- Email: [email protected]
On the role of the residual iron growth catalyst in the gasification of multi-walled carbon nanotubes with carbon dioxide
Catalysis Today 186 (2012) 128–133
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
On the role of the residual iron growth catalyst in the gasification of multi-walled carbon nanotubes with carbon dioxide Chen Jin, Wei Xia ∗ , Peirong Chen, Martin Muhler ∗ Laboratory of Industrial Chemistry, Ruhr-University Bochum, D-44780 Bochum, Germany
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 21 December 2011 Accepted 24 February 2012 Available online 7 April 2012 Keywords: Carbon nanotubes Gasification with CO2 Residual growth catalyst Temperature-programmed desorption Temperature-programmed surface reaction Boudouard reaction
a b s t r a c t The gasification of carbon with CO2 was applied to examine the role of the residual iron growth catalyst in multi-walled carbon nanotubes (CNTs), which were pre-treated either by refluxing in nitric acid at 120 ◦ C or by nitric acid vapor at 200 ◦ C. Temperature-programmed desorption (TPD) and surface reaction (TPSR) experiments were performed in He and CO2 , respectively. The Fe nanoparticles were retained after the treatment in HNO3 vapor, whereas the liquid HNO3 treatment was able to remove the accessible residual Fe catalyst. The exposed Fe nanoparticles were found to catalyze the gasification of CNTs with CO2 according to the reverse Boudouard reaction C + CO2 = 2CO. In case of the CNTs pretreated in HNO3 vapor, evolving CO2 formed due to the decomposition of oxygen-containing functional groups during the TPD experiments was fully converted above 750 ◦ C into desorbing CO, and the addition of 2000 ppm CO2 in the feed gas during the TPSR experiments resulted in full conversion at 1000 ◦ C. X-ray photoelectron spectroscopy studies show that the treatment in HNO3 vapor at 200 ◦ C favors the formation of oxygen species doubly bound to carbon (C O groups). During the TPSR experiments, CO2 as a weak oxidant partially oxidized the CNTs leading to the formation of C O groups, and a much higher amount of these groups was detected on HNO3 vapor-treated CNTs with residual Fe catalyst. Their presence suggests that C O groups are reaction intermediates of the CNT gasification with CO2 , which is considered an effective test reaction for the presence of residual catalytically active nanoparticles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The synthesis of multi-walled carbon nanotubes (CNTs) usually involves metallic catalysts, such as Fe, Co, or Ni, and in most cases a support like silica, magnesia, or carbon materials [1–3]. The as-synthesized CNTs always contain a certain amount of metallic impurities, which depends on the CNT yield obtained with the catalysts [4,5]. The removal of the support and the metallic impurities from the CNTs often requires tedious purification procedures involving sequential washing by different solvents [6]. The difficulties in removing the metallic impurities by acid treatment are attributed to the sheathing of the metal nanoparticles by several graphene layers [7]. Therefore, the complete removal of the metal catalysts used for the CNT growth turns out to be very challenging [8]. A small amount of metallic impurities may be negligible for many applications such as in polymer composites. For certain CNT applications, however, even a small amount of residual metal nanoparticles can have significant disadvantages. For example, Fe
∗ Corresponding authors. Tel.: +49 234 32 28754; fax: +49 234 32 14115. E-mail addresses: [email protected] (W. Xia), [email protected] (M. Muhler). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.02.052
oxide impurities are responsible for the observed activity in hydrogen peroxide reduction by CNTs [9], and Cu impurities are active for the reduction of halothane [10]. It was also reported that residual Co and Fe catalysts showed superior activities in NH3 decomposition [11]. In addition to the catalytic properties of the impurities, ferromagnetic properties of the residual catalysts were reported [12], which were used for magnetic capturing and self-assembly of individual CNTs [13]. In a biocompatibility study the residual Fe nanoparticles were found to be cytotoxic for brain cells [14]. The residual metallic impurities in CNTs can be removed by washing with acids such as nitric acid. The washing can lead to the formation of oxygen-containing functional groups on the CNT surfaces [15]. Alternatively, these groups can be created by the treatment with HNO3 vapor [16], during which the residual metal catalysts are not removed. The CNT surfaces become hydrophilic due to the acid treatment, and the improved wettability can enhance the degree of dispersion of CNTs in polar solvents. Moreover, the oxygen functional groups can be further chemically modified or directly used as anchoring sites for the immobilization of metal particles to be applied in heterogeneous catalysis [17,18]. The reverse Boudouard reaction (Eq. (1)) is the gasification of carbon materials with CO2 forming CO with an endothermic enthalpy change of ca. 171 kJ mol−1 at 298 K [19]. Baker and
C. Jin et al. / Catalysis Today 186 (2012) 128–133
coworkers [20] applied in situ transmission electron microscopy (TEM) to investigate the nickel-catalyzed growth and gasification of carbon filaments. It was found that these processes can be reversed suggesting that they both involve the diffusion of carbon through the metal nanoparticles. Serp, Figueiredo and coworkers [21,22] performed detailed studies on the surface properties of vaporgrown carbon fibers after gasification with CO2 or air, which were found to be strongly influenced by the presence of traces of iron. Green and coworkers made use of the Boudouard reaction to open and cut CNTs [23]. Group VIII metals, alkali and alkaline-earth salts are known to be highly active catalysts for the reaction. Among them, Fe is reported to be the most suitable catalyst because of its high activity and low cost [24]. The redox cycle consisting of Eqs. (2) and (3) over Fe catalysts was proposed, and the reduction of higher-valent Fe oxide to metallic Fe or lower-valent Fe oxides (Eq. (3)) was reported to be the rate-determining step [25]: CO2 + C 2CO
(1)
Fem On + CO2 Fem On+1 + CO
(2)
Fem On+1 + C Fem On + CO
(3)
Here, we report on the catalytic and non-catalytic gasification of CNTs with CO2 , where CO2 was either used as feed gas during temperature-programmed surface reaction (TPSR) experiments or generated by the decomposition of oxygen-containing surface functional groups during temperature-programmed desorption (TPD) experiments. It was found that the residual iron nanoparticles present in the CNTs treated with HNO3 vapor catalyze the gasification of the CNTs with CO2 . After removal of the residual catalyst by treatment in liquid HNO3 , coordinatively unsaturated carbon species such as amorphous carbon or carbon at defect sites react preferably with CO2 yielding CO above 770 ◦ C. 2. Experimental
129
2.2. Temperature programmed experiments TPD and TPSR experiments were performed in He and in CO2 , respectively. A horizontal quartz tube reactor with an inner diameter of 12 mm was used for the TPD. The evolved gases were passed through a calibrated infrared detector (Leybold BINOS 1001) with two channels for quantitative analysis of CO and CO2 . The following temperature program was employed for the TPD: (i) heat to 50 ◦ C, and keep for 20 min; (ii) heat to 1000 ◦ C at 2 K min−1 , and keep for 110 min; (iii) cool down. Typically, 100–200 mg of CNTs were applied for the TPD in He using a flow of 30 ml min−1 . For the TPSR in CO2 (gasification with CO2 ), 200 mg of CNTs were always applied in a flow of CO2 diluted in He (cCO2,0 = 2000 ppm, total flow
rate 100 ml min−1 ). All the TPD profiles shown were normalized to 1 g of CNTs starting after 20 min at 50 ◦ C. The onset temperature of the gasification reaction was determined, when the conversion of CO2 (XCO2 ) had reached 1%: XCO2 =
cCO2,0 − cCO2 cCO2,0
(4)
2.3. Characterization Elemental analysis was performed with an ICP-OES instrument (type PU701) from Philips-Unicam. Samples were dried at 120 ◦ C for 30 min in order to remove the moisture before the analysis. Xray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum set-up equipped with a GammadataScienta SES 2002 analyzer. The base pressure in the measurement chamber was 2 × 10−10 mbar. Monochromatic Al Ka (1486.6 eV; 14.5 kV; 45 mA) was used as incident radiation, and a pass energy of 200 eV was chosen resulting in an energy resolution better than 0.5 eV. A flood gun was used to compensate for charging effects. The binding energies were calibrated based on positioning the main C 1s peak at 284.5 eV.
2.1. Materials 3. Results and discussion Multi-walled CNTs (Pyrograf® -III) with inner diameters of 20–50 nm and outer diameters of 70–200 nm were obtained from Applied Sciences Inc. (Ohio, USA), for which Fe had been applied as growth catalyst. The CNTs are relatively large in diameter, and they have the typical structural features of CNTs as shown by recent TEM studies [3]. The following chemicals and gases were used in this study: nitric acid (65%, J.T. Baker), helium (99.9999%), and hydrogen (99.9999%). The as-received CNTs were first treated at 800 ◦ C in flowing He in order to remove the adsorbed polyaromatics. The HNO3 treatment was performed using two different methods: (1) refluxing in liquid HNO3 for 90 min at 120 ◦ C (2 g CNT, 150 mL HNO3 ), followed by filtration, washing, and drying to obtain sample OCNT-L; (2) treatment in nitric acid vapor at 200 ◦ C for 24 h (1 g CNT, 200 mL HNO3 ) to obtain sample OCNT-V. The details of the treatment and their differences were described elsewhere [16]. Briefly, both methods can create oxygen-containing functional groups on CNTs. Method (1) can also remove the residual catalysts used for CNT growth, whereas method (2) can create more oxygen-containing groups, but cannot remove the residual catalysts. Subsequently, OCNT-L was treated in a horizontal quartz tube reactor at 800 ◦ C (heating ramp 10 K min−1 ) in flowing He (200 ml min−1 ) for 2 h, and the obtained sample is denoted as HT-OCNT-L. Furthermore, OCNT-L and OCNT-V were treated at 500 ◦ C (heating ramp 2 K min−1 ) in flowing He (200 ml min−1 ) for 210 min, and the obtained samples are labeled as LT-OCNT-L and LT-OCNT-V, respectively. Additionally, the sample OCNT-L was also reduced at 800 ◦ C (ramp 10 K min−1 ) in a mixture of H2 and He (total 200 ml min−1 , 1:3) for 2 h to obtain R-OCNT-L.
3.1. Elemental analysis Recent TEM studies found that many of the as-received CNTs are open-ended without catalyst nanoparticles [3]. However, catalyst particles were observed at the tips of some CNTs with dimensions comparable with the inner diameter of the CNTs, which is in the range from 20 to 50 nm. Elemental analysis detected 1.19 wt% Fe in the as-received CNTs. A very similar amount (1.20 wt%) of Fe was found in the CNTs treated by HNO3 vapor indicating that the gas-phase method did not cause the loss of Fe. Refluxing in liquid HNO3 led to the removal of all the accessible Fe nanoparticles, and the residual content of 0.60 wt% Fe detected by elemental analysis is assumed to be inaccessible due to a dense coverage by carbon [6]. 3.2. Temperature-programmed desorption The TPD profiles of the samples OCNT-L and OCNT-V in He are shown in Fig. 1. The desorption of CO2 below 340 ◦ C can be attributed to the decomposition of carboxylic acid groups, and the peaks at around 480 ◦ C and 640 ◦ C are due to the decomposition of more stable groups like carboxylic anhydrides and lactones, respectively. In the CO profile, desorption below 300 ◦ C can be assigned to aldehydes. The decomposition of carboxylic anhydride groups also gives rise to CO production at the same temperature of 480 ◦ C as observed for CO2 . The desorption of CO at 700 ◦ C is related to the decomposition of phenol and ether groups. Carbonyls decompose at an even higher temperature of around 830 ◦ C [15,26]. The
130
C. Jin et al. / Catalysis Today 186 (2012) 128–133
Fig. 1. TPD profiles of OCNT-L (a) and OCNT-V (b) performed in He at a heating rate of 2 K min−1 . (1) CO2 ; (2) CO.
quantitative analysis showed that the desorption of both CO and CO2 from sample OCNT-L (CO 1.0 mmol g−1 , CO2 0.62 mmol g−1 ) is significantly lower than that from sample OCNT-V (CO 1.56 mmol g−1 , CO2 1.07 mmol g−1 ) as summarized in Table 1. CO2 desorption was not observed above 750 ◦ C in both samples. A sharp CO desorption peak at 730 ◦ C was obtained with sample OCNT-V, which did not appear for OCNT-L. In case the gasification reaction (Eq. (1)) of CNTs by CO2 released from the oxygen functional groups had occurred, the increase in the CO concentration should be accompanied by a decrease in the CO2 concentration. As this is indeed the case, the CO peak observed for sample OCNT-V at ca. 730 ◦ C can be assigned to a pronounced gasification reaction due to the evolved CO2 . 3.3. Temperature-programmed surface reaction In order to determine the onset temperature of the gasification reaction, the CO2 gasification of OCNT-L and OCNT-V was studied by heating the samples in a constant diluted flow of CO2 . The resulting TPSR profiles are shown in Fig. 2. There are at least two contributions to the variation of the CO and CO2 concentrations during heating: (1) the desorption of CO and CO2 from oxygen-containing surface functional groups on CNTs, and (2) the conversion of CO2 to CO at high temperatures due to the gasification reaction (Eq. (1)). The desorption of oxygen-containing groups can be seen in both samples as indicated by the increased CO2 concentrations in the low-temperature range (Fig. 2). Obviously, sample OCNT-V released more CO2 than OCNT-L, which is in good agreement with the TPD results shown in Fig. 1. The onset temperature of gasification was determined at 770 ◦ C for OCNT-L. For OCNT-V, it is difficult to determine the onset due to the continuous increase of the CO concentration in the range from 450 ◦ C to 800 ◦ C. It was found that the concentration of CO2 dropped below the amount of ca. 2000 ppm in the feed at about 500 ◦ C. It is known from Fig. 1b that at 500 ◦ C the desorption of CO2 from the CNT surfaces was still occurring indicating that the gasification actually started below 500 ◦ C, which is far
below the temperature of 770 ◦ C observed for OCNT-L. As the Fe catalyst was not removed by the HNO3 vapor treatment, the presence of accessible residual Fe nanoparticles in OCNT-V is responsible for its low gasification temperature. The higher onset gasification temperature of OCNT-L without Fe nanoparticles is obviously related to surface defects and not to the residual catalyst [23]. It is shown in Fig. 1 that for sample OCNT-V CO2 desorption was not observed above 750 ◦ C. Although there is still CO desorption from the surface above 750 ◦ C, its amount is believed to be very small as compared to the amount of CO produced by carbon gasification in the CO2 flow. By neglecting the desorption of CO from surface oxygen groups in the temperature range from 750 ◦ C to 1000 ◦ C, the theoretical CO concentration, i.e., the amount of CO generated by gasification, can be calculated from the consumed amount of CO2 according to Eq. (5), that is, one molecule CO2 can generate 2 molecules of CO through the gasification reaction of carbon. In the mass balance, cco,calc is the calculated CO concentration, cCO2,0 is the detected CO2 concentration in the feed gas at the initial stage (50 ◦ C) of each experiment (the concentration varies slightly in different experiments), and cCO2 is the detected CO2 concentration during heating from 750 ◦ C to 1000 ◦ C. cCO,calc = (cCO2,0 − cCO2 ) × 2
(5)
The calculated CO evolution based on the CO2 evolution in the range between 750 ◦ C and 1000 ◦ C is shown in trace 3 in Fig. 2a and b. Sample OCNT-L shows good agreement between traces 2 and 3, but the actual CO evolution (trace 2 in Fig. 2b) of sample OCNT-V is less than the theoretical amount of CO (trace 3 in Fig. 2b) between 750 ◦ C and 1000 ◦ C. The deviation is assumed to be related to the
Table 1 Amount of desorbing CO and CO2 derived from the TPD studies in He and the surface atomic concentrations of oxygen (O in C O, and O in C–O) derived from the XPS measurements. Samples
CO2 (mmol g−1 )
CO (mmol g−1 )
C O (at.%)
C–O (at.%)
OCNT-L OCNT-V OCNT-L (2nd run)a OCNT-V (2nd run)a
0.62 1.07 0.00 0.10
1.00 1.56 0.34 0.64
6.5 10.4 0.8 4.4
9.4 6.2 2.1 1.3
a
After 2nd CO2 gasification run (TPSR in CO2 ).
Fig. 2. TPSR profiles in 2000 ppm CO2 of (a) OCNT-L and (b) OCNT-V. (1) CO2 ; (2) CO; (3) calculated theoretical concentration of CO from CO2 based on the reverse Boudouard reaction in the temperature range from 750 ◦ C to 1000 ◦ C.
C. Jin et al. / Catalysis Today 186 (2012) 128–133
131
LT-OCNT-V with accessible Fe catalyst displays desorption and gasification in the first run (Fig. 4c), and the conversion remained at 9% after keeping the temperature at 700 ◦ C for 210 min. In the second run, the initial temperature of gasification was found to be at 600 ◦ C, and a constant conversion of 8.5% was reached at 700 ◦ C. Compared to the as-prepared sample (OCNT-V, Fig. 2b), the onset temperature of gasification is about 100 ◦ C higher presumably due to the consumption of the Ccus species during the first gasification run. 3.4. TPD studies subsequent to TPSR experiments
Fig. 3. TPSR profiles in 2000 ppm CO2 of (a) HT-OCNT-L and (b) R-OCNT-L, which had been heated at 800 ◦ C for 2 h in He and H2 , respectively. (1) CO2 ; (2) CO; (3) calculated theoretical concentration of CO from CO2 based on the reverse Boudouard reaction in the temperature range from 50 ◦ C to 1000 ◦ C.
interaction of CO with the Fe nanoparticles. Sample OCNT-V shows a high and constant CO2 conversion of 94% for 60 min at 1000 ◦ C. It is known that coordinatively unsaturated carbon (Ccus ) species like amorphous carbon or carbon at defect sites such as edges or steps can preferably react with CO2 as compared to graphitic carbon [24]. The Ccus species on OCNT-V were consumed before reaching 1000 ◦ C because of the high activity of the residual Fe catalyst. Therefore, at 1000 ◦ C, the main carbon source is graphitic carbon. In contrast, the conversion of CO2 over OCNT-L decreased from 37% to 31% after 60 min at 1000 ◦ C, likely due to the consumption of Ccus species. We reported on the effect of thermal treatment in He or H2 on the removal of oxygen-containing functional groups [15]. To avoid the influence of evolved CO and CO2 originating from the decomposition of oxygen groups on the gasification, a thermal treatment of the samples was performed in He or H2 at 800 ◦ C. The TPSR profiles in CO2 of the heat-treated OCNT-L sample are shown in Fig. 3. As expected, the CO2 desorption from the functional groups on CNTs cannot be observed in the whole temperature range. The calculated CO evolution (trace 3 in Fig. 3) overlaps with the actual evolution of CO in trace 2 in Fig. 3 in the full temperature range above 750 ◦ C. These results further confirm that the liquid HNO3 treatment is sufficient to remove all the accessible Fe nanoparticles. The onset gasification temperature of sample HT-OCNT-L (740 ◦ C, Fig. 3) with the pretreatment at 800 ◦ C in He is slightly lower than that of OCNT-L (770 ◦ C, Fig. 2) without the heat pretreatment. The differences in the onset temperatures of the non-catalytic gasification are presumably due to the heat treatment at 800 ◦ C creating more Ccus species such as surface defects from the decomposition of the functional groups on the CNT surface [27]. The Ccus species react preferably with CO2 compared with graphitic carbon. Therefore, the conversion of CO2 on HT-OCNT-L decreased from 54% to 45% due to the consumption of Ccus species. OCNT-L treated in H2 (R-OCNT-L) shows the same onset temperature of gasification as OCNT-L with a constant conversion of 36% for 60 min at 1000 ◦ C indicating that the Ccus species had been removed more efficiently by the H2 treatment. In order to further study the gasification activity of the residual Fe catalyst, OCNT-L and OCNT-V were heat-treated at 500 ◦ C to remove the low-temperature CO2 desorption groups, and then heated to 700 ◦ C in CO2 (Fig. 4). During the first TPSR run with sample LT-OCNT-L, further desorption of CO and CO2 in the temperature range between 500 ◦ C and 700 ◦ C can be observed (Fig. 4a). The second TPSR run (Fig. 4b) with the samples resulting from the first run led to a completely flat CO2 profile. The results show that no gasification reaction occurred below 700 ◦ C for the sample LT-OCNT-L.
In order to further investigate the strong gasification peak at 730 ◦ C detected in Fig. 1b, the samples LT-OCNT-L and LT-OCNTV obtained after the second CO2 gasification experiment (TPSR in CO2 ) at 700 ◦ C were further characterized by TPD in He up to 1000 ◦ C. The quantitative results are summarized in Table 1. The TPD profile of LT-OCNT-L after the second TPSR run disclosed the presence of high temperature-stable CO desorption groups, as indicated by the CO peak between 700 ◦ C and 1000 ◦ C (trace 2 in Fig. 5a). The amount of desorbed CO was determined to be 0.34 mmol g−1 (Table 1), whereas only traces of CO2 were detected (trace 1 in Fig. 5a). For the sample LT-OCNT-V after the second TPSR run (Fig. 5b), a significantly higher amount of desorbed CO (0.64 mmol g−1 ) was detected, and there was still CO desorption even after 1 h at 1000 ◦ C indicating that the vapor HNO3 treatment generates more high-temperature stable oxygen functional groups such as pyrone or quinone on the CNTs. Moreover, a sharp CO desorption peak at 720 ◦ C was observed correlated with a sudden decrease of CO2 at the same temperature. The sharp CO peak fits well to the one at 730 ◦ C in trace 2 in Fig. 1b. These results confirm a strong gasification occurring at about 720–730 ◦ C catalyzed by the accessible residual Fe nanoparticles. 3.5. XPS studies The TPD and TPSR studies clearly demonstrate that the treatments in liquid HNO3 and in HNO3 vapor result in a clearly different CO2 gasification behavior. To study the difference in the oxygen functional groups generated by the liquid and the vapor method, XPS was applied to the samples before and after CO2 gasification. The surface atomic concentrations derived from the C 1s and O 1s spectra are summarized in Table 1. The O 1s spectra can be deconvoluted into two peaks corresponding to oxygen doubly bound to carbon (C O) in aldehydes, ketones, carboxyls, anhydrides, lactones, quinones and pyrones at ca. 531.5 eV, and oxygen singly bound to carbon (C–O) in carboxyls, anhydrides, lactones, phenols, ethers and pyrones at a binding energy of ca. 533.4 eV [15]. From Fig. 6a it can be seen that the HNO3 vapor treatment can generate more oxygen functional groups especially of the C O type (10.4 at.%) compared with the liquid HNO3 treatment (6.5 at.% C O) (Table 1), which is in good agreement with our previous results [16]. The O 1s spectra of the samples after the TPSR experiments at 700 ◦ C are shown in Fig. 6b. It is obvious that CO2 as a weak oxidant can partially oxidize CNTs at 700 ◦ C leading to the formation of oxygen functional groups on CNT surfaces. As discussed above, both C O and C–O species can be high-temperature stable. The C O groups are still the dominant species in the HNO3 vapor-treated sample, whereas the C–O groups are the major species in the liquid HNO3 -treated sample after the second gasification run (Fig. 6b and Table 1). Furthermore, a much higher amount of oxygen groups was detected on the vapor HNO3 -treated CNTs with residual nanoparticles compared with the purified liquid HNO3 -treated CNTs. These results show that mainly C O groups were formed on the CNT surfaces in the presence of CO2 suggesting that the C O groups are intermediates of the Boudouard reaction catalyzed by the residual
132
C. Jin et al. / Catalysis Today 186 (2012) 128–133
Fig. 4. TPSR profiles in 2000 ppm CO2 of LT-OCNT-L (first run (a), second run (b)) and LT-OCNT-V (first run (c), second run (d)). (1) CO2 ; (2) CO.
Fig. 5. TPD profiles obtained in helium at a heating rate of 2 K min−1 after the 2nd TPSR run in CO2 . (a) LT-OCNT-L, (b) LT-OCNT-V. (1) CO2 ; (2) CO.
Fig. 6. XP O 1s spectra of CNTs treated by HNO3 in the liquid phase (1) and gas phase (2). (a) subsequent to the HNO3 treatment; (b) after further treatment at 500 ◦ C in He and the 2nd TPSR run in CO2 .
C. Jin et al. / Catalysis Today 186 (2012) 128–133
Fe catalyst. In summary, the gasification with CO2 is considered an effective test reaction for the presence of residual catalytically active nanoparticles in CNTs. 4. Conclusions Refluxing in liquid HNO3 and the treatment in HNO3 vapor were employed as two different methods for the oxygen functionalization of CNTs. Residual Fe nanoparticles were retained after the vapor HNO3 treatment, while the liquid HNO3 treatment washed out the accessible residual Fe catalyst originating from the growth of the CNTs. The accessible Fe nanoparticles in the HNO3 vapor-treated sample catalyzed the gasification of CNTs by CO2 above 500 ◦ C either generated due to the decomposition of the oxygen functional groups on the CNT surface during the TPD experiments or by added CO2 during the TPSR experiments. The non-catalytic gasification reaction is assumed to preferably consume coordinatively unsaturated carbon species, which also originate from the removal of oxygen functional groups upon heating. The XPS studies disclosed that CO2 as a weak oxidant was able to oxidize CNTs at high temperatures leading to the formation of oxygen functional groups on CNT surfaces. A much higher amount of oxygen groups, especially of C O groups, was detected after the TPSR experiments on vapor HNO3 -treated CNTs with residual nanoparticles compared with purified liquid HNO3 -treated CNTs. These formed C O groups on the CNTs are presumably reaction intermediates of the CO2 gasification catalyzed by the residual Fe nanoparticles. Acknowledgement Chen Jin thanks the International Max Planck Research School Surface and Interface Engineering in Advanced Materials (SurMat) for a research grant.
133
References [1] J.H. Bitter, J. Mater. Chem. 20 (2010) 7312–7321. [2] K.L. Klein, A.V. Melechko, T.E. McKnight, S.T. Retterer, P.D. Rack, J.D. Fowlkes, D.C. Joy, M.L. Simpson, J. Appl. Phys. 103 (2008) 061301–061326. [3] W. Xia, X. Chen, S. Kundu, X. Wang, G. Grundmeier, Y. Wang, M. Bron, W. Schuhmann, M. Muhler, Surf. Coatings Technol. 201 (2007) 9232–9237. [4] J.-P. Tessonnier, D. Rosenthal, T.W. Hansen, C. Hess, M.E. Schuster, R. Blume, F. Girgsdies, N. Pfänder, O. Timpe, D.S. Su, R. Schlögl, Carbon 47 (2009) 1779–1798. [5] L. Guo, D.G. Morris, X. Liu, C. Vaslet, R.H. Hurt, A.B. Kane, Chem. Mater. 19 (2007) 3472–3478. [6] M. Pumera, H. Iwai, J. Phys. Chem. C 113 (2009) 4401–4405. [7] J.L. Lyon, K.J. Stevenson, Langmuir 23 (2007) 11311–11318. [8] S. Kundu, T.C. Nagaiah, W. Xia, Y. Wang, S.V. Dommele, J.H. Bitter, M. Santa, G. Grundmeier, M. Bron, W. Schuhmann, M. Muhler, J. Phys. Chem. C 113 (2009) 14302–14310. [9] B. Sljukic, C.E. Banks, R.G. Compton, Nano Lett. 6 (2006) 1556–1558. [10] X. Dai, G.G. Wildgoose, R.G. Compton, Analyst 131 (2006) 901–906. [11] J. Zhang, M. Comotti, F. Schüth, R. Schlögl, D.S. Su, Chem. Commun. (2007) 1916–1918. [12] Y.-H. Lee, D.H. Kim, D.H. Kim, B.-K. Ju, Appl. Phys. Lett. 89 (2006) 083113. [13] S.S. Joon, Y.H. Yun, M.J. Rust, J. Do, V. Shanov, M.J. Schulz, C.H. Ahn, Nanotechnology 20 (2009) 325607. ˜ S. Jin, Biomaterials 28 [14] T.R. Pisanic Ii, J.D. Blackwell, V.I. Shubayev, R.R. Finones, (2007) 2572–2581. [15] S. Kundu, Y. Wang, W. Xia, M. Muhler, J. Phys. Chem. C 112 (2008) 16869–16878. [16] W. Xia, C. Jin, S. Kundu, M. Muhler, Carbon 47 (2009) 919–922. [17] C.H. Liang, W. Xia, M. van den Berg, Y.M. Wang, H. Soltani-Ahmadi, O. Schluter, R.A. Fischer, M. Muhler, Chem. Mater. 21 (2009) 2360–2366. [18] W. Xia, V. Hagen, S. Kundu, Y.M. Wang, C. Somsen, G. Eggeler, G.G. Sun, G. Grundmeier, M. Stratmann, M. Muhler, Adv. Mater. 19 (2007) 3648–3652. [19] T. Osaki, T. Mori, React. Kinet. Catal. Lett. 89 (2006) 333–339. [20] J.L. Figueiredo, C.A. Bernardo, J.J. Chludzinski Jr., R.T.K. Baker, J. Catal. 110 (1988) 127–138. [21] P. Serp, J.L. Figueiredo, P. Bertrand, J.-P. Issi, Carbon 36 (1998) 1791–1799. [22] J.L. Figueiredo, P. Serp, B. Nysten, J.-P. Issi, Carbon 37 (1999) 1809–1816. [23] S.C. Tsang, P.J.F. Harris, M.L.H. Green, Nature 362 (1993) 520–522. [24] H. Ohme, T. Suzuki, Energy Fuels 10 (1996) 980–987. [25] G. Hermann, K.J. Hüttinger, Carbon 24 (1986) 429–435. [26] H.F. Gorgulho, J.P. Mesquita, F. Gonc¸alves, M.F.R. Pereira, J.L. Figueiredo, Carbon 46 (2008) 1544–1555. [27] Y. Luo, Y. Heng, X. Dai, W. Chen, J. Li, J. Solid State Chem. 182 (2009) 2521–2525.
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
On the role of the residual iron growth catalyst in the gasification of multi-walled carbon nanotubes with carbon dioxide Chen Jin, Wei Xia ∗ , Peirong Chen, Martin Muhler ∗ Laboratory of Industrial Chemistry, Ruhr-University Bochum, D-44780 Bochum, Germany
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 21 December 2011 Accepted 24 February 2012 Available online 7 April 2012 Keywords: Carbon nanotubes Gasification with CO2 Residual growth catalyst Temperature-programmed desorption Temperature-programmed surface reaction Boudouard reaction
a b s t r a c t The gasification of carbon with CO2 was applied to examine the role of the residual iron growth catalyst in multi-walled carbon nanotubes (CNTs), which were pre-treated either by refluxing in nitric acid at 120 ◦ C or by nitric acid vapor at 200 ◦ C. Temperature-programmed desorption (TPD) and surface reaction (TPSR) experiments were performed in He and CO2 , respectively. The Fe nanoparticles were retained after the treatment in HNO3 vapor, whereas the liquid HNO3 treatment was able to remove the accessible residual Fe catalyst. The exposed Fe nanoparticles were found to catalyze the gasification of CNTs with CO2 according to the reverse Boudouard reaction C + CO2 = 2CO. In case of the CNTs pretreated in HNO3 vapor, evolving CO2 formed due to the decomposition of oxygen-containing functional groups during the TPD experiments was fully converted above 750 ◦ C into desorbing CO, and the addition of 2000 ppm CO2 in the feed gas during the TPSR experiments resulted in full conversion at 1000 ◦ C. X-ray photoelectron spectroscopy studies show that the treatment in HNO3 vapor at 200 ◦ C favors the formation of oxygen species doubly bound to carbon (C O groups). During the TPSR experiments, CO2 as a weak oxidant partially oxidized the CNTs leading to the formation of C O groups, and a much higher amount of these groups was detected on HNO3 vapor-treated CNTs with residual Fe catalyst. Their presence suggests that C O groups are reaction intermediates of the CNT gasification with CO2 , which is considered an effective test reaction for the presence of residual catalytically active nanoparticles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The synthesis of multi-walled carbon nanotubes (CNTs) usually involves metallic catalysts, such as Fe, Co, or Ni, and in most cases a support like silica, magnesia, or carbon materials [1–3]. The as-synthesized CNTs always contain a certain amount of metallic impurities, which depends on the CNT yield obtained with the catalysts [4,5]. The removal of the support and the metallic impurities from the CNTs often requires tedious purification procedures involving sequential washing by different solvents [6]. The difficulties in removing the metallic impurities by acid treatment are attributed to the sheathing of the metal nanoparticles by several graphene layers [7]. Therefore, the complete removal of the metal catalysts used for the CNT growth turns out to be very challenging [8]. A small amount of metallic impurities may be negligible for many applications such as in polymer composites. For certain CNT applications, however, even a small amount of residual metal nanoparticles can have significant disadvantages. For example, Fe
∗ Corresponding authors. Tel.: +49 234 32 28754; fax: +49 234 32 14115. E-mail addresses: [email protected] (W. Xia), [email protected] (M. Muhler). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.02.052
oxide impurities are responsible for the observed activity in hydrogen peroxide reduction by CNTs [9], and Cu impurities are active for the reduction of halothane [10]. It was also reported that residual Co and Fe catalysts showed superior activities in NH3 decomposition [11]. In addition to the catalytic properties of the impurities, ferromagnetic properties of the residual catalysts were reported [12], which were used for magnetic capturing and self-assembly of individual CNTs [13]. In a biocompatibility study the residual Fe nanoparticles were found to be cytotoxic for brain cells [14]. The residual metallic impurities in CNTs can be removed by washing with acids such as nitric acid. The washing can lead to the formation of oxygen-containing functional groups on the CNT surfaces [15]. Alternatively, these groups can be created by the treatment with HNO3 vapor [16], during which the residual metal catalysts are not removed. The CNT surfaces become hydrophilic due to the acid treatment, and the improved wettability can enhance the degree of dispersion of CNTs in polar solvents. Moreover, the oxygen functional groups can be further chemically modified or directly used as anchoring sites for the immobilization of metal particles to be applied in heterogeneous catalysis [17,18]. The reverse Boudouard reaction (Eq. (1)) is the gasification of carbon materials with CO2 forming CO with an endothermic enthalpy change of ca. 171 kJ mol−1 at 298 K [19]. Baker and
C. Jin et al. / Catalysis Today 186 (2012) 128–133
coworkers [20] applied in situ transmission electron microscopy (TEM) to investigate the nickel-catalyzed growth and gasification of carbon filaments. It was found that these processes can be reversed suggesting that they both involve the diffusion of carbon through the metal nanoparticles. Serp, Figueiredo and coworkers [21,22] performed detailed studies on the surface properties of vaporgrown carbon fibers after gasification with CO2 or air, which were found to be strongly influenced by the presence of traces of iron. Green and coworkers made use of the Boudouard reaction to open and cut CNTs [23]. Group VIII metals, alkali and alkaline-earth salts are known to be highly active catalysts for the reaction. Among them, Fe is reported to be the most suitable catalyst because of its high activity and low cost [24]. The redox cycle consisting of Eqs. (2) and (3) over Fe catalysts was proposed, and the reduction of higher-valent Fe oxide to metallic Fe or lower-valent Fe oxides (Eq. (3)) was reported to be the rate-determining step [25]: CO2 + C 2CO
(1)
Fem On + CO2 Fem On+1 + CO
(2)
Fem On+1 + C Fem On + CO
(3)
Here, we report on the catalytic and non-catalytic gasification of CNTs with CO2 , where CO2 was either used as feed gas during temperature-programmed surface reaction (TPSR) experiments or generated by the decomposition of oxygen-containing surface functional groups during temperature-programmed desorption (TPD) experiments. It was found that the residual iron nanoparticles present in the CNTs treated with HNO3 vapor catalyze the gasification of the CNTs with CO2 . After removal of the residual catalyst by treatment in liquid HNO3 , coordinatively unsaturated carbon species such as amorphous carbon or carbon at defect sites react preferably with CO2 yielding CO above 770 ◦ C. 2. Experimental
129
2.2. Temperature programmed experiments TPD and TPSR experiments were performed in He and in CO2 , respectively. A horizontal quartz tube reactor with an inner diameter of 12 mm was used for the TPD. The evolved gases were passed through a calibrated infrared detector (Leybold BINOS 1001) with two channels for quantitative analysis of CO and CO2 . The following temperature program was employed for the TPD: (i) heat to 50 ◦ C, and keep for 20 min; (ii) heat to 1000 ◦ C at 2 K min−1 , and keep for 110 min; (iii) cool down. Typically, 100–200 mg of CNTs were applied for the TPD in He using a flow of 30 ml min−1 . For the TPSR in CO2 (gasification with CO2 ), 200 mg of CNTs were always applied in a flow of CO2 diluted in He (cCO2,0 = 2000 ppm, total flow
rate 100 ml min−1 ). All the TPD profiles shown were normalized to 1 g of CNTs starting after 20 min at 50 ◦ C. The onset temperature of the gasification reaction was determined, when the conversion of CO2 (XCO2 ) had reached 1%: XCO2 =
cCO2,0 − cCO2 cCO2,0
(4)
2.3. Characterization Elemental analysis was performed with an ICP-OES instrument (type PU701) from Philips-Unicam. Samples were dried at 120 ◦ C for 30 min in order to remove the moisture before the analysis. Xray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum set-up equipped with a GammadataScienta SES 2002 analyzer. The base pressure in the measurement chamber was 2 × 10−10 mbar. Monochromatic Al Ka (1486.6 eV; 14.5 kV; 45 mA) was used as incident radiation, and a pass energy of 200 eV was chosen resulting in an energy resolution better than 0.5 eV. A flood gun was used to compensate for charging effects. The binding energies were calibrated based on positioning the main C 1s peak at 284.5 eV.
2.1. Materials 3. Results and discussion Multi-walled CNTs (Pyrograf® -III) with inner diameters of 20–50 nm and outer diameters of 70–200 nm were obtained from Applied Sciences Inc. (Ohio, USA), for which Fe had been applied as growth catalyst. The CNTs are relatively large in diameter, and they have the typical structural features of CNTs as shown by recent TEM studies [3]. The following chemicals and gases were used in this study: nitric acid (65%, J.T. Baker), helium (99.9999%), and hydrogen (99.9999%). The as-received CNTs were first treated at 800 ◦ C in flowing He in order to remove the adsorbed polyaromatics. The HNO3 treatment was performed using two different methods: (1) refluxing in liquid HNO3 for 90 min at 120 ◦ C (2 g CNT, 150 mL HNO3 ), followed by filtration, washing, and drying to obtain sample OCNT-L; (2) treatment in nitric acid vapor at 200 ◦ C for 24 h (1 g CNT, 200 mL HNO3 ) to obtain sample OCNT-V. The details of the treatment and their differences were described elsewhere [16]. Briefly, both methods can create oxygen-containing functional groups on CNTs. Method (1) can also remove the residual catalysts used for CNT growth, whereas method (2) can create more oxygen-containing groups, but cannot remove the residual catalysts. Subsequently, OCNT-L was treated in a horizontal quartz tube reactor at 800 ◦ C (heating ramp 10 K min−1 ) in flowing He (200 ml min−1 ) for 2 h, and the obtained sample is denoted as HT-OCNT-L. Furthermore, OCNT-L and OCNT-V were treated at 500 ◦ C (heating ramp 2 K min−1 ) in flowing He (200 ml min−1 ) for 210 min, and the obtained samples are labeled as LT-OCNT-L and LT-OCNT-V, respectively. Additionally, the sample OCNT-L was also reduced at 800 ◦ C (ramp 10 K min−1 ) in a mixture of H2 and He (total 200 ml min−1 , 1:3) for 2 h to obtain R-OCNT-L.
3.1. Elemental analysis Recent TEM studies found that many of the as-received CNTs are open-ended without catalyst nanoparticles [3]. However, catalyst particles were observed at the tips of some CNTs with dimensions comparable with the inner diameter of the CNTs, which is in the range from 20 to 50 nm. Elemental analysis detected 1.19 wt% Fe in the as-received CNTs. A very similar amount (1.20 wt%) of Fe was found in the CNTs treated by HNO3 vapor indicating that the gas-phase method did not cause the loss of Fe. Refluxing in liquid HNO3 led to the removal of all the accessible Fe nanoparticles, and the residual content of 0.60 wt% Fe detected by elemental analysis is assumed to be inaccessible due to a dense coverage by carbon [6]. 3.2. Temperature-programmed desorption The TPD profiles of the samples OCNT-L and OCNT-V in He are shown in Fig. 1. The desorption of CO2 below 340 ◦ C can be attributed to the decomposition of carboxylic acid groups, and the peaks at around 480 ◦ C and 640 ◦ C are due to the decomposition of more stable groups like carboxylic anhydrides and lactones, respectively. In the CO profile, desorption below 300 ◦ C can be assigned to aldehydes. The decomposition of carboxylic anhydride groups also gives rise to CO production at the same temperature of 480 ◦ C as observed for CO2 . The desorption of CO at 700 ◦ C is related to the decomposition of phenol and ether groups. Carbonyls decompose at an even higher temperature of around 830 ◦ C [15,26]. The
130
C. Jin et al. / Catalysis Today 186 (2012) 128–133
Fig. 1. TPD profiles of OCNT-L (a) and OCNT-V (b) performed in He at a heating rate of 2 K min−1 . (1) CO2 ; (2) CO.
quantitative analysis showed that the desorption of both CO and CO2 from sample OCNT-L (CO 1.0 mmol g−1 , CO2 0.62 mmol g−1 ) is significantly lower than that from sample OCNT-V (CO 1.56 mmol g−1 , CO2 1.07 mmol g−1 ) as summarized in Table 1. CO2 desorption was not observed above 750 ◦ C in both samples. A sharp CO desorption peak at 730 ◦ C was obtained with sample OCNT-V, which did not appear for OCNT-L. In case the gasification reaction (Eq. (1)) of CNTs by CO2 released from the oxygen functional groups had occurred, the increase in the CO concentration should be accompanied by a decrease in the CO2 concentration. As this is indeed the case, the CO peak observed for sample OCNT-V at ca. 730 ◦ C can be assigned to a pronounced gasification reaction due to the evolved CO2 . 3.3. Temperature-programmed surface reaction In order to determine the onset temperature of the gasification reaction, the CO2 gasification of OCNT-L and OCNT-V was studied by heating the samples in a constant diluted flow of CO2 . The resulting TPSR profiles are shown in Fig. 2. There are at least two contributions to the variation of the CO and CO2 concentrations during heating: (1) the desorption of CO and CO2 from oxygen-containing surface functional groups on CNTs, and (2) the conversion of CO2 to CO at high temperatures due to the gasification reaction (Eq. (1)). The desorption of oxygen-containing groups can be seen in both samples as indicated by the increased CO2 concentrations in the low-temperature range (Fig. 2). Obviously, sample OCNT-V released more CO2 than OCNT-L, which is in good agreement with the TPD results shown in Fig. 1. The onset temperature of gasification was determined at 770 ◦ C for OCNT-L. For OCNT-V, it is difficult to determine the onset due to the continuous increase of the CO concentration in the range from 450 ◦ C to 800 ◦ C. It was found that the concentration of CO2 dropped below the amount of ca. 2000 ppm in the feed at about 500 ◦ C. It is known from Fig. 1b that at 500 ◦ C the desorption of CO2 from the CNT surfaces was still occurring indicating that the gasification actually started below 500 ◦ C, which is far
below the temperature of 770 ◦ C observed for OCNT-L. As the Fe catalyst was not removed by the HNO3 vapor treatment, the presence of accessible residual Fe nanoparticles in OCNT-V is responsible for its low gasification temperature. The higher onset gasification temperature of OCNT-L without Fe nanoparticles is obviously related to surface defects and not to the residual catalyst [23]. It is shown in Fig. 1 that for sample OCNT-V CO2 desorption was not observed above 750 ◦ C. Although there is still CO desorption from the surface above 750 ◦ C, its amount is believed to be very small as compared to the amount of CO produced by carbon gasification in the CO2 flow. By neglecting the desorption of CO from surface oxygen groups in the temperature range from 750 ◦ C to 1000 ◦ C, the theoretical CO concentration, i.e., the amount of CO generated by gasification, can be calculated from the consumed amount of CO2 according to Eq. (5), that is, one molecule CO2 can generate 2 molecules of CO through the gasification reaction of carbon. In the mass balance, cco,calc is the calculated CO concentration, cCO2,0 is the detected CO2 concentration in the feed gas at the initial stage (50 ◦ C) of each experiment (the concentration varies slightly in different experiments), and cCO2 is the detected CO2 concentration during heating from 750 ◦ C to 1000 ◦ C. cCO,calc = (cCO2,0 − cCO2 ) × 2
(5)
The calculated CO evolution based on the CO2 evolution in the range between 750 ◦ C and 1000 ◦ C is shown in trace 3 in Fig. 2a and b. Sample OCNT-L shows good agreement between traces 2 and 3, but the actual CO evolution (trace 2 in Fig. 2b) of sample OCNT-V is less than the theoretical amount of CO (trace 3 in Fig. 2b) between 750 ◦ C and 1000 ◦ C. The deviation is assumed to be related to the
Table 1 Amount of desorbing CO and CO2 derived from the TPD studies in He and the surface atomic concentrations of oxygen (O in C O, and O in C–O) derived from the XPS measurements. Samples
CO2 (mmol g−1 )
CO (mmol g−1 )
C O (at.%)
C–O (at.%)
OCNT-L OCNT-V OCNT-L (2nd run)a OCNT-V (2nd run)a
0.62 1.07 0.00 0.10
1.00 1.56 0.34 0.64
6.5 10.4 0.8 4.4
9.4 6.2 2.1 1.3
a
After 2nd CO2 gasification run (TPSR in CO2 ).
Fig. 2. TPSR profiles in 2000 ppm CO2 of (a) OCNT-L and (b) OCNT-V. (1) CO2 ; (2) CO; (3) calculated theoretical concentration of CO from CO2 based on the reverse Boudouard reaction in the temperature range from 750 ◦ C to 1000 ◦ C.
C. Jin et al. / Catalysis Today 186 (2012) 128–133
131
LT-OCNT-V with accessible Fe catalyst displays desorption and gasification in the first run (Fig. 4c), and the conversion remained at 9% after keeping the temperature at 700 ◦ C for 210 min. In the second run, the initial temperature of gasification was found to be at 600 ◦ C, and a constant conversion of 8.5% was reached at 700 ◦ C. Compared to the as-prepared sample (OCNT-V, Fig. 2b), the onset temperature of gasification is about 100 ◦ C higher presumably due to the consumption of the Ccus species during the first gasification run. 3.4. TPD studies subsequent to TPSR experiments
Fig. 3. TPSR profiles in 2000 ppm CO2 of (a) HT-OCNT-L and (b) R-OCNT-L, which had been heated at 800 ◦ C for 2 h in He and H2 , respectively. (1) CO2 ; (2) CO; (3) calculated theoretical concentration of CO from CO2 based on the reverse Boudouard reaction in the temperature range from 50 ◦ C to 1000 ◦ C.
interaction of CO with the Fe nanoparticles. Sample OCNT-V shows a high and constant CO2 conversion of 94% for 60 min at 1000 ◦ C. It is known that coordinatively unsaturated carbon (Ccus ) species like amorphous carbon or carbon at defect sites such as edges or steps can preferably react with CO2 as compared to graphitic carbon [24]. The Ccus species on OCNT-V were consumed before reaching 1000 ◦ C because of the high activity of the residual Fe catalyst. Therefore, at 1000 ◦ C, the main carbon source is graphitic carbon. In contrast, the conversion of CO2 over OCNT-L decreased from 37% to 31% after 60 min at 1000 ◦ C, likely due to the consumption of Ccus species. We reported on the effect of thermal treatment in He or H2 on the removal of oxygen-containing functional groups [15]. To avoid the influence of evolved CO and CO2 originating from the decomposition of oxygen groups on the gasification, a thermal treatment of the samples was performed in He or H2 at 800 ◦ C. The TPSR profiles in CO2 of the heat-treated OCNT-L sample are shown in Fig. 3. As expected, the CO2 desorption from the functional groups on CNTs cannot be observed in the whole temperature range. The calculated CO evolution (trace 3 in Fig. 3) overlaps with the actual evolution of CO in trace 2 in Fig. 3 in the full temperature range above 750 ◦ C. These results further confirm that the liquid HNO3 treatment is sufficient to remove all the accessible Fe nanoparticles. The onset gasification temperature of sample HT-OCNT-L (740 ◦ C, Fig. 3) with the pretreatment at 800 ◦ C in He is slightly lower than that of OCNT-L (770 ◦ C, Fig. 2) without the heat pretreatment. The differences in the onset temperatures of the non-catalytic gasification are presumably due to the heat treatment at 800 ◦ C creating more Ccus species such as surface defects from the decomposition of the functional groups on the CNT surface [27]. The Ccus species react preferably with CO2 compared with graphitic carbon. Therefore, the conversion of CO2 on HT-OCNT-L decreased from 54% to 45% due to the consumption of Ccus species. OCNT-L treated in H2 (R-OCNT-L) shows the same onset temperature of gasification as OCNT-L with a constant conversion of 36% for 60 min at 1000 ◦ C indicating that the Ccus species had been removed more efficiently by the H2 treatment. In order to further study the gasification activity of the residual Fe catalyst, OCNT-L and OCNT-V were heat-treated at 500 ◦ C to remove the low-temperature CO2 desorption groups, and then heated to 700 ◦ C in CO2 (Fig. 4). During the first TPSR run with sample LT-OCNT-L, further desorption of CO and CO2 in the temperature range between 500 ◦ C and 700 ◦ C can be observed (Fig. 4a). The second TPSR run (Fig. 4b) with the samples resulting from the first run led to a completely flat CO2 profile. The results show that no gasification reaction occurred below 700 ◦ C for the sample LT-OCNT-L.
In order to further investigate the strong gasification peak at 730 ◦ C detected in Fig. 1b, the samples LT-OCNT-L and LT-OCNTV obtained after the second CO2 gasification experiment (TPSR in CO2 ) at 700 ◦ C were further characterized by TPD in He up to 1000 ◦ C. The quantitative results are summarized in Table 1. The TPD profile of LT-OCNT-L after the second TPSR run disclosed the presence of high temperature-stable CO desorption groups, as indicated by the CO peak between 700 ◦ C and 1000 ◦ C (trace 2 in Fig. 5a). The amount of desorbed CO was determined to be 0.34 mmol g−1 (Table 1), whereas only traces of CO2 were detected (trace 1 in Fig. 5a). For the sample LT-OCNT-V after the second TPSR run (Fig. 5b), a significantly higher amount of desorbed CO (0.64 mmol g−1 ) was detected, and there was still CO desorption even after 1 h at 1000 ◦ C indicating that the vapor HNO3 treatment generates more high-temperature stable oxygen functional groups such as pyrone or quinone on the CNTs. Moreover, a sharp CO desorption peak at 720 ◦ C was observed correlated with a sudden decrease of CO2 at the same temperature. The sharp CO peak fits well to the one at 730 ◦ C in trace 2 in Fig. 1b. These results confirm a strong gasification occurring at about 720–730 ◦ C catalyzed by the accessible residual Fe nanoparticles. 3.5. XPS studies The TPD and TPSR studies clearly demonstrate that the treatments in liquid HNO3 and in HNO3 vapor result in a clearly different CO2 gasification behavior. To study the difference in the oxygen functional groups generated by the liquid and the vapor method, XPS was applied to the samples before and after CO2 gasification. The surface atomic concentrations derived from the C 1s and O 1s spectra are summarized in Table 1. The O 1s spectra can be deconvoluted into two peaks corresponding to oxygen doubly bound to carbon (C O) in aldehydes, ketones, carboxyls, anhydrides, lactones, quinones and pyrones at ca. 531.5 eV, and oxygen singly bound to carbon (C–O) in carboxyls, anhydrides, lactones, phenols, ethers and pyrones at a binding energy of ca. 533.4 eV [15]. From Fig. 6a it can be seen that the HNO3 vapor treatment can generate more oxygen functional groups especially of the C O type (10.4 at.%) compared with the liquid HNO3 treatment (6.5 at.% C O) (Table 1), which is in good agreement with our previous results [16]. The O 1s spectra of the samples after the TPSR experiments at 700 ◦ C are shown in Fig. 6b. It is obvious that CO2 as a weak oxidant can partially oxidize CNTs at 700 ◦ C leading to the formation of oxygen functional groups on CNT surfaces. As discussed above, both C O and C–O species can be high-temperature stable. The C O groups are still the dominant species in the HNO3 vapor-treated sample, whereas the C–O groups are the major species in the liquid HNO3 -treated sample after the second gasification run (Fig. 6b and Table 1). Furthermore, a much higher amount of oxygen groups was detected on the vapor HNO3 -treated CNTs with residual nanoparticles compared with the purified liquid HNO3 -treated CNTs. These results show that mainly C O groups were formed on the CNT surfaces in the presence of CO2 suggesting that the C O groups are intermediates of the Boudouard reaction catalyzed by the residual
132
C. Jin et al. / Catalysis Today 186 (2012) 128–133
Fig. 4. TPSR profiles in 2000 ppm CO2 of LT-OCNT-L (first run (a), second run (b)) and LT-OCNT-V (first run (c), second run (d)). (1) CO2 ; (2) CO.
Fig. 5. TPD profiles obtained in helium at a heating rate of 2 K min−1 after the 2nd TPSR run in CO2 . (a) LT-OCNT-L, (b) LT-OCNT-V. (1) CO2 ; (2) CO.
Fig. 6. XP O 1s spectra of CNTs treated by HNO3 in the liquid phase (1) and gas phase (2). (a) subsequent to the HNO3 treatment; (b) after further treatment at 500 ◦ C in He and the 2nd TPSR run in CO2 .
C. Jin et al. / Catalysis Today 186 (2012) 128–133
Fe catalyst. In summary, the gasification with CO2 is considered an effective test reaction for the presence of residual catalytically active nanoparticles in CNTs. 4. Conclusions Refluxing in liquid HNO3 and the treatment in HNO3 vapor were employed as two different methods for the oxygen functionalization of CNTs. Residual Fe nanoparticles were retained after the vapor HNO3 treatment, while the liquid HNO3 treatment washed out the accessible residual Fe catalyst originating from the growth of the CNTs. The accessible Fe nanoparticles in the HNO3 vapor-treated sample catalyzed the gasification of CNTs by CO2 above 500 ◦ C either generated due to the decomposition of the oxygen functional groups on the CNT surface during the TPD experiments or by added CO2 during the TPSR experiments. The non-catalytic gasification reaction is assumed to preferably consume coordinatively unsaturated carbon species, which also originate from the removal of oxygen functional groups upon heating. The XPS studies disclosed that CO2 as a weak oxidant was able to oxidize CNTs at high temperatures leading to the formation of oxygen functional groups on CNT surfaces. A much higher amount of oxygen groups, especially of C O groups, was detected after the TPSR experiments on vapor HNO3 -treated CNTs with residual nanoparticles compared with purified liquid HNO3 -treated CNTs. These formed C O groups on the CNTs are presumably reaction intermediates of the CO2 gasification catalyzed by the residual Fe nanoparticles. Acknowledgement Chen Jin thanks the International Max Planck Research School Surface and Interface Engineering in Advanced Materials (SurMat) for a research grant.
133
References [1] J.H. Bitter, J. Mater. Chem. 20 (2010) 7312–7321. [2] K.L. Klein, A.V. Melechko, T.E. McKnight, S.T. Retterer, P.D. Rack, J.D. Fowlkes, D.C. Joy, M.L. Simpson, J. Appl. Phys. 103 (2008) 061301–061326. [3] W. Xia, X. Chen, S. Kundu, X. Wang, G. Grundmeier, Y. Wang, M. Bron, W. Schuhmann, M. Muhler, Surf. Coatings Technol. 201 (2007) 9232–9237. [4] J.-P. Tessonnier, D. Rosenthal, T.W. Hansen, C. Hess, M.E. Schuster, R. Blume, F. Girgsdies, N. Pfänder, O. Timpe, D.S. Su, R. Schlögl, Carbon 47 (2009) 1779–1798. [5] L. Guo, D.G. Morris, X. Liu, C. Vaslet, R.H. Hurt, A.B. Kane, Chem. Mater. 19 (2007) 3472–3478. [6] M. Pumera, H. Iwai, J. Phys. Chem. C 113 (2009) 4401–4405. [7] J.L. Lyon, K.J. Stevenson, Langmuir 23 (2007) 11311–11318. [8] S. Kundu, T.C. Nagaiah, W. Xia, Y. Wang, S.V. Dommele, J.H. Bitter, M. Santa, G. Grundmeier, M. Bron, W. Schuhmann, M. Muhler, J. Phys. Chem. C 113 (2009) 14302–14310. [9] B. Sljukic, C.E. Banks, R.G. Compton, Nano Lett. 6 (2006) 1556–1558. [10] X. Dai, G.G. Wildgoose, R.G. Compton, Analyst 131 (2006) 901–906. [11] J. Zhang, M. Comotti, F. Schüth, R. Schlögl, D.S. Su, Chem. Commun. (2007) 1916–1918. [12] Y.-H. Lee, D.H. Kim, D.H. Kim, B.-K. Ju, Appl. Phys. Lett. 89 (2006) 083113. [13] S.S. Joon, Y.H. Yun, M.J. Rust, J. Do, V. Shanov, M.J. Schulz, C.H. Ahn, Nanotechnology 20 (2009) 325607. ˜ S. Jin, Biomaterials 28 [14] T.R. Pisanic Ii, J.D. Blackwell, V.I. Shubayev, R.R. Finones, (2007) 2572–2581. [15] S. Kundu, Y. Wang, W. Xia, M. Muhler, J. Phys. Chem. C 112 (2008) 16869–16878. [16] W. Xia, C. Jin, S. Kundu, M. Muhler, Carbon 47 (2009) 919–922. [17] C.H. Liang, W. Xia, M. van den Berg, Y.M. Wang, H. Soltani-Ahmadi, O. Schluter, R.A. Fischer, M. Muhler, Chem. Mater. 21 (2009) 2360–2366. [18] W. Xia, V. Hagen, S. Kundu, Y.M. Wang, C. Somsen, G. Eggeler, G.G. Sun, G. Grundmeier, M. Stratmann, M. Muhler, Adv. Mater. 19 (2007) 3648–3652. [19] T. Osaki, T. Mori, React. Kinet. Catal. Lett. 89 (2006) 333–339. [20] J.L. Figueiredo, C.A. Bernardo, J.J. Chludzinski Jr., R.T.K. Baker, J. Catal. 110 (1988) 127–138. [21] P. Serp, J.L. Figueiredo, P. Bertrand, J.-P. Issi, Carbon 36 (1998) 1791–1799. [22] J.L. Figueiredo, P. Serp, B. Nysten, J.-P. Issi, Carbon 37 (1999) 1809–1816. [23] S.C. Tsang, P.J.F. Harris, M.L.H. Green, Nature 362 (1993) 520–522. [24] H. Ohme, T. Suzuki, Energy Fuels 10 (1996) 980–987. [25] G. Hermann, K.J. Hüttinger, Carbon 24 (1986) 429–435. [26] H.F. Gorgulho, J.P. Mesquita, F. Gonc¸alves, M.F.R. Pereira, J.L. Figueiredo, Carbon 46 (2008) 1544–1555. [27] Y. Luo, Y. Heng, X. Dai, W. Chen, J. Li, J. Solid State Chem. 182 (2009) 2521–2525.