NMR Study of Preferential Endohedral Adsorption of Methanol in

Article pubs.acs.org/JPCC

NMR Study of Preferential Endohedral Adsorption of Methanol in Multiwalled Carbon Nanotubes Xin Liu, Xiulian Pan, Wanling Shen, Pengju Ren, Xiuwen Han, and Xinhe Bao* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China S Supporting Information *

ABSTRACT: Adsorption of 13C-enriched methanol in multiwalled carbon nanotubes (MWNTs) was systematically studied under well-controlled environment by 13C MAS NMR. The results of variable-pressure experiments indicate that the endohedral adsorption of methanol in carbon nanotubes (CNTs) is preferential over the exohedral adsorption. The exohedral adsorption does not occur until the endohedral adsorption saturates. The 13C chemical shift of endohedral methanol exhibits a large upfield shift due to the strong spatial diamagnetic shielding effect induced by the delocalized electrons of nanotubes under the influence of external magnetic field. The exohedral methanol is also shielded, but to a lesser extent. DFT calculations support these experimental results. The 13C spin−lattice relaxation times, T1, of endohedral and exohedral methanol were also measured, and they have different vapor pressure dependence at room temperature. Furthermore, variable-temperature experiments suggest that methanol molecules inside CNTs may form a layered structure at low temperature with one layer close to the wall and the second layer near the center of the nanotubes.

1. INTRODUCTION Carbon nanotubes (CNTs) offer unique quasi-one-dimensional nanopore structures, and the encapsulation of molecules in CNTs is an important topic in the field. Because of their large adsorptive capacity and high electron and thermal conductivity, CNTs have many potential applications as, for example, heterogeneous catalysts,1−3 storage media,4,5 and gas sensors,6 for which understanding of the gas−nanotube interactions is rather important. Adsorption isotherm,7,8 infrared (IR) spectroscopy,9,10 temperature programmed desorption (TPD),11,12 and Raman spectroscopy13,14 have been widely employed to study the adsorption of the gas molecules in CNTs. As compared to these methods, nuclear magnetic resonance (NMR) is a nondestructive technique and can provide bulk information on the adsorption sites, mechanisms, and strengths of adsorbates on nanotubes.15 Most importantly, theoretical studies show that NMR spectroscopy and relaxometry of the molecules encapsulated within nanotubes has the potential to provide information on the local electronic structures of surrounding nanotubes.16−18 Sekhaneh et al.19 and Chen et al.20 employed the 1H MAS NMR technique and assigned two proton chemical shift ranges to the water adsorbed inside and outside the nanotubes. Pietrass et al.21 reported the 1H NMR studies of hydrogen adsorption on single-walled carbon nanotubes (SWNTs) and identified the interstitial sites and the tube interior as strong adsorption sites. In addition, several studies22,23 using 129Xe NMR spectroscopy showed that © 2012 American Chemical Society

xenon can penetrate the interior of the nanotubes. Among different nuclei, 13C NMR is a sensitive probe to the local electronic environment of CNTs.16,24,25 However, a wealth of literature26−29 has so far focused only on the direct measurement of CNTs using 13C isotope enrichment technique. To the best of our knowledge, there is no systematic 13C NMR investigation that involves the study of 13C-enriched guest molecules confined inside CNTs. In this Article, we report a well-controlled NMR study of 13 C-enriched methanol adsorption in multiwalled carbon nanotubes (MWNTs). The results show that methanol can enter the cavities of CNTs. The endohedral and exohedral adsorption is well identified, and the endohedral adsorption is preferential. Our experimental results are complemented by the theoretical calculations, which bring to light the identification of 13 C NMR chemical shifts of the endohedral and exohedral methanol.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Materials, Sample Preparation, and Pretreatment. CNTs, synthesized by chemical vapor deposition (CVD), were purchased from Chengdu Organic Chemicals Co., Ltd. The raw materials were purified by concentrated HNO3 to remove the residual catalysts and amorphous carbon impurities. The Received: January 5, 2012 Revised: March 21, 2012 Published: March 21, 2012 7803

dx.doi.org/10.1021/jp300138x | J. Phys. Chem. C 2012, 116, 7803−7809

The Journal of Physical Chemistry C

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resulting CNTs had at least at one end opened. The average inner tube diameter was approximately 4−8 nm and the average tube length was several micrometers, as determined by transmission electron microscopy (TEM) (see Supporting Information, Figure S1). This sample was denoted as O-CNTs. Another bamboo-like MWNTs sample with the same tube diameter and tube length had closed tips at both ends and was denoted as C-CNTs. To remove the oxygen functionalities that might limit the access to endohedral adsorption sites through the dipole-induced dipole interactions,30 the purified CNTs were heated at 1173 K under Ar atmosphere for 30 min. Electronic spin resonance (ESR) demonstrated that the final CNTs samples for NMR analysis exhibited negligible paramagnetism (see Supporting Information, Figure S2). 2.2. NMR Studies. 13C MAS NMR experiments were performed on a Varian 400 MHz spectrometer at a resonance frequency of 100.5 MHz with a sample spinning rate of 7 kHz using a 5 mm zirconia rotor. The spectra were acquired using single pulse excitation with a π/2 pulse length of 2.6 μs and a recycle delay of 8 s. The spectra were recorded by accumulating 1000 scans. The 13C chemical shift was referenced with respect to the 13C resonance of admantane. For spin−lattice relaxation time (T1) measurements (inversion recovery method), 100 scans were accumulated for each data point. A special homemade quartz device31 was used for in situ pretreatment, methanol adsorption, and NMR rotor filling. MWNTs samples were first treated overnight at 673 K under dynamic vacuum (10−6 Torr) to remove the residual water before methanol adsorption. All of the adsorption experiments were carried out at room temperature. 13C-enriched methanol (99% purity, Sigma-Aldrich) vapor was introduced into the system, and the vapor pressure was controlled by a pressure gauge. After the pressure has stabilized and remained constant within 15 min, the 5 mm NMR rotor was in situ filled up with 40 mg of CNTs and sealed by a polytetrafluoroethene cap. Saturation adsorption measurement was conducted under a vapor pressure of 16 kPa. At this pressure, the sample had adsorbed methanol for several times until the pressure did not change. Before NMR measurement, the samples were allowed to thermally equilibrate for at least 1 h after being sealed. For variabletemperature experiments, the temperature was measured with a thermocouple close to the sample with an accuracy of ±0.5 K. At each temperature, the samples were allowed to re-equilibrate for at least 15 min before acquiring NMR data. 2.3. Computational Methods. All isotropic nuclear shielding calculations were carried out with the gauge-including atomic orbitals (GIAO) methodology32,33 implemented within the Gaussian 09 package.34 Density functional theory (DFT) with the M062X exchange-correlation functional35 was used to compute nuclear shielding, which has been well accepted as a good choice for the dispersion interaction.36 A Gaussian-type basis set of 6-31G* was implemented for all atoms. Metallic (6,6) and semiconducting (11,0) SWNTs with similar diameters of 8.2 and 8.6 Å were used for the calculation and were modeled with finite hydrogen capped tubes. All of the structures were optimized using the self-consistent charge Density Functional based Tight Binding by dispersion correction (SCC-DFTB-D) method37,38 with edge C and H atoms fixed, as shown in Supporting Information, Figure S3.

Figure 1. (a) 13C MAS NMR spectra of the methanol adsorbed in OCNTs as a function of methanol vapor pressure at room temperature. (b) 13C MAS NMR spectra of the methanol adsorbed in C-CNTs at a vapor pressure of 9 kPa. The peak labeled “*” is the spinning sidebands at 7 kHz. The peak at 112 ppm is the background signal of the probe. The exponential line broadening for each spectra is 100 Hz. Each spectrum was accumulated for 1000 scans to guarantee the signals with acceptable signal-to-noise ratios. For example, the signalto-noise ratios for the peaks at 32 and 44 ppm under 15 kPa are 131 and 173, respectively.

MAS NMR spectra of the O-CNTs exposed to 13C-enriched methanol at various vapor pressures. At the lowest pressure (i.e., 6 kPa), there is only one peak at around 32 ppm. As the vapor pressure is elevated to 9 kPa, the peak at 32 ppm grows in intensity and a new peak appears at about 44 ppm, whose chemical shift is close to that of liquid methanol (50 ppm). The intensity of the 44 ppm peak becomes stronger with the increasing loading of methanol, and it clearly surpasses the other peak at very high methanol vapor pressure (i.e., 15 kPa). This is a strong hint that the signal at 44 ppm results from the methanol outside the CNTs. The saturated adsorption measurement in Figure 1a shows that the intensity of the 44 ppm signal is absolutely stronger than that at 32 ppm. In comparison, bamboo-like C-CNTs with closed tips at both ends only exhibit one peak centered at about 41 ppm upon exposure to 9 kPa methanol vapor, as shown in Figure 1b. This signal can only be attributed to methanol outside of the nanotubes because both ends are closed and no methanol can enter the channels. This confirms that the 44 ppm signal in Figure 1a arises from the outside methanol, while the upfield peak at 32 ppm is from the endohedrally adsorbed methanol molecules. To gain insight into the molecular adsorption on MWNTs, we need to consider the structure of the adsorption sites available to the adsorbate molecule. Generally, two adsorption sites can be identified in MWNTs:39 1-D nanoscale cavities corresponding to the endohedral adsorption sites, and aggregated pores referred to as the exohedral adsorption sites. Aggregated pores are formed by isolated tubes of different

3. RESULTS AND DISCUSSION 3.1. Identification of Endohedral and Exohedral Adsorption. Figure 1a shows the room-temperature 13C 7804



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diameters and orientations. Because of the concave nature of graphene walls, the endohedral adsorption sites are expected to have a higher binding energy toward adsorbing molecules as compared to the exohedral adsorption sites.40 Thus, methanol molecules are preferentially adsorbed in the CNTs interior. Several theoretical studies predicted that for most nuclei in a variety of guest molecules, there is an upfield shift upon encapsulation in the nanotubes as a result of the diamagnetic shielding from ring currents induced in the tubes by the external field.18,24,41 It was observed by 1H NMR that the broad peaks for methane and ethane confined in the interior of SWNTs both moved upfield relative to the free gas resonance.42 The 1H MAS NMR studies by Sekhaneh et al.19 and Chen et al.20 also found that the confined water signal shifted upfield relative to free bulk water by 3.3 and 4.4 ppm, respectively. Matsuda et al.43 showed a large proton upfield shift (−15 ppm) for the confined water relative to free bulk water. In addition, the diamagnetic shielding arising from the ring current effect was also observed by 1H NMR for hydrogen adsorption in narrow-pore activated carbon.44 Given that the chemical shift of free liquid methanol is 50 ppm, we observed that the methanol molecules adsorbed inside CNTs exhibit a large (−18 ppm) upfield shift, and that the methanol outside CNTs experience a relatively small (−6 ppm) upfield shift. To shed more light on the underlying mechanisms of the above-mentioned experimental observations, theoretical calculations were carried out on two SWNTs with a similar diameter (i.e., (11,0) tube and (6,6) tube), and the results are listed in Table 1. The methanol encapsulated inside the (11,0)

Figure 2. Pressure dependence of the intensity of the two peaks (32 ppm, 44 ppm) of the 13C spectra (see Figure 1a) of O-CNTs at room temperature. Each spectrum is deconvoluted, and the integral intensity is averaged by the number of scan. Black circles represent the endohedral adsorption and are fit to the heterogeneous Langmuir− Freundlich equation: n = n∞bPm/(1 + bPm), indicated by the solid line. Blue squares represent the exohedral adsorption and are fit to a linear regression equation (>8 kPa), also indicated by the solid line. The regression coefficients for the least-squares data fittings of endohedral and exohedral adsorption are 0.94 and 0.93, respectively. Two dashed lines are the tangent for the solid fitting curve of endohedral adsorption and the extrapolating line for the solid fitting line of exohedral adsorption, and the initial adsorption pressure points at Xcoordinate are 1.08 and 7.07 kPa, respectively.

Table 1. 13C Isotropic Chemical Shift (ppm) for Methanol Inside and Outside (11,0) and (6,6) CNTs Referenced to Isolated Methanol isolated methanol methanol inside CNT methanol outside CNT a

(11,0)

(6,6)

0.00a −25.15 −0.29

0.00a −7.19 −1.13

neous Langmuir−Freundlich model45,46 n = n∞bPm/(1 + bPm) is employed to fit the endohedral adsorption data, where n is the number of absorbed molecules, n∞ is the number of adsorption sites, b is a variable relevant to the binding energy, and m is the heterogeneity index, which varies from 0 to 1. A pressure of 1.08 kPa, at which the endohedral adsorption is projected to occur, is determined by extrapolating the fitting curve. Similarly, the exohedral adsorption is fitted with a linear regression equation; by extrapolating the fitting curve, we obtain the initial exohedral adsorption point (pressure = 7.07 kPa). Obviously, the endohedral adsorption occurs at a lower methanol vapor pressure than the exohedral adsorption. These results demonstrate that CNTs exhibit unique molecular adsorption ability and that the endohedral adsorption is unambiguously preferential. This finding is important for applications such as catalysis and sensing. 3.3. Relaxation Properties for Endohedral and Exohedral Methanol. The 13C spin−lattice relaxation time was measured, and Figure 3 shows the resulting 13C T1 values of both endohedral and exohedral adsorption peaks in Figure 1a as a function of pressure. The T1 of the exohedral methanol increases linearly with increasing pressure, indicating fast molecular motion. When extrapolated to zero pressure the curve does not go through the origin, which suggests that the methanol molecules also collide with the walls of MWNTs. Similar results were also reported previously in ref 42. In contrast, the T1 for endohedral methanol exhibits little dependence on pressure, different from the behavior in gas phase. Furthermore, the T1 of the endohedral methanol remains shorter than that of the exohedral methanol at any given pressure, which is another manifestation of the strong

The 13C chemical shift for isolated methanol was set to 0 ppm.

tube shows an upfield shift of −25.15 ppm, which is greater than that outside the tube (i.e., an upfield shift of −0.29 ppm). A similar trend is also observed on the (6,6) tube. This is consistent with the experimental observation in Figure 1a. The difference between the calculated and observed chemical shifts could be caused by the models used for calculation, that is, SWNTs with a small diameter of 0.8 nm. 3.2. Endohedral Adsorption Is Preferential. Figure 2 highlights the integral intensities of the two peaks in Figure 1a as a function of methanol vapor pressure. For exohedral adsorption, its intensity is almost proportional to the pressure (>8 kPa). In contrast, the endohedral adsorption shows different behavior. The intensity first increases nonlinearly with pressure, and then reaches a maximum at a pressure higher than 8 kPa. This is similar to the observation by Wu et al. on the 1H NMR studies of methane and ethane adsorption in SWNTs.42 In addition, it is estimated from Figure 2 that the integral intensity for exohedral adsorption under saturation adsorption condition contributes 63% of the total adsorption amount, which is also consistent with the previously reported value of 78%.39 It is interesting to note that the exohedral adsorption does not take place until the endohedral adsorption saturates, which has never been reported yet. The heteroge7805



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adsorption (32 ppm) at 298 K, and this peak moves slightly downfield as the temperature decreases. When the temperature is increased from 153 to 298 K, the peak due to the endohedral adsorption moves from 42 back to 32 ppm (Figure 4b). As the temperature is further increased, a new peak centered at 47 ppm appears at 353 K. Its intensity grows with increasing temperature, indicating the appearance of exohedral methanol. This demonstrates that the encapsulated methanol molecules diffuse outside of the tubes above 353 K. The same variable-temperature experiments were performed on O-CNTs after methanol adsorption under 9 kPa. As demonstrated in Figure 5b, the heating process is similar to that for methanol adsorbed under 6 kPa in Figure 4b. Only one peak for endohedral adsorption is observed in the temperature range of 153−353 K, and the exohedral signal shows up when the temperature reaches above 353 K. However, in the cooling process (Figure 5a), the situation is different from the heating process. The endohedral and exohedral methanol coexist at 298 K, and the exohedral adsorption peak disappears at 273 K leaving only one peak at 32 ppm attributed to endohedral adsorption. It indicates that the methanol molecules outside of CNTs diffuse into the channels with decreasing temperature, analogous to the capillary condensation phenomenon at low temperature that has been observed for Xe.22 These results also show that it is easier (i.e., 273 K) for methanol to diffuse from outside to inside the tubes, whereas it is more difficult to diffuse outside (i.e., 353 K) once it is inside. Below 273 K, a new peak at 44 ppm (labeled Endo II in Figure 5a) shows up as the temperature is further lowered. At the same time, the other signal at 32 ppm moves downfield (labeled Endo I in Figure 5a). These two well-distinguished signals represent two different forms of endohedral methanol. Finally, at 173 K, Endo I and EndoII overlap at 41 ppm.

Figure 3. Pressure dependence of 13C spin−lattice relaxation time (T1) of methanol adsorbed in O-CNTs. Black circles represent the T1 values of the endohedral methanol, and blue squares represent the T1 values of the exohedral methanol. The lines present linear fits to these data points, and the regression coefficients for the least-squares data fittings of endohedral and exohedral relaxation times are 0.86 and 0.94, respectively.

interaction between the endohedral methanol and the nanotubes. 3.4. Temperature-Dependent Behavior of Endohedral and Exohedral Methanol. Variable-temperature experiments were carried out to investigate the behavior of methanol in CNTs. The temperature was first decreased from 298 to 153 K, and then increased to 383 K with an interval of 10 K, and the sample was thermally equilibrated for 15 min at each temperature. The 13C MAS NMR spectra in Figure 4a demonstrate that under 6 kPa there is only endohedral

Figure 4. Variable-temperature 13C MAS NMR spectra of O-CNTs after methanol adsorbed under the vapor pressure of 6 kPa at room temperature. (a) Spectra recorded in the range 298−153 K with decreasing steps of 10 K. (b) Spectra recorded in the range 153−383 K with increasing steps of 10 K. Each spectrum was accumulated by 100 scans, and the spinning rate was 7 kHz. The exponential line broadening for the spectra is 100 Hz. 7806



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Figure 5. Variable-temperature 13C MAS NMR spectra of O-CNTs after methanol adsorbed under vapor pressure 9 kPa at room temperature. (a) Spectra recorded in the range 298−153 K with decreasing steps of 10 K. (b) Spectra recorded in the range 153−383 K with increasing steps of 10 K. Each spectrum was accumulated by 100 scans, and the spinning rate was 7 kHz. The exponential line broadening for the spectra is 100 Hz.

Table 2 lists the 13C peak width for the endohedral methanol signals in Figures 4a and 5a as a function of temperature. The

tubes decreases as the distance apart from the walls of CNTs increases, a relative upfield shift is expected for molecules close to the walls of CNTs as compared to molecules near the center of CNTs. Thus, at 9 kPa, methanol inside CNTs may form a layered structure at low temperature (i.e., 273 K), indicating that there is fast exchange between the two endohedral methanols on the NMR time scale. This exchange slows as the temperature decreases so that these two signals are well distinguished. The chemical shifts of these two signals depend on both the temperature and the exchange. In addition, the populations of these two methanols evolve differently with temperature. The signal for Endo II first grows in intensity as the exchange slows, and then at 173 K two peaks

Table 2. 13C Peak Full-Width at Half-Height (ppm) for the Endohedral Methanol Signals of Both Types in Figures 4a and 5a in the Temperature Ranges from 273 to 153 K temperature/K 273 K endohedral (6 kPa) Endo I (9 kPa) Endo II (9 kPa) a

173 K

153 K

7.9

253 K 8.7

233 K 10

213 K 193 K 11.7

13.2

15.5

17.5

8.1 −a

8.7 9.0

10 7.0

11.7 6.9

13.2 7.3

15.5

17.5

Too weak to be measured.

corresponding full-width at half-height (fwhh) for methanol adsorption under 6 kPa in Figure 4a is almost identical to that of Endo I methanol in Figure 5a. It is therefore clear from Figure 4a, Figure 5a, and Table 2 that Endo I is the same endohedral methanol in Figure 4a, because they exhibit a similar chemical shift and fwhh. Moreover, the NMR linewidths of Endo I increase on cooling from 273 K, while the fwhh of the Endo II methanol in Figure 5a is narrower and remains constant for about 7 ppm, indicating that the molecular motion of the former is slower as compared to the latter. This hindering of motion is reflective of the strong interaction between Endo I methanol and CNTs, and it also indicates that this interaction increases with decreasing temperature. The 13C chemical shift reflects the chemical environment of methanol in CNTs, so it is useful to help us understand the distribution of endohedral methanol. Because the shielding induced in the 7807



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Table 3. T1 Valuesa (s) for Both Types of Endohedral Methanol Signals in Figure 5a as a Function of Temperature temperature/K Endo I (9 kPa) Endo II (9 kPa)

298 K

273 K

253 K

233 K

213 K

193 K

173 K

153 K

0.195

0.251

0.292

0.355 0.842

0.396 1.758

0.508 1.976

0.540

0.567

Inversion recovery T1 data at each temperature point were obtained by fitting the 13C signal integral intensity after variable delay time τ. The regression coefficients for the least-squares data fittings of each T1 are higher than 0.90.

a

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for Endo I and Endo II overlap at 41 ppm, indicating that endohedral methanol molecules may condense at this temperature. However, it is found that the cooling process (Figure 5a) is irreversible as compared to the heating process (Figure 5b), which may reflect different chemistry experienced by endohedral methanol through these two routes. Further T1 measurements (Table 3) show that the T1 values increase with decreasing the temperature. In addition, T1 for Endo I is shorter than that for Endo II, indicating that Endo I methanol has a relatively strong interaction with the walls of CNTs and is from methanol close to the walls of CNTs.

Corresponding Author

*Tel.: (+86) 411-84379128. Fax: (+86) 411-84694447. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Yining Huang from the University of Western Ontario and Dr. Cheng Sun for fruitful discussion. This work was supported by grants from the National Natural Science Foundation of China (11079005, 21033009, and 21103183) and the Ministry of Science and Technology of China (2009CB623507).

4. CONCLUSIONS 13 C MAS NMR has been demonstrated to be a useful tool for investigating the molecular adsorption and confinement behaviors in CNTs. 13C-enriched methanol adsorption on OCNTs with opened ends at a variety of vapor pressures leads to two peaks at 32 and 44 ppm, which are assigned to endohedral and exohedral adsorption, respectively. Furthermore, preferential endohedral adsorption is observed, and the exohedral adsorption does not take place until the endohedral adsorption reaches saturation. Both experimental study and theoretical calculations show that the confined methanol molecules experience a relatively large upfield chemical shift due to the diamagnetic shielding induced in the tubes by the external magnetic field. The molecules absorbed in aggregated pores are shielded to a much less extent. 13C spin−lattice relaxation values of methanol inside CNTs show little dependence on pressure and are always smaller than those of the methanol molecules outside CNTs, indicating a strong interaction between CNTs and the encapsulated methanol. Furthermore, methanol molecules outside CNTs diffuse into the cavities of CNTs with decreasing the temperature and do not diffuse out of the tubes until the temperature reaches as high as 353 K. The confined methanol may form a layered structure at low temperature, one layer close to the walls of CNTs and the second near the center of CNTs. The former has significant interaction with the walls of the nanotubes as compared to the latter. Because CNTs are a family with different diameters and electronic structures, we anticipate that further study on gas adsorption in double-walled carbon nanotubes (DWNTs) and SWNTs will shed more light on the gas−CNT interaction and its dependence on the unique electronic structure of CNTs. This finding is important in promoting design of CNT-based catalysts and sensors.

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AUTHOR INFORMATION

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ASSOCIATED CONTENT

S Supporting Information *

TEM images and ESR result of O-CNTs, and optimized structures for DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org. 7808



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