Synthesis of Reduced Graphene Oxide-Carbon Nanotubes (rGOâCNT

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Industrial & Engineering Chemistry Research

Synthesis of reduced graphene oxide-carbon nanotubes (rGO-CNT) composite and its use as a novel catalyst support for hydro-purification of crude terephthalic acid

S. Tourani1, A.M. Rashidi2, A.A. Safekordi1,3, H. R. Aghabozorg4, and F. Khorasheh3,*

1

Faculty of Engineering, Islamic Azad University, Science and Research Branch, Tehran, Iran

2

Nanotechnology Research Center, Research Institute of the Petroleum Industry, Tehran, Iran

3

Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran

4

Catalysis Department, Research Institute of the Petroleum Industry, Tehran, Iran

*

Author for correspondence, e-mail: [email protected]

Abstract Palladium nanocatalysts supported on reduced graphene oxide (rGO), multi walled carbon nanotubes (MWCNT), and rGO/CNT composite were synthesized by a wet impregnation method using PdCl2 as precursor. Palladium loading was 0.3 wt. % and the catalysts were reduced at 300oC. The catalysts were characterized by ICP, BET, FTIR, XRD, TEM, TPR, TPD, and Raman Spectroscopy. The performance of the catalysts was investigated for hydro-purification of crude terephthalic acid (CTA) containing 2100 ppm 4carboxybenzaldehyde (4-CBA) as impurity. The reaction products were analyzed by HPLC to determine the amounts of 4-CBA, benzoic acid, and p-toluic acid. Pd/rGO-CNT catalyst had excellent performance in terms of both selectivity and 4-CBA conversion. All catalysts exhibited more than 99% removal of 4-CBA. The most desired selectivity, however, was obtained for the catalyst with rGO-CNT as support. Comparison with the performance of the commercial catalyst (0.5 wt. % palladium on activated carbon) indicated that Pd/rGO-CNT catalyst had a better performance.

Keywords: Crude terephthalic acid; 4-carboxybenzaldehyde; Hydro-purification; Palladium catalyst, Graphene oxide- carbon nanotube composite

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1. Introduction Terephthalic acid (TA) is an important industrial chemical for the synthesis of plastics, dyes, pesticides and many types of polyesters with different physical properties for a variety of applications.1 Three major polyester products synthesized from TA include polyethylene terephthalate, polytrimethylene terephthalate and polybutylene terephthalate among which polyethylene terephthalate has the most applications and the greatest demand for TA. The benefits of polyethylene terephthalate, such as flexibility, resistance to chemicals and weathering, adhesion, and clarity, allow it to be used in different home and industrial products including food and drink containers, polyester fibers for packaging, clothing, furnishings, polyester films and tapes, coatings and glues.1, 2 The most common method for synthesis and purification of TA is the Amoco process in which p-xylene is oxidized by air at a temperature of about 176-225°C and pressure of 15-30 bar.1, 3 Residence time is about 2 hours using a soluble Co/Mn catalyst with Br as a promoter and acetic acid as solvent. After oxidation, the final cake called CTA contains about 3000 ppm of 4-carboxybenzoaldehyde (4-CBA) and 500 ppm of p-toluic acid (p-TA) as the main impurities.4 The permissible concentrations of 4-CBA in TA as a polyester feedstock is usually less than 25 ppm.5 The presence of 4-CBA impurity results in a lower polymerization rate and a reduction in the average molecular weight of the resulting polymer . Furthermore, the yellow color of 4-CBA would result in a colored polymer that is undesired for fiber manufacturing. Purification processes are therefore needed to obtain purified terephthalic acid (PTA). In the Amoco purification process two major steps, namely hydrogenation and crystallization are used to achieve the desired purity for PTA. Hydrogenation of 4-CBA to p-TA can be carried out over a suitable catalyst. The most common catalysts for this purpose are noble metal catalysts where Pd-based catalysts are used commercially.6 The general reaction chemistry is hydrogenation of 4-CBA to 4-hydroxymethylbenzoic acid (4-HMBA) that is subsequently converted to p-toluic acid. Simultaneously, 4-CBA is decarbonylated to benzoic acid (BA) producing an equimolar amount of carbon monoxide which is a well-known poison for palladium hydrogenation catalysts. The unfavorable hydrogenation reaction, however, can occur over Pd catalysts.

1, 7-9

The catalyst selectivity towards each hydrogenation path is

affected by many factors including the interaction between active phase and support, the cluster size of active sites, and the type of support. In this study, we investigated the effect of support in hydrogenation of crude terephthalic acid (CTA). Among the different types of supports which are used in hydrogenation processes, carbon materials attract a growing 2 ACS Paragon Plus Environment

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interest due to their specific characteristics including inertness to acid/basic media, possibility to control, up to certain limits, the porosity and surface chemistry, and easy recovery of precious metals by burning of the support. The structure and properties of carbon supports, such as surface functional groups, graphitization structure, and surface area, have a significant effect on the activity and selectivity of the catalyst thus attracting research interests in recent years in many fields including capacitors, catalysts, fuel cells, and batteries. Various carbon allotropes, including graphite, diamond, fullerenes, graphene, and carbon nanotubes (CNTs), have very different electrochemical properties.10 Carbon nanotubes are the most widely investigated carbon nanostructures as alternative for carbon black as a support for hydrogenation catalysts.11 The high crystallinity of CNTs makes these materials highly conductive and their high surface area and significant fraction of mesopores results in a catalyst with high metal dispersion. These characteristics result in a higher catalytic activity for catalysts supported on CNTs compared with those supported on activated carbon.11 Furthermore, the inert graphitic basal plane surface of CNTs generally requires functionalization, which may significantly reduce its electrical conductivity, to achieve a higher dispersion of Pd nanoparticles. In contrast to CNTs, graphene nanosheet (GNS) could offer the high surface area (theoretical value of 2630 m2/g) conductivity and outstanding electrical conductivity

13, 14

12

, good thermal

, unique graphitized basal plane

structure, and potential low manufacturing cost 12, 15 making it a promising candidate for lowtemperature fuel cell catalyst support. In comparison with CNTs, graphene not only possesses similar stable physical properties but also larger edge planes. Additionally, production cost of GNS in large quantities is much lower than that of CNTs. Furthermore, it has been suggested that the heterogeneity of carbon support with edge planes can better stabilize and enhance the catalytic activity of the Pd catalyst.16 The one-dimensional (1D) CNTs and two-dimensional (2D) GNS are both inclined to aggregate or stack with each other due to strong van der Waals forces thereby hindering the full utilization of the surface area and active sites for catalytic reactions. In fact, the integration of GNS and CNTs into a hybrid material is quite a promising strategy to enhance the dispersion of GNS and CNTs, to inherit the advantages of both GNS and CNTs, and to obtain an efficient and effective electronic and thermal conductive 3D network. Furthermore, combination of GNS with high charge density

17-19

and CNTs with large surface area

generates a versatile 3D GNS-CNT hybrid network with synergic properties.16 Thus the combination of GNS and CNTs as a novel catalyst support may increase the catalytic activity 3 ACS Paragon Plus Environment


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of palladium nanoparticles. Several studies have focused on the use of GNS-CNT composites as support for palladium nanoparticles for different catalytic applications and reported that catalysts supported on the composites had better performance than those prepared on a single support. 16, 20-24 Some studies have also reported that palladium nanoparticles supported on CNT or rGO had high catalytic activity for hydrogenation reactions.25-28 In this work, we synthesized a 3D porous rGO-CNT hydrogel by the hydrothermal treatment of an aqueous dispersion of graphene oxide (GO) and CNTs and subsequently synthesized a Pd/rGO-CNT nanocomposite catalyst by the wet impregnation method for hydro-purification of CTA. The performance of the catalyst was compared with those of Pd/rGO, Pd/functionalized multiwalled CNTs (Pd/FMWCNTs), and a commercial activated carbon supported Pd catalyst. This is also the first time that the performance of such materials is reported for hydropurification of CTA.

2. Experimental 2.1. Catalyst preparation and characterization 2.1.1. Synthesis of graphene oxide (GO) GO was synthesized by oxidation of high purity graphite powder according to the modified Hummer’s method.29 2 g of natural graphite (Merck) and 1 g of NaNO3 were added to 45 ml of concentrated sulfuric acid (98%) under stirring in a flask immersed in an icewater bath and cooled to 0oC. While maintaining vigorous mixing, 6.0 g of KMnO4 was gradually added to the suspension. The rate of addition was controlled carefully to prevent the temperature of the suspension from exceeding 20oC. The ice-bath was then removed and the temperature of the suspension was brought to 30oC where it was maintained for 2 h under the vigorous stirring. 92 ml of deionized (DI) water was then added and the mixture was further stirred for 30 min at 95oC. Finally, 185ml of DI water was added to the mixture followed by drop by drop addition of 6 ml of H2O2 (5%) to reduce the residual KMnO4 and terminate the reaction with the color of the solution turning from dark-brown to yellow.23 The resulting solid graphite oxide was separated and washed with 1:10(v/v) solution of choloridric acid (37%) and DI water to remove metal ions. The resulting mixture was filtered and washed several times with DI water until the pH of the filtrate was neutral. The resulting cake was then dried in a vacuum oven at 40oC for 24h.30


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2.1.2. Purification of CNT and oxidation of CNT The mutli-walled carbon nanotubes (MWCNTs) used in this study (96% purity, 10-30 nm in OD) was prepared in-house at the Research Institute of the Petroleum Industry (RIPI), Iran, by a special CVD method over a catalyst consisting of cobalt-molybdenum nanoparticles supported on nanoporous magnesium oxide (Co-Mo/MgO) at a temperature of 1000oC.

31-33

The reaction of methane decomposition was conducted at atmospheric pressure

with a retention time of 20-50 min. The purification procedure was as follows. A pristine MWCNTs sample was added to 1:1 (v/v) solution of HCl (37%) and DI water and mixed for about 16 h at 60oC. The resulting mixture was filtered and washed several times with DI water until the pH of the filtrate was neutral. For further purification, the prepared materials were dissolved in 1:3 (v/v) solution of nitric acid (65%) and DI water for 3 h at 60oC with the resulting mixture filtered and washed several times with DI water until the pH of the filtrate was neutral. The resulting cake was dried in an oven at 60oC for 24h, and to eliminate the amorphous carbons, the oven temperature was increased to 250oC and held for 30 min.34, 35 The purity of the MWCNTs was about 96%, and their diameters and lengths were between 10-30 nm and 5-15 μm, respectively. Functional groups such as carboxylic (–COOH), carbonyl (–C=O), and hydroxyl (–COH) groups on CNTs are commonly made by treating CNTs in strong oxidants such as sulfuric acid (H2SO4) and nitric acid (HNO3). Functionalization of the MWCNTs would increase the surface area by opening the MWCNTs ends and introducing oxygenated surface groups into their walls.29 This modification would also improve the dispersion of the MWCNTs in H2O and polar solvents during catalyst synthesis.36 These functional groups have been demonstrated to provide favorable nucleation sites for metal nanoparticles growth, and to stabilize the nanoparticles by increasing the nanoparticles-CNTs interaction.37-39 The MWCNTs were oxidized by a sono-chemical

treatment procedure in a 1:3 (v/v) solution of HNO3 (65%) and H2SO4 (98%) in an ultrasonic bath maintained at 60oC for 3 h. The solution was then diluted in an ice bath for 8h, filtered, and washed under vacuum using DI water to remove excess acid and solvent until the pH of the filtrate was neutral. The resulting cake was dried in an oven at 60oC for 24 h.

2.1.3. Synthesis of rGO-CNTs composite and rGO Among different methods commonly used for the preparation of rGO-CNT composites, we prepared the rGO-CNT composites by addition of purified-MWCNTs to a homogeneous GO aqueous dispersion in the 1:1 weight ratio under sonication for approximately 30 min. Purified-MWCNTs are attracted on the surfaces of GO due to strong 5 ACS Paragon Plus Environment


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π–π interaction.20,

23

The mixture was then transferred into a 500 ml autoclave for

hydrothermal treatment at 180°C for 12 h.23, 40 The mixture was then cooled in air at room temperature with natural convection. A black gel-like 3D cylinder was obtained and the solid products were separated by centrifuge and washed with distilled water. The composites were obtained by freeze drying.17, 23, 41 The rGO sample was made by the same procedure without the addition of CNTs.23, 41

2.1.4. Catalyst preparation The Pd/rGO-CNTs catalysts were prepared using rGO-CNTs as support and PdCl2 (Merck) as Pd precursor. Supported Pd catalysts with 0.3 wt. % Pd loading were prepared by a wet impregnation technique with the resulting suspension sonicated for 1 h, followed by evaporation of water in a rotary evaporator at 75oC, drying at 110oC in an oven overnight, and reduction at 300oC with H2 as a carrier gas at a flow rate of 30 ml/min for 2 h. The same procedure was used for the synthesis of Pd/rGO and Pd/FMWCNTs.42 The catalyst synthesis scheme is illustrated in Figure 1.43

2.1.5. Catalyst characterization techniques The metal loadings of the nanocatalysts were determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) using a Perkin Elmer Optima 3000DV instrument. The morphology of the catalysts was characterized by Transmission Electron Microscopy (TEM). TEM images were obtained using a Zeiss EM900 instrument operated with an accelerating voltage of 120 kV to investigate the dispersion and metal particle size of the catalysts. X-ray diffraction (XRD) measurements were obtained using a Philips type PW1840 diffractometer with cobalt Kα radiation and wavelength of 1.79 Å to determine the crystalline phases of the catalysts and to obtain the average crystallite size of the nanoparticles using Scherrer’s equation. The voltage and anode current were 40 kV and 30 mA, respectively, and the scanning range was 2θ=1-60o with a step size of 0.01oC. FTIR spectra for the Purified-MWCNTs, FMWCNTs, GO, rGO and rGO-CNT were collected on a Thermo Nicolet Nexus-670 instrument using KBr as dispersing medium. Raman spectra were obtained using a micro Raman setup (Almega Thermo Nicolet Dispersive Raman Spectrometer). A laser of 532nm wavelength was used as excitation source with a maximum power of 30 mW. Ammonia-TPD measurements were obtained on a Micromeritics TPD2900 apparatus. A sample of 200 mg was heated at 500oC at 10oC/min ramp under helium 6 ACS Paragon Plus Environment

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flow of 40 ml/min for 60 min. After the cooling process, adsorption of the NH3 was carried out at 150oC for 1 h. This was followed by purging the reactor with helium for 60 min at the same temperature. The samples were then heated from 150 to 900oC at a rate of 10oC/min under a helium flow of 40 ml/min while the concentration of the NH3 in the exit gas was monitored simultaneously and continuously by a thermal conductivity detector (TCD). H2temperature-programmed reduction (TPR) experiments were carried out with a Micromeritics 2900 instrument equipped with a thermal conductivity detector. The catalyst samples (70 mg) were first purged under a flow of argon at 300oC to remove traces of water and subsequently cooled to 40oC. TPR was performed using a gas mixture containing 5% hydrogen in argon at a flow rate of 40 ml/min. The samples were heated from 50 to 1000oC with a heating rate of 10oC/min. Textural properties of the nanocatalysts were obtained using a Micromeritics ASAP-2010 nitrogen sorption instrument. Prior to determination of the adsorption isotherm, the sample was de-gassed and heated under vacuum at 200ºC for 12 h. The BET equation was used to determine the specific surface area and the total pore volume.

2.2. Reactor tests The performance of Pd/FMWCNTs, Pd/rGO, and Pd/rGO-CNTs at the GO to CNTs weight ratio of 1:1 was evaluated for hydro-purification of crude terephthalic acid (CTA). The reactor tests were carried out in a batch autoclave reactor (Parr Corporation type 4848) under similar conditions employed in the industrial process. CTA (120 g) containing about 2100 ppm of 4-carboxybenzaldehyde (4-CBA), 234 ppm of p-toluic acid, and 50 ppm BA as impurities with 280 ml of water was charged into the reactor along with 1.3 g of catalyst. Hydro-purification of CTA was conducted under 10 bar hydrogen and 2.5 bar nitrogen partial pressure with stirring at 800 rpm. The reactor was heated to the desired reaction temperature of 290oC and was held at this temperature for one hour. The reaction vessel was then cooled to ambient temperature and the resulting solid and liquid products were separated by filtration and subsequently analyzed by high performance liquid chromatography (HPLC) to determine the concentration of 4-CBA, BA, and p-toluic acid in both the liquid and solid reaction products. HPLC was performed with a Waters Model 600E instrument equipped with 150 mm long C18 column and a UV detector with wavelength set at 230 nm (for BA and p-toluic acid) and 260 nm (for 4-CBA). The mobile phase consisted of 21% (by volume) acetonitrile, 0.1% tri-fluoro acetic acid, and 78.9% HPLC grade water. The catalytic performance was



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evaluated in terms of 4-CBA conversion, product yield, and selectivity for each hydrogenation path.

3. Results and discussion 3.1. Catalysts Characterization

A typical ICP-AES result indicated that Pd loading for catalysts on FMWCNTs and rGO-CNT composite reduced at 300oC, specified as Pd/FMWCNT and Pd/rGO-CNT composite, respectively, were 0.29 wt. % of the support. Residual impurities of Co and Mo (from catalyst used for preparation of CNTs) were 0.29 and 2.9 wt. %, respectively.44 The BET specific area and the total pore volume of Pd/FMWCNT, Pd/rGO, and Pd/rGO-CNT composite are reported in Table 1. The specific surface area of Pd/rGO-CNT composite was between those of Pd/rGO and Pd/FMWCNT.21, 45, 46 The adsorption analysis revealed that there was a reduction of the total surface area and pore volume for the composite material in comparison with the FMWCNT and thus any improved performance achieved with the composite material cannot be explained by an increase in the surface area or changes in the pore structure. The BET results obtained in this investigation are in good agreement with those reported in the literature.21, 45 The improvement in catalytic performance for composite structures is known to occur when the combination of two complementary materials can result in a synergistic effect on important properties.45

Table 1. BET surface area and pore volume of Pd/FMWCNTs, Pd/rGO and Pd/rGO-CNT at 0.3wt% loading and reduction temperature of 300oC Catalysts

SBET(m2/g)

Total pore volume(cm3/g)

Pd/rGO

62.6

0.1043

Pd/FMWCNT

212.8

0.671

Pd/rGO-CNT

142.6

0.39

Figure 2 shows the morphologies of rGO and rGO-CNT composite. In Figure 2(a), it is observed that the sample is composed of wef graphene layers. In addition, in Figure 2(b), the TEM image reveals that graphene nanosheets and the CNTs had formed a uniform composite where rGO sheets had adsorbed on the CNTs or CNTs had filled the space between graphene nanosheets thus providing an intimate contact between the two phases. The TEM images of Pd/CNT and Pd/rGO-CNT composite prepared using PdCl2 as Pd precursors 8 ACS Paragon Plus Environment

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are presented in Figures 2(c) and 2(d), respectively, demonstrating the uniformity of the particles made by the wet impregnation technique. TEM images also reveal that the Pd nanoparticles distribute more densely along the edges of the graphene sheets for the rGOCNT composite. Based on statistical analyses, metal crystallites size distributions were obtained. The results are summarized in Table 2 in terms of the average diameter and the standard deviation for the two catalysts. TEM images also demonstrated that palladium nanoparticles with a uniform size were well-dispersed on the rGO-CNT surface .GO contains a

large number of oxygen-containing functional groups, which allow GO to be well-dispersed in aqueous solutions. The amphiphilic nature of GO sheets can serve as surfactant; CNTs can be dispersed into individual particles after ultrasonication and be adsorbed onto the GO surface through π–π attractions. Hydrothermal treatment of the GO and CNT aqueous dispersion leads to the formation of macroscopic rGO–CNT cylinder hydrogels via weak interactions, such as van der Waals forces, hydrogen bonding, π–π stacking, and inclusion interactions.21, 23, 47, 48

Table 2. Palladium crystallite size and standard deviation from TEM results

Catalysts

Average diameter(nm)

Standard deviation

Diameter range(nm)

Pd/FMWCNT

8.1

1.84

3-12

Pd/rGO-CNT

9.5

2.35

5-13

The X-ray diffraction patterns of GO, rGO, MWCNT and rGO-CNT composite are presented in Figure 3. The diffraction peak of GO at 14.45o, which is corresponding to (001) plane of GO

45, 49

is observed with a interlayer space (d-spacing) of 7.11 Å. The expanded

interlayer spacing of GO compared with graphite (3.35 Å) has been attributed to the presence of functional groups such as epoxy, hydroxyl, and ethers at the basal plane.45,

50-52

rGO

o

exhibited a single peak at 27.41 with a interlayer space (d-spacing) of 3.71 Å, confirming that reduction of GO and the formation of layered graphene nanosheets.23, 45, 53 MWCNTs exhibit peaks at 29.8 and 44.8o.45 Two peaks are also observed for rGO-CNTs composite at 28.8 and 44.1o.45, 47, 49, 54 Compared with MWCNTs, the characteristic broad diffraction peak of the C (0 0 2) is slightly shifted to a lower angle for rGO-CNTs, which is attributed to the effect from the MWCNTs.20 The supported catalysts were also analyzed by XRD (Figure 3) but due to the low Pd loading on FMWCNT (below 0.3 wt%), no Pd peaks were detected. 9 ACS Paragon Plus Environment


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The FTIR spectra for GO, purified-MWCNTs, FMWCNTs, rGO, and rGO-CNT are presented in Figure 4 illustrating the stretching of hydroxyl groups at 3450 and 1390 cm-1 and the C=O carbonyl stretching at 1728 cm-1. The peaks at 1615.45 and 1055.9 cm−1 were assigned to C=C double bonds and the C-O epoxide group stretching, respectively.55As shown in Figure 4(b), the characteristic absorption bands of oxide groups decreased dramatically for the rGO-CNT composite and rGO after reduction. Comparison of the two spectra for purified-MWCNT and FMWCNT in Figure 4(a) indicated that oxygenated groups were formed on the FMWCNTs after treatment with H2SO4 /HNO3 solution. The two peaks at 1728 and 3450 cm-1 had resulted from purification of pristine MWCNTs. The introduction of oxygenated groups over CNTs has been reported to increase the wetting characteristics of CNTs17, 37, 42 and increase the dispersion of metal particles on their surface. These oxygenated groups may also serve as anchoring sites for uniform deposition and growth of Pd on the surface of CNTs .36, 37 The electronic and crystallographic structures of the samples were evaluated by Raman spectroscopy. The Raman spectra of carbon materials show three well known peaks called D, G, and 2D observed at ~1340, ~1540, ~2650 cm−1, respectively. The G peak corresponds to the optical E2g phonons at the Brillouin zone center and represents the bond stretching of sp2 carbon pairs in both rings and chains.56, 57 The D peak corresponds to the breathing mode of sp2 atoms in aromatic rings indicating the presence of defects

56, 57

and 2D

band corresponds to second-order dispersion, a characteristic feature of two-phonon double resonance process.58 A defect-activated peak called D+G is also readily visible at 2950 cm-1. 56, 57

Figure 5 illustrates the Raman spectra of purified-MWCNT, FMWCNTs, GO, rGO, and

rGO-MWCNTs samples. Three distinct peaks are observed in each sample corresponding to D, G, and 2D band, respectively, as mentioned earlier. In the Raman spectra the D/G intensity ratio (𝐼D/𝐼G) is a measure of disorder degree and defect densities in graphene sheets and is inversely proportional to the average size of the sp2 clusters.59 As it is seen in Table 3, the D/G intensity ratio for FMWCNTs, rGO and rGO-CNT composite are larger than those for GO and purified-MWCNTs.56 The increase in the ratio for FMWCNTs, rGO and rGO-CNT composite values are attributed to the ultra-high density of exposed graphene edges for rGO alone and along the rGO-CNT surfaces and the small size of the graphene sheets.16 rGO–CNT composite shows a D/G intensity ratio between rGO and purified-MWCNTs, which suggests the formation of rGO–CNT composite.60 The shift in the G peak position from 1593 (for rGO) to 1591 cm-1 (for rGO-CNT) also indicated interaction between graphene and CNT or


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the formation of rGO-CNT composite structures. A similar behavior for 2D peak was observed.60 The details of Raman spectroscopy results are provided in Table 3.

Table 3. Detailed Raman features of purified-MWCNTs, FMWCNTs, GO, rGO, and rGOCNT composite Sample

D

G

2D

D+G

ID/IG

I2D/IG

Purified –MWCNT FMWCNT GO rGO rGO-CNT

1339 1340 1345 1346 1345

1578 1581 1584 1593 1591

2682 2691 2680 2600 2672

2929 2930 2942 2910 2937

0.7 1.7 0.57 1.55 1.50

0.7 0.63 0.08 0.13 0.2

The ammonia-TPD results presented in Figure 6 indicated that the temperatures for the desorption peaks were quite similar for the three catalysts. NH3-TPD quantitative results are presented in Table 4. The functionalization of MWCNT resulted in the formation of oxygenated surface groups including phenolic and carboxylic groups. According to NH3-TPD profile for three catalyst, the surface acidic groups were the primary locations over the carbonaceous surface which the ammonia molecules were adsorbed by hydrogen bonds and desorbed at a low temperature of about 250oC.61 Acidic oxygen functional groups, such as anhydrides and carboxylic acids, on the FMWCNTs surface play an important role in the metal–support interaction.62 The Pd valence band is mainly formed by 4d states partially hybridized with 5s orbitals.63 Thereafter, the lone pair of electrons around C=O bonds in anhydrides or carboxylic acids can be shared with the 4d orbitals of Pd, thus decreasing the and tendency of Pd to accept electrons from NH3 leading to lower adsorption of NH3 on Pd/FMCNTs. But in Pd/rGO and Pd/rGO-CNT catalysts, on the other hand, most of the palladium is anchored on edge planes and surface defects and located at the edges without functional groups leading to a higher tendency of palladium in these positions to adsorb NH3.62, 64 Table 4. Results of ammonia-TPD for different catalysts Catalyst Pd/FMWCNT Pd/rGO Pd/rGO-CNT

Desorption temperature for first peak (oC) 250

Desorption temperature for second peak (oC) 693

Total ammonia desorption (mmol/g ) 1.57

247

698

6.2

250

696

4.4



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The reducibility of the catalysts under H2 atmosphere was determined by TPR experiments. The TPR results for Pd/FMWCNT, Pd/rGO and Pd/rGO-CNT are presented in Figures 7(a) to 7(c), respectively. For the catalyst with FMWCNT as the support, the two peaks located at 279oC and 406oC are related to the reduction of PdOx species.44 Two feasible explanations for these two reduction peaks could be i) the two step reduction of PdO, and ii) that there are two PdO species which have different interactions with the support.65, 66 For the Pd/rGO and Pd/rGO-CNT catalysts the reduction temperatures for the first and second peaks attributed to Pd species increased from 279oC to 294oC and from 406oC to 450oC, respectively, in comparison with the Pd/FMWCNT catalyst. The observed results are consistent with those reported by Machado et al. (2014) who also suggested that palladium on the oxidized CNT surface is reduced at a lower temperature compared with palladium particles anchored on zig-zag edges of rGO without any oxygenated groups.38 The results suggest that for rGO and rGO-CNT supports, some of the PdOx particles are anchored on edge planes that do not contain oxygen groups. The third peak in Pd/FMWCNT, and Pd/rGOCNT catalysts located at 640-670oC was attributed to the reduction of Mo and Co44, 67, while the peak appearing above 700oC was related to oxygen surface groups.68, 69 The width of the last peak for catalyst with rGO and rGO-CNT support was wider compared with that for the catalyst with FMWCNT support which is likely attributed to the presence of more functional groups on the rGO and rGO-CNT support compared with the FMWCNT support.70 3.2. Catalytic performance of Pd tn oe idee t no carbon structures as support The catalytic performance of Pd/rGO, Pd/FMWCNT, and Pd/rGO-CNT in hydrogenation of CTA was investigated and the results were compared with those for a commercial palladium catalyst supported on a high surface area activated carbon. All catalysts were synthesized using PdCl2 as precursor, had a 0.3 wt. % palladium loading (except for run 3 for which a catalyst with palladium loading of 0.05 wt. % was used), and were reduced at 300oC. Run 2 was also repeated to provide a measure of reproducibility of reactor data. Reactor tests were conducted with the same feed at identical operating conditions for all catalysts. ICP analysis of the liquid products from the reactor indicated that Pd was present in only trace amounts suggesting that no significant leaching of Pd occurred during the course of the reaction. The HPLC analyses for the content of 4-CBA, BA and ptoluic acid (ppm in the reaction products) are presented in Table 5. 4-CBA removal, product selectivity, and product yield are defined as follows: 12 ACS Paragon Plus Environment

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4-CBA conversion (%) =

[ 4  CBA ] in  [ 4  CBA ] out  100 [ 4  CBA ] in

(1)

Selectivity of path 2 to path 1 =

(moles of 4 - CBA consumed) - (moles of BA produced) (2) (moles of BA produced)

Yield of p-toluic acid and BA =

moles of p - toluic acid and BA produced moles of 4 - CBA consumed

(3)

Table 5. Comparison of the catalyst activity tests for different prepared Pd-based catalysts

Run

Catalysts

4-CBA

BA

(ppm)

(ppm)

p-toluic acid (ppm)

4-CBA % conversion

Selectivity of path 2 to path 1

Yield of p-toluic acid and BA

1

Pd/rGO

17.7

505

1584

99.15

2.71

0.983

2

Pd(0.3wt.%)/FMWCNT

5.5

303.6

807.6

99.7

5.71

0.451

2R

Pd(0.3wt.%)/FMWCNT

7.8

289

828

99.6

6.11

0.453

3

Pd(0.05 wt.%)/FMWCNT

1254

56

50.1

40.3

113.56

-0.231

4

Pd/rGO-CNT

3.4

132.6

1396

99.8

19.62

0.659

5

FMWCNT

1951

53

560

7.1

39.35

2.438

6

Non

1874

78

351

10.8

5.55

0.723

23

531

1855

98.9

2.50

1.146

7

Commercial catalyst(Pd/AC)

The proposed paths for hydrogenation of CTA are shown in Figure 8. Selectivity of path 2 to path 1 greater than 1 would indicate that hydrogenation of 4-CBA along path 2 is more favorable compared with path 1. This is advantageous because although BA is more soluble in water compared with p-toluic acid, hydrogenation along path 1 leads to the formation of CO that is considered a poison for palladium catalysts. A products yield above 1 points to higher catalyst hydrogenation activity as TA is converted to 4-CBA. On the other hand, product yields below 1 point to the formation of 4-HMBA (not detected by HPLC) as an intermediate in path 2 hydrogenation and formation of p-tolualdehyde (not detected by HPLC) from hydrogenation of p-toluic acid as the end product in path 2 hydrogenation. Other minor products resulting from hydrogenation of TA include cyclohexane dicarboxylic acid 13 ACS Paragon Plus Environment


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(CHDA) and cyclohexane carboxylic acid (CHCA).71-76 For the catalyst with 0.05 wt.% palladium loading, the hydrogenation activity was too low such that more than half of the 4CBA in the feed had remained unreacted. The low hydrogenation activity resulted in only a slight increase in BA content of the products (from hydrogenation of 4-CBA via path 1). For this catalyst, hydrogenation of p-toluic acid to p-tolualdehade, reaction (1), and hydrogenation of 4-CBA along path 2 to 4-HMBA, resulted in a net decrease in the p-toluic acid content of the reaction products giving rise to very high selectivity values and negative yields. It should be noted that the feed contained 234 ppm and 50 ppm of p-toluic acid and BA, respectively. An appropriate catalyst must therefore have high enough hydrogenation performance to reduce 4-CBA contents to acceptable values (below 25 ppm), product yields below 1 with no significant TA loss, and desired selectivity favoring hydrogenation along path 2. Data presented in Table 5 indicate that all prepared catalysts with 0.3 wt. % palladium loading exhibited a good performance in terms of 4-CBA conversion for hydrogenation of CTA. The highest conversion for 4-CBA and the best selectivity of path 2 to path 1 was obtained for Pd/rGO-CNT catalyst. The yield of p-toluic acid and BA for this catalyst was also below 1 (0.659) indicating no significant TA losses. The performance of Pd/rGO-CNT was also better than the commercial catalyst in terms of both selectivity of path 2 to path 1 and the yield of p-toluic acid and BA. For the commercial catalyst, the yield of p-toluic acid and BA was above 1 (1.146) indicating a high hydrogenation performance leading to TA losses. The highest activity (4-CBA conversion), selectivity, and a yield of p-toluic acid and BA below 1 for the Pd/rGO-CNT catalyst is indicative of best accessibility of catalyst active sites which could be attributed, according to Sun et al. (2013), to the unique hierarchical structure of the reduced graphene oxide-CNTs composite.23 Moreover, the larger particle size of the palladium nanoparticles may also be a positive factor for CTA hydrogenation.

The results for the reactor test with Pd/rGO as catalyst indicated that the product yield was close to 1. TA was hydrogenated under the reaction conditions employed and 4-CBA was hydrogenated via both path 1 and path 2. Pd/rGO had higher activity compared with catalysts supported on CNT and rGO-CNT composite as hydrogenation of 4-CBA was enhanced along both paths 1 and 2. Hydrogenation along path 1 leads to the formation of CO that can act as a poison for palladium active sites. Selectivity of path 2 to path 1 for Pd/rGO was the lowest among all the prepared catalysts. The lower selectivity for path 2 compared with path 1 for Pd/rGO suggests that enhanced CO production would preferentially poison palladium sites responsible for hydrogenation via path 2. In accordance with ammonia-TPD 14 ACS Paragon Plus Environment

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results, the high activity of this catalyst could due to a large number of surface edge-plane of few layer graphene sheets resulting in higher dispersion of palladium particles on the surface and the higher tendency of reactant molecules with free electron pair to be adsorbed on these particles. For the Pd/FMWCNT catalyst, on the other hand, the tendency to produce methyl and hydroxyl methyl groups was greater thus resulting in path 2 to be more favorable than path 1. The lower observed hydrogenation activity could also be due to the fact that a certain population of the metal active phase could be localized inside the carbon nanotube channel and thereby becoming less accessible to the reactants, especially in liquid-phase.77 For Pd/rGO-CNT catalyst, however, both effects could be observed (high dispersion and tendency for production of methyl and hydroxyl methyl groups). The TEM and TPR results also support the reactor test results in that proper size and availability of Pd nanoparticles was achieved for Pd/rGO-CNT catalyst. The higher catalyst dispersion, together with the enhanced properties of the composite support, results in a high performance of the composite supported catalysts. Also, an increase in the catalyst performance (4-CBA conversion) and selectivity for catalysts supported on a composite has commonly been ascribed to the synergic effect of the high electron conductivity of the carbon nanotubes and graphene, with the co-catalytic properties.16, 20, 78 A single-layer graphene sheet has two different structural regions; namely the basal plane and the edge. The edge of a single-layer graphene sheet is different from the surface edge-plane of highly oriented pyrolytic graphite and the end of CNTs mainly due to its linear atom-thick structure. It was found that the graphene edge has much larger specific capacitance, faster electron transfer rate and stronger electrocatalytic activity than those of its basal plane.79 Therefore, at high magnification, it is clear that the composite offers a high density of exposed edges, which is not possible for ordinary CNT where inert basal graphitic planes are exposed instead. When rGO is used as a catalyst support for Pd clusters, the high density of graphene edges could offer numerous anchor sites for the Pd catalyst nanoparticles resulting in a high dispersion that could affect the product yield. The rGO-CNT composite structure combines the advantages of both graphene and CNT, including the ultra-high density of the active graphene edges and the porous CNT structure. The composite possesses a high graphitic content. Characteristics of the composite match well with the criteria for a good carbon catalyst support in the hydrogenation process. The composite has distributed the particles densely and uniformly while retaining good porosity. These characteristics are highly valuable for the catalyst applications, offering a high external surface area catalyst support while ensuring efficient mass transport and good catalyst accessibility.16 15 ACS Paragon Plus Environment


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Reactor test results with the commercial catalyst (0.5 wt.% palladium supported on activated carbon) indicated that both Pd/CNT and especially Pd/rGO-CNT catalysts had a better performance than the commercial catalyst in terms of both 4-CBA conversion and selectivity. Furthermore, the calculated yield for the commercial catalyst was slightly above 1 indicating some TA losses. The BET results for the commercial catalyst indicated that the total surface area, total pore volume, and micropore volume were 1165 m2/g, 0.52 cm3/g, and 0.41 cm3/g, respectively, suggesting much larger surface area and lower pore volume for the commercial catalyst could be due to the larger micropore volume which could increase the residence time of the reactants and products within the catalyst pores thus enhancing the hydrogenation performance (4-CBA conversion) and lowering the selectivity. This greater hydrogenation performance is not necessarily advantageous as it would lead to TA losses. The results from a blank test (no catalyst) and a test using FMWCNT without Pd loading indicated negligible hydrogenation performance. According to ICP and TPR analyses, the CNT that was used as the catalyst support in the present study contained impurities of about 4% Co and Mo after the purification process. We can therefore conclude these impurities on the FMWCNT support do not contribute significantly to the hydrogenation of 4-CBA. The results for the blank test indicated that under hydrothermal conditions (without any catalyst), some 4-CBA (10.8% conversion) is being converted to BA and p-toluic acid with the yield below 1. In the case of the test with FMWCNT, however, hydrogenation of CTA also occurred leading to the formation of 4-CBA and its subsequent conversion to BA and p-toluic acid led to a product yield well above 1. Hydrogenation of CTA to 4-CBA and conversion of 4-CBA resulted in net overall 4-CBA conversion of 7.1%. There have also been reports in the literature indicating the hydrogenation activity carbocatalysts including graphene and CNT.80 We do not have any quantitative data to differentiate between adsorption and reaction of 4-CBA. However, we feel that the contribution of adsorption on overall 4-CBA removal is minimal. With the reaction carried over FMWCNT, a small conversion of 4-CBA was obtained with further reaction leading to BA and p-toluic acid resulting in product yields well in excess of 1. When the reaction was carried out in the presence of 0.05 wt. % Pd/FMWCNT, 4-CBA conversion was enhanced (from about 7% to about 40%), but more important, conversion of 4-CBA to 4-HMBA and conversion of p-toluic acid to p-tolualdehyde resulted in a significant change in selectivity indicating that enhanced 4-CBA conversions are not merely due to adsorption. The selectivity of path 2 to path 1 could be attributed to several factors: (i) the existence of a peculiar metal-support interaction between the palladium metal crystallites and 16 ACS Paragon Plus Environment

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carbon support with electronic modification that could occur through the electron transfer between them thus modifying the adsorption mode and selectivity of path 2 to 1 (ii) high external surface area and the complete absence of any microporosity of the support could decrease the contact time of the reactants and products leading to successive hydrogenations (iii) the presence of oxygenated groups on the carbon surface could significantly influence the catalytic selectivity by modifying the adsorption mode of the reactants. 81-84

4. Conclusions We have successfully synthesized reduced graphene oxide/carbon nanotubes composite materials with a 1:1 mass ratio of GO to CNTs using a hydrothermal treatment of an aqueous dispersion of GO and CNTs without using any chemical reagent. Pd/rGO-CNT was used for hydro-purification of CTA containing 2100 ppm of 4-CBA as impurity. The reactor test results indicated that compared with Pd/rGO, Pd/FMWCNT, and a commercial Pd/activated carbon catalyst, Pd/rGO-CNT catalyst had a superior performance in terms of selectivity that could be attributed to its homogeneous three-dimensional structure and the synergistic effect of the combination of rGO and CNTs. The rGO-CNT composite possesses a high graphitic content and combines the advantages of both graphene and CNT, including the ultra-high density of the active graphene edges and the porous CNT structure. The most important conclusions of this work are as follows: 1. CNT was functionalized before Pd was loaded. The inert graphitic basal plane surface of CNT generally requires functionalization, which may significantly reduce its electrical conductivity, to achieve a higher dispersion of Pd nanoparticles. In comparison with CNTs, graphene not only possesses similar stable physical properties but also larger edge planes. Furthermore, it has been suggested that the heterogeneity of carbon support with edge planes can better stabilize and enhance the catalytic performance of the Pd catalyst. The combination of graphene and CNTs generated a versatile 3D rGO-CNT composite network with synergic properties. 2. The catalysts prepared from rGO-CNT support had better performance in terms of a 4-CBA conversion and selectivity than those with rGO and CNTs as support. The yield of p-toluic acid and BA for Pd/rGO-CNT was below 1 (0.659) indicating no significant TA losses. For the commercial catalyst, the yield of p-toluic acid and BA was above 1 (1.146) indicating a high hydrogenation activity (of yield of products) leading to TA losses.



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3. The high hydrogenation performance (4-CBA conversion) and selectivity of Pd/rGOCNT was attributed to ultra-high density of the active graphene edges and the porous CNT structure. 4. Co and Mo impurities in purified CNTs had insignificant effects on the hydrogenation performance of supported Pd on CNT and rGO-CNT catalysts. 5. The performance of 0.3 wt.% Pd on rGO-CNT, CNTs and even rGO supports was slightly better than the commercial catalyst containing 0.5 wt.% Pd on activated carbon. The reduction in palladium loading while maintaining comparable or better performance than the commercial catalyst would justify the use of rGO-CNT as an alternative catalyst support.

Acknowledgment The financial support by the Research Institute of the Petroleum Industry (RIPI) is gratefully acknowledged.

References: (1) Chou, T. Li. Kinetics of Salt Formation Using Terphethalic Acid and N-methyl-2Pyrolidinone. M.Sc. Thesis, University of Montana State, Bozeman, Montana, 2004. (2) Scheirs, J.; Long, T. E. (Eds) Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters. (Wiley Series in Polymer Science) John Wiley and Sons Ltd, England, 2003.

(3) Tourani, S.; Baghalha, M.; Khorasheh, F.; Behvandi, A. Equilibrium Modeling of Xylene Adsorption on Molecular Sieves. Fluid Phase Equilib. 2010, 298, 54. (4) Azarpour, A.; Zahedi, G. Performance Analyses of Crude Terephthalic Acid Hydropurification in an Industrial Trickle-bed Reactor Experiencing Catalyst Deactivation. Chem. Eng. J. 2012, 209, 180. (5) Kwak , J. W.; Lee, J. S.; Lee, K. H. Co-Oxidation of P-xylene and P-toluic acid to Terephthalic Acid in Water Solvent: Kinetics and Additive Effects. Appl. Catal. A 2009, 358, 54. (6) Gao, J.; Wang, J.; Lin, W. D. Reaction Laws and Macro-Kinetics of 4Carboxybenzaldehyde Hydrogenation over Pd/TiO2 Catalyst. Adv. Mater. Res. 2013, 634, 594. 18 ACS Paragon Plus Environment

Page 19 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Jhung, S.; Park, Y. Hydropurification of Crude Terephthalic Acid over PdRu/Carbon Composite Catalyst. J. Korean Chem. Soc. 2002, 46, 57. (8) Schroeder, H.; James, D. E. Purification of crude terephthalic acid. U.S. Patent 4,892,972 A, 1990. (9) Serp, P.; Corrias, M.; Kalck, P. Carbon Nanotubes and Nanofibers in Catalysis. Appl. Catal. A 2003, 253, 337. (10) Galal, A.; Atta, N. F.; Hassan, H. K. Graphene Supported-Pt-M (M = Ru or Pd) for Electrocatalytic Methanol Oxidation. Int. J. Electrochem. Sci. 2012, 7, 768. (11) Antolini, E. Graphene as a New Carbon Support for Low-Temperature Fuel Cell Catalysts. Appl. Catal. B 2009, 88, 1. (12) Stankovich. S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282. (13) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183. (14) Toh, S. Y.; Loh, K. S.; Kamarudin, S. K.; Daud, W. R. W. Graphene Production via Electrochemical Reduction of Graphene Oxide: Synthesis and Characterisation. Chem. Eng. J. 2014, 251, 422. (15) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. Flexible Graphene Films via the Filtration of Water-Soluble Noncovalent Functionalized Graphene Sheets. J. Am. Chem. Soc. 2008, 130, 5856. (16) Pham, K. C.; Chua, D. H. C.; McPhail, D. S.; Wee, A. T. S. The Direct Growth of Graphene-Carbon Nanotube Hybrids as Catalyst Support for High-Performance PEM Fuel Cells. ECS Electrochem. Lett. 2014, 3, F37. (17) Mani, V.; Chen, S. M.; Lou, B. S. Three Dimensional Graphene Oxide-Carbon Nanotubes and Graphene-Carbon Nanotubes Hybrids. Int. J. Electrochem. Sci. 2013, 8, 11641. (18) Zhao, M. Q.; Zhang, Q.; Huang, J. Q.; Tian, G. L.; Chen, T.Ch.; Qian, W. Zh.; Wei, F. Towards High Purity Graphene/Single-Walled Carbon Nanotube Hybrids with Improved Electrochemical Capacitive Performance. Carbon 2013, 54, 403. (19) Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170. (20) Yang, S.; Shen, C.; Lu, X.; Tong, H.; Zhu, J.; Zhang, X.; Gao, H. J. Preparation and Electrochemistry of Graphene Nanosheets–Multiwalled Carbon Nanotubes Hybrid 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nanomaterials as Pd Electrocatalyst Support for Formic Acid Oxidation. Electrochim. Acta 2012, 62, 242. (21) Machado, B. F.; Marchionni, A.; Bacsa, R. R.; Bellini, M.; Beausoleil, J.; Oberhauser, W.; Vizza, F.; Serp, P. Synergistic Effect Between Few Layer Graphene and Carbon Nanotube Supports for Palladium Catalyzing Electrochemical Oxidation of Alcohols. J. Energy Chem. 2013, 22, 296. (22) Hu, F.; Chen, S.; Wang, C.; Yuan, R. Study on the Application of Reduced Graphene Oxide and Multiwall Carbon Nanotubes Hybrid Materials for Simultaneous Determination of Catechol, Hydroquinone, P-cresol and Nitrite. Anal. Chim. Acta 2012, 724, 40. (23) Sun, T.; Zhang, Z.; Xiao, J.; Chen, C.; Xiao, F.; Wang, S. Facile and Green Synthesis of Palladium Nanoparticles-Graphene-Carbon Nanotube Material with High Catalytic Activity. Sci. Rep. 2013, 3, 2527. (24) Ye, F.; Cao, X.; Yu, L.; Chen, S.; Lin, W. Synthesis and Catalytic Performance of PtRuMo Nanoparticles Supported on Graphene-Carbon Nanotubes Nanocomposites for Methanol Electro-Oxidation. Int. J. Electrochem. Sci. 2012, 7, 1251. (25) Zhang, A. M.; Dong, J. L.; Xu, J. L. Palladium Cluster Filled in Inner of Carbon Nanotubes and Their Catalytic Properties in Liquid Phase Benzene Hydrogenation. Catal. Today 2004, 93, 347. (26) Cano, M.; Benito, A. M.; Maser,W. K.; Urriolabeitia, E. P. High Catalytic Performance of Palladium Nanoparticles Supported on Multiwalled Carbon Nanotubes in Alkane Hydrogenation Reactions. New J. Chem. 2013, 37, 1968. (27) Gao, Y.; Ma, D.; Wang, C.; Guan, J.; Bao, X. Reduced Graphene Oxide as a Catalyst for Hydrogenation of Nitrobenzene at Room Temperature. Chem. Commun. 2011, 47, 2432. (28) Cano, M.; Benito, A. M.; Urriolabeitia, E. P.; Arenal, R.; Maser, W. K. Reduced Graphene Oxide: Firm Support for Catalytically Active Palladium Nanoparticles and Game Changer in Selective Hydrogenations. Nanoscale 2013, 5, 10189. (29) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (30) Luo, Y.; Cheng, J. S.; Ma, Q.; Feng, Y. Q.; Li, J. H. Graphene-Polymer Composite: Extraction of Polycyclic Aromatic Hydrocarbons from Water Samples by Stir Rod Sorptive Extraction. Anal. Methods 2011, 3, 92. (31) Arasteh, R.; Masoumi, M.; Rashidi, A. M.; Moradi, L.; Samimi, V.; Mostafavi, S. T. Adsorption of 2-nitrophenol by Multi-Wall Carbon Nanotubes from Aqueous Solutions. 20 ACS Paragon Plus Environment

Page 20 of 35

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Appl. Surf. Sci. 2010, 256, 4447. (32) Rashidi, A. M.; Akbarnejad, M. M.; Khodadadi, A. A.; Mortazavi, Y.; Ahmadpour, A. Single-Wall Carbon Nanotubes Synthesized Using Organic Additives to Co–Mo Catalysts Supported on Nanoporous MgO. Nanotechnology 2007, 18, 315605. (33) Rashidi, A. M.; Amini, B.; Mohajeri, A.; Jozani, K. J. Continuous process for producing carbon nanotubes .U.S. Patent 20,080,274,277 A1, 2008. (34) Rashidi, A. M.; Nouralishahi, A.; Khodadadi, A. A.; Mortazavi, Y.; Karimi, A.; Kashefi, K. Modification of Single Wall Carbon Nanotubes (SWCNT) for Hydrogen Storage. Int. J. Hydrogen Energy 2010, 35, 9489. (35) Karimi, A.; Nasernejad, B.; Rashidi, A. M. Particle Size Control Effect on Activity and Selectivity of Functionalized CNT-Supported Cobalt Catalyst in Fischer-Tropsch Synthesis. Korean J. Chem. Eng. 2012, 29, 1516. (36) Buang, N. A.; Fadil, F.; Majid, Z. A. Characteristic of Mild Acid Functionalized Multiwalled Carbon Nanotubes Towards High Dispersion with Low Structural Defects. Dig. J. Nanomater. Bios. 2012, 7, 33. (37) Hou, Y. Design and Synthesis of Hierarchical MnO2 Nanospheres Carbon Nanotubes Conducting Polymer Ternary Composite for High Performance Electrochemical Electrodes. Nano Lett. 2010, 10, 2727. (38) Machado, B. F.; Oubenali, M.; Axet, M. R.; Nguyen, T. T.; Tunckol, M.; Girleanu, M.; Ersen, O.; Gerber, I. C.; Serp, P. Understanding the Surface Chemistry of Carbon Nanotubes: Toward a Rational Design of Ru Nanocatalysts. J. Catal. 2014, 309, 185. (39) Oosthuizen, R. S.; Nyamori, V. O. Carbon Nanotubes as Supports for Palladium and Bimetallic Catalysts for Use in Hydrogenation Reactions. Platin. Met. Rev. 2011, 55, 154. (40) Chen, P.; Xiao, T. Y.; Qian, Y.; Li, S. A Nitrogen-Doped Graphene/Carbon Nanotube Nanocomposite with Synergistically Enhanced Electrochemical Activity. Adv. Mater. 2013, 25, 3192. (41) Fang. Q.; Shen, Y.; Chen, B. Synthesis, Decoration and Properties of Three-Dimensional Graphene-Based Macrostructures. Chem. Eng. J. 2015, 264, 753. (42) Kumar, M. K.; Ramaprabhu, S. Palladium Dispersed Multiwalled Carbon Nanotube Based Hydrogen Sensor for Fuel Cell Applications. Int. J. Hydrogen Energy 2007, 32, 2518. (43) Chao, Z.; Xi, L. T. A Review on Hybridization Modification of Graphene and Its Polymer Nanocomposites. Chin. Sci. Bull. 2012, 57, 3010. (44) Esmaeili, E.; Mortazavi, Y.; Khodadadi, A.; Rashidi, A. M.; Rashidzadeh, M. The Role 21 ACS Paragon Plus Environment


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of Tin-Promoted Pd/MWNTs via the Management of Carbonaceous Species in Selective Hydrogenation of High Concentration Acetylene. Appl. Surf. Sci. 2012, 263, 513. (45) Jha, N.; Ramesh, P.; Bekyarova, E.; Itkis, M. E.; Haddon, R. C. High Energy Density Supercapacitor Based on a Hybrid Carbon Nanotube–Reduced Graphite Oxide Architecture. Adv. Energy Mater. 2012, 2, 438. (46) Kim, H. J. Microwave Assisted Synthesis of Three Dimensional Graphene-NanotubePalladium Nanostructures and Their Electro-Chemical Characteristics. M.Sc. Thesis, Kaist Institute for the NanoCentury, Daejeon, South Korea, 2012. (47) Shahriary, L.; Ghourchian, H.; Athawale, A. A. Graphene-Multiwalled Carbon Nanotube Hybrids Synthesized by Gamma Radiations: Application as a Glucose Sensor. J. Nanotechnol. 2014, 2014, 1. (48) Mani, V.; Vilian, A. T. E.; Chen, S. M. Graphene Oxide Dispersed Carbon Nanotube and Iron Phthalocyanine Composite Modified Electrode for the Electrocatalytic Determination of Hydrazine. Int. J. Electrochem. Sci. 2012, 7, 12774. (49) Park, S.; An, J. H.; Jung, I. W.; Piner, R. D.; An, S. J.; Li, X. S.; Velamakanni, A.; Ruoff, R. S. Colloidal Suspensions of Highly Reduced Graphene Oxide in a Wide Variety of Organic Solvents. Nano Lett. 2009, 9, 1593. (50) Lerf, A.; Buchsteiner, A.; Pieper, J.; Schottl, S.; Dekany, I.; Szabo, T.; Boehm, H. P. Hydration Behavior and Dynamics of Water Molecules in Graphite Oxide. J. Phys. Chem. Solids 2006, 67, 1106. (51) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457. (52) Park, S.; Lee, K. S.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. Graphene Oxide Papers Modified by Divalent Ions-Enhancing Mechanical Properties via Chemical CrossLinking. ACS Nano 2008, 2, 572. (53) Chen, Sh.; Yeoh, W.; Liu, Q.; Wang, G. Chemical-Free Synthesis of Graphene–Carbon Nanotube Hybrid Materials for Reversible Lithium Storage in Lithium-Ion Batteries. Carbon 2012, 50, 4557. (54) Fan, Z.; Yan, J.; Zhi, L.; Zhang, Q.; Wei, T.; Feng, J.; Zhang, M.; Qian, W.; Wei, F. A. Three-Dimensional Carbon Nanotube/Graphene Sandwich and Its Application as Electrode in Supercapacitors. Adv. Mater. 2010, 22, 3723. (55) Wu, N.; She, X.; Yang, D.; Wu, X.; Su, F.; Chen, Y. Synthesis of Network Reduced 22 ACS Paragon Plus Environment

Page 22 of 35

Page 23 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphene Oxide in Polystyrene Matrix by a Two-Step Reduction Method for Superior Conductivity of the Composite. J. Mater. Chem. 2012, 22, 17254. (56) Chen, J.; Zheng, X.; Miao, F.; Zhang, J. X.; Zheng, C. W. Engineering Graphene/Carbon Nanotube Hybrid for Direct Electron Transfer of Glucose Oxidase and Glucose Biosensor. J. Appl. Electrochem. 2012, 42, 875. (57) Shi, L.; Yang, J.; Huang, Z.; Li, J.; Tang, Z.; Li, Y.; Zheng, Q. Fabrication of Transparent, Flexible Conducing Graphene Thin Films via Soft Transfer Printing Method. Appl. Surf. Sci. 2013, 276, 437. (58) Dong, X.; Ma, Y.; Zhu, G.; Huang, Y.; Wang, J.; Park, M. B. C.; Wang, L.; Huang, W.; Chen, P. Synthesis of Graphene–Carbon Nanotube Hybrid Foam and Its Use as a Novel Three-Dimensional Electrode for Electrochemical Sensing. J. Mater. Chem. 2012, 22, 17044. (59) Maio, A.; Botta, L.; Tito, A. C.; Pellegrino, L.; Daghetta, M.; Scaffaro, R. Statistical Study of the Influence of CNTs Purification and Plasma Functionalization on the Properties of Polycarbonate-CNTs Nanocomposites. Plasma Processes Polym. 2014, 11, 664. (60) Kamaliya, R.; Singh, B. P.; Gupta, B. K.; Singh, V. N.; Gupta, T. K.; Gupta, R .; Kumar, P.; Mathur, R. B. Large Scale Production of Three Dimensional Carbon Nanotube Pillared Graphene Network for Bi-functional Optical Properties. Carbon 2014, 78, 147. (61) Ellison, M. D.; Crotty, M. J.; Koh, D.; Spray. R. L.; Tate, K. E. Adsorption of NH3 and

NO2 on Single-Walled Carbon Nanotubes. J. Phys. Chem. B 2004, 108. (62) Zhang, B.; Shao, L.; Zhang, W.; Sun, X.; Pan, X.; Su, D. S. Interaction Between Palladium Nanoparticles and Surface-Modified Carbon Nanotubes: Role of Surface Functionalities. ChemCatChem. 2014, 6, 1. (63) Felten, A.; Ghijsen, J.; Pireaux, J. J.; Drube, W.; Johnson, R. L.; Liang, D.; Hecq, M.; Tendeloo, G. V.; Bittencourt, C. Electronic Structure of Pd Nanoparticles on Carbon Nanotubes. Micron 2009, 40, 74. (64) Bazzazzadegan, H.; Kazemeini, M.; Rashidi, A. M. Effects of Functionalization and Catalyst Selective Behavior of Multi-Walled Carbon Nanotube-Supported Palladium Catalysts In Hydrogenation of Acetylene. Res. Chem. Intermed. 2015, 41, 1023. (65) Ngamsom, B.; Bogdanchikova, N.; Borja, M. A.; Praserthdam, P. Characterizations of Pd–Ag/Al2O3 Catalysts for Selective Acetylene Hydrogenation: Effect of Pretreatment with NO and N2O. Catal. Commun. 2004, 5, 243. (66) Ge, C.; Li, Y.; Zhao, J.; Zhou, R. Carbon Nanotubes Supported Palladium Catalysts for Selective Hydrogenation of Formaldehyde Under Atmospheric Pressure. Indian J. Chem.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2010, 49, 281. (67) Shang, H.; Liu, C.; Xu, Y.; Qiu, J.; Wei, F. States of Carbon Nanotube Supported MoBased HDS Catalysts. Fuel Process. Technol. 2007, 88, 117. (68) Ramos, A. L. D.; Alves, P. S.; Aranda, D. A. G.; Schmal, M. Characterization of Carbon Supported Palladium Catalysts: Inference of Electronic and Particle Size Effects Using Reaction Probes. Appl. Catal. A 2004, 277, 71. (69) Escribano, A. S.; Coloma, F.; Reinoso, F. R. Platinum Catalysts Supported on Carbon Blacks with Different Surface Chemical Properties, Appl. Catal. A 1998, 173, 247. (70) Hu, H.; Xin, J. H.; Hu, H.; Wang, X.; Kong, Y. Metal-Free Graphene-Based CatalystInsight into the Catalytic Activity: A Short Review, Appl. Catal. A 2015, 492, 1. (71) Sumner, C. E. Method of purifying aromatic dicarboxylic acids. E.P. Patent 1,091,922 B1, 2006. (72) Konishi, M. Catalysts for hydrogenation of carboxylic acid. U.S. Patent 6,495,730 B1, 2002. (73) Brunner, M. Method for hydrogenating benzene polycarboxylic acids or derivatives thereof by using a catalyst containing macropores. U.S. 6,284,917 B1, 2001. (74) Nagayama, K.; Shimizu, I.; Yamamoto, A. Direct Hydrogenation of Carboxylic Acids to Corresponding Aldehydes Catalyzed by Palladium Complexes. Bull. Chem. Soc. Jpn. 2001, 74, 1803. (75) Lee, D.; Kim, H.; Kang, M.; Kim, J. M. Efficient Hydrogenation Catalysts of Ni or Pd on Nanoporous Carbon Workable in an Acidic Condition. Bull. Korean Chem. Soc. 2007, 28, 2034. (76) Lee, D.; Cho, S.; Kim, G.; Kim, H. Efficient and Selective Hydrogenation of Carboxylic Acid Catalyzed by Ni or Pd on ZSM-5. J. Ind. Eng. Chem. 2007, 13, 1067. (77) Huu, T. T.; Chizari, K.; Janowsk, Z.; Moldovan, M. S.; Ersen, O.; Nguyen, L.D.; Ledoux, M. J.; Huu, C. P.; Begin, D. Few-Layer Graphene Supporting Palladium Nanoparticles with a Fully Accessible Effective Surface for Liquid-Phase Hydrogenation Reaction. Catal. Today 2012, 189, 77. )78) Antolini, E. Graphene as a New Carbon Support for Low-Temperature Fuel Cell Catalysts. Appl. Catal. B 2012, 123, 52. (79) Yuan, W.; Zhou, Y.; Li, Y.; Li, C.; Peng, H.; Zhang, J.; Liu, Z. F.; Dai, L.; Shi, G. The Edge- and Basal-Plane-Specific Electrochemistry of a Single-Layer Graphene Sheet, Sci. Rep. 2013, 3, 2248.


Page 24 of 35

Page 25 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(80) Primo, A.; Neatu, F.; Florea, M.; Parvulescu, V.; Garcia, H. Graphenes in the Absence of Metals as Carbocatalysts for Selective Acetylene Hydrogenation and Alkene Hydrogenation. Nat. Commun. 2014, 5, 5291. (81) Tessonnier, J. P.; Pesant, L.; Ehret, G.; Ledoux, M. J.; Huu, C. P. Pd Nanoparticles Introduced Inside Multi-walled Carbon Nanotubes for Selective Hydrogenation of Cinnamaldehyde into Hydrocinnamaldehyde. Appl. Catal. A 2005, 288, 203. (82) Huu, C. P.; Keller, N.; Ehret, G.; Charbonniere, L.; Ziessel, R.; Ledoux, M. J. Carbon Nanofiber Supported Palladium Catalyst for Liquid-Phase Reactions as an Active and Selective Catalyst for Hydrogenation of Cinnamaldehyde into Hydrocinnamaldehyde. J. Mol. Catal. A: Chem. 2001, 170, 155. (83) Ma, H.; Wang, L.; Chen, L.; Dong, C.; Yu, W.; Huang, T.; Qian, Y. Pt Nanoparticles Deposited over Carbon Nanotubes for Selective Hydrogenation of Cinnamaldehyde. Catal. Commun. 2007, 8, 452. (84) Toebes, M. L.; Prinsloo, F. F.; Bitter, J. H. Influence of Oxygen-Containing Surface Groups on the Activity and Selectivity of Carbon nanofiber Supported Ruthenium Catalysts in the Hydrogenation of Cinnamaldehyde. J. Catal. 2003, 214, 78.



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List of Figures Figure 1. The synthesis scheme for Pd/rGO-CNT Figure 2. TEM micrographs for (a) rGO, (b) rGO-CNT composite, (c) Pd/FMWCNT, and (d) Pd/rGO-CNT Figure 3. XRD patterns of GO, rGO, MWCNT, rGO/MWCNT (1:1)(w/w), and Pd/FMWCNT Figure 4. FTIR spectra for (a) Purified-MWCNT, FMWCNT and (b)GO , rGO , rGO-CNT Figure 5. Raman spectra of (a) Purified-MWCNT, (b) FMWCNT, (c) GO, (d) rGO, and (e) rGO-CNT composite Figure 6. NH3-TPD profiles of (a) Pd/FMWCNT, (b) Pd/rGO, and (c) Pd/rGO-CNT Figure 7. TPR profiles of (a) Pd/FMWCNT, (b) Pd/rGO, and (c) Pd/rGO-CNT Figure 8. Possible reaction scheme in the hydro-purification of CTA


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GO

+

Hydrothermal treatment

Ultrasonication

rGO-CNT composite

CNTs

GO-CNT dispersion

Impregnation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reduction with H2 with H2

Pd based complex/rGo-CNT

Pd/rGO-CNT

Figure 1. The synthesis scheme for Pd/rGO-CNT



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

b

C c

d


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0.40 0.35

0.40

(c)

0.35

0.30

0.30

0.25

0.25

Fraction ) Fraction(%

Fraction Fraction(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.20 0.15 0.10

(d)

0.20 0.15 0.10 0.05

0.05

0.00

0.00 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity(a.u.)


1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

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Pd/FMWCNT(0.3 wt%,Treduced=300oC)

rGO/ MWCNT (1:1)(w/w) MWCNT

rGO

GO 10

20

30

40

50

60

70

2o

Figure 3. XRD patterns of GO, rGO, MWCNT, rGO/MWCNT (1:1)(w/w), and Pd/FMWCNT


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(a)

4.0

Purified-MWCNT

Transmittance

3.5

FMWCNT

3.0 2.5 2.0 1.5 0

1000

2000

3000

4000

Wave number(cm-1) 22 20

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

GO

18 16 14 12 rGO

10 8

rGO-CNT

6 0

1000

2000

3000 Wave number(cm-1)

4000

Figure 4. FTIR spectra for (a) Purified-MWCNT and FMWCNT and (b) GO, rGO, and rGO-CNT



G

(a)

Purified-MWCNT

(b)

D

FMWCNT

Intensity (a.u.)

D

Intensity (a.u.)

2D

D+G

1500

2000

2500

Raman Shift(cm-1)

(c)

G

2D

1000

(e)

1500

2000

D

(d)

D+G

2500

Raman Shift(cm-1)

2D

1000

GO

Intensity (a.u.)

D

3000

G

1500

2500

3000

D

rGO G

2D D+G

1000

3000

2000

Raman Shift(cm-1)

Intensity (a.u.)

1000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1500

2000

2500

3000

3500

Raman Shift(cm-1)

rGO-CNT G

2D D+G

1000 1500 2000 2500 3000 3500

Raman Shift(cm-1)

Figure 5. Raman spectra of (a) Purified-MWCNT, (b) FMWCNT, (c) GO, (d) rGO, and (e) rGO-CNT composite


600

(a)

Intensity (a.u.)

500 400 300 200 100 0 0

600

200

400

600

800

200

400

600

800

Temperature oC

(b)

500

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 300 200 100 0 0

TemperatureoC

600

(c)

500

Intensity (a.u.)

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400 300 200 100 0 0

200

400

600

800

TemperatureoC

Figure 6. NH3-TPD profiles of (a) Pd/FMWCNT, (b) Pd/rGO, and (c) Pd/rGO-CNT



Intensity (a.u.)

110 100 90 80 70 60 50 40 30 20 10 0

(a)

Intensity (a.u.)

0

110 100 90 80 70 60 50 40 30 20 10 0

200

400

600

800

1000

200

400

600

800

1000

Temperature oC

(b)

0

TemperatureoC

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

110 100 (c) 90 80 70 60 50 40 30 20 10 0 0 200

400

600

800

TemperatureoC

1000

Figure 7. TPR profiles of (a) Pd/FMWCNT, (b) Pd/rGO, and (c) Pd/rGO-CNT


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Possible reaction scheme in the hydro-purification of CTA


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