Journal of Hazardous Materials 274 (2014) 32–40

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Aqueous hydrodechlorination of 4-chlorophenol over an Rh/reduced graphene oxide synthesized by a facile one-pot solvothermal process under mild conditions Yanlin Ren a , Guangyin Fan a,∗ , Chenyu Wang b a Chemical Synthesis and Pollution Control, Key Laboratory of Sichuan Province, College of Chemistry and Chemical Industry, China West Normal University, Nanchong 637002, China b Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902, USA

h i g h l i g h t s

g r a p h i c a l

• Rh/RGO was synthesized through a

The Rh nanoparticles/reduced graphene oxide (Rh NPs/RGO) nanocatalyst synthesized by a solvothermal technique showed high activity and stability for the hydrodechlorination of 4-chlorophenol under mild conditions.

one-pot polyol reduction of GO and RhCl3 . • Complete HDC of 4-chlorophenol was obtained in aqueous phase without any additive. • The Rh/RGO exhibited an excellent catalytic performance for HDC reaction.

a r t i c l e

i n f o

Article history: Received 29 December 2013 Received in revised form 14 March 2014 Accepted 2 April 2014 Available online 13 April 2014 Keywords: Rhodium Chlorophenol Hydrodechlorination Reduced-graphene-oxide

a b s t r a c t

a b s t r a c t Reduced graphene oxide (RGO) supported rhodium nanoparticles (Rh-NPs/RGO) was synthesized through one-pot polyol co-reduction of graphene oxide (GO) and rhodium chloride. The catalytic property of Rh-NPs/RGO was investigated for the aqueous phase hydrodechlorination (HDC) of 4-chlorophenol (4CP). A complete conversion of 4-CP into high valued products of cyclohexanone (selectivity: 23.2%) and cyclohexanol (selectivity: 76.8%) was successfully achieved at 303 K and balloon hydrogen pressure in a short reaction time of 50 min when 1.5 g/L of 4-CP was introduced. By comparing with Rh-NPs deposited on the other supports, Rh-NPs/RGO delivered the highest initial rate (111.4 mmol/gRh min) for 4-CP HDC reaction under the identical conditions. The substantial catalytic activity of Rh-NPs/RGO can be ascribed to the small and uniform particle size of Rh (average particle size was 1.7 ± 0.14 nm) on the surface of the RGO sheets and an electron-deficient state of Rh in the catalyst as a result of the strong interaction between the active sites and the surface function groups of RGO. © 2014 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author at: Chemical Synthesis and Pollution Control, Key Laboratory of Sichuan Province, College of Chemistry and Chemical Industry, China West Normal University, Nanchong 637002, China. Tel.: +86 817 2568081; fax: +86 817 2568081. E-mail address: [email protected] (G. Fan). http://dx.doi.org/10.1016/j.jhazmat.2014.04.005 0304-3894/© 2014 Elsevier B.V. All rights reserved.

Catalytic hydrodechlorination (HDC) has been widely investigated as an effective technique to detoxify the chlorinated aromatic compounds due to their high toxicity and resistance to biodegradation [1,2]. With the advantages of high efficiency, relative mild conditions and production of reduced toxic products, the HDC of these compounds catalyzed by heterogeneous catalysts

Y. Ren et al. / Journal of Hazardous Materials 274 (2014) 32–40

especially supported catalysts such as Pd [3–9], Pt [10–12], and Rh [13,14] nanoparticles (NPs) have attracted more attention in the last two decades. In spite of its high cost, the HDC of chlorinated aromatic compounds such as chlorophenols catalyzed by Rh catalysts is widely investigated owing to their high catalytic activity, resistance to the attack of acid and base, and relative stability under severe conditions, which enabled the transformation of toxic chlorophenols to high valued products such as cyclohexanone and cyclohexanol [15–19]. A variety of Rh catalysts deposited on different supports such as active carbon (AC), Al2 O3 and pillared clays have been synthesized. However, the catalytic properties of these catalysts are not satisfied enough. Recently, it has been reported that the catalytic properties of the Rh catalysts can be improved by adjusting the particle size of the Rh metallic phase. The catalysts with smaller Rh particle size hold high surface-to-volume ratio and are resistant to deactivation from the depositing Cl through the scission of the C Cl bonds [16]. Besides the catalytic nature of metal nanoparticles themselves, it has been proposed that the support also plays a critical role in the catalytic performance of metal catalysts in a number of ways such as the acid/base properties of the carrier and metal-support interaction. Ascribed to the acid/base properties, the supports with basic property such as alumina show an enhanced catalytic performance, which is probably attributed to the capture of the halogen atoms by the basic sites of the aluminum, favoring the removal of the organic molecule [20]. On the contrary, the carriers with acidic properties are adversely to gain a higher catalytic performance. For example, the carbon materials with carboxylic groups from the HNO3 washing had a disadvantageous effect on the catalytic of the catalysts in the HDC reaction as a result of the interaction of the reactant and the surface functional groups [21]. Apart from acid/base properties of the supports, the metal-support interaction is also benefitted to protect the activity of the catalysts [22]. Especially, the interaction between the metallic phases and the functional groups of the supports plays a key role in the metallic dispersion, the resistance to sintering and the catalytic behavior of the supported catalysts [23,24]. Specifically, the dispersion of the metallic phase could be tuned by the coordination of the active sites and the functional groups on the surface of support via preventing the aggregation of the nanoparticles during the reducing process. Therefore tremendous effect has been exerted to modify the supports with various kinds of techniques to control the particle size and dispersion of the metallic phases with the aim of improving the catalytic properties of the supported catalysts [9,25–29] by changing the hydrogen uptake/release [30]. For example, the catalytic activity and stability of Pd NPs can be improved through the modification of the carbon nanofibers with pyridinic basic species [9]. However, the treatment of the support with the use of NH3 as a modifier must be conducted at very high temperature. Up to now, it still remains as a big challenge to seek appropriate supports with fine stability in an acidic environment and strong capability to interact with metal catalysts for recyclable conversion of chlorinated phenols under mild conditions. To this end, the graphene oxide (GO), with a large amount of functional group from the oxidation of graphite, was selected as a support to synthesize Rh NPs/RGO catalyst by a solvothermal technique and investigated its catalytic property toward the HDC of 4-CP in aqueous phase without any additives such as NaOH as reported by most of the literature. 2. Experiment 2.1. Materials Rhodium chloride was provided by the Kunming Institute of Precious Metals, China. Ethylene glycol (EG, >99%) and graphite

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powder were purchased from Aladdin Industrial Corporation, China, and used as received without further purification. Others solvents were analytical grade and used as received. 2.2. Instruments The specific surface area measurement was performed on a SSA-4200 automatic surface analyzer (Builder, China). The sample was treated under vacuum at 573 K for 3 h, and then applied to the measure the surface area by cooling the sample to 77 K using liquid N2 . The morphology and particle size of the catalysts were characterized by transmission electron microscopy (TEM) measurements, which were carried out on a JEOL model 2010 instrument operated at an accelerating voltage of 200 kV. The samples were prepared by dispersing the samples in acetone and deposited on a Cu based TEM grid. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku X-ray diffractometer D/max-2200/PC equipped with Cu K␣ radiation (40 kV, 20 mA). The samples were scanned at a rate of 0.02 step−1 over the range of 5–90◦ . X-ray photoelectron spectroscopy (XPS, Kratos XSAM800) spectra was obtained by using Al Ka radiation (12 kV and 15 mA) as an excitation source (hv = 1486.6 eV) and Au (BE Au4f = 84.0 eV) and Ag (BE Ag3d = 386.3 eV) as reference. All binding energy (BE) values were referenced to C1s peak of contaminant carbon at 284.6 eV. A Fourier transform infrared spectrum was recorded with a Nicolet 6700 (resolution 0.4 cm−1 ) infrared spectrometer. The HDC process was monitored from evolution of the concentration of 4-CP and products, which was measured by GC (Agilent 7890A) with a FID detector and PEG-20M capillary column (30 m × 0.25 mm, 0.25 ␮m film) and nitrogen was used as a carrier gas. From the experiment results, we can find that the removal percentage increases depending on the reaction time and phenol, cyclohexanol and cyclohexanone are the products of the reaction. The trend was observed in all the experiments with no exception. 2.3. Synthesis of GO GO used in the experiments was synthesized by oxidation of graphite according to the improved Hummer’s method [31]. Graphite powder (3.0 g) and KMnO4 (18.0 g) were mixed in a 500 mL round-bottom-flask at room temperature and a mixture of concentrated H2 SO4 /H3 PO4 (360:40 mL) was slowly dropped with vigorous stirring and resulted in an increase of temperature to 308–313 K. The mixtures were kept at 323 K under stirring for 12 h. The reaction was stopped by poured onto ice (400 mL) containing 30% H2 O2 (3 mL). Finally, the resulting suspension was filtered and washed with 5% HCl aqueous solution. The obtained solid was vacuum-dried overnight at room temperature for further use. 2.4. Preparation of the Rh-NPs/RGO In order to obtain well-dispersed Rh NPs on the surface of RGO, a co-reduction of starting materials graphite oxide (GO) and rhodium chloride (RhCl3 ) synthetic route has been designed and carried out. In the first stage, by pre-mixing Rh precursors and GO thoroughly Rh3+ was anchored by functional groups on the surface of GO sheets. Subsequently, Rh NPs immobilized on RGO were harvested via the reduction of Rh3+ –GO mixture in the presence of EG as a reducing agent (Scheme 1). In a typical preparation experiment, 0.3 g of GO and 20 mL EG were mixed under ultrasonication (200 W) for 3 h before transferred into a 50 mL autoclave to ensure most of GO being fully exfoliated. Then NaOH (0.1 g) was added to the autoclave under vigorous stirring, followed by the addition of rhodium chloride (8.0 mg). Then the autoclave was sealed and purged with N2 for several times to remove the air, then the autoclave was put into an oil bath at 433 K for 12 h under stirring. Finally, the black

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Scheme 1. Synthetic route of Rh-NPs/RGO composite.

precipitate was separated by filtration and washed several times with ethanol. Then the resultant catalyst was dried at 303 K under vacuum for 24 h. (The Rh loading was 0.94 wt.% measured by ICP and the surface area of the catalyst was 167.7 m2 /g measured by a BET method.) 2.5. Activity test To clarify the effects affecting the hydrodechlorination of chlorinated phenols, HDC of 4-CP was chosen as the model reaction, since 4-CP is a priority pollutant applied for producing pesticides, herbicides, and disinfectants, which is difficult to biological degradation in the environment. The series of experiment with molecular

hydrogen over Rh-NPs/RGO in liquid solution were carried out in a 25 mL two-necked round-bottom flask with a hydrogen balloon operating in water bath, under continuous stirring. The molar ratio of Cl/Rh was arranged from 141 to 424. The reaction time was set at zero, when the temperature of reactant solution was stable. During the course of the reaction, samples were withdrawn from the reaction medium periodically through a syringe and the catalyst was separated by centrifugation. The reaction was repeated under the same conditions with the usage of the different batch of catalysts or the withdraw sample analyzed twice showed that they had a good repeatability and analytical reproducibility (Fig. S2 and S3). The chloride balance of all the samples is between 95% and 100%.

Fig. 1. TEM micrographs of the fresh Rh-NPs/RGO at different scale bars (a–c) and the catalyst recycled five times (d). Insets are the particle size distribution of the fresh and recycled catalysts.

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Fig. 2. XRD patterns of graphite (a), GO (b) and Rh-NPs/RGO (c). Fig. 3. FTIR patterns of Rh-NPs/RGO (a) and GO (b).

3. Results and discussion 3.1. Characterization of Rh-NPs/RGO Fig. 1 depicts the TEM images of the generated Rh-NPs/RGO catalyst at different magnifications. As exhibited in Fig. 1a, the RGO flakes seem to be scrolled and entangled with each other, resembling crumpled silk cloth. In comparison, we also prepared RGO sheets in the absence of Rh and the result illustrated in Fig. S1 indicates that the RGO flakes are aggregated with each other without the immobilization of Rh NPs on the RGO surface. Rh NPs were revealed to be uniformly dispersed on the surface of RGO sheets, as can be seen from Fig. 1b. The average size of these NPs, determined with the high resolution transmission electron microscopy (HRTEM) characterization (Fig. 1c), was measured as 1.7 ± 0.14 nm with a narrow distribution from 1.0 nm to 2.8 nm (insert of Fig. 1b). The highly uniform size and dispersity of Rh NPs on RGO sheets can be ascribed to the interaction between the Rh ions and functional groups of GO, preventing the aggregation of Rh clusters during the solvothermal process. To further detect the details of as-synthesized nanocomposites, XRD spectra of graphite, graphite oxide (GO) and Rh-NPs/RGO were recorded and presented in Fig. 2. As can be seen in Fig. 2a, a strong peak at the position of 2 = 26.4◦ was detected which belongs to the (0 0 2) crystal plane of graphite. After the oxidation of the raw graphite to GO, such diffraction peak disappeared and a new peak at 10.0◦ was observed with sharply decreased intensity as illustrated in Fig. 2b, which can be attributed to the introduction of various oxygen-containing functional groups (epoxy, hydroxyl, carboxyl and carbonyl) [32]. This peak eventually vanished in the spectrum for Rh-NPs/RGO as shown by Fig. 2c, indicative of the disappearance of oxygen-containing groups and the formation of RGO support. The two new peaks at the position of 25.4◦ and 43.1◦ were assigned to the (0 0 2) and (1 0 0) reflection of RGO respectively. In contrast, no XRD signals for metallic Rh phase were detected, which is probably attributed to the low loading of Rh as well as the well dispersed Rh nanoparticles with no agglomeration. The FTIR spectra reflect the different micro-structure of RGO support with respect to GO. In the spectrum of Rh NPs/RGO (Fig. 3a), there are only two FTIR responses for Rh-NPs/RGO present at 3439 and 1635 cm−1 arising from the O H and aromatic C C stretching vibrations. In terms of GO (Fig. 3b), the broad characteristic band at 3433 cm−1 is ascribed to the O H stretching vibration. The adsorption bands at 1740 cm−1 and 1631 cm−1 are assigned to the C O stretching vibration in COOH groups and C C from unoxidized sp2 C C bonds. The adsorption peaks at 1229 cm−1 and 1064 cm−1 are attributed to the C O vibrations and alkoxy C O groups situated at the edges of the GO nanosheets as has been reported previously

[33]. The FTIR characterization confirmed the presence of different types of oxygen functionalities on the GO, which provided reactive anchoring sites for Rh ions and generated particles. Comparatively, the intensity of the peaks for GO was diminished which further demonstrates the formation of RGO with a deoxygenation process on the GO sheets. X-ray photoelectron spectra (XPS) performed on GO and RhNPs/RGO also implies the tendency of decrease of oxygenated functional groups during the synthetic reaction. As can be seen from Fig. 4a andc, the composition of carbon underwent a considerable increase from 61.49 to 81.77% after the reduction by ethylene glycol with a specific oxygen composition decreasing from 35.66 to 16.40%. The C1s spectrum is deconvoluted into several peaks (Fig. 4b and d) which corresponds to various types of carbon atoms including the C C, C OH, C O C and HO C O, respectively. In general, the peak intensities for oxygen-containing functionalities are substantially reduced along with the reduction of GO, nevertheless the hydroxyl group (C OH) signal exceptionally increases due to the reduction of the functional groups such as C O C and HO C O to form much more hydroxyl groups. The retention of hydroxyl functionalities on the RGO sheets not only modifies the hydrophilic of support but also serves as anchoring sites for depositing the Rh NPs, which are beneficial for the dispersion and stability of the catalyst in the heterogeneous system. Moreover, the binding energy value of Rh 3d5/2 was 308.8 eV, which could be assigned to an Rhn+ state [34], indicating that the Rh3+ has been partly reduced during the solvothermal process. The presence of electro-deficient metallic species was probably ascribed to the interaction between the oxygen groups of the RGO and the Rh nanoparticles because the high electronegetivity of oxygen was supposed to reduce the electron density of Rh atoms [35]. More importantly, recently research revealed that the presence of electro-deficient metallic species played a key role in improving the catalytic properties toward the HDC of the chlorinated compounds [16,36]. The high catalytic activity of the Rh NPs/RGO for the HDC of 4-CP in the presence work further confirmed the argument. 3.2. Consideration of mass transport limitation The possible transport limitations were firstly considered before investigating the catalytic performance of the catalyst. It has been proposed that three kinds of transport limitations should be evaluated including bulk solvent, the liquid film at the external catalyst surface and within the pores [37]. Firstly, the mass transport limitation between 4-CP and water can be eliminated since the reactant was soluble in water. Moreover, the reaction in water was not

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Fig. 4. XPS patterns of GO (a, b) and Rh-NPs/RGO (c–e).

limited as estimated by equation provided by Keane et al. [37]. The mass transfer between the liquids and catalyst surface estimated by the Carberry (Ca) number were below the critical limiting value (0.15), indicating that the transport limitation can be eliminated. Finally, the possible mass transfer between the internal diffusion were also calculated, similarly the calculated value is closed to unity. Consequently, the results confirmed that the experimental data were collected under conditions of kinetic control. 3.3. Effect of 4-CP concentration The catalytic property of the catalyst Rh NPs/RGO was tested by the HDC of 4-CP as a model reaction. Blank test or using RGO as the catalyst showed no conversion of 4-CP, conforming that Rh NPs was the active species for the HDC of 4-CP. Phenol, cyclohexanone and cyclohexnanol were detected as the main products during the reaction process. Effect of 4-CP concentration on the HDC of 4-CP catalyzed by Rh/RGO also was studied and the results were illustrated in Fig. 5. A decrease of conversion rate was observed with the increasing concentration of 4-CP. The reaction time for complete conversion of 4-CP was 30 and 50 min when 1.5 g/L and 2.0 g/L of 4-CP was introduced respectively. However, the conversion of 4-CP

Fig. 5. Effect of concentration of 4-CP on the conversion of 4-CP by Rh-NPs/RGO. Reaction conditions: catalyst, 5.0 mg; temperature, 303 K; water, 5 mL; H2 balloon pressure; time, 1 h.

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3.5. Effect of solvent

Fig. 6. A plot of ln(1 − C/C0 ) vs. time for the reduction of 4-CP by Rh-NPs/RGO. Reaction conditions: catalyst, 5.0 mg; 4-CP concentration (2.5 g/L, 5 mL); H2 balloon pressure.

was only 95.9 and 66.0% in 60 min when the concentration of 4-CP was 2.5 and 3.0 g/L. Moreover, the products distribution also was affected by 4-CP concentration, the lower concentration of 4-CP, the lower selectivity of phenol. Interestingly, the totally conversion of 4-CP to the high valued products of cyclohexanone and cyclohexanol was detected in 50 min with the use of 1.5 g/L 4-CP, while the selectivity of phenol was still remained at 54.8% when the concentration of 4-CP was 3.0 g/L (Fig. S4). Consequently, the catalytic HDC of 4-CP catalyzed by our Rh/RGO catalyst not only holds the high activity but also yields high value products of cyclohexanone and cyclohexanol exclusively under mild conditions without any additives.

3.4. Effect of reaction temperature To kinetically study the catalytic HDC of 4-CP, a set of control experiments were carried out under different temperatures while maintaining other conditions the same. A gradual enhancement in the conversion of 4-CP with the increase of the reaction temperature was observed at the range from 298 to 308 K. However, the product distributions did not vary obviously regardless of the temperature variation. To quantify the catalytic activity of HDC reaction over Rh-NPs/RGO, a kinetic model on the basis of the pseudo-firstorder dependence on 4-CP has been postulated [38]. On the basis of previous studies, the HDC of 4-CP followed a pseudo-first-order reaction. The reaction was conducted at different temperatures range from 298 to 303 K in order to evaluate the activation energy for the HDC of 4-CP with Rh-NPs/RGO. The activation energy (Ea) was calculated using the Arrhenius equation as follows: kobs = Ae−Ea /RT where kobs reveals the estimated first-order rate constant, A represents the frequency factor, R and T express the ideal gas constant and temperature, respectively. Fig. 6 illustrates the example of pseudo-first-order regression in the HDC of 4-CP reaction with RhNPs/RGO at a temperature range from 298 to 308 K. Rate constants obtained from the slope of the straight line are 0.027, 0.041 and 0.062 min−1 at 298, 303 and 308 K respectively. The coefficient of determination (R2 ) values was >0.99 in all tests. Consequently, the activation energy of HDC of 4-CP by Rh-NPs/RGO is approximately 63 kJ/mol.

It is well-known that solvent property has a considerable influence on the HDC rate and product distributions of dechlorination reaction [37,39]. As an inert reaction media, the solvent not only contributes to the heat transfer but also benefits the contact of reactants on the surface of catalysts during the HDC process. Therefore, we also investigated the catalytic properties of the catalyst Rh NPs/RGO in different reaction medium including water, ethanol, i-propanol, methanol, HF and cyclohexane. As shown in Table 1, the initial reaction rate of 4-CP was followed the order: water > methanol > ethanol > ipropanol > THF > cyclohexane. Under the standard experimental conditions, the dechlorination of 4-CP reached as high as 95.9% within 60 min in water. While using protic solvents such as methanol, ethanol and i-propanol, the removal percentages were obviously decreased and even lower removal percentages were obtained in aprotic solvents. In view of the product distributions, the phenol was the main product in THF and methanol, while the excess of cychohexanone was detected as the primary product with a relative low selectivity to phenol in the other solvents. Among the solvent properties, dielectric constant (ε) and molar volume (v) were considered to be the mostly important properties that can affect the catalytic performance during the HDC procedure. Keane et al. demonstrated that the initial rate for the HDC of 2,4-DCP was depended on the solvent polarity and structure which was corresponded to the dielectric constant (ε) and molar volume (v) [37,39]. It was found that initial rate of 2,4-DCP can be enhanced with the increased strength in the ionic forces because of the salvation. On the other hand, it also can be strengthened using the solvent with lower molar volume, where the greater number of solvent available to interact with the charge reaction intermediate [39]. The highest catalytic activity for the HDC reaction conducted in water was attributed to the properties of water, which can form well organized structures through the formation of an H-bonding between the dissolving ions and water with the highest/lowest values of ε and v. The lower catalytic of the catalyst in the solvent such as cyclohexane and THF was ascribed to their low salvation ability and H-bonding (if any) [37]. In the present work, we obtained similar results as reported by Keane et al., which further confirmed the conclusion to some extent. Therefore, to obtain a higher reaction rate, a proper choice of solvent with a higher value of ε and a lower value of v should be taken into consideration. 3.6. Effect of support The dependence of activity of catalysts on the supports prompted us to reveal the effect of RGO on the HDC reaction by comparing its catalytic performance with that of Rh NPs deposited on other supports. For comparison, Rh/TiO2 , Rh/AC, Rh/Fe3 O4 and Rh/Al2 O3 were prepared by a similar method with the same loading of Rh. It can be apparently seen that these supports performed differently toward the HDC of 4-CP with the same loading of Rh among which the Rh-NPs/RGO catalyst displayed the highest initial rate (Table 2). Relatively lower initial rates in the liquid phase HDC were obtained over the other catalysts, following the sequence with respect to supports: Rh/TiO2 > Rh/AC > Rh/Al2 O3 > Rh/Fe3 O4 . Furthermore, the selectivity was also demonstrated to be depending on the type of supports. RGO and TiO2 supported catalysts exhibited higher selectivities toward the cyclohexanol and cyclohexanone as a result of the further hydrogenation of phenol due to their high reactivity. Contrarily, the main product was phenol when Rh NPs was deposited on the carriers such as AC, Al2 O3 , and Fe3 O4 . With respect to the supports, it is generally accepted that there are several factors that can affect the catalytic performance of the catalyst for HDC reaction. Firstly, the interaction between the

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Table 1 Effect of solvents on the HDC of 4-CP catalyzed by catalyst Rh/RGO. Solvents

k (10−2 )a

Initial rateb

Conversion (%)a

SPhOH a

SCL a

SCE

H2 O Methanol Ethanol i-Propanol THF Cyclohexane

5.4 0.6 0.1 0.1 0.03 –

111.4 12.4 2.1 2.1 0.6

95.9 31.3 8.5 7.9 2.7 –

16.9 92.1 39.5 49.5 80.1 –

18.2 1.4 7.7 9.6 1.4 –

64.9 6.5 52.8 40.9 18.5 –

a Specific rate constant k (units:/min), initial rate ((units: mmol/gRh min)), conversions of 4-CP and selectivities to phenol (SPhOH ), cyclohexanol (SCL ) and cyclohexanone (SCE ) in a reaction time of 60 min. b initial rate: gRh min−1 .

Table 2 Effect of supports on the HDC of 4-CP catalyzed by Rh-based catalysts. Supports

k (10−2 )

Initial rate

Conversion (%)

SPhOH

SCL

SCE

RGO TiO2 AC Al2 O3 Fe3 O4

5.4 3.2 2.2 0.7 0.09

111.4 66.0 45.4 14.4 1.9

95.9 85.5 64.5 40.3 6.7

16.9 18.7 71.1 60.7 81.9

18.2 17.1 4.2 5.7 0

64.9 64.2 24.7 33.6 18.1

a Specific rate constant k (units: /min), initial rate((units: mmol/gRh min)), conversions of 4-CP and selectivities to phenol (SPhOH ), cyclohexanol (SCL ) and cyclohexanone (SCE ) in a reaction time of 60 min.

support and metallic NPs is crucial for obtaining a high catalytic activity. Especially, the interaction between the functional groups and the metallic NPs which made the Rh in an electron-deficient state resulting in a high catalytic performance [16]. Second, the acid–base property of the support was another factor that can affect the catalytic property of the catalyst [21]. Generally, the metallic NPs deposited on basic carriers exhibited higher catalytic property than that of the acidic ones, which was probably ascribed to their strong capability of capture the halogen atoms via the break of the

carbon-halogen bond [40]. In terms of the acidic support, Keane et al. proposed that the presence of carboxylic groups from the modification of carbon by HNO3 can prohibit the HDC process via stabilizing the interaction with the reactant which resulted in a low HDC reaction rate [21]. Specially, the Lewis and Bronsted acidity was reported to play an important role in the catalytic properties for HDC reaction. Hashimoto et al. [41] discovered that the carrier with Lewis acidic property exhibited much higher activity than that of the one with Bronsted acidic property, since the presence of Lewis

Fig. 7. TEM images of Rh/Al2 O3 (a), Rh/Fe3 O4 (b), Rh/RGO (c), Rh/AC (d), and Rh/TiO2 (e).

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acid sites was facilitated to the adsorption of reactant compared to the Brønsted acid sites. However, Urbano et al. [26] suggested that the acid–base properties of the supports have no obvious effect on the catalytic activity of the supported palladium catalysts. Thirdly, the most important factor affected the catalytic property of catalyst was metal particle size. In the present work, the RGO support was acidic (3.7 mol/g) characterized by NH3 -TPD, which was attributed to the residue of the carboxyl groups on the surface of the RGO. The high catalytic activity of Rh NPs/RGO toward 4-CP HDC indicate that the acidity property did not affect the catalytic property of the catalyst, which made further support to the conclusion proposed by Urbano et al. [26]. Moreover, we also examined the Rh particle size of the other catalyst and the results are shown in Fig. 7. It can be seen that a smaller particle size of Rh and better dispersion were obtained from the support TiO2 and RGO, while the Rh particle on the other supports were aggregated on the surface of supports which resulted in a poor dispersion of the Rh NPs. The high catalytic activities of Rh NPs/RGO and Rh NPs/TiO2 confirmed the fact that a smaller particle size of the metal was benefitted to the catalytic performance of the catalysts. On the basis of our experiment, we tentatively attribute the excellent catalytic performance of Rh-NPs/RGO in catalyzing HDC reaction to both small particle size with a high dispersion and strong metal-support interaction as compared to the other supports used in the present work. The high dispersion of the Rh NPs on the surface of the RGO support facilitated the occurrence of dechlorination between the active sites and surface absorbed 4-CP. Moreover, it has been claimed that the existence of surface hydroxyl groups on a support plays an important role in avoiding the deactivation of the catalyst in which the surface functional groups served as a adsorption sites to deliver the chlorine from the active sites of the catalyst, alleviating the detrimental effects from the chlorine adsorption to ensure a higher catalytic stability [3]. In case of RGO, which holds a large amount of hydroxyl group (C OH) from the reduction of the functional groups such as C O C and HO C O, may also have the same effect to prohibit the deactivation of the Rh surface and lead to a higher catalytic stability. Such catalytic properties indicate the great potential of RGO to support metals in the catalytic HDC of chlorophenols for yielding non-toxicity and high valued products. 3.7. Stability of the catalyst The as-prepared Rh-NPs/RGO composite exhibited excellent catalytic stability which was verified with successive runs under the identical conditions. The catalytic activity of the catalyst was maintained with no decrease from the first to the fourth run and only a slight loss of the activity was observed in the fifth run. Such phenomenon proved the good stability of Rh-NPs/RGO for the catalytic HDC of 4-CP. The TEM image and the particle size measurement of the catalyst after successive runs confirmed the retained homogeneous dispersion of metal Rh NPs on the surface of RGO sheets, as presented in Fig. 1d. The majority of Rh NPs were in a range from 1.2 nm to 2.8 nm with a mean particle size of 2.1 nm (Fig. 1d inset) which was slightly larger than that of original catalyst. The increase of particle size may be ascribed to the adsorption of other species on the active sites of Rh metal and responsible for the reduction of catalytic activity in the fifth run [2]. On the basis of our experimental data, for the catalytic HDC of 4-CP, we also compared our work with the literature reported in the Rh-based catalysts supported on a variety of materials. For example, Diaz et al. [42] achieved an initial rate of 20.6 and 38.0 mmol/gRh min for 4-CP HDC over a 0.5 wt% Rh/Al2 O3 catalyst at temperatures of 20 and 40 ◦ C, respectively and no reusability of the catalyst was available. Molina et al. [18] reported the HDC of 4-CP over a 0.92 wt.% Rh–Al pillared clays prepared by impregnation of RhCl3 and pillared clays, with an initial rate of 73.5 mmol/gRh min and a TOF of 0.24 s−1

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and the catalyst Rh-Al pillared clays could be reused three times. Calvo et al. obtained the catalytic activity in the range from 1.7 to 29.4 mmol/gRh ·min with the TOF value from 2.9 to 50.5 s−1 over the poly(N-vinyl-2-pyrrolidone) stabilized Rh catalyst with different particle sizes in the HDC of 4-CP and the catalyst can be reused at three times with a gradual loss of activity [16]. In the present work, we can achieve an initial HDC rate of 111.4 mmol/gRh min with a TOF of 11.9 s−1 for the HDC of 4-CP at 303 K with a balloon hydrogen pressure; moreover, the Rh/RGO exhibited excellent stability and could be recycled five times. 4. Conclusion In summary, we have prepared the Rh-NPs/RGO nanocomposite catalyst through a co-reduction process and its catalytic property for the HDC of 4-CP was systematically investigated in this work. It was observed that the Rh NPs were of a narrow size distribution and uniformly dispersed on the surface of RGO sheets. The assynthesized catalyst exhibited better catalytic performance toward the HDC of 4-CP compared to the counterparts Rh NPs deposited on the commonly used supports such as AC, Al2 O3 , TiO2 , Fe3 O4 . The high catalytic activity and selectivity can be attributed to a smaller particle size of Rh combined with the functional groups on the surface of RGO, an electron-deficient of Rh from the strong metal-support interaction and the presence of hydroxyl groups on the surface of the support. In addition, the generated Rh-NPs/RGO catalyst was quite stable and can be reused at least four times without significant deactivation. This advanced composite material provides a kind of novel and effective catalyst with great promise for catalytic HDC of 4-CP in practical application. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21207109), Scientific Research Fund of Sichuan Provincial Education Department (11ZA034), and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1205). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2014.04.005. References [1] M.A. Keane, A review of catalytic approaches to waste minimization: case study—liquid-phase catalytic treatment of chlorophenols, J. Chem. Technol. Biotechnol. 80 (2005) 1211–1222. [2] M.A. Keane, Supported transition metal catalysts for hydrodechlorination reactions, ChemCatChem 3 (2011) 800–821. [3] Y. Shao, Z. Xu, H. Wan, H. Chen, F. Liu, L. Li, S. Zheng, Influence of ZrO2 properties on catalytic hydrodechlorination of chlorobenzene over Pd/ZrO2 catalysts, J. Hazard. Mater. 179 (2010) 135–140. [4] Z. Jin, C. Yu, X. Wang, Y. Wan, D. Li, G. Lu, Hydrodechlorination of chlorophenols at low temperature on a novel Pd catalyst, Chem. Commun. (2009) 4438–4440. [5] J. Baeza, L. Calvo, M. Gilarranz, A. Mohedano, J. Casas, J. Rodriguez, Catalytic behavior of size-controlled palladium nanoparticles in the hydrodechlorination of 4-chlorophenol in aqueous phase, J. Catal. 293 (2012) 85–93. [6] T. Hara, T. Kaneta, K. Mori, T. Mitsudome, T. Mizugaki, K. Ebitani, K. Kaneda, Magnetically recoverable heterogeneous catalyst: palladium nanocluster supported on hydroxyapatite-encapsulated ␥-Fe2 O3 nanocrystallites for highly efficient dehalogenation with molecular hydrogen, Green Chem. 9 (2007) 1246–1251. [7] G. Yuan, M.A. Keane, Role of base addition in the liquid-phase hydrodechlorination of 2,4-dichlorophenol over Pd/Al2 O3 and Pd/C, J. Catal. 225 (2004) 510–522. [8] H. Hildebrand, K. Mackenzie, F.-D. Kopinke, Highly active Pd-on-magnetite nanocatalysts for aqueous phase hydrodechlorination reactions, Environ. Sci. Technol. 43 (2009) 3254–3259.

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