaction between the active sites and the oxygen of the oxygen-containing groups on the rGO-Fe₃O₄ support.

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1. Introduction

Chlorinated phenols are valuable chemicals with wide applications in the fields of agriculture and industry. However, residual chlorinated phenols show significant associations with adverse effects on the environment and human beings, due to their high toxicity and resistance to biodegradation.¹ Among the technologies for detoxifying these poisonous pollutants, catalytic hydrodechlorination (HDC) has attracted great attention because this method is known to be highly efficient, occur under relatively mild conditions and yield less-toxic products such as phenol, cyclohexanol, and cyclohexanone. In particular, the HDC of chlorinated phenols catalyzed by supported noble metal catalysts, through the scission of carbon–chlorine bonds on the metal surface using molecular hydrogen as a hydrogen source, is among the most investigated in recent years. When in the metallic phase, palladium has been the subject of intensive research because of its special activity in the HDC reaction.² However, the HDC activity is always prohibited by severe catalyst deactivation, caused by the surface adsorption of chlorine from the carbon–chlorine bonds³ and the aggregation of the active Pd particles.^4,5 To overcome these drawbacks, a large amount of additives, such as bases, were introduced into the reaction systems, meaning extra work is needed to separate the Pd particles from the reaction mixture in order to avoid secondary pollution. Up to now, it has still remained a tremendous challenge to seek stable and recyclable catalysts for detoxifying chlorinated phenols under mild conditions without any additives.

Besides the catalytic nature of the metal nanoparticles (NPs) themselves, it has been proposed that the support also plays a critical role in enhancing the catalytic performance of the metal catalyst. Bearing this in mind, a variety of supports including ZrO₂,⁶ Al₂O₃,^7–11 and carbon^4,5,11–16 have been investigated and applied in the detoxification of wastewater containing chlorinated phenols. Among these supports, metal oxides can improve the stability of the active sites of the catalysts. Nevertheless, they are more sensitive to the HCl produced from the HDC reaction, resulting in a loss of catalytic activity. Comparatively, carbon materials as supports are more resistant to acid and thus have provoked extensive interest from researchers in assessing their usefulness for the catalytic decomposition of chlorophenols. Activated carbon (AC), as a typical carbon carrier, has been demonstrated to have a strong ability to uniformly distribute the metal active sites, due to its high surface area and porosity. However, the application of AC as a support in the HDC reaction, depending on its pore size distribution and surface composition, may cause the adsorption of the substrate as well, which results in poor recycling properties. Several additives were also employed to assist these carbon materials in achieving a better dispersion of the metal NPs and thus a better catalytic performance. For example, by modifying the carbon nanofibers with a pyridinic basic species via the treatment of carbon nanofibers in a stream of NH₃ gas, Liu et al. successfully improved the catalytic activity and stability of Pd NPs on carbon nanofibers for the HDC of chlorophenol. However, very high temperature is required to introduce such basic species.¹⁷ Afterwards, Jin and co-workers reported a Pd/MSCN (mesoporous silica–carbon nano-composite) catalyst which exhibited pronounced activity for the catalytic HDC of 4-CP at room temperature under ordinary hydrogen, but this used triethylamine (Et₃N) as a base additive.¹⁸ Therefore, it is a big challenge to explore appropriate supports with a good stability in an acidic environment and a strong capability to interact with metal catalysts for the recyclable conversion of chlorinated phenols, under mild conditions and without additives.

In a previous work, we found that the catalyst Pd/rGO exhibited an excellent performance for the HDC of 4-chlorophenol (4-CP).¹⁹ However, lengthy centrifugation is required to separate the catalyst. To this end, we prepared a magnetic rGO-supported palladium catalyst and applied it for the catalytic HDC of 4-CP. The as-prepared catalyst showed an excellent catalytic performance with a yield of phenol as the sole product. The catalyst can be isolated from the reaction mixture by an external magnet and can be reused four times without any obvious loss of activity.

2. Experimental

2.1 Materials

Na₂PdCl₄ was purchased from the Kunming Institute of Precious Metals, China. FeCl₂·4H₂O, FeCl₃·6H₂O and graphite powder were provided by Chengdu Kelong Chemical Reagent Factory. Other chemicals and solvents (analytical grade reagents) were supplied from the Aladdin Industrial Corporation, China, and were used as received without further purification.

2.2 Catalyst preparation

The GO used in this procedure was synthesized by the oxidation of graphite, according to the improved Hummer's method.²⁰ For the synthesis of rGO-Fe₃O₄, FeCl₂·4H₂O (411 mg) and FeCl₃·6H₂O (1118 mg) were dissolved in deionized water (25 mL). Then, the as-prepared mixture was transferred into a yellow-brown suspension of GO, which had been firstly prepared through the addition of GO (100 mg) into deionized water (75 mL) under ultrasonication. Immediately, the pH value of the mixture was adjusted to 11 by adding ammonia, and this was followed by the reduction of the mixture at 363 K for 4 h using hydrazine hydrate (4 mL) as a reducing agent. After cooling down to room temperature, the black solid was washed with water and ethanol several times, and then dried under vacuum at 343 K for 12 h, to yield the rGO-Fe₃O₄.

The rGO-Fe₃O₄-stabilized Pd NPs (Pd/rGO-Fe₃O₄) were fabricated as follows: the desired amount of rGO-Fe₃O₄ (100 mg) and an aqueous solution of Na₂PdCl₄ (2.78 mg mL⁻¹, 5 mL) were mixed in a 50 mL round-bottomed flask under ultrasonication for 30 min. Afterwards, the flask was put into an ice-water bath, and an aqueous solution of NaBH₄ (5.6 mg mL⁻¹, 5 mL) was slowly added into the mixture under vigorous stirring. After stirring for 4 h, the black solid produced was collected with a magnet, washed with deionized water several times, and finally dried under vacuum at 343 K for 12 h. (The content of Pd was 5.0 wt%, estimated by inductively coupled plasma spectroscopy (ICP)).

2.3 Catalyst characterization

The morphology and particle sizes of the catalysts were characterized using transmission electron microscopy (TEM), which was carried out on a JEOL model 2010 instrument operated at an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku X-ray diffractometer D/max-2200/PC equipped with Cu Kα radiation (40 kV, 20 mA). The samples were scanned over the range of 10–90°. X-ray photoelectron spectroscopy (XPS, Kratos XSAM800) spectra were obtained using Al Kα radiation (12 kV and 15 mA) as an excitation source (hν = 1486.6 eV) and Au (BE Au4f = 84.0 eV) and Ag (BE Ag3d = 386.3 eV) as references. All binding energy (BE) values were referenced to the C1s peak of contaminant carbon at 284.6 eV.

2.4 Activity test

The HDC of 4-CP was chosen as a model reaction, to estimate the catalytic properties of the Pd/rGO-Fe₃O₄ nanocomposites. Typically, the HDC reaction was conducted in a two-necked round-bottomed flask equipped with a hydrogen balloon. The reaction temperature was controlled by a thermal controller. Specifically, an appropriate amount of catalyst (ca. 5.0 mg) and an aqueous solution of 4-CP (ca. 2.5 g L⁻¹, 5.0 mL) were transferred into the flask, followed by the treatment of the reactor under vacuum and the charging of it with highly pure hydrogen. After that, the reactor was put into a water bath. The stirring rate was adjusted to 1200 rpm and the reaction time was noted when the setting reaction temperature was achieved. To monitor the process of the HDC reaction, samples were periodically withdrawn from the reactor with a syringe. The removal efficiency of Pd/rGO-Fe₃O₄ for the 4-CP was analyzed by GC (Agilent 7890A) with a FID detector and a PEG-20M capillary column (30 m × 0.25 mm, 0.25 μm film), and nitrogen was used as a carrier gas.

3. Results and discussion

3.1 Characterization of Pd/RGO-Fe₃O₄

The particle sizes and morphologies of the rGO-Fe₃O₄ and Pd/rGO-Fe₃O₄ nanocomposites were characterized by TEM, and the results are illustrated in Fig. 1. As shown in Fig. 1a and c, the Fe₃O₄ NPs have been successfully immobilized on the surface of the rGO with various kinds of shapes, such as spheres and quadrilaterals. The particle sizes of the spheres were in a range from about 4 to 15 nm, and the average edge length of the quadrilaterals was about 15 nm. From the EDX result, it can be seen that carbon, oxygen and Fe were detected. During the process of depositing the Pd NPs on the rGO-Fe₃O₄, the smaller spherical Fe₃O₄ particles disappeared and were replaced by larger ones, because of the aggregation of particles during the reduction of the Pd precursor. Interestingly, the Pd NPs were well dispersed, not only on the surface of the rGO but also on the Fe₃O₄, with an average particle size of 1.8 nm and a narrow size distribution of 1.1–2.8 nm (Fig. 1b and d). The EDX characterization showed that the Pd NPs were successfully deposited on the surface of the rGO-Fe₃O₄ through the reduction of Pd²⁺ using NaBH₄ as a reducing agent, which was confirmed by elemental mapping (Fig. 1g).

Fig. 1 TEM images of rGO-Fe₃O₄ (a and c) and Pd/rGO-Fe₃O₄ (b and d); EDX spectra of rGO-Fe₃O₄ (e) and Pd/rGO-Fe₃O₄ (f); elemental mapping of Pd/rGO-Fe₃O₄ (g).

The XRD patterns of graphite, GO, rGO-Fe₃O₄ and Pd/rGO-Fe₃O₄ are shown in Fig. 2. The graphite exhibited a very strong and sharp peak at 26.4° which was assigned to the (002) crystal plane of graphite. As can be observed, this diffraction peak disappeared, accompanied by the appearance of a new peak at 10.0°, after the oxidation of graphite to GO. This phenomenon could possibly be attributed to the insertion of various oxygen-containing functional groups (epoxy, hydroxyl, carboxyl and carbonyl) between the layers of the GO.²¹ As shown in Fig. 2c, this peak was eventually removed though the treatment of the GO and iron precursors using hydrazine hydrate as the reducing agent. Meanwhile, the peaks located at 30.1°, 35.5°, 43.1°, 53.5°, 57.0°, and 62.6° matched well with the (220), (311), (400), (422), (511), and (440) Bragg diffractions of Fe₃O₄ (JCPDS no. 76-0956), respectively, confirming the formation of the Fe₃O₄ crystalline phase. However, weak peaks at 32.9 and 49.3° which were attributed to hematite, were also detected,²² indicating an impurity in the sample. Nevertheless, the presence of hematite also can enhance the magnetic properties of the catalyst to some extent. A new peak at 40.1°, which belonged to the (111) Bragg diffraction of Pd (JCPDS no. 88-2335), demonstrated the successful immobilization of palladium on the rGO-Fe₃O₄ support.

Fig. 2 XRD patterns for graphite (a), GO (b), rGO-Fe₃O₄ (c) and Pd/rGO-Fe₃O₄ (d).

XPS profiles of GO and the Pd/rGO-Fe₃O₄ were documented and are illustrated in Fig. 3. An XPS survey scan performed on the sample of GO showed the existence of oxygen and carbon on the surface of the GO sheets. The C1s spectrum is deconvoluted into four peaks at 284.5, 285.5, 287.0 and 288.3 eV (Fig. 3a) which are attributed to the C–C, C–OH, C–O–C and HO–C=O groups, respectively.²¹ In the case of the Pd-RGO-Fe₃O₄, aside from carbon and oxygen, iron and palladium were also distinctively observed. Compared to the C1s spectrum of the GO, it can be seen that the peak intensities for the oxygen-containing functionalities are substantially reduced, indicating that GO was successfully reduced during the procedure. In particular, it should be noted that the peak corresponding to the C–O–C groups disappeared, resulting in an enhancement of the intensity of the C–C groups. The residues of hydroxyl groups, such as OH and COOH, on the surface of the catalyst not only improve its hydrophilicity but also stabilize the Pd NPs as anchoring sites, ensuring a good dispersion of the Pd NPs on the rGO-Fe₃O₄.

Fig. 3 XPS spectra of GO (a), rGO-Fe₃O₄ (b) and Pd/rGO-Fe₃O₄ (c).

As shown in Fig. 3c, the XPS spectrum of Pd was also characterized, to ascertain the oxidation state of Pd. As can be observed, the Pd binding energy of the Pd/rGO-Fe₃O₄ showed two strong peaks centered at 341.3 and 336.0 eV, which were attributed to the Pd3d_3/2 and Pd3d_5/2 peaks respectively. This indicated that the Pd²⁺ has been completely reduced during the process. However, it should be noted that the metallic Pd was in an electron-deficient state, since the binding energy of Pd3d_5/2 was 0.6 eV higher than the standard spectrum. This was ascribed to a reduction of the electron density of the Pd atoms, caused by an interaction between the metallic Pd phase and the high electronegativity of the oxygen of the oxygen-containing groups in the GO. Interestingly, the electrondeficient Pd was assumed to be responsible for the improved performance of the catalysts for the HDC reaction,^3,23 which was proved by the excellent catalytic activity of the Pd/rGO-Fe₃O₄ at room temperature and balloon hydrogen pressure in our work.

The magnetic properties of the samples of rGO-Fe₃O₄ and Pd/rGO-Fe₃O₄ were also investigated, and the hysteresis loops of the rGO-Fe₃O₄ and Pd/rGO-Fe₃O₄ are illustrated in Fig. 4. It can be seen that the magnetic saturation value of the rGO-Fe₃O₄ was 91.4 emu g⁻¹, while the value of the Pd/rGO-Fe₃O₄ decreased to 82.2 emu g⁻¹, due to the immobilization of Pd NPs on the surface of the support. This is in agreement with the literature, as reported by Ma et al.²⁴ However, the decrease in saturation magnetization did not affect the separation of the catalyst from the reaction system, since the catalyst can be easily separated from the reaction mixture by a magnet.

Fig. 4 Room temperature magnetization curves of rGO-Fe₃O₄ and Pd/rGO-Fe₃O₄.

3.2 Catalytic properties of Pd/rGO-Fe₃O₄

The HDC of 4-CP was chosen as a model reaction to investigate the catalytic properties of the Pd/rGO-Fe₃O₄ catalyst. Apart from the main product, phenol, only trace amounts of cyclohexanol and cyclohexanone were detected during the reaction process. From the results listed in Table 1, it can be seen that no conversion of 4-CP was observed using rGO or rGO-Fe₃O₄ as a catalyst at 303 K and balloon hydrogen pressure. This indicates that the Pd provided the active sites for the HDC of 4-CP. For comparison, a Pd/rGO catalyst without magnetic properties was also prepared, and used for the HDC reaction. It exhibited a comparable activity to the Pd/rGO-Fe₃O₄ catalyst towards the HDC of 4-CP. However, high speed centrifugation is required to separate the catalyst from the reaction mixture, due to the hydrophilicity of the Pd/rGO ascribed to the functional oxygen-containing groups on the surface of the rGO. Therefore, the Pd/rGO-Fe₃O₄ nanocomposite was chosen as the catalyst for HDC.

Table 1 Hydrodechlorination of chlorophenols catalyzed by different catalysts^a

Catalysts	Time (min)	Substrates	Conversion (%)	Selectivity (%)
a Reaction conditions: catalyst, 5.0 mg; temperature, 303 K; pressure, 1 atm; solvent, water (2.5 g L^−1, substrate concentration).
rGO	25	4-CP	—	—
rGO-Fe3O4	25	4-CP	—	—
Pd/rGO	25	4-CP	99.8	99.0
Pd/rGO-Fe3O4	25	4-CP	100.0	99.2
Pd/rGO-Fe3O4	30	2-CP	100.0	99.5
Pd/rGO-Fe3O4	20	3-CP	100.0	98.1
Pd/rGO-Fe3O4	40	2,4-DCP	100.0	>99.8
Pd/rGO-Fe3O4	50	2,6-DCP	100.0	99.9
Pd/rGO-Fe3O4	120	2,4,6-TCP	100.0	99.9

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The effect of catalyst loading on the catalytic performance of the Pd/rGO-Fe₃O₄ catalyst was firstly investigated at 303 K and balloon hydrogen pressure. From the results illustrated in Fig. 5, it can be seen that the conversion of 4-CP to phenol increased with an increase in Pd loading. Specifically, the times for complete conversion of 4-CP were 50 and 30 min when the Pd loading was 3.0 wt% and 5.0 wt%, respectively, while only 76.4% conversion of 4-CP was obtained in 60 min with a Pd loading of 1.0 wt%. Therefore, 5 wt% loading of Pd was selected to further investigate the catalytic properties of the Pd/rGO-Fe₃O₄ catalyst towards the HDC of 4-CP.

Fig. 5 Effect of Pd loading on the HDC of 4-CP catalyzed by Pd/rGO-Fe₃O₄.

The effect of catalyst concentration on the HDC reaction efficiency with the Pd/rGO-Fe₃O₄ catalyst was studied, with the variation of the catalyst concentration from 0.5 to 1.5 g L⁻¹. The results are shown in Fig. 6. One can see that the conversion of 4-CP increased with an increase in catalyst concentration. For instance, the conversion of 4-CP was 93.7% in 60 min with a low catalyst concentration of 0.5 g L⁻¹. To our surprise, a complete conversion of 4-CP to phenol was achieved in 30 min when the catalyst concentration was 1.0 g L⁻¹. The reaction time for complete conversion of 4-CP was 15 min when the catalyst concentration was increased to 1.5 g L⁻¹. With regards to the product distributions, the increase in catalyst concentration did not change the final product, in which phenol was detected as the main product. Therefore, the catalyst concentration was set to 1.0 g L⁻¹ to further investigate other factors that affected the catalytic performance of the Pd/rGO-Fe₃O₄ catalyst.

Fig. 6 Effect of catalyst concentration on the HDC of 4-CP catalyzed by Pd/rGO-Fe₃O₄.

The effect of 4-CP concentration on the HDC of 4-CP with the Pd/rGO-Fe₃O₄ catalyst was explored. As shown in Fig. 7, an increase in 4-CP concentration led to a decrease in conversion rate. The conversion of 4-CP was accomplished at short reaction times of 15 and 20 min when the concentration of 4-CP was 1.5 and 2.0 g L⁻¹, respectively. The reaction times for complete conversion of 4-CP increased to 25 and 40 min when 2.5 and 3.0 g L⁻¹ of 4-CP were introduced to the reaction system. Moreover, the change of 4-CP concentration did not affect the product distributions, since only phenol was observed with an increase of 4-CP concentration from 1.5 to 3.0 g L⁻¹. Consequently, the HDC of 4-CP catalyzed by our Pd/rGO-Fe₃O₄ catalyst maintains high activity under mild conditions without any additives.

Fig. 7 Effect of 4-CP concentration on the performance of the Pd/rGO-Fe₃O₄ catalyst.

The effect of temperature on the catalytic properties of the Pd/rGO-Fe₃O₄ catalyst was investigated under different temperatures, keeping other reaction parameters the same. The results are illustrated in Fig. 8. As expected, the conversion of 4-CP increased gradually with an elevation of reaction temperature from 293 to 308 K. In terms of the selectivity, the product distributions did not obviously change in the investigated temperatures. It has been reported that the catalytic HDC of 4-CP is a pseudo-first-order reaction. Thus, the activation energy for the HDC of 4-CP with the use of Pd/rGO-Fe₃O₄ as a catalyst can be evaluated. According to the Arrhenius equation as follows,

k_obs = Ae^−E_a/RT

the activation energy (E_a) was calculated. In the equation, k_obs represents the estimated first-order rate constant, A reveals the frequency factor, and R and T express the ideal gas constant and temperature, respectively. As shown in Fig. 8 inset, the rate constants originating from the slope of the straight line were 0.1118, 0.1725, 0.2488, and 0.3138 at 293, 298, 303, and 308 K respectively. The coefficient of determination (R²) value was >0.99 in all tests. Consequently, the activation energy for the HDC of 4-CP over the Pd/rGO-Fe₃O₄ catalyst is approximately 52.0 ± 4.2 kJ mol⁻¹.

Fig. 8 Effect of reaction temperature on the HDC of 4-CP with the Pd/rGO-Fe₃O₄ catalyst.

Fig. 9 Effect of solvents on the HDC of 4-CP with the Pd/rGO-Fe₃O₄ catalyst.

As reported in previous literature, the solvent plays an important role in the catalytic properties of the catalyst, as well as the product distributions of the HDC reaction.^25,26 To this end, different solvents such as water, ethanol, i-propanol, methanol, HF and hexane were introduced to test the catalytic properties of the Pd/rGO-Fe₃O₄ catalyst, and the results are illustrated in Fig. 9. It was observed that the initial reaction rate of the 4-CP followed the order: water > methanol > ethanol > i-propanol > THF > hexane. In particular, the HDC of 4-CP reached as high as 100% within 40 min in water under the same reaction conditions. When the water was replaced by protic solvents such as methanol, ethanol and i-propanol, the conversion of 4-CP was obviously decreased, and even lower 4-CP conversion was observed in aprotic solvents such as THF and hexane. It has been reported that the dielectric constant (ε) and molar volume (v) are two of the most important properties which can affect the catalytic performance of the catalyst in the HDC reaction. For example, the initial rate of the HDC of 2,4-DCP can be influenced by the solvent polarity and structure, which corresponds to the dielectric constant (ε) and molar volume (v).²⁶ The initial rate of the 2,4-DCP was higher with an increased strength in the ionic forces due to solvation. On the other hand, a lower molar volume of the solvent enables more solvent molecules to interact with the charge reaction intermediate, strengthening the HDC reaction.²⁶ In particular, water has the highest and lowest values of ε and v respectively, and can form well organized structures through the formation of an H-bond between the dissolving ions, thus exhibiting the highest catalytic activity for the HDC reaction. The solvents, such as THF and hexane, which have a lower solvation ability and H-bonding (if any), or are without these properties, reached a lower catalytic activity.²⁶ Similarly, our results as discussed above confirm this argument. Therefore, a higher value of ε, and a lower value of v should be taken into consideration to achieve a higher reaction rate.

We also investigated the catalytic performance of the Pd/rGO-Fe₃O₄ catalyst, and the results are shown in Table 1. It can be seen that the complete conversions of 3-CP, 2-CP, and 4-CP could be accomplished in different reaction times due to the electronic and steric hindrance of the substrates. Moreover, the catalyst also showed excellent catalytic properties for the HDC of dichlorophenols (2,4-DCP and 2,6-DCP) and trichlorophenol (2,4,6-TCP) with an increase in reaction time. Under the same conditions, the HDC rate follows the order: monochlorophenols > dichlorophenols > trichorophenol. The reusability of the catalyst was also investigated. As shown in Fig. 10, the results indicate that the catalyst was quite stable and could be recycled four times without any loss of activity. Moreover, the catalyst can be easily separated from the reaction medium with a magnet and Pd leaching was negligible, as analyzed by ICP (<0.1%). Such phenomena proved the good stability of Pd/rGO-Fe₃O₄ for the catalytic HDC of 4-CP. The excellent catalytic properties of the Pd/rGO-Fe₃O₄ can probably be attributed to the following points. Firstly, the electron-deficient state of Pd, caused by the interaction between the metallic Pd and the oxygen of the oxygen-containing groups in the rGO, plays an important role in improving the catalytic properties of the catalyst towards the HDC of the chlorinated compounds, in agreement with reported literature.²⁷ Moreover, a smaller Pd particle size is more tolerable to the HCl during the HDC process. Finally, the magnetic properties of the catalyst, when it was recycled in a facile collection of catalysts, were immune from the loss usually caused by mechanical recycling. This advanced composite material provides a novel kind of effective catalyst, with great promise for the catalytic HDC of 4-CP in practical applications.

Fig. 10 Reusability of the Pd/rGO-Fe₃O₄ catalyst.

4. Conclusion

In summary, a Pd/rGO-Fe₃O₄ catalyst was synthesized and applied for the HDC of 4-CP in aqueous phase, at room temperature and balloon hydrogen pressure. Through characterization, it can be seen that the Pd NPs were of a narrow size distribution and were dispersed well on the surface of the magnetic rGO-Fe₃O₄ support. The results indicate that the magnetic catalyst Pd/rGO-Fe₃O₄ showed excellent catalytic properties for 4-CP HDC. This high catalytic activity could be attributed to the smaller particle size of the Pd combined with the functional groups on the surface of rGO, which resulted in an electron-deficient Pd from the strong metal-support interaction and the presence of hydroxyl groups on the surface of the support. Furthermore, the catalyst exhibited quite stable properties and can be recycled at least four times without significant deactivation. It was easily separated from the reaction mixture without filtration, due to its magnetic properties.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21207109), the Applied Basic Research Programs of Science and Technology Department of Sichuan Province (no. 2014JY0107), and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (no. LZJ1205).

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