J. Phys. Chem. C 2008, 112, 1199-1203
1199
Highly Active and Stable Material for Catalytic Hydrodechlorination Using Ammonia-Treated Carbon Nanofibers as Pd Supports Qiang Liu, Zhi-Min Cui, Zhuo Ma, Shao-Wei Bian, and Wei-Guo Song* Beijing National Laboratory of Molecular Sciences (BNLMS) Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100080, P. R. China ReceiVed: September 17, 2007; In Final Form: NoVember 7, 2007
Carbon nanofibers (CNFs) are treated with ammonia at elevated temperatures to generate pyridinic-type basic species on the CNFs surface. The basic species act as anchors for Pd nanoparticles, resulting in good dispersion and stability of the Pd nanoparticles. These ammonia-treated carbon nanofibers as Pd supports have been used for catalytic hydrodechlorination. It shows high activity and impressive time-on-stream stability in the gas- and liquid-phase catalytic hydrodechlorination of chlorobenzene. The presence of pyridine-type species helps to remove the byproduct HCl from the Pd catalyst, thus preventing the inhibition effect of HCl. It also keeps Pd nanoparticles from aggregation, which is another reason for catalyst deactivation.
1. Introduction
2. Experimental Section
Chlorinated organic compounds are highly hazardous and toxic environmental pollutants and are partially blamed for global warming and ozone depletion.1 Therefore, the disposal of these organic wastes is a vital environmental issue. Several methods have been developed for their elimination, including incineration,2-4 biodegradation,5-7 and photocatalytic oxidation using titania with UV light.8-11 These methods, with various degrees of success in degrading the chlorinated compounds, are still restricted for low conversion and difficulty in scale-up. The catalytic hydrodechlorination (HDC) reaction, which can be carried out in both the liquid and the gas phase, is a promising route to abate the impact of chlorinated compounds due to lowenergy demands and the fact that no other toxic products are produced.12-16 Catalytic HDC is known to be promoted by supported noble metals, especially Pd.17 Various carbon materials17 and other inorganic materials such as Al2O318 and MgO19 have been used as supports. The carbon-supported Pd catalyst is one of the most efficient catalysts for HDC reactions. Its high activity is attributed to the hydrogen spill over from Pd nanoparticles to the carbon surface.12,20 Keane et al. reviewed the literature on HDC over carbon-supported Pd catalysts, and they found a correlation between the activity and the nature of the carbon support, as Pd/CNFs (carbon nanofibers) > Pd/AC (activated carbon) > Pd/graphite.17 However, all of these catalysts suffer rapid deactivation by HCl,21-25 which is the byproduct of the reaction, and coking.26-28 These have become the main factors preventing the use of the catalytic HDC of chloroaromatic compounds. This paper describes our recent work to modify the surface of the CNF support with ammonia, which generates pyridinic basic species to counteract the factors that cause catalyst deactivation during the reaction, and reports a catalyst material with high activity as well as stability in HDC reactions. A mechanism is proposed to elucidate the catalyst activity and stability.
All reagents were of analytical grade and used as received. The CNFs were prepared according to our previous report.29 Briefly, the colloidal suspensions (ca. 40 µL) of Ni-oleate (0.1 mM) in hexane (1 mL) were deposited directly on 5 × 20 mm2 silicon substrates. The catalyst was dried overnight at 80 °C. The catalytic decomposition of acetylene was carried out on nickel catalysts at the desired temperature inside an 8 mm i.d. quartz tube that housed the silicon substrates. In a typical synthesis procedure, the furnace was heated to the reaction temperature at 10 °C/minute with 80 cm3 (STP) min-1 H2 flow. At 580 °C, 5 cm3 (STP) min-1 C2H2 was added to the hydrogen flow for 30 min. The chlorinated CNFs were synthesized by bubbler CH2Cl2 using 15 cm3 (STP) min-1 N2 without changing the other conditions. The NH3 modified CNFs were obtained by treating the HNO3 purified CNFs in NH3 (60 cm3 (STP) min-1) flow for 100 min at 850 °C. The reactor was then cooled to ambient temperature under H2 /NH3 flow. As-prepared CNFs were treated with 4 M HNO3 and sonicated overnight to remove the nickel catalysts. After that, the CNFs were washed thoroughly with deionized water. Pd particles loaded on various carbon supports were prepared as follows. CNFs were added to distilled water containing aqueous PdCl2 solution in a three-neck bottle. After agitation of the slurry for a few minutes, NaOH solution was added dropwise until pH ) 9.5 and the mixture were stirred overnight at room temperature. The resulting suspension was centrifuged, washed with distilled water several times, and dried at 80 °C overnight. The catalysts were activated in H2 flow at 200 °C for 2 h before HDC reaction. The gas-phase catalytic HDC reactions were carried out under atmospheric pressure in a fixed-bed down flow glass reactor (8 mm i.d.). In a typical experiment, 3 mg Pd/CNFs and 500 mg sea sands were mixed as the catalyst and the experiment was carried out from room temperature to 300 °C under 70 cm3 (STP) min-1 flows of H2, which also introduces chlorobenzene over the catalysts by bubbling. In a separate control experiment, the fresh Pd/CN catalyst was first treated with dilute HNO3 (0.5 M) overnight to neutralize pyridinic nitrogen species, centrifuged, washed with distilled water several times, dried at
* Corresponding author. Phone & Fax: (86)10-62557908. E-mail:
[email protected].
10.1021/jp077496s CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008
1200 J. Phys. Chem. C, Vol. 112, No. 4, 2008
Figure 1. (a) SEM image of sample CN; FTIR spectra corresponding to (b) sample C0 and (c) sample CN; (d) curve-fitted N 1s peak by XPS analysis of sample CN.
80 °C overnight, and then used in gas-phase HDC. The gas sample was taken periodically and analyzed by Agilent 6890 GC equipped with a FID detector. The liquid-phase catalytic HDC reactions were carried out in a 50 mL flask with magnetic stirring at room temperature, in which 10 µL chlorobenzene, 3 mg Pd/CNFs catalysts, and 30 mL methanol were added. Toluene was used as an internal standard, and 40 cm3 (STP) min-1 hydrogen was continuously bubbled into the solution during the reaction. Periodically, the reaction mixture was sampled (2 µL) and analyzed by Agilent 6890 GC equipped with a FID detector. The structure and morphology of the synthesized products were characterized by scanning electron microscopy (SEM, HITACHI S-4300), transmission electron microscopy (TEM, JEOL 2010, 200kV) equipped with an energy dispersive X-ray (EDX) analyzer (Phoenix), Fourier transform infrared spectrometry (FTIR, Bruker Tensor 27), X-ray photoelectron spectroscopy (XPS, ESCALab220I-XL with Al/Mg cathode radiation), and Brunauer-Emmett-Teller (BET, ASAP 2010). The thermal gravity analysis (TGA, NETZSCH STA 409 PC/ PG) result was obtained in N2 atmosphere from room temperature to 600 °C (10 °C/min), and inductively coupled plasma (ICP, Profile-ICP, LEEMAN) was used to determine the loading of Pd. 3. Results and Discussion The original CNFs (C0) were synthesized by catalytic chemical vapor deposition (CCVD) using Ni as the catalyst and acetylene as the carbon source. NH3 functionalized CNFs (CN) were prepared by flowing ammonia through the purified CNFs at 850 °C for 100 min. Figure 1a shows the SEM images of CN. It has the same morphology as C0, and no Ni catalyst particle is found. Figure 1b and c illustrates the FTIR spectra of the samples C0 and CN. The as-synthesized sample C0 showed no obvious FTIR adsorption features, indicating the mostly amorphous nature of the CNFs, whereas sample CN exhibited absorption peaks that were due to the stretching vibration of C-N (1260 cm-1) and the bending vibration of N-H (1595 cm-1), which indicated that the NH3 treatment has generated certain basic nitrogen-containing groups on CNFs. These functional groups were hydrophilic and made the CN easier to disperse in aqueous medium than C0. To further investigate the electronic structure of N atoms, we carried out XPS analysis out on sample CN. According to the XPS result, the nitrogen concentration was about 4.0
Liu et al.
Figure 2. TEM images of 10 wt % Pd dispersed on sample Pd/C0: (a) as-synthesized; (b) after HDC reaction; and sample Pd/CN: (c) assynthesized; d) after reaction; insets are the EDX analysis results.
atom %. Treating CNFs with NH3 removed the acidic functional groups on the CNFs, which were predominant on untreated CNF surfaces,30 and introduced basic nitrogen-containing groups on the CNFs.31 The splitting of the N 1s peak as shown in Figure 1d indicates that the doped N atoms are in three different electron states, similar to the report of Choi et al.32 The bands at 398.5 eV corresponds to pyridine-like nitrogen atoms that contribute to the π systems, and the band at 401.2 eV corresponds to graphitic nitrogen species, referring to N atoms that substitute carbon atoms in the graphite layers. The pyridinic structure is enriched in these materials, and this is consistent with a previous study showing that the pyridinic structure was common for CNFs treated with ammonia at temperatures below 1000 °C.33 The HDC of chlorobenzene (CB) was carried out over Pd supported on C0 and CN to probe the role of surface characteristics and to find a promising HDC catalyst. Figure 2 shows the nature of the Pd morphologies and dispersion states on the two supports. The Pd particles with indistinct spherical geometries tend to be located on the basal plane of CNFs, suggesting a strong metal-support interaction.34 The Pd particle size distribution on C0 exhibits a narrow size range of about 2-3.5 nm as measured from TEM images. The Pd particles on CN are too small to be identified from their TEM images (Figure 2c), but their existences are confirmed by surface EDX analysis (inset of Figure 2c). A previous work has shown that well-dispersed Pt nanoparticles on carbon nanotubes were obtained when modifying the metal nanoparticles with the organic molecule triphenylphosphine.35 The free electron pair on the P atom of the triphenylphosphine interacts with the empty orbital of Pt to form a stable complex. In the present study, the interaction between the lone electron pair of the pyridinic nitrogen and the Pd nanoparticles is similar, which enables high dispersion states of the metal nanoparticles on the CN support. This method can be applied to the preparation of a variety of well-dispersed mono/bimetallic metal/carbon composite materials with small metal particle sizes. A commercial catalyst with Pd 10 wt % on activated carbon (Pd/CM, Alfa Aesar) was also studied for comparison. Table 1 lists the details of the Pd nanoparticles on these carbon supporters. The gas-phase HDC of CB using supported Pd catalysts (C0, CN, and CM) under hydrogen flow produces benzene and HCl as the predominant products, with trace amounts of cyclohexane (selectivity 1000
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