reduced graphite oxide nanocomposite: An efficient catalyst for nitrobenzene hydrogenation

Journal of Colloid and Interface Science 374 (2012) 83–88

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Pt/titania/reduced graphite oxide nanocomposite: An efficient catalyst for nitrobenzene hydrogenation Yanfei Zhao, Hongye Zhang, Changliang Huang, Sha Chen, Zhimin Liu ⇑ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry Chinese Academy of Sciences, Beijing 100080, China

a r t i c l e

i n f o

Article history: Received 9 November 2011 Accepted 22 January 2012 Available online 30 January 2012 Keywords: TiO2 Reduced graphite oxide Platinum Nitrobenzene hydrogenation

a b s t r a c t In this work, a ternary composite, Pt/TiO2/RGO (reduced graphite oxide), was prepared via immobilizing Pt particles onto the TiO2/RGO composite that was obtained via redox reaction of TiCl3 and GO. The composite was characterized by different techniques including X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The TiO2 particles with size less than 10 nm were uniformly distributed throughout the RGO, and almost each Pt particle with size around 3 nm adhered to TiO2 particles, resulting in high dispersion of all Pt particles on the support. The Pt particles were in the electron-deficient state due to the strong interactions with the TiO2 particles and the RGO support. The catalytic performance of the composite for nitrobenzene hydrogenation was investigated under solvent-free condition. It was indicated that the Pt/TiO2/RGO catalyst exhibited high activity with a turnover frequency (e.g., 59,000 h 1) as well as superior selectivity to aniline (e.g., >99%). Moreover, the catalyst can be reused for six times without any activity loss, which resulted from the stable structure of the catalyst. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction As an important chemical raw material, aniline (AN) is mainly used for the production of methylene diphenyl diisocyanate (MDI) and additives for rubber process, intermediates dyes and pigments, pesticides and herbicides [1]. AN can be produced via catalytic hydrogenation of nitrobenzene (NB), reduction in NB by iron powder, and phenol catalytic ammoniation, etc. [2]. The catalytic hydrogenation of NB is commonly used to manufacture amines due to its environmentally friendly production process, which can be carried out either in gas or liquid phase. Various metal supported catalysts, such as Cu, Ru, Ni, Pd, Pt, have been applied in NB hydrogenation [1–16], and their catalytic performances are affected by many factors including metal type, metal particle size and composition, support, solvent, H2 pressure [5–7,11,13,14]. Pt or Pd supported nanocatalysts are reported as effective catalysts in liquid-phase hydrogenation under milder conditions with better activity and selectivity [5–8,12–16]. Especially, Pt supported catalysts have been identified as the best catalyst for NB hydrogenation to date [14]. Catalyst support is an important factor to influence the activity of the catalysts, and various supports, such as alumina, activated carbon and carbon nanotubes, montmorillonite, SiO2, MCM-41 meso-materials, have been investigated to immobilize metal nanoparticles for NB hydrogenation [1–3,5–8,11–16].

⇑ Corresponding author. Fax: +86 10 6256 2821. E-mail address: [email protected] (Z. Liu). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2012.01.046

However, most of them are concerned with the geometric effects of the support materials on the metal nanoparticle. In other words, the main roles of the support are usually limited to dispersing and stabilizing small particles. Little attention has been paid to the electronic effect of the support on the metal nanoparticles. It has been reported that the combination of small-crystal-size metal and metal oxide support with nanostructures has turned out to exhibit excellent catalytic performance [17,18]. For example, nanocrystalline ceria (5 nm) supported gold with size less than 5 nm showed high activity, superior selectivity, and good recyclability for the oxidation of alcohols into aldehydes and ketones [17]. However, it is difficult to prepare this kind of catalysts because the nanostructured support is apt to aggregate, which results in the decrease in the performances of the catalyst. As an emerging class of materials, graphene and graphene-based nanocomposites have sparked enormous research interest in recent years owing to its unique structure, high surface area, and high electrical conductivity [19–30]. In catalysis, graphene or reduced graphite oxide (RGO) as catalyst support often exhibits synergistic and/or extra contributions with the supported active components to catalytic reactions. For example, graphene modified with gold nanoparticles displayed an excellent visible-light photocatalytic performance in degrading dyes in water [25]. Co3O4/RGO hybrid exhibited high performance toward both the oxygen reduction reaction and oxygen evolution reaction due to synergistic chemical coupling effects between Co3O4 and graphene [31]. Therefore, as a functional support, graphene or RGO has attracted much attention and shown promising applications in catalyst designing.

Y. Zhao et al. / Journal of Colloid and Interface Science 374 (2012) 83–88

In this work, we designed a ternary catalyst, Pt/TiO2/RGO, in which the two-dimensional RGO acted as support to disperse TiO2 nanoparticles, and TiO2 particles were used to support metal particles. This composite was prepared via immobilizing Pt particles onto the TiO2/RGO composite that was obtained via redox reaction of TiCl3 and GO. The resultant composites were characterized by different techniques including X-ray diffraction (XRD), high resolution TEM (HRTEM), and X-ray photoelectron spectroscopy (XPS). The catalytic performance of the Pt/TiO2/RGO catalyst for liquid-phase NB hydrogenation was investigated with focus on the influence of the support.

(101)

Pt/TiO2 /RGO

(004) (200) (105)(201)

Intensity (a.u.)

84

TiO2 /RGO

GO

2. Experimental 2.1. Materials All chemicals used in this work were of analytical grade and used as supplied. Graphite powder and Titanium (III) chloride (20% in 3% HCl aqueous solution) were purchased from Alfa Aesar. Nitrobenzene, aniline, and other chemicals were provided by Beijing Chemical Reagent Company. Graphite oxide was prepared based on the procedures reported by Hummers and Offeman [32]. 2.2. Synthesis of catalysts In a typical experiment to synthesize TiO2/RGO, 60 mg of GO was dispersed in 40 mL of distilled water under the tip sonication (Vibra Cell CVX, 500 W, 20 kHz, 30% amplitude) for 10 min, and then 5 mL of 0.4% TiCl3 aqueous solution was mixed with the GO dispersion and ultrasonicated for about 5 min under Ar protection. Subsequently, the mixture was loaded into a Teflon-lined autoclave and kept at 150 °C for 10 h. Finally, the solid sample was separated via centrifugation (5000 r/min) and washing with ethanol for several times. The TiO2 content in the TiO2/RGO sample was about 35.1 wt%, determined by TG analysis. For comparison, TiO2 nanorods were prepared via TiCl3 hydrolysis (20 g of 5% TiCl3 aqueous solution) at 150 °C for 10 h. The RGO support was prepared via hydrothermally treating GO suspension (60 mg of GO dispersed in 40 mL of distilled water) at 150 °C for 10 h. In a typical experiment to synthesize Pt/TiO2/RGO, 15 mg of the as-prepared TiO2/RGO was initially dispersed in 50 mL H2PtCl6
6H2O ethanol solution at a designated concentration to form a uniform suspension via tip sonication (500 W, 20 kHz, 30% amplitude power output) for 3 min. Then, 1 mL NaBH4 (0.6 mg/mL) ethanol solution was dropped into the suspension under tip sonication within 3 min. Subsequently, the obtained sample was ultracentrifuged (5000 r/min), washed repeatedly with absolute ethanol and distilled water, and then vacuum dried at 60 °C for 6 h. For comparison, samples Pt/TiO2 and Pt/RGO were prepared via reduction reaction between H2PtCl6
6H2O and NaBH4 by using TiO2 nanorods and RGO as supports, respectively.

10

20

30

40

50

60

2-Theta (Degree) Fig. 1. XRD patterns of GO, TiO2/RGO, and Pt/TiO2/RGO.

2.3. Characterization of the composites XRD analysis was carried out on a D/MAX-RC diffractometer operated at 30 kV and 100 mA with Cu Ka radiation. XPS was performed on an ESCAL Lab 220i-XL spectrometer at a pressure of 3  10 9 mbar (1 mbar = 100 Pa) using Al Ka as the excitation source (hm = 1486.6 eV) and operated at 15 kV and 20 mA. The morphology and microstructure of the products were examined by TEM on a transmission electron microscope (JEOL JEM-2010) equipped with an energy dispersive X-ray spectrometer with 200 kV accelerating voltage. All the samples for TEM, HRTEM and EDS, observation were dispersed on the lacey carbon film with Cu grids. TG measurements were performed on a thermal analyzer (NETZSCH STA 409 PC/PG) with a heating rate of 10 °C/min under air. 2.4. Hydrogenation of nitrobenzene Hydrogenation reactions of nitrobenzene were carried out in a high pressure stainless steel reactor with a magnetic stirrer. Typically, the desired amount of catalyst and NB were placed in the reactor; the reactor was then sealed and flushed with H2 three times to remove the air inside. Subsequently, the reactor was moved to an oil bath set at a desired temperature, and H2 was introduced up to the required pressure. The H2 pressure was kept constant by replenishing H2 as the reaction proceeded. After desired reaction time, the reactor was cooled in ice water and the gas inside was vented slowly. The product and catalyst were separated by centrifugation (16,000 rpm) after reaction. The composition of the reaction mixture was analyzed by means of GC

Scheme 1. Representation of the fabrication of Pt/TiO2/RGO hybrid.

Y. Zhao et al. / Journal of Colloid and Interface Science 374 (2012) 83–88

RGO was first prepared via redox reaction between TiCl3 and GO, which was subsequently redispersed in an H2PtCl6.6H2O ethanol solution, followed by dropping NaBH4 aqueous solution under intense tip ultrasonication, resulting in Pt/TiO2/RGO composite. For comparison, Pt/RGO and Pt/TiO2 composites with the same Pt loading were also prepared, respectively. These catalysts were characterized by means of different techniques. Fig. 1 shows the XRD patterns of GO, TiO2/RGO, and Pt/TiO2/ RGO. In the XRD pattern of GO, a sharp peak appeared at 11.86°, indicating the layered structure of GO with a larger interlayer distance of approximately 0.75 nm. This peak disappeared in the XRD pattern of TiO2/RGO, suggesting the layered structure of GO was destroyed after it was reduced by TiCl3. In this work, the TiO2/

(Agilent 4890D) with a FID detector and a nonpolar capillary column (DB-5) (30 m  0.25 mm  0.25 lm). The column oven was temperature-programmed with a 2 min initial hold at 323 K, followed by the temperature increase to 538 K at a rate of 15 K/min and kept at 538 K for 10 min. High purity nitrogen was used as a carrier gas. 3. Results and discussion In this work, the ternary catalyst, Pt/TiO2/RGO hybrid, was designed, and its synthesis process is schematically illustrated in Scheme 1. TiCl3, GO, H2PtCl6, and NaBH4 were used as the starting materials for the synthesis of the Pt/TiO2/RGO composites. TiO2/

2

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85

4

Size (nm)

2

3

4

Size (nm)

2

3

4

Size (nm) Fig. 2. (a) TEM image of Pt/TiO2/RGO, inset: the size histogram of Pt particles in Pt/TiO2/RGO, (b) EDX profile of Pt/TiO2/RGO, (c) HRTEM image of Pt/TiO2/RGO, (d) TEM image of Pt/RGO, inset: the size histogram of Pt particles in Pt/RGO, and (e) TEM image of Pt/TiO2, inset: the size histogram of Pt particles in Pt/TiO2.

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Y. Zhao et al. / Journal of Colloid and Interface Science 374 (2012) 83–88

RGO composite was constructed by redox reaction between TiCl3 and GO accompanied with subsequent hydrothermal treatment. From the XRD result, it can be concluded that GO was reduced to RGO. In addition, the presence of the diffraction peaks assigning to pure anatase phase (JCPDS, No. 21-1272) suggested that TiCl3 converted into anatase during the formation of the composites. As compared with the XRD patterns of TiO2/RGO, there is no characteristic diffraction peak of Pt species in the XRD pattern of Pt/TiO2/RGO, which implies the low metal loading and/or tiny metal particle size. TEM observation was carried out to gather information about the morphology of the resultant Pt/TiO2/RGO, Pt/RGO, and Pt/TiO2 catalysts. Fig. 2a–e shows typical TEM images together with Pt particle size distribution histograms of the as-prepared samples with the same Pt loading (2.06%). The mean diameter of the Pt NPs was estimated from counting about 100 metal particles. The TEM image of Pt/TiO2/RGO shows the uniform distribution of TiO2 particles throughout the RGO surface and the particle size was around 7 nm (Fig. 2a). In the TEM image of Pt/TiO2/RGO, numerous particles with average size of 3 nm appeared besides TiO2 particles, which should be Pt particles confirmed by EDS analysis shown in Fig. 2b. As shown in Fig. 2c, it should be pointed out

286.2

284.4

287.2 288.6

290

284.3

b Intensity (a.u.)

Intensity (a.u.)

a

that majority of the Pt nanoparticles adhered to the TiO2 particles, and only a few were distributed on the RGO support, free of TiO2 particles. This special structure might provide the composite with unique property in catalysis. The TEM images of Pt/RGO and Pt/ TiO2 shown in Fig. 2d and e exhibited that the Pt particles were uniformly distributed on the support and the average size of Pt particles was around 3 nm. To explore the oxidation states of the various species in the asprepared samples, XPS measurements were conducted on the asprepared composites. As shown in Fig. 3a, the C1s core level XPS spectrum of the GO material can be deconvoluted into four peaks at 284.4, 286.2, 287.2, and 288.6 eV, which are assigned to C@C/ CAC, CAO in hydroxyl or epoxy forms, C@O and O@CAO, respectively, suggesting the presence of these groups in the GO material. Interestingly, compared to the spectrum of GO, in the C1s XPS spectrum of the sample TiO2/RGO, the peaks at 286.5 and 287.2 eV decreased remarkably relative to that at 284.4 eV (Fig. 3a), indicating that considerable amount of CAO and C@O groups on the GO was removed due to the reaction between TiCl3 and GO. However, the relative intensity of carboxylic group to graphitic carbon changed a little, suggesting that some ACOOH groups still remained in the RGO surface. It is worth noticing that a new

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Binding Energy (eV)

Intensity (a.u.)

Intensity (a.u.)

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Ti2p

Pt4f

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72.4 eV

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Binding energy (eV)

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e Intensity (a.u.)

Intensity (a.u.)

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Pt4f7/2

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Binding energy (eV)

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Binding energy (eV)

Pt4f7/2

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Pt4f7/2

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c

O1s

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Binding Energy (eV)

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71.1 eV

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82

Binding energy (eV)

Fig. 3. (a) C1s XPS spectra of GO, (b) C1s XPS spectra of TiO2/RGO, (c) survey XPS spectrum of Pt/TiO2/RGO, (d) Pt 4f core level XPS pattern of Pt/TiO2/RGO, (e) Pt 4f core level XPS pattern of Pt/RGO, and (f) Pt 4f core level XPS pattern of Pt/TiO2.

Y. Zhao et al. / Journal of Colloid and Interface Science 374 (2012) 83–88

peak appeared at 285.5 eV, which can be attributed to defectcontaining sp2-hybridized carbon [33,34]. This indicates that defects remained after removal of the oxygen-containing groups, which may provide the nucleating sites for TiO2 produced via hydrolysis of Ti4+. The above findings are identical to those reported for the GO reduction by NaBH4 [35]. The wide survey XPS spectrum of Pt/TiO2/RGO (Fig. 3c) shows the coexistence of elements such as Ti, O, C, and Pt, which were originated from the TiO2, RGO, and Pt species; no heteroelements, including Cl, were detectable, ruling out the presence of unreacted precursor or the formation of by-products in Pt/TiO2/RGO. The Pt 4f core level XPS spectrum of Pt/TiO2/RGO can be deconvoluted into two sets of doublet components, as illustrated in Fig. 3d. The peaks of Pt 4f7/2 appeared at the binding energy (BE) of 72.4 eV, which is close to, but much higher than that of bulk Pt (0) (70.7 eV) [14,36]. It should be assigned to Pt (0) because the Pt species were produced 2 by reducing PtCl6 with excessive NaBH4 during the synthetic process. The other weak doublets indicated the presence of a small amount of Pt oxides, which may be attributed to the partial oxidation of the metal after exposing to air [14,36]. For comparison, we also made XPS measurements on the as-prepared Pt/RGO and Pt/TiO2 composites. As illustrated in Fig. 3e and f, the BEs of Pt in the two composites were centered at 72.1 eV and 71.1 eV, respectively. Compared to the BE of the bulk Pt (70.7 eV), the BEs of Pt particles in these three composites shifted to higher values, suggesting that the Pt particles were in the electron-deficient states. The small sizes of Pt particles as well as the interaction between the Pt particles and the support should be responsible for the shifts. Note that the BE of Pt shifts to higher values followed the order: Pt/TiO2/RGO > Pt/RGO > Pt/TiO2. Since the average sizes of Pt particles in these composites were similar, the differences of BE shifts may mainly result from the interactions between the Pt particles and the supports. The as-prepared catalysts, Pt/TiO2/RGO, Pt/TiO2, and Pt/RGO, were used to catalyze NB hydrogenation under mild and solventfree conditions, and the results are listed in Table 1. All the supported Pt catalysts were very active for catalyzing NB hydrogenation with good selectivity to AN at 333 K and H2 pressure of 4.0 MPa, though the supports TiO2, RGO, and TiO2/RGO were inactive to this reaction. This indicates the Pt particles were the actual active component for catalyzing NB hydrogenation, which was consistent with the reported results [14]. From Table 1, note that the catalytic activity of these three catalysts increased in the order: Pt/TiO2 < Pt/RGO < Pt/TiO2/RGO. Moreover, compared to Pt/TiO2 and Pt/RGO, Pt/TiO2/RGO showed highly enhanced activities (Table 1, entries 1–3) with a TOF of 59,000 h 1 at the NB conversion of 100% and selectivity toward AN of 99.9%. Since the average sizes of Pt particles in these catalysts were similar, the differences in their catalytic activity should originate from the influence of the supports. It was reported that the rate determining step of NB hydrogenation is a nucleophilic attack of hydride ion, produced

Table 1 NB hydrogenation over the as-prepared catalysts.a Entry

Catalyst

Time (min)

NB conversion (%)

AN selectivity (%)

TOFb (h 1)

1 2 3

Pt/TiO2 Pt/RGO Pt/TiO2/ RGO Pt/TiO2/ RGO

20 14 8

98.4 99.9 100

98.1 99.9 99.9

23,000 34,000 59,000

8

100

99.9

59,000

4c a b c

Substrate (10 mmol), 12 mg catalyst, 333 K, 4 MPa H2. TOF of NB conversion (mol NB converted per mol Pt per h). Catalyst reused for the sixth time.

87

by dissociative adsorption of H2 molecule on the metal surface, on the nitrogen atom of the nitro group [37]. Moreover, the changes in the electronic structure within a supported Pt particle affected the chemisorption of H2 on its surface [38]. Consequently, the alterations in the electronic structure induced by changing the support have an important influence on the catalytic behavior of the Pt particles. XPS analysis showed that the Pt nanoparticles in these catalysts were in the electron-deficient state, and the electron transfer occurred from the Pt particles to the supports. However, due to the inherent nature of the supports, the Pt particles deposited on these supports exhibited different electron-deficient states. Among these three catalysts, the Pt particles in Pt/TiO2/ RGO displayed the most electron-deficient state due to the simultaneous interactions with TiO2 and RGO, which may be the most favorable to the sorption of H2 on the Pt particle surfaces. This resulted in the production of more hydride ion and further enhanced the catalytic properties of the Pt/TiO2/RGO catalyst. For Pt/TiO2 catalyst, its catalytic activity decreased about 40% for the second run. The Pt/RGO catalyst kept a good catalytic activity after reused for three times; however, it had much lower activity than the Pt/TiO2/RGO catalyst. Pt/TiO2/RGO catalyst can be reused for six times without obvious activity loss (Table 1, entry 4), suggesting the stable nature of the Pt/TiO2/RGO catalyst, which should result from its stable structure. In this catalyst, the TiO2 particles with size less than 10 nm were uniformly distributed through the RGO support, and almost each Pt particle adhered to TiO2 particles. This means that the Pt particles were highly dispersed on the support; meanwhile, the TiO2 particles could prevent them from aggregation during the reaction process. Moreover, the sizes of Pt and TiO2 particles as well as the morphology of the catalyst almost kept unchanged after the catalyst was reused for six times, confirmed by the TEM observation. 4. Conclusion A ternary nanocomposite, Pt/TiO2/RGO, was obtained by immobilizing Pt particles onto the TiO2/RGO composite that was prepared via the redox reaction between TiCl3 and GO. The Pt particles with size around 3 nm were highly dispersed, and they were in the electron-deficient state due to the strong interactions with TiO2 particles and the RGO support. The resultant Pt/TiO2/RGO composite exhibited high activity with a turnover frequency (e.g., 59,000 h 1) as well as superior selectivity to aniline (e.g., >99%) and can be reused for six times without any activity loss originated from its stable structure. The approach to prepare Pt/TiO2/RGO composite may be extended to the synthesis of other metal/TiO2/ RGO nanocomposites such as Ru/TiO2/RGO, AuPt/TiO2/RGO, AuPd/ CeO2/RGO, and PtRu/CeO2/RGO, with the similar morphology to Pt/TiO2/RGO. The further work to design multi-component nanocatalyst is under way. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 20903105, 21073202) and the Chinese Academy of Sciences (KJXC2-YW-H30). References [1] C.H. Li, Z.X. Yu, K.F. Yao, S.F. Ji, J. Liang, J. Mol. Catal. A: Chem. 226 (2005) 101– 105. [2] X.M. Fang, S.L. Yao, Z. Qing, F.Y. Li, Appl. Catal. A: Gen. 161 (1997) 129–135. [3] S.G. Diao, W.Z. Qian, G.H. Luo, F. Wei, Y. Wang, Appl. Catal. A: Gen. 286 (2005) 30–35. [4] K.V.R. Chary, C.S. Srikanth, Catal. Lett. 128 (2009) 164–170. [5] E.A. Gelder, S.D. Jackson, C.M. Lok, Catal. Lett. 84 (2002) 205–208. [6] X. Yu, M. Wang, H. Li, Appl. Catal. A: Gen. 202 (2000) 17–22. [7] F.Y. Zhao, Y. Ikushima, M. Arai, J. Catal. 224 (2004) 479–483.

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